Research ArticleNanoflake Manganese Oxide and Nickel-Manganese OxideSynthesized by Electrodeposition for Electrochemical Capacitor
Man Van Tran1 An The Ha2 and Phung My Loan Le12
1Department of Physical Chemistry Faculty of Chemistry University of Science VNUHCM 227 Nguyen Van Cu StreetDistrict 5 Ho Chi Minh City 70000 Vietnam2Applied Physical Chemistry Laboratory Faculty of Chemistry University of Science VNUHCM 227 Nguyen Van Cu StreetDistrict 5 Ho Chi Minh City 70000 Vietnam
Correspondence should be addressed to Phung My Loan Le lmlphunghcmuseduvn
Received 15 January 2015 Accepted 24 March 2015
Academic Editor Jumi Yun
Copyright copy 2015 Man Van Tran et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited
Nanoflake structures of electrochemical manganese oxide (EMD) and nickel mixed manganese oxide (NiMD) were directlydeposited on a stainless steel by using Chronoamperometry and Cyclic Voltammetry (CV) techniquesThe structure morphologyand capacitive behavior of EMDorNIMDnanoflake were affected by the electrodepositionmodes and deposition timeThe highestspecific capacitance (119862sp) was obtained for only two-minute deposition by both methods EMD nanoflakes electrodeposited byCV technique show higher specific capacitance values than those prepared by Chronoamperometry owing to its homogenous andhighly porous surface All EMD samples exhibited excellent cycle behavior less than 5 capacitance loss after 1000 cycles Ni mixedMnO
2was prepared at different Mn2+Ni2+ ratios for 2 minutes of electrodeposition The presence of Ni2+ ion enhanced the 119862sp
value at high charge-discharge rate due to the decrease of the charge transfer resistance The supercapacitor prototype of 2 cm times2 cm was assembled using EMD and NiMD as electrode material and tested at 1 Asdotgminus1
1 Introduction
Nanomaterials such as nanowires nanorods nanotubes andnanoparticles have attracted great interest due to their uniquephysical properties (electrical optical and magnetic) andcatalyst and chemical properties [1 2] Mostly used materialsfor supercapacitors include carbon materials conductingpolymers and transitions metal oxides [1] Manganese oxideshows excellent pseudocapacitive behavior with the large spe-cific capacitance over 700 Fg in spite of the theoretical valueof 1100 Fg [1 3 4] Manganese oxide is a promising elec-trode material in supercapacitor due to its low cost naturalabundance and being environmentally friendly compared toother pseudocapacitor materials [3] In recent years nanos-tructured manganese oxides have been thoroughly investi-gated for different potential applications such as catalysisfor synthetic organic chemistry water treatment and energystorage devices [1ndash3] Nanostructured manganese oxide wassynthesized by chemical methods and physical methods
However chemical methods have more benefits due to sim-plicity economical effectiveness and ability to control vari-ous structures andmorphologies Several methods have beensuggested like chemical reduction hydrothermal sonochem-istry electrochemical deposition and sol-gel method [3]Thestructural parameters such as crystal form defect chemistrymorphology porosity and texture play a crucial role indetermining and optimizing the electrochemical propertieswhen using MnO
2as electrode materials (Table 1) [3]
One of the most common methods is electrochemicaldeposition due to its abilities to control the film thicknessstructure and morphology [5ndash9] Pang et al potentiostati-cally prepared the first electrodeposited thin filmMnO
2elec-
trode for electrochemical capacitor (ECs) in 2000 and theirfinding had sparked strong interest among energy researchcommunity for its application in supercapacitor electrode [56] Such high specific capacitance value arises from the ionsinsertiondesertion within MnO
2structure and it depends
crucially on the particle size surface and porosity Most of
Hindawi Publishing CorporationJournal of NanomaterialsVolume 2015 Article ID 609273 12 pageshttpdxdoiorg1011552015609273
2 Journal of Nanomaterials
Table 1 Synthesis method physicochemical features and subsequent specific capacitance of crystalline MnO2
Hydrothermal [17] Platelike nanorod 120572-MnO2 100ndash150 72ndash160 (200mAsdotgminus1)High viscosity process [18] Rod-shaped 120572-MnO2 120574-MnO2 mdash 389 (10mVsdotsminus1)Room temperature precipitation [19] Rod-shaped 120575-MnO2 mdash 201Low temperature reduction [20] Nanoflower Cubic MnO2 (Fd3m) 2259 1215 (1000mAsdotgminus1)Sol-gel process [21] Nanorods 120574-MnO2 317 (100mAsdotgminus1)
following studies have focused on varying the depositionparameters in order to achieve the enhanced electrochemicalperformance [7ndash13]TheMnO
2thin films can be prepared by
anodiccathodic electrodeposition Cationic Mn2+ precursoris used in anodic oxidation while anionic MnO
4
minus (Mn7+)is used in cathodic reduction In comparison cathodicreduction offers more versatility as various metals could becodeposited during the deposition process Oxidation of themetallic substrate during anodic deposition could also beavoided [14ndash16]
In this workMnO2(EMD) andNimixedMnO
2(NiMD)
were electrochemically synthesized by Cyclic Voltammetry(CV) and Chronoamperometry (CA) and characterized forsupercapacitor application For Ni mixed MnO
2 the Ni
content of binary oxide was investigated to find out theoptimized capacitive value high power density and electro-chemical stability Material characterization was carried outby Transmission Electron Microscopy (TEM) Raman spec-troscopy Infrared Spectroscopy Cyclic Voltammetry (CV)and charge-discharge cycling Excellent capacitor values andstability of binary oxide Ni-MnO
2were demonstrated
2 Experimental
21 Synthesis Process All chemical reagents are analyticalgrades MnSO
4sdotH2O (99 Sigma Aldrich) Na
2SO4(99
Prolabo Chemical) and Ni(NO3)2sdot6H2O (Sigma Aldrich)
For MnO2electrochemical deposition aqueous solutions of
03MMnSO4and 005MNa
2SO4(solutionA)were prepared
in distilled water just before use The pH of solution A isneutral (pH sim 7)
Stainless steel (SS) sheet of grade 214with 2 cmtimes 2 cm sur-face and thickness of 05mm was used as substrate for elec-trodepositionThe EMD andNiMD samples were performedby using Biologic-MPG 2 potentiogalvanostat system atroom temperature in three-electrode cell The electrochem-ical cell includes a stainless steel as working electrode (WE)a titanium mesh as auxiliary electrode (AE) and a saturatedAgAgCl (in 3M KCl) electrode as reference electrode (RE)
EMD and NiMD materials were synthesized by Chrono-amperometry (CA) and Cyclic Voltammetry (CV) methodBy using CA method MnO
2thin film was deposited in
galvanostatic mode at 12 V versus AgAgCl on SS substrateIn CV method MnO
2thin film was formed during potential
sweep in range of 0-1 V versus AgAgCl (sat) at scan rateof 50mVs The deposition time of EMD samples wasinvestigated from 2 to 12 minutes for each method (Table 2)
Table 2 Abbreviation of EMD samples prepared by two electrode-position methods
Time (minutes) CV method CA method2 EMD-CV2 EMD-C24 EMD-CV4 EMD-C46 EMD-CV6 EMD-C68 EMD-CV8 EMD-C810 EMD-CV10 EMD-C10
22 Characterization The structure of electrodeposited sam-ples was characterized by using Raman spectroscopy andFourier Transform Infrared Spectrometer (FTIR) IFS 28Brucker The IR absorption was carried out over the wave-length range 400ndash4000 cmminus1 The surface morphology andparticle size were determined using a Scanning ElectronMicroscopy (SEM) JSM 6480 LV and Transmission ElectronMicroscopy (TEM) JEOL JEM 1400 Stylus Profilometer wascarried out by Dektak 6M equipment in order to determinethickness of MnO
2films The contact angle of thin film was
measured using Data Physics Optical Contact Angle OCA20in order to check the wetting property in aqueous solution
The capacitive behavior of thin film electrodes was eval-uated in 1M Na
2SO4electrolyte The electronic resistance of
thin film as well as assembled supercapacitor prototype wasstudied by electrochemical impedance spectroscopy (EIS) ina frequency range of 10minus5 to 10minus2Hz The three-electrode cellconsisted of thin film MnO
2on SS as working electrode
platinum wire as auxiliary electrode and electrode reference(AgAgCl for aqueous solution)The CVmeasurements werecarried out at in 0-1 V versus AgAgCl (sat) The 119862sp wascalculated by using the following equation
119862sp =119876
119898Δ119881 (1)
where 119876 charge area calculated from integration of half CVcurve 119898 the mass of electrode material and Δ119881 the widthof potential window
The specific capacitance (119862sp) of supercapacitor was alsoevaluated byGalvanostatic charge-dischargemethodThe 2 times2 cm supercapacitor prototypes containing electrodepositedmaterial on SS substrates used as current collector wereused for cycling test and were charged-discharged at currentdensity of 1 Asdotgminus1
Journal of Nanomaterials 3
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cmminus1)Tr
ansm
ittan
ceEMD-CV2EMD-C2
MnndashO
OHndashMn
ndashOH
Figure 1 Infrared spectra of EMD-C2 and EMD-CV2
40
35
30
25
20
15
10
5
0100 200 300 400 500 600 700 800 900
Wavenumber (cmminus1)
RS in
tens
ity (c
ps)
222
264
337
379
491 520
572
631
665
738
LIOH67PRN120574-MnO2
(EMD Pr = 29)
(a)
144
145
147
150
287
287
288
291
412
413
415
415
507
508
511
511
577
579
580
580
646
647
651
651
736
738
738
740
Sample 3Sample 1
Sample 2
200 600400 800 1000
Sample 4
Inte
nsity
(cou
nts)
Raman shift (cmminus1)
MnO2
Mn Ni = 3
Mn Ni = 1
Mn Ni = 1 3
(b)
Figure 2 Raman spectra of electrodeposited MnO2(EMD-C2) and NiMD with different Mn2+ Ni2+ ratios (3 1 1 1 1 3)
3 Results and Discussion
31 FTIR and Raman Characterization The structure of thinfilms EMD and NiMD was characterized by using IR andRaman spectroscopy Figure 1 showed the spectra of EMDdeposited in 2 minutes by two methods in 400ndash4000 cmminus1The samples were abbreviated as EMD-CV2 and EMD-C2
The vibration at 3400 cmminus1 indicated ndashOH vibrationmode of the water As thin film was prepared in the aqueoussolution a tunnel structure MnO
2can be formed during
the electrodeposited process Thus H2O molecules can be
intercalated in the tunnel of thin film [8]The vibration bandsat 1600 cmminus1 and 500 cmminus1 were assigned toMnndashO stretchingmodes of MnO
6octahedral sites These vibration modes are
similar to those of natural mineral nsutite samples (120574-MnO2
structure complex tunnel) but some features are not clearlydetermined [8]
An examination of Raman spectroscopy is presentedin Figure 2 for EMD and NiMD thin films with variousMn Ni ratios Compared to the study of Julien et al [22] thevibrationmode ofMnO
2thin film at 577 cmminus1 (]3) 640 cmminus1
(]2) and 740 cmminus1 (]1) corresponds to the stretching modes
of the MnO6octahedral The corresponding antisymmetry
stretching modes are recorded in the infrared spectrum at517 cmminus1 and 621 cmminus1 (Figure 2) The weak Raman bands at511 415 297 and 145 cmminus1 are assigned to the deformationmodes of themetal-oxygen chain ofMnndashOndashMn in theMnO
2
octahedral lattice The Ni mixed MnO2(NiMD) sample
exhibited similarly Raman vibration bands However theshifted modes were clearly observed for example the blueshift for (]2) and (]3) vibration and the red shift for (]1)vibration Also the vibration peaks at lower wavenumberwere shifted (Figure 2) The results showed that the presenceof Ni ion affected the local structure of MnO
2 typically
the short range environment of oxygen coordination aroundtransition metal cations in MnO
2lattice
32 Morphology and Wettability Studies Figures 3 and 4show SEM images of EMD and NiMD thin filmsThe surfaceporosity and nanoflakes dimension were also affected bydeposit time and the depositionmode A slight difference wasobserved when the electrodepositionmode changed (Figures3(a) and 3(c)) Generally the surface morphology of thinfilms is composed entirely of nanoscale fiber (nanoflakes)
4 Journal of Nanomaterials
(a) (b)
(c) (d)
Figure 3 SEM images of EMD-C12 (a) EMD-CV2 (b) EMD-CV12 (c) and EMD-C2 (d)
(a) (b)
(c) (d)
Figure 4 SEM images of NiMD prepared by CA (a b) and by CV (c d) in 2 minutes
Journal of Nanomaterials 5
(a) (b)
(c) (d)
Figure 5 TEM images of EMD and NiMD prepared by CA method (a c) and by CV method (b d) in two minutes
An irregular interconnection of nanofibers formed a highlyporous network By using CV method the nanofiber slightlygrows up The nanoflakes dimension is about 100ndash120 nmin length compared to 40ndash60 nm of which prepared by CAmethod As the deposition time is long the nanofibers growup quickly and the interconnection points seem to be a starform (Figures 3(c) and 3(d) for 12minutes electrodeposition)Due to the variation of surface porosity and nanofibermorphology the specific capacitance behavior is expectedlychanged [1]
In case of Ni mixed MnO2 the surface morphologies
seem to be similar to those of MnO2films (Figure 4) Using
CV method exhibits homogenous surface covered of well-formed nanoflakes
TEM images were performed for Ni mixed MnO2
(NiMD) at the same electrodeposition conditions (Figure 5)In general it was observed that film structure prepared in twominutes is dense The nanoflakes seem to be very fine andtend to interweave to make a network
Wettability test is carried out in order to investigatethe interaction between liquid and thin film surface As
the wettability is high the small contact angle (120579) resultedand the surface is hydrophilic In contrast the surface ishydrophobic Both hydrophilic and hydrophobic propertiescan be applied to many fields from medicine to engineeringespecially in energy storages [1] Figure 6 shows the contactangle measured for MnO
2films prepared by CA and CV
method Both films are expectedly hydrophilic due to thepresence of ndashOH group on the surface and in the structureIndeed the results in Figure 6 indicated that contact angleis less than 90∘ In addition EMD-CV2 thin film is morehydrophilic than EMD-C2The hydrophilicity is essential forelectrochemical reaction on the electrodeaqueous electrolyteinterface
33 Electrochemical Properties of Electrodeposited Thin FilmThe capacitance behavior of MnO
2and NiMD thin films was
tested using CV in potential range of 0-1 V at 100mVs Fig-ure 7 shows CV curves of MnO
2thin films prepared by CA
and CV method in 1M Na2SO4
The specific capacitance (119862sp) was calculated by using(1) EMD was prepared by CA method EMD-CA exhibited
6 Journal of Nanomaterials
CA left 195∘CA right 188∘ EMD-CV2
(a)
CA left 314∘CA right 319∘ EMD-C2
(b)
Figure 6 Water contact angle of EMD-CV2 (a) and EMD-C2 (b)
Curr
ent d
ensit
y (m
Ac
m2)
EMD-C2EMD-C4EMD-C6
EMD-C8EMD-C10EMD-C12
6
4
2
0
minus2
minus4
minus60 02 04 06 08 1
Voltage versus AgAgCl (3M) (V)
(a)
EMD-CV2
EMD-CV4
EMD-CV6
EMD-CV8
EMD-CV10
EMD-CV12
6
4
2
0
minus2
minus4
minus60 02 04 06 08 1
Curr
ent d
ensit
y (m
Ac
m2)
Voltage versus AgAgCl (3M) (V)
(b)
0
50
100
150
200
250
300
350
05 1 15 2 25 3
EMD-CV EMD
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
Film thickness (120583m)
2min
4min
6min
10min
12min
(c)
Figure 7 CV curves of EMD-C and EMD-CV samples in 2M Na2SO4at 100mVs (a b) Plot of 119862sp and thickness of EMD thin films
corresponding to deposition time (c)
Journal of Nanomaterials 7
350
300
250
200
150
1000 200 400 600 800 1000
Number of cycles
Spec
ific c
apac
itanc
e (F
g)
EMD-C2EMD-CV2
Figure 8 Electrochemical stability of EMD-C2 and EMD-CV2 at scan rate 100mVs
the119862sp values that are 250 138 91 79 62 and 48 Fg for EMD-C2 EMD-C4 EMD-C6 EMD-C8 EMD-C10 and EMD-C12 respectively With CV method the 119862sp values are 317112 95 79 60 and 53 Fg for EMD-CV2 EMD-CV4 EMD-CV6 EMD-CV8 EMD-CV10 and EMD-CV12 respectivelyPrevious studies [2 3 6] show that capacitances ofmanganeseoxide synthesized by electrochemical deposition were about200ndash300 Fg
The highest 119862sp was obtained at two minutes of elec-trodeposition for both methods (Figure 7(c)) The thicknessofMnO
2film is about 055 120583mThus it can be concluded that
the thin and porous film was only obtained in a short depo-sition time When the deposition time is long the nanoflakesgrew up fast and their interconnection makes the film morecompact than a short-time deposition Thus penetration ofelectrolyte into the film as well as the specific capacitancedecreases due to the increase of electric resistance The smallthickness of MnO
2film is necessarily required in order to
obtain the good electric conductivity and specific capacitance[4 12]
The electrochemical stability of thin films was evaluatedFigure 8 shows the specific capacitance (119862sp) of EMD-C2and EMD-CV2 measured at a scan rate of 100mVs AmongEMD-C samples the highest initial capacitance was obtainedfor EMD-C2 The 119862sp value keeps constant about 170 Fgafter 200 cycles For EMD-CV2 the 119862sp decreases slightlyafter 100 cycles before increasing significantly to 330 Fg inthe following cycles After 1000 cycles the fading of 119862sp ofEMD-C2 and EMD-CV2 was 23 and 5 respectively (seeFigure 10) Thus the nanostructured EMD-CV2 seems to bebetter for supercapacitor application than EMD-C2 In factthis fade observed for EMD samples can be explained bythe structure evolution in aqueous solution so the metalion doping into MnO
2lattice would improve the structural
stability
The two minutes electrodeposition was chosen to studythe Ni mixed MnO
2(NiMD) at different Mn2+ Ni2+ ratios
Figures 9(a) and 9(b) show theCVelectrochemical propertiesof NiMD samples prepared by two deposition methods Incomparison to the EMD samples the NiMD exhibited thesame shape type of voltammogram but larger magnitudecurrent It can be observed that the CVs are more rectangularand symmetrical but lower in current magnitude at highMn2+Ni2+ ratio than at the low ratio Thus the charac-teristics indicate the ideal capacitive behavior with highreversibility of NiMD sample The highest 119862sp was obtainedfor NiMD samples at Mn2+Ni2+ = 1 1 at all scan rates (Fig-ures 9(c) and 9(d))
The electrochemical stability of NiMD thin films pre-pared at Mn2+Ni2+ = 1 1 was also evaluated (Figure 11) Incomparison to the EMD at the same deposition conditionNiMD samples exhibited higher 119862sp about 500 Fg ver-sus 300 Fg By changing the electrodeposition modes theNiMD-CV shows the 119862sp increase after 500 cycles from430 Fg to 500 Fg while NiMD-CA keeps constant during800 cycles after that a slight fading is observed at the finalcycles After 1000 cycles at 100mVs the fading of 119862sp is lessthan 7 for NiMD samples The fact that Ni ion doping intoMnO2lattice increases119862sp as well as electrochemical stability
was briefly reported by Rajendra Prasad and Miura [23] Inthis work the highest 119862sp was obtained about 480 Fg at100mVs for NiMD sample (65 wt MnO
2 35 wt NiO)
34 Electrochemical Properties of Supercapacitor PrototypeElectrochemical impedance spectroscopy (EIS) measure-ments were also carried out in the frequency range of100 kHzndash01Hz Figure 11 shows the Nyquist plots of EMDthin films prepared at various deposition times by twomethods
8 Journal of Nanomaterials
minus100
minus50
0
50
100
0 02 04 06 08 1
Spec
ific c
urre
nt (A
middotgminus1)
E versus AgAgClNaCl (V)
3 13 2 1 0 (EMD-CV2)1 1
1 2
(a)
minus30
minus20
minus10
0
10
20
30
0 02 04 06 08 1E versus AgAgClNaCl (V)
Spec
ific c
urre
nt (A
middotgminus1)
3 13 2 1 0 (EMD-C2)1 1
1 2
(b)
100
200
300
400
500
600
0 50 100 150 200 250
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
3 13 2 1 21 0
1 1
Scan rate (mVmiddotsminus1)
(c)
100
200
300
400
500
600
0 50 100 150 200 250
3 13 2 1 21 0
1 1
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
Scan rate (mVmiddotsminus1)
(d)
Figure 9 CV curves of Ni mixed MnO2(NiMD samples) prepared by CV method (a) and CA method (b) in 2M Na
2SO4at 50mVs Plot
of 119862sp as a function of scan rate (c d)
TheNyquist plot shows a high-frequency intercept on thereal axis corresponding to the electrolyte resistance (119877
Ω) a
semicircle is considered as a parallel combination of chargetransfer resistance (119877ct) and double-layer capacitance (119862dl)with a linear region at low-frequency range In the low-frequency region linear part of the plot exhibits an anglebetween 45 and 90∘
The EIS data was analyzed by the electrical equivalentcircuit which consists of (119877
Ω) the electrolyte resistance (119877ct)
charge transfer resistance constant phase element (CPE)
used instead of double-layer capacitance (119862dl) Warburg(W) arising from a diffusion controlled process at low-frequency and CPE assigned to pseudocapacitance of thematerial in the low-frequency because of nonideal capacitivebehavior The starting nonzero intercept at Z1015840 at beginningof semicircle is identical in all the curves and its electricalresistance of Na
2SO4around 1Ω For CAmethod the charge
transfer values of EMD-C2 EMD-C6 EMD-C8 and EMD-C12 are 43Ω 102Ω 116Ω and 126Ω respectively For CVmethod the charge transfer values of EMD-CV2 EMD-CV4
Journal of Nanomaterials 9
100
200
300
400
500
600
700
0 200 400 600 800 1000
CV methodChronoamperometry
Number of cycles
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
Figure 10 Electrochemical stability of NiMD samples prepared at Mn2+Ni2+ = 1 1 by two methods at scan rate 100mVs
0
10
20
30
40
50
60
70
80
0 10 20 30 40 50 60 70 80
EMD-CV2EMD-CV4
EMD-CV6EMD-CV12
0
20
40
60
80
100
120
0 20 40 60 80 100 120 160140
EMD-C2EMD-C6
EMD-C8EMD-C12
CPE1
CPE2
W
Real (Z998400) (Ohm) Real (Z998400) (Ohm)
Rs
Rct
minuslm
(Z998400998400
) (O
hm)
minuslm
(Z998400998400
) (O
hm)
Figure 11 Nyquist plot of EMD thin films prepared by two methods and equivalent circuit model used for data fitting
EMD-CV8 and EMD-CV12 are 40Ω 42Ω 47Ω and 55Ωrespectively It means that the increase of deposition time (orfilm thickness) decreases the charge transfer resistanceThusa high resistance caused a difficult charge transfer electronbetween interface electrolyte and interface electrode
At the same deposition conditions the charge transferresistance ofNiMD samples decreases significantly comparedto EMD (Figure 12)
As the Mn2+Ni2+ increase the semicircle diameterbecomes smaller The lowest resistance was obtained forNiIMD at Mn2+Ni2+ = 1 1 about 4Ω versus 10Ω at Mn2+Ni2+ = 1 2 (Figure 13)
The Ni ion addition to the MnO2lattice would improve
the electrical resistivity of MnO2film in addition to enhance
the specific capacitance It maybe also enhanced the powerdensity as well as reversibility of the supercapacitior at
10 Journal of Nanomaterials
0
10
20
30
40
50
60
0 10 20 30 40 50 60Real (Z) (Ohm)
minuslm
(Z) (
Ohm
)
3 13 2 1 01 2
1 1
Figure 12 Nyquist plot of NiMD samples prepared by CA method compared to EMD-C2
0
2
4
6
8
10
0 2 4 6 8 10 12
minuslm
(Z) (
Ohm
)
Real (Z) (Ohm)
1 21 1
Figure 13 Nyquist plot (zoom part) of NiMD samples at Mn2+ Ni2+ = 1 1 and 1 2
high charge-discharge rate Symmetrical capacitor prototypesof 2 cm times 2 cm were assembled with two thin films ofelectrodes EMD-CV2 and NiMD-CV at Mn2+Ni2+ = 1 1and characterized by charge-discharge cycling at very highcurrent 1 Asdotgminus1
Figure 14 shows charge-discharge values of EMD andNIMD sample at current 1 Asdotgminus1 The efficiency of charge-discharge cycling is about 09 Discharge 119862sp values werecalculated by using the current density discharge time andthe weight of material 119862sp values obtained from Figure 14for EMD-CV2 and NiMD-CV are 10 Fsdotgminus1 and 25 Fsdotgminus1
As expected supercapacitor of NiMD-CV based electrodeexhibits a higher power density than EMD-CV2
The stability of supercapacitor based on NiMD-CV andEMD-CV2 was also tested for a large number of cycles (upto 10000 cycles) at 2 Asdotgminus1 A 119862sp fading of 25 and 14was observedwithNiMD-CVorEMD-CV2based capacitorsrespectively after 10000 charge-discharge cycles There is aninitial large 119862sp decrease and then the 119862sp remained almostconstantTheNiMD-CVexhibited expectedly higher stabilitythan EMD-CV2 based capacitorThus it would be promisingfor the long-term capacitor application
Journal of Nanomaterials 11
(a)
0
5
10
15
20
25
30
35
40
0 20 40 60 80 100Cycles number
NiMD-CVEMD-CV
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
(b)
Figure 14 The prototype (left) and the charge-discharge test at 1 Asdotgminus1 using thin film electrode of NiMD-CV and EMD-CV2 (right)
4 Conclusions
Nanoflakes EMD and NiMD with high porosity were pre-pared by two electrochemical depositions modes The thinfilms exhibited the 120574-MnO
2based structure The Ni addition
to MnO2lattice affected the local structure of 120574-MnO
2that
was evidently observed by using Raman spectroscopy TheEMD-CV exhibited better electrochemical behavior (119862sp andcharge-discharge stability) than EMD-C due to the homoge-nous film forming and high porosity during the reduction-oxidation sweep With short-time deposition (2 minutes)EMD thin films show the highest 119862sp and lowest chargetransfer resistance Compared to EMD electrodes preparedat the same deposition NiMD films enhance significantly the119862sp values as well as electrochemical stability The highest119862sp value was 500 Fsdotgminus1 compared to 300 Fsdotgminus1 of EMDThe enhanced electrectrochemical performance of NiMDcan be explained by the increase of thin film conductivitywith the presence of ion Ni2+ Moreover high power densityand excellent stability of assembled supercapacitor based onNiMDelectrodematerials were demonstrated (119862sp of 25 Fsdotg
minus1
at 1 Asdotgminus1 14 capacitance fade for 10000 cycles)
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors acknowledge funding from VNU-HCM underGrant HS2013-76-01 The authors would like to thank Officeof Naval Research Global (ONRG) for Grant N62909-13-1-N235 The authors would like to thank the technical supportof Horibarsquos Representative Company in Vietnam
References
[1] H Xia M O Lai and L Lu ldquoNanostructured manganese oxidethin films as electrode material for supercapacitorsrdquo JOM vol63 no 1 pp 54ndash59 2011
[2] X Zhao B M Sanchez P J Dobson and P S Grant ldquoThe roleof nanomaterials in redox-based supercapacitors for next gen-eration energy storage devicesrdquoNanoscale vol 3 no 3 pp 839ndash855 2011
[3] W Wei X Cui W Chen and D G Ivey ldquoManganese oxide-based materials as electrochemical supercapacitor electrodesrdquoChemical Society Reviews vol 40 no 3 pp 1697ndash1721 2011
[4] J E Post ldquoManganese oxide minerals crystal structures andeconomic and environmental significancerdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 96 no 7 pp 3447ndash3454 1999
[5] S C Pang M A Anderson and T W Chapman ldquoNovel elec-trode materials for thin-film ultracapacitors comparison ofelectrochemical properties of sol-gel-derived and electrode-posited manganese dioxiderdquo Journal of the ElectrochemicalSociety vol 147 no 2 pp 444ndash450 2000
[6] S C Pang and M A Anderson ldquoNovel electrode materials forelectrochemical capacitors Part II Material characterization ofsol-gel-derived and electrodeposited manganese dioxide thinfilmsrdquo Journal of Materials Research vol 15 no 10 pp 2096ndash2106 2000
[7] N Nagarajan H Humadi and I Zhitomirsky ldquoCathodic elec-trodeposition of MnO
ior of manganese dioxidestainless steel electrodes at differentdeposition currentsrdquoAmerican Journal ofMaterials Science vol2 no 2 pp 11ndash14 2012
[9] C Xu B Li H Du F Kang and Y Zeng ldquoElectrochemicalproperties of nanosized hydrous manganese dioxide synthe-sized by a self-reacting microemulsion methodrdquo Journal ofPower Sources vol 180 no 1 pp 664ndash670 2008
12 Journal of Nanomaterials
[10] X Hu X Lin Z Ling Y Li and X Fu ldquoFabrication and char-acteristics of galvanostatic electrodeposited MnO
2on porous
nickel from etched aluminiumrdquo Electrochimica Acta vol 138pp 132ndash138 2014
[11] S Hassan M Suzuki and A A El-Moneim ldquoEffect of Ag-doping on the capacitive behavior of amorphous manganesedioxide electrodesrdquo Electrical and Electronic Engineering vol 2no 2 pp 18ndash22 2012
[12] T Shinomiya V Gupta and N Miura ldquoEffects of electrochem-ical-deposition method and microstructure on the capacitivecharacteristics of nano-sized manganese oxiderdquo ElectrochimicaActa vol 51 no 21 pp 4412ndash4419 2006
[13] M Minakshi P Singh T B Issa S Thurgate and R De MarcoldquoLithium insertion into manganese dioxide electrode inMnO
2
Zn aqueous battery Part II Comparison of the behavior ofEMD and battery grade MnO
2in ZnmdashMnO
2mdash aqueous LiOH
electrolyterdquo Journal of Power Sources vol 138 no 1-2 pp 319ndash322 2004
[14] T Yousefi R Davarkhah A N Golikand and M H Mash-hadizadeh ldquoSynthesis characterization and supercapacitorstudies of manganese (IV) oxide nanowiresrdquo Materials Sciencein Semiconductor Processing vol 16 no 3 pp 868ndash876 2013
[15] J Li and I Zhitomirsky ldquoCathodic electrophoretic depositionof manganese dioxide filmsrdquo Colloids and Surfaces A Physico-chemical and Engineering Aspects vol 348 no 1ndash3 pp 248ndash2532009
[16] J Wei M Cheong N Nagarajan and I Zhitomirsky ldquoCathodicelectrodeposition of manganese oxides for electrochemicalsupercapacitorsrdquo ECS Transactions vol 3 no 37 pp 1ndash9 2007
[17] V Subramanian H Zhu R Vajtai P M Ajayan and B WeildquoHydrothermal synthesis and pseudocapacitance properties ofMnO
2nanostructuresrdquoThe Journal of Physical Chemistry B vol
109 no 43 pp 20207ndash20214 2005[18] C Ye Z M Lin and S Z Hui ldquoElectrochemical and capac-
itance properties of rod-shaped MnO2for supercapacitorrdquo
Journal of the Electrochemical Society vol 152 no 6 pp A1272ndashA1278 2005
[19] Q T Qu P Zhang B Wang et al ldquoElectrochemical perform-ance of MnO
2nanorods in neutral aqueous electrolytes as a
cathode for asymmetric supercapacitorsrdquo The Journal of Phys-ical Chemistry C vol 113 no 31 pp 14020ndash14027 2009
[20] J P Ni W C Lu L M Zhang B H Yue X F Shang and Y LvldquoLow-temperature synthesis of monodisperse 3D manganeseoxide nanoflowers and their pseudocapacitance propertiesrdquoTheJournal of Physical Chemistry C vol 113 no 1 pp 54ndash60 2009
[21] X Wang A Yuan and Y Wang ldquoSupercapacitive behaviorsand their temperature dependence of sol-gel synthesized nanos-tructured manganese dioxide in lithium hydroxide electrolyterdquoJournal of Power Sources vol 172 no 2 pp 1007ndash1011 2007
[22] C Julien M Massot S Rangan M Lemal and D GuyomardldquoStudy of structural defects in 120574-MnO
2byRaman spectroscopyrdquo
Journal of Raman Spectroscopy vol 33 no 4 pp 223ndash228 2002[23] K Rajendra Prasad and N Miura ldquoElectrochemically synthe-
sized MnO2-based mixed oxides for high performance redox
Hydrothermal [17] Platelike nanorod 120572-MnO2 100ndash150 72ndash160 (200mAsdotgminus1)High viscosity process [18] Rod-shaped 120572-MnO2 120574-MnO2 mdash 389 (10mVsdotsminus1)Room temperature precipitation [19] Rod-shaped 120575-MnO2 mdash 201Low temperature reduction [20] Nanoflower Cubic MnO2 (Fd3m) 2259 1215 (1000mAsdotgminus1)Sol-gel process [21] Nanorods 120574-MnO2 317 (100mAsdotgminus1)
following studies have focused on varying the depositionparameters in order to achieve the enhanced electrochemicalperformance [7ndash13]TheMnO
2thin films can be prepared by
anodiccathodic electrodeposition Cationic Mn2+ precursoris used in anodic oxidation while anionic MnO
4
minus (Mn7+)is used in cathodic reduction In comparison cathodicreduction offers more versatility as various metals could becodeposited during the deposition process Oxidation of themetallic substrate during anodic deposition could also beavoided [14ndash16]
In this workMnO2(EMD) andNimixedMnO
2(NiMD)
were electrochemically synthesized by Cyclic Voltammetry(CV) and Chronoamperometry (CA) and characterized forsupercapacitor application For Ni mixed MnO
2 the Ni
content of binary oxide was investigated to find out theoptimized capacitive value high power density and electro-chemical stability Material characterization was carried outby Transmission Electron Microscopy (TEM) Raman spec-troscopy Infrared Spectroscopy Cyclic Voltammetry (CV)and charge-discharge cycling Excellent capacitor values andstability of binary oxide Ni-MnO
2were demonstrated
2 Experimental
21 Synthesis Process All chemical reagents are analyticalgrades MnSO
4sdotH2O (99 Sigma Aldrich) Na
2SO4(99
Prolabo Chemical) and Ni(NO3)2sdot6H2O (Sigma Aldrich)
For MnO2electrochemical deposition aqueous solutions of
03MMnSO4and 005MNa
2SO4(solutionA)were prepared
in distilled water just before use The pH of solution A isneutral (pH sim 7)
Stainless steel (SS) sheet of grade 214with 2 cmtimes 2 cm sur-face and thickness of 05mm was used as substrate for elec-trodepositionThe EMD andNiMD samples were performedby using Biologic-MPG 2 potentiogalvanostat system atroom temperature in three-electrode cell The electrochem-ical cell includes a stainless steel as working electrode (WE)a titanium mesh as auxiliary electrode (AE) and a saturatedAgAgCl (in 3M KCl) electrode as reference electrode (RE)
EMD and NiMD materials were synthesized by Chrono-amperometry (CA) and Cyclic Voltammetry (CV) methodBy using CA method MnO
2thin film was deposited in
galvanostatic mode at 12 V versus AgAgCl on SS substrateIn CV method MnO
2thin film was formed during potential
sweep in range of 0-1 V versus AgAgCl (sat) at scan rateof 50mVs The deposition time of EMD samples wasinvestigated from 2 to 12 minutes for each method (Table 2)
Table 2 Abbreviation of EMD samples prepared by two electrode-position methods
Time (minutes) CV method CA method2 EMD-CV2 EMD-C24 EMD-CV4 EMD-C46 EMD-CV6 EMD-C68 EMD-CV8 EMD-C810 EMD-CV10 EMD-C10
22 Characterization The structure of electrodeposited sam-ples was characterized by using Raman spectroscopy andFourier Transform Infrared Spectrometer (FTIR) IFS 28Brucker The IR absorption was carried out over the wave-length range 400ndash4000 cmminus1 The surface morphology andparticle size were determined using a Scanning ElectronMicroscopy (SEM) JSM 6480 LV and Transmission ElectronMicroscopy (TEM) JEOL JEM 1400 Stylus Profilometer wascarried out by Dektak 6M equipment in order to determinethickness of MnO
2films The contact angle of thin film was
measured using Data Physics Optical Contact Angle OCA20in order to check the wetting property in aqueous solution
The capacitive behavior of thin film electrodes was eval-uated in 1M Na
2SO4electrolyte The electronic resistance of
thin film as well as assembled supercapacitor prototype wasstudied by electrochemical impedance spectroscopy (EIS) ina frequency range of 10minus5 to 10minus2Hz The three-electrode cellconsisted of thin film MnO
2on SS as working electrode
platinum wire as auxiliary electrode and electrode reference(AgAgCl for aqueous solution)The CVmeasurements werecarried out at in 0-1 V versus AgAgCl (sat) The 119862sp wascalculated by using the following equation
119862sp =119876
119898Δ119881 (1)
where 119876 charge area calculated from integration of half CVcurve 119898 the mass of electrode material and Δ119881 the widthof potential window
The specific capacitance (119862sp) of supercapacitor was alsoevaluated byGalvanostatic charge-dischargemethodThe 2 times2 cm supercapacitor prototypes containing electrodepositedmaterial on SS substrates used as current collector wereused for cycling test and were charged-discharged at currentdensity of 1 Asdotgminus1
Journal of Nanomaterials 3
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cmminus1)Tr
ansm
ittan
ceEMD-CV2EMD-C2
MnndashO
OHndashMn
ndashOH
Figure 1 Infrared spectra of EMD-C2 and EMD-CV2
40
35
30
25
20
15
10
5
0100 200 300 400 500 600 700 800 900
Wavenumber (cmminus1)
RS in
tens
ity (c
ps)
222
264
337
379
491 520
572
631
665
738
LIOH67PRN120574-MnO2
(EMD Pr = 29)
(a)
144
145
147
150
287
287
288
291
412
413
415
415
507
508
511
511
577
579
580
580
646
647
651
651
736
738
738
740
Sample 3Sample 1
Sample 2
200 600400 800 1000
Sample 4
Inte
nsity
(cou
nts)
Raman shift (cmminus1)
MnO2
Mn Ni = 3
Mn Ni = 1
Mn Ni = 1 3
(b)
Figure 2 Raman spectra of electrodeposited MnO2(EMD-C2) and NiMD with different Mn2+ Ni2+ ratios (3 1 1 1 1 3)
3 Results and Discussion
31 FTIR and Raman Characterization The structure of thinfilms EMD and NiMD was characterized by using IR andRaman spectroscopy Figure 1 showed the spectra of EMDdeposited in 2 minutes by two methods in 400ndash4000 cmminus1The samples were abbreviated as EMD-CV2 and EMD-C2
The vibration at 3400 cmminus1 indicated ndashOH vibrationmode of the water As thin film was prepared in the aqueoussolution a tunnel structure MnO
2can be formed during
the electrodeposited process Thus H2O molecules can be
intercalated in the tunnel of thin film [8]The vibration bandsat 1600 cmminus1 and 500 cmminus1 were assigned toMnndashO stretchingmodes of MnO
6octahedral sites These vibration modes are
similar to those of natural mineral nsutite samples (120574-MnO2
structure complex tunnel) but some features are not clearlydetermined [8]
An examination of Raman spectroscopy is presentedin Figure 2 for EMD and NiMD thin films with variousMn Ni ratios Compared to the study of Julien et al [22] thevibrationmode ofMnO
2thin film at 577 cmminus1 (]3) 640 cmminus1
(]2) and 740 cmminus1 (]1) corresponds to the stretching modes
of the MnO6octahedral The corresponding antisymmetry
stretching modes are recorded in the infrared spectrum at517 cmminus1 and 621 cmminus1 (Figure 2) The weak Raman bands at511 415 297 and 145 cmminus1 are assigned to the deformationmodes of themetal-oxygen chain ofMnndashOndashMn in theMnO
2
octahedral lattice The Ni mixed MnO2(NiMD) sample
exhibited similarly Raman vibration bands However theshifted modes were clearly observed for example the blueshift for (]2) and (]3) vibration and the red shift for (]1)vibration Also the vibration peaks at lower wavenumberwere shifted (Figure 2) The results showed that the presenceof Ni ion affected the local structure of MnO
2 typically
the short range environment of oxygen coordination aroundtransition metal cations in MnO
2lattice
32 Morphology and Wettability Studies Figures 3 and 4show SEM images of EMD and NiMD thin filmsThe surfaceporosity and nanoflakes dimension were also affected bydeposit time and the depositionmode A slight difference wasobserved when the electrodepositionmode changed (Figures3(a) and 3(c)) Generally the surface morphology of thinfilms is composed entirely of nanoscale fiber (nanoflakes)
4 Journal of Nanomaterials
(a) (b)
(c) (d)
Figure 3 SEM images of EMD-C12 (a) EMD-CV2 (b) EMD-CV12 (c) and EMD-C2 (d)
(a) (b)
(c) (d)
Figure 4 SEM images of NiMD prepared by CA (a b) and by CV (c d) in 2 minutes
Journal of Nanomaterials 5
(a) (b)
(c) (d)
Figure 5 TEM images of EMD and NiMD prepared by CA method (a c) and by CV method (b d) in two minutes
An irregular interconnection of nanofibers formed a highlyporous network By using CV method the nanofiber slightlygrows up The nanoflakes dimension is about 100ndash120 nmin length compared to 40ndash60 nm of which prepared by CAmethod As the deposition time is long the nanofibers growup quickly and the interconnection points seem to be a starform (Figures 3(c) and 3(d) for 12minutes electrodeposition)Due to the variation of surface porosity and nanofibermorphology the specific capacitance behavior is expectedlychanged [1]
In case of Ni mixed MnO2 the surface morphologies
seem to be similar to those of MnO2films (Figure 4) Using
CV method exhibits homogenous surface covered of well-formed nanoflakes
TEM images were performed for Ni mixed MnO2
(NiMD) at the same electrodeposition conditions (Figure 5)In general it was observed that film structure prepared in twominutes is dense The nanoflakes seem to be very fine andtend to interweave to make a network
Wettability test is carried out in order to investigatethe interaction between liquid and thin film surface As
the wettability is high the small contact angle (120579) resultedand the surface is hydrophilic In contrast the surface ishydrophobic Both hydrophilic and hydrophobic propertiescan be applied to many fields from medicine to engineeringespecially in energy storages [1] Figure 6 shows the contactangle measured for MnO
2films prepared by CA and CV
method Both films are expectedly hydrophilic due to thepresence of ndashOH group on the surface and in the structureIndeed the results in Figure 6 indicated that contact angleis less than 90∘ In addition EMD-CV2 thin film is morehydrophilic than EMD-C2The hydrophilicity is essential forelectrochemical reaction on the electrodeaqueous electrolyteinterface
33 Electrochemical Properties of Electrodeposited Thin FilmThe capacitance behavior of MnO
2and NiMD thin films was
tested using CV in potential range of 0-1 V at 100mVs Fig-ure 7 shows CV curves of MnO
2thin films prepared by CA
and CV method in 1M Na2SO4
The specific capacitance (119862sp) was calculated by using(1) EMD was prepared by CA method EMD-CA exhibited
6 Journal of Nanomaterials
CA left 195∘CA right 188∘ EMD-CV2
(a)
CA left 314∘CA right 319∘ EMD-C2
(b)
Figure 6 Water contact angle of EMD-CV2 (a) and EMD-C2 (b)
Curr
ent d
ensit
y (m
Ac
m2)
EMD-C2EMD-C4EMD-C6
EMD-C8EMD-C10EMD-C12
6
4
2
0
minus2
minus4
minus60 02 04 06 08 1
Voltage versus AgAgCl (3M) (V)
(a)
EMD-CV2
EMD-CV4
EMD-CV6
EMD-CV8
EMD-CV10
EMD-CV12
6
4
2
0
minus2
minus4
minus60 02 04 06 08 1
Curr
ent d
ensit
y (m
Ac
m2)
Voltage versus AgAgCl (3M) (V)
(b)
0
50
100
150
200
250
300
350
05 1 15 2 25 3
EMD-CV EMD
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
Film thickness (120583m)
2min
4min
6min
10min
12min
(c)
Figure 7 CV curves of EMD-C and EMD-CV samples in 2M Na2SO4at 100mVs (a b) Plot of 119862sp and thickness of EMD thin films
corresponding to deposition time (c)
Journal of Nanomaterials 7
350
300
250
200
150
1000 200 400 600 800 1000
Number of cycles
Spec
ific c
apac
itanc
e (F
g)
EMD-C2EMD-CV2
Figure 8 Electrochemical stability of EMD-C2 and EMD-CV2 at scan rate 100mVs
the119862sp values that are 250 138 91 79 62 and 48 Fg for EMD-C2 EMD-C4 EMD-C6 EMD-C8 EMD-C10 and EMD-C12 respectively With CV method the 119862sp values are 317112 95 79 60 and 53 Fg for EMD-CV2 EMD-CV4 EMD-CV6 EMD-CV8 EMD-CV10 and EMD-CV12 respectivelyPrevious studies [2 3 6] show that capacitances ofmanganeseoxide synthesized by electrochemical deposition were about200ndash300 Fg
The highest 119862sp was obtained at two minutes of elec-trodeposition for both methods (Figure 7(c)) The thicknessofMnO
2film is about 055 120583mThus it can be concluded that
the thin and porous film was only obtained in a short depo-sition time When the deposition time is long the nanoflakesgrew up fast and their interconnection makes the film morecompact than a short-time deposition Thus penetration ofelectrolyte into the film as well as the specific capacitancedecreases due to the increase of electric resistance The smallthickness of MnO
2film is necessarily required in order to
obtain the good electric conductivity and specific capacitance[4 12]
The electrochemical stability of thin films was evaluatedFigure 8 shows the specific capacitance (119862sp) of EMD-C2and EMD-CV2 measured at a scan rate of 100mVs AmongEMD-C samples the highest initial capacitance was obtainedfor EMD-C2 The 119862sp value keeps constant about 170 Fgafter 200 cycles For EMD-CV2 the 119862sp decreases slightlyafter 100 cycles before increasing significantly to 330 Fg inthe following cycles After 1000 cycles the fading of 119862sp ofEMD-C2 and EMD-CV2 was 23 and 5 respectively (seeFigure 10) Thus the nanostructured EMD-CV2 seems to bebetter for supercapacitor application than EMD-C2 In factthis fade observed for EMD samples can be explained bythe structure evolution in aqueous solution so the metalion doping into MnO
2lattice would improve the structural
stability
The two minutes electrodeposition was chosen to studythe Ni mixed MnO
2(NiMD) at different Mn2+ Ni2+ ratios
Figures 9(a) and 9(b) show theCVelectrochemical propertiesof NiMD samples prepared by two deposition methods Incomparison to the EMD samples the NiMD exhibited thesame shape type of voltammogram but larger magnitudecurrent It can be observed that the CVs are more rectangularand symmetrical but lower in current magnitude at highMn2+Ni2+ ratio than at the low ratio Thus the charac-teristics indicate the ideal capacitive behavior with highreversibility of NiMD sample The highest 119862sp was obtainedfor NiMD samples at Mn2+Ni2+ = 1 1 at all scan rates (Fig-ures 9(c) and 9(d))
The electrochemical stability of NiMD thin films pre-pared at Mn2+Ni2+ = 1 1 was also evaluated (Figure 11) Incomparison to the EMD at the same deposition conditionNiMD samples exhibited higher 119862sp about 500 Fg ver-sus 300 Fg By changing the electrodeposition modes theNiMD-CV shows the 119862sp increase after 500 cycles from430 Fg to 500 Fg while NiMD-CA keeps constant during800 cycles after that a slight fading is observed at the finalcycles After 1000 cycles at 100mVs the fading of 119862sp is lessthan 7 for NiMD samples The fact that Ni ion doping intoMnO2lattice increases119862sp as well as electrochemical stability
was briefly reported by Rajendra Prasad and Miura [23] Inthis work the highest 119862sp was obtained about 480 Fg at100mVs for NiMD sample (65 wt MnO
2 35 wt NiO)
34 Electrochemical Properties of Supercapacitor PrototypeElectrochemical impedance spectroscopy (EIS) measure-ments were also carried out in the frequency range of100 kHzndash01Hz Figure 11 shows the Nyquist plots of EMDthin films prepared at various deposition times by twomethods
8 Journal of Nanomaterials
minus100
minus50
0
50
100
0 02 04 06 08 1
Spec
ific c
urre
nt (A
middotgminus1)
E versus AgAgClNaCl (V)
3 13 2 1 0 (EMD-CV2)1 1
1 2
(a)
minus30
minus20
minus10
0
10
20
30
0 02 04 06 08 1E versus AgAgClNaCl (V)
Spec
ific c
urre
nt (A
middotgminus1)
3 13 2 1 0 (EMD-C2)1 1
1 2
(b)
100
200
300
400
500
600
0 50 100 150 200 250
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
3 13 2 1 21 0
1 1
Scan rate (mVmiddotsminus1)
(c)
100
200
300
400
500
600
0 50 100 150 200 250
3 13 2 1 21 0
1 1
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
Scan rate (mVmiddotsminus1)
(d)
Figure 9 CV curves of Ni mixed MnO2(NiMD samples) prepared by CV method (a) and CA method (b) in 2M Na
2SO4at 50mVs Plot
of 119862sp as a function of scan rate (c d)
TheNyquist plot shows a high-frequency intercept on thereal axis corresponding to the electrolyte resistance (119877
Ω) a
semicircle is considered as a parallel combination of chargetransfer resistance (119877ct) and double-layer capacitance (119862dl)with a linear region at low-frequency range In the low-frequency region linear part of the plot exhibits an anglebetween 45 and 90∘
The EIS data was analyzed by the electrical equivalentcircuit which consists of (119877
Ω) the electrolyte resistance (119877ct)
charge transfer resistance constant phase element (CPE)
used instead of double-layer capacitance (119862dl) Warburg(W) arising from a diffusion controlled process at low-frequency and CPE assigned to pseudocapacitance of thematerial in the low-frequency because of nonideal capacitivebehavior The starting nonzero intercept at Z1015840 at beginningof semicircle is identical in all the curves and its electricalresistance of Na
2SO4around 1Ω For CAmethod the charge
transfer values of EMD-C2 EMD-C6 EMD-C8 and EMD-C12 are 43Ω 102Ω 116Ω and 126Ω respectively For CVmethod the charge transfer values of EMD-CV2 EMD-CV4
Journal of Nanomaterials 9
100
200
300
400
500
600
700
0 200 400 600 800 1000
CV methodChronoamperometry
Number of cycles
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
Figure 10 Electrochemical stability of NiMD samples prepared at Mn2+Ni2+ = 1 1 by two methods at scan rate 100mVs
0
10
20
30
40
50
60
70
80
0 10 20 30 40 50 60 70 80
EMD-CV2EMD-CV4
EMD-CV6EMD-CV12
0
20
40
60
80
100
120
0 20 40 60 80 100 120 160140
EMD-C2EMD-C6
EMD-C8EMD-C12
CPE1
CPE2
W
Real (Z998400) (Ohm) Real (Z998400) (Ohm)
Rs
Rct
minuslm
(Z998400998400
) (O
hm)
minuslm
(Z998400998400
) (O
hm)
Figure 11 Nyquist plot of EMD thin films prepared by two methods and equivalent circuit model used for data fitting
EMD-CV8 and EMD-CV12 are 40Ω 42Ω 47Ω and 55Ωrespectively It means that the increase of deposition time (orfilm thickness) decreases the charge transfer resistanceThusa high resistance caused a difficult charge transfer electronbetween interface electrolyte and interface electrode
At the same deposition conditions the charge transferresistance ofNiMD samples decreases significantly comparedto EMD (Figure 12)
As the Mn2+Ni2+ increase the semicircle diameterbecomes smaller The lowest resistance was obtained forNiIMD at Mn2+Ni2+ = 1 1 about 4Ω versus 10Ω at Mn2+Ni2+ = 1 2 (Figure 13)
The Ni ion addition to the MnO2lattice would improve
the electrical resistivity of MnO2film in addition to enhance
the specific capacitance It maybe also enhanced the powerdensity as well as reversibility of the supercapacitior at
10 Journal of Nanomaterials
0
10
20
30
40
50
60
0 10 20 30 40 50 60Real (Z) (Ohm)
minuslm
(Z) (
Ohm
)
3 13 2 1 01 2
1 1
Figure 12 Nyquist plot of NiMD samples prepared by CA method compared to EMD-C2
0
2
4
6
8
10
0 2 4 6 8 10 12
minuslm
(Z) (
Ohm
)
Real (Z) (Ohm)
1 21 1
Figure 13 Nyquist plot (zoom part) of NiMD samples at Mn2+ Ni2+ = 1 1 and 1 2
high charge-discharge rate Symmetrical capacitor prototypesof 2 cm times 2 cm were assembled with two thin films ofelectrodes EMD-CV2 and NiMD-CV at Mn2+Ni2+ = 1 1and characterized by charge-discharge cycling at very highcurrent 1 Asdotgminus1
Figure 14 shows charge-discharge values of EMD andNIMD sample at current 1 Asdotgminus1 The efficiency of charge-discharge cycling is about 09 Discharge 119862sp values werecalculated by using the current density discharge time andthe weight of material 119862sp values obtained from Figure 14for EMD-CV2 and NiMD-CV are 10 Fsdotgminus1 and 25 Fsdotgminus1
As expected supercapacitor of NiMD-CV based electrodeexhibits a higher power density than EMD-CV2
The stability of supercapacitor based on NiMD-CV andEMD-CV2 was also tested for a large number of cycles (upto 10000 cycles) at 2 Asdotgminus1 A 119862sp fading of 25 and 14was observedwithNiMD-CVorEMD-CV2based capacitorsrespectively after 10000 charge-discharge cycles There is aninitial large 119862sp decrease and then the 119862sp remained almostconstantTheNiMD-CVexhibited expectedly higher stabilitythan EMD-CV2 based capacitorThus it would be promisingfor the long-term capacitor application
Journal of Nanomaterials 11
(a)
0
5
10
15
20
25
30
35
40
0 20 40 60 80 100Cycles number
NiMD-CVEMD-CV
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
(b)
Figure 14 The prototype (left) and the charge-discharge test at 1 Asdotgminus1 using thin film electrode of NiMD-CV and EMD-CV2 (right)
4 Conclusions
Nanoflakes EMD and NiMD with high porosity were pre-pared by two electrochemical depositions modes The thinfilms exhibited the 120574-MnO
2based structure The Ni addition
to MnO2lattice affected the local structure of 120574-MnO
2that
was evidently observed by using Raman spectroscopy TheEMD-CV exhibited better electrochemical behavior (119862sp andcharge-discharge stability) than EMD-C due to the homoge-nous film forming and high porosity during the reduction-oxidation sweep With short-time deposition (2 minutes)EMD thin films show the highest 119862sp and lowest chargetransfer resistance Compared to EMD electrodes preparedat the same deposition NiMD films enhance significantly the119862sp values as well as electrochemical stability The highest119862sp value was 500 Fsdotgminus1 compared to 300 Fsdotgminus1 of EMDThe enhanced electrectrochemical performance of NiMDcan be explained by the increase of thin film conductivitywith the presence of ion Ni2+ Moreover high power densityand excellent stability of assembled supercapacitor based onNiMDelectrodematerials were demonstrated (119862sp of 25 Fsdotg
minus1
at 1 Asdotgminus1 14 capacitance fade for 10000 cycles)
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors acknowledge funding from VNU-HCM underGrant HS2013-76-01 The authors would like to thank Officeof Naval Research Global (ONRG) for Grant N62909-13-1-N235 The authors would like to thank the technical supportof Horibarsquos Representative Company in Vietnam
References
[1] H Xia M O Lai and L Lu ldquoNanostructured manganese oxidethin films as electrode material for supercapacitorsrdquo JOM vol63 no 1 pp 54ndash59 2011
[2] X Zhao B M Sanchez P J Dobson and P S Grant ldquoThe roleof nanomaterials in redox-based supercapacitors for next gen-eration energy storage devicesrdquoNanoscale vol 3 no 3 pp 839ndash855 2011
[3] W Wei X Cui W Chen and D G Ivey ldquoManganese oxide-based materials as electrochemical supercapacitor electrodesrdquoChemical Society Reviews vol 40 no 3 pp 1697ndash1721 2011
[4] J E Post ldquoManganese oxide minerals crystal structures andeconomic and environmental significancerdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 96 no 7 pp 3447ndash3454 1999
[5] S C Pang M A Anderson and T W Chapman ldquoNovel elec-trode materials for thin-film ultracapacitors comparison ofelectrochemical properties of sol-gel-derived and electrode-posited manganese dioxiderdquo Journal of the ElectrochemicalSociety vol 147 no 2 pp 444ndash450 2000
[6] S C Pang and M A Anderson ldquoNovel electrode materials forelectrochemical capacitors Part II Material characterization ofsol-gel-derived and electrodeposited manganese dioxide thinfilmsrdquo Journal of Materials Research vol 15 no 10 pp 2096ndash2106 2000
[7] N Nagarajan H Humadi and I Zhitomirsky ldquoCathodic elec-trodeposition of MnO
ior of manganese dioxidestainless steel electrodes at differentdeposition currentsrdquoAmerican Journal ofMaterials Science vol2 no 2 pp 11ndash14 2012
[9] C Xu B Li H Du F Kang and Y Zeng ldquoElectrochemicalproperties of nanosized hydrous manganese dioxide synthe-sized by a self-reacting microemulsion methodrdquo Journal ofPower Sources vol 180 no 1 pp 664ndash670 2008
12 Journal of Nanomaterials
[10] X Hu X Lin Z Ling Y Li and X Fu ldquoFabrication and char-acteristics of galvanostatic electrodeposited MnO
2on porous
nickel from etched aluminiumrdquo Electrochimica Acta vol 138pp 132ndash138 2014
[11] S Hassan M Suzuki and A A El-Moneim ldquoEffect of Ag-doping on the capacitive behavior of amorphous manganesedioxide electrodesrdquo Electrical and Electronic Engineering vol 2no 2 pp 18ndash22 2012
[12] T Shinomiya V Gupta and N Miura ldquoEffects of electrochem-ical-deposition method and microstructure on the capacitivecharacteristics of nano-sized manganese oxiderdquo ElectrochimicaActa vol 51 no 21 pp 4412ndash4419 2006
[13] M Minakshi P Singh T B Issa S Thurgate and R De MarcoldquoLithium insertion into manganese dioxide electrode inMnO
2
Zn aqueous battery Part II Comparison of the behavior ofEMD and battery grade MnO
2in ZnmdashMnO
2mdash aqueous LiOH
electrolyterdquo Journal of Power Sources vol 138 no 1-2 pp 319ndash322 2004
[14] T Yousefi R Davarkhah A N Golikand and M H Mash-hadizadeh ldquoSynthesis characterization and supercapacitorstudies of manganese (IV) oxide nanowiresrdquo Materials Sciencein Semiconductor Processing vol 16 no 3 pp 868ndash876 2013
[15] J Li and I Zhitomirsky ldquoCathodic electrophoretic depositionof manganese dioxide filmsrdquo Colloids and Surfaces A Physico-chemical and Engineering Aspects vol 348 no 1ndash3 pp 248ndash2532009
[16] J Wei M Cheong N Nagarajan and I Zhitomirsky ldquoCathodicelectrodeposition of manganese oxides for electrochemicalsupercapacitorsrdquo ECS Transactions vol 3 no 37 pp 1ndash9 2007
[17] V Subramanian H Zhu R Vajtai P M Ajayan and B WeildquoHydrothermal synthesis and pseudocapacitance properties ofMnO
2nanostructuresrdquoThe Journal of Physical Chemistry B vol
109 no 43 pp 20207ndash20214 2005[18] C Ye Z M Lin and S Z Hui ldquoElectrochemical and capac-
itance properties of rod-shaped MnO2for supercapacitorrdquo
Journal of the Electrochemical Society vol 152 no 6 pp A1272ndashA1278 2005
[19] Q T Qu P Zhang B Wang et al ldquoElectrochemical perform-ance of MnO
2nanorods in neutral aqueous electrolytes as a
cathode for asymmetric supercapacitorsrdquo The Journal of Phys-ical Chemistry C vol 113 no 31 pp 14020ndash14027 2009
[20] J P Ni W C Lu L M Zhang B H Yue X F Shang and Y LvldquoLow-temperature synthesis of monodisperse 3D manganeseoxide nanoflowers and their pseudocapacitance propertiesrdquoTheJournal of Physical Chemistry C vol 113 no 1 pp 54ndash60 2009
[21] X Wang A Yuan and Y Wang ldquoSupercapacitive behaviorsand their temperature dependence of sol-gel synthesized nanos-tructured manganese dioxide in lithium hydroxide electrolyterdquoJournal of Power Sources vol 172 no 2 pp 1007ndash1011 2007
[22] C Julien M Massot S Rangan M Lemal and D GuyomardldquoStudy of structural defects in 120574-MnO
2byRaman spectroscopyrdquo
Journal of Raman Spectroscopy vol 33 no 4 pp 223ndash228 2002[23] K Rajendra Prasad and N Miura ldquoElectrochemically synthe-
sized MnO2-based mixed oxides for high performance redox
Figure 2 Raman spectra of electrodeposited MnO2(EMD-C2) and NiMD with different Mn2+ Ni2+ ratios (3 1 1 1 1 3)
3 Results and Discussion
31 FTIR and Raman Characterization The structure of thinfilms EMD and NiMD was characterized by using IR andRaman spectroscopy Figure 1 showed the spectra of EMDdeposited in 2 minutes by two methods in 400ndash4000 cmminus1The samples were abbreviated as EMD-CV2 and EMD-C2
The vibration at 3400 cmminus1 indicated ndashOH vibrationmode of the water As thin film was prepared in the aqueoussolution a tunnel structure MnO
2can be formed during
the electrodeposited process Thus H2O molecules can be
intercalated in the tunnel of thin film [8]The vibration bandsat 1600 cmminus1 and 500 cmminus1 were assigned toMnndashO stretchingmodes of MnO
6octahedral sites These vibration modes are
similar to those of natural mineral nsutite samples (120574-MnO2
structure complex tunnel) but some features are not clearlydetermined [8]
An examination of Raman spectroscopy is presentedin Figure 2 for EMD and NiMD thin films with variousMn Ni ratios Compared to the study of Julien et al [22] thevibrationmode ofMnO
2thin film at 577 cmminus1 (]3) 640 cmminus1
(]2) and 740 cmminus1 (]1) corresponds to the stretching modes
of the MnO6octahedral The corresponding antisymmetry
stretching modes are recorded in the infrared spectrum at517 cmminus1 and 621 cmminus1 (Figure 2) The weak Raman bands at511 415 297 and 145 cmminus1 are assigned to the deformationmodes of themetal-oxygen chain ofMnndashOndashMn in theMnO
2
octahedral lattice The Ni mixed MnO2(NiMD) sample
exhibited similarly Raman vibration bands However theshifted modes were clearly observed for example the blueshift for (]2) and (]3) vibration and the red shift for (]1)vibration Also the vibration peaks at lower wavenumberwere shifted (Figure 2) The results showed that the presenceof Ni ion affected the local structure of MnO
2 typically
the short range environment of oxygen coordination aroundtransition metal cations in MnO
2lattice
32 Morphology and Wettability Studies Figures 3 and 4show SEM images of EMD and NiMD thin filmsThe surfaceporosity and nanoflakes dimension were also affected bydeposit time and the depositionmode A slight difference wasobserved when the electrodepositionmode changed (Figures3(a) and 3(c)) Generally the surface morphology of thinfilms is composed entirely of nanoscale fiber (nanoflakes)
4 Journal of Nanomaterials
(a) (b)
(c) (d)
Figure 3 SEM images of EMD-C12 (a) EMD-CV2 (b) EMD-CV12 (c) and EMD-C2 (d)
(a) (b)
(c) (d)
Figure 4 SEM images of NiMD prepared by CA (a b) and by CV (c d) in 2 minutes
Journal of Nanomaterials 5
(a) (b)
(c) (d)
Figure 5 TEM images of EMD and NiMD prepared by CA method (a c) and by CV method (b d) in two minutes
An irregular interconnection of nanofibers formed a highlyporous network By using CV method the nanofiber slightlygrows up The nanoflakes dimension is about 100ndash120 nmin length compared to 40ndash60 nm of which prepared by CAmethod As the deposition time is long the nanofibers growup quickly and the interconnection points seem to be a starform (Figures 3(c) and 3(d) for 12minutes electrodeposition)Due to the variation of surface porosity and nanofibermorphology the specific capacitance behavior is expectedlychanged [1]
In case of Ni mixed MnO2 the surface morphologies
seem to be similar to those of MnO2films (Figure 4) Using
CV method exhibits homogenous surface covered of well-formed nanoflakes
TEM images were performed for Ni mixed MnO2
(NiMD) at the same electrodeposition conditions (Figure 5)In general it was observed that film structure prepared in twominutes is dense The nanoflakes seem to be very fine andtend to interweave to make a network
Wettability test is carried out in order to investigatethe interaction between liquid and thin film surface As
the wettability is high the small contact angle (120579) resultedand the surface is hydrophilic In contrast the surface ishydrophobic Both hydrophilic and hydrophobic propertiescan be applied to many fields from medicine to engineeringespecially in energy storages [1] Figure 6 shows the contactangle measured for MnO
2films prepared by CA and CV
method Both films are expectedly hydrophilic due to thepresence of ndashOH group on the surface and in the structureIndeed the results in Figure 6 indicated that contact angleis less than 90∘ In addition EMD-CV2 thin film is morehydrophilic than EMD-C2The hydrophilicity is essential forelectrochemical reaction on the electrodeaqueous electrolyteinterface
33 Electrochemical Properties of Electrodeposited Thin FilmThe capacitance behavior of MnO
2and NiMD thin films was
tested using CV in potential range of 0-1 V at 100mVs Fig-ure 7 shows CV curves of MnO
2thin films prepared by CA
and CV method in 1M Na2SO4
The specific capacitance (119862sp) was calculated by using(1) EMD was prepared by CA method EMD-CA exhibited
6 Journal of Nanomaterials
CA left 195∘CA right 188∘ EMD-CV2
(a)
CA left 314∘CA right 319∘ EMD-C2
(b)
Figure 6 Water contact angle of EMD-CV2 (a) and EMD-C2 (b)
Curr
ent d
ensit
y (m
Ac
m2)
EMD-C2EMD-C4EMD-C6
EMD-C8EMD-C10EMD-C12
6
4
2
0
minus2
minus4
minus60 02 04 06 08 1
Voltage versus AgAgCl (3M) (V)
(a)
EMD-CV2
EMD-CV4
EMD-CV6
EMD-CV8
EMD-CV10
EMD-CV12
6
4
2
0
minus2
minus4
minus60 02 04 06 08 1
Curr
ent d
ensit
y (m
Ac
m2)
Voltage versus AgAgCl (3M) (V)
(b)
0
50
100
150
200
250
300
350
05 1 15 2 25 3
EMD-CV EMD
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
Film thickness (120583m)
2min
4min
6min
10min
12min
(c)
Figure 7 CV curves of EMD-C and EMD-CV samples in 2M Na2SO4at 100mVs (a b) Plot of 119862sp and thickness of EMD thin films
corresponding to deposition time (c)
Journal of Nanomaterials 7
350
300
250
200
150
1000 200 400 600 800 1000
Number of cycles
Spec
ific c
apac
itanc
e (F
g)
EMD-C2EMD-CV2
Figure 8 Electrochemical stability of EMD-C2 and EMD-CV2 at scan rate 100mVs
the119862sp values that are 250 138 91 79 62 and 48 Fg for EMD-C2 EMD-C4 EMD-C6 EMD-C8 EMD-C10 and EMD-C12 respectively With CV method the 119862sp values are 317112 95 79 60 and 53 Fg for EMD-CV2 EMD-CV4 EMD-CV6 EMD-CV8 EMD-CV10 and EMD-CV12 respectivelyPrevious studies [2 3 6] show that capacitances ofmanganeseoxide synthesized by electrochemical deposition were about200ndash300 Fg
The highest 119862sp was obtained at two minutes of elec-trodeposition for both methods (Figure 7(c)) The thicknessofMnO
2film is about 055 120583mThus it can be concluded that
the thin and porous film was only obtained in a short depo-sition time When the deposition time is long the nanoflakesgrew up fast and their interconnection makes the film morecompact than a short-time deposition Thus penetration ofelectrolyte into the film as well as the specific capacitancedecreases due to the increase of electric resistance The smallthickness of MnO
2film is necessarily required in order to
obtain the good electric conductivity and specific capacitance[4 12]
The electrochemical stability of thin films was evaluatedFigure 8 shows the specific capacitance (119862sp) of EMD-C2and EMD-CV2 measured at a scan rate of 100mVs AmongEMD-C samples the highest initial capacitance was obtainedfor EMD-C2 The 119862sp value keeps constant about 170 Fgafter 200 cycles For EMD-CV2 the 119862sp decreases slightlyafter 100 cycles before increasing significantly to 330 Fg inthe following cycles After 1000 cycles the fading of 119862sp ofEMD-C2 and EMD-CV2 was 23 and 5 respectively (seeFigure 10) Thus the nanostructured EMD-CV2 seems to bebetter for supercapacitor application than EMD-C2 In factthis fade observed for EMD samples can be explained bythe structure evolution in aqueous solution so the metalion doping into MnO
2lattice would improve the structural
stability
The two minutes electrodeposition was chosen to studythe Ni mixed MnO
2(NiMD) at different Mn2+ Ni2+ ratios
Figures 9(a) and 9(b) show theCVelectrochemical propertiesof NiMD samples prepared by two deposition methods Incomparison to the EMD samples the NiMD exhibited thesame shape type of voltammogram but larger magnitudecurrent It can be observed that the CVs are more rectangularand symmetrical but lower in current magnitude at highMn2+Ni2+ ratio than at the low ratio Thus the charac-teristics indicate the ideal capacitive behavior with highreversibility of NiMD sample The highest 119862sp was obtainedfor NiMD samples at Mn2+Ni2+ = 1 1 at all scan rates (Fig-ures 9(c) and 9(d))
The electrochemical stability of NiMD thin films pre-pared at Mn2+Ni2+ = 1 1 was also evaluated (Figure 11) Incomparison to the EMD at the same deposition conditionNiMD samples exhibited higher 119862sp about 500 Fg ver-sus 300 Fg By changing the electrodeposition modes theNiMD-CV shows the 119862sp increase after 500 cycles from430 Fg to 500 Fg while NiMD-CA keeps constant during800 cycles after that a slight fading is observed at the finalcycles After 1000 cycles at 100mVs the fading of 119862sp is lessthan 7 for NiMD samples The fact that Ni ion doping intoMnO2lattice increases119862sp as well as electrochemical stability
was briefly reported by Rajendra Prasad and Miura [23] Inthis work the highest 119862sp was obtained about 480 Fg at100mVs for NiMD sample (65 wt MnO
2 35 wt NiO)
34 Electrochemical Properties of Supercapacitor PrototypeElectrochemical impedance spectroscopy (EIS) measure-ments were also carried out in the frequency range of100 kHzndash01Hz Figure 11 shows the Nyquist plots of EMDthin films prepared at various deposition times by twomethods
8 Journal of Nanomaterials
minus100
minus50
0
50
100
0 02 04 06 08 1
Spec
ific c
urre
nt (A
middotgminus1)
E versus AgAgClNaCl (V)
3 13 2 1 0 (EMD-CV2)1 1
1 2
(a)
minus30
minus20
minus10
0
10
20
30
0 02 04 06 08 1E versus AgAgClNaCl (V)
Spec
ific c
urre
nt (A
middotgminus1)
3 13 2 1 0 (EMD-C2)1 1
1 2
(b)
100
200
300
400
500
600
0 50 100 150 200 250
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
3 13 2 1 21 0
1 1
Scan rate (mVmiddotsminus1)
(c)
100
200
300
400
500
600
0 50 100 150 200 250
3 13 2 1 21 0
1 1
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
Scan rate (mVmiddotsminus1)
(d)
Figure 9 CV curves of Ni mixed MnO2(NiMD samples) prepared by CV method (a) and CA method (b) in 2M Na
2SO4at 50mVs Plot
of 119862sp as a function of scan rate (c d)
TheNyquist plot shows a high-frequency intercept on thereal axis corresponding to the electrolyte resistance (119877
Ω) a
semicircle is considered as a parallel combination of chargetransfer resistance (119877ct) and double-layer capacitance (119862dl)with a linear region at low-frequency range In the low-frequency region linear part of the plot exhibits an anglebetween 45 and 90∘
The EIS data was analyzed by the electrical equivalentcircuit which consists of (119877
Ω) the electrolyte resistance (119877ct)
charge transfer resistance constant phase element (CPE)
used instead of double-layer capacitance (119862dl) Warburg(W) arising from a diffusion controlled process at low-frequency and CPE assigned to pseudocapacitance of thematerial in the low-frequency because of nonideal capacitivebehavior The starting nonzero intercept at Z1015840 at beginningof semicircle is identical in all the curves and its electricalresistance of Na
2SO4around 1Ω For CAmethod the charge
transfer values of EMD-C2 EMD-C6 EMD-C8 and EMD-C12 are 43Ω 102Ω 116Ω and 126Ω respectively For CVmethod the charge transfer values of EMD-CV2 EMD-CV4
Journal of Nanomaterials 9
100
200
300
400
500
600
700
0 200 400 600 800 1000
CV methodChronoamperometry
Number of cycles
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
Figure 10 Electrochemical stability of NiMD samples prepared at Mn2+Ni2+ = 1 1 by two methods at scan rate 100mVs
0
10
20
30
40
50
60
70
80
0 10 20 30 40 50 60 70 80
EMD-CV2EMD-CV4
EMD-CV6EMD-CV12
0
20
40
60
80
100
120
0 20 40 60 80 100 120 160140
EMD-C2EMD-C6
EMD-C8EMD-C12
CPE1
CPE2
W
Real (Z998400) (Ohm) Real (Z998400) (Ohm)
Rs
Rct
minuslm
(Z998400998400
) (O
hm)
minuslm
(Z998400998400
) (O
hm)
Figure 11 Nyquist plot of EMD thin films prepared by two methods and equivalent circuit model used for data fitting
EMD-CV8 and EMD-CV12 are 40Ω 42Ω 47Ω and 55Ωrespectively It means that the increase of deposition time (orfilm thickness) decreases the charge transfer resistanceThusa high resistance caused a difficult charge transfer electronbetween interface electrolyte and interface electrode
At the same deposition conditions the charge transferresistance ofNiMD samples decreases significantly comparedto EMD (Figure 12)
As the Mn2+Ni2+ increase the semicircle diameterbecomes smaller The lowest resistance was obtained forNiIMD at Mn2+Ni2+ = 1 1 about 4Ω versus 10Ω at Mn2+Ni2+ = 1 2 (Figure 13)
The Ni ion addition to the MnO2lattice would improve
the electrical resistivity of MnO2film in addition to enhance
the specific capacitance It maybe also enhanced the powerdensity as well as reversibility of the supercapacitior at
10 Journal of Nanomaterials
0
10
20
30
40
50
60
0 10 20 30 40 50 60Real (Z) (Ohm)
minuslm
(Z) (
Ohm
)
3 13 2 1 01 2
1 1
Figure 12 Nyquist plot of NiMD samples prepared by CA method compared to EMD-C2
0
2
4
6
8
10
0 2 4 6 8 10 12
minuslm
(Z) (
Ohm
)
Real (Z) (Ohm)
1 21 1
Figure 13 Nyquist plot (zoom part) of NiMD samples at Mn2+ Ni2+ = 1 1 and 1 2
high charge-discharge rate Symmetrical capacitor prototypesof 2 cm times 2 cm were assembled with two thin films ofelectrodes EMD-CV2 and NiMD-CV at Mn2+Ni2+ = 1 1and characterized by charge-discharge cycling at very highcurrent 1 Asdotgminus1
Figure 14 shows charge-discharge values of EMD andNIMD sample at current 1 Asdotgminus1 The efficiency of charge-discharge cycling is about 09 Discharge 119862sp values werecalculated by using the current density discharge time andthe weight of material 119862sp values obtained from Figure 14for EMD-CV2 and NiMD-CV are 10 Fsdotgminus1 and 25 Fsdotgminus1
As expected supercapacitor of NiMD-CV based electrodeexhibits a higher power density than EMD-CV2
The stability of supercapacitor based on NiMD-CV andEMD-CV2 was also tested for a large number of cycles (upto 10000 cycles) at 2 Asdotgminus1 A 119862sp fading of 25 and 14was observedwithNiMD-CVorEMD-CV2based capacitorsrespectively after 10000 charge-discharge cycles There is aninitial large 119862sp decrease and then the 119862sp remained almostconstantTheNiMD-CVexhibited expectedly higher stabilitythan EMD-CV2 based capacitorThus it would be promisingfor the long-term capacitor application
Journal of Nanomaterials 11
(a)
0
5
10
15
20
25
30
35
40
0 20 40 60 80 100Cycles number
NiMD-CVEMD-CV
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
(b)
Figure 14 The prototype (left) and the charge-discharge test at 1 Asdotgminus1 using thin film electrode of NiMD-CV and EMD-CV2 (right)
4 Conclusions
Nanoflakes EMD and NiMD with high porosity were pre-pared by two electrochemical depositions modes The thinfilms exhibited the 120574-MnO
2based structure The Ni addition
to MnO2lattice affected the local structure of 120574-MnO
2that
was evidently observed by using Raman spectroscopy TheEMD-CV exhibited better electrochemical behavior (119862sp andcharge-discharge stability) than EMD-C due to the homoge-nous film forming and high porosity during the reduction-oxidation sweep With short-time deposition (2 minutes)EMD thin films show the highest 119862sp and lowest chargetransfer resistance Compared to EMD electrodes preparedat the same deposition NiMD films enhance significantly the119862sp values as well as electrochemical stability The highest119862sp value was 500 Fsdotgminus1 compared to 300 Fsdotgminus1 of EMDThe enhanced electrectrochemical performance of NiMDcan be explained by the increase of thin film conductivitywith the presence of ion Ni2+ Moreover high power densityand excellent stability of assembled supercapacitor based onNiMDelectrodematerials were demonstrated (119862sp of 25 Fsdotg
minus1
at 1 Asdotgminus1 14 capacitance fade for 10000 cycles)
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors acknowledge funding from VNU-HCM underGrant HS2013-76-01 The authors would like to thank Officeof Naval Research Global (ONRG) for Grant N62909-13-1-N235 The authors would like to thank the technical supportof Horibarsquos Representative Company in Vietnam
References
[1] H Xia M O Lai and L Lu ldquoNanostructured manganese oxidethin films as electrode material for supercapacitorsrdquo JOM vol63 no 1 pp 54ndash59 2011
[2] X Zhao B M Sanchez P J Dobson and P S Grant ldquoThe roleof nanomaterials in redox-based supercapacitors for next gen-eration energy storage devicesrdquoNanoscale vol 3 no 3 pp 839ndash855 2011
[3] W Wei X Cui W Chen and D G Ivey ldquoManganese oxide-based materials as electrochemical supercapacitor electrodesrdquoChemical Society Reviews vol 40 no 3 pp 1697ndash1721 2011
[4] J E Post ldquoManganese oxide minerals crystal structures andeconomic and environmental significancerdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 96 no 7 pp 3447ndash3454 1999
[5] S C Pang M A Anderson and T W Chapman ldquoNovel elec-trode materials for thin-film ultracapacitors comparison ofelectrochemical properties of sol-gel-derived and electrode-posited manganese dioxiderdquo Journal of the ElectrochemicalSociety vol 147 no 2 pp 444ndash450 2000
[6] S C Pang and M A Anderson ldquoNovel electrode materials forelectrochemical capacitors Part II Material characterization ofsol-gel-derived and electrodeposited manganese dioxide thinfilmsrdquo Journal of Materials Research vol 15 no 10 pp 2096ndash2106 2000
[7] N Nagarajan H Humadi and I Zhitomirsky ldquoCathodic elec-trodeposition of MnO
ior of manganese dioxidestainless steel electrodes at differentdeposition currentsrdquoAmerican Journal ofMaterials Science vol2 no 2 pp 11ndash14 2012
[9] C Xu B Li H Du F Kang and Y Zeng ldquoElectrochemicalproperties of nanosized hydrous manganese dioxide synthe-sized by a self-reacting microemulsion methodrdquo Journal ofPower Sources vol 180 no 1 pp 664ndash670 2008
12 Journal of Nanomaterials
[10] X Hu X Lin Z Ling Y Li and X Fu ldquoFabrication and char-acteristics of galvanostatic electrodeposited MnO
2on porous
nickel from etched aluminiumrdquo Electrochimica Acta vol 138pp 132ndash138 2014
[11] S Hassan M Suzuki and A A El-Moneim ldquoEffect of Ag-doping on the capacitive behavior of amorphous manganesedioxide electrodesrdquo Electrical and Electronic Engineering vol 2no 2 pp 18ndash22 2012
[12] T Shinomiya V Gupta and N Miura ldquoEffects of electrochem-ical-deposition method and microstructure on the capacitivecharacteristics of nano-sized manganese oxiderdquo ElectrochimicaActa vol 51 no 21 pp 4412ndash4419 2006
[13] M Minakshi P Singh T B Issa S Thurgate and R De MarcoldquoLithium insertion into manganese dioxide electrode inMnO
2
Zn aqueous battery Part II Comparison of the behavior ofEMD and battery grade MnO
2in ZnmdashMnO
2mdash aqueous LiOH
electrolyterdquo Journal of Power Sources vol 138 no 1-2 pp 319ndash322 2004
[14] T Yousefi R Davarkhah A N Golikand and M H Mash-hadizadeh ldquoSynthesis characterization and supercapacitorstudies of manganese (IV) oxide nanowiresrdquo Materials Sciencein Semiconductor Processing vol 16 no 3 pp 868ndash876 2013
[15] J Li and I Zhitomirsky ldquoCathodic electrophoretic depositionof manganese dioxide filmsrdquo Colloids and Surfaces A Physico-chemical and Engineering Aspects vol 348 no 1ndash3 pp 248ndash2532009
[16] J Wei M Cheong N Nagarajan and I Zhitomirsky ldquoCathodicelectrodeposition of manganese oxides for electrochemicalsupercapacitorsrdquo ECS Transactions vol 3 no 37 pp 1ndash9 2007
[17] V Subramanian H Zhu R Vajtai P M Ajayan and B WeildquoHydrothermal synthesis and pseudocapacitance properties ofMnO
2nanostructuresrdquoThe Journal of Physical Chemistry B vol
109 no 43 pp 20207ndash20214 2005[18] C Ye Z M Lin and S Z Hui ldquoElectrochemical and capac-
itance properties of rod-shaped MnO2for supercapacitorrdquo
Journal of the Electrochemical Society vol 152 no 6 pp A1272ndashA1278 2005
[19] Q T Qu P Zhang B Wang et al ldquoElectrochemical perform-ance of MnO
2nanorods in neutral aqueous electrolytes as a
cathode for asymmetric supercapacitorsrdquo The Journal of Phys-ical Chemistry C vol 113 no 31 pp 14020ndash14027 2009
[20] J P Ni W C Lu L M Zhang B H Yue X F Shang and Y LvldquoLow-temperature synthesis of monodisperse 3D manganeseoxide nanoflowers and their pseudocapacitance propertiesrdquoTheJournal of Physical Chemistry C vol 113 no 1 pp 54ndash60 2009
[21] X Wang A Yuan and Y Wang ldquoSupercapacitive behaviorsand their temperature dependence of sol-gel synthesized nanos-tructured manganese dioxide in lithium hydroxide electrolyterdquoJournal of Power Sources vol 172 no 2 pp 1007ndash1011 2007
[22] C Julien M Massot S Rangan M Lemal and D GuyomardldquoStudy of structural defects in 120574-MnO
2byRaman spectroscopyrdquo
Journal of Raman Spectroscopy vol 33 no 4 pp 223ndash228 2002[23] K Rajendra Prasad and N Miura ldquoElectrochemically synthe-
sized MnO2-based mixed oxides for high performance redox
Figure 3 SEM images of EMD-C12 (a) EMD-CV2 (b) EMD-CV12 (c) and EMD-C2 (d)
(a) (b)
(c) (d)
Figure 4 SEM images of NiMD prepared by CA (a b) and by CV (c d) in 2 minutes
Journal of Nanomaterials 5
(a) (b)
(c) (d)
Figure 5 TEM images of EMD and NiMD prepared by CA method (a c) and by CV method (b d) in two minutes
An irregular interconnection of nanofibers formed a highlyporous network By using CV method the nanofiber slightlygrows up The nanoflakes dimension is about 100ndash120 nmin length compared to 40ndash60 nm of which prepared by CAmethod As the deposition time is long the nanofibers growup quickly and the interconnection points seem to be a starform (Figures 3(c) and 3(d) for 12minutes electrodeposition)Due to the variation of surface porosity and nanofibermorphology the specific capacitance behavior is expectedlychanged [1]
In case of Ni mixed MnO2 the surface morphologies
seem to be similar to those of MnO2films (Figure 4) Using
CV method exhibits homogenous surface covered of well-formed nanoflakes
TEM images were performed for Ni mixed MnO2
(NiMD) at the same electrodeposition conditions (Figure 5)In general it was observed that film structure prepared in twominutes is dense The nanoflakes seem to be very fine andtend to interweave to make a network
Wettability test is carried out in order to investigatethe interaction between liquid and thin film surface As
the wettability is high the small contact angle (120579) resultedand the surface is hydrophilic In contrast the surface ishydrophobic Both hydrophilic and hydrophobic propertiescan be applied to many fields from medicine to engineeringespecially in energy storages [1] Figure 6 shows the contactangle measured for MnO
2films prepared by CA and CV
method Both films are expectedly hydrophilic due to thepresence of ndashOH group on the surface and in the structureIndeed the results in Figure 6 indicated that contact angleis less than 90∘ In addition EMD-CV2 thin film is morehydrophilic than EMD-C2The hydrophilicity is essential forelectrochemical reaction on the electrodeaqueous electrolyteinterface
33 Electrochemical Properties of Electrodeposited Thin FilmThe capacitance behavior of MnO
2and NiMD thin films was
tested using CV in potential range of 0-1 V at 100mVs Fig-ure 7 shows CV curves of MnO
2thin films prepared by CA
and CV method in 1M Na2SO4
The specific capacitance (119862sp) was calculated by using(1) EMD was prepared by CA method EMD-CA exhibited
6 Journal of Nanomaterials
CA left 195∘CA right 188∘ EMD-CV2
(a)
CA left 314∘CA right 319∘ EMD-C2
(b)
Figure 6 Water contact angle of EMD-CV2 (a) and EMD-C2 (b)
Curr
ent d
ensit
y (m
Ac
m2)
EMD-C2EMD-C4EMD-C6
EMD-C8EMD-C10EMD-C12
6
4
2
0
minus2
minus4
minus60 02 04 06 08 1
Voltage versus AgAgCl (3M) (V)
(a)
EMD-CV2
EMD-CV4
EMD-CV6
EMD-CV8
EMD-CV10
EMD-CV12
6
4
2
0
minus2
minus4
minus60 02 04 06 08 1
Curr
ent d
ensit
y (m
Ac
m2)
Voltage versus AgAgCl (3M) (V)
(b)
0
50
100
150
200
250
300
350
05 1 15 2 25 3
EMD-CV EMD
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
Film thickness (120583m)
2min
4min
6min
10min
12min
(c)
Figure 7 CV curves of EMD-C and EMD-CV samples in 2M Na2SO4at 100mVs (a b) Plot of 119862sp and thickness of EMD thin films
corresponding to deposition time (c)
Journal of Nanomaterials 7
350
300
250
200
150
1000 200 400 600 800 1000
Number of cycles
Spec
ific c
apac
itanc
e (F
g)
EMD-C2EMD-CV2
Figure 8 Electrochemical stability of EMD-C2 and EMD-CV2 at scan rate 100mVs
the119862sp values that are 250 138 91 79 62 and 48 Fg for EMD-C2 EMD-C4 EMD-C6 EMD-C8 EMD-C10 and EMD-C12 respectively With CV method the 119862sp values are 317112 95 79 60 and 53 Fg for EMD-CV2 EMD-CV4 EMD-CV6 EMD-CV8 EMD-CV10 and EMD-CV12 respectivelyPrevious studies [2 3 6] show that capacitances ofmanganeseoxide synthesized by electrochemical deposition were about200ndash300 Fg
The highest 119862sp was obtained at two minutes of elec-trodeposition for both methods (Figure 7(c)) The thicknessofMnO
2film is about 055 120583mThus it can be concluded that
the thin and porous film was only obtained in a short depo-sition time When the deposition time is long the nanoflakesgrew up fast and their interconnection makes the film morecompact than a short-time deposition Thus penetration ofelectrolyte into the film as well as the specific capacitancedecreases due to the increase of electric resistance The smallthickness of MnO
2film is necessarily required in order to
obtain the good electric conductivity and specific capacitance[4 12]
The electrochemical stability of thin films was evaluatedFigure 8 shows the specific capacitance (119862sp) of EMD-C2and EMD-CV2 measured at a scan rate of 100mVs AmongEMD-C samples the highest initial capacitance was obtainedfor EMD-C2 The 119862sp value keeps constant about 170 Fgafter 200 cycles For EMD-CV2 the 119862sp decreases slightlyafter 100 cycles before increasing significantly to 330 Fg inthe following cycles After 1000 cycles the fading of 119862sp ofEMD-C2 and EMD-CV2 was 23 and 5 respectively (seeFigure 10) Thus the nanostructured EMD-CV2 seems to bebetter for supercapacitor application than EMD-C2 In factthis fade observed for EMD samples can be explained bythe structure evolution in aqueous solution so the metalion doping into MnO
2lattice would improve the structural
stability
The two minutes electrodeposition was chosen to studythe Ni mixed MnO
2(NiMD) at different Mn2+ Ni2+ ratios
Figures 9(a) and 9(b) show theCVelectrochemical propertiesof NiMD samples prepared by two deposition methods Incomparison to the EMD samples the NiMD exhibited thesame shape type of voltammogram but larger magnitudecurrent It can be observed that the CVs are more rectangularand symmetrical but lower in current magnitude at highMn2+Ni2+ ratio than at the low ratio Thus the charac-teristics indicate the ideal capacitive behavior with highreversibility of NiMD sample The highest 119862sp was obtainedfor NiMD samples at Mn2+Ni2+ = 1 1 at all scan rates (Fig-ures 9(c) and 9(d))
The electrochemical stability of NiMD thin films pre-pared at Mn2+Ni2+ = 1 1 was also evaluated (Figure 11) Incomparison to the EMD at the same deposition conditionNiMD samples exhibited higher 119862sp about 500 Fg ver-sus 300 Fg By changing the electrodeposition modes theNiMD-CV shows the 119862sp increase after 500 cycles from430 Fg to 500 Fg while NiMD-CA keeps constant during800 cycles after that a slight fading is observed at the finalcycles After 1000 cycles at 100mVs the fading of 119862sp is lessthan 7 for NiMD samples The fact that Ni ion doping intoMnO2lattice increases119862sp as well as electrochemical stability
was briefly reported by Rajendra Prasad and Miura [23] Inthis work the highest 119862sp was obtained about 480 Fg at100mVs for NiMD sample (65 wt MnO
2 35 wt NiO)
34 Electrochemical Properties of Supercapacitor PrototypeElectrochemical impedance spectroscopy (EIS) measure-ments were also carried out in the frequency range of100 kHzndash01Hz Figure 11 shows the Nyquist plots of EMDthin films prepared at various deposition times by twomethods
8 Journal of Nanomaterials
minus100
minus50
0
50
100
0 02 04 06 08 1
Spec
ific c
urre
nt (A
middotgminus1)
E versus AgAgClNaCl (V)
3 13 2 1 0 (EMD-CV2)1 1
1 2
(a)
minus30
minus20
minus10
0
10
20
30
0 02 04 06 08 1E versus AgAgClNaCl (V)
Spec
ific c
urre
nt (A
middotgminus1)
3 13 2 1 0 (EMD-C2)1 1
1 2
(b)
100
200
300
400
500
600
0 50 100 150 200 250
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
3 13 2 1 21 0
1 1
Scan rate (mVmiddotsminus1)
(c)
100
200
300
400
500
600
0 50 100 150 200 250
3 13 2 1 21 0
1 1
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
Scan rate (mVmiddotsminus1)
(d)
Figure 9 CV curves of Ni mixed MnO2(NiMD samples) prepared by CV method (a) and CA method (b) in 2M Na
2SO4at 50mVs Plot
of 119862sp as a function of scan rate (c d)
TheNyquist plot shows a high-frequency intercept on thereal axis corresponding to the electrolyte resistance (119877
Ω) a
semicircle is considered as a parallel combination of chargetransfer resistance (119877ct) and double-layer capacitance (119862dl)with a linear region at low-frequency range In the low-frequency region linear part of the plot exhibits an anglebetween 45 and 90∘
The EIS data was analyzed by the electrical equivalentcircuit which consists of (119877
Ω) the electrolyte resistance (119877ct)
charge transfer resistance constant phase element (CPE)
used instead of double-layer capacitance (119862dl) Warburg(W) arising from a diffusion controlled process at low-frequency and CPE assigned to pseudocapacitance of thematerial in the low-frequency because of nonideal capacitivebehavior The starting nonzero intercept at Z1015840 at beginningof semicircle is identical in all the curves and its electricalresistance of Na
2SO4around 1Ω For CAmethod the charge
transfer values of EMD-C2 EMD-C6 EMD-C8 and EMD-C12 are 43Ω 102Ω 116Ω and 126Ω respectively For CVmethod the charge transfer values of EMD-CV2 EMD-CV4
Journal of Nanomaterials 9
100
200
300
400
500
600
700
0 200 400 600 800 1000
CV methodChronoamperometry
Number of cycles
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
Figure 10 Electrochemical stability of NiMD samples prepared at Mn2+Ni2+ = 1 1 by two methods at scan rate 100mVs
0
10
20
30
40
50
60
70
80
0 10 20 30 40 50 60 70 80
EMD-CV2EMD-CV4
EMD-CV6EMD-CV12
0
20
40
60
80
100
120
0 20 40 60 80 100 120 160140
EMD-C2EMD-C6
EMD-C8EMD-C12
CPE1
CPE2
W
Real (Z998400) (Ohm) Real (Z998400) (Ohm)
Rs
Rct
minuslm
(Z998400998400
) (O
hm)
minuslm
(Z998400998400
) (O
hm)
Figure 11 Nyquist plot of EMD thin films prepared by two methods and equivalent circuit model used for data fitting
EMD-CV8 and EMD-CV12 are 40Ω 42Ω 47Ω and 55Ωrespectively It means that the increase of deposition time (orfilm thickness) decreases the charge transfer resistanceThusa high resistance caused a difficult charge transfer electronbetween interface electrolyte and interface electrode
At the same deposition conditions the charge transferresistance ofNiMD samples decreases significantly comparedto EMD (Figure 12)
As the Mn2+Ni2+ increase the semicircle diameterbecomes smaller The lowest resistance was obtained forNiIMD at Mn2+Ni2+ = 1 1 about 4Ω versus 10Ω at Mn2+Ni2+ = 1 2 (Figure 13)
The Ni ion addition to the MnO2lattice would improve
the electrical resistivity of MnO2film in addition to enhance
the specific capacitance It maybe also enhanced the powerdensity as well as reversibility of the supercapacitior at
10 Journal of Nanomaterials
0
10
20
30
40
50
60
0 10 20 30 40 50 60Real (Z) (Ohm)
minuslm
(Z) (
Ohm
)
3 13 2 1 01 2
1 1
Figure 12 Nyquist plot of NiMD samples prepared by CA method compared to EMD-C2
0
2
4
6
8
10
0 2 4 6 8 10 12
minuslm
(Z) (
Ohm
)
Real (Z) (Ohm)
1 21 1
Figure 13 Nyquist plot (zoom part) of NiMD samples at Mn2+ Ni2+ = 1 1 and 1 2
high charge-discharge rate Symmetrical capacitor prototypesof 2 cm times 2 cm were assembled with two thin films ofelectrodes EMD-CV2 and NiMD-CV at Mn2+Ni2+ = 1 1and characterized by charge-discharge cycling at very highcurrent 1 Asdotgminus1
Figure 14 shows charge-discharge values of EMD andNIMD sample at current 1 Asdotgminus1 The efficiency of charge-discharge cycling is about 09 Discharge 119862sp values werecalculated by using the current density discharge time andthe weight of material 119862sp values obtained from Figure 14for EMD-CV2 and NiMD-CV are 10 Fsdotgminus1 and 25 Fsdotgminus1
As expected supercapacitor of NiMD-CV based electrodeexhibits a higher power density than EMD-CV2
The stability of supercapacitor based on NiMD-CV andEMD-CV2 was also tested for a large number of cycles (upto 10000 cycles) at 2 Asdotgminus1 A 119862sp fading of 25 and 14was observedwithNiMD-CVorEMD-CV2based capacitorsrespectively after 10000 charge-discharge cycles There is aninitial large 119862sp decrease and then the 119862sp remained almostconstantTheNiMD-CVexhibited expectedly higher stabilitythan EMD-CV2 based capacitorThus it would be promisingfor the long-term capacitor application
Journal of Nanomaterials 11
(a)
0
5
10
15
20
25
30
35
40
0 20 40 60 80 100Cycles number
NiMD-CVEMD-CV
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
(b)
Figure 14 The prototype (left) and the charge-discharge test at 1 Asdotgminus1 using thin film electrode of NiMD-CV and EMD-CV2 (right)
4 Conclusions
Nanoflakes EMD and NiMD with high porosity were pre-pared by two electrochemical depositions modes The thinfilms exhibited the 120574-MnO
2based structure The Ni addition
to MnO2lattice affected the local structure of 120574-MnO
2that
was evidently observed by using Raman spectroscopy TheEMD-CV exhibited better electrochemical behavior (119862sp andcharge-discharge stability) than EMD-C due to the homoge-nous film forming and high porosity during the reduction-oxidation sweep With short-time deposition (2 minutes)EMD thin films show the highest 119862sp and lowest chargetransfer resistance Compared to EMD electrodes preparedat the same deposition NiMD films enhance significantly the119862sp values as well as electrochemical stability The highest119862sp value was 500 Fsdotgminus1 compared to 300 Fsdotgminus1 of EMDThe enhanced electrectrochemical performance of NiMDcan be explained by the increase of thin film conductivitywith the presence of ion Ni2+ Moreover high power densityand excellent stability of assembled supercapacitor based onNiMDelectrodematerials were demonstrated (119862sp of 25 Fsdotg
minus1
at 1 Asdotgminus1 14 capacitance fade for 10000 cycles)
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors acknowledge funding from VNU-HCM underGrant HS2013-76-01 The authors would like to thank Officeof Naval Research Global (ONRG) for Grant N62909-13-1-N235 The authors would like to thank the technical supportof Horibarsquos Representative Company in Vietnam
References
[1] H Xia M O Lai and L Lu ldquoNanostructured manganese oxidethin films as electrode material for supercapacitorsrdquo JOM vol63 no 1 pp 54ndash59 2011
[2] X Zhao B M Sanchez P J Dobson and P S Grant ldquoThe roleof nanomaterials in redox-based supercapacitors for next gen-eration energy storage devicesrdquoNanoscale vol 3 no 3 pp 839ndash855 2011
[3] W Wei X Cui W Chen and D G Ivey ldquoManganese oxide-based materials as electrochemical supercapacitor electrodesrdquoChemical Society Reviews vol 40 no 3 pp 1697ndash1721 2011
[4] J E Post ldquoManganese oxide minerals crystal structures andeconomic and environmental significancerdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 96 no 7 pp 3447ndash3454 1999
[5] S C Pang M A Anderson and T W Chapman ldquoNovel elec-trode materials for thin-film ultracapacitors comparison ofelectrochemical properties of sol-gel-derived and electrode-posited manganese dioxiderdquo Journal of the ElectrochemicalSociety vol 147 no 2 pp 444ndash450 2000
[6] S C Pang and M A Anderson ldquoNovel electrode materials forelectrochemical capacitors Part II Material characterization ofsol-gel-derived and electrodeposited manganese dioxide thinfilmsrdquo Journal of Materials Research vol 15 no 10 pp 2096ndash2106 2000
[7] N Nagarajan H Humadi and I Zhitomirsky ldquoCathodic elec-trodeposition of MnO
ior of manganese dioxidestainless steel electrodes at differentdeposition currentsrdquoAmerican Journal ofMaterials Science vol2 no 2 pp 11ndash14 2012
[9] C Xu B Li H Du F Kang and Y Zeng ldquoElectrochemicalproperties of nanosized hydrous manganese dioxide synthe-sized by a self-reacting microemulsion methodrdquo Journal ofPower Sources vol 180 no 1 pp 664ndash670 2008
12 Journal of Nanomaterials
[10] X Hu X Lin Z Ling Y Li and X Fu ldquoFabrication and char-acteristics of galvanostatic electrodeposited MnO
2on porous
nickel from etched aluminiumrdquo Electrochimica Acta vol 138pp 132ndash138 2014
[11] S Hassan M Suzuki and A A El-Moneim ldquoEffect of Ag-doping on the capacitive behavior of amorphous manganesedioxide electrodesrdquo Electrical and Electronic Engineering vol 2no 2 pp 18ndash22 2012
[12] T Shinomiya V Gupta and N Miura ldquoEffects of electrochem-ical-deposition method and microstructure on the capacitivecharacteristics of nano-sized manganese oxiderdquo ElectrochimicaActa vol 51 no 21 pp 4412ndash4419 2006
[13] M Minakshi P Singh T B Issa S Thurgate and R De MarcoldquoLithium insertion into manganese dioxide electrode inMnO
2
Zn aqueous battery Part II Comparison of the behavior ofEMD and battery grade MnO
2in ZnmdashMnO
2mdash aqueous LiOH
electrolyterdquo Journal of Power Sources vol 138 no 1-2 pp 319ndash322 2004
[14] T Yousefi R Davarkhah A N Golikand and M H Mash-hadizadeh ldquoSynthesis characterization and supercapacitorstudies of manganese (IV) oxide nanowiresrdquo Materials Sciencein Semiconductor Processing vol 16 no 3 pp 868ndash876 2013
[15] J Li and I Zhitomirsky ldquoCathodic electrophoretic depositionof manganese dioxide filmsrdquo Colloids and Surfaces A Physico-chemical and Engineering Aspects vol 348 no 1ndash3 pp 248ndash2532009
[16] J Wei M Cheong N Nagarajan and I Zhitomirsky ldquoCathodicelectrodeposition of manganese oxides for electrochemicalsupercapacitorsrdquo ECS Transactions vol 3 no 37 pp 1ndash9 2007
[17] V Subramanian H Zhu R Vajtai P M Ajayan and B WeildquoHydrothermal synthesis and pseudocapacitance properties ofMnO
2nanostructuresrdquoThe Journal of Physical Chemistry B vol
109 no 43 pp 20207ndash20214 2005[18] C Ye Z M Lin and S Z Hui ldquoElectrochemical and capac-
itance properties of rod-shaped MnO2for supercapacitorrdquo
Journal of the Electrochemical Society vol 152 no 6 pp A1272ndashA1278 2005
[19] Q T Qu P Zhang B Wang et al ldquoElectrochemical perform-ance of MnO
2nanorods in neutral aqueous electrolytes as a
cathode for asymmetric supercapacitorsrdquo The Journal of Phys-ical Chemistry C vol 113 no 31 pp 14020ndash14027 2009
[20] J P Ni W C Lu L M Zhang B H Yue X F Shang and Y LvldquoLow-temperature synthesis of monodisperse 3D manganeseoxide nanoflowers and their pseudocapacitance propertiesrdquoTheJournal of Physical Chemistry C vol 113 no 1 pp 54ndash60 2009
[21] X Wang A Yuan and Y Wang ldquoSupercapacitive behaviorsand their temperature dependence of sol-gel synthesized nanos-tructured manganese dioxide in lithium hydroxide electrolyterdquoJournal of Power Sources vol 172 no 2 pp 1007ndash1011 2007
[22] C Julien M Massot S Rangan M Lemal and D GuyomardldquoStudy of structural defects in 120574-MnO
2byRaman spectroscopyrdquo
Journal of Raman Spectroscopy vol 33 no 4 pp 223ndash228 2002[23] K Rajendra Prasad and N Miura ldquoElectrochemically synthe-
sized MnO2-based mixed oxides for high performance redox
Figure 5 TEM images of EMD and NiMD prepared by CA method (a c) and by CV method (b d) in two minutes
An irregular interconnection of nanofibers formed a highlyporous network By using CV method the nanofiber slightlygrows up The nanoflakes dimension is about 100ndash120 nmin length compared to 40ndash60 nm of which prepared by CAmethod As the deposition time is long the nanofibers growup quickly and the interconnection points seem to be a starform (Figures 3(c) and 3(d) for 12minutes electrodeposition)Due to the variation of surface porosity and nanofibermorphology the specific capacitance behavior is expectedlychanged [1]
In case of Ni mixed MnO2 the surface morphologies
seem to be similar to those of MnO2films (Figure 4) Using
CV method exhibits homogenous surface covered of well-formed nanoflakes
TEM images were performed for Ni mixed MnO2
(NiMD) at the same electrodeposition conditions (Figure 5)In general it was observed that film structure prepared in twominutes is dense The nanoflakes seem to be very fine andtend to interweave to make a network
Wettability test is carried out in order to investigatethe interaction between liquid and thin film surface As
the wettability is high the small contact angle (120579) resultedand the surface is hydrophilic In contrast the surface ishydrophobic Both hydrophilic and hydrophobic propertiescan be applied to many fields from medicine to engineeringespecially in energy storages [1] Figure 6 shows the contactangle measured for MnO
2films prepared by CA and CV
method Both films are expectedly hydrophilic due to thepresence of ndashOH group on the surface and in the structureIndeed the results in Figure 6 indicated that contact angleis less than 90∘ In addition EMD-CV2 thin film is morehydrophilic than EMD-C2The hydrophilicity is essential forelectrochemical reaction on the electrodeaqueous electrolyteinterface
33 Electrochemical Properties of Electrodeposited Thin FilmThe capacitance behavior of MnO
2and NiMD thin films was
tested using CV in potential range of 0-1 V at 100mVs Fig-ure 7 shows CV curves of MnO
2thin films prepared by CA
and CV method in 1M Na2SO4
The specific capacitance (119862sp) was calculated by using(1) EMD was prepared by CA method EMD-CA exhibited
6 Journal of Nanomaterials
CA left 195∘CA right 188∘ EMD-CV2
(a)
CA left 314∘CA right 319∘ EMD-C2
(b)
Figure 6 Water contact angle of EMD-CV2 (a) and EMD-C2 (b)
Curr
ent d
ensit
y (m
Ac
m2)
EMD-C2EMD-C4EMD-C6
EMD-C8EMD-C10EMD-C12
6
4
2
0
minus2
minus4
minus60 02 04 06 08 1
Voltage versus AgAgCl (3M) (V)
(a)
EMD-CV2
EMD-CV4
EMD-CV6
EMD-CV8
EMD-CV10
EMD-CV12
6
4
2
0
minus2
minus4
minus60 02 04 06 08 1
Curr
ent d
ensit
y (m
Ac
m2)
Voltage versus AgAgCl (3M) (V)
(b)
0
50
100
150
200
250
300
350
05 1 15 2 25 3
EMD-CV EMD
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
Film thickness (120583m)
2min
4min
6min
10min
12min
(c)
Figure 7 CV curves of EMD-C and EMD-CV samples in 2M Na2SO4at 100mVs (a b) Plot of 119862sp and thickness of EMD thin films
corresponding to deposition time (c)
Journal of Nanomaterials 7
350
300
250
200
150
1000 200 400 600 800 1000
Number of cycles
Spec
ific c
apac
itanc
e (F
g)
EMD-C2EMD-CV2
Figure 8 Electrochemical stability of EMD-C2 and EMD-CV2 at scan rate 100mVs
the119862sp values that are 250 138 91 79 62 and 48 Fg for EMD-C2 EMD-C4 EMD-C6 EMD-C8 EMD-C10 and EMD-C12 respectively With CV method the 119862sp values are 317112 95 79 60 and 53 Fg for EMD-CV2 EMD-CV4 EMD-CV6 EMD-CV8 EMD-CV10 and EMD-CV12 respectivelyPrevious studies [2 3 6] show that capacitances ofmanganeseoxide synthesized by electrochemical deposition were about200ndash300 Fg
The highest 119862sp was obtained at two minutes of elec-trodeposition for both methods (Figure 7(c)) The thicknessofMnO
2film is about 055 120583mThus it can be concluded that
the thin and porous film was only obtained in a short depo-sition time When the deposition time is long the nanoflakesgrew up fast and their interconnection makes the film morecompact than a short-time deposition Thus penetration ofelectrolyte into the film as well as the specific capacitancedecreases due to the increase of electric resistance The smallthickness of MnO
2film is necessarily required in order to
obtain the good electric conductivity and specific capacitance[4 12]
The electrochemical stability of thin films was evaluatedFigure 8 shows the specific capacitance (119862sp) of EMD-C2and EMD-CV2 measured at a scan rate of 100mVs AmongEMD-C samples the highest initial capacitance was obtainedfor EMD-C2 The 119862sp value keeps constant about 170 Fgafter 200 cycles For EMD-CV2 the 119862sp decreases slightlyafter 100 cycles before increasing significantly to 330 Fg inthe following cycles After 1000 cycles the fading of 119862sp ofEMD-C2 and EMD-CV2 was 23 and 5 respectively (seeFigure 10) Thus the nanostructured EMD-CV2 seems to bebetter for supercapacitor application than EMD-C2 In factthis fade observed for EMD samples can be explained bythe structure evolution in aqueous solution so the metalion doping into MnO
2lattice would improve the structural
stability
The two minutes electrodeposition was chosen to studythe Ni mixed MnO
2(NiMD) at different Mn2+ Ni2+ ratios
Figures 9(a) and 9(b) show theCVelectrochemical propertiesof NiMD samples prepared by two deposition methods Incomparison to the EMD samples the NiMD exhibited thesame shape type of voltammogram but larger magnitudecurrent It can be observed that the CVs are more rectangularand symmetrical but lower in current magnitude at highMn2+Ni2+ ratio than at the low ratio Thus the charac-teristics indicate the ideal capacitive behavior with highreversibility of NiMD sample The highest 119862sp was obtainedfor NiMD samples at Mn2+Ni2+ = 1 1 at all scan rates (Fig-ures 9(c) and 9(d))
The electrochemical stability of NiMD thin films pre-pared at Mn2+Ni2+ = 1 1 was also evaluated (Figure 11) Incomparison to the EMD at the same deposition conditionNiMD samples exhibited higher 119862sp about 500 Fg ver-sus 300 Fg By changing the electrodeposition modes theNiMD-CV shows the 119862sp increase after 500 cycles from430 Fg to 500 Fg while NiMD-CA keeps constant during800 cycles after that a slight fading is observed at the finalcycles After 1000 cycles at 100mVs the fading of 119862sp is lessthan 7 for NiMD samples The fact that Ni ion doping intoMnO2lattice increases119862sp as well as electrochemical stability
was briefly reported by Rajendra Prasad and Miura [23] Inthis work the highest 119862sp was obtained about 480 Fg at100mVs for NiMD sample (65 wt MnO
2 35 wt NiO)
34 Electrochemical Properties of Supercapacitor PrototypeElectrochemical impedance spectroscopy (EIS) measure-ments were also carried out in the frequency range of100 kHzndash01Hz Figure 11 shows the Nyquist plots of EMDthin films prepared at various deposition times by twomethods
8 Journal of Nanomaterials
minus100
minus50
0
50
100
0 02 04 06 08 1
Spec
ific c
urre
nt (A
middotgminus1)
E versus AgAgClNaCl (V)
3 13 2 1 0 (EMD-CV2)1 1
1 2
(a)
minus30
minus20
minus10
0
10
20
30
0 02 04 06 08 1E versus AgAgClNaCl (V)
Spec
ific c
urre
nt (A
middotgminus1)
3 13 2 1 0 (EMD-C2)1 1
1 2
(b)
100
200
300
400
500
600
0 50 100 150 200 250
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
3 13 2 1 21 0
1 1
Scan rate (mVmiddotsminus1)
(c)
100
200
300
400
500
600
0 50 100 150 200 250
3 13 2 1 21 0
1 1
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
Scan rate (mVmiddotsminus1)
(d)
Figure 9 CV curves of Ni mixed MnO2(NiMD samples) prepared by CV method (a) and CA method (b) in 2M Na
2SO4at 50mVs Plot
of 119862sp as a function of scan rate (c d)
TheNyquist plot shows a high-frequency intercept on thereal axis corresponding to the electrolyte resistance (119877
Ω) a
semicircle is considered as a parallel combination of chargetransfer resistance (119877ct) and double-layer capacitance (119862dl)with a linear region at low-frequency range In the low-frequency region linear part of the plot exhibits an anglebetween 45 and 90∘
The EIS data was analyzed by the electrical equivalentcircuit which consists of (119877
Ω) the electrolyte resistance (119877ct)
charge transfer resistance constant phase element (CPE)
used instead of double-layer capacitance (119862dl) Warburg(W) arising from a diffusion controlled process at low-frequency and CPE assigned to pseudocapacitance of thematerial in the low-frequency because of nonideal capacitivebehavior The starting nonzero intercept at Z1015840 at beginningof semicircle is identical in all the curves and its electricalresistance of Na
2SO4around 1Ω For CAmethod the charge
transfer values of EMD-C2 EMD-C6 EMD-C8 and EMD-C12 are 43Ω 102Ω 116Ω and 126Ω respectively For CVmethod the charge transfer values of EMD-CV2 EMD-CV4
Journal of Nanomaterials 9
100
200
300
400
500
600
700
0 200 400 600 800 1000
CV methodChronoamperometry
Number of cycles
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
Figure 10 Electrochemical stability of NiMD samples prepared at Mn2+Ni2+ = 1 1 by two methods at scan rate 100mVs
0
10
20
30
40
50
60
70
80
0 10 20 30 40 50 60 70 80
EMD-CV2EMD-CV4
EMD-CV6EMD-CV12
0
20
40
60
80
100
120
0 20 40 60 80 100 120 160140
EMD-C2EMD-C6
EMD-C8EMD-C12
CPE1
CPE2
W
Real (Z998400) (Ohm) Real (Z998400) (Ohm)
Rs
Rct
minuslm
(Z998400998400
) (O
hm)
minuslm
(Z998400998400
) (O
hm)
Figure 11 Nyquist plot of EMD thin films prepared by two methods and equivalent circuit model used for data fitting
EMD-CV8 and EMD-CV12 are 40Ω 42Ω 47Ω and 55Ωrespectively It means that the increase of deposition time (orfilm thickness) decreases the charge transfer resistanceThusa high resistance caused a difficult charge transfer electronbetween interface electrolyte and interface electrode
At the same deposition conditions the charge transferresistance ofNiMD samples decreases significantly comparedto EMD (Figure 12)
As the Mn2+Ni2+ increase the semicircle diameterbecomes smaller The lowest resistance was obtained forNiIMD at Mn2+Ni2+ = 1 1 about 4Ω versus 10Ω at Mn2+Ni2+ = 1 2 (Figure 13)
The Ni ion addition to the MnO2lattice would improve
the electrical resistivity of MnO2film in addition to enhance
the specific capacitance It maybe also enhanced the powerdensity as well as reversibility of the supercapacitior at
10 Journal of Nanomaterials
0
10
20
30
40
50
60
0 10 20 30 40 50 60Real (Z) (Ohm)
minuslm
(Z) (
Ohm
)
3 13 2 1 01 2
1 1
Figure 12 Nyquist plot of NiMD samples prepared by CA method compared to EMD-C2
0
2
4
6
8
10
0 2 4 6 8 10 12
minuslm
(Z) (
Ohm
)
Real (Z) (Ohm)
1 21 1
Figure 13 Nyquist plot (zoom part) of NiMD samples at Mn2+ Ni2+ = 1 1 and 1 2
high charge-discharge rate Symmetrical capacitor prototypesof 2 cm times 2 cm were assembled with two thin films ofelectrodes EMD-CV2 and NiMD-CV at Mn2+Ni2+ = 1 1and characterized by charge-discharge cycling at very highcurrent 1 Asdotgminus1
Figure 14 shows charge-discharge values of EMD andNIMD sample at current 1 Asdotgminus1 The efficiency of charge-discharge cycling is about 09 Discharge 119862sp values werecalculated by using the current density discharge time andthe weight of material 119862sp values obtained from Figure 14for EMD-CV2 and NiMD-CV are 10 Fsdotgminus1 and 25 Fsdotgminus1
As expected supercapacitor of NiMD-CV based electrodeexhibits a higher power density than EMD-CV2
The stability of supercapacitor based on NiMD-CV andEMD-CV2 was also tested for a large number of cycles (upto 10000 cycles) at 2 Asdotgminus1 A 119862sp fading of 25 and 14was observedwithNiMD-CVorEMD-CV2based capacitorsrespectively after 10000 charge-discharge cycles There is aninitial large 119862sp decrease and then the 119862sp remained almostconstantTheNiMD-CVexhibited expectedly higher stabilitythan EMD-CV2 based capacitorThus it would be promisingfor the long-term capacitor application
Journal of Nanomaterials 11
(a)
0
5
10
15
20
25
30
35
40
0 20 40 60 80 100Cycles number
NiMD-CVEMD-CV
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
(b)
Figure 14 The prototype (left) and the charge-discharge test at 1 Asdotgminus1 using thin film electrode of NiMD-CV and EMD-CV2 (right)
4 Conclusions
Nanoflakes EMD and NiMD with high porosity were pre-pared by two electrochemical depositions modes The thinfilms exhibited the 120574-MnO
2based structure The Ni addition
to MnO2lattice affected the local structure of 120574-MnO
2that
was evidently observed by using Raman spectroscopy TheEMD-CV exhibited better electrochemical behavior (119862sp andcharge-discharge stability) than EMD-C due to the homoge-nous film forming and high porosity during the reduction-oxidation sweep With short-time deposition (2 minutes)EMD thin films show the highest 119862sp and lowest chargetransfer resistance Compared to EMD electrodes preparedat the same deposition NiMD films enhance significantly the119862sp values as well as electrochemical stability The highest119862sp value was 500 Fsdotgminus1 compared to 300 Fsdotgminus1 of EMDThe enhanced electrectrochemical performance of NiMDcan be explained by the increase of thin film conductivitywith the presence of ion Ni2+ Moreover high power densityand excellent stability of assembled supercapacitor based onNiMDelectrodematerials were demonstrated (119862sp of 25 Fsdotg
minus1
at 1 Asdotgminus1 14 capacitance fade for 10000 cycles)
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors acknowledge funding from VNU-HCM underGrant HS2013-76-01 The authors would like to thank Officeof Naval Research Global (ONRG) for Grant N62909-13-1-N235 The authors would like to thank the technical supportof Horibarsquos Representative Company in Vietnam
References
[1] H Xia M O Lai and L Lu ldquoNanostructured manganese oxidethin films as electrode material for supercapacitorsrdquo JOM vol63 no 1 pp 54ndash59 2011
[2] X Zhao B M Sanchez P J Dobson and P S Grant ldquoThe roleof nanomaterials in redox-based supercapacitors for next gen-eration energy storage devicesrdquoNanoscale vol 3 no 3 pp 839ndash855 2011
[3] W Wei X Cui W Chen and D G Ivey ldquoManganese oxide-based materials as electrochemical supercapacitor electrodesrdquoChemical Society Reviews vol 40 no 3 pp 1697ndash1721 2011
[4] J E Post ldquoManganese oxide minerals crystal structures andeconomic and environmental significancerdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 96 no 7 pp 3447ndash3454 1999
[5] S C Pang M A Anderson and T W Chapman ldquoNovel elec-trode materials for thin-film ultracapacitors comparison ofelectrochemical properties of sol-gel-derived and electrode-posited manganese dioxiderdquo Journal of the ElectrochemicalSociety vol 147 no 2 pp 444ndash450 2000
[6] S C Pang and M A Anderson ldquoNovel electrode materials forelectrochemical capacitors Part II Material characterization ofsol-gel-derived and electrodeposited manganese dioxide thinfilmsrdquo Journal of Materials Research vol 15 no 10 pp 2096ndash2106 2000
[7] N Nagarajan H Humadi and I Zhitomirsky ldquoCathodic elec-trodeposition of MnO
ior of manganese dioxidestainless steel electrodes at differentdeposition currentsrdquoAmerican Journal ofMaterials Science vol2 no 2 pp 11ndash14 2012
[9] C Xu B Li H Du F Kang and Y Zeng ldquoElectrochemicalproperties of nanosized hydrous manganese dioxide synthe-sized by a self-reacting microemulsion methodrdquo Journal ofPower Sources vol 180 no 1 pp 664ndash670 2008
12 Journal of Nanomaterials
[10] X Hu X Lin Z Ling Y Li and X Fu ldquoFabrication and char-acteristics of galvanostatic electrodeposited MnO
2on porous
nickel from etched aluminiumrdquo Electrochimica Acta vol 138pp 132ndash138 2014
[11] S Hassan M Suzuki and A A El-Moneim ldquoEffect of Ag-doping on the capacitive behavior of amorphous manganesedioxide electrodesrdquo Electrical and Electronic Engineering vol 2no 2 pp 18ndash22 2012
[12] T Shinomiya V Gupta and N Miura ldquoEffects of electrochem-ical-deposition method and microstructure on the capacitivecharacteristics of nano-sized manganese oxiderdquo ElectrochimicaActa vol 51 no 21 pp 4412ndash4419 2006
[13] M Minakshi P Singh T B Issa S Thurgate and R De MarcoldquoLithium insertion into manganese dioxide electrode inMnO
2
Zn aqueous battery Part II Comparison of the behavior ofEMD and battery grade MnO
2in ZnmdashMnO
2mdash aqueous LiOH
electrolyterdquo Journal of Power Sources vol 138 no 1-2 pp 319ndash322 2004
[14] T Yousefi R Davarkhah A N Golikand and M H Mash-hadizadeh ldquoSynthesis characterization and supercapacitorstudies of manganese (IV) oxide nanowiresrdquo Materials Sciencein Semiconductor Processing vol 16 no 3 pp 868ndash876 2013
[15] J Li and I Zhitomirsky ldquoCathodic electrophoretic depositionof manganese dioxide filmsrdquo Colloids and Surfaces A Physico-chemical and Engineering Aspects vol 348 no 1ndash3 pp 248ndash2532009
[16] J Wei M Cheong N Nagarajan and I Zhitomirsky ldquoCathodicelectrodeposition of manganese oxides for electrochemicalsupercapacitorsrdquo ECS Transactions vol 3 no 37 pp 1ndash9 2007
[17] V Subramanian H Zhu R Vajtai P M Ajayan and B WeildquoHydrothermal synthesis and pseudocapacitance properties ofMnO
2nanostructuresrdquoThe Journal of Physical Chemistry B vol
109 no 43 pp 20207ndash20214 2005[18] C Ye Z M Lin and S Z Hui ldquoElectrochemical and capac-
itance properties of rod-shaped MnO2for supercapacitorrdquo
Journal of the Electrochemical Society vol 152 no 6 pp A1272ndashA1278 2005
[19] Q T Qu P Zhang B Wang et al ldquoElectrochemical perform-ance of MnO
2nanorods in neutral aqueous electrolytes as a
cathode for asymmetric supercapacitorsrdquo The Journal of Phys-ical Chemistry C vol 113 no 31 pp 14020ndash14027 2009
[20] J P Ni W C Lu L M Zhang B H Yue X F Shang and Y LvldquoLow-temperature synthesis of monodisperse 3D manganeseoxide nanoflowers and their pseudocapacitance propertiesrdquoTheJournal of Physical Chemistry C vol 113 no 1 pp 54ndash60 2009
[21] X Wang A Yuan and Y Wang ldquoSupercapacitive behaviorsand their temperature dependence of sol-gel synthesized nanos-tructured manganese dioxide in lithium hydroxide electrolyterdquoJournal of Power Sources vol 172 no 2 pp 1007ndash1011 2007
[22] C Julien M Massot S Rangan M Lemal and D GuyomardldquoStudy of structural defects in 120574-MnO
2byRaman spectroscopyrdquo
Journal of Raman Spectroscopy vol 33 no 4 pp 223ndash228 2002[23] K Rajendra Prasad and N Miura ldquoElectrochemically synthe-
sized MnO2-based mixed oxides for high performance redox
Figure 6 Water contact angle of EMD-CV2 (a) and EMD-C2 (b)
Curr
ent d
ensit
y (m
Ac
m2)
EMD-C2EMD-C4EMD-C6
EMD-C8EMD-C10EMD-C12
6
4
2
0
minus2
minus4
minus60 02 04 06 08 1
Voltage versus AgAgCl (3M) (V)
(a)
EMD-CV2
EMD-CV4
EMD-CV6
EMD-CV8
EMD-CV10
EMD-CV12
6
4
2
0
minus2
minus4
minus60 02 04 06 08 1
Curr
ent d
ensit
y (m
Ac
m2)
Voltage versus AgAgCl (3M) (V)
(b)
0
50
100
150
200
250
300
350
05 1 15 2 25 3
EMD-CV EMD
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
Film thickness (120583m)
2min
4min
6min
10min
12min
(c)
Figure 7 CV curves of EMD-C and EMD-CV samples in 2M Na2SO4at 100mVs (a b) Plot of 119862sp and thickness of EMD thin films
corresponding to deposition time (c)
Journal of Nanomaterials 7
350
300
250
200
150
1000 200 400 600 800 1000
Number of cycles
Spec
ific c
apac
itanc
e (F
g)
EMD-C2EMD-CV2
Figure 8 Electrochemical stability of EMD-C2 and EMD-CV2 at scan rate 100mVs
the119862sp values that are 250 138 91 79 62 and 48 Fg for EMD-C2 EMD-C4 EMD-C6 EMD-C8 EMD-C10 and EMD-C12 respectively With CV method the 119862sp values are 317112 95 79 60 and 53 Fg for EMD-CV2 EMD-CV4 EMD-CV6 EMD-CV8 EMD-CV10 and EMD-CV12 respectivelyPrevious studies [2 3 6] show that capacitances ofmanganeseoxide synthesized by electrochemical deposition were about200ndash300 Fg
The highest 119862sp was obtained at two minutes of elec-trodeposition for both methods (Figure 7(c)) The thicknessofMnO
2film is about 055 120583mThus it can be concluded that
the thin and porous film was only obtained in a short depo-sition time When the deposition time is long the nanoflakesgrew up fast and their interconnection makes the film morecompact than a short-time deposition Thus penetration ofelectrolyte into the film as well as the specific capacitancedecreases due to the increase of electric resistance The smallthickness of MnO
2film is necessarily required in order to
obtain the good electric conductivity and specific capacitance[4 12]
The electrochemical stability of thin films was evaluatedFigure 8 shows the specific capacitance (119862sp) of EMD-C2and EMD-CV2 measured at a scan rate of 100mVs AmongEMD-C samples the highest initial capacitance was obtainedfor EMD-C2 The 119862sp value keeps constant about 170 Fgafter 200 cycles For EMD-CV2 the 119862sp decreases slightlyafter 100 cycles before increasing significantly to 330 Fg inthe following cycles After 1000 cycles the fading of 119862sp ofEMD-C2 and EMD-CV2 was 23 and 5 respectively (seeFigure 10) Thus the nanostructured EMD-CV2 seems to bebetter for supercapacitor application than EMD-C2 In factthis fade observed for EMD samples can be explained bythe structure evolution in aqueous solution so the metalion doping into MnO
2lattice would improve the structural
stability
The two minutes electrodeposition was chosen to studythe Ni mixed MnO
2(NiMD) at different Mn2+ Ni2+ ratios
Figures 9(a) and 9(b) show theCVelectrochemical propertiesof NiMD samples prepared by two deposition methods Incomparison to the EMD samples the NiMD exhibited thesame shape type of voltammogram but larger magnitudecurrent It can be observed that the CVs are more rectangularand symmetrical but lower in current magnitude at highMn2+Ni2+ ratio than at the low ratio Thus the charac-teristics indicate the ideal capacitive behavior with highreversibility of NiMD sample The highest 119862sp was obtainedfor NiMD samples at Mn2+Ni2+ = 1 1 at all scan rates (Fig-ures 9(c) and 9(d))
The electrochemical stability of NiMD thin films pre-pared at Mn2+Ni2+ = 1 1 was also evaluated (Figure 11) Incomparison to the EMD at the same deposition conditionNiMD samples exhibited higher 119862sp about 500 Fg ver-sus 300 Fg By changing the electrodeposition modes theNiMD-CV shows the 119862sp increase after 500 cycles from430 Fg to 500 Fg while NiMD-CA keeps constant during800 cycles after that a slight fading is observed at the finalcycles After 1000 cycles at 100mVs the fading of 119862sp is lessthan 7 for NiMD samples The fact that Ni ion doping intoMnO2lattice increases119862sp as well as electrochemical stability
was briefly reported by Rajendra Prasad and Miura [23] Inthis work the highest 119862sp was obtained about 480 Fg at100mVs for NiMD sample (65 wt MnO
2 35 wt NiO)
34 Electrochemical Properties of Supercapacitor PrototypeElectrochemical impedance spectroscopy (EIS) measure-ments were also carried out in the frequency range of100 kHzndash01Hz Figure 11 shows the Nyquist plots of EMDthin films prepared at various deposition times by twomethods
8 Journal of Nanomaterials
minus100
minus50
0
50
100
0 02 04 06 08 1
Spec
ific c
urre
nt (A
middotgminus1)
E versus AgAgClNaCl (V)
3 13 2 1 0 (EMD-CV2)1 1
1 2
(a)
minus30
minus20
minus10
0
10
20
30
0 02 04 06 08 1E versus AgAgClNaCl (V)
Spec
ific c
urre
nt (A
middotgminus1)
3 13 2 1 0 (EMD-C2)1 1
1 2
(b)
100
200
300
400
500
600
0 50 100 150 200 250
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
3 13 2 1 21 0
1 1
Scan rate (mVmiddotsminus1)
(c)
100
200
300
400
500
600
0 50 100 150 200 250
3 13 2 1 21 0
1 1
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
Scan rate (mVmiddotsminus1)
(d)
Figure 9 CV curves of Ni mixed MnO2(NiMD samples) prepared by CV method (a) and CA method (b) in 2M Na
2SO4at 50mVs Plot
of 119862sp as a function of scan rate (c d)
TheNyquist plot shows a high-frequency intercept on thereal axis corresponding to the electrolyte resistance (119877
Ω) a
semicircle is considered as a parallel combination of chargetransfer resistance (119877ct) and double-layer capacitance (119862dl)with a linear region at low-frequency range In the low-frequency region linear part of the plot exhibits an anglebetween 45 and 90∘
The EIS data was analyzed by the electrical equivalentcircuit which consists of (119877
Ω) the electrolyte resistance (119877ct)
charge transfer resistance constant phase element (CPE)
used instead of double-layer capacitance (119862dl) Warburg(W) arising from a diffusion controlled process at low-frequency and CPE assigned to pseudocapacitance of thematerial in the low-frequency because of nonideal capacitivebehavior The starting nonzero intercept at Z1015840 at beginningof semicircle is identical in all the curves and its electricalresistance of Na
2SO4around 1Ω For CAmethod the charge
transfer values of EMD-C2 EMD-C6 EMD-C8 and EMD-C12 are 43Ω 102Ω 116Ω and 126Ω respectively For CVmethod the charge transfer values of EMD-CV2 EMD-CV4
Journal of Nanomaterials 9
100
200
300
400
500
600
700
0 200 400 600 800 1000
CV methodChronoamperometry
Number of cycles
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
Figure 10 Electrochemical stability of NiMD samples prepared at Mn2+Ni2+ = 1 1 by two methods at scan rate 100mVs
0
10
20
30
40
50
60
70
80
0 10 20 30 40 50 60 70 80
EMD-CV2EMD-CV4
EMD-CV6EMD-CV12
0
20
40
60
80
100
120
0 20 40 60 80 100 120 160140
EMD-C2EMD-C6
EMD-C8EMD-C12
CPE1
CPE2
W
Real (Z998400) (Ohm) Real (Z998400) (Ohm)
Rs
Rct
minuslm
(Z998400998400
) (O
hm)
minuslm
(Z998400998400
) (O
hm)
Figure 11 Nyquist plot of EMD thin films prepared by two methods and equivalent circuit model used for data fitting
EMD-CV8 and EMD-CV12 are 40Ω 42Ω 47Ω and 55Ωrespectively It means that the increase of deposition time (orfilm thickness) decreases the charge transfer resistanceThusa high resistance caused a difficult charge transfer electronbetween interface electrolyte and interface electrode
At the same deposition conditions the charge transferresistance ofNiMD samples decreases significantly comparedto EMD (Figure 12)
As the Mn2+Ni2+ increase the semicircle diameterbecomes smaller The lowest resistance was obtained forNiIMD at Mn2+Ni2+ = 1 1 about 4Ω versus 10Ω at Mn2+Ni2+ = 1 2 (Figure 13)
The Ni ion addition to the MnO2lattice would improve
the electrical resistivity of MnO2film in addition to enhance
the specific capacitance It maybe also enhanced the powerdensity as well as reversibility of the supercapacitior at
10 Journal of Nanomaterials
0
10
20
30
40
50
60
0 10 20 30 40 50 60Real (Z) (Ohm)
minuslm
(Z) (
Ohm
)
3 13 2 1 01 2
1 1
Figure 12 Nyquist plot of NiMD samples prepared by CA method compared to EMD-C2
0
2
4
6
8
10
0 2 4 6 8 10 12
minuslm
(Z) (
Ohm
)
Real (Z) (Ohm)
1 21 1
Figure 13 Nyquist plot (zoom part) of NiMD samples at Mn2+ Ni2+ = 1 1 and 1 2
high charge-discharge rate Symmetrical capacitor prototypesof 2 cm times 2 cm were assembled with two thin films ofelectrodes EMD-CV2 and NiMD-CV at Mn2+Ni2+ = 1 1and characterized by charge-discharge cycling at very highcurrent 1 Asdotgminus1
Figure 14 shows charge-discharge values of EMD andNIMD sample at current 1 Asdotgminus1 The efficiency of charge-discharge cycling is about 09 Discharge 119862sp values werecalculated by using the current density discharge time andthe weight of material 119862sp values obtained from Figure 14for EMD-CV2 and NiMD-CV are 10 Fsdotgminus1 and 25 Fsdotgminus1
As expected supercapacitor of NiMD-CV based electrodeexhibits a higher power density than EMD-CV2
The stability of supercapacitor based on NiMD-CV andEMD-CV2 was also tested for a large number of cycles (upto 10000 cycles) at 2 Asdotgminus1 A 119862sp fading of 25 and 14was observedwithNiMD-CVorEMD-CV2based capacitorsrespectively after 10000 charge-discharge cycles There is aninitial large 119862sp decrease and then the 119862sp remained almostconstantTheNiMD-CVexhibited expectedly higher stabilitythan EMD-CV2 based capacitorThus it would be promisingfor the long-term capacitor application
Journal of Nanomaterials 11
(a)
0
5
10
15
20
25
30
35
40
0 20 40 60 80 100Cycles number
NiMD-CVEMD-CV
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
(b)
Figure 14 The prototype (left) and the charge-discharge test at 1 Asdotgminus1 using thin film electrode of NiMD-CV and EMD-CV2 (right)
4 Conclusions
Nanoflakes EMD and NiMD with high porosity were pre-pared by two electrochemical depositions modes The thinfilms exhibited the 120574-MnO
2based structure The Ni addition
to MnO2lattice affected the local structure of 120574-MnO
2that
was evidently observed by using Raman spectroscopy TheEMD-CV exhibited better electrochemical behavior (119862sp andcharge-discharge stability) than EMD-C due to the homoge-nous film forming and high porosity during the reduction-oxidation sweep With short-time deposition (2 minutes)EMD thin films show the highest 119862sp and lowest chargetransfer resistance Compared to EMD electrodes preparedat the same deposition NiMD films enhance significantly the119862sp values as well as electrochemical stability The highest119862sp value was 500 Fsdotgminus1 compared to 300 Fsdotgminus1 of EMDThe enhanced electrectrochemical performance of NiMDcan be explained by the increase of thin film conductivitywith the presence of ion Ni2+ Moreover high power densityand excellent stability of assembled supercapacitor based onNiMDelectrodematerials were demonstrated (119862sp of 25 Fsdotg
minus1
at 1 Asdotgminus1 14 capacitance fade for 10000 cycles)
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors acknowledge funding from VNU-HCM underGrant HS2013-76-01 The authors would like to thank Officeof Naval Research Global (ONRG) for Grant N62909-13-1-N235 The authors would like to thank the technical supportof Horibarsquos Representative Company in Vietnam
References
[1] H Xia M O Lai and L Lu ldquoNanostructured manganese oxidethin films as electrode material for supercapacitorsrdquo JOM vol63 no 1 pp 54ndash59 2011
[2] X Zhao B M Sanchez P J Dobson and P S Grant ldquoThe roleof nanomaterials in redox-based supercapacitors for next gen-eration energy storage devicesrdquoNanoscale vol 3 no 3 pp 839ndash855 2011
[3] W Wei X Cui W Chen and D G Ivey ldquoManganese oxide-based materials as electrochemical supercapacitor electrodesrdquoChemical Society Reviews vol 40 no 3 pp 1697ndash1721 2011
[4] J E Post ldquoManganese oxide minerals crystal structures andeconomic and environmental significancerdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 96 no 7 pp 3447ndash3454 1999
[5] S C Pang M A Anderson and T W Chapman ldquoNovel elec-trode materials for thin-film ultracapacitors comparison ofelectrochemical properties of sol-gel-derived and electrode-posited manganese dioxiderdquo Journal of the ElectrochemicalSociety vol 147 no 2 pp 444ndash450 2000
[6] S C Pang and M A Anderson ldquoNovel electrode materials forelectrochemical capacitors Part II Material characterization ofsol-gel-derived and electrodeposited manganese dioxide thinfilmsrdquo Journal of Materials Research vol 15 no 10 pp 2096ndash2106 2000
[7] N Nagarajan H Humadi and I Zhitomirsky ldquoCathodic elec-trodeposition of MnO
ior of manganese dioxidestainless steel electrodes at differentdeposition currentsrdquoAmerican Journal ofMaterials Science vol2 no 2 pp 11ndash14 2012
[9] C Xu B Li H Du F Kang and Y Zeng ldquoElectrochemicalproperties of nanosized hydrous manganese dioxide synthe-sized by a self-reacting microemulsion methodrdquo Journal ofPower Sources vol 180 no 1 pp 664ndash670 2008
12 Journal of Nanomaterials
[10] X Hu X Lin Z Ling Y Li and X Fu ldquoFabrication and char-acteristics of galvanostatic electrodeposited MnO
2on porous
nickel from etched aluminiumrdquo Electrochimica Acta vol 138pp 132ndash138 2014
[11] S Hassan M Suzuki and A A El-Moneim ldquoEffect of Ag-doping on the capacitive behavior of amorphous manganesedioxide electrodesrdquo Electrical and Electronic Engineering vol 2no 2 pp 18ndash22 2012
[12] T Shinomiya V Gupta and N Miura ldquoEffects of electrochem-ical-deposition method and microstructure on the capacitivecharacteristics of nano-sized manganese oxiderdquo ElectrochimicaActa vol 51 no 21 pp 4412ndash4419 2006
[13] M Minakshi P Singh T B Issa S Thurgate and R De MarcoldquoLithium insertion into manganese dioxide electrode inMnO
2
Zn aqueous battery Part II Comparison of the behavior ofEMD and battery grade MnO
2in ZnmdashMnO
2mdash aqueous LiOH
electrolyterdquo Journal of Power Sources vol 138 no 1-2 pp 319ndash322 2004
[14] T Yousefi R Davarkhah A N Golikand and M H Mash-hadizadeh ldquoSynthesis characterization and supercapacitorstudies of manganese (IV) oxide nanowiresrdquo Materials Sciencein Semiconductor Processing vol 16 no 3 pp 868ndash876 2013
[15] J Li and I Zhitomirsky ldquoCathodic electrophoretic depositionof manganese dioxide filmsrdquo Colloids and Surfaces A Physico-chemical and Engineering Aspects vol 348 no 1ndash3 pp 248ndash2532009
[16] J Wei M Cheong N Nagarajan and I Zhitomirsky ldquoCathodicelectrodeposition of manganese oxides for electrochemicalsupercapacitorsrdquo ECS Transactions vol 3 no 37 pp 1ndash9 2007
[17] V Subramanian H Zhu R Vajtai P M Ajayan and B WeildquoHydrothermal synthesis and pseudocapacitance properties ofMnO
2nanostructuresrdquoThe Journal of Physical Chemistry B vol
109 no 43 pp 20207ndash20214 2005[18] C Ye Z M Lin and S Z Hui ldquoElectrochemical and capac-
itance properties of rod-shaped MnO2for supercapacitorrdquo
Journal of the Electrochemical Society vol 152 no 6 pp A1272ndashA1278 2005
[19] Q T Qu P Zhang B Wang et al ldquoElectrochemical perform-ance of MnO
2nanorods in neutral aqueous electrolytes as a
cathode for asymmetric supercapacitorsrdquo The Journal of Phys-ical Chemistry C vol 113 no 31 pp 14020ndash14027 2009
[20] J P Ni W C Lu L M Zhang B H Yue X F Shang and Y LvldquoLow-temperature synthesis of monodisperse 3D manganeseoxide nanoflowers and their pseudocapacitance propertiesrdquoTheJournal of Physical Chemistry C vol 113 no 1 pp 54ndash60 2009
[21] X Wang A Yuan and Y Wang ldquoSupercapacitive behaviorsand their temperature dependence of sol-gel synthesized nanos-tructured manganese dioxide in lithium hydroxide electrolyterdquoJournal of Power Sources vol 172 no 2 pp 1007ndash1011 2007
[22] C Julien M Massot S Rangan M Lemal and D GuyomardldquoStudy of structural defects in 120574-MnO
2byRaman spectroscopyrdquo
Journal of Raman Spectroscopy vol 33 no 4 pp 223ndash228 2002[23] K Rajendra Prasad and N Miura ldquoElectrochemically synthe-
sized MnO2-based mixed oxides for high performance redox
Figure 8 Electrochemical stability of EMD-C2 and EMD-CV2 at scan rate 100mVs
the119862sp values that are 250 138 91 79 62 and 48 Fg for EMD-C2 EMD-C4 EMD-C6 EMD-C8 EMD-C10 and EMD-C12 respectively With CV method the 119862sp values are 317112 95 79 60 and 53 Fg for EMD-CV2 EMD-CV4 EMD-CV6 EMD-CV8 EMD-CV10 and EMD-CV12 respectivelyPrevious studies [2 3 6] show that capacitances ofmanganeseoxide synthesized by electrochemical deposition were about200ndash300 Fg
The highest 119862sp was obtained at two minutes of elec-trodeposition for both methods (Figure 7(c)) The thicknessofMnO
2film is about 055 120583mThus it can be concluded that
the thin and porous film was only obtained in a short depo-sition time When the deposition time is long the nanoflakesgrew up fast and their interconnection makes the film morecompact than a short-time deposition Thus penetration ofelectrolyte into the film as well as the specific capacitancedecreases due to the increase of electric resistance The smallthickness of MnO
2film is necessarily required in order to
obtain the good electric conductivity and specific capacitance[4 12]
The electrochemical stability of thin films was evaluatedFigure 8 shows the specific capacitance (119862sp) of EMD-C2and EMD-CV2 measured at a scan rate of 100mVs AmongEMD-C samples the highest initial capacitance was obtainedfor EMD-C2 The 119862sp value keeps constant about 170 Fgafter 200 cycles For EMD-CV2 the 119862sp decreases slightlyafter 100 cycles before increasing significantly to 330 Fg inthe following cycles After 1000 cycles the fading of 119862sp ofEMD-C2 and EMD-CV2 was 23 and 5 respectively (seeFigure 10) Thus the nanostructured EMD-CV2 seems to bebetter for supercapacitor application than EMD-C2 In factthis fade observed for EMD samples can be explained bythe structure evolution in aqueous solution so the metalion doping into MnO
2lattice would improve the structural
stability
The two minutes electrodeposition was chosen to studythe Ni mixed MnO
2(NiMD) at different Mn2+ Ni2+ ratios
Figures 9(a) and 9(b) show theCVelectrochemical propertiesof NiMD samples prepared by two deposition methods Incomparison to the EMD samples the NiMD exhibited thesame shape type of voltammogram but larger magnitudecurrent It can be observed that the CVs are more rectangularand symmetrical but lower in current magnitude at highMn2+Ni2+ ratio than at the low ratio Thus the charac-teristics indicate the ideal capacitive behavior with highreversibility of NiMD sample The highest 119862sp was obtainedfor NiMD samples at Mn2+Ni2+ = 1 1 at all scan rates (Fig-ures 9(c) and 9(d))
The electrochemical stability of NiMD thin films pre-pared at Mn2+Ni2+ = 1 1 was also evaluated (Figure 11) Incomparison to the EMD at the same deposition conditionNiMD samples exhibited higher 119862sp about 500 Fg ver-sus 300 Fg By changing the electrodeposition modes theNiMD-CV shows the 119862sp increase after 500 cycles from430 Fg to 500 Fg while NiMD-CA keeps constant during800 cycles after that a slight fading is observed at the finalcycles After 1000 cycles at 100mVs the fading of 119862sp is lessthan 7 for NiMD samples The fact that Ni ion doping intoMnO2lattice increases119862sp as well as electrochemical stability
was briefly reported by Rajendra Prasad and Miura [23] Inthis work the highest 119862sp was obtained about 480 Fg at100mVs for NiMD sample (65 wt MnO
2 35 wt NiO)
34 Electrochemical Properties of Supercapacitor PrototypeElectrochemical impedance spectroscopy (EIS) measure-ments were also carried out in the frequency range of100 kHzndash01Hz Figure 11 shows the Nyquist plots of EMDthin films prepared at various deposition times by twomethods
8 Journal of Nanomaterials
minus100
minus50
0
50
100
0 02 04 06 08 1
Spec
ific c
urre
nt (A
middotgminus1)
E versus AgAgClNaCl (V)
3 13 2 1 0 (EMD-CV2)1 1
1 2
(a)
minus30
minus20
minus10
0
10
20
30
0 02 04 06 08 1E versus AgAgClNaCl (V)
Spec
ific c
urre
nt (A
middotgminus1)
3 13 2 1 0 (EMD-C2)1 1
1 2
(b)
100
200
300
400
500
600
0 50 100 150 200 250
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
3 13 2 1 21 0
1 1
Scan rate (mVmiddotsminus1)
(c)
100
200
300
400
500
600
0 50 100 150 200 250
3 13 2 1 21 0
1 1
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
Scan rate (mVmiddotsminus1)
(d)
Figure 9 CV curves of Ni mixed MnO2(NiMD samples) prepared by CV method (a) and CA method (b) in 2M Na
2SO4at 50mVs Plot
of 119862sp as a function of scan rate (c d)
TheNyquist plot shows a high-frequency intercept on thereal axis corresponding to the electrolyte resistance (119877
Ω) a
semicircle is considered as a parallel combination of chargetransfer resistance (119877ct) and double-layer capacitance (119862dl)with a linear region at low-frequency range In the low-frequency region linear part of the plot exhibits an anglebetween 45 and 90∘
The EIS data was analyzed by the electrical equivalentcircuit which consists of (119877
Ω) the electrolyte resistance (119877ct)
charge transfer resistance constant phase element (CPE)
used instead of double-layer capacitance (119862dl) Warburg(W) arising from a diffusion controlled process at low-frequency and CPE assigned to pseudocapacitance of thematerial in the low-frequency because of nonideal capacitivebehavior The starting nonzero intercept at Z1015840 at beginningof semicircle is identical in all the curves and its electricalresistance of Na
2SO4around 1Ω For CAmethod the charge
transfer values of EMD-C2 EMD-C6 EMD-C8 and EMD-C12 are 43Ω 102Ω 116Ω and 126Ω respectively For CVmethod the charge transfer values of EMD-CV2 EMD-CV4
Journal of Nanomaterials 9
100
200
300
400
500
600
700
0 200 400 600 800 1000
CV methodChronoamperometry
Number of cycles
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
Figure 10 Electrochemical stability of NiMD samples prepared at Mn2+Ni2+ = 1 1 by two methods at scan rate 100mVs
0
10
20
30
40
50
60
70
80
0 10 20 30 40 50 60 70 80
EMD-CV2EMD-CV4
EMD-CV6EMD-CV12
0
20
40
60
80
100
120
0 20 40 60 80 100 120 160140
EMD-C2EMD-C6
EMD-C8EMD-C12
CPE1
CPE2
W
Real (Z998400) (Ohm) Real (Z998400) (Ohm)
Rs
Rct
minuslm
(Z998400998400
) (O
hm)
minuslm
(Z998400998400
) (O
hm)
Figure 11 Nyquist plot of EMD thin films prepared by two methods and equivalent circuit model used for data fitting
EMD-CV8 and EMD-CV12 are 40Ω 42Ω 47Ω and 55Ωrespectively It means that the increase of deposition time (orfilm thickness) decreases the charge transfer resistanceThusa high resistance caused a difficult charge transfer electronbetween interface electrolyte and interface electrode
At the same deposition conditions the charge transferresistance ofNiMD samples decreases significantly comparedto EMD (Figure 12)
As the Mn2+Ni2+ increase the semicircle diameterbecomes smaller The lowest resistance was obtained forNiIMD at Mn2+Ni2+ = 1 1 about 4Ω versus 10Ω at Mn2+Ni2+ = 1 2 (Figure 13)
The Ni ion addition to the MnO2lattice would improve
the electrical resistivity of MnO2film in addition to enhance
the specific capacitance It maybe also enhanced the powerdensity as well as reversibility of the supercapacitior at
10 Journal of Nanomaterials
0
10
20
30
40
50
60
0 10 20 30 40 50 60Real (Z) (Ohm)
minuslm
(Z) (
Ohm
)
3 13 2 1 01 2
1 1
Figure 12 Nyquist plot of NiMD samples prepared by CA method compared to EMD-C2
0
2
4
6
8
10
0 2 4 6 8 10 12
minuslm
(Z) (
Ohm
)
Real (Z) (Ohm)
1 21 1
Figure 13 Nyquist plot (zoom part) of NiMD samples at Mn2+ Ni2+ = 1 1 and 1 2
high charge-discharge rate Symmetrical capacitor prototypesof 2 cm times 2 cm were assembled with two thin films ofelectrodes EMD-CV2 and NiMD-CV at Mn2+Ni2+ = 1 1and characterized by charge-discharge cycling at very highcurrent 1 Asdotgminus1
Figure 14 shows charge-discharge values of EMD andNIMD sample at current 1 Asdotgminus1 The efficiency of charge-discharge cycling is about 09 Discharge 119862sp values werecalculated by using the current density discharge time andthe weight of material 119862sp values obtained from Figure 14for EMD-CV2 and NiMD-CV are 10 Fsdotgminus1 and 25 Fsdotgminus1
As expected supercapacitor of NiMD-CV based electrodeexhibits a higher power density than EMD-CV2
The stability of supercapacitor based on NiMD-CV andEMD-CV2 was also tested for a large number of cycles (upto 10000 cycles) at 2 Asdotgminus1 A 119862sp fading of 25 and 14was observedwithNiMD-CVorEMD-CV2based capacitorsrespectively after 10000 charge-discharge cycles There is aninitial large 119862sp decrease and then the 119862sp remained almostconstantTheNiMD-CVexhibited expectedly higher stabilitythan EMD-CV2 based capacitorThus it would be promisingfor the long-term capacitor application
Journal of Nanomaterials 11
(a)
0
5
10
15
20
25
30
35
40
0 20 40 60 80 100Cycles number
NiMD-CVEMD-CV
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
(b)
Figure 14 The prototype (left) and the charge-discharge test at 1 Asdotgminus1 using thin film electrode of NiMD-CV and EMD-CV2 (right)
4 Conclusions
Nanoflakes EMD and NiMD with high porosity were pre-pared by two electrochemical depositions modes The thinfilms exhibited the 120574-MnO
2based structure The Ni addition
to MnO2lattice affected the local structure of 120574-MnO
2that
was evidently observed by using Raman spectroscopy TheEMD-CV exhibited better electrochemical behavior (119862sp andcharge-discharge stability) than EMD-C due to the homoge-nous film forming and high porosity during the reduction-oxidation sweep With short-time deposition (2 minutes)EMD thin films show the highest 119862sp and lowest chargetransfer resistance Compared to EMD electrodes preparedat the same deposition NiMD films enhance significantly the119862sp values as well as electrochemical stability The highest119862sp value was 500 Fsdotgminus1 compared to 300 Fsdotgminus1 of EMDThe enhanced electrectrochemical performance of NiMDcan be explained by the increase of thin film conductivitywith the presence of ion Ni2+ Moreover high power densityand excellent stability of assembled supercapacitor based onNiMDelectrodematerials were demonstrated (119862sp of 25 Fsdotg
minus1
at 1 Asdotgminus1 14 capacitance fade for 10000 cycles)
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors acknowledge funding from VNU-HCM underGrant HS2013-76-01 The authors would like to thank Officeof Naval Research Global (ONRG) for Grant N62909-13-1-N235 The authors would like to thank the technical supportof Horibarsquos Representative Company in Vietnam
References
[1] H Xia M O Lai and L Lu ldquoNanostructured manganese oxidethin films as electrode material for supercapacitorsrdquo JOM vol63 no 1 pp 54ndash59 2011
[2] X Zhao B M Sanchez P J Dobson and P S Grant ldquoThe roleof nanomaterials in redox-based supercapacitors for next gen-eration energy storage devicesrdquoNanoscale vol 3 no 3 pp 839ndash855 2011
[3] W Wei X Cui W Chen and D G Ivey ldquoManganese oxide-based materials as electrochemical supercapacitor electrodesrdquoChemical Society Reviews vol 40 no 3 pp 1697ndash1721 2011
[4] J E Post ldquoManganese oxide minerals crystal structures andeconomic and environmental significancerdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 96 no 7 pp 3447ndash3454 1999
[5] S C Pang M A Anderson and T W Chapman ldquoNovel elec-trode materials for thin-film ultracapacitors comparison ofelectrochemical properties of sol-gel-derived and electrode-posited manganese dioxiderdquo Journal of the ElectrochemicalSociety vol 147 no 2 pp 444ndash450 2000
[6] S C Pang and M A Anderson ldquoNovel electrode materials forelectrochemical capacitors Part II Material characterization ofsol-gel-derived and electrodeposited manganese dioxide thinfilmsrdquo Journal of Materials Research vol 15 no 10 pp 2096ndash2106 2000
[7] N Nagarajan H Humadi and I Zhitomirsky ldquoCathodic elec-trodeposition of MnO
ior of manganese dioxidestainless steel electrodes at differentdeposition currentsrdquoAmerican Journal ofMaterials Science vol2 no 2 pp 11ndash14 2012
[9] C Xu B Li H Du F Kang and Y Zeng ldquoElectrochemicalproperties of nanosized hydrous manganese dioxide synthe-sized by a self-reacting microemulsion methodrdquo Journal ofPower Sources vol 180 no 1 pp 664ndash670 2008
12 Journal of Nanomaterials
[10] X Hu X Lin Z Ling Y Li and X Fu ldquoFabrication and char-acteristics of galvanostatic electrodeposited MnO
2on porous
nickel from etched aluminiumrdquo Electrochimica Acta vol 138pp 132ndash138 2014
[11] S Hassan M Suzuki and A A El-Moneim ldquoEffect of Ag-doping on the capacitive behavior of amorphous manganesedioxide electrodesrdquo Electrical and Electronic Engineering vol 2no 2 pp 18ndash22 2012
[12] T Shinomiya V Gupta and N Miura ldquoEffects of electrochem-ical-deposition method and microstructure on the capacitivecharacteristics of nano-sized manganese oxiderdquo ElectrochimicaActa vol 51 no 21 pp 4412ndash4419 2006
[13] M Minakshi P Singh T B Issa S Thurgate and R De MarcoldquoLithium insertion into manganese dioxide electrode inMnO
2
Zn aqueous battery Part II Comparison of the behavior ofEMD and battery grade MnO
2in ZnmdashMnO
2mdash aqueous LiOH
electrolyterdquo Journal of Power Sources vol 138 no 1-2 pp 319ndash322 2004
[14] T Yousefi R Davarkhah A N Golikand and M H Mash-hadizadeh ldquoSynthesis characterization and supercapacitorstudies of manganese (IV) oxide nanowiresrdquo Materials Sciencein Semiconductor Processing vol 16 no 3 pp 868ndash876 2013
[15] J Li and I Zhitomirsky ldquoCathodic electrophoretic depositionof manganese dioxide filmsrdquo Colloids and Surfaces A Physico-chemical and Engineering Aspects vol 348 no 1ndash3 pp 248ndash2532009
[16] J Wei M Cheong N Nagarajan and I Zhitomirsky ldquoCathodicelectrodeposition of manganese oxides for electrochemicalsupercapacitorsrdquo ECS Transactions vol 3 no 37 pp 1ndash9 2007
[17] V Subramanian H Zhu R Vajtai P M Ajayan and B WeildquoHydrothermal synthesis and pseudocapacitance properties ofMnO
2nanostructuresrdquoThe Journal of Physical Chemistry B vol
109 no 43 pp 20207ndash20214 2005[18] C Ye Z M Lin and S Z Hui ldquoElectrochemical and capac-
itance properties of rod-shaped MnO2for supercapacitorrdquo
Journal of the Electrochemical Society vol 152 no 6 pp A1272ndashA1278 2005
[19] Q T Qu P Zhang B Wang et al ldquoElectrochemical perform-ance of MnO
2nanorods in neutral aqueous electrolytes as a
cathode for asymmetric supercapacitorsrdquo The Journal of Phys-ical Chemistry C vol 113 no 31 pp 14020ndash14027 2009
[20] J P Ni W C Lu L M Zhang B H Yue X F Shang and Y LvldquoLow-temperature synthesis of monodisperse 3D manganeseoxide nanoflowers and their pseudocapacitance propertiesrdquoTheJournal of Physical Chemistry C vol 113 no 1 pp 54ndash60 2009
[21] X Wang A Yuan and Y Wang ldquoSupercapacitive behaviorsand their temperature dependence of sol-gel synthesized nanos-tructured manganese dioxide in lithium hydroxide electrolyterdquoJournal of Power Sources vol 172 no 2 pp 1007ndash1011 2007
[22] C Julien M Massot S Rangan M Lemal and D GuyomardldquoStudy of structural defects in 120574-MnO
2byRaman spectroscopyrdquo
Journal of Raman Spectroscopy vol 33 no 4 pp 223ndash228 2002[23] K Rajendra Prasad and N Miura ldquoElectrochemically synthe-
sized MnO2-based mixed oxides for high performance redox
Figure 9 CV curves of Ni mixed MnO2(NiMD samples) prepared by CV method (a) and CA method (b) in 2M Na
2SO4at 50mVs Plot
of 119862sp as a function of scan rate (c d)
TheNyquist plot shows a high-frequency intercept on thereal axis corresponding to the electrolyte resistance (119877
Ω) a
semicircle is considered as a parallel combination of chargetransfer resistance (119877ct) and double-layer capacitance (119862dl)with a linear region at low-frequency range In the low-frequency region linear part of the plot exhibits an anglebetween 45 and 90∘
The EIS data was analyzed by the electrical equivalentcircuit which consists of (119877
Ω) the electrolyte resistance (119877ct)
charge transfer resistance constant phase element (CPE)
used instead of double-layer capacitance (119862dl) Warburg(W) arising from a diffusion controlled process at low-frequency and CPE assigned to pseudocapacitance of thematerial in the low-frequency because of nonideal capacitivebehavior The starting nonzero intercept at Z1015840 at beginningof semicircle is identical in all the curves and its electricalresistance of Na
2SO4around 1Ω For CAmethod the charge
transfer values of EMD-C2 EMD-C6 EMD-C8 and EMD-C12 are 43Ω 102Ω 116Ω and 126Ω respectively For CVmethod the charge transfer values of EMD-CV2 EMD-CV4
Journal of Nanomaterials 9
100
200
300
400
500
600
700
0 200 400 600 800 1000
CV methodChronoamperometry
Number of cycles
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
Figure 10 Electrochemical stability of NiMD samples prepared at Mn2+Ni2+ = 1 1 by two methods at scan rate 100mVs
0
10
20
30
40
50
60
70
80
0 10 20 30 40 50 60 70 80
EMD-CV2EMD-CV4
EMD-CV6EMD-CV12
0
20
40
60
80
100
120
0 20 40 60 80 100 120 160140
EMD-C2EMD-C6
EMD-C8EMD-C12
CPE1
CPE2
W
Real (Z998400) (Ohm) Real (Z998400) (Ohm)
Rs
Rct
minuslm
(Z998400998400
) (O
hm)
minuslm
(Z998400998400
) (O
hm)
Figure 11 Nyquist plot of EMD thin films prepared by two methods and equivalent circuit model used for data fitting
EMD-CV8 and EMD-CV12 are 40Ω 42Ω 47Ω and 55Ωrespectively It means that the increase of deposition time (orfilm thickness) decreases the charge transfer resistanceThusa high resistance caused a difficult charge transfer electronbetween interface electrolyte and interface electrode
At the same deposition conditions the charge transferresistance ofNiMD samples decreases significantly comparedto EMD (Figure 12)
As the Mn2+Ni2+ increase the semicircle diameterbecomes smaller The lowest resistance was obtained forNiIMD at Mn2+Ni2+ = 1 1 about 4Ω versus 10Ω at Mn2+Ni2+ = 1 2 (Figure 13)
The Ni ion addition to the MnO2lattice would improve
the electrical resistivity of MnO2film in addition to enhance
the specific capacitance It maybe also enhanced the powerdensity as well as reversibility of the supercapacitior at
10 Journal of Nanomaterials
0
10
20
30
40
50
60
0 10 20 30 40 50 60Real (Z) (Ohm)
minuslm
(Z) (
Ohm
)
3 13 2 1 01 2
1 1
Figure 12 Nyquist plot of NiMD samples prepared by CA method compared to EMD-C2
0
2
4
6
8
10
0 2 4 6 8 10 12
minuslm
(Z) (
Ohm
)
Real (Z) (Ohm)
1 21 1
Figure 13 Nyquist plot (zoom part) of NiMD samples at Mn2+ Ni2+ = 1 1 and 1 2
high charge-discharge rate Symmetrical capacitor prototypesof 2 cm times 2 cm were assembled with two thin films ofelectrodes EMD-CV2 and NiMD-CV at Mn2+Ni2+ = 1 1and characterized by charge-discharge cycling at very highcurrent 1 Asdotgminus1
Figure 14 shows charge-discharge values of EMD andNIMD sample at current 1 Asdotgminus1 The efficiency of charge-discharge cycling is about 09 Discharge 119862sp values werecalculated by using the current density discharge time andthe weight of material 119862sp values obtained from Figure 14for EMD-CV2 and NiMD-CV are 10 Fsdotgminus1 and 25 Fsdotgminus1
As expected supercapacitor of NiMD-CV based electrodeexhibits a higher power density than EMD-CV2
The stability of supercapacitor based on NiMD-CV andEMD-CV2 was also tested for a large number of cycles (upto 10000 cycles) at 2 Asdotgminus1 A 119862sp fading of 25 and 14was observedwithNiMD-CVorEMD-CV2based capacitorsrespectively after 10000 charge-discharge cycles There is aninitial large 119862sp decrease and then the 119862sp remained almostconstantTheNiMD-CVexhibited expectedly higher stabilitythan EMD-CV2 based capacitorThus it would be promisingfor the long-term capacitor application
Journal of Nanomaterials 11
(a)
0
5
10
15
20
25
30
35
40
0 20 40 60 80 100Cycles number
NiMD-CVEMD-CV
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
(b)
Figure 14 The prototype (left) and the charge-discharge test at 1 Asdotgminus1 using thin film electrode of NiMD-CV and EMD-CV2 (right)
4 Conclusions
Nanoflakes EMD and NiMD with high porosity were pre-pared by two electrochemical depositions modes The thinfilms exhibited the 120574-MnO
2based structure The Ni addition
to MnO2lattice affected the local structure of 120574-MnO
2that
was evidently observed by using Raman spectroscopy TheEMD-CV exhibited better electrochemical behavior (119862sp andcharge-discharge stability) than EMD-C due to the homoge-nous film forming and high porosity during the reduction-oxidation sweep With short-time deposition (2 minutes)EMD thin films show the highest 119862sp and lowest chargetransfer resistance Compared to EMD electrodes preparedat the same deposition NiMD films enhance significantly the119862sp values as well as electrochemical stability The highest119862sp value was 500 Fsdotgminus1 compared to 300 Fsdotgminus1 of EMDThe enhanced electrectrochemical performance of NiMDcan be explained by the increase of thin film conductivitywith the presence of ion Ni2+ Moreover high power densityand excellent stability of assembled supercapacitor based onNiMDelectrodematerials were demonstrated (119862sp of 25 Fsdotg
minus1
at 1 Asdotgminus1 14 capacitance fade for 10000 cycles)
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors acknowledge funding from VNU-HCM underGrant HS2013-76-01 The authors would like to thank Officeof Naval Research Global (ONRG) for Grant N62909-13-1-N235 The authors would like to thank the technical supportof Horibarsquos Representative Company in Vietnam
References
[1] H Xia M O Lai and L Lu ldquoNanostructured manganese oxidethin films as electrode material for supercapacitorsrdquo JOM vol63 no 1 pp 54ndash59 2011
[2] X Zhao B M Sanchez P J Dobson and P S Grant ldquoThe roleof nanomaterials in redox-based supercapacitors for next gen-eration energy storage devicesrdquoNanoscale vol 3 no 3 pp 839ndash855 2011
[3] W Wei X Cui W Chen and D G Ivey ldquoManganese oxide-based materials as electrochemical supercapacitor electrodesrdquoChemical Society Reviews vol 40 no 3 pp 1697ndash1721 2011
[4] J E Post ldquoManganese oxide minerals crystal structures andeconomic and environmental significancerdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 96 no 7 pp 3447ndash3454 1999
[5] S C Pang M A Anderson and T W Chapman ldquoNovel elec-trode materials for thin-film ultracapacitors comparison ofelectrochemical properties of sol-gel-derived and electrode-posited manganese dioxiderdquo Journal of the ElectrochemicalSociety vol 147 no 2 pp 444ndash450 2000
[6] S C Pang and M A Anderson ldquoNovel electrode materials forelectrochemical capacitors Part II Material characterization ofsol-gel-derived and electrodeposited manganese dioxide thinfilmsrdquo Journal of Materials Research vol 15 no 10 pp 2096ndash2106 2000
[7] N Nagarajan H Humadi and I Zhitomirsky ldquoCathodic elec-trodeposition of MnO
ior of manganese dioxidestainless steel electrodes at differentdeposition currentsrdquoAmerican Journal ofMaterials Science vol2 no 2 pp 11ndash14 2012
[9] C Xu B Li H Du F Kang and Y Zeng ldquoElectrochemicalproperties of nanosized hydrous manganese dioxide synthe-sized by a self-reacting microemulsion methodrdquo Journal ofPower Sources vol 180 no 1 pp 664ndash670 2008
12 Journal of Nanomaterials
[10] X Hu X Lin Z Ling Y Li and X Fu ldquoFabrication and char-acteristics of galvanostatic electrodeposited MnO
2on porous
nickel from etched aluminiumrdquo Electrochimica Acta vol 138pp 132ndash138 2014
[11] S Hassan M Suzuki and A A El-Moneim ldquoEffect of Ag-doping on the capacitive behavior of amorphous manganesedioxide electrodesrdquo Electrical and Electronic Engineering vol 2no 2 pp 18ndash22 2012
[12] T Shinomiya V Gupta and N Miura ldquoEffects of electrochem-ical-deposition method and microstructure on the capacitivecharacteristics of nano-sized manganese oxiderdquo ElectrochimicaActa vol 51 no 21 pp 4412ndash4419 2006
[13] M Minakshi P Singh T B Issa S Thurgate and R De MarcoldquoLithium insertion into manganese dioxide electrode inMnO
2
Zn aqueous battery Part II Comparison of the behavior ofEMD and battery grade MnO
2in ZnmdashMnO
2mdash aqueous LiOH
electrolyterdquo Journal of Power Sources vol 138 no 1-2 pp 319ndash322 2004
[14] T Yousefi R Davarkhah A N Golikand and M H Mash-hadizadeh ldquoSynthesis characterization and supercapacitorstudies of manganese (IV) oxide nanowiresrdquo Materials Sciencein Semiconductor Processing vol 16 no 3 pp 868ndash876 2013
[15] J Li and I Zhitomirsky ldquoCathodic electrophoretic depositionof manganese dioxide filmsrdquo Colloids and Surfaces A Physico-chemical and Engineering Aspects vol 348 no 1ndash3 pp 248ndash2532009
[16] J Wei M Cheong N Nagarajan and I Zhitomirsky ldquoCathodicelectrodeposition of manganese oxides for electrochemicalsupercapacitorsrdquo ECS Transactions vol 3 no 37 pp 1ndash9 2007
[17] V Subramanian H Zhu R Vajtai P M Ajayan and B WeildquoHydrothermal synthesis and pseudocapacitance properties ofMnO
2nanostructuresrdquoThe Journal of Physical Chemistry B vol
109 no 43 pp 20207ndash20214 2005[18] C Ye Z M Lin and S Z Hui ldquoElectrochemical and capac-
itance properties of rod-shaped MnO2for supercapacitorrdquo
Journal of the Electrochemical Society vol 152 no 6 pp A1272ndashA1278 2005
[19] Q T Qu P Zhang B Wang et al ldquoElectrochemical perform-ance of MnO
2nanorods in neutral aqueous electrolytes as a
cathode for asymmetric supercapacitorsrdquo The Journal of Phys-ical Chemistry C vol 113 no 31 pp 14020ndash14027 2009
[20] J P Ni W C Lu L M Zhang B H Yue X F Shang and Y LvldquoLow-temperature synthesis of monodisperse 3D manganeseoxide nanoflowers and their pseudocapacitance propertiesrdquoTheJournal of Physical Chemistry C vol 113 no 1 pp 54ndash60 2009
[21] X Wang A Yuan and Y Wang ldquoSupercapacitive behaviorsand their temperature dependence of sol-gel synthesized nanos-tructured manganese dioxide in lithium hydroxide electrolyterdquoJournal of Power Sources vol 172 no 2 pp 1007ndash1011 2007
[22] C Julien M Massot S Rangan M Lemal and D GuyomardldquoStudy of structural defects in 120574-MnO
2byRaman spectroscopyrdquo
Journal of Raman Spectroscopy vol 33 no 4 pp 223ndash228 2002[23] K Rajendra Prasad and N Miura ldquoElectrochemically synthe-
sized MnO2-based mixed oxides for high performance redox
Figure 10 Electrochemical stability of NiMD samples prepared at Mn2+Ni2+ = 1 1 by two methods at scan rate 100mVs
0
10
20
30
40
50
60
70
80
0 10 20 30 40 50 60 70 80
EMD-CV2EMD-CV4
EMD-CV6EMD-CV12
0
20
40
60
80
100
120
0 20 40 60 80 100 120 160140
EMD-C2EMD-C6
EMD-C8EMD-C12
CPE1
CPE2
W
Real (Z998400) (Ohm) Real (Z998400) (Ohm)
Rs
Rct
minuslm
(Z998400998400
) (O
hm)
minuslm
(Z998400998400
) (O
hm)
Figure 11 Nyquist plot of EMD thin films prepared by two methods and equivalent circuit model used for data fitting
EMD-CV8 and EMD-CV12 are 40Ω 42Ω 47Ω and 55Ωrespectively It means that the increase of deposition time (orfilm thickness) decreases the charge transfer resistanceThusa high resistance caused a difficult charge transfer electronbetween interface electrolyte and interface electrode
At the same deposition conditions the charge transferresistance ofNiMD samples decreases significantly comparedto EMD (Figure 12)
As the Mn2+Ni2+ increase the semicircle diameterbecomes smaller The lowest resistance was obtained forNiIMD at Mn2+Ni2+ = 1 1 about 4Ω versus 10Ω at Mn2+Ni2+ = 1 2 (Figure 13)
The Ni ion addition to the MnO2lattice would improve
the electrical resistivity of MnO2film in addition to enhance
the specific capacitance It maybe also enhanced the powerdensity as well as reversibility of the supercapacitior at
10 Journal of Nanomaterials
0
10
20
30
40
50
60
0 10 20 30 40 50 60Real (Z) (Ohm)
minuslm
(Z) (
Ohm
)
3 13 2 1 01 2
1 1
Figure 12 Nyquist plot of NiMD samples prepared by CA method compared to EMD-C2
0
2
4
6
8
10
0 2 4 6 8 10 12
minuslm
(Z) (
Ohm
)
Real (Z) (Ohm)
1 21 1
Figure 13 Nyquist plot (zoom part) of NiMD samples at Mn2+ Ni2+ = 1 1 and 1 2
high charge-discharge rate Symmetrical capacitor prototypesof 2 cm times 2 cm were assembled with two thin films ofelectrodes EMD-CV2 and NiMD-CV at Mn2+Ni2+ = 1 1and characterized by charge-discharge cycling at very highcurrent 1 Asdotgminus1
Figure 14 shows charge-discharge values of EMD andNIMD sample at current 1 Asdotgminus1 The efficiency of charge-discharge cycling is about 09 Discharge 119862sp values werecalculated by using the current density discharge time andthe weight of material 119862sp values obtained from Figure 14for EMD-CV2 and NiMD-CV are 10 Fsdotgminus1 and 25 Fsdotgminus1
As expected supercapacitor of NiMD-CV based electrodeexhibits a higher power density than EMD-CV2
The stability of supercapacitor based on NiMD-CV andEMD-CV2 was also tested for a large number of cycles (upto 10000 cycles) at 2 Asdotgminus1 A 119862sp fading of 25 and 14was observedwithNiMD-CVorEMD-CV2based capacitorsrespectively after 10000 charge-discharge cycles There is aninitial large 119862sp decrease and then the 119862sp remained almostconstantTheNiMD-CVexhibited expectedly higher stabilitythan EMD-CV2 based capacitorThus it would be promisingfor the long-term capacitor application
Journal of Nanomaterials 11
(a)
0
5
10
15
20
25
30
35
40
0 20 40 60 80 100Cycles number
NiMD-CVEMD-CV
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
(b)
Figure 14 The prototype (left) and the charge-discharge test at 1 Asdotgminus1 using thin film electrode of NiMD-CV and EMD-CV2 (right)
4 Conclusions
Nanoflakes EMD and NiMD with high porosity were pre-pared by two electrochemical depositions modes The thinfilms exhibited the 120574-MnO
2based structure The Ni addition
to MnO2lattice affected the local structure of 120574-MnO
2that
was evidently observed by using Raman spectroscopy TheEMD-CV exhibited better electrochemical behavior (119862sp andcharge-discharge stability) than EMD-C due to the homoge-nous film forming and high porosity during the reduction-oxidation sweep With short-time deposition (2 minutes)EMD thin films show the highest 119862sp and lowest chargetransfer resistance Compared to EMD electrodes preparedat the same deposition NiMD films enhance significantly the119862sp values as well as electrochemical stability The highest119862sp value was 500 Fsdotgminus1 compared to 300 Fsdotgminus1 of EMDThe enhanced electrectrochemical performance of NiMDcan be explained by the increase of thin film conductivitywith the presence of ion Ni2+ Moreover high power densityand excellent stability of assembled supercapacitor based onNiMDelectrodematerials were demonstrated (119862sp of 25 Fsdotg
minus1
at 1 Asdotgminus1 14 capacitance fade for 10000 cycles)
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors acknowledge funding from VNU-HCM underGrant HS2013-76-01 The authors would like to thank Officeof Naval Research Global (ONRG) for Grant N62909-13-1-N235 The authors would like to thank the technical supportof Horibarsquos Representative Company in Vietnam
References
[1] H Xia M O Lai and L Lu ldquoNanostructured manganese oxidethin films as electrode material for supercapacitorsrdquo JOM vol63 no 1 pp 54ndash59 2011
[2] X Zhao B M Sanchez P J Dobson and P S Grant ldquoThe roleof nanomaterials in redox-based supercapacitors for next gen-eration energy storage devicesrdquoNanoscale vol 3 no 3 pp 839ndash855 2011
[3] W Wei X Cui W Chen and D G Ivey ldquoManganese oxide-based materials as electrochemical supercapacitor electrodesrdquoChemical Society Reviews vol 40 no 3 pp 1697ndash1721 2011
[4] J E Post ldquoManganese oxide minerals crystal structures andeconomic and environmental significancerdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 96 no 7 pp 3447ndash3454 1999
[5] S C Pang M A Anderson and T W Chapman ldquoNovel elec-trode materials for thin-film ultracapacitors comparison ofelectrochemical properties of sol-gel-derived and electrode-posited manganese dioxiderdquo Journal of the ElectrochemicalSociety vol 147 no 2 pp 444ndash450 2000
[6] S C Pang and M A Anderson ldquoNovel electrode materials forelectrochemical capacitors Part II Material characterization ofsol-gel-derived and electrodeposited manganese dioxide thinfilmsrdquo Journal of Materials Research vol 15 no 10 pp 2096ndash2106 2000
[7] N Nagarajan H Humadi and I Zhitomirsky ldquoCathodic elec-trodeposition of MnO
ior of manganese dioxidestainless steel electrodes at differentdeposition currentsrdquoAmerican Journal ofMaterials Science vol2 no 2 pp 11ndash14 2012
[9] C Xu B Li H Du F Kang and Y Zeng ldquoElectrochemicalproperties of nanosized hydrous manganese dioxide synthe-sized by a self-reacting microemulsion methodrdquo Journal ofPower Sources vol 180 no 1 pp 664ndash670 2008
12 Journal of Nanomaterials
[10] X Hu X Lin Z Ling Y Li and X Fu ldquoFabrication and char-acteristics of galvanostatic electrodeposited MnO
2on porous
nickel from etched aluminiumrdquo Electrochimica Acta vol 138pp 132ndash138 2014
[11] S Hassan M Suzuki and A A El-Moneim ldquoEffect of Ag-doping on the capacitive behavior of amorphous manganesedioxide electrodesrdquo Electrical and Electronic Engineering vol 2no 2 pp 18ndash22 2012
[12] T Shinomiya V Gupta and N Miura ldquoEffects of electrochem-ical-deposition method and microstructure on the capacitivecharacteristics of nano-sized manganese oxiderdquo ElectrochimicaActa vol 51 no 21 pp 4412ndash4419 2006
[13] M Minakshi P Singh T B Issa S Thurgate and R De MarcoldquoLithium insertion into manganese dioxide electrode inMnO
2
Zn aqueous battery Part II Comparison of the behavior ofEMD and battery grade MnO
2in ZnmdashMnO
2mdash aqueous LiOH
electrolyterdquo Journal of Power Sources vol 138 no 1-2 pp 319ndash322 2004
[14] T Yousefi R Davarkhah A N Golikand and M H Mash-hadizadeh ldquoSynthesis characterization and supercapacitorstudies of manganese (IV) oxide nanowiresrdquo Materials Sciencein Semiconductor Processing vol 16 no 3 pp 868ndash876 2013
[15] J Li and I Zhitomirsky ldquoCathodic electrophoretic depositionof manganese dioxide filmsrdquo Colloids and Surfaces A Physico-chemical and Engineering Aspects vol 348 no 1ndash3 pp 248ndash2532009
[16] J Wei M Cheong N Nagarajan and I Zhitomirsky ldquoCathodicelectrodeposition of manganese oxides for electrochemicalsupercapacitorsrdquo ECS Transactions vol 3 no 37 pp 1ndash9 2007
[17] V Subramanian H Zhu R Vajtai P M Ajayan and B WeildquoHydrothermal synthesis and pseudocapacitance properties ofMnO
2nanostructuresrdquoThe Journal of Physical Chemistry B vol
109 no 43 pp 20207ndash20214 2005[18] C Ye Z M Lin and S Z Hui ldquoElectrochemical and capac-
itance properties of rod-shaped MnO2for supercapacitorrdquo
Journal of the Electrochemical Society vol 152 no 6 pp A1272ndashA1278 2005
[19] Q T Qu P Zhang B Wang et al ldquoElectrochemical perform-ance of MnO
2nanorods in neutral aqueous electrolytes as a
cathode for asymmetric supercapacitorsrdquo The Journal of Phys-ical Chemistry C vol 113 no 31 pp 14020ndash14027 2009
[20] J P Ni W C Lu L M Zhang B H Yue X F Shang and Y LvldquoLow-temperature synthesis of monodisperse 3D manganeseoxide nanoflowers and their pseudocapacitance propertiesrdquoTheJournal of Physical Chemistry C vol 113 no 1 pp 54ndash60 2009
[21] X Wang A Yuan and Y Wang ldquoSupercapacitive behaviorsand their temperature dependence of sol-gel synthesized nanos-tructured manganese dioxide in lithium hydroxide electrolyterdquoJournal of Power Sources vol 172 no 2 pp 1007ndash1011 2007
[22] C Julien M Massot S Rangan M Lemal and D GuyomardldquoStudy of structural defects in 120574-MnO
2byRaman spectroscopyrdquo
Journal of Raman Spectroscopy vol 33 no 4 pp 223ndash228 2002[23] K Rajendra Prasad and N Miura ldquoElectrochemically synthe-
sized MnO2-based mixed oxides for high performance redox
Figure 12 Nyquist plot of NiMD samples prepared by CA method compared to EMD-C2
0
2
4
6
8
10
0 2 4 6 8 10 12
minuslm
(Z) (
Ohm
)
Real (Z) (Ohm)
1 21 1
Figure 13 Nyquist plot (zoom part) of NiMD samples at Mn2+ Ni2+ = 1 1 and 1 2
high charge-discharge rate Symmetrical capacitor prototypesof 2 cm times 2 cm were assembled with two thin films ofelectrodes EMD-CV2 and NiMD-CV at Mn2+Ni2+ = 1 1and characterized by charge-discharge cycling at very highcurrent 1 Asdotgminus1
Figure 14 shows charge-discharge values of EMD andNIMD sample at current 1 Asdotgminus1 The efficiency of charge-discharge cycling is about 09 Discharge 119862sp values werecalculated by using the current density discharge time andthe weight of material 119862sp values obtained from Figure 14for EMD-CV2 and NiMD-CV are 10 Fsdotgminus1 and 25 Fsdotgminus1
As expected supercapacitor of NiMD-CV based electrodeexhibits a higher power density than EMD-CV2
The stability of supercapacitor based on NiMD-CV andEMD-CV2 was also tested for a large number of cycles (upto 10000 cycles) at 2 Asdotgminus1 A 119862sp fading of 25 and 14was observedwithNiMD-CVorEMD-CV2based capacitorsrespectively after 10000 charge-discharge cycles There is aninitial large 119862sp decrease and then the 119862sp remained almostconstantTheNiMD-CVexhibited expectedly higher stabilitythan EMD-CV2 based capacitorThus it would be promisingfor the long-term capacitor application
Journal of Nanomaterials 11
(a)
0
5
10
15
20
25
30
35
40
0 20 40 60 80 100Cycles number
NiMD-CVEMD-CV
Spec
ific c
apac
itanc
e (Fmiddot
gminus1)
(b)
Figure 14 The prototype (left) and the charge-discharge test at 1 Asdotgminus1 using thin film electrode of NiMD-CV and EMD-CV2 (right)
4 Conclusions
Nanoflakes EMD and NiMD with high porosity were pre-pared by two electrochemical depositions modes The thinfilms exhibited the 120574-MnO
2based structure The Ni addition
to MnO2lattice affected the local structure of 120574-MnO
2that
was evidently observed by using Raman spectroscopy TheEMD-CV exhibited better electrochemical behavior (119862sp andcharge-discharge stability) than EMD-C due to the homoge-nous film forming and high porosity during the reduction-oxidation sweep With short-time deposition (2 minutes)EMD thin films show the highest 119862sp and lowest chargetransfer resistance Compared to EMD electrodes preparedat the same deposition NiMD films enhance significantly the119862sp values as well as electrochemical stability The highest119862sp value was 500 Fsdotgminus1 compared to 300 Fsdotgminus1 of EMDThe enhanced electrectrochemical performance of NiMDcan be explained by the increase of thin film conductivitywith the presence of ion Ni2+ Moreover high power densityand excellent stability of assembled supercapacitor based onNiMDelectrodematerials were demonstrated (119862sp of 25 Fsdotg
minus1
at 1 Asdotgminus1 14 capacitance fade for 10000 cycles)
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors acknowledge funding from VNU-HCM underGrant HS2013-76-01 The authors would like to thank Officeof Naval Research Global (ONRG) for Grant N62909-13-1-N235 The authors would like to thank the technical supportof Horibarsquos Representative Company in Vietnam
References
[1] H Xia M O Lai and L Lu ldquoNanostructured manganese oxidethin films as electrode material for supercapacitorsrdquo JOM vol63 no 1 pp 54ndash59 2011
[2] X Zhao B M Sanchez P J Dobson and P S Grant ldquoThe roleof nanomaterials in redox-based supercapacitors for next gen-eration energy storage devicesrdquoNanoscale vol 3 no 3 pp 839ndash855 2011
[3] W Wei X Cui W Chen and D G Ivey ldquoManganese oxide-based materials as electrochemical supercapacitor electrodesrdquoChemical Society Reviews vol 40 no 3 pp 1697ndash1721 2011
[4] J E Post ldquoManganese oxide minerals crystal structures andeconomic and environmental significancerdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 96 no 7 pp 3447ndash3454 1999
[5] S C Pang M A Anderson and T W Chapman ldquoNovel elec-trode materials for thin-film ultracapacitors comparison ofelectrochemical properties of sol-gel-derived and electrode-posited manganese dioxiderdquo Journal of the ElectrochemicalSociety vol 147 no 2 pp 444ndash450 2000
[6] S C Pang and M A Anderson ldquoNovel electrode materials forelectrochemical capacitors Part II Material characterization ofsol-gel-derived and electrodeposited manganese dioxide thinfilmsrdquo Journal of Materials Research vol 15 no 10 pp 2096ndash2106 2000
[7] N Nagarajan H Humadi and I Zhitomirsky ldquoCathodic elec-trodeposition of MnO
ior of manganese dioxidestainless steel electrodes at differentdeposition currentsrdquoAmerican Journal ofMaterials Science vol2 no 2 pp 11ndash14 2012
[9] C Xu B Li H Du F Kang and Y Zeng ldquoElectrochemicalproperties of nanosized hydrous manganese dioxide synthe-sized by a self-reacting microemulsion methodrdquo Journal ofPower Sources vol 180 no 1 pp 664ndash670 2008
12 Journal of Nanomaterials
[10] X Hu X Lin Z Ling Y Li and X Fu ldquoFabrication and char-acteristics of galvanostatic electrodeposited MnO
2on porous
nickel from etched aluminiumrdquo Electrochimica Acta vol 138pp 132ndash138 2014
[11] S Hassan M Suzuki and A A El-Moneim ldquoEffect of Ag-doping on the capacitive behavior of amorphous manganesedioxide electrodesrdquo Electrical and Electronic Engineering vol 2no 2 pp 18ndash22 2012
[12] T Shinomiya V Gupta and N Miura ldquoEffects of electrochem-ical-deposition method and microstructure on the capacitivecharacteristics of nano-sized manganese oxiderdquo ElectrochimicaActa vol 51 no 21 pp 4412ndash4419 2006
[13] M Minakshi P Singh T B Issa S Thurgate and R De MarcoldquoLithium insertion into manganese dioxide electrode inMnO
2
Zn aqueous battery Part II Comparison of the behavior ofEMD and battery grade MnO
2in ZnmdashMnO
2mdash aqueous LiOH
electrolyterdquo Journal of Power Sources vol 138 no 1-2 pp 319ndash322 2004
[14] T Yousefi R Davarkhah A N Golikand and M H Mash-hadizadeh ldquoSynthesis characterization and supercapacitorstudies of manganese (IV) oxide nanowiresrdquo Materials Sciencein Semiconductor Processing vol 16 no 3 pp 868ndash876 2013
[15] J Li and I Zhitomirsky ldquoCathodic electrophoretic depositionof manganese dioxide filmsrdquo Colloids and Surfaces A Physico-chemical and Engineering Aspects vol 348 no 1ndash3 pp 248ndash2532009
[16] J Wei M Cheong N Nagarajan and I Zhitomirsky ldquoCathodicelectrodeposition of manganese oxides for electrochemicalsupercapacitorsrdquo ECS Transactions vol 3 no 37 pp 1ndash9 2007
[17] V Subramanian H Zhu R Vajtai P M Ajayan and B WeildquoHydrothermal synthesis and pseudocapacitance properties ofMnO
2nanostructuresrdquoThe Journal of Physical Chemistry B vol
109 no 43 pp 20207ndash20214 2005[18] C Ye Z M Lin and S Z Hui ldquoElectrochemical and capac-
itance properties of rod-shaped MnO2for supercapacitorrdquo
Journal of the Electrochemical Society vol 152 no 6 pp A1272ndashA1278 2005
[19] Q T Qu P Zhang B Wang et al ldquoElectrochemical perform-ance of MnO
2nanorods in neutral aqueous electrolytes as a
cathode for asymmetric supercapacitorsrdquo The Journal of Phys-ical Chemistry C vol 113 no 31 pp 14020ndash14027 2009
[20] J P Ni W C Lu L M Zhang B H Yue X F Shang and Y LvldquoLow-temperature synthesis of monodisperse 3D manganeseoxide nanoflowers and their pseudocapacitance propertiesrdquoTheJournal of Physical Chemistry C vol 113 no 1 pp 54ndash60 2009
[21] X Wang A Yuan and Y Wang ldquoSupercapacitive behaviorsand their temperature dependence of sol-gel synthesized nanos-tructured manganese dioxide in lithium hydroxide electrolyterdquoJournal of Power Sources vol 172 no 2 pp 1007ndash1011 2007
[22] C Julien M Massot S Rangan M Lemal and D GuyomardldquoStudy of structural defects in 120574-MnO
2byRaman spectroscopyrdquo
Journal of Raman Spectroscopy vol 33 no 4 pp 223ndash228 2002[23] K Rajendra Prasad and N Miura ldquoElectrochemically synthe-
sized MnO2-based mixed oxides for high performance redox
Figure 14 The prototype (left) and the charge-discharge test at 1 Asdotgminus1 using thin film electrode of NiMD-CV and EMD-CV2 (right)
4 Conclusions
Nanoflakes EMD and NiMD with high porosity were pre-pared by two electrochemical depositions modes The thinfilms exhibited the 120574-MnO
2based structure The Ni addition
to MnO2lattice affected the local structure of 120574-MnO
2that
was evidently observed by using Raman spectroscopy TheEMD-CV exhibited better electrochemical behavior (119862sp andcharge-discharge stability) than EMD-C due to the homoge-nous film forming and high porosity during the reduction-oxidation sweep With short-time deposition (2 minutes)EMD thin films show the highest 119862sp and lowest chargetransfer resistance Compared to EMD electrodes preparedat the same deposition NiMD films enhance significantly the119862sp values as well as electrochemical stability The highest119862sp value was 500 Fsdotgminus1 compared to 300 Fsdotgminus1 of EMDThe enhanced electrectrochemical performance of NiMDcan be explained by the increase of thin film conductivitywith the presence of ion Ni2+ Moreover high power densityand excellent stability of assembled supercapacitor based onNiMDelectrodematerials were demonstrated (119862sp of 25 Fsdotg
minus1
at 1 Asdotgminus1 14 capacitance fade for 10000 cycles)
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors acknowledge funding from VNU-HCM underGrant HS2013-76-01 The authors would like to thank Officeof Naval Research Global (ONRG) for Grant N62909-13-1-N235 The authors would like to thank the technical supportof Horibarsquos Representative Company in Vietnam
References
[1] H Xia M O Lai and L Lu ldquoNanostructured manganese oxidethin films as electrode material for supercapacitorsrdquo JOM vol63 no 1 pp 54ndash59 2011
[2] X Zhao B M Sanchez P J Dobson and P S Grant ldquoThe roleof nanomaterials in redox-based supercapacitors for next gen-eration energy storage devicesrdquoNanoscale vol 3 no 3 pp 839ndash855 2011
[3] W Wei X Cui W Chen and D G Ivey ldquoManganese oxide-based materials as electrochemical supercapacitor electrodesrdquoChemical Society Reviews vol 40 no 3 pp 1697ndash1721 2011
[4] J E Post ldquoManganese oxide minerals crystal structures andeconomic and environmental significancerdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 96 no 7 pp 3447ndash3454 1999
[5] S C Pang M A Anderson and T W Chapman ldquoNovel elec-trode materials for thin-film ultracapacitors comparison ofelectrochemical properties of sol-gel-derived and electrode-posited manganese dioxiderdquo Journal of the ElectrochemicalSociety vol 147 no 2 pp 444ndash450 2000
[6] S C Pang and M A Anderson ldquoNovel electrode materials forelectrochemical capacitors Part II Material characterization ofsol-gel-derived and electrodeposited manganese dioxide thinfilmsrdquo Journal of Materials Research vol 15 no 10 pp 2096ndash2106 2000
[7] N Nagarajan H Humadi and I Zhitomirsky ldquoCathodic elec-trodeposition of MnO
ior of manganese dioxidestainless steel electrodes at differentdeposition currentsrdquoAmerican Journal ofMaterials Science vol2 no 2 pp 11ndash14 2012
[9] C Xu B Li H Du F Kang and Y Zeng ldquoElectrochemicalproperties of nanosized hydrous manganese dioxide synthe-sized by a self-reacting microemulsion methodrdquo Journal ofPower Sources vol 180 no 1 pp 664ndash670 2008
12 Journal of Nanomaterials
[10] X Hu X Lin Z Ling Y Li and X Fu ldquoFabrication and char-acteristics of galvanostatic electrodeposited MnO
2on porous
nickel from etched aluminiumrdquo Electrochimica Acta vol 138pp 132ndash138 2014
[11] S Hassan M Suzuki and A A El-Moneim ldquoEffect of Ag-doping on the capacitive behavior of amorphous manganesedioxide electrodesrdquo Electrical and Electronic Engineering vol 2no 2 pp 18ndash22 2012
[12] T Shinomiya V Gupta and N Miura ldquoEffects of electrochem-ical-deposition method and microstructure on the capacitivecharacteristics of nano-sized manganese oxiderdquo ElectrochimicaActa vol 51 no 21 pp 4412ndash4419 2006
[13] M Minakshi P Singh T B Issa S Thurgate and R De MarcoldquoLithium insertion into manganese dioxide electrode inMnO
2
Zn aqueous battery Part II Comparison of the behavior ofEMD and battery grade MnO
2in ZnmdashMnO
2mdash aqueous LiOH
electrolyterdquo Journal of Power Sources vol 138 no 1-2 pp 319ndash322 2004
[14] T Yousefi R Davarkhah A N Golikand and M H Mash-hadizadeh ldquoSynthesis characterization and supercapacitorstudies of manganese (IV) oxide nanowiresrdquo Materials Sciencein Semiconductor Processing vol 16 no 3 pp 868ndash876 2013
[15] J Li and I Zhitomirsky ldquoCathodic electrophoretic depositionof manganese dioxide filmsrdquo Colloids and Surfaces A Physico-chemical and Engineering Aspects vol 348 no 1ndash3 pp 248ndash2532009
[16] J Wei M Cheong N Nagarajan and I Zhitomirsky ldquoCathodicelectrodeposition of manganese oxides for electrochemicalsupercapacitorsrdquo ECS Transactions vol 3 no 37 pp 1ndash9 2007
[17] V Subramanian H Zhu R Vajtai P M Ajayan and B WeildquoHydrothermal synthesis and pseudocapacitance properties ofMnO
2nanostructuresrdquoThe Journal of Physical Chemistry B vol
109 no 43 pp 20207ndash20214 2005[18] C Ye Z M Lin and S Z Hui ldquoElectrochemical and capac-
itance properties of rod-shaped MnO2for supercapacitorrdquo
Journal of the Electrochemical Society vol 152 no 6 pp A1272ndashA1278 2005
[19] Q T Qu P Zhang B Wang et al ldquoElectrochemical perform-ance of MnO
2nanorods in neutral aqueous electrolytes as a
cathode for asymmetric supercapacitorsrdquo The Journal of Phys-ical Chemistry C vol 113 no 31 pp 14020ndash14027 2009
[20] J P Ni W C Lu L M Zhang B H Yue X F Shang and Y LvldquoLow-temperature synthesis of monodisperse 3D manganeseoxide nanoflowers and their pseudocapacitance propertiesrdquoTheJournal of Physical Chemistry C vol 113 no 1 pp 54ndash60 2009
[21] X Wang A Yuan and Y Wang ldquoSupercapacitive behaviorsand their temperature dependence of sol-gel synthesized nanos-tructured manganese dioxide in lithium hydroxide electrolyterdquoJournal of Power Sources vol 172 no 2 pp 1007ndash1011 2007
[22] C Julien M Massot S Rangan M Lemal and D GuyomardldquoStudy of structural defects in 120574-MnO
2byRaman spectroscopyrdquo
Journal of Raman Spectroscopy vol 33 no 4 pp 223ndash228 2002[23] K Rajendra Prasad and N Miura ldquoElectrochemically synthe-
sized MnO2-based mixed oxides for high performance redox
[10] X Hu X Lin Z Ling Y Li and X Fu ldquoFabrication and char-acteristics of galvanostatic electrodeposited MnO
2on porous
nickel from etched aluminiumrdquo Electrochimica Acta vol 138pp 132ndash138 2014
[11] S Hassan M Suzuki and A A El-Moneim ldquoEffect of Ag-doping on the capacitive behavior of amorphous manganesedioxide electrodesrdquo Electrical and Electronic Engineering vol 2no 2 pp 18ndash22 2012
[12] T Shinomiya V Gupta and N Miura ldquoEffects of electrochem-ical-deposition method and microstructure on the capacitivecharacteristics of nano-sized manganese oxiderdquo ElectrochimicaActa vol 51 no 21 pp 4412ndash4419 2006
[13] M Minakshi P Singh T B Issa S Thurgate and R De MarcoldquoLithium insertion into manganese dioxide electrode inMnO
2
Zn aqueous battery Part II Comparison of the behavior ofEMD and battery grade MnO
2in ZnmdashMnO
2mdash aqueous LiOH
electrolyterdquo Journal of Power Sources vol 138 no 1-2 pp 319ndash322 2004
[14] T Yousefi R Davarkhah A N Golikand and M H Mash-hadizadeh ldquoSynthesis characterization and supercapacitorstudies of manganese (IV) oxide nanowiresrdquo Materials Sciencein Semiconductor Processing vol 16 no 3 pp 868ndash876 2013
[15] J Li and I Zhitomirsky ldquoCathodic electrophoretic depositionof manganese dioxide filmsrdquo Colloids and Surfaces A Physico-chemical and Engineering Aspects vol 348 no 1ndash3 pp 248ndash2532009
[16] J Wei M Cheong N Nagarajan and I Zhitomirsky ldquoCathodicelectrodeposition of manganese oxides for electrochemicalsupercapacitorsrdquo ECS Transactions vol 3 no 37 pp 1ndash9 2007
[17] V Subramanian H Zhu R Vajtai P M Ajayan and B WeildquoHydrothermal synthesis and pseudocapacitance properties ofMnO
2nanostructuresrdquoThe Journal of Physical Chemistry B vol
109 no 43 pp 20207ndash20214 2005[18] C Ye Z M Lin and S Z Hui ldquoElectrochemical and capac-
itance properties of rod-shaped MnO2for supercapacitorrdquo
Journal of the Electrochemical Society vol 152 no 6 pp A1272ndashA1278 2005
[19] Q T Qu P Zhang B Wang et al ldquoElectrochemical perform-ance of MnO
2nanorods in neutral aqueous electrolytes as a
cathode for asymmetric supercapacitorsrdquo The Journal of Phys-ical Chemistry C vol 113 no 31 pp 14020ndash14027 2009
[20] J P Ni W C Lu L M Zhang B H Yue X F Shang and Y LvldquoLow-temperature synthesis of monodisperse 3D manganeseoxide nanoflowers and their pseudocapacitance propertiesrdquoTheJournal of Physical Chemistry C vol 113 no 1 pp 54ndash60 2009
[21] X Wang A Yuan and Y Wang ldquoSupercapacitive behaviorsand their temperature dependence of sol-gel synthesized nanos-tructured manganese dioxide in lithium hydroxide electrolyterdquoJournal of Power Sources vol 172 no 2 pp 1007ndash1011 2007
[22] C Julien M Massot S Rangan M Lemal and D GuyomardldquoStudy of structural defects in 120574-MnO
2byRaman spectroscopyrdquo
Journal of Raman Spectroscopy vol 33 no 4 pp 223ndash228 2002[23] K Rajendra Prasad and N Miura ldquoElectrochemically synthe-
sized MnO2-based mixed oxides for high performance redox
