Download - Effects of sodium dodecyl sulfate on the electrochemical behavior of supercapacitor electrode MnO2

ORIGINAL PAPER

Effects of sodium dodecyl sulfate on the electrochemicalbehavior of supercapacitor electrode MnO2

Huaihao Zhang & Jiangna Gu & Yuanyuan Jiang &

Jing Zhao & Xiaoxing Zhang & Chengyin Wang

Received: 17 July 2013 /Revised: 10 September 2013 /Accepted: 18 September 2013 /Published online: 4 October 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract Supercapacitor electrode material MnO2 was pre-pared by liquid co-precipitation with different concentrationof anionic surfactant sodium dodecyl sulfate (SDS). Asevidenced by X-ray diffraction, the obtained MnO2 are alltypical amorphous α-MnO2 with poor crystallinity. Scanningelectron microscopy reveals that the dispersity of MnO2 ini-tially get better and then worse with the increase of SDS, andthe particle sizes first become smaller then larger as well. It isworthwhile noting that the morphology of MnO2 tested bytransmission electron microscopy undergoes a changeableprocess: fibrous, pine needle like, cotton like, round bubblelike, flocculent, and nervous tissue like as SDS increases.Through cyclic voltammetry and galvanostatic charge/discharge tests, SDS addition amount 0.2 g (0.017 mol L−1)is found to be the optimal effect value, and the as-preparedMn-0.2 obtains the highest specific capacitance (C sp) of154.5 F g−1 at a current density of 500 mA g−1. Comparedwith the sample Mn-0 synthesized without SDS, the C sp

increases by about 50 % (±5 %), which can be attributed toits largest Brunauer–Emmett–Teller–specific surface area of255.9 m2 g−1, best particle dispersity, and smallest particle sizeof approximately 50–80 nm. Meanwhile, the rate capabilityand cycle stability of Mn-0.2 also improves obviously, and theequivalent series resistance decreases a lot, only 0.120 Ω.

Keywords MnO2. Capacitance . Sodium dodecyl sulfate

(SDS) . Supercapacitor

Introduction

Supercapacitor, a high-efficiency energy storage device,has fast charging rate, superior large current charge/discharge performance, wide operating temperature range,high-power density, and excellent cycle life in comparisonwith other kinds of batteries [1–3]. Evidently, given theseprominent characteristics, it has lots of potential applica-tions, including electric vehicles, cardiac pacemakers, mo-bile phones, airbags, engine start, solar cell power storage,etc. [4–8]. Nonetheless, it has a defect: the storage capacityis much smaller than batteries with the same volumes,namely the energy density is relatively low [9]. As weknow, electrode material plays an important role in deter-mining the performance of supercapacitors. Thus, at thepresent stage, developing an electrode material havinghigh-energy density is very crucial in enhancing deeplythe applications of supercapacitors. Transition metal oxideMnO2, a kind of low-priced, abundant, environmentalfriendly material with good electrochemical performance,is regarded as a promising material in supercapacitor fields[10–13]. It is well known that the particle properties, suchas size, morphology, surface area, crystalline structure, etc.,have a profound effect on the performance of MnO2 mate-rials. However, MnO2 may agglomerate easily to formsecondary particles because of high surface tension andsurface energy, which may worsen particle dispersity, en-large particle size, change morphology, and minimize sur-face area. Finally, the special nanometer functions of MnO2

would disappear. Undoubtedly, how to overcome the ag-glomeration effect in the synthesis process and post-processing is the key to maintain the excellent performanceof MnO2 [14, 15].

Surfactants are fine chemical products, known as industrialmonosodium glutamate. Owing to its unique amphiphilicstructure, it can significantly reduce the surface tension and

H. Zhang (*) : J. Gu (*) :Y. Jiang : J. Zhao :X. Zhang :C. WangCollege of Chemistry and Chemical Engineering, YangzhouUniversity, Yangzhou 225002, People’s Republic of Chinae-mail: [email protected]: [email protected]

J Solid State Electrochem (2014) 18:235–247DOI 10.1007/s10008-013-2266-1

surface energy of nanometer particles. The steric effect ofsurfactant long molecular chains can prevent them fromgetting together. Moreover, surfactants in solutions canself-assemble to form some ordered aggregates, such asmicelles, reverse micelles, microemulsions, vesicles, andliquid crystals, which can be used as microreactors ortemplates to realize the regulation and control morphologyand structure of materials. Recently, there have been manyreports on the use of surfactants in electrode materials field.Zhang et al. [16] modified the activated carbon (AC) sur-face by adding sodium dodecyl benzene sulfonate (SDBS).When the ratio of SDBS to AC was 4 % (weight percent),the specific capacitance (C sp) increased by 20.62 %(5 mA cm−2) compared with the sample without SDBS.Zhao et al. [17] deposited cobalt hydroxide (Co(OH)2) withnonionic surfactant N -methyl-2-pyrrolidone (NMP), find-ing that Co(OH)2 had denser and thinner layered structure,greater specific surface area, shorter ion diffusion path inthe presence of 20 % (volume percent) NMP, and the C sp

reached 651 F g−1 in −0.1 to 0.45 V. Jiang et al. [18]prepared MnO2 with Pluronic P123. At the optimum P123addition amount of 0.02 % (weight percent), MnO2 showedloose clew shapes and C sp was 176 F g−1, whereas thesample without P123 appeared as spherical particles andC sp only 77 F g−1. Lee et al. [19] used Pluronic F127 asdispersant to prepare adjustable porous Mn3O4. As F127increased, the Brunauer–Emmett–Teller (BET)–specificsurface area of Mn3O4 became larger, the particle dispersityand material power storage performance were more prefer-able. Zhang et al. [20] added cetyl trimethyl ammoniumbromide (CTAB) to prepare multilayered porous carbon. AtCTAB concentration of 0.27 mol L−1, the sample had thelargest specific surface area at 689 m2 g−1 and largest C sp

272 F g−1 (1 mV s−1). Zhou et al. [21] used F127 to enhancethe mesoporosity of carbon aerogel (CA), showing that themesopore volume of CA prepared with 0.6 % (weightpercent) F127 increased to 0.90 cm3 g−1, which occupied86 % of the total pore volume. The C sp reached 130.8 F g−1,45 % higher than that of CAwithout F127.

Based on previous studies, we select a familiar anionicsurfactant sodium dodecyl sulfate (SDS) as a structure-directing agent to regulate and control MnO2 morphology,structure, particle size, etc. We added a series of differentconcentrations of SDS and mainly emphasized on studyingthe SDS influence mechanism and law on the above prop-erties of MnO2 electrodes. We conducted in-depth analysisand explanations using scanning electron microscopy(SEM), transmission electron microscopy (TEM), and N2

physical adsorption aspects. Through a variety of charac-terizations and tests, we finally reached the aim to improveelectrode MnO2 electrochemical performance. Under thecircumstance of the optimal SDS adding amount, bothmacroscopic and microscopic performances of MnO2 allachieved the most ideal results.

Experimental

Preparation of MnO2 electrodes

Figure 1 shows the preparation process of MnO2 electrodes.Potassium sulfate (K2SO4), potassium permanganate(KMnO4), manganese sulfate (MnSO4·H2O), SDS, and de-ionized water were employed. SDS addition amount were 0,0.05, 0.1, 0.2, 0.5, 0.8, 1.2, 1.5, and 3.0 g, and correspond-ingly the prepared MnO2 were denoted as Mn-0, Mn-0.05,Mn-0.1, Mn-0.2, Mn-0.5, Mn-0.8, Mn-1.2, Mn-1.5, and Mn-3.0, respectively.

Structural characterization

German Bruker D8 super speed X-ray diffractometer (XRD)was used to test MnO2 crystalline. Test conditions: CuKα

radiation λ =1.5406 Å; tube voltage, 40 kV; tube current,200 mA; and scan range, 3–60°. Fourier transform infrared(FT-IR) spectra of MnO2 were recorded on Bruker Tensor 27spectrometer using potassium bromide as dispersant at a scan-ning wave number of 4,000 to 400 cm−1. The morphology of

dry

magnetic stirringSamplesCentrifuge

Vacuum filtration

Washing85%

dry

stirring

0.15 mol L-1

KMnO4

graphite10 %

Paste likemixture

Smear on

1cm2nickel foam

Products

Electrode

START

Pressed into sheetsunder10 MPa

END

SDS dissolvedin 20 ml water

0.4 mol L-1

MnSO4 H2O

PTFE5 %

Fig. 1 Preparation process ofMnO2 electrode materials

236 J Solid State Electrochem (2014) 18:235–247

MnO2 was observed by S-4800 II field emission scanningelectron microscope and Philips TECNAI 12 TEM. The spe-cific surface area and pore size distribution was tested byASAP 2020 M physical adsorption analyzer.

Electrochemical characterization

Electrochemical measurements were conducted in 0.5 mol L−1

K2SO4 system by a three-electrode system that MnO2 was

used as a working electrode, AC as auxiliary electrode, andsaturated calomel electrode (SCE) as the reference electrode.Cyclic voltammetry (CV), scanning rates from 5 to 100mV s−1

within 0–0.8 V versus SCE, are performed on CHI660Aelectrochemical workstation (Chenghua, Shanghai China).Galvanostatic charge/discharge tests are also conducted atcurrent densities from 300 to 5,000 mA g−1 between 0 and1.0 V versus SCE. Electrochemical impedance spectroscopy(EIS) is tested by analyzer AutoLab-PGSTAT30. The potential

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Mn-0.05

Mn-0

wavenumber(cm-1)

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Fig. 2 a FT-IR spectra and bXRD patterns of MnO2 samples

J Solid State Electrochem (2014) 18:235–247 237

Fig. 3 SEM images of MnO2

samples


amplitude was set to 5 mV at open-circuit, peak-to-peak (ACsignal), with 6 points per decade, and the frequency range wasset from 0.01 to 105 Hz.

Results and discussion

FT-IR and XRD

Figure 2a shows the FT-IR spectra of MnO2 samples. Itclearly shows that all the samples spectra are almost thesame, indicating that SDS addition amount has no signifi-cant effects on MnO2 composition. Meanwhile, SDS orother organic functional groups characteristics absorptionpeaks are not observed, suggesting that the residual SDShas been washed away [18]. The adsorption band appearingat 3,400 cm−1 is the H–O–H stretching and bending vibra-tion absorption peak. The broad one located at 539.71 cm−1

is assigned to the characteristic peak of MnO6 octahedron,and the two peaks appeared at 1,631.5 and 1,378.83 cm−1

are attributed to the hydroxyl bending vibrations of physi-cal adsorption water and crystalline water [22–25].Figure 2b shows the XRD patterns of Mn-0, Mn-0.1, andMn-0.5. The three samples peak shapes are basically sim-ilar, suggesting that SDS has no effects on MnO2 crystal-line. Based on the standard JCPDS card (no. 44-0141),there is a broad and weak diffraction peak at 2θ =37.2°,indicating that the samples are typical amorphous α-MnO2

with poor crystallinity [26–28]. Amorphous structure ma-terials are excellent ideal materials for supercapacitors [29].Not only is it conducive for protons and K+ to intercalateand de-intercalate in the tunnel of α-MnO2 rapidly [30], butalso it can accelerate reversible adsorption/desorption and

oxidation/reduction reactions on the electrode material sur-face or within the bulk phase, generat ing greatpseudocapacitance. Moreover, the amorphous structurewill not have any serious deformations and may have littleinfluences on the electrode performance.

SEM and TEM

Figure 3 is the SEM images of MnO2 samples. All thesamples present irregular aggregates. The dispersity ofMnO2 first becomes better and then worse compared withMn-0. As can be seen from the figure, the agglomeration ofMn-0 is serious and the particle dispersity is uneven. This isto a large extent related to the growth process of MnO2:MnSO4 and KMnO4 initially react to generate MnO2 crystalnucleus in the absence of SDS, and then the newly gener-ated MnO2 molecules continue to grow on the nucleussurface as reaction goes on, finally forming large, rough,and irregular particles about 200 to 300 nm. When it comesto sample Mn-0.05, the dispersity improves a little. As SDSaddition amount is low, it usually presents as single mole-cules in aqueous solution in Fig. 4a, which has limitedparticle dispersion effects for MnO2 particles. With theincrease of SDS, it starts to self-assemble into critical mi-celle, as shown in Fig. 4b. From literature [31], SDS criticalmicelle concentration (CMC) at 298 K in aqueous mediumis 6×10−6 mol cm−3, which is about 0.07 g SDS in ourreaction system. There is no doubt that the CMC of asurfactant and the number of micelles presents in the reac-tion system play important roles in the particle nucleationprocess. At CMCs, given the limited number of micelles,only a few can really play dispersion effects for MnO2

particles. Mn-0.1 and Mn-0.2, over SDS CMC, display

Mn2+countra-ion

hydrophilic group

hydrophobic group

O

O

O

O-S

(a)C<CMC

(e)

(b)C=CMC (d)C>>CMC(c)C>CMC

Single molecules Critical Micelles Spherical or rod-like micelles Vesicles

Fig. 4 Existing form and rod-likemicelle structure model of SDS inaqueous solution


good dispersity obviously and the particle size of is about50–80 nm. This is primarily because: SDS present in theform of spherical or rod-like micelles, just like in Fig. 4c.MnO2 particles can grow along the micelles surfacedirectionally. From the Mn-0.2 SEM illustration under highmagnification, we can see that the sample is consisted ofrod-like particles. Conversely, the steric hindrance arisingfrom electrostatic repulsion among SDS micelles can pre-vent further agglomeration of the MnO2 particles. Howev-er, with the further increase of the SDS addition amount, suchasMn-0.5, Mn-0.8, Mn-1.0, Mn-1.2, Mn-1.5, andMn-3.0, theparticle size increases and the dispersity deteriorates whichowing to the formation of larger SDS aggregates.

Considering the limit in the SEM visual fields, TEM wasconducted to study MnO2 morphology deeply, as is shown inFig. 5 TEM images of MnO2 samples

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dV/d

w /

cm3 ·

g-1·n

m-1

Mn-0 Mn-0.05 Mn-0.2 Mn-0.5 Mn-1.2

b

Fig. 6 a N2 physical adsorption isotherms and b pore size distribution ofMnO2 samples


Fig. 5. Sample Mn-0 shows plush-shaped sphere about200 nm, without clear boundaries among them. In the absenceof SDS, given the high surface tension and energy of thenewly generated MnO2 nucleus, a large number of MnO2

particles tend to agglomerate together spontaneously. Sam-ple Mn-0.05 appears to be a 150-nm fibrous solid sphere withshort burrs on the surface. The dispersity appears to be a littlebetter than Mn-0. Although SDS addition amount is verysmall, its unique amphiphilic structure still has a certain pos-itive dispersion effect on the nanometer particles. Mn-0.1 ismuch similar to Mn-0.05 except for the smaller particles. Asmentioned above in SEM images analysis, SDS in solutionsstarts to self assemble into micelles for Mn-0.1, whose disper-sive ability and structure-oriented ability have been muchimproved. Of special interest is the fact that Mn-0.2 morphol-ogy is much different from the samples mentioned above,showing obvious pine needle-like aggregates gathered bynumerous nanowires. Combining with Mn-0.2 SEM illustra-tion, the nanowires can be described as the structure model inFig. 4e. Considering the counter ion effects, Mn2+ combineswith SDS micelles hydrophilic groups when the MnSO4 so-lution was added into SDS solution and SO4

2− exist aroundthe micelles. Then the MnO4

− was added drop by drop,reacting with Mn2+ to generate MnO2. Finally, the newlygenerated MnO2 grow along the micelles directionally. Toreduce the surface tension and energy, the nanowires sponta-neously aggregate together. Mn-0.5 and Mn-0.8 are largeirregular cotton-like entities. This may be because, the sizeof the micelles acting as structure directing agent becomelarger as the SDS addition amount increases. As SDS concen-tration increases further, Mn-1.2 shows aggregation of vesi-cles (Fig. 4d) about 100 nm. Considering the special structureof vesicles, MnO2 particles could only be adsorbed on theouter wall surface, thus the agglomeration does not get obvi-ous improvement. Mn-1.5 is like floccules, and Mn-3.0

appears as cell neural tissue-like distribution. In a word, itseems that the TEM results coincide with the ideas mentionedin SEM.

Surface area and pore structure

Figure 6a is N2 physical adsorption isotherms of MnO2 sam-ples, showing typical IV type isotherm feature [32–34]. Atlow relative pressure, the adsorption amount increases slowlyand there is an unconspicuous protrusion at the beginning ofthe adsorption branch, suggesting that the micropore contentis low. As the relative pressure P /P0 increases, N2 adsorptionincreases linearly, indicating a large number of mesopores.Moreover, the existence of a large hysteresis loop demon-strates that MnO2 has obvious characteristic of mesoporousmaterials [35–37]. Obviously, the adsorption amount of theSDS-assisted samples increases and the hysteresis loop areasare larger than that of Mn-0, implying that the presence ofSDS contributes to the development of materials mesopore.The adsorption amount increases rapidly when P /P0 is closeto 1, which is attributed to the capillary condensations of N2 inmicropores or gaps among particles [32]. Figure 6b is the poresize distribution (PSD) of MnO2 samples. It is evident that allthe MnO2 samples possess unimodal PSD at (2.8–4.2 nm)with high mesoporosity. The unimodal areas of MnO2 pre-pared with SDS have different degrees of growth. That is tosay, SDS ameliorates the particle-packing situation, in-creasing mesopore content. For Mn-0.2, the adsorptionamount in the range of the most probable pore size (2.8–4.2 nm) is the highest.

It is well known that surface area is one of the importantphysical characteristics affecting electrochemical properties ofmaterials. The surface area and pore structure parameters arelisted in Table 1. With SDS addition amount rising, the BET-specific surface area (SBET) increases from 213.3 to

Table 1 Specific surface area and pore structure parameters of MnO2

Sample SBET/m2 g−1a Smeso/m

2 g−1b Smicro/m2 g−1c V t/cm

3 g−1d Vmicro/cm3 g−1e Vmeso/cm

3 g−1f D /nmg

Mn-0 213.3 222.6 57.8 0.228 0.018 0.210 4.28

Mn-0.05 232.3 261.7 13.8 0.289 0.013 0.276 4.98

Mn-0.2 255.9 273.6 30.2 0.283 0.014 0.269 4.42

Mn-0.5 238.9 263.9 11.2 0.311 0.011 0.300 5.20

Mn-1.2 206.8 230.3 9.9 0.26 0.010 0.250 5.05

a Brunauer–Emmett–Teller (BET)-specific surface areabMesopore-specific surface area, calculated using Barrett–Joyner–Halender method from desorption curvecMicropore-specific surface area, derived from t-plot methodd Total pore volume, measured at P /P0 =0.993eMicropore pore volume, derived from t-plot methodfMesopore volume, calculated by subtraction of micropore volume from total pore volumegAverage pore diameter of MnO2, calculated by 4V t/SBET


255.9 m2 g−1, then drops to 206.8 m2 g−1. Mesopore-specific surface area (Smeso) follows the same tendency.Based on MnO2 electricity storage mechanism, the largerthe specific surface area is, the higher the C sp will be. It iswell known that mesopore is also important in electrodematerials because it is conducive to the adsorption/desorptionor migration of protons and electrolyte ions K+. Thus, mate-rials with high specific surface area and mesoporosity are verypopular in supercapacitor applications.

In addition, after adding SDS, the micropore surface area(Smicro) and volume (Vmicro) decrease; total pore volume(V t) and mesopore volume (Vmeso) increase in general,which is due to the steric hindrance arising from electro-static repulsion among SDS. It enlarges the space or gapsamong MnO2 particles. In addition, the fact that averagepore sizes of SDS-assisted samples are all larger than that ofMn-0 verifies the opinion. Generally, Mn-0.2 with highSBET, 255.9 m2 g−1; Smeso, 273.6 m2 g−1; and Vmeso,0.269 cm−3 g−1, is the preferable electrode material thatwe desire to obtain.

Capacitance performance

Figure 7a shows the CV curves of MnO2 at the scan rate of5 mV s−1 over the voltage range of 0–0.8 V versus SCE in0.5 mol L−1 K2SO4 solution. The CV curves are substan-tially rectangular in shape, showing good capacitive char-acteristics [38]. Obvious redox peaks do not exist and thecathodic and anodic processes are mirror symmetry, whichindicate that the electrodes are charged and discharged at aconstant rate and the potential changes have no effects oncapacitance. Usually, the larger the area of CV curvessurround, the greater the average capacitance will be. Theareas of CV curve first increase gradually then decrease,meaning that the average capacitance follows the sametrend. Figure 7b is the constant current charge–dischargecurve of MnO2 samples at 500 mA g−1. The charge/discharge curves are basically isosceles triangle symmetri-cal distribution. The relationship between potential andtime is linear, which means that the slope of dV /dt isconstant and the samples have good capacitance perfor-mance and reversibility. There is a significant voltage dropat the beginning of the discharge process because of thesystem resistance [39].

Figure 7c, the constant discharge curves of MnO2 elec-trodes, shows that the discharge time of samples first increasesand then decreases. C sp can be calculated by the followingequation:

Csp ¼ IΔt= mΔVð Þ

Where I is the charge/discharge current (milliamperes), Δtis the discharge time (seconds), m is the active material mass

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/ V

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Mn-0 Mn-0.05 Mn-0.1 Mn-0.2 Mn-0.5 Mn-0.8 Mn-1.2 Mn-1.5 Mn-3.0

c

Fig. 7 a CV curves (scan rate, 5 mV s−1), b constant current charge–discharge curves, and c constant current discharge curves (current densi-ty, 500 mA g−1, 0.5 mol L−1 K2SO4)


of working electrode (grams), ΔV is the voltage change(volts), and C sp is electrode-specific capacitance (French pergram).

Table 2 shows the C sp of MnO2 at current densities from300 to 5,000 mA g−1. Clearly, SDS addition amount hassignificant impacts on MnO2 capacitance properties. Here-in, as for 500 mA g−1, the C sp of Mn-0.05 is 111.3 F g−1,only 8 F g−1 higher than sample Mn-0, having little positiveeffects on the C sp when SDS addition amount is relativelylow. The C sp of Mn-0.2 reaches the maximum value of154.5 F g−1, increases by about 50 % (±5 %) in comparisonwith Mn-0. However, Mn-0.5 sharply declines to86.3 F g−1, even lower than Mn-0, meaning that excessiveSDS amount has negative effects on electricity storageperformance. The capacitance of samples Mn-0.8, Mn-1.2, Mn-1.5, and Mn-3.0 verify the conclusion. Therefore,in terms of charge storage ability, SDS addition amount of0.2 g (0.017 mol L−1) is the optimum impact value, which isconsistent with the results obtained from surface area andpore structure.

Rate capability

We conduct CV test at different scan rates to explore therate characteristic of electrodes Mn-0, Mn-0.2, and Mn-1.2,as is shown in Fig. 8. Mn-0 and Mn-0.2 exhibit idealcapacitor properties at low scan rates, displaying standardsymmetrical square curves [40, 41]. As the scan rate in-creases, Mn-0 changes from regular rectangle to deformedrectangle and finally an inclined oval, the degree deviatedfrom rectangle is serious, showing poor rate capability.While Mn-0.2 prepared under the optimal condition main-tains good square shape and symmetry even at high scanrates of 30 and 50 mV s−1, exhibiting good rate capability,power characteristics, and good dynamic characteristics. Asfor Mn-1.2, its CV curves have displayed poor standard

rectangle shape at low scan rate, much less at high ones,implying that excessive SDS has negative impact on therate capability of the electrode materials. Generally, the ratecapability of the electrode materials have much to do withmesopore content, because a large amount of mesopores areconducive to the adsorption/desorption or migration ofprotons and electrolyte K+, even at high scanning rates.Combining with the mesopore-specific surface area datafrom Table 1, it is evident that the Smeso of Mn-0.2 is thelargest, thus which rate capability is also the best.

EIS

Figure 9 shows the experimental and simulated Nyquist,Bode, Bode phrase plots, and equivalent circuit of MnO2

electrodes. The fitted parameters of EIS spectra (Fig. 9a)obtained by Zview software are listed in Table 3. Theproposed equivalent circuit used to fit the experimental datais shown in Fig. 9d. R s represents the equivalent seriesresistance (ESR), and R ct is charge transfer resistance be-tween the active material and electrolyte. Moreover, CPE isthe constant phase element (representing the double layerand the SEI capacitances), while Zw is deemed as diffusionresistance, namely Warburg impedance [42, 43]. As forFig. 9a, seeing from the enlarged EIS plot of the middleand high-frequency region in the upper right, the curvesintercept in the x -axis represents the ESR, including elec-trolyte resistance, intrinsic resistance of active material,contact resistance between electrolyte and current collectorand so on. ESR is one of the main electrical parameters ofsupercapacitors. When a capacitor is modeled as an equiv-alent circuit (Fig. 9d), including inductors, capacitors, andresistors, the resistive part in the series with the capacitorsare deemed as ESR, representing the power consumed bythe internal heat of capacitors, which will have great influ-ences on the capacitors charging and discharging process.

Table 2 Csp of MnO2 at differentcurrent densities Csp (F g−1)

Sample (error)

Mn-0(±2 %)

Mn-0.05(±3 %)

Mn-0.1(±6 %)

Mn-0.2(±7 %)

Mn-0.5(±2 %)

Mn-0.8(±4 %)

Mn-1.2(±1 %)

Mn-1.5(±1 %)

Mn-3.0(±2 %)

Current density (mA g−1)

300 113.0 122.7 155.7 182.4 114.6 123.6 107.1 114.6 80.7

500 103.0 111.3 140.5 154.5 86.3 89.3 86.5 89.0 59.5

1,000 79.5 78.9 102.0 106.0 60.4 56.5 57.0 59.1 37.0

2,000 56.2 57.0 67.4 82.0 38.4 33.9 33.0 35.4 19.6

3,000 46.8 46.2 51.3 68.4 28.5 24.0 23.8 26.4 12.9

5,000 37.5 34.5 27.0 55.5 18.5 15.5 18.6 15.0 8.5


Obviously, whether from the intercepts of the enlarged plotor Table 3, the ESR of Mn-0.2 and Mn-0.5 decreases, whilethat of Mn-1.2 and Mn-3.0 with excessive SDS increases.Thus, Mn-0.2 has the smallest value of about 0.120 Ω,which is primarily because under the optimal SDS additionamount, the agglomeration of MnO2 particles improvesevidently and the specific surface area of materials in-creases effectively.

Combining with the Nyquist (Fig. 9a) and Bode plots(Fig. 9b), the impedance and Bode curves can be divided intohigh-, intermediate-, and low-frequency regions. In the high-frequency region, the EIS impedance curve is an irregularsemicircle controlled by electrochemical polarization, show-ing R ct. The smaller the semicircle diameter is, the smaller theRct will be. As can be seen from Table 3, among the fivesamples, the Rct of Mn-0.2 is also the smallest. This is to alarge extent related with its better dispersity and abundantmesopore content evidenced by N2 adsorption data. Corre-spondingly, as for bode plot in Fig. 9b, the curves tend to bemild between 104 and 105 Hz, indicating the/Z /values havenothing to do with frequency. The modulus is equal to theESR [44, 45]. Evidently, the results obtained from the bodeplot in this region are consistent with that of the Nyquist plot.In the intermediate-frequency area, the electrodes reactionprocess become controlled by the diffusion procedure. TheNyquist plots gradually evolved into an oblique line with aslope of about 1, and the more the curves are near to the 45°line, the easier the diffusion process will be conducted.Through Zview software fitting, except Mn-3.0, the EIScurves for the rest of the four samples can be well fitted withthe equivalent circuit in this frequency region, as can be seenin Fig. 9e. In Table 3, the value of ZW-R represents theelectrolyte K+ and protons diffusion resistance. Clearly, theZW-R decreases sharply at first and then increases converselywhen the SDS added amount increases. As we know, thediffusion Warburg impedance is mainly dominated by theparticle size, dispersity, morphology, and pore structure ofthe electrode materials. Evidenced by the previous character-ization methods, adding suitable amount of SDS is beneficialto improve the packing situation of the MnO2 particles,change the particle size, ameliorate the pore structure, andaccelerate electrolyte ions migration/immigration in electrodematerials, thus decreasing the Warburg impedance. However,the excessive SDS will lead to negative effects. In addition,the capacitance performance is closely linked with the War-burg impedance. In Table 2, it is clear that C sp variationtendency is inversely proportional to ZW-R in all the examinedcurrent densities. For Fig. 9b, at intermediate frequencies(between 10 and 104 Hz), compared with the similar tendencyand/Z /values of Mn-0, Mn-0.5, Mn-1.2, and Mn-3.0, Mn-0.2shows more distinct double-layer characteristics [44, 46],which are conducive to the capacitance performance of theelectrode. Furthermore, from Table 3, CPE-T value of Mn-0.2

0.0 0.2 0.4 0.6 0.8-12

-8

-4

0

4

8

i /A·g

-1

E /V

5 mV·s-110 mV·s-1

30 mV·s-150 mV·s-1

100 mV·s-1Mn-0

0.0 0.2 0.4 0.6 0.8

-10

-5

0

5

10

i /A·g

-1

E /V

100 mV·s-1

50 mV·s-1

30 mV·s-1

10 mV·s-1

5 mV·s-1

Mn-0.2

0.0 0.2 0.4 0.6 0.8-6

-4

-2

0

2

4

6

i /A·g

-1

E /V

100 mV·s-1

50 mV·s-1

10 mV·s-130 mV·s-1

5 mV·s-1

Mn-1.2

Fig. 8 CV curves of Mn-0, Mn-0.2, and Mn-1.2 in 0.5 mol L−1 K2SO4

solution at different scan rates


is the highest, representing the electrolyte K+ or protonsdiffusion capability and the rate of forming electric doublelayer with the active material is the largest. In the low-frequency region (Fig. 9a), the inclination of straight portionrepresents the proximity degree with the ideal capacitor.

Evidently, Mn-0.2 impedance curve is more perpendicular tothe real axis, meaning that the idealization degree is higher. Asfor the bode plot shown in Fig. 9b between 10−2 and 100 Hz, itis a straight line with a slope of about −1. It means that theinfluence of the resistances can be mostly ignored.

Figure 9c depicts the phase angle as a function of frequen-cy. In the phase angle plot, an approach to pure capacitivebehavior at low frequency can usually be identified by ap-proaching the −90° [47]. The value of the phase angle can beused to evaluate the effectiveness of ion diffusion inmesopores at the medium–high-frequency region (between102 and 104 Hz). The smaller the phase angle is, the fasterthe ions diffuse. As shown in Fig. 9c, the sample presents thesmallest phase angle, about −56° at 100 Hz, indicating thesamples possess better capacitance performance.

-10 0 10 20 30 40 50 60 70 80 90 100-10

0

10

20

30

40

50

60

-Z'/O

hm

Mn-0 Mn-0.2 Mn-0.5 Mn-1.2 Mn-3.0

-Z"/

Ohm

Z'/Ohm

0.1 Hz

0.1 Hz

0.1 Hz0.1 Hz

0.1 Hz0.0 0.2 0.4 0.6 0.8 1.0

0.0

0.2

0.4

0.6

0.8

1.0

Z'/Ohm

Mn-0 Mn-0.2 Mn-0.5 Mn-1.2 Mn-3.0

100 Hz

slope=1

100 Hz100 Hz

100 Hz100 Hz

a

10-3 10-2 10-1 100 101 102 103 104 105

10-1

100

101

102

Mod

ulus

of

Impe

danc

e (/

Z/)

/Ohm

Frequency ( f ) / Hz

Mn-0 Mn-0.2 Mn-0.5 Mn-1.2 Mn-3.0

b

10-3 10-2 10-1 100 101 102 103 104 105 1060

-10

-20

-30

-40

-50

-60

-70

Mn-0 Mn-0.2 Mn-0.5 Mn-1.2 Mn-3.0

Phas

e an

gle

()

/ deg

rees

Frequency ( f ) / Hz

c

Rs: equivalent series resistance;Rct: electron transfer resistance;CPE: constant phase element;Zw: diffusion Warburg impedance

dRs

Rct

Zw

CPE

-10 0 10 20 30 40 50 60 70 80 90 100 110 120-10

0

10

20

30

40

50

60

70-Z

''/O

hm

Z'/Ohm

Mn-0 Mn-0.2 Mn-0.5 Mn-1.2 Mn-3.0

e

Fig. 9 a Nyquist, b Bode, c Bode phrase, d equivalent circuit, and e simulated EIS plots of MnO2 electrodes

Table 3 Fitted results of EIS spectra for MnO2 samples

Sample Rs/Ω cm−2 Rct/Ω cm−2 CPE-T/F cm−2 ZW-R/Ω cm−2

Mn-0 0.164 2.92 (±0.13) 0.0283 (±0.0024) 144.3 (±29.8)

Mn-0.2 0.120 2.61 (±0.36) 0.0760 (±0.0011) 40.7 (±4.3)

Mn-0.5 0.160 3.69 (±0.24) 0.0308 (±0.0019) 227.1 (±56.2)

Mn-1.2 0.182 5.08 (±0.47) 0.0145 (±0.0013) 359.6 (±71.9)

Mn-3.0 0.231 14.85 (±2.51) 0.0209 (±0.0021) –


At a frequency of 0.1 Hz, the moduli of impedance of thefive samples are 16.6, 18.2, 21.5, 25.5, and 33.1Ω. In a word,the above experimental impedance parameters obtained atdifferent frequency region provide a clear indication that theMnO2 prepared under the situation of optimal additionamount can be related to better electrochemical behavior whencompared with the results of other examined samples [42].

Cycle performance

Figure 10 exhibits cycle performance of Mn-0.2. As can beseen from Fig. 10a, the ten times charge/discharge curve keepsgood symmetry and reproducibility and the E-t relationship islinear, indicating good reversibility [26]. After ten cycles, thespecific capacitance decreases by only about 2 % (±0.1 %).Figure 10b is the tendency of discharge capacitance with cycletimes. After 500 cycles, the C sp tends to be stabilized and stillhas 87 % (±0.5 %) capacitance retention rate, demonstratingthat Mn-0.2 has high charge and discharge efficiency andcapacitance retention.

Conclusions

In summary, adding suitable amount of SDS can improve thedispersity of MnO2 particles, change the morphologies, re-duce the particle size, and increase the specific surface area aswell as improve the electrochemical behavior of the MnO2

samples. As SDS addition amount increases, it is interestingthat the morphology of MnO2 is various: fibrous, pine needle-like, cotton-like, round bubble-like, flocculent, and nervoustissue-like shape. This is because SDS forms different formsof aggregates in aqueous solutions, acting as structure direc-tion agent in the preparation process. In addition, the

electrochemical and N2 adsorption tests show that the C sp

and BET-specific surface area of MnO2 samples have thesame tendency: first increases then decreases. Corresponding-ly, the C sp of Mn-0, Mn-0.05, Mn-0.2, Mn-0.5, and Mn-1.2are 103.0, 111.3, 154.5, 86.3, and 86.5 F g−1, and the SBET are213.3, 232.3, 255.9, 238.9, and 206.8 m2 g−1, respectively.Thus, when SDS addition amount is too low, it has limitedimprovement of capacitance, specific surface area, etc. How-ever, with excessive SDS added, it has negative effects on theabove properties. Obviously, there is an optimum impact SDSaddition amount of 0.2 g (0.017 mol L−1). Whether from themicrostructure or macroscopic perspective, Mn-0.2 is thepromising excellent supercapacitor electrode material, whosecapacitance and energy density increased a lot.

Acknowledgments This work was financially supported by the NaturalScience Foundation of China (no. 21106124 and no. 21375116). Therelated instrument of testing for this work was supported by the TestingCenter of Yangzhou University.

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