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Electrochimica Acta 278 (2018) 51e60

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Electrochimica Acta

journal homepage: www.elsevier .com/locate/electacta

N/S co-doped three-dimensional graphene hydrogel for highperformance supercapacitor

Weijie Zhang a, Zhongtao Chen a, Xinli Guo a, *, Kai Jin a, YiXuan Wang a, Long Li b,Yao Zhang a, Zengmei Wang a, Litao Sun c, Tong Zhang c, **

a Jiangsu Key Laboratory of Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, Nanjing 211189, Chinab Yinbang Clad Material Co., Ltd, Wuxi 214145, Chinac School of Electronics Science and Engineering, Southeast University, Nanjing 210096, China

a r t i c l e i n f o

Article history:Received 5 March 2018Received in revised form27 April 2018Accepted 2 May 2018Available online 4 May 2018

Keywords:N/S co-doped three dimensional (3D)graphene hydrogel (N/S-3DGH)Electrode materialsSupercapacitive performanceAll-symmetric solid-state supercapacitor

* Corresponding author.** Corresponding author.

E-mail addresses: guo.xinli@seu.edu.cn (X. Guo), t

https://doi.org/10.1016/j.electacta.2018.05.0180013-4686/© 2018 Elsevier Ltd. All rights reserved.

a b s t r a c t

Doping and high specific area are essential to the performance of supercapacitor electrode materials.However, the conventional single doping in two dimensional electrode materials is still unable to gethigh supercapacitive performance. Here we report a facile hydrothermal process using ammonia as asource of nitrogen (N) and thiourea as a sulfur (S) source to prepare N/S co-doped three dimensional(3D) graphene hydrogel (N/S-3DGH) for supercapacitor electrode application. The as-prepared N/S-3DGH is uniform and stable. The N/S co-doped 3DGH electrode material exhibits a high specific ca-pacity of 1063 Cg-1 at a current density of 1 Ag-1. Even at a density of 20 Ag-1, it can still hold anexcellent charge and discharge cycling stability, and with 76% of initial capacity retained after 6000charge and discharge cycles at a density of 10 Ag-1. Moreover, the all-symmetric solid-state super-capacitor fabricated by N/S-3DGH without any binders owned an energy density of 6.25Wh kg�1 at apower density of 500W kg�1 showing very promising applications for portable power and flexibleenergy storage devices.

© 2018 Elsevier Ltd. All rights reserved.

1. Introduction

As oil becomes scarce and pollution is becoming more and moreserious, more research is needed in the field of new energy to createa new energy storage device that provides higher energy and po-wer, as well as being environmentally friendly [1e4]. Super-capacitor is recognized as a promising energy storage device withhigh power density, short charging time, long cycle life, beingenvironmentally friendly etc. [5]. It can be divided into electricdouble layer capacitance and pseudo capacitance. The properties ofelectrode materials depend on reaction mechanisms of super-capacitors [6]. The specific area of electrode materials is vital to theperformance of supercapacitor according to the double electricallayer model and equation of C ¼ εA=d, where A is the surface area,d is the distance between thewall of electrode and the center of ion,and εis the local dielectric constant of electrolyte [7,8]. Graphenehas recently attracted increasing interests in using as electrode

zhang@seu.edu.cn (T. Zhang).

materials of supercapacitor due to its large surface area, mechanicalstability and good conductivity. However, the irreversible agglom-eration or restacking of graphene sheets, because of the strong vander Waals interactions and high inter-sheet junction contactresistance between isolated graphene sheets, severely suppress theintrinsically high conductivity and mechanical strength of indi-vidual graphene sheets and diminish its accessible surface area.Compared with the two-dimensional (2D) graphene, the threedimensional (3D) graphene especially substrate-free 3D graphenehydrogels shows porous structure, high surface area and lowresistance but without severely sheets aggregation. Furthermore,when 3D graphene hydrogel was used as supercapacitor electrode,its unique 3D porous microstructure promoted an ideal interfacebetween electrolyte and electrodes, and could provide more activesites shorter transport path for electrons and ions and enlargeelectrode/electrolyte contact area, thus leading to improved elec-trochemical performance [9e11].

In addition, recently, the introduction of heteroatoms into gra-phene has shown a promising way to increase the capacitive per-formance of graphene electrode by influencing spin density and thecharge distribution of carbon atoms, which increases the activation

W. Zhang et al. / Electrochimica Acta 278 (2018) 51e6052

region on graphene surface thus improving the energy storageability [12e15]. For example, single N doping and single S doping,respectively, have improved the specific capacitance and cycleperformance of graphene in supercapacitors [16e18]. N singledoped graphene has modified electronic structure delivery toelectrochemical sites with a minimized change of conjugationlength due to charge polarization. Besides it also introduce N-containing functional groups such as pyridine and pyrrole intographene to increase wettability and pseudo capacitance. By singledoping sulfur into graphene, the sulfur atom has unique effectsincluding of bond polarization, lattice distortion and tailoring ofcharge density distribution due to its large size. In addition, dopingsulfur is likely to induce structure defects in thematrix of graphene,which endows these materials with unique electronic structure,high specific surface area and numerous active site [19e21].However, compared with single doping, multiple doping is a ver-satile synthetic approach, which can further tune the properties ofsingle doped graphene [13,22,23]. Among the doped carbon ma-terials, the electronegative sulfur atom features p orbitals in itsoutermost shell and possesses a unique electronic structure whichsimilar to that of nitrogen atom. Therefore, nitrogen and sulfur canbe doped into graphene simultaneously. After co-doping nitrogenand sulfur into graphene, the graphene not only has unique elec-tronic structure, defected morphology but also has a series redoxfaradic reactions occurred at pyrrole and pyridine groups andadditional sulfone and sulfoxide species to increase the pseudocapacitance. It is confirmed that by introducing the advantages of Nand S into graphene simultaneously is effective for enhancingcapacitive performance of graphene's capacitive performance.Although co-doped graphene potentially possess promising prop-erties for supercapacitor application, there aren't many reportsabout them that are simultaneously doped with S and N. NgocQuang Tran et al. [24]doped N and S into graphene and get highcapacitance up to 536 Fg-1. TaoWang [25] prepared N/S-3DGHwiththe specific capacitance 566 Fg-1 at 0.5 Ag-1 via a one-pot hydro-thermal route using graphene oxide as starting material and L-cysteine, an amino acid containing both N and S, as the dopingagent. Wee Siang Vincent Lee [26] prepared the N/S co-dopedgraphene with high capacitance 1089 Fg�1at 1 Ag-1 by one pothydrothermal method.

In this research, we report our works on preparing N/S co-dopedgraphene hydrogel via a facile one-step hydrothermal method forhigh-performance supercapacitor application. We systematicallytested the electrochemical properties of the material and assem-bled it into an all-symmetric solid state supercapacitor withexcellent supercapacitive performance.

2. Experiment

2.1. Materials preparation

Graphite, sulfuric acid (H2SO4), phosphoric acid (H3PO4), hy-drochloric acid (HCl) and potassium permanganate (KMnO4) wereanalytically pure and used without further purification. Polyvinylalcohol (PVA, DP¼ 1750), Ammonia (NH3$H2O), thiourea (CH4N2S),potassium hydroxide(KOH), ethanol were purchased from ChinaNational Pharmaceutical Group Corporation (SINOPHARM). Nickelfoam (NF) was purchased from Nanjing WanQing Chemical Glass-ware Instrument CO. LTD.

2.2. Preparation of GO

Graphene oxide was prepared by improved Hummers method[27]. Briefly, graphite was oxidized by concentrated H2SO4, H3PO4and KMnO4 and the resulting GO was purified and isolated via a

centrifuge with HCl and diluted water. The colloid solution wasdiluted and sonicated for 30 mints.

2.3. Preparation of N/S co-doped three dimensional graphenehydrogel (N/S-3DGH)

The ammonia and thiourea (CN2H4S) were used as a precursorfor N source and both the N and S, respectively. During the syn-thesis, 300 ml ammonia was added to 30ml graphene oxide collidesolution (of concentration 3mg/ml) slowly. The mixed solutionwasstirred magnetically for 30min. Then, the thiourea solution withdifferent concentrations (12.5, 25, 37.5, 50, 62.5mg/ml) was addedto the dispersed solution and stirred magnetically for 30min andtransferred to Teflon-lined stainless steel autoclave with the vol-ume of 40ml. Hydrothermal treatment was then conducted on themixed solution at 180 �C for 24 h. The autoclave was then naturallycooled to room temperature and the as-prepared product wastaken out with tweezers and cleaned with ethanol and deionizedwater. The N doped graphene hydrogel (N-GH) and un-dopedgraphene hydrogel (GH) has been prepared at the same experi-ment condition respectively.

2.4. Fabrication of electrodes and all-symmetric solid-statesupercapacitor (ASSC)

The electrode was prepared according to the reported method[24,26]. Firstly, graphene hydrogel was cut into slices with a dryweight of 2mg, then pressed into nickel foam under a pressure of20MPa before moving to the vacuum oven at 80 �C for 6 h for usingas an electrode.

All-symmetric solid-state supercapacitor (ASSC) was assembledby using N/S-3DGH as the electrode materials and PVA/KOH poly-mer as the gel electrolyte. The mass of active materials on negativeand positive electrode is 2.5mg respectively. In addition, a kind ofPVA/KOH gel polymer electrolyte was prepared by followingmethod [27,28]. In brief, 6 g of PVA was mixed with 60ml ofdeionized water. The mixture was under stirring at 85 �C until itturned homogeneous gel. 6 g KOH was added slowly to homoge-neous gel after it cooled down to room temperature and the PVA/KOH gel polymer electrolyte was obtained.

2.5. Materials characterization

Morphology and uniformity of graphene oxide was investigatedby tapping mode atomic force microscopy (AFM, Dimension ICON).The morphologies, microstructures and chemical constitution of N/S-3DGHwere characterized by scanning electron microscopy (SEM,Sirion), X-ray diffraction (XRD, D8-Discover) with Cu Ka radiation inthe range of 10e80�, X-ray photoelectron microscope (XPS, PHI500) and Raman spectra (Thermo Fish), respectively. The hydrogelhas been cold dried at �40 �C for 48 h, after cold drying to aerogel,specific surface area and pore size of N/S-3DGH were performed byN2 adsorption/desorption method. The surface area was calculatedusing the Brunauere Emmett Teller (BET) model. The pore volumeand total pore size distributionwere determined by Barrette-JoynerHalenda (BJH) method.

2.6. Electrochemical measurements

The electrochemical performance of N/S-3DGHwasmeasured inthree-electrode measure system by the electrochemical worksta-tion (CHI 550, Shanghai). The electrochemical performance ofsamples was tested in an aqueous 6M KOH solution with platinumcounter electrode and mercury/mercury oxide (Hg/HgO) referenceelectrode. The cyclic voltammetry (CV) was conducted at scan rates

Fig. 1. Schematic illustration of the formation process of N/S-3DGH.

W. Zhang et al. / Electrochimica Acta 278 (2018) 51e60 53

of 5mVs-1 to 100mVs-1 in the voltage range from �1 to 0 V. Gal-vanostatic charge-discharge (GCD) and circle performance werecarried out in the voltage range from �1 and 0 V. The electro-chemical impedance spectroscopy (EIS) was performed at ACvoltage with 5mV amplitude in a frequency range from 0.01 Hz to100 KHz.

The electrochemical performance of ASSC was performed by theelectrochemical workstation. The CV test was conducted at scanrates of 5mVs-1 to 100mVs-1 in the voltage range from 0 to 1.6 V.GCD test and circle performance were carried out in the voltagerange from 0 to 1 V. The EIS test was performed at AC voltage with5mV amplitude in a frequency range from 0.01 Hz to 100 KHz.

The specific energy and power are two important factors toevaluate the performance of a symmetric supercapacitor. Since thecharge-discharge is a non-linear function, the specific energy andpower of ASSC were calculated using the following equations[31,32]:

E ¼ IZtðVminÞ

tðVmaxÞVðtÞdt (1)

P ¼ E=t (2)

here, E (Wh kg�1), I (Ag�1) V(v), P (W kg�1) and t (s) are the specificenergy, the specific current, the discharge potential window, thespecific power and the discharge time, respectively.

Fig. 2. (a) CV curves, (b) GCD curves, (c) EIS of N

3. Results and discussion

3.1. Morphological and structural characterizations

The quality of graphene oxide is vital for the performance ofgraphene hydrogel. The atomic force microscopy (AFM) test wasused to perform the quality of GO. Fig. S1 shows the AFM image ofgraphene oxide (GO) which was used as precursor of N/S-3DGH onsilicon substrate. The graphene oxide possesses large sheet butwith certain aggregation. The thickness of the representative gra-phene oxide is about 4.1 nm, which is equivalent to 6 or 8 grapheneoxide layers [33,34].

Fig. 1 shows a schematic illustration of synthesis of N/S-3DGH.In a typical synthesis, ammonia and thiourea were used as nitro-gen and sulfur sources respectively. The mixture was heated at180 �C for 24 h to form graphene hydrogel self-assembly. Duringthe hydrothermal process graphene oxidewas reduced to grapheneand N, and the S functional group was chemically grafted ontographene. Finally, the N/S-3DGH was washed with deionized waterand was further washed by ethanol to remove the possible elementN and S.

Fig. 2 shows the electrochemical properties of N/S-3DGH whichwas synthesized with different thiourea concentration. Accordingto Fig. 2a, the CV curves with peaks are similar since they have sameelectrochemical mechanism, and the area of CV curves changed alittle with increase of thiourea concentration. Fig. 2b is GCD curvesof N/S-3DGH with different thiourea concentration under 1 A/gcurrent density charge and discharge test. It is apparently thatwhen the thiourea concentration is 25mg/ml, the N/S-3DGH hasmaximum specific capacity (~832.6C/g). Fig. 2c probably reveals thecause of high capacitance. When the thiourea concentration is

/S-3DGH at different thiourea concentration.

Fig. 3. (a) Digital photos of a 20mg N/S-3DGH pillar with the diameter of 1.3 cm and the height of 1.5 cm supporting a 300 g counterpoise, more than 15000 times its own weight.(b, c) SEM images with different magnifications of the N/S-3DGH interior micro-structures. (d) Electron image of the selected area of N/S-3DGH for EDS mapping. (e)e(h) Elementalmapping of C, O, N, S.

W. Zhang et al. / Electrochimica Acta 278 (2018) 51e6054

25mg/ml, the N/S-3DGH has much low resistance and bettercapative properties. Therefore, after getting the optimal amount ofthiourea (25mg/ml) added to graphene, we further tested andcharacterized N/S-3DGH (thiourea 25mg/ml).

Fig. 3a shows the digital photos of a 20mg graphene hydrogelpillar with the diameter of 1.3 cm and the height of 1.5 cm sup-porting a 300 g counterpoise, more than 15000 times of its ownweight. After being pressed by weight, it can restore its originalshape or structure. The goodmechanical properties of graphene arealso important for the fabrication of capacitor electrodes[18,35e38]. Fig. 3dec shows SEM images of N/S-3DGH at differentmagnifications. The three-dimensional interconnected structurewith the pore size about 1 mm to 3 mm can be observed clearly. Theporous structure favors electrolyte permeation and ion transport[5,11]. Energy dispersive x-ray spectroscopy (EDS) elementalmapping was conducted for the N/S-3DGH. Fig. 3 eeh show the

Fig. 4. (a) N2 sorption isotherms. (b) Pore size

detection of carbon, oxygen, nitrogen and sulfur elements in the N/S-3DGH, respectively which indicates that N and S are doped intographene successfully.

Fig. 4a and b shows the BET of N adsorption-desorption corre-sponding to GH, N-GH and N/S-3DGH samples, respectively. Asshown in Fig. 4a, the curve of GH is similar to N-GH at relativepressures. Due to the doping of N element, the absorption volumehas increased, indicating that N-doping has a certain role inimproving the specific surface area. As for the BET curve of N/S-3DGH, IV isotherm type with hysteresis loops were observed,indicating a mesoporous structure. The N/S-3DGH has a specificarea of up to 419.4 m2g-1 larger than specific area of GH(187.475m2g�1) and N-GH (340.232 m2g-1), which indicates thatthe presence of large atom size S atom in the graphene induced thegraphene defects, thereby increasing the specific surface area,which is confirmed by the pore size distribution derived from BJH

distribution of GH, N-GH and N/S-3DGH.

Fig. 5. (a) XRD patterns of GH, N-GH and N/S-3DGH. (b) Raman spectra of GH, N-GH and N/S-3DGH.

W. Zhang et al. / Electrochimica Acta 278 (2018) 51e60 55

model. As Fig. 4b shows, the pore size of N/S-3DGH is much smallerthan GH and N-GH and the pore size distribution is mainly between1.6 and 5 nm. The high surface area and porosity of N/S-3DGH withmain pore size of 1.6e5 nm are of advantageous in providing morespace and permeability to facilitate the diffusion of ions in elec-trolyte, thereby enhancing the electrochemical properties of elec-trode materials [5,11].

Fig. 5a and b shows typical XRD patterns and Raman spectra ofgraphene hydrogel (GH), N-doped graphene hydrogel (N-GH) andN/S co-doped three dimensional graphene hydrogel (N/S-3DGH).From Fig. 5a we can see a broad diffraction peak at 2 theta of

Fig. 6. XPS spectra of N/S-3DGH. (a) Full sc

approximately 24.2�, corresponding to the (002) planes of graphite,which shows that the graphene hydrogel was prepared successfullyand graphene oxidationwas reduced. It is notable that after dopingN and S, the main peak is shifted to the left when compared tographene hydrogel, which is due to the fact that the lattice constantof graphene becomes larger [35,36]. Fig. 5b shows the Ramanspectra of GH, N-GH and N/S-3DGH. Compared with the Ramanspectrum of GH in Fig. 5b, the ID/IG value of N/S-3DGH (1.3135) islarger than that of N-GH (1.1678) and GH (0.9542). This also in-dicates the doping of N and S elements enhances the voids anddefects of the graphene.

an spectrum. (b) C1s. (c) N1s. (d) S2p.

W. Zhang et al. / Electrochimica Acta 278 (2018) 51e6056

The surface chemical composition and the chemical state of thedoped N and S in graphene was investigated by XPS. From Fig. 6a, afull scan spectrum, C1s, O1s, N1s and S2p peaks can be clearlyobserved. This indicates the existence of N and S elements in the N/S-3DGH sample, conforming that N and S are successfully dopedinto graphene. Fig. 6b shows the C1s high resolution XPS spectrumof N/S-3DGH with the curve fitted with species of C-C (284.6eV), C-O (285.8eV) and C¼O (287.8eV) [44]. Fig. 6c shows the N1s high-resolution spectrum, it can be observed from the curves that in-cludes four types of N-functional groups: Pyridine-N (398.6eV),Sp3-C and N (399.5eV), Pyrrolic-N (400.2), Graphitic N (401.4eV)[39,40]. Those types of N group are typically observed in the Ndoped carbonmaterials. As pyridine N usually locates at the edge ofthe flake like layer and contributes a p-electron to the conjugatedsystem in the graphene layers, which can enhance hydrophilicity ofthe carbon materials. Pyrrolic N refers to the N atom contributingtwo p-electrons to the psystem of graphene, which is assigned tothe contribution of pyrrole functionalities [41]. The last type of Nfunctional group is graphitic N, which is derived from N atom thatreplaces the C atom in the graphene hexagonal-ring. This graphiticN is important for improving the wettability and hydrophilicity ofthe carbon materials. Owing to that N functional group, the as-prepared electrode materials exhibit low resistance and goodcapacitive characteristic.

Fig. 6d represents the high resolution S2p spectrum of N/S-3DGH, the curve has four kind of peaks which corresponding toS2�(161.8eV), C-S-C (163.7eV), C-S (165eV) and sulphone SOx(167e172eV) [26], respectively. The S2�peak presented was due tothe H2S produced from thiourea decomposition, which is adsorbedonto the graphene. Spin-orbit coupling, S atoms bonds with

Fig. 7. (a) CV curves of GH, N-GH, N/S-3DGH at scan rate 20mV/S (b) CV curves of N/S-3DGH1 A/g. (d) Specific gravimetric capacitance of GH, N-GH and N/S-3DGH at different current

neighboring carbon atoms form C-S-C and C-S. It was reported[26,42e44]that the introduction of dopant S provides a morepolarized surface thus moderating the electronic structure of gra-phene and offering more defects that facilitate the electrochemicalproperties. Moreover, it can observed that there are many sulphoneC-SOx-C groups, which are believed to be very important forgenerating pseudo-capacitance. Due to these pseudo-site specif-ically, a reversible redox reaction occurs between C-S-C and C-SOx-C during charge and discharge process.

3.2. Electrochemical performance of N/S-3DGH

Fig. 7a shows the CV curves of graphene hydrogel; N-dopedgraphene hydrogel and N/S co-doped graphene hydrogel at ascanning rate of 20mVs-1. The CV curves of GH exhibit symmetricand approximately rectangular shape indicating the pure doublelayer capacitance behavior.While CV curve for N-GH and N/S-3DGHshows peaks shape at about �0.35and �0.2 V respectively whichcorresponding to pseudocapacitance behavior. Moreover, the peaksfor N/S-3DGH is bigger than N-GH electrode, indicating the highercapacitance. The high intensity peak for the N/S-3DGH electrodecan be due to N and S groups caused by the co-doping [26]. It in-dicates that after doping S element into the electrode materials, itnot only exhibits double layer capacitance but also pseudo-capacitance Fig. 7b shows the CV curves for N/S-3DGH atdifferent scan rates from 5 to 200mV/s and the potential rangefrom�1 to 0 V. It can be seen that, at low scan rate (5e50mV/s) theCV curve exhibit peaks shape indicating pseudocapacitancebehavior. A clear redox peak can be observed in the CV curves of N/S-3DGH at�0.2 V and�0.7 Vwhen scan rates value from5mVs-1 to

at different scan rates from 5mV/S-200mV/S (c) GCD curves of GH, N-GH, N/S-3DGH atdensity.

Fig. 8. (a) EIS spectra of GH, N-GH and N/S-3DGH and equivalent circuit. (b) The cycle tests of N/S-3DGH at 10 A/g.

W. Zhang et al. / Electrochimica Acta 278 (2018) 51e60 57

50mVs-1, which indicates strong presence of pseudo-capacitivereaction. When the scan rates increase from 50mV/s to 200mVs-1, the peaks are disappeared, and this because at high scan ratethere is no time for reaction (redox reaction) to occur. Therefore, athigh scan rate only one charge storage mechanism (EDLC) isobserved [45,46].

The GCD method was used to evaluate the specific capacity ofthe electrode materials for supercapacitors. Fig. 7c shows thecomparative GCD curves at 1 A/g GH, N-GH and N/S-3DGH. Thelinear CD curve was observed for GH electrode indicating pureEDLC behavior. However, N-GH and N/S-3DGH show nonlinear CDcurves with plateau region indicating the pseudocapacitancebehavior. Moreover, the initial drop at the begging of dischargingfor N/S-3DGH electrode indicate the low resistance as seen inFig. S2.

The discharging time for N/S-3DGH electrode is longer thanother electrodes indicating the higher specific capacity. To estimatethe amount of charge stored in the electrode appropriately, thespecific capacity(Cs) especially in terms of total charge stored perunit mass(Cg�1/mAhg�1) is used and defined as [29,30].

Cs ¼ It=3:6m (3)

where, I is the charge-discharge current, t is the time elapsed forthe discharge, andm is the weight of active materials on electrode.Based on this equation, the specific capacity of 1063 Cg-1

(295.3mA hg�1), 442 Cg-1(123mA hg�1) and 210 Fg-1 (189Cg-1)were calculated for N/S-3DGH, N-GH and GH respectively at 1 Ag-1.

Based on this equation of N/S-3DGH, the specific capacitycalculated fromGCD curves is ~1063 Cg-1 at the current density of 1Ag-1, which is much higher than that of the single element N-doped(442Cg-1) and un-doped graphene hydrogel (189Cg-1). This in-dicates that the high capacitive performance of N/S-3DGH isattributed not only to the porous structure and high specific surfacearea but also to the co-doped N and S elements.

The average specific capacities of GH, N-GH and NS-3DGHcalculated at different discharge current densities (1-10 Ag-1) arepresented in Fig. 7 d. It can be seen that the Cs shows the decreasingtrend with the increase of current density. Moreover, the N/S-3DGHshows the highest capacity (389Cg-1) compared with N-GH (210Cg-1) and GH (175Cg-1) even at high discharge current density.

Fig. 8a shows the Electrochemical Impedance Spectra (EIS) ofthe electrode materials. At the high frequency region the Nyquistplots of N/S-3DGH show the lowest equivalent series resistance(Rs). The smallest semicircle at mid-high frequency region implies

the lowest charge transfer resistance (Rct) of the electrodematerial,while the GH has the highest charge transfer resistance with thelongest radius of the semicircle. At low frequency processcontrolled by diffusion, the straight lines of N/S-3DGH have goodlinearity and show an ideal capacitive characteristic. The Nyquistplots are simulated using ZView software. Fig. 8b shows thesimulated values of Rs and Rct are 4.7U and 4.3U for GH, 1.7U and4.1U for N-GH, 1.42U and 1.58U for N/S-3DGH, respectively. Thesmall charge transfer resistance of N/S-3DGH attributes theenhanced polarization of the carbon surface by N and S doping,which will improve the wetting of graphene, and thus reduce thecharge transfer resistance [47,48].

Fig. 8b shows the cyclic stability of N/S-3DGH at 10 Ag-1. Thespecific capacity of N/S-3DGH decreases with increasing number ofcycles and after cycle test, the capacity retained above 76%compared to that of the first cycle, which indicates a high electro-chemical stability and reversibility of N/S-3DGH.

As the N, O, S-groups make large pseudo-capacitance contri-bution to overall capacity of materials. The redox mechanisms of N/S-3DGH materials in KOH electrolyte has been proposed[18,20,26,49e51]. N-containing functional groups are predomi-nantly in the form of pyridine and pyrrole, both of which exhibitredox reactions as shown in Formulas (4) and (5). In the redoxprocess, only produces an electron gain and loss, so only the N-doped pseudo-capacitance is low. After the N/S co-doping, theredox reaction of Formulas (6) also occurs.

�C � N � C �þH2Oþ e�4� C � NH � C �þOH� (4)

�C ¼ N � C �þH2Oþ e�4� C � NH � C �þOH� (5)

�C � S� C �þH2Oþ 3e�4� C � SO2 � C �þ3OH� (6)

During this reaction, thiocarboxylic acid ester (C-S-C) is oxidizedto sulphone (C-SO2-C) when charged, and the correspondingreduction process is carried out. There are three electrons in thisredox process, so the resulting pseudo-capacitance is much higherthan the N doping. Whereas the N/S co-doping can effectivelyimprove the capacitance of the N/S-3DGH.

3.3. Electrochemical performance of ASSC

In this work, the N/S-3DGH materials was directly used as theASSC electrode materials without any binders, and the nickel foamwas pressed as current collector, as shown in Fig. 9a. Fig. 9b illus-trates the CV curves of the ASSC at different scan rates in the range

Fig. 9. (a) Schematic illustration of the all solid symmetric supercapacitor (ASSC). (b) CV curves of ASSC at different scan rates from 5mV/S-200mV/S (c) GCD curves of ASSC atdifferent current density from 1 A/g to 20 A/g. (d) The cycle tests of ASSC at 2 A/g. (d) EIS spectra of ASSC.

W. Zhang et al. / Electrochimica Acta 278 (2018) 51e6058

of 5e100mVs-1. As the scan rate increases, the CV curves canmaintain an intact shape, demonstrating the device's good capac-itive properties and excellent reversibility. GCD tests has been usedto test the ratability of the ASSC, Fig. 9c shows the GCD curves ofASSC at different current densities. All the curves exhibited goodsymmetry shape, indicating the well capacitive behavior. The spe-cific capacitance was calculated by equation (3) with the full massof ASSC. The specific capacity of ASSC was 45C/g�1 at 1 A/g�1 andremained at 63% at a current density of 20 A/g�1, indicating wellrate performance of the device. Fig. 9d shows the cycling perfor-mance of ASSC at a current density of 2 A/g�1 for 4000 cycles. It canbe seen that the ASSC maintained a capacitance retention of 69%after 4000 cycles. The solid-state supercapacitor's specific capacityis not good when comparing the three-electrode electrochemicaltests, since the solid-state electrolytes could avoid leakage of liquidelectrolyte but result in the increase of diffusion resistance of ionand insufficient oxidation-reduction reaction [52,53]. The ASSC'scompetitive values of energy density and power density werecalculated by equations (1) and (2), respectively. The device candeliver a maximum energy density of 6.25Wh kg-1 at a powerdensity of 500Wkg�1 and remain 3.75Wh kg�1 at a power densityof 9 kWkg�1. Fig. 9e shows the Nyquist plot of ASSC device. The EISspectra exhibits a small semicircle at the high-frequency region,suggesting low charge transfer resistance, at low frequency region,the straight lines means ASSC have good capacitive performance.The excellent performance of ASSC device could be ascribed to wellporous structure, high surface area and the pseudo capative effectof nitrogen and sulfur co-doped into graphene. This kind of mate-rials can be used to other energy storage device to improve theelectrochemical performance.

4. Conclusions

In summary, by a facile hydrothermal process using ammonia as

a source of N and thiourea as S source to synthesis N/S co-dopedthree dimensional graphene hydrogel(N/S-3DGH) for super-capacitor electrode application. It exhibits a high specific capacityof ~1063 Cg-1 at a current density of 1 Ag-1. Even at a density of 10Ag-1, it also exhibits good charge and discharge cycling stabilitywith 76% of initial capacitance retained after 6000 charge anddischarge cycles and good mechanical properties under the weightof 300 g. Moreover, all-symmetric solid-state supercapacitor wasfabricated by N/S-3DGH without any binders and it owned an en-ergy density of 6.25Wh kg�1 at a power density of 500Wkg�1. Thehigh supercapacitive performance is believed to be attributed tohigh specific area, porous structure and both N functional groupand S functional group to co-enhance pseudo-capacitive active site.The results indicate that as-prepared N/S-3DGH electrode materialcan significantly enhance the supercapacitor performance andshowing very promising applications for portable power and next-generation energy storage devices.

Acknowledgement

The authors would like to thank the financial supports by MOSTunder Grant Number 2017YFA0205800, National Natural ScienceFoundation of China (21173041), the Opening Project of Jiangsu KeyLaboratory of Advanced Metallic Materials, China and the Project ofJiangsu Key Laboratory for Clad Materials, China (BM2014006). Theauthors will also thank Mr. Drew Cannon from Washington StateUniversity, USA for his linguistic assistance during the preparationof this manuscript.

Appendix A. Supplementary data

Supplementary data related to this article can be found athttps://doi.org/10.1016/j.electacta.2018.05.018.

W. Zhang et al. / Electrochimica Acta 278 (2018) 51e60 59

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