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Electrochimica Acta 305 (2019) 81e89

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Electrochimica Acta

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

Hierarchical CoNi2S4 nanosheet/nanotube array structure on carbonfiber cloth for high-performance hybrid supercapacitors

Chen Su a, Shusheng Xu a, Lu Zhang c, Xinwei Chen a, Guoqin Guan a, Nantao Hu a,Yanjie Su a, Zhihua Zhou a, Hao Wei a, Zhi Yang a, *, Yong Qin b, **

a Key Laboratory of Thin Film and Microfabrication (Ministry of Education), Department of Micro/Nano Electronics, School of Electronic Information andElectrical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, PR Chinab Institute of Nanoscience and Nanotechnology, School of Physical Science and Technology, Lanzhou University, Gansu, 730000, PR Chinac School of Advanced Materials and Nanotechnology, Xidian University, Xi'an, 710071, PR China

a r t i c l e i n f o

Article history:Received 24 December 2018Received in revised form26 February 2019Accepted 3 March 2019Available online 4 March 2019

Keywords:CoNi2S4 nanosheet/nanotube arraysCarbon fiber clothElectrodepositionHybrid supercapacitorEnergy density

* Corresponding author.** Corresponding author.

E-mail addresses: zhiyang@sjtu.edu.cn (Z. Yang), q

https://doi.org/10.1016/j.electacta.2019.03.0130013-4686/© 2019 Elsevier Ltd. All rights reserved.

a b s t r a c t

Well-defined nanostructures are attractive due to their excellent advantages in enhancing the perfor-mance of electrochemical energy storage. In this work, a kind of hybrid structure of hierarchical CoNi2S4nanosheet/nanotube array directly assembled on carbon fiber cloth has been designed and developed forhigh-performance supercapacitors. CoNi2S4 nanosheet/nanotube arrays are fabricated through orderlyelectrodeposition of ZnO nanorod arrays and CoNi2S4 nanosheets, followed by removing ZnO nanorodstemplate. Benefiting from the unique hollow nanostructure with abundant electrochemical active sites,the specific capacitance of this electrode can reach up to 995.8 C g�1 at a current density of 2 A g�1, alongwith excellent rate capability (740 C g�1 at 50 A g�1). Moreover, the hybrid supercapacitors are preparedby using hierarchical CoNi2S4 nanosheet/nanotube arrays as positive electrode and reduced grapheneoxide-carbon nanotubes as negative electrode for energy storage application, which demonstrate a highenergy density of 35Wh kg�1 at a power density of 3 kW kg�1 and excellent cycle stability with 96.9%capacitance retention after 10000 cycles. This work provides a feasible and a practical approach tofabricate CoNi2S4 hollow nanostructures and its huge potential in energy storage.

© 2019 Elsevier Ltd. All rights reserved.

1. Introduction

With the ever growing of energy crisis and environment prob-lems, it is urgent to develop low cost and eco-friendly energystorage devices [1e3]. Supercapacitors (SCs) are efficient energystorage devices with excellent properties such as high powerdensity, fast charge-discharge process and long cycle life [4e6].However, compared with lithium ion batteries, the lower energydensity of SCs limits its practical application. It is expected that SCsshould have higher energy density and power density. It has beenproved that hybrid supercapacitors (HSCs) taking the advantages ofboth the electrochemical double-layer and faradaic reactions tostore electrical energy are the promising energy storage devices[7e9].

Transition metal oxides/hydroxides are the common used active

inyong@lzu.edu.cn (Y. Qin).

electrode materials for battery-type faradaic electrode which storeelectrical energy through the reversible faradaic reactions [10].However, the performance of metal oxides/hydroxides basedpesudocapacitors is limited due to the poor electrochemical sta-bility and low conductivity. These problems can be solved either bydesigning new electrode active materials with higher conductivityor novel structures with more active sites exposed. Recently,transition metal sulfides have been regarded as the potentialelectrode active materials for pseudocapacitive energy storage. Forexample, Xu and co-workers prepared Co3S4 nanosheets (NSs) witha capacitance of 1037 F g�1 at 1 A g�1 [11]; Dai and co-workers re-ported NiS2 hollow prisms with a high capacitance of 1725 F g�1 at5 A g�1 [12]; Guo and co-workers fabricated double-shell CuSnanocages through anion exchange reaction, with electrochemicalperformance of 843 F g�1 at 1 A g�1 [13]. It has been reported thattransition metal sulfides such as Co3S4, NiS2, and CuS have loweroptical band gap energy and higher conductivity than transitionmetal oxides [14]. Since the electronegativity of sulfur is lower thanthat of oxygen, the substitution of oxygenwith sulfur can effectively

mailto:zhiyang@sjtu.edu.cnmailto:qinyong@lzu.edu.cnhttp://crossmark.crossref.org/dialog/?doi=10.1016/j.electacta.2019.03.013&domain=pdfwww.sciencedirect.com/science/journal/00134686www.elsevier.com/locate/electactahttps://doi.org/10.1016/j.electacta.2019.03.013https://doi.org/10.1016/j.electacta.2019.03.013https://doi.org/10.1016/j.electacta.2019.03.013

C. Su et al. / Electrochimica Acta 305 (2019) 81e8982

prevent the structural damage caused by the elongation betweenlayers and facilitate the electron transport in the structure [15].Moreover, bimetallic NieCo sulfides show more active sites for theredox reactions, higher electrical conductivity for charge transferand more stable than their corresponding single metal sulfides,which results in enhanced electrochemical performance [16,17].Encouraged by these findings, various nanostructures of NieCosulfides have been reported, such as CoNi2S4 nanoparticles (NPs)[18], CoNi2S4 nanowires [16], CoNi2S4 nanosheets [19], CoNi2S4mushroom-like arrays [20], NiCo2S4 nanotube arrays (NTAs) [17]and NiCo2S4 urchin-like nanostructures [14]. Among them, well-defined NTAs have drawn great attention due to their high sur-face area. It has been demonstrated that the well-oriented NTAsalso contribute to the ion diffusion and substantially facilitate theelectrolyte penetration [21]. Xiao and co-workers reported NiCo2S4NTAs grew on carbon fiber paper [17]. However, the preparationprocedure is complicated, including multistep of the synthesis ofNieCo precursor, vulcanization of precursor at high temperatureand acid etching to remove the interior metal oxide. Xing and co-workers have synthesized Ni3S2 NTAs by sacrificial ZnO nanorodarrays (NRAs) template [22], but this sample has the contactproblems between the conductive substrate and active materialwith the removal of ZnO seed layer. Therefore, it is imperative todevelop a simple method to synthesize stable CoNi2S4 nano-structures with high surface area which show great potential inenhancing the electrochemical performance. Here, inspired byChen and co-workers’ work to synthesize CoNi2S4 NSs in one step[23], we make use of electrodeposition method to synthesize aunique stable hierarchical CoNi2S4 nanosheet/nanotube array (NS/NTA) structure and further validate its application in hybridsupercapacitors.

In this work, we have successfully synthesized well aligned hi-erarchical CoNi2S4 NS/NTAs by employing electrodeposited ZnONRAs as template. This novel hybrid nanostructure of hierarchicalNSs/NTAs intimately attached to carbon fiber cloth (CFC) caneffectively improve the specific surface area and provide moreactive sites which are benefit for improving specific capacitance.This binder-free electrode has an ultrahigh specific capacitance of

Fig. 1. (aed) Schematic illustration for the preparation of CoNi2S4 NS/NTAs, and correspondinand (h) hierarchical CoNi2S4 NS/NTAs-5 on CFC.

995.8 C g�1 at 2 A g�1, superior rate capability (740 C g�1 at50 A g�1), and good cycling stability (can retain 77.2% after 2000charge-discharge cycles). Furthermore, HSCs based on the hierar-chical CoNi2S4 NS/NTAs and reduced graphene oxide-carbonnanotube (rGO-CNT) film was assembled and exhibited a high en-ergy density of 35Wh kg�1 at a power density of 3 kWkg�1 withgood stability. This result outperforms most of the NieCo basedmaterials, indicating that our new developed CoNi2S4 NS/NTAs arethe promising candidate for energy storage devices.

2. Experimental section

2.1. Synthesis of ZnO NRAs on CFC

Two different methods including the electrodeposition and thehydrothermal process were employed for the preparation of ZnONRAs on CFC for comparison. CFC was treated in concentratedHNO3 for 30min at 60 �C in advance to improve the surface hy-drophilicity. For the electrodeposition method, the electrolyte wasprepared by dissolving 5mM zinc nitrate hexahydrate(Zn(NO3)2$6H2O) and 5mM hexamethylenetetramine (HMTA) into100mL deionized (DI) water. The electrodeposition was carried outin a conventional three-electrode system, where CFC (1� 1 cm2)acted as the working electrode, 1� 1 cm2 platinum plate as thecounter electrode and Ag/AgCl as the reference electrode, respec-tively. ZnO NRAs were continuously deposited at a constant po-tential of �1 V versus the reference electrode for 3600 s at 90 �C.For the hydrothermal method, firstly ZnO seed layer was sputteredon CFC, then ZnO NRAs template was grown on CFC (1� 1 cm2) in asolution of 0.01M Zn(NO3)2$6H2O and 0.01M HMTA at 90 �C for6 h.

2.2. Preparation of hierarchical CoNi2S4 NS/NTAs on CFC

Both of the two types ZnO NRAs were used as the templates forthe electrodeposition of CoNi2S4. The electrodeposition of CoNi2S4on ZnO NRAs were conducted by cyclic voltammetry (CV) scan atselected scan rate and CV cycles in a potential range of �1.2 to 0.2 V

g SEM images of (e) bare CFC, (f) ZnO NRAs on CFC, (g) ZnO/CoNi2S4 NS/NRAs-5 on CFC,

C. Su et al. / Electrochimica Acta 305 (2019) 81e89 83

vs Ag/AgCl at 40 �C in a three-electrode system mentioned above.The electrolyte was prepared by dissolving 5mM CoCl2$6H2O,7.5mM NiCl2$6H2O and 0.75M thiourea (CS(NH2)2) into 100mL DIwater. The working electrodes were labeled as ZnO/CoNi2S4 NS/NRAs-x (x¼ number of CV cycles, i.e., 3, 5, 7 and 11 cycles). Afterthe electrodeposition of CoNi2S4, the working electrodes wereetched in 1M KOH for 6 h for the removal of ZnO NRAs template,followed by drying at 60 �C for 10 h. The as-synthesized sampleswere labeled as CoNi2S4 NS/NTAs-x, and directly used as theworking electrode for the evaluation of energy storage perfor-mance. The loading mass of the CoNi2S4 on CFC for optimal samplewas 0.4mg cm�2 by comparing the mass of the CFC/CoNi2S4 NS/NTAs with that of bare CFC using a microbalance.

2.3. Characterizations

The morphologies and nanostructures of the as-synthesizedelectrodes were observed by using a field emission scanning

Fig. 2. Fine structure and chemical composition characterization. (a) TEM image of CoNi2S4(cef) XPS analysis of the electrodeposition growth of CoNi2S4 NT.

electronmicroscope (FE-SEM, Ultra Plus, Carl Zeiss, Germany) and ahigh resolution transmission electron microscopy (HRTEM, FEITecnai G2 F30). X-ray diffraction (XRD) patterns were recorded onan advanced X-ray diffractometer (D8 Advance, Bruker, Germany)with Cu-Ka radiation. X-ray photoelectron spectroscopy (XPS,Japan Kratos Axis UltraDLD spectrometer) was applied to analyzethe elemental composition and chemical state of the as-synthesized materials. The specific surface area was estimated bythe Brunauer-Emmett-Teller (BET, Autosorb-iQ, Quantachrome,USA) through measuring N2 adsorption-desorption isothermals at77 K. The electrochemical properties of as-synthesized materialswere measured with a CHI760E electrochemical workstation. CV,galvanic charge-discharge (GCD) and electrochemical impedancespectroscopy (EIS) measurements were conducted using a three-electrode set-up in 6M KOH aqueous electrolyte. A Pt plate andthe Hg/HgO were used as the counter electrode and the referenceelectrode, respectively.

NT. The inset image is the SAED pattern. (b) HRTEM image of as prepared CoNi2S4 NT.

C. Su et al. / Electrochimica Acta 305 (2019) 81e8984

3. Results and discussion

The synthetic process of the hierarchical CoNi2S4 NS/NTAs isillustrated in Fig. 1. CFC (Fig. 1a) was treated in concentrated HNO3to improve the surface hydrophilicity and used as the support forthe electrodeposition of ZnO NRAs (Fig. 1b). The ZnO NRAs werethen used as the template for the electrodeposition of CoNi2S4(Fig. 1c). After the removal of ZnO NRAs, hierarchical CoNi2S4 NS/NTAs on CFC were obtained (Fig. 1d). As shown in Fig. 1f, theelectrochemical deposition method derived ZnO NRAs are uni-formly coated on the CFC surface. With the further electrochemicaldeposition of CoNi2S4, the smooth surface of ZnO NRAs becomerather rough with conformally coated NSs (Fig. 1g). There is nomorphology change observed with the removal of ZnO NRAs(Fig. 1h). XRD patterns in Fig. S1 show no diffraction peaks of ZnO,which means the complete removal of ZnO by the etching process.

It must be mentioned that we adopted the electrodepositionmethod for the preparation of the ZnO NRAs template rather thanthe common used hydrothermal method. The hydrothermalmethod usually needs the assistance of ZnO seed layer, which leadsto observable voids after the removal of ZnO NRAs as evidenced bySEM images in Fig. S2a and b. The ZnO NRAs from the electro-chemical deposition have intimate contact with the CFC substrate(Fig. S2c and d). The Nyquist plots in Fig. S3 indicate that theCoNi2S4 NS/NTAs prepared by using the hydrothermal methodderived ZnONRAs template hasmuch higher resistance than that ofthe sample from electrochemical deposition. The electrochemicaldeposition of CoNi2S4 was optimized by changing the scan rate ofthe CV scans. SEM images of the four samples with different scanrates (5, 10, 15 and 20mV s�1) for 5 CV cycles are shown in Fig. S4.The sample deposited at 15mV s�1 has the most NSs on the surfaceof NRAs, implying its more active sites. And the electrochemicalproperties showed in Fig. S5 indicate that the optimal deposition

Fig. 3. Electrochemical characterization of CoNi2S4 NS/NTAs-5 electrode. (a) CV curves. (b) Gby using the discharge curves in Fig. 3b. (d) Capacitance retention as a function of cycles a

scan rate is 15mV s�1.The nanotube (NT) structure of the CoNi2S4 is confirmed by TEM

characterization (Fig. 2a). The wall thickness of CoNi2S4 NT is about20 nm. The insert image of the selected area electron diffraction(SAED) pattern in Fig. 2a shows the ring-pattern, indicating thepolycrystalline structure of the CoNi2S4 NT. The diffraction rings areindexed to the (220), (311), (331) and (511) planes of cubic phaseCoNi2S4 [20]. The measured distance of the lattice fringes in theHRTEM image in Fig. 2b is 0.282 nm and 0.236 nm corresponding tothe (311) and (400) planes of CoNi2S4, respectively [24]. Theelemental mapping images (Fig. S6) further demonstrate the uni-form distribution of Ni, Co and S elements throughout CoNi2S4 NT.As shown in Fig. S7, the specific surface area of bare CFC is only1.23m2 g�1, and after grow CoNi2S4 NS/NTAs on the CFC surface, thespecific surface area increased to 13.01m2 g�1. This further illus-trates that the specific surface area can be greatly increased bysynthesizing this hierarchical structure.

The composition and chemical states of CoNi2S4 was analyzedby XPS, as shown in Fig. 2cef. The survey scan spectrum shows theexistence of Ni, Co, S, C and O. The Ni 2p and Co 2p spectrum isfurther studied by using peak-differentiation-imitating method. Asshown in Fig. 2d, the well fitted Ni 2p3/2 orbit can be separated intotwo peaks with binding energies of 856.3 and 857.2 eV, that rep-resenting Ni2þ 2p3/2 and Ni3þ 2p3/2, respectively. Meanwhile, the Ni2p1/2 can also be separated into two peaks at 874.1 and 875.9 eV,represent Ni2þ 2p1/2 and Ni3þ 2p1/2, respectively [25,26]. Similarly,in the Co 2p spectra (Fig. 2e), the peaks at 781.7 and 783.0 eVrepresenting Co3þ 2p3/2 and Co2þ 2p3/2, and the peaks at 796.9 and798.4 eV denoted Co3þ 2p1/2 and Co2þ 2p1/2 [27,28]. For the S 2pspectrum in Fig. 2f, the peaks at 161.8 eV for S 2p1/2 and 163.1 eV forS 2p3/2 are associated with metalesulfur bonds (NieS and CoeS),while the peak at 168.2 eV corresponds to the shakeup satellite[29].

CD curves. (c) Specific capacitance and area capacitance as a function of current densityt a current density of 40 A g�1.

Fig. 4. Electrochemical characterization of as-prepared rGO-CNTs electrode. (a) Cross-section SEM image of rGO-CNTs film. (b) CV curves of rGO-CNTs at different scan rates. (c) GCDcurves of rGO-CNTs at different current densities. (d) Specific capacitance of rGO-CNTs at different current densities calculated from GCD curves. (e) Nyquist plots of rGO-CNTs. (f)Cycle stability of rGO-CNTs at 10 A g�1.

C. Su et al. / Electrochimica Acta 305 (2019) 81e89 85

The electrochemical performance was investigated and the as-prepared hierarchical CoNi2S4 NS/NTAs on CFC were directly usedas the binder-free supercapacitor electrode. The electrochemicalmeasurements were conducted using a three-electrode configura-tion in 6M KOH aqueous solution. It was found that the perfor-mance was highly related to the film thickness (details seen in thesupporting information). The bare CFC electrode shows negligiblecapacitance (the black curve in Fig. S9a). While at 5 cycles, the as-prepared CoNi2S4 NS/NTAs sample shows the best electro-chemical performance (the pink curve in Fig. S9a). Except other-wise defined, the following results were obtained by using thesample prepared at the optimal conditions, i.e., 15mV s�1 and 5cycles of CV scan, and the obtained results are shown in Fig. 3. Theprofiles of the CV curves for at different scan rates in Fig. 3a showsymmetrical anodic and cathodic peaks which are the typicalcharacteristic of pesudocapacitors. The peaks are indexed to theredox reactions of Ni2þ/Ni3þ and Co2þ/Co3þ in KOH solution [23]:

CoNi2S4 þ 2OH�4CoS2xOH þ Ni2S4�2xOH þ 2e�

GCD curves for CoNi2S4 NS/NTAs-5 based on different currentdensities are presented in Fig. 3b. Both of the charge and thedischarge curves show symmetric profiles with voltage plateauswhich is consistent with the results of CV scans, i.e., reversibleredox reactions are responsible for the energy storage. It is foundthat the CoNi2S4 NS/NTAs-5 electrode shows high specific capaci-tance of 995.8 C g�1 at a current density of 2 A g�1, and even at ahigh current density of 50 A g�1, the specific capacitance is still ashigh as 740 C g�1 (Fig. 3c). The energy storage performance of ourmaterial is better than those of recently developed materials(Table S1). The CoNi2S4 NS/NTAs-5 electrode also displays goodlong-term stability, evidenced by the high capacitance retention of77.2% after 2000 cycles of charge/discharge at the high currentdensity of 40 A g�1 (Fig. 3d). SEM images in Fig. S10a and b showthat the CoNi2S4 NS/NTAs-5 electrode also has excellent structuralstability during the fast charge/discharge process. The Nyquist plotsof CoNi2S4 NS/NTAs-5 electrode before and after cycle test wereshown in Fig. S11. It shows low initial Rs, which indicates excellentionic and electronic conductivity of CoNi2S4 NS/NTAs-5 electrode.

C. Su et al. / Electrochimica Acta 305 (2019) 81e8986

Even after 2000 cycles the Rs is still 0.87U ensuring the high cyclingstability of this electrode. These results demonstrate that the as-prepared CoNi2S4 NS/NTAs is an excellent energy storage materialwith high specific capacitance, good long-term stability andstructural stability.

It has been well accepted that pesudocapacitors usually showhigh energy density, while electrochemical double-layer super-capacitors display high power density and fast charge/dischargeproperty. To further increase the energy storage performance, wehave proposed to assemble HSCs with rGO-CNT film which takesthe advantages of both pesudocapacitors and electrochemicaldouble-layer capacitors. rGO is widely used as the supercapacitorelectrode material due to its high specific surface area, high con-ductivity and good stability [30]. However, in the reduction process,rGO sheets tend to restack because of the strong Van Der Waalsinteraction, leading to an obvious decrease of the specific capaci-tance [31]. Therefore, we used CNTs as spacers to prevent rGO NSsfrom restacking, as evidenced by the SEM image of rGO-CNT film(Fig. 4a). The detailed preparation process of rGO-CNT film is shownin supporting information.

Before the assembling of the HSCs, we have investigated theelectrochemical energy storage performance of the rGO-CNT filmusing a three-electrode set-up in 6M KOH solution (Fig. 4bef). TheCV curves show rectangular profiles at all scan rates, and thecharge/discharge curves display symmetrical triangle profiles at allcurrent densities, indicating the good energy storage performance.The specific capacitance calculated from the discharge curves is160.3, 132.8, 119.6, 116, 109.6, 106 and 94 F g�1 at 1, 2, 4, 5, 8, 10 and20 A g�1, respectively. Fig. 4e shows the Nyquist plot and theequivalent series resistance is as low as 0.794U. The rGO-CNT filmalso shows high cycling stability supported by the high retention of

Fig. 5. . Schematic illustration and electrochemical characterization of the as-prepared CoNi2of CoNi2S4 NTAs-5 and rGO-CNTs at a scan rate of 10mV s�1. (c) CV curves of HSCs underpotential windows at a current density of 4 A g�1.

98.1% of the specific capacitance after 3000 cycles.Since the excellent energy storage performance of both the

CoNi2S4 NS/NTAs and the rGO-CNTs, we assembled HSCs using theCoNi2S4 NS/NTAs as the positive electrode and rGO-CNTs as thenegative electrode (as shown in Fig. 5a). Based on the measuredspecific capacitance of each electrode, the mass ratio of the CoNi2S4NS/NTAs to the rGO-CNTs electrode is calculated to be 0.18. Theelectrolyte for the HSCs is 6M KOH. The voltage window has criticalimportance to the energy density and powder density of super-capacitors devices. The HSCs in this study consists of electro-chemical double-layer capacitance and pseudocapacitance inseries, as shown in Fig. 5b. Therefore, the voltage window is thesuperposition of the voltage window of the electrochemicaldouble-layer capacitor and the pseudocapacitor. To avoid thesplitting of water in the aqueous electrolyte, we investigated thevoltage window of our supercapacitor devices in 6M KOH firstly.Fig. 5c shows the CV curves recorded at 30mV s�1 of the HSCs atdifferent voltage windows. It is found that with the increase ofvoltage window, the area inside CV curves increases. Obviousoxidation current appears when the voltage window reaches to1.6 V, implying the possible splitting of water at 1.6 V of the voltagewindow. The CGD curves of the as-assembled HSC device underpotential windows ranging from 1.1 to 1.5 V at 4 A g�1 were testedto further explore the potential window. As shown in Fig. 5d, noevident polarization appeared as the potential increased to 1.5 V.Therefore, the optimal voltage window is 1.5 V.

Fig. 6a shows the CV curves of the HSCs at different scan rates.All CV curves exhibit quasi-rectangular shapes, which ascribes tothe combined contributions of the electrochemical double-layercapacitance and the pseudocapacitance. The GCD curves in Fig. 6bexhibit nearly symmetrical triangular shape, demonstrating the

S4 NS/NTAs//rGOeCNTs HSCs. (a) Schematic illustration of the HSC device. (b) CV curvesvarious voltage window at 30mV s�1. (d) GCD curves of HSCs measured at different

Fig. 6. Electrochemical characterization of the as-prepared CoNi2S4 NS/NTAs//rGOeCNTs HSC. (a) CV curves of HSC device at different scan rates. (b) GCD curves of HSCs at differentcurrent densities. (c) Specific capacitance and area capacitance of the HSCs as a function of current densities calculated from GCD curves. (inset: optical photo shows a red LEDlighted powered by two aqueous state HSCs of CoNi2S4 NS/NTAs//rGOeCNTs connected in series). (d) Cycle performance of HSCs at 10 A g�1. Inset is the GCD curves of last 10 cycles.(e) Nyquist plots of HSCs before and after 10000 cycle tests. (f) Ragone plots of HSCs. (For interpretation of the references to colour in this figure legend, the reader is referred to theWeb version of this article.)

C. Su et al. / Electrochimica Acta 305 (2019) 81e89 87

well-balanced loading mass of the active materials at both elec-trodes. The specific capacitance of the HSCs calculated from thedischarge curve is 112 F g�1 (277.5mF cm�2 in areal capacitance) ata current density of 4 A g�1, and remains at 56 F g�1 (146.2mF cm�2

in areal capacitance) at a high current density of 20 A g�1 (Fig. 6c).The performance of our device is better than the reported values[32e34]. As shown in inset picture of Fig. 6c, two HSCs of CoNi2S4NS/NTAs//rGOeCNTs connected in series can power a red light-emitting-diode (LED), demonstrating their practical applications.

The HSCs also show high cycling stability. The charge/dischargecurves almost keep unchanged and the capacitance retention is ashigh as 96.9% even after 10000 charge/discharge cycles at a highcurrent density of 10 A g�1 (Fig. 6d), which demonstrates theexcellent cycling stability. The Nyquist plots of HSCs device beforeand after stability test were shown in Fig. 6e. It shows that theinitial Rs is very low indicating the assembling quality of the

supercapacitor device, and even after 10000 cycles the Rs is still aslow as 3.9U ensuring the high cycling stability of our device. Asshown in Fig. S12, the SEM images and EDS analysis of CoNi2S4 NS/NTAs after 10000 cycles were done to further confirms the goodcycle stability. Seen from the SEM images, the CoNi2S4 NS/NTAs stilltightly attached to CFC. The EDX spectras show that the CoNi2S4were partially oxidized after long-time cycling test.

Energy density and power density are two important parame-ters for of supercapacitors. Fig. 6f shows the Ragone plots of theHSCs. The highest gravimetric energy density is 35Wh kg�1 at apower density of 3 kWkg�1, and remains 17.5Wh kg�1 at a powerdensity of 15 kWkg�1. These performances are higher than those ofpreviously reported NieCo sulfides including NiCo2S4 NTs//rGOHSCs (16.6Wh kg�1 at 2.35 kWkg�1) [35], CoNi2S4 nanosheet ar-rays (NSAs)//AC HSCs (27.2Wh kg�1 at 2.45 kWkg�1) [36], NiCo2S4NPs//AC HSCs (28.3Wh kg�1 at 245Wkg�1) [37], NiCo2S4

C. Su et al. / Electrochimica Acta 305 (2019) 81e8988

NT@NiCo2S4 NSAs//rGO HSCs (24.9Wh kg�1 at 334Wkg�1) [38],and NiCo2S4@NiO//AC HSCs (30.38Wh kg�1 at 288Wkg�1) [39].

4. Conclusions

In summary, the hierarchical CoNi2S4 NS/NTAs tightly connectedwith CFC were synthesized through electrodeposition method forthe purpose of providing more active sites and decreasing thecontact resistance. The developed synthesis method ensures theintimate contact of the hierarchical CoNi2S4 NS/NTAs with the CFCsubstrate after the removal of the ZnO NRAs template. The hierar-chical CoNi2S4 NS/NTAs on CFC were directly used as the binder-free supercapacitor electrode which exhibits the ultrahigh specificcapacitance of 995.8 C g�1 at a current density of 2 A g�1 and su-perior cycling stability. The hierarchical CoNi2S4 NS/NTAs on CFCwere used to assemble HSCs with rGO-CNT film as the counterelectrode. The HSCs display high specific capacitance of 112 F g�1 ata current density of 4 A g�1, with the energy density of 35Wh kg�1

at a power density of 3 kWkg�1. The present study provides apromising way for constructing high performance energy storagedevice.

Acknowledgments

We gratefully acknowledge the financial support of the NationalNatural Science Foundation of China (61671299), Shanghai Scienceand Technology Grant (16JC1402000 and 17ZR1414100), and theProgram for Professor of Special Appointment (Eastern Scholar) atShanghai Institutions of Higher Learning (GZ2016005). We alsoacknowledge support from the Instrumental Analysis Center ofShanghai Jiao Tong University and the Center for Advanced Elec-tronic Materials and Devices of Shanghai Jiao Tong University.

Appendix A. Supplementary data

Supplementary data related to this article can be found athttps://doi.org/10.1016/j.electacta.2019.03.013.

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1

Supporting information

Hierarchical CoNi2S4 nanosheet/nanotube array structure on carbon fiber cloth

for high-performance hybrid supercapacitors

Chen Sua, Shusheng Xua, Lu Zhangc, Xinwei Chena, Guoqin Guana, Nantao Hua,

Yanjie Sua, Zhihua Zhoua Hao Weia, Zhi Yanga,*, Yong Qinb,*

aKey Laboratory of Thin Film and Microfabrication (Ministry of Education),

Department of Micro/Nano Electronics, School of Electronic Information and

Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China

bInstitute of Nanoscience and Nanotechnology, School of Physical Science and

Technology, Lanzhou University, Gansu 730000, P. R. China

cSchool of Advanced Materials and Nanotechnology, Xidian University, Xi’an 710071,

P. R. China

* To whom correspondence should be addressed:

zhiyang@sjtu.edu.cn, qinyong@lzu.edu.cn

2

1. Preparation of rGO-CNT film

The rGO-CNT film was prepared according to our previous report [1].

Specifically, 50 mg GO and 10 mg CNTs were dispersed in 100 mL DI water and

sonicated for 2 h. Then added 0.1 g KMnO4 into this solution and continue stirred for

10 min. The mixed solution was heated by a microwave oven for 5 min. After cooled

to room temperature, the material was washed with oxalic acid, hydrochloric acid (v/v,

1:1), and DI water, respectively. Then, the material was dispersed in 90 mL DI water,

followed by adding 90 μL hydrazine hydrate and 350 μL ammonium hydroxide into

this solution. The prepared solution was stirred for 20 min and then putted into a

100 °C oven for 3 h. Finally the suspension was vacuum-filtrated through a mixed

cellulose filter membrane to obtain rGO-CNTs film.

2. Calculation

The formulae for calculating the specific capacitance (C, C g–1) of CFC/CoNi2S4

NS/NTAs electrode is given below [2]:

(1)C = IΔt / m

Where I (A) is current density, Δt (s) refers to discharge time, m (g) is the mass of

active material.

To optimize the electrochemical performance of these HSCs, the charges stored

between two electrodes should follow equations [3]:

(2)× × (3)

+ -

± ± ± ±

Q = QQ = m C ΔV

Where Q (C) is the quantity of electric charge, m (g) represents the loading mass, C (F

g–1) stands for specific capacitance and ΔV (V) is the voltage window.

The specific capacitance (C, F g–1), energy density (E, Wh kg−1) and power

density (P, W kg−1) of the ASC device calculated from GCD curves are given below

[4]:

3

(4)× (5)

(6)

2

C = IΔt / mΔVW = CV / (2 3.6)P = 3600W / t

Where I (A) is current density, Δt (s) refers to discharge time, m (g) is the mass of

active material, and V stands for the voltage window (V).

Fig. S1. XRD patterns of CFC/ZnO, CFC/ZnO/CoNi2S4 and CFC/CoNi2S4 NS/NTAs.

4

Fig. S2. Cross section SEM images of (a) ZnO/CoNi2S4 NSs/NRAs-5 and (b) CoNi2S4

NS/NTAs-5 synthesized by using hydrothermal ZnO NRAs as template, (c) ZnO/CoNi2S4

NSs/NRAs-5 and (d) CoNi2S4 NS/NTAs-5 synthesized by using electrodeposition ZnO NRAs as

template.

Fig. S3. Nyquist plots of CoNi2S4 NS/NTAs-5 synthesized by using electrodeposition ZnO (black

5

curve) and hydrothermal ZnO (red curve) as template.

Fig. S4. SEM images of CoNi2S4 NSs deposited on ZnO NRAs under different scan rate of (a) 5

mV s−1, (b) 10 mV s−1, (c) 15 mV s−1 and (d) 20 mV s−1for 5 CV cycles.

6

Fig. S5. Electrochemical characterization of CoNi2S4 NS/NTAs electrode deposited under

different scan rate for 5 CV cycles. (a) CV curves at 10 mV s–1. (b) GCD curves at 10 A g–1. (c)

Specific capacitance as a function of current density. (d) Nyquist plots.

Fig. S6. HRTEM images of (a) CoNi2S4 NS/NTAs-5 and (b) the partial enlarged view of CoNi2S4

NS/NTAs-5. Elemental mapping images of (c) Ni, (d) Co, and (e) S.

7

Fig. S7. N2 adsorption-desorption isotherms curves for (a) bare CFC and (b) CoNi2S4 NS/NTAs-5

electrode.

3. Optimization of the thickness for the electrodeposition of CoNi2S4 NSs on ZnO

The electrochemical performances of hierarchical CoNi2S4 NS/NTAs with

different electrodeposition cycles were also investigated to determine the optimal

thickness. Fig. S8 is the SEM images of samples before and after removing ZnO

NRAs template with 3, 7, 11 CV cycles, respectively. Fig. S9a shows CV curves of

bare CFC, CoNi2S4 NSs, CoNi2S4 NS/NTAs-3 to CoNi2S4 NS/NTAs-11 at scan rate of

10 mV s–1. The CV curve of bare CFC electrode is almost a straight line, which

indicates that the capacitance contributed from CFC is negligible. It can be seen that

the CV area increases to a maximum value when the electrodeposition cycles reach 5

and then decreases, indicating that the optimized electrodeposition sample is CoNi2S4

NS/NTAs-5. GCD measurements for different samples mentioned above are further

performed at a current density of 10 A g–1 as shown in Fig. S9b. The results show that

CoNi2S4 NS/NTAs-5 electrode exhibits the longest discharge time among these tested

electrodes, which agrees well with the comparison result of CV areas. As seen in Fig.

S9c, CoNi2S4 NS/NTAs-3 electrode exhibits a poor rate capability. This is mainly

8

because that the ultrathin CoNi2S4 NSs covered on ZnO NRAs incompletely after 3

CV cycles. Hence, after removing the ZnO NRAs template, there are just a few

scattered NTs remained (as shown in Fig. S8b). With the deposition cycles increased,

the thickness of deposited CoNi2S4 NSs will increase and the synthesized CoNi2S4

NTs become more stable. As a consequence, the performance of rate capacity and

specific capacity reaches maximum when the deposition cycles are 5 CV. However,

when the thickness of deposited CoNi2S4 NSs increased to 7 CV and 11 CV cycles,

both specific capacity and rate capability decreased for the reason that the top of NTs

congested together at some areas with the increase of CoNi2S4 NSs. There are even

some NSs clusters agglomerated on the top of NRAs, which may hinder electrolyte

diffusing into CoNi2S4 NS/NTAs and in turn increase the bad volume of active

materials. The specific capacitor of CoNi2S4 NS/NTAs-5 electrode is higher than

CoNi2S4 NSs electrode that demonstrates hierarchical CoNi2S4 NSs/NTA structure can

efficiently increase the specific capacitor by increasing the specific area. The

corresponding Nyquist plots are shown in Fig. S9d. In the high-frequency region,

there is no distinct semicircle shape, which demonstrates the fast charge transfer

inside the electrode materials during the charge-discharge process. In the

low-frequency region, the straight lines lean more toward the imaginary axis,

indicating all hierarchical CoNi2S4 nanostructure electrodes have a lower diffusion

resistance, which can be ascribed to the electrolyte ions diffusing and transporting

into the electrode materials.

9

Fig. S8. SEM images of (a) ZnO/CoNi2S4 NS/NRAs-3, (b) CoNi2S4 NS/NTAs-3, (c) ZnO/CoNi2S4

NS/NRAs-7, (d) CoNi2S4 NS/NTAs-7, (e) ZnO/CoNi2S4 NS/NRAs-11, and (f) CoNi2S4

NS/NTAs-11.

10

Fig. S9. Electrochemical characterization of CoNi2S4 NSs, CoNi2S4 NS/NTAs-3, CoNi2S4

NS/NTAs-5, CoNi2S4 NS/NTAs-7 and CoNi2S4 NS/NTAs-11 electrode. (a) CV curves at 10 mV

s–1. (b) GCD curves at 10 A g–1. (c) Specific capacitance as a function of current density. (d)

Nyquist plots.

11

Table S1. Comparison of capacitance and retention rate performance for hierarchical CoNi2S4

NS/NTAs and other reported active electrodes

Sample Capacitance Capacitance retention Electrolyte Reference

Hierarchical

CoNi2S4 NS/NTAs

995.8 C g‒1

(2 A g‒1)

74.3%

(50 A g‒1) 6 M KOH This work

Porous Ni-Co-S NSs 521.6 C g‒1

(2 A g‒1)

85.6%

(20 A g‒1) 6 M KOH [5]

CoNi2S4 NSAs 709.0 C g‒1

(5 A g‒1)

90.6%

(100 A g‒1) 1 M KOH [6]

NiCo2S4 NTAs 313.2 C g‒1

(2 A g‒1)

73.6%

(32 A g‒1) 6 M KOH [7]

Reduced CoNi2S4

NSs

1117.0 C g‒1

(2 A g‒1)

78.0%

(40 A g‒1) 6 M KOH [8]

NiCo2S4/NCF 615.5 C g‒1

(2 A g‒1)

71.2%

(20 A g‒1) 6 M KOH [9]

NiCo2S4/RGO 749.0 C g‒1

(1 A g‒1)

72.3%

(40 A g‒1) 1 M KOH [10]

CoNi2S4–G–MoSe2 456.4 C g‒1

(1 A g‒1)

50.8%

(20 A g‒1) 6 M KOH [11]

Ni-Co-S/G 746.0 C g‒1

(1 A g‒1)

96.0%

(50 A g‒1) 6 M KOH [12]

12

Fig. S10. SEM images of (a) and (b) CoNi2S4 NS/NTAs-5 after 2000 cycles test, (c) and (d)

CoNi2S4 NSs deposited under 10 mV s‒1 for 5 CV cycles.

Fig. S11. Nyquist plots of CoNi2S4 NS/NTAs-5 before and after 2000 cycles at room temperature.

13

Fig. S12. (a) and (b) SEM images of CoNi2S4 NS/NTAs-5 after 10000 cycles test. (c) EDX spectra

of CoNi2S4 NS/NTAs-5 before cycles test. (d) EDX spectra of CoNi2S4 NS/NTAs-5 after 10000

cycles test.

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Hierarchical CoNi2S4 nanosheet/nanotube array structure on carbon fiber cloth for high-performance hybrid supercapacitors1. Introduction2. Experimental section2.1. Synthesis of ZnO NRAs on CFC2.2. Preparation of hierarchical CoNi2S4 NS/NTAs on CFC2.3. Characterizations

3. Results and discussion4. ConclusionsAcknowledgmentsAppendix A. Supplementary dataReferences