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Electrochimica Acta 278 (2018) 33e41

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Electrochimica Acta

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Reduced CoNi2S4 nanosheets with enhanced conductivity forhigh-performance supercapacitors

Zhipeng Li a, 1, Dian Zhao a, 1, Chunyang Xu a, Jiqiang Ning b, Yijun Zhong a, Ziyang Zhang b,Yongjiang Wang b, Yong Hu a, *

a Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Department of Chemistry, Zhejiang Normal University, Jinhua, 321004, PRChinab Vacuum Interconnected Nanotech Workstation, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, PR China

a r t i c l e i n f o

Article history:Received 3 April 2018Received in revised form2 May 2018Accepted 3 May 2018Available online 4 May 2018

Keywords:Reduced CoNi2S4NanosheetsSulfur vacanciesEnergy densityAsymmetric supercapacitors

* Corresponding author.E-mail address: yonghu@zjnu.edu.cn (Y. Hu).

1 Z. Li and D. Zhao contributed equally.

https://doi.org/10.1016/j.electacta.2018.05.0300013-4686/© 2018 Elsevier Ltd. All rights reserved.

a b s t r a c t

Defect engineering on transition metal dichalcogenides has been regarded as an effective method toimprove electrochemical properties in terms of generating active sites and enhancing the intrinsicconductivity. This study reports a new high-performance electrochemical supercapacitor made ofreduced CoNi2S4 (r-CoNi2S4) nanosheets, which are synthesized via a facile moderate-reduction process.The sulfur-deficient r-CoNi2S4 nanosheets exhibit significantly enhanced conductivity which is inducedby abundant sulfur vacancies formed in the reduction reaction. Compared with the pristine CoNi2S4nanosheets, the r-CoNi2S4 nanosheets are characterized with a higher specific capacity (1117C g�1 atcurrent density of 2 A g�1) as well as excellent rate capability and stable cycling performance. First-principle analysis confirms that the sulfur vacancies originating from the reduction lead to improvehybridization between the Ni and Co d states and the S p states close to the fermi level, and consequentlyenhance conductivity with the CoNi2S4 nanostructure. Moreover, an ultrahigh energy density of 55.4Whkg�1 at the power density of 8 kW kg�1 is obtained in an asymmetric supercapacitor configuration, and80% capacitance of the supercapacitor remains even after 10000 cycles.

© 2018 Elsevier Ltd. All rights reserved.

1. Introduction

Nickel cobalt sulfide (Ni-Co-S), a unique ternary sulfide ofmetallic nature, is well known for its high conductivity, favorablecorrosion stability, and excellent redox activity [1e4]. Its low-costand simple fabrication make it promising for electrochemical en-ergy storage and conversion, especially for realizing practicallithium-ion batteries, fuel cells, supercapacitors, electrochemicalwater splitting/oxygen reduction reactions, etc. [][5e14]. Since theNi-Co-S microstructure is a main factor determining its electro-chemical performance, a variety of Ni-Co-S nanostructures withdifferent morphologies have been investigated. The typical struc-tures include urchin-like [15] and cauliflower-like [16] structures,as well as spheres [17,18], which can facilitate charge transfer andthus enhance the performance of supercapacitors. In addition, inorder to reduce contact resistance and enhance electrochemical

performance, direct growth of Ni-Co-S onto conductive substratessuch as nickel foam, carbon cloth and graphene has also beenconsidered [19e24]. Furthermore, to promote charge transfer insupercapacitors, the improvement of the intrinsic conductivity ofNi-Co-S compounds is also an effective but challenging strategy. Ithas been reported that defect engineering on transition metal ox-ides can be an effective method for generating active sites andenhancing the intrinsic conductivity [25e29]. If the defects, such assulfur vacancies [30,31], can be introduced into themetallic Ni-Co-Scompounds, more active sites and higher conductivity are expectedto be obtained and the electrochemical performance of the Ni-Co-Ssystem can be further improved.

To generate surface sulfur vacancies in transition metal sulfidematerials, such as MoS2 [32,33] and Zn-Cd-S [34], a variety ofmethods have been explored, including annealing in the reducingatmosphere, and argon or hydrogen plasma exposure. However, theannealing method requires relatively high temperature and vastquantities of hydrogen, while plasma sputtering demands expen-sive inductively coupled plasma systems and vacuum conditions.These peculiarities are not capable of energy saving and restrict the

Z. Li et al. / Electrochimica Acta 278 (2018) 33e4134

general utility of these approaches. Recently, a moderate solutionprocess was developed, by employing NaBH4 solution, to reduceFe1Co1-O nanosheets [35] and Co3O4 nanowires [36], whichgenerated surface oxygen vacancies and thus improved the elec-trical conductivity and electrochemical performance of these ma-terials. Because of its safety, convenience, as well as low-cost andlow-energy features, this solution method is attractive for pro-ducing metal sulfides with surface defects. To the best of ourknowledge, there is still no research work reported on metallic Ni-Co-S nanosheets synthesized via the reducing treatment, and theelectrical conductivity and electrochemical properties have notbeen explored yet.

Herein, we report the reduction of CoNi2S4 nanosheets withNaBH4 solution treatment and the superior supercapacitor perfor-mance obtainedwith the reduced products. The ultrathin structuralnature of the CoNi2S4 nanosheets allows efficient reduction treat-ment and charge carrier transport at the surface. Compared withthe pristine CoNi2S4 nanosheets, the reduced CoNi2S4 (r-CoNi2S4)nanosheets exhibit a much higher capacity of 1117C g�1 at 2 A g�1,along with excellent rate capability and cycling performance. First-principle analysis reveals that the introduction of sulfur vacanciescan effectively enhance the orbit hybridization between the Ni andCo d states, and the S p states near the Fermi level, leading toenhancement of the conductivity of CoNi2S4. In addition, anasymmetric supercapacitor device, with r-CoNi2S4 nanosheets asanode and activated carbon (AC) as cathode, exhibits ultrahighenergy density of 55.4Wh kg�1 at a power density of 8 kWkg�1, aswell as excellent electrochemical stability. Our results suggest thatthe r-CoNi2S4 nanosheets are a type of excellent electrode materialfor supercapacitor applications.

2. Experimental section

2.1. Materials

All reagents and solvents were of analytical grade. Cobalt(Ⅱ)nitrate hexahydrate (Co(NO3)2$6H2O, 99.5%), Nickel(Ⅱ) nitratehexahydrate (Ni(NO3)2$6H2O, 98%), Acetic acid sodium salt(CH3COONa, 99%), Sodium tetrahydroborate (NaBH4, 98%), Thio-acetamide (CH3CSNH2, 99%), Ethanol (EtOH, 99.7%), Ethylene glycol(EG, 99%), and Polyethylene glycol 200 (PEG, 99.9%) were pur-chased from Sinopharm Chemical Reagent Co., Ltd (China), andused as received without further purification.

2.2. Preparation of the nickel cobalt (Ni-Co) precursor

According to the reported method, a modified solvothermalprocess was developed to synthesize the Ni-Co precursor [38].Typically, a mixture of Ni(NO3)2$6H2O (5mmol, 1.454 g), Co(N-O3)2$6H2O (5mmol, 1.455 g) and CH3COONa (10mmol, 1.6406 g)was dissolved in 20mL of polyethylene glycol 200 (PEG-200) and20mL of ethylene glycol (EG) under stirring to form a transparentpink solution. The solution was then transferred into a 50mLTeflon-lined stainless steel autoclave and then placed in an oven at200 �C for 16 h. The resulting precipitate was collected by centri-fugation and washed with water and ethanol for several times, andfinally dried at 80 �C for 12 h to have the Ni-Co based precursor.

2.3. Preparation of the CoNi2S4 nanosheets

A solution sulfidation process was utilized to convert the nickelcobalt precursors into the CoNi2S4 nanosheets. Briefly, 60mg of theas-synthesized nickel cobalt precursors was dispersed into 40mLethanol, followed by the addition of 120mg thioacetamide (TAA).The mixture was transferred into a Teflon-lined stainless steel

autoclave and heated at 140 �C for 1 h. The product was collected bycentrifugation and washed with water and ethanol for severaltimes, and finally dried at 80 �C for 12 h to obtain the CoNi2S4nanosheets.

2.4. Preparation of the r-CoNi2S4 nanosheets with sulfur vacancies

The CoNi2S4 nanosheets were simply immersed in 0.9M NaBH4solution at room temperature for 1.5 h to yield the reduced CoNi2S4nanosheets. The products were collected by centrifugation, washedwith distilled water for several times, and then dried in a vacuumoven at 80 �C for 12 h.

2.5. Characterization

The morphology, microstructure of the products were investi-gated by field-emission scanning electron microscopy (FE-SEM,Hitachi S-4800), transmission electron microscopy (TEM, JEM-2100F) and energy-dispersive X-ray spectroscope (EDX) attach tothe TEM. The crystallinity of the samples was characterized bypowder X-ray diffraction (XRD) on a Philips PW3040/60 X-raydiffractometer using Cu-Ka radiation at a scanning rate of 0.06� s�1.X-ray photoelectron spectroscopy (XPS) measurements were per-formed using an ESCALab MKII X-ray photoelectron spectrometerwith a Mg Ka X-ray radiation.

2.6. Electrochemical measurements

The electrochemical properties of the CoNi2S4 and r-CoNi2S4electrode were investigated under a three-electrode configurationin 6.0M KOH aqueous solution using Pt foil as a counter electrodeand Hg/HgO electrode as reference electrode. The working elec-trodes were prepared by first mixing the as-obtained active ma-terials, carbon black (super P), and polyvinylidene difluoride with amass ratio of 70:20:10, and the mixture was then pressed onto afoam nickel and dried at 80 �C for 12 h. The mass loading of theactivate materials was about 1.5e2.0mg cm�2. All the testsincluding cyclic voltammetry (CV), galvanostatic charge-dischargeand electrochemical impedance spectroscopy (EIS) were conduct-ed on a Zennium E (Zahner, Germany) electrochemical workstation.EIS measurements were obtained by employing an AC voltage with5mV amplitude in the frequency range of 10mHze100 kHz at opencircuit potential. The specific capacitances (Cs) were obtained fromthe galvanostatic discharge curves according to the followingformula:

Cs ¼ ðI� DtÞ=ðm� DVÞ (1)

where I is the discharge current, Dt the discharge time, DV thevoltage range upon discharging, and m the mass of the activematerial.

The electrochemical performance of the r-CoNi2S4 was furtherevaluated through an asymmetric supercapacitor. The asymmetricsupercapacitor was assembled into a cell device by using r-CoNi2S4nanosheets as the positive electrode, active carbon (AC) as thenegative electrode, and one piece of cellulose paper as the sepa-rator in 6.0M KOH electrolyte. The negative electrodewas preparedby first mixing AC, carbon black, and poly (tetrafluoroethylene)with N-methyl-2-pyrrolidone with a weight ratio of 80:10:10, andthemixturewas then casted onto nickel foam. The mass ratio of ther-CoNi2S4 to AC was determined to be 0.1 to obtain the chargebalance between the two electrodes. The mass loading of AC and r-CoNi2S4 is about 12.0mg and 1.2mg, respectively. The energydensity (E) and power density (P) were obtained based on the totalweight of the active materials in the cell device according to the

Z. Li et al. / Electrochimica Acta 278 (2018) 33e41 35

following formulas:

E ¼ 0:5� C� DV2 (2)

P ¼ E=Dt (3)

where C is the cell capacitance, DV the voltage range during thecharge-discharge measurement, and Dt the discharge time.

2.7. Computational details

First-principle density functional theory (DFT) analysis wereperformed using the Vienna ab initio simulation package (VASP)and the projector aug-mented wave (PAW) potential method. Thegeneralized gradient approximation (GGA) with the Perdew-Burker-Ernzerhof (PBE) functional was employed to describe theexchange and correlation effects. We adopted the GGAþU, whereUeff (U-J) was set to 6.4 and 4.0 eV for Ni and Co atom, respectively.The CoNi2S4 crystal structure was modeled by the conventionalspinel cell containing 56 at-oms, i.e., 16 Ni, 8 Co, and 32 S atoms. Thesulfur vacancy was introduced within the conventional cell ofCoNi2S4 to obtain the reduced CoNi2S4 structure. In the calculation,the plane-wave cut off is set to be 450 eV, and a k-point grid of7� 7� 7 is used for the relaxation of the crystal structure. All atomsare allowed to relax until the force on the atom is smaller than0.02 eV Å�1, and the energy change per atom is lower than 10�5 eV.

3. Results and discussion

The synthesis process of r-CoNi2S4 nanosheets is presented inScheme 1. The Ni-Co-based precursor nanosheets were prepared byan improved solvothermal method [37], followed by a solutionsulfidation process with the thioacetamide (TAA), to first obtain thepristine CoNi2S4 nanosheets. A NaBH4 reduction treatment at roomtemperature was applied to reduce the pristine CoNi2S4 nano-sheets. The morphology and microstructure of the so-obtainedsamples were characterized using scanning electron microscopy(SEM) and transmission electron microscopy (TEM). As shown bythe SEM images in Figs. S1a and b (see Supplementary Materials),the Ni-Co precursors are uniform petal-like nanosheets with anaverage thickness of about 20 nm, which are interconnected toform an open-up network structure. After sulfuration, the Ni-Coprecursors are converted into CoNi2S4 and the twisting sheet-likemorphology (Figs. S1c and 1d) is well maintained, and the TEMimage (inset in Fig. S1d) further reveal their ultrathin feature. After

Scheme 1. Schematic illustration of the

the NaBH4 reduction, the morphology of the pristine CoNi2S4nanosheets is preserved and the obtained r-CoNi2S4 nanosheetsstill retain the 3D interconnected open-up network structure(Fig. 1a and b). The TEM image (Fig. 1c) depicts the ultrathin char-acteristic of the r-CoNi2S4 nanosheets. As shown in the inset ofFig. 1c, the observed lattice spacing is determined to be 0.283 nm,which can be ascribed to the (311) lattice planes of the cubicCoNi2S4 phase. The element mapping analysis (Fig. 1deg) furtherdemonstrates the uniform distribution of Ni (blue), Co (green) and S(red) elements throughout r-CoNi2S4 nanosheet.

The as-prepared Ni-Co precursor, CoNi2S4 and r-CoNi2S4 nano-sheets were characterized by X-ray diffraction (XRD). In Fig. S2, thediffraction peaks indicate that the precursor is of nickel cobalt hy-droxide hydrate [38]. From the XRD patterns in Fig. 2a, it can beclear seen that both the pristine CoNi2S4 and the r-CoNi2S4 nano-sheets display peaks at 26.6�, 31.4�, 50.2� and 55.1�, which can beindexed to the (220), (311), (511) and (440) planes of the cubic typeCoNi2S4 (JCPDS card NO. 20-0334), respectively. Moreover, thewell-matched XRD peaks of these two samples demonstrate thepreservation of the CoNi2S4 crystal structure after treated withNaBH4. In addition, the EDS patterns (Fig. S2) clearly reveal that theNi/Co:S molar ratio is expressively increased after the NaBH4treatment, compliancewith the presence of an extensive number ofsulfur vacancies [30]. We employed X-ray photoemission spec-troscopy (XPS) to investigate the chemical valence states of the Niand Co in the pristine CoNi2S4 and the r-CoNi2S4 nanosheets.Fig. 2bec depicts the typical Co 2p and Ni 2p XPS spectra of eachsample, obtained based on the Gaussian fitting method. In the Co2p XPS spectrum of CoNi2S4 (Fig. 2b), the peaks at 778.8 eV for Co2p3/2 and 793.9 eV for Co 2p1/2 are spin-orbit characteristics ofCo3þ, while the existing peaks at 781.4 eV for Co 2p3/2 and 797.9 forCo 2p1/2 correspond to the spin-orbit characteristics of Co2þ

[39e41]. The weak satellite peaks indicate that the Co2þ state ispredominant in the Co 2p spectrum. In the Ni 2p XPS spectrum ofCoNi2S4 (Fig. 2c), strong peaks at 853.2 and 856.2 eV for Ni 2p3/2and 873.2 and 874.8 eV for Ni 2p1/2 are observed, suggesting theexistence of both Ni2þ and Ni3þ [42e44]. Likewise, The intensesatellite peaks imply that the majority of Ni atoms were in the Ni3þ

state. These results demonstrate that the chemical composition ofthe pristine CoNi2S4 contain Ni2þ, Ni3þ, Co2þ and Co3þ, which isconsistent with the reported CoNi2S4 [45e47]. After the NaBH4treatment, the bonding energies of Ni and Co in r-CoNi2S4 do notexhibit pronounced change, but the relative spectral intensity ofCo2þ shows an obvious increase, suggesting that a portion of theCo3þ ions was reduced to Co2þ and sulfur vacancies were formed[48]. To further confirm the existence of sulfur vacancies, we also

synthesis of r-NiCo2S4 nanosheets.

Fig. 1. (a) SEM, (b) High-resolution SEM images, and (c) TEM image of the as-prepared r-CoNi2S4 nanosheets, with the HRTEM image as inset; (d) STEM image and elementalmappings for (e) nickel, (f) cobalt, and (g) sulfur of the r-CoNi2S4 nanosheets.

Fig. 2. (a) XRD pattern of the as-prepared pristine CoNi2S4 and r-CoNi2S4 nanosheets. XPS spectra of (b) Co 2p, (c) Ni 2p, (d) S 2p for the CoNi2S4 and r-CoNi2S4 nanosheets.

Z. Li et al. / Electrochimica Acta 278 (2018) 33e4136

investigated the S 2p XPS spectra of the pristine CoNi2S4 and r-CoNi2S4 nanosheets. In the S 2p XPS spectrum of the pristineCoNi2S4 (Fig. 2d), the binding energies at 162.3 and 163.9 eVcorrespond to S 2p3/2 and S 2p1/2 core levels, respectively. In detail,

the peak at 162.3 eV is typical of a metal-sulfur bond in this CoNi2S4material [2,49], while the other peak at 163.9 eV is assigned to thesulfur ions in low coordination at the surface, which is generallyrelate to sulfur vacancies in the structure [50e52]. Compared with

Fig. 4. Site projected density of states of the CoNi2S4 and r-CoNi2S4 (inset: the crystalmodel of r-CoNi2S4).

Z. Li et al. / Electrochimica Acta 278 (2018) 33e41 37

the pristine CoNi2S4 nanosheets, the energy bands of S 2p3/2 and S2p1/2 in the r-CoNi2S4 nanosheets exhibits shifts to 161.6 and162.9 eV, respectively. More importantly, the relative intensity of2p1/2 in S 2p XPS spectrum for r-CoNi2S4 nanosheets is determinedto be 57.7%, which is much higher than that of the pristine CoNi2S4(43.3%), revealing a higher sulfur vacancy concentration in r-CoNi2S4 nanosheets, which was caused by the reduction process ofCoNi2S4 treated by NaBH4.

The electrochemical performances of the as-prepared sampleswere investigated by using a three-electrode system in 6.0M KOHaqueous electrolyte. A pair of strong redox peaks can be observed inthe cyclic voltammetry (CV) curves of both CoNi2S4 and r-CoNi2S4nanosheets electrodes at various scan rates ranging from 2 to40mV s�1 in the voltage window of 0e0.5 V versus a standard Hg/HgO electrode (Figs. S4a and S4c), reflecting the common behaviorof battery-type electrodes [7]. The well-defined peaks reveal theexistence of the reversible and fast Faradaic redox reactions duringthe electrochemical process with both CoNi2S4 and r-CoNi2S4nanosheets, mainly involving the Ni2þ/Ni3þ and Co2þ/Co3þ/Co4þ

redox couples, and can be clarified by the following equations [53]:

CoNi2S4 þ 2OH­4CoSxOHþNi2S4­xOHþ 2e­ (4)

CoS2xOHþ OH­4CoS2xOþ H2Oþ e­ (5)

In addition, Fig. 3a shows a representative CV curve of the r-CoNi2S4 nanosheets at a scan rate of 10mV s�1 along with that ofthe pristine CoNi2S4 and bare Ni foam for comparison. For bare Nifoam, the CV area are negligible compared with that of r-CoNi2S4and CoNi2S4, indicating the r-CoNi2S4 and CoNi2S4 are the maincapacitor contributor. Clearly, the r-CoNi2S4 nanosheets possess alarger enclosed region of the CV curve, indicating that more chargescan be stored compare with the corresponding CoNi2S4. The gal-vanostatic charge-discharge (GCD) curves of both CoNi2S4 and r-

Fig. 3. (a) CV curves of the bare Ni foam, r-CoNi2S4 and CoNi2S4 nanosheets electrodes at a s2 A g�1 for the r-CoNi2S4 and CoNi2S4 nanosheets electrodes, (c) Specific capacity at differeperformance at a current density of 10 A g�1 for the r-CoNi2S4 and CoNi2S4 nanosheets elec

CoNi2S4 were measured at various current densities ranging from 2to 40 A g�1 and presented in Figs. S4b and S4d. Fig. 3b reveals thedirect GCD comparison of the r-CoNi2S4 and CoNi2S4 electrodescollected at 2 A g�1, which indicates that the r-CoNi2S4 exhibitsmuch longer discharge time and higher capacity after reducingtreatment. Consistent with the CV analysis, the distinct voltageplateaus in the charge-discharge curves at different current den-sities prove the existence of Faradaic processes [54]. Moreover, thenearly symmetric charge-discharge profiles further demonstratethe highly reversible of the Faradaic redox reactions [23].

The specific capacity of the CoNi2S4 and r-CoNi2S4 nanosheetscan be obtained from the charge-discharge profiles. As shown in

can rate of 10mV s�1, (b) Galvanostatic charge-discharge curves at a current density ofnt current densities for the r-CoNi2S4 and CoNi2S4 nanosheets electrodes, (d) Cyclingtrodes.

Z. Li et al. / Electrochimica Acta 278 (2018) 33e4138

Fig. 3c, the specific capacity of r-CoNi2S4 nanosheets are deter-mined to be 1117,1070,1002, 973, 913 and 872C g�1 at 2, 5,10,15, 20and 40 A g�1, respectively. Notably, about 78% of the initial specificcapacity can be retained even for a 20-fold increase in the charge-discharge current density, demonstrating the excellent rate per-formance of the r-CoNi2S4 nanosheets. In contrast, the pristineCoNi2S4 exhibits similar rate performance but much lower specificcapacity of 882, 847, 806, 792, 763, and 737C g�1 at the same seriesof current densities. It is necessary to note that, both CoNi2S4 and r-CoNi2S4 nanosheets electrode show high specific capacity and goodrate performance, which are much higher than those of the pre-viously reported CoNi2S4 or NiCo2S4 counterparts (Table S1). More

Fig. 5. Electrochemical performance of the r-CoNi2S4//AC asymmetric supercapacitor device.AC with a cellulose paper separator in 6.0M KOH solution. (b) CV curves at scan rates fromfrom 10 to 80 A g�1, (d) Specific capacitance at different current densities, (e) Cycling performdensities, (g) Twenty-six LEDs powered by the r-CoNi2S4//AC asymmetric supercapacitor de

importantly, the r-CoNi2S4 nanosheets exhibit superior specificcapacity in comparison with that of CoNi2S4, which can be attrib-uted to the improved electric contact and conductivity of the r-CoNi2S4 nanosheets due to the induction of sulfur vacancies [48].The cycling stability of both CoNi2S4 and r-CoNi2S4 nanosheets wasinvestigated by the repeated charge-discharge cycling measured ata constant current density of 10 A g�1 (Fig. 3d). Impressively, after4000 charge-discharge cycles, about 81% of the initial specific ca-pacity of the CoNi2S4 is retained whereas the r-CoNi2S4 shows aslightly lower capacity retention ratio of 79%, indicating theexcellent electrical stability of these two samples. After electro-chemical cycling test, the specific capacity of the r-CoNi2S4

(a) Schematic diagram of the fabricated asymmetric supercapacitor with r-CoNi2S4 and2 to 40mV s�1, (c) Galvanostatic charge-discharge curves at different current densitiesance at a current density of 10 A g�1, (f) The Ragone plot related to energy and powervice.

Z. Li et al. / Electrochimica Acta 278 (2018) 33e41 39

nanosheets is 883C g�1 after 4000 cycles and the structure exhibitsa inconspicuous variation (SEM and TEM images, Fig. S7), demon-strating that sulfur vacancies are relatively stable [51].

Electrochemical impedance spectroscopy (EIS) was performedto have further evidence for the superior electrochemical perfor-mance of r-CoNi2S4. Fig. S5 shows the representative Nyquist plotsof the r-CoNi2S4 and CoNi2S4 electrodes, and the EIS data werefitted based on an equivalent circuit model (Fig. S5 inset image)involves bulk solution resistance Rs, charge transfer resistance Rct,Warburg impedance W, and double-layer capacitance Cd. Typically,the semicircle at the high frequency range represents the chargetransfer resistance (Rct) corresponded to the active materials/elec-trolyte interfaces. For the r-CoNi2S4 nanosheets electrode, a smallersemicircle was observed than that of the CoNi2S4 nanosheetselectrode, revealing that a lower interfacial Rct was carried with ther-CoNi2S4 nanosheets electrode. The obtained Rct value for the r-CoNi2S4 electrode is 0.18U, apparently smaller than that of CoNi2S4electrode (0.37U), indicating that the charge transport of CoNi2S4nanosheets can be improved by the NaBH4 reduction treatment[55e57]. In addition, the resistance of the pressed r-CoNi2S4 andCoNi2S4 nanosheets pellets in the frequency range from 40 Hz to110MHz were tested and the results are presented in Fig. S6.Clearly, the resistance of r-CoNi2S4 nanosheets is much small thanthat of the pristine CoNi2S4 at a given frequency. These resultsreveal that the introduction of sulfur vacancies in metallic CoNi2S4nanosheets can significantly improve their electronic conductivityand charge transfer, which is further verified by the first-principlecalculation. As previously reported, the crystal structure ofCoNi2S4 is normal spinel [58]. The optimized lattice constant ofCoNi2S4 is 9.293Å, which is in good agreement with the experi-ment value 9.424Å. According to the projected density of states(PDOSs) results (shown in Fig. 4), the CoNi2S4 is truly metallic sinceits electronic bands cross the Fermi level (0 eV), consistent with anexisting report [1]. For the reduced CoNi2S4 (with sulfur vacanciesconcentration of 3.125%), the much stronger hybridization betweenthe Ni and Co d states and the S p states near the Fermi level thanthat of the pristine CoNi2S4, suggesting superior conductivity of thereduced CoNi2S4 [59], which is in accord with the experimentalresults.

To further investigate the electrochemical performance of the r-CoNi2S4 nanosheets, an asymmetric supercapacitor device isfabricated by employing the r-CoNi2S4 nanosheets as the positiveelectrode, active carbon (AC) as the negative electrode (electro-chemical performance, Fig. S8) and one piece of cellulose paper asthe separator in KOH aqueous electrolyte. Fig. 5a shows the sche-matic diagram of r-CoNi2S4//AC asymmetric supercapacitor. Themass loading of AC and r-CoNi2S4 nanosheets was balanced beforepreparing the hybrid full cell device r-CoNi2S4//AC [7], and thevoltagewindowof the cell devicewas determined to be 1.6 V on thebasis of their individual electrochemical behaviors (Fig. S9). Fig. 5bshows typical CV curves of the cell device obtained with variousscan rates in the voltage window of 0e1.6 V. The CV profiles clearlyreveal the electric double-layer capacitive and Faradaic redox be-haviors corresponding to the AC and r-CoNi2S4 nanosheets elec-trodes, respectively. With increased scan rate, no apparentdistortion in the CV profiles can be observed, suggesting practicallyinstant current response and excellent capacitive behavior of thesupercapacitor device. The nearly symmetric GCD curves withsmall voltage drops at the current densities 10e80 A g�1 furtherconfirm excellent electrochemical reversibility (Fig. 5c). Accordingto the GCD curves, the specific capacitance of the cell device is155.8 F g-1 at the current density of 10 A g�1, and retains 101.5 F g-1 at a higher current density of 80 A g�1 (Fig. 5d). In addition, thelow Rct value (0.71U) calculated from the Niquist plot (Fig. S10) ofthe r-CoNi2S4//AC device further exhibits its favorable electrical

conductivity. Fig. 5e presents cycling performance of the asym-metric supercapacitor at a current density of 10 A g�1. Impressively,the cell device exhibits very high cycling stability and 80% of theinitial capacitance even after 10000 consecutive charge-dischargecycles.

We have further investigated the energy density and powerdensity of the r-CoNi2S4//AC asymmetric supercapacitor. As sum-marized in the Ragone plot (Fig. 5f), the r-CoNi2S4//AC device showsan ultrahigh energy density of 55.4Wh kg�1 at a power density of8 kWkg�1. Even at an outstanding power density of 61.1 kWkg�1,the energy density is still up to 36.1Wh kg�1. The obtained energydensity and power density of this r-CoNi2S4//AC device is superiorto that of many reported asymmetric supercapacitors, such as Ni-Co-S/G//PCNS [60], Ni-Co-Fe hydroxide//AC [61], Ni(OH)2//AC [62],RGO-MNCO//RGO [63], and MnO2@OMCRs//OMCRs [64]. Further-more, two asymmetric supercapacitor cells in series can easilypower 26 parallel light-emitting diodes (LEDs) with a workingvoltage of 1.5 V, as shown in Fig. 5g, demonstrating potentialapplication of the r-CoNi2S4//AC supercapacitor as energy storagedevices.

4. Conclusions

In summary, a facile moderate solution reduction method hasbeen demonstrated to prepare sulfur vacancies at CoNi2S4 nano-sheets surface for enhance conductivity and significantly improvedFaradaic redox behaviors have been demonstrated. Employed as asupercapacitor electrode, the reduced r-CoNi2S4 nanosheets exhibita specific capacity of 1117C g�1 at a current density of 2 A g�1, muchhigher than that of the pristine CoNi2S4 (882C g�1), and excellentrate capability (872C g�1 at 40 A g�1) and good cycling stability,manifested as capacity retention of about 79% after 4000 cycles at acurrent density of 10 A g�1. The superior electrochemical propertiesof the r-CoNi2S4 nanosheets can be related to the NaBH4 reductiontreatment in which the sulfur vacancies are generated. The intro-duced vacancies can not only produce more active sites for theelectrochemical reactions but also further enhance the conductivityof metallic CoNi2S4 originating from the strengthened hybridiza-tion between the Ni and Co d states and the S p states near theFermi level. Furthermore, the asymmetric supercapacitor cell de-vice fabricatedwith r-CoNi2S4 nanosheets exhibits ultrahigh energyand power densities, as well as excellent long-term cycling stability.Our results suggest that the sulfur-deficient CoNi2S4 nanosheetsprepared by the solution reduction method have great potential insupercapacitor applications, and will promote further research in-terest in the transition metal sulfides.

Notes

The authors declare no competing financial interest.

Acknowledgements

Y. Hu acknowledges financial support from the Natural ScienceFoundation of China (21671173). J. Q. Ning acknowledges thefinancial support from National Key Technologies R&D Program ofChina (2016YFA0201101), Key Frontier Scientific Research Programof the Chinese Academy of Sciences (QYZDB-SSW-JSC014), andHundred Talents Program of Chinese Academy of Sciences and theNANO-X Workstation of SINANO, CAS. And Z. Y. Zhang acknowl-edges the financial support from The Thousand Youth Talents Plan.

Appendix A. Supplementary data

Supplementary data related to this article can be found at

Z. Li et al. / Electrochimica Acta 278 (2018) 33e4140

https://doi.org/10.1016/j.electacta.2018.05.030.

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