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School of Optical and Electronic Informati

Technology (HUST), Wuhan 430074, China.

† Electronic supplementary information (Ethe electrochemical performances of10.1039/c3nr02958a

Cite this: DOI: 10.1039/c3nr02958a

Received 7th June 2013Accepted 14th July 2013

DOI: 10.1039/c3nr02958a

www.rsc.org/nanoscale

This journal is ª The Royal Society of

Highly conductive NiCo2S4 urchin-like nanostructuresfor high-rate pseudocapacitors†

Haichao Chen, Jianjun Jiang,* Li Zhang, Houzhao Wan, Tong Qi and Dandan Xia

A 3D highly conductive urchin-like NiCo2S4 nanostructure has

been successfully prepared using a facile precursor transformation

method. Remarkably, the NiCo2S4 electroactive material demon-

strates superior electrochemical performance with ultrahigh high-

rate capacitance, very high specific capacitance, and excellent cycling

stability.

Electrochemical capacitors (also called supercapacitors) havebeen intensely studied recently because of their higher powerdensity, long cycling lifespan, short charging time, and lowmaintenance cost.1–4 These signicant advantages make themessential power resources for various applications, includinghighpowerelectricdevices, electric vehicles andmilitarydevices.5

Despite the signicant advances achieved in supercapacitors, thehigh-rate capacitance is still insufficient. The high speciccapacitance cannot be maintained under high rates includinghigh scan rate, high current density and high power operation,which are commonly found for most electroactive materials.Simultaneously increasing the rates of ion diffusion and electrontransfer is an effective way to improve the rate capability of elec-trode materials. Preparing active materials with various nano-structures can undoubtedly contribute to the optimization ofhigh-rate electrode performances because of the increased activesurface areas, reduced diffusion resistance of electrolytes, shortelectron and ion transport pathways and so on. Especially, thehigh conductivity of the active materials is benecial for thetransfer of electrons, which will therefore facilitate the rapidcharging–discharging of active materials. Therefore, the rationaldesign of various electroactive materials with high conductivityand desirablemicro-/nanostructures is imperative for the furtherenhancement of the high-rate electrochemical properties.

on, Huazhong University of Science and

E-mail: jiangjj@mail.hust.edu.cn

SI) available: Experimental details, andNiCo2O4, Co9S8 and NiS. See DOI:

Chemistry 2013

To date, a few active materials with excellent high-ratecapacitance have been reported by designing a conductiveactive material with various novel nanostructures. By designand tailoring of hydrous RuO2 with a three-dimensional meso-porous nanotubular arrayed architecture to enhance the facilityof electrolyte penetration, up to 800 F g�1 of the speciccapacitance of the as-obtained active material is achieved atscan rates as high as 1000 mV s�1 with a capacitance retentionof 60% for a 100 times increase in scan rate from 10 to 1000 mVs�1.6 The high-rate capacitance is obviously superior andimproved for highly conductive RuO2 because of the facility ofelectrolyte ion diffusion. Furthermore, a 3D aperiodic hierar-chical porous graphitic carbon (HPGC) that combines macro-pores, mesoporous walls, and micropores was designed andsynthesized using an inorganic multitemplate method. As highas 90% of the specic capacitance retention is obtained for a20-time increase in scan rate.7 Despite the excellent high-rateperformances of the as-discussed nanotubular arrayed elec-trodes and the 3D aperiodic HPGC, their practical applicationsare still seriously hindered by the sheer high cost of RuO2 andthe low specic capacitance in carbon based materials.8,9 So it isdesirable to seek alternative inexpensive active materials withgood capacitive characteristics, high conductivity and ease ofpreparation into various nanostructures.

Recently, a ternary metallic oxide, spinel nickel cobaltite(NiCo2O4), has drawn much research interest. The higher elec-trochemical capacitive performances may mainly derive fromthe superior electrochemical activity of NiCo2O4.10 It is reportedthat NiCo2O4 possesses at least two orders of magnitude higherelectronic conductivity than nickel oxides and cobalt oxides.11,12

Meanwhile, NiCo2O4 has various ion diffusion-favoured shapesand structures, such as nanowires,13 porous aerogels,11 urchin-like structures,14,15 and various nanostructures on conductivesubstrates.16–20 All of these features enable the nickel cobaltite toexhibit excellent high-rate performances. It is reported that upto 80% specic capacitance can still be retained when thecurrent density increases 15 times from 1 to 15 A g�1.13

Although the ternary nickel cobalt oxide has been intensely

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researched recently, to the best of our knowledge, no work hasbeen reported about the ternary nickel cobalt sulphide. By wayof the UV-vis absorption spectrum and current–voltage (I–V)measurements, we rstly found that NiCo2S4 shows much loweroptical band gap energy and much higher conductivitycompared to NiCo2O4. Similar to NiCo2O4, due to the contri-butions from both nickel and cobalt ions, it is expected to offerricher redox reactions than the two corresponding singlecomponent sulphides or the two corresponding single compo-nent oxides. By elaborately designing the NiCo2S4 with variousion diffusion-favoured structures, such as the urchin-likestructure, excellent high-rate performances can be expected.The present work provides a facile method to construct theconductive NiCo2S4 with urchin-like nanostructure. The as-prepared urchin-like NiCo2S4 sample shows ultrahigh speciccapacitances, outstanding capacitance retention at high-ratesand ultralong lifespan, which make it a promising electrodecandidate material for supercapacitors.

In this work, by a facile precursor transformation method,we successfully synthesized a NiCo2S4 urchin-like nano-structure. The detailed synthesis procedure can be found in theESI.† Briey, the Ni and Co-based precursor was rstly synthe-sized by a simple hydrothermal route. In this process, NiCl2 andCoCl2 are used to provide Ni2+ and Co2+ for the precursor, H2Ois used as the solvent, and urea serves as a source for thegeneration of both carbonate and hydroxyl anions. Themorphology and composition were characterized by scanningelectron microscopy (SEM) and the X-ray diffraction (XRD)pattern with the results shown in Fig. 1. Obviously, theprecursor is composed of an urchin-like structure withnumerous small nanobelts radially grown from their commoncentre. The XRD pattern of the precursor shows nearly the samepattern as that of Co(CO3)0.5OH, except that all the diffractionpeaks slightly shi to the low diffraction direction. Since Co2+ orNi2+ always forms carbonate hydroxide in a hydrothermalenvironment in the presence of urea,21,22 the sample must beformed from the Ni and Co-based carbonate hydroxide. The

Fig. 1 (a) SEM image and (b) XRD pattern of the NiCo2S4 precursor. (c) XRFspectrum and (d) XRD pattern of the NiCo2S4.

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partial substitution of Co ions by Ni ions does not change thecrystal structure, but only slightly changes the lattice parame-ters, as shown in Fig. 1b.

The nanostructure is in situ chemically converted into itsNiCo2S4 counterpart in a hydrothermal environment by reactingwith sodium sulphide (Na2S). Fig. 1c shows the X-ray uores-cence probe (XRF) pattern of the as-fabricated nickel cobaltsulphide. The existence of S element in the sample signies thesuccessful conversion of the precursor into its sulphide coun-terpart. The element ratio of Ni, Co and S is 1 : 2.06 : 3.911,which is very close to the formula NiCo2S4. Fig. 1d shows theXRD pattern of the as-synthesized sample. The broad peaks withlow intensity indicate the poor crystallization of the sample. Thediffraction peaks at 26.8�, 31.5�, 38.1�, 50.4� and 55.2� corre-spond to the respective (220), (311), (400), (511) and (440) planesof the nanostructured cubic type NiCo2S4 (JCPDS Card no. 43-1477). The diffraction peaks show a similar XRD pattern to thecubic Co3S4 (JCPDS Card no. 75-1561), except for a slight shi ofthe diffraction peaks towards the low angle direction, which canfurther conrm that the substitution of Co ions by Ni ions onlyslightly changes the lattice parameters while maintaining thecrystal structure. Two other diffraction peaks at 29.9� and 52.1�

can also be found, as shown in Fig. 1d. These two peaks have thesame diffraction angle with the (311) and (440) plane diffractionof Co9S8 (JCPDS Card no. 73-1442). The substitution of Co ionsby Ni ions does not change the crystal structure of the Co-basednanocrystal, which can be proved by the XRD patterns of Ni andCo-based precursor and the NiCo2S4. So it can be conjecturedthat these two diffraction peaks are attributed to the NixCo9�xS8.Assuming that the formula of the as-obtained sample isNiCo2S4$aNixCo9�xS8, according to the element ratioobtained from the XRF test, the formula can be calculated to beNiCo2S4$0.05Ni2.5Co6.5S8. Obviously, the sample is almost solelyconstituted by NiCo2S4.

To further evaluate the near-surface elemental compositionand the chemical state in the as-prepared NiCo2S4 urchin-likenanostructure, X-ray photoelectron spectroscopy (XPS)measurements were conducted and the results are presented inFig. 2. The Co 2p and Ni 2p spectra are presented in Fig. 2a andb, respectively. By using a Gaussian tting method, the Co 2pspectrum can be best tted with two spin-orbit doublets, char-acteristic of Co2+ and Co3+, and two shake-up satellites (iden-tied as “Sat.”). The Ni 2p spectrum can also be best tted byconsidering two spin-orbit doublets characteristic of Ni2+ andNi3+ and two shake-up satellites. These results match well withthe reported data of Co 2p and Ni 2p spectra in NiCo2O4.16,23 TheS 2p spectrum can be divided into two main peaks and oneshake-up satellite. The component at 163.8 eV is typical of

Fig. 2 XPS spectra of (a) Co 2p, (b) Ni 2p, and (c) S 2p for the NiCo2S4.

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metal–sulphur bonds,24 while the component at 162.6 eV can beattributed to the sulphur ion in low coordination at the surface.According to the XPS analysis, the near-surface of the NiCo2S4sample has a composition of Co2+, Co3+, Ni2+, Ni3+ and S2�,which is in good agreement with the NiCo2S4.

The morphology and the structure of the NiCo2S4 arecharacterized by the SEM and high-resolution transmissionelectron microscopy (HRTEM) measurements. Fig. 3a and bshow the representative SEM images of the sample. Obviously,the NiCo2S4 sample completely maintains the urchin-likemorphology and structure of the Ni and Co-based precursor.The NiCo2S4 is composed of various nanobelts radially grownfrom the center. HRTEM measurement is also employed tocharacterize the as-synthesized NiCo2S4. Apparently, thenanobelts in the urchin-like structure are composed of

Fig. 3 (a and b) SEM images, (c–e) TEM images, (f) HRTEM image, (g) the cor-responding SAED pattern and (h) EDS spectrum of the NiCo2S4.

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domains of nanocrystallites, as shown in Fig. 3c–e. Theaverage crystallite size can be calculated from the XRDpatterns using the Scherrer equation. The mean crystallitedimension is calculated to be 17.4 nm from the (311) diffrac-tion, which matches very well with the TEM test. The latticefringes shown in Fig. 3f can be indexed to the (111) and (400)crystal planes of the cubic phase, which further conrms theformation of crystalline NiCo2S4. In addition, the correspond-ing selected area electron diffraction (SAED) pattern indicatesthe polycrystalline nature of the nanobelts, and the diffractionrings can be readily indexed to the (111), (220), (311), (400),(511) and (440) planes of the NiCo2S4 phases, which isconsistent with the above analysis. By the TEM analysis, nophase can be attributed to (NixCo1�x)9S8, which can in turnindicate that the as-synthesized sample is mainly composed ofthe NiCo2S4. The (NixCo1�x)9S8 phase in the XRD pattern mayarise from the incomplete sulfurizing of the precursor in thedeep center of the urchin structure because of the low contactwith the S2� in the sulfurizing process.

To evaluate the band gap of the as-synthesized NiCo2S4, theUV–vis absorption spectrum is measured, as shown in the insetof Fig. 4a. Generally, the absorption band gap energy, Eg, can bedetermined by the following equation:23,25

(ahn)n ¼ K(hn � Eg) (1)

where hn is the photoenergy, a is the absorption coefficient, K isa constant relative to the material, and n is either 2 for a directtransition or 1/2 for an indirect transition. The band gap ener-gies of NiCo2S4 can be calculated to be 1.2 and 2.5 eV for n ¼ 2by the extrapolation of eqn (1), as shown in Fig. 4a. No linearrelation was found for n ¼ 1/2, indicating that the as-preparedNiCo2S4 is semiconducting with direct transition. The NiCo2S4sample shows much lower Eg than NiCo2O4, that is, 2.4 and3.6 eV, so much higher conductivity is expected for the NiCo2S4sample. Fig. 4b shows the I–V curves of the as-synthesizedNiCo2S4 and NiCo2O4 samples tested by linear sweep voltam-metry. Obviously, NiCo2S4 has a much higher conductivity thanNiCo2O4. The Brunauer–Emmett–Teller (BET) surface area ofNiCo2S4 is calculated to be 20.33 m2 g�1 from the nitrogenadsorption–desorption isotherm (Fig. S1, ESI†). The highconductivity of NiCo2S4 is benecial for the fast transfer ofelectrons. The urchin-like structure can greatly enhance thefacility of electrolyte penetration. These features make the as-synthesized NiCo2S4 sample an excellent electrode material forsupercapacitors.

Fig. 4 (a) (ahn)2 hn curves and UV–vis adsorption spectra (inset), and (b) I–Vcurves of NiCo2S4 and NiCo2O4.

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Typical cyclic voltammograms (CVs) were tested in 6 M KOHaqueous electrolyte at various scan rates ranging from 5 to60 mV s�1, as shown in Fig. 5a. Apparently, the CV curves of theNiCo2S4 hybrid electrode suggest typical pseudocapacitivecharacteristics of the active material, which is obviously distinctfrom the electric double-layer capacitance characterized bynearly rectangular CV curves. Clearly, a pair of redox peaks isfound at the high scan rates of 30, 40, 50 and 60 mV s�1.However, the anodic peaks and the cathodic peaks are respec-tively separated into two peaks as the sweep rates decrease,which indicate the rich redox reactions of NiCo2S4 in the cyclingprocess. Comparing the CV curves of NiCo2S4 with those of NiSand CoS2,26,27 the obvious redox peaks in all CV curves can beattributed to the redox reactions related to M–S/M–S–OH, whereM represents Ni and Co ions.

To further evaluate the capacitance performances of the as-synthesized NiCo2S4 sample, the galvanostatic charge–discharge measurements were conducted in 6 M KOH solutionwith a potential window of 0 to 0.565 V (vs. Hg/HgO referenceelectrode) at various current densities ranging from 1 to 50 A

Fig. 5 Electrochemical performances of the ternary NiCo2S4 urchin-like nano-structure: (a) CV curves, (b and c) galvanostatic charge–discharge curves, (d) thecorresponding specific capacitance versus discharge current density, (e) cyclingperformance at progressively varying current densities, (f) cycling stability at aconstant current density of 20 A g�1, (g) EIS measured before and after 5000charge–discharge cycles, and (h) CV curves measured before and after 5000charge–discharge cycles with a scan rate of 5 mV s�1.

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g�1. Fig. 5b and c show the representative plots of the charge–discharge curves with various current densities of 1, 2, 3, 4, 5, 7,10, 15, 20, 30, 40 and 50 A g�1, respectively. Consistent with theCV results, the plateaus in the charge–discharge curves indicatethe existence of Faradaic processes. Moreover, there are twoseparate plateaus in the charge or discharge process, whichdemonstrate the dual redox processes of the active materialwhile charging or discharging. The galvanostatic charge–discharge curves of the composite are highly symmetricalwithout an obvious iR drop at low current densities, indicating arapid I–V response and an excellent electrochemical revers-ibility.28 Based on the galvanostatic charge–discharge curves,the specic capacitances can be calculated to be 1149, 1056,1065, 1062, 1056, 1032, 1018, 977, 888, 850, 807 and 761 F g�1 atthe current densities of 1, 2, 3, 4, 5, 7, 10, 15, 20, 30, 40 and 50 Ag�1, respectively, as plotted in Fig. 5d. The capacitive perfor-mance is evidently superior to those of the cognate spinel nickelcobaltite (NiCo2O4) and the corresponding binary sulphides,Co9S8 and NiS (Fig. S2–S4, ESI†). The reduction of speciccapacitance at high rate can be attributed to the low diffusion ofthe electrolyte ion. The ionic motion in the electrolyte is alwayslimited by diffusion because of the time constraint during thehigh-rate charge–discharge process, and only the outer activesurface is utilized for charge storage, so there is an obviousreduction in the electrochemical utilization of the electroactivematerials.29,30 However, the 3D urchin-like structure provides ashort path length and good accessibility for the electrolyte. Thenanobelts in the urchin-like structure provide a large specicsurface area andmore active sites, which can effectively improvethe utilization and the pseudocapacitance of electroactivematerials even at high rates. The high conductivity of theNiCo2S4 facilitates a pathway for electron transfer, which isbenecial for the fast charging and discharging. Besides, thehigh conductivity can minimize the resistance for electrontransfer, which can further improve the power operation.Therefore, large high-rate capacitance can be achieved using theunique ternary NiCo2S4 urchin-like structure. The NiCo2S4active material demonstrates a remarkable specic capacitanceof 761 F g�1 even when the current density is as high as 50 A g�1.Remarkably, up to 77.3% of the capacitance is retained whenthe charge–discharge rate is increased 20-times from 1 to 20 Ag�1, and about 66.2% of the capacitance can still be retainedeven for a 50-time increase in the charge–discharge currentdensities, which is much higher than those of NiCo2O4, Co9S8and NiS counterparts (Fig. S5, ESI†). These results indicate theexcellent high-rate properties of the as-synthesized NiCo2S4nanostructure.

The cycling stability is also evaluated by the repeatedcharging–discharging measurements at progressivelyincreasing current densities and subsequently at a constantcurrent density of 20 A g�1, as shown in Fig. 5e and f. The initialspecic capacitance is as high as 1050 F g�1 at the currentdensity of 2 A g�1. No step-like reduction is found when thecurrent density turns to 5 A g�1, which is consistent with thespecic capacitances calculated from the charge–dischargecurves, as shown in Fig. 5d. It can be observed that the speciccapacitance rst slightly decreases at the current densities of 2 A

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g�1 and 5 A g�1 in the course of the rst 200 cycles, and thenbecomes much stable. Aer continuous cycling for 500 cycles atsuccessively increased current densities of 2, 5, 10, 15 and 20 Ag�1, the current density is reduced back to 2 A g�1. Encourag-ingly, 96.7% of the initial specic capacitance of 2 A g�1 can stillbe recovered. Aer several cycling cycles, the specic capaci-tance is increased to 97.6% of the initial specic capacitance(1025 F g�1) and maintained for numerous cycles withoutnoticeable diminishment. The hybrid electrode is subsequentlytested by repeated charging–discharging measurements at avery high discharge current density of 20 A g�1 for another 5000cycles, as shown in Fig. 5f. Remarkably, 91.4% capacitance isstill retained aer 5000 cycles, which is remarkably higher thanthose of numerous binary sulphides, such as NiS and CoS2,26,27

reported previously. So the as-obtained NiCo2S4 sample showsremarkable cycling stability. To demonstrate the evolution ofelectrochemical behaviour of the NiCo2S4 hybrid electrode,electrochemical impedance spectroscopy (EIS) and CVmeasurements were performed before and aer 5000 charge–discharge cycles, as shown in Fig. 5g and h. The remarkablysimilar EIS curves before and aer cycling indicate the highstability of the present electrode. Meanwhile, the CV curvesbefore and aer cycling are presented in Fig. 5h. Interestingly,there is an apparent attenuation of the redox peaks from theredox reaction of Co3+/Co4+. Simultaneously, the redox peakscorresponding to the redox reaction of Ni2+/Ni3+ are greatlyenhanced aer charge–discharge cycling for 5000 cycles. So thedecay in the capacitive performance with cycling can be attrib-uted to the decay of the redox reaction of Co3+/Co4+. However,the simultaneous enhancement of the redox reaction of Ni2+/Ni3+ can effectively remedy the capacitive performance reduc-tion, resulting in a high cycling stability.

Conclusions

In summary, a 3D urchin-like NiCo2S4 nanostructure wassuccessfully prepared by sulfurizing the Ni and Co-basedprecursor. The as-synthesized NiCo2S4 sample shows muchlower optical band gap energy and much higher conductivity.The ion diffusion favoured urchin-like structure and the highconductivity make the as-synthesized NiCo2S4 sample anexcellent electroactive material for high-rate supercapacitors.The NiCo2S4 hybrid electrode exhibits outstanding electro-chemical performance with very high specic capacitance,improved cycling stability and excellent capacitance retention.This work provides a novel electroactive material for the designof next-generation supercapacitors.

Acknowledgements

This work was supported by the National Natural ScienceFoundation (no. 61172003). The authors thank the Analysis andTesting Center of HUST for sample testing support. We alsothank Dr C. Fan for assistance in the XPS measurements. Webeneted from the fruitful discussions with Q. S. Jiang.

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