Truncated NiCo2S4 cubohexa-octahedral nanostructures for high-performance supercapacitor

Materials Letters 189 (2017) 21–24

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Truncated NiCo2S4 cubohexa-octahedral nanostructures for highperformance supercapacitor ⁎

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Shahid Hussaina,b, , Tianmo Liua,b, , Nimra Aslamc, Yangyang Zhanga,b, Shuoqing Zhaoa,b a b c

College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China National Engineering Research Centre for Magnesium Alloys, Chongqing 400030, China Department of Physics, University of Sargodha, Sargodha 40100, Pakistan

A R T I C L E I N F O

A BS T RAC T

Keywords: Crystal growth Nanocomposites NiCo2S4 Cubohexa-octahedral Supercapacitor

Simple hydrothermal method was adopted to synthesize zero dimensional 14-facets (6 squares and 8 cuboids) NiCo2S4 cubohexa-octahedral (NCS-COH) shaped nanostructures for high performance supercapacitor properties. The as-prepared nanostructures were characterized using X-ray powder diffraction, scanning and transmission electron microscopy. A plausible formation mechanism was also proposed along with crystal structure of NCS-COH. The exceptional architectures fashioned an ultrafast electron transfer during electrochemical reaction. The NCS-COH electrode exhibited a high specific capacitance of 835 at 1 A/g and capacitance retention of 92.7% at 10 A/g after 5000 cycles.

1. Introduction Recent trends in energy storage devices has motivated researchers to develop and produce potential nanomaterials that can exhibit high tendency towards industrial applications. To evaluate the best physical and chemical properties of a crystal, polyhedral crystals enclosed by multiple facets are of great importance due to unique size, composition, and crystal phase and crystallinity distribution. The modulation of crystal structure enables researcher to engineer desirable morphologies that may anticipate the nanomaterial for actual nanotechnology applications. Transition metal chalcogenides (TMCs) are emerging class of electrode materials for superior electrochemical devices. Binary TMCs such as MoS2, NiS, CoS has already fascinated due to their unique structures and properties in diverse applications, but ternary metal sulfides (AxBxS) show richer redox reactions from double metal ions and possess higher electronic conductivity as compared to single element sulfides. Amongst TMCs, NiCo2S4 being captivating pseudocapacitive materials has provoked probing attention due to magnanimous electrochemical conductivity [1–3] contains versatile nanostructures including nanoflowers [4], cauliflower-like [5], urchin-like [6] and nanotubes [7], nano-network [8], etc. So, it is viable to develop ternary metal sulfides that could deliver electrochemical devices with high energy and power capability and strong long cyclic stability, flexibility and low material deformation [17–19]. In present study, we reported synthesis of zero dimensional 14-facets NiCo2S4 cubohexa-

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octahedral (NCS-COH) shaped nanostructures for high performance supercapacitor properties via simple low cost hydrothermal method. The as-prepared unique nanostructures exhibited an ultrafast electron transfer during electrochemical reaction and delivered a high specific capacitance of 835 at 1 A/g and capacitance retention of 92.7% at 10 A/g after 5000 cycles. 2. Experimental Typical synthesis of NCS-COH includes1 mM cobalt nitrate hexahydrate [Co(NO3)2·6H2O], 1 mM nickel nitrate hexahydrate [Ni(NO3)2·6H2O], 1 mM thaioethioamide (C2H5N·S) and 0.7 g sodium dodecyl sulfate (SDS) dissolved in 50 ml deionized water and vigorously stirred for 1 h. The dark brown solution was poured into a 50 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 24 h. The final products were cleaned and washed using distilled water, ethanol, and air-dried at 60 °C for 8 h. 2.1. Electrochemical measurements The electrochemical tests were conducted in a three-electrode cell electrochemical workstation (RST 5100F) with 6 M KOH aqueous solution as the electrolyte. The working electrode was fabricated by mixture of as-prepared NCS-COH powders (70 wt%), with acetylene black (20 wt%) and slurry of polyvinylidene difluoride (PVDF) binder (10 wt%) binder in N-methyl-pyrrolidone (NMP). The mixture, then

Corresponding authors at: College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China. E-mail address: [email protected] (S. Hussain).

http://dx.doi.org/10.1016/j.matlet.2016.11.056 Received 8 October 2016; Received in revised form 10 November 2016; Accepted 14 November 2016 Available online 16 November 2016 0167-577X/ © 2016 Elsevier B.V. All rights reserved.

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Fig. 1. XRD pattern of as-prepared NCS-COH (a), EDS spectra for single particle (b, c) and crystal structure of NiCo2S4 (d).

was coated onto a Ni foam (1×1) cm as current collector and finally dried at 60 °C for 6 h. Saturated calomel electrode (SCE) and platinum plate were used as counter and reference electrodes, respectively.

octahedral (with eight hexagons and six quadrangles) high homogeneity of nanostructures (Fig. 2c). The diameter of NCS-COH nanostructures is approximately 800 nm. The high magnification FESEM images shown in Fig. 2c narrates two different orientations of cuboctahedra configurations. The NCS cuboctahedra structure consisting of fourteen faces with six {100} square surfaces and eight hexagonal {111} surfaces. Interestingly square {100} surfaces are very smooth while hexagonal {111} surfaces are cubical in shape, reveals the selective etching process occurred only at {111} surfaces. It implies the successful anisotropic growth of NCS crystal. For polyhedral crystals, the general order of stability is {100} & $2gt;{111} & $2gt;{110}, hence {110} facets may generate at the expense of the selective-dissolution of {1111} surfaces, however the final stability of crystal shapes is due to {100} surfaces [9]. Under hydrothermal reaction, Ni2+ and Co2+ in aqueous solution react with C2H5N·S–SDS ions to produce final products. The role of anionic SDS is to disrupt non-covalent bonds and electrostatic binder during the nucleation while enhancing the static interactions at polar and nonpolar surfaces [10]. At the same time C2H5N·S–SDS both serve as S− ions reservoir. The formation of unique NCS-COH nanostructures is mainly attributed to following factors: the hydrothermal reaction system provides initial nuclei (Ni2+, Co2+ and S−) by dissociation of parent nuclei with full contact of ions leading to rapid formation of Ni/Co/S nanostructures. At the same time, SDS redirects the polar and nonpolar surfaces interlacing each other to form quadrangles and cubical hexagons with high exposed surface area. Then, the important factor is use of slow vulcanization

3. Results and discussion 3.1. Structure characterization The surface morphological and structural characterizations of asprepared NCS-COH nanostructures were accomplished using X-ray diffraction (XRD) in a Rigaku D/Max-1200X diffractometer with Cu Kα radiation (λ=1.5406 Å) operated at 30 kV and 100 mA, and Hitachi S4300 field-emission scanning electron microscope (FESEM). Fig. 1a shows XRD pattern of the as-prepared NCS-COH. The sharp diffractions peaks are observed at 17.2°, 26.8°, 31.2°, 38.2°, 47.5°, 50.1°, and 55.0°, corresponding to (111), (220), (311), (400), (422), (511), and (440) planes, respectively. All peaks indexed to standard cubic phase of NiCo2S4 with space group Fd-3m (227), JCDP card No. 20-0782 exhibiting high crystallinity and no other impurity is found in the product. The one-particle selected EDS spectra and atomic (%) distribution (1:2:4) confirms the presence of Ni, Co and S atoms, depicted in Fig. 1b and c. The TEM, HRTEM and SAED images of the as-prepared NCS-COH are shown in Fig. 2e and f. The crystal structure of NiCo2S4 product is shown in Fig. 1d. Low and high resolution FESEM images of unique NCS-COH are shown in Fig. 2a and b. It exhibited truncated polyhedral cubohexa-

Fig. 2. SEM images of 14-facets NCS cubo-octahedral nanostructures (a, b), different growth facets (c), BET surface analysis (d), TEM, HRTEM (e) and SAED (inset) (f) and growth mechanism of nanostructures (g).

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Fig. 3. Electrochemical characterization of NiCo2S4 electrode material: (a) the CV curves at different scan rates of 5，10，20，50, and 100 mV/s; (b) charge–discharge curves at different current densities from 1，2， 3，5, 10 and 20 A/g; (c) specific capacitances at different current densities from 1 to 20 A/g; and (d) cyclic performance at 10 A/g (corresponding charge/discharge curves of 20 cycle test of the NiCo2S4 electrode in inset).

The high specific capacitance behavior is attributed to the intercalation/de-intercalation of ions at activate electrode material through inner-ions movements, accumulating the active species inside the electrode materials, resulting rise in the specific capacitance [15]. Another important factor in supercapacitor application is excellent cycling performance with good stability. Fig. 3d shows cyclic of NCSCOH electrode examined at 10 A/g for 5000 cycles. The as-prepared NCS-COH electrodes exhibited superior cycling performance with a small loss of 7.3% after 5000 cycles. Furthermore, the galvanostatic charge–discharge curves of NCSCOH electrode for 20 cycles were tested (Fig. 3d (inset)). All the curve exhibited analogous quasi-rectangular redox behavior. The following factors might be attribute to the superior capacitive performance of NCS-electrode. The wide and unique surface area with multi-junctions at the facets-edges in the structure provided more continuous charge transport and diffusion of OH−1 ions in the electrolyte, resulting in a low resistance redox process. Then, the nano-sized unique multi-faced complex units with uniform dispersion enhance the utilization rate of electroactive sites during the reaction system [16]. Absolutely, all these factors make NCS-COH a promising materials with high specific capacitance and long cyclic stability.

process. In addition, the surface-to-volume ratios of NCS-COH (Fig. 2d) determined using BET isotherms by N2 adsorption–desorption and pore-size distribution exhibited surface area of 75.54 m2 g−1 and pore volume of 0.96 cm3 g−1. The high surface area of NCS-COH nanostructures enables multichannel for the transfer of electron-ion species in super electrochemical activities. During the ion-exchange mechanism, interchange of CO32− and OH− to S2− can slightly take place while the escaping of CO32− and OH− interact with H+ initiating from the hydrolysis of C2H5N·S–SDS to produce CO2 and H2O, keeping the original structure stabilize. Besides, the vulcanization process aids in morphology shrinkage [11,12]. The perfect combination of all the factor give a rise to construct distinctive NiCo2S4 nanostructures. 3.2. Electrochemical properties In comparison to binary TMCs such as CoS, MoS2, NiS, ternary sulfides are considered performing richer redox reaction due to the manifestation of Ni and Co ions. The electrochemical performances of the NCS-COH supercapacitor were tested in three electrode system. Cyclic voltammetry (CV) for NCS-COH carried out at scan rates of 5, 10, 20, 50 and 100 mV S−1 in 6 M KOH aqueous solution are shown in Fig. 3a. No obvious shape variations are observed showing a high current capability. The CV curves exhibit a pair of well-defined redox peaks implying that the capacitance attributes were mainly directed by Faradaic redox reactions. The redox reaction of NCS-COH in alkaline electrolyte is attributed to reversible redox reaction of Ni2+/Ni3+, Co2+/ Co3+ and Co3+/Co4+ transitions to form NiSOH, CoSOH, and CoSO, described in the following equations [13,14].

NiCo 2 S4 +OH−+H2 O ↔ NiS4 OH + 2CoS2 OH + e− CoSOH +

OH−→CoSO

+ H2 O +

e−

4. Conclusions Simple hydrothermal method was adopted to synthesize zero dimensional 14-facets NiCo2S4 cubohexa-octahedral (NCS-COH) shaped nanostructures for high performance supercapacitor properties. The as-prepared nanostructures were characterized using XRD, FESEM and TEM. A plausible formation mechanism was also proposed along with crystal structure of NCS-COH. The exceptional architectures fashioned an ultrafast electron transfer during electrochemical reaction. The NCS-COH electrode exhibited a high specific capacitance of 835 at 1 A/g and capacitance retention of 92.7% at 10 A/g after 5000 cycles. Hence, unique NiCo2S4 cubohexa-octahedral nanostructures are promising pseudocapacitive materials for supercapacitors applications.

(1) (2)

CoS + OH−→CoS + H2 O + e−

(3)

NiS + OH−→NiSOH + e−

(4)

Acknowledgements

The low shift in both cathodic and anodic peak potentials is attributed to internal resistance of the material. The narrow peak to peak separation indicates fast electron transportation between the electrode and electrolyte due the unique NCS-COH high surface area nanostructures and the high conductivity of NiCo2S4, which is important for high-current capacitive performances. Fig. 3b shows galvanostatic charge–discharge (GCD) curves at current densities of 1, 2, 3, 5 and 20 A/g for potential window between 0 and 0.40 V. The rectangular shapes with linear slopes at high current densities indicate pseudocapacitive behavior of NCS-COH. The relationship between specific capacitance and current density is shown in Fig. 3c. The specific capacitances calculated for current densities of 1, 3, 5, 10 and 20 A/g were 832, 827, 818, 805 and 735 F/g, respectively.

The authors gratefully acknowledge the financial support for this work from the National Natural Science Foundation under the Grant No. 11332013 and Fundamental Research Projects for Central Universities under the Grant No. 106112016CDJCR131203. References [1] X.H. Xiong, G. Waller, D. Ding, D.C. Chen, Nano Energy 16 (2015) 71–80. [2] Z.B. Wu, X.L. Pu, X.B. Ji, Y.R. Zhu, M.J. Jing, Electrochim. Acta 174 (2015) 238–245. [3] Y.F. Zhang, M.Z. Ma, J. Yang, C.C. Sun, H.Q. Su, Nanoscale 6 (2014) 9824–9830. [4] S. Hussain, T.M. Liu, Ceram. Int. 42 (2016) 11851–11857. [5] Y.L. Xiao, Y. Lei, B.Z. Zheng, L. Gu, Y.Y. Wang, RSC Adv. 5 (2015) 21604–21613.

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