One-step synthesis of CoNi₂S₄ nanoparticles for supercapacitor electrodes

Abstract

A one-step solvothermal process is developed to synthesize novel CoNi₂S₄ nanoparticles. When applied as electrode materials of supercapacitors, CoNi₂S₄ nanoparticles with lower cost of production exhibit better electrochemical performances, i.e.: higher specific capacitance, better rate capability, and higher energy density which make it a promising electrode candidate material for next generation supercapacitors.

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As a new form of energy store, supercapacitors (SCs), also known as ultracapacitors, have been widely applied in hybrid electric vehicles, large industrial equipment, memory back-up devices and renewable energy power plants due to their fast charge and discharge process, large lifespan, and high dynamic of charge propagation, etc.^1,2 However, the practical applications of SCs are greatly hindered due to the lack of high-performance electrode materials available from a facile synthesis or at a reasonable cost. Therefore, seeking ideal electrode materials is always the research focus in the past decades. Up to date, three kinds of solid state substances have been selected as the electrode materials of SCs which includes carbon and its derivatives (e.g.: particles,³ graphene,⁴ carbon nanotubes (CNTs),⁵ etc.), transition metal compounds and their derivatives (e.g.: oxides,^6,7 hydroxides,^8,9 sulfides,¹⁰ phosphates,¹¹ etc.), conducting polymers and their derivatives (e.g.: polyaniline,¹² polypyrrole,¹³ etc.). However, lower specific capacitance of carbon materials and the mechanical degradation of conducting polymers largely restrict their application as electrode materials. Although transition metal compounds also have some drawbacks (e.g.: poor electrical conductivity), they can not only store energy like electrostatic carbon materials but also exhibit electrochemical Faradaic reactions between electrode materials and ions. As a result, transition metal compounds as the electrode materials of SCs have been devoted massive efforts in recent decade. Early transition metal compound used as electrode materials of SCs is ruthenium oxide.¹⁴ But the higher cost has prevented this material from commercial applications. Therefore, some cheaper compounds of cobalt, nickel, or manganese become a preferred substitute. Detailed research advances about transition metal compounds applied in SCs have been reviewed in a latest paper.¹⁵

Recently, ternary nickel cobalt oxides (NiCo₂O₄) have attracted more and more attentions due to the richer redox chemistry, higher electrochemical activity, low-cost and at least two orders of magnitude higher electronic conductivity than simple oxides. So, single-phase materials or composite of NiCo₂O₄ have been synthesized and some excellent electrochemical properties were reported, e.g.: nanowires,¹⁶ microbelts,¹⁷ NiCo₂O₄–CNT,¹⁸ and NiCo₂O₄–graphene oxide,¹⁹ etc. Similar to NiCo₂O₄, due to the contributions from nickel and cobalt ions, ternary cobalt nickel sulfides (CoNi₂S₄ or NiCo₂S₄) are expected to offer richer redox reactions than single component sulfides. Furthermore, ternary cobalt nickel sulfides have the even higher electronic conductivity than NiCo₂O₄.²⁰ Higher electrical conductivity can decrease both the sheet resistance and the charge transfer resistance of the electrode which causes a smaller interior resistance (IR) loss at a higher current density. As a result, higher power density and rate capability can be achieved.²¹ Thus, two works about single-phase NiCo₂S₄ have been explored, i.e.: NiCo₂S₄ porous nanotubes synthesized via sacrificial templates showing a specific capacitance of 933 F g⁻¹ at 1 A g⁻¹,²² urchin-like NiCo₂S₄ obtained by a precursor transformation method with a high value of 1050 F g⁻¹ at 2 A g⁻¹.²⁰ Nevertheless, the needs for premade templates or two-step transformation would largely increase the production cost which isn't beneficial to the commercial application. More recently, NiCo₂S₄ nanosheets-graphene composites were successfully fabricated by a facile hydrothermal method which exhibited higher specific capacitance of 1161 F g⁻¹ at 5 A g⁻¹.²³ However, to the best of our knowledge, no report is available on the synthesis and supercapacitive properties of CoNi₂S₄ nanostructures up to now. Hence one can see that it is significant and challenging to exploit CoNi₂S₄ nanomaterials for supercapacitor electrodes. The present work provides a one-step method to construct CoNi₂S₄ nanoparticles and investigate the corresponding electrochemical properties. Compared with the NiCo₂S₄ nanostructures or other nanomaterials with single phase, CoNi₂S₄ nanoparticles exhibited lower production cost and better supercapacitive performances, which make it a promising electrode candidate material of supercapacitors in the future.

Typical CoNi₂S₄ nanoparticles was synthesized by a improved solvothermal process which is similar to that of ternary chalcogenides in our previous work.²⁴ Detailed experimental parameters are listed as the following: metal source: 0.04 g Co(AC)₂·4H₂O and 0.08 g Ni(AC)₂·4H₂O; sulfur source: 0.021 g sulfur powder; surfactant: 3 mL oleylamine; solvent: 20 mL anisole; reaction temperature: 200 °C; reaction time: 24 hours. Crystal structures of products were characterized on a Rigaku Ultima III X-ray diffractometer (XRD) equipped with a Cu Kα radiation source (λ = 0.15418 nm). Inductively coupled plasma optical emission spectrometer (ICP, Perkin Elmer Optima 8000) was used to analyze the actual composition of CoNi₂S₄ nanoparticles. Transmission electron microscopy (TEM) images and high-resolution transmission electron microscopy (HRTEM) images were captured on the JSM JEM-2100 at an acceleration voltage of 200 kV. Surface area and pore size distribution of the products were determined by Brunauer–Emmett–Teller (BET) nitrogen adsorption–desorption and Barrett–Joyner–Halenda (BJH) methods (Mike, Gemini VII 2390). Optical properties were determined by measuring solid UV-Vis spectra using UV-Vis spectrophotometer (Shimadzu, Uv-2550).

XRD pattern (Fig. 1a) shows that all the diffraction peaks of the as-synthesized products can be indexed to the cubic phase of CoNi₂S₄ structure (JCPDS no. 24-334) with the Fd-3m space group and a primitive hexagonal unit cell a = 9.4279 Å. Five characteristic peaks at 26.68°, 31.53°, 38.18°, 50.12°, and 55.04° correspond to the (220), (311), (400), (511), and (440) diffraction planes, respectively. No other impurity phases were detected, e.g.: CoS, NiS, oxides or organic compounds related to the reactants indicating the obtained product as single phase. For confirming the actual elemental composition, the obtained products were further dissolved by chloroazotic acid and characterized by ICP. Results show that the atom ratio between cobalt and nickel of products was 1 : 1.98 which is very consist with the chemical formula. Hence, the sample is constituted by CoNi₂S₄ according to the experimental data obtained from XRD and ICP. The crystal structure in Fig 1b clearly indicates that CoNi₂S₄ belongs to the spinel structure. TEM image in Fig. 1c shows that resulted CoNi₂S₄ appeared quasi-spherical morphology and the average size of the CoNi₂S₄ nanoparticles is about 8–15 nm. HRTEM reveals that the lattice spacing is about 2.82 Å, which can be assigned to the (311) planes of the cubic CoNi₂S₄ phase. Clear stripes indicate that the as-prepared nanoparticles are in single-crystalline structure. Different sulphur sources have important roles on the phase of products. The as-synthesized sample with CS₂ as sulfur source contained some impurity phases (See ESI, Fig. S1†). Although there are not obvious impure diffraction peaks in the XRD patterns of the as-prepared products by adding sulfourea or thioacetamide, the ICP data indicates that the atom ratio between cobalt and nickel was 1 : 1.90 or 1 : 2.2, respectively. Thus, sulfur powder is the best sulfur source in present work.

Fig. 1 (a) XRD pattern (the upper is the typical sample and the below is the standard data of JCPDS no. 24-334), (b) crystal structure, (c) TEM image, and (d) HRTEM image of CoNi₂S₄ nanoparticles.

A better electrode material of SCs should have a large surface area. Therefore, surface area and pore size distribution of CoNi₂S₄ nanoparticles were determined by Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods. Nitrogen adsorption–desorption isotherm curve and the corresponding pore size distribution plot was shown in Fig. 2a. The BET surface area was 21.64 m² g⁻¹. The overall surface area was smaller than the reported research which is due to the aggregation of CoNi₂S₄ nanoparticles.²⁵ But according to a previous review,¹⁵ specific capacitance of various materials does not linearly increase with the specific surface area. The pore size distribution curve inserted in Fig 2a demonstrates that the pore size is about 3.5 nm which are formed by the pileup of CoNi₂S₄ nanoparticles. To evaluate the band gap, the converted UV-Vis absorption spectrum was shown in the inset of Fig. 2b. CoNi₂S₄ nanoparticles have better light absorption in the visible range which is consistent with the black appearance. Generally, the absorption band gap energy, E_g, can be determined by the following equation: (Ahν)ⁿ = K(hν − E_g), where hν is the photoenergy, A is the absorbance, K is a constant relative to the material, and n is either 2 for a direct transition or 1/2 for an indirect transition. CoNi₂S₄ is a semiconductor with direct transition and n = 2.²⁰ Hence, the band gap energies can be calculated to be 1.71 eV, as shown in Fig. 2b. CoNi₂S₄ samples show much lower E_g than NiCo₂O₄, that is, 2.4 and 3.6 eV. So much higher conductivity of CoNi₂S₄ is beneficial for the fast transfer of the electron.

Fig. 2 (a) Nitrogen adsorption–desorption isotherms and pore size distribution curve (inset) (b) (Ahν)² − hν curves and solid UV-vis adsorption spectra which is Kubelka–Munk converted from diffuse reflectance spectra (inset) of CoNi₂S₄ nanoparticles.

To investigate the electrochemical performance, the as-prepared CoNi₂S₄ nanoparticles were fabricated as supercapacitor electrodes. Working electrodes were prepared by mixing active material (85 wt%) with acetylene black (10 wt%) and poly(tetrafluoroethylene) (5 wt%), coating on a piece of foamed nickel of about 1 cm² and pressing to be a thin foil at a pressure of 10 MPa. The weight of active material is about 5 mg. All electrochemical properties of self-made electrode, including cyclic voltammetry (CV), galvanostatic charge–discharge (CD) and cycle stability, were carried out at room temperature in a three-electrode system equipped with platinum electrode and a saturated calomel electrode (SCE) as counter and reference electrodes, respectively. The electrolyte was 3.0 M KOH solution. CV and CD tests were measured by an electrochemical working station (CHI660E, Shanghai, China) with the potential window from 0 V to 0.5 V (vs. SCE). Cycle stability tests were carried out with an Arbin electrochemical instrument. Electrochemical impedance spectroscopy (EIS) measurements were conducted at open-circuit voltage in the frequency range of 100 kHz to 0.01 Hz with AC voltage amplitude of 5 mV using PARSTAT2273 advanced electrochemical system.

Electrochemical properties of CoNi₂S₄ nanoparticle electrodes were evaluated by cyclic voltammetry (CV) and galvanostatic charge–discharge (CD) measurements. Typical CV curves were tested at various scan rates ranging from 1 mV s⁻¹ to 20 mV s⁻¹, (Fig. 3a). Apparently, the CV curves of the CoNi₂S₄ nanoparticle electrodes suggest two pairs of well-defined redox peaks within 0–0.5 V (vs. SCE) and typical pseudocapacitive characteristics of the active material, which is obviously distinct from the electric double-layer capacitance characterized by nearly rectangular CV curves. These peaks mainly originate from Faradaic redox reactions related to Co²⁺/Co³⁺ and Ni²⁺/Ni³⁺ redox couples, and probably mediated by the OH⁻ ions in the alkaline electrolyte.²³ The reduction peak current and the oxidation peak current were constrained between −0.1 A and +0.1 A. With increasing scan rates, the potential of the oxidation peak shifts in the positive direction and that of the reduction peak shifts in the negative direction, which is mainly related to the internal resistance of the electrode.²⁶ The specific capacitance of the active materials can be calculated based on the following equation:

where C (F g⁻¹), m (g), v(V s⁻¹), V_c (V) and V_a(V), and I (A) are the specific capacitance, the mass of the active materials in the electrode, scan rate, high and low potential limits of the CV tests, and the instant current on CV curves, respectively.¹¹ The average specific capacitance of CoNi₂S₄ nanoparticles was calculated to be 1330.2, 916.7, 711.8, and 480.5 F g⁻¹ at scan rates of 1, 5, 10, and 20 mV s⁻¹, respectively. With increasing scan rate, specific capacitance decreases gradually which can be ascribed to the diffusion effect limiting the diffusion and migration of the electrolyte ions within the electrode at high scan rates, resulting in lower electrochemical utilization of CoNi₂S₄ nanoparticles.⁹

Fig. 3 (a) cyclic voltammetry (CV) curves, (b) galvanostatic charge–discharge (CD) curves, (c) cycle life and coulombic efficiency and (d) Ragone plot of the CoNi₂S₄ nanoparticle electrodes.

To further evaluate the application potential of CoNi₂S₄ nanoparticles for supercapacitor electrodes, galvanostatic charge–discharge (CD) measurements were carried out between 0 and 0.4 V (vs. SCE) at various current densities (Fig. 3b). As can be seen from the CD curves, the shapes of five curves are very similar and show ideal pseudocapacitive behaviour with sharp responses and small internal resistance (IR) drop. Moreover, there is a potential platform in every charge–discharge curve. It is the typical pseudo-capacitance characterization of transition metal compounds which is in agreement with the result obtained from CV curves in Fig 3a. This phenomenon is caused by a charge transfer reaction or electrochemical absorption–desorption process at the electrode–electrolyte interface.²⁷ Specific capacitances of CoNi₂S₄ nanoparticles can be calculated based on the charge–discharge curves and the following equation: C = I × Δt/(ΔV × m), where C (F g⁻¹) is the specific capacitance, I (A) is the discharge current, Δt (s) represents the discharge time, ΔV (V) is the potential change during discharge, and m (g) is the mass of the active material. Encouragingly, CoNi₂S₄ nanoparticles exhibited excellent pseudo-capacitances of 1169, 972.2, 843.3, 770.3 and 702.3 F g⁻¹ at current densities of 1, 2, 3, 4 and 5 A g⁻¹. Compared with urchin-like NiCo₂S₄ obtained by precursor transformation method and NiCo₂S₄ porous nanotubes prepared via sacrificial templates, CoNi₂S₄ nanoparticles synthesized by one-step route exhibited similar or better pseudo-capacitances but with simplified synthetic process and lower cost of production. Moreover, CoNi₂S₄ nanoparticles have better rate capability which is very important for the electrode materials of a higher power density supercapacitor.

To investigate the stability of the CoNi₂S₄ nanoparticles, the specific capacitance with respect to the cycle numbers at a current density of 4 A g⁻¹ between 0 and 0.4 V are measured and the corresponding result is shown in Fig. 3c. The specific capacitance gradually decreased with the increase of cycle number, and a higher specific capacitance of 377.7 F g⁻¹ was still maintained after 2000 cycles. In addition, the charge–discharge efficiency (η), also called Coulombic efficiency, can be estimated according to following equation: η = t_c/t_d × 100%. Where t_c and t_d are the charge and discharge intervals, respectively. The CoNi₂S₄ nanoparticle electrode exhibited a higher charge–discharge efficiency of ∼99% over the entire cycling test. The last 20 cycles of 2000 charge–discharge runs at 4 A g⁻¹ were presented in Fig. S2, ESI.† It was found that the shapes of the charge–discharge curves were almost unchanged indicating better reversibility of CoNi₂S₄ nanoparticle electrodes. Hence, CoNi₂S₄ nanoparticles exhibit long cycle life and higher degree of reversibility which are the crucial requirements of supercapacitors. Although cycling stability of CoNi₂S₄ nanoparticles is not much better than some ternary cobalt nickel compounds, this work provided a novel electrode material with excellent electrochemical performances. Further work will focus on how to enhance the specific capacitance and cycling stability of CoNi₂S₄ nanoparticles.

Specific energy and specific power are two key factors for evaluating the power applications of supercapacitors. A good supercapacitor can provide high energy density and specific capacitance. The power density and energy density values were estimated from charge–discharge curves by the following equations:²⁸

E = (CΔV²)/2, P = (QΔV)/(2t) = E/t

where P is the average power density (W kg⁻¹), C is specific capacitance based on the mass of the electroactive material (F g⁻¹), Q is total charge delivered (C), ΔV is the potential window of discharge (V), t is the discharge time (s) and E is the average energy density (W h kg⁻¹). As shown in Fig. 3d, the energy density of CoNi₂S₄ nanoparticles was able to reach 46.9 W h kg⁻¹ at a power density of 446.6 W kg⁻¹, and still remained at 28.2 W h kg⁻¹ at a power density of 2358.4 W kg⁻¹. Compared with other nanomaterials with single phase (e.g.: hierarchical carbon, graphene, Co₃O₄ thin films, and NiO nanoparticles) applied as electrode of supercapacitors, CoNi₂S₄ nanoparticles exhibited higher specific capacitance and energy density (See Table S1 ESI†). Therefore, CoNi₂S₄ nanoparticles are shown to be a promising electrode material of SCs in hybrid vehicle systems in the future.

To identify the exact electrical conductivity of CoNi₂S₄ nanoparticle electrode, EIS was measured at room temperature in the frequency range 0.01–10⁵ Hz under open-circuit conditions (See Fig. S3, ESI†). An equivalent circuit used to fit the impedance curve is inserted in Fig. S3,† which is similar to the circuit employed for the working electrode of supercapacitor. The electrochemical impedance spectroscopic spectrum data can be fitted by a bulk system resistance R_s, a charge-transfer R_ct and a pseudo-capacitive element C_p from redox process of electrode materials, and a constant phase element (CPE) to account for the double-layer capacitance. Among these parameters, the charge-transfer resistance R_ct, also called Faraday resistance, is a limiting factor for the specific power of the supercapacitor. R_ct was calculated by ZSimpWin software and the calculated value is 1.524 Ω cm⁻². Thus, it is the lower Faraday resistance that results in the high specific energy of CoNi₂S₄ nanoparticle electrode.

Conclusions

In summary, CoNi₂S₄ nanoparticles were successfully synthesized by a one-step solvothermal method. The as-prepared CoNi₂S₄ nanoparticles have a uniform size of 8–15 mm and better crystallinity demonstrated by HRTEM. When applied as electrode materials for SCs, CoNi₂S₄ nanoparticles exhibited better electrochemical performances, i.e.: higher specific capacitance, better rate capability, and higher energy density. Compared with the reported NiCo₂S₄ nanostructures or other nanomaterials with single-phase, CoNi₂S₄ nanoparticles with lower production cost and better supercapacitive performances are a promising electrode candidate material of SCs in the future.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21201010), Key Research Projects of Science and Technology Department of Henan Province, P. R. China (092102210253) and Science and Technology Problem of Science and Technology Bureau of Anyang City, (Anke[2009]-34#-Industrial research-91).

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Footnote

† Electronic supplementary information (ESI) available: XRD patterns of the as-prepared samples with different sulfur sources; the last 20 cycles of 2000 charge–discharge runs at 4 A g⁻¹; Nyquist plots of experimental impedance data of the CoNi₂S₄ nanoparticle electrode; compared electrochemical performance between CoNi₂S₄ nanoparticles and other nanomaterials. See DOI: 10.1039/c3ra46805d

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