Hybrid structured CoNi2S4/Ni3S2 nanowires with multifunctional performance for hybrid capacitor electrodes and overall water splitting

Rational design of electrode materials plays a significant role in potential applications such as energy storage and conversion. In this work, CoNi2S4/Ni3S2 nanowires grown on Ni foam were synthesized through a facile hydrothermal approach, revealing a large capacitance of 997.2 F g−1 and cycling stability with 80.3% capacitance retention after 5000 cycles. The device was prepared using CoNi2S4/Ni3S2//AC as the positive electrode and active carbon as the negative electrode, and delivered an energy density of 0.4 mW h cm−3 at a power density of 3.99 mW cm−3 and an excellent cycle life with 79.2% capacitance retention after 10 000 cycles. In addition, the hybrid CoNi2S4/Ni3S2 nanowires demonstrate excellent OER performance with low overpotential of 360 mV at 30 mA cm−2 and overpotential of 173.8 mV at −10 mA cm−2 for the HER, a cell voltage of 1.43 V, and excellent cycle stability.


Introduction
In recent years, various transition metal-based catalysts such as phosphides, oxides, nitrides, carbides, and suldes have been widely investigated as excellent electrochemical catalysts but the improvement of bi-functional electrocatalysts with excellent HER and OER activity is still difficult and slow to implement. [1][2][3][4] To date, as demonstrated by experiments, electrocatalysts with vertically well-aligned arrays on 1D nanostructures consistently lead to improved HER and OER activity and promise efficient overall water splitting. 5,6 Nanostructures possess distinctive advantages, including easier electrolyte diffusion and ionic transportation, better electrical connection with substrates, and offer larger surface area to provide contact between the electrode and electrolyte. 7 Special nanostructures possess interconnected networks; these hierarchical heterostructures allow for efficient electron/ion transportation and accommodate volume variation nicely. Meanwhile, heterostructures consist of highly conductive materials as the "substratum" and transition metal oxides as the "superstratum"; thus, the combination of the two types of active materials create heterostructures that inherit advantages from both the "substratum" and "superstratum". [8][9][10] As highlighted in recent reports, Ni(OH) 2 /Ni 3 S 2 nanostructure arrays on Ni foam show excellent electrochemical activity with a low overpotential of 270 mV (OER) at 20 mA cm À2 in 1.0 M KOH but exhibited a low HER activity (211 mV at 10 mA cm À2 ). 11 Zhao et al. prepared nitrogen-doped carbon nanomaterials as efficient oxygen evolution electrocatalysts, which exhibited an overpotential of 380 mV at a current density of 10 mA cm À2 in alkaline media. 12 An effective design of catalysts integrating HER and OER activity materials is the key for overall water splitting. 13 Recently, benets from cobalt include better redox activity under mild reaction conditions, modest overpotential, and the extremely high theoretical rich polymorphism of molybdenum, Co-based electrode materials have demonstrated vastly improved electrochemical performance. 14,15 Among various electrode materials, spinel-structured CoNi 2 O 4 has been investigated due to the high theoretical capacitance, high electrochemical performance and excellent electrocatalytic performance. 16,17 For example, Tong et al. reported CoNi 2 O 4 nanoneedles through a simple hydrothermal method. The as-prepared samples exhibited a specic capacitance of 1192 mF cm À2 at 2 mA cm À2 and cycle stability. 18 Liu and co-workers prepared NiCo 2 O 4 nanowires through a simple hydrothermal process; the as-prepared samples showed excellent electrocatalytic performance with an overpotential of 320 mV at 10 mA cm À2 . 19 However, single metal oxide electrode materials oen present poor electrical conductivity, which limits the electrochemical performance. To improve the electrical conductivity, constructing a hybrid structure with metal suldes has been considered an efficient method due to the improved electrical conductivity (about 100 times) over corresponding single metal oxides and binary materials. 20 In the literature, Li et al. fabricated NiCo 2 S 4 @Co(OH) 2 core-shell nanotube arrays, which showed an area capacitance of 9.6 F cm À2 at 2 mA cm À2 . 21 Jiang et al. prepared NiCo 2 S 4 @NiFe LDH heterostructures through a simple method, and the as-prepared electrocatalysts showed an overpotential of 201 mV and excellent cycle stability. 22 Herein, we successfully synthesize CoNi 2 O 4 /Ni 3 S 2 nanowires on Ni foam through a facile hydrothermal route. The asfabricated samples deliver a high specic capacitance of 997.2 F g À1 at 1 A g À1 and cycle stability. In addition, the device exhibits an energy density of 0.4 mW h cm À3 at a power density of 3.99 mW cm À3 and an excellent cycle life with 79.2% capacitance retention aer 10 000 cycles. As electrocatalysts, the hybrid CoNi 2 S 4 /Ni 3 S 2 nanowires show excellent OER performance with a low overpotential of 360 mV and a small Tafel slope of 69.7 mV dec À1 , and overpotential of 173.8 mV for the HER and Tafel slope of 98.8 mV dec À1 and excellent cycle stability.

Material preparation
Firstly, Ni foam was pre-treated via immersion in 0.1 M HCl, absolute ethanol and deionized water, respectively. Then, 1 mM Ni(NO 3 ) 2 $6H 2 O, 2 mM Co(NO 3 ) 2 $6H 2 O, 4 mM NH 4 F and 10 mM urea were dissolved in 60 mL deionized water. Then, the solution was transferred into an 80 mL Teon-lined autoclave and kept at 120 C for 8 h. Then, the samples were washed several times. Finally, the CoNi 2 O 4 nanowires were obtained aer calcination at 350 C for 2 h (the average mass loading was 1.8 mg cm À2 ). Secondly, 0.35 g Na 2 S was dissolved into 40 mL deionized water and kept at 120 C for 4 h. Aer cooling down to room temperature, the as-fabricated products were washed and dried at 60 C. The average mass loading was 3.2 mg cm À2 .

Materials characterization
The crystallographic structure of the as-fabricated samples was measured through X-ray diffraction. The morphology and structure of the samples were characterized by using scanning electron microscopy (SEM, Hitachi-4800) and high resolution transmission electron microscopy (HRTEM, JEM-2100 PLUS). Xray photoelectron spectroscopy (XPS) measurements were conducted to investigate the element composition using an ESCA-LAB250 with Al Ka sources.

Electrochemical measurement
Electrochemical measurements of the as-prepared products including cyclic voltammetry (CV) curves, galvanostatic chargedischarge (GCD) and electrochemical impedance spectroscopy (EIS) were carried out by a CHI660e electrochemical workstation in a three-electrode system in 3.0 M KOH solutions. The asfabricated samples (d ¼ 1 cm) were used as the working electrode, Pt foil served as the counter electrode and Hg/HgO as the reference electrode. Electrochemical impedance spectroscopy (EIS) measurements were conducted in a frequency range from 0.01 to 100 kHz at a 5 mV open circuit potential. The specic capacitance of the as-prepared samples was calculated from GCD curves via the following equations; where C s , I, Dt, V and m are the specic capacitance (F g À1 ), current density (A), discharge time (s), voltage (V) and mass loading (mg), respectively.

Fabrication of the hybrid battery
A hybrid battery was assembled with active carbon as the negative electrode, the as-fabricated CoNi 2 S 4 /Ni 3 S 2 nanowires as the positive electrode and PVA-KOH gel as the electrolyte. PVA-KOH gel as the electrolyte was prepared as follows: 3 g PVA was added in 25 mL deionized water at 65 C. Then, 3 g KOH was dissolved in 5 mL deionized water and added to the above solution.

Electrocatalytic performance of the hybrid battery
OER and HER performance tests of the as-prepared device were conducted in the three-electrode system. The as-fabricated samples were the working electrodes, and Ag/AgCl was the reference electrode. A graphite rod was the counter electrode.
Overall water splitting was measured in a two-electrode system. All potentials were converted to reversible hydrogen electrode (RHE) though the Nernst equation  211) and (300) crystal planes of Ni 3 S 2 (JCPDS no. 44-1418), respectively, which can be attributed to the vulcanization of the Ni foam. XPS analyses were used to further investigate the chemical composition and valence. Fig. 2b shows the XPS full survey spectrum of the CoNi 2 S 4 //Ni 3 S 2 nanowires, and the products consisted of Ni, Co and S. The Co 2p spectrum (Fig. 2c) contains two spin-orbit doublets and two shakeup satellites, the binding energy at 779.2 and 795.1 eV could be ascribed to Co 3+ , and the binding energies at 780.9 and 795.3 eV belong to the Co 2+ phase. 23 In addition, the satellite peaks reveal that Co 3+ is present. 24 Fig. 2d shows the Ni 2p spectrum, which can be divided into two spin-orbit doublets and two shakeup satellites. The binding energies at 854.8 and 871.6 eV can be assigned to Ni 3+ and 854.3 and 871.2 eV are Ni 2+ . 25 The sharp intense satellite peaks indicate that Ni is present as Ni 2+ . 26 The S 2p spectrum is presented in Fig. 2e. The peak at 161.2 eV is S 2p 1/2 and the peak at 162.3 eV belongs to a typical metal-sulfur bond, which improves the electrochemical activity of the as-prepared samples. 27,28 Energy-dispersive X-ray spectrometry (EDX) measurements of the as-prepared CoNi 2 S 4 / Ni 3 S 2 nanowires indicate that the as-fabricated products are composed of Ni, Co and S, as shown in Fig. 2f, and the three elements are uniformly distributed on the surface of the asprepared samples, revealing that CoNi 2 S 4 /Ni 3 S 2 nanowires are uniformed deposited on the surface of Ni foam.

Results and discussion
The morphology characteristics of the as-synthesized CoNi 2 S 4 /Ni 3 S 2 nanowires were rst characterized through SEM. From the low magnication SEM images in Fig. 3a, it is clearly found that CoNi 2 S 4 /Ni 3 S 2 nanowires are uniformly located on the surface of Ni foam. From Fig. 3b, it can be found that adjacent nanowires are connected to each other to form a network structure. Fig. 3c presents high magnication SEM images, indicating that the as-prepared CoNi 2 S 4 /Ni 3 S 2 nanowires possess an average diameter of 75 nm. TEM measurements were used to further characterize the microstructures of CoNi 2 S 4 /Ni 3 S 2 nanowires. Fig. 3d exhibits the low-magnication TEM images of the as-fabricated samples, in which a single nanowire is made of some tiny nanoparticles and the samples show the average diameter of 70 nm. The HRTEM image shows that the lattice spacing of 0.321 nm matches well with the (220) plane of Ni 3 S 2 phase, and the lattice spacing of 0.283 nm can be indexed to the (311) plane of CoNi 2 S 4 spinel structure. As can be seen from Fig. 3e, it can further conrmed that the as-prepared samples contain CoNi 2 S 4 and Ni 3 S 2 phases, which is consistent with the XRD patterns. The SAED pattern illustrated in Fig. 3f, shows that the as-fabricated samples present polycrystalline features that possess more active sites and energy barriers for adsorbing water molecules.
To investigate the electrochemical performances of the asprepared samples, Fig. 4a exhibits the CV curves of CoNi 2 O 4 samples at scan rates from 5 to 50 mV s À1 . It is seen that CV curves show a pair of redox peaks, demonstrating that the products possess typical pseudocapacitive characteristics. Fig. 4b shows the GCD curves of the CoNi 2 S 4 /Ni 3 S 2 samples, and it is seen that the  samples possess a specic capacitance of 997.2 F g À1 at a current density of 1 A g À1 . CV curves of the hybrid CoNi 2 S 4 /Ni 3 S 2 samples are depicted in Fig. 4c, and it is observed that the integrated areas increase as the scan sweep increases, indicating that the asfabricated samples possess a fast ion and electron transfer rate. Based on previous reports, Ni foam contributes little to the   Fig. 4d shows the GCD curves of the as-prepared electrode at different current densities from 1 A g À1 to 10 A g À1 . It is found that the as-fabricated samples exhibit a high specic capacitance of 997.2 F g À1 at 1 A g À1 . To further analyze the electrochemical kinetics of the as-prepared samples, the capacitance can be calculated according to following equation: where a and b are constants. The b value of 0.5 and 1.0 represent capacitive behavior and battery behavior, respectively. The b value (Fig. 4e) for reduction peaks of CoNi 2 O 4 and CoNi 2 S 4 / Ni 3 S 2 electrodes is 0.64 and 0.56, respectively. In addition, the capacitive contribution ratio can be obtained through the following equation: 30 where i, n, k 1 and k 2 represent the current, scan rates and the constant, respectively. Fig. 4f shows the calculated pseudocapacitance and diffusion-controlled capacitance at scan rates from 5 to 50 mV s À1 . It is seen that the contribution of diffusioncontrolled capacity gradually decreases with increasing scan rate, indicating that the samples show fast transmission for OH À . At the same time, the pseudocapacitance arrives at the highest value of 74.8% at 50 mV s À1 . EIS is used to further investigate the electrochemical performance, as shown in Fig. 4g. It is obvious that the intercept at the real axis shows the R s value of CoNi 2 S 4 /Ni 3 S 2 nanowires of 0.61 U. The underlying mechanism of differences in the capacitance performance of different samples was explored in the corresponding Nyquist curves, as shown in the inset of Fig. 4g. The CoNi 2 S 4 /Ni 3 S 2 electrode showed lower contact resistance and charge transfer resistance than the CoNi 2 O 4 nanowires, further conrming that the hybrid structure possesses excellent electrochemical performance. Simultaneously, in order to study the reaction kinetics in the low frequency region, the spectrum can be t using the following equation: where s w is the Warburg factor and u is angular frequency. Z can be attributed to the diffusive resistance of OH À . The values of CoNi 2 O 4 and CoNi 2 S 4 /Ni 3 S 2 electrodes are 16.54 and 9.49, respectively, indicating that CoNi 2 S 4 /Ni 3 S 2 samples possess a fast transmission for OH À (Fig. 4g). The cycling stability of the as-fabricated samples is measured by GCD measurements, as illustrated in Fig. 4i, revealing that the as-prepared CoNi 2 S 4 / Ni 3 S 2 sample shows more excellent stability with 80.3% capacitance retention than CoNi 2 O 4 samples (64.2%) aer 5000 cycles.
To further investigate the practical application of the asfabricated electrode, the device is assembled with the CoNi 2 S 4 /Ni 3 S 2 nanowires electrode as the positive electrode and active carbon as the negative electrode. The composition of CV curves of the CoNi 2 S 4 /Ni 3 S 2 nanowires and active carbon at a current density of 50 mV s À1 are depicted in Fig. 5a. It is found that the electrode materials exhibit single potential windows of À1.0-0 V and 0-0.6 V, respectively, indicating that the device can reach a total potential of 1.6 V. Fig. 5b exhibits the CV curves of the fabricated CoNi 2 S 4 /Ni 3 S 2 nanowires//AC device at scan rates from 2 to 50 mV s À1 . It can be seen that the device can work in a voltage window of 0-1.5 V. With increasing scan rates, the CV shapes are maintained, indicating that the capacitors possess excellent rate performance. CV curves of the CoNi 2 S 4 / Ni 3 S 2 nanowires//AC device with different potential windows at a scan rate of 50 mV s À1 are collected to investigate the extended working voltage, as illustrated in Fig. 5c, and the device shows a stable capacitive behavior with electric double-layer capacitive properties even at a voltage of up to 1.5 V. Fig. 5d shows the GCD measurement of the device, which showed that the areal capacitance of the CoNi 2 S 4 /Ni 3 S 2 nanowires//AC device reaches 136.05 mF cm À2 at a current density of 2 mA cm À2 . As the current density increased to 10 mA cm À2 , the areal capacitance was maintained at 56 mF cm À2 . The Ragone plots of the asymmetric supercapacitors are shown in Fig. 5e. The CoNi 2 S 4 / Ni 3 S 2 nanowires//AC supercapacitors deliver a maximum energy density of 0.4 mW h cm À3 at a power density of 3.99 mW cm À3 , which is comparable to previously reported ASC devices. [31][32][33][34] The long-term cycling stability of the CoNi 2 S 4 /Ni 3 S 2 nanowires//AC device is also evaluated by repeating charge-discharge measurements, as shown in Fig. 5f. Aer 10 000 cycles, there is still 90% retention of the original capacitance at a current density of 5 mA cm À2 .
The as-fabricated samples possess excellent cycle stability, which can be ascribed to the following aspects, as shown in Fig. 6. First, the as-fabricated CoNi 2 S 4 /Ni 3 S 2 nanowires are directly on Ni foam, which provides a lot of active sites. Secondly, the hybrid CoNi 2 S 4 /Ni 3 S 2 structure alleviates the volume change during the electrochemical reaction process. Finally, the adjacent nanowires possess enough reaction space, which leads to a rapid electrochemical reaction.
The electrocatalytic performance of the as-prepared catalysts is estimated through LSV and Tafel curves in 1.0 M KOH solution. As shown in Fig. 7a, the hybrid CoNi 2 S 4 /Ni 3 S 2 nanowires show a lower overpotential of 360 mV compared to that of CoNi 2 O 4 nanowires (380 mV), Ni 3 S 2 samples (410) and Ni foam (459 mV) at a current density of 30 mA cm À2 , which demonstrates that CoNi 2 S 4 /Ni 3 S 2 nanowires possess better electrocatalytic performance. To give insight into the OER kinetic mechanism, Fig. 7b presents the Tafel slope of the catalyst. Hybrid CoNi 2 S 4 /Ni 3 S 2 nanowires exhibit a small Tafel slope of 69.7 mV dec À1 , which is lower than that of the CoNi 2 O 4 nanowires (71.5 mV dec À1 ), Ni 3 S 2 samples (83.2 mV dec À1 ) and Ni foam (135.3 mV dec À1 ). The low Tafel slope increases the charge transfer rate and provides active sites for OH adsorption. 35 To further study the outstanding OER activity, the electrochemical active surface area (ECSA) was measured through the double-  This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 33428-33435 | 33433 layer capacitance (C dl ). The value of the electrochemical double layer capacitance is shown in Fig. 7c. CoNi 2 S 4 /Ni 3 S 2 nanowires exhibit a higher ECSA (0.039 mF cm À2 ) than CoNi 2 O 4 nanowires (0.0168 mF cm À2 ) and Ni 3 S 2 samples (0.0177 mF cm À2 ). The HER performance of the as-prepared samples is also tested at 5 mV s À1 . The comparison of the LSV curves of catalysts is depicted in Fig. 7d. It is clearly found that hybrid CoNi 2 S 4 /Ni 3 S 2 nanowires exhibit a small Tafel slope of 173.8 mV at À10 mA cm À2 , which is lower than that of CoNi 2 O 4 nanowires (192.6 mV) and Ni 3 S 2 samples (210.8 mV). Fig. 7e shows the corresponding Tafel plots, which shows that hybrid CoNi 2 S 4 /Ni 3 S 2 nanowires exhibit a small Tafel slope of 98.8 mV dec À1 , which is lower than the CoNi 2 O 4 nanowires (117.2 mV dec À1 ) and Ni 3 S 2 samples (148.9 mV dec À1 ). The Tafel slope of CoNi 2 S 4 /Ni 3 S 2 nanowires is smallest, revealing the fast reaction kinetics. Since the Tafel slope is in the range from 40 to 120 mV dec À1 , this implies that the HER reaction of CoNi 2 S 4 /Ni 3 S 2 nanowires obey the Volmer-Heyrovsky mechanism, and the Volmer step is ratedetermining. 36 The long-term stability measurements of the three electrode materials are conducted at a constant potential for 13 h, as shown in Fig. 7f. The results demonstrate that the as-fabricated catalysts present excellent cycle stability.
To further investigate the potential practical applications, overall water splitting tests are conducted through a two-electrode system with the hybrid structured catalysts as both the cathode and anode. Fig. 8a presents the LSV curve of overall water splitting with a scan rate of 5 mV s À1 . It is seen that hybrid CoNi 2 S 4 /Ni 3 S 2 samples show a small cell voltage of 1.43 V at 30 mA cm À2 , which is lower than that of CoNi 2 S 4 nanowires (1.51 V) and Ni 3 S 2 samples (1.65 V). In order to evaluate the stability of the as-prepared catalysts, chronoamperometry measurements were conducted, as depicted in Fig. 8b. It was found that the hybrid structured CoNi 2 S 4 /Ni 3 S 2 samples possess excellent long durability. At the same time, a large number of bubbles were formed from both the cathode and anode as the electrocatalytic test proceeded.

Conclusion
In summary, hybrid structured CoNi 2 S 4 /Ni 3 S 2 nanowires were fabricated through a facile two-step hydrothermal method. The as-prepared products directly act as a supercapacitor electrode with a specic capacitance of 997.2 F g À1 at 1 A g À1 and cycle stability of 80.3% retention aer 5000 cycles. The asymmetric supercapacitor shows a high energy density of 0.4 mW h cm À3 at a power density of 3.99 mW cm À3 and cycling stability of 79.2% retention aer 10 000 cycles. The as-fabricated CoNi 2 S 4 /Ni 3 S 2 nanowires also exhibit excellent electrocatalytic performance with a low overpotential of 360 mV for the OER and a small Tafel slope of 98.8 mV dec À1 and a cell voltage of 1.43 V.

Conflicts of interest
The authors declare no conict of interest.