In Situ Construction of ZnO/Ni2S3 Composite on Ni Foam by Combing Potentiostatic Deposition with Cyclic Voltammetric Electrodeposition

The ZnO/Ni2S3 composite has been designed and in situ synthesized on Ni foam substrate by two steps of electrodeposition. ZnO was achieved on Ni foam by a traditional potentiostatic deposition, followed by cyclic voltammetric (CV) electrodeposition, to generate Ni2S3, where the introduction of ZnO provides abundant active sites for the subsequent Ni2S3 electrodeposition. The amount of deposit during CV electrodeposition can be adjusted by setting the number of sweep segment and scan rate, and the electrochemical characteristics of the products can be readily optimized. The synergistic effect between the ZnO as backbones and the deposited Ni2S3 as the shell enhances the electrochemical properties of the sample significantly, including a highly specific capacitance of 2.19 F cm−2 at 2 mA cm−2, good coulombic efficiency of 98%, and long-term cyclic stability at 82.35% (4000 cycles).


Introduction
As an important energy storage device, the supercapacitor has attracted more and more attention by virtue of the advantages of its fast charging speed, long cycle life, large current discharge capacity, high power density, and friendly environment [1]. As the most critical part of the supercapacitor, electrode materials are the primary factor to determine the performance of the device. Researchers concentrate their efforts on the design and regulation of the composition, structure and morphology of the electrode materials, so as to realize the optimization of the properties of the device [2,3].
There are two main ways to regulate the structure and morphology of electrode materials. The first is to exploit different preparation methods, such as hydrothermal reaction, electrodeposition technique, and in situ polymerization processes. It also includes the selection of different electrode substrates, such as Ni foam, Cu foam, and carbon cloth, to directly control the structure and morphology of the electrode materials [4]. Another way is to select an appropriate component to construct the composite structure [5], which can be divided into two cases. In one case, the components with capacitance characteristics constitute the composite materials, and this can generate a synergistic effect between the two active materials and the performance of materials can be improved thereby. In the other case, the component without capacitance characteristics acts as an effective additive ingredient to increase the conductivity, or as a backbone to provide sufficient surface area for the recombination of other components. For example, He et al. fabricated CuO@Ni-Fe-layered double hydroxide (LDH) nanorods arrays on Cu foam, by two-step in situ electrochemical processes. The CuO nanorods provide support for the subsequent electrodeposition of Ni-Fe LDH, and also participate in the electrochemical reaction as an active component, which reveals an improved specific capacitance of 2.682 F cm −2 , responding to the scan rates of 2 mV s −1 [6]. Yuksel et al. designed a Ag nanowire@Ni(OH) 2 coaxial nanocomposite electrode. The introduction of Ag improves the electrical conductivity of the material, and synergistically promotes ion and electron transport with Ni(OH) 2 , which reveals an enhanced specific capacitance of 1165.2 F g −1 at a current density of 3 A g −1 [7]. Both copper oxide and Ag have their own characteristics. For instance, copper has various valence states of oxides, such as Cu(OH) 2 , CuO, and Cu 2 O. Therefore, when copper oxides are used as electrode material, we should pay attention to analyze the valence state of copper, because their energy storage mechanisms are different. Although Ag can be used as an active component, to participate in the electrochemical reaction, the encapsulation by other components causes the decline in the electrical properties of Ag.
Based on the above analysis, ZnO has also been explored as a component of electrode materials, because of its convenient preparation and controllable morphology. Generally, researchers have prepared ZnO-based composite electrode materials by a two-step method, such as ZnO@Ni(OH) 2 [8], ZnO@MnO 2 [9], ZnO@Co(OH) 2 [10], and ZnO@CoFe 2 O 4 [11]. All of these achieved stable ZnO nanowires [8], nanorods [9], nanopets [10], or nanoplates [11], play the role of an active component of the composite electrode material, and provide large specific surface area for the following growth of other components. The typical preparation methods include the hydrothermal method, seed-layer growth, and chemical bath deposition [12,13]. On the other hand, Ni(OH) 2 is one of the most ideal electrode materials, with abundant storage and high theoretical specific capacitance [14]. Therefore, among ZnO-and Ni-based compounds, ZnO/Ni(OH) 2 is relatively more explored [15]. In order to expand the selection range of electrode materials, Ni-base sulfide has gradually attracted more and more attention. As a new electrode material, metal sulfide not only has a complex structure, outstanding physical properties, and large specific capacitance, but also has low electronegativity, which is beneficial for electron transport [16,17].
In this paper, the ZnO/Ni 2 S 3 composite was in situ grown on Ni foam substrate, by a two-step electrodeposition, including a traditional potentiostatic deposition and a new deposition strategy, cyclic voltammetric (CV) electrodeposition. Compared with the pure Ni 2 S 3 , the introduction of ZnO improves the electrical conductivity of the material and provides abundant active sites for the subsequent Ni 2 S 3 electrodeposition. As a binderfree electrode material for supercapacitor, the ZnO/Ni 2 S 3 composite exhibits enhanced storage properties.

Preparation of ZnO/Ni 2 S 3 Composite Electrode
In the first step, the mixed solution of 0.2 M Zn(NO 3 ) 2 and 4.8 M KCl was prepared and used as electrolyte to electrodeposit ZnO by exploiting a three-electrode system. Ni foam was used as working electrode, saturated calomel electrode (SCE) was used as reference electrode, and a Pt plate was used as counter electrode, respectively. Potentiostatic electrodeposition technique was adopted to perform 90 s at −10 V versus SCE.
The second step for Ni 2 S 3 electrodeposition was carried out with the same threeelectrode system, except the working electrode is Ni foam deposited by ZnO obtained in the previous step and the electrolyte is a mixture solution containing 0.05 M NiCl 2 and 1 M CH 4 N 2 S. The CV electrodeposition was performed by 10 cycles (abbreviated as 10-CV) in the range of −1.25 to 0.16 V at a scan rate of 0.005 V s −1 . The CV electrodeposition curves obtained from different cycles are labeled as 1-CV, 5-CV, 10-CV and 20-CV ( Figure 1). The corresponding products are to be tested after rinsing thoroughly.

Characterization
X-ray diffraction (XRD Cu Kα radiation with λ = 1.5406 Å), field-emission scannin electron microscopy FE-SEM (JSM-7610F, Tokyo, Japan), and X-ray photoelectron spec troscopy (XPS ESCALAB 250Xi, ThermoFisher Scientifific Co., Ltd, Waltham, MA, USA Al Kα as X-ray source) were used to characterize the product. A CHI 760E electrochemica workstation was used to conduct the electrochemical tests, including cyclic voltammetr (CV), galvanostatic charge-discharge (GCD) and cycling performance, at a three-electrod system in 2 M NaOH aqueous solution, in which the ZnO/Ni3S2 on Ni foam was served a working electrode, and a Pt plate and Hg/HgO electrode were served as the counter elec trode and reference electrode, respectively.

Results
The ZnO/Ni2S3 composite is synthesized on Ni foam substrate by a two-step electro deposition, and the construction process is illustrated in Figure 2. Firstly, potentiostati electrodeposition is adopted to deposit ZnO nanosheets on the surface of Ni foam sub strate. After that, CV electrodeposition is employed, by using a similar three-electrod system to achieve a Ni2S3 layer on the surface of ZnO, and the thickness of the Ni2S3 laye increases gradually with the increase in the number of cycles.

Characterization
X-ray diffraction (XRD Cu Kα radiation with λ = 1.5406 Å), field-emission scanning electron microscopy FE-SEM (JSM-7610F, Tokyo, Japan), and X-ray photoelectron spectroscopy (XPS ESCALAB 250Xi, ThermoFisher Scientifific Co., Ltd, Waltham, MA, USA Al Kα as X-ray source) were used to characterize the product. A CHI 760E electrochemical workstation was used to conduct the electrochemical tests, including cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and cycling performance, at a three-electrode system in 2 M NaOH aqueous solution, in which the ZnO/Ni 3 S 2 on Ni foam was served as working electrode, and a Pt plate and Hg/HgO electrode were served as the counter electrode and reference electrode, respectively.

Results
The ZnO/Ni 2 S 3 composite is synthesized on Ni foam substrate by a two-step electrodeposition, and the construction process is illustrated in Figure 2. Firstly, potentiostatic electrodeposition is adopted to deposit ZnO nanosheets on the surface of Ni foam substrate. After that, CV electrodeposition is employed, by using a similar three-electrode system to achieve a Ni 2 S 3 layer on the surface of ZnO, and the thickness of the Ni 2 S 3 layer increases gradually with the increase in the number of cycles.
The XRD pattern of the product that is electrodeposited on the Ni foam substrate by the potentiostatic deposition method is shown in Figure 3a. Ni foam shows three strong diffraction peaks, which are marked with an asterisk (     The ZnO/Ni 2 S 3 composite was further obtained through the subsequent CV electrodeposition, as shown in Figure 4a. The XRD pattern of the product exhibits three kinds of diffraction peaks. In addition to the peaks that are marked with asterisks and dots belong to the Ni foam substrate and ZnO, respectively, the peaks that are marked with diamonds belong to the Ni 2 S 3 obtained by CV electrodeposition (JCPDS card No. 44-1418). Figure 4b-d are the corresponding FE-SEM images, as seen from Figure 4b, ZnO nanosheets are completely covered and wrapped into ZnO/Ni 2 S 3 composite nanospheres. The enlarged image in Figure 4c confirms that these ZnO/Ni 2 S 3 composite nanospheres have rough surfaces, with approximately 1.4-1.8 µm in diameter. In fact, the rough surface is made up of tiny Ni 2 S 3 particles that have been electrodeposited layer by layer (Figure 4d).
Micromachines 2021, 12, x FOR PEER REVIEW 5 of 12 The ZnO/Ni2S3 composite was further obtained through the subsequent CV electrodeposition, as shown in Figure 4a. The XRD pattern of the product exhibits three kinds of diffraction peaks. In addition to the peaks that are marked with asterisks and dots belong to the Ni foam substrate and ZnO, respectively, the peaks that are marked with diamonds belong to the Ni2S3 obtained by CV electrodeposition (JCPDS card No. 44-1418). Figure  4b-d are the corresponding FE-SEM images, as seen from Figure 4b, ZnO nanosheets are completely covered and wrapped into ZnO/Ni2S3 composite nanospheres. The enlarged image in Figure 4c confirms that these ZnO/Ni2S3 composite nanospheres have rough surfaces, with approximately 1.4-1.8 µm in diameter. In fact, the rough surface is made up of tiny Ni2S3 particles that have been electrodeposited layer by layer (Figure 4d).   (Figure 5a), two peaks located at 1044.70 and 1021.40 eV belong to Zn 2p1/2 and Zn 2p3/2, respectively, confirming the existence of ZnO [10,18]. As for the Ni 2p spectrum in Figure 5b, two major peaks of the binding energies at 855.60 and 873.30 eV, with a spinenergy separation of 17.7 eV, correspond to Ni 2p3/2 and Ni 2p1/2, respectively, indicating the emergence of Ni 2+ and Ni 3+ [19][20][21]. Apart from that, both of them have a satellite peak at 861.20 and 879.80 eV [22]. In regard to the O 1s spectrum in Figure 5c, as previously reported, the binding energy of 531.20 eV is ascribed to the oxygen bond with Zn 2+ , and the binding energy of 529.90 eV is assigned to the oxygen of NiO, which was generated from the reduction in part of the Ni 3+ during electrodeposition [10,18]. The S 2p spectrum in Figure 5d can be divided into two peaks that are centered at 162.99 and 161.54 eV, corresponding to S 2p1/2 and S 2p3/2 of S 2− , respectively, which are in agreement with those of Ni2S3 in the reports [23,24].   (Figure 5a), two peaks located at 1044.70 and 1021.40 eV belong to Zn 2p 1/2 and Zn 2p 3/2 , respectively, confirming the existence of ZnO [10,18]. As for the Ni 2p spectrum in Figure 5b, two major peaks of the binding energies at 855.60 and 873.30 eV, with a spin-energy separation of 17.7 eV, correspond to Ni 2p 3/2 and Ni 2p 1/2 , respectively, indicating the emergence of Ni 2+ and Ni 3+ [19][20][21]. Apart from that, both of them have a satellite peak at 861.20 and 879.80 eV [22]. In regard to the O 1s spectrum in Figure 5c, as previously reported, the binding energy of 531.20 eV is ascribed to the oxygen bond with Zn 2+ , and the binding energy of 529.90 eV is assigned to the oxygen of NiO, which was generated from the reduction in part of the Ni 3+ during electrodeposition [10,18]. The S 2p spectrum in Figure 5d can be divided into two peaks that are centered at 162.99 and 161.54 eV, corresponding to S 2p 1/2 and S 2p 3/2 of S 2− , respectively, which are in agreement with those of Ni 2 S 3 in the reports [23,24].
In order to study the electrochemical properties of the ZnO/Ni 2 S 3 composite, CV and GCD measurements were carried out in a three-electrode system, in 2 M NaOH aqueous solution. As seen in Figure 6a, the CV curves for various electrodes are compared, at a scan rate of 20 mV s −1 , including ZnO/Ni 2 S 3 , Ni 2 S 3 , ZnO, and Ni foam substrate. The ZnO/Ni 2 S 3 composite has the largest enveloping area compared with the others, proving the maximum specific capacitance, while the ZnO and Ni foam have almost no capacitance characteristics. Further observation reveals that the CV curve of the ZnO/Ni 2 S 3 composite contains a pair of strong redox peaks in the potential range of 0-0.7 V, which is ascribed to Ni 2+ /Ni 3+ as the following electrochemical reaction [21,25]: In order to study the electrochemical properties of the ZnO/Ni2S3 composite, CV and GCD measurements were carried out in a three-electrode system, in 2 M NaOH aqueous solution. As seen in Figure 6a, the CV curves for various electrodes are compared, at a scan rate of 20 mV s −1 , including ZnO/Ni2S3, Ni2S3, ZnO, and Ni foam substrate. The ZnO/Ni2S3 composite has the largest enveloping area compared with the others, proving the maximum specific capacitance, while the ZnO and Ni foam have almost no capacitance characteristics. Further observation reveals that the CV curve of the ZnO/Ni2S3 composite contains a pair of strong redox peaks in the potential range of 0-0.7 V, which is ascribed to Ni 2+ /Ni 3+ as the following electrochemical reaction [21,25]: (1) Figure 6b depicts the GCD curves corresponding to the four electrodes at 2 mA cm −2 . Among them, the ZnO/Ni2S3 composite has the longest discharge time, and thus the largest specific capacitance. The test results are consistent with those of the CV measurement. Figure 6c compares the line diagrams of the specific capacitance of the three electrodes at different discharge current densities, revealing that the significant enhancement of the electrochemical properties of the ZnO/Ni2S3 composite is attributed to the introduction of ZnO, although ZnO has almost no capacitance characteristics.   Among them, the ZnO/Ni 2 S 3 composite has the longest discharge time, and thus the largest specific capacitance. The test results are consistent with those of the CV measurement. Figure 6c compares the line diagrams of the specific capacitance of the three electrodes at different discharge current densities, revealing that the significant enhancement of the electrochemical properties of the ZnO/Ni 2 S 3 composite is attributed to the introduction of ZnO, although ZnO has almost no capacitance characteristics.
ZnO is grown on Ni foam by potentiostatic electrodeposition, based on the following reaction [9]: In the absence of ZnO, Ni 2 S 3 nanoparticles are piled in disorder on the Ni foam substrate, and the surface is wrinkled (Figure 7a-c). In comparison, the pre-formed ZnO as backbones provides support and abundant active sites for the subsequent electrodeposition of Ni 2 S 3 . In the absence of ZnO, Ni2S3 nanoparticles are piled in disorder on the Ni foam substrate, and the surface is wrinkled (Figure 7a-c). In comparison, the pre-formed ZnO as backbones provides support and abundant active sites for the subsequent electrodeposition of Ni2S3.  In view of the role of Ni 2 S 3 , it is necessary to investigate the effects of different CV electrodeposition cycles of Ni 2 S 3 on the morphology and electrochemical properties of the ZnO/Ni 2 S 3 composite. The reaction of sulfidization of nickel salt can be described as follows [20]: When the CV electrodeposition was performed for one circle (1-CV), the ZnO nanosheets are covered with a thin layer of Ni 2 S 3 , but the original shape of the ZnO nanosheet is clearly visible (Figure 8a). Figure 8b presents the surface and edges of these ZnO nanosheets, which are roughened because of the coating of the Ni 2 S 3 nanoparticles. When the electrodeposition reaches 5-CV, as seen in Figure 8c,d, the morphology of these ZnO nanosheets is basically completely covered, and only the edges of these nanosheets, which become wider, due to the coverage of Ni 2 S 3 , can be vaguely observed. When the electrodeposition is extended to 10-CV, the nanospheres with a diameter of 1.4-1.8 µm are formed, because of the continuous deposition of Ni 2 S 3 , as shown in Figure 8e. The electrodeposition of up to 20-CV results in the disordered accumulation of Ni 2 S 3 particles.
When the electrodeposition reaches 5-CV, as seen in Figure 8c and d, the morphology of these ZnO nanosheets is basically completely covered, and only the edges of these nanosheets, which become wider, due to the coverage of Ni2S3, can be vaguely observed. When the electrodeposition is extended to 10-CV, the nanospheres with a diameter of 1.4-1.8 µm are formed, because of the continuous deposition of Ni2S3, as shown in Figure 8e. The electrodeposition of up to 20-CV results in the disordered accumulation of Ni2S3 particles.     Figure 9a-c display the CV curves, GCD curves, and the line diagrams of specific capacitance at different current densities. The test results confirm that the electrochemical performances of 1-CV to 20-CV are gradually enhanced with the increasing of the number of Ni 2 S 3 electrodeposition cycles. However, from the perspective of coulombic efficiency (Figure 9d), the efficiency of 1-CV and 10-CV is about 98% at a discharge current density of 2 mA cm −2 ; They are relatively high compared with the others. Furthermore, the line diagram of the average R ESR in Figure 9e, which is deduced from the voltage drops during the GCD tests, demonstrates that the R ESR of 10-CV is the lowest. From the above test data results, two trends can be inferred, with the increase in the number of electrodeposition cycles of Ni 2 S 3 . On one hand, the amount of Ni 2 S 3 that is electrodeposited inevitably increases, which reflects the enhanced CV and GCD characteristics. On the other hand, the skeleton supporting effect of ZnO is weakened, or even disappeared. Meanwhile, the excessive deposition leads to the disordered accumulation of particles. Therefore, we choose 10-CV as the electrodeposition cycles of Ni 2 S 3 for further in-depth study.
From the above test data results, two trends can be inferred, with the increase in the number of electrodeposition cycles of Ni2S3. On one hand, the amount of Ni2S3 that is electrodeposited inevitably increases, which reflects the enhanced CV and GCD characteristics. On the other hand, the skeleton supporting effect of ZnO is weakened, or even disappeared. Meanwhile, the excessive deposition leads to the disordered accumulation of particles. Therefore, we choose 10-CV as the electrodeposition cycles of Ni2S3 for further indepth study. For the ZnO/Ni2S3 composite (10-CV), further CV measurements were performed, at different scan rates of 2-50 mV s −1 in the same potential window, as shown in Figure 10a. The redox peaks are symmetrical, and the variation trend of the CV curves at different scan rates is basically the same. In addition, with the increase in the scan rate, the area For the ZnO/Ni 2 S 3 composite (10-CV), further CV measurements were performed, at different scan rates of 2-50 mV s −1 in the same potential window, as shown in Figure 10a. The redox peaks are symmetrical, and the variation trend of the CV curves at different scan rates is basically the same. In addition, with the increase in the scan rate, the area surrounded by the CV curves gradually increases, while the specific capacitance gradually decreases. This decrease is attributed to the fact that some active sites cannot fully participate in the redox reaction, due to the restriction of ion/electron diffusion at a larger scan rate [7]. Figure 10b reveals the GCD measurements at various current densities of 2-20 mA cm −2 , in a potential range of 0-0.55 V. The GCD curves show an obvious pseudocapacitance characteristic because of their nonlinearity. The coulombic efficiency is as high as 98% at 2 mA cm −2 , and detailed data have been recorded in Figure 9d. In addition, the specific capacitance of the ZnO/Ni 2 S 3 electrode can be calculated by the formula in the Supporting Information [26]. participate in the redox reaction, due to the restriction of ion/electron diffusion at a larger scan rate [7]. Figure 10b reveals the GCD measurements at various current densities of 2-20 mA cm −2 , in a potential range of 0-0.55 V. The GCD curves show an obvious pseudocapacitance characteristic because of their nonlinearity. The coulombic efficiency is as high as 98% at 2 mA cm −2 , and detailed data have been recorded in Figure 9d. In addition, the specific capacitance of the ZnO/Ni2S3 electrode can be calculated by the formula in the Supporting Information [26]. As seen from Figure 10c, (6), the average RESR is 1.11 Ω cm −2 , as shown in Figure 10d. In addition, the CV curves, and GCD curves, and the corresponding line diagram of the specific capacitance at different current densities of Ni2S3 and ZnO, were also carried out for comparison in Figure S1.
The cyclic stability of the ZnO/Ni2S3 composite electrode is revealed in Figure 11. The specific capacitance is kept at the original value of 88.10% after 1000 cycles, and stabilizes the value at 82.35% within the range of 4000 cycles. After the cycle test, the general structure of the ZnO/Ni2S3 composite remains almost unchanged ( Figure S2). As seen from Figure 10c, (6), the average R ESR is 1.11 Ω cm −2 , as shown in Figure 10d. In addition, the CV curves, and GCD curves, and the corresponding line diagram of the specific capacitance at different current densities of Ni 2 S 3 and ZnO, were also carried out for comparison in Figure S1.
The cyclic stability of the ZnO/Ni 2 S 3 composite electrode is revealed in Figure 11. The specific capacitance is kept at the original value of 88.10% after 1000 cycles, and stabilizes the value at 82.35% within the range of 4000 cycles. After the cycle test, the general structure of the ZnO/Ni 2 S 3 composite remains almost unchanged ( Figure S2). The in situ two-step electrodeposition of the ZnO/Ni2S3 composite presents two aspect distinct advantages. On one hand, from the perspective of the preparation method, compared with the hydrothermal method and seed-layer growth method, electrodeposition deposition technology has the advantages of a convenient operation process, shorter experiment period, and good repeatability. In particular, this CV electrodeposition The in situ two-step electrodeposition of the ZnO/Ni 2 S 3 composite presents two aspect distinct advantages. On one hand, from the perspective of the preparation method, compared with the hydrothermal method and seed-layer growth method, electrodeposition deposition technology has the advantages of a convenient operation process, shorter experiment period, and good repeatability. In particular, this CV electrodeposition method, compared with the traditional constant-voltage deposition and constant-current deposition, has more adjustable parameters and a more intuitive deposition curve [27]. At the same time, the in situ electrodeposition strategy decreases the contact resistance and provides an unobstructed path for the transmission of ions/electrons [10]. On the other hand, from the point of view of composition selection for electrode materials, ZnO has the characteristics of a controllable and diverse structure and morphology, and friendly environment [15]. The introduction of ZnO provides support for the subsequent Ni 2 S 3 electrodeposition. In addition, Ni 2 S 3 has more research space compared with the traditional Ni(OH) 2 , NiOand Ni-based double hydroxide [19,21,23]. Therefore, the facile method can be promoted to realize the efficient regulation of other metal oxide-based composite and binary metal sulfides electrode materials.

Conclusions
The ZnO/Ni 2 S 3 composite has been constructed and grown on Ni foam by combing potentiostatic electrodeposition and CV electrodeposition. Benefiting from the twostep in situ electrodeposition strategy, the ZnO/Ni 2 S 3 electrode illustrated a highly specific capacitance value of 2.19 F cm −2 at 2 mA cm −2 , and long-term cyclic stability at 82.35% (4000 cycles). This convenient method can be expanded for the preparation of other ZnO-based composite electrode materials, and binary or ternary metal sulfide electrode materials.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/mi12070829/s1, Figure S1: CV curves, GCD curves and line diagram of specific capacitance at different current densities of Ni 3 S 2 (a,c,e) and ZnO (b,d,f). Figure S2: FE-SEM images of the ZnO/Ni 3 S 2 composite at different magnifications after electrochemical test. Equations for calculating specific capacitance.