Design and Construction of Cu(OH)2/Ni3S2 Composite Electrode on Cu Foam by Two-Step Electrodeposition

A Cu(OH)2/Ni3S2 composite has been designed and in situ constructed on Cu foam substrate by facile two-step electrodeposition. Cu(OH)2 is achieved on Cu foam by galvanostatic electrodeposition, and the subsequent coating of Ni3S2 is realized by cyclic voltammetric (CV) electrodeposition. The introduction of Cu(OH)2 provides skeleton support and a large specific surface area for the Ni3S2 electrodeposition. Benefiting from the selection of different components and preparation technology, the Cu(OH)2/Ni3S2 composite exhibits enhanced electrochemical properties with a high specific capacitance of 4.85 F cm−2 at 2 mA cm−2 and long-term cyclic stability at 80.84% (4000 cycles).


Introduction
Recently, transition metal sulfides-especially nickel sulfide-such as NiS, Ni 3 S 2 , Ni 9 S 8 have attracted extensive attention as a promising cathode for supercapacitors [1][2][3]. Compared with their corresponding oxides and hydroxides, transition metal sulfides have better conductivity and electrochemical activity due to the lower electronegativity of sulfur [4,5]. However, these available nickel-sulfide-based electrode materials still have problems, such as low-rate capability and restricted electrochemical redox reaction kinetics at large current density. Many works have focused on constructing nickel sulfide-based composite to solve these problems, including hybridizing with high conductive agents such as carbon fiber and graphene, to increase the conductivity and compounding with other components to give full play to the synergy of each component [6][7][8]. In addition, researchers also designed the composite components from the perspective of regulating the material structure and growth process. For example, Xing et al. designed ZnO@Ni 3 S 2 array on Ni foam with a high specific capacitance of 1529 F g −1 at 2 A g −1 [9]. The enhanced capacitance is ascribed to backbone support of ZnO array, which provides a large specific surface area for subsequent growth of Ni 3 S 2 , and the uniform core/shell structure with good stability and facilitates the charges transport during the processes of charging and discharging. At the same time, ZnO was chosen because of its mature synthetic pathway, which facilitates the regulation of diverse structures and morphologies [10]. Taking a similar view, other ZnO-based materials, including ZnO@Co(OH) 2 [11], ZnO/Ni(OH) 2 [12], ZnO/CoS [13], and ZnO@CoFe 2 O 4 [14] have been explored. Although enhanced capacitance could be achieved through assistance with ZnO, ZnO has almost no capacitance characteristics, and it may partly dissolve in a long-term strong acidic or alkaline test environment. The most typical candidate with similar advantages to ZnO is Cu(OH) 2 . An especially prominent advantage of Cu(OH) 2 is that Cu foam can act as both electrode substrate and the only Cu source to achieve the in situ growth of Cu(OH) 2 , and the characteristic of this in situ growth is to avoid impurities, increase yield and reduce contact resistance. Moreover, Cu(OH) 2 can be calcined to obtain CuO, which can also be used as an active electrode component. In general, there are two ways to achieve the in situ growth of Cu(OH) 2 on Cu foam substrate. One is in situ oxidation treatment; that is, the Cu foam is oxidized by a mixture of (NH 4 ) 2 S 2 O 8 and NaOH aqueous solutions at room temperature [15,16]. The other is an electrodeposition technique, and the essence of the reaction is in situ electrochemical anodic oxidation [17,18]. The latter has the advantage of simple operation, a short experimental period and strong repeatability. In terms of the electrodeposition device, it usually adopts a two-electrode or three-electrode system. Compared with a three-electrode system, the two-electrode system has fewer influencing factors because no reference electrode is involved, and thus it is bound to have higher repeatability.
Based on the above analysis, herein, a two-electrode system was adopted to achieve in situ growth of Cu(OH) 2 nanostructures on Cu foam substrate followed by coating of Ni 3 S 2 through subsequent CV electrodeposition. Cu(OH) 2 , as an effective active component, participates in the electrochemical reaction, and provides a large specific surface area for the growth of Ni 3 S 2 . Compared with the single-component Ni 3 S 2 , the electrochemical properties of Cu(OH) 2 /Ni 3 S 2 composite are significantly improved.

Materials
Cu foam (70 PPI, 1.8 mm thick, Kunshan DESSCO Co., Ltd., Kunshan, China) was cut into 1 × 1.5 cm 2 slices, and then the slices were washed in dilute hydrochloric acid, acetone, ethanol and deionized water to remove the surface oxide layer. The chemical reagents used in the experiment included nickel chloride hexahydrate (NiCl 2 ·6H 2 O), thiourea (CH 4 N 2 S), KOH and NaOH.

Preparation of Cu(OH) 2 /Ni 3 S 2 Electrode
In the first step, a two-electrode setup was exploited to electrodeposit Cu(OH) 2 in 2 M KOH solution. Cu foam and Pt plate acted as the working and counter electrode, respectively. The electrochemical oxidation was carried out at a current density of 0.05 A cm −2 for 20 min.
In the second step, a three-electrode setup was adopted to achieve Ni 3 S 2 electrodeposition. The Cu(OH) 2 obtained in the previous step served as working electrode, and saturated calomel electrode (SCE) and Pt plate served as reference and counter electrode, respectively. The CV electrodeposition was carried out for 20 segments in the potential window of −1.245 to 0.156 V with a scan rate of 0.005 V s −1 in a mixture solution containing 0.48 g of NiCl 2 and 3.04 g of CH 4 N 2 S.

Characterization
XRD (Cu Kα radiation), FE-SEM (JSM-7610F) and XPS (ESCALAB 250Xi) were used to characterize the structure and morphology of the sample. CHI 760E Electrochemical workstation was used to complete the electrochemical tests, such as cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and cyclic stability. The test system consists of Cu(OH) 2 /Ni 3 S 2 composite, Hg/HgO and Pt plate, which served as working, reference and counter electrode, respectively. The electrolyte is 2 M NaOH aqueous solution. Figure 1 depicts the Cu(OH) 2 /Ni 3 S 2 composite constructed by two-step electrodeposition. Homogeneous and dense Cu(OH) 2 nanorods are firstly deposited on the Cu foam substrate by two-electrode galvanostatic deposition, and the Cu(OH) 2 /Ni 3 S 2 composite is eventually obtained by the continuous coating of Ni 3 S 2 on the above Cu(OH) 2 nanorods surface by CV electrodeposition in a three-electrode system.   Figure 2d gives a typical single Cu(OH) 2 nanorod, which clearly confirms the above conclusion.

Results
In order to study the growth process of Cu(OH) 2 on Cu foam substrate, the effects of current density and electrodeposition time on the morphology of Cu(OH) 2 were investigated in detail. The FE-SEM images of Cu(OH) 2 obtained at different current density ( Figure S2a-c) demonstrate bundles of Cu(OH) 2 nanorods are scattered on Cu substrate, leaving partially exposed Cu substrate visible when the current density is 0.01 A cm −2 . As the current density is increased to 0.025 A cm −2 , the number of Cu(OH) 2 nanorods increases significantly, and the Cu substrate is covered with dense and uniform Cu(OH) 2 nanorods ( Figure S2d-f). Further doubling of the current density leads to the generation of more dense Cu(OH) 2 nanorods, as shown in Figure 2b. However, as the current density continues to increase to 0.075 A cm −2 , the surface of the Cu substrate is damaged by the large current and obvious cracks are formed ( Figure S2g,f). The FE-SEM images of Cu(OH) 2 obtained with different electrodeposition time show only a small amount of Cu(OH) 2 nanorods with sharp tips emerging from the Cu substrate at the early stage of electrodeposition (1 min, Figure S3a,b). Upon increasing the electrodeposition time to 5 min, as seen in Figure S3c,d, the Cu substrate is completely covered with Cu(OH) 2 nanorods, and the nanorods continue generating when the electrodeposition reaches 20 min (Figure 2b). However, when the electrodeposition time is prolonged to 1 h ( Figure S3e,f), these Cu(OH) 2 nanorods collapse and crush each other as they grow longer. The Cu(OH) 2 /Ni 3 S 2 composite was further achieved after subsequent CV electrodeposition. The XRD pattern of the sample (Figure 3a) presents three types of diffraction peaks. Except that the peaks labeled with asterisks and dots are derived from the Cu foam and Cu(OH) 2 , respectively, the peaks labeled with squares are ascribed to Ni 3 S 2 obtained by (JCPDS card No. 44-1418). The corresponding FE-SEM image in Figure 3b demonstrates that the nanorod-shape of Cu(OH) 2 is unchanged, while the surface becomes rough due to the coverage and wrapping of Ni 3 S 2 . Besides, the diameter of these nanorods is increased to 240 ± 30 nm (Figure 3c). The enlarged image in Figure 3d confirms that these Ni 3 S 2 are bent nanoflakes with a thickness of ca. 50 nm.
The XPS spectra of Cu(OH) 2 /Ni 3 S 2 composite are presented in Figure 4, which are calibrated with C 1s at 284.8 eV (Figure 4b). The full spectrum in Figure 4a confirms that Cu, Ni, O, C and S are present, and no obvious impurity is detected. The Ni 2p spectrum in Figure 4c mainly contains two peaks at 856.50 and 874.26 eV, corresponding to Ni 2p 3/2 and Ni 2p 1/2 , respectively. Separation energy of 17.7 eV indicates the existence of Ni 2+ and Ni 3+ [19,20]. Both of them have a satellite peak at 861.70 and 880.00 eV. In regard to Cu 2p spectrum (Figure 4d), two main characteristic peaks located at 935.10 and 954.50 eV, with satellite peaks at 962.94, 944.80 and 941.90, correspond to Cu 2p 3/2 and Cu 2p 1/2 , respectively. The two separated small peaks at 952.56 and 932.82 eV are attributed to the exposed Cu substrate during the test [21]. The O 1s spectrum in Figure 4e can be divided into three peaks with binding energies at 530.90, 531.70 and 532.80 eV, corresponding to Cu(OH) 2 , OHand H 2 O molecule, respectively [22]. Figure 4f exhibits the S 2p spectrum, and the peaks centered at 162.30 and 161.40 eV belongs to S 2p 1/2 and S 2p 3/2 , respectively, which can be assigned to S 2-. In addition, the characteristic peak of S-O at 168.40 eV in the S 2p spectrum indicates the oxygen of the hydroxyl group is bonded to the S bond and adsorbed on the surface of the electrode material [23][24][25]. The hydroxyl group is derived from the hydrolysis of thiourea as follows:  CV and GCD measurements were carried out to explore the electrochemical performances of the Cu(OH) 2 /Ni 3 S 2 composite. Figure 5a records the CV curves of the four electrodes when the scan rate is set at 10 mV s −1 , including Cu(OH) 2 /Ni 3 S 2 , Ni 3 S 2 , Cu(OH) 2 electrodes and Cu foam substrate. Among them, Cu(OH) 2 /Ni 3 S 2 represents the largest enclosing area, and thus provides the maximum specific capacitance. Meanwhile, the CV curve of Cu(OH) 2 /Ni 3 S 2 shows a pair of typical redox peaks (0 to 0.7 V), which is attributed to Ni 2+ /Ni 3+ originating from the Faradaic reactions as Equation (3) [23]. For Cu(OH) 2 electrode, a pair of redox peak at about 0.46 and 0.32 V can be observed, corresponding to the electrochemical transformation between Cu 2+ and Cu + as described by Equation (4) [26,27]:  Figure 5b compares the GCD curves of the four electrodes mentioned above at 2 mA cm −2 . Obviously, the Cu(OH) 2 /Ni 3 S 2 reveals the longest discharge time, illustrating the largest specific capacitance [22]. The capacitance characteristics of Cu(OH) 2 and Cu foam are rather weak, while the electrochemical performance of Ni 3 S 2 is significantly improved by the addition of Cu(OH) 2 . In the absence of Cu(OH) 2 , Ni 3 S 2 appears as an irregular network structure scattered on the surface of the Cu foam ( Figure 6). Therefore, the Cu(OH) 2 nanorods function as skeletons and provide abundant active sites for the subsequent Ni 3 S 2 deposition. The Cu(OH) 2 /Ni 3 S 2 composite was further carried out via CV tests at various scan rates (2-40 mV s −1 ), as seen in Figure 7a. The pair of redox peaks exhibits the same variation trend and standard symmetry under the appropriate potential window. It is evident that, at all kinds of scan rates, the large scan rate is bound to a large enclosing area, while the specific capacitance decreases accordingly. This decrease is attributed to the fact that the participation of the internal electrochemical active sites is limited due to the restriction of ions/electrons diffusion at a relatively large scan rate [15]. Figure 7b describes the GCD measurements at different current densities of 2-20 mA cm −2 . The nonlinearity of the GCD curves reflects the obvious pseudocapacitance characteristic of the electrode material. The specific capacitance value of Cu(OH) 2 /Ni 3 S 2 electrode can be calculated using the equation in Supplementary Materials [28]. Figure 7c lists the specific capacitance up to 4.85, 4.48, 4.05, 3.81, 3.63 and 3.48 F cm −2 corresponding to discharge current densities of 2, 4, 8, 12, 16 and 20 mA cm −2 , respectively. In addition, the average RESR is calculated to be 0.90 Ω cm −2 [27], as shown in Figure 7d. Meanwhile, the CV curves, GCD curves and the corresponding line diagram of specific capacitance at different current densities of single Ni 2 S 3 and Cu(OH) 2 were also recorded for comparison in Figure 8.      Table 1 shows a comparison of the specific capacitance of the Cu(OH) 2 /Ni 3 S 2 composite in this work with those electrode materials containing Cu(OH) 2 or Ni 3 S 2 in the literature. It demonstrates our sample is comparable to or even better than the recently reported electrode materials. The enhanced electrochemical properties of the Cu(OH) 2 /Ni 3 S 2 composite can be attributed to both the composition of the material and the preparation technique. In terms of components, the selection of Cu(OH) 2 has three advantages: (1) Cu foam serves as both a current collector and the only Cu source to achieve the in situ growth of Cu(OH) 2 . The in situ growth strategy reduces the contact resistance and facilitates the transmission of ions/electrons. At the same time, the products can be directly used as a binder-free electrode for an electrochemical performance test [27]; (2) the generated rod-shaped Cu(OH) 2 provides skeleton support and large specific surface area for the subsequent Ni 3 S 2 electrodeposition [26]; (3) Cu(OH) 2 , also exhibits some pseudocapacitance characteristics as an effective active component [15]. In terms of preparation technology, the in situ electrodeposition technique is easy to manipulate and has a short experimental period. More importantly, the electrode to be measured can be obtained directly [17].

Conclusions
The Cu(OH) 2 /Ni 3 S 2 electrode has been designed and constructed on Cu foam substrate by two-step electrodeposition. The combination of the composition of the material and preparation technology advantages endows the Cu(OH) 2 /Ni 3 S 2 electrode superior electrochemical performance with a high specific capacitance of 4.85 F cm −2 at 2 mA cm −2 and long-term cyclic stability at 80.84% after 4000 cycles. This facile method provides an effective route to prepare other Cu(OH) 2 and CuO-based composite electrode materials.

Conflicts of Interest:
The authors declare no conflict of interest.