NiCo2S4 core-shell composites as efficient bifunctional electrocatalyst electrodes for overall water splitting

Electrochimica Acta 326 (2019) 135002

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3D CuCo2S4/NiCo2S4 core-shell composites as efficient bifunctional electrocatalyst electrodes for overall water splitting Li Ma a, Jiwei Liang a, Tian Chen a, Yongjie Liu a, Songzhan Li b, Guojia Fang a, * a Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education of China, School of Physics and Technology, Wuhan University, Wuhan, 430072, PR China b School of Electronic and Electrical Engineering, Wuhan Textile University, Wuhan, 430073, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 June 2019 Received in revised form 3 September 2019 Accepted 2 October 2019 Available online 3 October 2019

Water splitting is a vital reaction for the storage of renewable energy, which requires highly active and steady catalysts to produce hydrogen and oxygen. While the development of efficiently stable bifunctional electrocatalysts for the simultaneous production of hydrogen and oxygen remains challenge. Herein, we show a simple multi-step process to prepare porous CuCo2S4/NiCo2S4 core-shell materials directly as bifunctional electrocatalyst electrodes for water splitting. The porous NiCo2S4-sheets combined with CuCo2S4-rods on nickel foam not only can provide for more active sites and higher specific surface area, but also maintain their structural integrity. At a current density of 10 mA cm
2, the CuCo2S4/ NiCo2S4 core-shell electrodes deliver lower overpotentials of 271 mV and 206 mV for the oxygen and hydrogen evolution reaction, respectively. The CuCo2S4/NiCo2S4 core-shell electrodes are successfully used as bifunctional electrocatalyst towards overall water splitting and showed overpotential of 1.66 V at the current density of 10 mA cm
2. A z97% current density retention for 50 h is represented which illustrates the superior stability of this electrode material and water splitting device. Subsequently, a water splitting process is demonstrated using electrochemical cell based on CuCo2S4/NiCo2S4 core-shell electrodes driven by a perovskite solar cell module. H2 and O2 are generated. Our work here demonstrates the possibility of wide application of catalytic material to produce clean energy driven by perovskite solar cell. The work also illustrates the potential applications of CuCo2S4/NiCo2S4 core-shell nanometer materials for clean energy generation. © 2019 Elsevier Ltd. All rights reserved.

Keywords: CuCo2S4/NiCo2S4 core-shell Composite electrodes Water splitting Hydrogen evolution reaction Oxygen evolution reaction

1. Introduction The rapid growth of energy consumption and environmental problems associated with the burning of depleted fossil fuels are driving the sustainable development of environment-friendly alternative energy [1e3]. Due to the advantages of high capacity density, eminent energy conversion efficiency and environment protection, hydrogen molecule can be an ideal energy source to overcome the energy shortage in the future [4,5]. At present, electrochemical water splitting has attracted much attention, and it has gradually become a mature industrial technology to produce zero carbon clean hydrogen energy. Commercial Pt [6] or Ir/Ru [7e9] based materials exhibit the most desired catalytic performance for hydrogen evolution reaction (HER) [10,11] or oxygen

* Corresponding author. E-mail address: [email protected] (G. Fang). https://doi.org/10.1016/j.electacta.2019.135002 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

evolution reaction (OER) [12,13], but they are expensive and rare on the earth which hinder their wide adhibition. Hence, a series of transition metal-based oxides [14], phosphides [15e19] and chalcogenides [20e23] have been developed as available replacement of noble metal to serve as OER or HER materials. Therefore, designing single function catalysts require disparate materials and preparation processes, which are time-wasting and very complex process. At the same time, the single function catalysts are also difficult to carry out both OER and HER reactions in an integrated electrolytic cell because they can’t match for both purposes and demonstrate poor stability in different acid and alkaline solution. Therefore, a lot of efforts have been made to realize the goal that using transition metal-based bifunctional catalysts [24] for both HER and OER applications. Recently, we developed CuCo2O4 [25] with different topographic characteristics and substrate materials for electrocatalytic water splitting. However, the inherent of poor electrical conductivity hinders the wide application of this material. In contrast, nickel/

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copper chalcogenides have larger active site and better conductivity, but their applications as the electrocatalysts for water splitting have received little attention. Simultaneously, NiCo2S4 [26] has garnered much interest for the electrocatalytic HER and OER, and NiCo2S4 is a main choice because of its relatively low cost, high activity. Although these catalysts displayed promising properties, they must also exhibit stability if they are to be used on a large scale as durable catalysts in alkaline environment. Considering that the core-shell structure material synthesized by different materials shows better electrocatalytic activity than that of single material, and the Ni, Cu and Co chalcogenide [27] composite can effectively improve the electrocatalytic activity due to their chemical interaction. The CuCo2S4-rods grown on the flexible nickel foam are served as backbone conductive substrate for “core”, and CuCo2S4rods as the core ensures effective electron transport to the active site. There is no report about the CuCo2S4/NiCo2S4 core-shell electrode material for electrocatalytic water splitting. This study is innovation in that the CuCo2S4/NiCo2S4 core-shell electrodes are prepared through hydrothermal synthesis. The active sites and specific area are significantly increased by rationally constructing this three-dimensional CuCo2S4/NiCo2S4 coreshell structure on nickel foam substrate. Using transition metal cluster chalcogenides of CuCo2S4/NiCo2S4 core-shell electrodes benefit for carrier transfer. Moreover, the nanostructure composite material exhibited lower Tafel slopes of 57 mV/dec and 90 mV/dec for OER and HER, respectively, in 1.0 (M) KOH electrolyte solution. The CuCo2S4/NiCo2S4 core-shell electrodes exhibit about 108.7% current retention for OER, suggesting the decent stability. An overall water splitting device for producing both hydrogen and oxygen is setup by the optimization of the CuCo2S4/NiCo2S4 coreshell electrodes. We tested the stability of the material for 50 h, the composite materials demonstrate long-term stability without

rarely any degradation. Based on the above-mentioned advantages, the construction of core-shell structure electrode material is of great significance to realize highly efficient and durable water splitting reaction. 2. Experimental section 2.1. Synthesis of porous CuCo2S4-rods The CuCo2S4-rods were synthesized using a hydrothermal method. A piece of nickel foam (NF) was cleaned through acetone and ethanol under ultra-sonication for 15 min, respectively. The NF was transferred into a Teflon-lined stainless steel autoclave containing 80 ml pink solution (deionized water: anhydrous ethanol ¼ 1:1) with 1.164 g Co(NO3).26H2O, 0.1876g Cu(NO3).23H2O and 1.441 g urea. Subsequently, the autoclave was sealed and kept at 120  C for 8 h. The autoclave was allowed to cool naturally to room temperature, and then CueCo precursor was polled out and rinsed with deionized water (DI). Finally, the CueCo precursor material was placed into another Teflon-lined stainless steel autoclave containing 75 ml DI with 1.8 g Na2S.9H2O. Then the autoclave was sealed and kept at 150  C for 5 h. After cooling to room temperature, the sample was removed and rinsed with DI. Subsequently, the sample was dried in an oven at 60  C for 1 h and CuCo2S4-rods were obtained. 2.2. Synthesis of CuCo2S4/NiCo2S4 nanostructure 1.18 g Ni(NO3).26H2O, 0.1678 g Co(NO3).26H2O and 1.72 g thiourea were dissolved in 40 ml DI and 40 ml anhydrous ethanol under vigorous stirring to form a clear pink solution. Then, the solution was transferred to a Teflon-lined stainless steel autoclave. The NF

Scheme 1. Schematic diagram illustrating the synthesis process of CuCo2S4/NiCo2S4 electrodes directly grown on nickel foam.

L. Ma et al. / Electrochimica Acta 326 (2019) 135002

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100  C for 12 h. 2.3. Hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) tests All the electrochemical performance measurements (LSV, stability, EIS) were carried out on an CHI660D electrochemical workstation (Chenhua, Shanghai) at room temperature (25 ± 1  C) using 1.0 (M) KOH solution as the electrolyte. For the electrocatalytic HER and OER, the CuCo2S4/NiCo2S4 core-shell electrode (1  1 cm
2) was directly used as the working electrode. A saturated calomel electrode (SCE) was used as reference electrode. But for HER and OER, the counter electrode was a graphite rod and an Pt foil, respectively. In 1.0 (M) KOH, the potential values were corrected to the reversible hydrogen electrode (RHE) based on the following formula: E(RHE) ¼ E(SCE)þ0.241 V þ 0.059 PH [28]. The linear sweep voltammetry (LSV) was carried out under scan rate of 2 mV s
1. All the polarization curves were iR-corrected (90%) in regard to the ohmic resistance of the solution. Electrochemical impedance spectroscopy (EIS) experiments were performed with frequencies ranging from 100 kHz to 0.01 Hz. Fig. 1. XRD patterns of CuCo2S4/NiCo2S4 electrodes.

2.4. Overall water splitting tests with CuCo2S4-rods were put into the above solution, then the autoclave was sealed and kept at 90  C for 12 h to obtain CuCo2S4/ NiCo2S4 and at 120  C for 12 h to obtain CuCo2S4/NiCo2S4-1. After cooling to room temperature, the sample was removed and rinsed with DI. Subsequently, the sample was dried in an electric oven at

The overall water splitting was tested in a two-electrode system. And the CuCo2S4/NiCo2S4 core-shell electrode was used as the anode and cathode in 1 M KOH, simultaneously. Besides, in order to get the high open-circuit voltage (Voc) perovskite solar cells (PSCs), surpassing 1.66 V for electrolytic water, we assemble the device

Fig. 2. Characterization of the as-prepared CuCo2S4/NiCo2S4 core-shell electrodes. Low (inset) and high magnification (a) SEM image of CuCo2S4-rods. (b) SEM image of CuCo2S4/ NiCo2S4 electrodes. (c) TEM and HRTEM (the inset shows the corresponding SAED pattern) image showing the presence of CuCo2S4-rods and NiCo2S4-sheets.

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with a patterned mothed as reported in our previous work [29]. The etched FTO pattern and the preparation process of the PSCs are shown in Fig. S1, where we can clearly see that the series of three cells and a 0.16 cm2 active area of single cell were obtained. And then, we test the power conversion efficiency (PCE) of the devices and the results were presented in Fig. S2. A voltage of 3.025 V was achieved in the series of three cells. 2.5. Materials characterizations Scanning electron microscope (SEM) images were taken with a Hitachi S-4800 with an energy dispersive spectroscope (EDS), and transmission electron microscopy (TEM, JEOL, JEM-2100) were used to characterize the morphology of the samples. X-ray diffraction (XRD, Bruker AXS, D8) were employed to obtain the XRD patterns of the samples. X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCLAB 250Xi) were used to analyze the composition and valence states of the materials. The PSCs were measured using a CHI 660D electrochemical work station (Shanghai Chenhua Instruments, China) with a 450 W xenon lamp and an AM 1.5G filter (Newport, 94023 A). 3. Results and discussion Here, a multi-compositional hierarchical electrode was prepared by a simple hydrothermal method, and the water splitting ability of electrocatalysts was enhanced by the core-shell structure and porous surface and interface effects. As illustrated in Scheme 1, the synthetic course contains the direct growth of CuCo2S4-rods on

the surface of a NF. Then the growth of NiCo2S4-sheets over CuCo2S4-rods was conducted. In terms of support effects, though CuCo2S4-rods were poor in catalytic activity, they can provide large protrude area for the subsequent growth of the NiCo2S4-sheets. Therefore, the catalytic performance of the electrode can be greatly improved. For interface interaction, more activity center can be formed by synergistic interaction between CuCo2S4-rods and NiCo2S4-sheets compared with single component at the interface. At the same time, the protruding nanoarray structure provided for high specific surface area which appreciably increased the number of active sites. The Brunner-Emmer-Teller (BET) measurement to investigate the specific surface area of composite nanostructure array. In Fig. S3, the measurement result shows the CuCo2S4/ NiCo2S4 possesses a high specific surface area of 14.0798 m2 g
1which appreciably increased the number of active sites. The high electrical conductivity of the CuCo2S4 nanoneedle arrays could enhance the electron transfer rate, the CuCo2S4/NiCo2S4 composite structure could increase the contact area of electrolyte with the active materials and expose more active sites, thus improving the electrochemical properties. Therefore, the obtained composite as an efficient bifunctional electrocatalyst can be used in alkaline environments with good applicability. Fig. 1 displays X-ray diffraction (XRD) patterns for the CuCo2S4/ NiCo2S4 core-shell electrodes. And Fig. S4 reveals that XRD patterns of CuCo2S4-rods and NiCo2S4-sheets. The peaks in the XRD patterns confirm the co-existence of CuCo2S4-rods, NiCo2S4-sheets and NF. The diffraction peaks located at 2 q ¼ 26.6 , 31.27, 37.97, 46.9 , 49.9 and 54.8 could be indexed to the (022), (113), (004), (224), (115) and (044) signals from the CuCo2S4-rods, respectively (PDF

Fig. 3. Detailed XPS information for (a) Cu 2p, (b) Co 2p, (c) Ni 2p and (d) S 2p of CuCo2S4/NiCo2S4 electrodes.

L. Ma et al. / Electrochimica Acta 326 (2019) 135002

no. 42e1450) [30]. And additional signals positioned at 26.8 , 31.5 , 33 , 38.3 , 50.5 and 55.3 are observed in the XRD pattern and correspond to the (220), (311), (222), (400), (511) and (440) signals from single crystalline NiCo2S4-sheets, respectively (PDF no. 20e0782) [31]. It was noteworthy that the peak intensity of all samples was relatively weak, which was mainly ascribe to the highly crystalline NF and the samples obtained has low crystallinity. Scanning electron microscope (SEM) images of the CuCo2S4/ NiCo2S4 core-shell electrodes distributed over the entire NF are analyzed. Fig. 2a reveals the presence of CuCo2S4-rods grown on the NF. As shown in Fig. S5, it appears that the images of NiCo2S4sheets, forming a regular structure over the NF. Fig. 2b reveals that the whole NF substrate was covered by the CuCo2S4/NiCo2S4 closely and uniformly, forming a 3D hierarchical porous structure. SEMEDX mapping performed on a portion of the sample (Figs. S6e7) reveal that elements of Cu, Ni, Co and S consist of the sample. Inductively coupled plasma atomic emission spectrometry (ICPAES) analysis concludes an atomic ratio of 1:0.14:0.18:1.15 for Co: Cu: Ni: S. A high-resolution transmission electron microscopy (HRTEM) (Fig. 2c) image taken from the resulting CuCo2S4/NiCo2S4 core-shell electrodes (Fig. 2d and Fig. S8a) displays well-resolved lattice fringes with interplanar distance of 0.167 nm index to the (044) plane of CuCo2S4-rods. Fig. S8b reveals the morphology of the CuCo2S4-rods, with another 0.332 nm index to the (220) plane of NiCo2S4-sheets (Fig. S8c). In addition, the inset SAED image show two diffraction rings which can be indexed to polycrystalline structures as marked. The formation of CuCo2S4, NiCo2S4 and CuCo2S4/NiCo2S4 core-shell structures was further investigated by X-ray photoelectron spectroscopy (XPS) measurements (Fig. S9c).

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In Fig. S9a, the XPS spectrum of Cu 2p for CuCo2S4/NiCo2S4 exhibits about 0.7 eV shift toward low binding energy compared with CuCo2S4, indicating the strongly electron interaction between CoCo2S4 and NiCo2S4. Meanwhile, the increased binding energy of Ni 2p (Fig. S9b) spectrum occurs on CuCo2S4/NiCo2S4 [32]. The survey spectrum indicates the coexistence of copper, nickel, cobalt, and sulfur elements in the synthetic material. For the Cu 2p spectrum of CuCo2S4/NiCo2S4 core-shell electrodes, 2p3/2 and 2p1/2 peak were situated at 932.5 eV and 952.3 eV, revealing the presence of Cuþ (shown in Fig. 3a) [33]. At the same time, as revealed in Fig. 3b, two pairs of spin orbital peaks make up the XPS pattern of Co 2p, the existence of two satellite peaks also indicates Co has two valence states. The main major peaks can be decomposed into four peaks attributed to the Co 2p1/2 and Co 2p3/2 were related to Co2þ and Co3þ, respectively. The binding energy located at 850e886 eV [34], the Ni 2p (Fig. 3c) peak could be ascribed to the Ni 2p1/2 and Ni 2p3/ 2þ and Ni3þ [35]. In addition, 2, which showed the coexistence of Ni as shown in Fig. 3d, for S 2p XPS spectrum, there were two prime peaks and one satellite peaks in the S 2p region, the binding energies of 163.1 and 161.0 eV corresponding to S 2p1/2 and S 2p3/2, respectively [36]. Impressively, the electrocatalytic liveness properties of the CuCo2S4/NiCo2S4 core-shell electrodes were studied in 1.0 (M) KOH by a typical three-electrode system (Fig. 4). The effect of different hydrothermal temperatures on the electrocatalytic properties during the synthesis of CuCo2S4/NiCo2S4 electrocatalyst has been explored. From that we can conclude that the of CuCo2S4/NiCo2S4 has better performance than that of CuCo2S4/NiCo2S4-1 (Fig. S10). In order to study the effect of the porous structure of the full

Fig. 4. (a) OER polarization curves (iR-corrected) of CuCo2S4/NiCo2S4 core-shell electrodes, NiCo2S4-sheets, CuCo2S4-rods with a scan rate of 2 mV s
1. (b) The required overpotential to achieve a current density of 10 mA cm
2 for different electrocatalysts. (c) Corresponding OER Tafel plots. (d) Stability measurements at 10 mA cm
2 for CuCo2S4/NiCo2S4 electrodes.

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composite on the device properties, we also prepare CuCo2S4-rods and NiCo2S4-sheets, and test their OER activities under identical conditions. From Fig. 4a, linear sweep voltammetry (LSV) curves were measured for OER of synthetic samples with a scan rate of 2 mV s
1, which revealed that the CuCo2S4/NiCo2S4 core-shell electrodes exhibit a smaller OER onset potential compared to CuCo2S4-rods and NiCo2S4-sheets. And all potential values present in the above test have been conducted with iR-corrected to eliminate the ohmic potential drop. When a current density was reached at 10 mA cm
2, the overpotential value was defined as h10. And the Tafel slope was also an important parameter to evaluate the catalytic performances. We fit the liner regions in the LSV curves of all the samples according to the Tafel equation (h ¼ b log j þaÞ [37], where b was Tafel slope and j was the current density. CuCo2S4/ NiCo2S4 core-shell electrodes (Fig. 4b) present a much lower h10 of 270 mV. As for the single component of CuCo2S4-rods and NiCo2S4sheets, their h10 were 318 mV and 327 mV, respectively. In sharp contrast, the Tafel slope of CuCo2S4/NiCo2S4 core-shell electrodes (57 mV/dec) sample was lower than those of CuCo2S4-rods (84 mV/ dec) and NiCo2S4-sheets (87.9 mV/dec), manifesting superior OER electrocatalytic performance of the CuCo2S4/NiCo2S4 core-shell electrodes (Fig. 4c). The electrochemically active surface area (ECSA) of CuCo2S4/NiCo2S4 core-shell electrodes, CuCo2S4-rods and NiCo2S4-sheets were measured in the electrolyte solution (Fig. S11). The current density was recorded at different scan rates of CV curves. From the linear plots, we can the calculate the electrochemical double layer capacitance (Cdl) values of CuCo2S4-rods, NiCo2S4-sheets and CuCo2S4/NiCo2S4 core-shell electrodes. It clearly demonstrates that CuCo2S4/NiCo2S4 core-shell electrodes

(19.8 mF cm
2) show higher capacitance than those of CuCo2S4rods (5.1 mF cm
2) and NiCo2S4-sheets (2.39 mF cm
2). The extremely high electrochemical capacitance of CuCo2S4/NiCo2S4 core-shell electrodes indicates that the synthesized heteronanowire has provided for more active sites. Besides, electrochemical impedance spectroscopy (EIS) analyses were used to study the reaction kinetics of these samples. The Nyquist plots were present in Fig. S13a. The Nyquist traces were fitted with an equivalent circuit (Fig. S13b). From which we can see that the Rs of all catalysts reveals our experiment condition was consistent. The curves in the different frequency range of the Nyquist plots indicate a kinetic reaction. The stability of CuCo2S4/NiCo2S4 core-shell electrodes under alkaline media was further investigate by continuously measuring at a current density of 10 mA cm
2 for OER (Fig. 4d). The CuCo2S4/NiCo2S4 core-shell electrodes exhibit about 108.7% current retention for OER, suggesting the benign stability. The morphology of CuCo2S4/NiCo2S4 core-shell electrodes catalyst after stability test was observed and the SEM images were shown in Fig. S12 a. The above test results show that the CuCo2S4/NiCo2S4 core-shell electrode was an electrocatalyst with superior performance and remarkable charge transfer toward OER in 1.0 (M) KOH. The HER electrocatalytic performance of the CuCo2S4/NiCo2S4 active core-shell electrodes were evaluated using the linear sweep voltammetry (LSV) at a scan rate of 2 mV s
1 between
1.6 and
0.9 V vs RHE in a 1.0 (M) KOH aqueous solution. CuCo2S4/ NiCo2S4 core-shell electrodes on NF were directly used as the working electrode in a three-electrode system. Fig. 5a presents the polarization curves of CuCo2S4/NiCo2S4 core-shell electrodes, CuCo2S4-rods and NiCo2S4-sheets. From Fig. 5b, it was obvious that

Fig. 5. (a) HER polarization curves (iR-corrected) of CuCo2S4/NiCo2S4 core-shell electrodes, NiCo2S4-sheets, CuCo2S4-rods with a scan rate of 2 mV s
1. (b) The required overpotential to achieve a current density of 10 mA cm
2 for different electrocatalysts. (c) Corresponding HER Tafel plots. (d) Stability measurements at 10 mA cm
2 for CuCo2S4/NiCo2S4 core-shell electrodes for 15 h, in 1.0 (M) KOH.

L. Ma et al. / Electrochimica Acta 326 (2019) 135002

CuCo2S4/NiCo2S4 core-shell electrodes exhibit a low h10 of 206 mV at a current density of 10 mA cm
2. As for CuCo2S4-rods and NiCo2S4-sheets, their h10 were 226 mV and 272 mV, respectively. The HER was finished by CuCo2S4/NiCo2S4 core-shell electrodes in the alkaline medium under Tafel-Volmer-Heyrovsky mechanism (Equations (1)e(3)) [26, 38].

Mcat þ H2 O þ e
/Mcat H * þ OH


Volmer reaction

Mcat H* þ H2 O þ e
/H2 þ OH
þ Mcat

(1)

Heyrovsky reaction (2)

Mcat H* þ Mcat H * /H2 þ Mcat

Tafel reaction þ

(3)

It involves the absorption of H on the catalytic metal center (Volmer step, equation (1)), there were two ways to make hydrogen by Heyrovsky step (equation (2)), and Tafel step (equation (3)). (* represents the adsorbed active site). The Tafel slope was a determining parameter for HER property. However, the HER of CuCo2S4/ NiCo2S4 core-shell electrodes with a Tafel slope of 90 mV dec
1 was associated with the Heyrovsky mechanism (Fig. 5c). At the same time, the Tafel slope of CuCo2S4/NiCo2S4 core-shell electrodes (90 mV/dec) sample was lower than those of CuCo2S4-rods (149 mV/dec) sample and NiCo2S4-sheets (152 mV/dec) sample, indicating favorable HER electrocatalytic kinetics of the CuCo2S4/ NiCo2S4 core-shell electrodes. Therefore, the Tafel slope also shows the better HER performance of the CuCo2S4/NiCo2S4 core-shell electrodes, which was corresponding to that of the LSV data. During the electrolysis, the performance of CuCo2S4/NiCo2S4 core-shell

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electrodes were holding up very well, as shown in Fig. 5d. At the same time, the wettability of CuCo2S4/NiCo2S4 core-shell electrode was analyzed by using a water droplet (50 ml) and the static contact angle was measured (Fig. S14). The contact angle of the water droplet was affected by the roughness of the sample surface, it can prove the wettability degree of a catalyst. As the experiment showed, the contact angle of the CuCo2S4-rods and NiCo2S4-sheets were 60 and 77 (Fig. S14a and Fig. S14b), respectively. Under the same condition, the average contact angle of CuCo2S4/NiCo2S4 coreshell electrode was 35 (Fig. S14c), suggesting an extremely hydrophilic surface. Compared with the original nanocrystals, the morphology had changed after 15 h electrolysis for HER (Figure S12 b). Our result suggested that the catalyst on the CuCo2S4/NiCo2S4 core-shell electrodes surface was more accessible to the electrolyte, thus improving the OER and HER performance in the alkaline aqueous electrolyte. All of the experiments show that the CuCo2S4/NiCo2S4 core-shell electrode is a superior bifunctional electrocatalyst for both OER and HER in 1.0 (M) KOH. Hence, the CuCo2S4/NiCo2S4 core-shell electrodes on NF were directly assembled as anode and cathode to form a two-electrode electrolytic cell in 1.0 (M) KOH solution. And we have provided the real demonstration of the generation of gas bubbles from both electrodes during overall water splitting in supporting information. As shown in Fig. 6a, we connect the terminal vertex of the perovskite solar cells directly to a two-electrode water electrolysis device. In the end, when the digital multimeter displays a voltage of 1.58 V, water splitting began to occur, at the same time, we can observe the OER and HER reactions at the anode and the cathode, respectively (Fig. S15). Fig. 6b displays the

Fig. 6. (a) Solar energy derived water electrolysis. (b) LSV plots showing over all water splitting in two electrode system (without iR compensation) of CuCo2S4/NiCo2S4//CuCo2S4/ NiCo2S4 with a scan rate of 2 mV s
1. (c) Constant current stability study in 1 M KOH. The size of both the electrodes is 1 cm  1 cm.

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polarization (LSV) curves of the cell. The potential of the CuCo2S4/ NiCo2S4 structure//CuCo2S4/NiCo2S4 structure was 1.66 V and the stable current density for 50 h represents with z97% current density retention (Fig. 6c). The obtained overall voltage values here are lower or comparable with those reported in the literature such as NieCoeSeP device (1.61 V at j ¼ 10 mA cm
2), NCT-NiCo2S4 device (1.6 V at j ¼ 10 mA cm
2), 3D Se-(NiCo)Sx/(OH)x device (1.6 V at j ¼ 10 mA cm
2), Ni0.33Co0.67S2 device (1.6 V at j ¼ 10 mA cm
2) NiCo2S4 device (1.68 V at j ¼ 10 mA cm
2), NiCo2S4@NiFe device (1.6 V at j ¼ 10 mA cm
2), NieCoeS device (1.67 V at j ¼ 10 mA cm
2), NieCoeS//NieCoeP device (1.57 V at j ¼ 20 mA cm
2). Those NieCo based sulfides catalysts for water splitting are shown in Table S1. Finally, we used a perovskite solar cells which could provide a voltage of 3.025 V and was enough to drive the water splitting. Our work opens up the possibility of hydrogen production driven by solar energy. 4. Conclusions To sum up, a multi-compositional hierarchical nanostructured CuCo2S4/NiCo2S4 core-shell electrode on nickel foam is synthesized by a simple hydrothermal method. The structure enables the sample to have larger active sites and specific surface area. The CuCo2S4-rods as the core materials ensure effective electron transport to the active site. The results show that the porous structure is suitable for electrochemical application. Whether hydrogen evolution reaction or oxygen evolution reaction tests, this porous structure all shows high performance. The CuCo2S4/NiCo2S4 core-shell electrode materials present stability for OER and HER in 1.0 (M) KOH. At the same time, the electrochemical cell for water splitting based on CuCo2S4/NiCo2S4 core-shell electrodes can be driven by a perovskite solar cell-module and H2 and O2 are generated. The work illustrates the potential applications of CuCo2S4/NiCo2S4 core-shell porous materials for clean energy generation. In the meantime, this study demonstrated that our strategy may provide a new way for the synthesis of other kind material with multi-compositional hierarchical nanostructure. Acknowledgements This work was supported by the National High Technology Research and Development Program (2015AA050601), the National Natural Science Foundation of China (11674252), and the Fundamental Research Funds for the Central Universities (Grant No. 2042019kf0317). We thank the nano center of Wuhan University for XRD, XPS measurements. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.135002. References [1] J.H. Zhou, Z.G. Wang, J.B. Liu, Y.F. Chen, Enhanced hydrogen evolution performance by covalent-linked ultrafine, uniform Pt nanoparticles with doped sulfur atoms in three-dimensional graphene, Int. J. Hydrogen Energy 43 (2018) 23231e23238. [2] H.Q. Zhou, F. Yu, J.Y. Sun, H.T. Zhu, I.K. Mishra, S. Chen, Z.F. Reif, Highly efficient hydrogen evolution from edge-oriented WS2(1ex) Se2x particles on threedimensional porous NiSe2 foam, Nano Lett. 16 (2016) 7604e7609. [3] Y.F. Zhang, Y. Xia, S.S. Yan, J. Han, Y.F. Chen, W.Z. Zhai, Z.N. Gao, One-step green synthesis of composition-tunable PteCu alloy nanowire networks with high catalytic activity for 4-nitrophenol reduction, Dalton Trans. 47 (2018) 17461e17468. [4] T. Chen, S. Li, J. Wen, P. Gui, Y. Guo, C. Guan, J. Liu, G. Fang, Rational construction of hollow core-branch CoSe2 nanoarrays for high-performance asymmetric supercapacitor and efficient oxygen evolution, Small 14 (2018)

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