Self-Interconnected Porous Networks of NiCo Disulfide as Efficient

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Self-Interconnected Porous Networks of NiCo Disulfide as Efficient Bifunctional Electrocatalysts for Overall Water Splitting Qing Zhang, Cui Ye, Xiao Lin Li, Yang Hui Deng, Bai Xiang Tao, Wei Xiao, Ling Jie Li, Nian Bing Li, and Hong Qun Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 27, 2018

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ACS Applied Materials & Interfaces

Self-Interconnected Porous Networks of NiCo Disulfide as Efficient Bifunctional Electrocatalysts for Overall Water Splitting Qing Zhang†, Cui Ye†, Xiao Lin Li†, Yang Hui Deng†, Bai Xiang Tao†, Wei Xiao‡, Ling Jie Li§, Nian Bing Li*†, Hong Qun Luo*† †

School of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, People’s Republic of China

‡

Cengong County Agriculture Bureau, Cengong, Guizhou 557800, People’s Republic of China

§

School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, People’s Republic of China

∗Corresponding author. ∗Nian Bing Li and Hong Qun Luo Tel: +86-23-68253237; Fax: +86-23-68253237. *E-mail address: [email protected] (N. B. Li); [email protected] (H. Q. Luo)

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ABSTRACT Electrochemical splitting of water has been viewed as a highly efficient technique to produce clean hydrogen and oxygen energy. However, designing inexpensive multifunctional electrocatalysts with high performance is a great challenge. Here, a unique 3D catalyst of self-interconnected porous Ni-Co disulfide networks grown on carbon cloth ((Ni0.33Co0.67)S2 NWs/CC) was prepared by a facile hydrothermal method coupled with further low-temperature sulfuration strategy. As a bifunctional electrocatalyst, (Ni0.33Co0.67)S2 NWs/CC exhibits remarkable activity to catalyze both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). To drive a current density of 100 mA cm-2, (Ni0.33Co0.67)S2 NWs/CC needs the overpotentials of 156 mV in 0.5 M H2SO4 solution and 334 mV in 1.0 M KOH solution for HER, respectively. Moreover, when used as a catalyst of OER, (Ni0.33Co0.67)S2 NWs/CC needs an overpotential of 295 mV to produce a current density of 100 mA cm-2. The excellent electrochemical properties are mainly attributed to the synergetic catalysis of Ni-Co based bimetallic disulfide, the porous-network structure, and the high conduction of carbon cloth. Moreover, the two-electrode alkaline water-splitting system constructed by (Ni0.33Co0.67)S2 NWs/CC only needs a low cell voltage of 1.57 V to approach 10 mA cm-2. This work offers more new insights for the design and preparation of the non-noble metal catalysts based on transition metal sulfide with excellent electrocatalytic performance in overall water splitting. KEYWORDS: hydrothermal method, low-temperature sulfuration strategy, Ni-Co disulfide, overall water splitting, transition metal sulfide.

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1. INTRODUCTION The rapid exhaustion of fossil fuels and the associated environmental issues urgently demand for renewable and clean energy alternatives. Hydrogen has been viewed as one of the most promising future energy carriers due to its abundant, eco-friendly, and renewable nature.1,2 Electrochemically converting water to hydrogen and oxygen as a clean and promising technique has attracted more attention in recent years.3,4 However, extremely large overpotentials for water splitting is needed to achieve hydrogen evolution reaction (HER) at the cathode and oxygen evolution reaction (OER) at the anode simultaneously.5,6 The most efficient commercial electrocatalysts for HER and OER derive from infrequent and expensive noble metals such as Pt-group metals, IrO2, and RuO2.7,8 Thus, a great deal of effort has been paid to search highly active, highly durable, and low-cost bifunctional electrocatalysts based on earth-abundant elements, especially transition metal (e.g., Fe, Co, Ni, and Mo) compounds,9-11, including oxides/hydroxides,12-15 sulfides,16-21 selenides,22,23 carbides,24 and phosphides.25-28 In this respect, most of researches have been widely implemented in synthesis of nanostructured transition metal chalcogenides, such as CoS2,29 Co9S8,30 MoS2,31 FeS2,32 WS2,33 NiS2,34 Ni3S2,17 but higher activity and stability for HER and OER are still desired. Among the transition metal chalcogenides, Ni and Co sulfides display superior electrocatalytic capability toward water splitting due to their high corrosion resistant properties in alkaline solution and richly variable valence states.35 Compared to their monometal sulfides such as CoS2 and NiS2, the catalytic activity of bimetallic sulfide are significantly optimized, because the synergistic effects and complementary effects (typically, improved d-band states of the bimetallic active centers) are connected with different metal 3



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constituents.36 In addition, the performance of electrocatalyst strongly depends on its size and morphology. Electrocatalysts with the 0D morphologies like nanoparticles have been widely studied for HER and OER, but they are easily agglomerate, resulting in low-speed electron transfer and poor long-term

stability.37-39 Therefore, designing some anisotropic

nanostructures such as nanowires and nanosheets for electrocatalyst can provide effective electron/ion diffusion path.40,41 Meanwhile, the use of organic polymer binders (e.g., Nafion) fixing catalytic materials and current collectors usually increases the dead volume even buries the number of active sites. Furthermore, the extra contact resistance between the catalyst and collector generates due to insulating polymer binders.42 Therefore, modified electrodes prepared by conventional techniques often suffer from low load levels of active materials, resulting in an increase of the contact resistance at the interface between the active materials and the current collector. Fortunately, the 3D electrodes possess much inherent merits, such as a structural integrity derived from direct bonding, a small contact resistance, and a large amount of exposed active sites.40,43-45 These 3D electrodes can be obtained by loading active materials on the surface of substrates, such as nickel foam, carbon paper, and metal foils. Carbon cloth (CC), as a new candidate of 3D supporting substrates, exhibits outstanding properties including high conductivity, flexibility, and lower cost. Tong et al. built a metal-free porous N-doped CC electrocatalyst as the self-supporting anode, which drove OER with a considerably low overpotential in alkaline solution.46 Herein, we propose a facile two-step route including a hydrothermal synthesis and a low-temperature sulfuration process to obtain porous (Ni0.33Co0.67)S2 NWs/CC. Taking 4


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advantage of the high conduction of CC substrate and the unique porous 3D structure, (Ni0.33Co0.67)S2 NWs/CC provides well-distributed pathways for electron transfer throughout the integrated electrode, and displays excellent bifunctional activity and stability toward HER and OER. It demands the overpotentials of 81 mV in 0.5 M H2SO4 and 156 mV in 1.0 M KOH to achieve a current density of 10 mA cm-2 for HER. And it achieves a current density of 10 mA cm-2 at a low overpotential of 216 mV to in 1.0 M KOH for OER. Furthermore, we construct a two-electrode alkaline water-splitting system based on (Ni0.33Co0.67)S2 NWs/CC, which affords a current density of 10 mA cm-2 at a cell voltage as low as 1.57 V. 2. EXPERIMENTAL SECTION 2.1. Materials Carbon cloth (CC) was obtained from Ce Tech Co., Ltd (Taiwan, China). Nickelous chloride hexahydrate (NiCl2·6H2O), cobalt chloride hexahydrate (CoCl2 6H2O), sulfur sublimed, ammonium fluoride (NH4F), and potassium hydroxide (KOH) were purchased from Aladdin Co., Ltd. (Shanghai, China). Urea was purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). Sulphuric acid (H2SO4) was bought from Chongqing Chuandong Chemical Co., Ltd. (Chengdu, China). Pt/C (20 wt % Pt on Vulcan XC-72R), ruthenium (IV) oxide (RuO2), and Nafion (5 wt %) were purchased from Sigma-Aldrich Chemical Reagent Co., Ltd. (USA). Ultrapure water (18.2 MΩ cm) was used throughout the experiments. 2.2. Characterization The morphologies and element composition were sequentially investigated using scanning electron microscopy (FESEM, S4800, Hitachi, Japan), energy-dispersive X-ray spectroscopy (EDX, GENESIS, EDAX, USA), and transmission electron microscope (TEM, 5



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Tecnai G220, Fei Corporation, Japan). The powder X-Ray Diffraction (p-XRD, Shimadzu XRD-7000, Shimadzu, Japan) patterns were performed using an XRD-7000 with Cu Kα radiation at a scanning rate of 2°min−1. Raman spectrum source (Renishaw, InVia, UK) was recorded over the frequency range of 100-1000 cm-1 using a 532.8 nm laser. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method (Autosorb-IQ surface area analyzer). The pore size distribution was obtained by Barrett-Joyner-Halenda (BJH) method. The surface properties of samples were analyzed with X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi, Thermoelectricity Instruments, USA). Specially, the samples for XRD, TEM, and BET tests were scraped from carbon cloth. 2.3. Synthesis of the Ni-Co Precursor NNs /CC Typically, a piece of CC (2 cm×3 cm) was sequentially cleaned with concentrated HNO3, ultrapure water, and ethanol before use. The weight of CC was recorded after drying at 60 ℃ for 5 h. After that, 0.30 g of NiCl2·6H2O, 0.60 g of CoCl2·6H2O, 0.30 g of urea, and 0.085 g of NH4F were dissolved into 20 mL of ultrapure water under continuous stirring for 5 min. Then the red solution was transferred to a 50 mL Teflon-lined stainless steel autoclave. The clean CC was dipped in the autoclave, then the autoclave was maintained at 120 ℃ for 6 h in an electric oven. After the autoclave cooled to room temperature, the Ni-Co precursor NNs/CC was obtained by washing with ultrapure water and ethanol several times, followed by drying at 60 ℃ for 6 h. 2.4. Synthesis of the Porous (Ni0.33Co0.67)S2 NWs/CC To construct the porous (Ni0.33Co0.67)S2 NWs/CC, the Ni-Co precursor NNs/CC and 2.0 g of sulfur powder were placed in two ceramic boats and located at downstream and upstream 6


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respectively within the tube furnace. N2 gas was initially flowed into the tube for 0.5 h to remove the air. Then, the two ceramic boats were heated at 350 ℃ (450 and 550 ℃) for 2 h under N2 atmosphere, and then naturally cooled to ambient temperature. The loading for (Ni0.33Co0.67)S2 on CC was determined to be 3.0-3.8 mg cm-2. For comparison, NiS2 NSs/CC and CoS2 NNs/CC samples were synthesized by the same procedure by the same procedure individually adding CoCl2·6H2O or NiCl2·6H2O for the preparation of precursor. The preparation of (Ni0.33Co0.67)S2 powder by the same procedure without CC. 2.5. Synthesis of 20 wt% Pt/C/CC and RuO2/CC The commercial 20 wt% Pt/C powder was dispersed in 1 mL of solution with a ratio of V(H2O)/V(ethanol) = 1/2, followed by the addition of 80 µL of Nafion. The mixture was sonicated for 20 min to obtain a homogeneous ink. The ink was evenly coated onto a piece of clean carbon cloth, and then evaporated slowly under ambient conditions. RuO2/CC was prepared using RuO2 powder by the same procedure. 2.6. Electrochemical Characterization The electrochemical measurements were conducted on an electrochemical work station (CHI 660E, CH Instruments Inc, Shanghai, China) in a three-electrode setup coupled with (Ni0.33Co0.67)S2/CC (size: 1 cm × 1 cm) as a working electrode, a platinum wire as the counter electrode, Ag/AgCl electrode as the reference electrode. The long-time durability test was performed using chronoamperometric technique at fixed potentials using a graphite rod as the counter electrode. The HER experiment was performed in 0.5 M H2SO4 solution and 1.0 M KOH solution, respectively. The OER experiment was performed in 1.0 M KOH solution. Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) of (Ni0.33Co0.67)S2/CC for 7



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HER and OER were conducted with a scan rate of 2 mV s-1. The data presented in the polarization curves and corresponding Tafel plots were iR-corrected using the data of the electrochemical impedance spectroscopy (EIS) measurements. EIS was performed in the frequency range of 100 kHz to 0.01 Hz with an amplitude of 5 mV. For overall water splitting, the two-electrode configuration was constructed using (Ni0.33Co0.67)S2 NWs/CC both as cathode and anode. The recorded potentials were adjusted by the equation: E (RHE) = E(Ag/AgCl) + (0.2012 + 0.059 pH) V. 2.7. Turnover Frequency Calculations The turnover frequency (TOF) value is calculated from the equation (1):47 TOF (s-1) = (j×A) / (4×F×n).

(1)

where j (A cm-2) is the measured current density at overpotential of 330 mV. A (cm2) is the area of the carbon cloth electrode, F is Faraday constant (96485.3 C mol-1), the number 4 means 4 electrons to generate 1 mol of O2, and n is moles of coated metal atom. RESULTS AND DISCUSSION

The strategy to fabricate self-interconnected porous (Ni0.33Co0.67)S2 NWs/CC is illustrated in Scheme 1. The precursor was prepared through a hydrothermal treatment of Ni2+ and Co2+ at 120 ℃ for 6 h. During this process, urea acted as a homogeneous precipitation agent to keep a constant pH value of the reaction system by a slow release of ammonia,48 which promotes the homogeneous reaction of Ni2+ and Co2+. Therefore, high-crystalline Ni-Co precursor nanoneedles can uniformly grow on the surface of carbon cloth (Ni-Co precursor NNs/CC). Subsequent sulfurization can facilely convert the as-grown Ni-Co nanoneedles into porous (Ni0.33Co0.67)S2 networks at 350 ℃ for 2 h in sublimed sulfur 8


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atmosphere. In the sulfurization process, more and more voids formed in the inner of nanoneedles along with the variation of the crystal structure and the slow release of H2O and CO2,40 thereby the surface of nanoneedles became rougher. At the same time, the porous structure was formed with the shrinkage and cross-linking among adjacent nanoneedles. Through the above processes, the fabrication of conductive self-interconnected porous (Ni0.33Co0.67)S2 networks on CC was finalized. The optical images of initial carbon cloth, Ni-Co precursor NNs/CC and (Ni0.33Co0.67)S2 NWs/CC are shown in Figure S1. The color of the surface of carbon cloth changed from gray to pink through a hydrothermal process, and then converted to dark black after sulfidation treatment, which demonstrates (Ni0.33Co0.67)S2 networks successfully grew on the surface of flexible CC. The morphology of the obtained products was first characterized using scanning electron microscopy (SEM). From Figure 1a, bare CC has the smooth and clean surface, after a facile hydrothermal process, uniform Ni-Co precursor nanoneedles radially on the surface of the CC (Figure 1b). The high-magnification SEM image (inset of Figure 1b) clearly reveals that these nanoneedles with typical 3D acicular morphology possess diameters ranging from 100 to 200 nm and length of several micrometers. After the sulfurization reaction, the uniform Ni-Co precursor NNs shrink and transform into interconnected porous-network structure. The SEM image of (Ni0.33Co0.67)S2 NWs/CC (Figure 1c) shows the ordered honeycomb-like networks grown on the surface of CC. This change of structure is mainly attributed to regular polymerization of Ni-Co precursor NNs. From the high-magnification SEM image of (Ni0.33Co0.67)S2 NWs/CC (Figure 1d), it can be clearly observed that the thickness of the cross-linked walls is 0.8-1 µm and the pore diameter is about 1-2 µm, confirming the 9



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formation of porous-network structure after the thermal process. Energy-dispersive X-ray spectroscopy (EDX) elemental mapping images (Figure 1e) reveal the existence and well-distribution of Ni, Co, and S around the whole carbon cloth. However, as shown in Figure S2, NiS2 grown on CC shows the morphology of nanosheets (NiS2 NSs/CC) and CoS2 grown on CC reveals lots of disordered nanowires (CoS2 NWs/CC), respectively. This further indicates that the combination of Ni and Co is in favor of obtaining ideal porous structure. Without the existence of CC, the (Ni0.33Co0.67)S2 materials tend to aggregate into larger blocks (Figure S3). Thus, serving as a 3D electrode, (Ni0.33Co0.67)S2 NWs/CC avoids dense aggregation of active materials and the use of organic binders. Single Ni-Co precursor nanoneedle has a diameter of 80 nm and a smooth surface from the TEM image in Figure 1f. (Ni0.33Co0.67)S2 NWs/CC has a rough surface, moreover, stream-like inner structure is observed in the longitudinal direction of a nanoneedle (Figure 1g). Numerous voids formed inside the nanoneedle (Figure S4, yellow circles), mainly arising from the release of CO2 and H2O gases from interior of Ni-Co precursor and the partial transformation of the crystal structure during thermal decomposition process. The comparison of the low-magnification TEM images of Ni-Co precursor NNs and (Ni0.33Co0.67)S2 NWs (Figure S5) fully demonstrate the cross-linking among nanoneedles in (Ni0.33Co0.67)S2 NWs, which is consistent with the SEM image. The high-resolution TEM (HRTEM) image reveals the lattice fringes with plane spacings of 0.46 and 0.23 nm, which can be indexed to the (001) and (101) planes of Ni-Co hydroxide (Figure 1h). In the case of (Ni0.33Co0.67)S2, the well-resolved lattice fringes with two plane spacing values of 0.28 and 0.25 nm can be indexed to the (200) and (210) crystal planes of the cubic (Ni0.33Co0.67)S2 phase (Figure 1i). Moreover, the corresponding selected 10


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area electron diffraction (SAED) pattern shows the polycrystalline characteristics, and the diffraction rings are well indexed to (200) and (210) planes of (Ni0.33Co0.67)S2 (Figure 1j). These results confirmed the formation NiCo-based disulfide hybrid. The crystalline nature of the as-prepared products was investigated by X-ray diffraction (XRD). It is observed from Figure 2a that the positions of characteristic diffraction peaks of (Ni0.33Co0.67)S2 locate between those of CoS2 (JCPDS Card No. 41-1471) and NiS2 (JCPDS Card No. 11-0099), and there is no phase separation. The slight shifting of these diffraction peaks also indicates that partial Co ions were replaced by Ni ions and the bimetallic NiCo disulfide solid solution was formed.49 Thus, the chemical formula of sample is (NixCo1-x)S2. (Ni0.33Co0.67)S2 is named according to a 1 : 2 atomic ratio for Ni : Co from the EDX spectrum (Figure S6), agreeing with the initial molar ratio of Ni to Co used in the synthesis. The diffraction peaks of Ni-Co precursor are corresponding to NiCo hydroxides (Figure S7, JCPDS Card No. 30-0443 and No. 14-0117), which is in accordance with previous reports.50,51 These results further indicate the successful synthesis of (Ni0.33Co0.67)S2 by the transformation of NiCo hydroxides. The Raman spectrum exhibits that three main peaks at 278.5, 396.3, and 470.1 cm-1 of (Ni0.33Co0.67)S2 are overlapped with those of NiS2 and CoS2 (Figure 2b). The peaks at 278.5 and 470.1 cm-1 correspond to the characteristic active modes of libration (Eg) and in-phase stretch (Ag) for the chalcogenide dumbbells in NiS2.52 The peak at 396.3 cm-1 approximately corresponds to the characteristic active mode of Ag for CoS2.53 The Raman results further confirm the presence of the Ni-Co disulfide hybrid. According to the nitrogen adsorption-desorption isotherm plot of (Ni0.33Co0.67)S2 NWs (Figure 2c), a type-IV isotherm is observed, which is associated with the presence of 11



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mesopores. By comparing the Barrett-Joyner-Halenda (BJH) pore-size distribution curves (Figure 2d), the pore average size of Ni-Co precursor NNs is 1.2 nm, while that of (Ni0.33Co0.67)S2 NWs is 9.1 nm, further certifying the mesoporous character. This character is also consistent with the TEM analysis. The Brunauer-Emmett-Teller (BET) specific surface area of the (Ni0.33Co0.67)S2 NWs is 24.61 m2 g-1, which is significantly larger than that of Ni-Co precursor nanoneedles (5.57 m2 g-1). With mesoporous structure and higher surface area, a large number of active sites are exposed, the contact area between electrode and electrolyte is enlarged, the electron diffusion rate is improved. To further confirm the chemical composition and the surface electronic states of the Ni-Co precursor and (Ni0.33Co0.67)S2, X-ray photoelectron spectroscopy (XPS) measurements were carried out. Figure 3a shows the survey spectrum of (Ni0.33Co0.67)S2. High-resolution Ni 2p and Co 2p spectra are well fitted with two spin-orbit doublets 2p1/2 and 2p3/2. The high-resolution XPS spectrum of Ni 2p3/2 in the Ni-Co precursor (Figure 3b) distinctly shows a major peak around 855.8 eV and a satellite peak ascribed to bivalent state of Ni, indicating the major existence of Ni2+ in Ni(OH)2 phase.54 The high-resolution XPS spectrum of Co 2p3/2 in the Ni-Co precursor (Figure 3c) shows a major peak around 781.6 eV and a satellite peak, indicating that the major existence of Co2+ in Co(OH)2 phase.55 However, a new major peak around 857.8 eV in the high-resolution XPS spectrum of Ni 2p3/2 for the (Ni0.33Co0.67)S2 was observed, which can be attributed to the existence of trivalent state of Ni.20 Similarly, a distinct major peak around 779.2 eV in the high-resolution XPS spectrum of Co 2p3/2 for the (Ni0.33Co0.67)S2 also indicates that the existence of trivalent state of Co.20 The S 2p region can be fitted with two main peaks (163.1 and 163.9 eV) and a satellite peak (169.0 eV) (Figure 12


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3d). The peak at 163.1 eV is ascribed to S2-, and the peak at 163.9 eV is ascribed to the metal-sulfur bonds forming at the surface.56 According to the XPS analysis, the surface electronic states of (Ni0.33Co0.67)S2 have a component of Co2+, Co3+, Ni2+, Ni3+, and S2−. We first evaluated the electrocatalytic HER activity of the (Ni0.33Co0.67)S2 NWs/CC (catalyst loading: 3.5 mg cm−2) in acid (0.5 M H2SO4) and alkaline (1.0 M KOH) using a standard three-electrode setup (see details in the Experiment Section). The sample prepared at a low sulfuration temperature of 350 ℃ exhibits much superior HER activity in acidic electrolyte than other samples prepared at 450 and 550 ℃ (Figure S8). Therefore, 350 ℃ was selected as suitable sulfuration temperature to prepare the optimal (Ni0.33Co0.67)S2 NWs/CC in this work. The HER performances of Ni-Co precursor NNs/CC, NiS2 NSs/CC, CoS2 NWs/CC, bare CC, and 20 wt% Pt/C deposited on CC with the same loading were also tested for comparison. All initial data were presented after iR correction for assessing the intrinsic activity of the catalysts.57 As shown in Figure 4a, the catalytic performance of bare CC is poor. The as-prepared (Ni0.33Co0.67)S2 NWs/CC shows outstanding HER performance in 0.5 M H2SO4 solution with the current densities of 10 and 100 mA cm-2 at low overpotentials of 81 and 156 mV, superior to those of the NiS2 NSs/CC (η10 = 129 mV) and CoS2 NWs/CC (η10 = 115 mV). It is noted that Ni-Co precursor NNs/CC also exhibits low onset overpotential (η10 = 89 mV) close to that of (Ni0.33Co0.67)S2 NWs/CC (Figure 4b), whereas higher potential is needed to drive high current density (η100 = 284 mV). It reveals that the optimization for onset overpotential mainly owes to the synergistic effect of Ni-Co species, and the fast-rising current response mainly owes to introduction of sulfur active sites.58 The catalytic kinetics was further estimated by the Tafel plots (Figure 4c). The Tafel slope of (Ni0.33Co0.67)S2 13



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NWs/CC is 60 mV mV dec-1, which is much smaller than those of NiS2 NSs/CC (89 mV dec-1), CoS2 NWs/CC (100 mV dec-1), and Ni-Co precursor NNs/CC (318 mV dec-1), suggesting (Ni0.33Co0.67)S2 NWs/CC has markedly faster HER kinetics. Simultaneously, compared to the recently reported electrocatalysts for HER in acidic solution (Table S1), (Ni0.33Co0.67)S2 NWs/CC in this work exhibits a remarkable electrocatalytic activity. What’s more, the similar regularity is observed when (Ni0.33Co0.67)S2 NWs/CC, Ni-Co precursor NNs/CC, NiS2 NSs/CC, CoS2 NWs/CC, bare CC, and 20 wt% Pt/C/CC are carried out in 1.0 M KOH solution. As depicted in Figure 4d and 4e, (Ni0.33Co0.67)S2 NWs/CC displays current densities of 10 and 100 mA cm-2 at overpotentials of 156 and 334mV, respectively. Ni-Co precursor NNs/CC also shows lower onset overpotential than NiS2 NSs/CC and CoS2 NWs/CC. From Figure 4f, the corresponding Tafel slope of (Ni0.33Co0.67)S2 NWs/CC is 127 mV dec-1. The electrochemical double-layer capacitance (Cdl) was surveyed to evaluate the electrochemical active surface area (ECSA). We collected the CV curves in the regions of 0.125-0.225 V vs. RHE in 0.5 M H2SO4 and of 0.074-0.174 V vs. RHE in 1.0 M KOH, respectively (Figures S9 and S10). In 0.5 M H2SO4 solution, (Ni0.33Co0.67)S2 NWs/CC has the highest Cdl of 30.43 mF cm-2, which is 70.8, 14.5, and 4.3 times larger than Ni-Co precursor NNs/CC, NiS2 NSs/CC, and CoS2 NWs/CC, respectively. In 1.0 M KOH solution, (Ni0.33Co0.67)S2 NWs/CC also has the highest Cdl of 46.5 mF cm-2. Based on these comparison, (Ni0.33Co0.67)S2 NWs/CC catalyst exposes more active sites to promote its electrocatalytic activity. Furthermore, electrochemical impedance spectroscopy (EIS) analysis was performed to study the charge transfer kinetics. As shown in Figure S11, the Nyqusit plots reveal that 14


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(Ni0.33Co0.67)S2 NWs/CC has the lowest Faradaic impedance Rct (the diameter of the semicircle) among CoS2 NWs/CC, NiS2 NSs/CC, and Ni-Co precursor NNs/CC in both 0.5 M H2SO4 and 1 M 1.0 M KOH solutions, suggesting that the (Ni0.33Co0.67)S2 NWs/CC catalyst possesses the fastest charge-transfer capacity during the HER process. Meanwhile, the EIS data were fitted with an equivalent circuit model as illustrated in the inset of Figure S11. The electrochemical stability of (Ni0.33Co0.67)S2 NWs/CC was further evaluated using cycling tests and chronoamperometric response tests. After 2000 cycles of continuous CV scanning, (Ni0.33Co0.67)S2 NWs/CC exhibits insignificant degradation in 0.5 M H2SO4 and 1.0 M KOH solutions (Figure 5a and 5b). In addition, Figure 5c and 5d shows that (Ni0.33Co0.67)S2 NWs/CC has the small decreases of cathodic current density from 10 to 7.8 mA cm-2 under an overpotential of 87 mV in 0.5 M H2SO4 and from 10 to 8.1 mA cm-2 under an overpotential of 164 mV in 1.0 M KOH after testing for 24 h, demonstrating the outstanding durability of (Ni0.33Co0.67)S2 NWs/CC. The relevant SEM images of the (Ni0.33Co0.67)S2 NWs/CC (Figure S12) after 24 h HER stability test certify that the morphology was substantially retained. The electrocatalytic OER performances for (Ni0.33Co0.67)S2 NWs/CC, Ni-Co precursor NNs/CC, CoS2 NWs/CC, NiS2 NSs/CC, and RuO2/CC with the same loading were also assessed in 1.0 M KOH solution. It should be noted that the oxidation peak at 1.33 V in polarization curve of (Ni0.33Co0.67)S2 NWs/CC in Figure 6a is ascribed to oxidation peak of Ni2+ or Co2+ or both.59 As shown in Figure 6a and 6b, as expected, (Ni0.33Co0.67)S2 NWs/CC drives the current densities of 20 and 100 mA cm-2 at low potential of 1.446 V (overpotential of 216 mV) and 1.525 V (overpotential of 295 mV), respectively, which are slightly lower than that of RuO2/CC (η20 = 288 mV and η100 = 413 mV) and many recently reported 15



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state-of-the-art OER electrocatalysts (Table S2). Meanwhile, the overpotential of (Ni0.33Co0.67)S2 NWs/CC is also significantly lower than that of NiS2 NSs/CC (η20 = 253 mV), CoS2 NWs/CC (η20 = 324 mV) and Ni-Co precursor NNs/CC (η20 = 369 mV). Unlike the regularity in HER, NiS2 NSs/CC displays the highest OER activity than CoS2 NWs/CC, indicating nickel sulfides can more efficiently promote OER.60 XPS analysis was used to investigate the surface compositional change of (Ni0.33Co0.67)S2 NWs/CC after OER (Figure S13). After the OER catalysis, the XPS spectrum of Co 2p3/2 shows a new peak at 783.6 eV that can be assigned to CoOOH.61 The Ni 2p3/2 XPS spectrum after OER also shows a new peak at 858.5 eV that can be assigned to NiOOH.62 The Raman spectrum (Figure S14) of the post-OER catalyst reveals the major characteristic peaks (Eg and A1g) of CoOOH, further suggesting the formation of the surface CoOOH species.63 Thus, we may infer that (Ni0.33Co0.67)S2 NWs/CC serves as “precatalyst”during the OER process. With the active species of both (Ni0.33Co0.67)S2 and formed Ni/Co oxy-hydroxidea, the OER performance can be significantly improved. Figure 6c shows that the Tafel slope of (Ni0.33Co0.67)S2 NWs/CC is 78 mV dec-1, which is smaller than those of CoS2 NWs/CC (107 mV dec-1), NiS2 NSs/CC (126 mV dec-1), Ni-Co precursor NNs/CC (130 mV dec-1) and the RuO2/CC (159 mV dec-1). In the same way, the Cdl values of these products were obtained from CV curves in the regions of 0.925-1.025 V vs. RHE (Figure S15). (Ni0.33Co0.67)S2 NWs/CC possesses the highest Cdl values of 4.67 mF cm-2, higher than that of Ni-Co precursor NNs/CC (0.79 mF cm-2), CoS2 NWs/CC (1.42 mF cm-2) and NiS2 NSs/CC (3.57 mF cm-2). Nyquist plots for (Ni0.33Co0.67)S2 NWs/CC and other products are shown in Figure 6d, and the corresponding 16


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equivalent circuit diagram is sketched in the inset. Coincident with the HER activity, (Ni0.33Co0.67)S2 NWs/CC exhibits the smallest Rct among products, verifying the fastest charge transfer process on OER. Furthermore, the TOF values of these catalysts at an overpotential of 340 mV were also calculated (Figure S16), assuming that all metal atoms were involved in the catalysis. (Ni0.33Co0.67)S2 NWs/CC possesses much larger TOF value of 0.031 s-1 than those of Ni-Co precursor NNs/CC, CoS2 NWs/CC, and NiS2 NSs/CC. The OER durability of (Ni0.33Co0.67)S2 NWs/CC was investigated. The negligible variation is observed in polarization curve after 2000 cycles of continuous CV scanning, and a 92.8% of initial current density is retained after 24 h durability test at an overpotential of 275 mV, confirming the superior stability of (Ni0.33Co0.67)S2 NWs/CC (Figure 6e and 6f). The SEM and TEM images (Figure S17a and S17b) show that the morphology and structure of (Ni0.33Co0.67)S2 NWs/CC have no dramatic changes after OER stability test. Given that (Ni0.33Co0.67)S2 NWs/CC has impressive activity and stablity towaed both OER and HER, we then assembled a two-electrode configuration using (Ni0.33Co0.67)S2 NWs/CC as both anode and cathode for overall water splitting in 1.0 M KOH. Pt/C/CC || RuO2/CC were also tested for comparison. From the iR-corrected polarization curves in Figure 7a, (Ni0.33Co0.67)S2 NWs/CC exhibits high performance with a cell voltage of 1.57 V to attain 10 mA cm-2 close to that of Pt/C/CC || RuO2/CC (1.54 V at 10 mA cm-2) and even superior to that of previously reported catalysts (see Table S3).Abundant bubbles were produced at the surface of both the cathode and anode during electrolysis process (inset of Figure 7b and Movie S1). Moreover, (Ni0.33Co0.67)S2 NWs/CC exhibits high stability with a current retention of 95.6% at a cell voltage of 1.65 V for 30 h electrolysis (Figure 7b). It is 17



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observed no excessive changes in morphology from the SEM images (Figure S18) of (Ni0.33Co0.67)S2 NWs/CC before and after water splitting durability test. 3. CONCLUSION In summary, the self-interconnected porous (Ni0.33Co0.67)S2 networks directly grown on carbon cloth were successfully synthesized through a facile hydrothermal method followed by a low-temperature sulfuration process. Benefiting from the synergistic effect of the homologous Ni-Co based disulfide hybrid, unique porous-network structure and good conductivity of carbon cloth substrate, the flexible 3D (Ni0.33Co0.67)S2 NWs/CC electrode enables significantly enhanced activity and stability for HER and OER. We have demonstrated that the (Ni0.33Co0.67)S2 NWs/CC requires overpotentials as low as 156 mV under acidic condition and 334 mV under alkaline condition for HER to attain a current density of 100 mA cm-2, respectively. Moreover, to approach a current density of 100 mA cm-2 for OER, it only demands a low overpotential of 216 mV. As a bifunctional electrocatalyst for overall water splitting, it also possesses high catalytic performance with the capability to drive 10 mA cm-2 at a cell voltage of 1.57 V and a current retention of 95.6% after 30 h electrolysis. This work is propitious to broaden applications of low-cost, highly active transition metal sulfide catalysts to electrochemical energy conversion. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS publication Website.

18


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The digital photographs of carbon cloth, SEM images of NiS2 NSs/CC and CoS2 NWs/CC, SEM image of (Ni0.33Co0.67)S2 without CC substrate, EDX spectrum of (Ni0.33Co0.67)S2 NWs/CC, XRD patterns of Ni-Co precursor, polarization curves for optimal samples, comparison of the double-layer capacitance (Cdl) for all products, SEM images of (Ni0.33Co0.67)S2 NWs/CC after HER stability test, XPS spectra, Raman spectrum, SEM and TEM images of (Ni0.33Co0.67)S2 NWs/CC after OER test, comparison of the TOFs for all products, comparison of HER, OER and overall water splitting performance with reported non-precious metal catalysts AUTHOR INFORMATION Corresponding Author * Prof. Nian Bing Li, E-mail: [email protected]. * Prof. Hong Qun Luo, E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No.

21675131)

and

the

National

Science

Foundation

of

Chongqing

(No.

CSTC-2015jcyjB50001).

19



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Scheme 1. Schematic illustration of the fabrication procedure of (Ni0.33Co0.67)S2 NWs/CC.

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Figure 1. (a) SEM image of the bare CC. (b) Low-magnification SEM images of Ni-Co precursor NNs/CC, inset is the corresponding high-magnification SEM images. (c) Low- and (d) high-magnification SEM images of (Ni0.33Co0.67)S2 NWs/CC. (e) SEM and corresponding EDX elemental mapping images of Co, Ni and S for (Ni0.33Co0.67)S2 NWs/CC. (f) TEM images of single nanoneedle of Ni-Co precursor NNs. (g) TEM images of one forked nanoneedles of (Ni0.33Co0.67)S2 NWs. (h) HRTEM image of Ni-Co precursor NNs and (i) (Ni0.33Co0.67)S2 NWs. (j) the corresponding SAED pattern of (Ni0.33Co0.67)S2 NWs.

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Figure 2. (a) XRD pattern of (Ni0.33Co0.67)S2, CoS2 and NiS2. (b) Raman spectra of (Ni0.33Co0.67)S2, NiS2, and CoS2. (c) N2 adsorption-desorption isotherms and (d) BJH pore-size distributions of Ni-Co precursor and (Ni0.33Co0.67)S2.

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Figure 3. (a) XPS survey of (Ni0.33Co0.67)S2. (b-d) high resolution XPS spectra of Ni 2p, Co 2p, and S 2p.

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Figure 4. Polarization curves of bare CC, (Ni0.33Co0.67)S2 NWs/CC, Ni-Co precursor NNs/CC, NiS2 NSs/CC, CoS2 NWs/CC, and Pt/C/CC in 0.5 M H2SO4 (a) and in 1.0 M KOH (d). The column graph of overpotential at 10 mA cm-2 and 100 mA cm-2 for all samples in 0.5 M H2SO4 (b) and in 1.0 M KOH (e). Tafel plots of (Ni0.33Co0.67)S2 NWs/CC, Ni-Co precursor NNs/CC, NiS2 NSs/CC, CoS2 NWs/CC, and Pt/C/CC in 0.5 M H2SO4 (c) and in 1.0 M KOH (f).

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Figure 5. Polarization curves for (Ni0.33Co0.67)S2 NWs/CC before and after 2000 cycles CV scanning in 0.5 M H2SO4 (a) and in 1.0 M KOH (b). Time-dependent current density curve of (Ni0.33Co0.67)S2 NWs/CC under a fixed overpotential of 87 mV in 0.5 M H2SO4 (c) and under a fixed overpotential of 164 mV in 1.0 M KOH (d) for 24 h.

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Figure 6. OER electrocatalysis: (a) Polarization curves of (Ni0.33Co0.67)S2 NWs/CC, Ni-Co precursor NNs/CC, NiS2 NSs/CC, CoS2 NWs/CC, and RuO2/CC. (b) The column graph of overpotential at 20 mA cm-2 and 100 mA cm-2 for all samples. (c) Tafel plots of (Ni0.33Co0.67)S2 NWs/CC, Ni-Co precursor NNs/CC, NiS2 NSs/CC, CoS2 NWs/CC, and RuO2/CC. (d) EIS of (Ni0.33Co0.67)S2 NWs/CC, Ni-Co precursor NNs/CC, NiS2 NSs/CC, CoS2 NWs/CC, and RuO2/CC at 295 mV vs. RHE. (e) Polarization curves for (Ni0.33Co0.67)S2 NWs/CC before and after 2000 cycles CV scanning. (f) Time-dependent current density curve of (Ni0.33Co0.67)S2 NWs/CC under a fixed overpotential of 275 mV for 24 h.

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Figure 7. (Ni0.33Co0.67)S2 NWs/CC for overall water splitting in 1.0 M KOH. (a) IR-corrected polarization curves. (b) Time-dependent current density curve carried out at 1.65 V. Inset: The digital photograph of the O2 and H2 evolution from overall water splitting on (Ni0.33Co0.67)S2 NWs/CC electrodes.

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Table of Contents

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Self-Interconnected Porous Networks of NiCo Disulfide as Efficient

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