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Dominating Role of Aligned MoS2/Ni3S2 Nanoarrays Supported on 3D Ni Foam with Hydrophilic Interface for Highly Enhanced Hydrogen Evolution Reaction Jiamu Cao, Jing Zhou, Yufeng Zhang, Yuxi Wang, and Xiaowei Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16407 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 23, 2017

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ACS Applied Materials & Interfaces

Dominating Role of Aligned MoS2/Ni3S2 Nanoarrays Supported on 3D Ni Foam with Hydrophilic Interface for Highly Enhanced Hydrogen Evolution Reaction Jiamu Caoa, Jing Zhoua, Yufeng Zhanga,b,*, Yuxi Wanga, Xiaowei Liua,b a

MEMS Center, Harbin Institute of Technology, 150001, China.

b

Key Laboratory of Micro-systems and Micro-Structures Manufacturing, Ministry of

Education, 150001, China. KEYWORDS hydrogen evolution reaction, electrocatalyst, MoS2, Ni3S2, hydrophilic property.

ABSTRACT

When using water splitting to achieve sustainable hydrogen production, low-cost, stable, and naturally abundant electrocatalysts are required to replace Pt-based ones for the hydrogen evolution reaction (HER). Herein, for the first time, a novel nanostructure with one-dimensional (1D) MoS2/Ni3S2 nanoarrays directly grow on a three-dimensional (3D) Ni foam is developed for this purpose, showing excellent catalytic activity and stability. The as-prepared 3D MoS2/Ni3S2/Ni composite has an onset overpotential as low as 13 mV in 1 M KOH, which is comparable to Pt-based electrocatalyst for HER. According to the classical theory, the Tafel 1 ACS Paragon Plus Environment

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slope of the new composite is relatively low, as it goes through a combined Volmer−Heyrovsky mechanism during hydrogen evolution. All the results attribute the excellent electrocatalytic activity of the nanostructure to the electrical coupling among Ni, Ni3S2, and MoS2; the Super hydrophilic interface; the synergistic catalytic effects produced by the MoS2/Ni3S2 nanoarrays; and abundant exposed active edge sites. These unique and previously undeveloped characteristics of the 3D MoS2/Ni3S2/Ni composite make it a very promising earth-abundant electrocatalyst for HER.

1. INTRODUCTION Hydrogen is an environmentally friendly, scalable, and renewable energy source with low climate impacts, especially over its entire conversion chain from production to employment.1 Numerous advantages of hydrogen over other energy sources make it a promising way to reduce the environmental emission from fossil fuels.2 One effective method to produce hydrogen is the electrochemical water splitting using solar energy.3 To match the solar photon flux to produce a high current density and increase the yield of the hydrogen evolution reaction (HER) in water splitting, active electrocatalysts are needed to reduce the overpotential of electrodes.4 To date, the most effective catalysts in HER are the Pt-based metals.5,6 However, the high cost and scarcity of Pt have hindered the adoption of hydrogen energy.7,8 Alternative low-cost catalysts are therefore in urgent demand.9,10 Molybdenum disulfide (MoS2) is a cheap and geographically ubiquitous graphene-like transition metal sulfide, which has a layered structure held together by weak van der Waals forces.11 Both density functional theory (DFT) calculations and experimental investigations indicated that the edges of MoS2 possess high chemisorption capability for hydrogen in its under-coordinated Mo-S sites, analogous to Pt.12-15 However, the poor intrinsic conductivity of MoS2 materials suppresses charge transport and hinders their use as effective 2 ACS Paragon Plus Environment

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electrocatalysts for HER.16,17 The best way to solve this problem is grafting the nanometerscaled MoS2 onto a conductive substrate.18 Meanwhile, earth-abundant Ni-based materials have recently been shown to have impressive potential for water splitting.19,20 The Ni3S2 nanostructures are believed to possess many attractive features as effective electrocatalysts for HER, including much lower costs, high abundance, and easy fabrication process.21 In particular, because of its high conductivity, the catalytic performance of Ni3S2 and Ni3S2-based core/shell nanostructures in HER is better than pure MoS2.22,23 Moreover, the exchange current density (which is a function of the Gibbs free energy of adsorbed atomic hydrogen) measured in an acidic solution and the overpotentials measured in an alkaline media were in the volcano plots and both of them showed that the value of Ni is only below those of noble metal.24,25 On the other hand, compared with two-dimensional (2D) planar structures, the three-dimensional (3D) electrode structures are reported to have a larger surface area for catalyst loading, which can significantly improve the electrocatalytic HER efficiency.26,27 To maximize the number of reaction sites, conductive 3D Ni foam is a good choice as catalyst substrate due to its high surface area, low cost, and the Gibbs free energy close to Pt.28,29 Considering these factors, to integrating three electrocatalysts be one composite electrocatalyst (utilizing the 3D Ni foam as a conductive substrate for supporting the hybrid structure of MoS2 and Ni3S2) to increase its conductivity and the surface area seems to be a promising method to enhance the electrocatalytic activity for HER. Herein, we present a facile and inexpensive method to synthesize a unique 3D combination of the MoS2/Ni3S2 hybrid structure of nanoarrays directly grown on Ni foam. The synthesized 3D MoS2/Ni3S2/Ni composited is played remarkable hydrogenation performance, due to the abundant active sites, first-class surface wettability, and excellent electrical conductivity. We believe that such

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rationally designed unique 3D composites will have potential applications in supercapacitors, lithium-ion batteries, energy conversion-storage devices, and superhydrophobic surfaces. 2. EXPERIMENTAL SECTION 2.1. Catalyst preparation. The 3D architecture of MoS2/Ni3S2 hybrid structure nanoarrays directly grown on Ni foam was fabricated by a simple one-step hydrothermal method. A piece of Ni foam (3 cm × 7.5 cm × 1.6 mm, 0.2 g/cm3) was reduced in an H2 flow of 100 sccm at 1050 °C for 30 min. In 100 mL DI water, 143 mg thiourea, and 121 mg sodium molybdate were added and sonicated for 1 h. The sonicated solution and the hydrogen-reduced Ni foam were placed into a Teflon-lined stainless steel autoclave (100 mL), which was then sealed before hydrothermal reaction at 180 °C for 24 h. Afterward, the stainless steel autoclave was allowed to cool down to 26 °C (room temperature) naturally. The obtained black samples were washed with DI water and ethanol for several times and dried in a vacuum oven at 60 °C. All chemicals in the experiments were purchased from Sinopharm Chemical Reagent Co., Ltd, and were used without further purification. 2.2. Catalyst characterizations. X-ray diffraction (XRD) analysis was carried out with an Empyrean diffractometer with Cu Kα radiation (λ = 0.15406 nm). The morphology and structure of the 3D MoS2/Ni3S2/Ni composite were recorded using a field emission scanning electron microscope (FESEM, Hitachi SU-70) with an energy dispersive spectroscopic (EDS) detector and a transmission electron microscope (TEM, JEOL JEM2100). Furthermore, the compositions and atomic valence states were examined by X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi). 2.3. Electrochemical evaluation. All electrochemical measurements were performed on an electrochemistry workstation (CHI 660D) at room temperature (about 26 °C). The electrocatalytic activity of the 3D MoS2/Ni3S2/Ni composite in 1 M KOH aqueous electrolyte 4 ACS Paragon Plus Environment

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was evaluated using a standard three-electrode system, in which the produced sample was directly used as the working electrode with a fixed geometric area of 1 cm × 1 cm, a Hg/HgO electrode as the reference electrode, and a graphite rod as the counter electrode. Before electrochemical measurements, we first degassed the electrolyte by bubbling Ar gas for 1 h. The scan rate in the linear sweep voltammetry (LSV) was 1 mV/s. Nyquist plots were obtained at the open circuit potential and various HER overpotentials versus the reversible hydrogen electrode (RHE) in the frequency range from 100 KHz to 10 MHz in 5 mV amplitude potential. All potential values were converted to values with reference to RHE according to the following equation: E (RHE) = E (Hg/HgO) + 0.099 V+ 0.059 × pH Long-term stability was assessed by continuous cyclic voltammetry at a scan rate of 50 mV s-1 for 2000 cycles. 3. RESULTS AND DISCUSSION Recently, it was recognized that the incorporation of catalytic materials with special structures, such as hollow, hierarchical, or uniformly distributed architectures with conductive networks, can improve the HER performance of electrocatalysts.30-40 Based on this insight, rationally-designed electrocatalysts, the prepared 3D MoS2/Ni3S2/Ni composite may exhibit enhanced catalytic activity and stability. As schematically displayed in Figure 1a, the selfassembled 3D MoS2/Ni3S2/Ni composite which the 1D MoS2/Ni3S2 hybrid structure nanoarrays grow on the Ni substrate were using a simple one-step hydrothermal method. In the initial stage of the reaction, the S ions as active particles are released from thiourea (CH4N2S). The S ions react with the exposed Ni foam to produce Ni3S2 particles, and with MoO42- ions released from the sodium molybdate (Na2MoO4) to produce MoS2 nanosheets. The Ni3S2 nanoparticles grow into nanorods to act as the backbone to guide the preferential 5 ACS Paragon Plus Environment


deposition of MoS2 nanosheets by a complex oriented attachment process. A large number of this nanostructure decorated on the Ni foam were formed the MoS2/Ni3S2 nanoarrays. During the reaction, the Ni foam acts as the skeleton for anchoring the nanostructured arrays, and as the nickel source for the growth of Ni3S2 nanorods. The completely black surface of the resulting Ni foam (Figure S1) suggests the uniform formation of the MoS2/Ni3S2 hybrid nanoarray structure. Figure 1b shows the XRD patterns of the as-prepared 3D MoS2/Ni3S2/Ni composite. Three strong peaks at 44.5, 51.8, and 76.4° arise from the Ni foam substrate. The diffraction peaks at 21.7, 31.1, 37.8, 44.4, 49.7, 50.1, and 55.1° correspond to the (101), (110), (003), (202), (113), (211), and (122) planes of Ni3S2 (JCPDS No. 44-1418); and those at 14.2, 38.2, 73, and 78.1° correspond to the (002), (104), (205), and (027) planes of MoS2 (JCPDS No. 37-1492), respectively. Because no other diffraction peaks were observed, the phase purity of the asprepared sample is assumed to be high. Both Ni3S2 and MoS2 were successfully fabricated, that is, the Ni foam was decorated with Ni3S2 and MoS2. The XPS spectra were analyzed to gain a deeper understanding of the bonding state and chemical nature of the 3D MoS2/Ni3S2/Ni composite. The spectral peaks in Figure 2a indicates that the major elements in the composite are Ni, Mo, and S. As depicted in Figure 2b, two main peaks with binding energies at 855.3 and 872.9 eV in the Ni 2p spectrum are respectively assigned to Ni 2p3/2 and Ni 2p1/2 according to the spin-orbit characteristics of Ni2+, accompanied by satellite peaks (denoted as "sat.").41 Moreover, two very weak peaks with the binding energies of 857.1 eV in Ni 2p3/2 and 874.7 eV in Ni 2p1/2 are in agreement with the characteristics of Ni3+.42 The peak at 852.5 eV is the characteristic peak of Ni3S2.43 Mo 3d3/2 and Mo 3d5/2 peaks with the respective binding energies of 231.8 and 228.6 eV are characteristic of Mo4+ in MoS2 (Figure 2c).44 In the spectra of S 2p (Figure 2d), the peaks at 162.2 and 163.5 eV and the satellite peak at 168.1 eV are attributed to the S2- states in MoS2 6 ACS Paragon Plus Environment

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and Ni3S2.45,37 These results are quite consistent with the XRD analysis of the 3D MoS2/Ni3S2/Ni composite. In Figure 3a, the 3D MoS2/Ni3S2/Ni composite has macroporous features according to the SEM images and appears black to the naked eyes (Figure S1). The grown MoS2/Ni3S2 hybrid structure nanoarrays are dense and uniform on the nickel foam, being almost vertical to the Ni surface (Figure 3b and Figure 3c). High-resolution SEM images demonstrate that the MoS2 nanosheets almost fully cover the entire Ni3S2 nanorod in a uniform manner, and the average diameter of the MoS2/Ni3S2 nanowires is about 400 nm (Figure 3d). In addition, no material loss occurred after ultra-sonicating the sample in solution for several minutes, thereby confirming the robust mechanical adhesion of the 3D MoS2/Ni3S2/Ni composite (Figure S2). To characterize the interior structure of the 3D MoS2/Ni3S2/Ni composite, the fracture surface was analyzed by SEM and EDS which showed in Figure 4a. The MoS2/Ni3S2 nanowires have an average length of about 4 μm. EDS mapping of Mo, Ni, and S elements (Figure 4b) indicates that MoS2/Ni3S2 nanowires are uniformly distributed on the Ni foam substrate, and the overlapping Ni, Mo, and S signals also point out the uniform distribution of Ni3S2 and MoS2. The microstructure of these nanowires was further characterized by TEM. Figure 4c displays the nanosheets of MoS2 being interconnected and almost fully supported on the entire Ni3S2. The side view indicates that the lateral scale is about 400 nm, which is in good agreement with the SEM observation. The further high-resolution TEM (HRTEM) result is shown in Figure 4d. The large lattice spacing of 0.62 nm belongs to the arched MoS2 (002) plane, which appears to have 3-15 MoS2 layers and particular lattice structure (the inset of Figure 4d). It is worth mentioning that the MoS2 nanosheets have hydrophilic properties. The surface energy will increase with the increase of the number of atomic layers and converge to a constant value over five layers.46 In the meanwhile, the relationship between the thickness and the wettability of a layer is the higher the thickness is, the lower the contact angle will 7 ACS Paragon Plus Environment


be.47 Moreover, the high surface energy of materials with special structures, such as rough, hollow and hierarchical, can highly enhance its hydrophilic properties.48 These imply that the abundant MoS2 nanosheets covered 3D MoS2/Ni3S2/Ni composite surface has a hydrophilic interface. Moreover, in the additional experiments, the 3D Ni3S2/Ni nanostructure prepared without sodium molybdate source, very dense Ni3S2 nanosheets instead of Ni3S2 nanowires were grown on the Ni foam surface. A possible reason is that, since no S ions were consumed by the molybdenum source, relatively excess S ions in the reaction solution eventually led to a large number of Ni3S2 nanosheets grown irregularly on the surface of Ni foam (Figure S3, Figure S4, and Figure S5, Supporting Information). It is worth mentioning that, no material loss occurred after ultra-sonicating the sample in solution for several minutes, thereby confirming the robust mechanical adhesion of the Ni3S2/Ni nanostructure (Figure S6). An optimum structure is very important for greater access of the electrolyte into the electrocatalyst to enhance the HER performance in an alkaline aqueous electrolyte. The behavior of a water droplet on the surface can be influenced by the surface roughness, which may amplify the wettability characteristics of a catalyst. Therefore, we tested the wettability of the 3D MoS2/Ni3S2/Ni composite electrode using a water droplet (100 μL) and the static contact angle method and compared the results to those of the bare Ni foam. As expected, in Figure 5a, the contact angle of the bare Ni foam was ≈120°, indicating its hydrophobic nature. In contrast, the small droplet entered the 3D MoS2/Ni3S2/Ni as soon as it contacted the surface (Figure 5b), suggesting an extremely hydrophilic surface. In addition, the gas-liquid twophase transport of different electrodes in HER are observed respectively. Lots of large hydrogen bubbles are adhering to the surface of the Ni foam and seriously hinder the entrance of water to the surface of the electrode (Figure 5c). As a sharp contrast, the surface of the 3D MoS2/Ni3S2/Ni composite is not adhered by large hydrogen bubbles (Figure 5d). It is 8 ACS Paragon Plus Environment

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attributed to the super-hydrophilic interface make the water entry rapidly thus the hydrogen bubbles are crowded out immediately when they are produced. In other words, the 3D MoS2/Ni3S2/Ni composite exhibit excellent gas-liquid two-phase transport. Therefore, the 3D MoS2/Ni3S2/Ni composite is certain more effective than Ni foam for enhancing the HER. To assess the suitability of the 3D MoS2/Ni3S2/Ni composite electrocatalysts over the entire water splitting process, the electrocatalytic HER performance of all samples was evaluated in 1 M KOH electrolyte solution using LSV in a three-electrode configuration purged with Ar gas. We applied iR correction to all our data, and Figure 6 presents the results. In Figure 6a, the polarization curves show that the onset overpotentials of the Ni3S2/Ni nanostructure and the Ni foam are respectively about 145 and 270 mV, while that of the 3D MoS2/Ni3S2/Ni composite is lower (ca. 13 mV, which approaches the value of the Pt/C electrocatalyst). The applied overpotential at 10 mA cm-2 is approximately 76 mV for the 3D MoS2/Ni3S2/Ni composite, substantially lower than those of the Ni3S2/Ni nanostructure (ca. 260 mV), the Ni foam (ca. 350 mV), and the previously reported Ni-based electrocatalysts (Table S1). Just as Figure S7 showed the electrocatalytic HER performance of variety catalysts was evaluated. The MoS2/Ni is used Nafion as a binder to fix the small-sized MoS2 nanosheets (with layer number from 1 to 10, diameter about 20-500nm, and purchased from JCNANO Co., Ltd.) and the Ni foam. An obvious trend is that the applied overpotential reduces gradually, as 350 mV, 300 mV, 260 mV, and 76 mV are the applied overpotential of Ni foam, MoS2/Ni, Ni3S2/Ni nanostructure and 3D MoS2/Ni3S2/Ni composite, respectively. Compared with Ni foam, the introduction of MoS2 nanosheets has no significant improvement in catalytic performance. This is because the binder will block some of the active sites and deteriorate the mass diffusion in HER process. The sharp contrast in catalytic performance with the MoS2/Ni and the Ni3S2/Ni is, the 3D MoS2/Ni3S2/Ni composite which the MoS2/Ni3S2 hybrid structure nanoarrays grow on the Ni foam without binders have rather low applied overpotential. This 9 ACS Paragon Plus Environment


is due to the 3D MoS2/Ni3S2/Ni composite have abundant exposed active edge sites and a novel structure which is hollow, hierarchical, large specific surface area and uniformly distributed architectures with conductive networks. Figure 6b provides the corresponding Tafel plots. The linear regions in the LSV curves of all samples were fitted with the Tafel equation η = b×log j + a, where j is the current density, and b is the Tafel slope. A Tafel slope of 56 mV dec–1 was obtained for the 3D MoS2/Ni3S2/Ni composite, which is lower than the values for Ni3S2/Ni nanostructure and Ni foam (95 and 115 mV dec–1, respectively). Thus, the Tafel slope also confirms the better HER performance of the 3D MoS2/Ni3S2/Ni composite, in good agreement with the LSV data. The comparison results of the electrocatalytic activities between the Ni3S2/Ni nanostructure and the 3D MoS2/Ni3S2/Ni composite indicated the importance of introducing the molybdenum source. Furthermore, the obtained activity for HER in 1M KOH aqueous electrolyte is also better than most reported Ni-based electrocatalysts, such as NiCo2S4/Ni,28 MoS2/Ni3S2,37 V-Ni3S2-NW,19 NiSe/NF,29 Ni-Co-P,35 NiCo2S4 NW/NF,39 and Ni-Fe/NC36 (Figure 6c). To further evaluate the long-term stability of the synthesized 3D MoS2/Ni3S2/Ni composite, the sample was exposed to 2000 continuous treatment cycles in an alkaline environment. The final I-V curves were very similar to the initial ones and exhibited negligible loss of the cathodic current (Figure 6d). We also tested the long-term HER electrochemical stability of the 3D MoS2/Ni3S2/Ni composite through 24 h of galvanostatic electrolysis at 10 mA cm-2, in which the composite showed excellent durability with negligible change (insert of Figure 6d). The superior durability than the noble metal Pt sheet (Figure S8) can be attributed to the distinguished gas-liquid two-phase transfer in HER of the composite (Figure S9). Additional test through 24 h of galvanostatic electrolysis at 20 mA cm-2 (Figure S10) and through continuous electrolysis at a static potential of 76 mV versus RHE (Figure S11) further exhibited the durability of the prepared electrocatalyst. The SEM images depicted in Figure 10 ACS Paragon Plus Environment

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S12 showed that the original morphology of the composite was well preserved after a longterm test. The ionic and transport resistances were also examined as essential factors affecting the HER activities in an electrolyte. The advantage of the 3D MoS2/Ni3S2/Ni composite became more apparent when we used electrochemical impedance spectroscopy (EIS) to further examine the electrode kinetics under HER operating conditions. Nyquist plots (Figure S13) revealed a dramatically decreased charge transfer resistance (RCT) for the 3D MoS2/Ni3S2/Ni composite (ca. 20 Ω) relative to the Ni3S2/Ni nanostructure (ca. 50 Ω) and Ni foam (ca. 140 Ω). These results demonstrate that the 3D MoS2/Ni3S2/Ni composite possesses faster electrode kinetics, a particularly useful feature in enhancing the HER catalytic activity. By the way, it was exhibited that the structure has a potential application of reduces corrosion in neutral media (Figures S14 and 15). In addition, the dominant mechanisms in the HER can often clarify by the corresponding Tafel slope. According to the classic theory for transition metal chalcogenides in the alkaline solution,19,29,49 the HER is achieved via the Tafel-Volmer-Heyrosky mechanism (different from that in the acid medium), which can be divided into two elementary reaction steps according to Equations 1 and 2 or 3. H2O + e- + C* → C*Hads + OH-

(1)

2C*Hads → H2 + 2C*

(2)

H+ + e- + C*Hads →H2 + C*

(3)

The first step is the Volmer mechanism of the electron transfer and adsorption reaction at the electrode. In Equation 1, C* denotes a free adsorption site on the catalyst, and C*Hads denotes a hydrogen atom adorned on the cathode surface. In the second step, the H2 molecule 11 ACS Paragon Plus Environment


is formed in a Langmuir-Hinshelwood-type reaction (Equation 2) or in a Heyrovsky mechanism (Equation 3). Therefore, the 3D MoS2/Ni3S2/Ni composite with a Tafel slope of 56 mV dec-1 goes through a combined Volmer-Heyrovsky mechanism in HER. To clarify the synergistic effects from the synthesized 3D MoS2/Ni3S2/Ni composite on the catalytic process, a simple model (Figure 7) can be considered. The obtained composite contains a large number of active HER catalytic sites, due to the abundantly accessible edges resulting from the small and highly dispersed MoS2 nanosheets on the surface of the Ni3S2 nanoarrays grown on the Ni foam. The electrical coupling among Ni, Ni3S2, and MoS2 also facilitates rapid electron transfer from Ni to MoS2/Ni3S2, forming an excess negative charge density to improve the Gibbs free energy and facilitate the HER route. The related HER process consisted of a combination of the Volmer reaction (involving an electrochemical sorption step that converts molecules into absorbed hydrogen atoms on the catalyst surface), and the Heyrovsky reaction (involving an electrochemical desorption step that forms surface hydrogen molecules). Therefore, the produced 3D MoS2/Ni3S2/Ni composite is believed to be an effective catalyst to reduce dissociated H+ ions and release the H2 molecules on a large number of active sites. 4. CONCLUSIONS In conclusion, we have used a facile hydrothermal method to fabricate a composite of 1D MoS2/Ni3S2 nanoarrays on 3D Ni foams as an efficient HER electrocatalyst in alkaline solution. By virtue of its large surface area, well-separated nanoarrays with uniform growth, abundant active edge sites, high conductivity, and super hydrophilic interface, the adhesivefree continent 3D MoS2/Ni3S2/Ni composite exhibited a low applied overpotential of 76 mV and a low Tafel slope of 56 mV dec–1. Moreover, the low applied overpotentials over long periods of time suggest that the 3D MoS2/Ni3S2/Ni electrocatalyst has superior stability for stable hydrogen evolution. With its structural advantages and facile synthesis, the rationally 12 ACS Paragon Plus Environment

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designed, binder-free, and self-assembled 3D MoS2/Ni3S2/Ni composite provides a new way for developing inexpensive and efficient HER electrocatalysts in electrochemical H2 production and will have potential applications in supercapacitors, lithium-ion batteries, energy conversion-storage devices, and superhydrophobic surfaces. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Comparison of the HER performances of representative non-platinum electrocatalysts in alkaline solution; Photographs of Ni foam without and with the MoS2/Ni3S2 coating; Photograph of the 3D MoS2/Ni3S2/Ni after ultrasonication in solution for 10 min; Photographs of the electrode surfaces of Ni foam and Ni3S2/Ni; SEM images of Ni3S2/Ni; XPS spectra of Ni 2p and S 2p of Ni3S2/Ni; Photograph of the Ni3S2/Ni after ultrasonication in solution for 10 min; Polarization curves of the Ni foam, MoS2/Ni, Ni3S2/Ni, and 3D MoS2/Ni3S2/Ni composite; Durable operation of the 3D MoS2/Ni3S2/Ni and Pt foil at 10 mA cm-2; Photos of the Pt foil and 3D MoS2/Ni3S2/Ni during the durable operation at 10 mA cm-2 in an alkaline electrolyzer; The long-term stability test result at 20 mA cm-2 in 1 M KOH; Time-dependent current density curves of 3D MoS2/Ni3S2/Ni at a static potential of 76mV versus RHE for 24 h in 1 M KOH; SEM images of 3D MoS2/Ni3S2/Ni after the long-term test; The Nyquist plots of bare Ni foam, Ni3S2/Ni, and 3D MoS2/Ni3S2/Ni electrodes; Contact angle photograph of surface modified 3D MoS2/Ni3S2/Ni composite; Potentiodynamic polarization curves for Ni foam, 3D MoS2/Ni3S2/Ni composite and surface modified 3D MoS2/Ni3S2/Ni composite in 3.5 wt.% NaCl solution. (PDF) AUTHOR INFORMATION Corresponding Author



*E-mail: [email protected]. ORCID Yufeng Zhang: 0000-0002-6241-1687 Jiamu Cao: 0000-0002-4631-3382 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The work described in this paper was financially supported by the National Natural Science Foundation of China (No.61404037 and No.61376113).


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Figure 1. (a) Schematic illustration of the formation of the 3D MoS2/Ni3S2/Ni composite by a facile one-step process. (b) XRD pattern of the composite.



Figure 2. (a) XPS survey spectra and (b-d) high-resolution XPS spectra of the 3D MoS2/Ni3S2/Ni composite.


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Figure 3. (a and b) Low-magnification and (c and d) high-magnification SEM images of the 3D MoS2/Ni3S2/Ni composite.



Figure 4. (a) SEM image of the cross-section of 3D MoS2/Ni3S2/Ni composite and (b) the corresponding EDX mapping data for Ni, Mo, and S. (c) TEM image, and (d) HRTEM images of the MoS2/Ni3S2 nanowire. The inset is the particular lattice structure of MoS2.


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Figure 5. Contact angle photograph of bare Ni foam from (a) and 3D MoS2/Ni3S2/Ni composite (b) the droplet experiment. Ni foam from (c) and 3D MoS2/Ni3S2/Ni composite (d) after 20 min long-term stability test (at 20 mA cm-2 in 1 M KOH aqueous solution).



Figure 6. (a) Polarization curves and (b) the corresponding Tafel plots of the Ni foam, Ni3S2/Ni nanostructure, 3D MoS2/Ni3S2/Ni composite, and Pt/C. (c) Performance comparison with selected state-of-the-art HER electrocatalysts. (d) Polarization curves of the 3D MoS2/Ni3S2/Ni before and after 2,000 cyclic voltammetry cycles. The inset is the long-term stability test result at 10 mA cm-2 in an alkaline electrolyte (1 M KOH aqueous solution).


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Figure 7. Schematic illustration of mechanism governing electrocatalytic HER on the 3D MoS2/Ni3S2/Ni composite.



Table of Contents: A novel nanostructure with one-dimensional (1D) MoS2/Ni3S2 nanoarrays directly grow on a three-dimensional (3D) Ni foam is an ideal electrocatalyst with the hydrophilic interface can accelerate the gas-liquid two-phase transport in HER so that the catalytic effect of the electrocatalyst is strengthened.


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Figure 1 132x175mm (300 x 300 DPI)

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Figure 2 160x122mm (300 x 300 DPI)


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Figure 3 180x154mm (300 x 300 DPI)



Figure 4 167x133mm (300 x 300 DPI)


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Figure 5 96x62mm (300 x 300 DPI)



Figure 6 154x114mm (300 x 300 DPI)


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Figure 7 75x56mm (300 x 300 DPI)


Ni3S2 Nanoarrays Supported on

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