Ni3S2 for Highly Efficient

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Super Hydrophilic Heteroporous MoS2/Ni3S2 for Highly Efficient Electrocatalytic Overall Water Splitting fang li, Dafeng Zhang, rongchen xu, Wen-Fu Fu, and Xiao-Jun Lv ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00665 • Publication Date (Web): 26 Jul 2018 Downloaded from http://pubs.acs.org on July 29, 2018

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ACS Applied Energy Materials

Super Hydrophilic Heteroporous MoS2/Ni3S2 for Highly Efficient Electrocatalytic Overall Water Splitting ∥

∥

Fang Li,†, § Dafeng Zhang,*, †, ‡ Rong-Chen Xu, †, Wen-Fu Fu*, †, and Xiao-Jun Lv*, † †

Key Laboratory of Photochemical Conversion and Optoelectronic Materials & CAS-HKU Joint

Laboratory on New Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R. China ‡

Department of Energy and Chemical Engineering, College of Chemistry and Chemical

Engineering, Henan Polytechnic University, Jiaozuo 454003, P. R. China ∥

College of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming

650092, P. R. China §

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

KEYWORDS: electrocatalysis, superhydrophilic, heteroporous, high activity, overall water splitting

ABSTRACT: Water molecular adsorption, intermediate species transformation, and product desorption are three main steps for the water splitting that are subject to the corresponding factors of surface wettability, exposed active sites and mass transfer, respectively. Suitable catalyst with the tailored architecture should be highly regarded to optimize the consistency and systematicness of these three procedures. Herein, highly hydrophilic heteroporous MoS2/Ni3S2 on nickel foam (p-MoS2/Ni3S2/NF) is fabricated through two-steps strategy including

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electrodeposition and solvothermal reaction. Intensive water affinity relevant with the porous structure and composition is identified by the contact angle test. Electrochemical surface area results demonstrate the upsurge of active sites for the porous structure in comparison with other samples. The fast mass diffusion feature of the p-MoS2/Ni3S2/NF catalyst is further proved by the multi-step chronoamperometric for HER and OER process. Attributed to the above nature superiorities, the electrochemical activity of this p-MoS2/Ni3S2/NF has been greatly enhanced. The overpotentials at a geometric current density of 10 mA cm-2 for OER and HER significantly reduce to the 185 mV and 99 mV in alkaline media, respectively. The as-prepared catalyst pMoS2/Ni3S2/NF only needs the cell voltages of 1.50, 1.62, and 1.71 V at current densities of 10, 20, and 50 mA cm−2 to drive water splitting and performs the good stability for at least 48 h totally. This fascinating material is designed by multiscale principles stemmed from a better insight on the structure-activity relationship and it can also afford constructive inspirations for the forward development of various catalytic reactions.

INTRODUCTION Electrocatalytic water splitting into H2 and O2 is one of the most attractive and competitive way to supply the renewable and sustainable hydrogen energy to reduce the exploitation and emission from the fossil fuels.1–5 However, the high dynamic overpotentials for both cathodic hydrogen evolution reaction (denoted as HER) and anodic oxygen evolution reaction (denoted as OER) have greatly limited the practical application of the electrocatalytic overall water splitting.6–8 Currently, state-of-the-art electrocatalysts used for water splitting are mainly Pt based materials for HER, and Ru or Ir based compounds for OER, respectively.9,10 However, their high cost, low earth storage and poor stability limit extensively practical utilization.11 Therefore, developing


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and designing highly efficient and economical electrocatalysts especially made from earthabundant elements still is one of the key factor for water splitting.12 Recently, metal sulfide has been reported that can be highly active to realize water splitting,9,13,14 and their catalytic activity shows a large dependence on its morphology and structure. Tremendous progresses focused on macromorphology have been achieved in high-efficiency metal sufide catalysts expecting to provide more contact interface between catalyst and electrolyte.15−18 Although the high energy ultilization efficiency can be actually obtained by the well-arranged active sites, some other features of the catalysts are also very important and profitable for water splitting system. The surface physicochemical properties such as adsorption/desorption ability have been demonstrated that an improved wettability can strongly enhance the water splitting performance.19-21 The water splitting reaction processes as expressed in the following Equations (1) ~ (4) further illustrate that both the adsorption of water molecular [(1) ~ (2)] and the fast desorption of the produced gas molecular from the corresponding sites [(3) ~ (4)] are pivotal steps. 22 HER : H2O + e− → OH− + H*

(1)

OER : OH− → OH* + e−

(2)

HER : H* + H2O + e− → H2 + OH−

(3)

OER : OOH* → O2 + H+ + e−

(4)

Therefore, a well-organized microporous structure can not only provide much more abundant reactive centers, but also more channels for the outward diffusion of the produced gas (H2 on cathode and O2 on anode) to make more room for the reactant molecules is needed for the high


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active water splitting.23−25 Unfortunately, until now few literatures have taken a panoramic view to build the interplay of these three aspects (adsorption ability/electroactive sites/desorption ability) and the reaction mechanism with the goal of largely promoting the electrocatalytic performance. Herein, for the first time we skillfully prepared the super hydrophilic heteroporous pMoS2/Ni3S2/NF for the overall water splitting. This heterostructure catalyst emerges for exhibiting extraordinary hydrohyphilic nature, exposing more abundant active sites and simultaneously constructing enough channels for the outward diffusion of the produced gas. Further, sufficient experimental evidences also prove the positive advantages to the catalytic activity. The p-MoS2/Ni3S2/NF has presented an ability to effectively reduce the overpotential to 185 mV for OER, and 99 mV for HER at a geometric current density of 10 mA cm-2 in alkaline media, respectively. Furthermore, quite low cell voltages of only 1.50, 1.62, and 1.71 V at the current density of 10, 20, and 50 mA cm-2, respectively, and a remarkable stability for more than 48 h are obtained for the overall water splitting. The distinguished OER, HER, and water splitting performance of this porous electrode is superior to that of the other reported electrocatalysts, such as the Ni3S2/NixCo3-xS4,26 Ni3S2/MoOx,6 Co9S8@MoS2 core-shell nanostructure,17,18 and NiFe-LDH27 etc. RESULTS The pure porous nickel layer is firstly electrodeposited on the surface of Ni foam without any other template, and that is demonstrated by X-ray diffraction (XRD) and scanning electron microscope (SEM, Figure S1 and S2). The higher resolution SEM images of Ni/NF show that the pores are surrounded by a plenty of small stacked Ni microspheres (Figure S3). Furthermore, the


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MoS2/Ni3S2 heterostructure was obtained after the porous Ni/NF sample directly react with (NH4)2MoS4 (ATM) under the solvothermal condition. As the XRD results shown in Figure 1a, the temperature of solvothermal process has a great influence on the growth of MoS2/Ni3S2 heterostructure. At the initial of 140 oC, we find that there are no appearance peaks except for the metal nickel phase. As the temperature goes up to 170 oC, the diffraction peaks at 31.1o, 37.8o, 44.3o, 49.7o, 55.3o are ascribed to the (110), (003), (202), (113), (300) lattice planes of Ni3S2 (JCPDS card no. 44-1418),28,29 respectively, that demonstrated pure Ni3S2 is firstly obtained,. When the temperature furtherly reaches to 200 oC, the MoS2 phase has also appeared at then, showing the XRD peaks at 32.7o, 35.9o, 58.3o, 70.1o and 72.8o which are assigned to the (100), (102), (110), (108), and (203) lattice planes of MoS2, respectively (JCPDS card no.650160).30,31 This indicates a different nucleation process of MoS2 and Ni3S2 phases. The morphology of the as-produced MoS2/Ni3S2 heterostructure is examined by SEM, which presents a distinctive porous feature similar with that of Ni/NF (Figure 1b) and the pore diameter distributes between 4 and 12 µm (Figure 1c). In sharp contrast, no specific porous morphology is observed on the MoS2/Ni3S2/NF catalyst that is prepared under the same condition except replacing the porous Ni/NF substrate with the smooth pristine NF in solvothermal process (Figure S4 and S5), demonstrating the advantage of electrodeposition method in creating porous heterostructures. The energy dispersive spectrum (EDS, Figure S6) of the p-MoS2/Ni3S2/NF verifies the existence of the Mo, Ni and S. And the corresponding elements mapping of the pMoS2/Ni3S2/NF is shown in the Figure 1d ~ 1g, manifesting the uniform distribution of the Mo, Ni and S across the catalyst. The as-prepared MoS2/Ni3S2 composite catalyst is further investigated by transmission electron microscopy (TEM, Figure 1h). The lattice fringe spacing of 0.24 nm is assigned to the (003) facet of Ni3S2 28,32 and 0.62nm is attributed to the (002) facet of


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MoS2.33,34 To further verify if the electrodeposited Ni is fully conversed to Ni3S2. The inductively coupled plasma atom emission spectrometry (ICP–AES) result indicates that the mole ratio of Ni : Mo : S equals to 56% : 7% : 37% (including the NF substrate). Further, based on the weight increment after the solvothermal reaction and ICP result, we can calculate the quality of the Ni3S2 (20.90 mg/ 0.5cm2) (detailed calculation part in the supporting information). Therefore, we can conclude that the sulfurated process can completely consume the electrodeposited metal nickel layer (9 mg on 0.5 cm2 NF, weighed by analytical balance) so as to transform to Ni3S2. The electrodeposited nickel plays the role of template for the formation of the porous pMoS2/Ni3S2/NF.

Figure 1 (a) the XRD of the catalyst prepared after solvothermal reaction at different temperature, (b) the SEM of the p-MoS2/Ni3S2/NF, (c) the corresponding pore size distribution the EDS of the MoS2/Ni3S2 composite, (d)~(g) SEM images of the p-MoS2/Ni3S2/NF and the corresponding elements mapping, the yellow: Mo; the green: Ni; the red: S, (h) the HRTEM of the catalyst MoS2/Ni3S2, and the scale bar is 3 nm. Furtherly, X-ray photoelectron spectroscopy (XPS) is used to analyze the chemical state and surface composition of the p-MoS2/Ni3S2/NF catalyst. From the core level XPS spectrum of Ni


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2p in Figure 2a, two strong peaks at around 873.4 eV and 855.8 eV can be attributed to Ni 2p1/2 and Ni 2p3/2, respectively, with the satellite peaks at 861.6 eV and 879.5 eV.35 At the Mo 3d region, the peaks at 232.7 eV and 228.8 eV are assigned to the 3d3/2 and 3d5/2 of Mo4+, whereas, the peaks at 235.2 eV and 232.0 eV are of the Mo6+ (Figure 2b). The existence of the Mo6+ is resulted from the surface oxidation of MoS2.16,36 In addition, the corresponding deconvolution spectrum of the S exhibits four peaks, of which 163.1 eV and 161.8 eV are ascribed to the S 2p1/2 and S 2p3/2 of Mo–S bond,37 and the peaks at 162.5 eV and 161.1 eV are the S 2p1/2 and S 2p3/2 for Ni–S bond (Figure 2c).38

Figure 2 (a) Core level XPS spectra of Ni (2p) in MoS2/Ni3S2 hybrid. (b) Core level XPS spectra of Mo (3d) in MoS2/Ni3S2 hybrid. (c) the corresponding deconvolution spectrum of the S (2p) The electrocatalytic activity of the p-MoS2/Ni3S2/NF catalyst are first evaluated for the water oxidation with electrochemical cyclic voltammetry (CV) method in 1.0 M KOH aqueous solution at a scan rate of 1 mV s−1 under a traditional three-electrode system. The optimized experiments show that the catalytic activity is affected by the electrodeposition time of porous Ni layer on NF and the quantity of the precursor (NH4)2MoS4 (Figure S7). When the electrodeposition time of porous Ni layer is 500 s and the quantity of (NH4)2MoS4 is 80 mg in solvothermal reaction, the catalysts shows the best electrocatalytic performance. All the samples


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for electrocatalytic property were the optimal electrodeposition time and precursor dosage unless otherwise stated. Figure 3a shows the iR–corrected CV curves of the p-MoS2/Ni3S2/NF, MoS2/Ni3S2/NF, MoS2/NF, Ni3S2/NF, 20% IrO2/C and NF. Note that, in forward potential scan, there is a strong oxidative peak overlapped with the initial OER process, inducing a difficulty to identify the onset feature of OER. Therefore, the following backward scan is adopted to evaluate the OER performances. The OER activities for different as-prepared catalysts are summarized in Table 1. The p-MoS2/Ni3S2/NF has shown a significantly lower overpotential of 185 mV, 210 mV, and 240 mV at current densities of 10, 20 and 50 mA cm-2 compared with both Ni3S2/NF and MoS2/NF, even better than the non-porous MoS2/Ni3S2/NF (230 mV, 260 mV, and 310 mV, respectively). The Tafel slops are used to evaluate the OER kinetics of the catalysts (Figure 3b). The smallest Tafel slop of 46 mV dec−1 is obtained for the p-MoS2/Ni3S2/NF which can be comparable to those of previously reported active catalysts.39,40 Other catalysts in this work show the slops larger than 100 mV dec−1, indicating the excellent intrinsic OER kinetic of the pMoS2/Ni3S2/NF. Furthermore, the rational measurements of double-layer capacitances (Cdl) are used to estimate the electrochemical surface area (ECSA) of catalysts. The corresponding CV curves at non-faradic region with various potential scan rates are displayed in Figure S8. The Cdl of p-MoS2/Ni3S2/NF is calculated to be 55.1 mF cm−2 much larger than that of MoS2/Ni3S2/NF (Figure 3c). The higher value of the Cdl indicates the larger ECSA of the catalyst (Table S1) which is in good agreement with the activity sequence of the OER. In addition, by normalizing the CV curves to the ECSA, a better intrinsic activity of p-MoS2/Ni3S2/NF over MoS2/Ni3S2/NF is obtained (Figure S9), designating that the porous structure can expose more active intrinsic sites. In order to testify that p-MoS2/Ni3S2/NF indeed obtains superior intrinsic catalytic activity


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than MoS2/Ni3S2/NF, the mass activities normalizing the current for both HER and OER are provided in Figure S10. The excellent OER property of p-MoS2/Ni3S2/NF is also confirmed by the electrochemical impedance spectroscopy (EIS) measurement. The Nyquist plots (Figure S11) of p-MoS2/Ni3S2/NF shows a smaller semicircle, which means a lower charge transfer resistance in comparison with the other catalysts.

Figure 3 (a) CV curves of OER for the p-MoS2/Ni3S2/NF, MoS2/Ni3S2/NF, Ni3S2/NF, MoS2/NF, 20% IrO2/C, and NF with iR–correction (b) Tafel plots of the above catalysts for the OER, (c) double-layer capacitance per geometric area (Cdl), (d) LSV curves of HER for the pMoS2/Ni3S2/NF, MoS2/Ni3S2/NF, Ni3S2/NF, MoS2/NF, 20% Pt/C, and NF with iR–correction. (e) Tafel plots of the above catalysts for the HER, (f) intrinsic HER activity of the MoS2/Ni3S2/NF and p-MoS2/Ni3S2/NF in 1.0 M KOH The HER performance of the as-prepared catalysts is further evaluated by the linear sweep voltammetry (LSV) in 1.0 M KOH solution for HER, as shown in Figure 3d and summarized in Table 1. The p-MoS2/Ni3S2/NF also exhibits the best HER activity in our system with the overpotential (η10) of only 99 mV at a current density of 10 mA cm−2. In contrast, the


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MoS2/Ni3S2/NF (η10 = 129 mV), MoS2/NF (η10 = 186 mV), Ni3S2/NF (η10 = 219 mV) and NF (η10= 305 mV) just show mediocre HER performances (Figure 3d). A smaller Tafel slope of 71 mV dec−1 is detected on the p-MoS2/Ni3S2/NF, demonstrating a fast kinetic for HER (Figure 3e). As for the Cdl, the p-MoS2/Ni3S2/NF catalyst shows the largest value (Figure S12 and S13). The specific HER activities of MoS2/Ni3S2/NF and p-MoS2/Ni3S2/NF also confirm the porous structure could provide more intrinsic active sites (Figure 3f), in a good agreement with the results of OER. Moreover, it is worth noting that the distinguished HER and OER activities of the p-MoS2/Ni3S2/NF catalyst is comparable with some other noble-metal-free catalysts in Table S2 and S3. Table 1. Comparison of HER and OER activities for different catalysts.

Catalyst

Reaction

η10 (mV))

Tafel slop (mV dec-1)

Cdl (mF cm-2)

p-MoS2/Ni3S2/NF

HER OER HER OER HER OER HER OER HER OER

99 185 129 230 219 285 186 321 305 370

71 46 81 101 151 143 139 161 169 167

202.1 55.1 178.4 32.3 9.9 12.1 29.9 3.46 3.6 1.93

MoS2/Ni3S2/NF Ni3S2/NF MoS2/NF NF

Long-term stability is another parameter to consider the industrial application.41 The overpotentials at the current density of 10 mA cm−2 of OER and HER were 200 mV and 105 mV, respectively, and showed negligible increase during the 35 h in Figure 4a. The morphology of pMoS2/Ni3S2/NF catalyst after long term HER and OER also shows no evident changes (Figure S14a and 14b), further demonstrating the stability of this porous catalyst. In view of the notable


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HER and OER properties, this porous p-MoS2/Ni3S2/NF catalyst could potentially be applied to efficiently drive the water-splitting. Consequently, the p-MoS2/Ni3S2/NF is exploited as both the anode and cathode with two-electrode water-splitting system in 1.0 M KOH electrolyte, as the optical photograph illustrated in Figure S15. The bubbles can be clearly seen on the surface of the two electrodes during the water splitting. Figure 4b shows a corresponding duration test result. The p-MoS2/Ni3S2/NF catalyst has exhibited excellent performance for water splitting with cell voltages of only 1.50, 1.62, and 1.71 V at current densities of 10, 20, and 50 mA cm−2, respectively, comparable with the well-known Pt/C/NF||RuO2/NF (~1.53 V at 10 mA cm−2) cell.13 Meanwhile there are almost no changes in cell voltage at different current density, manifesting the good stability of the catalyst. Table S4 shows the comparison of water-splitting performance of different materials. The catalyst in our work is much better than the other reported catalysts.

Figure 4 (a) Long term Chronopotentiometric measurements obtained over p-MoS2/Ni3S2/NF with iR–correction for 35 h of OER (the black line) and HER (the red line). The current density all remains at 10 mA cm-2. (b) The stability of water splitting at different current densities of 10, 20, and 50 mA cm-2 are for total 48 h, each period maintains 16 h with iR–correction. DISCUSSION


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It is widely accepted that the complex water splitting phenomena can be simplified as the following mechanisms proceeded at the electrode step by step: (i) the adsorption of water molecular on the deserved site, (ii) the transforming of intermediate reactive species to the expected products through the functional sites, (iii) the desorption of the produced gas from the sites.42 Herein, various experimental investigations disclose that the p-MoS2/Ni3S2/NF catalyst has a comprehensive optimization to the above procedures, ranging from coupling reactant, prolific active sites to mass transfer aiming at fully enhancing water splitting performance. Figure 5 integrally depicts the species evolution throughout the whole process on the porous electrode p-MoS2/Ni3S2/NF as cathode (for HER) and anode (for OER) with all around judicious regulation of macromorphology and interface physicochemical properties. Several features are strongly expected:

Figure 5. Schemetic diagram of the HER on cathode and OER on anode (1) A super hydrophilicity enhanced water molecular adsorption. As we aforementioned, water splitting requires the adsorption of water molecular as the primary and fundamental step which directly influences the subsequent progress. Therefore a better


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hydrophilicity of the catalyst will enhance its compatibility to the water. The wettability of the as-produced materials is analyzed by dropping water (10 µL) onto their surface. Figure 6 shows the contact angle (CA) of different catalysts changing with time. Clearly, the CA is larger than 90º for all of the Ni3S2/NF, MoS2/NF and NF catalysts, indicating a hydrophobic nature of them. By comparison, the wettability has undergone an obvious improvement on the MoS2/Ni3S2 heterosturcture. A water droplet with a smaller initial CA of 70º is observed on the non-porous MoS2/Ni3S2/NF, which has been quickly absorbed within 0.5 s (Figure 6d). More interestingly, a super hydrophilicity is further obtained on the porous p-MoS2/Ni3S2/NF, on which the water droplet is ready to spread out and be totally absorbed as soon as they contact with the catalyst surface (Figure 6e). The dynamic CA tests demonstrate that not only the composition but also the microporous morphology has contributed to the superb wettability of the p-MoS2/Ni3S2/NF heterostructure. This super hydrophilic nature has an intrinsic ability to induce more favorable contact points around the catalytic active sites and thus to improve the overall reaction process.43 Actually, during the reaction, lots of big gas bubbles of O2 and H2 have accumulated on the surface of non-porous MoS2/Ni3S2/NF and seriously prevented the adsorption of water (Figure 6f). In contrast, large gas bubbles have not been found on the porous p-MoS2/Ni3S2/NF catalyst, which is believed to be largely squeezed out by the quick adsorption of water (Figure 6g). (2) The high intrinsic activity and the reaction kinetics for the HER and OER. The reaction steps of the HER and OER in alkaline solution can be expressed in Equations (5) ~ (11).22,44,45 For HER reaction, it includes the Volmer (5), Heyrovsky (6) and Tafel reaction (7). As for the OER, it is a four-electron transferring steps [(8) ~ (11)]. HER : H2O + e− → OH− + H*

(5)


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H* + H2O + e−→ H2 + OH−

(6)

H* + H* → H2

(7)

OER : OH− → OH* + e−

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(8)

OH* → O* + H+ + e−

(9)

O* + OH− → OOH* + e−

(10)

OOH* → O2 + H+ + e−

(11)

In the case of the p-MoS2/Ni3S2/NF heterostructure, the nature of integrating two phases (MoS2 and Ni3S2) has made it being intrinsically active to drive the above OER and HER procedure. As shown in Figure 3, the p-MoS2/Ni3S2/NF shows the lower onset potential and smaller overpotential at geometric current density of the 10 mA cm-2. The intrinsic electrocatalytic activities (normalizing the current by the ECSA and the catalyst mass loading, Figure S9, S10 and Figure 3f) further testify that the porous characteristic not only guarantees a larger contact area of the electrocatalyst/electrolyte, but more importantly enhances the intrinsic activity of the individual interfacial active centers for the overall reactions. Notably, the Tafel slope value of the p-MoS2/Ni3S2/NF is the smallest compared to other catalysts. Importantly, the reaction Tafel slope can reflect the reaction mechanism and reaction kinetics. According to the Dai’s calculation of the Tafel slop, the Tafel slopes of the catalysts with Volmer reaction, Heyrovsky reaction and Tafel reaction as the rate-determining step are 120, 40 and 30 mV/decade, respectively.46 Durst proposed that these reactions are unchanged in alkaline.47 The Tafel slop of only 71 mV dec-1 of the p-MoS2/Ni3S2/NF in the current work suggests that the possible elementary parthway for HER is a Volmer [the Equations (5)] or the Heyrovsky [the Equations


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(6)] reaction mechanism, and hydrogen adsorption step or electrochemical desorption is the ratelimiting step, that is similar with the report reference.48 As for the OER, the Tafel slop of 46 mV dec-1 of p-MoS2/Ni3S2/NF in the current work is close to the 40 mV dec-1, indicating that the second electron transfer step is the rate-determining pathway [the Equations (9)].49 In addition, the p-MoS2/Ni3S2/NF has also been proved that the heteroporous structure has also better electron mobility capability by the Nyquist plots (Figure S11) compared to other samples, all these superiority guarantee more efficient electrocatalytic reaction.

Figure 6. the contact angel of the as-prepared catalysts, (a) NF, (b) MoS2/NF, (c) Ni3S2/NF, (d) MoS2/Ni3S2/NF, (e) p-MoS2/Ni3S2/NF, (f) the picture of MoS2/Ni3S2/NF during the reaction, (g) the picture of p-MoS2/Ni3S2/NF during the reaction under the same electrocatalytic condition with MoS2/Ni3S2/NF. (3) A fast mass diffusion enhanced electrocatalytic activity.


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The produced gas of H2 and O2 hold the active sites and impede the proximity of the water molecular. Considering that, although the super hydrophilic nature of p-MoS2/Ni3S2/NF has promoted a highly efficient water adsorption on reactive site to squeeze out the produced H2 and O2, the outward diffusion of the gas should be further accelerated to feed a fast kinetic of the overall reaction. Herein, multi-step chronoamperometric measurements for HER and OER have been used to evaluate the mass diffusion feature of the p-MoS2/Ni3S2/NF catalyst. As shown in Figure 7a and 7b, potential ranges of 0 V to –0.4 V for HER and 1.3 V to 1.7 V for OER with augmentation of 50 mV per 500 s are adopted. The current densities significantly increase with the potential stepping and maintain a steady increased current density, demonstrating an excellent mass transport property of the present catalyst.50 In fact, the larger numbers of micro channels embedded in the matrix have favored an interesting chance to release the gas from the porous structure.

Figure 7. (a, b) Multi-step chronoamperometric curve of p-MoS2/Ni3S2/NF obtained with 1 M KOH without the iR–correction, the potentials for (a) HER from 0 V to – 0.4 V vs. RHE, and for (b) OER from 1.3 V to 1.7 V vs. RHE. The augmentation: 50 mV per 500 s. CONCLUSION For the first time, we prepared a super hydrophilic porous p-MoS2/Ni3S2/NF catalyst that can simultaneously resolve the bottlenecks during the water splitting through multiscale tuning


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surface wettability, active sites and mass transportation. The super hydrophilic nature of the pMoS2/Ni3S2/NF realizes the reasonable water absorbance which is the foremost requirement for the overall reaction. Owing to the porous structure, abundant reactive sites have been created. Moreover, the macroporous morphology also provides a great number of micro channels for the produced gas diffusion which can in return accelerate the water adsorption. It is believed that the aforementioned material design merits have exactly contributed the distinguished superiority of p-MoS2/Ni3S2/NF catalyst in HER and OER, and also in the overall water splitting process. The low overpotential, large current density and long term stability of this porous catalyst are expected to be comparable to the most of state-of-the-art water splitting catalysts. This investigation has clarified the relationship between the effective strategies and tremendous optimizing reactivity. Further, this work will pave the way for the future development of a broad range of reactions. ASSOCIATED CONTENT Supporting information includes experimental materials, synthesis, instruments parameters, electrocatalytic measurements, XRD, SEM, EDS, electrocatalytic results and other corresponding Tables. AUTHOR INFORMATION Corresponding Authors * Xiao-Jun Lv. E-mail: [email protected] * Da-Feng Zhang. E-mail: [email protected] * Wen-Fu Fu. E-mail: [email protected] ORCID Xiao-Jun lv: 0000-0002-8040-881X


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Dafeng Zhang: 0000-0002-2124-3867 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the NSFC (Grant No. 21477136, 21471155), Beijing Natural Science Foundation (2182077), Henan Educational Committee Foundation (13B150024), and Natural Science Foundation of Henan Province (182300410196). We are also thankful for the financial support by the Strategic Priority Research Program of the Chinese Academy of Sciences, Grant No. XDB17030300. REFERENCES (1) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Norskov, J. K.; Jaramillo, T. F. Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design. Science 2017, 355, 6321−6332. (2) Zhao, S.; Wang, Y.; Dong, J.; He, C.-T.; Yin, H.; An, P.; Zhao, K.; Zhang, X.; Gao, C.; Zhang, L.; Lv, J.; Wang, J.; Zhang, J.; Khattak, A. M.; Khan, N. A.; Wei, Z.; Zhang, J.; Liu, S.; Zhao, H.; Tang, Z. Ultrathin Metal–Organic Framework Nanosheets for Electrocatalytic Oxygen Evolution. Nat. Energy 2016, 1, 16184−16193. (3) Roger, I.; Shipman, M. A.; Symes, M. D. Earth-Abundant Catalysts for Electrochemical and Photoelectrochemical Water Splitting. Nat. Rev. Chem. 2017, 1, 0003. (4) Stamenkovic, V. R.; Strmcnik, D.; Lopes, P. P.; Markovic, N. M. Energy and Fuels from Electrochemical Interfaces. Nat. Mater. 2016, 16, 57−69. (5) Hunter, B. M.; Gray, H. B.; Muller, A. M. Earth-abundant Heterogeneous Water Oxidation


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Ni3S2 for Highly Efficient

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