Ni as a Highly

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Hierarchical Nanoassembly of MoS2/Co9S8/Ni3S2/Ni as a Highly Efficient Wide-pH Range Electrocatalyst for Overall Water Splitting Yan Yang, Huiqin Yao, Zihuan Yu, Saiful M. Islam, Haiying He, Mengwei Yuan, Yonghai Yue, Kang Xu, Weichang Hao, Genban Sun, Huifeng Li, Shulan Ma, Peter Zapol, and Mercouri G. Kanatzidis J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 06 Jun 2019 Downloaded from http://pubs.acs.org on June 6, 2019

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Journal of the American Chemical Society

Hierarchical Nanoassembly of MoS2/Co9S8/Ni3S2/Ni as a Highly Efficient Wide-pH Range Electrocatalyst for Overall Water Splitting Yan Yang,a Huiqin Yao,b Zihuan Yu,a Saiful M. Islam,c,d Haiying He,e Mengwei Yuan,a Yonghai Yue,f Kang Xu,f Weichang Hao,f Genban Sun,a Huifeng Li,a Shulan Ma,a,c,* Peter Zapol,g,* and Mercouri G. Kanatzidisc,g,* aBeijing

Key Laboratory of Energy Conversion and Storage Materials and

College of Chemistry, Beijing Normal University, Beijing 100875, China. E-mail: [email protected] bSchool

of Basic Medical Sciences, Ningxia Medical University, Yinchuan 750004, China.

cDepartment

of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA. E-mail: [email protected]

dDepartment

of chemistry, Physics and Atmospheric Sciences, Jackson State University, Jackson, MS 39217, USA.

eDepartment

of Physics and Astronomy, Valparaiso University, Valparaiso, IN 46383, USA.

fSchool

of Physics and School of Chemistry, Beihang University, Beijing 100191, China.

gMaterials

Science Division, Argonne National Laboratory, Lemont, IL 60439, USA. E-mail: [email protected]

1

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ABSTRACT The design of low-cost while high-efficiency electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in wide pH is highly challenging. We now report a hierarchical co-assembly of interacting MoS2 and Co9S8 nanosheets attached on Ni3S2 nanorod arrays which are supported on nickel foam (NF). This tiered structure endows high performance towards HER and OER over a very broad pH range. By adjusting molar ratios of Co:Mo precursors, we have created CoMoNiS-NF-xy composites (x:y means Co:Mo molar ratios ranged 5:1 to 1:3) with controllable morphology and composition. The three dimensional composites own abundant active sites capable of universal pH catalytic HER and OER activity. The CoMoNiS-NF-31 demonstrates the best electrocatalytic activity, giving ultralow overpotentials (113, 103 and 117 mV for HER) and (166, 228, and 405 mV for OER) to achieve the current density of 10 mA cm−2 in alkaline, acidic and neutral electrolyte, respectively. It also shows a remarkable balance between electrocatalytic activity and stability. Based on distinguished catalytic performance of CoMoNiS-NF-31 towards HER and OER, we demonstrate a two-electrode electrolyzer performing water electrolysis over a wide pH range, with low cell voltages of 1.54, 1.45 and 1.80 V at 10 mA cm−2 in alkaline, acidic and neutral medias, respectively. First-principles calculations suggest high OER activity arises from electron transfer from Co9S8 to MoS2 at the interface, which alters binding energies of adsorbed species and decreases overpotentials. Our results demonstrate that hierarchical metal sulfides can serve as highly efficient all-pH (pH=0-14) electrocatalysts for overall water splitting.

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1. INTRODUCTION Hydrogen energy, as an alternative to conventional fossil fuels, is a long standing goal which if achieved will have a revolutionizing impact in all aspects of energy utilization.1 Efficient electrochemical water splitting driven by renewable energy sources can be a clean pathway to generate pure hydrogen.2,3 In general, water electrolysis proceeds via two half-cell reactions: anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER).4 Due to the unfavorable thermodynamics and sluggish kinetics of both OER and HER, we need highly efficient electrocatalysts that can simultaneously reduce overpotentials and accelerate the reaction rate.5 Noble metals and their compounds are known as the most efficient catalysts for HER (Pt-based materials) and OER (Ru-/Ir-based oxides), but their broad commercial application is largely hindered by the scarcity and high price.6 Decades of researches in developing ideal noble-metal free catalysts have shown how challenging it is to find stable and scalable electrocatalysts to accomplish cost-effective HER or OER. Generally, efforts are focused on different catalysts toward the HER and OER. In order to achieve practical applications of overall water splitting, low cost bifunctional electrocatalysts, meaning catalysts that can perform both HER and OER in the same electrolyte, are preferable, since they can simplify the operating system.6,7 Water electrolysis for industrial applications is mainly performed in alkaline solutions.8 A great number of non-noble metals (such as Co, Ni, Mn, Fe, Mo, and W) based materials have been investigated as bifunctional electrocatalysts for water splitting in alkaline media.9-15 Compared with alkaline electrolysis, water electrolysis in acidic medium sometimes has the advantages of simplicity and high-current density,16 and acid electrolysis can expand the application scope of water splitting. 3



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However, only a few non-noble metal catalysts (such as NC-CNT/CoP (hollow CoP nanosphere-embedded carbon nanotube/nitrogen-doped carbon)17 and Co-MoS218) have been studied for water splitting in acid electrolytes, and the overpotentials are found to be large and the working stability is poor. Hence the most effective electrocatalysts in acid are still limited to noble metals.19 Furthermore, extreme pH operating conditions in acid and alkali electrolytes often cause serious stability problems of the catalysts as well as corrosion problems of the devices, limiting the types of cell components and electrodes. Therefore, catalysts performing efficiently under benign conditions are also in great need for applications.20 In neutral media, some electrocatalysts, such as Co3S4 nanosheets,21 Li2Co2O422 and Li2MnP2O7,23 have been studied toward OER reactions, but exhibited large overpotentials. For overall water splitting in neutral media, investigations on bifunctional electrocatalysts (catalyze both HER and OER) are rather rare.17,24 In this respect, effective and pHuniversal bifunctional electrocatalysts for electrochemical water splitting are highly desirable. Currently,

MoS2

and

Mo-based

materials

are

promising

non-precious

electrocatalysts for HER, having catalytic edge sites with a low Gibbs free energy for hydrogen adsorption in the Volmer reaction, analogous to Pt.25 However, the OER activity of the Mo-containing materials needs to be improved.26 On the other hand, cobalt sulfides, such as CoS,27 CoS2,28 and Co9S8,29 can serve as efficient electrocatalysts for OER, while their HER performance is unsatisfactory. Integration of catalysts to achieve both HER and OER activity is an effective way to construct dual use catalysts.15,30 Furthermore, integrating active materials with conductive and robust substrates such as three-dimensional (3D) Ni foam (NF) can increase electron transfer rate and enhance the mechanical strength.31 4


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Motivated by the above considerations, we develop a hierarchical electrocatalytic structure made of 3D porous conductive NF covered with Ni3S2 nanorods which are then decorated with ultrathin MoS2 and Co9S8 nanosheets (~3 nm). By changing the molar ratios of CoSO4·7H2O to Na2MoO4·2H2O precursors, we obtained a series of CoMoNiS-NF-xy composites (x:y indicates Co:Mo ratios of 5:1, 4:1, 3:1, 2:1, 1:1, 1:2 and 1:3) presenting controllable morphology and compositions. Because the composites are supported on the conductive NF, they can work directly as electrodes. The CoMoNiS-NF is a synergistic hierarchical nanoassembly displaying excellent electrocatalytic performance (HER and OER), which is superior than the sum of its parts. The CoMoNiS-NF-31 (Co:Mo ratios is 3:1) exhibits the best catalytic activity achieving very low overpotentials of 113, 103, and 117 mV at a current density of 10 mA cm−2 (10) for HER, and low 10 values of 166, 228, and 405 mV for OER, in alkaline, acid, and neutral media, respectively. Moreover, we demonstrate overall water splitting achieved by this composition exhibits low cell voltages of 1.54, 1.45 and 1.80 V (at a current density of 10 mA cm−2) in alkaline, acidic and neutral conditions, respectively. Based on the activity-stability factor (ASF),32 CoMoNiS-NF31 displays an outstanding balance between the catalytic activity and stability. The electrocatalytic performance we report here outperforms the vast majority of reported materials. First-principles electronic structure calculations provide valuable insights on the origins of the enhanced activity in this unique integrated CoMoNiS-NF nanoassembly, whose superior activity is very promising for water electrolysis over a wide pH range. 2. EXPERIMENTAL SECTION 2.1 Synthesis of CoMoNiS-NF-xy electrocatalysts.

5



A series of CoMoNiS-NF-xy were prepared via a one-pot hydrothermal method, by adjusting the feeding ratio of Co:Mo ranging from 5:1 to 1:3. In a typical synthesis, a piece of commercial NF (6×10 cm) was treated with HCl solution (1 M ) for 20 min to get rid of the oxidation layer on the surface, followed by washing with water and ethanol. After that, the NF was weighed after dried in a vacuum oven. Taking the CoMoNiS-NF-11 (Co:Mo molar ratio of 1:1) as an example, 0.562 g (2 mmol) of CoSO4·7H2O, 0.482 g (2 mmol) of Na2MoO4·2H2O and 2 g (26.27 mmol) of CH4N2S were dissolved in 33 mL of deionized water and 15 mL of hydrazine hydrate under continuous magnetic stirring. The pretreated NF was immersed in the abovementioned solution, reacted in 80 mL Teflon-lined autoclave at 200 °C for 24 h. After cooled down naturally to 25°C, black product (named by CoMoNiS-NF-11) was obtained and rinsed with deionized water and ethanol, and dried under vacuum at 50 °C for 12 h. In addition to above Co:Mo molar ratio of 1:1 (2 mmol:2 mmol), we fixed the Na2MoO4·2H2O amount of 2 mmol (0.482 g) while varied the amount of Co source using Co:Mo molar ratios of 5:1, 4:1, 3:1 and 2:1, and carried out similar hydrothermal procedures, yielding products (denoted as CoMoNiS-NF-51, CoMoNiSNF-41, CoMoNiS-NF-31 and CoMoNiS-NF-21, respectively). Also, at fixed amount of 2 mmol CoSO4·7H2O (0.562 g), we varied the Mo dosage with Co:Mo molar ratios of 1:2 and 1:3, generating products recorded as CoMoNiS-NF-12 and CoMoNiS-NF13, respectively. As control samples, MoS2/Ni3S2/NF, Co9S8/Ni3S2/NF and Ni3S2/NF were synthesized with similar procedures to CoMoNiS-NF-xy, except not adding CoSO4·7H2O or/and Na2MoO4·2H2O. 2.2 Electrochemical measurements.

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Electrochemical measurements were conducted using a typical three electrode glass cell with a graphite rod as the counter electrode, the as-synthesized samples as the working electrode, Hg/HgO electrode (in 1 M KOH) and saturated calomel electrode (SCE, in 0.5 M H2SO4 and 1 M PBS) as reference electrodes. All potentials were converted to a reversible hydrogen electrode (RHE) via calibration (Figure S1). The mass loadings on NF were shown in Table S1. To make a more reliable comparison, RuO2 and Pt/C were loaded on NF with the same loading as that of CoMoNiS-NF-31. The polarization curves were recorded in 1.0 M KOH (pH = 13.7), 0.5 M H2SO4 (pH = 0.5), and 1.0 M phosphate buffered saline (PBS, pH = 7.0) with a scan rate of 5 mV s-1. 1 M PBS solution was prepared by dissolving 13.61 g of KH2PO4 in 100 mL water and adjusting pH to 7.0 with 1 M KOH. All polarization curves were iRcorrected. Electrochemical impedance spectroscopy (EIS) were carried out in 100 kHz to 0.01 Hz at the given potential with AC amplitude of 5 mV. 2.3 Computational Methods. The calculations within the framework of the density functional theory (DFT) with periodic boundary conditions were performed using the VASP program.33 The PBE exchange-correlation functional34 and the van der Waals (vdW) interactions described via a pair-wise force field using the DFT-D3 method of Grimme35 were used for all calculations. The projector augmented wave (PAW) method and plane wave basis sets were used with energy cutoff of 400 eV. Transition metal elements are treated by the PBE+U method with Ueff = 4.4 eV and 3.0 eV for Co and Mo, respectively.36 All atoms were allowed to relax during the structure optimization of MoS2/Co9S8 slabs. For geometry optimizations of slabs with adsorbed molecules, only adsorbed molecules and four top layers of Co9S8 were relaxed. The total energy was converged to 10-5 eV, and the geometry was relaxed until the force on each atom was below 0.03 7



eV/Å. Overpotentials were calculated using computational hydrogen electrode as described for OER.37 Bader charge analysis was done to analyze charge populations.38 3. RESULTS AND DISCUSSION 3.1 Characterization of the as-prepared electrode materials. The schematic illustration of the synthesis is shown in Figure 1A. The atomic ratios of Co:Mo in CoMoNiS-NF-xy determined with ICP-AES generally agree with their respective starting ratios (Table S2). The resulting products were first characterized by XRD (Figure 1). As shown in Figure 1B-a, three Bragg peaks emerge at 44.5°, 51.8° and 76.4° (marked by ‘□’), which correspond to (111), (200) and (220) planes of Ni (JCPDS No. 04-0850). The peaks at 21.7°, 31.1°, 37.8°, 50.1° and 55.3° (marked by ‘Δ’) can be identified as the (101), (110), (003), (211) and (300) planes of the hexagonal Ni3S2 phase (JCPDS No. 44-1418). The characteristic peaks of Ni3S2 observed in all as-obtained samples indicate the surface of NF is partially transformed into Ni3S2 during the hydrothermal treatment. The peaks located at 29.8°, 47.6° and 52.1° in Figure 1B-b and 1B-d (marked by ‘’) are assigned to Co9S8 (JCPDS No. 656801), corresponding to (311), (511) and (440) planes, respectively. The Bragg peaks at 33.1° and 39.7° in Figure 1B-c and 1B-d (marked by “ ”) are respectively indexed to (100) and (103) planes of MoS2 (JCPDS No. 37-1492). The absence of (002) diffraction peak (14.4°) could be ascribed to the less stacking MoS2 ultrathin nanosheets.39 Typical surface morphologies of all samples are shown in Figure 2. With the absence of Co and Mo sources, the as-obtained Ni3S2/NF is an interconnected, macroporous 3D framework (Figure 2a), similar to that of pristine NF (Figure S2a). A closer inspection of Ni3S2/NF shows a very rough surface (Figure 2bc), which is in sharp contrast to the smooth surface of NF itself (Figure S2b-c). After the introduction of the Co precursor, Ni3S2 nanorod arrays (with diameters of 300-600 8


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nm) are observed to align on the NF substate, and Co9S8 nanosheets uniformly cover the Ni3S2 nanorods (see Figure 2e, 2f). When only Mo source is introduced, the resulting MoS2/Ni3S2/NF exhibits a structure similar to Co9S8/Ni3S2/NF but with a slightly larger diameter (0.6-1 μm) and more nanosheets on the suface of Ni3S2 nanorod (Figure 2g-i). In the presence of Mo precursor with high Co content, the resulting

CoMoNiS-NF-51

exhibits

a

very

different

structure

from

the

Co9S8/Ni3S2/NF, and the nanorods are covered completely by densely aggregated Co9S8 nanosheets (Figure S3a-c). For fixed Mo amount and decreasing Co fraction, for instance CoMoNiS-NF-41, the aggregation is reduced and the nanorods are exposed (Figure S3d-f). When further decreasing the Co fraction, the resulting CoMoNiS-NF-31 shows that the Co9S8 and MoS2 nanosheets are directly grown on Ni3S2 nanorods and there are no redundant nanosheets to form aggregation (Figure 2jl). This effect enlarges the specific surface area of the system. The morphologies of CoMoNiS-NF-21 and CoMoNiS-NF-11 are almost the same as that of CoMoNiS-NF31 but the former two contain fewer nanosheets (Figure S3g-i) and shorter nanorods (Figure S3j-l), respectively. At fixed Co content but increased Mo fraction, such as CoMoNiS-NF-12 (Figure S3m-o) and CoMoNiS-NF-13 (Figure S3p-r), the nanorods gradually transform into nanobelts. These results clearly indicate that the amount of starting materials and the ratio of Mo:Co can effectively control the hierarchy in the nanoassembly, such as the number of nanosheets and the size of nanorods. As shown the CoMoNiS-NF-31 has a modest number of ultrathin nanosheets and the least aggregation, which remarkably increases the electrochemically active surface area with more accessible active sites. At the Co:Mo molar ratio of 3:1, we further conducted the reactions at reduced time of 8, 12, 16, 20 h (See Figure S4 for details). It was found even within 20 h (Figure S4j-l), the number of nanosheets was not 9



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sufficient to cover the nanorods. This confirms that the 24 h is an optimal time to ensure the formation of the catalytically active hierarchical co-assembly. TEM observasions (Figure 3b) confirm the hierarchical structure of CoMoNiS-NF31 that the nanosheets are densely and firmly grown on the nanorods. The highresolution TEM (HRTEM) image (Figure 3a) of the selected area of Figure 3b reveals lattice fringes of 0.28 and 0.19 nm, corresponding to the (222) and (511) plane of Co9S8, respectively. The HRTEM image of the nanorod is shown in the right part of Figure 3a, in which the lattice fringes have spacings of 0.41, 0.29, and 0.24 nm, correspond to the (101), (110) and (003) lattice planes of Ni3S2, respectively. Figure 3c (HRTEM image of the selected area of Figure 3b) shows the lattice fringe with spacing of 0.62 nm, which is assigned to the (002) crystal plane of MoS2. Meanwhile, the lattice fringes with spacing of 0.30 and 0.28 nm correspond to the (311) and (222) planes of Co9S8, respectively. From Figure 3c, the interfaces between MoS2 and Co9S8 are observable, which is an important feature and is considered to play a significant role in promoting the electrocatalytic performance for HER and OER. Figure 3d reveals the nanosheet-like structure (~3 nm) covering the surface of CoMoNiS-NF-31. AFM image and corresponding line-scan profiles of CoMoNiSNF-31 (Figure 3e-f) indicate the thickness of the nanosheets is about 3 nm. These results verify the formation of Co9S8 and MoS2 ultrathin nanosheets (~3 nm) in contact with one another, and lying on the Ni3S2 nanorod arrays. Figure

1A

depicted

the

fabrication

of

the

hierarchical

structure

of

Co9S8/MoS2/Ni3S2/NF. Sevearl reactions shown below would occur during the hydrothermal process. First, HSˉ ions as active particles are released from thiourea (NH2CSNH2) (reaction 1). The Ni3S2 particles generated from the reaction of the HSˉ ions with NF grow into nanorods (reaction 2). Meanwhile, MoO42- ions react with the 10


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HSˉ ions to form MoS2 nanosheets (reaction 3). Also, Co2+ ions react with HSˉ to yield the Co9S8 nanosheets (reaction 4). In the reactions, the Ni3S2 nanorods act as the backbone to guide the preferential deposition of MoS2 and Co9S8 nanosheets. The NF not only works as a robust support for the nanostructured arrays, but also provides Ni sources for the growth of Ni3S2 nanorods. NH2CSNH2 + 3H2O → 2NH4+ + HSˉ + HCO3ˉ

(reaction 1)

3Ni + 2HSˉ + 2H2O → Ni3S2 + 2OHˉ +2H2

(reaction 2)

4MoO42- + 12HSˉ + 15N2H4 + 4H2O→ 4MoS2 + 4S + 5N2 + 20NH3 + 20OHˉ (reaction 3) 36Co2+ + 36HSˉ+ 27N2H4 + 36OHˉ→ 4Co9S8 + 4S + 9N2 + 36NH3 + 36H2O (reaction 4) To further understand the composition and chemical valences of the composites, we performed XPS analysis of CoMoNiS-NF-31. As shown in Figure 4a, the signal from Co 2p for the CoMoNiS-NF-31 can be divided into two spin-orbit doublets and two shakeup satellites (identified as “Sat.”). The two peaks located at 779.9 and 795.1 eV correspond to the 2p3/2 and 2p1/2 of Co3+;29,40 while the two peaks at 781.8 and 796.4 eV originate from 2p3/2 and 2p1/2 of Co2+, respectively. Compared with the XPS spectra of Co9S8/Ni3S2/NF (779.6 and 794.9 eV, Figure 4b), the binding energies of Co 2p are up-shifted (~0.3 eV) to 779.9 and 795.1 eV, suggesting a strong interaction between the Co9S8 and MoS2. For the Mo 3d (Figure 4c), two obvious peaks located at 229.7 and 232.7 eV are attributed to MoIV 3d5/2 and MoIV 3d3/2, indicating a Mo4+ oxidation state of MoS2. Note that the Mo 3d peaks display ~1.1 eV down-shifts compared to the MoS2/Ni3S2/NF (230.8 and 233.8 eV, Figure 4d), further confirming the strong interaction between the MoS2 and Co9S8.41 For Ni, the two peaks centered 11



at 855.6 and 873.7 eV correspond to Ni 2p3/2 and Ni 2p1/2, respectively (see Figure 4e). In addition, a small peak at 852.8 eV can be observed, belonging to the Ni 2p signal of NF substrate.11 Figure 4f shows eight peaks located at 162.1, 162.6, 163.3, 163.8, 164.4, 164.9, 169.2 and 170.7 eV. The two peaks appearing at 162.1 and 163.8 eV correspond to S 2p3/2 and S 2p1/2 binding energies of S2- in Co9S8,41,42 respectively. Another two peaks at 162.6 and 164.4 eV are assigned to S 2p3/2 and S 2p1/2 binding energies of S2- in Ni3S2, respectively.43 The characteristic peaks of S 2p3/2 and S 2p1/2 of S2- in MoS2 are located at 163.3 and 164.9 eV, verifying the formation of MoS2.41 Moreover, the peaks at 169.2 and 170.7 eV correspond to residual sulfate groups or oxidized sulphur species due to surface oxidation. The XPS results not only confirm the successful assembly of the Co9S8/MoS2/Ni3S2/NF, but also reveal an interaction between Co9S8 to MoS2 via charge transfer as judged by the up-shift of Co 2p binding energy and the down-shift of Mo 3d binding energy.41,44 A charge transfer process will adjust the frontier orbital energy of metal sulfides and this could have an effect on the binding energies of intermediates and catalytic rates. First-principles calculations support this experimental obesrvation and suggest that transfer of electrons from Co9S8 to MoS2 at the interface is the origin of the observed enhanced performance as will be discussed below. 3.2. Overall water splitting in alkaline pH 3.2.1 HER activity To investigate the electrocatalytic activity of the CoMoNiS-NF-xy toward overall water splitting, we first performed experiments in Argon-saturated 1.0 M KOH using a three-electrode system. Co9S8/Ni3S2/NF, MoS2/Ni3S2/NF, Ni3S2/NF, and Pt/C were used as control samples. As displayed in Figure 5a, Figure S5a and Table S3, there is an

activity

trend

of

CoMoNiS-NF-31>CoMoNiS-NF-21>CoMoNiS-NF12


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41>CoMoNiS-NF-51>CoMoNiS-NF-12>CoMoNiS-NF-11>CoMoNiS-NF13>MoS2/Ni3S2/NF>Co9S8/Ni3S2/NF>Ni3S2/NF. The Ni3S2/NF shows the worst HER activity, suggesting the Ni3S2-containing NF itself (partially-sulfided NF is not very active for HER. The MoS2/Ni3S2/NF exhibits better HER catalytic activity, with a lower 10 of 203 mV than Co9S8/Ni3S2/NF (10 = 225 mV), consistent with previously reported results that MoS2-based materials have good HER activity.45,46 The CoMoNiS-NF-xy exhibit elevated HER activity than the control samples, possibly caused by synergistic effects originated from the interfacial areas between the Co9S8 and MoS2 phases. CoMoNiS-NF-31 displays the best HER performance (η10 = 113 mV, Figure 5a) among all of the samples, which may be attributed to its well-defined nanostructures with optimally distributed ultrathin nanosheets, scarce aggregation and largest surface area. The superior HER activity of CoMoNiS-NF-31 is comparable to the Pt/C catalyst (η10 = 92 mV), and well ahead of those reported for non-noble metal electrocatalysts, such as CoxMoy@NC (η10 = 218 mV),9 CoS2 (η10 =193 mV),47 Co1Mn1CH/NF (η10 = 180 mV),48 CoNiP@NF (η10 = 155 mV),49 Ni3Se2/NF (η10 = 203 mV),50 Co9S8-NixSy/NF (η10 = 163 mV).51 A more extensive comparison of the HER performance of CoMoNiS-NF-31 with other non-precious electrocatalysts are given in Table 1.9,10,24,41,47-68 The Tafel slope of 85 mV dec-1 for CoMoNiS-NF-31 (see Figure 5b) is much lower than the control samples, and implies more favorable kinetics and higher catalytic activity of CoMoNiS-NF-31 toward HER. Moreover, the exchange current density (j0) is an important factor reflecting the number of active sites and indicating the inherent performance of the catalyst.7 The j0 values are calculated by extrapolating the Tafel plots of all the samples (Figure S6). In detail, the j0 value of CoMoNiS-NF-31 is

13



calculated to be 0.65 mA cm−2, larger than those of Co9S8/Ni3S2/NF (0.17 mA cm−2), MoS2/Ni3S2/NF (0.18 mA cm−2) and Ni3S2/NF (0.10 mA cm−2). To better understand the HER mechanism, we evaluated the intrinsic properties of the as-prepared samples by performing EIS measurements at an overpotential of 150 mV (Figure S5b). The charge transfer resistance (Rct) is associated with the electrocatalytic kinetics at the interface of electrolyte and electrocatalyst. Generally, a smaller Rct value indicates a faster electron transfer. The CoMoNiS-NF-31 delivers a much lower Rct than Co9S8/Ni3S2/NF, MoS2/Ni3S2/NF and Ni3S2/NF, suggesting a higher electrical conductivity for our hierarchical composite. 3.2.2 OER activity OER activities of the as-synthesized samples were assessed in 1.0 M KOH electrolyte. The overall activity trend of the samples follows as CoMoNiS-NF-31 > CoMoNiSNF-41 > CoMoNiS-NF-51 > CoMoNiS-NF-21 > Co9S8/Ni3S2/NF > CoMoNiS-NF11 > CoMoNiS-NF-12 > CoMoNiS-NF-13 > MoS2/Ni3S2/NF > Ni3S2/NF (Figure 5c, Figure S5c, Table S4). As shown, the Co9S8/Ni3S2/NF (with only Co and Ni but no Mo) presents better OER activity than the composites CoMoNiS-NF-xy (x:y = 1:1, 1:2 and 1:3, with higher Mo amount) and MoS2/Ni3S2/NF (without Co). This highlights the higher OER activity of the Co9S8 nanocrystals in alkaline media. The η10 of 166 mV of CoMoNiS-NF-31 is much lower than Co9S8/Ni3S2/NF (192 mV), MoS2/Ni3S2/NF (239 mV), Ni3S2/NF (395 mV), and is merely 16 mV behind the commercial RuO2 (η10 = 150 mV). The OER activity of CoMoNiS-NF-31 is superior to other NF-loaded materials, for example Ni3Se2/NF (ca. 239 mV),50 FePO4/NF (ca. 218 mV),31 Co5Mo1.0O NSs@NF (ca. 270 mV),60 FeOOH/CeO2 HLNTs-NF (ca. 210 mV).69 Table 2 shows a detailed comparison of the OER performance of CoMoNiSNF-31 with other reported non-precious electrocatalysts.9,10,16,17,24,30,31,41,47,48,50,60,68-76 14


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The best OER activity of CoMoNiS-NF-31 may be attributed to its optimal morphology, composition and synergy of heterometallic active sites. As we pointed out above in the discussion of the XPS spectra, the special nature of the hierarchical CoMoNiS-NF-31 composite enables a charge transfer between the Co9S8 and MoS2 phases. This is a synergistic effect that results in a slightly more reduced Mo in the MoS2 nanosheets and slightly more oxidized Co in Co9S8 nanocrystals. Since MoS2 species are generally HER active and Co9S8 species are OER active, we proposed the enhanced catalytic activity derives from this charge transfer between the two nanophases. However, further increasing either the Co amount (such as in CoMoNiS-NF-51) or the Mo amount (such as in CoMoNiS-NF-13) leads to degraded OER activity (Figure S5c, Table S4). This may be explained by the enhanced aggregation of surplus Co9S8 nanosheets at high Co fraction or the transformation to nanobelts from nanorods at high Mo fraction. Similarly, as shown in Figure 5d, the Tafel plots indicate that CoMoNiS-NF-31 has the smallest slope (58 mV dec-1), indicating appreciable kinetics for OER. The outstanding OER performance of CoMoNiS-NF-31 in alkaline solution is also associated with the significant improvement of its conductivity, as indicated by its lower Rct mentioned above (Figure S5d). 3.2.3 HER and OER simultaneously Considering the good HER and OER performance, we constructed a two-electrode configuration with CoMoNiS-NF-31 as the both cathodic and anodic catalyst, for the water reduction and oxidation with 1 M KOH solution as the medium. Also, Co9S8/Ni3S2/NF, MoS2/Ni3S2/NF, Ni3S2/NF were used as control samples. In addtion, commercial electrodes (RuO2 as the anode and Pt/C as the cathode) were evaluated under the same conditions. The polarization curve (Figure 5e) for water electrolysis 15



indicates that a cell voltage of 1.540 V is required to achieve the current density of 10 mA cm-2 for CoMoNiS-NF-31, much lower than those observed in Co9S8/Ni3S2/NF (1.743 V), MoS2/Ni3S2/NF (1.697 V), Ni3S2/NF (1.786 V), and only 0.014 V (14 mV) higher than that (1.526 V) of precious metals (Pt/C||RuO2). Table 3 compares the overall water splitting performance of our present CoMoNiS-NF-31 with recently reported materials.15,17,18,24,26,48,60,75,77 The low cell voltage of 1.54 V of the CoMoNiSNF-31 is superior to the values of reported materials, such as 1.62 V for EG/Ni3Se2/Co9S8,77 1.68 V for NiCo2S4 NW/NF75 and Co5Mo1.0P NSs@NF,60 1.65 V for Co9S8/WS2/Ti foil,15 1.58 V for Co-MoS2.18 More importantly, the CoMoNiS-NF-31 exhibits greater long-term stability, without obvious degradation of the current density (20 mA cm-2) at 1.70 V for nearly 24 h (see Figure 5f). After the long-term stability test, cathodic (HER) and anodic (OER) substances were examined with SEM (Figure S7), showing no obvious change in surface morphology. The XPS spectra of the samples after HER and OER are shown in Figure S8. The Co 2p, Mo 3d, Ni 2p and S 2p spectrum after HER revealed no significant change in binding energies. This well maintained morphology and unchanged chemical state of CoMoNiS-NF-31 confirm the high stability. After the OER test, Co 2p and Mo 3d showed no obvious change of binding energies (Figure S8a,S8b). But concerning Ni 2p, the peaks of metal Ni disappear (Figure S8c), deriving from the surface oxidation of Ni0 into nickel (oxy)hydroxides during the OER test in alkaline electrolyte.14 The nickel (oxy)-hydroxide species on the surface of electrocatalysts is regarded as the active site to enhance the OER performance.78-80 Furthermore, as shown in Figure S8d, the intensitiy of metal-S bond decreased after OER testing because of partial surface oxidation of CoMoNiS-NF-31, which confirms the conversion of metal sulfides into metal (oxy)hydroxides.81 These metal 16


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(oxy)hydroxides species can lead to a remarkable electrocatalytic performance and durability for the OER.67 The above results demonstrate that the CoMoNiS-NF-31 can be used as both HER and OER electrocatalysts in alkline media with high activity and good stability. 3.3. Overall water splitting in acidic pH 3.3.1 HER activity Under acidic conditions (0.5 M H2SO4), the HER activity of CoMoNiS-NF-31 and its counterparts were investigated. All samples require lower overpotentials to drive HER in acid media due to more favorable kinetics under lower pH. As shown in Figure 6a, Figure S9a and Table S3, the HER activity follows a trend of CoMoNiS-NF31>CoMoNiS-NF-21>CoMoNiS-NF-11>MoS2/Ni3S2/NF~CoMoNiS-NF-41> CoMoNiS-NF-51~CoMoNiS-NF-12>CoMoNiS-NF-13>Co9S8/Ni3S2/NF>Ni3S2/NF. Here in acidic media, only some composites are comparable to MoS2/Ni3S2/NF. We observe that too high Co:Mo ratio (4:1 and 5:1) and too low Co:Mo ratio (1:2 and 1:3) both degraded the HER performance, suggesting there exists an optimal range of Co/Mo proportions (from 3:1 to 1:1) to ensure good HER activity. The aggregation of Co9S8 nanosheets at Co-rich and formation of nanobelts at Mo-rich ratios both decreases HER activity. Specifically, apart from being comparable to Pt/C (η10 = 75 mV), CoMoNiS-NF-31 achieves an η10 of 103 mV, which is much lower than Co9S8/Ni3S2/NF (η10 = 232 mV), MoS2/Ni3S2/NF (η10 = 176 mV), Ni3S2/NF (η10 = 291 mV) and other control samples (Figure S9a and Table S3). The performance of CoMoNiS-NF-31 is much better than Co5Mo1.0O NSs@NF (η10 =173 mV),60 MoP-C (η10 = 136 mV),61 Co9S8@MoS2/CNFs (η10 = 190 mV)41 and Ni−Sn@C (η10 = 350 mV).58 For CoMoNiS-NF-31, the overpotentials of 72 and 84 mV respectively at 2.5 and 5.0 mA cm−2 are much lower than those reported good catalysts of MoB2 (η2.5 = 17



230 mV)66 and CoMoSx (η5 = 210 mV)54 (see Table 1). In addition, the Tafel slope of 55 mV dec-1 (Figure 6b) suggests that hydrogen evolution is consistent with the Volmer–Heyrovsky mechanism, indicating the Volmer reaction (Mcat + H2O + e− ⇌ McatHad + OH−) works as the rate-determining step.82 The small Tafel slope and Rct (1.1 Ω, Figure S9b) of CoMoNiS-NF-31 demonstrate enhanced intrinsic activity and conductivity leading to better HER electrocatalytic ability. 3.3.2 OER activity OER performance in acidic media follows the order of CoMoNiS-NF-31>CoMoNiSNF-21>CoMoNiS-NF-41>CoMoNiS-NF-11>CoMoNiS-NF-51>CoMoNiS-NF12>CoMoNiS-NF-13>Co9S8/Ni3S2/NF>>MoS2/Ni3S2/NF>Ni3S2/NF (the details are shown in Figure 6c, Figure S9c and Table S4). CoMoNiS-NF-31 retains the best OER electrocatalytic performance with a very small η10 of 255 mV. Also, it displays the smallest Tafel slope (78 mV dec-1, Figure 6d) and lowest Rct value (19.4 Ω, Figure S9d) among all the tested samples. 3.3.3 HER and OER simultaneously When using CoMoNiS-NF-31 as the electrocatalyst for overall water splitting in a two-electrode electrolyzer, we observe a considerably small cell voltage (1.45 V) for reaching a catalytic current density of 10 mA cm-2 (Figure 6e). The cell voltage is much lower than those reported Co-MoS2 (1.90 V)18 and NC-CNT/CoP (1.66V),17 comparable to the N-WC/CFP (1.40 V)16 (see Table 3 for details). The stability of CoMoNiS-NF-31 for overall water splitting in acid, however, needs to be improved (Figure 6f), as the current density decreased from 21.7 to 17.4 mA cm-2 after 80 min of continuous water splitting at 1.53 V. 3.4. Overall water splitting in neutral condition 3.4.1 HER activity 18


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We further evaluated the electrocatalytic performance of CoMoNiS-NF-xy and the control samples in neutral media (1 M PBS, pH = 7.0). Here the activity trend is CoMoNiS-NF-31>CoMoNiS-NF-21>CoMoNiS-NF-11>CoMoNiS-NF-12> CoMoNiS-NF-13>CoMoNiS-NF-41>CoMoNiS-NF-51>MoS2/Ni3S2/NF> Co9S8/Ni3S2/NF>Ni3S2/NF (Figure 7a, Figure 8a, and Table S3). Figure 7a shows that CoMoNiS-NF-31 displays superior electrocatalytic behavior for HER (η10 = 117 mV), exceeding Co9S8/Ni3S2/NF (η10=330 mV), MoS2/Ni3S2/NF (η10=279 mV), Ni3S2/NF (η10=395 mV) and other CoMoNiS-NF-xy (Figure 8a, Table S3). The overpotential (117 mV) of CoMoNiS-NF-31 is even much lower than those of many reported promising materials such as S-NiFe2O4/NF (η10 = 197 mV),24 MoP NA/CC (η10 = 187 mV),64 WP NAs/CC (η10 = 200 mV)65 (see Table 1). The lower Tafel slope (Figure 7b) and Rct (Figure 8b) verify the superior HER activity of CoMoNiS-NF-31. 3.4.2 OER activity For OER, an activity trend of CoMoNiS-NF-31>CoMoNiS-NF-21>CoMoNiS-NF41>CoMoNiS-NF-51>CoMoNiS-NF-11>CoMoNiS-NF-12>CoMoNiS-NF13>Co9S8/Ni3S2/NF>>MoS2/Ni3S2/NF>Ni3S2/NF is displayed. To make a more reasonable comparison, we also synthesized Ni3S2 nanorod-NF (for details, see Figure S10, S11 in Supporting Information). We found the overpotentials of Ni3S2 nanorodNF are close to those of the present Ni3S2/NF for HER and OER in alkaline, acidic and neutral conditions, respectively (see Figure S12). This results validate that the Ni3S2/NF via the method we employed can be used as a reasonable control system. Notably, Figure 7c shows that the overpotential (405 mV) at 10 mA cm-2 (η10) of CoMoNiS-NF-31 is much lower than other promising reported materials such as SNiFe2O4/NF (494 mV)24 and Co−Se−S−O/CC (480 mV)76 (see Table 2). More importantly, under neutral pH, the overpotential of 405 mV at 10 mA cm-2 for 19



CoMoNiS-NF-31 is much lower than that of the NF-supported RuO2 (683 mV), demonstrating an outsatading OER performance. To better understand the outstanding OER performance of CoMoNiS-NF-31, we measured the double layer capacitance (Cdl) which can provide an estimate of the electrochemical surface area (ECSA) of electrocatalysts,83 as shown in Figure 9 and Figure S13, S14. The obtained Cdl value for CoMoNiS-NF-31 is 60.9 mF cm-2 (Figure 9b), which is the largest among the control samples (Figure S14). Besides, as seen in Figure 9c, by controlling the ratio of Co and Mo precursors, the Cdl values increase from 15 mF cm−2 for Co9S8/Ni3S2/NF to 30.9 mF cm−2 for CoMoNiS-NF-41 and climb to the maximum value of 60.9 mF cm−2 for CoMoNiS-NF-31, the Cdl then decreases to 10 mF cm−2 for MoS2/Ni3S2/NF. This suggests that regulating the Co/Mo ratio is effective in controlling the ECSA of CoMoNiS-NF-xy. What's more, the Tafel slopes (Figure 7d) and Rct values (Figure 8d) confirm the favorable structure and chemical property of CoMoNiS-NF-31 as electrocatalyst for OER. 3.4.3 HER and OER simultaneously Investigations on overall water splitting in neutral pH are very rare because of the scaricity of promising electrocatalysts, especially bifunctional ones. In view of the high performance of CoMoNiS-NF-31, we performed this investigation. As described above for overall water splitting, the electolyzer employed two electrodes (an anode and a cathode) both consisting of CoMoNiS-NF-31. The polarization curve displayed a relative low potential of 1.80 V to achieve a current density of 10 mA cm-2 (Figure 7e), which is even lower than that (1.85 V) of NF-supported precious metals (Pt/C||RuO2). More importantly, the present value (1.80 V) is significantly lower than the known high performing materials of S-NiFe2O4/NF (1.95 V),24 comparable to the NC-CNT/CoP (1.69 V).17 The NC-CNT/CoP material is the rarest example whose 20


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overall water-splitting performance had been tested under all-pH conditions,17 but its cell votages of 1.63 V in alkaline pH and 1.66 V in acidic pH are much higher than our CoMoNiS-NF-31 material (1.54 V in alkaline media and 1.45 V in acid, see Table 3). Furthermore, the CoMoNiS-NF-31 based device shows excellent durability after 20 h at 1.80 V with negligible degradation of the current density (Figure 7f). These results demonstrate that CoMoNiS-NF-31 is a superior electrocatalyst for overall water splitting in neutral electrolytes and has potential for development in applications on splitting of river or ocean water. 3.5. Balance between HER/OER activity and material stability For a electrocatalyst, proper description of activity and stability can use the ratio between hydrogen/oxygen production rate (current density J) and metal dissolution rate during HER/OER (equivalent dissolution current density, S). This activitystability metric is denoted as the activity-stability factor, that is ASF (Eq. 1),32 ASF 

J S S

|

η

(Eq. 1)

where at a constant overpotential (η), the best electrocatalyst for the HER/OER would have the highest ASF values. Table S5 contains representative ASF values for CoMoNiS-NF-xy samples (x:y = 2:1, 3:1 and 4:1) evaluated under the same conditions and over broad pH conditions. Three trends are evident as shown in Table S5. First, CoMoNiS-NF-31 has the largest ASF, suggesting the best balance between HER/OER activity and stability among these composites. Second, for CoMoNiS-NF31, in alkaline conditions, the ASF of HER are higher than in acidic and neutral conditions, with an order of HER (base) > HER (neutral) > HER (acid). Third, for OER, the order is OER(base) > OER(neutral) > OER(acid) for the ASF factor. The lower ASF value under acidic conditions than in alkaline and nuetral environments 21



may be due to the poor stability under oxidizing potentials in acidic media during OER. As depicted above, for the assemblies containing binary metal sulfiles such as Co9S8/Ni3S2/NF and MoS2/Ni3S2/NF, the Ni3S2 exists in both of them, while their catalytic performance is not compelling, however, the integrated CoMoNiS-NF-31 displays superior electrocatalytic activity in any medium (alkaline, acidic or neutral). This suggests the two synergistic external phases of Co9S8 and MoS2 make the main contribution for the activity enhancement, and the role of the interior Ni3S2 can be overlooked in some ways. Thus, four key points are noteworthy upon the catalytic performance: (1) Regardless of pH conditions, for HER, MoS2 is always more active than Co9S8, while for OER, Co9S8 is more active than MoS2, and the optimized integration of the two metal sulfides can significantly improve the HER and OER performance; (2) For OER, all materials follow an activity trend of alkaline>acidic>neutral; (3) CoMoNiS-NF-31 shows the best electrocatalytic activity, especially in neutral media, with a catalytic performance remarkably outperforming comercial RuO2; (4) synergistic metal sulfides of Co9S8 and MoS2 contribute dominatingly to the enhanced electrocatalytic activity. The results presented above confirm that, unlike the most non-noble metal electrocatalyts that are active and stable over a narrow pH range, the optimized CoMoNiS-NF-31 hierarchical material can serve as a highly active and stable pHuniversal bifunctional electrocatalyst. 3.6. First-principles calculations As described above, the co-present two phases of Co9S8 and MoS2 take the main contributions to the improvement of electrocatalytic activity. There would exsit strong coupling interactions at the interface of Co9S8 and MoS2, thus we focus on this 22


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interface to conduct the first-principles calculations. A higher activity of Co9S8/MoS2 for HER compared to MoS2 by itself was already established in DFT studies of this reaction on the edge of MoS2 layer normal to Co9S8 surface.84 Therefore, we performed DFT calculations in order to address the effect of Co9S8(100)/MoS2 interface on the activity of Co9S8 for the OER reaction. Our TEM results (see Figure 3c) indicate that the MoS2 interlayer distance is nearly the same as in MoS2 bulk, and the interface orientation between the two phases is non-specific. Therefore, we have calculated interface stability depending on Co9S8(100) termination and MoS2 orientation (Figure S15 and Table S6). As a result, two interface structures shown in Figure 10a,b with MoS2 layers parallel and normal to the interface with Co-terminated Co9S8 were selected to investigate charge transfer and activity in OER. Both have CoS termination of the opposite Co9S8(100) surface shown in Figure 10c. Calculated charge transfer between MoS2 and Co9S8 using Bader charges (Table S6) indicates an overall strong transfer of electrons from Co9S8 to MoS2 at the Co terminated interface due to the formation of Co-S bonds across this interface. Calculated density of states (DOS) in Figure 10d shows a decrease at the Fermi level upon binding of Co9S8 to MoS2, which indicates that electrons are withdrawn from Co to MoS2 (Figure S16 and S17 for additional DOS plots). These results are consistent with the XPS results presented above which show the slight oxidation of Co and reduction of Mo in this hierarchical nanocomposite, respectively. Gibbs free energies of four electrochemical reaction steps for OER (2H2O→ O2+4H++4e-) on the Co9S8 surfaces of Co9S8(100)/MoS2 structures were estimated using calculated O*, OH* and OOH* intermediates on different surface sites (see Figure S16-S20 for geometries of adsorbed species, computational methods, and Table S7 for computational details) and given in Figure 10e. Without MoS2, 23



formation of O-O bond in OOH* is the rate limiting step, with calculated overpotential of 1.45 V. The overpotential required for this step decreases dramatically to 0.86 V for Co9S8/MoS2 structure shown in Figure 10b, whereas for the structure on Figure 10a the rate limiting step changes to the initial OH* adsorption with the overpotential of 0.52 V. While the overpotentials are dependent on pH, the relative values in this approach will show the same trend at all pH values. As Co9S8 loses electrons to MoS2, the binding energies of adsorbed species change and the overpotential decreases. Our experimental results, as well as previous experimental and theoretical results indicate that Co9S8 activity in OER is strongly dependent on the presence of interfaces to other transition metal sulfides.41,84,85 Similar to our results, in the case of core-shell Co9S8/MoS2 the structure formed with MoS2 layers parallel to the Co9S8(100) surface resulting in enhanced OER activity.41 Our calculations therefore support the experimental results as they suggest that Co9S8/MoS2 has enhanced electrochemical activity than Co9S8 by itself because of (a) charge transfer from Co to MoS2; (b) resulting higher oxidation state of Co and (c) different binding energies of reaction intermediates. 4. CONCLUSION A hydrothermal process which employs nickel foam as a conductive support produces a controllable hierarchical nanoassembled structure of ultrathin Co9S8 and MoS2 nanosheets grown on Ni3S2 nanorod arrays which themseleves are coating the nickel foam without the use of a binder. This multifunctional assembly exhibits synergistic effects deriving from charge transfer processes between the phases, yielding a superior electrocatalyst than the sum of its parts. The CoMoNiS-NF-31 is a highly efficient optimal bifunctional electrocatalyst for both HER and OER under alkaline, acidic and neutral conditions (all pH range). At current density of 10 mA cm–2, 24


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CoMoNiS-NF-31 achieves extremely low overpotentials of 103, 113 and 117 mV for HER in alkaline, acidic and neutral medias, respectively. The catalyst also requires untralow overpotentials of 166, 405 and 228 mV for OER, respectively in alkaline, acidic and neutral media. The ASF for CoMoNiS-NF-31 indicates that it possesses an outstanding balance between the HER/OER activity and stability. The theoretical calculations support the experimental results that Co9S8/MoS2 has enhanced electrochemical activity than Co9S8 by itself. This hierarchical structure displays highly efficient and all-pH bifunctional electrocatalytic performance for overall water splitting. The superior electrocatalytic performance of CoMoNiS-NF-31 is attributed to the following features: (1) intimate contact of 2D active ultrathin nanosheets on nanorod arrays directly grown on 3D highly conductive substrate NF enabling the full use of active sites of the electrocatalyst and efficient electron and mass transfer; (2) synergistic effects of two active metal sulfides of Co9S8 and MoS2 via charge transfer; (3) complete cover of the pristine NF surface by the Co9S8, MoS2 and Ni3S2, which prevents corrosion and enhances durability of the 3D frame structures. These findings and insights encourage the further design and exploration of synergistic bifunctional electrocatalysts for water splitting in wide pH range using this type of hierarchical approach. ACKNOWLEDGMENTS. Experimental work is supported by National Science Foundation of China (No. U1832152, 21665021, 21771024, 21871028), Beijing Municipal Natural Science Foundation (2182029), Natural Science Foundation of Ningxia (NZ17052). Computational study and work of MGK were supported by the US Department of Energy, BES Materials Sciences under Contract DEAC02-06CH11357 with UChicago Argonne, LLC, operator of Argonne National Laboratory. We thank Dr 25



Nenad Markovic and Dr. Pedro Lopes at Argonne National Laboratory for fruitful discussions and suggestions. Supporting Information: SEM images, HER and OER polarization curves for CoMoNiS-NF-xy,CoMoNiS-NF-31 (in reaction time of 8, 12, 16 and 20 h) and Ni3S2 nanorod-NF, Nyquist plots, XPS spectra, calculation of exchange current density, average mass loading of catalysts on NF, HER and OER of as-prepared samples in different electrolytes, activity-stability factor (ASF) of composites, and some DFT calculation results. This material is available free of charge via the Internet at http://pubs.acs.org. References (1)

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35



Page 36 of 46

Table 1. Comparison of HER performance of CoMoS-NF-31 with reported electrocatalysts. Electrolyte

Catalysts

1 M KOH

CoMoNiS-NF-31 CoxMoy@NC NixCo3−xS4/Ni3S2/NF CoS2 Co1Mn1CH/NF Ni3S2/NF High-index faceted Ni3S2 nanosheet arrays/NF CoNiP@NF Co9S8-NixSy/NF Ni3Se2 /NF Ni0.7Fe0.3S2/NF Cu@CoSx/CF S-NiFe2O4/NF CoMoSx

0.1 M KOH 0.5 M H2SO4

0.1 M HClO4 1.0 M PBS

Loading (mg/cm2 ) 1.86 0.56 1.5 5.6 1.6

η2.5 (mV)

η5 (mV)

η10 (mV)

51

77

113 218 136 193 180 116 223

This work J. Mater. Chem. A 2017.9 Nano Energy 2017.67 Nanoscale 2018.47 J. Am. Chem. Soc. 2017.48 J. Am. Chem. Soc. 2016.52 J. Am. Chem. Soc. 2015.68

155 163 203 155 134 138

J. Mater. Chem. A 2016.49 J. Mater. Chem. A, 2016.51 Nano Energy 2016.50 J. Mater. Chem. A 2017.10 Adv. Mater. 2017.53 Nano Energy 2017.24 Nat. Mater. 2016.54

103

200

This work Angew. Chem. Int. Ed. 2017.66 J. Am. Chem. Soc. 2015.55 ACS Catal. 2018.56 ACS Appl. Mater. Interfaces 2017.57 Adv. Mater. 2015.41 ACS Appl. Mater. Interfaces 2015.58 Angew. Chem. Int. Ed. 2014.59 Nano Energy 2018.60 Nano Energy 2017.61 Nat. Mater. 2016.54 J. Am. Chem. Soc. 2013.62

117

This work

1.0 9.0 8.87 3.0 3.9 -

CoMoNiS-NF-31 MoB2 Au-MoS2 MoS2/hydrogenated graphene Co9S8/NC@MoS2

1.86 0.2-0.3 0.13 0.28

200 72 230

84

120 124 117

References

Co9S8@MoS2/CNFs Ni−Sn@C

0.21 0.1

190 350

Co-NRCNTs Co5Mo1.0O MoP-C CoMoSx Co0.6Mo1.4N2

0.28 0.84 0.24

260 173 136

CoMoNiS-NF-31

1.86

Co9S8/NC@MoS2

0.28

261

ACS Appl. Mater. Interfaces 2017.57

-

197

Nano Energy 2017.24

1.6

170

J. Am. Chem. Soc. 2015.68

MoP NA/CC

2.5

187

Appl. Catal. B: Environ. 2016.64

WP NAs/CC

2.0

200


CoNiP@NF

1.0

120

J. Mater. Chem. A 2016.49

S-NiFe2O4/NF High-index faceted nanosheet arrays/NF

Ni3S2

210 77

36


91

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Table 2. Comparison of OER performance of CoMoNiS-NF-31 with reported electrocatalysts. Electrolyte

Catalysts

1 M KOH

CoMoNiS-NF-31 Co9S8@MoS2/CNFs

Loading

η10 (mV)

References

1.86

166

This work

0.212

430

Adv. Mater. 2015.41

MoS2/Ni3S2

9.7

218

Angew. Chem. Int. Ed. 2016.30

High-index faceted Ni3S2

1.6

260


-

330


CoS2

1.5

290

Nanoscale 2018.47

Ni3Se2 /NF

8.87

239

Nano Energy 2016.50

Ni0.7Fe0.3S2/NF

3.0

198


FePO4/NF

0.29

218

Adv. Mater. 2017.31

S-NiFe2O4/NF

-

267

Nano Energy 2017.24

Co5Mo1.0O NSs@NF

-

270

Nano Energy 2018.60

0.44

210

Adv. Mater. 2016.69

NC/NiMo/NiMoOx/NF

-

284

Small 2017.70

Co-doped NiO/NiFe2O4

4.0

186


Ni2P/Ni/NF

-

200

ACS Catal. 2016.72

NiFe/Ni(OH)2/NiAl foil

-

246

Adv. Sci. 2017.86

Fe–Pi/NF

-

215


NiFe-LDH/NF

-

240

Science 2014.74

NiCo2S4 NW/NF

-

260

Adv. Funct. Mater. 2016.75

CoMoNiS-NF-31

1.86

228

This work

N-WC/CFP

10.0

220

Nat. Commun. 2018.16

NC-CNT/CoP/CC

1.3

350


CoMoNiS-NF-31

1.86

405

This work

-

494

Nano Energy 2017.24

2.0

480


(mg/cm2)

nanosheet arrays/NF CoxMoy@NC

FeOOH/CeO2 HLNTs-NF

0.5 M H2SO4

1.0 M PBS

S-NiFe2O4/NF Co−Se−S−O/CC

37



Page 38 of 46

Table 3. Comparison of overall water splitting performance with reported materials. Cell votages

Electrolytes

Catalysts

1 M KOH

CoMoNiS-NF-31

1.54

This work

EG/Ni3Se2/Co9S8

1.62

Nano Lett. 2017.77

Co1Mn1CH

1.68


NC-CNT/CoP

1.63


NiCo2S4 NW/NF

1.68

Adv. Funct. Mater. 2016.75

Co9S8/WS2/Ti foil

1.65


Co-MoS2

1.58

Chem. Commun. 2018.18

Co5Mo1.0P NSs@NF

1.68

Nano Energy 2018.60

CoMoNiS-NF-31

1.45

This work

N-WC/CFP

1.40

Nat. Commun. 2018.16

NC-CNT/CoP

1.66


Co-MoS2

1.90

Chem. Commun. 2018.18

CoMoNiS-NF-31

1.80

This work

S-NiFe2O4/NF

1.95

Nano Energy 2017.24

NC-CNT/CoP

1.69


0.5 M H2SO4

1 M PBS

at 10 mA cm-2 (V)

38


References

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Figure 1. (A) Schematic illustration of synthesis and growth of hierarchical CoMoNiS-NF-xy composites via a one-pot hydrothermal method. (B) XRD patterns of (a) Ni3S2/NF, (b) Co9S8/Ni3S2/NF, (c) MoS2/Ni3S2/NF and (d) CoMoNiS-NF-31.

Figure 2. SEM images of (a-c) Ni3S2/NF, (d-f) Co9S8/Ni3S2/NF, (g-i) MoS2/Ni3S2/NF, (j-l) CoMoNiS-NF-31 at various magnifications.

39



Figure 3 ． (a, c) HRTEM images and (b, d) TEM images of CoMoNiS-NF-31, (e) AFM image and (f) line-scan profiles of CoMoNiS-NF-31.

Figure 4. X-ray photoelectron spectra with deconvolution of XPS peaks of (a) Co 2p and, (c) Mo 3d, (e) Ni 2p, and (f) S 2p of CoMoNiSNF-31, and (b) Co 2p of Co9S8/Ni3S2/NF and (d) Mo 3d of MoS2/Ni3S2/NF.

40


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Figure 5. (a) HER polarization curves and (b) Tafel curves of CoMoNiS-NF-31, Co9S8/Ni3S2/NF, MoS2/Ni3S2/NF, and Ni3S2/NF; (c) OER polarization curves and (d) Tafel curves of CoMoNiS-NF-31, Co9S8/Ni3S2/NF, MoS2/Ni3S2/NF and Ni3S2/NF; (e) Polarization curves of CoMoNiS-NF-31, Co9S8/Ni3S2/NF, MoS2/Ni3S2/NF and Ni3S2/NF for overall water splitting in a two-electrode configuration; (f) Chronoamperometric curve of CoMoNiS-NF-31 for water electrolysis at 1.70 V. The electrolyte is 1.0 M KOH.

41



Figure 6. (a) HER polarization curves and (b) Tafel curves of CoMoNiS-NF-31, Co9S8/Ni3S2/NF, MoS2/Ni3S2/NF and Ni3S2/NF; (c) OER polarization curves and (d) Tafel curves of CoMoNiS-NF-31, Co9S8/Ni3S2/NF, MoS2/Ni3S2/NF, and Ni3S2/NF; (e) Polarization curves of CoMoNiS-NF-31, Co9S8/Ni3S2/NF, MoS2/Ni3S2/NF and Ni3S2/NF for overall water splitting in a two-electrode configuration; (f) Chronoamperometric curve of CoMoNiS-NF-31 for water electrolysis at 1.53 V. The electrolyte is 0.5 M H2SO4.

42


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Figure 7. (a) HER polarization curves and (b) Tafel curves of CoMoNiS-NF-31, Co9S8/Ni3S2/NF, MoS2/Ni3S2/NF, Ni3S2/NF; (c) OER polarization curves and (d) Tafel curves of CoMoNiS-NF-31, Co9S8/Ni3S2/NF, MoS2/Ni3S2/NF and Ni3S2/NF; (e) Polarization curves of CoMoNiS-NF-31, Co9S8/Ni3S2/NF, MoS2/Ni3S2/NF, and Ni3S2/NF for overall water splitting in a two-electrode configuration; (f) Chronoamperometric curve of CoMoNiS-NF-31 for water electrolysis at 1.80 V. The electrolyte is 1 M PBS (pH7).

43



Figure 8. (a) HER polarization curves for CoMoNiS-NF-xy at a scan rate of 5 mV s-1; (b) Nyquist plots of CoMoNiS-NF-31, Co9S8/Ni3S2/NF, MoS2/Ni3S2/NF, and Ni3S2/NF tested at -0.2 V vs. RHE; (c) OER polarization curves for CoMoNiS-NF-xy at a scan rate of 5 mV s-1; (d) Nyquist plots of CoMoNiS-NF-31, Co9S8/Ni3S2/NF, MoS2/Ni3S2/NF, Ni3S2/NF tested at 1.6 V vs. RHE. The electrolyte is 1 M PBS.

Figure 9. (a) CV curves at different scan rates of CoMoNiS-NF-31; (b) Plots of current density as a function of scan rate for CoMoNiS-NF-31; (c) Influence of Co/Mo ratio on the double layer capacitance (Cdl) for CoMoNiS-NF-xy.

44


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Figure 10. (a, b) DFT-optimized structures of MoS2/Co9S8 interfaces: (a) MoS2(001) parallel to Co9S8(001). (b) (001)MoS2 perpendicular to Co9S8(001). Atom colors: Mo black, Co blue and S yellow. Co-termination at the interface results in the most stable structure. (c) Top view of Co9S8(001) surface sites used for DFT calculations of adsorbed O*, OH* and OOH* species. (d) Calculated densities of electronic states for MoS2/Co9S8 interfaces with Co termination and with Co9S8. (e) Gibbs free energy changes for four steps of OER at 0 V vs standard hydrogen electrode.

45



TOC Figure

46


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Ni as a Highly

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