Introduction

With the urgent need of energy demand and the gradual depletion of non-renewable energy, people are committed to looking for benign, sustainable, ecological friendly energy to replace the exhausted fossil fuel [1, 2]. H2 is a clean energy and ideal energy carrier. Among various production methods, the alkaline water electrolysis technology has been proved to be a promising technique [3, 4]. The advantages of the alkaline water electrolysis technology include that the availability of raw water is relatively unlimited and can be easily obtained. In addition, a large amount of high-purity H2 can be obtained by this method [5]. Noble metal Pt is the most efficient hydrogen evolution catalyst, but the high price makes it necessary to explore cheap non-noble metal hydrogen evolution catalyst.

At present, a variety of transition metal-based HER catalysts, including alloys [6,7,8,9], carbides, [10,11,12,13] phosphides, [14,15,16,17] nitrides, [18,19,20,21] sulfides [22, 23] and borides [24,25,26,27] have been proved to be possible to replace Pt. Among them, alloys, especially Mo–Ni alloys, are excellent HER catalysts in alkaline electrolyte. This is because Mo–Ni alloys have a Pt-like adjustable electron density state, which can promote the adsorption and desorption process of Hads [28, 29]. However, the Mo–Ni alloys catalysts still have some problems, such as high hydrogen evolution overpotential and sluggish reaction kinetics (high Tafel slope) in alkaline solution, which makes it still unable to replace Pt. In recent years, the grafting of H2O dissociation promoters (metal oxides or hydroxides) onto the surface of Pt or transition metal HER catalysts can promote the dissociation of H2O, thus accelerating the slow kinetics of Volmer reaction in alkaline solutions [30, 31]. Recent studies show that the combination of Mo–Ni alloys with metal oxide (MoOx) can effectively accelerate the Volmer reaction kinetics and improve the catalytic activity. However, most of the reported Mo–Ni alloys grafting with metal oxide only have simple one-dimensional (1D) structures, such as cuboids, [32] nanowires [33] and nanorods, [34] which cannot further improve the HER performance. Compared with 1D nanomaterial, two-dimensional (2D) nanomaterials have higher specific surface area, electrical conductivity, more exposed active sites and mass transfer channels, which make them attract extensive attention in the field of electrocatalysis [35]. If the composite with two-dimensional nanosheet structure can be prepared, it is expected to further improve the catalytic activity. However, how to integrate various active components (Mo–Ni alloys and metal oxides) into 2D nanosheets structure is a big challenge.

Herein, a novel 2D nanosheets self-supporting composite electrocatalyst MoNi4/MoO2.5-NF was reported for the first time. This self-supporting catalyst is composed of MoO2.5 nanosheets arrays embedded with MoNi4 alloy nanoparticles. Benefiting from the 2D nanosheets structure and synergistic effect of the components, this electrocatalyst shows high HER activity in alkaline solution. In 1 M KOH solution, the MoNi4/MoO2.5-NF needs overpotential of 49 mV to reach 10 mA cm−2. The HER activity of MoNi4/MoO2.5-NF can be further improved through electrochemical-cycling activation (CV) process. After 10 CV cycles, the overpotential to reach 10 mA cm−2 of the activated catalyst (EA-MoNi4/MoO2.5-NF) decreased from 49 to 27 mV and even exceeded 20% Pt/C (32 mV). The improvement of HER performance was attributed to the increase in the electrochemically active surface area (ECSA) and the decreased impedance and Tafel slope after CV process.

Results and discussion

The preparation process of the MoNi4/MoO2.5-NF electrocatalyst is shown in Scheme 1 and supplementary information. The first step is to grow the 2D NiMoO4 nanosheets arrays precursor on Ni foam by hydrothermal method. After the precursor was reduced in H2, the 2D MoO2.5 nanosheets embedded with MoNi4 alloy nanoparticles can be obtained. The XRD patterns of precursor and reduction product are shown in Fig. 1. As shown in Fig. 1a, the diffraction peaks at 14.2°, 19.0°, 23.8°, 25.4°, 28.9°, 32.6°, 39.1°, 41.1°, 43.8°, 47.3° and 53.4° correspond to the (110), (101), (− 121), (− 112), (220), (022), (− 402), (400), (330), (− 204) and (− 510) planes of the NiMoO4 phase (JCPDS card no. 33–0948), respectively. The XRD patterns of the reduction product are shown in Fig. 1b, which match well with the MoNi4 (JCPDS card no. 65–5480). Except for the XRD patterns of the MoNi4, no other diffraction peaks can be observed, which indicated that the Mo–O species in the reduction products may exist in amorphous state. The surface chemical states of oxide precursors NiMoO4-NF and the reduction products MoNi4/MoO2.5-NF were further studied by XPS. The survey XPS shows that the catalyst contained Ni, Mo and O elements (Fig. S1). Figure 1c shows the HR-XPS spectra of Ni 2p before and after H2 reduction. The oxide precursor NiMoO4-NF only shows Ni2+ signal (857.1 eV). After H2 reduction, the reduction product MoNi4/MoO2.5-NF shows a strong Ni0 signal (854.2 eV) and a weak Ni2+ signal. The results show that the Ni2+ in precursor oxide are almost completely reduced to Ni0 after hydrogen reduction, and a small amount of Ni2+ belongs to the surface oxidized Ni species in contact with air. Figure 1d shows the HR-XPS spectra of Mo 3d before and after H2 reduction. Only Mo6+ signal (232.0 eV) can be found in the precursors NiMoO4-NF. After H2 reduction, Mo with low valence of Mo4+ (230.5 eV, 229.0 eV) and Mo0 (226.1 eV) can be observed. Combined with the XRD results, the Mo0 signal should be related to the MoNi4 alloy. The Mo6+ signal can still be observed in the reduction products, and the ratio of Mo6+: Mo4+ is about 1:1. The XPS results show that after hydrogen reduction, almost all of the Ni2+ species is reduced to Ni0 (MoNi4 alloy), while Mo6+ species is only partially reduced, part of which is reduced to Mo0 (MoNi4 alloy), the other part is reduced to low valence oxide Mo4+, and some Mo6+ still exists in the highest valence state.

Scheme 1
scheme 1

Synthetic process of MoNi4/MoO2.5-NF electrocatalyst

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Figure 1
figure 1

XRD patterns of a NiMoO4-NF; b MoNi4/MoO2.5-NF; HR-XPS spectra of c Ni 2p and d Mo 3d

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The SEM image of NF shows that the surface is very smooth and there are micron-sized pores in the NF framework (Fig. 2a). Figure 2b shows that the ultrathin NiMoO4 nanosheets arrays are grown on the NF and the spacing between the nanosheets is about 200 nm. The SEM image of hydrogen reduction products MoNi4/MoO2.5-NF is shown in Fig. 2c. After hydrogen reduction, the material still maintains the nanosheet structure, but the surface of the nanosheet becomes rough obviously. The TEM image of hydrogenated reduction product shows that the MoNi4 nanoparticles with a size of about 8 nm are evenly distributed on the MoO2.5 nanosheets (Fig. S2). In addition, hydrogenated holes with diameter about 4 nm can be observed on the nanosheets (Fig. 2d). The HRTEM image of hydrogenated reduction products shows that the lattice spacing of diffraction fringes is 0.21 nm, corresponding to (121) crystal plane of MoNi4 (Fig. 2e). No other lattice diffraction fringes were observed except for the MoNi4 nanoparticles. This result is consistent with the XRD results, which indicate that the hydrogen-reduced nanosheets exist as amorphous Mo–O species with low valence. Since the XPS result shows that the ratio of Mo6+ to Mo4+ is 1:1, the chemical formula of the nanosheets after hydrogen reduction can be determined as MoO2.5. The EDS mapping images of MoNi4/MoO2.5-NF are shown in Fig. 2f–i. Ni, Mo and O elements can be observed. The BET surface area of the MoNi4/MoO2.5-NF is 40 m2/g (Fig. 3a). The pore size distribution of the MoNi4/MoO2.5-NF is between 3 and 10 nm (Fig. 3b). These nanopores mainly belong to the pores on the nanosheets caused by hydrogenation.

Figure 2
figure 2

SEM image of a NF; b NiMoO4-NF; c MoNi4/MoO2.5-NF; d–e TEM and HRTEM image of MoNi4/MoO2.5-NF; f–i EDAX elemental mapping of MoNi4/MoO2.5-NF

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Figure 3
figure 3

The BET surface area (a) and pore size distribution (b)

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The electrocatalytic HER performances of MoNi4/MoO2.5-NF and control samples are studied in 1.0 M KOH solution. The LSV curves of samples are shown in Fig. 4a. The HER activity of NF and NiMoO4-NF is very poor, which require high overpotential of 295 mV and 173 mV to reach the current density of 10 mA cm−2. The hydrogenated product MoNi4/MoO2.5-NF displays superior HER activity compared to that of NiMoO4-NF, which only requires a very low overpotential of 49 mV to reach the same current density. The HER activity of MoNi4/MoO2.5-NF can be further improved through electrochemical-cycling activation (CV) process. After 10 CV cycles, the overpotential to reach 10 mA cm−2 of the catalyst (EA-MoNi4/MoO2.5-NF) decreased from 49 to 27 mV and even exceeded that of 20% Pt/C (32 mV). This electrochemical activation has been widely reported and proved to be an effective strategy to enhance the activity of catalysts [36,37,38,39]. The Tafel slopes are tested to study the reaction kinetics of samples. As shown in Fig. 4b, the Tafel slopes of NiMoO4-NF and NF are 157 mV dec−1 and 225 mV dec−1, respectively, indicating their very sluggish HER reaction kinetics. The Tafel slope of MoNi4/MoO2.5-NF is 39 mV dec−1. This Tafel slope value indicates that the rate-determining step of MoNi4/MoO2.5-NF was Tafel reaction, which can be related to the acceleration of sluggish Volmer reaction by the MoO2.5. After CV activation, the Tafel slope of the EA-MoNi4/MoO2.5-NF drops to 31 mV dec−1, even lower than that of 20% Pt/C (32 mV dec−1). This result indicates that the EA-MoNi4/MoO2.5-NF has more favorable HER reaction kinetics than that of the MoNi4/MoO2.5-NF. The double-layer capacitances (Cdl) of samples are tested by CV measurements. As shown in Fig. 4c, the Cdl of NiMoO4-NF and NF is 11.5 mF cm−2 and 0.4 mF cm−2, respectively, indicating their very low electrochemically active surface area (ECSA). The Cdl of MoNi4/MoO2.5-NF is 34.7 mF cm−2, which is significantly higher than that of NiMoO4-NF. After CV activation, the Cdl of the EA-MoNi4/MoO2.5-NF is increased dramatically to 105.4 mF cm−2. This result indicates that electrochemical activation can effectively increase the electrochemically active surface area of the catalyst and improve the electrocatalytic performance. The electrochemical impedance spectroscopy (EIS) of samples are tested to study the HER process kinetics. As shown in Fig. 4d, the charge transfer resistance (Rct) of EA-MoNi4/MoO2.5-NF, MoNi4/MoO2.5-NF, NiMoO4-NF and NF is 4, 13, 26 and 83 Ω, respectively. The EA-MoNi4/MoO2.5-NF has the smallest Rct, which shows rapid electron transport and HER kinetics. This result indicates that electrochemical activation can effectively reduce the impedance of the catalyst and improve the electrocatalytic performance. The electrocatalytic Faraday efficiency of EA-MoNi4/MoO2.5-NF is nearly 100% (Fig. 4e), which indicates that the electron utilization efficiency is very high and no other side reactions occur. The chronoamperometry test shows that the catalyst can maintain high efficiency and stable electrolysis for at least 50 h (Fig. 4f). Compared with the one-dimensional electrocatalytic materials with similar composition (i.e., Mo–O species and Ni–Mo alloy complexes) previously reported, these two-dimensional layered electrocatalysts reported here show better electrocatalytic properties. The comparison of electrocatalytic performance between the 2D nanosheets MoNi4/MoO2.5-NF and the 1D composite catalysts (composed of alloy and metal oxide/ hydroxide) is shown in Table S1.

Figure 4
figure 4

The HER performance of MoNi4/MoO2.5-NF and control samples in 1 M KOH. a LSV curves, b Tafel plots, c capacitive currents density as a function of scan rate, d EIS Nyquist plots, e Faradaic efficiency at different overpotentials, f Chronopotentiometry curves

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In order to further explore the reason for the improvement of the performance of the catalyst after electrochemical activation, the XRD, SEM, TEM and BET of the MoNi4/MoO2.5-NF before and after the electrochemical activation were characterized. The XRD diffraction patterns of MoNi4/MoO2.5-NF before and after the electrochemical activation are shown in Fig. 5a. The XRD patterns of the EA-MoNi4/MoO2.5-NF sample match well with the MoNi4 phase. The SEM and TEM images of the EA-MoNi4/MoO2.5-NF also show no change compared with that of MoNi4/MoO2.5-NF (Fig. 5b,c). The BET specific surface area of the EA-MoNi4/MoO2.5-NF is 43 m2/g, which is similar to the MoNi4/MoO2.5-NF (40 m2/g, Fig. 3a). In addition, the double-layer capacitances (Cdl), Tafel slope and charge transfer resistances (Rct) of EA-MoNi4/MoO2.5-NF are significantly higher than that of the MoNi4/MoO2.5-NF (Fig. 4b,d). Therefore, the reason for the improvement of the HER performance of the catalyst after electrochemical activation can be associated with the enhanced electrochemically active surface area (ECSA), the decreased Tafel slope and charge transfer resistance (Rct).

Figure 5
figure 5

The XRD (a), SEM (b) and TEM (c) images of MoNi4/MoO2.5-NF before and after the electrochemical activation; d and e the bubble adhesion force and contact angles of NF and MoNi4/MoO2.5-NF, f the bubbles adhesion behaviors diagram. Inset: sectional views of the discontinuous three-phase contact line (TPLC) on MoNi4/MoO2.5-NF

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The bubble adhesion force and contact angle between bubbles and the catalyst surface have important effects on the release of H2 from the catalyst surface and the promotion of the HER reaction. As shown in Fig. 5d, the bubble adhesion force and contact angle of NF are 24 μN and 128.4°. As a result, the bubbles formed on the electrode are pinned on the electrode, which is difficult to escape in the process of electrocatalysis. These bubbles will occupy the catalytic active sites and hinder the further development of hydrogen evolution reaction. The bubble adhesion force and contact angle of EA-MoNi4/MoO2.5-NF are 0.6 μN and 155° (Fig. 5e). The MoNi4/MoO2.5 nanosheet arrays reduce continuance of the three-phase lines and the area of contact between electrocatalyst and bubbles, resulting in aerophobic feature of EA-MoNi4/MoO2.5-NF electrodes. The nanosheet arrays structure endows the electrocatalyst a superaerophobic feature (Fig. 5f).

In conclusion, a novel 2D layered self-supporting composite electrocatalyst MoNi4/MoO2.5-NF was reported for the first time. Compared with the one-dimensional electrocatalytic materials with similar composition (i.e., Mo–O species and Ni–Mo alloy complexes) previously reported, this 2D nanosheets electrocatalyst reported here shows better electrocatalytic properties. The reason for the high catalytic activity of EA-MoNi4/MoO3-x/NiCo@NF is revealed. (1) The superior 2D nanosheets morphology and structure endow the catalyst with high specific surface area, electrical conductivity, more exposed active sites and mass transfer channels; (2) The synergistic effect between metal oxides and alloys. The MoO2.5 can effectively promote the cleavage of H–OH bond and formed Hads. The Hads adsorbed on the MoNi4 particles combines rapidly to release H2. The synergistic effect of MoO2.5 and MoNi4 particles greatly reduces the overpotential and Tafel slope of HER process. (3) The electrochemical-cycling activation (CV) process can further improve the HER activity. After 10 CV cycles, the HER electrocatalytic properties even exceeded that of 20% Pt/C. This study provides a reference for the design and synthesis of alkaline electrocatalysts to replace Pt in the future.