Accelerated alkaline hydrogen evolution on M(OH)_x/M-MoPO_x (M = Ni, Co, Fe, Mn) electrocatalysts by coupling water dissociation and hydrogen ad-desorption steps

Abstract

Developing efficient and cheap electrocatalysts for the alkaline hydrogen evolution reaction is still a big challenge due to the sluggish water dissociation kinetics as well as poor M–H_ad energetics. Herein, hydroxide modification and element incorporation have been demonstrated to realize a synergistic modulation on a new class of M(OH)_x/M-MoPO_x catalysts for accelerating water dissociation and hydrogen ad-desorption steps in the HER. Theoretical and experimental results disclosed that in situ modification with hydroxide endowed M(OH)_x/M-MoPO_x with a strong ability to dissociate water, and meanwhile, oxygen incorporation effectively optimized the M–H_ad energetics of the NiMoP catalyst. Moreover, the interaction between M(OH)_x and M-MoPO_x components in M(OH)_x/M-MoPO_x further enhances their ability to catalyze the two elementary steps in alkaline hydrogen evolution, providing a wide avenue for efficiently catalyzing hydrogen evolution. In general, the optimized Ni(OH)₂/NiMoPO_x catalyst exhibits excellent alkaline HER activity and durability, superior to the state-of-the-art Pt/C catalyst when the overpotential exceeds 65 mV.

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Introduction

Hydrogen has been extensively researched as an alternative fuel source to fossil fuels. Water electrolysis is an appealing, but challenging method for large-scale production of H₂. In a water-alkali electrolyser, there are two half-cell reactions occurring at the cathode and anode, i.e. the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). The kinetics of these two electrochemical reactions under typical operation conditions are inherently slow, which necessitates the use of catalysts with high activity and stability to reduce electricity consumption.^1–4 Nowadays, the most effective HER catalysts are Pt-based metals, but their industrial application is limited by their scarcity and high cost. Although some studies have been conducted to replace Pt with Ru compounds (that are relatively low in cost), the intrinsic limitation of noble metals has still not been overcome completely.^5,6 Thus, it is especially important to use highly active HER electrocatalysts derived from low-cost materials.

In recent few years, abundant non-noble metal-based materials have been developed as promising electrocatalysts for hydrogen evolution under alkaline conditions.^7,8 Among these transition metal catalysts, molybdenum-based compounds,⁹ such as alloys,¹⁰ carbides,^11–15 chalcogenides,^16–18 and phosphides,^19–21 have recently attracted significant research interest due to their desired chemical and physical properties. For example, as a prototypical model, MoS₂ is the first reported transition metal-based HER catalyst, in which only the surface edges are catalytically active.^22–24 Unlike MoS₂, which only shows HER activity at surface edges, all the sites of bulk MoP show high HER activity. Calculations of P sites on MoP indicate that P acts like a ‘hydrogen deliverer’ as hydrogen adsorbs on P at low coverage whilst it desorbs at high coverage.^25,26 Accordingly, molybdenum phosphides usually exhibit higher HER current density than MoS₂, as more active sites are available for the intensive electrocatalytic reaction. However, even though these MoP-based catalysts have abundant active sites for catalyzing protons to hydrogen, their HER activities are still unsatisfactory, especially in alkaline solution.

Under alkaline conditions, the HER process inevitably undergoes the first water dissociation step (Volmer step), because there are few protons existing in alkaline solutions.^27,28 In 2012, Markovic et al. probed the surface HER reactivity of the materials with near-optimal M–H_ad energetics (such as Pt) and found that it can be enhanced by modifying them with more efficient active sites (such as 3d metal hydroxides) for water molecule dissociation.²⁹ Recently, our group has found that there is a strong interaction occurring at the interface of Ni@Ni(OH)₂–Pd, which is effective for catalysing hydrogen generation in alkaline solution compared with the pure Pd catalyst.³⁰ With this in mind, we speculate that two types of active sites are required for efficient alkaline hydrogen evolution, i.e., one with optimized M–H_ad energetics (ΔG(H)) and another with the capability of accelerating the dissociation of water. However, all the above-mentioned HER catalysts contain noble metals (e.g. Pt and Pd) which seem to act as indispensable parts for the high HER activity. Pure metal oxides or hydroxides are generally considered as HER inactive materials, although they facilitate the water dissociation step.^31,32 Such strong dependency on noble metals is undesirable for future massive applications. Therefore, achieving a satisfactory synergism in accelerating both water dissociation and hydrogen ad-desorption (i.e. optimal M–H_ad energetics) steps on a non-noble material is urgent but challenging for an efficient HER.

Based on the above observation, we put forward that hydroxide modification and element incorporation can help realize a synergistic modulation for accelerating both water dissociation and hydrogen ad-desorption steps in the HER. And a new class of noble-metal free catalysts, M-MoPO_x-based nanomaterials in situ modified with hydroxide species on the surface (M(OH)_x/M-MoPO_x, M = Ni, Co, Fe, Mn), served as a platform to elaborate our proposed view. Combining experiments with DFT calculations, we demonstrated that on the one hand, hydroxide modification generates more active sites in catalysts for water dissociation, thus accelerating the Volmer step during the HER process. On the other hand, oxygen incorporation can regulate the electronic structure of M-MoP, resulting in modulated M–H_ad energetics. More interestingly, we also revealed that the interaction between M(OH)_x and M-MoPO_x further enhances their catalytic abilities, i.e., M(OH)_x and M-MoPO_x components in M(OH)_x/M-MoPO_x catalysts exhibit greater abilities in catalysing water dissociation and hydrogen ad-desorption steps than their corresponding single component catalysts, respectively. Benefiting from the hydroxide modification, element incorporation and component interaction, efficient HER activity in an alkaline environment was achieved, proving the validity of coupling water dissociation and hydrogen ad-desorption steps for efficient alkaline hydrogen evolution.

Results and discussion

The typical preparation procedure of the hydroxide species modified M-MoPO_x based nanomaterials is schematically described in Scheme S1.† Taking Ni(OH)₂/NiMoPO_x/NF as the representative example, a flower-like structure composed of regular NiMoO₄ nanocuboids on the surface of Ni foam was first acquired via a facile and scalable hydrothermal method.¹⁵ Field-emission scanning electron microscopy (FESEM) indicates that the NiMoO₄ nanocuboids uniformly grown on Ni foam have an average width of about 300 nm and a very smooth surface (Fig. S1a†), and the 3D-network structure of Ni foam is of great benefit for loading the active species. Then the as-prepared NiMoO₄/NF was transformed into NiMoPO_x/NF by phosphatizing with NaH₂PO₂ at 400 °C for 2 h under an Ar gas flow. No obvious change in the morphology can be observed when NiMoO₄/NF was converted into NiMoPO_x/NF (Fig. S1b–d†), while the crystal structure changes from the high-crystallinity NiMoO₄ phase (JCPDS no. 86-0361) to the Ni₂P₄O₁₂ phase (JCPDS no. 76-1557) (Fig. 1b). As depicted in Fig. S1e,† the NiMoPO_x nanocuboids are composed of numerous nanocrystallites with a small size of about 10 nm, uniformly embedded in the amorphous phase. High-resolution TEM (HRTEM) confirmed the crystalline nature of the nanocrystallites. The lattice fringes with an interplanar distance of 0.23 nm in Fig. S1f† correspond to the (−133) planes of Ni₂P₄O₁₂. Energy dispersive X-ray spectra (Fig. S1g†) show that Ni, Mo, P and O are distributed over the NiMoPO_x nanocuboids. The atomic contents of Ni, Mo, P and O are 12.22, 13.66, 14.33 and 59.80, respectively (Fig. S1h†), and the exact chemical formula of MMoPO_x is NiMo_1.12P_1.17O_4.89. Considering the P-XRD results and EDX-mapping results, it can be speculated that the NiMoPO_x material contains crystalline Ni₂P₄O₁₂ and an amorphous Mo-containing compound.

Fig. 1 (a) Schematic illustration of the microstructure of M(OH)_x/M-MoPO_x. (b) P-XRD patterns of NiMoO₄, NiMoPO_x and Ni(OH)₂/NiMoPO_x powders scraped from Ni foam. (c and d) SEM images, (e) elemental mapping and (f) EDX spectrum of Ni(OH)₂/NiMoPO_x. Morphology comparison of (g and h) NiMoPO_x and (i and j) Ni(OH)₂/NiMoPO_x on the same scale. (k) HR-TEM image of Ni(OH)₂/NiMoPO_x.

After electrochemical transformation in 1 M NaOH, tiny and flexible Ni(OH)₂ nanosheets were generated through redox of Ni₂P₄O₁₂, and were uniformly attached to the surface of pre-synthesized NiMoPO_x nanocuboids to produce Ni(OH)₂/NiMoPO_x/NF hybrid nanocuboids (named Ni(OH)₂/NiMoPO_x). Fig. S2a† displays the CV curves at different scans for the presented NiMoPO_x/NF. Obviously, the current density of NiMoPO_x/NF continuously increases with CV scans until 8000 cycles, as well as the area of the redox zone (−0.15 to 0.2 V). These results reveal that the catalytic properties of NiMoPO_x/NF change constantly with the CV scans. On the basis of the general electrochemical redox states of elements under potential bias,³³ it is speculated that the Ni species on the surface of NiMoPO_x/NF varied during electrochemical activation. The growing area of the redox zone implies that more Ni species were involved in the reaction during the redox switching scan. In addition, the existence of PO₄³⁻ ions in the electrolytes used for electrochemical activation of M-MoPO_x was detected by UV-vis-NIR spectroscopy (Fig. S3a†). XRD results show that a crystal structure transition occurred after 8000 cycles, as crystalline Ni₂P₄O₁₂ disappeared and NiMoPO_x/NF became an amorphous structure (Fig. 1b). It has been reported that Co-based phosphate would leach out and transform into hydroxide during the electrochemical redox process in alkaline solution,³⁴ which shows us that an in situ transformation of crystalline Ni₂P₄O₁₂ into amorphous Ni(OH)₂ occurred on the NiMoPO_x/NF surface via the following pathways:

Ni₂P₄O₁₂ + 12OH⁻ = 2Ni(OH)₂ + P₄O₁₂⁴⁻ + 4H₂O (1)

Electron microscopy observations together with X-ray photon spectroscopy (XPS) of NiMoPO_x/NF prove the evolution of local structures during electrochemical activation. SEM images (Fig. 1c and d) reveal that the morphology of NiMoPO_x/NF is maintained in activated Ni(OH)₂/NiMoPO_x/NF, while a large number of nanosheets appear on the surface of NiMoPO_x/NF. The diameter of Ni(OH)₂/NiMoPO_x/NF nanocuboids gets smaller and their surface becomes more rough than that of NiMoPO_x/NF (Fig. 1g–j). Although the mapping images (Fig. 1e) show that the elements are still uniformly distributed over the nanocuboids, the decreased atomic content of the P element measured by EDX spectroscopy (Fig. 1f) further reveals the dissolution of P-species during electrochemical activation. The HR-TEM image (Fig. 1k) verifies the existence of newly formed amorphous Ni(OH)₂ nanosheets with a short-order (001) plane on the Ni(OH)₂/NiMoPO_x/NF surface and the disappearance of crystalline Ni₂P₄O₁₂ nanoparticles inside the catalyst. The variation of element valences and their contents on the catalyst surface was analysed by XPS (Fig. 2). A higher Ni content in XPS than in EDX in contrast to the lower P and Mo contents in XPS than in EDX (Table S1,†Fig. 1f) demonstrates the relatively outer position of Ni and inner position of P and Mo in the transformed nanostructure. Ni species have three states (Fig. 2a), i.e. Ni⁰ (851.8 eV), Ni²⁺ (855.9 eV), and Ni³⁺ (858.5 eV). Compared with NiMoPO_x/NF, Ni(OH)₂/NiMoPO_x/NF has a higher content of Ni²⁺ species and a decreased Ni³⁺ content. Meanwhile, an additional peak of OH⁻ groups is observed in the O 1s spectrum of Ni(OH)₂/NiMoPO_x/NF, indicating the new formation of Ni(OH)₂ on the surface of the activated catalysts (Fig. 2d). The sharply decreased P and Ni³⁺ content together with the intensively increased Ni²⁺ content measured by XPS further confirms that the Ni₂P₄O₁₂ first leached out and the Ni ions are then re-deposited to form the hydroxide during the electrochemical redox process. The overall element contents of Ni(OH)₂/NiMoPO_x in Fig. 1f reveal that the core component is amorphous NiMoPO_x. The relative contents of Mo species with different valences are stably retained in the Ni(OH)₂/NiMoPO_x/NF hybrids, verifying that the amorphous NiMoPO_x below the thin Ni(OH)₂ layer retains its original state after electrochemical activation. Based on the above evidence, Ni(OH)₂/NiMoPO_x/NF is shown to be composed of amorphous NiMoPO_x in the core and a thin Ni(OH)₂ layer on its surface. Beyond Ni(OH)₂/NiMoPO_x/NF, a series of 3D-nanocuboid hybrids composed of other 3d metals (Co, Fe, and Mn) were obtained. SEM images show that CoMoPO_x/NF and MnMoPO_x/NF also have a nanocuboid-like structure while the structure of FeMoPO_x/NF is nanoflake-like (Fig. S4†). The element valences and contents of M-MoPO_x and activated M(OH)_x/M-MoPO_x were measured by XPS (Fig. S4–S7 and Table S2†). The variation trend of element content in M-MoPO_x and corresponding M(OH)_x/M-MoPO_x is similar to that of NiMoPO_x, demonstrating the universality of electrochemical activation in modulating the catalyst surface structure.

Fig. 2 XPS spectra of NiMoPO₄ and Ni(OH)₂/NiMoPO_x; (a) Ni 2p region, (b) Mo 3d region, (c) P 2p region and (d) O 1s region.

The catalytic performance of the synthesized electrocatalysts in the alkaline HER was evaluated in N₂-saturated 1.0 M NaOH. The polarization curves of Ni(OH)₂/NiMoPO_x/NF along with those of NiMoPO_x/NF, NiMoP₂/NF, NiMoO₄/NF, Ni foam, and commercial Pt/C loaded on NF (20 wt% Pt/C/NF) without iR correction are shown in Fig. 3a. NiMoP₂/NF exhibits a good HER activity, while Ni foam exhibits relatively poor HER activity, and NiMoO₄/NF and Ni(OH)₂/NF are even worse. As expected, the O incorporation greatly improves the HER performance of NiMoPO_x/NF due to optimized hydrogen binding energy (H_ad).³⁵ Furthermore, the generation of Ni(OH)₂ on the NiMoPO_x nanocuboids results in a sharply improved HER reactivity, which even outperformed that of the benchmark Pt/C. In the iR corrected polarization curves (Fig. 3b), Ni(OH)₂/NiMoPO_x/NF exhibits a rapid enhancement of the cathodic current along with negative potential and surpasses the current density of Pt/C when the overpotential exceeds 65 mV. Moreover, to drive a current density of 10 mA cm⁻², Ni(OH)₂/NiMoPO_x/NF needs an extremely low overpotential of 51 mV, which is markedly lower than that of NiMoO₄/NF (234 mV), NiMoPO_x/NF (77 mV), and the most advanced HER electrocatalysts reported recently, such as Ni(OH)₂/MoS₂ (80 mV),³⁶ NiCo₂P_x/CF (58 mV),³⁷ Cu NDs/Ni₃S₂ NTs–CFs (128 mV)³⁸ and CoMoP@C (81 mV)³⁹ (more details in Table S6†). To obtain a larger cathodic current density of 100 mA cm⁻², Ni(OH)₂/NiMoPO_x/NF needs an overpotential of 72 mV, approaching half the overpotential of 20% Pt/C/NF (154 mV, Table S3†). Such an excellent HER activity of Ni(OH)₂/NiMoPO_x/NF is impressive and shows its potential for industrial application.

Fig. 3 (a) LSV curves, (b) LSV curves with iR correction and (c) Tafel slopes of NiMoO_x/NF, NiMoPO_x/NF, Ni(OH)₂/NiMoPO_x/NF, 20% Pt/C/NF and NF with 5 mV s⁻¹ in 1 M NaOH. (d) Chronopotentiometry curve of Ni(OH)₂/NiMoPO_x/NF at a constant potential of −100 mV for 90 hours. The inset in (d) is the LSV curves of Ni(OH)₂/NiMoPO_x/NF before and after 4000 CV scans at −0.3 V ∼ +0.2 V vs. RHE at 100 mV s⁻¹.

The Tafel plots of these electrocatalysts were measured to provide a deep insight into the HER reaction pathways on the electrocatalysts (Fig. 3c, Table S3†). The Tafel slope of Ni(OH)₂/NiMoPO_x/NF is 33 mV dec⁻¹, much lower than that of Ni foam (140 mV dec⁻¹), NiMoO₄/NF (101 mV dec⁻¹), NiMoPO_x/NF (72 mV dec⁻¹), and 20 wt% Pt/C/NF (34 mV dec⁻¹). Such a low value of the Tafel slope suggests that the HER kinetics on Ni(OH)₂/NiMoPO_x/NF is determined by the Tafel step rather than by the Volmer step. Furthermore, the electrochemical impedance spectroscopy results (EIS, Fig. S8a†) of Ni(OH)₂/NiMoPO_x/NF further show a faster HER kinetics process than that on the NiMoPO_x/NF. Compared with the NiMoPO_x/NF, which undergoes the Volmer–Heyrovsky mechanism during the HER process, it is easy to infer that the formation of Ni(OH)₂ on its surface significantly accelerates the HER reaction kinetics by optimizing the water dissociation step. In addition, the electrochemical specific surface area (ECSA, Fig. S8b and S9†) of Ni(OH)₂/NiMoPO_x/NF is 56.4 mF cm⁻², which is similar to that of NiMoPO_x/NF (48.6 mF cm⁻²), revealing that the enhancement of the HER activity is not attributed to the enlargement of the ECSA, but the improvement of the intrinsic activity.

Stability is another important evaluation index for the performance of catalysts in practical applications, especially in strongly alkaline media. The long-term stability of the best catalyst Ni(OH)₂/NiMoPO_x/NF was studied by continuous electrolysis at an overpotential of 100 mV for over 90 h. A steady current curve without notable degradation was observed during the 90 h aging test in an alkaline environment (Fig. 3d). Besides, the LSV curves (inset in Fig. 3d) of Ni(OH)₂/NiMoPO_x/NF show almost no change after 4000 CV cycles, revealing that Ni(OH)₂/NiMoPO_x/NF possesses good corrosion resistance in a wider potential region. The Ni(OH)₂/NiMoPO_x/NF also exhibits a comparable HER activity and stability to the industrial PtRuNiP/Ni electrode when integrated into a practical water electrolyzer (Fig. S10†). It is not difficult to understand the excellent stability of Ni(OH)₂/NiMoPO_x/NF: (i) the unique three-dimensional flower-like structures comprising regular nanocuboids can provide patulous architectures and open spaces that are beneficial to promote the release of generated gas bubbles. (ii) The in situ growth technology enhances the mechanical adhesion between catalysts and substrates. (iii) Moreover, the hydroxides formed on the surface of the catalyst and appropriate phosphate species can serve as protective agents to enhance the stability of Ni(OH)₂/NiMoPO_x/NF in alkaline solution.^40,41

In order to confirm the synergistic effect of the hybrid structure for coupling the water dissociation and hydrogen ad-desorption processes during the alkaline HER, we studied the structural properties and HER activities of NiMoPO_x, Ni(OH)₂/NiMoPO_x and Ni(OH)₂/NiMoPO_x-acid in 1 M HClO₄ and 1 M NaOH (Fig. 4a, b and S11†). As for the Raman spectrum of the NiMoPO_x sample, the absorption bands at ∼200, 330, 710, 880 and 960 cm⁻¹ can be assigned to the phosphomolybdate anion.^42,43 After electrochemical activation, two new broad bands at 365 and 470 cm⁻¹ appear, corresponding to the E_g(T) mode of the Ni-OH lattice vibration and the A_1g(T) mode due to υNi-OH, respectively.^44,45 The band at 550 cm⁻¹ is consistent with the structural defects.^46,47 These results indicate the generation of amorphous Ni(OH)₂ on the surface of NiMoPO_x during electrochemical activation. The band of P–O–P for Ni(OH)₂/NiMoPO_x is red-shifted in comparison with that for the NiMoPO_x, due to the interaction between Ni(OH)₂ and NiMoPO_x. The Ni(OH)₂/NiMoPO_x was then treated by acid etching in 0.5 M H₂SO₄ to remove the generated Ni(OH)₂ on its surface and its Raman spectrum changes back to that of the NiMoPO_x sample, revealing that only the surfacial component of NiMoPO_x changes but the inside does not change during electrochemical activation.

Fig. 4 (a) Raman spectra and (b) comparison between activities for the HER, expressed as the overpotential required for a 10 mA cm⁻² current density, in 1 M HClO₄ and 1 M NaOH of the NiMoPO_x, Ni(OH)₂/NiMoPO_x and Ni(OH)₂/NiMoPO_x-acid; (c) LSV curves of M(Ni, Co, Fe, Mn)-MoPO_x/NF and their activated M(OH)₂/M-MoPO_x/NF electrodes without iR correction; (d) comparison with M(Ni, Co, Fe, Mn)-MoPO_x/NF electrocatalysts before and after electrochemical activation in alkaline solutions.

It should be pointed out that the HER activity at high pH values is determined using M–H_ad bond strength and the energy required for water dissociation simultaneously. While in acid electrolyte, the HER activity can only be determined using M–H_ad bond strength, as the H protons are abundant.⁴⁸ Therefore, the alkaline activities of HER catalysts are always worse than their acid activities due to the additional energy required for water dissociation. As shown in Fig. 4b, the observed activities of the three catalysts in alkaline solutions are obviously lower than those in acid solutions. The alkaline HER activity of Ni(OH)₂/NiMoPO_x is significantly superior to that of NiMoPO_x in alkaline electrolyte, and is similar to that of NiMoPO_x in acid electrolyte. When the generated Ni(OH)₂ on the surface of Ni(OH)₂/NiMoPO_x is removed by acid etching, the HER activity of Ni(OH)₂/NiMoPO_x-acid in both acid and alkaline electrolytes reduces back to that of the original NiMoPO_x. These results confirm that the obstruction of water dissociation for NiMoPO_x in alkaline electrolyte can be effectively resolved by the generated Ni(OH)₂.

The HER activities of other 3d M(OH)_x/M-MoPO_x (M = Ni, Co, Fe, Mn) hybrids were systemically investigated to confirm the universality of coupling water dissociation and hydrogen ad-desorption for excellent alkaline HER activity. The HER polarization curves in 1 M NaOH are shown in Fig. 4c. A clear alkaline HER activity trend is observed in the order of Fe < Mn < Co < Ni for both M-MoPO_x catalysts and the derived M(OH)_x/M-MoPO_x hybrids. Besides, all these derived M(OH)_x/M-MoPO_x hybrids exhibit an obvious improvement in activity to different degrees compared with the original M-MoPO_x catalysts (Fig. 4d). This result reveals that the in situ formed hydroxide is commonly useful for improving the HER activity of these M-MoPO_x-based catalysts. The Tafel slopes, EIS spectra, ECSA and ECSA-normalized HER polarization curves of these catalysts were further measured (Fig. S12–S14, Table S4†). The Tafel slopes of Ni(OH)₂/NiMoPO_x/NF and Co(OH)₂/CoMoPO_x/NF sharply reduce from around 70 mV dec⁻¹ to below 40 mV dec⁻¹ as well as that of Mn(OH)_x/MnMoPO_x/NF, revealing that the sluggish Volmer step (i.e. water dissociation) was thoroughly accelerated by the newly formed M(OH)_x on the catalyst surface. The relatively larger Tafel slope of Fe(OH)_x/FeMoPO_x/NF is ascribed to the very strong adsorption of the Fe based hydroxide. The much smaller reaction resistances (R_ct) of these M(OH)_x/M-MoPO_x catalysts further confirm the faster HER kinetics process on the hybrid catalysts. Therefore, it is speculated that the hybridized M(OH)_x side is favourable for producing H* and OH⁻via accelerating the water dissociation step (H₂O → H* + OH⁻ + e⁻).^49,50 On the other hand, all these M-MoPO_x catalysts are weak in water dissociation, and the distinct difference in their HER activities should be ascribed to the ability of catalyzing the H₂ ad-desorption step. Despite the strong water dissociation ability of Ni(OH)₂, the poor HER activity of the Ni(OH)₂/NF compared with that of NiMoPO_x and Ni(OH)₂/NiMoPO_x further confirms the crucial role of the hydrogen ad-desorption step in the overall HER process. Based on the experimental results above, we can easily assume that superior alkaline HER catalysts should be capable of catalysing both water dissociation and hydrogen ad-desorption steps efficiently.

To gain further insights into the origin of the hybrid heterostructure for the excellent HER activity, DFT calculations on the H₂O dissociation step (ΔG(H₂O), Volmer step) and the H ad-desorption step (ΔG(H), Tafel step) were performed (Fig. 5a). The energy change in the Volmer reaction (ΔG_R, Fig. 5b) for Ni(OH)₂/NiMoPO_x (0.60 eV) is lower than that of Ni*MoPO_x (1.31 eV) and Ni*(OH)₂ (1.38 eV), revealing that the Ni*(OH)₂/NiMoPO_x hybrid is more favourable for H₂O dissociation thermodynamically. Moreover, the water dissociation kinetic energy barrier (ΔG_TS) of Ni*(OH)₂/NiMoPO_x dramatically decreases from the 2.89 eV of Ni*MoPO_x to 1.01 eV, suggesting that the sluggish Volmer step on NiMoPO_x was greatly accelerated after the in situ generation of the Ni(OH)₂ component. As for the concomitant Tafel step (Fig. 5c), the Ni(OH)₂ has a very negative ΔG(H) (−0.37 eV), indicating a very high H adsorption strength, while all these NiMoP based catalysts present modulated ΔG(H) closer to the thermoneutral. The ΔG(H) of Ni*MoPO_x is further decreased by 0.04 eV when O atoms are doped into the Ni*MoP, suggesting that the incorporation of O can modulate the H adsorption on the surface of the NiMoP catalyst. Interestingly, the final Ni(OH)₂/Ni*MoPO_x obtains an optimal ΔG(H) of 0.14 eV, which is even close to the absolute value of ΔG(H) for Pt* (−0.09 eV).^51,52 Note that the water dissociation kinetic energy barrier (ΔG_TS) and hydrogen adsorption free energy (ΔG(H)) of Ni(OH)₂/NiMoPO_x are superior to the ΔG_TS of pure Ni(OH)₂ and the ΔG(H) of pure NiMoPO_x. This result reveals that the interaction between the two components in Ni(OH)₂/NiMoPO_x could further enhance their catalytic ability, i.e., the Ni(OH)₂ and NiMoPO_x components in Ni(OH)₂/NiMoPO_x catalysts possess greater ability to catalyze the water dissociation and hydrogen ad-desorption steps, respectively. Thus, the two types of active sites required for alkaline hydrogen evolution coexist adjacently in the Ni(OH)₂/NiMoPO_x catalysts, providing a new and smoother mechanism for hydrogen evolution via the synergy of Ni(OH)₂ and NiMoPO_x components. That is, H* is formed by H₂O* dissociation on the metal hydroxide first and then transferred to the adjacent NiMoPO_x sites to form H₂. These results combined with experimental observations reveal that the excellent HER activity of Ni(OH)₂/NiMoPO_x is attributed to the synergistic effect in the hybrid components.

Fig. 5 (a) Chemisorption models of H and OH intermediates on the surfaces of NiMoPO_x and the Ni(OH)₂/NiMoPO_x hybrid; calculated adsorption energy diagram of (b) the water dissociation step and (c) hydrogen ad-desorption for Ni(OH)₂, NiMoPO_x and the Ni(OH)₂/NiMoPO_x hybrid. The symbol * in the sample name represents the active site for DFT calculations. Color codes: Mo, cyan; P, pink; Ni, blue; O, red; H, white.

Conclusions

In summary, synergistic modulation of both Volmer and Tafel steps for alkaline hydrogen evolution was achieved in NiMoP nanocuboids by hydroxide modification and element incorporation. By in situ modification with the hydroxide, the HER catalysts are endowed with the strong ability to dissociate water. Besides, the M–H_ad energetics of the NiMoP catalyst was optimized by oxygen incorporation, accelerating the hydrogen ad-desorption process. The interaction between the two components in the Ni(OH)₂/NiMoPO_x further enhances their individual catalytic ability. Therefore, the optimized oxygen incorporated NiMoP catalyst with a thin hydroxide layer modification possesses high reactivities for both water dissociation and hydrogen ad-desorption steps, thus exhibiting a superior HER performance in an alkaline environment. As demonstrated, the optimal Ni(OH)₂/NiMoPO_x possesses a remarkable HER activity with a low overpotential of 51 mV at 10 mA cm⁻² as well as an excellent long-term stability for over 90 h, making it a prominent alternative for Pt-based catalysts. This work provides a direction to design efficient electrocatalysts by strengthening the reactivity of each elementary step.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research work was financially sponsored by the National Natural Science Foundation of China (Grant nos. 21822803, and 91834301). Special thanks to Lianqiao Tan and Jiao Yang for their contributions during the revision process.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9sc04603h

‡ These authors contributed equally to this work.

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