Modulating the electronic interactions via heterostructure engineering for energy-saving hydrogen production at high current densities

Abstract

Advanced alkaline hydrogen evolution reaction (HER) catalysts should exhibit a low water dissociation energy barrier and fast reaction kinetics. However, single-component transition metal catalysts typically show insufficient activation of water, leading to unsatisfactory electrocatalytic activity. Rationally modulating the composition and constructing heterogeneous interfaces can effectively regulate the interfacial electronic structure of catalysts, thereby improving the kinetics and catalytic activity of the alkaline HER. Herein, Ni_0.2Mo_0.8N/F,N–C@NF heterogeneous catalyst was prepared, demonstrating excellent alkaline HER activity (281 mV @ 1000 mA cm⁻²). Experimental results and density functional theory (DFT) calculations indicate that heterogeneous engineering can effectively modulate the surface charge structure of catalysts, enhancing the adsorption and activation of reactive intermediates. Simultaneously, heterogeneous engineering accelerates reaction kinetics by further reducing the energy barrier of the Tafel step. The Ni_0.2Mo_0.8N/F,N–C@NF catalyst also exhibits satisfactory catalytic activity towards the HER when integrated with a methanol oxidation reaction. This work provides an effective guide for the rational design of high-performance alkaline HER electrocatalysts for energy-saving hydrogen production.

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Dongxing Tan

Dr Dongxing Tan received his PhD degree from the School of Chemical Sciences, University of Chinese Academy of Sciences, in 2020. After completing his postdoctoral research at the School of Chemistry and Chemical Engineering, Shandong University, he joined the School of Chemistry and Chemical Engineering at Qufu Normal University (China) in 2023. His research interests focus on functional design of advanced energy materials for the catalytic conversion of clean energy, including electrocatalytic/photocatalytic reduction of carbon dioxide, water splitting and organic electrocatalytic conversion.

Introduction

Hydrogen (H₂), as a renewable and sustainable resource, is an ideal alternative energy to traditional fossil fuels.^1–3 It is both promising and crucial to produce H₂ through environmentally friendly and cost-effective means. The production of green H₂via overall water splitting (OWS) has gained significant attention. OWS consists of a hydrogen evolution reaction (HER) at the cathode and an oxygen evolution reaction (OER) with a high thermodynamic equilibrium potential (1.23 V versus reversible hydrogen electrode, RHE) at the anode.^4–6 However, the kinetically sluggish OER consumes about 90% of the input electricity, severely limiting the efficiency of H₂ production and increasing the overall cost.^7,8 To maximize energy utilization, it is urgent and more meaningful to combine HER with thermodynamically favorable oxidation reactions.⁹ Electrocatalytic oxidation of organic molecules has been proposed as a replacement for the traditional OER. These organic molecular oxidation reactions can significantly reduce energy consumption, and the value-added oxidative products are key intermediates in the pharmaceutical and chemical industries.^10–15

Methanol, a cheap and widely available platform chemical, has been extensively investigated for use in the electrocatalytic methanol oxidation reaction (MOR) integrated with the HER. MOR has a low thermodynamic equilibrium potential (CH₃OH + 5OH⁻ → HCOO⁻ + 4H₂O + 4e⁻, 0.103 V vs. RHE) and can produce high value-added formate.^16–18 Various catalysts have been designed to enhance the combined HER and methanol-to-formate process. For example, bifunctional 2D exfoliated boron nanosheets have been used for HER integrated with MOR to enable energy-efficient H₂ production.¹² The hybrid electrolyzer (HER + MOR) requires a significantly lower voltage (@10 mA cm⁻²) compared to a conventional electrolyzer (HER + OER). Furthermore, the optimized local coordination environment of NiOOH with oxy-anions can regulate the adsorption of OH* and methanol, improving the MOR activity.¹⁶ Hence, utilizing the MOR to replace the traditional OER not only offers a promising approach for energy-saving H₂ production but also generates high-valued oxidative products at the anode.

Here, we synthesized a heterogeneous catalyst of bimetal nitride (Ni_0.2Mo_0.8N) embedded in F,N-doped carbon nanoarrays supported on the nickel foam (NF) substrate (denoted as Ni_0.2Mo_0.8N/F,N–C@NF) to achieve efficient MOR-assisted energy-saving H₂ production. Experimental results and DFT calculations show that the interactions between bimetal nitride and F,N-doped carbon greatly facilitate the adsorption and activation of water molecules and accelerate the charge transfer process. Excitingly, the optimized Ni_0.2Mo_0.8N/F,N–C@NF shows superior activity for HER and MOR in an alkaline solution. Even at the industrial-level current density of 1000 mA cm⁻², the Ni_0.2Mo_0.8N/F,N–C@NF catalyst presents low potentials of 281 and 1483 mV for HER and MOR, respectively. Impressively, simultaneous H₂ and formate generation can be achieved when the Ni_0.2Mo_0.8N/F,N–C@NF acts as both the cathode and the anode for the HER and the MOR.

Results and discussion

Compared to acidic HER, HER in alkaline electrolytes is more cost-effective and stable, especially at an industrial-grade high current density. However, the alkaline HER exhibits a sluggish kinetic process due to the additional Volmer step (H₂O + e⁻ → H_ad + OH⁻) for water dissociation.¹⁹ Hence, an efficient alkaline HER catalyst should enhance water dissociation kinetics. The composite catalysts with different functional components could accelerate water dissociation kinetics to enhance alkaline HER activity.²⁰ Constructing a heterogeneous interface between bimetallic nitride and F,N-doped carbon, regulating the interface electronic structure, and improving the alkaline HER activity through the interface synergy effect between bimetallic nitride and doped carbon are feasible strategies for designing highly efficient alkaline HER catalysts (Fig. 1a). Furthermore, compared with nickel nitride, the synergistic effect of Ni and Mo in bimetallic nitride could reduce the reaction energy barrier of the Volmer step to improve the alkaline HER activity.²¹ Consequently, bimetallic nitride/F,N-doped carbon heterogeneous catalyst delivers superior intrinsic electrocatalytic activity.

Fig. 1 (a) Illustration of alkaline HER over Ni₃N/N–C@NF and Ni_0.2Mo_0.8N/F,N–C@NF. (b) SEM image of NF. (c and d) SEM images, (e) TEM image, (f) HRTEM image, (g) SAED and inverse FFT HRTEM images, and (h) EDX mapping of Ni_0.2Mo_0.8N/F,N–C@NF catalyst.

The metal nitride embedded in F,N-doped carbon nanoarrays supported on the three-dimensional porous nickel foam (NF) substrate was constructed through the solvent-thermal combined with a post-treatment strategy (Fig. 1b). The magnified scanning electron microscopy (SEM) images show that the nanoarrays have a 3D honeycomb-like structure (Fig. 1c, d and S1†). This special network structure could expose more active sites and facilitate the desorption of generated gases, thereby accelerating the electrocatalytic process.²² From the transmission electron microscopy (TEM) image, metal nitride particles are embedded in the F,N-doped carbon layer (Fig. 1e). The diffraction rings of the selected area electron diffraction (SAED) pattern and the lattice fringes of high-resolution transmission electron microscopy (HRTEM) images of optimized bimetallic nitride/F,N–C@NF catalyst indicate that there are Ni_0.2Mo_0.8N and Ni phases from NF in the catalyst (Fig. 1f and g).^23,24 Energy-dispersive X-ray spectroscopy (EDS) mapping images indicate that Mo, Ni, N, C and F elements are uniformly distributed in the catalyst (Fig. 1h).

The chemical compositions and electronic structure of the pre-prepared catalysts were investigated by X-ray diffraction (XRD) pattern and X-ray photoelectron spectroscopy (XPS). The characteristic peaks located at 32.2°, 36.5° and 49.4° are indexed to the (001), (100), and (101) planes of Ni_0.2Mo_0.8N, respectively; the peaks centered at 38.9°, 42.1°, 58.5° and 70.6° are assigned to the (110), (002), (112) and (300) planes of Ni₃N, respectively (Fig. S2a†).^25,26 The remaining diffraction peaks at 44.5° and 51.8° can be assigned to the (111) and (200) planes of NF substrate.²⁷ In the high-resolution Mo 3d spectra (Fig. S2b†), the binding energies at 229.4 and 232.8 eV, 230.8 and 234.0 eV, as well as 232.3 and 235.5 eV correspond to Mo³⁺, Mo⁴⁺ and Mo⁶⁺, respectively.^28,29 Specifically, the Mo oxo species with an unusual oxidation state (most likely Mo³⁺) on the Ni–Mo catalyst was responsible for the high HER activity.^30–32 In the high-resolution Ni 2p spectra (Fig. S2c†), the binding energies at 852.7 and 870.1 eV, 855.9 and 873.5 eV, as well as 858.5 and 876.9 eV correspond to Ni–N, Ni²⁺ and Ni³⁺, respectively.^33,34 It is noteworthy that the binding energies of the Ni 2p in Ni_0.2Mo_0.8N/F,N–C@NF shifted to the lower binding energies compared with that of Ni₃N/N–C@NF (Fig. S3†). This confirms the existence of electronic interactions between the bimetallic nitride and F,N-doped carbon. More importantly, electron-rich active sites could optimize the adsorption energies of H₂O and H* to improve HER activities.³⁵ In the high-resolution N 1s spectra (Fig. S2d†), the binding energies at 395.3, 397.5 and 399.2 eV correspond to Mo 3p, metal–N and N–H, respectively.²⁸ The five carbon species in the high-resolution C 1s spectra (Fig. S2e†) correspond to C–C/C=C (B.E. 284.8 eV), C–N (B.E. 286.3 eV), C–O (B.E. 287.3 eV), C–F (B.E. 288.3 eV) and Ph–π* species (B.E. 289.2 eV), respectively.^36,37 Meanwhile, the C–F bond was further confirmed by the high-resolution F 1s spectra (Fig. S2f†) with the binding energy at 688.9 eV.³⁸

The electrocatalytic performances of the prepared catalysts and commercial Pt/C for the HER were evaluated in three-electrode cells with a 1.0 M KOH solution. As observed from the LSV curves in Fig. 2a, the Ni_0.2Mo_0.8N/F,N–C@NF exhibits significantly improved HER activity compared to the Ni_0.2Mo_0.8N/N–C@NF, Ni₃N/F,N–C@NF, Ni₃N/N–C@NF and commercial Pt/C. To be more specific, the potential required for the Ni_0.2Mo_0.8N/F,N–C@NF catalyst to drive a current density of 100 mA cm⁻² is only 83 mV, which is comparable to the commercial Pt/C@NF. To drive the higher current densities (500 and 1000 mA cm⁻²), the potentials required for Ni_0.2Mo_0.8N/F,N–C@NF catalyst are significantly lower than those of the reference catalysts (Fig. 2b). Moreover, the Ni_0.2Mo_0.8N/F,N–C@NF catalyst has a smaller Tafel slope (65.6 mV dec⁻¹) than that of the Ni₃N/N–C@NF catalyst (130.2 mV dec⁻¹), indicating a fast HER kinetic process (Fig. 2c).³⁹ These results indicate that the introduction of F and Mo can accelerate the reaction kinetics and improve the electrocatalytic activity. In addition, the large double layer capacitance (C_dl) of the Ni_0.2Mo_0.8N/F,N–C@NF catalyst indicates that Ni_0.2Mo_0.8N/F,N–C@NF has a larger electrochemical surface area (ECSA), which can expose more active sites (Fig. S4†). In addition, the ECSA-normalized HER polarization curves of catalysts indicate that the high catalytic performance of Ni_0.2Mo_0.8N/F,N–C@NF mainly depends on the improvement of the intrinsic activity of the metal sites rather than the size of the active area (Fig. S5†). The charge transfer during the HER process was evaluated by electrochemical impedance spectroscopy (EIS) (Fig. S6†). The Ni_0.2Mo_0.8N/F,N–C@NF catalyst has the smallest charge transfer resistance, indicating rapid charge transfer during the electrocatalytic process. Due to the favour kinetics process, large ECSA and fast charge transfer, the Ni_0.2Mo_0.8N/F,N–C@NF has a large turnover frequency (TOF) (Fig. 2d). The TOFs of Ni_0.2Mo_0.8N/F,N–C@NF at −0.10 and −0.20 V are 0.82 and 2.83 s⁻¹, respectively, which are 10.6 and 10.3 times those of the Ni₃N/N–C@NF. This further proves that rationally modulating the composition and constructing heterogeneous interfaces is a feasible means of improving the activity of alkaline HER.

Fig. 2 (a) LSV curves of the prepared catalysts and commercial Pt/C in a 1.0 M KOH solution at a scan rate of 5 mV s⁻¹. (b) Comparison of the potentials at 100, 500 and 1000 mA cm⁻², (c) the corresponding Tafel slopes and (d) TOF of the catalysts and commercial Pt/C@NF. The potential-dependent (e) Nyquist and (f) Bode plots of Ni_0.2Mo_0.8N/F,N–C@NF for HER.

The in situ potential-dependent Nyquist and Bode plots were used to further explore the charge transfer kinetics of the catalytic processes. The radius of the semicircular arc gradually decreases with a decrease in the applied potential, indicating a decrease in charge transfer impedance (Fig. 2e and S7a†). In the Bode plots, the phase angle (θ) in the low-frequency region corresponds to underpotentially deposited hydrogen, while the θ in the high-frequency region corresponds to charge transfer from the inner layer of the catalyst to the surface active site.^40–42 There is a significant difference in the decreasing trend in the different frequency regions as the applied potential changes for Ni_0.2Mo_0.8N/F,N–C@NF and Ni₃N/N–C@NF (Fig. 2f and S7b†). Ni_0.2Mo_0.8N/F,N–C@NF displays a low θ in the high frequency than that of Ni₃N/N–C@NF, indicating a favourable kinetic process. Meanwhile, as the potential decreases, the θ in the low-frequency region gradually decreases, implying that the Ni_0.2Mo_0.8N/F,N–C@NF can accelerate the dissociation of water and promote the transfer of electrons to active sites. Due to the favourable kinetics and rapid charge transfer, the HER catalytic performance of Ni_0.2Mo_0.8N/F,N–C@NF is superior or comparable to that of the currently reported state-of-the-art electrocatalysts in the literature (Table S1†).

According to the experimental results of the structural analysis, Ni_0.2Mo_0.8N/F,N–C@NF, Ni_0.2Mo_0.8N/N–C@NF, Ni₃N/F,N–C@NF and Ni₃N/N–C@NF models were constructed to reveal the synergistic impacts of heterogeneous engineering through DFT calculations (Fig. 3a). As shown in Fig. 3b, there are obvious charge transfers at the heterogeneous interface between the Ni_0.2Mo_0.8N or Ni₃N and doped carbon nanoarrays. The charge accumulation on Ni_0.2Mo_0.8N and Ni₃N indicates that Ni_0.2Mo_0.8N and Ni₃N have acceptor properties, while F,N-doped carbon and N-doped carbon have donor properties. It is noteworthy that the charge distributions for the atoms in the top layer of Ni_0.2Mo_0.8N/F,N–C@NF and Ni₃N/F,N–C@NF change more significantly compared to Ni_0.2Mo_0.8N/N–C@NF and Ni₃N/N–C@NF. This demonstrates that the introduction of F atoms further enhances the interactions between the bimetal nitride and carbon nanoarrays. Subsequently, the projected density of state (PDOS) analysis shows that the Mo-3d orbitals in Ni_0.2Mo_0.8N/F,N–C@NF and Ni_0.2Mo_0.8N/N–C@NF exhibit higher electron density near the Fermi level than Ni-3d orbitals in Ni₃N/F,N–C@NF and Ni₃N/N–C@NF. The corresponding d-band center (ε_d) values of Mo atoms of Ni_0.2Mo_0.8N/F,N–C@NF, Ni_0.2Mo_0.8N/N–C@NF, and Ni atoms of Ni₃N/F,N–C@NF and Ni₃N/N–C@NF are −0.71, −0.79, −1.28 and −1.72 eV, respectively (Fig. 3c and S8†). In particular, the ε_d value of Ni_0.2Mo_0.8N/F,N–C@NF is located closer to the Fermi level, which may make it more activated and favored for the adsorption of reactive intermediates.³⁵

Fig. 3 (a) Structural models and (b) charge density difference plots of Ni_0.2Mo_0.8N/F,N–C@NF, Ni_0.2Mo_0.8N/N–C@NF, Ni₃N/F,N–C@NF and Ni₃N/N–C@NF. (c) PDOS of Ni_0.2Mo_0.8N/F,N–C@NF and Ni_0.2Mo_0.8N/N–C@NF. (d) Adsorption energies of *OH₂ on the surface of Ni_0.2Mo_0.8N/F,N–C@NF, Ni_0.2Mo_0.8N/N–C@NF, Ni₃N/F,N–C@NF and Ni₃N/N–C@NF. (e) The COHP and the corresponding ICOHP values between the O atom of *OH₂ and the active sites of Ni_0.2Mo_0.8N/F,N–C@NF, Ni_0.2Mo_0.8N/N–C@NF, Ni₃N/F,N–C@NF and Ni₃N/N–C@NF. The dotted lines indicate the Fermi level. (f) Calculated H* binding energies (ΔG_H*) for Ni_0.2Mo_0.8N/F,N–C@NF, Ni_0.2Mo_0.8N/N–C@NF, Ni₃N/F,N–C@NF and Ni₃N/N–C@NF.

To further understand the mechanism of HER electrocatalytic processes and gain insights into the excellent HER activity of the Ni_0.2Mo_0.8N/F,N–C@NF heterogeneous catalyst, the adsorption of water molecules on catalysts was further evaluated (Fig. S9†). Because the Volmer step (H₂O + e⁻ → H_ad + OH⁻) for water dissociation limits the activity of alkaline HER, the effective adsorption and activation of water molecules on the catalyst surface are crucial for improving alkaline HER activity.¹⁹Fig. 3d shows that the adsorption energy of the Ni_0.2Mo_0.8N/F,N–C@NF for water molecules is −1.43 eV, which is more negative compared to that of Ni_0.2Mo_0.8N/N–C@NF (−1.19 eV), Ni₃N/F,N–C@NF (−0.86 eV) and Ni₃N/N–C@NF (−0.70 eV). Then, the bonding-antibonding strengths between Mo sites of Ni_0.2Mo_0.8N/F,N–C@NF, Ni_0.2Mo_0.8N/N–C@NF, and Ni sites of Ni₃N/F,N–C@NF and Ni₃N/N–C@NF sites and the O atom of the *OH₂ were quantitatively examined using the COHP method.^40,41 As shown in Fig. 3e, the value of ICOHP (−0.81 eV) of Ni_0.2Mo_0.8N/F,N–C@NF interacting with *OH₂ is smaller than that of Ni_0.2Mo_0.8N/N–C@NF (−0.66 eV), Ni₃N/F,N–C@NF (−0.49 eV) and Ni₃N/N–C@NF (−0.32 eV), which implies that Ni_0.2Mo_0.8N/F,N–C@NF has a stronger bonding strength with *OH₂.⁴³ This further indicates that the synergy of heterogeneous engineering can promote the adsorption and activation of water molecules, thereby enhancing the alkaline HER activity. The ΔG_H* is an important parameter for evaluating HER activity. The ΔG_H* of Ni_0.2Mo_0.8N/F,N–C@NF is much closer to 0 eV compared to the Ni_0.2Mo_0.8N/N–C@NF, Ni₃N/F,N–C@NF and Ni₃N/N–C@NF (Fig. 3f). This result is consistent with the experimental results, suggesting that heterogeneous engineering can lower the thermodynamic barrier of the reaction process and boost catalytic activity.⁴⁴

The electrocatalytic activities of OER for catalysts were also evaluated. Although the OER activity of the Ni_0.2Mo_0.8N/F,N–C@NF catalyst has been improved compared to the reference catalysts, it is still slightly inferior to commercial RuO₂ at low current densities (Fig. S10†). This may be attributed to the weak adsorption of oxygen containing intermediates on the catalyst surface. The sluggish OER activity leads to high energy consumption for electrocatalytic H₂ production.⁴⁵ Therefore, the MOR was used to replace the traditional OER. The electrocatalytic MOR performances of the catalysts were investigated in 1.0 M KOH with a 0.5 M methanol solution. As illustrated in Fig. 4a, the Ni_0.2Mo_0.8N/F,N–C@NF displays an impressive MOR catalytic performance among all the catalysts. To drive the current densities of 100, 500 and 1000 mA cm⁻², the required potentials for Ni_0.2Mo_0.8N/F,N–C@NF are 1387, 1430 and 1483 mV, respectively, which are significantly lower than those of Ni₃N/N–C@NF and commercial RuO₂@NF (Fig. 4b). The excellent reaction kinetics of Ni_0.2Mo_0.8N/F,N–C@NF for MOR was also verified by a lower Tafel slope of 17.7 mV dec⁻¹ than those of Ni₃N/N–C@NF (22.6 mV dec⁻¹) and commercial RuO₂@NF (66.3 mV dec⁻¹) (Fig. 4c). In addition to a low Tafel slope, the Ni_0.2Mo_0.8N/F,N–C@NF also presents a smaller charge transfer impedance confirmed by EIS (Fig. 4d). The formate produced from the MOR was identified by ¹H nuclear magnetic resonance (NMR) (Fig. 4e). After 1000 CV cycles, there is no obvious change in the LSV curve, indicating the good stability of catalytic performance (Fig. 4f).

Fig. 4 (a) LSV curves of catalysts and commercial RuO₂ in 1.0 M KOH with 0.5 M methanol solution with a scan rate of 5 mV s⁻¹. (b) Comparison of the potentials at 100, 500 and 1000 mA cm⁻²; (c) the corresponding Tafel slopes. (d) EIS Nyquist plots of the catalysts and commercial RuO₂@NF. (e) ¹H NMR spectra of Ni_0.2Mo_0.8N/F,N–C@NF before and after MOR. (f) LSV curves of Ni_0.2Mo_0.8N/F,N–C@NF before and after 1000 CV cycles in 1.0 M KOH with 0.5 M methanol solution.

Compared with OER, the MOR activity of the Ni_0.2Mo_0.8N/F,N–C@NF was significantly improved (Fig. 5a). This is mainly attributed to the low thermodynamic equilibrium potential and favourable reaction kinetics of MOR. In particular, the Tafel slope of MOR and the potentials required to drive different current densities (100, 500 and 1000 mA cm⁻²) for Ni_0.2Mo_0.8N/F,N–C@NF are significantly lower than those of OER (Fig. 5b). To further investigate the in-depth reason for the superior MOR activity of Ni_0.2Mo_0.8N/F,N–C@NF, the open circuit potential (OCP) was measured to evaluate the methanol adsorption behavior on the surface of Ni_0.2Mo_0.8N/F,N–C@NF. The change in the adsorbed species in the Helmholtz layer affects the OCP. Compared to the Ni₃N/N–C@NF, the significant decrease in OCP for Ni_0.2Mo_0.8N/F,N–C@NF was observed, suggesting a stronger surface adsorption ability of methanol on the surface of Ni_0.2Mo_0.8N/F,N–C@NF (Fig. S11†).⁴⁶ The in situ potential-dependent Nyquist and Bode plots were used to confirm the electron transfer during MOR and OER. With the varying positive-going potentials, the disappearance of the phase angle in the high-frequency region indicates the surface oxidation of the catalyst, where low conductivity Ni(OH)₂ is converted to high conductivity NiOOH (Fig. 5c).^47–49 However, the phase angle in the low-frequency region sharply decreases at a potential higher than 1.4 V, indicating that the occurrence of OER and the charge transfer resistance gradually decrease, which is consistent with the potential-dependent Nyquist plots (Fig. S12†). For UOR on Ni_0.2Mo_0.8N/F,N–C@NF, the fast potential-dependent decrease in the θ in the low-frequency region and semicircle in Nyquist plots validates that the electron transfer between the active site and the adsorbed methanol molecules is nearly barrier-free, indicating that MOR is a favour kinetics process (Fig. 5d and S13†).⁵⁰

Fig. 5 (a) LSV curves and (b) the potentials at 100, 500 and 1000 mA cm⁻² and Tafel slope for MOR and OER. The potential-dependent Bode plots of Ni_0.2Mo_0.8N/F,N–C@NF for (c) OER and (d) MOR. (e) LSV curves and (f) the cell voltages of Ni_0.2Mo_0.8N/F,N–C@NF at different current densities for electrocatalytic HER integrated with MOR and OWS. (g) Schematic illustration of a solar-driven electrolysis cell of HER integrated with MOR in one electrolysis system. (h) Current–voltage curves of the photovoltaic cell and integrated electrolysis cell. (i) Current–time curve of the solar-driven electrocatalytic HER integrated with the MOR.

Encouraged by the excellent electrocatalytic performance, the Ni_0.2Mo_0.8N/F,N–C@NF was used as both anode and cathode for electrocatalytic OWS and HER integrated with MOR. Due to the more favourable kinetics process of MOR compared to OER, the activity of electrocatalytic HER integrated with MOR was superior to that of the electrocatalytic OWS (Fig. 5e). To drive the current densities of 50, 100, 150, 200 and 250 mA cm⁻², electrocatalytic HER integrated with MOR requires only the low potentials of 1.57, 1.77, 1.96, 2.13 and 2.31 V, respectively, outperforming electrocatalytic OWS (Fig. 5f). This fully demonstrates that the combination of HER with MOR can not only maximize the utilization of input electricity but also obtain value-added products at both the cathode and anode. Furthermore, the Ni_0.2Mo_0.8N/F,N–C@NF catalyst displays good durability under the current density at 100 mA cm⁻² (Fig. S14†). The reused Ni_0.2Mo_0.8N/F,N–C@NF catalyst was able to maintain the initial morphology and phase structure (Fig. S15†). Further assembling the photovoltaic cell with an electrolysis cell is undoubtedly a sustainable approach to converting solar energy into chemical energy (Fig. 5g).^51–53 According to the intersection point of the current–voltage curves of the photovoltaic cell and integrated electrolysis cell, the operating voltage and current are 2.0 V and 161.2 mA, respectively (Fig. 5h). The electrolysis cell exhibits rapid and uniform current responses upon the cycling light on and light off. Furthermore, the current of the solar-driven electrolysis cell has no obvious attenuation under long-term light illumination operation, which indicates that the device has good stability (Fig. 5i).

Conclusions

In summary, we prepared Ni_0.2Mo_0.8N/F,N–C@NF heterogeneous catalyst for energy-saving H₂ production assisted with a methanol oxidation reaction. The interfacial synergistic effect between bimetal nitride and F,N-doped carbon layer can promote the dissociation of water and accelerate the reaction kinetics, thus improving the alkaline HER activity. Consequently, the optimized Ni_0.2Mo_0.8N/F,N–C@NF requires overpotentials of 83 and 281 mV to yield current densities of 100 and 1000 mA cm⁻² for alkaline HER, respectively. The constructed two-electrode electrolyzer using Ni_0.2Mo_0.8N/F,N–C@NF as anode for MOR and cathode for HER delivers a current density of 100 mA cm⁻² at an especially low potential of 1.77 V. This work possibly provides feasible guidance for the design and preparation of the state-of-the-art alkaline HER catalyst through heterogeneous engineering.

Data availability

All relevant data are within the manuscript and its ESI.†

Conflicts of interest

The authors of this manuscript have no conflicts of interest.

Acknowledgements

We thank the financial supports from the National Natural Science Foundation of China (22072070) and the Natural Science Foundation of Shandong Province (ZR2020MB112, ZR2024MB078 and ZR2024QB034).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: SEM images; XRD patterns; XPS spectra; CV curves; Nyquist plots; density of states and so on. See DOI: https://doi.org/10.1039/d4ta07348g

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