Synergistic effects boosting hydrogen evolution performance of transition metal oxides at ultralow Ru loading levels

Abstract

In this study, ultralow ruthenium nanoparticles on the nickel molybdate nanorods grown on nickel foam (Ru–NiMoO₄–NF) were synthesized. The Ru–NiMoO₄–NF exhibited outstanding hydrogen evolution reaction performances in alkaline with overpotential of 52 mV at the current density of 10 mA cm⁻². And, it maintains excellent stability for 20 h at the current density of 20 mA cm⁻². The mass activity of Ru–NiMoO₄–NF is 0.21 A mg_Ru⁻¹, which is higher than that of Pt/C. Lots of exposed heterojunction interfaces and synergistic effects between Ru nanoparticles and NiMoO₄ nanorods were regarded as the reasons for excellent performance. This work provides an innovative route for developing low-cost catalysts based on the transition metal oxides and trace precious metal with unique heterostructures for hydrogen production through water splitting.

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Introduction

Hydrogen (H₂) is one of the most promising clean energy due to its high energy density and zero carbon dioxide (CO₂) emissions after combustion.^1,2 Nowadays, hydrogen production via electrochemical water splitting can effectively minimize environmental pollution and energy consumption, which is widely recognized as a promising carbon-neutral technology.^3–6 The water electrolysis consists of hydrogen evolution reactions (HER) and oxygen evolution reactions (OER), which can operate in either alkaline or acid conditions.^7–9 While, industry-compatible large-scale hydrogen production is based on alkaline water electrolysis.^10–13 However, the alkaline HER is more sluggish than that in acidic media, which requires considerable energy to break the HO–H bond to produce protons.^14–17 Therefore, developing efficient electrocatalysts to enhance water dissociation and hydrogen desorption is highly desirable.^18,19

Noble metal platinum-based catalysts are regarded as the state-of-the-art electrocatalysts for HER, ascribing to its optimal hydrogen adsorption free energy.^20,21 But, the low reserves, high cost, and inferior activity for water dissociation limited their application in alkaline conditions.²² As cheaper alternatives to platinum, ruthenium-based electrocatalysts with similar metal–hydrogen binding energy and relatively higher reserves have attracted considerable research interest.^14,23,24 The ultrafine Ru nanoclusters anchored on the N-doped nanoporous carbon afford unprecedented performance for HER.²⁵ To study the sized-dependent activity of Ru nanocrystals, a series of Ru nanoparticles and clusters were synthesized by He's group.¹⁴ They discovered that the subnanometric Ru clusters with unique physicochemical features displayed excellent HER electrocatalytic performance. However, the pure ruthenium catalyst delivered inferior hydrogen evolution in alkaline environment because of the poor water decomposition ability and strong hydrogen adsorption. Therefore, exploring Ru nanomaterials with other strong water dissociation ability components are desired to achieve the perfect hydrogen evolution activity. The latest research shows that non-noble transition metal oxides (TMOs) are abundant reserves and effective for the HO–H bond rupture in water.^26–30 Hence, combining with the TMOs and Ru-based materials should be the active electrocatalysts for HER in alkaline. For instance, the Ru–MoO₂ nanocomposites reported by Q. Chen et al. exhibited Pt-like electrocatalytic behavior with very low overpotential of 29 mV at 10 mA cm⁻² in 1 M KOH.³¹ Shi Hu and coworkers synthesized densely packed NiO@Ru nanosheets, which exhibited intimate interface contact of NiO and Ru, super-hydrophilic surface structure, and low charge transfer resistance for excellent HER performance.³² Besides, it was also shown that integration of NiFe LDH with sub-nanoscale Ru species can dramatically optimize the adsorption energy of H* and improve the HER kinetics.³³ Considering these above views, the appropriate design of electrocatalysts with synergistic effects would achieve superb HER performance in alkaline.

A particular challenge is how to construct TMOs and Ru-based materials in intimate contact and expose large areas. Generally speaking, α-NiMoO₄ has high stability, poor conductivity, and low catalytic activity, while β-NiMoO₄ is metastable but has high catalytic activity. How to improve the activity of α-NiMoO₄ catalyst is a great challenge at present.^28,30,34 In this work, ultralow Ru was introduced into α-NiMoO₄-NF nanorods, and the catalytic activity was improved through the synergistic effect of two components and the improvement of the conductivity. Meantime, the nanorods array structure could promote the directional transfer of electron along the array direction. Ru nanoparticles were grown on the α-NiMoO₄-NF nanorods by ion-exchange strategy (shorthand for Ru–NiMoO₄–NF). The unique heterostructures between Ru nanoparticles and α-NiMoO₄ nanorods resulted in remarkable HER catalytic activity in alkaline solutions. Notably, the overpotential of Ru–NiMoO₄–NF is only 52 mV@10 mA cm⁻² for alkaline HER, with Tafel slope of 45 mV dec⁻¹ and mass activity of 0.21 A mg_Ru⁻¹. Thus, this work establishes an innovative strategy to boost HER activity with ultralow Ru loading in TMOs.

Results and discussion

The Ru–NiMoO₄–NF catalyst was synthesized via two-step solvothermal approach (details in the Experimental section in ESI†). Briefly, nickel molybdate hydrate nanorods precursor supported on nickel foam (abbreviation for NiMoO₄–NF) was firstly grown on the nickel foam via hydrothermal method.³⁴ The synthesized solid nickel molybdate hydrate nanorod arrays were immersed in RuCl₃ ethanol solution at 100 °C for 15 h to obtain Ru–NiMoO₄–NF nanorods through an ion-exchange strategy. The scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to deeply investigate the morphologies and microstructures of these catalysts. As illustrated in ESI Fig. S1,† the pure nickel form with a three-dimensional (3D) skeleton structure could expose more surface area and increase the contact interface between the electrolyte and electrode. After the hydrothermal reaction in Ni(NO₃)₂ and NaMoO₂ aqueous solution, the NiMoO₄ nanorods with smooth surfaces were vertically grown on the nickel foam as shown in Fig. 1a, b, and S2.† After the introduction of Ru, the nanorod arrays kept unchanged, but the surface became rough, with uniform decoration of Ru nanoparticles (Fig. 1c and d). As displayed in Fig. 2a–h, high-resolution TEM images revealed that the nanoparticles anchored on the nanorod were the Ru nanoparticles. The magnified images of Ru–NiMoO₄ exhibit clear lattice fringes with an interplanar spacing of 0.873 nm in nanorod, which was assigned to the (001) plane of NiMoO₄. As shown in Fig. 2b and c, the intimate contact between NiMoO₄ nanorod and Ru nanoparticles could efficiently accelerate electron transfer by the interface mutual interaction during the electrocatalytic process. In addition, elemental mappings based on the HADDF-STEM images (Fig. 2d–h) demonstrated the elements of Mo, O, and Ni distribute uniformly, while the element of Ru was mainly located in the part of nanoparticles. Concomitantly, as shown in Fig. S3,† the energy-dispersive spectrometer (EDS) illustrated the element content of Ru–NiMoO₄. The weight ratio of O, Ni, Mo, and Ru was 18.83%, 34.75%, 44.06%, and 2.36%, respectively. Furthermore, the element content of Ru–NiMoO₄–NF was collected by inductively coupled plasma-optical emission spectrometer (ICP-OES). The results indicated that the mass content of Ru is about 2.1 wt%, confirming the successful doping of Ru. N₂ adsorption–desorption isotherms of Ru–NiMoO₄–NF and NiMoO₄–NF (Fig S4†) were tested. The BET surface area of Ru–NiMoO₄–NF and NiMoO₄–NF are 5.03 m² g⁻¹ and 3.43 m² g⁻¹, indicating the increase of specific surface area after the introduction of Ru.

Fig. 1 SEM images of (a, b) NiMoO₄–NF and (c, d) of Ru–NiMoO₄–NF.

Fig. 2 (a and b) TEM; (c) HRTEM; (d–h) HAADF and elemental mapping images of Ru–NiMoO₄–NF.

X-ray powder diffraction (XRD) and X-ray photoelectron spectra (XPS) of the prepared samples were obtained to analyze the crystal structure and surface electronic structures. Fig. 3 illustrated the XRD patterns of NiMoO₄ and Ru–NiMoO₄ catalysts scraped from the Nickel Foam surface. The XRD patterns indicated that the catalysts of α-NiMoO₄ (PDF No. 97-024-7435) were successfully synthesized on Nickel Foam. However, with the introduction of Ru, XRD diffraction peaks of the catalysts did not change, indicating that no crystalized Ru metals and oxides were formed. Ru may exist in an amorphous state. The survey XPS spectrum (Fig. 4a) of Ru–NiMoO₄–NF shows the coexistence of Ru, Ni, Mo, and O elements. As depicted in Fig. 4b–f, the high-resolution XPS spectra of Ru, Ni, Mo, and O elements are provided. The high-resolution Ni 2p spectrum can be deconvoluted into two group peaks. The peaks at around 856.20 eV and 874.33 eV were the characteristic peaks of Ni²⁺.³⁵ And, the peaks at 862.14 eV and 880.85 eV belonged to satellite peaks, which may derive from the surface oxidation of the catalyst.³⁶ The fitted curve of Mo 3d spectrum was deconvoluted into 232.16 eV and 235.28 eV, which corresponded to Mo⁶⁺.^28,34,37 The binding energy of Ru 3p_3/2 and 3p_1/2 were located 463.39 eV and 485.62 eV.^14,23 Meanwhile, the high-resolution XPS spectra of Ru 3d and C 1s, the peaks with the energy of 284.75 eV and 281.18 eV could be assigned to Ru 3d_3/2 and Ru 3d_5/2,^35,38 indicating that Ru have been successfully incorporated into NiMoO₄–NF products. Besides, the binding energies of O 1s spectrum located at 532.47 eV and 530.68 eV, which were assigned to O–H of adsorbed water molecules and lattice oxygen.^28,39 By comparing the XPS of Ru–NiMoO₄–NF and NiMoO₄–NF (Fig. S5†), it is found that the binding energy of Ni, Mo, and O positively shift after the introduction of Ru. The results indicate that electrons are transferred from Ni, Mo, and O to Ru. Therefore, the interaction between Ru and substrate is the strong electron coupling effect.⁴⁰

Fig. 3 XRD patterns of NiMoO₄ and Ru–NiMoO₄.

Fig. 4 XPS spectra of Ru–NiMoO₄–NF (a) full spectrum, (b) Ni 2p, (c) Mo 3d, (d) O 1s, (e) Ru 3d, and (f) Ru 3p.

The HER electrocatalytic performances of Ru–NiMoO₄–NF, NiMoO₄–NF, and NF catalysts were evaluated in 1 M KOH solution under room temperature. As displayed in Fig. 5a, linear sweep voltammetric (LSV) curves of these three electrocatalysts indicated that the nickel foam was inert to HER. While NiMoO₄–NF performed poor HER activity. Remarkably, the Ru–NiMoO₄–NF catalyst exhibited enhanced remarkable HER activity in alkaline conditions after introducing Ru into NiMoO₄–NF. The overpotential of Ru–NiMoO–NF at 10 mA cm⁻² is only 52 mV, which was considerably smaller than those of NiMoO₄–NF (249 mV), and NF (374 mV). Meanwhile, the electrocatalytic performance of Ru + NiMoO₄–NF was much worse than that of Ru–NiMoO₄–NF (Fig. S6†), indicating that forming heterojunctions can improve the catalytic activity, but not the simple drop coating on the surface. The HER kinetic behaviors and rate-determining step of the electrocatalysts were evaluated using Tafel plots.^4,14 The outstanding HER performance of Ru–NiMoO₄–NF was further confirmed by its much smaller Tafel slope (45 mV dec⁻¹) than those of NiMoO₄–NF (104 mV dec⁻¹), and NF (132 mV dec⁻¹) (Fig. 5b). This can be attributed to the favorable HER reaction kinetics on Ru–NiMoO–NF catalyst and Volmer–Heyrovsky mechanism. Meanwhile, the rate-determining step was the Heyrovsky step in the reaction process.

Fig. 5 (a) LSV curves, (b) Tafel slop curves, (c) EIS Nyquist plots, and (d) C_dl of different samples in 1 mol L⁻¹ KOH.

Furthermore, electrochemical impedance spectroscopy (EIS) was performed in order to better probe the charge transfer mechanism towards HER. The Nyquist plots were shown in Fig. 5c, the Ru–NiMoO₄–NF electrode presented a smaller semicircle and a lower charge transfer resistance (R_ct) value (0.42 Ω) than NiMoO₄–NF catalyst (37.78 Ω) in the low-frequency range, implying the faster of charge transfer and favorable reaction rate of HER.

The electrochemically active surface area (ECSA) was a good indicator to in-depth study the origin of the excellent activity, which was positively correlated with the electrochemical double-layer capacitance (C_dl). The C_dl value can be derived from the electrochemical cyclic voltammograms (CVs) measurement in the non-faradaic regions (Fig. S9†).^41,42 As displayed in Fig. 5d, Ru–NiMoO₄–NF had higher C_dl (74.77 mF cm⁻²) than that of NiMoO₄–NF (1.09 mF cm⁻²), implying Ru–NiMoO₄–NF had more active sites than NiMoO₄–NF. The mass activity of Ru–NiMoO₄–NF is 0.23 A mg_Ru⁻¹, which is larger than commercial Pt/C (0.18 A mg_pt⁻¹) at the overpotential of 70 mV.^4,22 In addition, long-time durability is a vital parameter for the catalyst in an electrochemical reaction.^28,43 Accordingly, the chronoamperometric measurement was carried out to further evaluate the electrocatalytic stability of Ru–NiMoO₄–NF and NiMoO₄–NF. As shown in Fig. 6a, the current density of Ru–NiMoO₄–NF was no apparent degradation even after 20 hours test. Simultaneously, the polarization curve did not show significant degradation after 3000 cyclic voltammetry cycles test (Fig. 6b). Furthermore, the morphology of Ru–NiMoO₄–NF after the stability test were well maintained (Fig. S11†). Meanwhile, the high resolution XPS of Ni, Mo, O, and Ru indicates no obvious change after HER tests (Fig. S12†). All these results demonstrate the excellent electrocatalytic stability of Ru–NiMoO₄–NF in alkaline.

Fig. 6 Stability tests of Ru–NiMoO₄–NF and NiMoO₄–NF. (a) Chronoamperometry curves and (b) polarization curves.

Conclusions

In summary, ultralow Ru nanoparticles dispersed on α-NiMoO₄–NF nanorods electrocatalysts were successfully fabricated and exhibited highly efficient HER performance in alkaline media. The obtained Ru–NiMoO₄–NF showed an overpotential of 52 mV at the current density of 10 mA cm⁻² and maintained stability over 20 h. The mass activity of Ru–NiMoO₄–NF is 0.23 A mg_Ru⁻¹, which is larger than commercial Pt/C (0.18 A mg_pt⁻¹). The unique heterostructures combined with Ru and α-NiMoO₄ boost the water dissociation kinetics and hydrogen evolution. This work provides an innovative way to construct transition metal oxide catalysts with trace noble metal content for HER.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (No. 2021L544), the Applied Basic Research Programs of Science and Technology Department of Shanxi Province (No. 20210302124470) and the Fund for Shanxi “1331” Project (Collaborative Innovation Center of CO₂ Conversion and Utilization).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ra01501g

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