Insights into alloy/oxide or hydroxide interfaces in Ni-Mo-based electrocatalysts for hydrogen evolution under alkaline conditions

Abstract

Nickel-molybdenum (Ni-Mo) alloys are promising non-noble metal electrocatalysts for the hydrogen evolution reaction (HER) in alkaline water; however, the kinetic origins of their catalytic activities still remain under debate. In this perspective, we systematically summarize the structural characteristics of Ni-Mo-based electrocatalysts recently reported and find that highly active catalysts generally have alloy-oxide or alloy-hydroxide interface structures. Based on the two-step reaction mechanism under alkaline conditions, water dissociation to form adsorbed hydrogen and combination of adsorbed hydrogen into molecular hydrogen, we discuss in detail the relationship between the two types of interface structures obtained by different synthesis methods and their HER performance in Ni-Mo based catalysts. For the alloy-oxide interfaces, the Ni₄Mo/MoO _x composites produced by electrodeposition or hydrothermal combined with thermal reduction exhibit activities close to that of platinum. For only the alloy or oxide, their activities are significantly lower than that of composite structures, indicating the synergistic catalytic effect of binary components. For the alloy-hydroxide interfaces, the activity of the Ni _x Mo _y alloy with different Ni/Mo ratios is greatly improved by constructing heterostructures with hydroxides such as Ni(OH)₂ or Co(OH)₂. In particular, pure alloys obtained by metallurgy must be activated to produce a layer of mixed Ni(OH)₂ and MoO _x on the surface to achieve high activity. Therefore, the activity of Ni-Mo catalysts probably originates from the interfaces of alloy-oxide or alloy-hydroxide, in which the oxide or hydroxide promotes water dissociation and the alloy accelerates hydrogen combination. These new understandings will provide valuable guidance for the further exploration of advanced HER electrocatalysts.

Fig. 1. (A) The reaction mechanism for the HER in alkaline electrolytes and two pathways are presented in the forms of the Volmer–Heyrovsky mechanism and the Volmer–Tafel mechanism. (B) Schematic diagram of synergetic catalysis of alloy–oxide or alloy–hydroxide interface structures, in which the oxide or hydroxide promotes water dissociation and the alloy accelerates hydrogen combination.

Fig. 2. (A) Schematic of the Ni₄Mo/MoO_x synthesis route on Cu foam. (i) Induced codeposition; (ii) formation of nanointerfaces with the dispersion of the alloy nanoparticles. (B and C) TEM images, (D) HRTEM image, and (E) SAED pattern of Ni₄Mo nanoparticles on amorphous MoO_x nanosheets. (F) Polarization curves of Pt/C, bare Cu foam, Ni, Ni₄Mo alloy, and Ni₄Mo/MoO_x. (G) Electronic properties of interface model systems. Brown balls = Cu; cyan balls = Mo; blue balls = Ni; and red balls = O. (H) Free energy pathway, chemisorption, and formation energy for the HER. Reproduced with permission. Copyright 2019, Wiley-VCH.

Fig. 3. (A) Schematic illustration of the formation of MoNi₄/MoO_3−x nanorod arrays on Ni foam. (B) HRTEM image for MoNi₄/MoO_3−x (inset: SAED pattern). (C) Raman spectra of NiMoO₄ (black) and MoNi₄/MoO_3−x (green). (D) Mo 3d XPS spectra of NiMoO₄ (black) and MoNi₄/MoO_3−x (green). (E) Polarization curves for Pt/C, Ni foam, NiMoO₄, and MoNi₄/MoO_3−x. (F) Polarization curves of MoNi₄/MoO_3−x for the HER before and after electrochemical oxidation. (G) Polarization curves of MoNi₄/MoO_3−x and NiMoO₄ annealed in an Ar atmosphere (sample-Ar). Scan rate: 2 mV s⁻¹. Reproduced with permission. Copyright 2017, Wiley-VCH.

Fig. 4. (A–C) Typical SEM images of MoNi₄/MoO₂@Ni. Scale bars, (A) 20 μm; (B) 1 μm; (C) 100 nm. (D–F) HRTEM images of MoNi₄/MoO₂@Ni. The inset image in (D) is the related selected-area electron diffraction pattern of the MoNi₄ electrocatalyst and the MoO₂ cuboids. Scale bars, (D–F) 2 nm; inset in (D), 1 1/nm. (G) Corresponding elemental mapping images of the MoNi₄ electrocatalyst and the MoO₂ cuboids. Scale bars: 20 nm. (H) Polarization curves of the MoNi₄ electrocatalyst supported by the MoO₂ cuboids, pure Ni nanosheets, and MoO₂ cuboids on nickel foam. (I) Polarization curves of the MoO₂ nanosheets and the MoNi₄ electrocatalyst supported by the MoO₂ cuboids on carbon cloth in different electrolytes. Reproduced with permission. Copyright 2017, Springer Nature.

Fig. 5. SEM images of the (A) as-prepared Ni₄Mo nanorod arrays and (B) Ni₄Mo nanorod arrays after the HER test for 12 h. (C) Polarization curves of the Ni₄Mo alloy and Pt/C. (D) Time-dependent concentration of dissolved Mo and Ni in the electrolyte of the Ni₄Mo alloy. (E) Mo 3d and (F) Ni 2p XPS spectra of Ni₄Mo before and after the HER test. (G) Mo K-edge and (H) Ni K-edge XANES spectra of Ni₄Mo before and after the HER test. (I) Potential-dependent in situ Raman spectra of Ni₄Mo during the alkaline HER process. (J) Free energy diagrams of the HER on bare Ni(111), Ni(111) + MoO₄, and Ni(111) + Mo₂O₇. Reproduced with permission. Copyright 2021, Wiley-VCH.

Fig. 6. (A) Schematic illustration of the preparation of the Mo_0.84Ni_0.16@Ni(OH)₂ heterostructure. (B) HRTEM image of Mo_0.84Ni_0.16@Ni(OH)₂. (C) The high-resolution XPS spectra of O 1s for NiMoO₄–Mo_0.84Ni_0.16 composites and the Mo_0.84Ni_0.16@Ni(OH)₂ heterostructure. (D and E) Polarization curves and the corresponding Tafel plots of NiMoO₄–Mo_0.84Ni_0.16, and Mo_0.84Ni_0.16@Ni(OH)₂ with different deposition times of 5, 10, and 15 min, and Pt/C. (F) The chronoamperometric curve at an overpotential of 34 mV for Mo_0.84Ni_0.16@Ni(OH)₂. The inset is the SEM image after the stability test. Reproduced with permission. Copyright 2020, Royal Society of Chemistry. (G and H) SAED pattern and HRTEM image of P–Mo–Ni(OH)₂ NSAs. (I) Polarization curves of Ni foam, P–Ni foam, Ni(OH)₂ NSAs, Mo–Ni(OH)₂ NSAs, P–Ni(OH)₂ NSAs, P–Mo–Ni(OH)₂ NSAs, and Pt/C. Reproduced with permission. Copyright 2020, Elsevier.

Fig. 7. (A) TEM image of the h-NiMoFe catalyst. (B and C) HRTEM images of h-NiMoFe. (B) An enlarged view of h-NiMoFe, lattice fringes of a MoO₂ nanosheet (blue box) and Ni₄Mo nanoparticles (red box), scale bars in the insets are 5 1/nm; (C) side view of the MoO₂ nanosheet. (D) Polarization curves of h-NiMoFe and its control samples. (E) Relative percentages of surface Ni species from the Ni 3s XPS spectra on the Ni, NiMo, and h-NiMoFe samples before and after the HER test. (F) FT-EXAFS of the h-NiMoFe catalyst and control samples at Fe K-edges. (G) Calculated relaxed configuration of an Fe–(OH)₄–Ni₄ motif on a Ni₄Mo (002) slab and the corresponding charge density difference in this configuration. (H and I) Adsorption energies for H and dissociated H₂O on h-NiMoFe and control samples. Reproduced with permission. Copyright 2021, Royal Society of Chemistry.

Fig. 8. (A) Synthetic illustration of the fabrication process of Co(OH)₂/NiMo CA@CC. (B) HRTEM image of NiMo alloy particles from Co(OH)₂/NiMo CA@CC. (C–E) XPS spectra of Ni 2p, Mo 3d, and Co 2p of Co(OH)₂/NiMo CA@CC, Co(OH)₂/NiMo@CC, NiMo@CC, and Co(OH)₂@CC. (F) Polarization curves of bare CC, Co(OH)₂/NiMo CA@CC, Co(OH)₂/NiMo@CC, NiMo@CC, Co(OH)₂@CC, and Pt/C@CC toward the HER in 1.0 M KOH. (G) The free energy diagram for the HER on the surface of Ni₅₈Mo₆ and Co(OH)₂, and the interface of Co(OH)₂/Ni₄₄Mo₄. (H) The projected density-of-states of d orbitals of Ni₅₈Mo₆, Co(OH)₂, and Co(OH)₂/Ni₄₄Mo₄ with aligned Fermi levels. Reproduced with permission. Copyright 2021, Wiley-VCH.

Fig. 9. (A) Schematic diagram of the fabrication process for a nanosponge-like NiMo solid solution prepared by high-temperature sintering. (B and C) HRTEM images of Ni_0.33Mo_0.67-900 before and after the HER test. (D–F) Ni 2p, Mo 3d, and O 1s XPS spectra of Ni_0.33Mo_0.67-900 before and after aging. (G) Polarization curves of pure Ni-1000, pure Mo-1000, Ni_0.94Mo_0.06-1000, Ni_0.5Mo_0.5-1000, Ni_0.33Mo_0.67-900, and Pt/C electrodes in a 1 M KOH solution. (H) Comparison of polarization curves of Ni_0.33Mo_0.67-900 obtained under different conditions. (I) Bath voltage variations at a current density of 2 A cm⁻² in 1 M KOH solution. Reproduced with permission. Copyright 2020, American Chemical Society.