Alkaline Hydrogen Evolution Reaction Electrocatalysts for Anion Exchange Membrane Water Electrolyzers: Progress and Perspective

Abstract

For the aim of achieving the carbon-free energy scenario, green hydrogen (H₂) with non-CO₂ emission and high energy density is regarded as a potential alternative to traditional fossil fuels. Over the last decades, significant breakthroughs have been realized on the alkaline hydrogen evolution reaction (HER), which is a fundamental advancement and efficient process to generate high-purity H₂ in the laboratory. Based on this, the development of the practical industry-oriented anion exchange membrane water electrolyzer (AEMWE) is on the rise, showing competitiveness with the incumbent megawatt-scale H₂ production technologies. Still, great challenges lie in exploring the electrocatalysts with remarkable activity and stability for alkaline HER, as well as bridging the gap of performance difference between the three-electrode cell and AEMWE devices. In this perspective, we systematically discuss the in-depth mechanisms for activating alkaline HER electrocatalysts, including electronic modification, defect construction, morphology control, synergistic function, field effect, etc. In addition, the current status of AEMWE is reviewed, and the underlying bottlenecks that impede the application of HER electrocatalysts in AEMWE are summarized. Finally, we share our thoughts regarding the future development directions of electrocatalysts toward both alkaline HER and AEMWE, in the hope of advancing the commercialization of water electrolysis technology for green H₂ production.

Figure 1

Representative descriptors of HER. (a) The relationship between ΔG_H* and HER current densities on various metals. Reproduced with permission from ref (53). Copyright 2017 AAAS. (b) In situ electrochemical Raman spectra of the O–H stretching mode in 0.1 M NaOH at −0.04 V. These were fitted with three Gaussians (red: tetrahedrally coordinated water; blue: trihedrally coordinated water; green: dangling O–H bonds). (c) The relationship between the area ratio of the three peaks and the overpotentials at −0.04 V. Reproduced with permission from ref (58). Copyright 2020 Wiley-VCH. (d) The d band center vs overpotential on the studied catalysts. Reproduced with permission from ref (64). Copyright 2020 American Chemical Society. (e) Relationship between the coordination numbers and the current densities on Pt materials. Reproduced from ref (69). Available under a CC-BY-NC-ND license. Copyright 2017 The Authors. (f) Correlations between the A-site ionic electronegativity and HER overpotentials on perovskites. Reproduced from ref (72). Available under a CC-BY 4.0 license. Copyright 2019 The Authors. (g) Schematic energy band diagrams of these catalysts. (h) Volcano relationship between the HER activity and the unpaired d-electron number of different catalysts. Reproduced from ref (73). Available under a CC-BY 4.0 license. Copyright 2023.

Figure 2

Activation strategies for HER electrocatalysts. (a) Differential charge density distributions for selected single Ru atom in P,Mo–Ru cluster with a value of 0.002 e Å^–3. Reproduced with permission from ref (77). Copyright 2022 Wiley-VCH. (b) Schematic diagram of microenvironmental changes on Ru–Ni(OH)₂ with low crystallinity. Reproduced with permission from ref (81). Copyright 2024 Wiley-VCH. (c) ΔG value and scheme of alkaline HER on Ru/ac-CeO_2−δ. Reproduced with permission from ref (85). Copyright 2024 Wiley-VCH. (d) HAADF-STEM image of the ultrathin SA In–Pt NWs. Reproduced with permission from ref (90). Copyright 2020 Wiley-VCH. (e) The operando Pt L₃-edge EXAFS spectroscopy on Pt–Ru/RuO₂ during alkaline HER. The (f) H₂O* adsorption energy and (g) H* adsorption free energy values for Pt, Ru and RuO₂ sites in Pt–Ru/RuO₂. Reproduced from ref (97). Available under a CC-BY 4.0 license. Copyright 2024 The Authors. (h) The mechanism of the Pt_SA-NiO/Ni network as an efficient catalyst toward large-scale water electrolysis in alkaline media. Reproduced from ref (100). Available under a CC-BY 4.0 license. Copyright 2021 The Authors.

Figure 3

Components of AEMWE. (a) Schematic illustration of AWE, PEMWE, and AEMWE, and their performance comparison. Reproduced with permission from ref (103). Copyright 2024 Elsevier. (b) The AEMWE performances of Ni₃Fe-LDH//Pt/C in 1 M KOH. Reproduced with permission from ref (104). Copyright 2024 Wiley-VCH. (c) The polarization curves of the electrolyzer using Li₂Mn_0.85Ru_0.15O₃ or NiFe-LDH as the anode and Pt/C as the cathode. Reproduced with permission from ref (105). Copyright 2024 Springer Nature.

Figure 4

HER electrocatalysts in AEMWE. (a) Polarization curve of Ru SAs/WC_x//NiFeOH_x-NF and Pt/C//NiFeOH_x-NF in the AEMWE operated at 80 °C. Reproduced with permission from ref (106). Copyright 2024 American Chemical Society. (b) Cross-sectional FE-SEM image of Co₃S₄ NS/NF. (c) Comparison of the reported single cell AEMWE performance using nonprecious metal catalyst. Reproduced with permission from ref (108). Copyright 2020 Elsevier. (d) The challenges of catalyst stability for high-current-density water electrolysis. Reproduced with permission from ref (109). Copyright 2023 Royal Society of Chemistry. (e) TEM image of the carbon-shell-coated FeP nanoparticles. (f) Plots of overpotential vs potential cycle for the data in panel. Reproduced with permission from ref (111). Copyright 2017 American Chemical Society. (g) Chronopotentiometric curves of the porous Co–P//porous Co–P electrolyzer recorded at a constant current density of 1000 mA cm^–2 with iR-correction. Reproduced with permission from ref (112). Copyright 2020 American Chemical Society. (h) Price activities of NiFe LDH//Pt–Ru/RuO₂ and NiFe LDH//commercial Pt/C at various cell potentials (without iR correction). Reproduced from ref (97). Available under a CC-BY 4.0 license. Copyright 2024 The Authors. (i) Polarization curves for 10000 cycle tests of FeP with the carbon shell obtained by large-scale synthesis. The inset shows a picture of the product and its weight. Reproduced with permission from ref (111). Copyright 2017 American Chemical Society.

Figure 5

Outlook of the HER electrocatalysts for AEMWE: the challenges and perspectives regarding the materials, mechanism, and synthesis for the development of the HER catalysts for practical AEMWE.