Hydrogen production through water electrolysis is a promising method to utilize renewable energy in the context of urgent need to phase out fossil fuels. Nickel-molybdenum (NiMo) electrodes are among the best performing non-noble metal-based electrodes for hydrogen evolution reaction in alkaline media (alkaline HER). Albeit exhibiting stable performance in electrolysis at a constant power supply (i.e., constant electrolysis), NiMo electrodes suffer from performance degradation in electrolysis at an intermittent power supply (i.e., intermittent electrolysis), which is emblematic of electrolysis powered directly by renewable energy (such as wind and solar power sources). Here we reveal that NiMo electrodes were oxidized by dissolved oxygen during power interruption, leading to vanishing of metallic Ni active sites and loss of conductivity in MoOx substrate. Based on the understanding of the degradation mechanism, chromium (Cr) coating was successfully applied as a protective layer to inhibit oxygen reduction reaction (ORR) and significantly enhance the stability of NiMo electrodes in intermittent electrolysis. Further, combining experimental and Molecular Dynamics (MD) simulations, we demonstrate that the Cr coating served as a physical barrier inhibiting diffusion of oxygen, while still allowing other species to pass through. Our work offers insights into electrode behavior in intermittent electrolysis, as well as provides Cr coating as a valid method and corresponding deep understanding of the factors for stability enhancement, paving the way for the successful application of lab-scale electrodes in industrial electrolysis powered directly by renewable energy.
Keywords: alkaline hydrogen evolution reaction; chromium coating; dissolved oxygen; intermittent electrolysis; nickel−molybdenum; stability.