Non-noble metal catalysts for small-molecule-assisted hybrid electrolysis to produce green hydrogen from diverse water sources

Hybrid water electrolysis surpasses the efficiency limits of conventional alkaline water splitting by replacing the sluggish oxygen evolution reaction (OER), which requires overpotentials exceeding 350 mV at 500 mA cm −2 , with the oxidation of thermodynamically favorable small molecules such as alcohols, urea, hydrazine, and biomass derivatives. This approach routinely achieves cell voltages below 1.5 V at current densities of ≥1 A cm −2 while simultaneously co-producing valuable chemicals. The present review focuses on state-of-the-art earth-abundant non-noble metal-based electrocatalysts that compete with or exceed precious-metal benchmarks for key anodic reactions in both freshwater and seawater alkaline electrolytes. We systematically evaluate performance metrics including onset potentials, overpotentials at industrially relevant current densities (≥500 mA cm −2 ), Faradaic efficiencies, and long-term stability across major catalyst classes. The discussion highlights scalable synthesis routes, clarifies structure–activity relationships and reaction mechanisms through in-situ/operando spectroscopy, and critically examines chloride-repelling strategies. Many leading catalysts show comparable or even enhanced activity and durability in natural seawater relative to purified electrolytes. These insights establish clear, transferable design principles and engineering guidelines for developing robust, low-cost, seawater-compatible hybrid electrolyzers capable of simultaneously producing green hydrogen and enabling decentralized chemical manufacturing from the most abundant water resource on Earth.