Small-molecule electrooxidation catalysts for high energy-saving level hybrid water electrolysis

Unlike conventional water electrolysis (WE), hybrid water electrolysis (HWE) enables more energy-saving hydrogen production by replacing anodic oxygen evolution with the thermodynamically favorable oxidation of small molecules. Recent efforts have reduced the operating voltage of HWE by exploring substrates with low oxidation potentials and developing efficient electrocatalysts. Nevertheless, limited attention has been paid to actual energy savings and techno-economic viability. Specifically, the types and classification of high-energy-saving hybrid electrolyzers remain unclear, and recent advances in tailored catalysts have been insufficiently addressed. This review first introduces energy consumption and the levelized cost of hydrogen (LCOH) as primary evaluation criteria for HWE technologies. Based on these metrics, existing HWE systems are classified to identify pathways with high energy savings, and their hydrogen production costs are assessed through techno-economic analysis. These routes are further divided into two categories. The first includes potential reduction reaction systems, exemplified by hydrazine and sulfur oxidation at the anode. The second covers value-added reaction systems, such as formaldehyde and furfural oxidation at the anode. Subsequently, recent advances in electrocatalysts for both categories are systematically reviewed, with an emphasis on anodic product selectivity, faradaic efficiency (FE), and the resulting energy savings and techno-economic performance. The underlying catalytic mechanisms are analyzed in depth, and representative case studies are discussed to illustrate the advantages and limitations of different catalyst systems. Finally, key challenges and future directions are highlighted to guide research toward industrially viable, energy-efficient, and economically competitive HWE technologies.