Hybrid Water Electrolysis: Enabling Sustainable Hydrogen Production, Value-Added Chemicals, and Environmental Remediation

Abstract The global transition toward carbon neutrality is driven by the pressing need to mitigate fossil fuel depletion and climate instability. While green hydrogen production through water electrolysis is pivotal, its scalability is hindered by slow reaction kinetics, high overpotentials, and reliance on precious-metal catalysts (e.g., Ir, Ru) inherent to the oxygen evolution reaction (OER). Hybrid water electrolysis (HWE) offers a solution to these challenges by substituting the OER with the oxidation of more thermodynamically favorable substrates. This approach not only reduces cell voltage and improves energy efficiency but also enables the concurrent production of high-value chemicals and the remediation of pollutants. This review delves into the fundamental principles, reaction pathways, and catalyst design strategies that underpin HWE systems. It places particular emphasis on a diverse array of anodic substrates, including various bio-derived compounds and pollutants such as urea, hydrazine, ammonia, alcohols, polyols, sugars, sulfur-based compounds, dyes, and plastics, as well as recent advancements in their integration into HWE. Despite significant progress, challenges persist in areas such as reaction kinetics, product separation, catalyst stability, and operational durability. This review identifies these obstacles and underscores future research directions to advance HWE toward scalable and sustainable hydrogen production.