Hybrid water electrolysis: Replacing oxygen evolution reaction for energy-efficient hydrogen production and beyond

Renewable energy-driven hydrogen generation from water electrolysis has been widely recognized as a promising approach to utilize sustainable energy resources, reduce our dependence on legacy fossil fuels and alleviate net carbon dioxide emissions. However, conventional water electrolyzers suffer from the high overpotentials, mainly due to the sluggish kinetics of anodic oxygen evolution reaction (OER). This reaction also generates reactive oxygen species that could degrade the proton exchange membrane and oxygen that may mix with the cathodic hydrogen to form explosive gaseous mixtures. To address these issues, an innovative hybrid water electrolysis strategy which involves a certain alternative oxidation reaction to replace OER has been developed, and has led to a burgeoning area that sparks much research interest in finding available alternative reactions and their corresponding electrocatalysts. Herein, we summarize the alternative reactions into three groups: (1) the reagent-sacrificing type that can generate H2 with an ultra-low potential while the substrates are oxidized to valueless products; (2) the pollutant-degrading type at which environmental pollutants are used as substrates; (3) the value-added type that produces valuable products at the anode. Catalyst and electrolyzer designs for hybrid electrolysis are also briefly discussed, with an emphasis on the catalyst reconstruction phenomenon. Finally, the present challenges and perspectives are put forward.