Decoupled water splitting for hydrogen generation

Green hydrogen (H₂) has the potential to displace fossil fuels because it has a high energy density, and its consumption yields solely water (H₂O). Water electrolysis generates green H₂, but it is expensive to operate. In conventional water splitting (CWS), hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are closely correlated, producing H₂ and oxygen (O₂) concurrently. Decoupled water splitting (DWS) may geographically and temporally isolate HER from OER. The costly membrane may no longer be necessary. Since 2013, utilizing an auxiliary electrode or redox mediator to separate the rate, time frame, or location of H₂ and O₂ generation during water splitting has been extensively studied. This review provides a comprehensive overview of the latest developments in decoupled water splitting, which offers various benefits over conventional water splitting, including improved safety, higher purity, reduced membrane degradation, and greater design flexibility. The development history, electrolyzer configuration design, improved decoupled water splitting techniques, as well as challenges and future perspectives, are extensively reviewed. We also discuss H₂ generation mechanisms of different DWS technologies, i.e., electron-coupled proton buffers, proton-independent electron reservoirs, electrochemical-thermally activated chemical (E-TAC) water splitting, pseudocapacitors by using soluble molecular redox mediators, insoluble solid-state redox mediators, and hybrid water splitting by integrating oxidative biomass valorization in acidic, alkaline, and neutral environments. Overall, DWS offers promising applications for long-term, renewable energy-driven, viable hydrogen generation from water. Future hydrogen generation from renewable sources may be improved with the help of DWS due to its greater versatility and consistency.