Stable Tetrasubstituted Quinone Redox Reservoir for Enhancing Decoupled Hydrogen and Oxygen Evolution

Redox reservoirs (RRs) may be used to decouple the two half-reactions of water electrolysis, enabling spatial and temporal separation of hydrogen and oxygen evolution. Organic RRs are appealing candidates for this application; however, their instability limits their utility. Here, we show that a tetrathioether-substituted quinone, tetramercaptopropanesulfonate quinone (TMQ), exhibits significantly enhanced stability relative to anthraquinone-2,7-disulfonate (AQDS), the most effective organic RR reported previously. The enhanced stability, confirmed by symmetric flow battery experiments under relevant conditions, enables stable electrochemical production of H₂ and O₂ in a continuous flow electrolysis cell. The reduced RR, tetramercaptopropanesulfonate hydroquinone (TMHQ), is not susceptible to decomposition, while the oxidized state, TMQ, undergoes slow decomposition, evident only after sustained operation (>60 h). Analysis of the byproducts provides that basis for a decomposition mechanism, establishing a foundation for the design of new organic RRs with even better performance.