Decoupled Water Electrolysis with an Air-Stable Zwitterionic Azo Mediator

Abstract Decoupled water electrolysis (DWE) separates H2 and O2 evolution, improving safety and flexibility under intermittent renewable electricity, but requires durable, oxygen-tolerant redox mediators. Here we evaluate 4,4′-azo-bis(1-pyridinium-3-propane-sulfonate) (ABPS), a zwitterionic azo molecule, as an electron-coupled proton buffer for acidic DWE. Its azo/hydrazo core supports reversible two-electron/two-proton chemistry at 0.81 V vs RHE, while zwitterionic substituents provide high aqueous solubility and suppress crossover through low-cost sulfonated poly(ether ether ketone) membranes. Reduced H2ABPS shows no detectable oxidation after 90 days in air or during >5 h of continuous pure-O2 bubbling. Symmetric flow-cell cycling over 1000 h yields a capacity-fade rate below 1% year–1. With commercial catalysts at room temperature, decoupled flow cells reach 1.2 A cm–2 and operate for >200 h at 500 mA cm–2 with near-quantitative Faradaic efficiencies while producing high-purity H2 and O2. These results establish zwitterionic azo chemistry as an oxygen-tolerant, durable platform for high-rate decoupled hydrogen production.