Solvent-controlled O₂diffusion enables air-tolerant solar hydrogen generation

Solar water splitting into H2 and O2 is a promising approach to provide renewable fuels. However, the presence of O2 hampers H2 generation and most photocatalysts show a major drop in activity in air without synthetic modification. Here, we demonstrate efficient H2 evolution in air, simply enabled by controlling O2 diffusion in the solvent. We show that in deep eutectic solvents (DESs), photocatalysts retain up to 97% of their H2 evolution activity and quantum efficiency under aerobic conditions whereas in water, the same catalysts are almost entirely quenched. Solvent-induced O2 tolerance is achieved by H2 generation outcompeting O2-induced quenching due to low O2 diffusivities in DESs combined with low O2 solubilities. Using this mechanism, we derive design rules and demonstrate that applying these rules to H2 generation in water can enhance O2 tolerance to >34%. The simplicity and generality of this approach paves the way for enhancing water splitting without adding complexity.