Solvent-controlled O diffusion enables air-tolerant solar hydrogen generation

Solar water splitting into H 2 and O 2 is a promising approach to provide renewable fuels. However, the presence of O 2 hampers H 2 generation and most photocatalysts show a major drop in activity in air without synthetic modification. Here, we demonstrate efficient H 2 evolution in air, simply enabled by controlling O 2 diffusion in the solvent. We show that in deep eutectic solvents (DESs), photocatalysts retain up to 97% of their H 2 evolution activity and quantum efficiency under aerobic conditions whereas in water, the same catalysts are almost entirely quenched. Solvent-induced O 2 tolerance is achieved by H 2 generation outcompeting O 2 -induced quenching due to low O 2 diffusivities in DESs combined with low O 2 solubilities. Using this mechanism, we derive design rules and demonstrate that applying these rules to H 2 generation in water can enhance O 2 tolerance to >34%. The simplicity and generality of this approach paves the way for enhancing water splitting without adding complexity. Oxygen-tolerant photocatalytic H 2 evolution with O 2 -intolerant photocatalysts is achieved by designing solvents with low O 2 diffusivity.