Photocatalytic CO2 reduction into value-added fuels has garnered considerable attention as a strategy to mitigate global warming and fossil fuel depletion. However, under practical aerobic conditions, photocatalytic activity often declines dramatically due to undesirable O2-photoreduction. Here, we show that deep eutectic solvents (DESs) can provide a protective reaction field against O2 while maintaining robust CO2 reduction performance using a Ru(II)-complex/Ag/polymeric carbon nitride (PCN) ternary hybrid photocatalyst. The turnover number of formic acid reached 1300 with 96% selectivity, and the apparent quantum yield was 2.7% in ethaline, composed of choline chloride and ethylene glycol, under pure CO2 conditions. Notably, ethaline retained 84% of its formic acid productivity under aerobic conditions with high selectivity, whereas the same catalyst showed only 63%, 42%, 28%, and 4% productivity in DMSO, DMA, MeOH, and MeCN, respectively. The protective nature of ethaline against O2 was also found in another hybrid photocatalyst consisting of a binuclear Ru(II) complex and Ag/PCN. This superior protective reaction field against O2 stems primarily from the low oxygen solubility and the low oxygen diffusion coefficient of ethaline. At the same time, its high CO2 solubility, biodegradability, and nonvolatility make it a promising solvent for CO2 reduction in O2-containing environments─an important step toward practical photocatalytic applications.
Onodera et al. (2026) studied this question.