Photoelectrochemical hybrid cell for unbiased CO2 reduction coupled to alcohol oxidation

The reduction of CO2 to renewable fuels must be coupled to a sustainable oxidation process to devise a viable solar fuel-producing device. In photoelectrochemical cells, water oxidation to O2 is the predominant oxidation reaction and typically requires a pair of light absorbers or an applied bias voltage when coupled to CO2 reduction. Here, we report a bias-free photoelectrochemical device for simultaneous CO2 reduction to formate and alcohol oxidation to aldehyde in aqueous conditions. The photoanode is constructed by co-immobilisation of a diketopyrrolopyrrole-based chromophore and a nitroxyl-based alcohol oxidation catalyst on a mesoporous TiO2 scaffold, providing a precious metal-free dye-sensitised photoanode. The photoanode was wired to a biohybrid cathode consisting of the CO2 reduction enzyme formate dehydrogenase integrated into a mesoporous indium tin oxide electrode. The bias-free cell delivers sustained photocurrents of up to 30 µA cm−2 under visible-light irradiation, resulting in simultaneous aldehyde and formate production. Our results show that single light absorber photoelectrochemical cells can be used for parallel fuel production and chemical synthesis from CO2 and waste streams in the absence of an external bias.