Photoelectrochemical CO 2 Reduction to Formate with the Sacrificial Reagent Free System of Semiconductor Photocatalysts and Formate Dehydrogenase

Efficient sacrificial reagent free (and no external bias) photoelectrochemical CO 2 reduction to formate with a hybrid system consisting of semiconductor‐based photocatalytic TiO 2 nanoparticles, methylviologen (MV) as an electron carrier, and biocatalytic formate dehydrogenase (FDH) was investigated. To develop the sacrificial reagent free photoelectrochemical cell for the CO 2 reduction to formate, the photoanode based on TiO 2 film and cathode based on the carbon fabric paper (CFP) were separated into two compartments using a proton‐permeable Nafion film to prevent gas diffusion. When the anode was irradiated with Xenon lamp (>300 nm), MV reduction and CO 2 reduction to formate with FDH on the cathode side proceeded in this system. For MV reduction with photoelectrochemical cell, MV was reduced using TiO 2 film at each pH (6, 7 and 8) without bias, the reduction of MV did not progressed at pH 6. In contrast, MV reduction was progressed at pH 7 or 8. For CO 2 reduction to formate in the presence FDH on the cathode side with photoelectrochemical cell, the rate of formate production was estimated to be 31.9 nmol h −1 at pH 8 under continuous irradiation without bias. Moreover, the rates of formate production at pH 7 and 8 were estimated to be 56.5 and 168.8 nmol h −1 , respectively under continuous irradiation with bias (0.4 V vs . cathode). By using this system, we have succeeded in constructing CO 2 reduction to formate using water as an electron donor, which does not require a sacrificial reagent.