Selective CO Production by Photoelectrochemical CO₂ Reduction in an Aqueous Solution with Cobalt-Based Molecular Redox Catalysts

Light-driven CO2 reduction was performed in a two-electrode photoelectrochemical cell (PEC) composed of a Co4O4 cubane complex-modified BiVO4 photoanode and a cobalt phthalocyanine complex-modified carbon cloth (cc) cathode. The hybrid electrodes assembled by the simple physical absorption of hydrophobic molecular catalysts exhibit long-term stability in an aqueous solution. Under 1 sun AM 1.5 G illumination, simultaneous oxygen and CO evolution at an approximately 2:1 ratio were achieved in a CO2-saturated NaHCO3 aqueous solution with high faradic efficiency up to 87% for CO production. Control experiments revealed a crucial role of immobilized molecular catalysts in promoting the activity and selectivity for both half-reactions. A solar-to-CO conversion efficiency of 0.44% was realized at a cell potential of 0.8 V, which is the highest efficiency for CO2 to CO conversion in PEC devices based on noble-metal-free materials.