Codesign of an integrated metal-insulator-semiconductor photocathode for photoelectrochemical reduction of CO₂ to ethylene

Photoelectrochemical carbon-dioxide reduction (PEC CO2R) is a potentially attractive means for producing chemicals and fuels using sunlight, water, and carbon dioxide; however, this technology is in its infancy. To date, most studies of PEC CO2R have reported products containing one carbon atom (C-1 products) but the production of valuable products containing two or more carbons (C2+ products), such as ethylene, ethanol, etc., is rarely demonstrated. Metal-semiconductor-insulator (MIS) photocathode/catalyst structures offer a promising approach for this purpose, since they integrate the functions of light absorption, charge separation, and catalysis. In this study, we have investigated a Cu/TiO2/p-Si photocathode/catalyst structure with the aim of establishing the effects of semiconductor-insulator interactions on the performance of the photocathode and the influence of the direction of illumination of the MIS structure on the total current density and the distribution of products formed by on the Cu catalyst. We have also examined the influence of ionomer coatings deposited on the Cu surface on the total current density and the distribution of products formed. A major finding is that for a fixed Cu potential the distribution of products formed by PEC CO2R are the same, irrespective of the direction of illumination, and are identical to those obtained by electrochemical reduction of CO2 (EC CO2R). Another important finding is that the total current density and the faradaic efficiency to ethylene are enhanced significantly by deposition of a thin bilayer of Sustainion/Nafion onto the surface of the Cu.