Establishing the Role of Operating Potential and Mass Transfer in Multicarbon Product Generation for Photoelectrochemical CO₂ Reduction Cells Using a Cu Catalyst

There is increasing interest in the possibility of photoelectrochemical (PEC) reduction of CO2 to C2+ products; however, the criteria for maximizing PEC solar-to-C2+ (STC2+) rates are not well understood. We report here a continuum-scale model of PEC CO2 reduction (CO2R) on Cu in 0.1 M CsHCO3 and use it to optimize the design and operating conditions for generating C2+ products. We demonstrate that the potential-dependent product distribution of CO2R on Cu requires operating near the potential that maximizes C2+ generation rates (Vid), unlike PEC water splitting, which desires operation at the maximum photocurrent density. Because of this requirement, the criterion for a high STC2+ rate includes high-photocurrent semiconductors with photovoltages near Vid and low series resistance. The STC2+ rate in these systems is enhanced by optimal CO2 transport and exhibits low sensitivity to dirunal solar irradiance variations.