Establishing the Role of Operating Potential and Mass Transfer in Multicarbon Product Generation for Photoelectrochemical CO2

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.