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 CO<sub>2</sub> to C<sub>2+</sub> products; however, the criteria for maximizing PEC solar-to-C<sub>2+</sub> (STC<sub>2+</sub>) rates are not well understood. We report here a continuum-scale model of PEC CO<sub>2</sub> reduction (CO<sub>2</sub>R) on Cu in 0.1 M CsHCO<sub>3</sub> and use it to optimize the design and operating conditions for generating C<sub>2+</sub>products. Furthermore, we demonstrate that the potential-dependent product distribution of CO<sub>2</sub>R on Cu requires operating near the potential that maximizes C<sub>2+</sub> generation rates ($V$<sub>id</sub>), unlike PEC water splitting, which desires operation at the maximum photocurrent density. Because of this requirement, the criterion for a high STC<sub>2+</sub> rate includes high-photocurrent semiconductors with photovoltages near $V$<sub>id</sub> and low series resistance. The STC<sub>2+</sub> rate in these systems is enhanced by optimal CO<sub>2</sub> transport and exhibits low sensitivity to dirunal solar irradiance variations.