Molecular Additives Improve Selectivity of CO2 Photoelectrochemical Reduction over Gold Nanoparticles on Gallium Nitride

Photoelectrochemical CO2 reduction (CO2R) is an appealing solution to convert carbon dioxide into higher-value products. However, CO2R in aqueous electrolytes suffers from poor selectivity due to the competitive hydrogen evolution reaction dominant on semiconductor surfaces in aqueous electrolytes. We demonstrate that functionalizing gold/p-type gallium nitride devices with a film derived from diphenyliodonium triflate suppresses hydrogen generation from 90% to 18%. As a result, we observe an increase in the Faradaic efficiency and partial current density for carbon monoxide by 50% and 3x, respectively. Furthermore, we demonstrate through optical absorption measurements that the molecular film employed herein, regardless of thickness, does not affect the photocathode’s light absorption and, therefore, photocurrent. Together, this study provides a rigorous platform to elucidate catalytic structure-property relationships to enable engineering of active, stable, and selective materials for photoelectrochemical CO2 reduction.