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

Photoelectrochemical CO₂ reduction (CO₂R) is an appealing solution for converting carbon dioxide into higher-value products. However, CO₂R in aqueous electrolytes suffers from poor selectivity due to the competitive hydrogen evolution reaction that is 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 increases in the Faradaic efficiency and partial current density for carbon monoxide of 50% and 3-fold, 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. Altogether, this study provides a rigorous platform for elucidating the catalytic structure–property relationships to enable engineering of active, stable, and selective materials for photoelectrochemical CO₂R.