Molecular Coatings Improve the Selectivity and Durability of CO₂ Reduction Chalcogenide Photocathodes

The quest for solar-driven conversion of carbon dioxide to chemicals and fuels hinges upon the identification of an efficient, durable, and selective photocathode. Chalcogenide p-type semiconductors exemplified by chalcopyrite Cu(In,Ga)Se-2 (GIGS) have been effectively deployed as photocathodes. However, selectivity toward CO2 reduction and durability of the commonly used CdS adlayer remain primary challenges. Here, we demonstrate that for the wide band gap CuGa3Se5 chalcopyrite absorber these challenges are well addressed by an organic coating generated in situ from an N,N'-(1,4-phenylene)bispyridinium ditriflate salt in the electrolyte. The molecular additive provides a 30-fold increase in selectivity toward CO2R products compared to the unmodified system and lowers Cd corrosion at least 10-fold. This dual functionality highlights the promise of hybrid solid-state-molecular photocathodes for enabling durable and efficient solar fuel systems.