Efficient Cu₂O Photocathodes for Aqueous Photoelectrochemical CO₂ Reduction to Formate and Syngas

Photoelectrochemical carbon dioxide reduction (PEC-CO₂R) represents a promising approach for producing renewable fuels and chemicals using solar energy. However, attaining even modest solar-to-fuel (STF) conversion efficiency often necessitates the use of costly semiconductors and noble-metal catalysts. Herein, we present a Cu₂O/Ga₂O₃/TiO₂ photocathode modified with Sn/SnO_x catalysts through a simple photoelectrodeposition method. It achieves a remarkable half-cell STF efficiency of ∼0.31% for the CO₂R in aqueous KHCO₃ electrolyte, under AM 1.5 G illumination. The system enables efficient production of syngas (FE: ∼62%, CO/H₂ ≈ 1:2) and formate (FE: ∼38%) with a consistent selectivity over a wide potential range, from +0.34 to −0.16 V vs the reversible hydrogen electrode. We ascribe the observed performance to the favorable optoelectronic characteristics of our Cu₂O heterostructure and the efficient Sn/SnO_x catalysts incorporated in the PEC-CO₂R reactions. Through comprehensive experimental investigations, we elucidate the indispensable role of Cu₂O buried p-n junctions in generating a high photovoltage (∼1 V) and enabling efficient bulk charge separation (up to ∼70% efficiency). Meanwhile, we discover that the deposited Sn/SnO_x catalysts have critical dual effects on the overall performance of the PEC devices, serving as active CO₂R catalysts as well as the semiconductor front contact. It could facilitate interfacial electron transfer between the catalysts and the semiconductor device for CO₂R by establishing a barrier-free ohmic contact.