Efficient Cu 2 O Photocathodes for Aqueous Photoelectrochemical CO 2 Reduction to Formate and Syngas.

Photoelectrochemical carbon dioxide reduction (PEC-CO 2 R) 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 Cu 2 O/Ga 2 O 3 /TiO 2 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 CO 2 R in aqueous KHCO 3 electrolyte, under AM 1.5 G illumination. The system enables efficient production of syngas (FE: ∼62%, CO/H 2 ≈ 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 Cu 2 O heterostructure and the efficient Sn/SnO x catalysts incorporated in the PEC-CO 2 R reactions. Through comprehensive experimental investigations, we elucidate the indispensable role of Cu 2 O 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 CO 2 R catalysts as well as the semiconductor front contact. It could facilitate interfacial electron transfer between the catalysts and the semiconductor device for CO 2 R by establishing a barrier-free ohmic contact.