Efficient Cu<sub>2</sub>O Photocathodes for Aqueous Photoelectrochemical CO<sub>2</sub> Reduction to Formate and Syngas
Meng Xia, Linfeng Pan, Yongpeng Liu, Jing Gao, Jun Li, Mounir Mensi, Kevin Sivula, Shaik M. Zakeeruddin, Dan Ren, Michaël Grätzel
Abstract
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.