Silver Halide Catalysts on GaN Nanowires/Si Heterojunction Photocathodes for CO2 Reduction to Syngas at High Current Density

Abstract

Photocathodes consisting of semiconductors and cocatalysts have demonstrated promising performances for the solar-driven CO2 reduction reaction (CO2 RR) and the H2 evolution reaction. However, the performance of cocatalyst materials has been limited due to the degradation of semiconductors during the loading processes. Hence, a photocathode that withstands harsh reaction conditions can broaden the selection of cocatalyst materials and improve catalytic activity. Here, we have developed Ag halide (AgX, X = Cl, Br, I) cocatalysts, which are transformed by anodization of Ag on GaN nanowires (NWs)/n+–p Si heterojunction photocathodes, for CO2 RR to syngas with superior performance. N-terminated GaN NWs are chemically robust modifiers to load the cocatalysts of AgCl and AgBr. As a result, the photocathodes showed a superior faradaic efficiency of CO >80% at −0.4 VRHE and an onset potential of ∼0.2 VRHE because the halogen elements reduce the energy barrier for forming a *COOH intermediate from CO2. Moreover, a record high photocurrent density of ∼92 mA/cm2 was achieved under concentrated solar light (300 mW/cm2) without any noticeable change in the CO/H2 syngas ratio for 12 h operation. The present findings offer critical insights into the design and development of photocathodes for selective, productive, and stable solar fuel generation.