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

Photocathodes consisting of semiconductors and cocatalysts have demonstrated promising performances for the solar-driven CO 2 reduction reaction (CO 2 RR) and the H 2 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 CO 2 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 V RHE and an onset potential of ∼0.2 V RHE because the halogen elements reduce the energy barrier for forming a *COOH intermediate from CO 2 . Moreover, a record high photocurrent density of ∼92 mA/cm 2 was achieved under concentrated solar light (300 mW/cm 2 ) without any noticeable change in the CO/H 2 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.