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 CO2 reduction reaction (CO2 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 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 similar to 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 similar to 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.