Electron-rich Au nanocrystals/Co₃O₄ interface for enhanced electrochemical nitrate reduction into ammonia

Solar-driven electrochemical NO₃^- reduction reaction (NO₃^-RR) is a clean and sustainable strategy that can convert pollutant NO₃^- in wastewater to value-added NH₃. In recent years, cobalt oxides-based catalysts have shown their intrinsic catalytic properties toward NO₃^-RR but still have room for improvement through catalyst design. Coupling metal oxides with noble metal has been demonstrated to improve electrochemical catalytic efficiency. Here, we use Au species to tune the surface structure of Co₃O₄ and improve the efficiency of NO₃^-RR to NH₃. The obtained Au nanocrystals-Co₃O₄ catalyst exhibited an onset potential of 0.54 V vs RHE, NH₃ yield rate of 27.86 µg/h·cm², and Faradaic efficiency (FE) of 83.1% at 0.437 V vs RHE in an H-cell, which is much higher than Au small species (Au clusters or single atoms)-Co₃O₄ (15.12 µg/h·cm²) and pure Co₃O₄ (11.38 µg/h·cm²), respectively. Combined experiments with theory calculations, we attributed the enhanced performance of Au nanocrystals-Co₃O₄ to the reduced energy barrier of *NO hydrogenation to the *NHO and suppression of HER, which originated from the charge transfer from Au to Co₃O₄. Using an amorphous silicon triple-junction (a-Si TJ) as the solar cell and an anion exchange membrane electrolyzer (AME), an unassisted solar-driven NO₃^-RR to NH₃ prototype was realized with a yield rate of 4.65 mg/h and FE of 92.1%.