Rational Pathways Tuning Facilitates Photoelectrochemical Upcycling of Nitrite to Ammonia Using CuPd Nanoalloy on GaN/Si Photocathode.

Solar-driven photoelectrochemical conversion of nitrite to ammonia represents a sustainable yet unexplored approach for environmental remediation and resource recovery. Here, we demonstrate that Cu5Pd1 alloy nanoparticles, integrated with vertically grown GaN nanowires on an n+-p Si photocathode (Cu5Pd1/GaN/Si), enable highly efficient and selective nitrite reduction to ammonia. This photoelectrode achieves a Faradaic efficiency of 99.7% for NH3, with a yield rate of 162.2 μmol h-1 cm-2 and a nearly 100% selectivity. Additionally, the Cu5Pd1/GaN/Si photoelectrode maintains robust performance in the presence of various anions and can effectively remove ∼98% of nitrite even at low concentrations. Density functional theory calculations, supported by in situ spectroscopic techniques, reveal that Cu-Pd alloying fundamentally alters the nitrite reduction mechanisms. Unlike the *NOH-mediated pathway on Cu and Pd, which can lead to competing N2 formation, the CuPd alloy preferentially stabilizes the *NHO intermediate, making NH3 production thermodynamically preferred and highly selective. These findings highlight that a rational electrocatalyst design can effectively tune reaction pathways to enhance both the efficiency and selectivity of photoelectrocatalytic nitrite upcycling.