Competitive Non‐Radical Nucleophilic Attack Pathways for NH3 Oxidation and H2O Oxidation on Hematite Photoanodes

Abstract The sluggish H 2 O oxidation kinetics on photoanodes severely obstructs the overall solar‐to‐energy efficiency of photoelectrochemical (PEC) cells. Herein, we find a 10 to 55‐fold increase of photocurrent by conducting ammonia oxidation reaction (AOR) on hematite (α‐Fe 2 O 3 ) photoanodes under near‐neutral pH (9–11) and moderate applied potentials (1.0–1.4 V RHE ) compared to H 2 O oxidation. By rate law analysis and operando spectroscopic studies, we confirm the non‐radical nucleophilic attack of NH 3 molecules on high‐valent surface Fe−O species (e.g., Fe IV =O) and Fe−N species that produces NO x − and N 2 , respectively, which overwhelms the nucleophilic attack of H 2 O on surface Fe IV =O and contributes to a high Faradaic efficiency of above 80 % for AOR. This work reveals a novel non‐radical nucleophilic attack strategy, which is significantly different from the conventional indirect radical‐mediated AOR mechanism, for the rational design of high‐performance AOR photoelectrocatalysts.