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

The sluggish H2O 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‐Fe2O3) photoanodes under near‐neutral pH (9–11) and moderate applied potentials (1.0–1.4 VRHE) compared to H2O oxidation. By rate law analysis and operando spectroscopic studies, we confirm the non‐radical nucleophilic attack of NH3 molecules on high‐valent surface Fe−O species (e.g., FeIV=O) and Fe−N species that produces NOx− and N2, respectively, which overwhelms the nucleophilic attack of H2O on surface FeIV=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. α‐Fe2O3 is found to be a highly effective and stable catalyst for driving photoelectrochemical ammonia oxidation reaction via a novel non‐radical nucleophilic attack mechanism under near‐neutral pH. The selectivity of the ammonia oxidation reaction can be precisely tuned by manipulating the proportion of two surface species including Fe−O species and Fe−N species on α‐Fe2O3.