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 nonradical nucleophilic attack of NH 3 molecules on highvalent surface FeÀ O species (e.g., Fe IV =O) and FeÀ N species that produces NO x À 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 nonradical nucleophilic attack strategy, which is significantly different from the conventional indirect radical-mediated AOR mechanism, for the rational design of highperformance AOR photoelectrocatalysts.