Competitive Non-Radical Nucleophilic Attack Pathways for NH₃ Oxidation and H₂ O Oxidation on Hematite Photoanodes

The sluggish H₂ 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-Fe₂ O₃ ) photoanodes under near-neutral pH (9-11) and moderate applied potentials (1.0-1.4 V_RHE ) compared to H₂ O 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., Fe^IV =O) and Fe-N species that produces NO_x ^- and N₂ , respectively, which overwhelms the nucleophilic attack of H₂ 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.