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 (alpha-Fe2O3) photoanodes under near-neutral pH (9-11) and moderate applied potentials (1.0-1.4 V-RHE) 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., Fe-IV=O) and Fe-N species that produces NOx- and N-2, respectively, which overwhelms the nucleophilic attack of H2O 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.