Reconstructed low-valent Fe single-atom sites on deficient TiO ₂ enables electrocatalytic nitrate reduction to ammonia

Electrochemical nitrate reduction reaction (NO₃RR) offers a promising route for nitrate (NO₃^-) remediation and sustainable ammonia (NH₃) synthesis, yet its efficiency is often constrained by the hydrogenation of nitrogen-containing intermediates. Herein, we report Fe single atoms anchored on oxygen-vacancy-rich TiO₂ nanosheet assemblies (Fe_SA-TiO₂-O_v) for efficient NO₃^- to NH₃ conversion. The Fe_SA-TiO₂-O_v catalyst achieves a high NH₃ yield rate of 16.6 mg h^-1 cm^-2 at -0.5 V versus reversible hydrogen electrode (vs. RHE), accompanied by a maximum Faradaic efficiency of 92% and excellent durability over 40 hours. <em>Operando</em> XAFS spectroscopy reveals a gradual decrease in Fe valence and contraction of the Fe-O coordination shell, confirming the formation of reconstructed low-valent Fe single-atom active sites during NO₃RR. Theoretical calculations and spectroscopic analysis further indicate that Fe sites are effective for *NO hydrogenation to the *NOH intermediate, thereby promoting the efficient formation of NH₃. These findings identify the reconstructed low-valent Fe single atoms as the active sites for selective electrosynthesis of NH₃, providing a mechanistic framework for designing single-atom catalysts applicable to multistep electrocatalytic reduction reactions.