Atomic transition metal–nitrogen–carbon electrocatalysts exhibit outstanding activity in various electrocatalytic reactions. The challenge lies in predicting the structure of the active center, which may undergo changes under applied potential and interact with reactants or intermediates. Advanced characterization techniques, particularly in-situ X-ray absorption spectroscopy (XAS), provide crucial insights into the structural evolution of the metal active center during the reaction. In this study, nitrate reduction to ammonia (NO₃RR) was selected as a model reaction, and we introduced in-situ XAS to reveal the structural evolution during the catalytic process. A novel single atom catalyst of iron loaded on three-dimensional nitrogen–carbon nanonetwork (designated as Fe SAC/NC) was successfully synthesized. We unraveled the structural transformations occurring as pyrrole-N₄-Fe transitions to pyrrole-N₃-Fe throughout the NO₃RR process. Notably, the Fe SAC/NC catalyst exhibited excellent catalytic activity, achieving a Faradaic efficiency of 98.2% and an ammonia generation rate of 22,515 μg·h⁻¹·mg_cat⁻¹ at −0.8 V versus reversible hydrogen electrode. Theoretical calculations combined with in-situ spectroscopic characterization showed that pyrrole-N₃-Fe reduced the energy barrier from *NO to *NHO and improved the selectivity of ammonia. This provides a robust reference for the design of efficient nitrate-to-ammonia synthesis catalysts.

Atomic transition-metal-nitrogen-carbon electrocatalysts hold great promise as alternatives to benchmark Pt in the oxygen reduction reaction. The pristine metal centers with quasi square-planar D_4h configuration, however, still suffer from unfavorable energetics and thereby strong activity/selectivity trade-off during the catalytic process. Here we present a ligand-field engineering of single-atom Ni-N-C catalysts to boost the sluggish kinetics via rationally constructing prototypical asymmetrically ligated Ni-N₃O₁ sites. The as-obtained Ni-supported multi-walled carbon nanotubes with molten salt-treated (defined as Ni/CNS) catalyst delivered an excellent H₂O₂ selectivity (> 90%) within a wide potential window (0.2–0.7 V vs. reversible hydrogen electrode (RHE)) and robust stability (for 10 h) in alkaline medium. Combined electron paramagnetic resonance and theoretical analysis rationalize this finding and demonstrate that the broken symmetry facilitates the electron transfer of a σ* to O–O orbital as compared to the Ni-N₄ counterpart, playing an indispensable role in efficient O₂ activation.