Operando Cu Aggregation-Induced Spin State Modulation in Fe–Cu Single Atom Catalyst for Enhanced Tandem Electrochemical Nitrate Reduction Reaction

The electrocatalytic nitrate reduction reaction (NO₃RR) provides a sustainable pathway to convert excess nitrate into ammonia, yet realizing high selectivity requires a fundamental understanding of dynamic structural changes occuring at active sites during reactions. Here, we investigate how in situ Cu clustering dynamically activates dual catalytic sites in Fe-Cu bimetallic single-atom catalysts (FeCu-N-C) during NO₃RR, through combined density functional theory calculations and <i>operando</i> spectroscopy. Under reductive potentials, atomically dispersed Cu spontaneously aggregates into nanoclusters that efficiently activate NO₃^-. Concurrently, Cu clustering induces pronounced structural strain and electronic distortion in adjacent Fe-N_<i>x</i> moieties, triggering a spin-state transition in the Fe active site from low-spin to high-spin configuration. This spin modulation dramatically enhances the activity for subsequent NO₂^- conversion to NH₃. The synergistic coupling between Cu clusters and spin-modulated Fe establishes a highly effective tandem pathway, yielding superior NO₃RR activity and NH₃ selectivity, compared to Cu-N-C and Fe-N-C counterparts. These findings provide new insights into the rational design of advanced multicomponent electrocatalysts with dynamically tunable active site properties.