Synergistic Coupling of Iron Single Atoms and Nanoscale Clusters for Enhanced Nitrate Electroreduction to Ammonia

Developing efficient electrocatalysts for nitrate reduction to ammonia is critical for both environmental remediation and energy conservation, yet it demands highly active and selective catalysts to overcome competing pathways and sluggish kinetics. Herein, we report a Fe single-atom-cluster coupled catalyst synthesized via a controlled pyrolysis strategy, which features atomically dispersed Fe-N₄ sites coexisting with subnanometer Fe clusters embedded in a nitrogen-doped carbon matrix. This unique architecture significantly enhances the electrocatalytic NO₃RR performance, achieving a high NH₃ yield rate of 12.5 mg h^-1 ${\mathrm{cm}}_{\mathrm{cat}.}^{-2}$ and a Faradaic efficiency of ≈92% at -0.5 V versus reversible hydrogen electrode, substantially outperforming Fe₃O₄ nanoparticle and bare carbon paper benchmarks. Combined experimental characterizations and density functional theory calculations reveal that the adjacent Fe nanoclusters electronically modulate the Fe-N₄ sites, strengthening nitrate adsorption and facilitating the critical *NO to *NOH step by shifting the rate-determining step and lowering the overall energy barrier. Moreover, the catalyst exhibits exceptional long-term stability over 24 h and remarkable cyclability, attributed to the robust Fe-N coordination within the graphitic carbon framework. This work highlights the immense potential of synergistically coupling single atoms with clusters as a powerful design principle for advanced electrocatalysts in sustainable ammonia synthesis and beyond.