Single-atom Fe–Nx-C electrocatalysts for efficient nitrate reduction reaction

Iron–nitrogen–carbons (Fe–Nx–Cs) emerged as highly efficient single-atom electrocatalysts (ECs) for the nitrate reduction reaction (NO3RR), yet the structural evolution of their active sites during pyrolysis remains a critical area of investigation. In this work, we systematically investigate the thermal stability and transformation of Fe–Nx active sites derived from iron(ii)-phthalocyanine (FePc) supported over conductive carbon black (Ketjen Black ec-600JD) during pyrolysis from room temperature (RT) to 600 °C. The primary objective is to evaluate how the pyrolysis temperature, from RT to 600 °C in N2/H2 atmosphere, dictates the evolution of these sites and correlates with the electrochemical performance toward NO3RR in neutral pH electrolytes. Utilizing a combination of advanced spectroscopic and microscopic techniques (XAS, XPS, BF-STEM, XRD and Raman), we tracked the transition of iron species from molecular precursors to integrated active phases. While BF-STEM and XAS detect the formation of sub-nanometric clusters and metallic nanoparticles at 500–600 °C, the faradaic efficiency (FE) for ammonia remains consistently above 75% at −0.8 V vs. RHE. However, the best electrocatalytic activity was observed for the EC pyrolyzed at 300 °C. Principal Component Analysis (PCA) and quantum chemical simulations were used to unravel the complex correlations between the structural evolution of the iron sites and the electrochemical performance. The results identify the Fe–N3 configuration as the primary active site responsible for maintaining a high FE for NH4+ at high overpotentials, successfully outcompeting the concurrent hydrogen evolution reaction (HER). This study provides fundamental insights into the structure–activity relationships of single-atom Fe–Nx–C ECs derived from FePc for the NO3RR.