Controlling Surface Basicity to Balance Activity and Durability in Fe−N−C Catalysts

Ammonium halide-assisted treatments have been widely employed to enhance the performance of Fe−N−C catalysts. However, the underlying reconstruction chemistry and its impact on active site microenvironments remain poorly understood. Here, we present a post-synthetic reconstruction strategy in which controlled NH4Cl thermolysis enables simultaneous modulation of carbon microstructure and nitrogen surface chemistry in pre-formed Fe−N−C catalysts. In situ thermogravimetry−mass spectrometry reveals that NH4Cl decomposes into NH3 and HCl over distinct temperature regimes, driving coupled nitrogen functionalization and preferential carbon etching while preserving atomic Fe−N4 coordination. This balanced reconstruction generates hierarchical porosity and moderately increased nitrogen basicity, markedly enhancing Fe−N4 site accessibility and intrinsic activity without inducing the excessive basicity and structural degradation typically associated with NH3 treatment. As a result, NH4Cl-treated catalysts exhibit significantly improved oxygen reduction activity and durability in proton exchange membrane fuel cells under both H2/O2 and H2/air operation, outperforming the untreated Fe−N−C catalyst and the same material after pyrolysis under pure NH3 flow. By explicitly linking ammonium halide decomposition pathways to microstructure evolution, surface basicity, and operational stability, this work establishes post-synthetic ammonium halide thermolysis as a rational approach for optimizing the activity−durability balance in Fe−N−C and related M−N−C electrocatalysts.