Influence of amino group incorporation and ammonia-enriched pyrolysis environment on the performance and stability of Fe-N-C catalysts for proton exchange membrane fuel cells

Dual nitrogen-doping strategies effectively regulate both the activity and durability of Fe-N-C catalysts. • Amino-functionalized precursors generate lower-coordinated Fe N sites, leading to enhanced ORR kinetics. • Ammonia-assisted pyrolysis boosts initial PEMFC performance by modulating spin state and increasing specific surface area. • The catalytic activity and stability of PEMFCs are strongly correlated with the spin state of the Fe active sites, where high-spin configurations offer higher activity but inferior stability. Nitrogen species play a crucial role in regulating the electronic structure of Fe-N-C catalysts, thereby governing the adsorption of oxygenated intermediates and ultimately determining proton exchange membrane fuel cell (PEMFC) performance. However, the intrinsic linkage among nitrogen configuration and catalytic performance remains insufficiently understood. To address this, we design four Fe-N-C catalysts with comparable structures but distinct nitrogen components by using amino-functionalized precursors and controlling the pyrolysis atmosphere. PEMFC testing and characterization show that amino groups in the iron precursor generate lower coordinated Fe-N x site, which density functional theory calculations show to possess more favorable adsorption energetics and reduced free-energy barriers for key reaction intermediates. In contrast, ammonia-assisted pyrolysis enhances initial performance primarily by introducing higher spin-state sites, abundant carbon defects, enlarging surface area, and increasing the density of accessible Fe-N x sites. Importantly, we find that both catalytic activity and durability can be regulated by Fe-N coordination by manipulating spin state of the Fe center: high-spin sites deliver higher intrinsic activity but suffer from accelerated degradation, whereas intermediate/low-spin sites offer improved stability. These findings suggest that a rational combination of precursor functionalization and pyrolysis atmosphere can simultaneously optimize Fe-N x site spin state, coordination environment, and carbon microstructure, providing a practical strategy to achieve both high activity and long-term durability in Fe-N-C catalysts for PEMFCs. Download: Download high-res image (57KB) Download: Download full-size image