Enhanced four-electron oxygen reduction reaction selectivity in atomically dispersed Fe-N-C catalysts via microwave heating

Abstract

Atomically dispersed supported iron (Fe-N-C) catalysts have emerged as the most promising alternatives to Pt-based catalysts for oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs). Advances in their synthesis over the past decade have significantly improved their activity, with some catalysts demonstrating high initial activities approaching those of commercial Pt-based catalysts. However, their practical application is hindered by durability challenge, which is facilitated by undesired formation of H₂O₂ via the 2e⁻ ORR pathway. We report a strategy to improve the selectivity to the 4e⁻ ORR pathway using microwave (MW) heating. This approach is demonstrated using Fe-N-C catalysts derived from ZIF-8. Brief heating of the Fe-N-C catalyst at 900 ºC for durations ranging from 1 s to 3 min in a single-mode microwave reactor improves its selectivity by suppressing H₂O₂ formation to the lowest level reported so far, while maintaining catalytic activity in ORR experiments conducted in acidic electrolytes. Characterization data indicate that MW heating reduces N and O atoms while Fe content remained at approximately 1.3 wt% after heating for ≥ 1 s. These changes were accompanied by increased basicity of Fe-N-C catalyst beyond the level reported for Fe-N-C catalysts synthesized via pyrolysis at 1050 ºC under an Ar atmosphere. We propose that MW heating selectively removes metal-free N and O sites responsible for the 2e⁻ ORR pathway, as well as possibly Fe sites weakly bonded to the carbon framework, while preserving Fe–N_x sites held, thereby improving selectivity.