Elucidating the influence of secondary nitrogen precursors on the performance of Fe–N–C catalysts for proton exchange membrane fuel cells

Abstract

Proton exchange membrane fuel cells (PEMFCs) offer high efficiency, rapid refueling, and zero-carbon operation, but their commercialization is constrained by the cost of platinum-based oxygen reduction catalysts. Transition metal–nitrogen–carbon materials, particularly Fe–N–C, are promising platinum-free alternatives, although their activity and durability still require improvement. Ammonia is often introduced during synthesis to enhance nitrogen incorporation, but safer nitrogen sources are desirable to simplify processing and reduce associated hazards. Here, we investigate the use of urea, melamine, cyanoguanidine, and nicarbazin as nitrogen precursors during the thermal activation of ZIF-8-derived Fe–N–C catalysts, aiming to promote nitrogen incorporation and active-site formation without the use of ammonia. Structural analysis reveals that the use of urea, melamine, and cyanoguanidine during heat-treatment largely preserve the ZIF-8 morphology, while nicarbazin leads to the formation carbonaceous flakes. The high surface area of ZIF-8 (∼1600 m² g⁻¹) is partially retained after pyrolysis (∼1200 m² g⁻¹). X-ray photoelectron spectroscopy reveals enhanced nitrogen content and increased Fe–N_x species, most notably in urea-activated samples. Electrochemical testing in an acidic electrolyte confirms higher onset potentials and mass activities for urea- and melamine-activated catalysts compared to the control, with consistent trends observed in rotating disk electrode and single-cell PEMFC measurements. Despite their lower intrinsic activity compared to Pt/C, Fe–N–C catalysts exhibit enhanced ORR kinetics when secondary nitrogen precursors are used during synthesis. Despite elevated peroxide yields predicted from rotating ring disk electrode measurements, ion chromatography indicates a modest increase in ionomer degradation compared to Pt/C during fuel cell tests. Overall, nitrogen-rich molecular precursors enhance Fe–N–C activity while providing a safer and scalable pathway for nitrogen doping, advancing the development of cost-effective non-platinum catalysts for fuel cells.