Tailoring active sites of iron-nitrogen-carbon catalysts for oxygen reduction in alkaline environment: Effect of nitrogen-based organic precursor and pyrolysis atmosphere

Abstract

Fe-N-C catalysts were synthesized from a nitrogen and iron wet impregnation of carbon black pearls followed by pyrolysis steps. Three different nitrogen sources (dopamine, imidazole and benzimidazole), and two different pyrolysis atmospheres (Ar and NH3) were used. The obtained materials were characterized in terms of structure, morphology, surface chemistry, and electrochemical properties. Electrodes with a high porosity and accessible active sites were obtained tailoring the synthesis parameters, as indicated by Raman and X-ray photoelectron spectroscopies, and cyclic voltammetry with rotating ring disk electrode. Pyrolysis under ammonia atmosphere led to high electrochemical active surface area (ECSA) and the use of imidazole as nitrogen-rich organic precursor improved oxygen reduction reaction (ORR) activity in alkaline pH. This can be ascribed to the modification of surface chemistry of the electrocatalysts triggered by the N-rich organic precursor and pyrolysis atmosphere. The catalyst obtained by using imidazole and pyrolyzed in NH3 had a variety of iron-, oxygen- and nitrogen-functional groups, nitrogen being mainly distributed in imine-, pyridinic- and pyrrolic-N. In addition, durability tests showed a stable ECSA and ORR activity after cycling of the prepared electrocatalysts outperforming durability of Pt-based materials in alkaline environment and indicating applicability in anion exchange membrane fuel cells.