Modulating the Fe–N₄ Active Site Content by Nitrogen Source in Fe–N–C Aerogel Catalysts for Proton Exchange Membrane Fuel Cell

Fe–N–C material is regarded as a promising non-precious-metal catalyst for oxygen reduction reaction (ORR) to replace Pt-based catalysts, but its activity and mass transport remain problematic before a large-scale application in proton exchange membrane fuel cells (PEMFCs). Our previous research developed an Fe–N–C aerogel catalyst by pyrolyzing resorcinol–melamine–formaldehyde (RMF) aerogel containing iron precursors. The abundance of micro- and mesopores in aerogel is known to improve the mass transport properties of Fe–N–C cathodes in PEMFC, facilitating the diffusion of O2 to the Fe–N4 sites. Herein, to further improve the ORR activity while maintaining good mass transport properties, a series of Fe–N–C aerogel catalysts were synthesized by modulating the nitrogen source (melamine) content and the texture in the RMF aerogel precursor. The Fe content in catalysts presents a positive relationship with melamine content in the aerogel, with adequate texture, indicating the important function of nitrogen source in stabilizing Fe atoms during pyrolysis to form Fe–N4 active sites. 57Fe Mössbauer spectroscopy revealed a majority of O–Fe(III)N4C12 configuration of the active sites, which is consistent with the variation of pyrrolic N content with Fe derived from X-ray photoelectron spectroscopy. As a result, the mass activity of the series of catalysts exhibits a linear relationship with Fe content and reaches 3.0 A g–1 at 0.8 V vs reversible hydrogen electrode (RHE) in 0.05 M H2SO4 and rotating disk electrode (RDE) setup. Their performance in PEMFC exhibits the same tendency as the RDE setup. In addition, the H2/air PEMFC polarization curves do not show any diffusion-limited current density effects, even at 0.7 A cm–2, with a cathode based on an Fe–N–C catalyst prepared with high melamine content. This work reveals the importance of nitrogen sources to reach a high atomically dispersed Fe content in Fe–N–C catalysts with a low yield of Fe nanoparticles, and the mass transport properties in PEMFC are not affected by low mesopore volume for aerogel-based catalysts