Modulating the Fe-N-4 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 O-2 to the Fe-N-4 sites. Herein, to further improve the ORR activity while maintaining good mass transport properties, a series of Fe-NC 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-N-4 active sites. Fe-57 Mossbauer 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 H-2/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.