Designing the 3D Architecture of PGM-Free Cathodes for H₂/Air Proton Exchange Membrane Fuel Cells

Metal–nitrogen–carbon catalysts have emerged as the most promising platinum group metal-free catalysts toward oxygen reduction reaction for proton exchange membrane fuel cell (PEMFC) applications. However, their large-scale implementation in H2/air PEMFCs is still hindered by the low density of active sites in such materials, implying the need for thick active layers with inferior mass-transport properties. In this work, the coelectrospinning of nano-ZIF-8 (a zeolitic imidazolate framework) and polyacrylonitrile results in anisotropic and microporous FeNC fibers, offering an effective approach toward active layers with hierarchical micro-, meso-, and macroporosity. X-ray computed tomography performed on the cathode ex situ reveals enhanced macroporosity of fibrous FeNC layers compared to a nonfibrous one derived from nano-ZIF-8. Applied in operando in a PEMFC, X-ray tomography showed abundant water-free macroporous voids in the fibrous FeNC layer, beneficial for the transport of reactants and products toward and away from the active sites. The combination of the Fe precursor in the electrospun solution and the high voltage applied during electrospinning is however also shown to enhance the formation of metallic Fe particles after pyrolysis, which is detrimental to the density of atomically dispersed FeNx active sites. FeNC fibrous morphology with higher density of FeNx active sites, obtained with a modified electrospinning process or other techniques, therefore holds great potential to replace Pt/C with MNC cathodes in H2/air PEMFCs.