Design & structuring of electrospun nanofibers for non-Pt PEM Fuel Cell cathodes

The project aimed at combining the synthesis of non-noble metal catalysts for the oxygen reduction reaction (ORR) in PEM fuel cells with the making of self-standing porous catalyst layers from electrospun carbon nanofibers. Both components are carbon based and involve one or more pyrolysis steps during preparation. The catalyst is made from iron, nitrogen and carbon precursors which are pyrolyzed to obtain Fe ions complexed by pyrrolic or pyridinic nitrogen of a carbon-nitrogen structure (Fe-N-C catalysts). One of the challenges with Fe-N-C catalysts is to make efficient electrodes with adequate active sites without having a large transport resistance due to thickness and structure. Electrode structures can be made from electrospun polymer nanofibers subsequently carbonized by another pyrolysis process to ensure sufficient electronic conductivity.

Several strategies were explored with electrospinning as the core process of synthesis with nanofibers used either as a template for Fe-N-C surface decoration or as mostly explored, host species to FeN-C precursors. Fe sources and ZIF-8 were embedded in polymer nanofibers, yiedling active and self-standing catalyst layers obtained after a thermal activation. The most active catalysts were obtained when a pyrolysis at 1050°C was followed by a short thermal treatment in ammonia (NH₃). Catalysts synthesized this way showed enhanced performance compared to a benchmark commercial catalyst from Pajarito Powder as tested on a rotating disk electrode (RDE). The ORR activity of the best performing catalyst layer was demonstrated as measured in its self-standing form as well as a ground-to-powder catalyst using RDE. It was shown that the present synthesis results in sufficient electronic conductivity after pyrolysis without making the self standing catalyst layer too brittle to handle. Most importantly, it was shown that an integration of the catalyst synthesis process with electrode preparation is possible, yielding ORR active and self-standing electrodes.

With single PEM fuel cell performance comparable to that of a traditionally prepared electrode made from a commercial Fe-N-C powder, the potential of nanofibrous catalysts tested as selfstanding electrodes is high-lightened. No spraying, casting or other deposition method needs to be optimized with respect to the catalyst’s surface properties as electrospinning uniformly monitors material deposition to be used as a simultaneous Fe-N-C synthesis and electrode preparation method. Although significant optimization is still pending, synthesis and electrode preparation are shown to combine well to manufacture ORR active and self-standing catalyst layers for PEM fuel cells.