Fe-N-C Electrocatalysts' Durability: Effects of Single Atoms' Mobility and Clustering

Atomically dispersed (or single atom) iron-nitrogen-carbon (Fe-N-C) catalysts are promising alternatives to platinum group metal nanoparticles supported on dispersed carbon as a cathode material in proton-exchange membrane fuel cells. Here, the degradation mechanism of Fe-N-C catalysts, synthesized by the sacrificial support method (SSM), was investigated by conducting accelerated stress tests under the load cycling protocol (i.e. from 0.6 to 1.0 V vs the reversible hydrogen electrode). Electrocatalyst activity toward the oxygen reduction reaction (ORR) was studied for a SSM-derived material, obtained by a single pyrolysis under a 7% H-2 atmosphere (Fe-HT1) and juxtaposed to that of a catalyst derived from the same sample, but subjugated to a second pyrolysis under 10% NH3 (noted as Fe-HT2). Several findings can be highlighted: (i) the second pyrolysis results in the skewing of the mesopore size toward higher diameter, along with an increase in iron content and N-pyridinic moieties, leading to a combined benefit in terms of ORR activity and selectivity, (ii) the morphological changes of these catalysts during ageing are drastically different depending on whether they were exposed to a second pyrolysis as, for example, (iii) for Fe-HT2, the formation of Fe-clusters was observed after the load cycling ageing protocol performed at T = 80 degrees C, along with the partial corrosion of the amorphous domains. No clustering was observed at T = 60 degrees C concomitantly with a higher ORR mass activity retention providing some guidelines to improve the stability of Fe-N-C materials.

Automated summary

Second pyrolysis of Fe-N-C catalysts leads to larger mesopore size, higher iron content, and N-pyridinic moieties, resulting in improved ORR activity and selectivity. Aging morphology of catalysts varies significantly depending on whether they underwent a second pyrolysis, with Fe-clusters formation and partial corrosion observed in Fe-HT2 after load cycling aging at 80 degrees C. No clustering was observed at 60 degrees C, suggesting potential guidelines for enhancing stability of Fe-N-C materials.