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-HT 1 ) and juxtaposed to that of a catalyst derived from the same sample, but subjugated to a second pyrolysis under 10% NH 3 (noted as Fe-HT 2 ). 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-HT 2, the formation of Fe-clusters was observed after the load cycling ageing protocol performed at T = 80 °C, along with the partial corrosion of the amorphous domains. No clustering was observed at T = 60 °C concomitantly with a higher ORR mass activity retention providing some guidelines to improve the stability of Fe–N–C materials.