Fe–N–C Electrocatalysts’ Durability: Effects of Single Atoms’ Mobility and Clustering
Kavita Kumar, Tristan Asset, Xiaoyan Li, Yuanchao Liu, Xingxu Yan, Yechuan Chen, Michel Mermoux, Xiaoqing Pan, Plamen Atanassov, Frédéric Maillard, Laëtitia Dubau
Abstract
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% H2 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 °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.