Fe-containing polyimide-based high-performance ORR catalysts in acidic medium: a kinetic approach to study the durability of catalysts
The ORR activity and durability of Fe-containing non-precious N-doped carbon catalysts in acidic medium were studied using a rotating ring-disk electrode voltammetry and XPS technique. The catalysts (Fe/PI) were synthesised from the pyrolysis of the Fe(acac)3 and polyimide nanoparticle (PI) mixture. The catalytic activity and durability of Fe/PI are superior to that of the conventional phthalocyanine-based catalyst. The onset potential of ORR was 0.915 V vs. RHE in 0.5 M H2SO4 which is very close to that of a commercially available Pt/C catalyst. The Fe/PI catalyst sustains its activity and stability even after 11 110 repeating potential steps between 0.6 and 1.0 V vs. RHE in O2-saturated 0.5 M H2SO4 and 1110 potential cycles between 1.0 and 1.5 V vs. RHE in argon-saturated 0.5 M H2SO4, respectively. The kinetic and mechanistic analyses of the ORR on this catalyst indicate that the ORR, as a whole, follows a 4-electron (parallel) pathway. The N 1s XPS spectra before and after the durability test indicate that pyridinic and graphite-like nitrogens take part in improving the ORR activity, whereas nitrogen oxides have a negative role in the ORR. The loss of ORR activity was observed after the acid washing of the catalysts, which suggests the role of Fe in the overall 4-electron reduction of O2.