Fe-N-functionalized carbon electrocatalyst derived from a zeolitic imidazolate framework for oxygen reduction: Fe and NH₃ treatment effects

Non-precious iron-nitrogen-functionalized carbon (Fe-N-C) electrocatalysts for the oxygen reduction reaction (ORR) are prepared from ZIF-8 and iron acetylacetonate. The amount of Fe in the precursor mixture is varied to scrutinize the effect of Fe on the structural and catalytic properties of the resulting Fe-N-C. The electrocatalytic activity of the optimized sample in acidic electrolyte is comparable to that of commercial Tanaka Pt/C electrocatalyst. The catalysts were also assembled in a membrane electrode assembly as a cathode electrode for a single cell test, which shows remarkable fuel cell performance compared to other related studies of non-precious metal-based catalysts for the ORR. Such a high ORR catalytic performance can be ascribed to the synergistic effect of the Fe-N-C species, high-density pyridinic N species, N-doped carbon-encapsulated iron carbide (NC-Fe₃C) nanoparticles, and high electrical conductivity as well as the porous framework structure of the as-prepared ZIF-based Fe-N-C catalysts. In addition, different properties of catalysts prepared in a nitrogen or ammonia (NH₃) atmosphere are also investigated. Interestingly, it is found that NH₃ treatment helps not only to improve carbon porosity and nitrogen content but also to increase carbon graphitization and induce NC-Fe₃C formation, leading to enhanced ORR activity and durability of thus-prepared catalysts.