Fe loading of a carbon-supported Fe–N electrocatalyst and its effect on the oxygen reduction reaction

Abstract

Carbon-supported non-noble metal catalysts with Fe as the metal and tripyridyl triazine (TPTZ) as the ligand (Fe-TPTZ/C) were synthesized using a simple chemical method. How the Fe loading in this Fe-TPTZ/C catalyst affected the ORR activity was investigated using several Fe loadings: 0.2, 0.4, 0.7, 2.7, 4.7, 5.8 and 7.8 wt%. The as-prepared catalysts were then heat-treated at 800 °C in an N 2 environment to obtain catalysts of Fe–N/C. Energy dispersive X-ray spectroscopy (EDX) was used to identify the Fe–N/C catalysts. These Fe–N/C catalysts showed significant ORR activity improvement over the as-prepared Fe-TPTZ/C catalysts. The kinetics of the ORR catalyzed by the catalysts with different Fe loadings was studied using the rotating disk electrode technique. It was observed that a 4.7 wt% Fe loading yielded the best catalytic ORR activity. Regarding the overall ORR electron transfer number, it was found that as the catalyst's Fe loading increased, the overall ORR electron transfer number changed from 2.9 to 3.9, suggesting that increasing the Fe loading could alter the ORR mechanism from a 2-electron to a 4-electron transfer dominated process. The Tafel method was also used to obtain one important kinetic parameter: the exchange current density. A fuel cell was assembled using a membrane electrode assembly with 4.7 wt% Fe loaded Fe–N/C as the cathode catalyst, and the cell was tested for both performance and durability, yielding a 1000-h lifetime.