Performance enhancement and degradation mechanism identification of a single-atom Co-N-C catalyst for proton exchange membrane fuel cells

The development of catalysts free of platinum-group metals and with both a high activity and durability for the oxygen reduction reaction in proton exchange membrane fuel cells is a grand challenge. Here we report an atomically dispersed Co and N co-doped carbon (Co-N-C) catalyst with a high catalytic oxygen reduction reaction activity comparable to that of a similarly synthesized Fe-N-C catalyst but with a four-time enhanced durability. The Co-N-C catalyst achieved a current density of 0.022 A cm(-2) at 0.9 ViR-free (internal resistance-compensated voltage) and peak power density of 0.64 W cm(-2) in 1.0 bar H-2/O-2 fuel cells, higher than that of non-iron platinum-group-metal-free catalysts reported in the literature. Importantly, we identified two main degradation mechanisms for metal (M)-N-C catalysts: catalyst oxidation by radicals and active-site demetallation. The enhanced durability of Co-N-C relative to Fe-N-C is attributed to the lower activity of Co ions for Fenton reactions that produce radicals from the main oxygen reduction reaction by-product, H2O2, and the significantly enhanced resistance to demetallation of Co-N-C.