Operando detection of oxygen reduction reaction kinetics of Fe-N-C catalysts in proton exchange membrane fuel cells

Low-cost, high-performances and durable hydrogen fuel cells are crucial for the success of the hydrogen economy. While Fe-N-C structures containing Fe-Nx active sites are amongst the most promising platinum (Pt)-group metal (PGM)-free catalysts for the oxygen reduction reaction, their highest performances-to-date are still inferior to commercial Pt in real proton exchange membrane fuel cells. Herein, we shed light on this performance gap by using the distribution of relaxation times to quantify the proton transport and oxygen reduction reaction kinetics of a high-performance Fe-N-C catalyst (1.08 W cm(-2)) and a commercial Pt catalyst (1.7 W cm(-2)) in hydrogen fuel cell. This study unveils that the Fe-N-C catalyst has slower proton transport and oxygen reduction reaction kinetics than Pt as the Fe-N-C nanoporous carbon matrix limits active site accessibility. Furthermore, while increasing the Fe-N-C catalytic mass loading (from 1 to 3 mg(Fe-N-C) cm(-2)) enhances the power density in hydrogen fuel cells, it also slows down proton transport and oxygen reduction reaction kinetics by lengthening the gas, electron, and proton pathways to the active sites. This finding will drive the development of PGM-free catalysts for hydrogen fuel cells and of single-atom catalysts for electrochemical applications.