Tuning the thermal activation atmosphere breaks the activity-stability trade-off of Fe-N-C oxygen reduction fuel cell catalysts

Fe-N-C catalysts are the most promising platinum group metal-free oxygen-reduction catalysts, but they suffer from a low density of active metal sites and the so-called activity-stability trade-off. Here we report an Fe-N-C catalyst prepared by adding an optimal amount of H2 to the traditional inert atmosphere during the thermal activation. The presence of H2 significantly increases the total density of FeN4 sites, suppressing the unstable pyrrolic-N-coordinated S1 sites and favouring the stable pyridinic-N-coordinated S2 sites with shortened Fe-N bond lengths. We propose that the intrinsically stable S2 sites are probably arranged in well-graphitized carbon layers, and the S1 sites exist in less-graphitized carbon. H2 could remove unstable S1 sites and retain stable S2 sites during the pyrolysis to break the challenging activity-stability trade-off. The Fe-N-C catalyst in membrane electrode assemblies maintains a current density of 67 mA cm-2 at 0.8 V (H2-air) after 30,000 voltage cycles (0.60 to 0.95 V under H2-air), achieving encouraging durability and performance simultaneously. Fe-N-C catalysts are a promising alternative to precious metals in fuel cell cathodes, but they suffer from durability issues. Now, a preparation method is reported that allows increasing the active site density while also improving durability.