Influence of 3d transition-metal substitution on the oxygen reduction reaction electrocatalysis of ternary nitrides in acid

The development of non-precious, acid-stable, oxygen reduction reaction (ORR) electrocatalysts can significantly aid the commercialization of proton exchange membrane fuel cells (PEMFCs). We report a survey of the ORR electrocatalysis on 3d metal substituted (M = Mn, Fe, Co) molybdenum and tungsten nitrides in acidic environments. We find that molybdate catalysts are more active than tungstates, with the specific activity depending on the chemistry of the substituted 3d metal. In both families, more electronegative 3d metals led to higher ORR activity (i.e., Co > Fe > Mn). We attribute this result to the ability of the more electronegative 3d metal to withdraw electrons from the Mo- or W-based active sites, effectively oxidizing the metal centers of the catalysts. Based on our observation, future nitride ORR electrocatalysts can be further optimized by oxidizing the Mo sites further by, for example, adding even more electronegative dopant metals or incorporating anion vacancies.