Nanoscale analysis of structural and chemical changes in aged hybrid Pt/NbOx/C fuel cell catalysts

We characterize the structural and chemical changes that take place in an electrochemically tested proton-exchange fuel cell cathode material composed of platinum nanoparticles on a niobium oxide-carbon black hybrid support. Two hybrid catalysts with different niobium oxide content (5 wt% and 12 wt%) are compared at the beginning and end of potential cycling. We observe an overall increase in the particle size of the hybrid catalysts after potential cycling, mediated by Ostwald ripening process. The general nanostructure of the catalysts was composed of small Pt-rich particles that were linked to niobium oxide particles. Nanoscale and microscale spectroscopy of the pristine materials reveals several co-existing oxidized forms of niobium (5+, 4+, 2+) in the systems; the most predominant being Nb(V). The study of the energy loss near-edge structure of the Niobium L2,3 edge of catalysts after being subjected to accelerated stress test (AST) potential cycles provides clues on the evolution of niobium oxides (NbOx), in which the relative distribution of Nb(V) decreases, while the number of Nb particles in lower oxidation states slightly increases. Furthermore, energy-dispersive spectroscopy reveals that the content of Nb decreased after cycling, implying that the loss of NbOx eventually altered the fraction of linked Pt-NbOx sites. The observed nanoscale catalyst changes and the presence of the NbOx may have important implications for developing an alternative design for improved hybrid catalyst materials.