RESUMEN
Most nonplatinum group metal (non-PGM) catalysts for polymer electrolyte fuel cell cathodes have so far been limited to iron(cobalt)/nitrogen/carbon [Fe(Co)/N/C] composites owing to their high activity in both half-cell and single-cell cathode processes. Group IV and V metal oxides, another class of non-PGM catalysts, are stable in acidic media; however, their activities have been mostly evaluated for half-cells, with no single-cell performances comparable to those of Fe/N/C composites reported to date. Herein, we report successful syntheses of zirconium oxynitride catalysts on multiwalled carbon nanotubes, which show the highest oxygen reduction reaction activity among oxide-based catalysts. The single-cell performance of these catalysts reached 10 mA cm-2 at 0.9 V, being comparable to that of state-of-the-art Fe/N/C catalysts. This new record opens up a new pathway for reaching the year 2020 target set by the U.S. Department of Energy, that is, 44 mA cm-2 at 0.9 V.
RESUMEN
Titanium oxides crystals are widely used in a variety of fields, but little has been reported on the functionalities of noncrystalline intermediates formed in their structural transformation. We measured the oxygen reduction reaction activity of titanium oxide nanoparticles heat-treated for a different time and found that the activity abruptly increased at a certain time of the treatment. We analyzed their structures by using X-ray pair distribution functions with the help of high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy and ascertained that the abrupt increase in the activity corresponded to a structural transformation from a reduced lepidocrocite-type layered titanate to a disordered structure consisting of domains of brookite-like TiO6 octahedral linkages. The further treatment transformed these brookite-like domains into another phase having more edge-sharing sites like the TiO-type cubic structure. This finding would position noncrystalline, disordered structure as a possible origin of the catalytic activity, though nanocrystalline rutile particles might be also considered as the origin.