A partially oxidized TaC0.58N0.42 has been investigated as a new cathode for polymer electrolyte fuel cells. The catalytic activity for oxygen reduction reaction (ORR) significantly depended on an oxidation state of the TaC0.58N0.42. TaC0.58N0.42 and Ta2O5 had a poor catalytic activity for the ORR. The onset potential on the partially oxidized TaC0.58N0.42 for the ORR had a maximum value of 0.9 V versus reversible hydrogen electrode in 0.1moldm3H2SO4 at 30°C, which indicated that the partially oxidized TaC0.58N0.42 had definite catalytic activity for the ORR. Hard X-ray photoelectron spectroscopy and X-ray diffraction analyses revealed that the surfaces of the partially oxidized TaC0.58N0.42 were oxidized, although the inner parts of the partially oxidized TaC0.58N0.42 remained tantalum carbonitride. These results suggested that an appropriate oxidation of the TaC0.58N0.42 was essential to enhance the catalytic activity for the ORR.

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