PNNL

Abstract

In the present study, the properties of a series of non-platinum based nanoscale tantalum oxide/tungsten oxide-carbon composite catalysts was investigated for potential use in catalyzing the oxygen reduction reaction (ORR) on the cathode side of a PEM fuel cell membrane electrode assembly. Electrochemical performance was measured using a half-cell test set up with a rotating disc electrode and compared with a commercial platinum-on-carbon (Pt/C) catalyst. Overall, all of the oxide-based composite catalysts exhibit high ORR on-set potentials, comparable to that of the baseline Pt/C catalyst. The addition of tungsten oxide as a dopant to tantalum oxide greatly improved mass specific current density. Maximum performance was achieved with a catalyst containing 32 mol% of tungsten oxide, which exhibited a mass specific current density ~8% that of the Pt/C catalyst at 0.6 V vs. the normal hydrogen electrode (NHE) and ~35% that of the Pt/C catalyst at 0.2 V vs. NHE. Results from X-ray photoelectron spectroscopy analysis indicated that the tungsten cations in the composite catalysts exist in the +6 oxidation state, while the tantalum displays an average valence of +5, suggesting that the addition of tungsten likely creates an oxygen excess in the tantalum oxide structure that influences its oxygen absorption kinetics. When the 32mol% tungsten doped catalyst loading on the working electrode was increased to five times that of the original loading (which was equivalent to that of the baseline Pt/C catalyst), the area specific current density improved four fold, achieving an area specific current density ~35% that of the Pt/C catalyst at 0.6 V vs. NHE.