Effects of tungsten oxide addition on the electrochemical performance of nanoscale tantalum oxide-based electrocatalysts for proton exchange membrane (PEM) fuel cells

In the present study, the properties of non-platinum based nanoscale tantalum oxide/tungsten oxide-carbon composite catalysts were investigated for potential use in catalyzing the oxygen reduction reaction on the cathode side of a PEM fuel cell. All of the tantalum oxide-based catalysts exhibit high ORR on-set potentials, comparable with the commercial Pt/C catalyst even though oxygen reduction current was limited. The tungsten oxide doping to tantalum oxide improved catalytic performance. The performance enhancement was due to a decrease in resistance polarization with increasing tungsten content mainly due to the decrease in resistance polarization. XPS results indicate that the oxidation state of tungsten is +6 and that of the tantalum is +5, suggesting that excess oxygen is generated in the resulting oxide structure. This compositional effect seems to reduce resistance polarization by altering the surface chemistry of the tantalum oxide and enhancing the reaction steps such as surface diffusion. Maximum performance was achieved with a catalyst containing 32 mol% of tungsten oxide, reaching a mass specific current density of ∼7% that of the commercial Pt/C catalyst at 0.6 V vs. NHE and ∼35% at 0.2 V vs. NHE. In term of area-specific current density, five-fold increase in loading of the doped catalyst leads to a 4-4.5 fold increase in area specific current density at 0.6 V vs. NHE, reaching 66% that of the Pt/C catalyst at 100 rpm and 35% at 2400 rpm.