Oxygen Reduction on Teflon Bonded Pt/WO[sub 3]/C Electrode in Sulfuric Acid

Abstract

Highly dispersed Pt/WO 3 /C was prepared by freeze-drying followed by hydrogen reduction. The catalyst particle size was examined by transmission electron microscopy and X-ray diffraction line broadening technique as well as electrochemical Pt surface area measurement by H 2 and CO stripping. Oxygen reduction on Teflon bonded Pt/WO 3 /C electrode was investigated under O 2 and air in 0.5 M H 2 SO 4 at 25 and 70°C. Pt/WO 3 /C was more active for oxygen reduction in sulfuric acid than Pt/C catalyst. The dissolved tungsten compounds ͑peroxy-tungstate͒ from the WO 3 in the electrode participate in the oxygen reduction as a homogeneous catalyst for the decomposition of H 2 O 2 , leading to significantly higher activity of the Pt/WO 3 /C for oxygen reduction. By coating the electrode with a thin layer Nafion dispersion, continual loss of WO 3 by dissolution and diffusion to the bulk electrolyte is avoided.There is a need to improve the performance of oxygen electrode in acid media for fuel cell applications. 1,2 Pt is the most favored catalyst for this because most of the other metals are corroded in acid media. Attempts to improve the performance of the Pt catalyst have involved alloying it with transition metals 3-12 and mixing it with acid resistant oxides such as WO 3 . 13-16 Although all of these efforts resulted in improved performance, the mechanism of the improvement is not clear. Furthermore, it has been observed that such alloys are invariably corroded, releasing transition metal ions into the electrolyte. 6-11 Recent work in our group has confirmed the pivotal role of homogeneous catalysis 17,18 in the improvement of oxygen reduction on Pt in acid media, via the formation of transition metal-peroxy compounds and their subsequent decomposition at Pt surface. Typically, dissolved W or Fe ͑10-100 ppm͒ 17,18 in sulfuric acid increased the current density of oxygen reduction by 70% at 0.6 V vs. SHE, 70°C. However, the long-term performance is not satisfactory because the dissolved ions will be plated out at the counter electrode. It is necessary therefore to consider the provision of a replenishment pathway, together with a means of preventing the diffusion of the dissolved ions to the counter electrode. WO 3 has been widely used as a component in anode [19][20][21][22] and cathode 6-11 catalysts in acid media. However, it is slightly soluble in acidic media. 23 It would be useful to consider WO 3 as a reservoir for the supply of dissolved tungsten species and to use Nafion coating to prevent the diffusion of dissolved W species to the counter electrode. In order to see the effect of WO 3 , it is necessary to compare the performance of Pt/C and Pt/WO 3 /C electrode, taking into account the Pt loading and specific surface area of the Pt in the two different electrodes. ExperimentalPreparation of Pt/WO 3 /C catalyst.-Pt/WO 3 supported catalyst was prepared by freeze-drying. 24 Vulcan XC-72 carbon ͑Cabot͒ support was treated in a stream of N 2 at 900°C for 3 h to remove any surface contaminant. After treatment, the ...

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Highly Dispersed and Nano-sized Pt-based Electrocatalysts for Low-Temperature Fuel Cells

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