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 ...

Recent studies by Savadogo et al. and Kulesza et al. indicated that WO 3 is an active support for the reduction of oxygen on a Pt/C electrode in phosphoric and sulfuric acid. However, the mechanism involved is not clear. WO 3 is slightly soluble in acidic media. The addition of 10 ppm of dissolved tungsten in sulfuric acid at 70°C results in a 60% increase in oxygen reduction at a current density at 0.6 V vs. standard hydrogen electrode on a Teflon bonded Pt/C electrode (Pt loading is 1.0 mg/cm 2 ). A homogeneous H 2 O 2 decomposition mechanism involving the electrochemical reduction of dissolved peroxytungstate and its subsequent reoxidation by H 2 O 2 is proposed. © 1999 The Electrochemical Society. S1099-0062(99)03-042-4. All rights reserved.Manuscript received March 9, 1999. Available electronically May 20, 1999 In recent years, there has been considerable interest in the possible synergistic effect of Pt/WO 3 for the reduction of oxygen in (i) phosphoric acid by Savadogo et al., 1,2 and (ii) sulfuric acid by Kulesza et al. 3 Savadogo et al. prepared Pt/WO 3 /C by two methods. Method 1 involved the addition of different quantities of tungstic acid to a Pt/C mixture. Method 2 involved mechanically mixing WO 3 powder with a Pt/C catalyst. The Pt electrochemical surface area of the electrodes was measured by hydrogen stripping and results for the electrodes containing WO 3 were significantly higher. Savadogo et al. assumed that the Pt surface area had been increased, 2 which led to higher oxygen reduction activity. They did not consider that hydrogen tungsten bronze 4 will be formed during potentiostatic cycling, leading to a very much higher anodic current for the subsequent hydrogen oxidation step. 4 It is inconceivable that the Pt particles in the Pt/C mixture will decrease in size on the addition of tungsten trioxide powder. Therefore their proposed mechanism involving the increase in Pt area on addition of tungsten trioxide powder 2 could not be substantiated. Kulesza et al. prepared Pt/WO 3 film on a glassy carbon by cycling the electrode potentials between 0.8 and -0.4 V vs. saturated calomel electrode in WO 3 •2H 2 O solution. They attributed the enhanced performance to higher activity for peroxide reduction. This is not likely to be the reason for the enhancement of activity since the reduction of peroxide can take place According to the proposed reaction scheme, the HO 2 -intermediate formed during oxygen reduction on Co 3 O 4 /graphite electrode is homogeneously decomposed by HCoO 2 -, which in turn is oxidized back to cobalt oxides. As a result, the concentration of HCoO 2 -in solution is always kept to a very low level. It has been pointed out that the transfer reaction between HCoO 2 -in solution and Co 3+ in the oxide lattice sites is a heterogeneous process. Therefore, the hydrogen peroxide decomposition process follows a homogeneous/heterogeneous mechanism involving the dissolution of Co 3 O 4 , followed by reoxidation of the HCoO 2 -by HO 2 -to form cobalt oxide. The high catalytic activity o...

There is an increasing demand for portable electrochemical CO 2 sensors for agricultural, environmental, medical, and biological applications. Present electrochemical sensors are relatively cumbersome and require frequent changes of electrolyte. A novel method based on the reduction of CO 2 on Pt/WO 3 electrode, followed by the anodic oxidation of adsorbed CO in 0.5 M H 2 SO 4 gave a very good linear correlation between peak CO oxidation current and CO 2 concentration (0-4% CO 2 , R 2 = 0.9928; 4-20% CO 2 , R 2 = 0.9082).Portable sensors for CO 2 monitoring are increasingly required by agriculture, environmental, medical, and biological applications. Two main types of electrochemical sensors have been developed for the above applications. The first type is a coulometric sensor 1 based on the measurement of the reduction in pH value of the 1 M KCl/1 M BaCl 2 electrolyte, when the CO 2 in the gas sample reacts with OH -. After measurement, pulse coulometry is used to restore the pH value. For longer lifetimes, the electrolyte must be replaced. The detection range is limited by the maximum change in pH value (0.02-2% of CO 2 ). The second type is based on amperometric measurement. 2,3 The amperometric method is more complicated since it is necessary to remove the oxygen in the gas sample. Dimethylsulfoxide (DMSO) is used as the electrolyte to ensure that no hydrogen evolution takes place during measurement of CO 2 reduction current. However, there is a need to replace the DMSO electrolyte frequently since it adsorbs moisture. Such a sensor can measure CO 2 concentration over a wide rage (0-100%).Theoretical and Practical Considerations The above sensors are relatively complicated and there is a need to change the electrolyte frequently which is inconvenient. Therefore, a sensor which can measure CO 2 in air over a wide range of CO 2 concentration using invariant electrolyte will be a great improvement. If a stream of air containing CO 2 is first reduced on a suitable electrocatalyst to produce adsorbed CO and then subsequently oxidized at anodic potentials, it may be possible to measure the CO 2 concentration on the same electrode. When CO 2 is reduced on Pt surface, adsorbed CO is the main product. 4 Subsequent anodic oxidation of the adsorbed CO may give a correlation between CO 2 concentration and the peak CO oxidation current. However, since it is well known that Pt is strongly poisoned by adsorbed CO, 5,7 very high anodic potential is required to oxidize the adsorbed CO, leading to Pt oxide formation. Subsequent reduction of the Pt oxide at lower potential leads to changes in Pt area, affecting reproducibility of results. Therefore, it is necessary to use catalysts which can oxidize CO at lower overpotentials. Work by Tseung et al. [5][6][7][8][9][10][11] showed that coelectrodeposited Pt/WO 3 and Pt-Ru/WO 3 are bifunctional anode catalysts, enabling CO and other reaction intermediates to be readily oxidized at much lower overpotentials and these catalysts are relatively immune to poisoning. Experimental All el...

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