Anodic Activity of Pt and PtRu Catalysts on Ta-Doped SnO2 Supports for Direct-Fuel Fuel Cells Using a New Energy Carrier

Abstract

Polymer electrolyte fuel cells (PEFCs) directly fueled liquid fuels are suitable power source for portable energy applications. Liquid alcohol fuels such as methanol and ethanol are attractive energy carriers with good energy density and low cost. However, the problems of their low flash points (below room temperature) and/or toxicity should be overcome for the safe application of alcohol-fueled PEFCs. The liquid oligomers of poly oxymethylene dimethyl ether (POMMn: CH3-O-(CH2-O)n-CH3, n = 3~8) have been proposed as alternative fuels, due to their advantages, including their higher energy density, lower toxicity and higher flash point1-3). In this study, we synthesized Pt and PtRu catalysts supported on Ta-SnO2 (Pt/Ta-SnO2, PtRu/Ta-SnO2) for the direct oxidation of POMM and evaluated their performance in a PEFC. The Ta-SnO2 support4) was synthesized by the flame spray synthesis method. The oxide supports obtained were nanometer-sized particles with a carbon-like fused-aggregate structure. Pt and PtRu catalysts were loaded on the Ta-SnO2 by the colloidal method. The Pt/Ta-SnO2 (Pt loading amount, 14.0 wt%; particle size, 6.3 nm) and PtRu/Ta-SnO2 (Pt/Ru loading amount, 6.3/3.5 wt%; particle size, 2.2 nm) were obtained after heat treatment. The Pt and PtRu particles were highly dispersed on the Ta-SnO2 support. As shown in the high-angle annular dark-field STEM (HAADF-STEM) image and STEM-EDX line profile, Sn metal diffused into the Pt catalysts with the sintering process. X-ray photoemission spectroscopic (XPS) Sn 3d5/2 spectra for each catalyst indicated that part of the Sn was converted from Sn4+ to Sn0. We concluded that the catalysts formed binary Pt-Sn and ternary Pt-Ru-Sn alloys during the sintering procedure (Fig. 1). The onset potential for POMM2 oxidation on PtRu/Ta-SnO2, which was measured by a half-cell with a rotating disk electrode (RDE), was 0.3 V less positive than that of a commercial Pt2Ru3/CB catalyst (Fig. 2). The mass activity for POMM2 oxidation of PtRu/Ta-SnO2 at 0.5 V was also 3.5 times higher than that for a commercial Pt2Ru3/CB catalyst. The anodic activity of Pt/Ta-SnO2 was also higher than that of the commercial catalyst, by a factor of 2. The electrochemical performances of these anode catalysts were also evaluated by the use of membrane-electrode assemblies (MEAs, NRE117 membrane, commercial Pt/CB(TEC10E50E) 0.5 ± 0.025 mgPt cm2 cathode, Pt/Ta-SnO2 or PtRu/Ta-SnO2 0.25 or 0.27 mgPt cm2 anode, Fig. 3). The current densities of the MEAs using the Pt/Ta-SnO2 and PtRu/Ta-SnO2 catalysts as the anodes and Pt/CB as the cathode (POMM2 simulated fuel, O2 oxidant, 80oC operation temperature) were more than 10 times higher than that using the commercial Pt2Ru3/CB anode. The current density at 0.6 V for formaldehyde fuel supplied to an MEA applying the Pt/Ta-SnO2anode was 15 times higher than that for the commercial anode, even though the performance for methanol remained at the same level. We consider that the Sn on the surface of the Pt catalyst may suppress CO poisoning by weakening the adsorption strength of CO formed during the fuel oxidation processes, even at such low potentials as 0.2 to 0.4 V, and directly accelerate the oxidation of carbon monoxide at more positive potentials by a bifunctional mechanism, similar to that for Pt-Ru catalysts. Acknowledgements This work was partially supported by funds for the A-STEP project from the Japan Science and Technology Agency (JST), JSPS KAKENHI (B24350093) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), and the SPer-FC project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References 1) D. Devaux, H. Yano, H. Uchida, J.L. Dubois, M. Watanabe, Electrochim. Acta 56 (2011) 1460. 2) S. Baranton, H. Uchida, D.A. Tryk, J.L. Dubois, M. Watanabe, Electrochim. Acta 108 (2013) 350. 3) K. Kakinuma, I.T. Kim, Y. Senoo, H. Yano, M. Watanabe, and M. Uchida, ACS Appl. Mater. Interfaces, 6 (2014) 22138. 4) Y. Senoo, K. Taniguchi, K. Kakinuma, M. Uchida, H. Uchida, S. Deki, and M. Watanabe, Electrochem. Commun., 51 (2015) 37. Figure 1