PEFCs Using Metallic Ti and Sn Electrocatalyst Supports

Abstract

Introduction Currently, Pt-based catalyst on carbon black support (Pt/C) is widely used as the electrocatalyst for polymer electrolyte fuel cells (PEFCs). Carbon black support has high electronic conductivity and high specific surface area for supporting highly-dispersed Pt catalyst nanoparticles. However, under cathode condition of PEFCs e.g. during start-stop cycles, carbon black support can be electrochemically oxidized, causing detachment and aggregation of Pt catalyst nanoparticles (1). In order to solve this issue, we are developing SnO2 and TiO2 supports, which are relatively stable even under cathode conditions, coated on carbon-based materials as the conductive backbone, exhibiting high durability (2, 3). However, as far as we use carbon materials, carbon corrosion problem may still be remained. Here in this study, porous metallic Ti and Sn sheets are applied as catalyst support. Such electrodes may exhibit high durability under PEFC cathode conditions due to the formation of stable TiO2 and SnO2 layers on the outermost surface of Ti and Sn metals. Furthermore, metallic sheets with porous structures also have a potential to act as gas diffusion layers (GDL). This study aims to prepare such electrodes integrating electrocatalysts with high conductivity and high durability. Experimental For preparing metallic Ti supported electrodes, porous Ti sintered sheets were used. First, the porous Ti sintered sheets were chemically etched in NaOHaq to increase their surface area. The etched Ti sheets were then heat-treated at 400 °C in 5%H2-N2. Pt catalyst nanoparticles were then decorated on the Ti sheets by the arc plasma deposition (APD) or the acac method (4). We also prepared Nb-doped Ti sheets to improve their electronic conductivity. For preparing metallic Sn supported electrodes, cellulose-based filter paper was used as a template. First, SnO2 sol was impregnated into the filter papers in vacuum, followed by heat treatment in oxidizing atmosphere to remove the cellulose-based templates. After that, porous metallic Sn sheets were obtained by reducing heat-treatment. The Pt catalyst nanoparticles were decorated on the porous Sn sheets by the acac method (4). Microstructures of the porous Pt/Ti and Pt/Sn sheets were observed by FE-SEM and STEM-EDS. Their electrochemical activities were measured in a half-cell setup in HClO4 electrolyte. Results and Discussion Figure 1 shows an FE-SEM image of the porous Ti sheet after the surface treatment by NaOHaq. The surface of the Ti sheets after the surface treatment has needle-like TiO2 nanostructures with a diameter of several nm, with high surface area. Figure 2 shows the linear sweep voltammograms by the half-cell tests for the Pt/Ti sheet electrodes. The Pt/Ti sheet electrode prepared by the acac method may have higher contact resistance than the Pt/Ti sheet electrode by the APD method. However, such contact resistance may be reduced by Nb-doping the Ti sheets. Figure 3 shows an FE-SEM image of the prepared Sn sheet for catalyst support. We could prepare metallic Sn sheets with porous structures derived from the filter paper template. However, optimization of heat-treatment conditions is further required. It is because the reduction of SnO2 is difficult if the heat-treatment temperature is too low, but Sn will be melted if the temperature is too high. Acknowledgement Financial support from New Energy and Industrial Technology Development Organization (NEDO) is gratefully acknowledged (Contract No. 20001214-0). References L. M. Roen, C. H. Pail, and T. D. Jarvi, Electrochem. Solid-State Lett., 7 (1), A19 (2004). S. Matsumoto, M. Nagamine, Z. Noda, J. Matsuda, S. M. Lyth, A. Hayashi, and K. Sasaki, J. Electrochem. Soc., 165 (14), 1165 (2018). Y. Nakazato, D. Kawachino, Z. Noda, J. Matsuda, A. Hayashi, and K. Sasaki, ECS Trans., 80 (8), 897 (2017).A. Hayashi, H. Notsu, K. Kijima, J. Miyamoto, and I. Yagi, Electrochim. Acta, 53 (21), 6117 (2008). Figure 1