Ti-Porous-Sheet-Supported Pt Electrocatalysts for PEFCs

Abstract

Introduction Currently, Pt electrocatalysts supported on carbon black (Pt/C) are typically used in polymer electrolyte fuel cells (PEFCs). Carbon black has high conductivity for conducting electrons and high specific surface area for supporting Pt catalysts. However，during the PEFC start-stop cycles, the potential of the cathode can reach up to 1.5 V, resulting in a severe carbon corrosion (1,2) . In addition, in the fuel starvation, carbon corrosion may also occur at the anode. To solve this carbon support corrosion problem, we have developed SnO2 and TiO2 supports dispersed on carbon nanofibers acting as the conductive backbone, exhibiting high durability (3-5) . However, also in this case, the carbon material is still used and the possibility of carbon corrosion may remain. Metallic materials generally have higher conductivity, in comparison with carbon materials and oxide materials. Here, we selected titanium as catalyst supports. The surface of metallic titanium is naturally oxidized to form stable TiO2, so that titanium supports may exhibit high durability under the strongly-acid PEFC condition. In addition, if the porous Ti sheet also acts as the gas diffusion layer (GDL), membrane electrode assembly (MEA) can be prepared just by the heater pressing. In this study, we prepare a carbon-free all-metallicelectrode structure, consisting of Pt electrocatalyst, Ti-based catalyst support, and Ti-based GDL for improving PEFC electrodes with both high activity and durability by using the porous titanium sheet support. Experimental The porous Ti sheet was etched with 5M NaOH solution at 60°C for 1 hour. After that, heat treatment was performed at 500°C in air for 5 hours, or at 400°C in 5%H2-N2 for 30 minutes. Pt nanoparticles were decorated on the porous Ti sheet by the arc plasma deposition (APD). The microstructure of the Pt/Ti sheet was observed by SEM，STEM, and TEM. Electrochemical measurements were made with the half cell and the single cell. Results and discussion Figure 1 shows the micrograph of the porous Ti sheet. The surface area of the porous Ti sheet increased by the NaOH treatment. The amount of Pt on the porous Ti sheet was controlled by varying the number of charge/discharge cycles of the APD process. Figure 2 shows the STEM image of the Pt/Ti sheet after 10 charge/discharge cycles of the APD, exhibiting high dispersion of Pt particles of 2.4 nm in average diameter on the porous Ti sheet. Figure 3 shows electrochemical surface area (ECSA) after different numbers of APD charge/discharge cycles which were measured by the half cell test. The ECSA of APD 10 and 25 charge/discharge cycles was considerably high approaching 100 to 150 m2g-1. These results indicate a considerable reduction the Pt loading by applying the APD. During the APD process, the surface oxide film may be etched and Pt particles were supported directly on the metallic titanium. Reference [1] L. M. Roen, C. H. Pail, and T. D. Jarvi, Electrochem. Solid-State Lett., 7(1) , A19 (2004) . [2] C. A. Resiser, L. Bregoli, T. W. Patterson, J. S. Yi, J. D. Yang, N. L. Perry, and T. D. Jarvi, Electrochem. Solid-State Lett., 8(6) , A273 (2005) . [3] A. Masao, S. Noda, F. Takasaki, K. Ito, and K. Sasaki, Electrochem. Solid-State Lett., 12 (9) , B119 (2009) . [4] M. Iwami, D. Horiguchi, Z. Noda, A. Hayashi, and K. Sasaki, ECS Trans., 69(17) , 603 (2015) . [5] Y. Nakazato, M. Iwami, M. Okumura, Z. Noda, A.Hayashi, and K. Sasaki, ECS Trans., 75 (14) , 841 (2016) . Figure 1