Abstract
PEFC electrocatalysts used for e.g. fuel cell vehicles are subjected to a highly acidic environment and frequent potential fluctuation, which will lead to degradation of electrochemical performance1. Our research group has achieved high durability and high activity by depositing a stable oxide layer on mesoporous carbon (MC) and a Pt-based alloy catalyst on such oxide layer.2．However, start-stop cycle durability remains as an important issue due to oxidative corrosion of the MC support. Here in this study, highly graphitized MC was used as a catalyst support to prepare Pt-based electrocatalysts, and the effect of graphitization of carbon support on the catalytic performance is evaluated.A commercial MC, graphitized at 1800°C having 5-nm-diameter mesopores (MH-18,Toyo Tanso, Japan), was ball-milled and subjected to high-temperature heat treatment to re-graphitize surface defects generated during the milling process. The heat treatment temperatures were 1300°C or 1500°C. After the heat treatment, Pt catalysts were loaded by the acetylacetonate (acac) method3. The dependence on heat treatment temperature was evaluated by microstructural observation using FESEM and STEM, electron diffraction patterns, BET adsorption measurements, and half-cell electrochemical measurements.Particle size observation revealed that the primary particle size became smaller by ball-milling. However, certain agglomeration was observed after the high-temperature heat treatment. Microstructural observation confirmed the retention of mesopores with a diameter of ca. 5 nm and the presence of highly dispersed Pt particles in all these electrocatalysts prepared. But slight agglomeration of Pt catalysts was observed if the support was heat-treated at 1500°C. BET adsorption measurements of specific surface area and pore distribution revealed no significant difference in the microstructure or pore size of the supports before and after milling or high-temperature heat treatment, but a slight decrease in specific surface area and total pore volume was observed after milling. Evaluation of graphitization by electron diffraction pattern revealed that the degree of graphitization decreased due to the milling. However, it was confirmed that re-graphitization occurred after the high-temperature heat treatment.Half-cell electrochemical measurements exhibited the highest Pt electrochemical surface area (ECSA) on the catalyst using the ball-milling-only MC support, but the highest oxygen reduction reaction (ORR) activity was observed on the catalyst using the support heat-treated at 1300°C after the milling. Figure 1 shows that the catalyst with the support heat-treated at 1300°C exhibited the highest load cycle and start-stop cycle durability. This is considered to be due to the effective utilization of the mesopores of the MC and the improvement of the resistance against oxidative corrosion by increasing the graphitization of the MC support. In particular, the decrease in ECSA was suppressed after the heat-treatment at 1300°C and 1500°C in terms of start-stop cycle durability, suggesting that high-temperature heat treatment of MC support after milling is effective in improving start-stop cycle durability.AcknowledgementThis paper is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).References Ohma, K. Shinohara, A. Iiyama, T. Yoshida, and A. Daimaru, ECS Trans., 41 (1), 775 (2011). Sanami, R. Nishiizumi, M. Yasutake, Z. Noda, S. M. Lyth, J. Matsuda, A. Hayashi, and K. Sasaki, ECS Trans., 112 (4), 369 (2023).Hayashi, H. Notsu, K. Kimijima, J. Miyamoto, and I. Yagi, Electrochim. Acta, 53, 6117 (2008). Figure 1
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