Improvement of Mesoporous Carbon Supports for PEFC

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Introduction Polymer electrolyte fuel cells (PEFCs) are one of the energy technologies to realize a carbon neutral society, and further improvement of IV performance is essential in order to expand the application into heavy-duty vehicles. Catalyst layers with three dimensional porous structures have an important role since they mostly determine the IV performance. To make such three dimensional structures, carbon supports are important. An attention to mesoporous carbon (MC) supports has been increased since MC is used in 2nd generation MIRAI. Our group have also been working on MC materials, MC bulk1 and MC fibers (MCF) 2. Based on our previous studies, we have found that the size of carbon particles affects the IV performance of PEFC as well as the structure of mesopores. In this study, we have successfully synthesized new MC that is composed of bigger mesopores and smaller particles than conventional MC bulk we developed. Effect of this difference in MC materials was evaluated in terms of IV performance by developing MEAs made by two MC materials. Experimental New MC was similarly synthesized to our previous study2 with modification. Pluronic F127 was used as the template to build up mesopores in addition to carbon precursors being composed of phloroglucinol dihydrate, formaldehyde, and ethyl orthoacetate. After well mixing of the template and precursors in acidic water/ethanol solvent, PVA solution was added and stirred. The heat treatments for polymerization of precursors in the air and then for decomposing the template and carbonizing carbon precursors in N2 were performed. Resulting new MC was ball-milled into small particles and used for catalyst supports. Pt deposition was done in two ways. In the conventional way1, Pt(acac)2 was used as a Pt precursor and thermally reduced. On the other hand, in the new method3, Pt(NO2)2(NH3)2 with alcohol reduction was used. Then, membrane electrode assemblies (MEAs) were prepared by spray printing 0.3 mg Pt/cm² using TEC10E50E and Pt/MCs for the anode and cathode, respectively, within 1 cm² area of Nafion 212 membrane. Results and Discussion N2 sorption measurements were done for new MC. Resulting N2 sorption isotherms and corresponding pore size distributions were shown in Fig. 1 (a) and (b), respectively, with those of MCs previously developed in our group. As shown in Fig. 1, new MC have bigger mesopores, and the pore size distribution is wider than other MC supports. The specific surface area of mesopores and external results in 350 cm2/g, which is almost equivalent to MC bulk. This result suggests that new MC also has a capability of Pt deposition. Regarding to Pt deposition, two methods were used as described in Experimental. Slightly smaller Pt particles were found in the case of the alcohol reduction method. MEAs using Pt/MC cathode synthesized in different conditions were prepared and evaluated. The best IV performance was obtained with 45% Pt/new MC cathode made by the alcohol reduction method. In comparison to 30.6% Pt/MC bulk, ohmic overvoltage was reduced, and the increase of IV performance was found especially at high current density. At the high current density region, IV performance was even better than that of MEA with TEC10E50E cathode. In order to investigate which factors have contributed to improvement of IV performance, electrocatalysts were prepared by varying %Pt, Pt deposition methods, and the pore structures of MCs, and then evaluated in details. References A. Hayashi et al., Electrochim. Acta, 53(21), 6117 (2008).W. Huang et al., Molecules, 26(3), 30 (2021).M. Rahman et al., RSC Adv., 11, 20601 (2021). Figure 1