1. Introduction

Very high performance, especially on output power density (6 kW/L), for PEFCs in FCVs is targeted for 2030 in Japan [1]. Therefore, we need to improve not only ORR activity, but also oxygen diffusivity of Pt-based cathode catalysts for PEFCs. In this study, highly interconnected mesoporous carbon (MPC) [2] was selected as a support of Pd@Pt core-shell catalyst. The Pd@Pt core-shell catalyst NPs were supported on different particle size of the MPC and cell performance was investigated.

2. Experimental

We selected CNovel^®MH-18 (central mesopore diameter: 4 nm, S_BET: 1,334 m²/g, TOYO TANSO) as a MPC support. As-received MPC with an average size of ca. 2 μm was pulverized to ca. 800, 450, and 250 nm by a wet bead-milling process. The Pd core NPs were loaded on MPCs by an impregnation-thermal reduction method and the Pt shell was formed on the Pd core NPs by a direct displacement reaction method [3]. Characterizations of the MPC, the Pd/MPC cores and the Pt/Pd/MPC catalysts were conducted by N₂ gas adsorption, TG-DTA, XRD, XRF, TEM, SEM, CV and the ORR activity of the catalysts was evaluated by the RDE method. MEAs with 1×1 cm² were fabricated by a decal method using a reinforced membrane (12 µm in thickness, Gore) and a GDL (TORAY). The Pt metal loading at the cathode was 0.1 mg-Pt/cm². Nafion^® DE2020 was used as an ionomer for the catalyst ink preparation (I/C was set to 0.83). I-V performance was evaluated at 80^oC, 35-95% RH. H₂ gas (418 mL/min.) and air (998 mL/min.) were supplied to the anode and the cathode, respectively, and H₂ gas was pressurized to 150 kPa (ambient + 50 kPa) at gas-outlet.

3. Results and Discussion

N₂ gas adsorption data were analyzed by the t-plot method and the changes in the total, external and internal surface area of the MPCs pulverized for different times are shown in Figure 1. It was shown that the total and the external surface area increased, while the internal surface area decreased with pulverization time. These results suggest that fresh surface was newly formed by the pulverization, and the fraction of the internal surface was decreased. Figure 2 shows cross-sectional TEM images of the Pt/Pd/MPC catalysts using different sizes of MPC supports (ca. 2,000, 800, 450 and 250 nm), which indicates that the catalyst NPs were uniformly supported throughout the interior of MPC regardless of the particle size of MPC. The ECSA of the Pt/Pd/MPC catalysts changed little with the particle size of MPC (ca. 120 m²/g-Pt), while the ORR mass activity at 0.9 V was the highest (1,533 A/g-Pt) when the catalyst NPs were supported on relatively large MPC (800 nm).

I-V curves at 80^oC, 35% RH of the MEAs using the Pt/Pd/MPC cathode catalysts using different sizes of MPC supports are demonstrated in Figure 3. Interestingly, the cell voltage was higher over the entire current density range for Pd@Pt catalysts using MPCs with larger particle sizes (2000 and 800 nm). This tendency was also observed at a high relative humidity of 95% RH. These results suggest that the Pd@Pt catalyst NPs supported on larger MPC (2,000 and 800 nm) contributed to ORR in the high current density range even when the relative humidity was low. Figure 4 compares the connectivity of mesopores in Ketjen Black EC-600JD (KB-600JD, LION) and CNovel^® MH-18 visualized from 3D-TEM observation results. It is clear that the MPC has a higher mesopore’s connectivity than KB-600JD. Figure 5 shows cross-sectional SEM images of the Pt/Pd/MPC. Larger pores were observed inside of the MPC support. Therefore, it is considered that the high connectivity of the mesopores and the presence of larger pores inside of the MPC support contributed to the superior I-V performance observed in MEAs using the catalysts supported on MPCs with particle size of ca. 2,000 and 800 nm. At present, it is not clear why the catalysts supported on smaller MPCs (450 and 250 nm) exhibited inferior I-V performance, though the oxygen diffusion inside MPCs should be easier than that for larger ones.

This study was partly supported by NEDO, Japan. The CNovel^®MH-18 supports with different size were supplied by Dr. Shodai of TOYO TANSO.

References

[1] M. Suzuki, FCCJ, https://www.nedo.go.jp/content/100895111.pdf, June 2019, in Japanese.

[2] Y. Kamitaka et al., Catalysts, 8, 230 (2018).

[3] N. Aoki et al., J. Electrochem. Soc., 167, 044513 (2020).