1. Introduction

In Japan, an extremely high I-V performance is required for polymer electrolyte fuel cells (PEFCs) used in fuel cell electric vehicles after 2030 (red line in Figure 1) [1]. Therefore, we have to design Pt-based catalysts that have a high ORR activity and a durability as well as a high oxygen diffusivity [2]. In this study, the Pt-based catalysts were supported on a mesoporous carbon to increase oxygen diffusivity and the catalysts were decorated with melamine to improve ORR activity and durability [3, 4].

2. Experimental

Ketjen Black EC-300J (KB-300J: S_BET: 922 m²/g, LION), Ketjen Black EC-600JD (KB-600JD: S_BET: 1,345 m²/g, LION) and a mesoporous carbon (CNovel MH-18, S_BET 1,334 m²/g, TOYO TANSO [5]) were used as carbon supports in the synthesis of PtCo alloy and Pd@Pt core-shell catalysts. A commercially available Pt/KB-300J catalyst (TEC10E50E, Pt size: 2.5 nm, Pt metal loading: 46.3 wt.%, TKK) was used as a reference catalyst. The PtCo alloy catalyst was synthesized by an oleylamine reduction method [6] and the Pd@Pt core-shell catalyst was synthesized by a direct displacement reaction method [7]. The metal loadings of Pt and Co in the alloy catalysts were 41 wt.% and 3 wt.%, respectively, and those of Pt and Pd in the Pd@Pt core-shell catalyst were 20 wt.% and 30 wt.%, respectively. The particle sizes of the PtCo alloy and the Pd@Pt core-shell catalyst NPs were 3.0 nm and 5.5 nm, respectively. CV of the catalyst was recorded at 25^oC in Ar-saturated 0.1 M HClO₄ with a potential scan rate of 50 mV/s and LSV was measured by RDE method at 25^oC in O₂-saturated 0.1 M HClO₄ with a potential scan rate of 10 mV/s at a rotation speed of 1,600 rpm. An accelerated durability test (ADT) of the catalysts was conducted by a square wave potential cycling of 0.6 V (3 s)-1.0 V (3 s) vs. RHE performed in Ar-saturated 0.1 M HClO₄ at 80^oC for 10,000 cycles with and without melamine addition (10 µM to the electrolyte). MEA was fabricated by a decal method using a reinforced membrane (12 µm in thickness, Gore) and a GDL (TORAY). Nafion^® DE2020 was used as an ionomer for the catalyst ink preparation (I/C: 0.83). I-V performance of a single cell was evaluated at 80^oC, 75% RH by using the MEA with an active area of 1 cm² in which the Pt loading in the cathode catalyst layer was set to 0.1 mg-PGM/cm². H₂ gas (418 NmL) and air (998 NmL) were supplied to the anode and the cathode, respectively, and the H₂ gas was pressurized to 150 kPa at the gas-outlet (H₂ utilization: 5% at a current density of 3.0 A/cm²).

3. Results and Discussion

Pore size distribution of the carbon supports is summarized in Figure 2. KB-300J has no clear mesopores, while KB-600JD and CNovel MH-18 have the mesopores. Figure 1 shows I-V performance of Pd@Pt core-shell catalysts, indicating that Pt/Pd/CNovel MH-18 catalyst exhibits a superior cell performance to that of Pt/Pd/KB-600JD catalyst. Connectivity of mesopores in KB-600JD and CNovel MH-18 supports analyzed by 3D-TEM is shown in Figure 3. It is clear that the connectivity in CNovel MH-18 support is much higher than that in KB-600JD support, which is considered to contribute to the superior I-V performance of Pt/Pd/CNovel MH-18 catalyst observed in Figure 1.

Figures 4 depicts morphological change of PtCo/CNovel MH-18 alloy catalyst by ADT performed at 80^oC for 10,000 cycles. A very small amount of melamine addition to the electrolyte (10 µM) notably suppressed the catalyst NPs agglomeration in ADT, which significantly mitigated ECSA decay of the catalysts. More importantly, Co dealloying of the PtCo alloy catalyst in ADT was remarkably suppressed by the melamine addition, and thus the alloy catalyst retained a high ORR mass activity even after ADT. Therefore, we believe that the target cell performance as well as the high durability of the Pt-based catalysts can be realized by combination of the mesoporous carbon support and the decoration with melamine or melamine derivatives.

This study was partly supported by NEDO, Japan.

References

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

[2] V. Yarlagadda et al., ACS Energy Lett., 3, 618 (2018).

[3] M. Asahi et al., J. Electrochem. Soc., 166, F498 (2019).

[4] S. Yamazaki et al., ACS Catal., 10, 14567 (2020).

[5] H. Orikasa, T. Morishita, Tanso, 254, 153 (2012). In Japanese.

[6] J. Li et al., J. Am. Chem. Soc., 142, 19209 (2020).

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