Introduction In Japan, extremely high cell performance is required for polymer electrolyte fuel cells (PEFCs) used in fuel cell electric vehicles after 2030 [1]. Therefore, it is of great importance to improve not only ORR activity, but also oxygen diffusivity of Pt and Pt-based cathode catalysts in PEFCs. In this study, a spherical mesoporous carbon (SMPC) with central mesopore’s diameter of 7 nm was synthesized and used for support of the Pt NP catalyst. Electrochemical properties of the Pt/SMPC catalyst were investigated. Experimental The SMPC support was synthesized by silica template method in which SiO2 precursor was in-situ formed [2]. 7 mL of tetrapropyl orthosilicate was added in a mixed solution (6 mL conc. NH3, 150 mL EtOH and 10 mL H2O) and stirred for 15 min. Then, 0.8 g resorcinol and 1.12 mL formaldehyde (37 wt.% aq.) were added and stirred at room temperature for 24 h. The precipitate was filtered and dried at 60°C overnight in air. The powder was heat-treated in a tube furnace at 850°C for 3 h in Ar atmosphere to obtain SiO2/SMPC. The SiO2/SMPC was dispersed in 10% HF aq. and stirred at room temperature for 2 h to remove silica template. The Pt/SMPC catalyst was synthesized by alcohol reduction method [3]. 300 mg SMPC was dispersed in a EtOH/H2O mixed solution (37.5 mL/250 mL), and Pt(NO2)2(NH3)2 (corresponding to 300 mg metallic Pt) dissolved in aqueous HNO3 was added, followed by refluxing at 90°C for 4 h in N2 atmosphere. The catalyst was filtered, washed and dried at 60°C overnight in air. Characterization of SMPC and Pt/SMPC was conducted by N2 gas adsorption, TG-DTA, XRD, TEM and TEM-EDX. The catalyst was loaded on GC-RDE. CV was recorded in Ar-saturated 0.1 M HClO4 at 25oC and at 50 mV/s. The ORR activity was evaluated by LSV performed in O2-saturated 0.1 M HClO4 at 25oC at 10 mV/s in the positive direction while rotating the GC-RDE at 1,600 rpm. Results and Discussion The weight percentage of silica template was 70% in SiO2/SMPC. Figure 1 shows SEM and TEM images of SiO2/SMPC before and after HF treatment. The SEM images clearly show that SiO2/SMPC and SMPC have spherical shape with mean diameter of 200 nm. In TEM images of SiO2/SMPC, center of the sphere is darker than edge and the edge became darker than the center after HF treatment, suggesting that the silica template NPs resided at the center of the sphere. TEM-EDX composition analysis of SiO2/SMPC is demonstrated in Fig. 2. Before HF treatment, Si was detected at the center of the sphere. After HF treatment, only C was detected with high intensities at the edge of the sphere. These results suggest that the SiO2/SMPC has a silica rich core-carbon rich shell structure. The thickness of the carbon rich shell was ca. 60 nm. The surface area of SiO2/SMPC was 510 m2/g and the area increased to 1,590 m2/g after HF treatment. Figure 3 shows pore size distribution of SiO2/SMPC. After HF treatment, SPMC support exhibited clear existence of mesopores with a central diameter of 7 nm.A cross-sectional TEM image of Pt/SMPC catalyst is depicted in Fig. 4. Pt NPs with Scherrer’s diameter of 2.3 nm were well dispersed over the carbon-rich shell, which indicates that mesopores in the carbon shell were well interconnected. The ECSA and the ORR activity of the Pt/SMPC catalyst were 92 m2/g and 450 A/g@0.9 V, respectively, the latter of which was similar to that of a commercially available Pt/C (TKK, TEC10E50E). Single cell tests using Pt/SMPC is now underway, and the results will be presented at the meeting.This study was partly supported by NEDO, Japan. Reference [1] M. Suzuki, https://www.nedo.go.jp/content/100888556.pdf (in Japanese).[2] Z. Deng et al., Nano Lett., 22, 9551 (2022).[3] M. M. Rahman et al., RSC Adv., 11, 20601 (2021). Figure 1
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Sep 27, 2024
Hiroki Doi + 2
Open Access