Synthesis of Mesoporous Carbon Support Using Two Different Sizes of SiO2 Hard Templates
Authors: Kim Minyoung , Inoue Hideo , Daimon Hideo , Doi Takayuki , Inaba Minoru 
Published: December 2024

Introduction

After 2030, extremely high cell performance is required for PEFCs used in fuel cell electric vehicles (FCEVs) in Japan [1]. For FCEVs, cost reduction is an urgent issue, and it is necessary to reduce the amount of Pt catalyst used in PEFCs by enhancing oxygen reduction reaction (ORR) activity. On the other hand, in order to improve cell performance, not only the ORR activity enhancement, but also suppression of ionomer poisoning and increase in oxygen diffusivity are required [2]. Therefore, development of a new carbon support is necessary for the latter. In this study, a mesoporous carbon (MPC) support was synthesized by using two different sizes of SiO2 hard templates. Porosity and morphology of the MPC support were characterized by N2 gas adsorption, SEM and TEM analyses and electrochemical properties of Pt/MPC catalyst were investigated.

Experimental

MPC support was synthesized by a SiO2 hard template method using resorcinol and formaldehyde as carbon sources [3]. In a mixture of 20 mL H2O and 80 mL EtOH, 150 mg of hexadecyltrimethylammonium bromide was added as a surfactant. The solution was stirred at 25℃ for 0.5 h. Subsequently, two kinds of SiO2 hard templates (5 nm: 600 mg and 50 nm: 300 mg) and 15 mg of 3-aminopropyltriethoxysilane as a silane coupling agent were added simultaneously to the solution and the solution was further stirred at 50℃ for 2.5 h. Then 650 mg resorcinol, 910 μL formaldehyde and 600 μL NH3 aq. (28%) were added to the solution, which was stirred at 25℃ for 24 h to cause polymerization reaction. After stirring, the solution was transferred to a 300 mL eggplant flask and the solvent was removed using an evaporator at 30 hPa and 30℃ for 3 h. The brown precipitate was dried in an oven at 60°C in air, and heat-treated at 850℃ for 3 h in Ar atmosphere. The obtained SiO2/MPC was dispersed in a HF solution (10%) to remove the SiO2 hard templates.The MPC support (300 mg) obtained was re-dispersed in a mixed solvent (37.5 mL EtOH/250 mL H2O) and Pt(NO2)2(NH3)2corresponding to 300 mg of metallic Pt was added. The solution was refluxed at about 90°C in N2 gas atmosphere for 4 h [4]. The resulting Pt/MPC catalyst was filtered, washed and dried at 60°C in air. The SiO2/MPC, MPC support and Pt/MPC catalyst were analyzed by N2 gas adsorption, TG-DTA, XRD, SEM, TEM and TEM-EDX. CV was recorded in Ar-saturated 0.1 M HClO4 at 25℃ at a scan rate of 50 mV/s and ORR activity was evaluated by LSV performed in O2-saturated 0.1 M HClO4 at 25℃ at a positive potential scan rate of 10 mV/s and at 1600 rpm.

Results and Discussion

TEM image and TEM-EDX compositional analysis of the SiO2/MPC are shown in Fig. 1. From the SEM images of Figure 2 and Figure 3, the primary particle size of MPC was about 60 nm and the secondary particle size was about 8 µm. Although presence of the 5-nm SiO2hard templates was not observed, the 50-nm ones were coexisted with carbon. Figure 2 depicts SEM and TEM images of the SiO2/MPC before and after HF treatment. The SiO2 templates were removed by the HF treatment and macropores about 50 nm in size were formed. Figure 4 demonstrates pore size distribution of MPC support after the HF treatment. The MPC had peak pore diameters of approximately 4 and 60 nm, reflecting the particle sizes of the SiO2 templates. The specific surface area of the SiO2/MPC was increased from 350 to 1170 m2/g after the HF treatment. The cross-sectional TEM images of Pt/MPC catalyst are shown in Fig. 5, indicating that Pt NPs are uniformly deposited throughout the MPC support. The Scherrer diameter of the Pt NPs was 2.6 nm. Electrochemical measurements revealed that electrochemical surface area and ORR mass activity were 84 m2/g and 350 A/g at 0.9 V vs. RHE, respectively. The low ORR mass activity of the Pt/MPC catalyst (350 A/g) can be attributed to the large secondary particle size of the MPC of 8 μm. At the conference, we will present the electrochemical properties and a single cell performance of the Pt/MPC catalyst synthesized by using pulverized MPC support.
This study was partly supported by NEDO, Japan.

Reference

[1] M. Suzuki, https://www.nedo.go.jp/content/100895111.pdf, in Japanese.
[2] V. Yarlagadda et al., ACS Energy Lett., 3, 618 (2018).
[3] S. Li et al., Mater. Sci.-Poland, 37, 585 (2019).
[4] M. M. Rahman et al., RSC Adv., 11, 20601 (2021).

Figure 1