Development of Mesoporous Carbon Fibers As Catalyst Supports for PEFC
Authors: Morinaka Yuma , Hutapea Yasir Arafat , Hayashi Akari 
Published: December 2024

Introduction

Polymer electrolyte fuel cells (PEFCs) have being considered for adaptation to heavy duty vehicles, and the optimization of catalyst layers is essential for further improvement of IV performance. Recently, mesoporous carbon (MC) supports have been used in second-generation MIRAI and have attracted much attention owing to several advantages, such as reducing the direct contact between Pt particles and Nafion ionomer and suppressing Pt particle growth. Our laboratory is currently developing mesoporous carbon fibers (MCF), which are fibers containing mesopores. For the purpose of controlling both nano- and macrostructures, MCF sheet electrodes were investigated. In this study, the effect of structures of MCF sheet on IV performance was investigated by developing membrane electrode assemblies (MEAs) with MCF sheet electrodes.

Experimental

MCF sheet was synthesized by an electrospinning method. The precursor solution was prepared by mixing and stirring Pluronic F127, phloroglucinol dehydrate, formaldehyde, and trimethyl orthoacetate in acidic water/ethanol, followed by addition of polyvinyl alcohol (PVA) according to our previous study1. Fiber sheet was prepared by electrospinning the precursor solution and by heating in the air to complete polymerization of precursors. Then, the sheet was cut into 2 cm x 2 cm pieces. Those pieces were sandwiched between two pieces of quartz plate and heat treated stepwise under a nitrogen atmosphere in order to finally obtain MCF sheet electrodes. Material characterization of MCF sheet electrodes was done by nitrogen adsorption measurements and SEM observations. For applying the cathode of MEAs, MCF sheet electrodes were first cut into a 1 cm x 1 cm piece. Pt was deposited by using Pt(acac)2 as a precursor, and then the Pt loading process was repeated until the required amount of Pt was loaded. Then, Nafion was introduced by spraying Nafion solution.
To fabricate a MEA, first, 0.3 mgPt/cm2 of 46.3% Pt/KB (TEC10E50E) was spray printed on Nafion 212 membrane within the area of 1 cm x 1 cm as the anode side. On the cathode side of the Nafion membrane, 0.03 mgPt/cm2, 1/10 of the amount used in the anode, was spray printed as a contact layer. Here, the contact layer was used to suppress the contact resistance between the Nafion film and the MCF sheet. The Pt/MCF sheet was then placed on the cathode side and hot pressed together with the GDL to complete our cathode. Resulting MEA was evaluated electrochemically.

Results and Discussion

MCF sheet electrodes with different pore diameters were prepared by varying the amounts of ethanol, pure water, and PVA in the precursor solution and named as A and B. In comparison to MCF sheet A, MCF sheet B was made under slightly lower concentration of precursors. An SEM image of MCF sheet B is shown in Figure 1 showing the fiber structure. For the characterization of pore structures, nitrogen sorption measurements were performed. MCF sheet B showed a shift toward smaller pore size distribution and a decrease in pore volume as compared to MCF sheet A.
MEAs were fabricated using prepared Pt/MCF sheet electrodes as cathodes, and their IV performance was evaluated. In the low current density region, IV performance was improved when MCF sheet B was used. This result suggests that structures of MCF sheet influence on IV performance.

References

[1] Ting-Wei Huang et. al., Molecules, Vol. 26(3), pp.30 (2021).

Figure 1