Development of MEAs Using Mesoporous Carbon Fibers as Cathode Carbon Supports

Abstract

Introduction Mesoporous carbon (MC) has recently been paid more attention to as a carbon support for polymer electrolyte fuel cells (PEFCs), which are promising energy technologies for a future decarbonized society, since it is used in TOYOTA MIRAI. We have also been developing MC materials, whose diameter is slightly less than 10 nm, since 2006. More recently, we have developed MC fibers (MCFs) using an electrospinning method. Although MCF can form a sheet, it can be milled to powder. Comparing to our MC, MCF was found to be easily milled to smaller carbon particles. Then, we are interested in a correlation between IV performance and three dimensional structure of cathode layers made of different MC based powder. The aim of this study is to find important parameters to develop the cathode layer to achieve high IV performance. In this study, we synthesized MCF followed by ball-milling to the smaller carbon particles and evaluated the IV performance and structure of MCF based cathode layers. The detailed comparison to MC cathode layers1 was performed. Experimental MCFs were made by electrospinning of the precursor solution containing a surfactant (Pluronic F127) and carbon precursors (phloroglucinol dihydrate, formaldehyde, and triethyl orthoacetate) dissolved in the acid water/ethanol solution, and PVA solution. The obtained MCF precursor sheet was heated in the oven for polymerization of carbon precursors, and then heated up to 8 h by stepwise in the oven under the nitrogen atmosphere in order to decompose Pluronic F127 and carbonized precursors. MCF sheets were milled to small particles by hands and then zirconia balls at 450 rpm and 800 rpm. Pt nanoparticles were deposited on MCF powder by using Pt(acac)2 as a precursor. MEAs with 1cm2 electrodes were fabricated on the Nafion212 membrane. Prepared MCF based catalysts were used for the cathode, and 46.5% Pt/KB (TEC10E50E) was used for the anode. Results and discussion IV performance of MEAs with different cathodes, such as 46.5% Pt/KB (TEC10E50E), 33%Pt/MC, and 32%Pt/MCF, were evaluated and compared. When each overvoltage was evaluated separately, high ohmic overvoltage was seen for MEA with 33%Pt/MC. However, it was reduced for MEA with 32%Pt/MCF. When carbon particles size was compared between the two, MCF was found to be milled into much smaller particles as seen Figure 1. The resulting smaller particles most likely reduced the contact resistance between the particles.Diffusion overvoltage was found to be lowered when 33%Pt/MC and 32%Pt/MCF were used in the cathode. In order to find out a mechanism of the resulting low diffusion overvoltage, the cathode structure was three dimensionally evaluated using FIB-SEM and 3D imaging software. Relatively larger pores were seen in 33%Pt/MC and 32%Pt/MCF cathodes than in 46.5% Pt/KB (TEC10E50E) cathode. Such large pores or/and mesopores might be positively work for mass diffusion, but the mechanism for low diffusion overvoltage is still under the study.