Development of Mesoporous Carbon Fibers for PEFC Catalyst Supports

Abstract

Introduction PEFCs have been known as an energy technology with high efficiency and low carbon emission. Among the components of PEFCs, the electrocatalyst layer is an important component to determine PEFC performance and mainly made by noble metal particles dispersed on carbon supports. In order to increase the efficiency and durability of electrocatalysts, our group has been focusing on developing novel carbon supports, and we have previously demonstrated the improvement of durability by utilizing mesoporous carbon (MC) with 6-8 nm pore size [1]. Even though the nanostructure of electrocatalysts has been successfully controlled, further control of the microstructure of electrocatalyst layers is required to improve efficiency of PEFC. In this study, we are trying to control the microstructure though making our MC into fibers with an electrospinning technique. Experimental MC fibers (MCFs) were synthesized by electrospinning the solution containing the resol-formaldehyde type resin polymer and Pluronic F127, after adding polyvinyl alcohol (PVA) as a polymer additive. Then, Pluronic F127 was removed by heat treatments to generate mesopores. MCFs were characterized by N2 sorption in order to analyze their mesoporosity. To make comparison on mechanical strength and conductivity among Vulcan® carbon black, MC, and MCF, FIB-SEM observation and electro-impedance measurements were performed, respectively, after spray printing carbon dispersions on the substrate. Electrochemical analyses were performed by both half-cell measurements in the solution and full-cell measurements with membrane-electrode assemblies (MEAs), after successful Pt deposition into mesopores of MCF. Results and discussion The morphology of MCF is shown in Figure 1, and both the fiber structure and mesopores were confirmed. The N2 adsorption measurement indicated the pore size of MCF is around 4-6 nm, which is slightly smaller than the pore size of MC. Regarding to the mechanical strength, MCF showed smaller change in the thickness after compression in comparison to the regular MC and Vulcan® carbon black. This probably comes from higher elasticity provided by the fiber structure. Moreover, according to the conductivity measurements, MCF showed higher conductivity than MC and Vulcan® carbon black. The higher conductivity is probably due to the fact that the fiber-fiber contact provides a better conductive path than the particle-particle contact. From electrochemical analyses through half-cell measurements, Pt deposited MCF catalyst (Pt/MCF) showed similar ECSA and ORR activity to Pt deposited MC catalyst (Pt/MC). However, based on better mechanical strength and conductivity of MCF, advantages on full-cell measurements with MEAs are expected. Details of preparation and evaluation of MEAs will be discussed.