Designing the Cathode in MEA Using Mesoporous Carbon Fibers

Abstract

Introduction PEFCs have been known as one of environment-friendly power sources and been used for both mobile and stationary applications. However, the high cost, which mostly comes from the fabrication of MEAs made by carbon, Pt and polymer electrolytes, has been a great wall on the way of wide commercialization of PEFCs. For electrocatalysts, which mostly determine PEFC performance, the better performance and high durability with low noble metal usage are highly desired. In our research group, mesoporous carbon (MC) materials have been studied as catalyst supports, and encapsulation of Pt into controlled nm-space has led to improvement in durability [1]. However, we have found a problem in the disordered μm-structure. Therefore, we are now focusing on developing the fiberized MC, mesoporous carbon fibers (MCFs), where both the nm- and μm-structures are controllable. In our previous study, fundamental electrochemical properties of MCFs powder catalysts were evaluated with half-cell measurements in solution [2]. In this study, MCF sheets were newly used as catalyst supports. MEAs with Pt deposited MCF sheet cathode and related cathodes were made and electrochemically evaluated . Experimental MCFs with mesoporous diameter of 6-7 nm were synthesized according to our previous study [2], which was based on combining the self-assembly of the surfactant-carbon precursor and an electrospinning deposition process. MCFs sheets were cut into 1 x 1 cm2, but a part of them were further ball-milled into the μm-scale powder. After the Pt deposition on MCFs described in our previous study [1], three different cathodes, Pt deposited milled MCFs, TKK Pt/KB (TEC10E50E), and with TKK Pt/KB were prepared by keeping the total Pt loading as 0.3 mg/cm2. MCF sheet with TKK Pt/KB was built by spray printing a Pt/KB layer on the Nafion212 first and then on the MCFs sheet, which was placed on the first layer of Pt/KB. Results and discussion A cross-sectional image of MCFs sheet was taken using a FIB fabrication method and shown in Fig.1. The thickness of a single layer of MCFs was 1.0 to 1.3 μm. In order to keep the total weight of carbon close to Pt/milled MCF cathode, two layers of MCFs were used as a sheet in this study. IV performance, corresponding power density, and ohmic overvoltage of MEAs with different cathodes are shown in Fig. 2. The current density at 0.8 V in IV curves was However, lower ohmic overvoltage was observed in MEA made by MCFs sheet with TKK Pt/KB. The obtained resistance was about 70 mΩ, which was 20 mΩ less and 30 mΩ less comparing to MEA with Pt/KB and MEA with Pt/milled MCFs, respectively. We have found that the usage of MCF sheets leads to thin electrodes, resulting in reduction in ohmic resistance. In order to further improve the activation and concentration overvoltage, Pt deposited MCFs sheet with Pt/KB cathode is under the study and will be reported.