Development of MEAs by Controlling Carbon Structures in Cathode Layers

Abstract

Introduction Catalyst layers in PEFCs are the most important components affecting their performance. Such catalyst layers are mostly composed by carbon materials. Therefore, optimization of the carbon structure in the catalyst layers is essential. In our study, we have successfully improved PEFC durability by suppressing the aggregation of Pt particles through the encapsulation in nano-channels of mesoporous carbon (MC).1 However, the improvement on the mass transfer, such as diffusion of the fuel gases and the removal of the produced water, still required. Deficient mass transfer is mainly caused by the insufficient structure of carbon materials in the macro scale. Therefore, the purpose of this study is to improve the performance and further durability of PEFCs by controlling carbon structures over the nanometer to micrometer scales. Experimental method s For the MEA preparation, the anode and the cathode were spray-printed using a standard catalyst made by TKK Co., Ltd. (TEC10E50E 46%Pt/KB) and our original MC made in the laboratory, respectively. IV characteristics of MEAs were evaluated while changing the cathode conditions. In order to control the structure over the nanometer to micrometer scales, two separate approaches were tried. As approach 1 to control the structure in the micrometer scale, cathode layers were made by original MC with the addition of carbon nanofibers (CNF). Then, as approach 2 to control the nanometer scale, new MC whose mesopores were expended, was in used. IV performance under the conditions of 80 oC-RH100% was evaluated by supplying 100 cc/min of hydrogen to the anode and 100 cc/min of the air to the cathode. Furthermore, overvoltages were carefully separated into ohmic, activation, and concentration overvoltages. Regarding to the cathode layer structures, the porosity was quantitatively analyzed by measuring the thickness of the cathode layer form cross-section SEM images. Results and discussion As a result form approach 1, the addition of CNF to original MC lead to improvement on IV performance owing to reduced concentration overvoltage. In order to see the structure change in the cathode layer by the addition of CNF, SEM observation was done, and images are shown in Fig. 1. If two images are compared, MC +CNF (Fig. 1(b)) clearly shows the more suitable structure for the mass transfer even from two-dimensional images. Furthermore, based on quantitative analyses of the porosity, the porosity was found to be improved by about 4%. Additionally, activation overvoltage resulted in lower most likely owing to improved conductive path in cathode layer. In the case of approach 2, mesopores of new MC resulted in larger (20~25 nm). IV performance of MEA with this new MC was also improved as a result of reduced concentration overvoltage. Based on the observation of the cathode structure, two factors, enlargement of mesopores and refinement of carbon particles, have most likely influence on PEFC performance. (1) Y. Minamida et al., ECS Trans., vol.64(3), pp.137-144, 2014. Figure 1