Catalyst layer/MEA performance evaluation

Abstract

The membrane electrode assembly (MEA) is a key unit of proton exchange membrane (PEM) fuel cells, including direct methanol fuel cells (DMFCs). In general, the MEA is composed of an anode gas diffusion layer (GDL), an anode catalyst layer, a membrane (the PEM), a cathode catalyst layer, and a cathode gas diffusion layer. The MEA materials, structures, components and fabrication technologies have strong effects on the corresponding fuel cell performance. In particular, the catalyst layers, where the electrochemical reactions take place, are the most important components. Theoretically, in an ideal catalyst layer all catalyst particle sites would be accessible to the reactant gas (H 2 or O2), protons, and electrons. In order to achieve this, the distributions of the electron conductor, proton conductor, catalyst sites, and gas pores should be uniform in a catalyst layer. Over the past several decades, many efforts have been made to develop high performance PEM fuel cells. MEA performance with advanced catalyst layers has been significantly improved by employing different fabrication methods [1-4], changing the catalyst layer structures [5-11], and using different components [5-8]. During PEM fuel cell (PEMFC) performance optimization, how to evaluate catalyst layers and their corresponding MEAs becomes critical. The major purpose of such an evaluation is to understand the relationship between fuel cell performance and MEA component structures/compositions. Based on this understanding, catalyst layer/MEA optimization with respect to performance can be carried out in terms of materials used, component compositions, and fabrication parameters. One example is identifying various cell voltage losses in order to determine whether improvements can be achieved via optimization of the electrode/membrane structures and the diffusion medium properties. Through optimizing the catalyst layers and MEAs, catalyst utilization can be improved, gas diffusion overpotential reduced, and membrane ohmic losses decreased, while water management inside the catalyst layers/MEAs can also be improved. Therefore, catalyst layer/MEA evaluation is a necessary step in fuel cell development. Accordingly, many physical-chemical and electrochemical methods have been developed to evaluate the performance of the catalyst layer/MEA. In this chapter, the principles and methods of catalyst layer/MEA evaluation will be introduced, with some detailed analysis. © 2008 Springer-Verlag.