PEM Fuel Cell Catalyst Layers and MEAs

Zhang, H., Wang, X., Zhang, J., and Zhang, J. Conventional catalyst ink, catalyst layer, and MEA preparation. In PEM fuel cell electrocatalysts and catalyst layers Fundamentals and applications, ed. J. Zhang. London Springer, 2008. 

Figure 2.1 shows a schematic structure of the fuel cell membrane electrode assembly (MEA), including both anode and cathode sides. Each side includes a catalyst layer and a gas diffusion layer. Between the two sides is a proton exchange membrane (PEM) conducting protons from the anode to the cathode. 

Basically, there are two methods to form the MEA of a PEM fuel cell. One alternative is using appropriate techniques to add the carbon-supported catalyst to a porous and conductive material, such as carbon cloth or carbon paper, called a gas diffusion layer (GDL). Normally, polytetrafluoroethylene (PTFE) and Nafion... 

The membrane electrode assembly (MEA) in a proton exchange membrane (PEM) fuel cell has been identified as the key component that is probably most affected by the contamination process [1]. An MEA consists of anode and cathode catalyst layers (CLs), gas diffusion layers (GDLs), as well as a proton exchange membrane, among which the CLs present the most important challenges due to their complexity and heterogeneity. The CL is several micrometers thick and either covers the surface of the carbon base layer of the GDL or is coated on the surface of the membrane. The CL consists of (1) an ionic conductor (ionomer) to provide a passage for proton transport ... 

FIGURE 3.9 XPS of the C Is spectra of MEAs constructed with Nafion membrane and Pt/C catalyst before and after 300 h of operation. (Reprinted from Electrochimica Acta, 54, Zhang, F. Y. et al. Quantitative characterization of catalyst layer degradation in PEM fuel cells by x-ray photoelectron spectroscopy, 4025-4030, Copyright (2009), with permission from Elsevier.)... 

To produce a structure capable of performing the above-mentioned electrochemical task, an anode and cathode, each containing catalyst particles, are adhered to opposite sides of the PEM to form a layered composite structure. This composite structure is responsible for the electrochemical conversions and directed flow of fuel, byproducts, ions, electrolytes and electrons requisite for the electrochemical functioning of a fuel cell. This layered composite structure is also referred to as a membrane electrode assembly (MEA). 

The MEA consists of an anodic electrode, PEM, and a cathodic electrode. Because the electrode reactions take place in the MEA, it is the heart of a H2/air PEM fuel cell. The components of an MEA include the anode gas diffusion medium (A-GDM), anode microporous layer (A-MPL), anode catalyst layer (A-CL), PEM, cathode catalyst layer (C-CL), cathode microporous layer (C-MPL), and cathode gas diffusion medium (C-GDM), as shown in Fig. 2.1. A commonly used term is gas diffusion layer (GDL), which actuaUy contains the gas-diffusion-medium layer and the microporous layer. Each component shown in Fig. 2.1 has specific characteristics and functions in fuel cell operation and performance. Therefore, they differ significantly in their design and fabrication. These topics wUl be addressed in the following sections. 

The CL is where the electrochemical reactions occur, which makes it another key component inside the MEA of PEM fuel cells. The CL is a uniform layer with a thickness of 10-100 pm (usually <50 pm), composed of electrocatalyst powders, proton-conducting ionomer (e.g. Nafion ), and/or binder (e.g. PTPE). Almost all the important challenges in PEM fuel ceU development, such as high cost and low durability, arise from the CLs because they are complex, heterogeneous, contain expensive Pt-based catalysts, and have low stability. The reactions in PEM fuel cells have three phases, involving the reactant gases (e.g. H2 or O2), proton conductive ionomer (e.g. Nafion ), and electron conductor (e.g. carbon-supported Pt catalyst). Therefore, when designing a CL, it is desirable to extend and maximize the three-phase reaction zone to optimize fuel cell performance. 

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