PEM materials

First, we describe what the PEM material is, and where to get it. Then we cover the steps necessary for preparing the membrane to use it in a fuel cell. 

David Booth, Alternative Energy Engineering, Redway, CA PEM Materials... 

Proton exchange membranes (PEMs) are a key component in PEM fuel cells (PEMECs) and an area of active research in commercial, government, and academic institutions. In this chapter, the review of PEM materials is divided into two sections. The first will cover the most important properties of a membrane in order for it to perform adequately within a PEMFC. The latter part of this chapter will then provide an overview of existing PEM materials from both academic and industrial research facilities. Wherever possible, the membranes will also be discussed with respect to known structure-property relationships. 

In this section, highlights from the large array of existing PEM materials will be presented, with examples from both academic and industrial research laboratories. The section is divided based on the chemical structure of fhe PEM ... 

Other high performance polymer backbones have been explored as PEM materials in addition to poly-(arylene ether)s and polyimides. Ductile copolymers with high modulus and glass transition values are desirable PEM candidates. The hydrolytic and oxidative stability of many of these materials remains to be determined. Nevertheless, interesting synthetic methodologies have been employed to investigate these materials, which have been instructive in the search for new PEM candidates. 

Even more important is the fact that most PEM materials (see below) are also quite permeable for water and methanol. Thus, thin membranes lead to substantial transport of these molecules from the anode side to the cathode (e. g., [25-29]). The permeation of methanol in the DMFC is undesirable for the obvious reason that it reduces the cell power ( mixed potential formation ), because no electrical work is generated in a cathodic oxidation reaction. Furthermore methanol on the cathode is unfavorable because it can block adsorption sites needed for the oxygen reduction reaction. The presence of methanol may even alter the rate constant of the oxygen reduction reaction. A typical solution to the problem of methanol transport is the use of dilute aqueous solutions of methanol, which assures almost complete oxidation when the anodic catalyst loading is high enough [30],... 

In the following sections, the key properties of PEM materials used in fuel cells are briefly reviewed and theoretical and simulation results on proton transport are discussed. 

Thus, by changing pH and solvent, it is possible to achieve controllable loading of low-molecular substances into PEMs. Materials obtained within such method can be used in biosensor control and pharmaceutics. 

Superior mechanical, thermal, and fllm-forming ability of the soluble polyaramids made them attractive material for membrane-based applications [29]. Incorporation of specific groups into the PA backbone leads to high-performance polymeric materials that can be used as gas separation, PV, and PEM materials. 

In the light of the above discussion, soluble and tractable fluorinated polymers have been synthesized, and they have shown their potential in gas separation, in PV application, and as PEM materials. However, there seems to be room for further research in synthesizing new tailored fluorinated PAs and exploring them for various industrial applications. 

One research area of particular interest is new proton-conducting solid polymer electrolyte membrane (PEM) materials possessing the desired properties, namely, (1) high proton conductanee at high temperature (up to 120°C), (2) effectively no co-transport of molecular species with proton, (3) reduction of electrode overpotential, and (4) good mechanical strength and chemical stability. 

Kreuer etal. [21] provided an in-depth review of the basic mechanisms of transport in proton conductors. Transport of the proton can occur by two mechanisms structural diffusion and vehicular diSusion. Vehicular diffusion is the classical Einstein diffusive motion. The structural diffusion is associated with hopping of the proton along water molecules (the so-called Grotthuss mechanism). In the nanosized confined hydrophihc spaces within the membrane, both mechanisms are operative. What is important here is that the underlying mechanism of transport in PEMs changes as a function the level of hydration. Understanding the nature of these mechanisms and their dependence on the level of hydration and molecular structure is important in the development of advanced PEM materials that are more tolerant of higher temperatures and lower levels of saturation. 

Nafion produced by Dupont is the most commonly used PEM material. Nation is a hydrated perfluorosulfonic acid (PFSA) pol5uner. The Nafion backbone is hydrophobic, while side-chain sulfonic acid groups are hydrophilic. The repeat unit of Nafion is given as structure (2). 

Proton exchange membranes (PEMs) are one of the key materials in low-temperature fuel cells proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMECs). Especially, recent trend in the research and development of low-temperamre fuel cells focuses on PEMFCs for transportation (electric vehicle) applications due to the impact on economy and environment. The most important role of PEMs is to transport protons formed as a product of oxidation reaction of fuels at the anode to the cathode, where oxygen reduction reaction takes place to produce water. In addition to this, there are a number of requirements for PEM materials for the practical fuel cell applications, which include... 

The CL should keep the phase equilibrium with other PEM materials [13], The CLs applied onto both sides of the PEM are highly intercoimected with the membrane by their content of proton-conducting PFSA ionomer. The typical MEA structures show strains of PFSA ionomer running through the CLs, connecting catalyst particles to the membrane on the ionic conductor level. These strains form an ionic connection to the membrane not only for mobile protons, but also for all species which can enter the pores within these materials. In the normal case, the phase equilibrium at the interface between the CLs and the PEM material is always assumed to be established. 

Kreuer et al. [66] first proposed heterocycles such as imidazole and pyrazole as proton solvents. The unprotonated and protonated nitrogen atoms, instead of water, are expected to act as fluctuating acceptors and donors in proton transfer. Imidazole at the melting point of 90 °C has long been known to possess proton conductivity as high as 10 S cm [67] via the mechanism of the intermolecular proton transfer [68] and has received much attention in the recent exploration of novel PEM materials [69-72]. Other basic solvents include pyrazole [64, 73], triazole [74, 75], and tetrazoles [76] as shown in Fig. 3.5, where melting points and p Ta values of their protonated forms are also listed. 

Figure 19.4a shows examples for thermogra-vimetric analysis (TGA) analysis for state-of-the-art HT-PEM plates of type BPP4 and low temperature LT-PEM materials as a reference. Recently developed HT-PEM materials with high temperature resistant thermoplastic binder polymers (PPS and PSU Ultrason ) are shown in Fig. 19.4b. 

Fig. 19.4 Thermogravimetric analysis of state-of-the-art HT-PEM material BPP4 compared to LT-PEM bipolar plate (a) and the advanced HT-PEM bipolar plates based...

What all of the work described so far has in common is that ratio of polycation to polyanion did not vary significantly in the investigated materials. In PEM materials this ratio cannot be tuned, because in multilayer formation the polyelectrolyte stoichiometry is controlled by self-assembly. The work of Michaels on PEC was devoted to 1 1 complexes, which are almost completely intrinsically charge-compensated. In the following sections we will review recent systematic studies on conductivity spectra of PEC, where the composition was varied. We will present the influences of PEC composition, type of alkali ions present in the PEC, temperature and RH on the dynamic conductivity of solid PEC. Based on the presented analysis, we will be able to arrive at further conclusions about how the microscopic ion dynamics in PEC can be visualized and how it is influenced by different parameters. With the help... 

Ex situ ADT methods, for example by using half-cells or rotating disk electrodes (RDEs), have also been developed to study materials aging mechanisms. " A typical example of an ex situ ADT is holding the electrode potential at high values (e.g. 1.4 V) in order to study carbon degradation. Based on the experimental data obtained under these conditions, it is currently concluded that state-of-the-art Pt, carbon and PEM materials will never reach durabihty requirements and that alternative materials are necessary for future CL materials. " " These conclusions merit reconsideration, as potentials higher than 1.2 V (the zero-... 

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