Polymer membranes for electrolytes

Supported Protic Ionic Liquids in Polymer Membranes for Electrolytes of Nonhumidified Fuel Cells... 

S.-P. S. Yen, S. R. Narayanan, G. Halpert, E. Graham, and A. Yavrouian. Polymer material for electrolytic membranes in fuel cells. US Patent 5 795 496, assigned to California Institute of Technology (Pasadena, CA), August 18, 1998. 

PEMFGs use a proton-conducting polymer membrane as electrolyte. The membrane is squeezed between two porous electrodes [catalyst layers (CLs)]. The electrodes consist of a network of carbon-supported catalyst for the electron transport (soHd matrix), partly filled with ionomer for the proton transport. This network, together with the reactants, forms a three-phase boundary where the reaction takes place. The unit of anode catalyst layer (ACL), membrane, and cathode catalyst layer (CCL) is called the membrane-electrode assembly (MEA). The MEA is sandwiched between porous, electrically conductive GDLs, typically made of carbon doth or carbon paper. The GDL provides a good lateral delivery of the reactants to the CL and removal of products towards the channel of the flow plates, which form the outer layers of a single cell. Single cells are connected in series to form a fuel-cell stack. The anode flow plate with structured channels is on one side and the cathode flow plate with structured channels is on the other side. This so-called bipolar plate... 

HT-PEFCs and PEFCs mainly differ in the nature of the polymer membrane and electrolyte. In an HT-PEFC, a combination of PBf, phosphoric acid, and water is used, whereas in a PEFC, Nafion and water are employed. Although phosphoric acid has been used for a longer time in PAFCs, many aspects of this substance remain unknown. The most important facts are summarized below to give an overview of this fairly complex matter which is discussed in greater detail in a separate chapter of this book [47]. 

High-T mp rature Polymer Electrolyte Fuel Cells, Table 1 Physicochmiical properties of different HT polymer membranes for fuel cells... 

HT-PEM fuel cells operate with phosphoric acid doped polymer membrane as electrolyte. The acid is physically adsorbed to the membrane. The phosphoric acid distribution within the fuel cell components, such as membrane, catalyst layers, microporous layer, gas diffusion layers, and bipolar plates, is known to be a critical parameter for performance and life time of this type of fuel cells [10]. There are no defined specifications about phosphoric acid uptake of the bipolar plate because its impact on the fuel cell performance strongly depends on several parameters and always has to be considered in a context of the overall fuel cell design. 

By the time the next overview of electrical properties of polymers was published (Blythe 1979), besides a detailed treatment of dielectric properties it included a chapter on conduction, both ionic and electronic. To take ionic conduction first, ion-exchange membranes as separation tools for electrolytes go back a long way historically, to the beginning of the twentieth century a polymeric membrane semipermeable to ions was first used in 1950 for the desalination of water (Jusa and McRae 1950). This kind of membrane is surveyed in detail by Strathmann (1994). Much more recently, highly developed polymeric membranes began to be used as electrolytes for experimental rechargeable batteries and, with particular success, for fuel cells. This important use is further discussed in Chapter 11. 

A completely separate family of conducting polymers is based on ionic conduction polymers of this kind (Section 11.3.1.2) are used to make solid electrolyte membranes for advanced batteries and some kinds of fuel cell. 

State-of-the-art thin film Li" cells comprise carbon-based anodes (non-graphitic or graphite), solid polymer electrolytes (such as those formed by solvent-free membranes, for example, polyethylene oxide, PEO, and a lithium salt like LiPFe or LiCFsSOs), and metal oxide based cathodes, in particular mixed or doped oxides... 

Gubler, L., S. A. Giirsel, and G. G. Scherer, Radiation-grafted membranes for polymer electrolyte fuel cells. Journal Fuel Cells, August 2005. 

Alkaline solutions are generally known to lead to better catal5Tic activities than acidic solutions for many relevant electrode reactions. However, owing to the paucity in the development of suitable electrolyte materials, such as alkaline membranes, there has been much less fundamental work in the area of fuel cell catalysis in alkaline media. Nevertheless, there are a few hopeful developments in new alkaline polymer membranes [Varcoe and Slade, 2005] that are currently stirring up interest in smdying fuel cell catalytic reactions in alkalme solution. 

Shen M, Roy S, Kuhlmann JW, Scott K, Lovell K, Horsfall JA. 2004. Grafted polymer electrolyte membrane for direct methanol fuel cells. J Memb Sci 251 121-130. 

Most automotive fuel cells use a thin, fluorocarbon-based polymer to separate the electrodes. This is the proton exchange membrane (PEM) that gives this type of fuel cell its name. The polymer provides the electrolyte for charge transport as well as the physical barrier to block the mixing... 

Okada, T., Xie, G. and Meeg, M. 1998. Simulation for water management in membranes for polymer electrolyte fuel cells. Electrochimica Acta 43 2141-2155. 

Wieser, C. 2004. Novel polymer electrolyte membranes for automotive applications—Requirements and benefits. Fuel Cells 4 245-250. 

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