Fuel cell membranes proton transport

The polymer electrolytes used for low-temperature proton exchange membrane fuel cells (PEMFCs) are fundamentally different from the polymer electrolytes used in batteries. Here, the polymer is a medium for a solvent, normally water, and it is mainly in the solvent that ion transport occurs. The polymer serves several functions, of which the most important is to provide mechanical stability and electrode separation in the fuel cell application. Since the fuel cell needs proton transport from the anode to the cathode, the polymer also contains proton donating groups, often sul-phonic acid (-SO3H). The prototype PEMFC membrane materials have been perfluorosulphonic acids (PFSAs), of which the most established membrane material is Nafion (Fig. 8.8). These consist of hydrophobic teflon -CF2-CF2- backbones, with fluorinated hydrophilic and acidic side-chains for Nafion -0CF2CF(CF3)0CF2Cp2S03H. 

Ford Motor Company. (1997). Direct Ilydrogcn-Fuclcd Proton Exchange Membrane Fuel Cell System for Transportation Applications Hydrogen Vehicle... 

Ferrel, J., Kotar, A. and Stern, S. (1996). Direct Hydrogen Fuelled Proton Exchange Membrane (PEM) Fuel Cell System for Transportation Applications. Final report, Section 3 Hydrogen Infrastructure Report. Prepared for the Ford Motor Company and the Department of Energy. 

Ford Motor Co., Direct-hydrogen-fueled proton-exchange membrane fuel cell system for transportation applications hydrogen vehicle safety report, D.T. Inc., ed., Arlington,VA (1997). 

Fig. 13.25. Preferential oxidation and re-former-shifter test bench experiment. (Reprinted with permission from Research and Development of Proton-Exchange-Membrane (PEM) Fuel Cell System for Transportation Applications, Phase I, Final Report, prepared for the U.S. Dept, of Energy by General Motors, 1996, Fig. 2.2.3.1.)...

In a H2/air fuel cell, the protons produced at the anode side need to be transferred to the cathode side to react with 02. This requires a proton transport electrolyte. Nafion membranes, composed of a perfluorosulfonated polymer, are the most commonly used polymer electrolyte membranes to conduct protons. The structure of the Nafion membrane is shown in Figure 1.5. Nafion can take on a... 

PROTON TRANSPORT IN POLYMER ELECTROLYTE FUEL CELL MEMBRANES... 

Proton transport in polymer electrolyte fuel cell membranes... 

The end use defines the quality and quantity of the gas needed. For our purposes, the end use for the hydrogen generated is a fuel for fuel cells. Most commercial hydrogen generating units incorporate a drying mechanism that removes much of the moisture from the gas. This is not appropriate for a fuel cell system. Free water will be abundant and should be removed by a water filter or series of water filters, but it is not necessary to remove very fine aerosols by coalescer or to use water absorption techniques. The reason is that the hydrogen side of the fuel cell membrane needs to remain hydrated (moist at a certain level) to aid and maintain proton transport for the operation of the fuel cell. 

While Nafion , a perfluorinated polymer developed by DuPont, is the most commonly used proton conductive polymer electrolyte membrane it is an insufficient solution in a number of areas. It has high cationic transport (approximately 9.56 5/cm) [8] but also has high levels of methanol fuel crossover, slow anode kinetics and very high cost [12]. Fuel cell membrane performance can be estimated from the ratio of proton conductivity (a) to methanol permeability (P). The higher the value of a/P, the better the membrane performance would be [13]. Chitosan has been shown to have a much lower methanol permeability than Nafion [14], and as such, a great deal of attention focused on developing chitosan membranes with high levels of ionic conduction and low methanol permeability as delineated in Table 3.1. 

Current research is centred on making compact cells of high efficiency. They are described in terms of the electrolyte that is used. The principle types are alkali fuel cells, described above, with aqueous KOH as electrolyte, MCFCs (molten carbonate fuel cells), with a molten alkali metal or alkaline earth carbonate electrolyte, PAFCs (phosphoric acid fuel cells), PEMs (proton exchange membranes), using a solid polymer electrolyte that conducts ions, and SOFCs, (solid oxide fuel cells), with solid electrolytes that allow oxide ion, 0 , transport The... 

Proton exchange membrane fuel cell (PEMFC) Proton conductive polymer membrane H2 O2 (in air) 60-90 Transportation vehicles, stationary power plants, cogeneration plants, portable power supplies... 

The main characteristic to consider for a PEM to be used in potential fuel cell is proton conductivity. To achieve good performance of a PEM fuel cell, high proton conductivity is essential, especially at a high current density. To understand proton transport at a molecular level in hydrated polymeric membranes, there are two principal proton transport mechanisms (1) the Grotthus mechanism or proton hopping mechanism, and (2) the vehicular mechanism or diffusion mechanism [243-245]. 

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