Polymer Membrane Fuel Cell performance loss

An important aspect in PBI/IL composite membranes is the IL lixiviation rate. When the IL were not effectively immobilized, a progressive release of the IL components during a long period of fuel cell operation took place, thus resulting in the decline of fuel cell performance [63]. In order to overcome this drawback, the srdfonation of polymer matrices led to a better dispersion of ionic domains and the retention of IL, along with a reduction of the proton conductivity loss, as shown by Ye et al. [68]. PolymerizatirMi techniques by introducing IL molecules in the polymer matrix were an actual alternative to overcome this drawback in polymer/IL composites. 

Abstract Most of the transport processes of a fuel cell take place in the gas diffusion media and flow fields. The task of the flow fleld is to uniformly distribute the reactant gases across the electrochemically active area and at the same time ensure an adequate removal of the reactant products, which is water on the cathode side in both polymer electrolyte membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC). Gas diffusion media are required to supply the reactant under the land areas of the flow fleld at the same time, the gas diffusion media has to ensure a good thermal as well as water management to avoid any non-optimum conditions. Characterization tools for gas diffusion media are presented, flow fleld types and design criteria are discussed and the effect of both components on the performance of a fuel cell are highlighted. System aspects for different fuels (hydrogen, vapor-fed DMFCS, liquid fed DMFCs) are compiled and the different loss contributions and factors determining the performance of a fuel cell system are shown. 

A limiting factor in PEMFCs is the membrane that serves as a structural framework to support the electrodes and transport protons from the anode to the cathode. The limitations to large-scale commercial use include poor ionic conductivities at low humidities and/or elevated temperatures, a susceptibility to chemical degradation at elevated temperatures and finally, membrane cost. These factors can adversely affect fuel cell performance and tend to limit the conditions under which a fuel cell may be operated. For example, the conductivity of Nafion reaches up to 1(T S cm in its fully hydrated state but dramatically decreases with temperature above the boiling temperature of water because of the loss of absorbed water in the membranes. Consequently, the developments of new solid polymer electrolytes, which are cheap materials and possess sufficient electrochemical properties, have become one of the most important areas for research in PEMFC. 

Addition of hydroscopic metal oxides such as silica, zirconia, or titania to a proton-conducting polymer is the most obvious way to improve water retention at elevated temperatures (Aparicio et al., 2003). Unfortunately, due to the negligible proton conductivity of these oxides, an increase in the overall resistance of the composite membrane is observed, especially at low temperatures. However, as the temperature is increased, the conductivity gain due to better hydration offsets the loss due to the excluded conducting volume, and the net fuel cell performance is improved, as compared to an unmodified membrane (Adjemian et al., 2002a,b). It should be stressed that there are limits to the water sorption capability of the oxides. While these membranes retain more water than traditional PEM materials such as Nafion at high temperature and low RH, water uptake is insufficient and ohmic losses are still unacceptably high for PEMFC applications. 

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