Fuel cell applications, membrane requirements

Such bimetallic alloys display higher tolerance to the presence of methanol, as shown in Fig. 11.12, where Pt-Cr/C is compared with Pt/C. However, an increase in alcohol concentration leads to a decrease in the tolerance of the catalyst [Koffi et al., 2005 Coutanceau et ah, 2006]. Low power densities are currently obtained in DMFCs working at low temperature [Hogarth and Ralph, 2002] because it is difficult to activate the oxidation reaction of the alcohol and the reduction reaction of molecular oxygen at room temperature. To counterbalance the loss of performance of the cell due to low reaction rates, the membrane thickness can be reduced in order to increase its conductance [Shen et al., 2004]. As a result, methanol crossover is strongly increased. This could be detrimental to the fuel cell s electrical performance, as methanol acts as a poison for conventional Pt-based catalysts present in fuel cell cathodes, especially in the case of mini or micro fuel cell applications, where high methanol concentrations are required (5-10 M). 

Physiochemical Requirements for the Membranes in Fuel Cell Applications.761... 

PHYSIOCHEMICAL REQUIREMENTS FOR THE MEMBRANES IN FUEL CELL APPLICATIONS... 

The protonic conductivity of a polymeric membrane is strongly dependent on membrane structure and membrane water content. A central challenge in the evaluation of ionomeric membranes for fuel cell applications has thus been the analysis of combined structural and water uptake characteristics required to achieve the highest protonic conductivity in an operating PEFC. Section 5.3.1 will address water uptake by ionomeric membranes employed in PEFCs, the state of water in such membranes and the resulting protonic conductivities obtained. 

Fuel cells, due to their higher efficiency in the conversion of chemical into electrical energy vhth respect to thermo-mechanical cycles, are another major area of R D that has emerged in the last decade. Their effective use, ho vever, still requires an intense effort to develop ne v materials and catalysts. Many relevant contributions from catalysis (increase in efficiency of the chemical to electrical energy conversion and the stability of operations, reduce costs of electrocatalysts) are necessary to make a step for vard in the application of fuel cells out of niche areas. This objective also requires the development of efficient fuel cells fuelled directly vith non-toxic liquid chemicals (ethanol, in particular, but also other chemicals such as ethylene glycol are possible). Together vith improvement in other fuel cell components (membranes, in particular), ethanol direct fuel cells require the development of ne v more active and stable electrocatalysts. 

Required Membrane Properties for Fuel Cell Application. 8... 

The required properties of solid polymer electrolyte membranes may be divided into interfacial and bulk properties [9]. As described above, the interfacial characteristics of these membrane materials are important for the optimum formation of the three-phase boundary. Hence, flow properties, gas solubility, wetting of carbon supported catalyst surfaces by the polymer, etc. are of paramount importance. The bulk properties concern proton conductivity, gas separation, and mechanical properties. This whole ensemble of properties has to be considered and balanced in the development of novel proton-exchange membranes for fuel cell application. 

In a similar work, the synthesis of a composite with iron oxide particles and sul-fonated cross-linked polystyrene (SXLPS) for application in the PEMs for fuel cells was described (Brijmohan and Shaw 2007). The technique used for the polymerization was similar to the miniemulsion polymerization (Ramirez and Landfester 2003). However, some modification to the procedure was required to make the cross-linked and functional polymer-iron oxide composites. Also reported was the membrane fabrication process, which inclndes the alignment of synthesized particles in a high-performance snlfonated poly(etherketoneketone) (SPEKK) matrix (Gasa et al. 2006), and the properties of such PEMs for fuel cell applications. The final properties of the membrane depend on various factors, such as the lEC, the matrix, and the size of the particles. However, the main emphasis of the research was to demonstrate a useful membrane-fabrication technique that can be utilized to enhance the conductivity of the PEMs. 

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