Direct membrane fuel cells cell configurations

Heteropolyacid-modified polymer silica membranes for Direct Methanol Fuel Cells have been prepared and tested under high temperature operation conditions (145°C) in single cell configuration. A maximum power density of 0.4 W/cm in oxygen with 2 M methanol has been obtained with air at the cathode, this value decreased to 0.25 W/cm. The higher performance of the heteropolyacid-Nafion-silica membrane, with respect to Nafion-silica, is attributed to its better ion transport properties, since the measured cell resistance value is similar for both membranes. 

The catalytic (supported or unsupported) interface in the vast majority of direct liquid fuel cell studies is realized in practice either as a catalyst coated membrane (CCM) or catalyst coated diffusion layer (CCDL). Both configurations in essence are part of the electrode design category, which is referred to as a gas diffusion electrode, characterized by a macroporous gas diffusion and distribution zone (thickness 100-300 pm) and a mainly mesoporous, thin reaction layer (thickness 5-50 pm). The various layers are typically hot pressed, forming the gas diffusion electrode-membrane assembly. Extensive experimental and mathematical modeling research has been performed on the gas diffusion electrode-membrane assembly, especially with respect to the H2-O2 fuel cell. It has been established fliat the catalyst utilization efficiency (defined as the electrochemically available surface area vs. total catalyst surface area measured by BET) in a typieal gas diffusion electrode is only between 10-50%, hence, flie fuel utilization eflfieieney can be low in such electrodes. Furthermore, the low fuel utilization efficiency contributes to an increased crossover rate through the membrane, which deteriorates the cathode performance. 

Conventional reference electrodes consist of a solid reversible electrode and an aqueous electrolyte solution. To measure the individual contributions from the anode and the cathode of a PEM fuel cell, the electrolyte solution of the reference electrode must either be in direct contact with one side of the solid proton exchange membrane or be located in a separate compartment with electrical contact between the reference electrode and the solid membrane by means of a salt bridge [66], As a result, two different types of reference electrode configurations are employed for the study of fuel cells internal and external. 

In the internal type, the reference electrode is in direct contact with the polymer electrolyte membrane of the MEA, as depicted in Figure 5.44. This configuration requires extra attention when assembling and disassembling the fuel cell. 

There is a class of nonporous materials called proton conductors which are made from mixed oxides and do not involve transport of molecular or ionic species (other than proton) through the membrane. Conduction of protons can enhance the reaction rate and selectivity of the reaction involved. Unlike oxygen conductors, proton conductors used in a fuel cell configuration have the advantage of avoiding dilution of the fuel with the reaction products [Iwahara ct al., 1986]. Furthermore, by eliminating direct contact of fuel with oxygen, safety concern is reduced and selectivity of the chemical products can be improved. The subject, however, will not be covered in this book. 

A unit fuel cell can be constructed using a membrane electrode assembly (MEA) and two bipolar plates as shown in Fig. 1(a). Multiple cells can be stacked up to obtain more power and such a configuration is shown in Fig. 1(b). The charge carriers and their flow direction are identified in Fig. l(a, b). 

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