Membrane electrode assemblies properties

The properties of Nafion at freezing temperatures can be quite relevant, for example, within the context of fuel cells in vehicles with regard to cold-starting, as well as the degradation of membrane/electrode assemblies due to the freezing of in situ water. 

New membrane materials for PEM fuel cells must be fabricated into a well-bonded, robust membrane electrode assembly (MEA) as depicted in Figure 1. In addition to the material requirements of the proton exchange membrane itself as outlined above, the ease of membrane electrode assembly fabrication and the resulting properties of the MEA are also... 

In PEMFCs working at low temperatures (20-90 °C), several problems need to be solved before the technological development of fuel cell stacks for different applications. This concerns the properties of the components of the elementary cell, that is, the proton exchange membrane, the electrode (anode and cathode) catalysts, the membrane-electrode assemblies and the bipolar plates [19, 20]. This also concerns the overall system vdth its control and management equipment (circulation of reactants and water, heat exhaust, membrane humidification, etc.). 

The membrane conductivity was measured in HCl(aq) solutions of different concentrations and in 2 M HC1 + 0.2 M CuCl solution to model the catholyte and anolyte solutions in the electrolyser. All membranes were equilibrated in the same solutions for 20 hours before starting the measurements. Detailed characterisation data for a number of commercial anion exchange membranes are published elsewhere (Gong, 2009). The AHA membrane, which demonstrated the highest conductivity in HC1 (12.61 mS/cm) compared to other membranes with similar IEC and water uptake, was selected to prepare a membrane electrode assembly (MEA) and carry out electrolysis tests with this MEA. The ACM membrane with lower conductivity values was also chosen for the electrolysis tests due to its proton blocking properties and high Cl- selectivity. 

The membrane electrode assembly (MEA), which consists of three components (two gas diffusion electrodes with a proton exchange membrane in between), is the most important component of the PEMFC. The MEA exerts the largest influence on the performance of a fuel cell, and the properties of each of its parts in turn play significant roles in that performance. Although all the components in the MEA are important, the gas diffusion electrode attracts more attention because of its complexity and functions. In AC impedance spectra, the proton exchange membrane usually exhibits resistance characteristics the features of these spectra reflect the properties of the gas diffusion electrode. In order to better understand the behaviour of a gas diffusion electrode, we introduce the thin-film/flooded agglomerate model, which has been successfully applied by many researchers to... 

Clearly, a fundamental understanding of the key strac-ture/property relationships, particularly membrane morphology and conductivity as a function of polymer electrolyte architecture and water content - both in the bulk hydrated membrane and at the various interfaces within the membrane electrode assembly (MEA), can provide guidance in the synthesis of novel materials or MEA manufacturing techniques that lead to the improvement in the efficiency and/or operating range of PEMFCs. 

In experiments performed with different membrane-electrode-assembly fabrication techniques, and containing a PSSA-PVOF membrane with different properties than the ones previously discussed, the overall performance was similar to that previously obtained, as shown in Fig. 1.80. However the MEA tested displayed distinctly different behavior in contrast to samples previously discussed in that it showed less sensitivity to oxygen flow rate, as illustrated in Fig. 1.81. This behavior can partly be rationalized by increased methanol crossover rates observed for this MEA, which can contribute to aid in proper hydration of the cathode. However, since the increase in methanol crossover compared with the earlier samples is not dramatic ( 25% greater), other factors such as the concentration of sulfonic acid groups present at the membrane surface available for participation in the interfacial reaction zone as well as the concentration of perfluorocarbon binder contribute to produce conditions less sensitive to water management problems. 

This introductory chapter provides a brief outline and history of the PEFC technology, and important requirements and aspects in the development of polymer membranes for fuel cells. To obtain materials that meet specific requirements, the relationship of composition/structure and the properties has to be estabUshed. For the preparation of the membrane electrode assembly, it is important to understand the interfacial properties between membrane and electrodes. 

The development of membranes for fuel cells is a highly complex task. The primary functionalities, (i) transport of protons and (ii) separation of reactants and electrons, have to be provided and sustained for the required operating time. Optimization of the composition and structure of the material to maximize conductivity and mechanical robustness involves careful balancing of synthesis and process parameters. The ultimate membrane qualification test is the fuel cell experiment. It is evident that the membrane is not a stand-alone component, but is combined with the electrodes in the membrane electrode assembly (MEA). Interfacial properties, influence on anode and cathode electrocatalysis, and water management are the key aspects to be considered and optimized in this ensemble. 

The identification of membrane properties relevant to fuel cells (ion exchange capacity, water uptake, conductivity), aspects of membrane electrode assembly fabrication, and fuel cell performance are described in detail in this review. 

Fundamental properties of the materials such as polymer electrolyte membranes, catalysts and gas diffusion layers making up the so called Membrane Electrode Assembly (MEA) as well as requirements to bipolar plates and sealing concepts necessary for stack integration are discussed. 

Alcohol crossover and cell resistance are the relevant properties determining the DAFC performance, which are closely related to the membrane used in the preparation of the membrane-electrode assembly (MEA). Mechanical properties, as well as the chemical and thermal stability, of the membrane could also be important when durability is considered. 

Recent intensive studies have, however, been reported to lead to AEMs with high ionic conductivities, reportedly comparable to Nalion . These promising AEMs [11, 45-51] are still to be evaluated in AMFCs. Most hydrocarbon AEMs are soluble in various solvents, which is potentially useful for the formulation of alkaline ionomers required for the preparation of high-performance membrane electrode assemblies (MEAs). If the conductive properties reported can be translated into high power outputs, then AMEC performances comparable to those of PEMFCs can be expected in the near future. 

Water transport capability (high water flux) from the cathode to the anode These properties have to be assured under a wide range of temperature and humidity (—30-120°C, nominal 0-100% relative humidity (RH)) considering the fabrication of membrane electrode assemblies (MEAs)... 

Guo QH, Qi ZH (2006) Effect of freeze-thaw cycles on the properties and performance of membrane-electrode assemblies. J Power Sources 160 1269-1274... 

Debe MK, Haugen GM, Steinbach AJ, Thomas JH III, Ziegler RJ, inventors 3M Innovative Properties Company, assignee. Catalyst for membrane electrode assembly and method of making. United States patent 6040077. 2000 Mar 21. 

In general, PFSA membranes are characterized by excellent performance, electrochemical stability, suitable mechanical properties, and allow rapid startup. However, it appears necessary to ameliorate the PFSA membranes and ionomers to improve the operating efficiency of membrane-electrode assemblies of PEM electrolysers at practical current densities useful to reduce capital costs. PESA membranes used in electrochemical devices are essentially based on Nafion however, several alternative PSFA membranes with shorter pendant side chain have been developed by Dow, 3 M, Gore, Asahi Glass, Solvay Specialty Polymers, etc.. 

The molecular weight of PBI strongly influences the membrane properties and the lifetime of the membrane electrode assembly (MEA), as shown by Yang et al. [1]. Due to the poor solubility of PBI in common solvents such as tetrahydrofurane (THF) or acetonitrile, the most common method to evaluate the molecular weight of PBI is to measure the viscosity dissolved in 96 % sulfuric acid. Care needs to be taken not to heat sulfuric acid-based PBI solutions and not to store them for a long time, to avoid possible side reactions like suUbnation or cross-linking. Also, viscosity depends on temperature, and a strict control of the temperature is necessary. Impurities like dust should be removed by filtration. 

FIGURE 9.5 Effects of freezing-thaw cycles on the catalyst layer surface, (a) Fresh catalyst surface, (b) catalyst surface after freeze/thaw cycles. (Reprinted from /. Power Sources, 160, Guo, Q. and Qi, Z., Effect of freeze-thaw cycles on the properties and performance of membrane-electrode assemblies, 1269-1274, Copyright (2006), with... 

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