Membrane electrode assembly performance

Sol-gel techniques have been successfidly applied to form fuel cell components with enhanced microstructures for high-temperature fuel cells. The apphcations were recently extended to synthesis of hybrid electrolyte for PEMFC. Although die results look promising, the sol-gel processing needs further development to deposit micro-structured materials in a selective area such as the triple-phase boundary of a fuel cell. That is, in the case of PEMFC, the sol-gel techniques need to be expanded to form membrane-electrode-assembly with improved microstructures in addition to the synthesis of hybrid membranes to get higher fuel cell performance. 

Bose, A. B., Shaik, R., and Mawdsley, J. Optimization of the performance of polymer electrolyte fuel cell membrane electrode assemblies Roles of curing parameters on the catalyst and ionomer structures and morphology. Journal of Power Sources 2008 182 61-65. 

Bender, G., Zawodzinski, T. A., and Saab, A. P. Fabrication of high-precision PEFG membrane electrode assemblies. Journal of Power Sources 2003 124 114—117. Ihm, J. W., Ryu, H., Bae, J. S., Ghoo, W. K., and Ghoi, D. K. High performance of electrode with low Pt loading prepared by simplified direct screen printing process in PEM fuel cells. Journal of Materials Science 2004 39 4647--4649. 

T. Erey and M. Linardi. Effects of membrane electrode assembly preparation on the polymer electrolyte membrane fuel cell performance. Electrochimica Acta 50 (2004) 99-105. 

Advances in fuel cell technology over the last four decades have come primarily from improved electrocatalysts, membrane electrode assembly fabrication strategies, and cell/stack/system engineering. Apart from Nafion, new ion conducting polymeric materials have played only a minor role in significantly increasing cell performance. However, new materials... 

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... 

FIGURE 27.22 Performance for DuPont membrane electrode assembly three-layer (DuPont MEA-3L). (From DuPont 2002, www2. dupont.com.)... 

A publication by the Paul Scherrer Institute reports progress in preparing membrane/electrode assemblies for polymer electrolyte fuel cells based on radiation-grafted FEP PSSA membranes [95]. Hot-pressing with Nation was used to improve the interfaces. These improved MEAs showed performance data comparable to those of MEAs based on Nafion 112 (Figure 27.58) and an service-life in H2/O2 fuel cells of more than 200 h at 60°C and 500 mA cm. ... 

The catalyst can be bonded to the membrane surfaces by many different methods such as vacuum deposition (21). However, the performance of the membrane-electrode assembly produced by the vacuum deposition method is poor because a smooth metallic layer is formed while electrode material for an electrochemical cell should be rough (high surface area). 

Demonstrate proofs of concept for the large-scale preparation of high performance electrodes for membrane electrode assemblies (MEAs) and assess electrode and MEA architecture against FreedomCAR cost targets. 

Test the performance of ion-exchange membranes in a membrane electrode assembly (MEA). 

Demonstrate enhanced performance of membrane electrode assemblies (MEAs) with low Pt content towards the DOE goals of 0.6 g Pt/kW in automotive applications for the year 2005. 

Design and optimize membrane-electrode assemblies (MEAs) to enhance cell performance. 

Cyclic voltammetric studies indicated that the activity of the Pt-Ru films increased with operating temperature just as in conventional catalyst layers produced from unsupported catalyst inks. Membrane electrode assemblies were fabricated from Pt-Ru films of the most active compositions, and a power density of 800 mW/mg was realized for anodes that were deposited with about 0.1 mg/cm of Pt-Ru (see Figure 1). Applying the catalyst layers by sputter deposition on the electrode was found to yield better performance than applying them on the membrane. This was attributed to the enhanced electrical connectivity achieved when the catalyst layer is applied on the electrode. However, this is only true for very thin films. When thicker composite films are produced, such as those planned later in this project, good electrical connectivity may be achieved even with membrane deposition. 

Electrodes were fabricated with catalyst layers containing platinum-ruthenium alloys and platinum-ruthenium oxide. Membrane electrode assemblies were fabricated with such cells, and the performance was evaluated in a full cell configuration. Although ruthenium oxide is a proton conductor and is expected to enhance the rate of proton transport from the interface during methanol oxidation, no noticeable improvement in activity of the catalyst layer was observed by addition of ruthenium oxide. The role of other metal oxides such as tungsten oxide will be investigated next year, along with evaluation of non-noble metal catalysts based on nickel, titanium, and zirconium. 

Recently, taking advantage of the very narrow size distribution of the metal particles obtained, microemulsion has been used to prepare electrocatalysts for polymer electrolyte membrane fuel cells (PEMFCs) Catalysts containing 40 % Pt Ru (1 1) and 40% Pt Pd (1 1) on charcoal were prepared by mixing aqueous solutions of chloroplatinic acid, ruthenium chloride and palladium chloride with Berol 050 as surfactant in iso-octane. Reduction of the metal salts was complete after addition of hydrazine. In order to support the particles, the microemulsion was destabilised with tetrahydrofurane in the presence of charcoal. Both isolated particles in the range of 2-5 nm and aggregates of about 20 nm were detected by transmission electron microscopy. The electrochemical performance of membrane electrode assemblies, MEAs, prepared using this catalyst was comparable to that of the MEAs prepared with a commercial catalyst. 

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. 

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