Membrane electrode assembly Micro

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. 

J. Xie, D. L. Wood III, K. L. More, P. Atanassov, and R. L. Borup, Micro-structural Changes of Membrane Electrode Assemblies During PEFC Durability Testing at High Humidity Conditions, /. Electrochem. Soc., 12, AlOl 1 (2005). 

Chu KL et al (2006) A nanoporous silicon membrane electrode assembly for on-chip micro fuel cell applications. J Microelectromech Syst 15(3) 671... 

Mocoteguy P, Ludwig B, Scholia J, Nedellec Y, Jones DJ, Roziere J (2010) Long-term testing in dynamic mode of HT-PEMFC H3PO4/PBI Celtec-P based membrane electrode assemblies for micro-CHP applications. Fuel Cells 10(2) 299-311... 

Cheng, X., Chen, L., Peng, C., Chen, Z., Zhang, Y., Fan, Q. 2004. Catalyst micro-structure examination of PEMFC membrane electrode assemblies vs. time. Journal of the Electrochemical Society, 151, A48-A52. 

In the two preceding chapters (2 and 3), different aspects of state-of-the-art catalysts and membranes as well as new approaches for these two components were discussed. Starting with the central part of low temperature fuel cells, adjacent to the membrane electrode assembly (MEA) is the gas difiusion media, which ensures a uniform distribution of reactant ses and the transport of products away from the electrodes on a micro- and millimeter scale. The fuel cell is then accomplished by the flow field which is required for a uniform distribution of the reactant across the whole electrochemically active area The chapter starts with a discussion of gas diftiision media and their impact on performance. Different flow field stmctures and requirements for stable and long-term operation of fuel cells are presented. The chapter concludes with an introduction to system-level aspects and overall considerations of system efficiency including other components such as humidifiers and cooling strategies. 

Kim S and Mench M M (2007), Physical degradation of membrane electrode assemblies undeigoing freeze/thaw cycling Micro-structure effects . Journal of Power Sources, 174,206-220. 

The transport of energy, mass, and charge is at the heart of proton exchange membrane fuel cell (PEMFC) operation. The porous layers in modern membrane electrode assemblies (MEAs) lie at the Interface between the macroscopic phenomena occurring in the flow channels and the micro- and nanoscopic processes in the catalyst layers (see Figure 5.1). These layers must deliver the reactants and remove the products from the electrochemical reactions at the fuel cell electrodes. They must also provide connections to the current collecting plates with minimal thermal and electrical resistances. 

FIGURE 9.6 MEA with layer delamination after freeze-thaw cycles. (Reprinted from /. Power Sources, 174, Kim, S. and Mench, M. M., Physical degradation of membrane electrode assemblies undergoing freeze/thaw cycling Micro-structure effects, 206 220, Copyright (2007), with permission from Elsevier.)... 

Morikawa, H., Mitsui, T., Hamagami, J. and Kanamura, K. (2002) Fabrication of membrane electrode assembly for micro fuel cell by using electrophoretic deposition process. Electrochemistry 70, 937-939. 

Chu, K.-L., Gold, S., Subramanian, V., Lu, C., Shannon, M.A., Masel, R.l. (2006) A nanoporous silicon membrane electrode assembly for on-chip micro fuel cell applications. Journal of Microelectromechanical Systems, 15, 671-677. 

Conventional fuel cell stack mainly comprises of (a) membrane electrode assemblies (MEAs) for achieving the electrochemical energy conversion process, (b) bipolar plates for the supply of reactant (fuel and oxidant) gases to MEAs in addition to providing cell to cell electronic conduction path and removal of heat and (c) auxiliary components for the reactant supply and product removal. Table 1 provides some of the essential differences between DMFC and micro fuel cell. 

Chan et al. (2005), have realised micro fuel cells through an approach that combines thin film materials with MEMS (micro-electro-mechanical system) technology. The membrane electrode assembly was embedded in a polymeric substrate (PMMA) which was micromachined through laser ablation to form gas flow channels. The micro gas channels were sputtered with gold to serve as current collectors. This cell utilized the water generated by the reaction for the humidification of dry reactants (H2 and O2). The peak power density achieved was 315 mW cm (901 mA cm" at 0.35 V) for the H2-O2 system with 20 ml min" O2 supply and H2 at 10 psi in dead ended mode of operation. A Y shaped microfluidic channel is depicted in Fig. 21. 

No smart solution is available to store the gaseous hydrogen used in miniature fuel cell (PEMFC) and hence DMFC is receiving enormous interest due to system simplicity. However, specialised air-breathing DMFC components have to be developed. New materials have to be developed in addition to optimisation of structure and operating conditions to take care of performance decay modes. New membrane/electrode assemblies appropriate for the microscale to be developed exploiting the enhanced heat and mass transfer on the microscale for improved performance, and developing microfluidic components for micro fuel cells. 

In this chapter, the electrode construction of a high temperature (HT) polymer electrolyte membrane (PEM) electrode assembly (MEA) will be explained. The different functionalities of the electrode layers like the gas diffusion layer (GDL), the micro porous layer (MPL), and the... 

Esquivel et al. (2010) present an all-polymer micro-DMFC fabricated with a SU-8 photoresistor. This development exploits the capability of SU-8 components to bond to each other by a hot-pressing process and obtain a compact device. The device is formed by a MEA sandwiched between two current collectors. The MEA consists of a porous SU-8 membrane filled with a proton-exchange polymer and covered by a thin layer of carbon-based electrodes with a low catalyst loading (1.0 mg/cm ). The current collectors consist of two metalhzed SU-8 plates provided with a grid of through-holes that make it possible to deliver the reactants to the MEA by diffusion. The components were then bonded to obtain a compact micro-DMFC. With this assembly, using a 4 M methanol concentration at a temperature of 40°C, a maximum power density of 4.15 mW/cm was obtained. 

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