Gas-diffusion media

In PEMFCs, the membrane electrode assembly (MEA, Eig. 15.2a) is a multilayer sandwich composed of catalytic layers (CLs) where electrochemical reactions take place, gas-diffusion media providing access of gases to the CLs, and a proton exchange membrane (PEM) such as Nafion . The CL is a multiphase multicomponent medium comprising ... 

C. Ji, G. J. Fleming, M. Fleming, and M. Mathias. Acrylic fiber bonded carbon fiber paper as gas diffusion media for fuel cell. US Patent 2007/0218346 (2007). 

F. Y. Zhang, A. K. Prasad, and S. G. Advani. Investigation of a copper etching technique to fabricate metallic gas diffusion media. Journal of Micromechanics and Microengineering 16 (2006) N23-N27. 

G. Lim and C. Y. Wang. Effects of hydrophobic polymer content in GDL on power performance of a PEM fuel cell. Electrochimica Acta 49 (2004) 4149-4156. G. G. Park, Y. J. Sohn, T. H. Yang, et al. Effect of PTFE contents in the gas diffusion media on the performance of PEMFC. Journal of Power Sources 131 (2004) 182-187. 

G. Velayutham, J. Kaushik, N. Rajalakshmi, and K. S. Dhathathreyan. Effect of PTFE content in gas diffusion media and microlayer on the performance of PEMFC tested under ambient pressure. Fuel Cells 7 (2007) 314—318. 

U. Pasaogullari, C. Y. Wang, and K. S. Chen. Two-phase transport in polymer electrolyte fuel cells with bilayer cathode gas diffusion media. Journal of the Electrochemical Society 152 (2005) A1574-A1582. 

J. G. Pharoah. On the permeability of gas diffusion media used in PEM fuel cells. Journal of Power Sources 144 (2005) 77-82. 

The coarse texture of the fibrous gas diffusion media can further amplify the contact stress exerted on the MEA. Figure 3 shows the relative size of a carbon fiber with respect to the typical thickness of the electrode and the electrolyte membrane. It can be seen that the diameter of the carbon fiber in the gas diffusion media is comparable to the thickness of the electrode. The rigid carbon fiber pressed onto the porous electrode layer can produce in-prints which can later become a stress-concentration and defect-initiation sites at the electrode-electrolyte interface. A microporous layer, if used, tends to smooth out the surface of the GDM and reduces fiber inprint. Thicker electrode layer also offers protection against fiber in-prints. 

During cell/stack operation, water content in the membrane is affected by the local intensive variables, such as local temperature, water vapor concentration in the gas phase, gas temperature and velocity in the channel, and the properties of the electrode and gas diffusion media. The power fluctuation can result in temperature variation inside the cell/stack, which will subsequently change the local membrane water content. As the water content in the membrane tends to be non-uniform and unsteady, this results in operation stresses. When the membrane uptakes water from a dry state, it tends to expand as there is no space for it to extend in plane and it can wrinkle up as schematically shown in Fig. 4 when the membrane dries out, the wrinkled part may not flatten out, and this ratcheting effect can cause the pile up of wrinkles at regions where membrane can find space to fold. The operation stress is typically cyclic in nature due to startup-shutdown cycles, freeze-thaw cycles, and power output cycles. 

Material cost reduction (catalysts, gas diffusion media, proton exchange... 

A plentitude of different flow field structures have been described in the literature starting from simple arrays of fence posts via arrays of straight parallel chaimels to complex serpentine type stmctures. In addition foam type structures, expanded metals and woven designs have been proposed as well. Under ordinary circumstances, gas flow will be maintained in the channels of the flow field while transport to the reaction site is by diffusion through the porous gas diffusion media next to the catalyst layer. 

In addition to loss of the platinum, the carlxm support that anchors the platinum crystallites and provides electrical coimectivity to the gas-diffusion media and bipolar plates is also subject to degradation. In phosphoric acid fuel cell, graphitized carbons are the standard because of the need for corrosion resistance in high-temperature acid environments [129], but PEM fuel cells have not employed fully graphitized carbons in the catalyst layers, due in large part to the belief that the extra cost could be avoided. Electrochemical corrosion of carbon materials as catalyst supports will cause electrical isolation of the catalyst particles as they are separated from the support or lead to aggregation of catalyst particles, both of which result in a decrease in the electrochemical active surface area of the catalyst and an increase in the hydrophUicity of the surface, which can, in turn, result in a decrease in gas permeability as the pores become more likely to be filled with liquid water films that can hinder gas transport. 

M or components of a PEMFC single cell (1 = end plates 2 = current collectors 3 = flow-field plates 4 = gaskets 5 = gas diffusion media 6 = membrane electrode assembly). 

Harkness, I.R., Hussain, N., Smith, L, and Sharman, ).D.B. (2009) The use of a novel water porosimeter to predict the water handling behaviour of gas diffusion media used in polymer electrolyte fuel cells. /. Power Sources, 193, 122-129. 

Sole, J.D. and Ellis, M.W. (2008) Determination of the relationship between capillary pressure and saturation in PEMFC gas diffusion media. Proceedings of Fuel Cell 2008, the Sixth International Conference on Fuel Cell Science, Engineering and Technology, Denver, CO. 

Membrane electrode assembly is the result of joining two electrodes and the electrolytic membrane together. Usually, the gas diffusion media are considered to be part of the MEA. 

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