Fuel Cells - Green Power - Martin's Marine Engineering Page

Membrane/electrode assembly with backing layers.AnodebackingPathways forgas accessto electrodeBacking layerPolymerElectrolyteMembraneMembrane/electrode assemblyCathodebackingElectrodeEnlarged cross-section of a membrane/electrode assembly showing structural details.TheBacking LayersThe hardware of the fuel cell, backing layers,flow fields and current collectors, is designed tomaximize the current that can be obtained from amembrane/electrode assembly. The so-called backinglayers, one next to the anode, the other next to thecathode, are usually made of a porous carbon paper orcarbon cloth, typically 100 to 300 microns thick (4 to 12sheets of paper). The backing layers have to be made ofa material, such as carbon, that can conduct the electronsexiting the anode and entering the cathode.Backing layerThe porous nature of the backing material ensureseffective diffusion of each reactant gas to the catalyst onthe membrane/electrode assembly. In this context,diffusion refers to the flow of gas molecules from aregion of high concentration, the outer side of thebacking layer where the gas is flowing by in the flowfields, to a region of low concentration, the inner side ofthe backing layer next to the catalyst layer where the gasis consumed by the reaction. The porous structure ofthe backing layers allows the gas to spread out as itdiffuses so that when it penetrates the backing, the gaswill be in contact with the entire surface area of thecatalyzed membrane.The backing layers also assist in watermanagement during the operation ofthe fuel cell; too little or too muchwater can cause the cell to cease operation.The correct backing materialallows the right amount of water vaporto reach the membrane/electrodeassembly to keep the membranehumidified. The backing material alsoallows the liquid water produced at thecathode to leave the cell so it doesn’t“flood”. The backing layers are oftenwet-proofed with Teflon to ensurethat at least some, and hopefully most,of the pores in the carbon cloth (orcarbon paper) don’t become cloggedwith water, which would prevent rapidgas diffusion necessary for a good rateof reaction to occur at the electrodes.This document, and more, is available for download at Martin's Marine Engineering Page - www.dieselduck.net11