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

Making a Membrane/Electrode AssemblyMembrane/electrode assembly construction varies greatly,but the following procedure is one of several used at LosAlamos National Laboratory where fuel cell research is activelypursued. The catalyst material is first prepared in liquid “ink”form by thoroughly mixing together appropriate amounts ofcatalyst (a powder of Pt dispersed on carbon) and a solution ofthe membrane material dissolved in alcohols. Once the ink isprepared, it is applied to the surface of the solid membrane ina number of different ways. The simplest method involvespainting the catalyst “ink” directly onto a dry, solid piece ofmembrane. The wet catalyst layer and the membrane areheated until the catalyst layer is dry. The membrane is thenturned over and the procedure is repeated on the other side.Catalyst layers are now on both sides of the membrane. Thedry membrane/electrode assembly is next rehydrated byimmersing in lightly boiling dilute acid solution to also ensurethat the membrane is in the H + form needed for fuel cell operation.The final step is a thorough rinsing in distilled water. Themembrane/electrode assembly is now ready for insertion intothe fuel cell hardware.TheMembrane/ElectrodeAssemblyThe combination of anode/membrane/cathode isreferred to as the membrane/electrode assembly.The evolution of membrane/electrode assemblies inpolymer electrolyte membrane fuel cells has passedthrough several generations. The original membrane/electrode assemblies were constructed in the 1960s forthe Gemini space program and used 4 milligrams ofplatinum per square centimeter of membrane area(4 mg/cm 2 ). Current technology varies with the manufacturer,but total platinum loading has decreased fromthe original 4 mg/cm 2 to about 0.5 mg/cm 2 . Laboratoryresearch now uses platinum loadings of 0.15mg/cm 2 .This corresponds to an improvement in fuel cell performancesince the Gemini program, as measured byamperes of current produced, from about 0.5 amperesper milligram of platinum to 15 amperes per milligramof platinum.Future OpportunitiesOptimization of membrane/electrodeassembly (MEA) construction is on-going.Fundamental research into the catalyst layer/membrane interface is needed to furtherunderstand the processes involved in currentgeneration. New MEA designs whichwill increase fuel cell performanceare needed. As always, the science andtechnology of MEAs are interconnected;whether improved understanding will leadto better MEA design or a different designwill lead to improved understanding remainsto be seen.Anode0.2mmThe thickness of the membrane in a membrane/electrodeassembly can vary with the type of membrane.The thickness of the catalyst layers depends upon howmuch platinum is used in each electrode. For catalystlayers containing about 0.15 mg Pt/cm 2 , the thickness ofthe catalyst layer is close to 10 microns, less than half thethickness of a sheet of paper. It is amazing that thismembrane/electrode assembly, with atotal thickness of about 200 microns or 0.2millimeters, can generate more than halfan ampere of current for every squarecentimeter of membrane/electrode assemblyat a voltage between the cathode andanode of 0.7 volts, but only when encasedCathodein well engineered components — backinglayers, flow fields, and current collectors.Polymer electrolyte membraneMembrane/electrode assemblyThis document, and more, is available for download at Martin's Marine Engineering Page - www.dieselduck.net