Fuel Cell Systems Explained - from and for SET students

4.2 How the polymer electrolyte worksThe basicly used material for the electrolyte is fluoroethylene. The established industrial polymerelectrolyte is Nafion from the company Dupont. The construction of the electroclyte materialconsists of the following steps. The starting material is the polymer polyethylene.1. The hydrogen in PE is substituted through fluorine, to get polytetrafluoroethylene PTFE,also known as Teflon. This process is called perfluorination.PTFE is durable and resistance against chemical attacks. It is also highly hydrophobic, waterrepellent.2. The PTFE polymer is sulphonated, that means a side chain, ending with sulphonic acidHSO 3 is added to the molecul. The resulting structure is caleed anionomer, due to the ionicalbond of the H + and SO 3 - ions. The ionic bondage results in a clustering of the side chains inthe overall structure.Another important property is, that HSO 3 is highly hydrophyllic,attracts water.In conclusion, we are creating water attracting areas in a water repellent material. Within thesehydrated regions, H + ions are relatively weakly bonded and are able to move (dilute acid). Even ifthese areas are seperated, it is still possible for the H + ions to move along the the supporting longmolecules.From the fuel cell use, the main features of Nafion are: chemically highly resistant mechanically strong, thin films are possible acidic absorb large quantities of water if well hydrated, H + ions are able to move quite freely through the material4.3 Electrodes and electrode structureThe electrodes in a PEMFC are essentially the same, a high porous conducting material withplatinum as catalyst. To built the single cells, first the platinum catalyst is formed of small platinumparticals, which are added onto bigger carbon-based particals. In the seperate electrode method, thispowder is fixed on corbon cloth or carbon paper, which builts the basic structure of the electrode.Also PTFE is added due to its hydrophobicproperty, so that the water as the reaction productis expelled from the surface. Additional thecarbon cloth/paper acts as a gas diffusion layerand diffuses the gas onto the catalysts. To formthe fuel cell, two of these electrodes are hotpressed on each side of the electrolyte membran.The result is a complete MEA.The alternative method involves building theelectrode directly onto the electrolyte. Theplatinum on carbon catalyst is fixed directly to theelectrolyte by rolling or spraying. The electrode isdirectly manufactured on the membrane.Then thegas diffusion layer is added ,which provides theelectrical connection between the carbonsupportedcatalyst and the bipolar plate, carriesthe water away and also forms a protective layerfor the thin catalyst layer. The final result is shown in figure 4.7, beginning from the left side:electrolyte molecules, platinum/carbon katalyst, carbon cloth or paper.In order to increase the performance the electrolyte material is spread out over the catalyst, topromote the three-phase contact between, the reactant gas, electrolyte abd electrode catalyst.