There is a growing demand for highly efficient, zero pollution fuel cell power sources for a wide variety of applications including automotive, portable systems, stationary, emergency back-up, and telecommunications systems, high temperature polymer electrolyte fuel cells (HT-PEMS),which operate at significantly higher temperatures (120-200°C) than conventional PEM fuel cells. Cellcontain polymer electrolyte membrane is basically of polybenzimidazole. Polybenzimidazole (PBI) fiber is a syntheticfiber with a very high meltingpoint. It has exceptional thermochemicalstability and does not readily ignite. PBI doped with phosphoricacid was utilized as a high temperature fuelcell electrolyte. Doping of phosphoric acid is donebecause PBI has low tensile stress to overcomethis issue we doped phosphoric acid to increase thermochemical stability,proton conductivity and to enhance the membrane efficiency.PBI-based HT-PEM, the water that is generated by reaction plays a major role in the degradation of these systems due to the fact that it limits obtaining of high current density, to overcome this issue by using the Microporous Layer (MPL) at the electrode (cathode) for more removal of excessiveliquid water.
Gas diffusion layer help to remove water fromoutside of catalyst layer and prevent water flooding . MPL contain pore that absorber the water, if pours diameter sizesmall is size it may causes water flooding(cathode catalytic layer) due to thislow voltage is gain but if pore diameter size is large(150nm) it show highercell voltage. By using MPL several things are observed (i) The ohmic losses can be reduced by using the MPL(ii) Mass transport of protons and electrons can be enhanced by using MPL,(iii)With MPL,more uniform water distribution profiles can be obtained,(iv) Thestrength of fuel cell can be enhanced with MPL since it acts as a buffer layerbetween the catalyst layer and diffusion layer.As microporous layer thickness increases, water content in MEA decreases. Ionic transport inPBI membranedepends on strongly phosphoric acidlevel and cell temperature. Since high water content in MEA causes phosphoric acid leakage from here, ionic conductivity of membrane decreases with the cell operating time. Therefore, the water content should be kept at a certain level.
In the case of with MPL, water content in MEA decreases with the increase of microporous layer thickness. In the gas diffusion layer, the distribution of water concentration declines by using an MPL.If water content in MEA is low, phosphoric acid leakage can be prevented and so the ionic conductivity of phosphoric acid doped PBI membrane canbe made stable. There is no remarkable effect of MPL on the cell performance. Polymer electrolyte membrane fuel cell aresuitable for small stationary application in power range is 1MW.
Polymerelectrolyte membrane fuel cell are the electrochemical converted withefficiency of more than 60% and no gaseous environmental pollutants areproduces.