Study On Performance Of High Temperature Proton Exchange Membrane Fuel Cells With Novel Sulfonated PBI Membranes

In recent decade, many attentions have been paid to high temperature protonexchange membrane fuel cells due to its advantages, such as less externalhumidification requirement, simple water management, fast reaction kinetics atelevated temperatures, easy waste heat recovery and high tolerance to CO poisoning.As a key component in fuel cells, high temperature proton exchange membranes havebecome a hot research topic. Acid doped polybenzimidazole (PBI) membranes showhigh proton conductivities under low relative humidity even in completely anhydrousstate. These doped PBI membranes exhibit excellent properties such as high protonconductivity and outstanding thermal stabilities. However, high doping level usuallyresults in poor mechanical strength. Excellent mechanical properties and better radicaloxidative stability can be obtained by introducing sulfonic acid groups into PBImembranes. Under such backgrounds, the novel sulfonated PBI membranes dopedwith phosphoric acid are synthesized.First of all, the absorption of phosphoric acid in sulfonated PBI membranes isstudied by experiments. The properties of thermal stability and micro-morphology offour different sulfonated PBI membranes are analyzed by thermo gravimetric analyzerand scanning electron microscope and atomic force microscope. The experimentalresults show that the higher temperature, the more phosphoric acid doped. When otherconditions are same, the membranes with higher sulfonated degree will absorb morephosphoric acid. The adsorption amount of phosphoric acid increases obviously in thefirst three days and then keeps stable. The four different sulfonated PBI membraneshave good thermal stability up to450oC. After doping, a weight loss begins at about200°C is observed. The surface roughness of phosphoric acid doped membranes isgenerally larger than that of undoped onesSecondly, a three-dimensional, steady-state, non-isothermal numerical model ofhigh temperature proton exchange membrane fuel cells operating with the novelsulfonated PBI membranes is developed. Also, FLUENT and user defined functions(UDFs) are used to solve this model. In this model, empirical correlations for theproton conductivity of phosphoric acid doped sulfonated PBI membranes withdifferent sulfonation degrees are formulated based on previously reported experimental data. The polarization curve predicted by the model reasonably agreeswith published experimental data.Thirdly, the model fully considers ohmic losses (including all components of thefuel cell) and effects of the temperature on the mass and charge transport to analyzethe voltage losses. The simulation results indicate that increasing the phosphoric aciddoping level or operating temperature is helpful to decrease ohmic losses. Increasingthe operating pressure can decrease activation losses. In addition, using oxygeninstead of air in the cathode feed gas also can decrease activation losses.Finally, the steady-state performances of fuel cell under different design andoperating conditions are simulated. It is found that the higher phosphoric acid dopinglevel of the membranes, the higher internal temperature of the fuel cell. The increaseof operating temperatures or operating pressures is beneficial to improve the fuel cellperformance. In addition, using oxygen instead of air in the cathode feed gas improvesthe fuel cell performance. Electrochemical reaction rates under the ribs of the bipolarplates are larger than the values under the flow channels; therefore the fuel cellperformance is mainly determined by the charge transport instead of the masstransport.