Study On Self-assembly Inorganic Proton Exchange Membrane For Elevated Temperature Fuel Cell

Contradiction between the shortage of precious Pt metals and the method for electrode catalyst has become the bottleneck of application in fuel cell industry. In 2007, the global output of Pt metal is only about 6.66 million ounce, and the demand and supply gap has already reached over 0.256 million ounce. Operation of PEMFCs at elevated temperature has been receiving increased attention because it will enhance reaction kinetics at both electrodes, improve the carbon monoxide tolerance of the platinum catalyst at the anode, and simplify heat and humidity managements of PEMFCs. However, at present, the most commonly used perfluorinated ionomer proton exchange membranes, e.g., Nafion, a polymer composed of a poly(fluorocarbon) backbone and fluorocarbon side chains with sulfonic acid end groups, are limited to being used at low temperature because of the evaporation of water. Inorganic electrolyte subjected to elevated temperature membrane for PEM fuel cells has attracted much of attention because of the high temperature stability and the low dependence of conductivity on liquid water. Here, we have reported our research about the highly ordered self-assembled HPA/meso-oxide electrolyte and the elevated-temperature proton conductive operation. Based on those research, we studied the electrochemical performance and durability of HPW and synthesized the periodic ordered Nafion-Silica-HPW electrolyte for elevated temperature fuel cell.(1) Using the homemade device to test the conductivity of inorganic powders, we have tested the proton conductivity of HPW, HPMo, HSiW. Finally we decide to use HPW as the inorganic acid to be anchored on the mesoporous silica, since HPW has higher conductivity and stability.(2) Periodic ordered Nafion-Silica-HPW electrolyte is synthesized through a facile multiphase self-assembly between the positively charged silica, negatively charged HPW acids and Nafion ionomers. The results exhibit uniform nanoarrays with long-range order of the electrolyte, the diametre of the channel is about 6nm. EDX mapping of the membrane demonstrated that the silica and Nafion ionomers dispersed uniformly in the electrolyte, with HPW molecules imbedded into the nanostructure. The Nafion-silica-HPW electrolyte displays desirable conductivity at both low and elevated temperature. The proton conductivity of Nafion-silica-HPW electrolyte at absolutely dry condition of 200℃is 0.044 S/cm.(3) Periodic ordered Nafion-Silica-HPW electrolyte with various Nafion content was synthesized through a facile multiphase self-assembly process. The results exhibit uniform nanoarrays with long-range order when Nafion content in the complex is lower than 30 wt %. When the content increases to 40%, the Nafion-Silica-HPW electrolyte can not be synthesized with an ordered structure.The water uptake of electrolyte decrease gradually with the increase of Nafion content. The highest value of water uptake is as high as 100%. The proton conductivity of Nafion-silica-HPW electrolyte with Nafion concentration lower than 30 wt% displays very stable conductivity at both low and elevated temperature conditions. The electrolyte with 40% Nafion content also has excellent performance at low temperature, but it can not bear high temperature near 200℃.