Preparation And Properties Of High Performance Sulfonated Polyimide Proton Exchange Membrane Materials

Proton exchange membrane fuel cells(PEMFCs)have attracted considerable attention for a high efficiencies and environmentally friendly energy system.One key constituent of a PEMFC system is the polymer electrolyte/exchange membrane(PEM)which directly affects the performance of fuel cell.Aromatic polyimide is a star polymer possessed higher mechanical property,excellent thermal dimensional stability and superior chemical resistance,which are similar ideal for PEMFC applications.However,high sulfonic acid often causes excessive degradation of the sulfonated polyimide(SPI)membranes in water,especially at high temperatures.To solve this problem,several methods have been applied to improve the properties of SPIs,such as changing chemical structures and cross-linking.In recent work by our group proposed,the highly branched sulfonated polymer exhibited considerable chemical stability and high proton conductivity.In order to further enhance the chemical stability of SPI membrane and compensate the shortcomings caused by cross-linking,we prepared different series of SPI membrane in this paper.The specific contents and results are as follows:1)Four kinds of branched PEMs based on SPIs were prepared by commercial available monomers.The effects of monomer and branched structure on the performance of PEM were investigated in detail.The results show that the introduction of the branched structure can enhance the oxidative stability of the SPI membrane,and the enhancement degree is related to the chemical structure of the polyimide itself.Based on rigid main chain structure,the proton conductivity of the PEM is improved by the introduction of the branched structure.The branched structure is detrimental to the mechanical properties of the membrane.2)The cross-linked PEM based on SPI containing 5 % branched structure was successful developed.Comparison with other corresponding membranes,this membranes show enlarged interchain spaces facilitating water uptake,which resulted in a significant improvement of proton conductivity.In addition,the final membrane shows excellent chemical stability,mechanical property and fuel cell performance.Those results demonstrated that branching is a new way to compensate for the reduction of electrochemical property of PEM causing by cross-linking.And we believe that this approach will provide new insights for development of high performance PEM materials for fuel cell application.3)A series of cross-linkable sulfonated polyimides with flexible pendant alkyl side chains containing trimethoxysilyl groups were successfully synthesized.Before the cross-linking reaction is finished,the cross-linkable sulfonated polyimides membranes can be reprocessed,and the recovery rate of the prepared films is in the acceptable range.Cross-linked membranes were obtained when the cross-linkable membranes were immersed in acid solution during proton exchange.The resulting membranes exhibit high hydrolytic and oxidative stabilities,excellent mechanical properties,and considerable proton conductivities.Note that CSPI-70 showed a remarkable power density in the DMFC test.All of these results suggest that cross-linked membranes with suitable cross-linking degrees and high sulfonation levels have great potential for PEMFC applications.