Synthesis and characterization of anhydrous conducting polyimide/ionic liquid complex membranes via a new route for high‐temperature fuel cells

The paper deals with the synthesis and characterization of a new series of anhydrous conducting acid‐doped complex membranes based on polyimide (PI) and ionic liquid (IL) for high‐temperature fuel cells via a new route. For this purpose, three imidazolium‐based ILs (RIm⁺BF₄^?) with different alkyl chain lengths (R=methyl, ethyl, and butyl) are added into polyamic acid (PAA) intermediate prepared from the reaction of benzophenonetetracarboxylic dianhydride and diaminodiphenylsulfone in different –COOH/imidazolium molar ratios (n = 0.5, 1, and 2). Then, the thermally imidized complex membrane was doped with H₂SO₄. The conductivities of acid‐doped PI/IL complex membranes prepared by taking n of 1 are found to be in the range of 10^?4?10^?5 S cm^?1 at 180°C, whereas the acid‐free PI/IL complex membranes show the conductivity at around 10^?9?10^?10 S cm^?1. Thermogravimetric analysis results reveal that the acid‐doped PI/IL complex membranes are thermally stable up to 250°C. Dynamic mechanical analysis results of the acid‐doped ionically interacted complex membrane show that the mechanical strengths of the PI/IL complex membranes including 1‐methyl imidazolium tetrafluoroborate (MeIm‐BF₄) and 1‐ethyl 3‐methyl imidazolium tetrafluoroborate (EtIm‐BF₄) are comparable with those of pristine PI until 200°C. Furthermore, it can be clearly emphasized that the ionic interaction between carboxylic acid groups of PAA's and IL's cations offers a positive role in long‐term conductivity stability by preventing the IL migration at high temperatures. On the other hand, preliminary methanol permeability tests of the acid‐doped membranes show that they can also be considered as an alternative for direct methanol fuel cells. Copyright © 2011 John Wiley & Sons, Ltd.