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

摘要

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+BF_4~-) 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 H_2SO_4. 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) Scm~(-1) at 180℃, whereas the acidfree 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℃. 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-BF_4) and 1-ethyl 3-methyl imidazolium tetrafluoroborate (EtIm-BF_4) are comparable with those of pristine PI until 200℃. 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.