New polymer electrolyte membrane for medium-temperature fuel cell applications based on cross-linked polyimide Matrimid and hydrophobic protic ionic liquid

New hydrophobic protic ionic liquid, 2-butylaminoimidazolinium bis(trifluoromethylsulfonyl)imide (BAIM-TFSI), has been synthesized. The ionic liquid showed good thermal stability to at least 350 degrees C. The conductivity of BAIM-TFSI determined by electrochemical impedance method was found to be 5.6 x 10(-2) S/cm at 140 degrees C. Homogeneous composite films based on commercial polyimide (PI) Matrimid and BAIM-TFSI containing 30-60 wt% of ionic liquid were prepared by casting from methylene chloride solutions. Thermogravimetric analysis data indicated an excellent thermal stability of PI/BAIMTFSI composites and thermal degradation points in the temperature range 377 degrees C-397 degrees C. The addition of ionic liquid up to 50 wt% in PI films does not lead to any significant deterioration of the tensile strength of the polymer. The dynamic mechanical analysis results indicated both an increase of storage modulus E' of PI/BAIM-TFSI composites at room temperature and a significant E' decrease with temperature compared with the neat polymer. The cross-linking of the PI with polyetheramine Jeffamine D-40 0 allowed to prepare PI/Jeffamine/BAIM-TFSI (50%) membrane with E' value of 300 MPa at 130 degrees C. The ionic conductivity of this cross-linked composite membrane reached the level of 10(-2) S/cm at 130 degrees C, suggesting, therefore, its potential use in medium-temperature fuel cells operating in water-free conditions. (C) 2021 Elsevier Ltd. All rights reserved.