Fabrication and characterization of phosphoric acid doped imidazolium ionic liquid polymer composite membranes

Abstract

Phosphoric acid (PA) doped imidazolium ionic liquid polymer composite membranes were fabricated using the approach of immersing the membranes based on imidazolium ionic liquids and polymers of sulfonated poly (ether ether) ketone (SPEEK) or polyvinylidenefluoride (PVDF) into pure phosphoric acid at room temperature. Imidazolium ionic liquids of 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6) or 1-butyl-3-methylimidazolium dihydrogenphosphate (BMIMH2PO4) link phosphoric acid molecules to polymer matrix. SPEEK is suitable to capture imidazolium ionic liquids owing to the strong interaction between imidazolium ionic liquid cations (BMIM+) and sulfonic acid group (SO3−). So the composite membranes based on SPEEK show promise as high temperature proton exchange membranes. Although all the components influence the conductivity values of these composite membranes, phosphoric acid molecular chains play a dominant role in the proton conduction. Proton conductivity of SPEEK/50%BMIMPF6/4.6PA membrane at a level of 3.0×10−2Scm−1 was achieved at 160°C under anhydrous conditions. Although the tensile stress at break decreased from 3.12MPa to 0.10MPa when the temperature increased from room temperature to 160°C, SPEEK/50%BMIMPF6/4.6PA membrane still possesses a stable conductivity of 2.0×10−2Scm−1 more than 600h at 160°C.