Ion and water transport in functionalized PEEK membranes

Water sorption, water transport and ion transport are compared for polyether–ether ketone polymers functionalized with sulfonic acid, quaternary amine hydroxide, and imidazolium hydroxide. Water sorption in all three ionomers increases modestly with increasing water activity up to aw=0.6 and then increases more rapidly between 0.60.6. The increase in ion conductivity with increasing water activity is 10 times greater for the proton exchanged polymer compared to the hydroxide exchanged polymers. Water permeation from partially saturated vapor streams is primarily diffusion limited for all three ionomers and the permeation rates were approximately the same. Water pervaporation from liquid water appeared to be interfacial transport limited for membranes<120 μm thick; the pervaporation rates at 40 and 60 °C were independent of membrane thickness. Water sorption and transport show little dependence on the mobile ion while the ion transport appears to depend on both ion size and hydrophilicity of the fixed ion.

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► Proton conductivity is ten times greater than hydroxide conductivity.
► Proton conduction occurs at a lower threshold water content than hydroxide conduction.
► Permeate interfacial transport limits water pervaporation across PEEK membranes.
► Water vapor permeation across PEEK membranes is diffusion controlled.