Free volume enhanced proton exchange membranes from sulfonated triptycene poly(ether ketone)

A triptycene based poly(ether ketone) (tripPEEK) was synthesized and sulfonated to form proton exchange membranes. The increase in intrinsic free volume resulting from the incorporation of sterically bulky triptycene moiety, imparts high affinity to water at all levels of relative humidity (RH) from 10% RH to 90% RH. S-tripPEEK membranes showed proton conductivities of 334 mS/cm at 85 °C at 90% RH and 0.37 mS/cm at 85 °C at 20% RH. Membranes of similar ion exchange capacity (IEC) are compared to deconvolute the effect of free volume from IEC in enhancing proton conductivity. Increasing the free volume of the membranes increases the proton conductivity and decreases the activation energy for proton conduction between 10% RH to 90% RH.