Free volume of poly(perfluorosulfonic acid)/SiO2 composite proton exchange membranes by 129Xe NMR

► This work provides the first investigation of free volume in composite PEM by 129Xe NMR spectroscopy and relaxometry. ► The free volume in PFSA/SiO2 membranes shows a maximum at 2 wt.% content in silica. ► The free volume is heterogeneous.