A grafted PEEK with graft chains of poly(sodium styrene sulfonate) (SSPEEK) has been prepared through atom transfer radical polymerization (ATRP) for use as a proton conducting membrane in vanadium redox flow battery (VRB).The Na-montmorillonite (Na-MMT) was introduced into the SSPEEK membrane by solution blending process to obtain a composite membrane.The composite membranes possess good thermal stability according to the TG analysis.The scanning electron microscopy indicates that the Na-MMT was uniformly dispersed in the polymer matrix.When the addition of Na-MMT reaches 9 phr

the permeability for vanadium ion of the composite membrane is 4.8810-6 cm2 min-1

which is lower than that of virgin SPFEK membrane (1.2410-5 cm2 min-1) and Nafion 117 (6.0410-6 cm2 min-1).The discharge time of the VRB assembled with the composite membrane is up to 215 min at 30 mA cm-2

which is higher than the Nafion's 198 min under the same measurement condition.The VRB single cell employing the SPFEK/Na-MMT membrane exhibits high coulombic efficiency which is about the same as that 8 Nafion 117.