SPEEK/PVDF binary membrane as an alternative proton-exchange membrane in vanadium redox flow battery application

Sulfonated polyether ether ketone (SPEEK)/polyvinylidene fluoride (PVDF) blends are employed to prepare the ion-exchange membranes for vanadium redox flow battery (VRB) application. The addition of the highly crystalline and hydrophobic PVDF effectively confines the swelling behavior of SPEEK. The vanadium ion permeability of SPEEK/PVDF membranes is one order of magnitude lower than that of Nafion117 membrane and SPEEK membrane. In VRB single-cell test, the SPEEK/PVDF binary membranes exhibit higher energy efficiency (EE) than recast SPEEK and Nafion117 at 30 mA cm⁻². The blend membrane with 30 wt% of PVDF (S_0.7P_0.3) shows EE of 83.2% at 30 mA cm⁻², which is superior to that of the Nafion117 and recast SPEEK membranes. In the self-discharge test, S_0.7P_0.3 membrane also possesses twice longer duration in open circuit decay test. These results indicate that the addition of PVDF is a simple and efficient way to improve the ion selectivity of SPEEK. With all the good properties and low cost, the SPEEK/PVDF membranes are expected to have excellent commercial prospects as ion-exchange membranes for VRB system.