A series of novel sulfonated poly(phthalazinone ether ketone)s containing pendant phenyl moieties (SPPEK-Ps) are synthesized and thoroughly characterized. The chemical structures of the polymers are confirmed by H-1 NMR and FTIR analysis. The physicochemical properties and single cell performance of SPPEK-P membranes are systematically evaluated, revealing that the membranes are thermally, chemically and mechanically stable. The area resistances of SPPEK-P-90 and SPPEK-P-100 are 0.75 Omega cm(2) and 0.34 Omega cm(2), respectively. SPPEK-P membranes are impermeable to the bulky hydrated VO2+ ion and exhibited low V3+ ion permeability (SPPEK-P-90, 2.53 x 10(-5) cm min(-1)) (Nafion 115 membrane: 9.0 x 10(-4) cm min(-1)). Tests of SPPEK-P-90 in vanadium redox flow batteries (VRFBs) demonstrate a comparable columbic efficiency (CE) and energy efficiency (EE) to that of Nafion 115, where the CE is 98% and the EE is 83% at 60 mA cm(-2). Moreover, the SPPEK-P-90 membrane exhibits stable performance in cell over 100 charge-discharge cycles (similar to 450 h). (C) 2017 Elsevier B.V. All rights reserved.