High-molecular-weight bulky-block poly(fluorenyl ether thioether ketone)s were successfully synthesized by a two steps one-pot protocol using N,N'-dimethy-S-carbamate masked dithiols for vanadium redox flow battery (VRB) application. The followed sulfonation procedure gave birth to novel sulfonated block poly(fluorenyl ether thioether ketone)s (SPFETKs) with controlled ionic exchange capacities (IEC). Membranes with proton conductivities higher than (IEC > 1.66 mequiv.g(-1)) or comparable to (IEC < 1.66 mequiv.g(-1)) that of Nafion117 membrane were achieved. The VO(2+) permeabilities of SPFETKs membranes were much lower than that of Nafion117 membrane. The thermal properties, mechanical properties, oxidative stability, water uptake, proton conductivity, VO(2+) permeability and cell performance were investigated in detail. (C) 2011 Elsevier Ltd. All rights reserved.