The need for electrochemical energy storage increases along with the growing share of fluctuating renewables for power generation. The all-vanadium redox flow battery (VRB) has experienced increasing attention in this context. Its electrolyte membrane is a key component, yet currently perfluorinated materials (e.g., Nafion) are mainly used in VRBs, which have neither been designed for this particular application, nor are they cost-effective. The cross-over of redox active vanadium species impairs cell efficiency, in particular in case of thin Nafion membranes with low ohmic resistance. Here, we present a potentially generic concept of a bifunctional ion-conducting membrane prepared by radiation grafting with sulfonic acid proton exchange sites and amidoxime moieties. The crossover of vanadium-ions is reduced four-fold in the presence of amidoxime groups without markedly impairing the conductivity of the membrane. A VRB cell containing such membrane shows increased energy efficiency and negligible capacity fading over 122 charge / discharge cycles compared to cells with Nafion membrane, which showed a loss of discharge capacity of around 35% after 35 cycles and considerable electrolyte imbalance. (C) The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: [email protected]. All rights reserved.