Vanadium/air redox flow batteries (VARFB) promise higher energy densities compared to all-vanadium redox flow batteries (VRFB). However, VARFB suffer from crossover processes through the membrane, i.e. vanadium crossover and oxygen permeation. The vanadium crossover causes ongoing capacity losses and therefore reduces the lifetime of the battery. Additionally, the coulombic efficiency is reduced due to vanadium crossover and oxygen permeation. In this contribution we propose a straightforward routine for Nafion 117 (N117) membrane modification to reduce both vanadium crossover and oxygen permeation. Layer-by-layer (LbL) deposited films of polyethylenimine (PEI) and Nafion ionomer are build up on the membrane by dipping the membrane alternatingly in solutions of the polyelectrolytes. The modification of the membranes is characterized with infrared (IR) spectroscopy, scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and film thickness measurements. The properties of the modified membranes are investigated by determining the proton conductivity, vanadium crossover and oxygen permeation. By the application of a LbL film of PEI/Nafion obtained after 10 LbL deposition repetitions, the selectivity (sigma(H)+/P-v2+) of the membrane towards protons is increased by factor 21. Using this membrane in a VARFB reveals a strongly reduced vanadium crossover (approx. -70%) during a cycle as determined with inductively coupled plasma mass spectroscopy (ICP-MS) analysis of the positive electrolyte. The coulombic efficiency increases from 81% to 93% and the energy efficiency from 41.5% to 45.2%. TGA and IR measurements of the membrane after VARFB operation indicated a vanadium ion uptake into the membrane and the stability of the LbL film under conditions of VARFB operation. (C) 2016 Elsevier B.V. All rights reserved.