Sub-5 nm Graphene Oxide Nanofilm with Exceptionally High H⁺/V Selectivity for Vanadium Redox Flow Battery

Highly H+/V selective membranes are desired in high-performance vanadium redox flow batteries (VFRBs) to overcome the crossover phenomena of vanadium species. Herein, we demonstrate the molecular-sieving nanochannels (∼0.84 nm) inside a graphene oxide (GO) laminate efficiently blocked the transport of vanadium ions, while allowing the transport of H+. Furthermore, an ultrathin (sub-5 nm) and highly selective GO nanofilm was successfully coated on a porous substrate to improve the H+ flux using a facile spin-coating method. The GO-coated thin-film composite (TFC) membrane showed much higher H+ flux with an exceptionally high H+/V selectivity (H+ permeation rate/VO2+ permeation rate, up to 850) due to the molecular-sieving nanochannels inside the GO nanofilm, leading to a much more enhanced VRFB performance in terms of energy efficiency (EE, 84.7%) compared to the benchmark Nafion membrane (EE, 69.2%), at 20 mA cm–2.