Sub-20 nm ultrathin perfluorosulfonic acid-grafted graphene oxide composite membranes for vanadium redox flow batteries

Abstract

Perfluorosulfonic acid (PFSA) membranes, such as Nafion, are widely used in vanadium redox flow batteries (VRFBs) because of their high proton conduction through the ion channels and excellent chemical stability. However, the high vanadium permeability of PFSA membranes induced by the randomly interconnected channels limits efficient cell operation. In this work, we demonstrate a sub-20 nm ultrathin PFSA-grafted graphene oxide/PFSA (PFSA-g-GO/PFSA) composite membrane with highly aligned ion channel morphology, which results in a 100-fold improvement in proton/vanadium ion selectivity compared to 25 μm-thick Nafion 211. In addition, the PFSA-g-GO nanosheets physically reinforce the ultrathin membrane while enabling the proton transport through the grafted PFSA ionomers, leading to stable cell operation at overall current densities from 40 to 200 mA cm−2. Especially, at a high current density of 200 mA cm−2, the PFSA-g-GO/PFSA composite membrane shows an energy efficiency (EE) of 78%, which is higher than that of Nafion 211, indicating its potential as an ion-selective membrane for VRFB.