Nafion/functionalized metal–organic framework composite membrane for vanadium redox flow battery

Abstract

Although vanadium redox flow batteries hold great promise for energy storage systems, Nafion (being a conventional membrane material) requires improvement in terms of the crossover phenomenon. Herein, a nanoporous metal–organic framework (MOF) is applied as a membrane for vanadium redox flow batteries via addition of the MOF to a Nafion solution to prepare Nafion/MOF composite membranes via casting. Composite membranes are fabricated into a Nafion matrix by introducing small amounts of CAU-10-X (X = –OH, –CH 3 , –OS1, –OS2), an Al-based MOF synthesized with ligands having hydroxyl, methyl, and a combination of hydroxyl and sulfonic acid functional groups. The membrane physicochemical properties (such as water uptake, swelling ratio, ion exchange capacity, proton conductivity, and electrochemical properties from charge–discharge experiments on a single cell) are compared and evaluated. In 100 charge–discharge cycles of the single cell at a current density of 160 mA cm −2 , the composite membrane N/CAU-10-OS1 with 0.6 wt% dispersed MOF and a combination of hydroxyl and sulfonic acid functional groups exhibits capacity retention of 80.34% and energy efficiency of 77.56% at approximately 50 μm thickness. These values are better those achieved by commercial separation membrane Nafion 115 with 127 μm thickness tested under identical conditions and the proposed membrane is thus expected to be economically advantageous. • Using casting, Nafion/metal organic framework (N/MOF) composite membrane prepared. • Small amount of functionalized MOF in Nafion improves membrane performance in energy efficiency and cycle retention. • N/MOF membrane has lower vanadium ion permeability and thickness than Nafion 115. • N/MOF membrane shows high capacity retention (80.34%) at 100 cycles and 160 mA cm −2 .