Design of polybenzimidazolium membranes for use in vanadium redox flow batteries

In recent years, polybenzimidazole (PBI) membranes have been proposed for vanadium redox flow batteries (VRFBs) as an alternative to perfluoroalkylsulfonic acid membranes such as Nafion™. Despite their excellent capacity retention, PBI membranes tend to suffer from a low ionic conductivity. The formation of a polybenzimidazolium through an N-alkylation of the benzimidazole core is shown to improve the ionic conductivity of the membrane, with this class of materials having found uses in alkaline fuel cell and water electrolysis systems. However, much less is known about their incorporation into a VRFB. This article describes the use of hexamethyl-p-terphenyl polybenzimidazolium (HMT-PMBI) membranes for a vanadium redox flow battery, with the membrane characteristics in acidic media being related to their performance in a single-cell VRFB setup. A change of the degree of methylation from 56 to 65, 75, and 89% leads to an increase in ionic conductivity, correlated with an increased fraction of free water in the ionomer. The corresponding increase in cell performance is, however, accompanied by a drop in capacity retention. The membrane with a degree of methylation of 65% shows balanced properties, with a 5% higher efficiency and a two times improved capacity retention compared to Nafion™ NR212 over 200 charge-discharge cycles at 200 mA cm^-2.