Design Principles for High‐Performance Meta‐Polybenzimidazole Membranes for Vanadium Redox Flow Batteries

The all‐vanadium redox flow battery (VRFB) plays an important role in the energy transition toward renewable technologies by providing grid‐scale energy storage. Their deployment, however, is limited by the lack of membranes that provide both a high energy efficiency and capacity retention. Typically, the improvement of the battery's energy efficiency comes at the cost of its capacity retention. Herein, novel N‐alkylated and N‐benzylated meta ‐polybenzimidazole ( m ‐PBI) membranes are used to understand the molecular requirements of the polymer electrolyte in a vanadium redox flow battery, providing an important toolbox for future research toward next‐generation membrane materials in energy storage devices. The addition of an ethyl side chain to the m ‐PBI backbone increases its affinity toward the acidic electrolyte, thereby increasing its ionic conductivity and the corresponding energy efficiency of the VRFB cell from 70% to 78% at a current density of 200 mA cm −2 . In addition, cells equipped with ethylated m ‐PBI showed better capacity retention than their pristine counterpart, respectively 91% versus 87%, over 200 cycles at 200 mA cm −2 . The outstanding VRFB cycling performance, together with the low‐cost and fluorine‐free chemistry of the N‐alkylated m ‐PBI polymer, makes this material a promising membrane to be used in next‐generation VRFB systems.