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.