Interfacial resistance and electric field modulation at the Nafion membrane–electrolyte interface in vanadium redox flow batteries

Abstract

Vanadium redox flow batteries are a leading technology for large-scale energy storage; however, high costs impede their widespread commercialization. A promising strategy to lower costs involves enhancing power density by reducing the ohmic resistance. Ion transport resistance across the membrane accounts for a large proportion of the total ohmic resistance; consequently, membranes as thin as several microns have been developed to reduce this resistance. However, the impact of membrane thickness on proton conductivity remains controversial, and the ion transport mechanism at the electrolyte-membrane interface remains unclear. In this study, we propose a thickness-difference-based method (TDM) to quantify the interfacial resistance at the liquid electrolyte/solid Nafion membrane interface. Our results reveal that the interfacial resistance constitutes approximately 28.9% of the overall ionic resistance in a vanadium redox flow battery utilizing a Nafion 212 membrane. Importantly, the proton conductivity of Nafion membranes is found to be independent of thickness after interfacial resistance is excluded. We also demonstrate that this interfacial resistance can be reduced by modulating the electric field at the membrane surface by depositing metal catalysts such as Bi and Ag. Furthermore, the incorporation of Bi significantly suppresses vanadium ion crossover through the Nafion membrane surface. This work presents methodologies for quantitatively analyzing proton transport resistance at the electrolyte-membrane interface and offers a strategy to reduce ionic resistance in flow batteries by manipulating the electric field at the membrane surface.