Unveiling Composition-Dependent Transport Properties in Imidazole—Levulinic Acid Systems for Redox Flow Batteries

Redox flow batteries (RFBs) require highly efficient electrolytes for optimal energy storage and release. One challenge in developing such electrolytes is the inverse relationship between viscosity and ionic conductivity, particularly in systems governed by vehicular diffusion mechanisms. This relationship can hinder the performance of RFBs, as increasing charge carrier concentrations often increases viscosity due to stronger intermolecular interactions, thus decreasing overall conductivity. However, enhancing proton-coupled electron transport (PCET) through the Grotthuss mechanism (proton hopping) can significantly boost the performance of electrolytes with low inherent conductivities, paving the way for better RFB systems.<br/>In this work, we explore concentrated hydrogen bond electrolytes (CoHBEs) systems composed of imidazole [Im] and levulinic acid [LA], which shows great potential as an RFB electrolyte. We provide a comprehensive analysis including thermal properties, densities, viscosities, and ionic conductivities, supplemented by molecular insights from dielectric spectroscopy and ¹H and ¹⁵N NMR and diffusion NMR.<br/>Our study reveals that the PCET mechanisms in the [Im][LA] system are highly dependent on composition. By leveraging eutectic melting point depression and hydrogen bonding networks, conditions favorable for the Grotthuss mechanism are created, enhancing proton transport. Through a detailed investigation of macroscopic transport properties and molecular dynamics, it has been demonstrated that the [Im][LA] system forms [LA]- and [Im]-rich Grotthuss chains at room temperature, resulting in superior PCET performance in a non-aqueous liquid electrolyte.