Non-aqueous organic redox active materials for a bicontinuous microemulsion-based redox flow battery

Abstract

To mitigate fluctuations in renewable energy sources, redox flow batteries (RFBs) attract much attention for stationary energy storage applications by taking advantage of decoupling power and energy. However, there are still some limitations for conventional RFBs, including low energy density, a narrow electrochemical window (∼1.23 V) for an aqueous electrolyte, and low ionic conductivity for a non-aqueous electrolyte. Herein, we demonstrate a bicontinuous microemulsion-based RFB using methyl-p-benzoquinone and 2,2,6,6-Tetramethyl-1-piperidinyloxy as anolyte and catholyte redox active materials, respectively. By stabilizing an aqueous and organic medium at a nanometer scale through a surfactant, the microemulsion electrolyte exhibits a wider electrochemical window and good ionic conductivity, a promising alternative to increase the energy density by applying non-aqueous redox species with high solubility and high redox potential. The resulting cell exhibits Coulombic efficiency of 80%–85% and capacity retention of 50% over 30 cycles. We provide a systemic analysis of performance limitation and capacity decay and proposed a possible dimerization mechanism for an anolyte through a density functional theorycalculation, ending with a discussion of the efforts needed to realize the application of microemulsion-based RFBs.