Performance Evaluation of the Non-Aqueous Vanadium Redox Flow Battery with Different Separators

Non-aqueous redox flow battery (RFB) systems are being considered for grid-level energy-storage applications. Non-aqueous electrolytes exhibit wide electrochemical potential windows, generally > 2 V, which has a direct impact on the system energy and power density prospects. The increased cell potential is considered the primary advantage of non-aqueous RFBs over aqueous RFBs, which are limited by the electrochemical stability of water. Nevertheless, the development of non-aqueous RFBs is still at an early stage and thus significant research efforts are needed to achieve similar performance as in aqueous RFBs, especially, at the cell component level.  The membrane or separator is one component that to a great extent affects the overall performance of RFB systems for practical applications.  However, few reports have been found in the literature on the development and evaluation of separator materials for non-aqueous RFBs to date.

In this work, a small-scale redox flow battery prototype was designed to simulate energy storage and to evaluate the performance parameters of non-aqueous redox electrolytes.  Cation exchanged Nafion® ionomer and microporous Daramic separators were prepared and investigated. A non-aqueous, single-metal redox flow battery based on vanadium (III) acetylacetonate (V(acac)₃) with a thermodynamic potential of  2.2 V was considered. At least a 70-fold increase in charge/discharge current density was achieved by using modified Nafion® or a microporous separator compared with current literature. Coulombic and round-trip energy efficiencies of 70-90% and of 65-80%, respectively, were obtained depending on the separator. Both the modified Nafion® and microporous separators are promising options for non-aqueous redox electrolyte systems.