(398h) Strategies for Performance Improvement of Polyoxometalates in Non-Aqueous Redox Flow Batteries

Polyoxometalates have been explored as multi-electron active species in non-aqueous redox flow batteries. Although non-aqueous systems offer a wider voltage window for redox flow battery operation, realization of this potential requires a judicious choice of solvent as well as polyoxometalate properties. We demonstrate here the superior performance of N,N-dimethylformamide (DMF) compared to acetonitrile as a solvent for redox flow batteries based on Li₃PMo₁₂O₄₀. This compound displays two 1-electron transfers in acetonitrile but can access an extra quasi-reversible 2-electron redox process in DMF. A cell containing 10 mM solution of Li₃PMo₁₂O₄₀ in DMF produced a cell voltage of 0.7 V with 2-electron transfers (State of Charge = 60%) and showed a good cyclability. As a strategy to boost energy density, operation of the redox flow battery at a higher concentration of 0.1 M Li₃PMo₁₂O₄₀ produced cells with cell voltage of 0.6 V in acetonitrile and cell voltage of 1.0 V in DMF; both showed excellent coulombic efficiencies of more than 90% over the course of 30 cycles. Another strategy to improve the performance is by employing an asymmetric cell with different polyoxometalates on each side to extend cell voltage. Li₆P₂W₁₈O₆₂ exhibited 3 quasi-reversible 2-electron transfers in the potential range between -2.05 V to -0.5 V vs. Ag/Ag⁺. 10 mM Li₆P₂W₁₈O₆₂/Li₃PMo₁₂O₄₀ in DMF produced a cell with cell voltage of 1.3 V involving 4-electron transfers (State of Charge = 50%) with coulombic efficiency of nearly 100% and energy efficiency of nearly 70% throughout the test with more than 20 cycles. These promising results demonstrate proof-of-concept approaches to improving the performance of polyoxometalates in non-aqueous redox flow batteries.