Investigations toward a Non-Aqueous Hybrid Redox-Flow Battery with a Manganese-Based Anolyte and Catholyte

A new all-Manganese flow battery (all-MFB) as a non-aqueous hybrid redox-flow battery is reported. The discharged active material [Cat](2)[(MnCl4)-Cl-II] (Cat = organic cation) utilized in both half-cells supports a long cycle life. The reversible oxidation of [(MnCl4)-Cl-II](2-) to [(MnCl5)-Cl-III](2-) at the positive electrode and manganese metal deposition from [(MnCl4)-Cl-II](2-) at the negative electrode give a cell voltage of 2.59 V. Suitable electrolytes are prepared and optimized, followed by a characterization in static battery cells and in a pumped flow-cell. Several electrode materials, solvents, and membranes are tested for their feasibility in the all-MFB. An electrolyte consisting of [EMP](2)[MnCl4] and some solvent gamma-butyrolactone is cycled 500 times, both in a static as well as a flow-cell, over a period of two months, with coulombic efficiencies up to 83%. With the electrolytes prepared in this work, energy densities up to 74 Wh L-1 are possible, exceeding the VRFB benchmark system, using solely the cheap and abundant element manganese as the active material. Although further optimizations are necessary, this system represents a new and promising setup toward sustainable stationary energy storage.