Development of a Redox-Flow-Battery Stack in Cascade Configuration

Recycling of vanadium is crucial to reduce the environmental impact and cost of all-vanadium redox-flow-batteries (VRFBs). There are various recycling processes, one being a vanadium extraction process, delivering V4+ as an intermediate product. In this work, a cascading flow stack is presented to partially reduce the intermediate with one pass through it. Two different electrode configurations are compared: the flow-by and the flow-through type. Therefore, single-cell tests were conducted, resulting in the selection of the flow-by cell for the cascading stack. The pressure drop through the flow-by cell was half as high as for the flow-through cell and symmetric on both half-cells. The ohmic resistance was measured through impedance spectroscopy. Resistances up to 6.2 Ω cm2 were measured for the flow-by cell, being lower than for the flow-through cell. The ohmic resistance for the flow-by cell, measured through polarization curves, was also lower, with 2 Ω cm2 to 2.3 Ω cm2 while charging and 2.4 Ω cm2 to 2.7 Ω cm2 while discharging. Lastly, the energy efficiency of the flow-by cell was higher, with up to 79.66 % at 50 mA cm−2. A four-cell stack was built to test the V4+ conversion. A conversion of 23 % with only one pass through the stack was reached at a current density of 70 mA cm−2 and a flow rate of 25 mL min−1. Charging 80 mL V4+ precursor with the same parameter setting by circulating it through the stack led to a 50 % conversion after 6.5 min. These results allowed the conclusion that a cell number of nine cells could be sufficient to reach the targeted 50 % conversion with one pass through the stack.