Rechargeable batteries play a vital role in effective utilization of solar and wind energy. Extreme cold condition reduces the performance of batteries severely. Several studies of the performance of Vanadium Redox Flow Batteries (VRFB) have been carried out on a VRFB cell with an active area of 426 cm2 at cell temperatures of 25, +10 and -10oC. Results indicate significant loss of discharge capacity and cell efficiency at low temperatures; this has been attributed to substantial increase in ohmic resistance and even higher increase in charge transfer resistance of the cell [1]. The present study is focussed on recouping the performance, at stack level, through design and operational measures. Two cell design parameters, namely, flow channel dimensions and electrode felt compression, have been optimized through experimental studies. At stack level, increased viscosity at low temperatures has been found to increase the pressure drop significantly leading to high parasitic power consumption for pumping electrolytes and also to lower diffusivity of the ionic species. The voltage window of operation has also been narrowed significantly at low temperatures. To counter these, two remedial measures, namely, decreasing the concentration of supporting electrolyte and using thermally activated electrode, have been investigated. Cell level studies have been carried out at two concentrations of sulphuric acid, 3M and 5M, with Vanadium concentration fixed at 1.6 M. The discharge capacity is found to have been increased from 6517 mAh to 7753 mAh at -10oC with 3M H2SO4 electrolyte. Viscosity of the electrolyte with 3M acid electrolyte is less than 10% compared to 5M acid electrolyte. Decrease in viscosity improved the diffusion of vanadium ions to the active sites and enhanced the performance of VRFB. Long cycling stability test carried out with 3M H2SO4 electrolyte has shown minimal capacity fade of 0.25% per cycle at -10oC compared to about 0.6% per cycle at 5oC for a stack with 5M H2SO4 electrolyte. Further to reduce the charge resistance of the electrode, studies carried out using activated felt at cell and stack level with 3M H2SO4 electrolyte. Energy efficiency of the 8-cell stack with thermally activated felt and 3M acid electrolyte increased by 12% compared to stack with bare felt and 5M acid electrolyte at -10oC. It is thus concluded that a VRFB stack designed for low temperature operations should have substantially different design features. With these measures, it is possible to retain a discharge capacity of 25 Wh/l even at sub-zero temperature operation of the VRFB system.





Reference: [1] Praphulla Rao, Sreenivas Jayanti, Influence of electrode design parameters on the performance of vanadium redox flow battery cells at low temperatures, Journal of Power Sources, 482(2021) 228988​