An Electrolyte Rebalancing Procedure for Vanadium Redox Flow Batteries
Authors: Poli Nicola , Schäffer Michael , Trovo Andrea , Noack Jens , Guarnieri Massimo , Fischer Peter 
Published: December 2020

A new method is proposed that restores the battery energy and capacity of a Vanadium Redox Flow Battery, by counteracting the charge imbalance caused by air-oxidation and hydrogen evolution in the negative electrolyte. The process makes use of a regeneration system specifically designed and successfully tested. This system is built around an electrolysis cell, that is called “regeneration cell”, where an electro-reduction reaction of V(V) is coupled with an oxygen evolution reaction catalyzed by a Ti-IrO2 electrode. The rebalancing of three different unbalanced electrolytes was tested: V3.55+, V3.60+ and V3.65+, namely: 45% V(III)/55% V(IV), 40% V(III)/60% V(IV) and 35% V(III)/65% V(IV) respectively. The behavior of the VRFB cell during the rebalancing process was investigated both numerically and experimentally. An end-point strategy for the rebalancing process without additional sensors has been developed by using of a specific mathematical model capable of simulating the current response and concentrations profile during the rebalancing process. This strategy allowed to improve the performance of the rebalancing process. After rebalancing, the resulting electrolyte oxidation states were respectively: V3.483+, V3.482+, V3.484+. Since a perfect balanced electrolyte is V3.50+ (50% V(III), 50 % (VIV) such results demonstrated that the electrolyte imbalance was drastically reduced and the rebalancing process was successful.

References

[1] A. Trovò, G. Marini , A. Sutto, P. Alotto, M. Giomo, F. Moro, M. Guarnieri, Standby thermal model of a vanadium redox flow battery stack with crossover and shunt-current effects. Appl Energy 240 (2019):893–906.
[2] M. Guarnieri, A. Trovò, G. Marini, A Sutto, P. Alotto, High current polarization tests on a 9 kw vanadium redoxflow battery, J. Power Sources 431 (2019): 239–249.
[3] N.V. Roznyatovskaya, V.A. Roznyatovsky, C.C Höhne, M. Fühl, T. Gerber, M. Küttinger, J. Noack, P. Fischer, K. Pinkwart, J. Tübke. The role of phosphate additive in stabilization of sulphuric-acid based vanadium(V) electrolyte for all-vanadium redox-flow batteries, J. Power Sources 363 (2017): 234–243.
[4] N. Roznyatovskaya, T. Herr, M. Küttinger, M. Fühl, J. Noack, K. Pinkwart, J. Tübke, Detection of capacity imbalance in vanadium electorlyte and its electrochemical regeneration for all-vanadium redox-flow battery, J. Power Sources 302 (2016): 79-83.
[5] J. Noack, N. Roznyatovskaya, K. Pinkwart, J. Tübke, Vanadium proton exchange membrane water electrolyser, J. Power Source 349 (2017): 144-151.
[6] J. Noack, P. Fischer, J. Tübke, K. Pinkwart, Method for regenerating the electrolyte solution of a rechargeable redox flow battery, Patent WO 2016092004 AI, 2016.
[7] A. Trovò, F. Picano, M. Guarnieri, Comparison of energy losses in a 9 kW vanadium redox flow battery, J. Power Sources 440 (2019): 227144.
[8] M. Guarnieri, A. Trovò, F. Picano, Enhancing the efficiency of kW-class vanadium redox flow batteries by flow factor modulation: An experimental method, Appl. Energy 262 (2020): 114532.
[9] T. Gerber, P. Fischer, K. Pinkwart, J. Tübke, Segmented printed circuit board electrode for locally-resolved current density measurements in all-vanadium redox flow batteries, Batteries 5 (2018) :38.
[10] A. Trovò, A. Saccardo, M. Giomo, M. Guarnieri, Thermal modeling of industrial-scale vanadium redox flow batteries in high current operations. J Power Sources 424 (2019): 204–14.

Figure 1