Redox flow batteries (RFBs) are open batteries offering scalable architecture and a promising technology for large-scale energy storage. The unique capability of decoupling energy storage capacity from power generation provides RFBs with flexibility for delivering desired capacity or output power. Among many chemistries developed for RFBs, all-vanadium redox flow batteries (VRFBs) currently show great potential for widespread commercialization [1, 2]. A unique attribute among all the chemistries developed for RFBs is that VRFBs utilize vanadium (in four different oxidation states) in the negative and positive electrolytes; this characteristic frees them from irreversible capacity decay as a function of electroactive species transport through the membrane (i.e. crossover). However, crossover of vanadium ions and water through the ion-exchange membrane is inevitable during the charge/discharge cycling. This undesired ionic and water crossover not only results in a lost discharge capacity (long-term impact) during cycling, but also has real-time influence on the cell performance (instantaneous impact). Current efforts to mitigate crossover are largely directed towards developing more highly selective membranes; however, an alternative approach can be pursued with deeper knowledge of the forces that drive crossover.

Numerous parameters affect solute (vanadium ions) and solvent (water) transport through the membrane; these include: membrane properties (polymer type, thickness, equivalent weight, and reinforcement), battery configuration (electrode morphology, flow field design), electrolyte composition and operating conditions (flow rate, temperature, and charge/discharge current) [3]. The concentration gradient and the electric field (via migration and electro-osmosis, respectively) are dominant driving forces for vanadium ion transport across the membrane [4, 5]. Water transport is due to multiple driving forces including electro-osmotic drag, thermo-osmosis, and osmotic and hydraulic pressure gradients [6, 7]. Therefore, it is necessary to understand these coincident forces affecting the transport of vanadium ions and water through the membrane.

In this talk, we will present extensive experimental data tailored to quantify the contributions to capacity decay stemming from ion-exchange membrane properties (thickness, equivalent weight, and degree of reinforcement), flow field design, electrode configuration and electrolyte properties. A major focus has been to understand the effect of the electrode/membrane interface on the overall capacity decay and contact resistance. Novel ex-situ conductivity cells have been prototyped to assess ionic conductivity of the ion-exchange membranes along with electrolytes leading to details on the impact of interfacial phenomena on ionic conductivity and crossover.

To quantify the long-term influence of crossover, a unique set-up has been designed and built that enables real-time measurement of ionic transport across the polymeric membrane using ultraviolet-visible (UV/Vis) spectroscopy. The set-up enables separation of contributions to crossover emerging from concentration and electrostatic potential gradients. Also, to investigate the instantaneous impact of crossover on the VRFB performance, a real-time current density distribution diagnostic has been implemented for measuring the in-plane current density distribution. The insights gained from this suite of experimental diagnostics have enabled identification and quantification of major contributors to capacity decay originating from the membrane, cell configuration, electrolyte composition, and operating conditions. These findings have instigated design of VRFBs with asymmetric configuration of the negative and positive sides. Such asymmetry in the reactor and external electrolytes passively mitigates vanadium ion and water crossover during long-term cycling, improving the energy storage efficiency of VRFBs. The novel cell configurations designed and engineered for this work are under invention disclosures at the University of Tennessee and provide an inexpensive and passive solution for further adopting the VRFBs as a safe and robust technology for grid-scale storage.

References:

1. D. S. Aaron, S. Yeom, K. Kihm, Y. Ashraf Gandomi, T. Ertugrul, M. M. Mench, Journal of Power Sources, 366, 241-248 (2017); https://doi.org/10.1016/j.jpowsour.2017.08.108.
2. Y. A. Gandomi, D. Aaron, T. Zawodzinski, and M. Mench, Journal of The Electrochemical Society, 163 (1), A5188-A5201 (2016); doi: 10.1149/2.0211601jes.
3. Y. Ashraf Gandomi, D. S. Aaron, M. M. Mench, Membranes, 7(2), 29 (2017); doi:10.3390/membranes7020029.
4. Y. A. Gandomi, D. Aaron, and M. Mench, Electrochimica Acta, 218, 174-190 (2016); http://dx.doi.org/10.1016/j.electacta.2016.09.087.
5. Y. A. Gandomi, T. A. Zawodzinski, and M. M. Mench, ECS Transactions, 61 (13); 23-32 (2014). doi:10.1149/06113.0023ecst.
6. Y. A. Gandomi, M. Edmundson, F. Busby, and M. M. Mench, Journal of The Electrochemical Society, 163 (8), F933-F944 (2016); doi: 10.1149/2.1331608jes.
7. Y. A. Gandomi and M. M. Mench, ECS Transactions, 58 (1), 1375-1382 (2013); doi: 10.1149/05801.1375ecst.