Delineating Contributions from Vanadium Crossover and Electrode Degradation to Capacity Decay in Vanadium Redox Flow Batteries
Authors: Daugherty M. C. , Ashraf Gandomi Yasser , Pezeshki Alan , Aaron Douglas , Mench Matthew M. 
Published: February 2020

All-vanadium redox flow batteries (VRFBs) are promising candidates for large-scale energy storage thanks to their scalability, relatively high-performance and safety 1. However, wide-spread integration of VRFBs requires mitigation of rapid capacity decay over long-term cycling. There are three major contributors to discharge capacity decay during cycling in VRFBs: 1) undesired transport of vanadium ions and water through the ion-exchange membrane 2) degradation of cell components (predominantly electrodes, and membrane) and 3) gas-generation from side reactions. With careful selection of upper and lower voltage limits for cycling, gas-generation can be mitigated to a great degree. However, capacity decay due to undesired vanadium ion and water crossover along with component degradation is inevitable.
It is important to note that the capacity decay over long-term cycling is cumulative and therefore, capacity decay due to vanadium crossover and degradation are combined in cycling experiments making it challenging to distinguish the contribution from these two sources independently.
In this work, we will present our latest experimental data focused on delineating contributions from vanadium crossover and electrode degradation. The data include two ion-exchange membranes and two electrodes with different degradation behavior.
To this end, we have designed and built a unique facility utilizing multiple electrochemical and flow cells equipped with UV/Vis spectroscopy to measure the vanadium ion crossover in real-time 2-6. To assess the capacity decay due to combined degradation effect of all components, we have developed another diagnostic in which we use a symmetric cell set-up for negative (V(II)/V(III)) and positive (V(IV)/V(V)) sides separately. The degradation diagnostics utilize electrochemical impedance spectroscopy to quantify the contributions of cell overpotential during cycling stemming from charge-transfer, ohmic and mass transport.
The results of this study should provide more in-depth insight to optimize VRFBs with enhanced performance and reduced ion-crossover and degradation.

References:

1. D. Aaron, Q. Liu, Z. Tang, G. Grim, A. Papandrew, A. Turhan, T. A. Zawodzinski, and M. M. Mench, Journal of Power sources,
206, 450-453 (2012); http://dx.doi.org/10.1016/j.jpowsour.2011.12.026
2. Y. A. Gandomi, D. Aaron, T. A. Zawodzinski, and M. M. Mench, Journal of The Electrochemical Society,
163 (1), A5188-A5201 (2016); doi: 10.1149/2.0211601jes.
3. Y. A. Gandomi, D. Aaron, and M. M. Mench, Electrochimica Acta,
218, 174-190 (2016); http://dx.doi.org/10.1016/j.electacta.2016.09.087.
4. 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.
5. Y. A. Gandomi and M. M. Mench, ECS Transactions,
58 (1), 1375-1382 (2013); doi: 10.1149/05801.1375ecst.
6. Y. A. Gandomi, T. A. Zawodzinski, and M. M. Mench, ECS Transactions,
61 (13); 23-32 (2014). doi: 10.1149/06113.0023ecst.

Figure 1