Investigation of Faradaic Contributions from Bipolar Plates to Areal Current Density in Vanadium Redox Flow Batteries
Authors: Daugherty Michael , Aaron Douglas , Mench Matthew M. 
Published: February 2020

All-vanadium redox flow batteries (VRFBs) have been identified as a promising technology for bridging the gap to integrating renewables onto grids as well as improving grid-level power generation efficiency. VRFBs are prime candidates for this role on account of their modularity, scalability, and relatively high performance. [1] There are still, however, many design parameters that can be more highly optmized to improve and specialize the aspects of this technology. Capacity fade is a parasitic loss that occurs in all electrochemical enery storage systems and is one of the parameters that can be most improved in VRFBs. Fading capacity is due to a host of variables in the system, such as membrane permeability and electroactive species concentration.[2] Degradation of cell components (electrodes and membranes) can also contribute to capacity decay in the form of increased overpotentials throughout the systems operating range.
The kinetics at the negative electrode (V(II) ↔ V(III)) are generally understood to be more sluggish than at the positive electrode [3] as well as being less stable (i.e. degrading more rapidly with respect to time). Several high performance materials have been identified for the negative electrode [4] [5], however extended cycling data on the order of years is uncommon in the literature. Mitigation of this degradation on this time scale is one such optimization that can improve voltage efficiency and mitigate capacity fade.
In this work we present our most recent experimental work in which the Faradaic current that can be supported by the plates is quantified on different sets of VRFB hardware and isolated from the current on the combined plates with GFD 2.5 mm carbon felt electrodes. For this work symmetric cells were utilized, to isolate crossover effects, in conjunction with electrochemical impedance spectroscopy and polarization curves to quantify the contributions from the different processes at each overpotential. Likewise, experiments were also performed on a symmetric cell where the plates served as the electrodes to quantify the parallel contribution. The data were assessed via statistical tests in order to determine significance.
The results of this study show that, graphitic bipolar plates can support 35-50% of the current density on the negative electrode at low overpotentials and 20 and 30% at higher overpotentials. The open circuit impedance data also show that the ASR of the cell with felt electrodes is increasing at a rate of around 25 mΩ cm2 min−1 at this stage in the electrode’s life. This degradation is nonlinear and asymptotes to a stable ASR, however, this implies that the percentage measured in this context is smaller than the steady state percentage. These combined results suggests a material oriented approach to improving the bipolar plate material in order to achieve a more stable negative electrode.
References
[1] D. S. Aaron, Q. Liu, Z. Tang, G. M. Grim, A. B. Papandrew, A. Turhan, T. A. Zawodzinski, and M. M. Mench. Dramatic performance gains in vanadium redox flow batteries through modified cell architecture. Journal of Power Sources, 206(2010):450–453, 2012.
[2] Y. Ashraf Gandomi, D. S. Aaron, J. R. Houser, M. C. Daugherty, J. T. Clement, A. M. Pezeshki, T. Y. Ertugrul, D. P. Moseley, and M. M. Mench. Critical Review — Experimental Diagnos- tics and Material Characterization Techniques Used on Redox Flow Batteries. Journal of The Electrochemical Society, 165(5):970–1010, 2018.
[3] H. Fink, J. Friedl, and U. Stimming. The faster half-cell in a Vanadium Redox Flow Battery is determined by the electrode composition. The Journal of Physical Chemistry, 2016.
[4] A. M. Pezeshki. Impedance-Resolved Performance and Durability in Redox Flow Batteries (Doc- toral Dissertation). (August), 2016.
[5] L. Wei, T.S. Zhao, G. Zhao, L. An, and L. Zeng. A high-performance dual-scale porous electrode for vanadium redox flow batteries. Journal of Power Sources, 325(2016):329–336, 2016.

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