Tandem Measurement of Ion and Water Transport Properties for Vanadium Redox Flow Batteries

The vanadium redox flow battery (VRFB) is a tool for energy storage that has been promising in applications of large grid storage. The battery is composed of positive and negative cells operating with VO²⁺/VO₂⁺ and V(II)/V(III) redox couples, respectively.  The two cells are separated by an ion exchange membrane, which allows proton conduction to maintain electrical balance during the charge/discharge cycles¹.  The membrane also allows water, vanadium and acid to crossover from one cell to the other, this can lead to concentration imbalances and self-discharge, ultimately decreasing the efficiency of the battery.^2,3

Nafion, a membrane commonly used to study this application, has been observed to allow vanadium ion crossover. This causes self-discharge of the battery and affects battery lifetime.³ By gaining a better understanding of the effects of the vanadium ions in membranes such as Nafion, methods can be extended to comparison of alternative membranes for uses in VRFBs, such as those with a hydrocarbon base.  EPR has also previously been used to further understand the environment of absorbed species in the membrane⁴, including alternative hydrocarbon membranes.^5,6

Here we report tandem UV/Vis and EPR measurements of vanadium ion transport across ion exchange membranes. A diagram of the experimental setup is shown in Figure 1. Previous studies have shown the VO²⁺ ion permeability to have a dependence on acid concentration in solution.^7,8 This study is extended to observe the effects of additional vanadium ions diffusing with and against each other while mimicking varying states of charge. The transport of one species is assumed to be intrinsically linked to the transport of the other vanadium species. Ultimately, the effects of convection on ion transport during battery charge/discharge cycles can be monitored with this tendem UV/vis / EPR method.

Figure 1. Diagram of the tandem EPR/UV/Vis method for monitoring water and ion transport in VRFB systems

References

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(2)           J. Xi; Z. Wu; X. Qiu ; Chen, L. J. Power Sources 2007, 166, 531.

(3)           Weidmann, E.; Heintz, E.; Lichtenthaler, R. N. J. Mem. Sci 1998, 141, 207.

(4)           Lawton, J.; Budil,D. J. Phys. Chem. B 2009, 113, 10679.

(5)           Lawton, J.; Budil, D. J. Membrane Sci. 2010, 357, 47.

(6)           Lawton, J.; Budil, D. Macromolecules 2010, 43, 652.

(7)           Lawton, J.; Jones, A.; Zawodzinski, T. A. J. Electrochem. Soc. 2013, 160, A697.

(8)           Lawton, J.; Aaron, D.; Tang, Z.; Zawodzinski, T. A.; J. Membrane Sci. 2013,428, 38.