All-iron flow batteries (IFBs) are being developed for low-cost large-scale energy storage applications. IFBs use abundantly available, non-toxic and low cost electrolyte materials^[1]. The all-iron flow battery involves plating and stripping of metallic iron in the negative half-cell and utilizes the Fe II/III (ferrous/ferric) redox couple reaction at its positive electrode^[2-3]. Hydrogen evolution occurs as the iron deposits at the negative electrode during the flow battery charging and even if the amount might be small, there will be an impact on the electrolyte balance over many cycles. Recently, Selverston et al.^[3] demonstrated a concept of hydrogen rebalance in an all-iron flow battery through designing a capillary-action galvanic reactor (CGR) to consume hydrogen and reduce excess ferric ions in the positive electrolyte reservoir. This design can maintain the balance of protons and reduce undesired precipitation reactions. A tube connects the negative tank headspace where hydrogen collects to the positive tank headspace where the CGR reactor is placed. An understanding of design factors on hydrogen transport through the tube between headspaces is desired and mathematical modeling^[4-7] is the approach we took to achieve this. In this poster, we report a mathematical model of hydrogen diffusion between the headspaces of the negative electrolyte tank and the positive electrolyte tank. The effects of dimensional parameters, including diameter of tank, height of tank, diameter of tube, length of tube and diffusion coefficient on hydrogen diffusion rates and partial pressure are investigated during both battery charge and battery standby cases. In Figure 1, we demonstrate that the mathematical model is reasonably validated by the previous published experimental data for a specific all-iron flow battery rebalance device.

Acknowledgements

This work is supported by the all-iron flow battery project (grant number: DE-AR0000352) funded by Department of Energy (DOE) of the United States.

References

[1] L.W. Hruska, R.F. Savinell, “Investigation of factors affecting performance of the iron-redox battery”, Journal of The Electrochemical Society, 128 (1981): 18-25

[2] K. L. Hawthorne, J.S. Wainright, R.F. Savinell, “Studies of iron-ligand complexes for an all-iron flow battery application”, Journal of The Electrochemical Society, 161 (2014): A1662-A1671

[3] S. Selverston, J.S. Wainright, R.F. Savinell, “In-tank hydrogen-ferric ion recombination”, Journal of Power Sources, 324 (2016): 674-678

[4] X. Ke, “CFD studies on mass transport in redox flow batteries”, Master Thesis, Case Western Reserve University, United States (2014), https://etd.ohiolink.edu

[5] X. Ke, J.I.D. Alexander, J.M. Prahl, R.F. Savinell, “Flow distribution and maximum current density studies in redox flow batteries with a single passage of the serpentine flow channel”, Journal of Power Sources, 270 (2014): 646-657

[6] X. Ke, J.I.D. Alexander, J.M. Prahl, R.F. Savinell, “A simple analytical model of coupled single flow channel over porous electrode in vanadium redox flow battery with serpentine flow channel”, Journal of Power Sources, 288 (2015): 308-313

[7] X. Ke, J.M. Prahl, J.I.D. Alexander, R.F. Savinell, “Mathematical modeling of electrolyte flow in a segment of flow channel over porous electrode layered system in vanadium flow battery with flow field design”, Electrochimica Acta, 223 (2017): 124-134