Conventional redox-flow batteries (RFBs), like all-vanadium, contain reactors with many individual cells stacked in series electrically and in parallel fluidically.  Because the positive and negative electrolytes are conductive (> 10 S/m), shunt currents flow in the fluidic manifolds connecting the individual cells.  This is a long recognized source of inefficiency in RFBs and in some fuel-cell [1, 2] and electrolyzer [3] stacks with circulating liquid electrolytes.  These undesirable shunt currents are typically managed by designing the manifold and port regions of the cell stacks to have large enough resistances to yield acceptable efficiency without causing excessive pressure drop [1, 4].  Equivalent-circuit models with resistor elements for the manifold and port are normally used to model the shunt and port currents when analyzing these tradeoffs (e.g., [2, 5]).

Considerably less attention has been paid to the electrochemical reactions that support the shunt currents.  For example, at the positive electrodes at the positive end of an all-vanadium RFB cell stack at open circuit, V(V) is discharged to V(IV) in the active area (drain current), while V(IV) is converted to V(V) in the vicinity of the port to support the shunt current.  The simultaneous occurrence of the V(IV)/V(V) reaction in the anodic and cathodic directions on the same electrode leads to interesting potential and current profiles.  More importantly, it is possible to develop large overpotentials at the junction between the edge of the electrically-conductive bipolar plate and the insulating port region that can cause deleterious reactions like carbon corrosion.

In this talk, steady-state and transient numerical simulations of an all-vanadium RFB stack with two cells will be presented.  The impact of the port and manifold resistances on the overpotential at the junction between the more positive bipolar plate and the adjacent insulating port will be discussed.  Figure 1 shows a nonlinear relationship between port resistance and overpotential at the junction.  High overpotentials, probably sufficient to cause carbon corrosion, are observed when the port resistance is low. The governing relationships and design criteria will be introduced.

References

1. C. Reiser, U.S. Patent 3,634,139 (1972).

2. M. Katz, J. Electrochem. Soc., 125, 515 (1978).

3. I. Rousar, J. Electrochem. Soc., 116, 676 (1969).

4. Q. Ye, J. Hu, P. Cheng, and Z. Ma, J. Power Sources, 296, 352 (2015).

5. E. A. Kaminski and R. F. Savinell, J. Electrochem. Soc., 130, 1103 (1983).