In the porous electrodes of a flow battery operating under high current densities, the mass transport resistance between the bulk solution in the pore and the electrode surface contributes a significant proportion to the voltage loss, especially at the end of charging/discharging process. It is generally recognized that high flow rate of electrolyte is beneficial in reducing voltage loss [1, 2], since the mass transport resistance is closely correlated with the electrolyte velocity. The aim of this work is to investigate the effect of electrolyte velocity on the mass transport in a typical flow battery structure by using a symmetric cell configuration with identical redox couples on both sides. The redox couple Fe²⁺/Fe³⁺, owning fast kinetics, is utilized to minimize the activation loss. The electrolyte containing equivalent concentrations of ferrous and ferric ions was stored in a single reservoir and circulated through both sides of the cell in parallel, which can keep the inlet concentrations invariant all the time.

The first experiment for investigating mass transport loss was conducted by feeding electrolyte containing 0.2M reactant at a low flow rate of 30 mL min^-1, and the stoichiometry of feeding reactant is about 1.9 at 200 mA cm^-2. As illustrated in Fig.1, the cell with thinner electrodes has significantly lower overpotential, partially due to its lower ohmic resistance. Operating at 200 mA cm^-2, the voltage difference between cells with 2 mm and 6 mm thick electrodes is reduced from 133 to 70 mV by increasing flow rate from 30 to 120 mL min^-1. These results suggest that in addition to the ohmic loss, mass transport loss also has significant disparity between different electrode thicknesses, especially at low flow rates.

Compared with altering electrode thickness, adjusting flow rate is a favored approach to investigating the effect of electrolyte velocity on mass transport due to the negligible variation of ohmic resistance. However, the stoichiometry of feeding reactant varies with the flow rate and this influence on mass transport loss should be minimized. In the following test, the cell with the 6 mm thick electrodes was fed with the electrolyte at fixed flow rates but different concentrations. It is worth noting that at 30 mL min^-1, increasing reactant concentration from 0.4 to 0.8 M improves the cell performance only a little and no improvements are found at 60mL min^-1. Thus, the improvement of cell performance by increasing flow rate is mainly attributed to the resulting increment of electrolyte velocity, if the inlet electrolyte concentration exceeds 0.8 M. The decreases of overpotential caused by doubling the velocity are listed in Table 1, where the largest difference exists at the lowest velocity range and the higher current density. At a velocity lower than 5 mm s^-1, significant mass transport loss was observed even though the stoichiometry of the reactant was high enough. However, extremely high velocity, higher than 20 mm s^-1for instance, is not recommended, since the reduction of voltage loss is limited and this slight benefit might be overweighed by the increment of pumping loss.

References

[1] A. Tang, J. Bao, M. Skyllas-Kazacos, Studies on pressure losses and flow rate optimization in vanadium redox flow battery, Journal of Power Sources, 248 (2014) 154-162.

[2] X.K. Ma, H.M. Zhang, C.X. Sun, Y. Zou, T. Zhang, An optimal strategy of electrolyte flow rate for vanadium redox flow battery, Journal of Power Sources, 203 (2012) 153-158.