Gas evolution induced vicious cycle between bubble trapping and flow choking in redox flow battery stacks

The uniform delivery of electrolyte to a parallelly fed redox flow battery stack might be impaired due to the maldistribution of evolved gas bubbles in each single electrode. Entrapped gas bubbles bring about flow choking, resulting in the decrease of liquid velocity, which in turn weakens bubble sweeping and aggravates bubble trapping. Thus, a vicious cycle between bubble trapping and flow choking occurs in porous electrodes. To quantitatively illustrate the development and hazard of this vicious cycle, we establish analytical models based on previous experimental data and present the correlation among gas saturation, liquid velocity, pressure drop and hydrogen evolution rate. When the liquid velocity drops from 10 mm s⁻¹ to 1 mm s⁻¹, the gas saturation exhibits more than fourfold increase at a given hydrogen evolution rate, showing that more bubbles tend to reside in the electrode and the flow path of liquid phase become narrower. With the decrease in the pressure drop, the liquid velocity becomes lower and more sensitive to the gas evolution rate and the electrode wettability. As the pressure drop further drops below a critical value, the liquid flow is very likely to be blocked, resulting in severe maldistribution of electrolyte feeding.