Effects of flow direction on electrolyte blockage induced by hydrogen evolution in a redox flow battery under stack feeding conditions

In the operation of redox flow battery stacks, chemically and physically induced gas evolution is almost inevitable. Evolved gas bubbles block flow paths in electrodes, and pose challenges to uniform electrolyte distribution. For flowthrough porous electrodes, the electrolyte flow direction is generally designed to be upward, so that the motion of bubbles aligns with the direction of buoyancy force, facilitating bubble discharge. However, with serpentine and interdigitated flow fields, the electrolyte flow direction in electrodes is not entirely upward, and horizontal and downward flows also exist. To investigate the impact of flow direction on electrolyte flow resistance in redox flow battery electrodes, we design an in-situ hydrogen evolution experiment using a single cell, and realize real-time measurement of electrolyte flow resistance under upward, horizontal, and downward flow directions. A gravity-driven electrolyte feeding system is employed, simulating constant pressure difference feeding conditions in real redox flow battery stacks. Results show that under the impact of bubble buoyancy force, the liquid-phase relative permeability of upward flow is higher than those of horizontal flow and downward flow. Under high feeding pressure differences, the impact of bubble buoyancy force is insignificant. However, under low feeding pressure differences, the liquid-phase relative permeability of upward flow is obviously higher than that of downward flow, with horizontal flow falling between the two. Under 5 kPa, the downward flow develops into complete blockage, whereas the liquid-phase relative permeability values of upward and horizontal flows remain above $\mathbf{0 . 6}$.