Effect of electrode compression and operating parameters on the performance of large vanadium redox flow battery cells

Studies in small cells of vanadium redox flow battery have shown electrode compression to have significant effect on the cell performance. However, the results are conflicting and the data have been obtained for very high electrolyte circulation rates. In the present study, experimental investigations have been conducted in vanadium redox flow battery cells of active area of 426 cm(2) and 936 cm(2) over a wide range of operating conditions typical of large cells and for electrode compressions of 20, 35, 50 and 70%. The influence of operating current density, electrolyte circulation rate and state of charge has been specifically studied for both cell sizes. Cell pressure drop studies have also been conducted. Results show that the beneficial effects of electrode compression are mostly nullified by the loss of permeability at current densities in the range of 30-60 mA cm(-2). At higher current densities, intermediate compressions of 35 and 50% are found to be the most optimal. Low state of charge tolerance is found to be better for the larger cell than the smaller cell, presumably due to better convective transport of electrolyte through the electrode arising from higher pressure drop.