Numerical studies of the effectiveness of electrodes with conductive dots in flow batteries

In flow batteries generally the entire surface of the electrodes are conductive. Due to the highly corrosive nature of the electrolytes of many types of flow batteries there exists a dilemma in choosing electrode materials: highly corrosion resistant materials have very high cost and low cost materials have low corrosion resistance and thus low durability. To overcome this difficulty, a novel design of electrodes with conductive dots and corrosion resistant film is proposed [1]. Such electrodes can be made of low-cost base metals covered with corrosion resistant film and evenly distributed conductive dots, which can be made of highly corrosion resistance material such as gold, titanium, etc. Since the area covered by the highly corrosion resistance metals can be only a small percentage of the total electrode area the cost of the electrode can be relatively low. In this study, numerical simulations have been performed to study the effectiveness of such a design. In this work, lead-acid flow batteries are used as a model for redox flow batteries. The results from the electrode with conductive dots are compared with those from conventional electrodes. When conventional electrodes are used, the entire electrode surface is active for electrochemical reactions. When redox reactions occur under charging and discharging cycles, the redox species are consumed very fast and the electrolyte concentrations on the electrode surface decrease sharply from those in the bulk flow, leading to sharp decrease in reaction rate along the flow direction. When electrodes with conductive dots are used, though the active area is much smaller, reaction rates in the down-stream are similar to those in the upstream. This is due to the recovery of electrolyte concentrations in the nonactive area surrounding the conductive dots, mainly from diffusion. The more uniform reaction rates from the upstream to the downstream results in much higher average current density per unit active area. Thus, even though the active area of the electrode with conductive dots is much smaller, the total reaction rate per unit apparent electrode area is not much lower. The modeling results show that the electrodes with conductive dots are highly effective even when the area covered by the conductive dots is only a few percent of the total electrode area.