Performance evaluation of redox flow batteries using major parameters summarizing effects of structure and operation conditions

The vanadium redox flow battery (VRFB) is expected as a large-capacity battery for leveling output fluctuation of variable renewable energy because of its characteristics: flexible design of charging and discharging capacities, superior responsiveness and safety, and other advantageous characteristics. In the previous research, the authors investigated the cell performance and the current density distribution in various operation conditions experimentally, and developed analysis models which can evaluate the effects of structure and operation conditions, based on the experimental results. The objective of this study is to propose an evaluation method of these effects on various overpotentials. First, electrochemical impedance spectroscopy was applied and it was shown that our conventional method are useful to evaluate the overpotentials during discharging where the method can divide measured overpotential into activation and concentration overpotentials with satisfactory accuracy. The analysis models were also developed using the experimental results. Then, two major parameters were introduced based on simple evaluation equations to calculate discharge performance under very low current density operations. The two major parameters summarize the effects of structure and operation conditions on the cell performance: one represents a ratio of through-plane ionic and electric resistances of porous electrode to activation overpotential effects, and the other represents a ratio of concentration overpotential effects at the electrode surface to in the electrolyte flow. This study showed that the major parameters and further introduced two dimensionless diagrams make it possible to evaluate various overpotentials visually even in high current density operations. Finally, the experimental validation of the proposed evaluation method was conducted, and the effectiveness for efficient design of the optimal structure and operation conditions to achieve high performance was demonstrated by showing some examples: evaluation of the effects of electrode thickness on the activation overpotential and the effects of SOC on the concentration overpotential.