Organic Redox Flow Batteries: Insights from Experimental and Numerical Study

Abstract

Secondary batteries are the key technology for the sustainable development of electrical grids with a high share of renewable power generation. Among different battery technologies, Organic Redox Flow Batteries (ORFBs) are considered one of the most promising solutions for low-cost large-scale energy storage systems thus avoiding the social and environmental issues associated with extraction of raw materials for large batteries. Mathematical modeling of ORFBs, that complements experimental testing, is a key tool that helps to understand and analyze the behavior of particular candidates, including electrolytes, membranes and geometrical parameters, on a battery level at operating conditions that are close to operational.In this study, we present a single-cell zero-dimensional model based on our previous works [1,2] extended for simulation of redox flow batteries with different organic redox-couples. The model is based on the general mass and charge conservation principles. Hence, it can simulate a wide range of RFB systems with different scales and electrolytes. The key feature of the model is the approach for the determination of integral mass-transfer coefficient on the electrode-electrolyte interface that allows describing non-linear polarization behavior at high current densities. Such approach makes the model very flexible and esilty extadable even to the other types of batteries with hybrid configurations (e.g. solid boosters). The kinetic parameters were also fitted for the effective single-step heterogeneous redox mechanisms at positive and negative electrodes in the case when activation losses treatment was needed to reproduce the experimental polarization curves. In addition, the model simulates the dynamic behavior of the cells working with different redox-couples in the wide range of loading currents and electrolyte flow rate and thus allowing to explore their performance at different operating conditions. Validation of the model showed a good agreement with experimental data (average error less than 5%) in the range of current decities of 20 – 150 mA cm-2.The model was also used for investigation of scalability of the considered ORFBs analyzing their performance on the level of industial-scale systems.The influence of internal processes on the battery behavior has been studied and the effect of mass-transport limitations has been estimated. The key parameters (coulombic, voltage, energy efficiencies and electrolyte utilization) of the batteries have been computed for the wide range of current densities and flow rates. The obtained results provide important insights for reserachers and ingeneers to develop reliable and efficient organic RFB systems applicable for grid-scale energy storage systems.