An electrochemical stack model for aqueous organic flow battery: The MV/TEMPTMA system

In recent decades, flow batteries have been rapidly developing for large-scale, low-cost energy storage, yet with concerns about the corrosivity and cost of metal-based electrolytes, such as vanadium. Recent research has shown great interest in seeking alternative redox couples for aqueous organic flow batteries (AOFB) due to their potentially improved safety, relatively lower cost and a wide diversity of tailored molecular structures. While most literature emphasises microscale or cell-scale experiments and modelling for AOFB, very little literature has reported on a detailed stack-level model. To this end, this work proposes an electrochemical stack model for the MV/TEMPTMA system, one of the MV/TEMPO derivatives, which claims high capacity and power density when maintaining excellent stability. The electrochemical stack model is based on the equivalent circuit method, which determines the flow battery's resistances, currents, voltages, efficiencies and capacity fade over multiple cycles, considering shunt currents, ion diffusion, self-discharge and degradation side reactions. The results show that ion diffusion and degradation play a predominant role in the capacity fade of the MV/TEMPTMA AOFB when the accumulation of diffused ions across the membrane suppresses the ion crossover and stabilises the coulombic capacity in the long run. The model uses MV/TEMPTMA as a demonstration but can also be adapted to other non-mixing AOFBs to predict and optimise battery performance. • An electrochemical stack model for the MV/TEMPTMA aqueous organic flow battery is developed. • The stack model is based on the equivalent circuit method to predict the battery performance over multiple cycles. • The effect of current density, ion diffusion across the membrane and degradation on the stack performance is revealed. • The ion diffusion and degradation play a predominant role in the capacity fade of the battery stack. • Diffused ions across the membrane suppress ion crossover and stabilise the capacity but may lead to cell voltage fluctuation.