Two-dimensional transient model of a Zn-Ce redox flow battery incorporating ion crossover

This work presents a transient two-dimensional model of a Zn-Ce redox flow battery that accounts for all three modes of transport (migration, diffusion and convection) coupled with the electrode kinetics of the main reactions as well as the hydrogen evolution (HER) and oxygen evolution (OER) side reactions. Importantly, the crossover of the various ionic species through the membrane and a correlation to estimate the mass transfer coefficients of Ce(III) and Ce(IV) ions at the positive electrode under different operating conditions have been included. After validation by fitting to experimental data and obtaining values of the diffusion coefficients of the ions in the membrane, exchange current density for hydrogen evolution and positive electrode reaction rate constant and charge transfer coefficients, the model is used to simulate battery performance at different electrolyte flow rates and when using positive electrodes with different geometries. These simulations reveal that the battery energy efficiency is improved by 9.7% when the flow rate is increased from 66 mL/min to 330 mL/min and 11.8% when the positive mesh electrode used in our experiments (1.5 mm fibre diameter; 645.33 m -1 specific surface area) is replaced with a micromesh electrode (0.2 mm fibre diameter; 9400 m -1 specific surface area) due to enhanced mass transfer of the Ce(III) and Ce(IV) ions. In addition, the model shows that this enhanced mass transfer when the Pt-Ti micromesh positive electrode is used would allow the battery to operate at the high current densities (>300 mA/cm 2 ) required of commercial-scale RFBs.