In this study, the V5+/V4+ redox couple has been replaced with the Fe3+/Fe2+, which results in significant cost reduction. The illustrated Fe-V battery employs an FeCl2 in HCl(aq) catholyte in one tank, and VCl3 in HCl(aq) anolyte in the other tank. A graphite electrochemical cell was equipped with a Nafion® 117 membrane, on which two carbon felt electrodes were attached. A galvanostat/potentiostat was used to apply constant currents and potentials. In this direction, an experimental and theoretical analysis of the iron-vanadium flow battery was carried out, investigating the effect of temperature, charge/discharge rate, and state of charge on the performance/efficiency of the energy storage system. To characterize the battery, voltage efficiency measurements and polarization curves were carried out at each point. In this way, the suitable operation SOC range for employed loads that the battery serves, can be defined for best voltage efficiency and power output.
The charge/discharge characteristics of the battery reveal an optimum operation temperature at ~47oC and an optimum charging current of ~50mA/cm2 for the state of charge range between 20% and 80%. Also, a small increase in the open circuit voltage with charging current was observed, indicating a slightly higher charging depth. The voltage efficiency as obtained by the finite-time galvanostatic charge and discharge cycles, strongly depends on the current J that is used. Efficiency measurements were performed for various SOC values using J from 300 to 800mA at 42oC. Moreover, voltage and coulombic efficiencies achieve up to 85%.
A mathematical model was built, based on the current exchange density obtained by the polarization curves for various operation conditions of the battery. The model concludes a negative asymmetry form to the energy barrier, to rate the determining half-reaction.