Energy storage is currently one of the major challenges for the deployment of renewable resources. A low cost, flexible, large-scale technology is required to store and buffer the fluctuated electricity production rate. The Redox Flow Battery (RFB) is a very promising candidate with long life-span, high efficiency, and suitability for large scale energy storage, enhancing the reliability of the power grid, and promoting energy independence of remote areas. One additional substantial advantage of RFBs is that power and energy can be independently adjusted. The current cost of conventional flow battery systems is relatively high. This is due to the use of expensive vanadium-base electrolytes, along with Nafion-based membranes.

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 ~47^oC 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 42^oC. 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.