2.4. Measurement of electrochemical parameters

The electrolyte solution was prepared by dissolving 1.61 M vanadium oxysulfate (VOSO₄. xH₂O, x = 2 ∼ 2.3) in 5 M sulfuric acid. The raw electrolyte solution contained vanadium in VO²⁺ ionic state. The redox couple VO²⁺/VO²₊and V²⁺/V³⁺ was obtained by two-step charging procedure as described in the reference number [3] with a pre-set voltage cut-off 1.8 V. Nitrogen gas was purged through the negative electrolyte. All experiments were conducted in an air-conditioned room maintained at 26 °C.

After completing dual-step charging process, the cell was programmed to run three life cycles for a given operating condition and the average of the three is considered in the processed data. Pre-set cut-off voltage limits 1.8 V for charging and 0.8 V for discharging were fixed for all the experiments. Electrochemical characteristics of a VRFB cell are described in terms of capacity, round-trip energy efficiency, coulombic efficiency, voltaic efficiency, net energy efficiency, energy density and power density. These terminologies are defined as follows

where I and E are the current and the voltage of the cell and the subscripts c and d denote charging and discharging phase of the cycle. From these, the system efficiency is calculated as follows:

Further, current density (j) and power density are defined based on unit nominal electrode area while the gravimetric or volumetric energy density of the electrolyte are defined based on the mass or the volume of the electrolyte on one side (anode/cathode) of the cell.