Effect of Pretreatment on the Rate of the VO2+/VO2+ and V2+/V3+ Reactions at a Carbon Electrode

Flow batteries offer a lot of promise for large scale energy storage applications.¹ In particular, Vanadium Redox Flow Batteries (VRFB) have received a lot of attention and significant commercialisation of the system has already begun.^2,3 VRFB cells can operate at coulombic efficiencies of over 90%^4,5 and their carbon electrodes have very good stability as long as the positive half-cell is not overcharged.^6,7 Since active species of the anolyte and catholyte are just different forms of vanadium in H₂SO₄, cross-contamination problems are effectively eliminated.⁸ In addition, because the active species are all in liquid form in solution, these batteries have long lifetimes.⁹ Furthermore the state of charge of the system may be obtained by monitoring the anolyte and catholyte using physical techniques.¹⁰

    It has previously been reported that the coulombic efficiency of a VRFB increases with increasing current density, and this has been attributed mainly to the transfer of vanadium ions across the proton exchange membrane,¹¹ leading to an imbalance in the state of charge of the two half-cells. However, hydrogen evolution at the negative electrode cannot be neglected and may also be a significant issue for VRFBs.¹²

    Charging and discharging experiments were carried out using 1.6 mol dm^-3 VOSO₄ (V^IV) in 3 mol dm^-3 H₂SO₄ as starting catholyte and 1.6 mol dm^-3 V₂(SO₄)₃ (V^III) in 3 mol dm^-3 H₂SO₄ as starting anolyte in a laboratory scale flow cell. The electrolytes were circulated by means of a peristaltic pump, typically at a flow rate of 0.3 cm³ s^-1. Carbon felt electrodes, 6 mm in thickness (compressed to 5 mm in the cell), were used in both the positive and negative half cell. The two half cells were separated by a 180-µm Nafion^® 117 membrane. The electrodes and the membrane were 50 mm × 50 mm in area and the electrodes were contacted by means of carbon-filled polymer contacts. The negative half cell potential, negative open circuit potential, positive half cell potential and the positive open circuit potential were measured relative to Hg/Hg₂SO₄ reference electrodes.

    We accurately determined the coulombic efficiency of each half cell by measuring its open circuit potential as the battery was charged and discharged. Experiments were performed in which cells were charged and discharged between states of charge of approximately 15% and 75%. The state of charge was determined from open circuit potentials in each half cell using the Nernst equation. The efficiency was then determined from the ratio of charge passed during charging and discharging between these endpoints. It was found that, in general, the positive half cell was more efficient than the negative half cell. Possible reasons for this imbalance in efficiency will be discussed.

    We will also present results of a more detailed study of coulombic efficiency of the full VRFB cell. Experiments were performed in which the charge rate was varied but the discharge rate has been kept constant. Similarly, experiments were performed in which the discharge rate was varied and the charge rate was held constant. It will be shown that while the coulombic efficiency increased at a higher rate of charging, the coulombic efficiency was not as significantly affected by the variation in the rate of discharging (see Fig. 1). This is attributed to the effect of H₂ formation at the negative electrode.  Results will be presented for various cell and charging conditions and will be discussed in detail.

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