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ACEP 12-03 | March 2012 When the vanadium battery is charged, the VO2+ ions in the positive half-cell are converted to VO2+ ions when electrons are removed from the positive terminal of the battery. Similarly in the negative half-cell, electrons are introduced converting the V3+ ions into V2+. The electrode reaction of the vanadium system can be expressed by: VO (tetravalent) + H2O ⇆ VO2 (pentavalent) +2H + e : E = 1.00 V............ (1) • Negative electrode: • Positive electrode: 2+ + +−0 3+ −2+ 0 V (trivalent) + e ⇆ V (bivalent): E = −0.26 V ............................................. (2) The reaction from left to right represents the reaction during charging in both equations. At the positive electrode, tetravalent V ions (VO2+) are oxidized to pentavalent V ions (VO2+) while at the negative electrode, trivalent V ions (V3+) are reduced to bivalent V ions (V2+). The hydrogen ions (H+) generated at the positive electrode during charging move to the negative electrode through the membrane to maintain the electrical neutrality of the electrolyte. Supplied electric power is thus stored in the form of the transformation of V ions of differing valence. During discharging, the stored power is delivered by the reverse reaction. The battery’s electromotive force calculated based on the standard oxidation reduction potential (E0) is 1.26 V. However, when the electrolytes and cells are prepared practically, the electromotive force is about 1.41 V (typical open-circuit voltage obtained at 25 °C). V + VO + H2O ⇆ V + VO2 + 2H ............................................................... (3) The discharged reactants are shown on the left and charged on the right. Because the electrolyte is returned to the same state at the end of every cycle it may be reused indefinitely (assuming it is not contaminated). 10 Oxidation States: Vanadium is present in both the positive and negative electrolytes, but in different oxidation states. The oxidation state is a measure of how far a reaction has proceeded. For example unreacted vanadium has an oxidation state of zero, when it is fully reacted it has an oxidation state of +5. The oxidation state may also have values in between if the vanadium is only partially reacted, although not every value is possible. Vanadium is an unusual metal in that it forms stable, concentrated solutions with four different oxidation states, a property shared only by uranium and some other heavy, radioactive elements. Charging and discharging the battery changes the average oxidation state of the vanadium in the electrolytes. ADVANCED ENERGY STORAGE RESEARCH 6 | A C E P • Overall reaction10: 3+ 2+ 2+ + +PDF Image | Advanced Battery Storage Systems Testing at ACEP VRB ESS
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