Thermodynamics of Vanadium Redox Flow Batteries - Electrochemical and Calorimetric Investigations

Thermodynamic properties of the so-called All-Vanadium battery used as energy storage system and other vanadium redox systems related to the All-Vanadium battery have been studied. An electrochemical cell has been constructed to measure the equilibrium cell voltages as function of the degree of charging in the temperature range from 278 K to 323 K. The first system studied is the disproportion reaction of VO²⁺ ions with the two half cell reactions VO²⁺ + H₂O ⇄ VO⁺₂ + 2H⁺ + e⁻ and VO²⁺ + 2H⁺ + e⁻ ⇄ V³⁺ + H₂O. The second system is the All-Vanadium battery reaction with the two half cell reactions VO²⁺ + H₂O ⇄ VO⁺₂ + 2H⁺ + e⁻ and V³⁺ + e⁻ ⇄ V²⁺. The third system is the disproportion reaction of V³⁺ ions with the half cell reactions V³⁺ + H₂O ⇄ VO²⁺ + 2H⁺ + e⁻ and V³⁺ + e⁻ ⇄ V²⁺. The molar reaction Gibbs energy, the molar reaction enthalpy, and the molar reaction entropy of each system has been obtained from these data. Additionally the molar reaction enthalpy of the two disproportion reactions and of the reaction V²⁺ + 2VO²⁺ + 2H⁺ → 3VO²⁺ + H₂O has been measured directly by titration calorimetry. The results agree with those obtained from the electrochemical data for the disproportion reactions. No direct calorimetric measurements of the molar reaction enthalpy of the All-Vanadium battery reaction is possible, but the sum of the molar reaction enthalpies of the two disproportion reactions gives the molar reaction enthalpy of the All-Vanadium battery reaction. Comparison with the electrochemically determined molar reaction enthalpy of the All-Vanadium battery reaction shows good agreement indicating satisfying thermodynamic consistency of the whole procedure.