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V2+ to balance VO2+. Figure 4.2 also reveals that the potential of the reference electrode is stable under operating conditions. The potential of the electrodes is given by the Nernst equation (4.1), where E0 is the standard potential, R is the gas constant, T is the temperature, n is the number of involved electrons, F is the Faraday constant and ared and aox are the activities of the reduced and oxidized species, respectively. (4.1) The redox reaction of the couple VO2+/VO2+ and V3+/V2+ are described in (2.17) and (2.18), respectively. The potential, versus standard hydrogen electrode (SHE), at the negative and positive electrode are given by (4.2) and (4.3), respectively. It shows the potential dependence at the positive electrode (VO2+/VO2+) on the protons activity, while the potential at the negative electrode only depend on the activities of vanadium species. (4.2) (4.3) At 50% of state of charge (SoC), the potential at the negative electrode is -0.26 V vs. SHE, whereas the potential at the positive electrode is pH-dependent as mentioned. Therefore, the potential of the reference electrode can be determined at open circuit at 50% of state of charge (SoC). The observed value of V3+/V2+ of - 0.625 V (vs. our Ag/Ag2SO4 reference electrode) should be equal to -0.26 V (vs. HER). Inserting these values in (4.4), (4.5) and (4.6) is obtained. Thus, the potential of the reference electrode is +0.365 V vs. NHE. Note that we represent all potential vs. NHE for easy comparison with other studies. The advantage of a reference electrode inserted in an all-vanadium flow battery is that the potential of the couple at the negative electrode, V2+/V3+, at 50% SoC can be employed as standard for the internal calibration of the reference potential. For every new reference electrode, its potential can be determined following the above described methodology, i.e. the potential of the reference electrode can be calibrated and corrected “in-situ”. The observed potential of the positive electrode of 1.15 V (vs. HER) is slightly higher than expected due to effects of side processes such as water evaporation or oxygen evolution. (4.4) 61PDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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