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3234 Paths tSouSsutastianianbalbeleEEnneergrgyy In order to enhance the VRB performance, the system behaviour along with its interactions with the different subsystems, typically between the stack and its auxiliaries (i.e. electrolyte circulation and electrolyte state of charge), and the electrical system it is being connected to, have to be understood and appropriately modeled. Obviously, modeling a VRB is a strongly multidisciplinary task based on electrochemistry and fluid mechanics. New control strategies, based on the knowledge of the VRB operating principles provided by the model, are proposed to enhance the overall performance of the battery. 2. Electrochemistry of the vanadium redox batteries Batteries are devices that store chemical energy and generate electricity by a reduction-oxidation (redox) reaction: i.e. a transformation of matter by electrons transfer. VRB differ from conventional batteries in two ways: 1) the reaction occurs between two electrolytes, rather than between an electrolyte and an electrode, therefore no electro-deposition or loss in electroactive substances takes place when the battery is repeatedly cycled. 2) The electrolytes are stored in external tanks and circulated through the stack (see Fig. 1). The electrochemical reactions occur at the VRB core: the cells. These cells are always composed of a bipolar or end plate - carbon felt - membrane - carbon felt - bipolar or end plates; they are then piled up to form a stack as illustrated in Fig. 1. In the VRB, two simultaneous reactions occur on both sides of the membrane as illustrated in Fig. 2. During the discharge, electrons are removed from the anolyte and transferred through the external circuit to the catholyte. The flow of electrons is reversed during the charge, the reduction is now taking place in the anolyte and the oxidation in the catholyte. Fig. 2. VRB redox reaction during the charge and discharge The VRB exploits the ability of vanadium to exist in 4 different oxidation states; the vanadium ions V4+ and V5+ are in fact vanadium oxide ions (respectively VO2+ and VO2+). Thus, the VRB chemical equations become (Sum & Skyllas-Kazacos, 1985; Sum et al., 1985): V O 2+ + 2 H + + e ā V O 2 + + H 2 O V2+ V3+ +eā V2+ +VO2+ +2H+ VO2+ +V3+ +H2O (1) where the water (H2O) and protons (H+) are required in the cathodic reaction to maintain the charge balance and the stoichiometry.PDF Image | Understanding the Vanadium Redox Flow Batteries
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