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2.6.1.3 All-vanadium In both of the systems described above, a chief of concern and liability is the incompatibility between, and sensitivity of, the two electrolyte streams to contamination from the other. If a species crosses over and reacts irreversibly with elements in the opposite stream, it comprises not just an efficiency loss on that particular charge/discharge cycle, but a loss of capacity and degradation in the overall performance of the system, which may result in expensive electrolyte separation and reactant recovery. To this end, it is helpful to develop a system with more than two oxidation states of the same element, wherein crossover only represents an efficiency loss as no species are irreversibly consumed or removed from their reactive electrolytic solution. The all-vanadium system employs the V(II)/V(III) redox couple at the negative electrode and the V(IV)/V(V) at the positive electrode, generally identified to exist in the form of VO2+ and VO2+. (2.5) (2.6) In this case, the 1.4 V cell voltage can generate a 25-35 WhL-1 of energy density. The current is maintained by the migration of protons across the membrane separator. While it is nominally the change in the oxidation state of vanadium on either side of the membrane that stores and releases charge, there is a change in the pH of the solution over the course of a charge and discharge cycle. While crossover of the different oxidation states of vanadium comprises an efficiency loss, the proper forms can be regenerated electrochemically, which eases the stringency of maintenance requirements. While exploratory research on vanadium as a redox couple began at NASA97, the all vanadium redox flow battery (VRFB) was invented and developed by Maria Skyllas- Kazacos and her co-workers at the University of New South Wales45,51,98. Research has continued on this technology since that time. As a promising technology for storing intermittent renewable energy, VRFB systems have received perhaps the most attention of all RFBs39,51,99. In fact, prototypes up to the range of MW in power and MWh in energy storage capacity have been demonstrated70,100,101. Skyllas- Kazacos developed a 5-10 kW VRFB stacks. While energy density is not necessarily a primary concern for stationary, grid applications, nonetheless, the VRFB energy density is limited by the solubility of vanadium in the electrolyte stream and precipitation can occur. The solubility limits depend upon both acid concentration and temperature102. 28PDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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