All-vanadium redox flow cells

Skyllas-Kazacos M, Rychcik M, Robins RG, Fane AG. New all vanadium redox flow cell. J Electrochem Soc 1986 133 1057-8. 

As arguably the most weU-known RFB chemistry, VRBs take advantage of the four oxidation states of vanadium within the stability window of water. This enables operation with the same element as an electroactive species as both negative and positive electrolytes and limits concerns about solution crossover and the associated permanent deleterious effects (e.g., capacity fade, irreversible side reactions). Since the initial electrochemical studies of the V(IV)A (V) and the V(II)A (III) redox couples in 1985 [48,49] and the first demonstration of an all-vanadium redox flow cell in 1986 [50] by Skyflas-Kazacos and co-workers, VRBs have been the focus of... 

Most commonly, the battery will be configured with a stack of bipolar cells (10 -100 cells per stack) to give a useful output voltage and with parallel flows for the electrolytes to each of the cells in the stack. Hence, the electrodes will be bipolar with a solid core from carbon, graphite, or a carbon/polymer composite and the three-dimensional elements bonded or pressed onto either side of the solid core. The composites are a blend of a chemically stable polymer and a micron-scaled carbon powder, most commonly an activated carbon Radford et al. [127] have considered the influence of the source of the carbon and the chemical and thermal treatments on the properties of such activated carbons, especially the pore size and distribution [126]. Even though reticulated vitreous carbon has been used for the three-dimensional elements [117], the predominant materials are graphite cloths or felts with a thickness of up to 5 mm, and it is clear that such layers are essential to scale the current density and thereby achieve an acceptable power density. Details of electrode performance in the more developed flow batteries are not available but, for example, Skyllas-Kazacos et al. [124] have tabulated an overview of the development of the all vanadium redox flow battery that includes the electrode materials and the chemical and thermal treatments used to enhance activity and stability. 

Some improvements in anionic commercial membranes were made possible by irradiation. For example, Hwang and Ohya [133] used accelerated electron radiation to cross-link a commercial membrane based on polysulfone (New-Selemion, Asahi Glass). They proved that these highly cross-linked anion exchange membranes showed a higher coulombic and energy efficiency than Nation membranes when used in an all-vanadium redox flow battery. Application of these membranes in an alkaline fuel cell is also conceivable. 

See colour insert.) Photograph of an all-vanadium redox flow battery showing the electrolyte reservoirs that store the two half-ceU solutions that are pumped through the cell stack where energy is generated by the electrochemical reactions of the vanadium redox couples at inert electrodes. 

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