Vanadium redox

Redox flow batteries, under development since the early 1970s, are stUl of interest primarily for utility load leveling applications (77). Such a battery is shown schematically in Figure 5. Unlike other batteries, the active materials are not contained within the battery itself but are stored in separate tanks. The reactants each flow into a half-ceU separated one from the other by a selective membrane. An oxidation and reduction electrochemical reaction occurs in each half-ceU to generate current. Examples of this technology include the iron—chromium, Fe—Cr, battery (79) and the vanadium redox cell (80). 

One of the most important requirements that must be met is the membrane s ability to prevent excessive transfer of water from one half cell to the other. The preferential transfer of water can be a problem in the vanadium battery as one half-cell (the negative half cell in the case of cation exchange membranes) is flooded and becomes diluted, while the other becomes more concentrated, adversely affecting the overall operation of the cell. Most of the membranes show good initial water transfer properties, but their performance deteriorates with exposure to the vanadium solutions. Sukkar et al. ° evaluated various polyelectrolytes to determine whether they could improve the selectivity and stability of the membranes in the vanadium redox cell solutions. Both the cationic and anionic polyelectrolytes evaluated improved the water transfer properties of the membranes, although upon extended exposure to the vanadium electrolyte the modified membranes did not maintain their improved water transfer properties. The solvent based Nuosperse 657 modified membrane displayed exceptional properties initially but also failed to maintain its performance with extended exposure to the vanadium solutions. 

A ubiquitous characteristic of vanadium chemistry is the fact that vanadium and many of its complexes readily enter into redox reactions. Adjustment of pH, concentration, and even temperature have often been employed in order to extend or maintain system integrity of a specific oxidation state. On the other hand, deliberate attempts to use redox properties, particularly in catalytic reactions, have been highly successful. Vanadium redox has also been successfully utilized in development of a redox battery. This battery employs the V(V)/V(IV) and V(III)AT(II) redox couples in 2.5 M sulfuric acid as the positive and negative half-cell electrolytes, respectively. Scheme 12.2 gives a representation of the battery. The vanadium components in both redox cells are prepared from vanadium pentoxide. There are two charge-discharge reactions occurring in the vanadium redox cells, as indicated in Equation 12.1 and Equation 12.2. The thermodynamics of the redox reactions involved have been extensively studied [8],... 

Heintz, A. and Ch. Illenberger. 1998. Thermodynamics of vanadium redox flow batteries Electrochemical and calorimetric investigations. Ber. Bunsenges. Phys. Chem. 102 1401-1409. 

This book does not follow a chronological sequence but rather builds up in a hierarchy of complexity. Some basic principles of 51V NMR spectroscopy are discussed this is followed by a description of the self-condensation reactions of vanadate itself. The reactions with simple monodentate ligands are then described, and this proceeds to more complicated systems such as diols, -hydroxy acids, amino acids, peptides, and so on. Aspects of this sequence are later revisited but with interest now directed toward the influence of ligand electronic properties on coordination and reactivity. The influences of ligands, particularly those of hydrogen peroxide and hydroxyl amine, on heteroligand reactivity are compared and contrasted. There is a brief discussion of the vanadium-dependent haloperoxidases and model systems. There is also some discussion of vanadium in the environment and of some technological applications. Because vanadium pollution is inextricably linked to vanadium(V) chemistry, some discussion of vanadium as a pollutant is provided. This book provides only a very brief discussion of vanadium oxidation states other than V(V) and also does not discuss vanadium redox activity, except in a peripheral manner where required. It does, however, briefly cover the catalytic reactions of peroxovanadates and haloperoxidases model compounds. 

Zn-bromine flow and vanadium redox flow are special cases of secondary batteries. Here, liquid electrode materials are used on one (Zn-Br flow) or both sides (V redox flow) of the electrochemical cell. In contrast to regular batteries, which are typically completely closed systems, the liquid electrode materials in flow batteries are circulated and replenished from tanks (Figure 3.5.5). Therefore, the flow batteries possess large electrodes, the effective size of which is just limited by the volume of those tanks. This partly decouples energy and power capabilities of the batteries, allowing one to optimize both separately. 

New perspectives arising from isothermal oxidation. The next chapter of this book describes the greatly altered perspective of the fuel cell industry, when Grove s ideas are updated. The second chapter describes the detail of Regenesys, or ESS-RGN. This system has changed hands, as noted above, and information is available from http //www.vrbpower.com/. (The initials VRB stand for Vanadium Redox Battery, a low-power alternative to Regenesys.) The new 2005 VRB Power Systems shorthand is ESS-VRB for 2.5 to 10 MW and ESS-RGN for 10 to 100 MW. In Chapter 2 the reader will be acquainted with ESS-RGN, one of the two VRB fuel cell systems (incompressible liquid based) which can be termed complete . The redox battery uses small pumps as circulators. 

The vanadium redox chemistry involves protons, since we have the equilibria... 

Other battery systems that have been suggested for use in RAPS facilities include zinc-bromine [7], vanadium redox [8], and aluminium-air [9]. It is considered unlikely, however, that these systems will be used in mainstream RAPS applications as their cost is still significantly higher than that of lead-acid alternatives, and their long-term reliability has yet to be proven. 

Because vanadium ions exist in four different oxidation states (as V2+, V3+, V02+ and V02+) in aqueous solution, redox couples can be formed by all vanadium ions. The emf of a vanadium redox battery is 1.4 V and the electrode kinetics are higher than those of the Fe-Cr battery. Also, the energy density of the battery can be increased due to the high solubility of vanadium salts.257 In the battery, vanadium sulfate solution is used,... 

M. Skyllas-Kazacos, M. Richcik, R.G. Robins, A.G. Fane and M.A. Green, New allvanadium redox flow cell, J. Electrochem. Soc., 1986, 133, 1057-1058 M. Kazacos, M. Cheng, M. Skyllas-Kazacos, Vanadium redox cell electrolyte optimization studies, J. Appl. Electrochem., 1990, 20, 463 M. Skyllas-Kazacos, and F. Grossmith, Efficient vanadium redox flow cell, J. Electrochem. Soc., 1987, 134, 2950-2953. 

M. Syllas-Kzacos, C. Mnictas and M. Kzacos, Thermal stability of concentrated V(V) electrolytes in the vanadium redox cell, J. Electrochem. Soc., 1996, 143, L86-87. 

G.-J. Hwang and H. Ohya, Cross-linking of anion exchange membrane by accelerated electron radiation as a separator for all-vanadium redox flow battery, J. Membr. Sci., 1997, 132, 55-61. 

In summary, the control of the different experimental parameters involved in the preparation step is crucial to develop new materials with improved catalytic properties. The optimal conditions found for the preparation of the VAION catalyst are pH= 5.5, V/Al ratio= 0.25 and [V]= 30 10 M. Under these conditions, the catalyst shows the maximum ACN productivity which is related to the existence of an optimum balance between the surface nitrogen species and the vanadium redox capacity [2],... 

In vanadium redox flow fuel cells, a redox system of penta- and tetravalent vanadium ions is used in the positive half-cell, and a redox system of di- and trivalent vanadium ions in the negative half-cell. When the fuel cell delivers charge, the following reactions take place ... 

Ferrigno R, Stroock AD, Clark TD, Mayer M, Whitesides GM (2002) Membraneless vanadium redox fuel cell using laminar flow. J Am Chem Soc 124 12930-12931... 

Another important breakthrough in this field was realized by a flow-through porous electrode architecture with vanadium redox electrolytes [6]. As shown in Fig. 4, the fuel and oxidant were supplied into the inlet ports and guided to flow through the porous carbon electrodes in the... 

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