Vanadium redox cell

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],... 

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. 

Liquid phase reactant chemistries originally developed for redox flow batteries can be exploited to great effect in microfluidic electrochemical cells. Most commonly, vanadium redox flow battery technology utilizes soluble vanadium redox couples in both half-cells for regenerative electrochemical energy storage units [53]. The combination of aqueous redox pairs in vanadium redox cells, and VO V... 

Kazacos M, Cheng M, Skyllas-Kazacos M (1990) Vanadium redox cell electrolyte optimization studies. J Appl Electrochem 20 463-467. doi 10.1007/BF01076057... 

M. Rychcik and M. Skyllas-Kazacos, Evaluation of electrode materials for vanadium redox cell, ]. Power Sources 19,1987, 45-54. 

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. 

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... 

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. 

Aaron DS, Liu Q, Tang Z et al (2012) Dramatic performance gains in vanadium redox flow batteries through modified cell architecmre. J Power Sources 206 450-453. doi 10.1016/j. jpowsour.2011.12.026... 

Flow cells are ideal for stors e systems in remote locations. Vanadium redox systems, for instance, deliver up to 500 kW for up to ten hours. Zinc-bromine systems have been produced for 50-kWh and 500-kWh systems to reinforce weak distribution networks or prevent power fluctuations. Hydrogen fuel cells can potentially do almost anything a battery can do provide backup power, perform power leveling, run handheld devices, and supply primary or auxiliary power to cars, trucks, buses, and boats. In many cases they are more efficient than petrochemical fuels. A hydrogen fuel cell in a vehicle that uses an electric motor, for example, can be 40 to 60 percent efficient, compared with the 35 percent peak efficiency of the internal combustion engine. 

Based on their early fundamental research of all-VRFBs, Skyllas-Kazacos et al. [25] also first developed some commercial products, for example, a 1 kW vanadium redox battery (VRB) cell stack. By employing 1.5-2 M vanadium sulphate, sulphuric acid in both half-cells, over 85% of theoretical capacity and 70-80% energy efficiency was obtained. Then in 1994, a 4 kW/12 kWh vanadium battery was evaluated in a demonstration solar house by Thai Gypsum Products Ltd. in Thailand under a license lirom the UNSW [26]. 

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

The operational principle of a vanadium-vanadium redox flow cell (vanadium redox battery or VRB) is illustrated in Figure 12.8. 

Microfluidic Fuel Cells, Rgure 4 Schematic of a microfluidic vanadium redox fuel cell with porous carbon electrodes, using aqueous V + fuel (purple, dark grey in the print version) and VO oxidant (yellow, light grey in the print version) (Reproduced with permission from [6])... 

A microfluidic fuel cell design based on soluble vanadium redox species was recently introduced (Fig. 4). Vanadium redox fuel cells utilize two different aqueous vanadium redox couples, and VO " "/VO, as fuel... 

Kjeang E, Proctor BT, Brolo AG, Harrington DA, Djilali N, Sin-ton D (2007) High-Paformance Microfluidic Vanadium Redox Fuel Cell. Electrochimica Acta 52(15) 4942-4946... 

The vanadium redox battery uses also a cell stack of bipolar plates and separate tanks for the positive and the negative electrolytes, similar to the design of Fig. 1.39. (cf., e.g. Fig. 7.9 in Ref. 76). An ion exchange membrane is used as separator in each cell. 

Figure 1.41 Charge/discharge curves of a vanadium redox battery. The cell stack consisted of 17 cells. Constant current of 100 Ah for charging and discharging (from Ref. 80). 

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