Redox flow cells

Fuel Cells Problems and Solutions, Second Edition. Vladimir S. Bagotsky. 

When the cell is recharged, this reaction occurs in the opposite (anodic) direction. 

An electrolyte solution containing divalent and trivalent chromium ions (i.e., the ions Cr and Cr ) flows through the negative half-cell. When the cell delivers charge, the anodic reaction occurring at the negative electrode is 

When the cell is recharged, this reaction occurs in the opposite (cathodic) direction. The two reactions occur simultaneously when the external circuit is closed. The overall current-generating reaction that occurs when the cell delivers charge is given by 

When the cell is recharged, it occurs in the opposite direction. The cathode and anode where reactions (9.1) and (9.2) take place are inert but catal5dicafly active (i.e., selected so that the reactions will be fast). 

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Fig. 9.24 Schematic cross-section of a redox flow cell...

Lastly, cells equipped of porous electrodes are useful in organic electrochemistry. Redox flow cells were efficiently developed by Moinet [55], and this concept can be considered as a great advantage toward conventional reduction and oxidation methods in organic chemistry. It is essentially because they can be particularly useful to obtain electroactive intermediates in reduction and oxidation multistep processes. A striking example had been mentioned in Sect. 6.5.1.1 in the synthesis of aryl nitroso compounds. However, the flow of the solution through the two porous electrodes is difficult to regulate. 

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. 

Figure 4.6. a) Diagram of redox-flow cell using LiFeP04 and Li4TisOi2 suspensions as the positive and negative electrode, respectively, and b) curves of discharge as well as power density of the system (source Journal of the Electrochemial Society)... 

Figure 4.7. Diagram of a redox-flow cell without a porous membrane...

Ponce de Leon C, Frias-Ferter A, Gonzalez-Garcia J et al (2006) Redox flow cells for energy conversion. J Power Sources 160 716-732. doi 10.1016/j.jpowsour.2006.02.095... 

Nikiforidis G, Berlouis L, Hall D, Hodgson D (2013) A study of different carbon composite materials for the negative half-cell reaction of the zinc cerium hybrid redox flow cell. Electrochim Acta 113 412-423. doi 10.1016/j.electacta.2013.09.061... 

Poon G (2008) Bromine complexing agents for use in vanadium bromide (V/Br) redox flow cell. The University of New South Wales... 

Flow cells (also redox flow cells, flow batteries) are similar to batteries, except that the electrodes are catalysts for the chemical reaction, which occurs as a microporous membrane allows ions to pass from one electrolyte solution to another. Among flow cells are types that use zinc and bromine, vanadium in two types with different states, or polysulfide and bromine as the pairs of electrolytes. The advantages of flow cells are that they are capable of a large number of cycles, and the electrolytes can be replenished. 

Ponce de Leon, C., Friar-Ferrer, A., Gonsalez-Garcia, J. et al. (2006) Redox flow cells for energy conversion. Journal of Power Sources, 160, 716-721. 

Figure 12 shows the basic concept of a redox flow cell stack. In practice, 10-200 unit cells with bipolar electrodes are stacked to form RFBs. Typically, increasing the... 

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. 

In iron-chromium redox flow cells, carbon materials are commonly used as the (inert) electrodes (carbon fiber cloth, felt, etc.). Johnson and Reid (1985) suggested depositing traces of lead and gold (p,g/cm ) on the carbon material of the electrode in the chromium redox system (the chromium electrode ) in order to accelerate the electrode reaction. The solution for the positive half-cell usually contains a certain concentration of hydrochloric acid (HCl) in addition to FeCls and FeCl2. 

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

More detailed data about the two systems described above and a number of other redox flow cells may be found in a detailed review by Ponce de Leon et al. (2006). 

Redox flow cells or regenerative fuel cells... 

Another type of cell sometimes called a Juel cell is the redox flow cell (Vincent and Scrosati, 1997). In this type of cell, the reactants are removed from the electrodes during charging and are stored in tanks. The capacity of such cells can thus be very large. They are discharged by resupplying the reactants to the electrodes. Because the operation of the cell involves supply of chemicals to the electrodes, these devices are sometimes called fuel cells. However, this is a misnomer, as will become clear. 

Figure 1.17 Diagram showing the principle of operation of a redox flow cell. See text for explanation. (Reproduced by kind permission of Regenesys Technology Ltd.)...

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