Redox flow battery electrolytes

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. 

At open circuit, electrode reactions that charge the electrodes lead to a slow oxidation of the electrolyte with H2 evolution at the anode and O2 evolution at the cathode. These reactions represent an irreversible self-discharge. Once the electrolyte is introduced, the battery has a poor shelf life. Under development are acidic aqueous electrolytes in which Pb(II) is soluble rather than condensing into the solid PbS04. This development of the lead-acid cell promises a flow battery not requiring a separation membrane. The separation membrane of redox-flow batteries (see last section) remains a challenging problem for the aqueous redox-flow technology. 

Li B, Nie Z, Vijayakumar M et al (2015) Ambipolar zinc-polyiodide electrolyte for a high-eneigy density aqueous redox flow battery. Nat Commun 6 6303. doi 10.1038/ ncomms7303... 

Wu M, Liu M, Long G et al (2014) A novel high-energy-density positive electrolyte with multiple redox couples for redox flow batteries. Appl Energy 136 576-581. doi 10.1016/j. apenergy.2014.09.076... 

The Zn/Br redox flow battery (RFB) is a modular system comprising a cell stack containing functional electrodes attached to current collectors (separated via membranes), electrolyte storage tanks/reservoirs, delivery pumps and pipes. The RFB relies on the electrolyte circulation system to deliver electrochemically active species to electrode surfaces in order to achieve charge transfer and cause electrical current to flow. A simple Zn/Br unit cell is illustrated in Fig. 2.1, with multiple such cells combined in series to create a practical battery. 

Nikiforidis G, Berlouis L, Hall D, Hodgson D (2013) Impact of electrolyte composition on the performance of the zinc-cerium redox flow battery system. J Power Sources 243 691-698. doi 10.1016/j.jpowsour.2013.06.045... 

Nikiforidis G, Daoud WA (2015) Indium modified graphite electrodes on highly zinc containing methanesulfonate electrolyte for zinc-ctaium redox flow battery. Electrochim Acta 168 394 02. doi 10.1016/j.electacta.2015.03.118... 

Based on the occurrence of phase transition, redox flow batteries can be classified as a true system or a hybrid system. In a true system, active species dissolve in the electrolytes all the time, and no second phase other than liquid is formed on the electrode. For a hybrid system, at least one kind of active species is insoluble solid or gas. In the next chapter, we will introduce different types of redox flow batteries using this method. 

Distinguished from true redox flow batteries, hybrid RFB systems employ partially soluble redox couples as active materials, either as a solid or a gas. Hybrid RFBs are more complicated than true RFBs because a new phase, different from the electrolytes, forms on the electrode. A zinc-bromine battery is considered as the prototypical hybrid RFB. 

Chakrabarti MH, Dryfe RAW, Roberts EPL. Evaluation of electrolytes for redox flow battery applications. Electrochim Acta 2007 52 2189-95. 

Liu QH, Sleightholme AES, Shinkle AA, Li YD, Thompson LT. Non-aqueous vanadium acetylacetonate electrolyte for redox flow batteries. Electrochem Commun 2009 11 2312-5. 

Zhang DP, Liu QH, Shi XS, Li YD. Tetrabutylammonium hexafluorophosphate and l-ethyl-3-methyl imidazohum hexafluorophosphate ionic liquids as supporting electrolytes for non-aqueous vanadium redox flow batteries. J Power Sources 2012 203 201-5. 

Leung PK, Ponce de Leon C, Low CTJ, Walsh FC. Ce(lll)/Ce(IV) in methane sulfonic acid as the positive half cell of a redox flow battery. Electrochim Acta 2011 56 6536-46. Hazza A, Pletcher D, Wills R. A novel flow battery a lead acid battery based on an electrolyte with soluble lead(II) Part I. Preliminary studies. Phys Chem Chem Phys 2004 6 1773-8. 

For all technical applications, small overpotentials are desirable with corrosion reactions being the notable exception. In industrial processes (electrolysers [2, 3]), energy storage systems (e.g., redox flow batteries [4, 5]) and further systems, mass transport is generally enhanced by circulating electrolyte solutions, using three-dimensional electrodes or applying other means of artificially enhanced convectimi. 

In redox flow batteries, the chemical compounds responsible for energy storage are liquid and remain in solution in the electrolyte. This mode of operation enables us to circumvent the limitation of conventional electrochemical batteries wherein electrochemical reactions create solid compounds which are stored directly in the electrodes where they are formed. The mass that it is possible to accumulate locally is therefore necessarily limited by the volume and mass of the electrodes, which sets a maximum capacity. 

This separation between the reactor and the tanks is one of the advantages of redox flow batteries in terms of transport, as the assembly of the batteries and electrolyte filling are done on site. Thus, during transport, the battery is not electrochemically active. 

Redox flow battery systems are promising devices, because the tendency is toward the development of low-cost systems, and future developments seem to be leaning toward choosing less toxic redox couples, more abundant materials, more stable membranes and effective recycling processes. Electrolytes can always be reused provided there is no precipitation of oxides - a phenomenon which occurs at low temperature. 

Not all systems can be described using these definitions, since there are also combined systems such as metal-air batteries [8-10] which contain a battery electrode (metal anode) and a fuel cell electrode (air cathode), or redox flow batteries [11,12] which are a form of rechargeable battery in which electrolyte containing one... 

The product solutions are kept separate, and the Fe can be oxidized by air back to Fe , whereas the Cr can be electrolytically reduced back to Cr. An Australian redox flow battery has been described which uses vanadium both as oxidant and reductant in the following reactions ... 

Organic electrode materials used in redox flow batteries still have low solubility in non-aqueous electrolytes and exhibit low energy density. This problem can be addressed by structural modifications with ionization or high polarizable groups, and selection of appropriate electrolytes. 

A classic example of a hybrid redox flow battery is the zinc-halogen system, such as zinc-bromine, which can be traced back to 1885 [103], and was systematically re-evaluated as a secondary battery in 1964 [104]. Similarly, zinc-chlorine was investigated in the late 1970s and 1980s but was found to be environmentally hazardous (chlorine gas formation) and more complicated in a system level than the zinc-bromine chemistiy. More recently, Lim et al. introduced a circulating aqueous electrolyte to the zinc-bromine battery (ZBB), which dramatically improved the performance of the system [105]. The electrode reactions of a ZBB are ... 

Cappillino PJ, Pratt HD, Hudak NS, Tomson NC, Anderson TM, Anstey MR (2014) Application of redox non-innocent ligands to non-aqueous flow battery electrolytes. Adv Energy Mater 4 n/a-n/a. doi 10.1002/aenm.201300566... 

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