Battery separators redox-flow

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). 

In redox flow batteries such as Zn/Cl2 and Zn/Br2, carbon plays a major role in the positive electrode where reactions involving Cl2 and Br2 occur. In these types of batteries, graphite is used as the bipolar separator, and a thin layer of high-surface-area carbon serves as an electrocatalyst. Two potential problems with carbon in redox flow batteries are (i) slow oxidation of carbon and (ii) intercalation of halogen molecules, particularly Br2 in graphite electrodes. The reversible redox potentials for the Cl2 and Br2 reactions [Eq. (8) and... 

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. 

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. 

The development of redox cells with circiilating flow (or redox-flow) is not new since it dates back to 1968 with the invention of the zinc-chloride (Zn-Cl) battery. Half-way between fuel cells and batteries, these systems involve two soluble circiilating redox couples separated by an ion exchange membrane. These redox couples, stored in two different reservoirs, called the catholyte and anolyte, are continuously injected using a pump inside the cell where... 

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. 

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. 

Zhang HZ, Zhang HM, Li XF, Mai ZS, Zhang JL. NanofUtration (NF) membranes the next generation separators for all vanadinm redox flow batteries (VRBs). Energy Environ Sci 2011 4 1676-9. 

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. 

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 ... 

The challenge of acid-alkaline hybrid power sources lies in how to separate the acid and alkaline electrolytes effectively while maintaining ion transport between these two electrolytes. As shown in Table 11.1, some conventional electrochemical power sources do operate in two chambers with two electrolytes separated by a membrane or an ionic interface, such as the Daniell cell and most flow batteries. In these well-known electrochemical cells with two electrolytes, the pEt value of both anolyte and catholyte are almost the same and there usually exists a common species to function as a charge carrier between the anolyte and catholyte. Examples include S04 ion for Daniell battery, and H+ ion for vanadium redox flow battery, in which there is a separator to avoid electrolyte mixing. An ion-exchange... 

Suppose you could separate the oxidation and reduction parts of a redox reaction and cause the electrons to flow through a wire. The flow of electrons in a particular direction is called an electrical current. In other words, you are using a redox reaction to produce an electrical current. This is what occurs in a battery—one form of an electrochemical cell in which chemical energy is converted to electrical energy. You can reverse the process and use a current to cause a redox reaction to occur. 

We have seen that in an oxidation electrons are released and that in a reduction they are acquired. In a battery, the release and acquisition are spatially separated. Electrons are released into an electrode, a metallic contact, in one region of the battery, travel through an external circuit, and then attach to the species undergoing reduction at a second electrode elsewhere in the battery. Thus, the redox reaction, the joint oxidation and reduction reactions, proceed, and in doing so, the flow of electrons from one electrode to the other is used to drive whatever electrical equipment is attached to the device. Modern batteries use a range of... 

By separating the two parts of the redox reaction, we have caused a current to flow through a wire. We have a simple battery, which we could use to... 

Redox Reactions. For many electrochemists the paramount concern of their discipline is the reduction and oxidation (redox) reaction that occurs in electrochemical cells, batteries, and many other devices and applications. Reduction takes place when an element or radical (an ionic group) gains electrons, such as when a double positive copper ion in solution gains two electrons to form metallic copper. Oxidation takes place when an element or radical loses electrons, such as when a zinc electrode loses two electrons to form a doubly positive zinc ion in solution. In electrochemical research and applications the sites of oxidation and reduction are spatially separated. The electrons produced by chemical processes can be forced to flow through a wire, and this... 

When Zn metal is placed into a Cu solution, Zn is oxidized and Cu is reduced—electrons are transferred directly from the Zn to the Cu . Suppose we separate the reactants and force the electrons to travel through a wire to get from the Zn to the Cu . The flowing electrons constitute an electrical current and can be used to do electrical work. This process is normally carried out in an electrochemical cell, a device that creates electrical current from a spontaneous redox reaction (or that uses electrical current to drive a nonspontaneous redox reaction). Electrochemical cells that create electrical current from spontaneous reactions are called voltaic cells or galvanic cells. A battery is a voltaic cell that (usually) has been designed for portability. 

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