Redox flow cell system

The second-generation 02" biosensors are mainly based on the electron transfer of SOD shuttled by surface-confined or solution-phase mediators, as shown in Scheme 2(b). In 1995, Ohsaka et al. found that methyl viologen could efficiently shuttle the electron transfer between SOD and the glassy carbon electrode and proposed that such a protocol could be useful for developing 02 biosensors [125], Recently, Endo et al. reported an 02, biosensor based on mediated electrochemistry of SOD [148], In that case, ferrocene-carboxaldehyde was used as the mediator for the redox process of SOD. The as-developed 02 biosensor showed a high sensitivity, reproducibility, and durability. A good linearity was obtained in the range of 0 100 pM. In the flow cell system, tissue-derived 02 was measured. 

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)... 

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). 

L. H. Thaller, Recent Advances in Redox Flow Cell Storage Systems, DOE/NASA/1002-79/4, NASA TM 79186, Aug. 1979. N. Hagedom, NASA Redox Storage System Development Project, U.S. Dept, of Energy, DOE/NASA/12726-24, Oct. 1984. 

Besides the zinc-halogen systems, other zinc-based hybrid chemistries have been explored. One example is the alkaline zinc-ferricyanide redox flow cell that was demonstrated by Adams in 1979 [108]. The advantage of this zinc-ferricyanide redox flow cell includes high efficiency, high cell voltage, and low toxicity [109]. The electrode reactions are ... 

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. 

A comprehensive review of spectrophotometric methods for the determination of ascorbic acid (1) was presented. Most of the methods are based on the reducing action of ascorbic acid, making use of an Fe(III)-Fe(II) redox system, and to a lesser extent Cu(II)-Cu(I), V(V)-V(IV) and phosphomolybdate/phosphotungstate-molybdenum/tungsten blue redox systems. A kinetic spectrophotometric method for the determination of L-ascorbic acid and thiols (RSH) was developed, whereby the absorbance of the Fe(II)-phen complex formed during the reaction of 1 or RSH with Fe(III)-phen was continuously measured at 510 nm by a double beam spectrophotometer equipped with a flow cell. The linearity range for 1 was 4-40 p,M and for RSH 8-80 xM. The method was validated for pharmaceutical dosage forms . 

A more complex biosensor for acetylcholine has been developed by Larsson et al. [154]. Three enzymes, AChE, ChOX, and HPR, have been coimmobilized in an Os-based redox polymer on solid graphite electrodes. After a careful optimization of the immobilization procedure, the biosensor, inserted into a flow cell of very small volume, was integrated into a flow injection system, and some samples of microdialysate, taken from rat brains before and after stimulation with KCl, were analysed. Even if a clear increase in signal could be noted, it was not possible to distinguish whether it was due to an increase in choline or in acetylcholine, since the biosensor responded to both metabolites. 

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... 

Carbon is a material commonly used in electrochemical energy devices, such as fuel cells, electrolysis cells, and redox flow and lithium-ion batteries. In these systems, it fulfills various functions, which can be as diverse as serving as catalytically active material, nanoparticles anchor, intercalating electrode, or electron-conducting additive. 

The failure criteria are defined by maximum tolerable temperature dependent DoO values. As stated earlier the limits are independent of the time of re-oxidation and air flow rate. However, these parameters may be important for system operation. In intended redox cycles upon system shut-down a well defined volume of air will be applied to the anode side of the cell until the system is cooled down. If air flow rate and time of re-oxidation can be controlled together with temperature, the DoO can be minimized. This shows Fig. 6. In two sets of experiments at 600 and 800°C samples based on 1.5 mm Coat-Mix substrates were re-oxidized with a total air volume of 18 1. The air volume was applied to the cells in various combinations of air flow rate and time of re-oxidation. The results show, that it is beneficial to apply the air in the shortest possible time with the highest possible flow. At 800 C the complete re-oxidation after 60 min with a flow of 300 ml/min could be reduced to less than 75% by applying the volume in 15 min with a flow of 1.2 1/min. At 600°C re-oxidation for 120 min and a flow rate of 150 ml/min resulted in a DoO of about 80%. After 15 min with 1.2 1/min the DoO stayed under 30%. So the choice of a reasonable combination of time of re-oxidation and air flow rate can be crucial for the mechanical integrity of the cell. In this sense both time of re-oxidation and air flow rate are important parameters for intended and controlled re-oxidation. Again an explanation for this will be given in the discussion section. 

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. 

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. 

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