Redox flow systems

Redox flow batteries (or systems) are present in ranges from ten to several himdred kW, and even beyond a MW for grid support or transport, 

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. 

The electroactive compounds are two soluble redox couples which are oxidized and reduced (section 2.2.4) in a reactor which forms the electrochemical cell. These two compounds have the ability to store electrical energy (during charge) and to restore this energy during the discharge phase. The core of the cell comprises two compartments separated by a proton exchange membrane (PEM), such as a Nafion membrane as 

15 That is, in disconnected mode, with all valves closed and pumps turned off. For rapid response capability, it needs to be left functioning. The internal losses and those due to the pumps cause an apparent self-discharge of around 10% per day. 

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. 

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Na2S2-Br2) redox flow systems, the active mass is in the electrolyte(s), which is stored externally and is pumped to the electrodes. In this case, the battery capacity (energy) can be changed independently of battery power by altering the size of the external storage tanks. 

Good performances can be obtained with the non-aqueous Li-ion redox-flow system however, a technical discussion of these results with RS2E s Industrial Partners immediately concluded that the economic viability of the system (for stationary applications) needed to focus on aqueous systems (for reasons of safety, toxicity and cost). This is the direction our research is currently taking. 

However, the issue of the stability of power supply grids is not a new one. Electrochemical systems for energy accmnulation have been installed on the grid for over 20 years, with lead-acid or nickel-cadmimn batteries. More recently, we are witnessing a significant development of sodium-sulfur batteries (which are discussed in detail in Chapter 12). Experiments have been performed with redox flow systems (also detailed in Chapter 12). The unitary powers range from several MW to several tens of MW, and the quantities of energy stored from several MWh to tens of MWh. 

Other types of batteries have been introduced to serve specific needs (electric vehicles, electricity storage for grid support, etc.). For this discussion, we have chosen to focus on sodium-sulfur (Na-S) batteries and nickel-chloride-based batteries, which are both so-called high temperature battery systems, and lastly redox flow systems. 

The same is true - with the exception of the inductances of the cables - for all immobile electrolyte batteries. It would not be true for redox flow systems, for which we would have to reverse the direction of the circulation of the reactants. 

However, similar to other hybrid redox flow systems where liquid-solid reactions occur, SLFBs suffer from Pb° dendrite formation on the negative electrode... 

Another system under investigation is the iron/ chromium redox flow battery (Fe/Cr RFB) developed by NASA. The performance requirements of the membrane for Fe/Cr RFB are severe. The membrane must readily permit the passage of chloride ions, but should not allow any mixing of the chromium and iron ions. An anionic permselective membrane CDIL-AA5-LC-397, developed by Ionics, Inc., performed well in this system. ° It was prepared by a free radical polymerization of vinylbenzyl chloride and dimethylaminoethyl methacrylate in a 1 1 molar ratio. One major issue with the anionic membranes was its increase in resistance during the time it was exposed to a ferric chloride solution. The resistance increase termed fouling is related to the ability of the ferric ion to form ferric chloride complexes, which are not electrically repelled by the anionic membrane. An experiment by Arnold and Assink indicated that... 

Constructed wetlands (CWs) can promote removal of PhCs through a number of different mechanisms, including photolysis, plant uptake, microbial degradation and sorption to the soil. The main benefits of horizontal and vertical subsurface flow systems are the existence of aerobic, anaerobic and anoxic redox conditions in proximity to plant rhizomes this provides an ideal environment for reducing... 

Not all redox titrations have a well-defined equivalence point, and amperometric titrations1, in which a potential corresponding normally to that necessary to attain the mass-transport-limited current is applied to the working (indicator) electrode, permit the calculation of the titration endpoint through measurements done far from the equivalence point. Titrations can be done in flow systems, and in this sense it is possible to alter the quantity of added titrant so as to obtain greater accuracy in the determination of the equivalence point2. 

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. 

Engesgaard, P., and Kipp, K. L., 1992, A geochemical transport model for redox-controlled movement of mineral fronts in groundwater flow systems A case of nitrate removal by oxidation ofpyrite Water Resources Research, v. 28, p. 2829-2843. 

Comparison of energy density (kWh/m ) for various storage systems of electric power such as pumped hydropower, redox-flow battery, lead battery, NAS battery and methylcyclohexane (MCH) and decalin (TEPCO = Tokyo Electric Power Company). 

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