Classes of Flow Batteries
Various classes of flow batteries exist including the redox (reduction-oxidation) flow battery, a reversible fuel cell in which all electroactive components are dissolved in the electrolyte. If one or more electroactive components are deposited as a solid layer the system is known as a hybrid flow battery; that is, the electrochemical cell contains one battery electrode and one fuel cell electrode. The main difference between these two types of flow batteries is that the energy of the redox flow battery, as with other fuel cells, is fully decoupled from the power, because the energy is related to the electrolyte volume (tank size) and the power to the electrode area (reactor size). The hybrid flow battery, similar to typical batteries, is limited in energy by the size of the battery electrode (reactor size).
Energy producing electrochemical cells are generally divided into two categories. Cells that can be discharged only, with irreversible electrochemical reactions, are termed primary cells, while rechargeable cells with reversible reactions are termed secondary cells (see also primary and secondary batteries). Using this historical convention, a redox flow battery is better described as a secondary fuel cell or regenerative fuel cell, with the fundamental difference between batteries and fuel cells being whether energy is stored in a solid state electrode material (batteries) or in the electrolyte (fuel cells). This difference leads to the decoupling of energy and power in a fuel cell described above.
The misnomer of "redox flow battery" rather than "reversible fuel cell" has led to a great deal of confusion in understanding and terminology. For example, the processes of solid-state diffusion and intercalation in a Lithium Ion Battery do not apply to redox flow batteries, but the heterogeneous electron transfer in a fuel cell does. In industrial practice, fuel cells are usually, and unnecessarily, considered to be primary cells, such as the H2 / O2 system, with limited examples of reversible systems (i.e., the unitized regenerative fuel cell on NASA's Helios Prototype). The European Patent Organisation classifies redox flow cells (H01M8/18C4) as a sub-class of regenerative fuel cells (H01M8/18).
Examples of redox flow batteries are the vanadium redox flow battery, polysulfide bromide battery (Regenesys), and uranium redox flow battery. Hybrid flow batteries include the zinc-bromine, zinc-cerium and lead-acid flow batteries. Redox fuel cells are less common commercially although many systems have been proposed.
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