9 Types of Flow Batteries and Characteristics Explained
Types of flow batteries are; full-flow, semi-flow, hybrid, membrane-less, organic redox, inorganic redox, redox-targeted solid, semi solid, and nano-network batteries.
They are classified based on structure, composition, and mode of operation.
This article discusses the types of flow batteries, as follows;
1). Full-Flow Battery (as one of the Types of Flow Batteries)
Full-flow batteries are flow batteries in which all elements actively involved in energy storage occur in liquid or gaseous form.
They are called ‘full-flow’ because this fluid phase encourages free flow of reactive species.
Free flow of electrolytes in full-flow batteries, leads to electron transfers which enable the cell to store and discharge energy repeatedly, in a rechargeable manner.
Full-flow batteries are heterogeneous in their electrolytic composition, meaning that they usually contain more than one electrolyte. The ionic charge difference between these electrolytes is what facilitates electron transfers.
The concept behind full-flow batteries is very similar to that behind fuel cell technology, differing only in the nature of materials used to store energy; where fuel cells depend on sources like liquid hydrogen, and full-flow batteries depend on acidic and alkaline electrolytes.
2). Semi-flow Battery
The semi-flow battery is a type of flow battery in which at least one of the electrolytes or electroactive elements is in the solid phase.
In most semi-flow batteries, this solid-state electrolyte is a highly conductive and reactive metal, like zinc 
The efficiency of energy storage in semi-flow batteries is generally lower than that of full-flow batteries. This lower energy efficiency is due to limitations in the flexibility of ion exchange in the semi-flow batteries, especially with regards to the interaction between solid and liquid phase electrolyte.
A halogen compound, alkali or acid usually serves as the liquid phase electrolyte.
Electrolyte combinations like iron-salt, zinc-polyiodide, zinc-cerium and zinc-halide are among those used in semi-flow batteries.
The electrolytes are usually separated from each other by a membrane, and exchange ions by the help of electrodes.
In each charge-discharge cycle, the solid electroactive element is deposited on the electrode and dissolved , as a result of ionic reactions that govern the flow of electric charges through the system.
Problems of semi-flow batteries include need for cooling in some, cathode electrolytic coating, loss of decoupled energy, and low energy density.
3). Hybrid Flow Battery (as one of the Types of Flow Batteries)
A hybrid flow battery is any energy storage system which combines the characteristics of multiple types pf flow batteries.
These characteristics could be inorganic and organic; solid and liquid state, containerized and membrane-less.
Based on the above, it can be deduced that the semi-flow battery is a hybrid flow battery, since it combines liquid and solid electroactive components
The goal behind developing hybrid flow batteries is to optimize performance by combining the favorable characteristics of different battery types into one.
4). Membrane-less Flow Battery
In membrane-less flow battery, the membrane which is used to physically separate the two half cell electrolytes of the battery, is removed.
In place of the membrane, a laminar flow mechanism is relied upon, to keep the electrolytes apart.
Membrane-less flow batteries have a simpler design and are generally more durable than other containerized types of flow batteries. They also tend to have a higher energy density as a result of more effective electrochemical reactions between electrolytes.
In membrane-less flow batteries, electron transfer is based on selective ion exchange between electrolyte fluids . They may also contain dissolved solid electroactive elements like zinc, and may combine organic and inorganic electrolytic reactants.
5). Organic Redox (as one of the Types of Flow Batteries)
All types of flow batteries can be described as redox flow batteries (RFBs) since they depend on reduction and oxidation reactions to facilitate energy storage through recharge and discharge, and electron transfer cycles.
However, the organic redox flow batteries are those which specifically use aqueous organic electrolytes to store energy.
They may also be referred to as Aqueous Organic Flow Batteries (AOFBs), and tend to have a relatively high specific capacity. This makes them a promising option for large-scale power storage .
AOFBs are also flexible in terms of the manner in which electron transfer occurs in the cells. As a result, they are suitable for integration with renewable technologies like solar panels, as well as with microgrids and smart grid networks. Lithium-ion batteries are however more prominent for such applications, because of their general better performance and simplicity, compared to flow batteries.
6). Inorganic Redox Flow Battery
Inorganic redox flow batteries are flow batteries that use inorganic electroactive materials to store energy.
Examples of materials used in this type of flow battery are metallic salts, metals, polysulfides, and halides .
Vanadium redox flow battery is a relatively common example of inorganic redox flow battery.
7). Redox-targeted Solid (ROTS) (as one of the Types of Flow Batteries)
Redox-targeted solid flow batteries can be alternatively referred to as solid energy boosters (SEBs) .
They comprise of redox fluids which exchange ions with externally-located electroactive solid materials that are usually placed in separate vessels.
The ROTS battery can be considered to be a hybrid type, because it combines the characteristics of solid state batteries with those of fluid batteries. It therefore possesses both energy decoupling and high specific capacity attributes, and is very durable and energy-dense, compared to other types of flow batteries.
8). Semi Solid Flow Battery
Semi-solid flow battery is a type of flow battery in which the catholyte and anolyte (positive and negative electrolytes) contain suspended electroactive solid materials that act as charge carriers to optimize the energy storage process .
It was developed in an effort to achieve the energy efficiency of lithium-ion batteries with less production materials and a simpler manufacturing process.
The presence of suspended solids in the electrolytes makes semi-solid flow batteries to operate with high specific power and efficiency.
9). Nano-network (as one of the Types of Flow Batteries)
The nano-network flow battery is a type of flow battery in which a network of nanoparticles or nanofibres is used to facilitate electron flow.
In some cases, the nanofibre is wrapped around an electrode, while in others, it is immersed in electrolyte.
Carbon in the form of graphite, is a common material used to make the nanofibres in this type of flow battery.
The nano framework creates an intricate multilayer structure in the battery, that increases energy density and facilitates effective electron flow.
Nano-network flow batteries are highly efficient compared to other types of flow batteries because they do not depend solely on electrode plates for charge conduction.
Types of flow batteries are;
5. Organic Redox
6. Inorganic Redox
7. Redox-targeted Solid (ROTS)
8. Semi Solid
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