text: 2.3 Single liquid flow battery
  2.4 Membraneless
  2.6 Metal hydride
  2.7 Metal-Organic
  2.8 Nano-network
  2.9 Other flow-type batteries
  2.9.1 Semi-solid
  Various types of flow cells (batteries) have been developed,[9] including redox, hybrid and membraneless. The fundamental difference between conventional batteries and flow cells is that energy is stored in the electrode material in conventional batteries, while in flow cells it is stored in the electrolyte.
  The redox (reduction–oxidation) cell is a reversible cell in which electrochemical components are dissolved in the electrolyte. Redox flow batteries are rechargeable (secondary cells).[10] Because they employ heterogeneous electron transfer rather than solid-state diffusion or intercalation they are more appropriately called fuel cells rather than batteries. The performance of a solid-state battery depends on the diffusion of ions within the electrolyte. Solid electrolytes must have high ionic conductivity, very low electronic conductivity and a high degree of chemical stability.[11] In industrial practice, fuel cells are usually, and unnecessarily, considered to be primary cells, such as the H
  Single liquid flow battery[edit]
  Standalone SLIQ Battery
  Single liquid flow battery (or "SLIQ") is a liquid battery that uses a rechargeable liquid and can be used for grid storage. SLIQ features include simplicity, low cost, durability, thermal stability (no thermal runaway), and a low carbon foot print. The technology eliminates the need of using rare earth minerals for storage. The battery converts the electrolyte and part of the cathode to a free flowing liquid.
  The technology has the potential to make a significant cost reduction to the capital cost of energy storage, due to its high efficiency (92%), high energy density, a millisecond response time, low cost, and a +20 year lifespan.[32][33]
  An artificial intelligence-based control system complements the SLIQ battery to optimise performance by storing renewable energy when it is available on the grid and delivering it when it is not. It learns usage behaviour to maximise profitability and uses machine learning to maximise the efficiency of the SLIQ battery.[34]
  The Single Liquid battery (SLIQ) technology was invented in 2013 by Pasidu Pallawela.[35][36] A 30kWh/8 kW prototype was installed in Scotland to support a remote community and has been running successfully since 2013.[37]
  A membraneless battery[38] relies on laminar flow in which two liquids are pumped through a channel, where they undergo electrochemical reactions to store or release energy. The solutions stream through in parallel, with little mixing. The flow naturally separates the liquids, eliminating the need for a membrane.[39]
  Membranes are often the most costly and least reliable components of batteries, as they can be corroded by repeated exposure to certain reactants. The absence of a membrane enables the use of a liquid bromine solution and hydrogen: this combination is problematic when membranes are used, because they form hydrobromic acid that can destroy the membrane. Both materials are available at low cost.[40]
  The design uses a small channel between two electrodes. Liquid bromine flows through the channel over a graphite cathode and hydrobromic acid flows under a porous anode. At the same time, hydrogen gas flows across the anode. The chemical reaction can be reversed to recharge the battery — a first for any membraneless design.[40] One such membraneless flow battery published in August 2013 produced a maximum power density of 7950 W/m2, three times as much power as other membraneless systems— and an order of magnitude higher than lithium-ion batteries.[40]
  Recently, a macroscale membraneless redox flow battery capable of recharging and recirculation of the same electrolyte streams for multiple cycles has been demonstrated. The battery is based on immiscible organic catholyte and aqueous anolyte liquids, which exhibits high capacity retention and Coulombic efficiency during cycling.[41]
  Organic redox flow batteries could be further classified into two categories: Aqueous Organic Redox Flow Batteries (AORFBs) and Non-aqueous Organic Redox Flow Batteries (NAORFBs).[42][43] AORFBs use water as solvent for electrolyte materials while NAORFBs employ organic solvents to dissolve redox active materials. Depending on using one or two organic redox active electrolytes as anode and/or cathode, AORFBs and NAORFBs can be further divided into total organic systems and hybrid organic systems that use inorganic materials for anode or cathode. In larger-scale energy storage, Due to lower solvent cost and higher conductivity AORFBs have greater commercial potential than NAORFBs , as well as the safety advantages of water-based electrolytes over non-aqueous electrolytes. The advantage of NAORFBs lies in their much larger voltage window and ability to possibly occupy less physical space for an installed storage facility. The content below lists materials advances for these organic based systems.
  Quinones and their derivatives are the basis of many organic redox systems including NARFBs and AORFBs.[44][45][46] In one study, 1,2-dihydrobenzoquinone-3,5-disulfonic acid (BQDS) and 1,4-dihydrobenzoquinone-2-sulfonic acid (BQS) were employed as cathodes, and conventional Pb/PbSO4 was the anolyte in an acid AORFB. These first AORFBs are hybrid systems as they use organic redox active materials only for the cathode side. The quinones accepts two units of electrical charge, compared with one in conventional catholyte, implying that such a battery could store twice as much energy in a given volume.
  9,10-Anthraquinone-2,7-disulfonic acid (AQDS), also a quinone, has been evaluated as well.[47] AQDS undergoes rapid, reversible two-electron/two-proton reduction on a glassy carbon electrode in sulfuric acid. An aqueous flow battery with inexpensive carbon electrodes, combining the quinone/hydroquinone couple with the Br
  redox couple, yields a peak galvanic power density exceeding 6,000 W/m2 at 13,000 A/m2. Cycling showed >99 % storage capacity retention per cycle. Volumetric energy density was over 20 Wh/l.[48] Anthraquinone-2-sulfonic acid and anthraquinone-2,6-disulfonic acid on the negative side and 1,2-dihydrobenzoquinone- 3,5-disulfonic acid on the positive side avoids the use of hazardous Br2. The battery was claimed to last for 1,000 cycles without degradation although no official data were published.[49] While this total organic system appears robust, it has a low cell voltage (ca. 0.55 V) and a low energy density (< 4 Wh/L).
  Hydrobromic acid used as an electrolyte has been replaced with a far less toxic alkaline solution (1M KOH) and ferrocyanide.[50] The higher pH is less corrosive, allowing the use of inexpensive polymer tanks. The increased electrical resistance in the membrane was compensated by increasing the voltage. The cell voltage was 1.2 V.[51][52] The cell's efficiency exceeded 99%, while round-trip efficiency measured 84%. The battery has an expected lifetime of at least 1,000 cycles. Its theoretic energy density was 19 Wh per liter.[53] Ferrocyanide's chemically stability in high pH KOH solution without forming Fe(OH)2 or Fe(OH)3 needs to be verified before scale-up.
  Another organic AORFB has been demonstrated methyl viologen as anolyte and 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl as catholyte, plus sodium chloride and a low-cost anion exchange membrane to enable charging and discharging. This MV/TEMPO system has the highest cell voltage, 1.25 V, and, possibly, lowest capital cost ($180/kWh) reported for AORFBs. The water-based liquid electrolytes were designed as a drop-in replacement for current systems without replacing existing infrastructure. A 600-milliwatt test battery was stable for 100 cycles with nearly 100 percent efficiency at current densities ranging from 20 to 100 mA per square centimeter, with optimal performance rated at 40-50 mA, at which about 70 percent of the battery's original voltage was retained.[54][55] The significance of the research is that neutral AORFBs can be more environmentally friendly than acid or alkaline AORFBs while showing electrochemical performance comparable to corrosive acidic or alkaline RFBs. The MV/TEMPO AORFB has an energy density of 8.4 Wh/L with the limitation on the TEMPO side. The next step is to identify a high capacity catholyte to match MV (ca. 3.5 M solubility in water, 93.8 Ah/L).
  One flow-battery concept is based on redox active, organic polymers employs viologen and TEMPO with dialysis membranes. The polymer-based redox-flow battery (pRFB) uses functionalized macromolecules (similar to acrylic glass or Styrofoam) being dissolved in water as active material for the anode as well as the cathode. Thereby, metals and strongly corrosive electrolytes – like vanadium salts in sulfuric acid – are avoided and simple dialysis membranes can be employed. The membrane, which separates the cathode and the anode of the flow cell, works like a strainer and is produced much more easily and at lower cost than conventional ion-selective membranes. It retains the big "spaghetti"-like polymer molecules, while allowing the small counterions to pass.[56] The concept may solve the high cost of traditional Nafion membrane but the design and synthesis of redox active polymer with high solubility in water is not trivial.
  Aligned with the tunability of the redox-active components as the main advantage of organic redox flow batteries, the idea of integrating both anolyte and catholyte in the same molecule has been developed. Those so called, bifunctional analytes[57] or combi-molecules[58] allow to use the same material in both tanks, which definitely has relevant advantages on the battery performance, as diminishing the effect of crossover.[59] Thus, diaminoanthraquinone,[59] also a quinone, and indigo[57] based molecules as well as TEMPO/phenazine[58] combining molecules have been presented as potential electrolytes for the development of symmetric redox-flow batteries (SRFB).
  Proton flow batteries (PFB) integrate a metal hydride storage electrode into a reversible proton exchange membrane (PEM) fuel cell. During charging, PFB combines hydrogen ions produced from splitting water with electrons and metal particles in one electrode of a fuel cell. The energy is stored in the form of a solid-state metal hydride. Discharge produces electricity and water when the process is reversed and the protons are combined with ambient oxygen. Metals less expensive than lithium can be used and provide greater energy density than lithium cells.[60][61]
  Metal-organic flow batteries use organic ligands to provide more favorable properties to redox-active metals. The ligands can be chelates like EDTA, and can enable electrolyte to be in neutral or alkaline pH, conditions under which metal aquo complexes would otherwise precipitate. By blocking the coordination of water to the metal, organic ligands can also inhibit metal-catalyzed water-splitting reactions, resulting in some of the highest voltage all-aqueous systems ever reported. For example, the use of chromium coordinated to 1,3-propanediaminetetraacetate (PDTA), gave cell potentials of 1.62 V vs. ferrocyanide and a record 2.13 V vs. bromine.[62] Metal-organic flow batteries are sometimes known as coordination chemistry flow batteries, which represents the technology behind Lockheed Martin's Gridstar Flow technology.[63]
  Lithium–sulfur system arranged in a network of nanoparticles eliminates the requirement that charge moves in and out of particles that are in direct contact with a conducting plate. Instead, the nanoparticle network allows electricity to flow throughout the liquid. This allows more energy to be extracted.[64]
  Semi-Solid Flow Battery[65]
  In a semi-solid flow cell, the positive and negative electrodes are composed of particles suspended in a carrier liquid. The positive and negative suspensions are stored in separate tanks and pumped through separate pipes into a stack of adjacent reaction chambers, where they are separated by a barrier such as a thin, porous membrane. The approach combines the basic structure of aqueous-flow batteries, which use electrode material suspended in a liquid electrolyte, with the chemistry of lithium-ion batteries in both carbon-free suspensions and slurries with conductive carbon network.[1][66][67] The carbon free semi-solid redox flow battery is also sometimes referred to as Solid Dispersion Redox Flow Battery.[68] Dissolving a material changes its chemical behavior significantly. However, suspending bits of solid material preserves the solid's characteristics. The result is a viscous suspension that flows like molasses.[69]
  Metal-Organic–ferrocyanide[62] 1.62 2000 21.7 Wh/L 75
  Metal-Organic–bromine[62] 2.13 3000 35 Wh/L 10
  Sodium–potassium[70]
  Sulfur-oxygen-salt[71]
  Redox flow batteries, and to a lesser extent hybrid flow batteries, have the advantages of flexible layout (due to separation of the power and energy components), long cycle life (because there are no solid-to-solid phase transitions), quick response times, no need for "equalisation" charging (the overcharging of a battery to ensure all cells have an equal charge) and no harmful emissions. Some types also offer easy state-of-charge determination (through voltage dependence on charge), low maintenance and tolerance to overcharge/overdischarge. They are safe and typically do not contain flammable electrolytes and due to the reason that electrolytes can be stored away from the power stack. These technical merits make redox flow batteries a well-suited option for large-scale energy storage.
  The two main disadvantages are their low energy density (you need large tanks of electrolyte to store useful amounts of energy) and their low charge and discharge rates (compared to other industrial electrode processes). The latter means that the electrodes and membrane separators need to be large, which increases costs.
  Flow batteries are normally considered for relatively large (1 kWh – 10 MWh) stationary applications.[72] These are for:
  Storing energy from renewable sources such as wind or solar for discharge during periods of peak demand.[73]
  Peak shaving, where spikes of demand are met by the battery.[74]
  Power conversion – because all cells share the same electrolyte(s). Therefore, the electrolyte(s) may be charged using a given number of cells and discharged with a different number. Because the voltage of the battery is proportional to the number of cells used the battery can therefore act as a very powerful DC–DC converter. In addition, if the number of cells is continuously changed (on the input and/or output side) power conversion can also be AC/DC, AC/AC, or DC–AC with the frequency limited by that of the switching gear.[75]
  Electric vehicles – Because flow batteries can be rapidly "recharged" by replacing the electrolyte, they can be used for applications where the vehicle needs to take on energy as fast as a combustion engined vehicle.[76][77] A common problem found with most RFB chemistries in the EV applications is their low energy density which translated into a short driving range. Flow batteries based on highly soluble halates are a notable exception.[78]
  Stand-alone power system – An example of this is in cellphone base stations where no grid power is available. The battery can be used alongside solar or wind power sources to compensate for their fluctuating power levels and alongside a generator to make the most efficient use of it to save fuel.[79][80] Currently, flow batteries are being used in solar micro grid applications throughout the Caribbean.[citation needed]
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