Vanadium redox battery

The vanadium redox battery (VRB), also known as the vanadium flow battery (VFB) or vanadium redox flow battery (VRFB), is a type of rechargeable flow battery. It employs vanadium ions as charge carriers.[5] The battery uses vanadium's ability to exist in a solution in four different oxidation states to make a battery with a single electroactive element instead of two.[6] For several reasons, including their relative bulkiness, vanadium batteries are typically used for grid energy storage, i.e., attached to power plants/electrical grids.

Vanadium redox battery
Specific energy10–20 Wh/kg (36–72 J/g)
Energy density15–25 Wh/L (54–65 kJ/L)
Charge/discharge efficiency75–80%<.[1][2]
Time durability20-30 years
Cycle durability>12,000-14,000 cycles[3]
Nominal cell voltage1.15–1.55 V
Schematic design of a vanadium redox flow battery system[4]
1 MW 4 MWh containerized vanadium flow battery owned by Avista Utilities and manufactured by UniEnergy Technologies
A vanadium redox flow battery located at the University of New South Wales, Sydney, Australia

Pissoort explored the possibility of VRFB's in the 1930s.[7] NASA researchers and Pellegri and Spaziante followed suit in the 1970s,[8] but neither was successful. Maria Skyllas-Kazacos presented the first successful demonstration of dissolved vanadium in a solution of sulfuric acid in the 1980s.[9][10] Her design used sulfuric acid electrolytes, and was patented by the University of New South Wales in Australia in 1986.[2]

Numerous companies and organizations are involved in funding and developing vanadium redox batteries.

Advantages and disadvantages


VRFB's main advantages over other types of battery:[11]

  • no limit on energy capacity
  • can remain discharged indefinitely without damage
  • mixing electrolytes causes no permanent damage
  • single charge state across the electrolytes avoids capacity degradation
  • safe, non-flammable aqueous electrolyte;[12]
  • wide operating temperature range including passive cooling[13][14]
  • long charge/discharge cycle lives: 15,000-20,000 cycles.
  • low levelized cost: (a few tens of cents), approaching the 2016 $0.05 target stated by the US Department of Energy and the European Commission Strategic Energy Technology Plan €0.05 target.[15]


VRFB's main disadvantages compared to other types of battery:[11]

  • high and volatile prices of vanadium minerals (i.e. the cost of VRFB energy) ;
  • relatively poor round trip efficiency (compared to lithium-ion batteries) ;
  • heavy weight of aqueous electrolyte ;
  • relatively poor energy-to-volume ratio compared to standard storage batteries ;
  • toxicity of vanadium (V) compounds.


Diagram of a vanadium flow battery

A vanadium redox battery consists of an assembly of power cells in which two electrolytes are separated by a proton exchange membrane. The electrodes in a VRB cell are carbon based. The most common types are carbon felt, carbon paper, carbon cloth, and graphite felt. Recently, carbon nanotube-based electrodes have attracted interest from the scientific community.[16][17][18]

Both electrolytes are vanadium-based. The electrolyte in the positive half-cells contains VO2+ and VO2+ ions, while the electrolyte in the negative half-cells consists of V3+ and V2+ ions. The electrolytes can be prepared by several processes, including electrolytically dissolving vanadium pentoxide (V2O5) in sulfuric acid (H2SO4). The solution remains strongly acidic in use.

The membrane is another critical component. The most common membrane material is perfluorinated sulfonic acid (PFSA) (Nafion). However, vanadium ions tend to penetrate the membrane and destabilize the cell. A 2021 Chinese study found that this penetration is reduced with hybrid sheets made by growing tungsten trioxide nanoparticles on the surface of single-layered graphene oxide sheets. These hybrid sheets are then embedded into a sandwich structured PFSA membrane reinforced with polytetrafluoroethylene (Teflon). The tungsten trioxide nanoparticles also promote proton transport, offering high Coulombic efficiency and energy efficiency of more than 98.1 percent and 88.9 percent, respectively.[19]


Cyclic voltammogram of vanadium (IV) solution in sulfuric acid solution

The reaction uses the half-reactions:[20]

VO+2 + 2H+ + eVO2+ + H2O ( = +1.00 V[21])
V3+ + e → V2+ ( = 0.26 V[22])

Other useful properties of vanadium flow batteries are their very fast response to changing loads and their extremely large overload capacities. Studies by the University of New South Wales have shown that they can achieve a response time of under half a millisecond for a 100% load change, and allowed overloads of as much as 400% for 10 seconds. The response time is mostly limited by the electrical equipment. Unless specifically designed for colder or warmer climates, most sulfuric acid-based vanadium batteries work only between about 10 and 40 °C. Below that temperature range, the ion-infused sulfuric acid crystallizes.[23] Round trip efficiency in practical applications is around 65–75 %.[24]

Proposed improvements

The original VRFB design by Skyllas-Kazacos employed sulfate (added as vanadium sulfate(s) and sulfuric acid) as the only anion in VRFB solutions, which limited the maximum vanadium concentration to 1.7 M of vanadium ions.[25] Around 2010 a team from Pacific Northwest National Laboratory proposed a mixed sulfate-chloride electrolyte, that allowed for the use in VRFBs solutions with the total vanadium concentration of 2.5 M over a whole temperature range between −20 and +50 °C.[26] It is worth noting, that the based on the standard equilibrium potential of the V(+5)/V(+4) couple it is expected to oxidize chloride, and for this reason chloride solutions were avoided in earlier VRFB studies. The surprising oxidative stability (albeit only at the state of charge below ca. 80%) of V(+5) solutions in the presence of chloride was explained on the basis of activity coefficients.[27] Nevetheless, because of a high vapor pressure of HCl solutions, such mixed electrolytes have not been widely adopted by the VRFB industry.

Another VRFB chemistry variation is the use of vanadium bromide salts. Since the redox potential of Br2/2Br- couple is more negative than that of V(5+/4+), the positive electrode operates via the bromine process. Such solutions were expected to approximately double the energy density and increase the temperature range in which the battery can operate.[28] However, due to the problems with volatility and corrosivity of Br2, they did not gain much popularity (see zinc-bromine battery for a similar problem). The vanadium/cerium flow battery has also been proposed .[29]

Typically, perfluorinated sulfonic acid (PFSA) is used as the separator membrane. However, vanadium ions can cross the membrane and destabilize the battery. Researchers grew tungsten trioxide nanoparticles on the surface of graphene oxide sheets and embedded them in a polytetrafluoroethylene-reinforced, sandwich-structured PFSA system. This reduced penetration and promoted proton transport, yielding Coulumbic efficiency and energy efficiency of greater than 98.1% and 88.9%, respectively.[30]

Specific energy and energy density

Current production vanadium redox batteries achieve a specific energy of about 20 Wh/kg (72 kJ/kg) of electrolyte. More recent research at UNSW indicates that the use of precipitation inhibitors can increase the density to about 35 Wh/kg (126 kJ/kg), with even higher densities made possible by controlling the electrolyte temperature. The current specific energy is quite low compared to other rechargeable battery types (e.g., lead–acid, 30–40 Wh/kg (108–144 kJ/kg); and lithium ion, 80–200 Wh/kg (288–720 kJ/kg)). However, using precipitation inhibitors would place vanadium redox batteries on par with lead-acid batteries.


VRFB's large potential capacity may be best-suited to buffer the irregular output of utility-scale wind and solar systems.[11]

Their reduced self-discharge makes them potentially appropriate in applications that require long-term battery storage with little maintenance as in military equipment, such as the sensor components of the GATOR mine system.[31][11]

They feature rapid response times well suited to uninterruptible power supply (UPS) applications, where they can replace lead–acid batteries or diesel generators. Fast response time is also appropriate for frequency regulation. These capabilities make VRBF's an effective "all-in-one" solution for microgrids, frequency regulation and load shifting.[11]

Largest vanadium grid batteries

Largest operational vanadium redox batteries
Name Commissioning date Energy (MWh) Power (MW) Duration (hours) Country
Minami Hayakita Substation[32][33]December 201560154Japan
Pfinztal, Baden-Württemberg[34][35][36] September 2019 20 2 10 Germany
Woniushi, Liaoning[37][38]1052China
Tomamae Wind Farm[39]2005641:30Japan
Zhangbei Project[40]2016824China
SnoPUD MESA 2 Project[41][42]March 2017824USA
San Miguel Substation[43]2017824USA
Pullman Washington[44]April 2015414USA
Dalian Battery[45] May 2021 (Final Capacity) 400 (800) 100 (200) 4 China

A 200 MW, 800 MWh (4 hours) vanadium redox battery is under construction in China; it was expected to be completed by 2018[46] and its 250 kW/ 1 MWh first stage was in operation in late 2018[47]

Companies funding or developing vanadium redox batteries

Companies include CellCube (Enerox),[48] UniEnergy Technologies,[49] StorEn Technologies,[50][51] Largo Energy[52] and Ashlawn Energy[53] in the United States; H2 in South Korea; Renewable Energy Dynamics Technology,[54] Invinity Energy[55] and VoltStorage[56] in Europe; Prudent Energy in China;[57] Australian Vanadium in Australia;[58] EverFlow Energy JV SABIC SCHMID Group in Saudi Arabia[59] and Bushveld Minerals in South Africa.[60]

See also


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