A flow battery, or redox flow battery, is a type of rechargeable battery where
rechargeability is provided by two chemical components dissolved in liquids
contained within the system and separated by a membrane. Ion exchange
occurs through the membrane while both liquids circulate in their own
respective space. Cell voltage is chemically determined by the Nernst
equation and ranges, in practical applications, from 1.0 to 2.2 volts.
A flow battery is technically akin both to a fuel cell and an electrochemical
accumulator cell. While it has technical advantages such as potentially separable
liquid tanks and near unlimited longevity over most conventional
rechargeables, current implementations are comparatively less powerful and
require more sophisticated electronics. Construction principle
A flow battery is a rechargeable fuel cell in which an electrolyte containing
one or more dissolved electroactive elements flow through an electrochemical
cell that reversibly converts chemical energy directly to electricity.
Additional electrolyte is stored externally, generally in tanks, and is
usually pumped through the cell of the reactor, although gravity feed systems
are also known. Flow batteries can be rapidly "recharged" by replacing the
electrolyte liquid while simultaneously recovering the spent material for
re-energization. In other words, a flow battery is just
like an electrochemical cell, with the exception that the ionic solution is not
stored in the cell around the electrodes. Rather, the ionic solution
is stored outside of the cell, and can be fed into the cell in order to
generate electricity. The total amount of electricity that can be generated
depends on the size of the storage tanks.
Types Different classes of flow cells have
been developed, including redox, hybrid and membraneless. The fundamental
difference between conventional batteries and flow cells is that energy
is stored as the electrode material in conventional batteries but as the
electrolyte in flow cells. = Redox =
The redox cell is a reversible fuel cell in which all electrochemical components
are dissolved in the electrolyte. The energy capacity of the redox flow
battery is fully independent of its power, because the energy available is
related to the electrolyte volume and the power to the surface area of the
electrodes. Redox flow batteries are rechargeable. Because they employ
heterogeneous electron transfer rather than solid-state diffusion or
intercalation they are more appropriately called fuel cells than
batteries. In industrial practice, fuel cells are usually, and unnecessarily,
considered to be primary cells, such as the H
2/O 2 system. The unitized regenerative fuel
cell on NASA's Helios Prototype is another reversible fuel cell. The
European Patent Organisation classifies redox flow cells as a sub-class of
regenerative fuel cells. Examples of redox flow batteries are the Vanadium
redox flow battery, polysulfide bromide battery, and uranium redox flow battery.
Redox fuel cells are less common commercially although many systems have
been proposed. Researchers announced a prototype,
zinc-polyiodide flow battery with an energy density of 167 Wh/l. Older
zinc-bromide cells reach 70 Wh/l. For comparison, lithium iron phosphate
batteries that store 233 Wh/l. The zinc-polyiodide battery is claimed to be
safer than other flow batteries given its absence of acidic electrolytes,
nonflammability and operating range of −4 to 122 °F that does not require
extensive cooling circuitry, which would add weight and occupy space. One
unresolved issue is zinc build-up that grew from the negative electrode and
permeated the membrane, reducing efficiency. Adding alcohol to the
electrolyte contained the problem. When the battery is fully discharged,
both tanks hold the same electrolyte solution: a mixture of positively
charged zinc ions and negatively charged iodide ion, I-. When charged, one tanks
holds another negative ion, polyiodide, I3-. The battery produces power by
pumping liquid from external tanks into the battery's stack area where the
liquids are mixed. Inside the stack, zinc ions pass through a selective
membrane and change into metallic zinc on the stack's negative side.
Traditional flow battery chemistries have both low specific energy and low
specific power. However, recently a high areal power of 1.4 W cm2 has been
demonstrated for hydrogen-bromine flow batteries, and a high specific energy
has been shown for hydrogen-bromate flow batteries
= Hybrid = The hybrid flow battery uses one or more
electroactive components deposited as a solid layer. In this case, the
electrochemical cell contains one battery electrode and one fuel cell
electrode. This type is limited in energy by the surface area of the
electrode. Hybrid flow batteries include the
zinc-bromine, zinc–cerium and lead–acid flow batteries.
= Membraneless = This battery employs a phenomenon called
laminar flow in which two liquids are pumped through a channel. 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.
Membranes are often the most costly component and the most unreliable
components of batteries, as they can corrode with repeated exposure to
certain reactants. The absence of a membrane enabled 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.
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. One such membraneless flow battery published in
August 2013 produced a maximum power density of 7,950 W/m2, three times as
much power as other membraneless systems— and an order of magnitude
higher than lithium-ion batteries. = Organic =
In 2013, researchers announced the use of 9,10-anthraquinone-2,7-disulphonic
acid, a quinone, as a charge carrier in metal-free flow batteries. Each of the
carbon-based molecules holds two units of electrical charge, compared with one
unit in conventional batteries, implying that a battery could store twice as much
energy in a given volume. AQDS undergoes rapid, reversible
two-electron/two-proton reduction on a glassy carbon electrode in sulphuric
acid. An aqueous flow battery with inexpensive carbon electrodes, combining
the quinone/hydroquinone couple with the Br
2/Br− redox couple, yields a peak galvanic power density exceeding 6,000
W/m2 at 13,000 A/m2. Cycling showed >99 per cent storage capacity retention per
cycle. Volumetric energy density was over 50 Wh/l and specific energy density
over 50 Wh/kg. The organic anthraquinone species can be synthesized from
inexpensive commodity chemicals. This organic approach permits tuning of the
reduction potential and solubility by adding functional groups. Adding two
hydroxy groups to AQDS increases the open circuit potential of the cell by
11%. In 2014, another example used
anthraquinone-2-sulfonic acid or anthraquinone-2,6-disulfonic acid on the
negative side and 1,2-dihydrobenzoquinone- 3,5-disulfonic
acid on the positive side. The battery was claimed to last for 5,000 cycles
without degradation. In 2015, researchers had improved the
chemistry and replaced the hydrobromic acid used as an electrolyte with a far
less toxic alkaline solution and ferrocyanide. The cell voltage was 1.2.
= Metal hydride = Proton flow batteries integrate a metal
hydride storage electrode into a reversible proton exchange membrane 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 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.
= Nano-network = In 2014 a technology was announced that
uses lithium–sulfur chemistry arranged in a network of nanoparticles. The
network 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. In August 2014, the Quant
e-Sportlimousine was approved for testing on public roads using the
nanoFLOWCELL® system with a claimed energy or power density of 600 Wh per
kilogram. = Semi-solid =
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 dissolved in a liquid
electrolyte, with the chemistry of lithium-ion batteries. 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. = Polymer =
A novel flow-battery concept that is based on redox active, organic polymers
and dialysis membranes was announced in 2015. The polymer-based redox-flow
battery uses functionalized macromolecules 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.
Chemistries There are a wide range of chemistries
that have been tried for flow batteries. Advantages and disadvantages
Redox flow batteries, and to a lesser extent hybrid flow batteries, have the
advantages of flexible layout, long cycle life, quick response times, no
need for "equalisation" charging and no harmful emissions. Some types also offer
easy state-of-charge determination, low maintenance and tolerance to
overcharge/overdischarge. On the negative side, flow batteries are
rather complicated in comparison with standard batteries as they may require
pumps, sensors, control units and secondary containment vessels. The
energy densities vary considerably but are, in general, rather low compared to
portable batteries, such as the Li-ion. Applications
Flow batteries are normally considered for relatively large stationary
applications. These are for Load balancing – where the battery is
connected to an electrical grid to store excess electrical power during off-peak
hours and release electrical power during peak demand periods. The common
problem limiting the use of most flow battery chemistries in this application
is their low areal power which translates into a high cost of power.
Storing energy from renewable sources such as wind or solar for discharge
during periods of peak demand. Peak shaving, where spikes of demand are
met by the battery. UPS, where the battery is used if the
main power fails to provide an uninterrupted supply.
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
power conversion can also be AC/DC, AC/AC, or DC–AC with the frequency
limited by that of the switching gear. 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. 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. Stand-alone power system – An example of
this is the telecom industry for use in cellphone base stations where there is
no grid power 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. Currently, flow
batteries are being used in solar micro grid applications throughout the
Caribbean. See also
References External links
Electropaedia on Flow Batteries Research on the uranium redox flow
battery How flow batteries work on YouTube