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Iron-based flow batteries to store renewable energies
Article in Environmental Chemistry Letters · February 2018
DOI: 10.1007/s10311-018-0709-8
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Environmental Chemistry Letters
https://doi.org/10.1007/s10311-018-0709-8
REVIEW ARTICLE
Iron‑based flow batteries to store renewable energies
Anarghya Dinesh1 · Sharon Olivera1 · Krishna Venkatesh1 · Mysore Sridhar Santosh1 · Murugesan Geetha Priya1 ·
Inamuddin2,3 · Abdullah M. Asiri2,3 · Handanahally Basavarajaiah Muralidhara1
Received: 22 January 2018 / Accepted: 24 January 2018
© Springer International Publishing AG, part of Springer Nature 2018
Abstract
The development of cost-effective and eco-friendly alternatives of energy storage systems is needed to solve the actual energy
crisis. Although technologies such as flywheels, supercapacitors, pumped hydropower and compressed air are efficient, they
have shortcomings because they require long planning horizons to be cost-effective. Renewable energy storage systems such
as redox flow batteries are actually of high interest for grid-level energy storage, in particular iron-based flow batteries. Here
we review all-iron redox flow battery alternatives for storing renewable energies. The role of components such as electrolyte,
electrode and membranes in the overall functioning of all-iron redox flow batteries is discussed. The effect of iron–ligand
chemistry on the performance of battery is highlighted. Additionally, a brief contextual background and fundamentals of
redox flow batteries are provided. The design aspects, progress in research, mathematical modeling, cost estimations and
future prospects of using all-iron energy systems are discussed in the context of future grid-level energy storage.
Keywords All-iron · Grid-scale · Redox flow battery · Renewable energy · Energy storage
Introduction
Energy is one of the most important prerequisites for global
economic activities. Every human being uses energy in
one or other form, and its use lies at the core of modern
society. As energy is the basic and indispensable input for
modern economy it can be termed as ‘life blood of modern
economy.’ The growth of world population and economic
standards has contributed to the increased consumption of
energy and hence the demand. However, the shrinkage of
availability of fossil energy resources demands the need for
development of renewable energy sources and their storage
* Inamuddin
inamuddin@rediffmail.com
* Handanahally Basavarajaiah Muralidhara
hb.murali@gmail.com
1
Centre for Incubation, Innovation, Research and Consultancy
(CIIRC), Jyothy Institute of Technology, Thataguni, Off
Kanakapura Road, Bangalore, Karnataka 560 082, India
2
Chemistry Department, Faculty of Science, King Abdulaziz
University, Jeddah 21589, Saudi Arabia
3
Centre of Excellence for Advanced Materials Research
(CEAMR), King Abdulaziz University, Jeddah 21589,
Saudi Arabia
methods so as to fulfill the demand in the future. In order
to reduce the reliance on non-renewable fossil fuels, the use
of energy from renewable sources such as wind and solar
is encouraged. However, it is not so easy to switch over to
use wind and solar energy for our entire daily needs. The
solution to this problem lies in energy storage. By storing
the energy generated from solar and wind power in reliable, powerful batteries, a power supply can be developed
with extended life span without putting pressure on earthly
resources. The existing energy storage technologies have
been classified as follows:
•
•
•
•
•
•
•
Pumped hydropower
Compressed air energy storage
Electrochemical batteries
Capacitors
Flywheel
Superconducting magnetic energy storage
Thermal energy storage.
The comparison of their performance is presented in
Table 1. The power and energy capacity of each energy
storage technology will vary. This variation in capacity
leads to numerous applications based on the requirements.
13
Vol.:(0123456789)
Environmental Chemistry Letters
Table 1 Comparison of performance of various energy storage systems. (Compiled from Mellentine 2011)
Storage technique
Power capacity
Discharge time
Pumped hydropower
Compressed air
Batteries
Flywheel
Capacitors
~ 400–200 MW
~ 2–300 MW
~ 7 KW–80 MW
~ 0.1–10 MW
~ 0.7–3 MW
~ 20–100 h
~ 2–10 h
~ Seconds–20 h
~ Few seconds–1 h
~ Fraction of
seconds–few
seconds
A comparison was made using a table based on the data
enrolled from installed storage system as of 2008 (Mellentine 2011).
Electrochemical storage devices particularly redox flow
batteries have been proposed as promising choices for
grid-scale storage systems (Wang et al. 2013). Redox flow
batteries are one of the classes of electrochemical energy
storage devices which are employed by the redox reactions. The name ‘redox’ refers to chemical reduction and
oxidation reactions which help to store energy in liquid
Fig. 1 Schematic representation of redox flow battery (reprinted with
permission of Weber et al. 2011). Diagrammatic representation of
flow battery employs two electrolyte tanks kept externally, and overall energy depends on the electrolyte. Analyte and catholyte take part
in redox reaction by flowing through anode and cathode, respectively.
13
electrolyte solutions which flow through a battery of electrochemical cells during charge and discharge.
During discharge, an electron is released via an oxidation reaction from a high chemical potential state on the
negative or anode side of the battery. The electron moves
through an external circuit to do useful work. Finally, the
electron is accepted via a reduction reaction at a lower
chemical potential state on the positive or cathode side of
the battery. The direction of the current and the chemical
reactions are reversed during charging. A schematic representation of redox flow battery is shown in Fig. 1.
There are different types of redox flow battery systems
such as iron–chromium, bromine–polysulfide, iron–vanadium, all-vanadium, vanadium–bromine, vanadium–oxygen, zinc–bromine that have been the topic of intense
investigations (Weber et al. 2011). In spite of being advantageous, these redox flow batteries face challenges in terms
of cost, availability and eco-friendliness (Viswanathan
et al. 2014). The all-iron redox flow batteries present an
attractive solution because of the use of inexpensive materials, abundantly available iron and non-toxic nature of
the system.
These electrolytes are drawn back into the external tanks upon completion of reaction. Whole process is favored by a pump which maintains the flow of electrolyte through the system. Ions migrate through
the membrane to favor the reaction to occur
Environmental Chemistry Letters
This work highlights the potential usefulness of all-iron
flow batteries by discussing the state-of-the-art technology
and the research development in the past few years.
Milestones in redox flow system history
The first reference to a vanadium (V) redox couple can be
found in the 1933 patent of P. A. Pissoort of France (Patent
754 065—1933). The patent work on titanium chloride flow
cell was first reported by Germany’s Walter Kango in 1954
(Bartolozzi 1989). The major breakthrough in the development of redox flow batteries was achieved in 1970s by the
works of Thaller of NASA’s space programs. His initial studies concentrated on Fe–Ti electrolytes. This program which
was completed in 1984 also contributed to the development
of Fe–Cr redox couple of 1 kW/13 kWh capacities for photovoltaic-array application (Thaller 1974, 1981; Giner et al.
1976; Reid and Gahn 1977; Thaller 1979; Hagedorn 1984).
Fe–Cr systems were also part of the extensive research carried out in Electrotechnical Laboratory in Japan in 1980
(Tanaka et al. 1990). In 1981, the idea of all-iron redox battery was conceived by (Hruska and Savinell 1981). Pioneering works on vanadium redox flow batteries were carried
out in University of New South Wales (UNSW) resulting
in subsequent commercial development and patenting of
the same (Sum et al. 1985; Sum and Skyllas-Kazacos 1985;
Rychcik and Skyllas-Kazacos 1988; Alotto et al. 2014). The
interest in VRB grew in countries like Japan and Australia
in the late 1980s which contributed greatly to its R&D. A
Fig. 2 Schematic of all-iron redox flow battery. Image represents the
schematic diagram of all-iron redox flow battery where analyte is a
mixture of both ferrous and ferric chloride and catholyte is the fer-
large number of test systems were developed in 1996–2000.
Large-scale redox flow battery systems were installed post2001, and the last decade saw a great deal of progress in
the research of redox flow battery chemistry. At present,
there are a number of producers of flow batteries across the
world, among which China, South Africa and Russia top the
list. Even though vanadium redox flow batteries have been a
subject of intensive investigation, there is constant search for
newer technologies which would help overcome the issues
related to the costs of vanadium redox batteries (Alotto
et al. 2014). In this regard, the all-iron redox batteries have
become promising as energy storage devices on account of
their cost-effective performance. An insurgence in the alliron redox systems took place after 2006. The last decade
has seen a rise in research investigations and development of
large-scale systems for grid-level applications. Companies
such as Energy Storage Systems (ESS) and Electric F
­ uel®
have become key players in the manufacturing of iron hybrid
redox batteries.
Fundamentals of all‑iron redox flow battery
Flow batteries are used to store electrical energy in the form
of chemical energy. Electrolytes in the flow batteries are usually made up of metal salts which are in ionized form. The
all-iron redox flow battery as represented in Fig. 2 employs
iron in different valence states for both the positive and
negative electrodes. The electrolytic solutions essentially
stored in the external storage tanks flow through the stacks
rous chloride alone. Graphite felt is used as electrode on both sides.
Redox reaction occurs at anode (Eq. 1), and plating/stripping occurs
at cathode (Eq. 2)
13
Environmental Chemistry Letters
of batteries and take part in the half-cell reactions given by
Eqs. 1 and 2. The overall reaction is shown in Eq. 3. The
cathode reaction involves the deposition and dissolution of
iron in the form of a solid plate, whereas the cathode utilizes
a carbon-based material to bring about the redox reaction
(Petek 2015). Like the conventional battery systems, the
redox flow battery can be constructed to obtain high-power
output by stacking the cells shown in Fig. 1 together.
Anode∕Redox electrode ∶
Cathode∕Plating electrode ∶
Total ∶
2Fe2+ ⇔ Fe3+ + 2e−
+ 0.77 V
(1)
Fe2+ + 2e− ⇔ Fe0
− 0.44 V
(2)
3Fe2+ ⇔ Fe0 + 2Fe3+
1.21 V
(3)
Research and development
The suitability of all-iron redox flow battery systems for
grid-level energy storage was researched highly by J. S.
Wainright and her colleagues of Case Western Reserve
University in the project works and research investigations.
Detailed analyses of the ways to counter the hydrogen evolution which was responsible for major reduction in coulombic
efficiency at the cathode of the all-iron redox flow battery
were listed by Hawthorne (2014). One of the promising
solutions is to raise the pH of the electrolyte which in turn
ensures the drop in diffusion-controlled current due to ­H2
evolution by shifting its equilibrium potential to the highly
negative region. Generally, this can be accomplished by the
use of complexing ligands such as EDTA, phenanthroline,
triethanolamine (Chen et al. 1981; Ibanez et al. 1987; Wen
et al. 2006; Modiba et al. 2012). The experiments concerning all-iron redox flow batteries included the screening of
organic ligands as complexing agents for Fe(III) ions at the
redox electrode in order to overcome the problem of latter’s
precipitation as ferric hydroxide at pH > 2. Glycine was
determined to be the best choice among other ligands tested
because the electrolyte containing 1:1 glycine-to-iron ratio
showed high open-circuit potential of 468 mV vs Ag/AgCl
which was the major deciding factor for use in all-iron redox
flow battery (Hawthorne et al. 2014a, b). K. Gong et al.
introduced iron–triethanolamine and iron–cyanide combinations in an all-iron redox flow battery. They demonstrated
a formal cell voltage output of 1.34 V (Gong et al. 2016).
The second method for keeping the H
­ 2 evolution in check
is to use suitable anionic species of supporting electrolyte
in optimum concentration. Plating efficiencies of 97% were
reported by using the electrolytes consisting of chloride anions instead of sulfate ions (Hawthorne et al. 2015). Tucker
­ 2/H+ couples and recorded
et al. examined F
­ e2+/Fe3+ and H
maximum power density values of 148, 207 and 234 mW
13
­cm−2 for iron sulfate, iron chloride and iron nitrate electrolytes, respectively (Tucker et al. 2013). They also showed
that an all-iron redox flow battery with 0.5 M ­Fe2(SO4)3
and 1.2 M NaCl as active material and supporting electrolyte, respectively, produced average power density of about
20 mW cm−2 and a maximum energy density of 11.5 Wh L−1
(Tucker et al. 2015).
In a separate study, suitability of novel electrode structures for use in iron hybrid flow battery was evaluated by
shallow charge–discharge cycles. The incorporation of bilayered electrode structure, namely the carbon felt containing
non-conducting felt configuration, retained voltaic efficiency
of 81% after six cycles of operation (Hawthorne et al. 2014a,
b).
The technical comparison of the different types of energy
storage devices is given in Table 2.
Components of all‑iron redox flow battery
The typical model of all-iron redox flow battery is depicted
in Fig. 2. It contains two tanks of negative and positive electrolyte. The negative electrolyte is mainly the ferrous chloride, and the positive one is the mixture of ferrous and ferric
chloride. Two pumps are attached to each tank in order to
generate a pressure for the flow of electrolyte through the
cell. The pressure of the pumps will be kept at the stoichiometric rate of reaction as negative effect of pump reduces
the battery efficiency.
Cell will be enclosed by bipolar plates, and it plays a role
as a structural supporter as well as a conductor. In stacked
cells, bipolar plates serve as anode for one cell and the cathode for the other. These plates also help in the transfer of
waste heat out of the cell which may lead to other chemical
changes in the cell.
Usually, graphite electrodes are used as electrodes in
many of the all-iron redox flow batteries. However, iron metals are also utilized in rare cases. Graphite electrodes do not
involve in the electrochemical reactions, but offer a surface
for it by providing a pathway for the electrons to enter and
leave electrolyte. In some cases the activated graphite felts
will be used to increase the surface area of the electrode. On
the positive side, the felts cause increases in reaction rate
at which ferrous ion can lose an electron (during charging)
and ferric ion can gain an electron (during discharging). On
the negative side, the rate of the reaction increases through
gaining two electrons by ferrous ion or losing two electrons
by metallic iron. Also, the increase in flow resistance in the
cell due to the felt increases the pressure within the cell and
hence the parasitic pump load (Mellentine 2011).
Between the two sides of the cell, there will be a membrane separator which may be macro- or microporous. In
the case of microporous separator, it will allow anions
Environmental Chemistry Letters
Table 2 Comparison of electrochemical energy storage devices. Reprinted with permission of Soloveichik (2015)
Technology
Typical
power
(MW)
Discharge time Storage capac- Lifetime (cycles/yr) Efficiency (%) Drawbacks
ity cost ($/
kWh)
Supercapacitors
0.25
< 1 min
500–3000
500,000/20
> 90
Regenerative fuel cells with
hydrogen storage
Lead-acid batteries
10a
>5h
–
13
40–50
0.5–20
3–5 h
65–120
1000–1200/
3–4
70–80
1–5 h
400–600
750–3000/
6–8
80–90
87
75
Lithium-ion batteries
Sodium sulfur battery
0.25–1
6–8 h
360–500
Flow battery (vanadium
redox battery)
All-iron redox flow battery
0.5–12
10 h
150–2500
2500–4500/
6–12
500–2000/10
1–6
–
250–400
730/15
(chloride in this case) to move through the membrane during charging and discharging processes in order to maintain
the electroneutrality, whereas in nanoporous Nafion (commonly used proton exchange membrane) separator it allows
positive charge to move through the membrane to attain
electroneutrality.
The flow frame provides a structure through which the
fluid electrolyte can flow in and out of the cell.
The design of all-iron redox flow battery plays a pivotal
role in deciding the total amount of energy that can be stored
in the system. The components of all-iron redox flow battery
and electrolyte solutions in the external storage tanks greatly
influence the performance and the costs of all-iron redox
flow battery. The ratio of anolyte to catholyte solutions is a
function of state of charge which represents residual energy
in each battery. The cost of all-iron redox flow battery is
directly proportional to the cost of metal salts used as the
electrolyte. Chemicals such as ammonium chloride ­(NH4Cl)
and boric acid (­ H3BO3) may be added to the electrolyte so as
to bring down the resistivity of the electrolyte and to inhibit
the hydrogen evolution, respectively. Pumps help in the
movement of electrolytes in and out of the cell stack. Bipolar plates provide structural support and act as a conductor.
In the stacked configuration, they work as positive electrode
for one cell and negative for adjacent cell. The electrodes
present next to the bipolar plates allow the electrochemical
reactions to occur on surface. The total energy output of alliron redox flow battery will depend on the amount of metal
deposited on the electrode surface. They enable the electrons
to move in or out of the electrolyte. The electrode used in
all-iron battery is usually graphite based. Other important
70
Explosion hazard, low energy
density, high cost
Low-density storage, high
cost, safety
Low energy density, short
lifetime, temperature sensitive
High cost, safety, short
lifetime, self-discharge,
temperature sensitive
High cost, high-temperature
operation, safety
Low energy density
Hydrogen evolution, low
cell voltage and current
efficiency—can overcome
these by suitable additives
parts of all-iron redox flow battery are membrane separator;
usually, proton exchange membranes are used to maintain
the charge balance by exchanging proton through it. Flow
frame and control system function as a boundary between
the electrolytes so that they do not mix, imparting a structure
for movement of electrolyte in and out of the stack and providing a computerized controlling of flow rates and charge as
well as discharge currents, respectively. Typically, the membrane separator employed is an ion exchange membrane such
as Nafion membrane (Mellentine 2011). The computer-aided
designs of assembled cell and components of all-iron redox
flow battery system are presented in Fig. 3a, b, respectively.
Electrolyte
The large percentage of the total cost of redox flow batteries
depends on the electrolytes. Generally, the ionized salts of
the metal in acidic condition have been used as electrolyte.
Large external tanks have been used to store the electrolyte
and are pumped through each side of the cell according to
the applied current. Hence, the amount of energy that can
be stored by a flow battery will determined by the extent
of solubility of the chemicals and also the size of the tank.
It was discussed that the coulombic efficiency of the battery depends on various factors, viz concentration and pH
of the electrolyte. It was clear from the literature that the
specific energy of the cell depends on the concentration of
the electrolyte. Hrushka and Savinell stated that low resistivity can be attained by maintaining the concentration of the
electrolyte in the range of 0.8–3.9 M and also the change
in pH will be moderate at that concentration. The same
13
Environmental Chemistry Letters
(a)
Nafion 117 membrane
Flow Field plate
Carbon Felt
Teflon Gasket
Silicon Gasket
End Plate
Teflon Gasket
Copper Plate
(b)
Fig. 3 Computer-aided designs of assembled cell (a) and components (b) of all-iron redox flow battery. Horizontal and vertical views of assembled redox flow battery cell are shown in a, and computer-aided designs of individual components are shown in b
referenced study also showed that the addition of ammonium
chloride to ­FeCl2 reduces the resistivity effectively and helps
to increase the plating characteristics. The addition of boric
acid will lower the energy efficiency and affects the pH by
hindering the evolution of hydrogen.
Further studies reported that utilization of ­NH4Cl with
chloride salts of iron reduces the voltaic loss from electrolyte
to around 4%. And also the deposition of iron was increased
with a decrease in poor adherence, powdery deposition of
which results in large voltaic loss at plating electrolyte during charging and discharging. The schematic reaction of
all-iron redox flow battery with ­NH4Cl as additive is represented in Fig. 4.
In order to avoid coulombic loses in the negative electrolyte, the pH has to be maintained from 2 to 3 where the
mitigation of hydrogen occurs, but that will cause precipitation of ferric ion in the positive electrolyte. The latter
can be avoided by the employment of complexing ligands
such as triethanolamine, ethylenediaminetetraacetic acid,
13
e-
eFe3+
Fe (s)
Cl
Fe 2+
NH4Cl
-
Fe 2+
NH4Cl
Fig. 4 Schematic reaction of all-iron redox flow battery with ­NH4Cl
as additive (modified after Hawthorne 2014). Diagrammatic representation explains the redox reaction, plating/stripping reaction and
ion exchange process employed in an all-iron redox flow battery with
ammonium chloride as an additive
Environmental Chemistry Letters
diethylenetriaminepentaacetic acid, nitrilotriacetic acid
which form Fe–ligand complex and shift reaction potential
in negative direction.
Hawthorne (2014) explained that additives will increase
the size of the ion in the electrolyte compared to iron hydration shield in the absence of ligands. This in turns decreases
the diffusion coefficient and increases the mass transfer over
potentials in the iron redox flow battery. The study revealed
that the decrease in the diffusion coefficient is due to the formation of iron–ligand complex and also due to the increase
in the viscosity of the electrolyte. It was stated that addition
of glycerol as supportive electrolyte will decrease the diffusion coefficient by 10% than the diffusion coefficient of
iron only. Among all the additives being used to reduce the
precipitation of ferric ions, glycine is the best, whereas the
others will decrease the diffusion coefficient of ferric and
ferrous ions. And also it increases the ferric ion solubility at
pH greater than 2.5.
It was documented in the literature that since glycine has
two states, positive and negative, and also the intermediate
zwitterion it will coordinate with the iron more elegantly
when compared to the other, so as the diffusion coefficient
increases. The coordination reaction of glycine with ferrous
and ferric ion is represented as shown in Eq. 4–10 (Hawthorne et al. 2014a, b).
NH2 CH2 COOH ↔ NH2 CH2 COO− + H+
(4)
NH2 CH2 COOH ↔ H3 N+ CH2 COOH
(5)
NH2 CH2 COO− ↔ H3 N+ CH2 COO− ↔ H3 N+ CH2 COOH
(6)
[ (
) ]2+
[ (
)
]+
Fe H2 O 6
↔ Fe H2 O 5 (OH) + H+
(7)
[ (
) ]3+
[ (
)
]2+
Fe H2 O 6
↔ Fe H2 O 5 (OH) + H+
(8)
)
[ (
)
]+
[ (
]2+
Fe H2 O 5 (OH)
↔ Fe H2 O 5 (OH)2 + H+
(9)
[ (
) ]3+
[ (
)
(
) ]4+
2 Fe H2 O 6
↔ Fe H2 O 4 (OH)2 Fe H2 O 4
+ 2H+
(10)
Reactions 4, 5 and 6 represent the protonation and
deprotonation of glycine into its negative and positive
states, respectively. Equation 6 represents the three equilibrium states of glycine (negative, zwitterion, and positive).
Table 3 represents the coordination constants for glycine
and protons.
It has been reported in the literature that the ­pKa value for
Fe(II) is 6.93 (Eq. 7) (Bolzan and Arvia 1963). The p­ Ka for
Fe(III) losing the first proton is 2.74 (Eq. 8) and for the second proton is 3.31(Eq. 9) (Perrin 1959). And also in addition
Table 3 Coordination constants for glycine and protons in the positively and negatively charged states. Reprinted with permission of
Hawthorne et al. (2014a, b)
Ion
Equilibrium
Log K
H+
H+
Fe2+
Fe2+
Fe3+
[HGly]/[H+][Gly]
[H2Gly]/[HGly][H+]
[FeGly]/[Fe2 +][Gly]
[FeGly2]/[Fe2+][Gly]2
[FeGly]/[Fe3 +][Gly]
9.54
2.36
3.83
7.65
10
to that there will be the chance that Fe(III) can also form a
dimer in aqueous solutions (Eq. 10). The p­ Ka of the Fe(III)
dimer is 2.91 (Knudsen et al. 1976).
It was also mentioned by Hawthorne (2014) that a model
for the pH as a function of iron and glycine concentration can
be derived using coordination constants and may be used for
the determination of ­pKa values of Fe(II) and Fe(III). And
the same model can be used to analyze the optimum concentration ratio of glycine to iron for flow battery operation.
The theoretical calculations of an ideal operating range
for an all-iron flow battery were reported to be between
0.5:1 and 1:1 glycine to total iron in the electrolyte, and an
electrolyte with a 1:1 ratio of glycine to total iron will be
stable at a pH of 2. The result suggested that the ratio should
not be less than 0.5:1 glycine to total iron. The electrolyte
ratio in between 0.5:1 and 1.85:1 glycine to total iron has
been reported for practical use in iron flow battery. With an
open-circuit potential of 468 mV versus Ag/AgCl and the
electrolyte pH of 2, a 1:1 glycine-to-iron ratio of electrolyte
is promising for use in an all-iron flow battery.
According to the literature, additives result in the conjugation of iron deposition reaction and also they hinder the
hydrogen evolution rate. Also high concentration of chloride
ions hinders the hydrogen evolution in negative electrode.
Membrane
Membranes have been used as separators in redox flow batteries. In order to get effective results the ideal membrane
has to possess following characteristics: Chemical stability
under acidic condition must be high; high resistivity has to
be shown for oxidizing environment of the positive half-cell
electrolyte; low electrical resistance; low permeability to
the iron or polyhalide ions; high permeability to the chargecarrying hydrogen ions; low cost; and good mechanical
strength. Most importantly, it must have the capacity of preventing the preferential transfer of water from one half-cell
to the other which results in flooding of one half-cell while
diluting the other (Sum et al. 1985).
From the literature it has been clear that developing a
cost-effective suitable membrane for redox flow battery is
13
Environmental Chemistry Letters
difficult because of the contamination of the two electrolytes when passed across the membrane. Hagedorn N. H
reported that this problem can be overcome by the use of
premixed reactants in two half-cell solutions. Researchers
have reported the fouling of anion-selective membranes
due to the complex formation of iron with halides or any
other ligands (Perez et al. 1991). Nafion 117 has been
tested best among the cationic membranes like Nafion
117 and NEOSEPTA CR-2, and microporous separator
Daramic.
Table 4 Diffusion coefficients for ­Fe2+ and ­Fe3+ iron–ligand complexes. Reprinted with permission of Hawthorne et al. (2014a, b)
Ligand
D ­Fe2+ ­(cm2 s−1)
D ­Fe3+ ­(cm2 s−1)
None
Citrate
DMSO
Glycerol
Glycine
Malic acid
Malonic acid
Xylitol
5.7 × 10−6
1.7 × 10−6
4.4 × 10−6
4.0 × 10−6
4.0 × 10−6
3.6 × 10−6
4.3 × 10−6
2.8 × 10−6
4.8 × 10−6
1.4 × 10−6
3.8 × 10−6
4.0 × 10−6
2.3 × 10−6
2.7 × 10−6
3.1 × 10−6
2.4 × 10−6
Diffusion coefficients and diffusivities
The influence of rate of diffusion of iron species on energy
storage capacity of an all-iron redox flow battery was investigated by using commercial-grade Nafion 117 and Daramic
250 membranes. The concentration gradient of membrane is
a function of rate of diffusion that is expressed by diffusion
coefficient ‘D,’ as well as equilibrium between membrane
and electrolyte which is effectively explained by using partition coefficient K. This can result in changes in local concentration in membrane which may be different compared to
bulk electrolyte. As there is no chemical interaction between
­Fe2+ and ­Fe3+ ions inside iron system, only the term effective diffusivity is used for describing the crossover of the
species across either side of the membrane. These values
become useful in designing concentration profiles in continuous cycling operations. The equation for is shown in
Eq. 11.
D = KD.
(11)
The result of the studies showed to be equal to 9.9 × 10−7
and 12.3 × 10−7 cm2 s−1, respectively, for Daramic 250,
while Nafion membrane showed values equal to 4.3 × 10−7
and 3.5 × 10−7 cm2 s−1, respectively (Petek 2015).
In another study, the diffusion coefficients which vary
inversely with mass transfer over potentials of all-iron redox
flow batteries were determined from limiting current data for
­Fe2+ and ­Fe3+ in the presence and absence of ligands. A total
of seven ligands were utilized, and the results were compiled
as shown in Table 4 for original and adjusted pH values.
The iron–ligand complex structures are shown in Fig. 5.
D value for F
­ e2+ was found to be 4.8 ± 0.2 × 10−6 cm2 s−1
when ligands were not used. This value is in concordance with that reported in the literature which is between
3 × 10−6 and 5.5 × 10−6 cm2 s−1. ­Fe3+ exhibited D value of
5.7 ± 0.2 × 10−6 cm2 s−1 which was lower than that in the
literature (1.1 × 10−6 cm2 s−1). However, that is justified for
the fact that F
­ eSO4 was used instead of F
­ eCl2 in the literature. As shown in Table 4, all of the ligands employed were
useful in bringing down the D values for both ­Fe2+ and ­Fe3+
systems (Hawthorne 2014; Hawthorne et al. 2014a, b).
13
Electrode
In all-iron redox flow batteries, the iron-based materials have
been made use of, where metal deposition takes place from
the solution of metal ions at both negative electrode and
positive electrode. As per the literature, redox couples which
have been soluble in both oxidized and reduced forms are
made use of as electrodes. Generally, ferric/ferrous redox
couple has been used as positive electrode and materials
plated from Fe(II) has been used as negative electrode
(Thaller 1974, 1981). Also it has been known from the literature that graphite can also be used as electrode which had
given comparatively good results like that of the traditional
one. The most important problem that has been discussed
so far is the precipitation of Fe(II) as ferric hydroxide when
the pH of the electrolyte increases more than the permissive
level due to high evolution of hydrogen.
Improving the capacity of all‑iron redox flow batteries
using slurry electrodes
The use of slurry electrodes is proposed as one of the best
means to enhancing the efficiency of all-iron redox flow batteries. Slurries are usually dispersed conductive particles in
the electrolytic solution. They serve the purpose of decoupling the energy capacity and power density so as to allow
the operation of all-iron redox flow batteries at large current
densities. This takes place when the iron gets plated on the
negative slurry electrode, resulting in their movement into
the external reservoir. Slurry electrodes are beneficial compared to their conventional counterparts in terms of freedom
to scale the surface area without taking into account the
separator area, ease of fabrication and replacement while
keeping the cell configuration intact, and easy recovery by
simple filtration (Petek 2015). A schematic of an electrochemical channel cell for the flow of slurry electrodes is
presented in Fig. 6. There are few reports already available
in the literature for the use of slurry electrodes in other electrochemical systems (Appleby and Jacquier 1976; Duduta
Environmental Chemistry Letters
OH
O
-
O
O
S
O
O
Fe
O
O
O
Cl
O
O
S
Fe
O
O
O
S
Cl
O
H2O
OH2
NH3
O
O-
Fe
S
H2O
O
NH3
OH2
O
O
O
HO
(a)
Fe
Fe
O
O
(b)
(c)
O
O
O
O
O
O
Fe
O
O
Fe
O
Fe
O
O
O
O
O
O
O
OH2
O
(d)
O
(e)
Fig. 5 Structures of iron–ligand complexes (modified after Hawthorne et al. 2014a, b). Complex of iron with a citrate, b DMSO, c
glycine, d malic acid and e malonic acid is represented. Supporting
electrolytes increase the diffusion coefficient of all-iron redox flow
batteries and help to increase the voltaic efficiency
et al. 2011; Zhao et al. 2014; Wu et al. 2015). The development of cost-effective slurry systems is of great scope in
all-iron redox flow battery research. Existing experimental
studies have revealed that multi-walled carbon nanotube
(MWCNT) slurries enhanced the voltaic efficiency with the
rise in state of charge of all-iron redox flow batteries (Petek
et al. 2015). Similar investigations concluded that a slurry
electrode containing 5.8% MWCNT flowing at 200 mL/min
through 1-mm channel gap yielded voltaic efficiency of 82%
at 200 mA/cm2 (Petek et al. 2016). The shunting and pumping losses were observed to be less than 5% of total capacity
when slurry electrodes were tested for a 5 kW all-iron redox
flow battery stack.
Drawing inspiration from the preliminary research done
in CWRU which modeled 5 kW all-iron redox flow battery
system, Energy Storage Systems Company has successfully
manufactured and commercialized all-iron redox flow batteries for large-scale applications. It has claimed that their
batteries can last for more than 10,000 cycles over a life
of about 25 years. ESS has patented various cost-effective
all-iron redox flow batteries (Zito 1973; Evans and Song
2013, 2016). Electric F
­ uel® has planned for the large-scale
demonstration of 100 kW all-iron redox flow battery in 2017
followed by its commercialization in 2020.
All‑iron redox flow batteries as low‑cost
alternatives
The primary reason for the viability of all-iron redox flow
batteries is their cost-effective nature (Hruska and Savinell
1981). The electrolyte whose cost decides its potential
application is cheaper when compared to expensive vanadium redox flow batteries. Utilization of graphite as an
electrode also impacts positively on cost-effective nature
of all-iron redox flow battery (Hruska and Savinell 1981).
Iron salts that are used for the preparation of electrolytes
are earth abundant with the cost lesser than $20/kWh
which is cheaper than the chemicals employed in most
other battery technologies. Estimated cost calculation of
13
Environmental Chemistry Letters
Redox flow battery modeling
The aim of flow battery modeling is basically to improve
fundamental understanding and to facilitate high-performance, low-cost designs of flow batteries through the
development of mathematical models implemented for
statistical simulation of electrochemically reactive flow.
Models offer a virtual laboratory for better design and
optimization capabilities, leading to
• Improvements in battery performance and safety
• Economic development cost
• New design developments using novel materials and
various configurations.
Fig. 6 A schematic of an electrochemical channel cell for the flow of
slurry electrodes (modified after (Petek et al. 2015). When the electrolyte flows through the negative slurry electrode which contains the
dispersed conductive particles, decoupling of energy capacity and
power density occurs so that the voltaic efficiency is enhanced
Table 5 Estimated cost data of all-iron redox flow battery components based on the size and performance characterization. Reprinted
with permission of Mellentine (2011)
Stack components
Estimated price
Activated felt
Bipolar plates
Flow frames
Gasket
Collector plate
End plate
Bolt
PVC set
Ferrous chloride solution (27%)
Ammonium chloride
Boric acid
Deionized water
Electrolyte preparation
Tanks
Pumps
Control system
$90/m2
$50/plate
$32/frame
$2/gasket
$125/plate
$175/plate
$12.50/bolt
$150/set
$0.129/L
$1.06/L
$1.96/L
$0.016/L
$100.00/batch
$276.97/pair
$200.00 each
$600.00 each
cell components of all-iron redox flow batteries based on
the size and performance characterization is represented
in Table 5.
Cost of battery including electrolyte, additives, membrane, pump, deionized water, electricity depends on the
size and compatibility of the battery design which is even
cheaper than the vanadium redox flow batteries.
13
Most of the models existing in the literature for flow
batteries include the basic models of transports of mass,
electrochemical kinetics, heat and charge, as well as the
momentum (Xu and Zhao 2015). It is not viable, on the
other hand, to integrate this level of detail in modeling
of redox flow battery stacks. Therefore, there is a necessity to develop control-oriented models that can rapidly
and precisely capture the performance of redox flow battery systems. These models focus on the cell performance
and system efficiency, rather than concentrating on the
detailed variables distribution. Such models are called
unit or stack-level network models. The network models
are usually simplified with several assumptions, including
cell/stack is uniform, tank is fully filled with electrolyte,
cell/stack resistance is constant over operating range. A
few such stack-level network models are available in the
literature for different flow batteries (Shah et al. 2011;
Tang et al. 2012a, b; Bromberger et al. 2014).
The gap in fundamental understanding of the transport
and electrochemical processes for flow batteries can be
bridged with the development of modeling technology.
The amalgamation of results from modeling and experimental finding can be used in addition to optimize the
specific design performance of battery. This will also help
to fine-tune the operating conditions for improved cell performance, leading ultimately to a reduction in cost. These
research directions can help to considerably speed up the
worldwide exploitation of flow battery technologies (Tang
et al. 2012b).
Future outlooks
The exploration of newer ligands is necessary for enhancing ­Fe2+ solubility without interfering with iron plating
at negative electrode of all-iron redox flow battery. Even
Environmental Chemistry Letters
though the ligands investigated so far are favorable in the
positive electrolyte reactions, they have faced problems by
hindering negative electrode reactions (Hawthorne et al.
2015).
The development of slurry negative electrodes is essential for all-iron redox flow batteries to reach their full
potential in grid-level energy storage market. The slurry
electrodes face challenges in terms of ohmic resistance
which is proportional to slurry electronic conductivity.
Even though efforts have been made to decrease ohmic
resistance of the electrode, such attempts gave rise to
inadvertent pressure drops leading to parasitic pumping
losses. Thus, the efficient slurry electrodes with enhanced
electronic conductivities must be developed in the future.
Efforts must be made to evaluate the effects of shape
and size of particles, surface treatments of particles and
electrolyte compositions on electronic conductivities of
slurry electrodes. Their dynamic and electrochemical performances need to be studied. The impact of the slurry
motion on mass transfer and conductivity must be understood. These factors would be helpful for understanding
and engineering slurry electrodes (Petek 2015).
Recently, utilization of 3D printing technology has
promised novel engineering approaches of implementation of flexible design methodology for fabrication of
components of redox flow batteries that is desirable for
the fulfillment of demands of cost-effective, long lifetime
of hardware for effective performance (Arenas et al. 2015;
Chang-Yong 2017; Marschewski et al. 2017). The 3D
printing has been used to make flow frames and end plates
and successfully tested in laboratory level. The study of
its use in manufacturing newer electrode materials and
configurations and integrated as well as miniaturized components can be undertaken. Optimization of flow and mass
transports in thus-produced small and large cells of redox
flow batteries must be carried out.
Conclusions
The all-iron batteries have been known to possess the
potential to transform area of energy storage by storing
energy cheaply for longer duration. In this review, the progress of research in this area using all-iron redox batteries
has been explored by providing the details of fundamentals
as well as components. They have been proposed as effective technologies for the future energy storage.
Acknowledgements We gratefully thank the Department of Science
and Technology (DST), India, for financial support under MES scheme,
DST/TMD/MES/2K16/83.
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