Journal of The Electrochemical Society, 153 共5兲 A929-A934 共2006兲
A929
0013-4651/2006/153共5兲/A929/6/$20.00 © The Electrochemical Society
Studies on Iron „Fe3+ /Fe2+…-Complex/Bromine „Br2 /Br−… Redox
Flow Cell in Sodium Acetate Solution
Y. H. Wen,a,b H. M. Zhang,a,z P. Qian,a H. T. Zhou,a P. Zhao,a B. L. Yi,a and
Y. S. Yangb
a
Full Cell R&D Center, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian,
Liaoning 116023, China
b
Research Institute of Chemical Defense, Beijing 100083, China
The formal potential of the Fe共III兲/Fe共II兲 couple shifts markedly in the negative direction by complexation with ethylenediamine
tetraacetate 共EDTA兲, oxalate, and citrate. The potentials of the complexes with EDTA and oxalate are less pH-dependent than with
citrate. But, the relatively high pH of around 6.0 is favorable electrochemically due to high corresponding currents. Complexation
of Fe共III兲/Fe共II兲 couple can provide fast electrode kinetics except for the complex with citrate. But, the solubility of the complex
with citrate is up to 0.8 M. Charge–discharge measurements were conducted with the iron-complex/Br2 redox cells. The results
show that performance of the cells with 0.1 M Fe共III兲/Fe共II兲-oxalate or Fe共III兲/Fe共II兲-citrate is relatively poor due to slow
kinetics for the Fe共III兲/Fe共II兲-citrate and the unstability of the ferric form for the Fe共III兲/Fe共II兲-oxalate, whereas performance of
the iron-citrate/Br2 cell is improved considerably by increasing concentration of the Fe共III兲-citrate complex. Also, energy efficiencies of up to approximately 80 and 70% could be obtained for the cell with 0.1 M Fe共III兲/Fe共II兲-EDTA and 0.8 M
Fe共III兲/Fe共II兲-citrate, respectively. The preliminary study shows that novel Br2/iron-complex cells are technically feasible in redox
flow batteries but need further investigation.
© 2006 The Electrochemical Society. 关DOI: 10.1149/1.2186040兴 All rights reserved.
Manuscript submitted August 5, 2005; revised manuscript received January 17, 2006. Available electronically March 31, 2006.
Redox flow batteries 共RFB兲 can be considered an electrochemical system intermediate classical secondary batteries and fuel
cells.1-3 Unlike conventional batteries, a redox flow battery stores
energy in liquid electrolytes 共in external tanks兲 containing different
redox couples that are pumped through the cell.4 Therefore, as the
active species, soluble redox couples have determining effects on
the performance of an RFB.
In the past few years, a number of redox couples have been
proposed and fabricated. Among the redox systems developed to
date, only a polysulfide/bromine system5,6 and an all-vanadium system have been successfully developed, showing the potential of
commercialization.1-4,7-11 However, a long-term existing problem in
the storage cells has not been solved perfectly: the possible intermixing of the components of two half-cells through an ion exchange
membrane. In addition, the long-term stability of Na2Sx and V共V兲
solutions in high vanadium concentrations and elevated temperatures is limited in application due to the formation of insoluble precipitates.
An alternative approach to minimize this intermixing problem
and to improve the stability of redox solutions is the use of metal ion
coordination compounds as redox couples in flow cells. Bard and
co-workers12,13 studied the possibility of Fe共III兲/Fe共II兲 and
Co共III兲/Co共II兲 couples complexed with o-phenanthroline and bipyridine ligands as positive half-cells. Murthy et al.14 further examined
the Fe共III兲/Fe共II兲 couple and shifted the redox potential to more
negative values by complexation with diethylenetriamine pentaacetic acid, nitrilotriacetic acid, and ethylenediaminetetraacetic acid
共EDTA兲 in 0.5 M K2SO4 medium, making a negative half-cell possible. However, these studies are only related to electrochemical
behaviors of the half-cells and the solubility of active materials is
rather low. The redox flow battery based on Fe共III兲/Fe共II兲 complexes has not been developed. In addition, the use of expensive
chelating agents such as o-phenanthroline and diethylenetriamine
pentaacetic acid also limited the application of the systems in some
ways.
We have developed a new electrolyte system, the iron-complex/
bromine redox system. In this system, the ligands with low cost are
employed to be capable of shifting the formal potential of
Fe共III兲/Fe共II兲 in the negative direction. The Fe共III兲/Fe共II兲-EDTA
complex couple was also tested under the same conditions for com-
z
E-mail: [email protected]
parison. Sodium acetate trihydrate 共BDH兲 is used as a supporting
electrolyte as well as a buffer solution. Then, the
Fe共III兲/Fe共II兲-complex redox couples can be used in conjunction
with the Br2 /Br− couple for a new redox flow cell combination. It is
preferable to use a sodium-based couple to allow the transport of
sodium to complete the circuit and minimize the crosscontamination problem during operation of the cell.
A new iron-complex/bromine redox flow cell is thus proposed
and employs the Fe共III兲/Fe共II兲-complex in the negative half-cell
electrolyte and the Br− /Br2 couple in the positive half-cell. Iron and
bromine species are chosen for this investigation because both of
them are abundant and inexpensive elements. Cyclic voltammetry is
used to study the electrochemical behaviors of the
Fe共III兲/Fe共II兲-complex couples, and the charge-discharge performance of a small iron-complex/bromine test cell is reported.
Experimental
Reagents sodium oxalate, sodium citrate·2H2O, disodium EDTA,
sodium acetate trihydrate, ferric sulfate, and sodium bromide were
all analytical grade. All solutions were prepared with distilled water.
Apparatus and procedure.— The supporting electrolyte was 1 M
aqueous sodium acetate 共NaAC兲. It can also be used as a buffer
solution to maintain the pH of the solution around 6.0. When necessary, 4 M NaOH or H2SO4 solution was added to the electrolyte to
adjust the pH to the desired value. The pH of the solution was
measured with a calibrated pH meter 共Shanghai instruments pHS25, China兲. Solutions were deaerated 15 min by bubbling with nitrogen before each measurement. The complexes were prepared by
mixing known concentrations of ferric sulfate and the ligand. In the
case of EDTA and citrate, a mole ratio of ligand/Fe共III兲 of slightly
more than 1 was used. In the case of oxalate, a mole ratio of 3 for
ligand/Fe共III兲 was used. The conductivity of various solutions was
measured by the DDS-12D electrical conductive meter 共Shanghai
LIDA Instrument Factory, China兲. The curves of current vs potential
were recorded in a three-compartment cell with a graphite rod 共area
0.103 cm2兲 as inert working electrodes and a large area graphite
sheet electrode as an auxiliary electrode. All potentials were expressed relative to an aqueous saturated calomel electrode 共SCE兲,
which was connected with the electrochemical cell through a salt
bridge full of saturated potassium chloride solution. The cyclic voltammogram was measured by the CHI660 electrochemical station
共CH Corporation, USA兲.
Journal of The Electrochemical Society, 153 共5兲 A929-A934 共2006兲
A930
Table I. Redox couples used for the constant current charge–
discharge tests.
Cell 1
Cell 2
Cell 3
Anolyte
Catholyte
1 M NaBr
1 M NaBr
1 M NaBr
0.1 M Fe共III兲-EDTA +1 M sodium acetate
0.1 M Fe共III兲-oxalate +1 M sodium acetate
0.1 M Fe共III兲-citrate +1 M sodium acetate
Small-redox-flow test cells were used for the constant current
charge–discharge tests with graphite felt electrodes, graphite plates
as the current collectors, and a series of different electrolytes. The
electrode and membrane areas were 5 and 6 cm2, respectively. The
Nafion 117 cation-exchange membrane in the H+ form was soaked
in 10% NaOH solution for 1 h at 80°C and then converted into the
Na+-form membrane. The two half-cell electrolytes were separated
by a sheet of Nafion 117 membrane in the Na+ form. Two Xishan
pumps 共China兲 were used to pump each half-cell electrolyte through
the corresponding half-cell cavity where the charge/discharge reactions occurred. Each cell compartment was filled with 50 mL of
these electrolytes. Prior to charging the cell, the negative electrolytes
were purged with nitrogen gas at 0.2 L min−1 for 30 min. The galvanostatic charge and discharge of the test cell were carried out
using a CT2001A multichannel generator 共Land instruments,
China兲. The applied current density for the charge and discharge was
10 mA cm−2. A range of combinations of redox couples was used in
the performance tests of the redox flow cells. These are shown in
Table I.
The permeation of Br2 across the membrane was determined. A
redox-flow-model cell was used for the measurement of the permeation of Br2. Br2 exists in the form of Br−3 in the concentrated NaBr
solution. The Br−3 solution was obtained by charging the cell employing 2.0 M NaBr as the positive electrolyte and 0.8 M
Fe共III兲/Fe共II兲-citrate complex as the negative electrolyte to the state
of 50% state of charge 共SOC兲 at a current density of 10 mA cm−2.
Then one reservoir was filled with the above Br−3 solution and the
other one with a solution of 0.1 M EDTA. Both solutions were
50 mL and circulated through the cell compartments, which were
separated by the Nafion 117 exchange membrane with effective area
of 5 cm2 at ambient temperature. Samples of 2 mL were taken at
regular intervals from the EDTA aqueous solution reservoir and analyzed to determine the Br− ion content by a method described in
detail in the literature,15 because the penetrated Br2 was reduced by
EDTA to Br− ions. This method is as follows: the mixing solution of
sodium acetate and acetate acid was used as a buffer to maintain the
pH of the solution in the range 4.5–4.7. Standard solutions containing 1–20 ␮g Br− were respectively added into a 10 mL flask, then
Figure 2. A standard curve determining the content of Br− by the UV-vis
absorption spectra.
0.4 mL of the buffer solution and 0.1 mL of 0.6 mM phenol red
were added. After shaking fully, 0.2 mL of 1.8 mM chloramine T
was added into the flask and maintained for 5 min. Finally, 0.1 mL
of 2 M sodium thiosulfate solution was added to reduce the excessive chloramines T. Then Br− was oxidized by the chloramine T to
Br2, reacting with the phenol red to the phenol blue. The blue solution can be analyzed by ultraviolet-visible 共UV-vis兲 absorption spectra obtained with a double-beam spectrometer 共Unico, UV-4802兲
referenced to Br2-free aqueous solutions, as shown in Fig. 1. A
standard curve was also obtained to determine the Br− ion content,
as shown in Fig. 2.
Results and Discussion
According to stable constants, it is qualitatively known that a
number of ligands can form highly stable complexes with Fe共III兲
and cause negative shift in the potential of the Fe共III兲/Fe共II兲 couple.
However, only a small quantity of chelating agents such as oxalate,
citrate, and EDTA can form Fe共III兲/Fe共II兲-complexes whose electrode reactions are reversible to some extent.
Solubilities.— Table II is a list of the solubilities of the complexes in aqueous 1 M sodium acetate. Uncomplexed Fe共III兲-sulfate
salts are quite soluble and yield solutions with metal-ion concentrations close to 1 M. The solubility of the complexes varies with the
nature of the ligand. Sodium citrate is quite soluble in water media
up to 1.8 M. The solubility of the Fe共III兲-citrate complex is approximately the same as uncomplexed Fe共III兲. However, the complex
with citrate in concentrations above 0.5 M forms relatively viscous
and deeply colored solutions. The solubilities of sodium oxalate acid
and disodium EDTA in water media are limited, at 0.4 and 0.3 M,
Table II. Estimated solubilities of Fe„III…-Fe„II… couplesa and
ligands.b
Figure 1. A typical UV-vis absorption spectra of the Br2-phenol bule solution containing 20 ug Br−.
a
b
Substance
Solubility
共g/100 mL兲
Concentration
共mol/L兲
Fe2共SO4兲3
Fe共III兲-EDTA
Fe共III兲-citrate
Fe共III兲-共oxalate兲3
Disodium EDTA
Sodium citrate·2H2O
Sodium oxalate
32
6.9
22
3.2
15
69.3
4
0.8
0.2
0.8
0.1
0.4
1.8
0.3
Measured in 1 M sodium acetate.
Measured in distilled water. T = 20°C ± 2°C.
Journal of The Electrochemical Society, 153 共5兲 A929-A934 共2006兲
Figure 3. CVs recorded at different potential scan rates at a graphite electrode for 50 mM Fe共III兲-complexes with EDTA 共a兲, oxalate 共b兲, and citrate
共c兲 in 1 M sodium acetate. Scan rate: 共1兲 10, 共2兲 25, 共3兲 50, 共4兲 100, and 共5兲
200 mV/s.
respectively, leading to low concentration of complexes. The complexes in low concentrations are transparent in the visible region,
similar to the uncomplexed Fe共III兲 ions in aqueous solution.
Cyclic voltammetry tests for Fe共 III兲 /Fe共 II兲-complexes redox processes.— As shown in Fig. 3, typical cyclic voltammograms 共CVs兲
recorded at a graphite electrode at a low scan rate of 10 mV s−1 in
50 mM of various Fe共III兲-complexes dissolved in 1 M sodium acetate buffer solution show well-formed oxidation and coupled reduction peaks below 0 V vs SCE. The formal potentials of the
Fe共II兲/Fe共III兲 complexes E0, was estimated from the CVs by taking
the mean of the average of the anodic and cathodic peak potentials,
Epa and Epc, namely, E0 = 共Epa + Epc兲/2. The formal potentials of
the complexes with EDTA, oxalate, and citrate are calculated to be
−0.104, −0.190, and −0.213 V 共vs SCE兲, respectively.
To investigate the kinetics of the Fe共III兲-ligands/Fe共II兲-ligands
redox reaction qualitatively, the CVs recorded at a range of potential
scan rates 共␯兲 for 50 mM Fe共III兲-ligands in 1 M sodium acetate
solution are shown in Fig. 3. The voltammograms have the classical
A931
form for quasi-reversible 1e transfer reactions with peak separations,
⌬Ep more than 60 mV, which increases with potential scan
rates.16-19 However, the Fe共III兲-EDTA complex shows the lowest
value of ⌬Ep 共⬃70 mV兲 close to a reversible system, which becomes wider slightly with potential scan rates, indicating that the
Fe共III兲-EDTA complex redox reaction would be very fast. The
Fe共III兲-citrate complex shows much lower responding currents and
wider peak potential separation 共⬃180 mV兲 even at the low sweep
of 10 mV s−1 than the other two complexes. The higher the potential
scan rates, the more round its reduction peak becomes. This indicates that the electrode process of Fe共III兲/Fe共II兲-citrate may be
slow, possessing relatively large polarization resistance. In addition,
the electrode process of the Fe共III兲-oxalate complex is more like
that of the Fe共III兲-EDTA complex in terms of the responding currents and peak potential separation.
The same reaction of the complex, i.e., the Fe共III兲-oxalate, is
quasi-reversible, as deduced by the proportionality between the anodic peak current density 共ipa兲 and v1/2 seen in Fig. 4a. Similar plots
of the peak current vs the square root of the scan rate were obtained
for the complex with citrate, whereas the plot of the peak current for
the oxidation process of the Fe共III兲-EDTA complex vs the square
root of the scan rate shown in Fig. 4b is an almost linear response,
indicative of relatively fast electrode kinetics and a diffusioncontrolled reaction.
The effects of pH on the CV of various Fe共III兲 complexes are
demonstrated in Fig. 5. It can seen that with an increase in pH over
1–7, the general appearance of the CVs improves, with higher responding currents and smaller peak potentials separation, indicating
a greater reversibility and improved kinetic characteristics, as well
as stabilization of Fe共III兲 complexed species. Also, the formal potentials shift in the negative direction. A similar observation was
made by Murthy et al.14 and Kolthoff et al.19 The negative shift of
the Fe共III兲/Fe共II兲 formal potential can be explained in terms of the
formation of the more stable complexes. However, the pH cannot be
too high. For example, when the pH is up to 6.60 in Fig. 5a, the
potential at which oxygen evolution takes place shifts in the negative direction. In addition, for EDTA and oxalate, there is little
change in the formal potentials of Fe共III兲-complexes when the pH is
above 3. In the case of citrate, the formal potential of the complex
cannot remain unchanged until the pH reaches 5.11. Therefore, the
half-cell potentials of the complexes with EDTA or oxalate are less
pH-dependent. However, the preferable pH range may be in the
neighborhood of 6 for the Fe共III兲/Fe共II兲 complexes due to higher
responding currents.
Comparison of charge–discharge performance of the test cell
battery employing a range of redox couples.— Performance of a
RFB employing the Br2 /Br− couple as anolyte-active species and the
Fe共III兲/Fe共II兲-complexes with EDTA, oxalate, and citrate as
Figure 4. Plots of the peak current density
共i p兲 vs the square root of scan rate 共 v1/2兲
for the oxidation processes of the
Fe共III兲-complexes with oxalate 共a兲 and
EDTA 共b兲.
A932
Journal of The Electrochemical Society, 153 共5兲 A929-A934 共2006兲
Figure 5. Effect of pH on the CVs of the Fe共III兲 complexes with different
ligands: 共a兲 EDTA, 共b兲 oxalate, and 共c兲 citrate.
catholyte-active species was evaluated with constant-current
charge–discharge tests and open-circuit voltage measurements, respectively. During charging, Br− is oxidized to Br2, which goes into
solution as tribromide ions, Br−3 , which are available to reoxidize the
Fe共II兲-complex to Fe共III兲-complex during discharge. Free bromine
is not very soluble in water, but it is very soluble in bromide salt
solutions in which it forms polybromide ions without separation of a
second liquid phase. A small quantity of Br−3 ions may cross the
membrane due to a large concentration difference between the positive and negative electrolytes. At the present stage, the concern is
mainly to demonstrate that the iron-complex/Br2 redox cell is still
technically feasible. Figure 6 shows the permeability of the Nafion
117 cationic exchange membrane for Br2. It is confirmed that in the
presence of bromine, the three complexing agents may be oxidized
and the bromine is reduced to Br− due to no Br2 determined. However, though the permeation of Br−3 ions through the membrane increases with time, the permeability of Br2 in concentrated Br− solution is rather low, on the order of parts per million, for as long as
Figure 7. Charge–discharge curves for the iron-ligand/Br2 test cells employing different Fe共III兲/Fe共II兲-complexes as negative couples: 共a兲 cell 1, with
Fe共III兲/Fe共II兲-/EDTA redox couples, 共b兲 cell 2, with Fe共III兲/Fe共II兲-oxalate
couples, and 共c兲 cell 3, with Fe共III兲/Fe共II兲-citrate couples. The initial concentrations of the electrolytes before charging for the three cells are given in
Table I. The electrolyte volume in each compartment was 50 mL and the
charge–discharge current was 50 mA.
548 min due to the Donnan exclusion effect of the cation-exchange
membrane to Br−3 anions.20,21 In addition, employing the thick
Nafion 117 membrane is also beneficial to prevent the penetration of
ions of two half-cells.
In Fig. 7, three charge–discharge curves are shown for the RFB
test cell with different negative Fe共III兲/Fe共II兲-complexes:
共a兲 cell 1, with Fe共III兲/Fe共II兲-EDTA redox couples; 共b兲 cell 2, with
Fe共III兲/Fe共II兲-oxalate couples; and 共c兲 cell 3, with
Fe共III兲/Fe共II兲-citrate couples. Initial concentrations of the electrolytes for the three cells are given in Table I. It is shown that cells 2
and 3 can be charged to higher voltage due to relatively low formal
potentials of the Fe共III兲/Fe共II兲-complexes with oxalate and citrate.
However, in the case of the Fe共III兲/Fe共II兲-EDTA/Br2 system 共cell
1兲, the initial voltage charged is much lower than that of the other
systems, exhibiting rather low internal iR drop. For the
Fe共III兲/Fe共II兲-oxalate/Br2 system 共cell 2兲, the initial voltage charged
is lower than that of the Fe共III兲/Fe共II兲-citrate system, but a larger
internal iR drop can be observed, probably due to the limited solubility of the Fe共II兲-oxalate complex.
Data obtained from the charge–discharge test of the cells are
summarized in Table III, where various cell efficiencies are the average values over ten charge/discharge cycles. It is found that the
three cells have similar open-circuit voltages under the present conditions. As expected, due to faster electrode kinetics, cell 1 with
Fe共III兲/Fe共II兲-EDTA has much higher voltage and energy efficiencies than the other two cells, while the average voltage and energy
efficiencies for cell 3 are a bit higher than that for cell 2.
Table III. Open-circuit voltage and efficiencies of the Fe-ligand/
Br2 test cells employing a series of Fe complexes with different
ligands as negative couples.
Figure 6. Permeability of Nafion 117 cationic exchange membrane for Br2
in the Br−3 solution obtained by charging the cell employing 2.0 M NaBr as
the positive electrolyte and 0.8 M Fe共III兲/Fe共II兲-citrate complex as the negative one to the state of 50% SOC at the current density of 10 mA cm−2.
Cells
Open circuit
voltage 共V兲
Current
efficiency
共%兲
Voltage
efficiency
共%兲
Energy
efficiency
共%兲
Cell 1
Cell 2
Cell 3
1.04
1.17
1.07
92.1
92.3
92.0
86.2
65.5
63.8
79.4
60.4
58.6
Journal of The Electrochemical Society, 153 共5兲 A929-A934 共2006兲
Figure 8. Cell capacity vs cycle number for the iron-ligand/Br2 test cells
employing
Fe共III兲/Fe共II兲-complexes
as
negative
couples:
共a兲
Fe共III兲/Fe共II兲-EDTA,
共b兲
Fe共III兲/Fe共II兲-oxalate,
and
共c兲
Fe共III兲/Fe共II兲-citrate.
Variation of the cell discharging capacity with cycling number is
given in Fig. 8. The capacity of cells 1 and 3 almost keeps constant
with an increase in cycle number, indicating that the intermixing of
two half-cells is minimized, while the capacity of cell 2 continues to
decrease with rising cycle number, possibly due to the instability of
the ferrous form in the oxalate solution.
As shown above, the solubility of the Fe共III兲-citrate complex in
1 M sodium acetate can be up to 0.8 M. Hence, dependence of cell
efficiencies of the Fe共III兲/Fe共II兲-citrate/Br2 system on concentration
of the Fe共III兲-citrate complex should be discussed as seen in Fig. 9,
where 2.0 M NaBr is employed as the positive electrolyte when
concentration of the Fe共III兲-citrate is beyond 0.5 M. The current
efficiency decreases a bit with concentration of the Fe共III兲-citrate
complex due to an increase in the charge–discharge time. However,
a considerable increase in both voltage and energy efficiency is observed for concentration of the Fe共III兲-citrate increased to 0.3 M,
followed by an almost unchanged trend after concentration of the
Fe共III兲-citrate continues to be raised to 0.7 M. When concentration
of the Fe共III兲-citrate is as high as 0.8 M, the voltage and energy
efficiencies start to show a relatively apparent drop. In order to give
an explanation for the above phenomenon, the conductivity of the
Fe共III兲-citrate electrolyte as a function of concentration was determined experimentally, as shown in Fig. 10. It is found that the
conductivity of the Fe共III兲-citrate electrolyte increases with concen-
Figure 9. Dependence of efficiencies of the Fe-citrate/Br2 cell on concentration of the Fe共III兲-citrate.
A933
Figure 10. Conductivity of the Fe共III兲-citrate complex solution as a function
of concentration of the Fe共III兲-citrate.
tration of the electrolyte up to 0.5 M. Afterward, the conductivity of
the electrolyte basically remains unchanged, possibly resulting from
an increase in viscosity. Since a high conductivity will minimize the
battery resistance and optimize the energy efficiency, variation of
cell performance of the Fe共III兲/Fe共II兲-citrate/Br2 system with concentration of the Fe共III兲-citrate lower than 0.7 M is mainly attributed to a change in conductivity of the Fe共III兲-citrate complex solution. When the concentration of the Fe共III兲-citrate is 0.8 M, the
cell efficiencies start to decay slightly. Figure 11 shows charge–
discharge curves of the Fe共III兲/Fe共II兲-citrate/Br2 cells employing
the 0.7 and 0.8 M Fe共III兲-citrate complexes, respectively. It can be
seen that compared with the cell with 0.7 M Fe共III兲-citrate, the discharging potential of the cell with 0.8 M Fe共III兲-citrate initially
drops more rapidly and the discharging reaction mainly takes place
at the lower potential indicative of a large polarization resistance,
possibly resulting from a high viscosity of the electrolyte. So the
maximal possible concentration of the Fe共III兲/Fe共II兲-citrate complex in 1 M sodium acetate feasible in the RFB can be up to 0.8 M.
Conversely, when the concentration of the Fe共III兲-EDTA complex is
increased to 0.2 M, the current efficiency is almost unchanged, but
the voltage efficiency decreases to 73.16%. Obviously, this also may
be attributed to an increase in viscosity of the electrolyte.
Figure 11. Charge–discharge curves for the iron-ligand/Br2 test cell employing the 0.7 M 共a兲 and 0.8 M 共b兲 Fe共III兲-citrate complex as negative couple
and 2 M NaBr as the positive electrolyte. The electrolyte volume in each
compartment was 50 mL and the charge–discharge current was 50 mA.
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Journal of The Electrochemical Society, 153 共5兲 A929-A934 共2006兲
Therefore, results of the charge/discharge tests are largely in
agreement with the CV experiments. Moreover, cell performance of
the Fe共III兲/Fe共II兲-citrate/Br2 system has been improved considerably by increasing concentration of the Fe共III兲-citrate complex.
While the electrolyte composition and cell component materials
have yet to be optimized and the solubility of the
Fe共III兲/Fe共II兲-complexes is still relatively low for redox flow cell
applications, the above results have demonstrated that novel iron
共Fe3+ /Fe2+兲-ligand/bromine共Br2 /Br−兲 cells are technically utilizable
in redox flow batteries. It is also suggested that the solubility of the
Fe共III兲-complexes might be improved by adding some additives or
elevating temperatures. Further investigations are needed.
with Fe共III兲/Fe共II兲-EDTA and Fe共III兲/Fe共II兲-citrate almost remains
unchanged with increasing cycle number, minimizing the crosscontamination problem. Although performance of the cell with the
0.1 M Fe共III兲/Fe共II兲-citrate is relatively poor, cell performance is
improved considerably by increasing concentration of the
Fe共III兲-citrate complex. The maximal possible concentration of the
Fe共III兲/Fe共II兲-citrate complex in 1 M sodium acetate feasible in the
RFB can be up to 0.8 M with energy efficiency of about 70%. However, further work is still needed to improve performance of the
iron-complex/bromine cells by optimizing the electrolyte composition and cell component materials and increasing the solubility of
the Fe共III兲/Fe共II兲-complexes.
Conclusion
Acknowledgments
In this study, the voltammetric behaviors of the
Fe共III兲/Fe共II兲-complexes as negative redox couples in 1 M sodium
acetate medium, influences of pH, electrode kinetics, and electrolytic solubility were investigated. The following conclusions can be
drawn.
Complexation of Fe共III兲/Fe共II兲 couples with EDTA, oxalate, and
citrate results in significant negative shifts in the potential of the
redox couple. The solubility of the Fe共III兲/Fe共II兲-citrate complex
appears more satisfactory. The potentials of the complexes with
EDTA or oxalate are less pH-dependent, but relatively high pH of
around 6.0 is electrochemically favorable. The medium employed
causes the systems to have a suitable pH value 共⬃6兲. The electrode
process for the Fe共III兲/Fe共II兲-complexes is electrochemically quasireversible. Nevertheless, complexation of the Fe共III兲/Fe共II兲 couple
can provide fast electrode kinetics, except for the complex with
citrate.
Results obtained from a comparison of the charge–discharge performance of test cells employing Br2 /Br− as anolyte-active species
and a variety of Fe共III兲/Fe共II兲-complexes as catholyte-active species
ones, demonstrate that novel iron 共Fe3+ /Fe2+兲-complex/bromine
共Br2 /Br−兲 cells are technically feasible for use in a RFBs. An opencircuit voltage of approximately 1.1 V and an energy efficiency of
approximately 80% could be obtained for the cell with
Fe共III兲/Fe共II兲-EDTA at a current density of 10 mA cm−2, while the
cells with Fe共III兲/Fe共II兲-oxalate or Fe共III兲/Fe共II兲-citrate exhibit
relatively
low
voltage
efficiencies.
The
cell
with
Fe共III兲/Fe共II兲-oxalate has a high initial discharge capacity, but the
instability of the ferrous form results in a rapid loss of capacity upon
a charge–discharge cycle test. Conversely, the capacity of the cells
The authors are grateful for financial support from the Scientific
Research and Innovation Fund of the Knowledge Innovation Program of the Chinese Academy of Science 共no. K2002D3兲.
Chinese Academy of Sciences assisted in meeting the publication costs of
this article.
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