Journal of Membrane Science 279 (2006) 394–402
Evaluation of membranes for the novel vanadium bromine redox flow cell
Helen Vafiadis, Maria Skyllas-Kazacos ∗
Centre for Electrochemistry and Mineral Processing, School of Chemical Engineering and Industrial Chemistry,
University of New South Wales, Sydney 2052, Australia
Received 24 October 2005; received in revised form 12 December 2005; accepted 12 December 2005
Available online 30 January 2006
Abstract
The novel vanadium bromide (V/Br) redox flow cell employs a vanadium(II)/vanadium(III) couple in the negative half-cell and a Br− /Br3 −
couple in the positive half-cell, using a mixture of vanadium bromide, hydrobromic acid and hydrochloric acid as the electrolyte in both half-cells.
An ion exchange membrane separates the two V/Br half-cell electrolytes. In this project, small test cell cycling experiments were conducted to
evaluate the performance of a range of ion exchange membranes in the V/Br redox flow cell. The features and charge/discharge results of the V/Br
redox flow cell will also be discussed in this paper.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Redox flow cells; Vanadium bromide redox cell; Vanadium electrolytes; Ion exchange membranes
1. Introduction
Batteries and fuel cells are regarded as being among the
most efficient devices for the conversion, storage and delivery of electrical energy [1]. A redox flow cell, also referred
to as the regenerative redox fuel cell, is an energy conversion device that directly converts the chemical energy of soluble reactants into low voltage dc electricity. Energy is stored
or released by means of a reversible electrochemical reaction
between two redox couple solutions with different electrochemical potentials, physically separated by an ion exchange
membrane. The vanadium redox flow cell, pioneered at the
University of NSW, employs vanadium(II)/vanadium(III) and
vanadium(V)/vanadium(IV) redox couples in sulphuric acid
supporting electrolyte, in the negative and positive half-cells,
respectively [2–5]. It possesses a number of features, which
makes it superior to a number of other energy storage systems.
The major advantage of the UNSW vanadium redox battery (VRB) is that it overcomes a major problem with other
redox flow batteries (e.g. Regenessys and the Fe/Cr system)
in which diffusion of ions through the membrane causes the
cross-contamination of the positive and negative half-cells [5].
Cross-contaminated electrolytes must be replaced or regener-
∗
Corresponding author. Tel.: +61 2 9385 4335.
E-mail address: [email protected] (M. Skyllas-Kazacos).
0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.memsci.2005.12.028
ated by external treatment [5]. In the VRB, this problem is
overcome by the use of the same element, vanadium, on each side
of the cell. If cross-mixing were to occur, the solution is readily regenerated upon charging. Occasional mixing of the two
discharged solutions is used to regain any capacity loss caused
by solution imbalance. Electrolyte service life is thus indefinite,
eliminating problems of waste disposal.
Despite its considerable advantages, the relatively low specific energy of the VRB, less than 25 W h kg−1 , currently limits
its use in some stationary applications as well as in electric vehicles. The specific energy of a redox flow system is related to the
concentration of redox ions in solution, the cell potential and to
the number of electrons transferred during discharge per mole
of active species. The concentration of vanadium in the vanadium redox flow battery is limited to 1.6–2 M due to its solubility
within the temperature ranges of 0–45 ◦ C [6]. In an attempt to
increase the energy density of the system, other redox couples
have been investigated.
A vanadium chloride/polyhalide redox flow battery was
recently described [7] employing the V(II)/V(III) redox couple
in the negative and a Br− /Br2 Cl− in the positive half-cell electrolyte. While this showed promising performance, the movement of ions across the membrane led to cross-contamination
and water transfer problems. This was overcome by adding vanadium bromide to both half-cells [8,9].
The vanadium bromide (V/Br) redox flow cell thus employs
a vanadium bromide solution in a mixture of hydrobromic and
H. Vafiadis, M. Skyllas-Kazacos / Journal of Membrane Science 279 (2006) 394–402
395
hydrochloric acid as its supporting electrolyte in each half-cell,
with electron transfer taking place at carbon felt electrodes.
As with the VRB, the V/Br cell employs the same elements
in both half-cell solutions so there are no problems of crosscontamination. In this system however, the V(IV) ions on the
positive side do not partake in the electrochemical reaction, i.e.
the oxidation of V(IV) to V(V) during cell charging does not
occur. Instead, the following overall reactions result [7].
At the negative electrode:
−
V3+
(aq) + e
Charge
V2+
(aq)
Fig. 1. Schematic diagram of the components of a half-cell.
Discharge
At the positive electrode:
−
2Br−
(aq) + Cl(aq)
Charge
−
ClBr−
2(aq) + 2e
Discharge
The V/Br redox system thus retains all the advantages of
the original vanadium redox flow cell, but the higher solubility of vanadium bromide means that it also has the potential to
produce higher energy densities. The possibility of employing
3–4 M vanadium solutions and the excess bromide ions in the
positive half-cell means that the energy densities can be effectively doubled.
The ion exchange membrane has been identified as being the
most critical component of a redox flow cell, defining its performance as well as its economic viability. It acts as a separator,
preventing the mixing and direct chemical reaction of the oxidant and reductant electrolytes. In addition, it provides an ionic
conduction path between the electrolytes [10]. The ideal membrane should be selective to the charge-carrying ions in order to
provide high ionic conductance [11]. In the V/Br flow cell, the
membrane must restrict the transfer of vanadium and polybromide ions from one half-cell to another without hindering the
transport of the charge-carrying hydrogen ions. It must also be
able to withstand attack from the highly oxidising polybromide
ions and be relatively inexpensive.
In this study, various types of experimentally and commercially available ion exchange membranes were assessed
to establish their suitability for use in the V/Br flow cell.
The initial behaviour of the ion exchange membranes during
small-scale cell cycling experiments was investigated. Selected
membranes were subsequently evaluated for properties of ion
exchange capacity, conductivity, diffusivity, dimensional stability and water content.
2. Experimental
The membranes evaluated in this study are listed in Table 1.
2.1. Cell performance
Cell cycling was performed at a constant current density of
20 mA cm−2 using a Repower Battery Testing Unit (China).
Unless otherwise specified, each cell was charged to 1.65 V
and discharged to 0.25 V. The electrolyte solutions used in the
cell consisted of 2 M V3.5 (1 M V(III) + 1 M V(IV)) in a supporting electrolyte of 6.4 M HBr and 2 M HCl. Fig. 1 shows
the overall setup of one half-cell. Each cell employed three,
5 cm × 5 cm × 0.3 cm graphite felt electrodes in each half-cell
and copper plates covered by glassy carbon sheets as current
collectors. The glassy carbon was used to ensure the copper
does not come into contact with the electrolyte. Iwaki pumps
(Japan) circulated the electrolyte through the cell via the flow
frames with a 5 mm cavity that contained the carbon felt porous
Table 1
Membranes studied
Membrane
Supplier
Type
Thickness (mm)
SELEMION®
ASAHI Glass Company, Japan
ASAHI Glass Company, Japan
Asahi Kasei, Japan
Tokuyama Corp., Japan
Tokuyama Corp., Japan
Australian Battery Tech. & Trading
Australian Battery Tech. & Trading
Australian Battery Tech. & Trading
Australian Battery Tech. & Trading
Australian Battery Tech. & Trading
LinAn Chemical, China
LinAn Chemical, China
Guangzhou Delong Technologies Pty Ltd, China
W.L Gore & Associates, USA
W.L Gore & Associates, USA
Cation
Cation
Microporous separator
Cation
Anion
Cation
Cation
Cation
Cation
Cation
Cation
Anion
Cation
Cation
Cation
0.12
0.12
0.62
0.12
0.12
0.14
0.16
0.02
0.04
0.06
0.42
0.42
0.13
0.03
0.04
HSV
SELEMION® HSF
HiporeTM
NEOSEPTA® CM-1
NEOSEPTA® AM-1
ABT-1
ABT-2
ABT-3
ABT-4
ABT-5
HZ cation
HZ anion
SZ
Gore Select L01854
Gore Select M04494
396
H. Vafiadis, M. Skyllas-Kazacos / Journal of Membrane Science 279 (2006) 394–402
electrodes. Rubber gaskets were placed between cell components to prevent leakage. The membrane was placed between the
two flow frames to separate each half-cell. Two PVC end plates
were bolted together to compress the cell components. Clear
polyethylene tubing was used to connect the solution reservoirs
to the pumps and the cell.
It must be noted that all membranes were pre-treated according to their manufacturing specifications. In addition, each membrane was soaked in the vanadium bromide electrolyte for a
minimum of 2 h before cycling to allow equilibrium to be established.
The performance of each membrane including chemical
stability in the battery electrolyte, cycling behaviour, as well
cycling efficiencies was evaluated. Efficiencies were calculated
using the following equations:
m−1 1
(Ij + Ij+1 )(tj+1 − tj )
ηc = 0n−1 12
(1)
0
2 (Ik + Ik+1 )(tk+1 − tk )
m−1 1
(Vj + Vj+1 )(tj+1 − tj )
ηe = 0n−1 12
(2)
0
2 (Vk + Vk+1 )(tk+1 − tk )
ηe = ηc ηv
(3)
where ηc is the coulombic efficiency, ηv the voltage efficiency, ηe the energy efficiency, Ij and Ik are the discharge
and charge currents respectively, tj and tk are the discharge
and charge times respectively, Vj and Vk are the discharge and
charge voltages respectively, m the total number of data points
in the discharge and n is the number of data points in the
charge.
2.2. Membrane properties
2.2.1. Ion exchange capacity (IEC)
The IEC of the membranes was determined by the method
described in Mohammadi and Skyllas-Kazacos [12].
2.2.2. Conductivity
The ionic conductivity of the membranes was determined by
impedance spectroscopy over a frequency range of 1 to 106 Hz
with AC amplitude of 0.2 V, using a Solartron 1255B FRA in
tandem with a Solartron Electrochemical Interface SI1287. Measurements were conducted in the area resistance test cell shown
in Fig. 2. The membrane was secured between two rubber gaskets containing a 0.985 cm diameter hole with silicon glue and
then placed between two half-cells. The cell was held together
with a G-clamp to prevent leakage. Fifty millilitres of a 2 M V3.5
in 6.4 M HBr, 2 M HCl solution was placed in each half-cell. Two
graphite discs, covered with epoxy to produce a single side with
constant surface area, were used as electrodes. One steel rod
was inserted into each disc and joined with silver epoxy resin
to provide electrical contact. The electrodes were held at a fixed
distance apart and a constant depth of immersion by an electrode
holder designed to fit over the cell.
It must be noted that each membrane was soaked in a solution
of 2 M V3.5 in 6.4 M HBr, 2 M HCl solution for 24 h before
Fig. 2. Conductivity test cell.
testing. The resistance of the conductivity cell with and without
the membrane was taken at the frequency where the phase shift
was minimal. The area resistivity of the membrane was thus
given by
R = (r1 − r2 ) × A
(4)
where R is the area resistance of the membrane ( cm2 ), A
the membrane area (cm2 ), r1 the resistance of the cell with
the membrane () and r2 is the resistance of the cell without the membrane (). It must be noted that the thickness
of the membrane is not taken into account in the calculation
of R.
2.2.3. Vanadium ion diffusion tests
The diffusivity of the membrane was determined by the
rate of diffusion of vanadium (IV) ions across the membrane
using the static diffusion test outlined in Grossmith et al. [14].
A 25 cm2 area of membrane was exposed to 50 ml of a 1 M
V(IV) in 6.4 M HBr, 2 M HCl solution on one side of the cell
and a 1 M ZnCl2 in 6.4 M HBr solution on the other. The 1 M
ZnCl2 was used to equalise the ionic strength of the solutions
and minimise the osmotic pressure effects on the rate of V(IV)
diffusion. The Zn2+ ions were found not to interfere with the
V(IV) spectra. A wavelength of 765.5 nm has been identified
as corresponding to the absorbance of V(IV) ions in sulfuric
acid [13]. Fig. 3 shows the absorbance of solutions containing different concentrations of V(IV) ions in the 6.4 M HBr,
2 M HCl mixtures until a peak absorbance of 1. The peak
Fig. 3. Absorption spectra of (a) 0.01 M V(IV), (b) 0.02 M V(IV), (c) 0.03 M
V(IV) and (d) 0.05 M V(IV) in 6.4 M HBr, 2 M HCl solution.
H. Vafiadis, M. Skyllas-Kazacos / Journal of Membrane Science 279 (2006) 394–402
397
In addition to the interruption test cell, the diffusion coefficient of the V(IV) ions in solution was calculated to establish
whether the diffusion of ions through the membrane is slower
than the diffusion of ions through the bulk. Cyclic voltammograms were thus performed over a range of scan rates using a
graphite counter electrode and a saturated calomel electrode as
the reference.
Fig. 4. Plot of V(IV) concentration vs. peak absorbance taken at a wavelength
of 748 nm.
absorbance of the V(IV) ions was found to be 748 nm. Fig. 4
shows the linear plot of peak absorbance at 748 nm versus V(IV)
concentration.
The rate of V(IV) ion diffusion was determined by the rate of
change in the absorbance according to the following derivation
[14]:
ln[absB◦ − 2absA] = ln[absB◦ ] − 2ks At/VA
(5)
where absB◦ is the initial absorbance of solution B (the 1 M
V(IV) solution), absA the absorbance of solution A (the 1 M
Zn2+ solution containing no V(IV) ions), A the area of the
membrane exposed, t is time and VA is the volume of solution A. The mass transfer coefficient (ks ) of the V(IV) ions
across the membranes was calculated from the slope of a plot of
ln[absB◦ − 2absA] versus t. The linear plot produced has slope
equal to −2ks A/VA from which ks can be determined. The diffusion coefficient, D was thus calculated using the following
equation:
D = ks dy
(6)
where y is the thickness of the membrane.
It has been suggested that the behaviour of a system in
which an ion exchange membrane separates two solutions is
not exclusively determined by the processes occurring within
the membrane [15]. The transfer of ions from one bulk solution
to another may be predominantly controlled by either membrane
diffusion (diffusion within the membrane) or film diffusion, from
concentration gradients within the solution diffusion layer on
either side of the membrane. In the bulk solutions, concentration levels may be kept constant by stirring, however, even
violent agitation may be insufficient to overcome film diffusion
[15]. To distinguish between membrane and film diffusion controls, an interruption test was conducted where two diffusion
tests containing the same membrane were run simultaneously.
Once an absorbance of approximately 0.5 was reached, the solutions contained in one cell were removed and were returned to
the cell 30 s later. Absorbance measurements resumed until an
absorbance of one (as previously). No change in the diffusion
rate would indicate that the diffusion was predominantly controlled by membrane processes, whilst an increase in diffusion
rate would imply film diffusion control.
2.2.4. Water content
A sample of each membrane was immersed in distilled water
for 24 h (membranes that were stored wet were rinsed with
distilled water before being immersed). Each membrane was
then removed, patted dry and placed on a piece of filter paper.
The membrane weight was measured until a stable weight was
established (i.e. until the surface of the membrane was dry).
Each membrane was then dried under vacuum at 60 ◦ C for and
weighed periodically until a constant weight had been obtained.
The water content of the membrane is taken as the percent weight
change before and after vacuum drying, i.e. weight percent of
water on dry weight according to the following equation:
Wc =
Wwet − Wdry
Wdry
(7)
2.2.5. Dimensional stability
To determine the dimensional stability of the ion exchange
membranes, samples were immersed in solutions of distilled
water, 2 M V3.5 in 6.4 M HBr, 2 M HCl and in 6.4 M HBr, 2 M
HCl for 15 days. Physical changes of length, width and thickness
were determined using a digital caliper.
3. Results and discussion
3.1. Cell performance
A summary of the cycling performance of the membranes
tested is given in Table 2. In general, the membranes that cycled
successfully were cation exchange membranes. Very few anion
exchange membranes were however examined and thus the
effect of exchange functionality is not yet apparent. As the
charge-carrying ions are the hydrogen ions, it was expected that
cation exchange membranes would give the lowest resistance
losses during battery cycling. During cycling, a preferential
transfer of solution from the positive to the negative half-cell
was observed with most membranes, the rate of which was
dependent on the type of membrane. As a result, the half-cell
with the lower volume was charged/discharged first, causing a
decrease in the capacity of the system. To recover the capacity of
the system, the re-balancing of the solution levels was required
periodically. During the cycling of the VRB, volumetric transfer
occurred in the opposite direction, as transfer from the negative
to the positive half-cells was found to occur with cation exchange
membranes, while transfer from the positive to the negative was
observed with anion exchange membranes [16–18].
As seen in Table 2, a number of membranes, namely the
SELEMION® HSV, NEOSEPTA® CM-1 and AM-1, HZ cation
and anion and the SZ, showed a very high resistance during
398
H. Vafiadis, M. Skyllas-Kazacos / Journal of Membrane Science 279 (2006) 394–402
Table 2
Summary of results obtained from the cell cycling experiments conducted
Membrane
No. of cycles
Average
coulombic
efficiency
Average
voltage
efficiency
Average
energy
efficiency
Cycling behaviour
HSV
9
61%
39%
24%
HSF
14
84%
60%
50%
CM1
AM1
ABT1
0
0
0
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
ABT2
ABT3
4
173
38%
80%
70%
72%
27%
56%
ABT4
ABT5
20 (0.5 A)
30
63%
45%
64%
66%
36%
30%
HZ cation
20 (0.5 A)
83%
43%
36%
HZ anion
40 (0.5 A)
90%
43%
36%
SZ
42
91%
44%
39%
L01854
159
86%
62%
53%
M04494
30
86%
77%
66%
HiporeTM
50
60%
63%
38%
Rapid capacity fade. Membrane surface blistered. Colour change from brown
to light brown
Low resistance, large transfer of solution from + to − side. Surface blistered,
opaque texture lost with browny/red staining on + side, blue/green on − side
High resistance, no cell cycling possible. No physical changes evident
High resistance, no cell cycling possible. No physical changes evident
Cell charged for triple theoretical time and did not reach upper limit Half the
solution crossed over from the + to − side. Two outer layers detached from
inner ion exchange layer
Two outer layers detached from inner ion exchange layer
Solution transfer from + to − side required solution remixing. Unusual discharge curve. No physical changes evident
Low coulombic efficiency. No physical changes evident
Low coulombic efficiency. Capacity decreased by factor of 10 over first 10
cycles. No physical changes evident
High resistance, upper limit increased to 1.75 V in attempt to improve efficiency. Membrane attacked by electrolyte, causing it to soften and tear. Colour
change from blue to green
High resistance, upper limit increased to 1.75 V in attempt to improve efficiency. Membrane attacked by electrolyte, causing it to soften and tear. Colour
change from yellow to green on negative side, brown on positive
High resistance, solution transfer from + to − side required remixing every
10 cycles. Slight colour loss black to red
Solution transfer from + to − side required solution remixing. When transfer
significant spike appears on charge. Membrane blistered
Solution transfer from + to − side required solution remixing. Membrane
blistered
Initially, frequent solution transfer from + to − side requiring remixing. Charge
times greater than theoretical
50
80%
70%
56%
cycling, characterised by voltage efficiencies less than 50%. The
resistance of the NEOSEPTA® was so high at a current of 1 A
that cycling was unsuccessful. The conductivity of a membrane
is determined by factors such as membrane thickness, the concentration of fixed ionic groups, the degree of cross-linking, the
size and valence of the counter ions [15]. Although all mentioned
membranes are of different composition, a relationship between
the resistance and membrane thickness was observed, as the
thicker membranes (range 0.12–0.15 mm) showed the higher
resistances.
The HiporeTM microporous separator was also subjected to
cell cycling experiments. It has been cycled for over 100 cycles
and is still continuing. Over the first 50 cycles, an average
coulombic efficiency of approximately 60% and average voltage
of 63%. Preferential crossover of solution from the positive to
the negative half-cell was occurring more often than other ion
exchange membranes tested. As a result, the cell required more
frequent solution re-balancing. The charge times recorded were
consistently over a third greater than theoretical, indicating that
the vanadium ions of different oxidation states were diffusing
through. This low ion selectivity was expected due to the porous
nature of the separator and its lack of ion exchange functionality.
At cycle number 50, the cell was pulled apart and reassembled
with fresh solution. No physical changes were observed for the
separator. During the subsequent 50 cycles, the performance of
the cell improved, as average coulombic and voltage efficiencies
recorded increased to 80% and 70%, respectively. It should also
be mentioned that the solution levels did not require re-balancing
during the last 50 cycles. At this stage, it is not clear what caused
the sudden change in behaviour of the cell, but further studies
are underway to optimise the separator properties.
The most promising ion exchange membranes tested in this
study were the ABT3, L01854, M04494 membranes and the
results obtained thus warrant further investigation. Fig. 5 shows
Fig. 5. A typical set of curves for a small-scale cell using the ABT3 membrane.
Charge/discharge current of 1 A with 35 ml of solution in each half-cell.
H. Vafiadis, M. Skyllas-Kazacos / Journal of Membrane Science 279 (2006) 394–402
Fig. 6. Rate of V(IV) ion diffusion of the ABT3 membrane in a static test cell
and an interruption test cell.
a typical charge/discharge curve taken from the cycling results
of the ABT3 membrane. Average coulombic and voltage efficiencies for the ABT3 membrane are 80% and 72%, while
corresponding values for the L01854 membrane are 86% and
62%, respectively. Although the M04494 membrane has not
been subjected to the same number of cycles as the ABT3 and
L01854 membranes, its average coulombic and voltage efficiencies are 86% and 78%, respectively. At this point it must be noted
that the values cited in this section represent the results obtained
from small-scale tests. They do not take into account any resistance losses from, for example, contact resistances, which can
be eliminated or greatly reduced with improved cell design. It
should also be mentioned that the 5 mm half-cell cavity used in
the membrane tests cell, is greater than what would be employed
in an optimised cell design. Considerable improvements in voltage efficiency, and thus the overall energy efficiency, can be
expected with a reduced half-cell cavity thickness (e.g. 2 mm)
and a single carbon felt electrode (3 mm thickness).
In an attempt to understand the behaviour of the membranes
during cycling, selected samples were analysed to determine
their properties.
3.2. Membrane properties
Fig. 6 compares the rate of diffusion of the V(IV) ions through
the ABT3 membrane in a static test cell and an interruption test
cell. The interruption of the diffusion test cell did not affect
the rate of V(IV) ion diffusion through the membrane. It can
thus be assumed that the diffusion processes are predominantly
controlled by membrane diffusion. To verify this, the diffusion
coefficient of the V(IV) ions in the vanadium bromide electrolyte
was determined using cyclic voltammetry.
The cyclic voltammograms of the 1 M V(IV) solution at different scan rates and switching potentials of −1.0 and 0.1 V are
shown in Fig. 7. The reverse peaks of V(IV)/V(II) were used in
the analysis. Fig. 8 shows a linear plot of the peak current versus
the square root of scan rate, indicating that the reaction is diffusion and not membrane controlled. The linear slope obtained
was used in the equation for irreversible reactions shown in the
following equation [19]:
1/2
ip = (2.99 × 105 ) n (αna )1/2 AC0 D0 v1/2
(8)
399
Fig. 7. Cyclic voltammograms of 1 M V(IV) at scan rates of (a) 0.05 V s−1 , (b)
0.10 V s−1 , (c) 0.20 V s−1 , (d) 0.30 V s−1 , (e) 0.40 V s−1 .
where ip is the peak current (A), n the number of electrons processes, α the transfer coefficient, A the electrode area (cm2 ), Co
the bulk concentration (mol cm−3 ), D the diffusion coefficient
(cm2 s−1 ) and ν is the scan rate (V s−1 ).
Assuming a α value of 0.5, from the slope of the linear plot in Fig. 8, a V(IV) ion diffusion coefficient of
1.77 × 10−3 cm2 min−1 was obtained. Compared to the values
listed in Table 3, it is evident that the rate of diffusion of V(IV)
ions through the solution is three to four times greater than
through the ion exchange membrane.
The IEC refers to the concentration of fixed charges in the
exchanger, analogous to the concentration of an electrolyte solution [20]. It is a measure of how many counter ions can be taken
up by the membrane. An ideal ion exchange membrane would
have a high IEC to allow the passage of the charge-carrying
H+ ions and thus providing a low resistance. Factors leading to
a high IEC include a low cross-linking density, flexible crosslinks rather than rigid, large number of functional groups and
a maximum number of active donor atoms per active functional group [11]. Although a high IEC may be desirable, a
low cross-linking density renders the membrane susceptible to
the effects of swelling. When immersed in an aqueous solution,
ion exchange membranes swell, absorbing water and ions. The
degree of swelling of the membrane significantly influences the
rate of ion diffusion as well as the ion selectivity. Generally, the
higher the degree of cross-linking, the higher the selectivity.
Table 3 shows the properties of the various ion exchange
membranes evaluated. The changes of length, width and thick-
Fig. 8. Plot of peak current (ip ) vs. the square root of the scan rate (ν1/2 ).
400
H. Vafiadis, M. Skyllas-Kazacos / Journal of Membrane Science 279 (2006) 394–402
Table 3
Properties of the membranes evaluated
Membrane
IEC (mmol g−1 )
Resistivity ( cm2 )
V(IV) diffusivity (10−7 cm2 /min)
Water content (%)
Gore L01854
Gore M04494
ABT3
ABT4
ABT5
SZ
HiporeTM
0.69
1.00
6.01
3.77
3.92
2.5
1.14
0.38
0.41
3.24
9.97
5.39
19.03
1.4
0.36
0.96
0.11
1.44
1.44
2.34
148
1.33
2.16
4.25
4.47
4.4
10.16
62.31
Table 4
Percentage changes in membrane length after 1 day and 15 days of soaking in each solution
Membrane
Gore M04494
Gore L01854
ABT3
ABT4
ABT5
SZ
Hipore
Distilled water (%)
2 M V3.5 in 2 M HCl + 6.4 M
HBr (%)
2 M HCl + 6.4 M HBr (%)
1 day
15 days
1 day
15 days
1 day
15 days
4.5
3.5
4.1
2.8
3.2
4.3
−0.4
6.5
2.5
4.5
2.5
3.6
3.1
0.6
0.9
−1.4
0.8
−1.8
1.1
3.1
0.2
1.3
1.6
−0.3
1.5
1.1
3.1
−0.2
1.8
1.6
−0.7
0.8
1.0
2.8
1.2
2.2
2.5
−1.4
0.6
2.6
3.7
−0.6
Table 5
Percentage changes in membrane width after 1 day and 15 days of soaking in each solution
Membrane
Gore M04494
Gore L01854
ABT3
ABT4
ABT5
SZ
Hipore
Distilled water (%)
2 M V3.5 in 2 M HCl + 6.4 M
HBr (%)
2 M HCl + 6.4 M HBr (%)
1 day
15 days
1 day
15 days
1 day
15 days
1.5
3.9
3.5
1.9
4.1
5.4
−0.5
6.2
4.4
3.3
2.2
4.4
5.3
−0.4
−0.1
−0.3
0.9
−1.1
1.2
4.0
−0.4
0.7
1.2
0.9
−0.7
1.5
4.0
−0.4
0.7
1.2
1.2
−0.7
2.1
3.8
−0.4
1.0
0.5
0.4
−0.7
1.5
3.8
−0.4
ness of the membranes during tests of dimensional stability are
shown in Tables 4–6. All membranes, except the HiporeTM ,
showed the most significant dimensional changes of length and
width in the distilled water. This result was expected, as the
concentration of ion exchange groups within the membrane is
higher than the distilled water, producing a marked difference in
osmotic pressure. Minimal degrees of swelling were expected
in the more concentrated solutions, as the osmotic pressure
difference between the external solution and the membrane is
decreased. Most membranes exhibited lowest levels of swelling
in the 2 M V3.5 electrolyte. The difference observed between the
final and initial dimensions indicates that the membranes generally swell or shrink by a certain degree then slowly stabilize and
come to equilibrium with the solution. Changes in membrane
Table 6
Changes in membrane thickness after 1 day and 15 days of soaking in each solution
Membrane
Gore M04494
Gore L01854
ABT3
ABT4
ABT5
SZ
Hipore
Distilled water (mm)
2 M V3.5 in 2 M HCl + 6.4 M
HBr (mm)
2 M HCl + 6.4 M HBr (mm)
1 day
15 days
1 day
15 days
1 day
15 days
0
0
0
−0.01
0
0
0
0
0
0
0
0.01
0
0.01
−0.01
−0.01
0
0
0.01
0
0.01
−0.005
0.005
0
0.01
0.02
0.01
0.03
−0.01
0
0
0.01
0.02
0.01
0.03
−0.01
0.005
0
0.01
0.02
0.01
0.03
H. Vafiadis, M. Skyllas-Kazacos / Journal of Membrane Science 279 (2006) 394–402
thickness (Table 6) appear to be more pronounced in the acidic
solutions of 2 M HCl, 6.4 M HBr and 2 M V3.5 . Changes detected
for most membranes were however very small and at the limits
of the measuring apparatus. Although changes in thickness have
been reported, they are merely for comparison as it is difficult
to draw valid conclusions.
From the relationships described above, it was assumed that
the ion exchange membranes with high IEC would have a high
rate of vanadium ion diffusion (due to their decreased selectivity), a low area resistivity and high degree of swelling. This general trend was not observed with all ion exchange membranes.
Ion exchange membranes ABT3, ABT4 and ABT5 differ only by
the number of functional groups on the surface and by thickness.
The IEC of the ABT3 was almost double, whilst measured resistivity was significantly lower than the ABT4 and ABT5 membranes (Table 3). This is consistent with the trend outlined above,
however the V(IV) diffusivity of the ABT3 membrane is not,
being an order of magnitude lower than the ABT4 and ABT5.
The behaviour of the ABT3, ABT4 and ABT5 membranes during the small-scale cycling experiments, outlined in Table 2,
support the results of the IEC, conductivity and diffusivity tests.
Compared to the ABT3 membrane, the ABT4 and ABT5 membranes produced lower voltage efficiencies, and the higher rates
of solution crossover resulted in lower coulombic efficiencies.
The swelling behaviour of the ABT range of membranes
is consistent with the properties listed in Table 3. The ABT3
membrane, exhibited one of the highest degrees of swelling in
distilled water (>4%). In the concentrated acid solutions however, the membrane length was less than the original value. This
can be attributed to the membrane’s high IEC and low resistivity. No change in ABT3 thickness was observed in any solution.
The thickness of the ABT4 increased only in the acidic solutions, whilst the ABT5 increased both in the distilled water and
the acid solutions.
The SZ membrane showed a high IEC, high rate of V(IV)
diffusion and a high area resistance. These properties are consistent with the low voltage efficiency (44%) and high rate of
solution crossover observed during the cycling of the membrane
(see Section 3.1). The SZ membrane showed high dimensional
changes, approximately 3%, which did not differ greatly in the
distilled water or the acids.
The Gore L01854 and M04494 membranes produced the lowest values of IEC and V(IV) ion diffusivity, whilst retaining the
lowest values of area resistance. Small dimensional changes
were observed in the acidic solutions, however the M04494
membrane showed the greatest dimensional changes in length
and width in distilled water (6.5%). Changes were consistently
larger than the L01854 membrane, possibly due to the greater
IEC (and thus higher diffusivity) of Gore M044994. Very small
thickness changes were detected for both membranes.
Although the HiporeTM microporous separator has no ion
exchange functionality, an IEC value of 1.14 mmol g−1 was
obtained. This can be attributed to the separators ability to uptake
solution into its pores, which can then readily diffuse into another
solution (as seen in the cell cycling results). The water content
calculated for the HiporeTM was 62% (Table 3), an order of magnitude greater than most other membranes. The highly porous
401
structure allowed a large uptake of water into its pores during
the soaking, which in turn was easily removed during the drying. It was thus established that the methods for calculating the
IEC and water content of the ion exchange membranes are not
as suitable for evaluating porous separators and thus cannot be
directly compared.
The dimensional changes of the HiporeTM microporous separator were expected to be significant as the ability of the separator
to uptake solution was demonstrated in the IEC and water content tests. Interestingly, the separator showed a slight decrease
in length and width, however showed a 0.03 mm increase in
thickness.
Table 3 lists the water content values calculated for the ion
exchange membranes. No direct relation between membrane
behaviour and water content was found. All ABT membranes
contained similar percentages of water, however their properties
and behaviour were quite different. It is however interesting to
note that the Gore M04494 membrane had higher IEC and water
content than the L04494.
3.3. Chemical stability
A major limiting factor in the lifetime of the V/Br system is the chemical stability of the ion exchange membrane
in the highly oxidising electrolyte. Membranes such as the
SELEMION® HSF, HZ cation and HZ anion showed good initial coulombic efficiencies (84%, 83% and 90%, respectively),
however their life in the cell was limited due to their poor chemical stability. The SELEMION® HSF membrane blistered within
14 cycles and lost its opaque colour and shinny texture. The HZ
cation and anion exchange membranes were broken down by
the electrolyte and became very fragile after 20 and 40 cycles,
respectively.
Other membranes such as the HSV, ABT1 and ABT2 did not
show good cycling properties or chemical stability. Although
their immersion time in the battery electrolyte was limited
(less than 10 cycles), it was sufficient to demonstrate their lack
of chemical resistance. The HSV membrane blistered and its
dark brown colour faded to a light brown. Both the ABT1 and
ABT2 membranes delaminated, as the backing material used
to increase the mechanical stability of the membranes detached
from the ion exchange layer. The ion exchange layer was too thin
and fragile to act as a separator and thus a large volumetric transfer of solution from the positive to the negative side occurred.
The ABT4 and ABT5 membranes were characterised by relatively low coulombic efficiencies (63% and 45%) and higher
rate of solution crossover. No physical changes after cycling
were however observed.
During long term cycling, however, the coulombic efficiency
of the ABT3 membrane increased, while the voltage efficiency
steadily decreased. This is thought to be the fouling of the
membrane, leading to the blockage of the pores and reduced
transfer rates of the redox ions across the membrane. Although
the L01854 membrane lasted at least 160 cycles, when removed
from the cell it was found to have blistered. It is not known
at what stage the blistering began, or how much longer the
membrane would last in this condition. Long term stability
402
H. Vafiadis, M. Skyllas-Kazacos / Journal of Membrane Science 279 (2006) 394–402
testing will review these questions. The M04494 membrane
was also found to have blistered at some stage during the 30
cycles completed thus far, however the effect of this on the
cycling behaviour has not yet been apparent. The cell testing is
still continuing for this membrane.
4. Conclusion
Most of the ion exchange membranes tested to date are
not suitable for use in a V/Br flow cell. A high resistance was
shown by a large number of membranes, some too high to
allow any cell cycling to occur with the set voltage limits. Other
membranes have shown a low chemical stability in the highly
oxidising electrolyte.
The membranes that have shown positive results in the V/Br
system are the ABT3, L01854 and M04494 membranes. Over
160 cycles, the ABT3 membrane has shown average coulombic
and voltage efficiencies of 80% and 64%, whilst the L01854
membrane, 86% and 62%, respectively. Considerable improvements in voltage efficiencies can be achieved by the optimisation
of cell design. The stability of the HiporeTM microporous separator in the cell electrolyte and the improved cycling behaviour
is promising. Studies will endeavour to optimise its selectivity
and IEC. Further investigations are also needed to understand
the unusual cycling behaviour seen with the discharge of the
ABT3 membrane and the blistering of the L01854 and M04494
membranes. These investigations are currently being conducted
in parallel to the screening of further membrane materials as
candidates for the V/Br redox flow cell.
Acknowledgments
The authors are grateful to W.L Gore and Associates for
the samples of M04494; to the Australian Battery and Technology Trading Company for the ABT membrane samples; to
Guangzhou Delong Technologies Pty Ltd for the SZ membrane
samples and to Mitsubishi Australia for the HiporeTM microporous separator.
References
[1] A. Landgrebe, F. McLarnon, Atmospheric impacts of fuel cell and battery technology, in: Proceedings of the Symposium on Fuel Cells, San
Francisco, California, 1989.
[2] E. Sum, M. Skyllas Kazacos, A study of the V(II)/V(III) redox couple for redox flow cell applications, J. Power Sources 15 (1985) 179–
190.
[3] E. Sum, M. Rychcik, M. Skyllas Kazacos, Investigation of the
V(V)/V(IV) system for use in the positive half-cell of a redox battery,
J. Power Sources 16 (1985) 85–95.
[4] M. Skyllas-Kazacos, M. Rychcik, R.G. Robins, A.G. Fane, M.A. Green,
New all-vanadium redox flow cell, J. Electrochem. Soc. 133 (1986)
1057–1058.
[5] M. Skyllas Kazacos, T.G. Robbins, US 4,786,567, 1986.
[6] M. Skyllas Kazacos, C. Menictas, M. Kazacos, Thermal stability of concentrated V(V) electrolytes in the vanadium redox cell, J. Electrochem.
Soc. 143 (1996) L86–L88.
[7] M. Skyllas-Kazacos, Novel vanadium chloride/polyhalide redox flow
battery, J. Power Sources 124 (2003) 299–302.
[8] M. Skyllas Kazacos, Vanadium/polyhalide redox flow battery, WO
03/019714 A1, 2003.
[9] M. Skyllas Kazacos, M. Kazacos, A. Mousa, Metal halide redox flow
battery, WO 03/092138 A2, 2003.
[10] D.G. Oei, Permeation of vanadium cations through anionic and cationic
membranes, J. Appl. Electrochem. 15 (1985) 231–235.
[11] S.C. Chieng, M. Kazacos, M. Skyllas-Kazacos, Preparation and evaluation of composite membrane for vanadium redox battery applications,
J. Power Sources 39 (1992) 11–19.
[12] T. Mohammadi, M. Skyllas Kazacos, Modification of anion-exchange
membranes for vanadium redox flow battery applications, J. Power
Sources 63 (1996) 179–186.
[13] E. Wiedemann, A. Heintz, R.N. Lichtenthaler, Transport properties of
vanadium ions in cation exchange membranes: determination of diffusion coefficients using a dialysis cell, J. Membr. Sci. 141 (1998) 215–
221.
[14] F. Grossmith, P. Llewellyn, A. Fane, M. Skyllas-Kazacos, Evaluation
of membranes for all-vanadium redox cell, in stationary energy storage,
load leveling, and remote applications, in: Proceedings of the Electrochemical Society, Honolulu, 1988, pp. 363–373.
[15] F.G. Helfferich (Ed.), Diffusion Across Membranes, McGraw-Hill Book
Company, Inc., New York, 1962, pp. 339–420.
[16] S.C. Chieng, Membrane Processes and Membrane Modification for
Redox Flow Battery Applications, PhD, Chemical Engineering, University of New South Wales, Sydney, 1993.
[17] T. Mohammadi, S.C. Chieng, M. Skyllas Kazacos, Water transport study
across commercial ion exchange membranes in the vanadium redox flow
battery, J. Membr. Sci. 133 (1997) 151–159.
[18] T. Sukkar, Membrane Studies for Vanadium Redox Flow Cell Applications, PhD, School of Chemical Engineering and Industrial Chemistry,
University of New South Wales, Sydney, 2002.
[19] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Fundamentals and
Applications, 2nd ed., John Wiley & Sons Inc., NJ, 2001.
[20] R. Paterson, An Introduction to Ion Exchange, Heyden and Son, Ltd.,
London, 1970.