Functionalised carbazole as a cathode for high voltage non-aqueous organic redox flow batteries

Article abstract of DOI:10.1039/d0nj02543g

Prospective high reduction potential cathode materials have been proposed that can be used in non-aqueous redox flow battery applications. A new class of material, 3,6-dibromo-9-(p-tolyl)-9H-carbazole(3)incorporating a carbazole core, showing a very good reversibility is employed as the cathode material and dissolved oxygen in the solvent mixture is used as the anolyte material. Labile positions of the carbazole have been substituted with electron withdrawing groups, which increases the reduction potential of the redox couple. Apart from substituting the labile positions, we have also explored the possible structural modification responsible for stabilizing the cation radical of the carbazole moiety and obtained the reversible behavior thereafter. From this, it is evident that a free radical is stabilized upon delocalization of the charge in the molecule. The mass-transport and redox parameters, diffusion coefficient and heterogeneous electron transfer rate coefficient values are high enough to realize good battery performance. A solvent mixture of acetonitrile and dichloromethane (4?:?1) has been used in this work in order to increase the solubility of electroactive materials in the medium.

Full text of DOI:10.1039/d0nj02543g

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Functionalised carbazole as cathode for high voltage non-aqueous
organic redox flow battery
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Chinmaya R. Mirle, Raja M, Vasudevarao P, Sankararaman S* and Kothandaraman R*
Department of Chemistry, Indian Institute of Technology Madras, Chennai, India
*Corresponding author’s E-mail: sanka@iitm.ac.in and rkraman@iitm.ac.in
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Abstract
Prospective high reduction potential cathode material has been proposed that can be used in non-
aqueous redox flow battery application. New class of material, 3,6-dibromo-9-(p-tolyl)-9H-
carbazole (3) incorporating carbazole core, showing a very good reversibility is employed as the
cathode material and dissolved oxygen in the solvent mixture is used as the anolyte material. Labile
positions of the carbazole have been substituted with the electron withdrawing groups, which
increases the reduction potential of the redox couple. Apart from substituting the labile positions,
we have also explored the possible structural modification responsible for stabilizing the cation
radical of carbazole moiety and obtained the reversible behavior thereafter. From this, it is evident
that free radical is stabilized upon delocalization of the charge in the molecule. The mass-transport
and redox parameters, diffusion coefficient and heterogeneous electron transfer rate coefficient
values are high enough to realize good battery performance. A solvent mixture of acetonitrile and
dichloromethane (4:1) has been used in this work in order to increase the solubility of electroactive
material in the medium.
Keywords: 3,6-dibromo-9-(p-tolyl)-9H-carbazole; oxygen/superoxide redox couple; acetonitrile
and dichloromethane electrolyte mixture
1

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1. Introduction
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There is a great demand for the storage of electrical energy generated by the renewable sources
due to intermittent nature of these sources. In flow batteries, energy is stored in the charged liquids
unlike conventional rechargeable batteries where energy-storing species are embedded on the
electrode structure and discharged as per demand as the charged solution flows through the
electrode. As a result, there is spatial separation between electrode structure and electroactive
material leading to decoupling of energy and power. So, energy and power can be independently
varied by modulating size of the solution tank for the former and the electrode area for the latter,
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1
respectively . So far, vanadium redox flow battery (VRFB) have undergone thorough research
and also been commercialized in some places 2. But, it also suffers from the inherent limitation of
offering a limited potential window for operation as water can be electrolyzed at the potentials of
3
operation and readily available vanadium which in turn leads to high cost of VRFB operation .
So, the use of non-aqueous medium having higher electrolyte decomposition voltage with suitable
redox couple offers larger operational potential window, which can increase the energy density of
the battery considerably.
In the case of metal containing batteries the lack of metal abundance, scarcity of technology
readiness and environmental concerns involving disposal of used battery materials capped the
growth at certain stage. So there is a growing demand for energy storage devices, which show least
impact on the environmental crisis. Non-aqueous organic redox flow battery (NAORFB) meets
this requirement since it uses less-polluting and bio-degradable electroactive organic materials for
its making and so requiring less attention for its disposal. In the current scenario, a number of class
of organic compounds (ketones, quinones, phenazines, phthalimides, 2,2,6,6-
Tetramethylpiperidin-1-yl)oxyl (TEMPO), alkoxyarenes etc.) showing reversible electrochemical
2

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behavior have been used 4–11. In a bid to expand the domain of materials capable of showing better
energy density with higher redox potential for NAORFB, we have come up with carbazole
substituted species as one of the electrode materials. Derivatives of carbazole look promising as
they form stable cation radicals due to planarity of the aromatic carbazole ring and are explored in
a number of applications like electrophotographic photoreceptors, photorefractive materials, light
emitting diodes and photovoltaic devices owing to the ease with which they can transport positive
charges 12–16. In all these applications, photoconductive properties of carbazole containing
polymers are used which exploit the ability of these polymers to transport positive charges, as the
formed positive charges are stable. Mikawa et al. have transformed insulating and non-conjugated
pendant poly(vinylcarbazole) polymer into electrically conducting polymer by electrochemical
doping as these are stable and used them as positive electrode material in Li-ion battery
applications 17. This was an enough motivation for us to look for carbazole derivatives as catholyte
material. Carbazoles are attractive for employing in organic redox flow battery as they can be
modified with suitable substituent to enhance their solubility in a given solvent which directly
affects capacity and so energy density of the battery 18,19. We can improve the reversibility of
electrode material by substituting the chemically labile positions with stable substituents which
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otherwise would yield a number of side reactions by reacting with the cation radical formed 20,21
.
Blocking 3 and 6 positions of carbazole yields quite a stable cation radical 22. Moreover, by
substituting with suitable functionality, redox potential could be varied towards higher open circuit
voltage of the battery 19. We have chosen (3), as carbazoles lend flexibility to tune properties such
as solubility, redox potential, reversibility of the redox species by introducing appropriate
substituents. Moreover, carbazole is an economically viable option as it is obtained as the by-
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product of distillation of coal tar, which in turn can be used to synthesis a number of carbazole
derivatives.
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2. Experimental
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2.1. Materials
(3) was synthesised from carbazole (1) as the starting material which was purchased from
Spectrochem chemicals. L-proline, p-bromotoluene, CuI, K2CO3, N-bromosucciniide, acetic acid
and 9-ethyl-9H-carbazole was purchased from TCI chemicals (India) Pvt. Ltd.
Tetrabutylammonium hexaflurophosphate (TBAPF6) was purchased from Sigma-Aldrich.
Dimethylformamide (DMF), toluene, dimethylsulfoxide (DMSO), acetonitrile (ACN) and
dichloromethane (DCM) were obtained from Fisher Scientific Ltd. Graphite felt was obtained from
Nickunj Eximp Entp Pvt. Ltd. and Celgard 2320, polypropylene based porous membranes were
procured from Celgard North Carolina. Nitrogen (N2) and oxygen (O2) gases were procured from
Indogas agency, Chennai, India.
2.2. Synthesis of 3,6-dibromo-9-(p-tolyl)-9H-carbazole
N-Arylation of (1) was carried out using Ullmann reaction condition. To the mixture of (1) (1.3 g,
7.77 mmol, 1 equiv), L-proline (90 mg,0.77 mmol, 0.1 equiv), p-bromotoulene (1.4 mL, 10.88
mmol, 1.4 equiv), CuI (148.1 mg,0.77 mmol, 0.1 equiv), K2CO3(2.15 g,15.55 mmol, 2 equiv) and
24 mL of DMSO as solvent was added and refluxed for 24 h. The product obtained from the
reaction mixture was filtered by adding excess of cold water. 9-(p-Tolyl)-9H-carbazole (2) was
purified from the mixture by silica gel column chromatography using hexane as solvent and then
dried to get pure compound. To chilled 10mL of 0.5 mM solution of (2) in DMF, N-
bromosuccinimide corresponding to 1.1 mM (0.2 g) was added. The mixture was stirred at room
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temperature for 3 hours. Ice was added to the mixture to give a white precipitate. After drying, the
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solid was recrystallized using toluene to obtain pure compound of (3).
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9-(p-Tolyl)-9H-carbazole (White solid) :1H NMR (400 MHz, CDCl3) δ 8.06 (d,2H), 7.36-7.28 (m,
8H), 7.21 – 7.16 (m, 2H), 2.40 (s, 3H); 13C NMR (100 MHz, CDCl3)  141.2, 137.5, 135.1, 130.6,
127.1, 125.9, 123.3, 120.4, 119.8, 109.9, 77.2; HRMS (ESI): Calcd. For C19H15N [M+H]+
258.1277; found = 258.1279 (Fig. S1 – S4).
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3,6-dibromo-9-(p-tolyl)-9H-carbazole (White solid) :1H NMR (400 MHz, CDCl3) δ 8.11 (s, 2H),
7.42 (d, 2H), 7.33, 7.29 (dd, 4H), 7.15 (d, 2H), 2.41 (s, 3H); 13C NMR (100 MHz, CDCl3)  140.2,
138.3, 134.2, 130.8, 129.4, 126.9, 123.9, 123.3, 113.0, 111.6, 77.2; HRMS (ESI): Calcd. For
C19H13Br2N [M+H]+ 413.9373; found = 414.9393 (Fig. S5 – S8).
2.3. Synthesis of 3,6-dibromo-9-ethyl-9H-carbazole (4)
A mixture of 9-ethyl-9H-carbazole (1 g, 1 equi), N-bromosuccinimide (1.8 g, 2 equi), toluene (1
ml, 1.75 V) and acetic acid (3 ml, 7 V) were stirred overnight. Reaction was monitored by thin
layer chromatographic technique to ensure the completeness. At the end of reaction, the mixture
was evaporated to remove Br2. The solid residue obtained upon evaporation was purified by
column chromatography on silica gel by using mixture of hexane/ethyl acetate (98 : 2) as an eluant.
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3,6-dibromo-9-ethyl-9H-carbazole (4) (White solid) ; H NMR (400 MHz, CDCl3) 8.16 (s, 2H),
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7.59, 7.57 (dd, 2H), 7.29 (d, 2H), 4.33 (q, 2H) and 1.42 (t, 3H) ; C NMR (100 MHz, CDCl3)
138.8, 129.0, 123.6, 123.3, 111.9, 110.2, 37.9, 13.7 ; HRMS (ESI): Calcd for C14H11Br2N [M+H]+
352.9312; found = 353.9302 (Fig. S9 – S12).
2.4. Electrolyte preparation and cell fabrication
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5 mM solution of (3) in 4:1 (v/v) solvent mixture of acetonitrile (ACN): dichloromethane (DCM)
with 0.1 M TBAPF6 was prepared to act as catholyte. Similarly O2 was dissolved in the solvent
mixture of 4:1 (v/v) of ACN : DCM to act as anolyte with 0.1 M TBAPF6 as the supporting
electrolyte as well as the stabilizer of superoxide 20. The cell was assembled as shown in the
schematic diagram (Fig. 1). The order of components of the half-cell: SS endplate, copper current
collector, carbon block with a well of 6 mL volume, silicone rubber gasket of 3 mm thickness with
a pocket of size 2.7 cm x 2.7 cm wherein activated carbon felt is to be placed. The well is connected
to the pocket with perforations to deliver the reactants from the well.
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Fig. 1 Schematic representation of the assembled cell using (3) as catholyte and O2 / TBAPF6 as
anolyte.
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Two such half cells are placed on either sides of the Celgard porous separator, which makes the
complete cell. The entire assembly is compacted using bolts and nuts. The solution in the anode
compartment was kept in contact with the oxygen atmosphere using the oxygen filled balloon.
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2.5. Electrochemical characterisation
Three electrode mode CV studies were carried out using Biologic VSP potentiostat. A glassy
carbon electrode of 0.196 cm2 area and Ag/Ag+ (10 mM AgNO3 in ACN) were used as working
and reference electrodes respectively. Pt mesh was used as the counter electrode. The cation
radicals of (1), (2) and (3) are labelled as (1+), (2+) and (3+) respectively. Similarly polarisation
studies were carried out by linear sweep voltammetry for the determination of exchange current
density (i0) of the redox couple (3)/(3+) using rotating disk electrode (RDE) as the working
electrode. The glassy carbon RDE with a disk diameter of 5 mm obtained from PINE instruments
was used. 5 mM solution (3) and oxygen saturated ACN:DCM (V/V 4:1) solutions were used to
record the Nyquist plot at 1.09 V and -1.24 V vs. Ag/Ag+ , respectively with a 5 mm diameter
glassy carbon working electrode. 0.1 M TBAPF6 was used as the supporting electrolyte.
Frequency range employed in recording Nyquist plot is 100 kHz to 0.1 Hz with a voltage
perturbation of 10 mV. The kinematic viscosity of the reaction medium was measured using
Anton-Paar Lovis2000 M/ME Microviscometer at 25°C for determining diffusion coefficient
using Levich equation.
3. Results and discussion
3.1 Cyclic voltammetry
Cyclic voltammetry was performed using (1), (2), (3), and (4) to understand the extent of
reversibility of the redox process. Fig. 2(a) and (b) compares the 3rd and 60th cycle of (1) and (2).
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Fig. 2 (a) and (b) shows the CV of (1) and (2) in 4 : 1 Solvent mixture of ACN : DCM containing
0.1 M TBAPF6 as supporting electrolyte respectively in their 3rd and 60th cycles.
(1) shows some redox activity for initial few cycles and loses it when cycled 60 times expectedly
due to the formation of unstable cation radical (1+.) and unknown polymer coating on the electrode
surface which blocks the availability of electrode surface for further reaction 21. When 9-H proton
of (1) was substituted with tolyl group (Fig. 2b) moderate retention of redox activity was witnessed
even after 60th cycle though there is decrement in the peak current due to the free availability of
chemically labile 3rd and 6th positions in the carbazole core for dimerization. The quasi-reversible
oxidation peak obtained for (2) around 0.95 V is due to removal of an electron from the lone pair
of N yielding cation radical which may undergo dimerization 22. The redox peak obtained around
0.6 V is due to the oxidation of dimer of (1) and (2). The 3,6 positions in (1) being highly
susceptible to electrophilic substitution can easily undergo electrochemical oxidative
polymerization during cycling making the redox reaction irreversible22. So, blocking these
positions is important to obtain electrochemical reversibility. Here bromine being an electron
withdrawing group shifts the onset of oxidation potential to more positive value 22.
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Fig. 3 (a) and (b) shows CV of (3) and (4) ) in 4 : 1 solvent mixture of ACN : DCM containing
0.1 M TBAPF6 as supporting electrolyte, respectively in their 1st, 2nd and 100th cycle.
(3) yields a stable reversible redox signature (3/3+.) for a very large number of cycles (100 cycles)
owing to the formation of stable cation radical (Fig. 3a). Since cation radical is formed initially at
the N centre of carbazole core, role of different N-H substituent in stabilizing the cation radical
formed can also be evaluated. Hence, 1st, 2nd and 100th cycles of (3) and (4) are compared in Fig.
3(a) and (b) respectively. CV curves of (3) in their 1st and 100th cycle are overlapping one over the
other indicating the reversible and stable cation radical formation. Formation of stable cation
radical can be attributed to the resonance stabilisation of cation radical by the tolyl group
substituent. There is gradual decrease in the peak current of (4) while moving from 1st cycle to
100th cycle indicating the relatively unstable nature of the carbocation formed 22. This is possibly
due to the absence of resonance stabilization by the N-ethyl group. Hence, an aromatic substituent
is the most suitable moiety for stabilizing the cation radical formed. Nitrogen of carbazole moiety
is capable of showing both inductive and mesomeric effect by virtue of being electronegative and
possessing lone pair of electron on it, respectively. Although the electronegative N atom in (3)
destabilizes the cation formed, the cation radical is stabilized more through mesomeric effect off-
setting the inductive effect. Substitution of tolyl group at the 9th position stabilizes the cation
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formed more compared to substitution by ethyl group further by resonance effect. Also,
substitution of more electron withdrawing group like tolyl group, imparts higher E1/2 (by 50 mV
s-1) than that of ethyl group as can be seen from Fig. S13 23. So, considering reversibility, stability
and higher E1/2 value of the redox couple, (3) was chosen as the catholyte material for
demonstrating an organic redox battery. The electrolyte containing 6 mM of (3) and 0.1 M TBAPF6
as supporting electrolyte was deaerated by N2 for 10 minutes to use as catholyte before starting the
experiment. Anolyte solution containing 0.1 M TBAPF6 was purged with O2 for 10 minutes before
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-.
starting the measurements. Fig. 4(a) and (b) shows the CV of redox couples O2 / O2 and (3) / (3+)
recorded at different scan rates for evaluating the diffusion coefficient of the respective redox
species. The anodic peak current of (3) / (3+.) and cathodic peak current of O2 / O2. redox couples
휈
were plotted against square root of scan rate ( ) as shown in Fig. 5(a) and (b) indicating diffusion
controlled mass transfer of the redox active species.
Fig. 4(a) shows the CV of (3) and (b) of O2/ TBAPF6 in in the 4:1 solvent mixture of ACN and
DCM at different scan rates.
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Fig. 5 (a) The plot of peak current vs. (scan rate)1/2 for (3) and (b) O2/ TBAPF6 in the 4:1 solvent
mixture of ACN and DCM.
Redox kinetics
Redox kinetics of charge transfer reaction at the electrode-electrolyte interface can be determined
based on Koutecky-Levich equation given by {1},
1
1 = + 1
{1}
푖
푖푘
푖푙
Where i is the total current, ik is the kinetically controlled current and il is the limiting current.
1
1
1 = +
{2}
0.62 푛 퐹 퐴 퐷2/3휔1/2 ―1/6 퐶0
푖
푖푘
μ
where F is faraday’s constant (96485 C mol-1), is rate of rotation (rad s-1) of RDE and is
휔
μ
kinematic viscosity (cm2 s-1) of ACN : DCM (4 : 1) medium from {2}.
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Fig. 6 (a) Polarisation plot for 2 mM of (3) in 0.1 M TBAPF6 supporting electrolyte in the 4:1
solvent mixture of ACN and DCM at different rotation rate of RDE. (b) Koutecky-levich plot. (c)
Tafel plot and (d) Levich plot
Fig. 6(a) shows the polarization plots for 2 mM of (3) at different rates of rotations. Fig. 6(b)
shows linear relationship for the plot of i-1 vs. ω-1/2 for a series of over-potentials (η = E applied – E
onset) of 10 mV, 20 mV, 30 mV, 40 mV, 50 mV, 60 mV, 70 mV and 80 mV. Y-axis intercept at
1
( = 0 provides the
, from which ik, corresponding to different potentials are calculated. A
푖푘
plot of log ik against η provides the exchange current density value from the y-axis intercept at η
= 0 (Fig. 6(c)). The kinetic rate constant is obtained from {3}.
i0  n F A k0 C0
{3}
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4
where i0 is the exchange current density (A cm-2). The value of k0 is determined to be 1.12
10-
×
5
6
3
cm s-1. Using the RDE data, diffusion coefficient has been determined employing the Levich
7
8
9
equation {4}. The estimated kinematic viscosity is 4.39
10-3 cm2 s-1. The diffusion coefficient
×
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36
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40
41
42
43
44
45
46
47
48
49
50
51
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53
54
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57
58
59
60
determined using {4} is 1.8
10-5 cm2 s-1 which is comparable to the value reported using the
×
CV studies (1.3
10-5 cm2 s-1).
×
2
1
-1
3
2
6
i
l
 0.62 n F A D  C ν
{4}
×
D = 1.8
10-5 cm2 s-1
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1
2
3
4
Table. 1 Heterogeneous rate constant and diffusion coefficient for redox active organic molecules.
5
6
7
8
Sl No
Molecule
Electrode
Electrolyte
Heterogeneous
Diffusion
Reference
rate constant, k0 coefficient, D
(cm s-1)
(cm2 s-1)
7.3 – 8.2
5
1
Glassy carbon
1 M LiTFSI
in MeCN
×
0.9
10-2
10-6
×
9
O
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
O
O
2,5-di-tert-butyl-1-
methoxy-4-
[2′methoxyethoxy]benzene
N
S
N
7
2
3
Glassy carbon
Glassy carbon
1 M LiTFSI
in MeCN
×
×
1.0
10-2
1.5 -1.7
10-5
2,1,3-benzothiadiazole
O
24
×
×
1.32
10-2
1.01 – 1.62 10-
5
Benzophenone
O
25
4
Glassy carbon
0.2 M Li2SO4
×
2
10-3
8.8
10-6
×
O
Benzoquinone
+
26 27
VO2 / VO2+
Graphite
1 M H2SO4
1 M H2SO4
0.5 M NaCl
,
1.2 10-6
5
6
7
×
×
3
4
10-7
10-3
×
×
V3+ / V2+
Graphite
,
27 31
0.16 10-6
×
×
18
Glassy carbon
1.4
10-2
3.1
10-6
N
Cl
Fe
N
Cl
bis((3trimethylammonio)propyl)f
errocene dichloride
18
Cl
8
9
Glassy carbon
Glassy carbon
0.5 M NaCl
×
×
×
2.2
10-2
3.3
1.8
10-6
10-5
Cl
N
N
N
N
Cl
Cl
bis(3-trimethylammonio)
propyl viologen tetrachloride
Br
0.1 M
TBAPF6
This work
×
1.12
10-3
N
Br
3,6-dibromo-9-(p-
tolyl)-9H-carbazole
14

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8
9
From Table. 1, the diffusion coefficient and heterogeneous rate constant values determined for the
organic molecule (3) in this work are comparable with the values of organic molecules used as
battery materials and better that the vanadium counterparts used in VRFB. Such high values of D
and k0 are ideal for fabricating good redox flow battery.
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60
Reaction schemes
The reaction at the positive and negative electrodes along with cell reaction is given below.
E0 = -1.25 V vs Ag/Ag+
Charge
O2 + e-
{5}
N
+
N
O2
Discharge
Reaction at negative electrode
Br
Br
Br
Br
E0 = 1.09 V vs. Ag/Ag+
Charge
N
N
+ e-
{6}
Discharge
Reaction at positive electrode
Net cell Reaction
Br
Br
Br
Br
N
Charge
N
+
+
N
+ O2
{7}
N
Discharge
O2
3.2. Charge discharge
15

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2
3
4
-.
In this study, the O2/O2 redox couple was used as anolyte along with (3)/(3+) as the catholyte{5}
5
6
-.
{6}{7}. Earlier, our group has reported O2/O2 redox couple for non-aqueous battery in N,N-
7
8
9
dimethylformamide medium 20,29. Fig. 7(a) shows the comparison CV of (3) with the dissolved
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42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
O2. Both electrolytes show reversible nature of the electron transfer reaction.
Fig. 7(a) CV of 5 mM of (3) and O2/TBAPF6 (b) galvanostatic charge-discharge carried out at 2.4
mA cm-2 in the 4:1 solvent mixture of ACN and DCM.
From Fig. 7(a), an open circuit voltage of 2.34 V is expected, if we consider E1/2 of (3) / (3+) and
-.
O2/O2 . Fig. 7(b) shows galvanostatic charge-discharge of the cell carried out at 2.4 mA cm-2 for
60 cycles with a charging cutoff voltage of 2.4 V. Fig. 7(b) also shows the charge-discharge curves
of the cell without (3) in the catholyte. As can be seen, there is polarization both during charge and
discharge indicating absence of any faradaic (charge transfer) reaction in the electrolyte devoid of
(3). The discharge capacity of the cell containing (3) increased from 90 mAh L-1 in the 1st cycle to
134 mAh L-1 in the 40th cycle and stabilized around this value beyond 40th cycle. The trend of
increasing discharge capacity can be due to accumulation of radicals (due to difference in the
charge and discharge capacity). For 6 mM of (3), the volumetric capacity of the cell is 160 mAh
L-1. The cell reaches a maximum discharge capacity of 152 mAh L-1 in the 56th cycle with a
16

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6
percentage utilization (ratio of experimental discharge capacity to theoretical discharge capacity)
of the reactant and coulombic efficiency (CE) being 95% and 53 % respectively.
7
8
9
10
11
12
13
14
15
16
17
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23
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52
53
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59
60
The cell charges to a higher capacity than the theoretical capacity with a voltage plateau beyond
-
2.3 V due to regeneration of (3) from (3+) upon its reaction with crossed-over O2 {8}.. Since a
porous separator is used, the cross-over becomes inevitable. It is to be noted here that O2 supply to
the anolyte is unlimited due to continuous exposure to O2. In order to explore the stability of (3)
in the presence of O2, the positive electrolyte was saturated with oxygen and 10 CV cycles were
performed to capture the redox signature of the (3) at 50 mV s-1 as shown in Fig. 8. Following this
experiment the working electrode was held at -1.24 V for 5 minutes in order to produce superoxide
in-situ. To study the reaction between superoxide and (3), a CV was recorded again to capture the
redox signature of the carbazole. Due to the reaction of superoxide with (3), the redox current of
the (3) has reduced which probably indicates decomposition of the carbazole. To confirm the loss
in redox activity is due to the reaction of superoxide with (3), the above sequence of experiment
was repeated saturating with N2 instead of O2 and there was no change in the redox activity of (3).
17

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7
8
9
10
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23
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50
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52
53
54
55
56
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58
59
60
Fig. 8 CV of 1st and 10th cycles of (3) before and after chronoamperometry (CA) for 300 s with
oxygen saturated electrolyte. Inset shows the CA plot for 300 s.
The cell suffers an irreversible capacity loss of 55 mAh L-1and 63 mAh L-1 at 40th and 50th cycle
numbers. Discharging the cell at 2.4 mA cm-2 leads to considerable amount of polarization due to
concentration polarization associated with low concentration of (3) and high ohmic drop of the
electrolyte due to its poor conductivity. To identify the sluggish reaction, electrochemical
impedance spectroscopy (Nyquist plot) of 5 mM solution of (3) and oxygen saturated ACN:DCM
(V/V 4:1) solutions were recorded at 1.09 V and -1.24 V vs. Ag/Ag+ , respectively as in Fig. 9.
Fig. 9 Nyquist plot of anolyte and catholyte along with fitted circuits in the inset.
18

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9
In the positive electrolyte, there is a semicircle corresponding to the charge transfer process
occurring at high frequency region and a sloped straight line in the low frequency region
corresponding to the diffusion of the (3). An equivalent circuit shown in Table 2 is used to fit the
Nyquist plot. A charge transfer resistance (RCT) of 44  was observed with a Warburg admittance
(Y) value of 2.6 x 10-3 mho s1/2 correspond to the diffusion characteristics of (3). Y is related to
10
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23
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29
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31
32
33
34
35
36
37
38
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42
43
44
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49
50
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54
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59
60
Warburg impedance
as shown in equation {9}, wherein ~ 0.16 Hz.
푍푊
휔
1
{9}
푍푊 =
푌.휔1/2
Similar analysis performed at the negative electrolyte indicates presence of two semicircles, one
probably corresponds to superoxide and the other correspond to the peroxide formation. RCT of
high frequency semicircle is 73.7  and the mid frequency one is 252 . Y of the dissolved oxygen
species is 2.6 x 10-3 mho s1/2. The solution resistance Rs was nearly same for both positive and
negative electrolyte as they contain same supporting electrolyte. Comparing the RCT values of the
positive and negative electrolyte, O2 / O2. redox couple seems to be limiting the performance of
the battery.
Table 2 Shows the parameters of Nyquist plot for anolyte and catholyte.
Electrolyt
e
RS (Ω) / RCT1 (Ω) Y1(mho×sn)
n1 /
%
Error
RCT2
(Ω) / %
Error
Warburg, Y
(mho×s1/2) /
% Error
C2 (F) / %
Error
2
% Error
/ %
/ % Error
Error
Negative 7.6 × 10-4
37.8/
1.9
73.74/
2.15
1.65× 10-5/
12.8
0.87 /
2.2
252.6/
2.28
2.6 × 10-3/
3.5
1.3 × 10-7/
3.15
Positive
3.3× 10-4
41.7/
1.1
44.3/1.2
-
-
-
3.2 × 10-3/
0.82
2.3 × 10-7/ 3.0
19

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9
Fig. 10 compares the charge and discharge capacity of the cell with respect to cycle number and
inset plot shows the coulombic efficiency of the cell measured at 2.4 mA cm-2. In general there is
a continuous increase in the discharge capacity of the cell as shown in Fig. 10. Enhancing the
solubility of the analyte and conductivity of the non-aqueous medium is paramount in realizing
battery with lesser overpotential and better performance. In order to improve the solubility, 3,6-
dibromo-9-phenyl-9H-carbazole may be functionalized in the para position (which is not
electrically in contact with the redox part of the carbazole) with methoxy and cyano functionality
which have high affinity for solvents such as ACN, DCM hence improved specific capacity of the
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50
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52
53
54
55
56
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59
60
Fig. 10 Capacity retention plot of the cell. Inset shows the coulombic efficiency with respect to
cycle number.
electrolyte. Improving solubility by suitable structural modification of organic core will lead to
enhancement in the capacity of the cell. Our aim in this work is to exemplify the working of
carbazole derived catholyte material in a flow battery, for which we have chosen oxygen as the
counter electrode material. The other materials which can be used in place of O2 are 9-fluorenone,
benzophenone and N-substituted phthalimides. Our future studies are in the above said directions.
In addition, finding a suitable ion-conducting membrane would reduce the crossover of the active
20

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species, thereby limiting the charging capacity and extending the discharge capacity to theoretical
capacity.
7
8
9
4. Conclusions
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We have designed an organic molecule capable of showing reversible redox behaviour at higher
potential thereby opening an avenue for better energy density of the battery. We have fabricated a
cell using (3) and O2/TBAPF6 as the catholyte and anolyte, respectively in the solvent mixture of
ACN : DCM (4 : 1). Labile positions of the carbazole core are suitably substituted to enhance the
redox potential and show good reversible redox behavior. This combination of electrode materials
offer an open circuit voltage of ~ 2.3 V. The diffusion coefficient and heterogeneous rate constant
×
of 1.8
10-5 cm2 s-1 and 1.12
10-3 cm s-1, respectively, for the (3), is on par with those already
×
reported in the literature. We believe that the concentration of analyte and its diffusion will play
vital role in bringing down the polarization rather than concentration of the supporting electrolyte
at the current densities employed in this work. Hence, improving solubility of the analytes would
be way forward to improvise this battery performance.
Acknowledgements
Authors thank IIT Madras for the infrastructure and Dr. Ramesh Gardas for the viscosity
measurement in his laboratory. The authors also thank Dr. P Venkatakrishnan and Mr. Sudhakar
for their valuable support and to author RM in completing the rebuttal amid the Covid-19
shutdown. SERB, DST, New Delhi (EMR/2016/004321) and CSIR (09/084(0695)/2016-EMR-I)
is gratefully acknowledged for funding and fellowship respectively. Author RM also acknowledge
DST-SERB-NPDF for the NPDF grant (No. PDF/2017/001756).
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