Evaluation of ionic liquids as electrolytes for vanadium redox flow batteries

Article abstract of DOI:10.1016/j.molliq.2020.114017

Non-aqueous redox flow batteries (NARFBs) are promising electrochemical energy storage devices due to their wide electrochemical potential windows, generally >2 V of organic solvents. This study aims to investigate the suitability of ionic liquids (ILs) as electrolytes for NARFBs containing a vanadium metal complex. The electrochemistry of a single-component NARFBs employing vanadium (III) acetylacetonate (V(acac)₃) was studied in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C₄mim][NTF₂], and 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, [C₄mpyr][NTF₂], electrolytes. The electrochemical kinetics of the anodic and cathodic reactions was measured using cyclic voltammetry. The V^II/V^III and V^III/V^IV couples were quasi-reversible and together yielded a cell potential of 2.2 V in both ILs. Charge/discharge characteristics show that a coulombic efficiency for cycles 1–50 ranged from 88 to 92% using a V(acac)₃/[C₄mpyr][NTF₂] cell.

Full text of DOI:10.1016/j.molliq.2020.114017

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Evaluation of ionic liquids as electrolytes for vanadium redox
flow batteries
L. Bahadori, R. Boyd, A. Warrington, M.S. Shafeeyan, P.
Nockemann
PII:
S0167-7322(20)33697-7
DOI:
https://doi.org/10.1016/j.molliq.2020.114017
MOLLIQ 114017
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
8 June 2020
20 July 2020
6 August 2020
Please cite this article as: L. Bahadori, R. Boyd, A. Warrington, et al., Evaluation of ionic
liquids as electrolytes for vanadium redox flow batteries, Journal of Molecular Liquids
(2020), https://doi.org/10.1016/j.molliq.2020.114017
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Evaluation of ionic liquids as electrolytes for vanadium
redox flow batteries
L. Bahadori a*, R. Boyd b, A. Warrington c, M. S. Shafeeyand, P. Nockemann b,
a Department of Chemical and Biomolecular Engineering, University of California, Los Angeles, CA 90095,
United states
b School of Chemistry and Chemical Engineering, The QUILL Research Centre, Queen's University Belfast,
Belfast BT9 5AG, United Kingdom
c Institute for Frontier Materials, Deakin University, Burwood, Victoria, 3122, Australia
d Department of Chemical Engineering, Faculty of Engineering, Golestan University, Aliabad Katoul, Iran
*Corresponding author:
Laleh Bahadori
Email: lalehbahadori@g.ucla.edu
Abstract
Non-aqueous redox flow batteries (NARFBs) are promising electrochemical energy storage
devices due to their wide electrochemical potential windows, generally >2 V of organic
solvents. This study aims to investigate the suitability of ionic liquids (ILs) as electrolytes for
NARFBs containing a vanadium metal complex. The electrochemistry of a single-component
NARFBs employing vanadium (III) acetylacetonate (V(acac)3) was studied in 1-butyl-3-
methylimidazolium bis(trifluoromethylsulfonyl)imide, [C4mim][NTF2], and 1-butyl-1-
methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, [C4mpyr][NTF2], electrolytes. The
electrochemical kinetics of the anodic and cathodic reactions were measured using cyclic
voltammetry. The VII/VIII and VIII/VIV couples were quasi-reversible and together yielded a
cell potential of 2.2 V in both ILs. Charge/discharge characteristics show that a coulombic
efficiency for cycles 1–50 ranged from 88-92% using a V(acac)3/[C4mpyr][NTF2] cell.
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Keywords: Redox flow battery; ionic liquid; all-vanadium electrolyte; electrochemistry; non-
aqueous electrolytes
1. Introduction
For the large amounts of energy produced by intermittent renewable sources such
as wind, solar, and hydroelectric to be efficiently used in grid-scale stationary applications of
the future, capable energy-storage solutions are essential to withstand power supply
fluctuations [1-3]. One potential solution is a redox flow battery (RFB), which can store
electrical energy through the electrochemical reactions of two redox couples in the
electrolytes [4, 5]. RFBs have attractive characteristics like decoupled energy storage and
power generation [6], and low self-discharge rates [7], therefore making them promising for
utility-scale market and long-duration energy storage [8].
The most crucial components of a RFB electrolyte are the redox-active materials
and the ionic conductors. Non-aqueous RFBs which are being developed as potentially
reliable candidate to aqueous RFBs [9, 10], use organic solvents or ionic liquids (ILs)
electrolytes that have broader stability windows, wider range of working temperature, and
higher energy densities than that of water [11, 12]. Since the rudimentary concept of non-
aqueous RFBs was proposed by Singh in 1984 [13], a large range of redox mediators have
been invented and studied, based on metallocene (Ferrocene and cobaltocene) [14],
metalbipyridyl complexes (Ru(bypridine)3[BF4]2 [9], [Ni(bypridine)3][BF4]2,
[Fe(bypridine)3][BF4]2 [15, 16], metal-acetylacetonate complexes (Mn(acac)3, V(acac)3,
Cr(acac)3) [17-19], Ti-ion complexes [20] and metal-free organic redox molecules [21].
In the selection of a competitive material for alternative redox couples, the active species
must be cost-effective and have the same number of rapid and reversible redox reactions with
no undesired electrochemical reactions, on both sides of the cell, within the voltage window
of the battery. Furthermore, for high energy density the open circuit potential (the
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electrochemical potential) should feature a significant voltage. Ultimately, the active species
should tend to have high solubility with the respective solvents. It should be noted that there
is a limited number of active species compatible with non-aqueous electrolytes with fast
reaction kinetics and high solubility [11, 22, 23].
Despite the benefits offered by organic compounds like acetonitrile to NARFBs,
they have some major obstacles like flammability, toxicity, and low solubility of redox
species to achieve their practical use. ILs have been exploited as non-aqueous electrolyte
solutions due to their electrochemical and thermal stabilities, intrinsic conductivity, low
volatility, and the large electrochemical window [24, 25]. The large amount of possible
anion/cation combinations offers various possibilities for the synthesis of ILs with specific
properties. The incorporation of functional groups can impart certain capabilities to an IL,
such as the ability to interact with a metal centre and lead to an enhanced solubility for metal
salts [26]. In comparison with conventional vanadium systems with a large quantity of
electrolytes consisting of sulfuric acid [27, 28], ILs are not corrosive, allow less usage of
robust materials in the RFB system, and reduce environmental impacts. Therefore, a proof-
of-concept is proposed that ILs can be employed as the non-aqueous electrolyte in RFBs.
However, only a few IL-based systems have been reported so far [29-32].
We investigated non-aqueous RFB systems based on 1-butyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide, [C4mim][NTF2], and 1-butyl-1-methylpyrrolidinium
bis(trifluoromethanesulfonyl)imide, [C4mpyr][NTF2], electrolytes. Among a variety of
aprotic ILs, [NTF2]-based ILs are preferable electrolytes for the metal complexes, since the
electron density in the anion is disassociated over the ion structure, reducing its ability to
coordinate to the metal centre resulting in stable formation of a metal complexes without
liberation of the ligands.
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Herein, we describe the potential application of these ILs in a single-element NARFBs
system contained of vanadium acetylacetonate (V(acac)3) active species. The results acquired
from our system are compared with acetylacetonate complex studies in the literature in order
to obtain trends that can be used for the design of non-aqueous electrolytes for RFBs. The
cell potential and the electrochemical kinetic behaviour of V(acac)3/ILs system were assessed
using a cyclic voltammetry (CV) technique. The charge/discharge measurements were
recorded in a flow battery.
2. Experimental Section
2.1. Preparation and characterization of electrolyte
1-methylpyrrolidine (C5H11N), 1-methylimidazole (C4H6N2), 1-bromobutane (CH3(CH2)3Br),
Lithium bis(trifluoromethylsulfonyl)amide (LiN(CF3SO2)2), dichloromethane (CH2Cl2),
acetonitrile (CH3CN) were purchased from Sigma-Aldrich with high purity (≥98%). All
chemicals were used as received and stored in an inert glove box purged with argon to avoid
further contamination with atmosphere humidity.
1-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C4mpyr][NTF2])-
1-methylpyrrolidine (47.94 g, 563.02 mmol) and 1-bromobutane (89.32 g, 651.88 mmol)
were heated in acetonitrile at 50 °C for 45 h . The solvent was evaporated and the resulting
solid was washed with diethyl ether (3 x 200 cm3) and filtered through a Büchner funnel. The
product was then dried in vacuo at 50 °C for 24 h to yield 1-butyl-1-methylpyrrolidinium
bromide ([C4mpyr]Br) as a white solid (111.87 g, 89.4 %). Found: C, 48.15; H, 8.41; N, 6.08;
S, <0.3%. Calc. for C9H20NBr: C, 48.66; H, 9.07; N, 6.30; S, <0.3%. ESI-MS: +ve mode: m/z
142.15 (M⁺ , 100%); calc. for C9H20N: 142.16. -ve mode: calc. for Br: 78.91. 1H NMR
(MeOD, 400 MHz):  1.05 (t, -CH2-CH3, 3H), 1.46 (sx, -CH2-CH3, 2H) 1.82 (p, N-CH2-CH2-
, 2H), 2.26 (m, -CH2-CH2-CH2-CH2-, 4H), 3.10 (s, -N-CH3, 3H), 3.40 (t, N-CH2-CH2-, 2H),
3.57 (m, -CH2-CH2-CH2-CH2-, 4H).
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[C4mpyr]Br (51.91 g, 233.65 mmol) was dissolved in dichloromethane (100 cm3). Lithium
bis(trifluoromethylsulfonyl)amide (73.10 g, 254.63 mmol) was dissolved in deionised water
(100 cm3) and the solution was slowly added to the stirring solution of [C4mpyr]Br. The
mixture was stirred at room temperature for 19.5 h. The resulting two layers were separated
and the lower ionic liquid phase was washed with water (3 x 30 cm3) before being over
anhydrous sodium sulfate. The solvent was then removed using a rotary evaporator and the
product was then dried in vacuo at 50 °C for 24 h, to yield 1-butyl-1-methylpyrrolidinium
bis(trifluoromethylsulfonyl)imide ([C4mpyr][NTF2]) as a yellow liquid (15.89 g, 94%).
Found: C, 31.37; H, 5.40; N, 6.42; S, 15.17%. Calc. for C11H20F6N2O4S2: C, 31.28; H, 4.77;
N, 6.63; S, 15.18%. ESI-MS: +ve mode: m/z 142.15 (M⁺ , 100%); calc. for C9H20N: 142.16.
-ve mode: m/z 279.91 (M-, 100%); calc. for C2F6NO4S2: 279.92. 1H NMR (MeOD, 400
MHz):  1.03 (t, -CH2-CH3, 3H, JHH= 14.72 Hz), 1.43 (sx, -CH2-CH3, 2H, JHH= 7.44 Hz),
1.80 (p, N-CH2-CH2-, 2H, JHH= 7.97 Hz), 2.23 (m, -CH2-CH2-CH2-CH2-, 4H), 3.07 (s, -N-
CH3, 3H), 3.37 (t, N-CH2-CH2-, JHH= 8.48 Hz, 2H), 3.54 (m, -CH2-CH2-CH2-CH2-, 4H).
13C{1H} NMR (MeOD, 1024 MHz):  12.51, 19.44, 21.21, 25.34, 49.00, 64.02.
1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim][NTF2])-
1-methylimidazole (15.28 g, 186.16 mmol) and 1-bromobutane (30.61 g, 223.41 mmol) was
heated in acetonitrile at 45 °C for 17 h. The solvent was then evaporated and product washed
with diethyl ether (3 x 100 cm3), which was then decanted off, and the product dried in vacuo
at 50 °C for 24 h to yield 1-butyl-3-methylimidazolium bromide ([C4mim]]Br) as a colourless
liquid (39.92 g, 97.9 %). Found: C, 43.33; H, 6.80; N, 12.78; S, <0.30%. Calc. for
C8H15BrN2: C, 43.85; H, 6.90; N, 12.78; S, 0.00%. ESI-MS: +ve mode: m/z 139.12 (M+,
100%); calc. for C8H15N2: 139.12. -ve mode calc. for Br: 78.91. 1H NMR (CDCl3, 400 MHz):
 0.96 (t, -CH2-CH3, 3H, JHH= 7.30 Hz), 1.39 (sx, -CH2-CH3, 2H, JHH= 7.44 Hz), 1.86 (p, N-
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CH2-CH2-, 2H, JHH= 7.44 Hz), 4.13 (s, -N-CH3, 3H), 4.35 (t, -N+-CH2-CH3-, 2H, JHH= 7.30
Hz), 7.51 (s, -N-CH=CH-N+-, 1H), 7.63 (s, -N-CH=CH-N+-, 1H), 10.35 (s, 1H).
[C4mim]Br (22.98 g, 104.87 mmol) was dissolved in dichloromethane (40 cm3). Lithium
bis(trifluoromethylsulfonyl) imide (33.10 g, 115.30 mmol) was dissolved in deionised water
(60 cm3) and the solution was slowly added to the stirring solution of [C4mim]Br. The
mixture was stirred at room temperature for 16 h. The lower ionic liquid phase was then wash
with water (3 x 30 cm3) before being over anhydrous sodium sulfate. The solvent was then
evaporated and the product was then dried in vacuo at 50 °C for 24 h, to yield 1-butyl-3-
methylimidazolium bis(trifluoromethylsulfonyl)imide [C4mim][NTF2] as a colourless liquid
(37.47 g, 91.8%). Found: C, 29.25; H, 3.19; N, 10.27; S, 14.79. Calc. for C10H15N3O4S2F6: C,
28.64; H, 3.61; N, 10.02; S, 15.29%. ESI-MS: +ve mode: m/z 139.11 (M+, 100%); calc. for
C8H15N2: 139.12. -ve mode: m/z 279.91 (M-, 100%); calc. for C2NO4S2F6: 279.92. 1H NMR
(MeOD, 400 MHz):  0.99 (t, -CH2-CH3, 3H, JHH= 7.36 Hz), 1.37 (sx, -CH2-CH3, 2H, 2H,
JHH= 7.72 Hz), 1.89 (p, N-CH2-CH2-, 2H, JHH= 9.72 Hz), 3.94 (s, -N-CH3, 3H), 4.20 (t, -N+-
CH2-CH3-, 2H, JHH= 7.34 Hz), 7.56 (t, -N-CH=CH-N+-, 2H, JHH= 1.72 Hz), 7.61 (t, -N-
CH=CH-N+-, 2H, JHH= 3.52 Hz ), 8.88 (s, -N-CH=N-, 1H). 13C{1H} NMR (MeOD, 1024
MHz):  13.67, 20.41, 33.05, 36.40, 50.57, 122.80, 123.62, 124.91.
2.2. Voltammetric measurements
All electrochemical experiments were performed under potentiostatic control using the
conventional electrochemical cell comprised of three-electrode set-up. Cyclic voltammetry
(CV) measurements were collected using a GC working electrode (3 mm diameter,
Metrohm), Pt rod counter electrode (2 mm diameter, Metrohm), and an Ag/Ag+ fritted
reference electrode with 3 M fill solution (BASi). All electrode surfaces were cleaned and
rinsed thoroughly with deionized water, and the working electrode was carefully polished
before each voltammetry experiment with 0.3 µm alumina suspension (Wirth Buehler),
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ultrasonically cleaned in distilled water to ensure complete removal of the alumina particles.
The assembly of the electrochemical cell was carried out inside a Faraday cage, which was
placed within the dry argon-filled glove box. The experiments were conducted using a
Potentiostat/Galvanostat (Autolab PGSTAT302N, Ecochemie, Netherlands) managed by a
Nova software. The system electrochemical behaviour was acquired by conducting a series of
various potential steps and storing the current-time response curves. The potential typically
changed linearly vs. time at different scan rates (ν) from 10 to 300 mVs-1 using conventional
electrodes. The approach allowed both oxidation and reduction processes to be evaluated
since the electrode polarization was reversed on the reverse scan.
2.3. Vanadium redox flow battery cell assembly
A single split unit vanadium redox flow cell (MTI Corporation., USA) with 25 cm2 active
area was utilized as the small-scale NARFBs test cell. A zero-gap configuration of electrolyte
flow in the flow cell resulted in the current collectors, membrane, and electrodes being in
direct contact. Fig.1 shows the configuration of a lab-scale NARFBs single cell experimental
set-up. Bipolar plates without flow field were employed to distribute the non-aqueous
electrolyte over the entire active area of the cell and to conduct current to the Cu current
collector plate. These flexible plates with 0.7 mm in thickness have similar performance but
offer higher resistance to breakage and have less defects compared to conventional graphite
plate. AMI 7001(Membranes International Inc, USA) served as the membrane. The positive
and negative electrodes comprised of graphite felt (1.12 mm thickness, Alfa Aesar, USA) and
Teflon-gaskets were utilized to attain the required electrode compression. The electrodes and
membrane were immersed in the test electrolyte before cell assembly for 22 h. Electrolyte
reservoir were filled with non-aqueous electrolyte, which was pumped through the flow cell
at 15 mL min-1 using a dual channel peristaltic pump (MTI Corporation., USA) with Tygon
tubing (Saint-Gobain, 1.6 mm inner diameter).
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Fig. 1. (a) Configuration of a single cell lab-scale NARFBs experimental set-up i: Computer,
ii: Battery analyser, iii: Single cell, iv: Compressing jig, v: Peristaltic pump, vi: reservoirs, (b)
Cut-away view of the half cell 1: Manifold Block, 2: Current collector, 3: Bipolar Plate, 4:
Gasket, 5: Frame, 6: Gasket 7: Graphite felt, 8: Membrane.
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2.4. Flow cell measurements
For charge-discharge characteristics, the negative and positive test electrolyte contained of a
solution of 0.1 M (V(acac)3, 98% Aldrich, US) dissolved in [C4mim][NTF2]. V(acac)3 was
stored under argon atmosphere as received and used without further drying processes or
purification. Each electrolyte reservoir consisted a volume of 125 mL. The test electrolyte
solution and the NARFBs cell were individually purged with ultra-high purity N2 prior to any
experiments for 30 min. During the flow cell operation, the catholyte and anolyte were
carried into their own chamber and then back to the feed reservoirs at ambient pressure and
temperature. Galvanostatic charge/discharge cycling tests were carried out using a multi-
channel battery testing system (BST8-WA, MTI Corporation., USA). During the
charge/discharge experiments, potential cut-off was 2.2 V for charging and 0 V for discharge.
Upper voltage cut-off was taken to remove accessing undesirable electrochemical reactions,
while lower voltage cut off was set to enable of the cell to completely discharge.
3. Result and discussion
3.1. Voltammetric behaviour of V(acac)3 in [C4mim][NTF2] and [C4mpyr][NTF2]
Fig. 2(a) indicates the CV recorded at GC for a solution comprising 0.01M V(acac)3 in
[C4mim][NTF2] and [C4mpyr][NTF2] cycled at 100 mVs-1. Two well-defined redox couples
were detected within the solvents potential window, at -1.48 V and 0.72 V for
[C4mim][NTF2] and -1.59 V and 0.61 V for [C4mpyr][NTF2] vs. Ag/Ag+ that are attributable
to the VII/VIII and VIII/VIV couples, respectively. However, absolute values of redox potentials
are difficult to compare with voltages reported in literature due to variations in the different
solvents and the reference electrodes. The potential difference, ΔE, between the VII/VIII and
VIII/VIV couples revealed that a NARFBs system based on the one-electron disproportionation
of V(acac)3 would yield a 2.2 V cell potential which compares to values of 2.18 V and 2.01 V
in acetonitrile and ethaline, respectively [18].
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(a)
(b)
Fig. 2. (a) CV recorded at a GC electrode in 0.01 M V(acac)3 in [C4mim][NTF2] and
[C4mpyr][NTF2] at a scan rate of 10 mV s-1; (b) potential windows of [C4mim][NTF2] and
[C4mpyr][NTF2].
Given the similarities, the redox couples observed in Fig.2(a) were attributed to the following
reactions [33]:
R1/O1 : VΙΙΙ(acac)3+ e-↔[VΙΙ(acac)3]-
(1)
R2/O2 : VΙΙΙ(acac)3↔[VIV(acac)3]++ e-
(2)
Formal potentials for each half-reaction were determined by calculating the average of the
cathodic and anodic peak potentials from the CV. There is also a third redox couple, labelled
R3/O3 at 0.85 V (vs. Ag/Ag+) can be seen in Fig. 2(a) which may attribute to the oxidation of
VIV to VV or oxidation of the V(acac)3 species, VO(acac)2 or some other species produced
during the reaction [33]. The electrochemical stability of the ILs was analysed by conducting
CV using a GC working electrode at room temperature and a scan rate of 10 mV s−1. The
cathodic and anodic limiting potentials for the ILs are shown in Fig 2(b). The anodic potential
window of ILs is mainly ascribed to the oxidation potential of the anionic part, while the
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cathodic potential window of ILs is affected by the reduction of the cation. The
electrochemical stability of V(acac)3 and the screened PWs of ILs indicated that VII/VIII and
VIII/VIV couples can be detected within the stability window.
3.2. Kinetics of electrode reactions with ILs
Fig.3 indicates the CVs recorded for V(acac)3/[ C4mpyr][NTF2] system at various scan rates
(focusing on the VII/VIII and VIII/VVI couples) using GC electrode at room temperature. In the
range 30 mV s-1 ≤ ν ≤ 150 mV s−1, the anodic to cathodic peak currents ratio, ipa/ipc, was close
to unity. The peak-to-peak separation, ΔEp, enhanced from 60 - 101 mV and 60-81 mV as the
scan rate was increased from 10 to 300 mV s−1 for the VII/VIII and VIII/IV redox couples,
respectively, suggesting that the electron transfer kinetics affected the response at higher scan
rates and kinetics were homogenous and/or that there were other complications in the
electrode processes.
Fig. 3. CVs recorded at a GC electrode in 0.01 M V(acac)3 in [C4mpyr][NTF2] at various
scan rates from 30-150 mV s-1.
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Given the quasi-reversible nature of the reactions is recorded by ΔEp >59 mV and ratio of
the peak current for oxidation and reduction components (ipa/ipc) of 1, and given that the peak
current density was linearly varying with the square root of the scan rate (v0.5) [34], Eq. (1-3)
can be used to determine the diffusion coefficients (D) and the standard heterogeneous rate
constants (k) of the active species. For the irreversible reactions (Eq. 1, and Eq. 2) plotting jp
versus v0.5 gives a straight line with slope proportional to the D of the active species, and
plotting ln jp vs. (Ep - E0) gives a slope proportional to transfer coefficient and an intercept
proportional to k [35, 36].
0.5
(1)
(2)
αF
Jp=4.958×10-4n1.5FC0D0.5v0.5
[
]
RT
-αnaF(Ep-E0)
RT
Jp=2.27×10-4nFC0k exp [
]
Where jp is the peak current density (A), n is the number of electron equivalents exchanged in
the electrode reaction, F is Faraday's constant (C mol-1), C0 is the bulk concentration of
primary reactant (mol cm-3), α is the transfer coefficient (under irreversible conditions, D is
the diffusion coefficient of the electroactive species (cm2 s-1), the transfer coefficient is
assumed to be 0.5 for a one-electron process, Ep is the peak potential (V), T is the absolute
temperature of electrolyte (K), ν is the voltage scan rate (V s-1), k is the standard
heterogeneous rate constants( cm s-1), R is the universal gas constant (J mol-1 K-1), and E0 is
the formal potential of the electrode (V).
As VII/VIII and VIII/VIV couples have shown quasi-reversible kinetics in ILs, the value for the
D of V(acac)3 would be located between the results calculated from reversible and
irreversible reactions.
For a reversible reaction, the peak current ip (A) is indicated by the Randles-Sevcik equation
[34]:
ip = 0.4463(nF)3/2(RT)−1/2AD1/2C0ν1/2
(3)
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The D value of VII/VIII and VIII/VIV redox couples measured in [C4mim][NTF2] and
[C4mpyr][NTF2] are shown in Table 1. They are an order of magnitude lower than in the
aqueous and acetonitrile systems due to lower viscosities of them, which were calculated as
5.7 × 10−6 cm2 s-1 [37] and 1.8 × 10−6 cm2 s-1 [33], respectively. These results are comparable
to that of V(acac)3 redox couples in deep eutectic solvents [18]. The low mass transport due
to the high viscosity of ILs could mitigated by evaluating operational temperature of RFB as
viscosity of ILs is temperature dependant [24]. Viscosities of selected ILs in the measured
temperature range from 293.15 to 353.15 K reduced 55-10 mPa.s for [C4mim][NTF2] and 76-
15 mPa.s in the case of [C4mpyr][NTF2] [38].
Table 1. Diffusion coefficients (D) and kinetic parameters of VIII/VVI and VII/VIII redox
VII/VIII
VIII/VIV
D / cm2 s-1
k / cm s-1
D / cm2 s-1
5.28× 10−8 (± 0.48) 5.41 x104(±0.87)
k / cm s-1
ILs
[C4mim][NTF2] 7.35 × 10−8 (±0.64) 2.24 x10-4(±0.59)
[C4mpyr][NTF2] 1.98 x10-7 (± 0.83) 8.00 x10-4(±0.76)
6.38 x10-8 (± 0.27) 1.61 x103(±0.94)
couples in [C4mim][NTF2] and [C4mpyr][NTF2] at a GC.
3.3 Charge/discharge performance of V(acac)3/[C4mpyr][NTF2]
Charge/discharge characteristics recorded in a lab-scale flow cell consisting 0.01 M V(acac)3
as the active species in [C4mpyr][NTF2] system were illustrated in Fig. 4a. Galvanostatic
conditions were employed with voltage cut-offs for both charge and discharge. Constant
current of 1 mA and 0.1 mA were utilized for charging and discharging, respectively. A
lower discharging current was employed to reduce over-potentials and ensure complete
discharge of the system.
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(a)
(b)
Fig. 4. (a) Charge/discharge curves for 0.01 M V(acac)3 in [C4mpyr][NTF2]. The charging
current was 1 mA and the discharging current was 0.1 mA. (b) Efficiencies as a function of
cycles.
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The charge and discharge potentials were respectively higher and lower than the expected
cell voltage determined by the voltammetry for the single-electron disproportionation of
V(acac)3 in [C4mpyr][NTF2]. The low discharge voltage may be due to ohmic effects;
particularly, the distance between the two electrodes in the flow cell could contribute to a
high cell resistance.
The voltage efficiency (VE), energy efficiency (EE), and coulombic efficiency (CE) of the
flow cell were estimated using Eq. (4-6):
IDtD
CE=
×100%
(4)
(5)
ICtC
EE=CE×VE
VD
VE=
×100%
(6)
VC
Where IC, ID, tC, and tD are charge and discharge currents and times and Vc is the average
charging voltage, VD is the average discharging voltage.
The coulombic efficiencies of 88-92% were obtained 50 cycles which are greater than
obtained values using a V(acac)3 in acetonitrile flow cell [18]. Fig. 4b shows there is no VE
and CE decay up to 50 cycles due to stability of AMI 7001 membrane in vanadium solution
at IL electrolyte condition and thus is capable to maintain the cell performance. As shown,
EE was reasonably consistent over the 50 cycles indicating that utilize of the voltage cut offs
limited the irreversible reductions.
Conclusion
The combination of V(acac)3 in [C4mpyr][NTF2] and [C4mim][NTF2] as non-aqueous
electrolytes is promising for the development of IL-based vanadium redox flow cell. The
cathodic and anodic half-cell reactions of V(acac)3 in [C4mpyr][NTF2] and [C4mim][NTF2]
have shown to be electrochemically quasi-reversible and the cell voltage for single-electron
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disproportion was 2.2 V. The charge/discharge performances for a simple
V(acac)3/[C4mpyr][NTF2] system NARFB were evaluated using a single unit vanadium redox
flow cell with an anion-exchange membrane separator. Energy and coulombic efficiencies of
the system were obtained as 88-92% and 61-71%, respectively. While the presented result
here illustrated the feasibility of V(acac)3/[C4mpyr][NTF2] as electrolyte for RFBs, the
dynamics of mass and charge transfer in ILs at different temperatures and flow cell
optimization must be considered.
Acknowledgements
The authors are grateful to the IERC Grant provided by the Queen’s University Belfast and
the International Energy Research Centre. LB shows special gratitude to Professor Carlos G.
Morales-Guio for allowing full access to facilities in University of California, Los Angeles.
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Authorship Statement:
The submission of the manuscript has been approved by all co-authors.
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Declaration of interests
Title: Evaluation of ionic liquids as electrolytes for vanadium redox flow batteries
Authors:
Laleh Bahadori *, Robert Boyd, Anna-Katrina Warrington, Mohammad Saleh
Shafeeyan, Peter Nockemann
*Corresponding Author:
Department of Chemical and Biomolecular Engineering,
University of California, Los Angeles, CA 90095,
United states
E-mail addresses: lalehbahadori@g.ucla.edu
☒ The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:
NA
Authorship Statement:
The submission of the manuscript has been approved by all co-authors.
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Highlights



Kinetic and electrochemical properties of V(acac)3 in ionic liquids is described.
[C4mpyr][NTF2] ionic liquid electrolyte stabilizes various Vn+ species.
Charge/discharge of a cell containing V(acac)3/[C4mpyr][NTF2] system shows CE of
92%.
21