A π-Conjugation Extended Viologen as a Two-Electron Storage Anolyte for Total Organic Aqueous Redox Flow Batteries

Article abstract of DOI:10.1002/anie.201710517

Extending the conjugation of viologen by a planar thiazolo[5,4-d]thiazole (TTz) framework and functionalizing the pyridinium with hydrophilic ammonium groups yielded a highly water-soluble π-conjugation extended viologen, 4,4′-(thiazolo[5,4-d]thiazole-2,5-diyl)bis(1-(3-(trimethylammonio)propyl)pyridin-1-ium) tetrachloride, [(NPr)₂TTz]Cl₄, as a novel two-electron storage anolyte for aqueous organic redox flow battery (AORFB) applications. Its physical and electrochemical properties were systematically investigated. Paired with 4-trimethylammonium-TEMPO (N^Me-TEMPO) as catholyte, [(NPr)₂TTz]Cl₄ enables a 1.44 V AORFB with a theoretical energy density of 53.7 Wh L^?1. A demonstrated [(NPr)₂TTz]Cl₄/N^Me-TEMPO AORFB delivered an energy efficiency of 70 % and 99.97 % capacity retention per cycle.

Full text of DOI:10.1002/anie.201710517

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Accepted Article
Title: A "π-Conjugation Extended Viologen" as Novel Two-Electron
Storage Anolyte for Total Organic Aqueous Redox Flow Battery
Authors: Jian Luo, Bo Hu, Camden Debruler, and Tianbiao Leo Liu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201710517
Angew. Chem. 10.1002/ange.201710517
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10.1002/anie.201710517
Angewandte Chemie International Edition
COMMUNICATION
A “π-Conjugation Extended Viologen” as Novel Two-Electron
Storage Anolyte for Total Organic Aqueous Redox Flow Battery
Jian Luo, Bo Hu, Camden Debruler, T. Leo Liu*
Dedicated to Professor Mei Wang on the occasion of her 62nd birthday
Abstract: Extending the conjugation of viologen by a planar
thiazolo[5,4-d]thiazole (TTz) framework and functionalizing the
pyridinium with hydrophilic ammonium group yielded a highly water-
soluble “π-conjugation extended viologen”, 4,4'-(thiazolo[5,4-
d]thiazole-2,5-diyl)bis(1-(3-(trimethylammonio)propyl)pyridin-1-ium)
air sensitive, which may affect the stability and energy efficiency
of battery.[4] The “extended viologens” are conjugation-extended
viologens in which two pyridinium moieties are linked by a
central π-conjugated framework.[5] With the extension of the
skeletal structure of 4,4’-bipyridinium of viologen, the reductive
potential gap between the 1st and 2nd electron reduction can be
reduced due to a larger π-conjugated system,[6] which would
enable the viologen-based batteries to deliver more uniform
battery voltages. More importantly, the stability of the reduced
species can be enhanced by the strategy of “π-conjugated
tetrachloride, [(NPr)2TTz]Cl4, as
anolyte for AORFB applications. Its physical and electrochemical
properties were systematically investigated. The [(NPr)2TTz]Cl4
a novel two-electron storage
/
NMe-TEMPO AORFB enables a 1.44 V battery voltage with 53.7
Wh/L theoretical energy density and delivered 70 % EE and 99.97 %
capacity retention per cycle battery stability.
extension”.[5a] The thiazolo[5,4-d]thiazole (TTz) is
a rigid
aromatic bicyclic framework that was widely applied in the
organic semiconductor, and it can be simply synthesized by
double condensation of aromatic aldehyde with dithiooxamide.[7]
The TTz backbone is a good extending framework for the
viologen due to its planar structure that is beneficial for the
With the advantages of decoupled energy and power, high
current and power performance, non-flammable and low cost
aqueous supporting electrolytes, as well as the tunable redox
potentials of the organic active materials, aqueous organic redox
flow batteries (AORFBs) have attracted increasing research and
technology development for large scale and dispatchable
storage (up to MW/MWh) of the intermittent renewable energy
including solar and wind energy.[1] In AORFBs, water-soluble
organic redox active materials or compounds were applied as
electrolyte materials. In the charge process, energy is stored by
the reduction of anolyte and oxidation of catholyte; in the
discharge process, the energy is outputted by the re-oxidation of
anolyte and re-reduction of catholyte. Organic compounds have
been used as electrolytes in AORFBs and non-aqueous organic
redox flow batteries (NAORFBs).[2] We and others have utilized
water-soluble viologen (anolyte),[2b, 2f-h, 2j] ferrocene (catholyte),[2g,
electronic communication of
“extended viologens”, N,N’-disubstituted
thiazolo[5,4-d]thiazole (TTz2+), were recently reported as long
life-time blue photo-luminescent materials.[8] Herein, we
a
conjugated system. The
dipyridinium
functionalize
the
2,5-di(pyridin-4-yl)thiazolo[5,4-d]thiazole
(Py2TTz) through rational molecular engineering to yield a new
water-soluble two-electron storage anolyte compound, 4,4'-
(thiazolo[5,4-d]thiazole-2,5-diyl)bis(1-(3-
(trimethylammonio)propyl)pyridin-1-ium)
tetrachloride,
[(NPr)2TTz]Cl4, for total organic ARFBs. The physical and
electrochemical properties of the as prepared [(NPr)2TTz]Cl4
were systematically studied. Paired with 4-trimethylammoinium-
TEMPO (NMe-TEMPO), the reliability of [(NPr)2TTz]Cl4 in
AORFB applications was demonstrated by full cell tests.
2h, 2j]
2f]
and TEMPO (catholyte)[2b,
compounds to demonstrate
high performance neutral AORFBs. Among the reported organic
compounds, only quinone or alloxazine based (anolyte)
compounds are capable of storing two electrons in AORFBs,
however, they have been applied in strong acidic or basic
AORFBs.[2a, 2c, 3] There is not a two-electron storage compound
that is applied in a neutral aqueous system. In addition, very few
total organic aqueous redox flow batteries with redox active
organic electrolytes in both anode and cathode sides were
reported.[2b, 2f, 2g, 2j, 3e]
In our previous research, the N,N’-dimethylated 4,4’-
bipyridinium (MV2+) showed reliable performance in AORFBs
application.[2b, 2g] So we first synthesized the N,N’-dimethylated
dipyridinium thiazolo[5,4-d]thiazole dichloride ((Me2TTz)Cl2) as
shown in Scheme 1. Unfortunately, due to the high rigidity and
hydrophobicity of the Py2TTz skeleton, (Me2TTz)Cl2 is poorly
water-soluble (< 10 mM in water). Functionality of highly
hydrophilic groups, such as ammonium, sulfonate, and
phosphate groups, has been suggested as an efficient strategy
to improve the water solubility of hydrophobic compounds.[2g, 9]
Herein, a highly hydrophilic functional group was applied to
functionalize the N atoms of the pyridine to overcome the
solubility issue of the TTz2+. Specifically, the Py2TTz was
functionalized with hydrophilic 3-(trimethylaminium)propyl (NPr)
pendant arms to get the water-soluble 4,4'-(thiazolo[5,4-
d]thiazole-2,5-diyl)bis(1-(3-(trimethylammonio)propyl)pyridin-1-
ium) tetrachloride ([(NPr)2TTz]Cl4). As displayed in Scheme 1,
Py2TTz was refluxed with (3-bromopropyl)trimethylammonium
bromide in DMF to precipitate the 4,4'-(thiazolo[5,4-d]thiazole-
2,5-diyl)bis(1-(3-(trimethylammonio)propyl)pyridin-1-ium)
Methyl viologen (MV2+) exhibits two single-electron
reductions at -0.45 V and -0.76 V (vs. NHE). However, only the
reversible MV2+/•+ redox couple was utilized in the battery
charge/discharge process, which is due to the insolubility of the
2f-h, 2j]
charge-neutral MV0 state in aqueous solution.[2b,
Meanwhile, the reduced states of MV (MV•+ and MV0) are highly
[a]
Dr. Jian Luo, Bo Hu, Camden Debruler, Dr. and Prof. Tianbiao Leo
Liu
Chemistry and Biochemistry
Utah State University
0300 Old Main Hill, Logan, Utah
E-mail: leo.liu@usu.edu
tetrabromide ([(NPr)2TTz]Br4). Then, anion exchange was
conducted to quantitatively convert [(NPr)2TTz]Br4 to the
chloride version [(NPr)2TTz]Cl4 to avoid the effect of redox
active bromine in the battery test. The synthesis of
Supporting information for this article is given via a link at the end of
the document.
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Scheme 1. Synthesis of the “π-conjugation extended viologen” compounds, (Me2TTz)Cl2 and [(NPr)2TTz]Cl4.
[(NPr)2TTz]Cl4 was demonstrated up to a 20 g scale through a
three-step synthetic route with 63% overall isolated yield. The
product was characterized by 1H-, 13C-NMR, and elemental
analysis. In comparison to inorganic Vanadium ARFBs, the
estimated cost of [(NPr)2TTz]Cl4 (ca. $2.3/kg, see Supporting
Information) is only one tenth of the V2O5 (ca. 24/kg).
test.[2f] As shown in Figure 1A, in a neutral NaCl solution,
[(NPr)2TTz]Cl4 exhibited two single-electron reductions with
average redox potential at -0.44 V (vs. NHE) and NMe-TEMPO
undergoes a single-electron oxidation at +1.0 V (vs. NHE). Both
of these potentials are within the water splitting window (the
potential gap between HER at -1.0 V and OER at +1.5 V in a
As expected, the highly charged ionic [(NPr)2TTz]Cl4
exhibited a high solubility in aqueous solution, 1.3 M in H2O
(69.7 Ah/L) or 1.1 M in 2.0 M NaCl (60.0 Ah/L). The cyclic
voltammogram (CV) of [(NPr)2TTz]Cl4 shows two reversible
redox waves at -0.38 V vs NHE for E1/2([(NPr)2TTz]4+/3+) and -
0.50 V vs NHE for E1/2([(NPr)2TTz]3+/2+) (Figures 1A and S1),
respectively. Under different scan rate, the peak currents of the
two reductions of [(NPr)2TTz]Cl4 show linear relationship with
the square root of scan rate (ν1/2, Figure S2), which indicates
both redox couples of [(NPr)2TTz]4+/3+ and [(NPr)2TTz]3+/2+ are
reversible and are diffusion controlled.
To further qualify the [(NPr)2TTz]Cl4 as an anolyte candidate
for AORFB, the diffusion coefficient (D) was investigated using
linear sweep voltammetry (LSV) with a glassy carbon rotating
disk electrode. LSV plots, and derived Levich plot for
[(NPr)2TTz]Cl4 are shown in Figure S3. Due to the near redox
potentials of the two single-electron reductions (-0.38 V for the
1st electron and -0.50 V for the 2nd electron), there was only one
plateau observed in the LSV curves (Figure S3A). The linear
Levich plot (Figure S3B) was constructed for the single-electron
reduction of [(NPr)2TTz]4+ and [(NPr)2TTz]3+ using limiting
currents (i) and the square root of rotation speeds (ω1/2). The
corresponding slope from the linear relationship was
transformed using the Levich equation (Equation S1, all
equations are given in the experimental section in the supporting
information) to calculate the average diffusion coefficient for
Figure 1. (A) Cyclic voltammograms of 4.0 mM [(NPr)2TTz]Cl4 and 4.0
mM NMe-TEMPO in 0.5 M NaCl solution. The gray dash curve is the cyclic
voltammogram of only the 0.5 M NaCl electrolyte, with labels for the onset
potentials for the hydrogen evolution reaction (HER, -1.00 V) and oxygen
evolution reaction (OER, +1.50 V). The red and green dash curves are
the fitted redox waves for the 1st and 2nd electron reduction, respectively.
(B) Schematic representation of the [(NPr)2TTz]4+/NMe-TEMPO AORFB
and its anodic and cathodic half-cell reactions.
[(NPr)2TTz]4+ and [(NPr)2TTz]3+ as 3.15
×
10-6 cm2/s.
Furthermore, the rate constants (k0) for the electron transfers of
[(NPr)2TTz]4+ and [(NPr)2TTz]3+ were estimated using
Nicholson’s method. The k0 for [(NPr)2TTz]4+ and [(NPr)2TTz]3+
were both greater than 0.28 cm/s, which indicates fast electron
transfer.
Regarding
the
high
water-solubility,
good
neutral NaCl supporting electrolyte), which ensures their
application in AORFBs. The combination of [(NPr)2TTz]Cl4 and
NMe-TEMPO enables a 1.44 V battery voltage with a 53.7 Wh/L
theoretical energy density, which is higher than most of reported
AORFBs. Equations in Figure 1B give the anodic and cathodic
half-cell reactions for the flow battery. In the anolyte side, the
yellow [(NPr)2TTz]4+ was first reduced to the cation radical,
[(NPr)2TTz]3+, at -0.38 V, and then further reduced to purple
electrochemical reversibility, and fast electron transfer, the
[(NPr)2TTz]Cl4 is a promising anolyte candidate for AORFBs
using a Cl- exchange mechanism.
To demonstrate the proof of concept of the two-electron
storage capability of [(NPr)2TTz]Cl4, it was paired with highly
water-soluble 4-trimethylammoinium-TEMPO (abbreviated as
NMe-TEMPO, 3.0 M solubility in water) for a redox flow battery
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[(NPr)2TTz]2+ with a neutral Py2TTz skeleton at -0.50 V. The two
electron reduced state, [(NPr)2TTz]2+, was synthetically
prepared and characterized by 1H- and 13C-NMR spectrum.
(EE) and voltage efficiencies (VE), for example, 70% EE and VE
at 40 mA/cm2 operational current density, and they decreased
linearly with the increasing of the operational current density
(Figure 2C). The long term cycling performance of the
[(NPr)2TTz]Cl4 / NMe-TEMPO AORFB was measured at 40
mA/cm2 for 300 cycles. As shown in Figure 2D, the flow battery
delivered rather stable cycling performance, i.e. more than 90%
1
Compared with the ground state [(Nr)2TTz]4+, both H-and 13C-
NMR signals of [(NPr)2TTz]2+ were upfield shifted due to its
higher electron density of the reduced state. It is worth noting
that the [(NPr)2TTz]2+ was oxygen insensitive (Figure S5), which
Figure 2. (A) Plot of battery capacity versus cycling numbers of the [(NPr)2TTz]Cl4 / NMe-TEMPO AORFB at current densities from 40 mA/cm2 to 100 mA/cm2; (B)
Representative charge and discharge curves at current densities from 40 mA/cm2 to 100 mA/cm2 for the AORFB; (C) Plots of average Coulombic efficiency (CE), energy
efficiency (EE), and voltage efficiency (VE) at different operational current densities; (D) Extended 300 cycle data of the AORFB showing charge capacity, discharge
capacity, and Coulombic efficiency versus cycle number at 40 mA/cm2 current density. Inset: Representative charge and discharge curve from the experiment. Conditions:
anolyte: 0.1 M [(NPr)2TTz]Cl4 in 2 M NaCl; catholyte: 0.2 M NMe-TEMPO in 2.0 M NaCl; AMV anion-exchange membrane, 25 oC.
confirms the stabilization of the reduced state species by
extending the conjugation of viologen molecules and would
make the TTz-based RFBs more tolerant. In the cathode side,
NMe-TEMPO was oxidized to the oxoammonium in the charge
state and recovered to the nitroxyl radical in the discharged state
through a single-electron redox process.
total capacity retention after 300 cycles, equivalent to 99.97%
capacity retention per cycle. It is worth noting that there are two
almost merged plateaus in the charge and discharge curve,
corresponding to the two single-electron redox processes
(Figure 2D inset, also see Figure S6 for a zoom-in image). The
CV post analysis showed no active material crossover in both
anolyte and catholyte (Figure S7).
A flow battery was constructed using 0.1 M [(NPr)2TTz]Cl4
and 0.2 M NMe-TEMPO in 2.0 M NaCl supporting electrolyte for
anolyte and catholyte (a charge capacity of 5.36 Ah/L and an
energy density of 3.86 Wh/L), respectively. Using a Selemion
AMV anion-exchange membrane as the separator, the battery
delivered outstanding performance as shown in Figure 2. The
current rate performance was investigated from 40 to 100
mA/cm2 with increments of 20 mA/cm2 with cutoff voltages at 1.8
V for the charge process and 0.2 V for the discharge process. In
five continuous cycles, stable capacity retention was observed at
each current density (Figure 2A). Upon increasing the current
density from 40 to 60, 80, and 100 mA/cm2, capacity retention
gradually decreased, which is due to the increased ohmic loss
and is also consistent with the increased voltage gap of the
charge/discharge curves (Figure 2B). The Coulombic efficiency
(CE) of the battery under each current density was nearly 100%.
Furthermore, the battery showed reasonable energy efficiencies
As shown in Figure S8, at higher concentrations of
electrolytes, i.e. 0.25 M [(NPr)2TTz]Cl4 and 0.5 M NMe-TEMPO
as catholyte (equivalent to 0.5 M electrons), the RFB (13.4 Ah/L
and 9.65 Wh/L) delivered similar rate performance and slightly
lower energy efficiency (68.6% EE at 40 mA/cm2) than the 0.1 M
[(NPr)2TTz]Cl4 and 0.2 M NMe-TEMPO battery. Same as the
2f, 2g, 2j]
previously reported viologen-based RFBs,[2b,
the
[(NPr)2TTz]Cl4 / NMe-TEMPO AORFB displayed concentration
dependent long-term cycling stability. Specifically, the 0.5 M
electron RFB delivered 99.94% capacity retention per cycle. A
mechanistic understanding of the AORFB is needed to elucidate
the concentration dependent battery performance.
In summary, we reported
a “π-conjugation extended
viologen” compound, [(NPr)2TTz]Cl4, as a novel two-electron
storage anolyte for total organic neutral AORFB applications.
Through the rational molecular engineering, planar TTz
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synthesis of [(NPr)2TTz]Cl4 was demonstrated through
straightforward reaction route from the commercially available
reagents with satisfactory isolated yield. [(NPr)2TTz]Cl4
a
a
exhibits a high solubility in water, reversible electrochemical
behaviors, and fast electron transfer rate constants. Paired with
the cathodic compound, NMe-TEMPO, the [(NPr)2TTz]Cl4 / NMe
-
TEMPO flow battery enables a 1.44 V battery voltage with a
theoretical energy density of 53.7 Wh/L. The demonstration of
[(NPr)2TTz]Cl4 / NMe-TEMPO cell delivered outstanding battery
performance, specifically, 70 % energy efficiency and 99.97 %
capacity retention per cycle. The results confirm the reliability of
the strategy of extending the π-conjugation of viologen
molecules to obtain new redox active compounds for the
AORFB application. It is anticipated that the concept of “π-
conjugation extension” can be also applied with other redox
active molecules and lead to the discovery of novel RFB
chemistry.
Supporting Information contains experimental details and
additional figures and tables. Supporting Information is available
from the Wiley Online Library or from the author.
Acknowledgements We thank Utah State University for
providing faculty startup funds to the PI (T. Leo Liu) and the
Utah Science Technology and Research initiative (USTAR)
UTAG award for supporting this study. Bo Hu is grateful for
China CSC Abroad Studying Fellowship and Utah Energy
Triangle Student Award supported by the Office of Energy of the
Utah State government, respectively. Camden DeBruler is
grateful for his USU Presidential Doctoral Research Fellowship
(PDRF) supported by USU.
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Keywords: “Extended viologen”
Molecular engineering • TEMPO
• Redox flow battery •
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Entry for the Table of Contents
A highly water-soluble “π-conjugation extended viologen” is designed as a novel two-electron
storage anolyte to enable a 1.44 V total organic neutral aqueous redox flow battery.
COMMUNICATION
Jian Luo, Bo Hu, Camden Debruler,
and T. Leo Liu*
Page No. – Page No.
A
“π-Conjugation
Extended
Viologen” as Novel Two-Electron
Storage Anolyte for Total Organic
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