All-PEGylated redox-active metal-free organic molecules in non-aqueous redox flow battery

Article abstract of DOI:10.1039/d0ta02303e

Non-aqueous organic material-based redox flow batteries (NAORFBs) possess the advantage of using organic solvents to achieve high electrochemical potential. However, regardless of the great progress made in this regard in the past decade, further development has been restricted by the lack of stable electroactive organic materials and highly selective separators. Here, we present a NAORFB with all-PEGylated, metal-free, organic compounds as electroactive materials. PEGylated phenothiazine and PEGylated viologen are utilized as the catholyte and anolyte, respectively. Combined with a composite nanoporous aramid nanofiber separator, the all-PEGylated NAORFB presents outstanding cyclability, with a capacity retention of 99.90% per cycle and average coulombic efficiency of 99.7%. By contrast, NAORFBs using half-PEGylated and non-PEGylated electrolytes display inferior cyclability owing to the crossover of non-PEGylated materials. An extended investigation was also performed on the batteries using non-PEGylated or half-PEGylated materials for mechanistic elucidation. This work validates the PEGylation strategy in NAORFBs for enhanced overall performance with respect to solubility, cyclability, and alleviated crossover. This journal is

Full text of DOI:10.1039/d0ta02303e

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Number 12
March 2018
Pages 4883-5230
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J Po lue ran sael od fo Mn aot te rai ad l js u Cs ht emm ai rs gt ri yn sA
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DOI: 10.1039/D0TA02303E
ARTICLE
All-PEGylated Redox-Active Metal-Free Organic Molecules in Non-
Aqueous Redox Flow Battery
a
a
a
a
a
Jingchao Chai, Amir Lashgari, Xiao Wang, Caroline K. Williams, Jianbing “Jimmy” Jiang*
Received 00th January 20xx,
Accepted 00th January 20xx
Non-aqueous organic material-based redox flow batteries (NAORFBs) possess the advantage of using organic solvents to
achieve high electrochemical potential. However, regardless of the great progress made in this regard in the past decade,
further development has been restricted by the lack of stable electroactive organic materials and highly selective separators.
Here, we present a NAORFB with all-PEGylated, metal-free, organic compounds as electroactive materials. PEGylated
phenothiazine and PEGylated viologen are utilized as the catholyte and anolyte, respectively. Combined with a composite
nanoporous aramid nanofiber separator, the all-PEGylated NAORFB presents outstanding cyclability, with a capacity
retention of 99.90% per cycle and average Coulombic efficiency of 99.7%. By contrast, NAORFBs using half-PEGylated and
non-PEGylated electrolytes display inferior cyclability owing to the crossover of non-PEGylated materials. An extended
investigation was also performed on the batteries using non-PEGylated or half-PEGylated materials for mechanistic
elucidation. This work validates the PEGylation strategy in NAORFBs for enhanced overall performance with respect to
solubility, cyclability, and alleviated crossover.
DOI: 10.1039/x0xx00000x
high ionic conductivity (> 100 mS/cm) in aqueous electrolytes,48-50
endowing the aqueous RFBs with superior power densities.
Unfortunately, merely using these membranes in non-aqueous
Introduction
Over the past century, there has been a significant increase in demand
for electricity. The continually increasing content of greenhouse gases
generated from traditional fossil fuels has prompted researchers
worldwide to develop clean energy, such as wind and solar energy.1-3
However, the intrinsic intermittence and fluctuation of these
renewable energy sources pose serious challenges. Energy storage
devices, such as lithium (metal),4-12 sodium,13,14 aluminum,15 and
magnesium ion batteries,16,17 have played an important role in
reducing peak shaving and valley filling in electric networks. Among
the emerging energy storage devices, redox flow batteries (RFBs)
have attracted much attention.18-21 RFBs store energy in liquid
electrolyte reservoirs, which decouple the energy and power
densities.22-25 Non-aqueous organic material-based redox flow
batteries (NAORFBs) have been studied in recent years owing to their
wide electrochemical window of non-aqueous electrolytes and vast
molecular diversity.26-38 However, a limiting factor that retards further
development of the NAORFBs is the crossover of electroactive
compounds through the membrane into the opposite compartments,
causing inferior cyclability performance.26,39,40 The crossover issue is
less severe in the conventional aqueous RFBs because of the
availability of ion-exchange membranes, such as the commercially
electrolytes typically induces new issues, such as inferior stability and
high resistance (< 100 mS/cm),27,51-53 resulting in low
charge/discharge rate and power density. The pretreatment of Nafion®
membranes to convert the proton of the sulfonic acid to lithium ion52
or tetraalkylammonium,54 such as tetrabutylammonium or
tetraethylammonium, does not substantially improve the
performance. Size exclusion-based porous membranes, such as
Daramic® and Celgard® membranes, have been used in some non-
aqueous symmetrical batteries or catholyte–anolyte-mixed batteries,
but typically in low Coulombic efficiencies (8095%)31,39,40,55-60 and
high self-discharge rates, owing to the large pore sizes (20–200 nm).
NAORFBs using inorganic glass ceramic separators display high
Coulombic efficiencies and high charge/discharge current density.29,61
However, their intrinsic brittleness and high cost prevent their wide
application.
Two strategies can be adopted to address the aforementioned
issues and improve the overall battery performance. One is to increase
the molecular size of electroactive compounds to suppress penetration
through the membrane based on a physical blocking mechanism. For
example, small electroactive compounds can be incorporated on
redox-innocent polymer backbones24,57,62,63 or decorated with long
tails.23,64,65 The second strategy is to design novel membranes with
high ion selectivity, low resistance, and high solvent
compatability.26,66 The porous membrane can be modified with
positively or negatively charged ions to introduce ion-exchange
features,66-69 thus suppressing crossover by a combination of steric
hindrance and Donnan exclusion mechanism.53,70 The Sanford group
®
®
®
®
available and widely used Nafion , Fumasep , AMV , and DSV
membranes,20,41-48 which can suppress the permeability of redox-
active species. These ion-exchange membranes usually possess super-
a. Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati,
Ohio 45221-0172, United States. Email: jianbing.jiang@uc.edu
†Electronic Supplementary Information (ESI) available: Synthesis of compounds,
reported a Fumasep® anion exchange membrane to separate the
positively charged anolyte and catholyte.67-69 Both strategies
NMR spectra, photographs of compounds, CV, battery performances. See
DOI: 10.1039/x0xx00000x
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(molecular engineering and membrane decoration) are used in a
View Article Online
DOI: 10.1039/D0TA02303E
synergistic manner to improve the overall battery performance.
Several types of organic redox-active materials have been
reported in NAORFBs, including (but not limited to): (1) Viologens.
The most promising feature of using viologens as the anolyte is the
two-electron redox activity for higher energy capacity compared to its
one-electron-active peers. In comparison to the intrinsically water-
soluble viologens,19,49,71,72 Liu et. al. reported a highly organic soluble
viologen by replacing its halogen anions with less polar
bis(trifluoromethanesulfonyl)imide (TFSI) units and successfully
applied it in NAORFB;30 (2) Ferrocene (Fc). As a highly
electrochemically stable organometallic complex, Fc is commonly
used as a catholyte in NAORFBs.29,73 However, the low redox
potential of Fc (0.69 V vs standard hydrogen electrode) does not take
full advantage of the wide electrochemical window of non-aqueous
solvents. In addition, the energy density of Fc is relatively low owing
to the limited solubility in organic solvents [200 mM in acetonitrile
Fig. 1 Venn diagram depicting the four key requirements for redox-
active species in NAORFBs.
Sigma-Aldrich. Methylene dichloride and N,N-dimethylformamide
were obtained from Fisher Chemical. Dimethyl sulfoxide was
purchased from TCI Chemicals. The aramid nanofiber was purchased
from the Thread Exchange. The Daramic separator was provided by
6
Daramic LLC. Tetrabutylammonium hexafluorophosphate (TBAPF )
(
ACN), and even lower in carbonate solvents].38,74 Derivatized Fc,
was recrystallized in absolute ethanol and dried in vacuo. The other
such as quaternized ferrocene30,38,75 and brominated ferrocene,73 were
also developed to improve the solubility and/or increase the redox
potential; and (3) Phenothiazine (PTZ). As catholyte
materials,44,57,76,77 PTZs can undergo facile molecular modification
on the N atom and aromatic ring to optimize solubility, stability, and
redox potentials, making PTZs attractive in NAORFBs.77 For
practical applications, an electroactive compound in RFB should
possess features including (1) high electrolyte stability, (2) wide
electrochemical window, (3) high electrolyte solubility, and (4)
mitigated crossover (Fig. 1). Few (if any) materials satisfy all four
criteria.
1
materials were used as purchased without any purification. The H
NMR and 13C NMR analyses were performed at room temperature
using a Bruker AV 400 MHz spectrometer. Chemical shifts are based
on the ppm unit. Electrospray ionization–mass spectrometry (ESI–
MS) analysis was performed on an Orbitrap Fusion Lumos mass
spectrometer from Thermo Scientific. Compounds PEGylated
viologen (PEG12-V, with 12 oxyethylene units),64 methyl viologen
(
Me-V)30,78 and N-propyl phenothiazine (C3-PTZ)79 were prepared
according to reported procedures.
Electrochemical measurement
Recently, we investigated a PEGylated strategy and used
PEGylated viologens as anolytes for the demonstration of proof-of- All the cyclic voltammetry (CV) tests were conducted in 0.1 M
concept in the NAORFB.64 The introduction of poly(ethylene glycol) TBAPF
-ACN solution in an argon-filled glovebox. The CV studies
PEG) increased the molecular size of viologen and improved were carried on a Bio-Logic potentiostat. For a CV study, a glassy
solubility in all the charged and discharged states of viologen in the carbon (3 mm in diameter) and a Pt wire electrode were used as the
organic solvent. In this paper, we present a PEGylated PTZ as the working and counter electrode, respectively. Ag/AgNO electrode
catholyte. PEGylated electroactive materials have been applied in with 0.01 M AgNO /ACN solution was used as reference electrode.
NAORFBs.23,58 However, the majority of the reported materials
Linear sweep voltammetry (LSV) studies were carried out using
6
(
3
3
employed lower degree of PEG units with limited capability for a Pine modulated speed rotator with CHI760e electrochemical
crossover suppression. The PEGylated viologen and PEGylated workstation. Rotating disk electrode (RDE, diameter: 5 mm), Pt wire
phenothiazine herein possess lower permeability due to the increases electrode and Ag/AgNO electrode were used as the working, counter
3
molecular size by the introduction of higher degree of PEG chains. and reference electrodes, respectively. Before testing, the samples
Paired with the PEGylated viologen, the all-PEGylated battery were purged with argon for 10 min to remove the dioxygen. LSV dates
exhibited substantially long cyclability. An extended investigation were collected at different rotation rates from 100 to 2500 rpm.
was also conducted on the NAORFBs using partially or non- Diffusion coefficient (D) of electroactive material was calculated
PEGylated compounds and illustrated that the all-PEGylated system from Levich plot:46,48,80
possesses superior cyclability and higher Coulombic efficiency. This i = 0.620nFAC
study validates that the PEGylation strategy can greatly improve the where i is limiting current density (A), n is the number of electrons in
0
D2/31/2–1/6
performance of redox-active materials in NAORFBs.
redox process, F is Faraday’s constant (96,485 C/mol), A is the area
2
of the glassy carbon electrode (0.196 cm ), C
0
is the concentration of
–
6
3
active material (1 × 10 mol/cm ), D is the diffusion coefficient
2
Experimental section
Materials
(cm /s),  is angular rotation rate (rad/s), and  is the kinematic
2
viscosity of 0.1 M TBAPF
6
-ACN (0.00442 cm /s).
k
The mass-transfer-independent kinetic current i was obtained
from the Koutecký-Levich plot for different overpotentials (the
reciprocal of the current at overpotentials vs the reciprocal of the
Sodium hydride, phenothiazine, sodium sulfate, potassium
hydroxide, 1-iodopropane, anhydrous acetonitrile (99.8%), and
tetrabutylammonium hexafluorophosphate were purchased from
square root of the rotation rate).20,62 Exchange current i
obtained from the Butler-Volmer equation41,62 via a Tafel plot [log(i
0
can be
k
)
2
| J. Name., 2012, 00, 1-3
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vs overpotential], which yields the kinetic oxidation rate constant k
cm/s) from following equation:
= FAk
The transfer coefficient () was obtained from Tafel analysis:
Tafel slope = (1–α)F/(2.3RT)
0
'06 2x global hybrid functional with 54% Hartree-FockVeiexwcAhraticnlge eOnalninde
DOI: 10.1039/D0TA02303E
6-31+G(d) basis sets. The universal solvation model SMD was
selected to optimize all geometries in ACN to include the solvation
effect to the free energies. The molecular orbitals (MOs) and molar
volume calculations were carried out at M06-2x/6-31+G(d) level.
(
i
0
0
C
0
where R is the universal gas constant (8.314 J/(K mol)) and T is
the absolute temperature (298 K).
Battery measurement
2
The flow battery possesses an active area of 28 cm , and is
composed of a metal plate, polytetrafluoroethylene plate, copper plate,
graphite current collector, polytetrafluoroethylene frame, and
graphite-felt electrodes. The CANF membrane was prepared
according to reported procedure,26,64 which was sandwiched between
Permeability measurement
The permeability measurements of active materials were carried
out following the procedures in the references.20,64 The samples of 15
mL 50 mM active material in 0.1 M TBAPF -ACN were added to one
6
side of the H-cells. The blank electrolyte solution without redox-
active materials was used in the other side. The H-cell was separated
by composite nanoporous aramid nanofiber (CANF) membrane. The
concentrations of active materials were calculated from the calibration
curve of the peak current density vs concentration. The permeability
two graphite-felts. For full batteries, 50 mM cathode materials and 10
mM anode materials were dissolved in 0.5 M TBAPF -ACN (8 mL).
6
The battery was galvanostatically charged/discharged at a current
2
density of 2 mA/cm within a potential range of 0–2.2 V.
Electrochemical impedance spectroscopy (EIS) of the RFBs
before and after cycling was conducted on a Bio-Logic potentiostat
under a frequency ranging from 200 kHz to 100 mHz and using a 10
mV AC amplitude signal.
(
P) is calculated from the following equation:20
2
Cr
Vl
ln 1 –
(–
)
(
)
C0
2A
P =
t
3
where C
r
is the concentration measured at the receiving side (mol/cm ),
Results and discussion
Molecular design and synthesis
C
0
is the active species concentration in the working chambers
3
3
(mol/cm ), V is the volume (cm ), l is the membrane thickness (cm),
2
A is the membrane area (cm ), and t is the test time (s).
A set of four compounds are synthesized for systematic investigation
of PEGylation effects on battery performance in an organic solvent
DFT calculation
(Fig. 2a). Among the four compounds, PEG12-V and Me-V function
All calculations were carried out by using Gaussian 09 software
for original and one-electron oxidation state.81 All oxidation states
were optimized at ground states with M06-2x functional: Minnesota
as anolytes, and N-PEG12 phenothiazine (PEG12-PTZ) and C3-PTZ
as catholytes (Fig. 2b). As the cathode redox material, compound PTZ
undergoes one-electron oxidation to become a stable radical cation.82
Fig. 2 (a) Molecular structures of compounds PEG12-V, Me-V, PEG12-PTZ, and C3-PTZ. (b) Redox reactions at the negative and positive sides
during charge and discharge. (c) CV scans of PEG12-PTZ and PEG12-V (5.0 mM of both) in 0.1 M TBAPF -ACN on a glassy carbon electrode at
a scan rate of 50 mV s . (d) HOMO/LUMO of PEG12-PTZ and C3-PTZ on the redox-neutral and one-electron oxidized states.
6
−1
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Different pairings of the four electrolytes give four battery systems for conjugation,
the
HOMO
View Article Online
thorough investigation. Instead of using KOH as the base for the orbitals show stable structures for C3-PTZDaOnId: 1P0E.10G3192/D-P0TTAZ02.3T0h3eE
deprotonation of PTZ to prepare C3-PTZ,79 a stronger base, sodium Table 1. Molecular sizes and permeability properties of the four
hydride, was used to afford a stronger PTZ nucleophile to react with compounds.
PEG methyl ether tosylate (Scheme S1 and Fig. S1), for the synthesis
of compound PEG12-PTZ; 66% yield of PEG12-PTZ was obtained
in high purity. The PEG12-PTZ compound is a purple liquid and is
miscible with ACN in any proportion, to be compared to C3-PTZ as
white crystal powders (Fig. S2). The density of PEG12-V and
PEG12-PTZ was 1.16 g/mL and 1.17 g/mL, corresponding to a
concentration of 0.79 M and 1.72 M, respectively. For two-electron-
active viologen, the electron concentration of PEG12-V is 1.58 M.
The high solubility of PTZ is required to improve the energy density
of the NAORFBs.
Charge
state
0
0
+2
+2
Molecular
diameter (nm)
1.20
Permeability
(cm /s  10 )
1.31
Compound
b
2
9
PEG12-PTZ
C3-PTZ
0.82
1.43
0.78
16.46
0.35
2.60
a
PEG12-V
Me-V
a
a
b
Data for PEG12-V and Me-V were derived from ref.64
The charge state refers to the viologen cations.
energy gaps (E ) for the different redox states of C3-PTZ and
g
PEG12-PTZ are almost identical, further confirming that the
Electrochemical properties
presence of the PEG unit does not affect the electrochemical
g
properties. The E for the original oxidation state of both C3-PTZ and
The electrochemical properties of the electrolytes were
PEG12-PTZ is higher than that for the one-electron oxidation state,
indicating higher energy is needed for electron transfer in redox
reactions.The crossover issue is one of the long-standing challenges
in NAORFBs.26,83 One of the advantages of PEGylation is that it can
increase the molecular size of organic electroactive compounds and
alleviate the crossover issue. A three-day permeability experiment
was conducted by monitoring the concentration of active materials in
the test chamber of a H-cell (Fig. S4). Compound PEG12-PTZ
presented a significant decreased crossover rate for the CANF
6
characterized by CV in ACN with 0.1 M TBAPF as the supporting
electrolyte (Fig. 2c). The single-electron oxidation of PEG12-PTZ
+
exhibits a reversible redox couple at 0.39 V vs Ag/Ag . The
compound PEG12-V presents two redox peaks at 0.74 V and 1.15
+
V vs Ag/Ag . The pairing of PEG12-V and PEG12-PTZ gives a
potential difference as high as 1.54 V. Moreover, the introduction of
the PEG chain had negligible effect on the redox potential of the
electroactive materials (Fig. S3). Molecular orbital (MO) Density
Functional Theory (DFT) calculations for PEG12-PTZ and C3-PTZ
in the two oxidation states were performed to evaluate the
corresponding redox potentials during the electrochemical processes.
The results show that for both C3-PTZ and PEG-PTZ, the HOMOs
and LUMOs are almost identical and mapped through three PTZ rings
symmetrically (Fig. 2d). Owing to the electron delocalization and
-9
2
membrane (1.31  10 cm /s), a tenth lower than that of C3-PTZ
-
9
2
(
1
16.46  10 cm /s, Fig. S5) owing to its bigger molecular size (Table
6
4
). A similar trend was observed for viologens. It is worth noting
that with similar molecular diameters (PEG12-PTZ vs PEG12-V,
and C3-PTZ vs Me-V), viologens presented a lower permeability
owing to their two positive charges and the Donnan exclusion of the
CANF separator.64
Fig. 3 (a) Linear sweep voltammetry of 1 mM of PEG12-PTZ in 0.1 M TBAPF
6
-ACN at different rotation rates from 100 to 2500 rpm. (b) Levich
4
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plot from the obtained limiting currents. (c) Koutecký–Levich plot (i1 vs 1/2) of 1 mM of PEG-PTZ. (d) Plot of log (i
) vs overpotential.
View Article Online
DOI: 10.1039/D0TA02303E
k
To investigate the effect of PEGylation on the stability of PTZ,
we repeated CV scans of PEG12-PTZ and C3-PTZ in 0.1 M
TBAPF
6
-ACN for 500 cycles. The overlapped traces showed the
electrochemical stability of PEG12-PTZ in a non-aqueous electrolyte
(
(
Fig. S6a). Meanwhile, C3-PTZ presented the same stability property
Fig. S6b). In addition, the PTZ and viologen CV scans at different
charge states showed that the redox potential and peak current density
had almost no change after 5 days (Fig. S7), which concluded that
PEGylation did not affect their electrochemical stability.
The electrochemical kinetics of PEG12-PTZ in 0.1 M TBAPF -
6
ACN were studied by the RDE method (Fig. 3a). The diffusion
coefficient (D) of PEG12-PTZ was calculated from Levich plot (Fig.
b),46,48,80 which yielded a diffusion coefficient of 1.23  10-5 cm /s,
2
3
consistent with that from the Randle–Sevcik equation (D
O
= 1.10 
= 9.88  10-6 cm /s) (Fig. S8 and Table S1). The
diffusion coefficient of PEG12-PTZ is slightly lower than that of C3-
0-5 cm /s and D
2
2
1
R
-5
2
PTZ (1.61  10 cm /s) (Fig. S9), presumably owing to the increased
viscosity from the PEG chains. Application of the Koutecký–Levich
plot gave the mass-transfer-independent kinetic current i
intercept of linear fitting (Fig. 3c).20,62 The Butler–Volmer
equation41,62 via a Tafel plot yielded the exchange current i
, and then
provided the kinetic oxidation rate constant k (k = 5.64  10 cm/s)
via i = FAk (Fig. 3d). The kinetic oxidation rate constant of C3-
PTZ in 0.1 M TBAPF
k
from the
0
-3
0
0
0
0 0
C
-3
6
-ACN is 9.88  10 cm/s, slightly higher than
Fig. 4 (a) Discharge capacity and Coulombic efficiency of the PEG12-
PTZ/PEG12-V battery at varied current densities. Inset:
charge/discharge profiles. (b) Average Coulombic efficiency, energy
that of PEG-PTZ, indicating that the introduction of PEG reduces the
reaction rate constant of PTZ. However, the transfer coefficient ()
of C3-PTZ, obtained from the Tafel analysis, is only 0.315, which is efficiency, and voltage efficiency at different charge/discharge
lower than that of PEG12-PTZ ( = 0.492). The latter value is close current densities.
to 0.5, which is the value for an ideally reversible redox reaction.62
Battery performance
4
.19 mAh, corresponding to 78.2% of the theoretical capacity. It
The battery was assembled for rate performance and long
cyclability tests using 50 mM PEG12-PTZ and 10 mM PEG12-V as
catholyte and anolyte, respectively. The excess PEG12-PTZ was
used to ensure the two-electron utilization of PEG12-V. The
investigation of the all-PEGylated RFB was initiated with open circuit
voltage (OCV) measurements at various states of charge (SOC) (Fig.
S10). The resulting all-PEGylated PEG12-PTZ/PEG12-V battery
presented an OCV increasing nearly linearly from 0.97 V at 5% SOC
to 1.17 V at 45% SOC, and from 1.42 V at 50% SOC to 1.59 V at 95%
SOC. The two charge platforms correspond to the two redox peaks of
PEG12-V in CV scans. Similarly, the discharge plot showed two
platforms. Additionally, the polarization voltage was as small as 75
mV, indicating the low internal impedance of the all-PEGylated
PEG12-PTZ/PEG12-V battery.
should be noted that the Coulombic efficiency increased from 97.7%
2
2
at 1 mA/cm to 99.7% at 5 mA/cm (Fig. 4b). The excellent rate
performance was rooted in the high diffusion coefficient and kinetic
oxidation rate constant of organic redox species in non-aqueous
electrolytes.
Given that the Coulombic efficiency of the battery is up to 99.0%
2
at a current density of 2 mA/cm (Fig. 4) and that the utilization of
active materials exceeded 91.4%, the PEG12-PTZ/PEG12-V battery
was galvanostatically charged/discharged at a current density of 2
mA/cm2 to investigate long cyclability. The battery possessed a
capacity of 4.84 mAh with a capacity retention of 69.0% after 300
cycles (99.9 % per cycle) (Fig. 5a) and an average Coulombic
efficiency of 99.7% (Fig. 5b). In addition, no evident change was
observed for the charge/discharge profiles at different cycle numbers
As the power density of a RFB is directly related to the rate
property, a PEG12-PTZ/PEG12-V battery was galvanostatically
(
Fig. S11), except for a slight reduction in capacity, exhibiting a stable
energy efficiency of 72.7%. CV scans after 300 cycles indicated that
the crossover of PEG12-PTZ through the CANF separator was
negligible (Fig. 5c). EIS plots of the battery before and after the 300
cycles showed that there was a slight increase in the high-frequency
2
charged/discharged at different current densities from 1 to 5 mA cm
2
with an interval of 1 mA/cm (Fig. 4a). At a current density of 1
2
mA/cm , the battery displayed a capacity of 5.03 mAh (93.4% of the
theoretic capacity) with a Coulombic efficiency of 97.7% and energy
efficiency of 88.3%. With increased current density, the overpotential
increases and discharge capacity decreases owing to the intrinsic
internal impedance of the battery. However, even at a current density
impedance (Fig. 5d). However, the charge transfer impedance of the
battery48 showed an evident decrease presumably because of the
prolonged soaking time of the CANF separator in the electrolyte. The
stable impedance caused stable charge/discharge profiles and a long
2
as high as 5 mA/cm the battery still presented a discharge capacity of
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operating lifetime.
View Article Online
DOI: 10.1039/D0TA02303E
Fig. 5 (a) Discharge capacity vs cycle number of the PEG12-PTZ/PEG12-V (red), C3-PTZ/PEG12-V (blue), PEG12-PTZ/Me-V (green), and C3-
PTZ/Me-V (violet) batteries. (b) Coulombic and energy efficiencies vs cycle number. (c) CV scans of catholyte and anolyte for PEG12-
PTZ/PEG12-V battery after 300 cycles. (d) AC impedance spectra of PEG12-PTZ/PEG12-V battery before cycling and after 300 cycles.
To investigate the structure–performance relationship, we C3-PTZ/Me-V batteries were charged/discharged for100 and 50
studied the half-PEGylated batteries (C3-PTZ/PEG12-V and cycles, respectively. Despite the fewer cycle numbers
PEG12-PTZ/Me-V) and non-PEGylated battery (C3-PTZ/Me-V). Table 2. Average Coulombic efficiency and capacity retention of the
The C3-PTZ/PEG12-V battery exhibited good capacity retention in four batteries.
the first 125 cycles (Fig. 5a), but with poor energy efficiency,
especially after 100 cycles (Fig. 5b). The poor energy efficiency was
attributed to the gradually increasing polarization during the
charge/discharge process (Fig. S12a). The redox peak for the
catholyte C3-PTZ was observed in the anolyte side (Fig. S12b) at
almost the same current intensity as that of the same species in the
catholyte, consistent with the permeability findings that C3-PTZ had
a much higher permeability than PEG-V. The catholyte C3-PTZ
may react with anolyte PEG12-V in the pores of the CANF
membrane, resulting in increased internal resistance of the battery
Capacity
retention
per cycle
(%)
Average
Coulombic
efficiency (%)
Overall
capacity
retention
Battery
PEG12-PTZ/
PEG12-V
C3-PTZ/
PEG12-V
PEG12-PTZ/
Me-V
68.6% after
300 cycles
55.3% after
200 cycles
62.9% after
100 cycles
65.4% after 50
cycles
9
9
9
9
9.71
9.41
9.48
8.99
99.90
99.78
99.63
99.37
(
Fig. S12c). The PEG12-PTZ/Me-V and C3-PTZ/Me-V batteries
displayed a stable energy efficiency but rapid capacity fading (Figs.
a, 5b, S12d, and S12g). The same batteries also showed relatively
C3-PTZ/
Me-V
5
low Coulombic efficiency (Table 2) due to the serious crossover of compared to those in the all PEGylated (300 cycles) and C3-
Me-V and C3-PTZ, which was confirmed by the CV scans after PTZ/PEG12-V (200 cycles) batteries, an evident internal impedance
battery cycling (Fig. S12e and S12h). The PEG12-PTZ/Me-V and increase of both batteries was still observed (Fig. S12f and S12i). The
6
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increase of impedance was presumably owing to the precipitates electrolysis of PEG12-PTZ and PEG12-V fVoierwmAertdicle Othnleinier
DOI: 10.1039/D0TA02303E
formed from the reaction of PTZs with viologens on the membranes corresponding radical anion and radical cation, respectively, and then
1
during the charge/discharge processes.
the two radical species were mixed in a stoichiometric ratio. The H
NMR spectrum of the resultant solution is the sum of the spectra of
PEG12-V and PEG12-PTZ (Fig. S18), suggesting neat redox
neutralization of the two radicals without forming side products. This
experiment result is against hypothesis C. Overall, the stability issue
of viologen molecules under oxidizing conditions is a possible cause
of battery degradation. The mechanistic analysis here not only
provides an explanation for the battery capacity decay, but also
emphasizes the importance of addressing the crossover issue by
designing the next generation of redox-active molecules and suited
membrane separators.
Mechanistic analysis
The pre- and post-cycling anolyte and catholyte solutions of all
the four batteries were analyzed by proton nuclear magnetic resonance
(1
H NMR, Fig. S13–S16) and CV to elucidate the possible
degradation mechanism. The anolyte solutions were comprised of the
anolyte species (PEG12-V or Me-V) and various amounts of
catholyte species from the crossover; no additional H NMR signals
were observed, indicating that both the anolyte and catholyte species
were electrochemically stable in the anolyte chamber. By contrast,
besides the anolyte species from the crossover, unexpected proton
signals (panels a in Fig. S13–S16) were detected in the catholyte
chamber, suggesting that side reactions occurred in the
1
Concentration dependence
The PEG12-PTZ/PEG12-V batteries with higher concentrations
electrochemically oxidizing catholyte chamber. We hypothesized were investigated to pursuit higher energy density. The battery was
three possible side reactions: (A) the catholyte molecules (PEG12- initially assembled with 480 mM PEG12-PTZ and 200 mM PEG12-
PTZ and C3-PTZ) underwent oxidative decomposition during the V mix electrolytes using a macroporous Daramic membrane as the
charging process; (B) viologen compounds crossed from the anolyte separator. Before cycling, the battery displayed an impedance of 17.5
side, decomposing under the oxidizing conditions in the catholyte Ω/cm2 (Fig.
6a).
The
battery
was
galvanostatically
2
chamber; and (C) the reduced viologens (radical anion) reacted with charged/discharged at a current density of 20 mA/cm and presented
oxidized PTZs (radical cation) in the catholyte chamber to form a new undesirable polarization and low Coulombic efficiency (55%, Fig.
chemical species, rather than returning to their neutral, original states. 6b). Then electrolytes were diluted to ¾ and afforded 360 mM
Extensive experiments were conducted to confirm or eliminate the PEG12-PTZ and 150 mM PEG12-V mix electrolytes. Both
hypotheses. (1) CV scans of PTZs for repetitive 500 cycles (Fig. S6) impedance and polarization of the battery reduced with the decreased
indicated that the pure PTZs were electrochemically stable for single- concentration with a Coulombic efficiency of 72%. Different
electron oxidation, thus eliminating hypothesis A. (2) Repetitive CV concentrations of electrolytes impose different viscosity effects on
scans of PEG12-V in different potential ranges were conducted. battery impedance; the higher electrolyte concentration, the more
Though the interconversion between PEG12-V and its radical anion viscous the electrolyte solution is; therefore, batteries with higher
+
is reversible (Fig. S17) from –1.5 to –0.2 V vs Ag/Ag , an evident electrolyte concentration present higher impedance. After further
current decrease was observed in the more positive scan range (from diluted to concentrations of 240 mM PEG12-PTZ and 100 mM
+
–
1.5 to 0.8 V vs Ag/Ag ), indicating the instability of PEG12-V under PEG12-V, the battery presented a stable Coulombic efficiency of 92%
oxidizing conditions, which supports hypothesis B. (3) Separate (Fig. 6c). The gradual decay of energy and voltage efficiencies were
Fig. 6 (a) EIS of the PEG12-PTZ/PEG12-V batteries at different concentrations. (b) First charge/discharge curves. (c) Discharge capacity and
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efficiencies of battery with 240 mM PEG12-PTZ and 100 mM PEG12-V mix electrolytes. (d) CV scans of catholyte and anolyte after cycling
View Article Online
and mix electrolyte before cycling.
DOI: 10.1039/D0TA02303E
observed during cycling. CV scans of the post-cycling electrolyte
indicated current density decrease for anolyte presumably due to the
interaction of PEG12-PTZ and the reduced viologen species. The
concentration-dependence study suggests that the high viscosity of
PEG units and the possible side reaction of the anolytes and catholytes
could be the two barriers to enhancing the energy density.
7
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Conclusions
In summary, this study demonstrated that PEGylation of organic
redox-active materials is a promising strategy to enhance the 13 P. He, Y. Fang, X. Y. Yu and X. W. Lou, Angew. Chem. Int. Ed.,
performance of NAORFBs by increasing the molecular size of
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The half-PEGylated (C3-PTZ/PEG12-V and PEG12-PTZ/Me-V)
and non-PEGylated (C3-PTZ/Me-V) batteries were also investigated
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Conflicts of interest
There are no conflicts to declare.
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The authors acknowledge the University of Cincinnati for startup 23 M. J. Baran, M. N. Braten, E. C. Montoto, Z. T. Gossage, L. Ma,
funding support, and Ohio Supercomputer Center for providing
the computational resources. NMR experiments were performed
on a Bruker AVANCE NEO 400 MHz NMR spectrometer
funded by NSF-MRI grant CHE-1726092.
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