Characterization of PEDOT-Quinone Conducting Redox Polymers for Water Based Secondary Batteries

Article abstract of DOI:10.1016/j.electacta.2017.03.068

Lithium-ion technologies show great promise to meet the demands that the transition towards renewable energy sources and the electrification of the transport sector put forward. However, concerns regarding lithium-ion batteries, including limited material

Full text of DOI:10.1016/j.electacta.2017.03.068

Accepted Manuscript
Title: Characterization of PEDOT-Quinone Conducting Redox
Polymers for Water Based Secondary Batteries
Authors: Mia Sterby, Rikard Emanuelsson, Xiao Huang,
Adolf Gogoll, Maria Strømme, Martin Sjo¨din
PII:
S0013-4686(17)30534-0
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.electacta.2017.03.068
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19-12-2016
8-3-2017
8-3-2017
Please cite this article as: Mia Sterby, Rikard Emanuelsson, Xiao Huang, Adolf
Gogoll, Maria Strømme, Martin Sjo¨din, Characterization of PEDOT-Quinone
Conducting Redox Polymers for Water Based Secondary Batteries, Electrochimica
Actahttp://dx.doi.org/10.1016/j.electacta.2017.03.068
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Characterization of PEDOT-Quinone Conducting Redox Polymers for
Water Based Secondary Batteries
Mia Sterby,a Rikard Emanuelsson,a Xiao Huang,b Adolf Gogoll,b Maria Strømme,a and Martin Sjödina
a Nanotechnology and Functional Materials, Department of Engineering Sciences, The Ångström Laboratory,
Uppsala University, Box 534, SE-751 21 Uppsala, Sweden
b Department of Chemistry - BMC, Biomedical Centre, Uppsala University, Box 576, SE-751 23 Uppsala,
Sweden
Graphical abstract
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Abstract
Lithium-ion technologies show great promise to meet the demands that the transition towards
renewable energy sources and the electrification of the transport sector put forward. However,
concerns regarding lithium-ion batteries, including limited material resources, high energy
consumption during production, and flammable electrolytes, necessitate research on
alternative technologies for electrochemical energy storage. Organic materials derived from
abundant building blocks and with tunable properties, together with water based electrolytes,
could provide safe, inexpensive and sustainable alternatives. In this study, two conducting
redox polymers based on poly(3,4-ethylenedioxythiophene) (PEDOT) and a hydroquinone
pendant group have been synthesized and characterized in an acidic aqueous electrolyte. The
polymers were characterized with regards to kinetics, pH dependence, and mass changes
during oxidation and reduction, as well as their conductance. Both polymers show redox
matching, i.e. the quinone redox reaction occurs within the potential region where the
polymer is conducting, and fast redox conversion that involves proton cycling during pendant
group redox conversion. These properties make the presented materials promising candidates
as electrode materials for water based all-organic batteries.
Keywords: Conducting Redox Polymer, Quinone, Organic Batteries, Proton
Batteries, Redox Matching
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1 Introduction
The need for electrical energy storage (EES) is steadily increasing and with that comes
increased requirements on EES technologies regarding safety, sustainability and reduced
environmental impact. Currently, the most promising EES-system that could meet future
energy storage demands is based on the lithium-ion technology. The primary concerns with
current lithium-ion batteries (LIBs) are linked to safety due to flammable electrolytes [1],
high costs, high energy consumption in their production and limited material resources due to
the lithium metal oxide materials used as cathodes [2, 3]. Commercially available LIBs are
using electrolytes containing mixtures of organic carbonates [4] which are flammable and/or
harmful. Future EES technologies would benefit from using non-flammable and non-harmful
electrolytes and to avoid the use of organic solvents, aqueous systems could be used.
Commercial LIBs have a cell voltage of around 3.8 V and the voltage of a water based EES
systems is limited by the water stability window of 1.23 V. However, Suo et al. have recently
increased the stability window of aqueous electrolytes to ~3.0 V using highly concentrated
salt solutions, enabling water based EES systems to operate at voltages approaching that of
LIBs [5].
Existing cathode materials most often include rare and expensive metals with high molecular
weights (e.g. cobalt, manganese, and nickel) that are extracted through energy-intensive
mining [6]. Organic materials, with their inexpensive, abundant and readily available building
blocks (H C O N S) and tunable properties, have therefore been proposed as replacements for
inorganic cathode materials [7, 8]. Many organic battery materials have previously been
studied, including carbonyl compounds [9, 10], conducting polymers [11-13], radicals [14-
16], and organosulfur compounds [7, 17, 18]. A unique class of carbonyl compounds that has
been extensively studied in the field of organic matter based EES is constituted by the
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quinones. Quinones are naturally occurring and they are involved in e.g. the electron transport
chains of photosynthesis and respiration [19]. Their high discharge capacity [9, 20, 21] and
reversible redox chemistry [21, 22], both in organic solvent and in aqueous solution, make
them attractive as components in EES applications. Moreover, quinones offer the possibility
of introducing various functional groups through chemical substitution, which allows for
tuning of the quinone physical properties. In particular the possibility to adjust the quinone
redox potential by substitution is of essence for the development of battery materials.
Batteries using quinones as anode and cathode material in the same device have been
demonstrated by virtue of this substitution induced voltage tuning [8, 23]. However, quinones
show low electronic conductivity and are easily dissolved in the electrolyte [24-26].
To solve these problems the quinone can be connected to a conducting polymer forming a
conducting redox polymer (CRP). A CRP consists of a conducting polymer backbone, a redox
active pendant group which can store charge, and a linker connecting the pendant group to the
backbone (Figure 1).
These types of battery materials have previously been investigated [27, 28]. They allow for
organic battery electrodes with both good conductivity, without the use of carbon additives,
and high charge storage capacity. In a CRP the conducting polymer backbone serves to
provide an electronic conductivity path through the material while the purpose of the pendant
group is to provide a well-defined redox reaction and a high charge storage capacity. That is,
the individual, unperturbed, properties of the conducting polymer and the redox group are
desirable in the CRP material. Moreover, as conducting polymers are only conducting when
in their oxidized or reduced states, i.e. in their doped states, the quinone redox reaction has to
occur in a potential region where the polymer is doped in order to benefit from polymer
conductivity, a condition referred to as redox matching.
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In a previous study with polypyrrole (PPy) as polymer backbone and a quinone pendant
group, the polymer conductivity was severely compromised by the redox activity of the
quinone [29]. This was concluded to be due to a twisting of the PPy backbone upon oxidation
of the quinone group, resulting in a localization of charge carriers. In the PPy-quinone system
the requirement for unperturbed properties of the active moieties was thus not met. In this
study we use the more rigid poly(3,4-ethylenedioxythiophene) (PEDOT) backbone, which
potentially can withstand the forces imposed on the polymer backbone during hydroquinone
to quinone redox conversion. The quinone functional groups are attached covalently to the
PEDOT backbone via two different linkers, thus forming two different CRPs (monomers
shown in Figure 2). We show that in these polymers the requirement of redox matching is met
and that the conductance of the polymer backbone is largely unaffected by the pendant group.
We also discuss the electrochemical characteristics of the materials within the framework of
cathode materials for water based quinone batteries.
2 Experimental
All reagents were purchased from Sigma-Aldrich and were used without further purifications
unless otherwise specified. Acetonitrile was dried over molecular sieves prior to use and
deionized water was used to prepare aqueous electrolytes. All electrochemical experiments
were conducted at ambient temperature and the electrolyte solutions were purged with N2
prior to measurements. Electrochemical measurements were performed on an Autolab
PGSTAT302N potentiostat (Ecochemie, The Netherlands) equipped with a bipotentiostat
module except for electrochemical quartz crystal microbalance (EQCM) measurements that
were performed on a VersaSTAT 3 potentiostat/galvanostat (Princeton Applied Research,
USA) together with a QCM922A quartz crystal microbalance (Princeton Applied Research,
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USA). All potentials are reported vs. standard hydrogen electrode (SHE) unless otherwise
specified.
2.1Monomer synthesis
Nuclear magnetic resonance (NMR) spectra were recorded on an Agilent MR (1H at 399.97
MHz, 13C at 100.58 MHz) spectrometer. Chemical shifts are reported in ppm relative to
tetramethylsilane (TMS) via the residual solvent signals: CDCl3, 1H at 7.26 and 13C at 77.0
ppm. The following abbreviations were used: s = singlet, d = doublet, t = triplet, q = quartet,
m = multiplet, dd = doublet of doublets, and br = broad. Solvents for extraction and
chromatography were of technical grade and were used without further purification. Flash
chromatography was performed using silica gel (200 ± 300 mesh). For mass spectra, samples
were dissolved in a mixture of water:methanol:formic acid 49:50:1, and 10 µL of this solution
was introduced via spray needle into an LTQ Orbitrap Velos Pro ETD mass spectrometer
using a spray voltage of 1.8 kV in the m/z range 100-1200. The resolution in the Fourier
Transform analyzer (Orbitrap) was 60 000. Spectra were collected for 2-3 min. Microwave
heating was performed with an Initiator+ microwave system (Biotage). See Supplementary
Figures S1-S7 for characterization spectra of M1.
2.1.1 M1 Synthesis
Bromohydroquinone (20 g, 0.106 mol) and 3,4-dihydro-2H-pyran (DHP) (50 mL) were
stirred in a round-bottom flask. Trifluoroacetic acid (TFA, 2-4 drops) was added to the
mixture and the solution was stirred at room temperature for 15 h. The reaction mixture was
diluted with diethyl ether (50 mL) and then poured into aqueous NH4Cl solution (saturated, 50
mL). After separation the organic layer was dried over MgSO4. The solvent was removed
under vacuum and the crude product was purified by chromatography (SiO2, pentane/ethyl
acetate, 10/1) yielding 1 as a white solid (33.32 g, 88%).
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1H NMR (CDCl3), δ = 7.28 (1H, dd, J = 2.8, 1.6 Hz), 7.06 (1H, dd, J = 9.0, 1.6 Hz), 6.94 (1H,
dd, J = 9.0, 2.8 HZ), 5.36 (1H, dt, J = 3.2, 3.1 Hz), 5.30 (1H, dt, J = 3.1, 2.6 Hz), 3.92 (2H,
m), 3.59 (2H, m), 2.07 (1H, m), 1.96 (2H, m), 1.90-1.80 (3H, m), 1.75-1.54 (6H, m).
13C NMR (CDCl3), δ = 152.20, 152.17, 148.4, 148.3, 121.5, 121.4, 117.9, 117.8, 116.5, 116.4,
113.49, 113.48, 97.6, 97.5, 97.2, 97.1, 62.0, 61.8, 30.31, 30.3, 30.24, 30.23, 25.3, 25.2, 18.67,
18.66, 18.42, 18.41 (Three diastereomers should give 48 signals, but due to overlapping peaks
only 28 are distinguishable).
m.p.: 65 °C.
HRMS, calculated for C16H21O4Na+: m/z = 379.0515, found: m/z = 379.0514.
Thiophene derivative 2 was prepared according to the procedure previously reported [30]. A
mixture of THP-protected 2-bromohydroquinone 1 (180 mg, 0.504 mmol), 2 (100 mg, 0.554
mmol), Pd(PPh3)2Cl2 (18 mg, 0.025 mmol), CuI (5 mg, 0.025 mmol), triphenylphosphine (26
mg, 0.101 mmol), diethylamine (0.8 mL, 7.73 mmol), and dimethylformamide (0.5 mL) was
stirred under nitrogen atmosphere in a heavy-walled glass Smith process vial at 120 °C for 25
min in the microwave cavity. The reaction mixture was then treated with diethyl ether,
filtered, and poured into 0.1 M aqueous HCl (5-10 mL). The organic layer was collected and
washed with brine (2 × 15 mL). It was then dried over MgSO4 and condensed under vacuum.
Following purification by chromatography (SiO2, pentane/ethyl acetate, 10/1), the crude
product 3 (100 mg) was used directly in the next step. After dissolution in a solvent mixture
(methanol/DCM, 2 mL/3 mL), TFA (2 drops) was added and the mixture was stirred at room
temperature for 12 h. Aqueous NaHCO3 (saturated, 2 mL) was added to the mixture that was
then separated between DCM and water. The organic layer was collected and dried over
MgSO4. After removing the organic solvent under vacuum the crude substrate was purified by
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chromatography (SiO2, pentane/ethyl acetate, 2/1), yielding M1 as a waxy yellow solid (50
mg, 35 %). The synthetic route is summarized in Scheme 1.
1H NMR (CDCl3), δ = 6.80 (1H, d, J = 8.7 Hz), 6.77 (1H, d, J = 3.1 Hz), 6.73 (1H, dd, J =
8.7, 3.1 Hz), 6.39 (1H, d, J = 3.8 Hz), 6.36 (1H, d, J = 3.8 Hz), 5.63 (1H, br s), 4.80 (1H, br
s), 4.40 (1H, m), 4.30 (1H, dd, J = 11.7, 2.2 Hz), 4.10 (1H, dd, J = 11.7, 7.1 Hz), 2.92 (1H,
dd, J = 17.2, 6.3 Hz), 2.85 (1H, dd, J = 17.2, 7.0 Hz).
13C NMR (CDCl3), δ = 151.3, 148.6, 141.1, 141.0, 117.8, 117.4, 115.5, 109.6, 100.2, 100.0,
91.2, 77.7, 71.6, 67.2, 22.1.
m.p.: 39 ⁰C.
HRMS calculated for C15H13O4S+: m/z = 289.0529, found: m/z = 289.0527.
2.1.2 M2 Synthesis
Monomer M2 was synthesized as previously reported [31] i.e. carboxylic acid 4 was reacted
with BBr3 in DCM at 0 °C, forming 5 and subsequently treated with NaOH to form 6. The
crude 6 was mixed with EDOT-MeBr and KHCO3 in anhydrous DMF and heated at 85 °C,
yielding M2. The synthetic route is summarized in Scheme 2.
2.2Polymerization
All polymerizations were performed using cyclic voltammetry where the potential was cycled
between -0.3 and 1.1 V (vs. Fc0/+) at 100 mV/s (25 mV/s for interdigitated array (IDA)
measurements). The number of cycles varied between 1 and 13. The organic electrolyte used
for polymerization was tetrabutylammonium hexafluorophosphate (TBAPF6, 0.1 M) in dry
acetonitrile (MeCN). A platinum wire was used as counter electrode and a Ag/Ag+ (10 mM
AgNO3, 0.1 M TBAPF6, -0.096 V vs. Fc0/+) electrode in a separate compartment was used as
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reference. Polymerizations were performed with a monomer concentration of 5 mM. After
polymerization the electrode was washed with MeCN and water. For EQCM measurements
the monomers were polymerized on a gold coated AT-cut quartz EQCM-crystal (8.95 MHz ±
30 kHz, Ø 5 mm) while the working electrode for IDA measurements was an IDA-electrode
with 90 pairs of Au bands on glass substrate (10 μm between bands, 150 nm high, MicruX
Technologies, Spain). For Scanning Electron Microscopy (SEM) images the monomers were
electropolymerized onto a graphite paper. For all other experiments a glassy carbon (GC)
electrode (3.0 mm diameter, BASi, USA) was used as working electrode.
2.3Electrochemical Characterization
The polymers were characterized by cyclic voltammetry in aqueous electrolyte with 1 M
sodium nitrate buffered with sodium acetate, monobasic sodium phosphate, and boric acid
(0.1 M each). A platinum wire was used as counter electrode and a Ag/AgCl (3 M NaCl,
+0.192 V vs. SHE) electrode was used as reference. The pH was adjusted with concentrated
HNO3 or NaOHaq and voltammograms were measured over the pH interval 0 to 10 for both
polymers at a scan rate of 10 mV/s. To obtain kinetic information, cyclic voltammetry
experiments at various scan rates, between 1 mV/s and 30 V/s, were performed. In these
experiments as well as for EQCM experiments the pH was adjusted to 0. Conductance
measurements were performed in situ during cyclic voltammetry as previously reported [32]
using a bipotentiostat with a voltage bias between the two working electrodes of 1mV for
polymerization and 10 mV for characterization. The conductance of the polymer was
determined from the bias voltage and the current passing between the two working electrodes.
The current, in turn, was evaluated from the sum of the absolute current responses (divided by
two in order to avoid double counting) from the two electrodes during the voltammetry
experiment. In order to expand the stability window of the electrolyte a “water-in-salt”
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electrolyte [5] was used consisting of a sodium chloride and hydrochloric acid (1 M) salt
slurry. SEM images were obtained using a Leo Gemini 1550 FEG SEM instrument (Zeiss,
Germany) operated at 7 kV with an in-lens secondary electron detector. No sputtering was
performed on the polymers prior to imaging as the material was found to be sufficiently
conducting to prevent charging.
3 Results and Discussion
3.1 Polymerization
During electropolymerization the irreversible oxidation peak seen above ~ 0.8 V (vs. Fc0/+)
(Figure 3) is related to polymer formation. For each scan an increased current at low
potentials can be seen that corresponds to an increased capacitive charge due to polymer
doping. The semi-reversible redox reaction centered at 0.34 V for polymer P1 and 0.36 V for
polymer P2 corresponds to the redox chemistry of the pendant group, showing a buildup of
quinone moieties concurrently with the polymer formation.
3.2 SEM Analysis
From the SEM images (Figure 4) of electropolymerized P1 and P2, open morphologies for
both CRPs can be seen with polymer grains aggregated to web-like structures. The grains
have diameters of about 500 nm and are separated by micrometer-sized pores. Since both
solvent molecules and the ions used in the electrochemical characterization experiments have
radii in the low-nm range and the pore size is around 1 µm both solvent molecules and ions
should easily be able to penetrate into the polymer pore structure. Moreover, since both
polymers show similar morphologies, no difference between the two polymers are expected
with respect to solvent and ion accessibility.
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3.3 Cyclic Voltammetry
The cyclic voltammograms (CVs) of polymers P1 and P2, seen in figure Figure 5a and Figure
5b respectively, are dominated by the chemically reversible hydroquinone/quinone redox
conversion peak centered at 0.70 V (for P1) and 0.63 V (for P2). In addition to the quinone
peak a capacitance increase, as compared to the bare electrode, can be seen at potentials on
both sides of the quinone peak. We attribute this capacitance to the doping of the PEDOT
backbone that is expected to provide a relatively featureless capacitance extending over the
entire potential interval studied, as judged from the CV of pristine PEDOT (dotted lines
Figure 5). Assuming a constant capacitance from the PEDOT backbone in the potential
interval investigated, the capacitance originating from the backbone was evaluated for P1 to
12 % of the total capacity and the remaining 88 % from the pendant group redox activity. For
P2 the capacitance originating from the backbone was evaluated to 22 % of the total capacity
and the remaining 78 % from the pendant group redox activity. P1 shows a slightly broader
quinone peak and a larger peak separation compared to P2 (P1 has a peak split of 75 mV and
P2 has a peak split of 16 mV, at 25 mV/s), which agrees with a lower apparent rate constant
for P1 (vide infra).
3.4 Kinetics
At scan rates (v) below 0.1 V/s the oxidation and reduction peak currents (ip) increase linearly
with scan rate and a plot of ln(ip) against ln(v) (Supplementary Figure S8) yields a linear
dependence with a slope of 1, indicating a reaction which is not diffusion-limited at these scan
rates. At higher scan rates, above 1 V/s, the oxidation and reduction peak potentials drift
apart. From the scan rate dependence of the peak potentials apparent rate constants (k0) can be
calculated using Equation 1 [33, 34],
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푅푇
푧퐹휈
푅푇푘0
퐸푝 = 퐸0′ ±
× ln (
)
(1)
훼푧퐹
where  is the transfer coefficient, z is the number of charges transferred, and E0’ is the formal
potential and where the plus sign signifies reduction and the minus sign oxidation. The
apparent rate constants were calculated from the linear regions (inset in Figure 6a) to 7.8 s-1
(oxidation) and 6.4 s-1 (reduction) for P1. For P2 the apparent rate constants were determined
to 20.3 s-1 (oxidation) and 26.7 s-1 (reduction) (see inset in Figure 6b). Both polymers show
fast kinetics compared to other battery materials, which might be partially due to the open
structures of the polymers (Figure 4) that allow for fast mass transport through the material. It
should, however, be notet that the polymers are both thin films (thickness in the low
micrometer range) and thus cannot be compared directly with most commercially available
battery materials. The rate constants for P2 are higher than those for P1 indicating faster
kinetics. This can also be perceived from the CVs (Figure 5), where the redox peaks for P2
are significantly narrower than those for P1. When increasing the scan rate the redox peaks
move apart and are broadend, which could either be due to mass transfer limitations or
sluggish redox conversion of the pendant group. The absence of a diffusion tail even at high
scan rates for both P1 and P2 (Figure 6) suggests that diffusion is not rate limiting, even at
scan rates above 1 V/s, and hence that the reaction must be limited by electron transfer. As the
two monomers building up the polymers differ only with respect to the linker it is tempting to
attribute the difference in rate constant to a rate limiting electron transfer between the pendant
and the polymer backbone. It is however unclear why the longer aliphatic linker in P2 should
be more favorable for fast electron transport than the short alkyne linker in P1. Alternatively,
the difference in rate might be due to subtle differences in polymer morphology. Noriega et al.
suggested that fast charge transport in conjugated polymers can be achieved in disordered
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materials through interconnected ordered regions in a predominantly amorphous matrix [35].
For the polymers in this work, i.e. CRPs, electron transfer between the ordered regions, where
charge transport predominantly occurs, and amorphous regions is required for full redox-
conversion of the material. We suggest that this electron transfer reaction is rate limiting and
hence that the difference in rate constants is due to the variations in the barriers or orbital
overlap involved in electron transport between amorphous and ordered regions of the
material.
3.5 pH-Dependence
The formal potential (E0ʹ) of the quinone reduction depends heavily on pH for both P1 and
P2. In the pH-interval between 0 and 10 (Figure 7, CVs for different pH of P1 in Figure S9
and of P2 in Figure S10) a slope of -61 mV/pH was seen for P1 and a slope of -59 mV/pH
was seen for P2. This indicates proton coupled redox reactions for both P1 and P2 with a one-
to-one stoichiometry between electrons and protons. As the quinone to hydroquinone redox
conversion involves two electrons (see Scheme 3) and since it is well known that the
semiquinone state is unstable with respect to disproportionation in water solution we attribute
the quinone peak to a 2e2H redox process.
The standard potentials (E0) for the quinone reduction in the polymers were evaluated by
extrapolating the pH-dependent lines in Figure 7 to pH 0. For P1 and P2 the formal potential
was calculated to 0.70 V and 0.63 V, respectively. The higher formal potential of P1 is likely
due to the alkyne in direct conjugation with the quinone moiety. An alkyne linker has
previously resulted in a higher formal potential for a different CRP [29]. A battery with P1 as
cathode material would have a higher voltage than with P2 as cathode material, if combined
with the same anode material.
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3.6 Mass changes during redox conversion
The frequency change recorded during cycling on the EQCM crystal was converted to mass
change using the Sauerbrey equation (Equation 2),
퐴(휌푞 × 휇푞)1/2
(2)
∆푚 = ∆푓
2(퐹푞2)
where Δm is the mass change, Δf is the frequency change, Fq is the reference frequency, A is
the area of the active surface, ρq is the quartz crystal density, and μq is the AT-cut quartz
constant. Thin films were used to ensure the validity of the Sauerbrey equation. The mass
change was plotted against charge, resulting in linear regions for which slopes could be
calculated. Multiplying the slope values (in g/C) with the Faraday constant (96 485 C/mol)
gave molecular weight values (g/mol).
The mass changes for both P1 and P2 show three linear regions per oxidation and reduction
sweep. Two regions correspond to polymer doping (gray fields in Figure 8a and Figure 8b)
and one region correlates with the quinone redox chemistry (blue fields in Figure 8a and
Figure 8b). The mass per charge changes in the different regions are listed in Table 1. During
polymer doping, i.e. in the potential regions on both sides of the quinone redox reaction, the
average apparent mass change per mole charge is 22 g/mol. None of the values in Table 1
correspond to the mass of nitrate (Mw = 62 g/mol) or any other anion in the electrolyte. The
charge compensating process during polymer doping must therefore involve a mixture of
nitrate uptake and cation expulsion. Given the acidic conditions used it is likely that proton
diffusion dominates the cation expulsion process. Under this assumption, and given that no
solvent molecules are involved in the mass transport, 37% of the charges are balanced by
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anion uptake while 63% are balanced by expulsion of protons initially present in the polymer
during oxidation. During polymer reduction the opposite mass transport processes are
occurring. In the potential region where the quinone is redox active the current is dominated
by the quinone redox chemistry and the mass is changing with ±3 grams per mole charge for
P1 and ±2 grams per mole charge for P2 (see Table 1). A mass decrease of ~1 g/mol
indicates, for oxidation, a proton leaving the polymer for each electron leaving, as would be
expected when the hydroquinone is oxidized to benzoquinone (see Scheme 3). Interestingly,
the small mass changes also indicate that the hydroquinone-to-quinone redox conversion
involves only very modest amounts of solvent motion. Together with the mixed anion-cation
mass transport during polymer doping it is therefore likely that the polymer film does not
experience large volume changes in the potential region investigated since i) solvent
molecules are unlikely to be involved during the pendant group redox chemistry and ii)
polymer doping involves a small net change in number of ions in the polymer matrix. Based
on the deposited mass on the EQCM electrode and the corresponding capacity of the polymer
layers the specific capacities of the polymers were estimated to 101 mAh/g (54 % of
theoretical capacity) for P1 and to 77 mAh/g (49 % of theoretical capacity) for P2. (Note,
however, that the mass used for normalization is based on the total deposited mass and energy
dispersive X-ray spectroscopy experiments (see Supplementary Data) indicate a high salt
concentration from polymerization that results in low specific capacities compared to the
theoretical capacities of the polymers).
3.7 In-situ Conductance
The use of a “water-in-salt” electrolyte enables conductivity measurements below the
traditional water stability window. The dependence of conductivity with polymer doping over
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an extended potential window, covering the onset of polymer doping (Figure 9c and Figure
9d), can thus be investigated. For both polymers there is a sharp sigmoidal increase in
conductance on going from the neutral state of the polymer to the oxidized/doped state, as is
commonly seen for unsubstituted PEDOT. The inflection points for oxidation (blue dashed
line in Figure 9c) and reduction (red dotted line in Figure 9c) of P1 are 0.094 V and -0.028 V,
respectively, and the inflection points for oxidation (blue dashed line in Figure 9d) and
reduction (red dotted line in Figure 9d) of P2 are 0.01 V and -0.03 V, respectively. The
similarity in conductivity onset potentials indicates that the polymer backbones in P1 and P2
are quite similar. However, the conductance hysteresis in P1 (0.12 V) is significantly larger
than the hysteresis in P2 (0.04 V) indicating that P1 undergoes more significant structural
changes upon polymer doping. Moreover, the conductance plateau, that follows the sigmoidal
increase at higher potentials, is also higher for P2 than for P1, indicating that P2 has a higher
intrinsic conductivity than P1. The increased conductance seen in P2 could plausibly account
for the faster kinetics seen for this polymer as a high conductance indicates an increased
degree of ordered regions. This would lead to an increased number of contacts between
ordered and amorphous regions in the polymer and hence result in an enhanced coupling
constant.
For both polymers a favorable polymer-pendant group redox matching is evident since the
quinone redox reaction (Figure 9a and Figure 9b) occurs in a potential region where the
polymer is conducting. Although the conductance is largely unaffected by the pendant group
redox chemistry, the conductivity of P1 displays a slight conductance decrease over the
quinone peak, Figure 9c. Similar, but much more dramatic effects of the pendant group redox
chemistry on polymer conductance have previously been reported for quinone substituted
PPy, and it was shown that this polymer-pendant group interaction was reduced by the use of
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a longer linker between the two moieties [29]. This effect also accounts for the results in this
work where P1, with a short linker, exhibits a small but noticeable perturbation of the
polymer conductance from the pendant group while P2 shows no effect of the pendant group
redox chemistry. With the longer and more flexible linker the conductivity of the conducting
polymer backbone in P2 has been decoupled from the quinone redox reaction, as can be seen
in Figure 9. We therefore propose that the more rigid PEDOT backbone, as compared to PPy,
is a better option for the construction of CRP based battery materials with quinone pendant
groups.
4 Conclusions
In this report we describe synthesis and electrochemical characterization of two PEDOT
based CRPs incorporating hydroquinone pendant groups, with particular emphasis of their use
as electrode materials in secondary battery applications. Successful combination of
conducting polymers with redox active pendant groups relies on the achievement of i) redox
matching between the polymer backbone and the pendant group and ii) the preservation of the
individual properties of the two active units, i.e. the intrinsic electronic conductivity of the
polymer backbone and the well-defined redox chemistry and high charge storage capacity of
the pendant group. In this report we show that both these criteria are met in both polymers. In
addition we demonstrate that the materials undergo quite modest mass changes due to a mixed
anion-cation mass transport during polymer doping and exclusive charge compensation by
proton transport during quinone redox conversion. Taken together we conclude that CRPs
based on PEDOT with quinone pendant groups provide a promising material for electrical
energy storage applications.
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Acknowledgements
Maria Vall is gratefully acknowledged for her assistance with the SEM images. This work
was founded by the Swedish Foundation for Strategic Research (SSF), the Swedish Research
Council (VR), the Carl Trygger Foundation, the Olle Engqvist Byggmästare Foundation, the
Swedish Energy Agency, and the European Union's Horizon 2020 Programme (H2020/2014-
2020) under grant agreement n° 644631 (ROLL-OUT).
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Figure 1. Schematic picture of a conducting redox polymer.
Figure 2. Chemical structures of the monomers (M1) and (M2), which subsequently where electropolymerized to polymer
P1 and P2, respectively.
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Figure 3. Electropolymerization of a) M1 (10 scans, forming polymer P1) and b) M2 (13 scans, forming polymer P2) onto
GC electrodes with a scan rate of 100 mV/s. Monomers (5 mM) were dissolved in dry MeCN containing 0.1 M TBAPF6.
Figure 4. SEM image at 10,000 x magnification of a) P1 and b) P2 electropolymerized on a graphite paper.
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Figure 5. CV of a) P1 and b) P2 characterized in aqueous NaNO3 buffer (pH 0) at a scan rate of 25 mV/s. Red lines
correspond to an empty GC electrode and black dashed lines correspond to a PEDOT covered electrode used for comparison.
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Figure 6. a) CVs of P1 at scan rates between 0.1 V/s and 30 V/s. b) CVs of P2 at scan rates between 0.3 V/s and 30 V/s. The
insets show the peak potential dependence on scan rate. From the linear regions the apparent rate constants were for P1
calculated to 7.8 s-1 for oxidation and to 6.4 s-1 for reduction and for P2 the rate constants were calculated to 20.3 s-1 for
oxidation and to 26.7 s-1 for reduction. Characterized in aqueous NaNO3 buffer (pH 0).
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Figure 7. pH dependence of P1 (red circles) and P2 (black squares). Characterized in aqueous NaNO3 buffer at a scan rate of
10 mV/s. pH adjusted with NaOH and HNO3.
Figure 8. a) EQCM data for P1. b) EQCM data for P2. Mass change during an oxidation sweep is shown. Gray regions show
oxidative doping of the PEDOT backbone (ca +22 g/mol) and blue region shows oxidation of the quinone pendant group (-2
g/mol). Characterized in aqueous NaNO3 buffer (pH 0) at a scan rate of 25 mV/s.
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Figure 9. CV of a) P1 and b) P2, and conductance data for c) P1 and d) P2. The red dotted line marks inflection points for
reduction and the blue dashed line marks inflection points for oxidation. Measurements performed in a sodium chloride and
hydrochloric acid (1 M) salt slurry at a scan rate of 10 mV/s at 22 °C, using an IDA electrode.
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Scheme 1. Synthetic route to M1.
Scheme 2. Synthetic route to M2.
Scheme 3. Oxidation/reduction of hydroquinone/benzoquinone.
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Table 1. Mean molecular weight of molecules added to and/or expelled from the polymer films during a CV experiment.
Mw (g/mol)
Lower potential region Quinone region Higher potential region
Oxidation of P1 +18
Reduction of P1 -6
Oxidation of P2 +20
Reduction of P2 -20
-3
+25
-27
+21
-36
+3
-2
+2
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