Unusual Internal Electron Transfer in Conjugated Radical Polymers

Article abstract of DOI:10.1002/anie.201705204

Nitroxide-containing organic radical polymers (ORPs) have captured attention for their high power and fast redox kinetics. Yet a major challenge is the polymer's aliphatic backbone, resulting in a low electronic conductivity. Recent attempts that replace the aliphatic backbone with a conjugated one have not met with success. The reason for this is not understood until now. We examine a family of polythiophenes bearing nitroxide radical groups, showing that while both species are electrochemically active, there exists an internal electron transfer mechanism that interferes with stabilization of the polymer's fully oxidized form. This finding directs the future design of conjugated radical polymers in energy storage and electronics, where careful attention to the redox potential of the backbone relative to the organic radical species is needed.

Full text of DOI:10.1002/anie.201705204

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Accepted Article
Title: Unusual Internal Electron Transfer in Conjugated Radical
Polymers
Authors: Fei Li, Danielle Gore, Shaoyang Wang, and Jodie Lutkenhaus
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Unusual Internal Electron Transfer in Conjugated Radical Polymers
Fei Li, Danielle N Gore, Shaoyang Wang and Jodie L Lutkenhaus*
Abstract: Nitroxide-containing organic radical polymers (ORPs) have
captured attention for their high power and fast redox kinetics. Yet a
major challenge is the polymer’s aliphatic backbone, resulting in a low
electronic conductivity. Recent attempts that replace the aliphatic
backbone with a conjugated one have not met with success. The
reason for this is not understood until now. We examine a family of
polythiophenes bearing nitroxide radical groups, showing that while
both species are electrochemically active, there exists an internal
electron transfer mechanism that interferes with stabilization of the
polymer’s fully oxidized form. This finding directs the future design of
conjugated radical polymers energy storage and electronics, where
careful attention to the redox potential of the backbone relative to the
organic radical species is needed.
Polythiophenes
bearing pendant
TEMPO
(2,2,6,6-
Tetramethylpiperidine 1-Oxyl) radicals with varying alkyl spacer
groups (CRP-4, CRP-6 and CRP-8) were synthesized according
to our prior report.[11] Poly(3-butylthiophene) (P3BT),
polythiophene bearing no pendant nitroxide radicals, was
synthesized electrochemically and used as a control (Figure 1a
and Figure S1).
a
Redox active polymeric battery electrodes have attracted a
great deal of attention due to their unique features in portable and
flexible electronic devices over recent years.[1-2] Organic radical
polymer (ORP) batteries are especially interesting because of
their exceptionally fast electron transfer kinetics.[3-5] For example,
PTMA (poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate)
has a theoretical capacity of 111 mAh/g[6] (comparable to
inorganic metal oxides such as LiCoO2), redox potential of 3.6 V
vs Li/Li+, nearly 100% Coulombic efficiency, and electrochemical
stability beyond 500 cycles.[7] However, success of PTMA and
similar ORPs as cathode materials depends on improving its
electronic conductivity. Otherwise, carbon additives are required,
which dilutes the active material.[8]
Recent approaches to improve the conductivity and capacity
of ORPs have focused upon replacing the aliphatic backbone with
conjugated units to form a conjugated radical polymer (CRP). The
conjugated backbone adds a second site for redox activity and an
electron-conducting pathway. However, polypyrrole[9] and
polythiophene[10] with tethered nitroxide radicals did not show
improved cathode performance or conductivity. The reason for
this was not immediately clear at the time.
Figure 1. (a) Chemical structures of polythiophene conjugated radical polymers
(CRPs) with alkyl spacers (n=4, 6, 8) and control polymer P3BT. (b) Cyclic
voltammograms (CVs) of CRPs and P3BT at 0.5 mV/s. (c) Peak current vs.
[scan rate]1/2 for CVs of CRPs. (NO: TEMPO radical; PT: polythiophene).
Measurements were taken using a three-electrode cell with the polymer as the
working electrode, lithium ribbons as counter and reference electrodes, and 0.5
M LiClO4 in propylene carbonate as the electrolyte.
Cyclic voltammetry at a scan rate of 0.5 mV/s (Figure 1b)
indicates little peak separation between the anodic and cathodic
peaks for the CRPs (~ 65 mV), suggestive of a very reversible
and facile reaction. The E1/2 for the CRPs was around 3.60 V vs.
Li/Li+, consistent with that of PTMA, and is, therefore, assigned to
the nitroxide radical.[12] In contrast, P3BT exhibited a higher E1/2
of 3.88 V[13] as well as broad and weak redox peaks, which is a
typical feature of polythiophenes.[14] It is noted that the CRP
oxidation potential increased slightly with increasing alkyl spacer
length, whereas the reduction potential remained constant. This
is probably due to the fact that the longer alkyl chains are more
electronically insulating, which impedes both electron transfer and
ion diffusion.[15-16] Higher oxidization potentials are required to go
from the insulating to the conductive form, but the reverse is not
required for the reduction process. As the scan rate increased, a
linear relationship between the square root of the scan rate and
the peak current was obtained for all polymers (Figure 1c and
Figure S2), confirming diffusion-limited processes.
Here, we hypothesized that interactions between the radical
unit and the conjugated backbone were responsible for the low
capacity and modest conductivity of CRPs. This questions
whether the radical unit and the conjugated backbone can be
treated as separate entities or whether there is coupling and
electron transfer between the two. This issue has not yet been
examined in CRPs, and is of utmost importance to the future
design of these promising materials.
[*]
Dr. F. Li, D. N Gore, S. Wang and Prof. J. L. Lutkenhaus
Artie McFerrin Department of Chemical Engineering and
Department of Materials Science and Engineering,
Texas A&M University
3122 TAMU, College Station, TX 77843-3122, United States
E-mail: jodie.lutkenhaus@tamu.edu
Supporting information for this article is given via a link at the end of
the document.
The galvanostatic charging response for each CRP (Figure 2
a-c, Figure S3) exhibited contributions from both the nitroxide
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radical unit and the polythiophene backbone at low current (e.g.,
2 A cm-2). Upon charging, a flat plateau around 3.5 – 3.7 V
appeared, which is a typical feature of the nitroxide radical.[6] As
charging proceeded further, the potential increased and then
plateaued around 4.1 V, which originates from the oxidation of the
polythiophene backbone.[14] This second plateau disappeared at
higher rates, which indicates sluggish electron transfer kinetics for
the polythiophene unit relative to the more reversible nitroxide
radical group. The P3BT control, on the other hand, exhibited
pseudocapacitive behavior with no distinguishable plateaus
(Figure 2d).
galvanostatic charging to varying potential cut-offs at a current
density of 5 A cm-2. For CRP-4, it is clearly seen in Figure 3a that
the nitroxide radical plateau around 3.5-3.6 V dominated the
charge storage process, where 86% of total charging capacity
was stored in this step. Further charging from 3.7 V to 4.2 V
accounted for only 16% of the overall capacity, indicating minimal
participation of the polythiophene backbone. Figure 3b shows the
normalized specific capacity, where it is noted that the longer octyl
spacer required a higher potential to initiate the charging process,
similar to the P3BT control. This shows that the nitroxide radical
dominates charge storage, and that the longer spacer group
indeed impacts charge transfer between the nitroxide radicals and
polythiophene backbone, as the charge/discharge curves of CRP-
8 partially resembles that of P3BT.
Figure 2. Galvanostatic charge-discharge response of polymers at different
currents. (a) CRP-4 (b) CRP-6, (c) CPR-8 and (d) P3BT; The arrows in panel
(a) apply to panels (b), (c), and (d). Dashed lines in panel (b) mark typical
respective charging plateaus for NO and PT in CRPs.
From the discharge curve, the capacities were calculated and
compared to theoretical values. At the lowest rate of 2 A cm-2,
the areal capacities for CRP-4, CRP-6, and CRP-8 were 1.70,
1.26 and 0.77 Ah cm-2, respectively, corresponding to specific
capacities of 68.0, 44.0, 21.5 mAh g-1; these are 38.2%, 27.7%,
and 14.6% of their corresponding theoretical capacities. In
comparison, the P3BT control had a capacity of 40.5 mAh g-1,
which was 21.1% of its theoretical value. This result is also on par
with earlier investigations.[17] Each of these capacities are lower
than that of PTMA (77-110 mAh g-1). This indicates that the CRP’s
polythiophene backbone either suppresses or does not enhance
redox activity.
Figure 3. (a) CRP-4 charged to different cutoff voltages, (b) Normalized specific
capacity of the CRPs and P3BT vs cut-off voltages. The current density was 5
A cm-2 (c) Illustration of charging process of CRP-4 at 5 A cm-2. The green
and the purple portions indicate contributions from the nitroxide radicals,
polythiophene backbones, respectively.
Open circuit potential (OCP) monitoring was conducted to
explain the charge transfer equilibrium and the previously
observed low capacity (Figure 4a and Figure S5-S6). After
equilibrating the CRPs at 4.2 V, the bias was removed and the
potential was allowed to drift. All CRPs displayed a sharp voltage
drop within the first 1000s, whereas P3BT displayed only a
gradual voltage drop. With further time (6 h), the OCP gradually
plateaued to around 3.6 – 3.7 V for the CRPs; this value is
consistent with that of the nitroxide radical’s potential. P3BT
exhibited an OCP decay to around 3.8 V, indicative of the higher
oxidation potential of the polythiophene.
Figure S4 shows the areal capacity of the CRPs and P3BT at
various current densities. It is noted polymer that CRP-6 had a
better rate capability than that of the other three polymers as
shown in Figure S4, although CRP-4 generally had the highest
capacity. CRP-8 and P3BT, on the other hand, had the poorest
rate capability and capacities. This suggests that the alkyl spacer
influences charge transfer, but the exact mechanism is beyond
the scope of this study. We speculate that the alkyl spacer affects
chain packing, radical-radical and radical-backbone proximity,
and dopant accessibility.
To isolate the contributions of the nitroxide radical group and
polythiophene backbone to the charging capacity, we carried out
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The rapid initial drop in OCP for the CRPs implies a possible
internal electron transfer from the nitroxide radical to the
polythiophene backbone (Figure 4b). At 4.2 V there exists both
oxidized nitroxide radicals and thiophene units, with some
minority not oxidized. This is because of the overlap of the redox
activity of the nitroxide radical and the polythiophene backbone
(Figure 1b). Once the bias is removed, any un-oxidized nitroxide
radicals transfer an electron to the oxidized thiophene unit, thus
reducing and dedoping the polythiophene backbone and oxidizing
the nitroxide radical. In this relaxed state, the OCP will reflect that
of the nitroxide radical, with little contribution from the
polythiophene backbone (Figure 4a inset). These results explain
the previously observed lower than theoretical capacities and
moderate conductivities.[9-10, 18] Conductivities of CRP-4, CRP-6,
and CRP-8 were 8.1 x 10-7, 3.6 x 10-7, and 6.2 x 10-9 S cm-1,[11]
respectively; whereas P3BT had a measured conductivity of 1.6
x 10-4 S cm-1. The conductivity differences between the CRPs and
P3BT is a consequence from the internal electron transfer
process. The lower conductivity in CRP-8 is possibly due to the
longer alkyl spacer that impacted the packing of the polymer
chains. After charging the electrode, the CRP potential decays as
the polythiophene backbone is reduced, leading to a dedoping of
the backbone and a reduction in conductivity.
indicates dedoping of the backbone during OCP decay. Similarly,
the intensity of the polythiophene cation (polarons) at 721 nm[19-
20]
also increased from 0.49 to 0.58, which suggests conversion
of bications (bipolarons) (> 1100 nm) to cations[21-22] (Figure 5d) –
further supporting reduction of the polythiophene backbone. In
comparison, P3BT showed a similar trend (Figure 5b), with more
pronounced polymer cation conversion as compared to CRP-6.
However, the intensity of the peak associated with
polythiophene’s neutral state for P3BT increased less than that of
CRP-6, indicative of a spontaneous reduction of polythiophene
cations by nitroxide radicals (Figure 4b). This is
thermodynamically favored because polythiophene (3.88 V) has
a higher oxidation potential than the nitroxide radical (3.60 V).
Such internal electron transfer is also suggested for
PTMA/LiFePO4 hybrid electrodes.[23]
Figure 5. In-situ spectroelectrochemistry of (a) CRP-6 and (b) P3BT. Absorption
for CRP-6 and P3BT at different wavelengths: (c) Neutral, de-doped
polythiophene (384 and 401 nm); (d) polythiophene cation (721 nm).
In conclusion, the low capacity and conductivity of CRPs
consisting of nitroxide radicals tethered to a polythiophene
backbone is caused by internal electron transfer occurring
between the two moieties. This is supported by evidence of a two-
step charging process, in which the nitroxide radicals contributed
to over 80% of the capacity. In situ spectroelectrochemistry during
open circuit potential decay revealed evidence of
a rapid
Figure 4. (a) OCP monitoring for 6 h. The electrodes were first charged by linear
sweep voltammetry to 4.2 V and held at constant potential for 50 s (inset:
schematic illustration of CRPs before and after open circuit potential decay); (b)
Schematic illustration of the internal electron transfer process occurring for the
CRPs.
dedoping of the polythiophene backbone caused by internal
transfer of an electron from the nitroxide group to the
polythiophene backbone. These results explain why conjugated
radical polymers to date have exhibited such poor capacity, yet
these results also bring to light interesting directions for future
research in their design. To achieve a stable conjugated radical
polymer, it will be essential to manipulate the redox potential of
both the radical unit in reference to the conjugated backbone. In
this study, the radical unit’s redox potential was below that of the
conjugated backbone; reversing or equalizing these potentials
might provide alternative handles to adjust the stability and
capacity of these materials. Alternatively, conjugated radical
polymers are potentially interesting as protective materials in
which they prevent overcharging by internally regulating an
To further reveal changes in the electronic structure of the
CRPs,
we
conducted
in-situ
spectroelectrochemical
measurements on CRP-6 and P3BT during the OCP decay
process (Figure 5). As the OCP decayed from its initial value of
4.2 V for CRP-6, the UV-vis absorption intensity at 384 nm
increased from 0.36 to 0.43 (a.u.) during the first 1000 s and
reached a plateau thereafter (Figure 5a and Figure 5c); this band
is attributed to the undoped form of polythiophene[19]
, and
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[9]
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Research supported by the U.S. Department of Energy (DOE),
Office of Science, Basic Energy Sciences (BES), under Award #
DE-SC0014006 (electroanalytical chemistry) and by the Welch
Foundation under Award # A-1766 (polymer synthesis and
characterization).
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Keywords: Polythiophene, conjugated radical polymer, lithium
polymer battery, organic radical polymer, TEMPO
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COMMUNICATION
Conjugated radical polymers:
Evidence for the internal transfer of an
electron from a nitroxide group to the
polythiophene backbone explains the
conductivity and redox activity of
polythiophenes bearing pendant
nitroxide radicals
Fei Li, Danielle N Gore, Shaoyang Wang
and Jodie L Lutkenhaus*
Page No. – Page No.
Unusual Internal Electron Transfer in
Conjugated Radical Polymers
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