Cathodic deposition of polypyrrole enabling the one-step assembly of metal-polymer hybrid electrodes

Article abstract of DOI:10.1002/anie.200903596

Deposit account: The generation of oxidizing agents in situ allows the cathodic deposition of polypyrrole (ppy) by oxidative polymerization. Conducting polymers are synthesized with new nanoscale morphologies (e.g., nanospheres creating a three-dimensiona

Full text of DOI:10.1002/anie.200903596

Angewandte
Chemie
DOI: 10.1002/anie.200903596
Conducting Polymers
Cathodic Deposition of Polypyrrole Enabling the One-Step Assembly
of Metal–Polymer Hybrid Electrodes**
Yongju Jung, Nikhilendra Singh, and Kyoung-Shin Choi*
Conducting polymers combining the advantages of organic
polymers and the electronic properties of semiconductors are
attractive materials for use in energy conversion/storage,
optoelectronics, coatings, and sensing applications.[1–7] The
polymerization process of conducting polymers initiates with
chemical or electrochemical oxidation of monomers to
radicals, which is followed by radical coupling and chain
propagation.[2] Chemical oxidation involves the use of oxidiz-
ing agents, such as FeCl3, while electrochemical oxidation is
achieved by applying an anodic bias to a conducting substrate
immersed in a monomer solution (anodic electropolymeriza-
tion).[2,3] The electropolymerization method has been pre-
dominantly used to prepare film- or electrode-type conduct-
ing polymers, as it allows for polymerization confined to the
working electrode with a facile control over film thickness
and morphology.[3]
Conducting polymers have also been utilized as a matrix
to embed or disperse metal particles (e.g., Cu, Au, Ag, Ni, Ru,
Ir, Pt, Co, Pd, Fe) to form metal–polymer hybrid electrodes
for use in sensors and electrocatalysts.[8–13] Typically, these
hybrid electrodes are prepared by a two-step electrodeposi-
tion process: electropolymerization followed by metal depo-
sition. This two-step process not only makes the synthesis
cumbersome and expensive but also limits the types and
qualities of the metal–polymer composite architectures that
can be assembled. However, no synthesis strategy that
enables one-step synthesis of metal–conducting polymer
hybrid films has been made available to date. This is because
electropolymerization and metal deposition require an oxi-
dation and a reduction reaction at the working electrode,
respectively, with a significantly different range of potentials.
Herein, we report an electrochemical method that allows
cathodic deposition of polypyrrole (ppy) for the first time.
The cathodic deposition method creates many new possibil-
ities for assembling conducting-polymer and conducting-
polymer-based hybrid electrodes that cannot be achieved by
conventional anodic polymerization. First, conducting-poly-
mer films or coatings can be deposited on substrates that are
not stable under anodic deposition conditions. Second, the
nucleation and growth processes of conducting polymers
during cathodic deposition are different from those of anodic
deposition, which results in new micro- and nanoscale
polymer morphologies. Third, one-step electrodeposition of
metal–conducting polymer hybrid electrodes becomes possi-
ble because both the polymerization and metal reduction
reactions can occur under the same cathodic conditions. In
this study, we demonstrate the use of cathodic polymerization
for the production of high-surface-area ppy electrodes and the
one-step synthesis of tin–ppy composite electrodes. The
resulting tin–ppy electrodes were characterized for use as
anodes in Li-ion batteries.
The cathodic deposition of conducting polymers was
achieved by coupling two redox reactions. The first reaction is
electrochemical generation of an oxidizing agent, the nitrosyl
ion (NO+). The production of NO+ ions involves reduction of
nitrate ions (NO3 ) to nitrous acid (HNO2) [Eq. (1)].[14,15]
ꢀ
HNO2 is amphoteric, and can generate various species in
solution depending on the pH. Under mildly acidic condi-
ꢀ
tions, HNO2 is the major species but it dissociates into NO2
and H+ as the pH increases (pKa = 3.3).[16] In strongly acidic
conditions, HNO2 reacts with H+ ions and generates the NO+
ion [Eq. (2), pKa of H2NO2+ = ꢀ7], which is a strong oxidizing
agent.[16–18]
NO3ꢀ þ 3 Hþ þ 2 eꢀ $ HNO2 þ H2O
ð1Þ
ð2Þ
HNO2 þ Hþ $ H2NO2 $ NOþ þ H2O
þ
The second reaction is chemical oxidation of pyrrole by
NO+ ions, which initiates the polymerization process. Since
the oxidizing agents are generated in situ only at the working
electrode, polymerization occurs predominantly on the work-
ing electrode, which results in deposition of electrode-type or
film-type conducting polymers at the cathode. Spectroscopic
detection of electrochemically generated NO+ ions and a
study of pH-dependent NO+ formation can be found in the
Supporting Information (Figure S1). We believe that this is
the first example of utilizing electrochemically generated
NO+ ions for cathodic deposition of a material that is typically
obtained by anodic deposition.
[*] Dr. Y. Jung,[+] N. Singh,[+] Prof. K.-S. Choi
Department of Chemistry, Purdue University
West Lafayette, IN 47907 (USA)
Fax: (+1)765-494-0239
E-mail: kchoi1@purdue.edu
The typical deposition conditions include the use of an
aqueous solution containing 0.4m HNO3, 0.5m NaNO3, and
0.2m pyrrole (the pH of the freshly made solution was 0.4) as
a plating solution. The working electrode was copper foil and
the counter electrode was 1000 ꢀ of platinum deposited on
200 ꢀ of titanium on a glass slide by sputter coating.
Electrodeposition was carried out at E = ꢀ0.65 V versus an
[+] These authors contributed equally to this work.
[**] This work was supported by the Basic Energy Sciences Program of
the US Department of Energy (DE-FG02-05ER15752) and made use
of the Life Science Microscopy Facility at Purdue University. We
thank Anna Kempa-Steczko for ICP–AES analysis.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200903596.
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Ag/AgCl/4m KCl reference electrode at room temperature.
phase as the only inorganic phase present in this film
(Supporting Information, Figure S2).
SEM images of tin–ppy hybrid electrodes show that the
hybrid film maintained the original ppy framework composed
of ppy nanospheres creating a porous network (Figure 2a).
Efficient deposition of ppy films was possible only when the
pH was lower than 1.5, because an appreciable amount of
NO+ species can be generated by electrochemical reduction
of NO3ꢀ only in a strongly acidic environment.
Scanning electron microscopy (SEM) shows that the
resulting ppy film contains spherical particles with diameters
ranging from 50 to 200 nm creating a three-dimensional
porous network, which can be highly beneficial for applica-
tions that require conducting-polymer electrodes with high
surface areas (Figure 1a). This morphology is unique in that
Figure 1. SEM images of ppy films deposited under a) cathodic
(E=ꢀ0.65 V) and b) anodic conditions (E=+0.80 V) using an aque-
ous solution containing 0.4m HNO3, 0.5m NaNO3, and 0.2m pyrrole
(pH 0.4).
Figure 2. SEM images of a) pure ppy spheres and b) tin–ppy hybrid
spheres. c) TEM image of a cross-sectioned tin–ppy hybrid sphere and
d) BSE image of a tin–ppy hybrid electrode.
anodically prepared ppy films typically possess two-dimen-
sional planar surface morphologies. An SEM image of a ppy
film deposited anodically (E = + 0.80 V vs. Ag/AgCl) using
the same plating solution is shown in Figure 1b for compar-
ison. (A platinum working electrode was used in this case, as
copper foil immediately oxidizes upon application of an
anodic potential.) This film displays similar spherical features
on the surface, but its surface is essentially two dimensional in
nature and lacks mesoporosity.
The new cathodic polymerization method described
above opens up a new possibility for preparing metal–
conducting polymer hybrid electrodes through one-step
deposition, because a broad range of metals can be cathodi-
cally deposited at the bias used to generate NO+. During such
a co-deposition process, new composite architectures can be
generated because metal deposition and polymer deposition
influence each other, thus altering their nucleation and
growth patterns. In addition, co-deposition will increase the
uniformity and degree of metal dispersion within the con-
ducting-polymer matrix compared to a two-step deposition
(anodic polymerization followed by metal deposition).
To demonstrate the effectiveness of this one-step syn-
thesis, we assembled tin–ppy hybrid electrodes, which can be
used as an anode material for Li-ion batteries.[19,20] The tin–
ppy hybrid films were prepared by simply adding 0.1m SnCl2
to the plating solution used to deposit ppy films. Cathodic
deposition was carried out at the identical potential used to
deposit ppy films with the bath temperature increased to 458C
to help dissolution of SnCl2. The X-ray diffraction study of the
resulting hybrid film showed the presence of a crystalline Sn
However, the surface of the ppy spheres became noticeably
rough because of the presence of tin nanoparticles (Fig-
ure 2b). A transmission electron microscopy (TEM) image of
a cross-sectioned tin–ppy sphere shows that Sn particles are
evenly coated on the surface of the ppy spheres (Figure 2c).
Analysis of multiple cross-sectional TEM images suggests
that the thickness of the tin coating layer on the ppy spheres
ranges from 25 to 100 nm. The uniformity of tin deposition on
the ppy spheres could also be confirmed by taking a back-
scattered electron (BSE) image (Figure 2d). In the BSE
image, tin particles with higher electron density should appear
brighter than ppy spheres. The fact that all spheres displayed
even contrast, instead of showing scattered and isolated
brighter spots on the ppy spheres, indicates that the tin layers
were formed uniformly on these spheres and no bare ppy
surfaces were exposed at the surface.
Tin metal has been reported as an attractive anode
material for high-energy-density Li-ion batteries because of
its high theoretical specific capacity for lithium (993 mAhgꢀ1,
corresponding to the formation of Li4.4Sn).[19,20] However, its
significant volume change upon insertion and extraction of
lithium (up to 300%) causes pulverization resulting in poor
cycle performance, and has limited the use of tin anodes in
commercial Li-ion batteries.[19,20] One of the most common
approaches to overcoming this problem is to combine tin with
an electrochemically inactive buffer matrix that can accom-
modate the volume change of tin during cycling.[21–24]
Decreasing the size and increasing the degree of dispersion
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of metallic tin in the matrix can be beneficial for improving
the cycle properties, because the volume changes caused by
smaller domains can be more easily accommodated by the
matrix.[24] The use of tin nanoparticles can also be advanta-
geous for increasing the rate capability, as it decreases the
diffusion length of Li+ ions to complete the alloying and de-
alloying processes.[24,25]
In this context, the tin–ppy hybrid morphologies shown in
Figure 2 look promising for improving both cycle properties
and rate capabilities. The ppy spheres can serve as an
excellent buffer matrix that can elastically accommodate the
volume expansion of nanoparticulate tin layers during
cycling. In addition, the thin coating layers of tin on a highly
porous ppy network will ensure facile Li-ion diffusion in and
out of the anode, thus resulting in high rate capabilities.
Another advantage of our hybrid electrode is the high weight
content of tin in the anode (two-component system). Typi-
cally, the preparation of tin anodes involves mixing tin
particles with a polymer binder and conducting additives
(three-component system). In our case, however, the as-
deposited tin–ppy electrode was used directly to assemble Li-
ion batteries without any additional binding material or
conducting additives, because tin particles were electrode-
posited with an excellent adhesion to the ppy spheres and
good electrical continuity between the particles within the tin
layers. It is worth mentioning that the ppy does not serve as a
conducting agent in the tin–ppy anode because ppy is not
conductive in the potential range used for electrochemical
evaluation of tin–ppy/Li cells (0.0 Vꢁ E vs. Li+/Li ꢁ
2.0 V).[26–28] As a result, the tin content in the hybrid electrode
could be increased up to 95 wt%. The tin content in the
hybrid electrodes used for electrochemical characterization
discussed below was 88 wt% (determined by inductively
coupled plasma–atomic emission spectroscopy).
Figure 3. a) First (c) and second (g) charge/discharge curves of
the as-prepared tin–ppy hybrid. b) Cycle properties of the tin–ppy
The potential profiles of the tin–ppy hybrid electrodes for
the initial two cycles (formation step) obtained at a rate of
0.2 C (1 C = 993 mAgꢀ1) are shown in Figure 3a (see the
Supporting Information for experimental details). The cou-
lombic efficiency for the first cycle (64%) was poor as a result
of the high irreversible capacity observed during the first
discharge process. This is typical behavior for systems
containing nanostructured electrochemically active materials
that create large electrode/electrolyte contact areas.[20] The
high coulombic efficiency for the second cycle (94%)
indicates that a stable solid-electrolyte interphase (SEI) was
formed during the first cycle. The potential profiles of a pure
tin electrode, which was electrochemically deposited using a
sulfate bath and contained the same amount of tin as the
hybrid electrode,[29,30] are shown in the Supporting Informa-
tion (Figures S3 and S4) for comparison. Unlike the tin–ppy
hybrid electrode, the pure tin electrode showed a drastic
capacity decrease during the second discharge process.
The cycle performance and coulombic efficiency of the
tin–ppy hybrid electrode up to 50 additional cycles after the
formation step is shown in Figure 3b. A rate of 1 C was used
for both charging and discharging processes. The initial
capacity of the hybrid electrode, 942 mAhgꢀ1 of Sn, corre-
sponds to 829 mAhgꢀ1 of composite (88 wt% of tin). This
value is approximately 2.5 times larger than that of commer-
hybrid electrode at a 1 C rate achieved after the formation step (charge
*
*
capacity ( ) and coulombic efficiency ( )). c) Rate capability of the
*
tin–ppy hybrid electrode ( ) achieved at various C rates after the
formation step. The C rates for charging were changed sequentially
through the cycles (0.2!0.5!1!2!5!10 C) whereas the discharge
rate was fixed at 0.2 C. Data obtained at a fixed discharge/charge rate
*
of 0.2 C through all cycles are also shown for comparison ( ).
cialized graphite anodes (ca. 330 mAhgꢀ1 of composite),[19]
which suggests that with proper optimization the tin–ppy
hybrid electrode may be used as anode material for future
high-energy-density Li-ion batteries. After 50 cycles, the tin–
ppy hybrid electrode showed a capacity retention of 47%.
This is a remarkably enhanced performance compared to
pure tin electrodes with a comparable thickness (ca. 10 mm),
which typically show a severe capacity fading within a few
cycles.[31] This finding suggests that the composite structure, in
which ppy nanospheres provided a high-surface-area sub-
strate to deposit tin as thin coating layers, efficiently sup-
pressed pulverization and enhanced the cycling property of
tin. Since the one-step production of tin–ppy electrodes did
not require any more time or effort than electrodeposition of
pure tin electrodes, the cathodic polymerization method
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enhance the performance of the tin anode.
The rate capabilities of the tin–ppy hybrid electrodes with
varying C rates are shown in Figure 3c. Data obtained with a
fixed discharge/charge rate of 0.2 C through all cycles are also
shown for comparison. These data indicate that when the
C rate was increased from 0.2 to 5 C, only an 18% reduction
of the charge capacity was observed (from 875 to
718 mAhgꢀ1), which suggests the possibility of using tin–ppy
hybrid electrodes as high-power-density as well as high-
energy-density anodes. This outstanding rate capability is a
result of the significant reduction of the diffusion length of Li
ions required for complete utilization of tin in the hybrid
structure. The results shown in Figure 3 were obtained from
as-deposited tin–ppy electrodes. Therefore, further enhance-
ments in capacity retention and rate capability are expected
with proper optimization efforts (e.g., composition and
morphology tuning in the hybrid electrodes, addition of a
protective coating on the tin layer).[25,32,33]
In summary, we have developed a new synthesis strategy
that enables cathodic deposition of conducting polymers by
utilizing cathodically generated oxidizing agents for oxidative
chemical polymerization of conducting polymers. This
method enabled not only the synthesis of conducting poly-
mers with new nanoscale morphologies but also the one-step
synthesis of metal–conducting polymer hybrid electrodes.
Because of the ability of cathodic polymerization to combine
a variety of conducting polymers and metals with new
composite architectures in
a simple and cost-effective
manner, the method described in this study will significantly
broaden our ability to fabricate conducting-polymer and
conducting-polymer-based composite electrodes for use in
various applications.
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Received: July 2, 2009
Published online: September 28, 2009
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Keywords: cathodic deposition · conducting materials ·
hybrid electrodes · polymers · tin
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