In situ synthesis of electroactive conjugated microporous fullerene films capable of supercapacitive energy storage

Article abstract of DOI:10.1039/c7cc01178d

We describe a general strategy for synthesizing conjugated microporous fullerene thin films via a high-throughput, efficient and controllable thiophene-based electropolymerization. By virtue of the ambipolar redox charge/discharge of the films, we showed the microporous fullerene films as outstanding pseudocapacitor materials with high capacity and wide potential windows.

Full text of DOI:10.1039/c7cc01178d

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
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In-Situ Synthesis of Electroactive Conjugated Microporous
Fullerene Films Capable for Supercapacitive Energy Storage
Mingxiao Sun,† Puxing Kuang,† Leiqiang Qin, Cheng Gu,* Zengqi Xie* and Yuguang Ma*
Received,
Accepted
DOI: 10.1039/x0xx00000x
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We describe
a general strategy for synthesizing conjugated based porous materials is still a synthetic challenge for
microporous fullerene thin films by a high-throughput, efficient functional explorations.
and controllable thiophene-based electropolymerization. By virtue
Conjugated microporous polymers (CMPs) are a unique
of ambipolar redox charge/discharge of the films, we showed the class of polymers that combine covalent π-networks with
microporous fullerene films as outstanding pseudocapacitor inherent porosity.13 CMPs are
a
class of emerging
materials with high capacity and wide potential windows.
multifunctional materials and show outstanding properties
and functions in various applications, such as gas storage,14
light emitting,15 energy transfer,16 catalysis,17 and energy
storage.18,19 They are robust in stability as a result of
crosslinked network structure and can be synthesized using a
variety of different π-units. Despite recent advances, the
synthesis of CMPs are still based on solution-phase
polymerization that gives rise to insoluble and unprocessable
solids, precluding the fabrication of thin films for applications.
Fullerene has sparked great interests of materials scientists to
use it as a building block for novel materials with intriguing
properties.1-3 Their highly conjugated structures are favorable
for ultra-fast electron transfer and therefore have been
intensively studied in solar cells, superconductors, and
ferromagnetic materials.4,5 The unique structures and
properties of fullerene also promote synthetic chemists to
explore the possibility of coupling fullerene into porous
frameworks to obtain functional fullerene-based porous
materials. Nevertheless, there are only four examples on the
fullerene-based porous materials, including fullerene-based
metal-organic frameworks (MOFs),6,7 covalent organic
frameworks (COFs)8 and porous aromatic frameworks (PAFs).9
The resulting materials were linked fullerene with peripheral
groups in non-conjugated way; these reports presented
extremely limited function of the fullerene-based porous
materials for applications. Other approaches to synthesize
crosslinked fullerene polymers include photopolymerization of
solely fullerene, addition of auxiliary monomers to incorporate
the fullerene core into a polymer chain, and post-crosslinking
of the self-assembled fullerene derivatives into two-
dimensional structures.10-12 However, these materials were
nonporous. Despite the synthetic efforts in the field of
fullerene-based materials, controlled synthesis of fullerene-
Fig. 1 (a) Complementary diagram for the rational design of
electroactive microporous π-networks with fullerene core and
thiophene peripheries, electropolymerization scheme (dotted
lines indicate the connection to the next thiophene unit), and
the bipolar charge/discharge of the C60Me5Th-CMP films. (b)
The photo of C60Me5Th powder. (c) The photo of the
State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer
Optoelectronic Materials and Devices, South China University of Technology,
Guangzhou 510640, P. R. China..
E-mail: gucheng0625@126.com; msxiez@scut.edu.cn; ygma@scut.edu.cn.
M. Sun and P. Kuang contributed equally to this work.
C60Me5Th-CMP film on ITO (1.0
photo of the freestanding CMP film.
×
1.0 cm2 in area). (d) The
† Electronic Supplementary Information (ESI) available: The theoretical calculations
on molecular configuration, the first cycle CV of the monomer, 13C solid-state NMR
spectra of the CMP, IR and absorption spectra, SEM and HR-TEM images of the
CMP film, CV of the CMP films, and the supercapacitive performance. See
DOI: 10.1039/x0xx00000x
Herein we for the first time report the controlled and direct
electrochemical synthesis of electroactive fullerene-based
CMP films, which are prepared in large area with high quality,
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and are compatible with device fabrication. We demonstrate
the controlled synthesis of thin films with different thicknesses
of nanometer precision, and highlight their superb properties,
including their porosities, band gaps, and their functions as
supercapacitive materials in energy storage.
The key concept in designing C60-based monomer is the
conjugated linkages of C60 with multiple electropolymerizable
units that allow for the polymerization via C–C bond
formation. We designed our monomer C60Me5Th, which bears
a methyl-substituted C60 with five 3-phenylthiphene units at
periphery (Fig. 1a, b, ESI).2,3 The 3-phenylthiphene group is
well established as a highly electroactive unit, which enables
effective coupling reaction between its oxidative species
(thienyl radical cation) and form highly conjugated
polythiophene (PTh) backbones.20,21 Both the C60 focal core
and PTh are highly electroactive moieties with revisable redox
behaviors, which endows the CMP frameworks with rapid and
stable charge/discharge property, an essence of the CMP films
for high-performance device applications. C60Me5Th was
designed to bear five 3-phenylthiphene groups to yield a
shuttlecock-shape structure (Fig. S1), which was rarely
reported in porous materials and upon polymerization
guarantee the growth of thin films with 3D conjugated
network, high porosity, and uniform pore distribution.
Fig. 2 (a) CV profiles of the 1st to 20th cycles of a solution of
C60Me5Th. (b) Plot of the film thickness vs. CV cycle number.
determines the compatibility with various device applications;
in this thiophene-based electropolymerization system, the film
thickness can be controlled by simply adjusting the number of
scan cycles. Using this technique, we prepared a series of films
with thickness ranged from several nanometers to several
micrometers. When the thickness reached micrometers, the
films can automatically peel off electrode to yield freestanding
CMP films (Fig. 1d). The freestanding films only grow on the
electrodes and are in the same size and shape as the
electrodes employed, and in our studies, films with size up to
1.0
×
1.0 cm2 were produced. Therefore, the present method
We set up a three-electrode electropolymerization system
using indium tin oxide (ITO) or glassy carbon as the working
electrodes, titanium plate as the counter electrode and Ag/Ag+
as the reference electrode (Fig. S2a).22-24 At the first cycle of
positive CV scan, the onset oxidative potential of 1.15 V was
attributed to the oxidation of thiophene (Fig. S2b).20,21 When
the scan potential is higher than 1.15 V, the anode current
increased rapidly, indicating more thiophenes were oxidized
and transformed into cationic radical, which couples each
other effectively to form polymeric thiophene cation.20,21
During negative scan, an obvious reductive peak at 0.29 V is
assignable to the reduction of polymeric thiophene cation.
Consequently, we set up a potential range between –0.2 and
1.3 V for the film preparation; in this case, a positive potential
in this range could only oxidize the thiophene units, whereas a
sufficiently low negative potential ensured the complete
reduction of the PTh cations to the neutral state and the
offers high quality films either on electrodes or in freestanding
forms, with synthetically controlled thickness, size, and shape.
Solid-state 13C NMR (Fig. S3) and FT-IR spectra (Fig. S4,
Table S1) revealed the formation of PTh backbones. The films
assumed a smooth and homogeneous surface morphology
without any large particles or aggregates, as revealed by field
emission scanning electron microscopy (Fig. S5). The high-
resolution transmission electron microscopy images show that
the CMP films consist of homogeneous microporous textures
with pore sizes less than 2 nm (Fig. S6). We investigated the
porosity of the C60Me5Th-CMPs by chemically synthesized
CMP powder samples (ESI), and then employed these samples
for N2 sorption isotherm measurements at 77 K. The
C60Me5Th-CMPs exhibited typical Type I adsorption profiles
(Fig. S7a) with a Brunauer-Emmett-Teller (BET) surface area of
695 m2 g–1. Usually the CMP monomers with high symmetry
are employed in order to obtain a homogeneously developed
network skeleton. Notably, the C60Me5Th monomer adopts a
relatively low symmetry, however the CMPs could achieve a
high BET surface area. These observations were rarely
reported and could provide a new opportunity for designing
CMPs with large variety of link ways and geometries. We
calculated the pore volume and pore size of the CMP. The
C60Me5Th-CMP possesses the pore volume of 0.66 cm3 g–1,
whereas the pore size shows three main distribution at 0.6, 0.7
and 1.2 nm, respectively (Fig. S7b, Table S2). In present stage,
it is extremely difficult to characterize the pore properties for
thin films by N2 sorption isotherm measurements, because the
collection of adequate samples is impossible. We conducted Kr
adsorption isotherm measurements and the results also
indicated a highly porous nature of the CMP films (Fig. S7c).
The C60Me5Th spin-coated film exhibited two peaks at 282
–
removal of the counterions (AsF6 ) from the films.22-25 From the
second cycle in continuous CV scan (Fig. 2a), a new oxidative
peak appears at 1.0 V, which is due to the oxidation of PTh on
electrode.22-25 A distinct feature is that the current increased
with the cycle numbers, which demonstrated that the
C60Me5Th-CMP film gradually grow on the electrode (Fig. 2a).
Polymerization occurs only at the solution-electrode interface;
this method precludes the formation of CMP particles in the
bulk solution. As the polymerization proceeded, the electrode
gradually became brown-yellow in color, as identified by the
naked eye. After 20-cycle scan, the CMP films are transparent
and exhibit brown-yellow color (Fig. 1c).
A distinct feature of electropolymerization to produce CMP
films is that this method enables well-defined control over the
thickness, whereas the thickness of the CMP films increase by
2.2 nm per cycle (Fig. 2b). The tunability of the film thickness
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and 360 nm, which were assigned to the π→π* transition of
thiophene and C60, respectively (Fig. S8). Another two peaks
with weak absorption were also observed at 406 and 482 nm,
which were attributed to the charge-transfer (CT) band from
thiophene to C60. In the CMP film, the PTh backbone displayed
an absorption band at 334 nm, which is 52-nm red-shifted
from the spin-coated C60Me5Th film. In this case, the peak due
to the focal C60 unit was covered by the broad band of PTh.
These results indicate an extended π-conjugation in the CMP
films. The CT bands in the CMP film were also red shifted to
472 and 586 nm, revealing the enhanced donor-acceptor CT
state in the films. The band gaps of the CMP film and
monomer were calculated as 1.84 and 2.13 eV, respectively,
on the basis of the onset edges of the absorption spectra.
The CV measurements of the CMP film showed the onset
oxidative and reductive potentials at 0.65 and -0.92 V,
respectively (Fig. S9a), which correspond to the highest
occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) levels of -5.62 and -3.74
eV, respectively (Fig. S9b, Table S2). Thus, the electrochemical
band gap is 1.88 eV. Using the same method, the HOMO and
LUMO levels of the monomer were evaluated to be -5.78 and -
Fig. 3 (a) CV of the pseudocapacitors at various scan rates
(acetonitrile as solvent, TBAAsF6 as electrolyte). (b) Impedance
3.50 eV, respectively (Table S2). Therefore, the C60Me5Th spectra of CMP films under the open circuit potential (OCP) of
the films, the oxidation potential of PTh at 0.9 V, and the
reduction potential of C60 at -1.3 V. (c) Galvanostatic charge–
discharge curves at different current densities. (d) The mass
specific capacitance at different current densities.
monomer has a band gap of 2.28 eV. The electrochemical band
gaps were identical with the optical band gaps and further
confirmed that the CMP films have a significantly reduced
band gap compared to the monomer, which was attributed to
the enhanced π-conjugation of PTh upon polymerization. The
low-band-gap CMP films is widely useful in semiconducting,
optoelectronic and energy-storage applications.
Electrochemical impedance spectroscopy (Fig. 3b) of the
CMP films were tested under the open circuit potential of the
films, the oxidation potential of PTh at 0.9 V and the reduction
potential of C60 at -1.3 V, respectively. Nyquist plots of the
three conditions showed intercepts with the Z’ axis (real
impedance axis) at almost the same position, at ~0.93 Ω. This
resistance represents the internal resistance that originates
from the resistance of the electrolyte and has therefore the
same value for all of the three conditions. The following
second semicircles, which are belonged to the charge transfer
process of the oxidation of PTh and reduction of C60,
respectively, are different: the reduction of C60 showed low
resistance, which revealed a rapid electron transfer process
compared to the oxidation of PTh. The map of the phase angle
vs. frequency of the Bode spectra (Fig. S13) revealed that there
are two significant peaks corresponding to the two time
constants which could also be found as two different RC
parallel combinations in the equivalent circuit diagram of the
impedance spectra (Fig. S14, fitting data see Table S3). The
equivalent series resistance (Rsaverage = 0.93 Ω), the charge
C60 and PTh are respectively typical n-type and p-type
moieties that not only hold unique electronic properties but
also feature redox ability for energy storage by reversibly
switching between the neutral PTh and its cation and the
neutral C60 and its anion (Fig. 1a).26 The rational integration of
these n-type and p-type units into one CMP skeletons yields an
ambipolar electroactive CMP scaffold, which endows the CMP
films with exceptional pseudocapacitive performance and wide
potential range capable for high-voltage applications. For this
purpose, we fabricated CMP thin films on ITO and directly used
them as pseudocapacitive electrodes. We firstly investigated
the effects of thickness on the charge/discharge performance
(Fig. S10). The optimal film thickness was 26.3 nm, which
corresponds to a mass of 0.53 µg (Fig. S11).
The CV of the CMP films showed two pairs of reversible
peaks at about 0.9 and -1.3 V (Fig. 3a), which were assigned to
the redox reactions of the PTh and C60, respectively. The peak
currents as functions of scan rate show linear relationships
(Fig. S12), indicating that the surface-confined redox reactions
have excellent reversibility and fast charge transfer process,
typical of the pseudocapacitive energy storage.27,28 Notably,
the CMP film-based pseudocapacitors could work under a wide
potential window of 2.6 V (-1.6 to 1.0 V), which was rare to
achieve in traditional supercapacitors especially water-phase
devices. The wide potential range of the CMP films would
endow the pseudocapacitors with high energy density and
power density, and was capable for high-voltage applications.
transfer resistance of the double electrode layer (Rct1average
=
54 Ω), the oxidative reaction (Rct2 = 1601 Ω) and the reductive
reaction (Rct2 = 869 Ω) can be gained from the equivalent
circuit (Table S3). The W component of the equivalent circuit
diagram is originated from the diffusion process in the low-
frequency region. The linear curve at low frequencies implies a
capacitive behavior, and the line slope reflects the ion
diffusion process, which is typical for an electrode material
that enables ideal ion transport in the pores.
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Based on these results, we conducted galvanostatic charge films able to work as outstanding electrode materials with high
and discharge tests to evaluate the energy-storage properties capacity and wide potential windows. Our results reveal the
(Fig. 3c). The CMP films exhibited two minor voltage plateaus enormous potential of the fullerene-based CMP films as an
at the potential of 0.61, and -1.11 V, which indicates the appealing platform for construction of electrode materials for
electron transfer to or from the C60 and PTh units. These energy storage. We believe that the multifunctional porous C60
capacitive energy storage based on the redox reaction of the films will introduce a new era of not only supercapacitors but
C60 and PTh units is consistent with the CV profiles. One also photocatalysis and optoelectronic devices.
significant feature is that the voltage plateaus retained at 0.61
and -1.11 V irrespective of the current density, indicating the
Notes and references
reversibility of the redox reaction for energy storage
throughout the charge and discharge processes. When the
current density increases, the charge and discharge time are
significantly shortened. The CMP films can be generally
operated at a current density of 5 A g−1 allowing high-rate
power supply. The specific capacitances are calculated from
the discharge parts of galvanostatic experiment. A maximum
capacitance of 573 F g−1 (117 F cm−3) is obtained at current
density of 5 A g−1. This is among the best values reported on
porous organic polymer based electrodes.19,27,28 This excellent
charging/discharging performance benefits from the highly
crosslinked network skeleton that immobilized the redox
build-blocks in the CMP films, and the microporous structures
that facilitated rapid electronic and ionic transportation
through the interface between electrode and electrolyte by
providing more contact sites. The capacitance could retain 49%
when the current density increased from 5 A g-1 to 50 A g-1
(Fig. 3d). The stability and the performance still have room to
improve (Fig. S15, S16). The control experiment of fullerene
polymer (PC60, Fig. S17) revealed that the CMP films were
much progressive than PC60 in capacitive performance. The
results indicated that the ambipolar CMP skeletons could
provide extra redox capability to enhance the capacity, while
the porous networks promoted ion transport within the pores,
thus leaded to superior capacitive performance compared to
PC60 films, which were nonporous and could only be n-doped.
To the best of our knowledge, this is the first example of
C60-based porous materials capable for device application and
functional exploration. We have developed a novel linkage
way and geometry for porous materials, which was rarely
reported. In ambipolar CMP films, the electroactive moieties
are covalently immobilized to assure an outstanding structural
stability, whereas the numerous pores promote ion transport,
and the high surface area offers a large interface for the access
to redox-active sites. These positive structural effects make
the CMP film an attractive material for energy storage. We
envisage that this strategy is promising for designing a large
number of stable and ambipolar CMP films with high capacity
and wide potential window for energy storage.
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In summary, we developed a general strategy for designing
highly electroactive conjugated microporous fullerene polymer
films by integrating fullerene core and electropolymerizable
peripheral thiophenes into well-defined porous π-networks.
This film synthesis method was high throughput and enabled
elaborate control over the film thickness with sub-nanometer
precision. The porous films possess large surface area, possess
low energy bandgap, and endow high-rate ion transport. The
reversible ambipolar charge/discharge capability renders the
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