Methane adsorption and methanol desorption of copper modified boron silicate

Article abstract of DOI:10.1039/c8ra08038k

Boron silicate (BS) with a chabazite framework structure was synthesised using a direct route and rigorously characterized before it was ion-exchanged with copper to form Cu-BS. Employing in situ infrared spectroscopy, we show that Cu-BS is capable of oxidising methane to methoxy species and methanol interacts with the boron sites without deprotonation.

Full text of DOI:10.1039/c8ra08038k

Nanoscale
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3D Ti3C2Tx aerogels with enhanced surface area
for high performance supercapacitors†
Cite this: Nanoscale, 2018, 10,
20828
Xinyu Wang,‡a Qishan Fu,‡a Jing Wen,a Xinzhi Ma,a Chuncheng Zhu,a
Xitian Zhang *a and Dianpeng Qi*b
Two-dimensional titanium carbide as a novel electrode material has been widely researched in the field
of energy storage in recent years. However, the restacking of Ti3C2Tx nanosheets is still a challenge, which
largely restricts their development. Here, we employ the 3D architecture of a Ti3C2Tx aerogel to restrict
the restacking of 2D Ti3C2Tx nanosheets. This special structure can not only effectively reduce the
restacking of 2D materials, but also accelerate the transport of electrolyte ions in the electrode. The as-
prepared Ti3C2Tx aerogel possessed a large special surface area of 108 m2 g−1 and achieved a special
capacitance of 349 F g−1 which is retained even at a high scan rate of 2000 mV s−1 in the 3 M H2SO4 elec-
trolyte, indicating an ultrahigh rate capability. Moreover, at a higher current density of 20 A g−1, 90% of the
initial capacitance is also retained after 20 000 charging–discharging cycles, revealing an excellent
cycling stability. Furthermore, the mechanism of charge storage was investigated.
Received 26th July 2018,
Accepted 14th October 2018
DOI: 10.1039/c8nr06014b
rsc.li/nanoscale
cycling stability is generally harmed due to the poor conduc-
tivity of composite electrodes.13
1. Introduction
Supercapacitors (SCs) as an important type of energy storage
MXenes, as novel 2D materials, have shown their fantastic
device have attracted considerable attention, due to their high potential in the field of energy storage devices, due to their
power density, long cycle-life and fast recharge capability.1 excellent electrical conductivity, large surface area and tunable
However, commercial SCs cannot meet the increasing energy surface properties.14 MXenes are produced by selective etching
demands for the usage of portable grids and electric vehicles of the “A” element from the MAX phases. The MAX phases
due to their low energy density.2 Therefore, numerous efforts have a general formula of Mn+1AXn (n = 1, 2, 3), where “M”
have been made in producing high-performance SCs with high stands for an early transition metal (M = Ti, V, Nb, etc.), “A”
specific capacitance, excellent rate capability and cycling stabi- stands for an A-group element (A = Al, Si, etc.), and “X” rep-
lity. Electrode materials are the key elements of SCs, and they resents C and/or N. In Mn+1XnTx, “T” denotes –O, –OH, and –F
play an important role in the electrochemical performance of functional groups terminated on the surface, and “x” stands
SCs.3 Carbon-based materials are usually used as electrodes of for the number (0 < x < 2) of termination groups.15,16
SCs due to their high surface area and good conductivity. Particularly, Ti3C2Tx has been widely studied as an electrode
However, they show a low specific capacitance. To obtain high- material for SCs and some inspiring achievements have been
performance SCs, transition metal oxides,4 transition metal obtained.17 In 2013, Gogotsi et al. demonstrated that the accor-
nitrides5–9 or conducting polymers10,11 with high theoretical dion-like Ti3C2Tx delivered a specific capacitance of 130 F g−1
capacitance combined with carbon-based materials have been at a scan rate of 2 mV s−1 18
A Ti3C2Tx “paper” electrode was
.
employed to form composite electrodes. The capacitance per- subsequently prepared through vacuum-assisted filtration of
formance of composite electrodes was significantly improved the Ti3C2Tx nanosheet suspension, and an improved capaci-
compared with carbon-based electrodes.12 However, their tance of 201 F g−1 at 2 mV s−1 was achieved.19 However, high
specific capacitances were only obtained at low scan rates or
low current densities according to most of the reports on
aKey Laboratory for Photonic and Electronic Bandgap Materials, Ministry of
Education, School of Physics and Electronic Engineering, Harbin Normal University,
Harbin 150025, P. R. China. E-mail: xtzhangzhang@hotmail.com
bSchool of Chemistry and Chemical Engineering, Harbin Institute of Technology,
Harbin, 150001, China. E-mail: dpqi@ntu.edu.sg
Ti3C2Tx, which limited the practical application of SCs. This
issue could be attributed to the restacking of the Ti3C2Tx
nanosheets, where there is not enough time for the ion
diffusion into the electrode materials.20 So far, several effective
strategies have been developed to avoid the restacking of
Ti3C2Tx nanosheets during synthesis processes.21 In 2015,
Zhao et al. designed a flexible and sandwich-like Ti3C2/CNT
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8nr06014b
‡These authors contributed equally to this work.
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composite paper electrode, which partly reduced the restack- a concentration of 10 mg mL−1 followed by sonication to get a
ing phenomenon and obtained an excellent specific capaci- colloidal solution. Then the mixture was sealed in a 5 mL vial
tance of 150 F g−1 at 2 mV s−1, and 117 F g−1 at 200 mV s−1 22
.
and maintained for 4 h at 90 °C. After the solution was cooled
More recently, a self-assembled Ti3C2Tx/SCNT composite elec- to room temperature, the as-formed hydrogel was immersed in
trode was reasonably prepared. The restacking of Ti3C2Tx ethanol for 24 h and washed with water three times to remove
nanosheets was further decreased and a specific surface area the residual impurities. Finally, the wet hydrogel was freeze-
of 55 m2 g−1 was achieved. The electrode showed a high areal dried for 24 h to get the Ti3C2Tx aerogel.
capacitance of 220 mF cm−2 and excellent cycling stability.21
Unlike the intercalation structures mentioned above, a novel
FL-Ti3C2Tx@Ni electrode prepared though an electrostatic
layer-by-layer self-assembly method has a specific surface area
of 67 m2 g−1 and showed a specific capacitance of 370 F g−1 at
2 mV s−1, and 117 F g−1 was also obtained at a much higher
2.3 Material characterization
The morphology and crystalline structure of the as-prepared
Ti3C2Tx aerogel were characterized by scanning electron
microscopy (SEM, SU70, Hitachi, Japan), transmission electron
microscopy (TEM; FEI, Tecnai TF20) and X-ray diffraction
(XRD) in the reflection mode (Cu Kα radiation, λ = 1.5418 Å)
on a Rigaku D/max2600 X-ray diffractometer, respectively.
X-ray photoelectron spectroscopy (XPS) measurements were
performed on a PHI 5700 ESCA System using Al Kα radiation
at 1486.6 eV. A nitrogen sorption isotherm was obtained by
using an automatic N2 adsorption/desorption instrument
(ASAP 2010, Micromeritics). The specific surface area was cal-
culated using the Brunauer–Emmett–Teller (BET) equation.
scan rate of 1000 mV s−1 23
However, these strategies could
.
partially avoid the restacking of Ti3C2Tx nanosheets, and in
fact many restacking layers also exist.
To overcome the restacking of Ti3C2Tx nanosheets as much
as possible and further enhance their electrochemical per-
formance, we employ Ti3C2Tx aerogel with a 3D architecture to
restrict the restacking of Ti3C2Tx nanosheets. Another merit of
the aerogel is that it consists of wrinkled Ti3C2Tx nanosheets
which can further reduce their restacking and enhance the
surface area. A special surface area of 108 m2 g−1 was obtained.
Additionally, the structure can provide more electroactive sites
and shorten the ion diffusion path, resulting in an excellent
specific capacitance of 438 F g−1 at 10 mV s−1. Ultrahigh rate
capability and excellent cycling performance were obtained,
indicating that the 3D Ti3C2Tx aerogel has promising potential
in SCs.
2.4 Electrode preparation and electrochemical
measurements
A commercially available glassy carbon electrode (GCE, 3 mm
diameter) was cleaned by polishing with Al2O3 paste on a
special polishing cloth until
a mirror-like surface was
obtained. After that, 5 mg Ti3C2Tx aerogel was added into
950 μL of ethanol and ultrasonically treated for 2 h, followed
by dropwise addition of 50 μL of Nafion (0.5 wt%) to ensure a
homogeneous solution was obtained. Then, 2 μL of the result-
ing suspension was carefully added dropwise onto a clean GCE
surface and dried in an oven at 30 °C for 12 h. The mass of the
Ti3C2Tx aerogel was around 0.01 mg. All electrochemical tests
were performed using an electrochemical workstation (VMP3,
France) with a standard three-electrode configuration. The as-
prepared electrode on a glassy carbon current collector was
used as the working electrode; a carbon rod and Ag/AgCl were
used as the counter electrode and reference electrode, respect-
ively, with 3 M H2SO4 aqueous solution as the electrolyte.
Cyclic voltammetry (CV) and galvanostatic charge–discharge
(GCD) profiles were obtained over the voltage range from −0.6
V to 0.2 V at different scan rates and current densities.
Electrochemical impedance spectroscopy (EIS) measurement
was performed in a frequency range of 10 mHz–200 kHz. The
specific capacitance (Cm) was calculated from the CV curves by
the following equation:24
2. Experimental
2.1 Synthesis of Ti3AlC2 powder
Ti3AlC2 powder was synthesized by mixing Ti powder (99.99%
purity, Aladdin), Al powder (99.80% purity, Aladdin) and C
powder (≥99.85% purity, Aladdin) in
3 : 1.1 : 1.8, followed by ball milling for 165 min at a speed of
650 rpm under an argon atmosphere. The bulk product was
ground with a pestle and mortar and then the milled powder
was sieved through a 400 mesh screen.
a molar ratio of
2.2 Synthesis of Ti3C2Tx aerogel
The Ti3C2Tx suspension was prepared by etching out the Al
layer from Ti3AlC2 in a mild etchant. As reported in the pre-
vious work,19 1 g Ti3AlC2 powder was added to a mixture solu-
tion of lithium fluoride (LiF, 1.56 g) and 12 M hydrochloric
acid (HCl, 20 mL) at 35 °C for 48 h. Then, the sediment was
centrifuged and washed with 1 M HCl and DI water several
times until the pH of the supernatant was approximately
neutral. To obtain the Ti3C2Tx suspension, DI water was added
into the sediment and magnetically stirred for 10 min; after
centrifugation for 5 min at 5000 rpm, the Ti3C2Tx suspension
Ð
IdU
m ꢀ v ꢀ ΔU
Cm
¼
ð1Þ
with a dark green color was collected. In order to prepare the where Cm is the specific capacitance (F g−1), I is the response
Ti3C2Tx aerogel, 30 mg thiourea dioxide and 150 μL ammonia current (A), m is the quantity of active materials (g), ν is the
solution (25 wt%) were added into the Ti3C2Tx suspension with scan rate (mV s−1) and ΔU is the potential window (V).
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could also give rise to nanosheet wrinkling.26 Therefore,
wrinkled nanosheets are observed in the experiment.
3. Results and discussion
The preparation procedure of the Ti3C2Tx aerogel is illus- Meanwhile, interconnected nanosheets could be formed by
trated in Fig. 1. Typically, a mixture of aqueous thiourea the strong chemical bonding between residual N and the ter-
dioxide and ammonia was added into the Ti3C2Tx suspen- minated O groups on the surface of the Ti3C2Tx nanosheets
sion. The thiourea dioxide has a strong reduction property in (see below). After freeze-drying the wet hydrogel for 24 h
alkaline solution to ensure that the Ti3C2Tx nanosheets are under vacuum conditions, a large number of H2O molecules
not oxidized after hydrolysis.25 The above mixture becomes a are released, and then a free-standing and ultralight porous
homodispersed colloidal solution by sonication. During the Ti3C2Tx aerogel with a density of ∼27 mg cm−3 is obtained.
thermal treatment process at a mild temperature of 90 °C
The morphology of the aerogel is quite uniform at low mag-
for 4 h, the ammonia located in the interlayer between the nification (as the SEM image shown in Fig. 2a), and mean-
Ti3C2Tx nanosheets could rapidly decompose in a very short while the Ti3C2Tx aerogel exhibits an interconnected and hier-
time. A number of NH3 gas molecules rapidly flow, causing a archical 3D network consisting of randomly oriented Ti3C2Tx
large impulse on the nanosheets. Different positions on the nanosheets with wrinkled texture. The macropores with a size
surface of the Ti3C2Tx sheets could suffer from different of several micrometers are closely stacked (Fig. 2b), and the
impulses due to uneven NH3 release. In addition, N-doping solid walls of these macropores are composed of inter-twisted
Ti3C2Tx nanosheets that may produce a lot of mesopores. The
wrinkled Ti3C2Tx nanosheets are very thin and transparent
(Fig. 2c), which also can be verified from the TEM image
(Fig. S1a and S1b†). Elemental mapping images of Ti3C2Tx
aerogel are shown in Fig. S1c.† All the Ti, C, O and N elements
are uniformly distributed, confirming the incorporation of N
element and the homogeneous structure of the sample.
Moreover, these wrinkled Ti3C2Tx nanosheets could signifi-
cantly improve the restacking of Ti3C2Tx nanosheets in terms
of the wrinkled graphene,27 indicating that more active sites
could be exposed. The special 3D aerogel structure is ben-
eficial for the electrochemical performance of the electrode.
XRD was used to further investigate the crystal structure of
the Ti3C2Tx aerogel as shown in Fig. 2d. It can be observed
that the sample exhibits strong ordering in the basal direc-
tions, which presents remarkable peaks (000l). Interestingly,
the distinct (110) peak around 61° appears in the Ti3C2Tx
aerogel, indicating that the Ti3C2Tx nanosheets are sheared
when they self-assemble into a 3D aerogel structure.28 This
Fig. 1 Schematics showing the preparation procedure of the Ti3C2Tx
aerogel.
Fig. 2 (a–c) SEM images of the Ti3C2Tx aerogel with different magnifications. (d) XRD pattern, (e) N2 adsorption–desorption isotherms, and (f) pore
size distribution plot of the Ti3C2Tx aerogel, respectively.
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phenomenon is also observed in the SEM images as men- 461.5 eV can be attributed to the Ti–C species in Ti3C2Tx
tioned above. nanosheets. The two pairs of peaks at 455.8/456.6 eV and
The increased specific surface area and porous properties 462.4/463.2 eV can be assigned to TiOx (1.5 < x < 2). The peaks
can be confirmed by the N2 adsorption/desorption test. As centered at 459.0 eV and 464.4 eV can correspond to TiO2. The
shown in Fig. 2e, the Ti3C2Tx aerogel possesses a typical type-IV peaks centered at 456.3 eV and 461.6 eV can be assigned to
isotherm characteristic with a distinct adsorption hysteresis Ti–N species, confirming that N is incorporated into the
loop between 0.45–1 relative pressure, indicating that there are Ti3C2Tx nanosheets during the synthesis of the Ti3C2Tx
a number of mesopores in the Ti3C2Tx aerogel. The pore size aerogel. The high-resolution of the O 1s signal can be deconvo-
distribution determined by the Barret–Joyner–Halenda method luted into five peaks at 530.2 eV, 531.1 eV, 532.2 eV, 532.3 eV,
shows that the pore volume lies mainly in the range of 2.9 to 532.5 eV, 533.4 eV, and 533.5 eV (Fig. 3c), corresponding to
8.2 nm and that the main peak is centered at ∼3.9 nm, as TiO2, TiOx, O–H, O–N, C–O, O–CvO and H–O–H, respectively.
shown in Fig. 2f. The BET specific surface area of the Ti3C2Tx The high-resolution XPS spectrum of N 1s can be deconvoluted
aerogel is calculated to be 108 m2 g−1. The value is almost into four peaks at 397.8 eV, 399.9 eV, 401.0 eV and 402.4 eV
7 times greater than 16.2 m2 g−1 of the pure Ti3C2Tx films.21 (Fig. 3d), corresponding to N–Ti, C–NH, C–NH2 and N–O,
This can probably be ascribed to the fact that the restacking of respectively. The existence of the N–O bonds is helpful to
Ti3C2Tx nanosheets is significantly improved and the formation improve the stability of the structure and charge transfer kine-
of the special 3D architecture gives rise to lots of macropores tics, which may enhance the electrochemical performance of
and mesopores. Consequently, this beneficial mesoporous dis- active materials.29 The appearance of the characteristic peak at
tribution and high specific surface area not only accelerate the 397.8 eV corresponding to N–Ti bond further confirms that the
transportation of electrolyte ions but also greatly enhance the N is doped into the Ti3C2Tx aerogel. Moreover, it can be
electrochemical utilization of Ti3C2Tx nanosheets.
expected that N-containing functionalities can enhance the
In order to investigate the elemental composition and wettability of the electrode and improve the capacitance per-
chemical bonding states of the Ti3C2Tx aerogel, XPS measure- formance due to their electron-donor properties.30
ments were carried out, as shown in Fig. 3. Their chemical
The electrochemical performance of the Ti3C2Tx aerogel as
states (Table S1†) are obtained by fitting the respective high- an electrode was investigated through a series of electro-
resolution XPS spectra. The survey spectrum indicates the pres- chemical tests. In this work, a GCE is used as the working elec-
ence of Ti, C, O, N, Li, and F elements (Fig. 3a). For the trode, due to the advantage of its specific overpotential for
Ti3C2Tx aerogel, the high-resolution Ti signals can be deconvo- hydrogen evolution reaction, which could expand the potential
luted into ten peaks (Fig. 3b). The peaks at 455.3 eV and window of the electrode.31
Fig. 3 (a) XPS survey spectrum of the Ti3C2Tx aerogel. (b–d) High-resolution XPS spectra of Ti 2p, O 1s and N 1s of the Ti3C2Tx aerogel, respectively.
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Fig. 4 (a) CV curves of the Ti3C2Tx aerogel and pure Ti3C2Tx electrode materials at a scan rate of 10 mV s−1. (b) CV curves of the Ti3C2Tx aerogel
electrode materials at different scan rates. (c) GCD plots of the Ti3C2Tx aerogel electrode materials at different current densities. (d) Specific capaci-
tance of the Ti3C2Tx aerogel and pure Ti3C2Tx electrode materials at various scan rates. (e) The corresponding diffusion time plots of the Ti3C2Tx
aerogel and pure Ti3C2Tx electrode materials. (f) Cycling performance of the Ti3C2Tx aerogel electrode materials at current densities of 10 A g−1
(black curve) and 20 A g−1 (red curve). Inset shows the GCD voltage profiles of the Ti3C2Tx aerogel electrode materials in the first five cycles (black)
and the last five cycles (red) at a current density of 20 A g−1, respectively.
Fig. 4a shows the CV curves of the Ti3C2Tx aerogel and pure ultrahigh coulombic efficiency (almost 100%) of the electrode
Ti3C2Tx electrodes at a low scan rate of 10 mV s−1 in a potential and the effective charge–discharge process of Ti3C2Tx aerogel
window ranging from −0.6 to 0.2 V. The CV curves in Fig. 4a electrode materials. Fig. 4d presents the specific capacitance
are pseudo-rectangular with obvious redox peaks. The redox of the Ti3C2Tx aerogel and pure Ti3C2Tx electrodes at different
peaks arise from the protonation of oxygen functional groups scan rates. The aerogel electrode achieves an ultrahigh capaci-
accompanying the changes in the titanium oxidation state.31 It tance of 438 F g−1 at a scan rate of 10 mV s−1, which is almost
is obvious that the aerogel electrode possesses a large inte- 2 times higher than the value of pure Ti3C2Tx (231 F g−1). The
grated area compared with pure Ti3C2Tx, indicating that a high capacitance of 349 F g−1 (80% of the initial specific capaci-
capacitance for the Ti3C2Tx aerogel electrode is achieved. The tance) is retained at a scan rate of 2000 mV s−1. Moreover, even
CV curves of the Ti3C2Tx aerogel electrode with wide scan rates at an ultrahigh scan rate of 20 000 mV s−1, a capacitance of
ranging from 50 to 2000 mV s−1 are shown in Fig. 4b. The 173 F g−1 is achieved, indicating that the Ti3C2Tx aerogel exhi-
shapes of CV curves are almost not distorted, showing an bits a higher specific capacitance and remarkable rate capa-
excellent rate capability of the electrode. The CV curves at bility compared with most of the MXene-based composites via
higher scan rates reaching 20 000 mV s−1 are also tested different test methods as shown in Table 1. As can be seen
(Fig. S2a†). Fig. 4c displays the GCD curves of the aerogel elec- from Fig. 4d, the specific capacitance decreases with the
trode at different current densities ranging from 1 to 20 A g−1 increase of the scan rate. This is a general characteristic of SC
in the corresponding potential window. The linear profile of due to that there is no enough time for the ion diffusion into
GCD plots and their symmetric triangular shape imply the the entire surface of the electrode materials, especially at
Table 1 Comparisons of the specific capacitance with previously reported MXene-based composite electrodes
Electrode
Electrolyte
Capacitance (F g−1
)
Scan rate
Ref.
Ti3C2Tx aerogel
Ti3C2Tx “clay”
Ti3C2Tx/MnO2
Ti3C2Tx/G-PDDA
P3@Ti3C2Tx
Ti3C2Tx–CF
Ti3C2Tx/PPy
Ti3C2Tx/glycine
Ti3C2Tx/MoO3
N–Ti3C2Tx
3 M H2SO4
1 M H2SO4
1 M Na2SO4
3 M H2SO4
1 M H2SO4
1 M H2SO4
1 M H2SO4
3 M H2SO4
1 M KOH
438
245
390
335
380
401
416
324
151
267
120
10 mV s−1
2 mV s−1
10 mV s−1
2 mV s−1
2 mV s−1
10 mV s−1
5 mV s−1
10 mV s−1
2 mV s−1
5 mV s−1
2 mV s−1
This work
28
32
33
34
35
36
37
38
39
40
6 M KOH
1 M KOH
Ti3C2Tx/ZnO
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higher scan rates. The comparison of the mass loading of the electron transmission and transport, and promoting the
Ti3C2Tx aerogel is also investigated and shown in Fig. S2b.† A Ti3C2Tx redox kinetics; (iii) more mesoporous structure
higher capacitance of 430 F g−1 is achieved at a scan rate of offering a rich inner surface for ion storage; (iv) N-doping not
10 mV s−1 although the mass loading is up to 1.82 mg cm−2
.
only increases the wettability of the material surface, but also
However, the rate performance of the electrode with a large enhances the additional pseudocapacitive effect. Therefore,
mass loading (1.82 mg cm−2) is much lower than that of the the Ti3C2Tx aerogel has enormous potential to be used as an
electrode with a low mass loading (0.14 mg cm−2). It is well electrode of SCs.
known that the diffusion and migration of electrolytic ions
To investigate the redox kinetics of Ti3C2Tx aerogel elec-
into the interior of the active materials are affected by mass trode materials, the CV data are analyzed at various scan rates
loading. The EIS plots of the three electrodes are shown in according to the following equation:41
Fig. S3.† In the low frequency region, the Ti3C2Tx aerogel elec-
i ¼ avb
ð2Þ
trode shows a vertical straight line, indicating a lower diffusion
resistance in the electrode material and more ideal capacitive
behavior. The real-axis intercept represents the equivalent
series resistance (Rs) in the high frequency region, as shown in
the inset of Fig. S3a,† showing the almost same resistance
values for both the electrodes. To gain further understanding
on the kinetics of the electrodes, the EIS Nyquist plots of the
Ti3C2Tx aerogel and pure Ti3C2Tx electrode are fitted by an
equivalent circuit, which is shown in Fig. S3b.† In order to
further understand the ion diffusion capability in Ti3C2Tx elec-
trode materials, the diffusion times of both electrodes are cal-
culated to be 0.11 s and 0.27 s for the Ti3C2Tx aerogel electrode
and pure Ti3C2Tx electrode as shown in Fig. 4e, respectively. As
a promising electrode material for SCs, it should possess not
only a high capacity and high-rate capability but also excellent
cycling stability. Fig. 4f shows the cycling behavior of the
Ti3C2Tx aerogel electrode. It is noted that the specific capaci-
tance retains 94% at a current density of 10 A g−1 over 20 000
cycles. Moreover, 90% of initial capacitance is also retained at
a much higher current density of 20 A g−1 after 20 000 cycles.
The SEM images and XRD patterns of the 3D Ti3C2Tx aerogel
electrode before and after the cycling test are shown in Fig. S4
and S5,† respectively. It can be seen that the restacking of
Ti3C2Tx nanosheets is restrained effectively, the morphology
and XRD patterns have scarcely changed after 20 000 cycles.
Besides, the stability of the Ti3C2Tx aerogel is much higher
than that of the pure Ti3C2Tx electrode, as shown in Fig. S6;†
the specific capacitance remains 87% at 10 A g−1 and 84% at
20 A g−1 for the pure Ti3C2Tx electrode over 20 000 cycles.
The improved cycling stability can be attributed to the
porous structure of the Ti3C2Tx aerogel, which not only acceler-
ates the transportation of electrolyte ions but also greatly
enhances the electrochemical utilization of Ti3C2Tx
nanosheets. Furthermore, a symmetric supercapacitor (SC) is
assembled based on the Ti3C2Tx aerogel electrode, which exhi-
bits a higher energy density of 6.1 W h kg−1 and a higher
power density of 1992 W kg−1, as shown in Fig. S7,† which is
comparable to other SCs reported previously.
where i, v, a and b represent the current, the scan rate, and
adjustable values, respectively. The plots of log(i) vs. log(v)
from 20 to 200 mV s−1 for the Ti3C2Tx aerogel and pure
Ti3C2Tx electrode materials at the potential of −0.3 V are
shown in Fig. 5a. The plots display a good linear relationship
of log(i) vs. log(v) and they are nearly parallel to each other.
The b-values of 0.94 and 0.95, which are close to 1, for the
Ti3C2Tx aerogel and pure Ti3C2Tx electrode materials are
obtained, respectively, indicating that they have the same
energy storage mechanism, in which the electrode redox kine-
tics is predominantly a non-diffusion-controlled process. The
b-values of the Ti3C2Tx aerogel electrode and pure Ti3C2Tx elec-
trode materials at different potentials are shown in Fig. S8.†
All the b-values are approximately 1, therefore revealing an
excellent rate capability. Moreover, it is also beneficial for the
charge storage of the Ti3C2Tx aerogel electrode materials
although the capacitive contribution from the diffusion-con-
trolled process is small. The ion diffusivity in the electrolyte
can offer important information about the rate capability and
cycling performance. The ion diffusion performance can be
explored based on the classical Randles Sevcik equation.42 The
slope of the fitted linear (i − v1/2) qualitatively represents the
ion diffusion rate. As shown in Fig. 5b, the anodic peak cur-
rents of the Ti3C2Tx aerogel and Ti3C2Tx electrode materials
are linear with the square root of the scan rates. It can be seen
that the Ti3C2Tx aerogel electrode materials shows much faster
diffusion and better reaction kinetics compared with Ti3C2Tx
electrode materials, indicating that the special 3D architecture
could enable highly efficient and fast intercalation of ions in
the electrolyte. To estimate the capacitive contribution of the
non-diffusion-controlled process and diffusion-controlled
process to the charge storage in Ti3C2Tx aerogel electrode
materials, the capacitive values are calculated, using the fol-
lowing formula:43
i ¼ k1v þ k2v1=2
ð3Þ
The excellent cycling stability, ultrahigh rate capability and where i and v represent the current and scan rate, respectively,
outstanding specific capacitance of the Ti3C2Tx aerogel elec- and k1 and k2 are constants. k1v and k2v1/2 correspond to the
trode may be attributed to the 3D porous network structure. currents produced from the non-diffusion-controlled process
The special 3D architecture has four advantages, including (i) and the diffusion-controlled intercalation process, respectively.
effectively preventing restacking of Ti3C2Tx nanosheets, enhan- Taking a scan rate of 50 mV s−1 for example, as shown in
cing the specific surface area and exposing abundant electroac- Fig. 5c, 91% of the charge storage could be assigned to the
tive sites; (ii) shortening the ion diffusion path, facilitating non-diffusion-controlled process by calculating the integrated
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Fig. 5 (a) Plots of log(i)–log(v) from 20 to 200 mV s−1 for the Ti3C2Tx aerogel and pure Ti3C2Tx electrode materials at the potential of −0.3 V. (b)
Linear relationship of peak current vs. square root of scan rates of the Ti3C2Tx aerogel and pure Ti3C2Tx electrode materials. (c) Currents’ contri-
bution from the non-diffusion-controlled process (blue) and diffusion-controlled process (red) to the charge storage of Ti3C2Tx aerogel electrode
materials at a scan rate of 50 mV s−1. (d) Ratio of the capacitive contribution from the non-diffusion-controlled process (blue) and diffusion-con-
trolled process (red) to the charge storage of the Ti3C2Tx aerogel electrode materials at different scan rates.
area. The capacitive contribution from the non-diffusion-con- cycling stability. This suggests that the Ti3C2Tx aerogel may be
trolled process gradually increases with the increase of the a promising electrode material for high performance SCs.
scan rate according to the ratio change, as shown in Fig. 5d.
However, the total capacitive value decreases with the increase
of the scan rate, mainly due to the decrease of the intercalation
capacity (Fig. S9†), because of this the scan rate has a signifi-
Conflicts of interest
cant effect on the intercalation capacity. The understanding of
the reaction kinetics and the charge storage mechanism in
Ti3C2Tx aerogel electrode materials is very important to further
develop Ti3C2Tx electrode materials with excellent electro-
chemical performance for SCs.
There are no conflicts to declare.
Acknowledgements
This work was partially supported by the National Natural
Science Foundation of China (no. 51772069, 51472066 and
51572064).
4. Conclusions
In summary, a Ti3C2Tx aerogel with a 3D network architecture
has been successfully synthesized by a facile method. This
special structure effectively prevents the restacking phenom-
enon of 2D materials, has a huge specific surface area and
enables faster transport of ions in the electrode. The Ti3C2Tx
aerogel electrode exhibits excellent electrochemical properties,
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