Cu(i)/{Nb6O19} catalyzed: N -acylation of arylacetic acids with amines under aerobic conditions

Article abstract of DOI:10.1039/c8cc06721j

The method described herein is a general, efficient and green approach to synthesize α-ketoamides from arylacetic acids and amines. Employing a simple copper(i)/{Nb₆O₁₉} catalyst system, the reaction offers a facile process to give functionalized α-ketoamides from readily available arylacetic acids under aerobic conditions. The merit of this new strategy is that it expands the syntheses of α-ketoamides from stable, inexpensive and widely available acylation reagents such as arylacetic acids in one step.

Full text of DOI:10.1039/c8cc06721j

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Flexible Ti3C2Tx/PEDOT:PSS films with outstanding
volumetric capacitance for asymmetric
Cite this: DOI: 10.1039/c8dt04374d
supercapacitors†
a
Lu Li,a,b Na Zhang,c Mingyi Zhang,b Xitian Zhang *b and Zhiguo Zhang
*
MXenes are two-dimensional transition metal carbides/nitrides, and they have shown exciting application
prospects for electrochemical energy storage in the future owing to their hydrophilicity, metallic conduc-
tivity and surface redox reactions, which are crucial for high-capacitance and high-rate electrode
materials. However, the strong tendency of adjacent MXene flakes to aggregate or self-restack under the
van der Waals force limits the electrochemical performance of MXene-based electrodes for practical
applications. In this study, we developed a simple and effective method to prepare Ti3C2Tx/PEDOT:PSS
hybrid films via filtering the Ti3C2Tx/Clevios PH1000 compound inks, followed by H2SO4 treatment.
H2SO4 treatment could remove part of the insulating PSS from the Ti3C2Tx/PEDOT:PSS hybrid film, result-
ing in significant conductivity enhancement of the composite. Furthermore, the conductive PEDOT not
only acted as a pillar between Ti3C2Tx sheets to expose more electroactive surfaces and reduce ion
diffusion pathways but also played a role as a conductive bridge to form multidimensional electronic
transport channels for accelerating the electrochemical reaction process. As a result, the as-prepared
H2SO4-treated Ti3C2Tx/PEDOT:PSS (Ti3C2Tx/P-100-H) hybrid film exhibited 4.5-fold increase in the
specific surface area and high volumetric capacitance of 1065 F cm−3 at 2 mV s−1 with superior rate per-
formance in 1 M H2SO4 electrolyte. Especially, we assembled an asymmetric supercapacitor (ASC) with
excellent flexibility based on a Ti3C2Tx/P-100-H hybrid negative electrode and rGO film positive elec-
trode, which delivered high energy density of 23 mW h cm−3 and high power density of 7659 mW cm−3
.
Received 3rd November 2018,
Accepted 22nd December 2018
Moreover, a simple luminous band was designed and powered by our two ASCs in series. The outstanding
volumetric electrochemical performance and energy density of the ASC based on the flexible Ti3C2Tx/
P-100-H hybrid film electrode demonstrated its promising potential as a strong power source for small
portable and wearable electronic devices.
DOI: 10.1039/c8dt04374d
rsc.li/dalton
exploring high-performance negative electrode materials, such
as metal oxides and metal sulfides,5,6 has been the goal of
Introduction
High-performance supercapacitors with high power density researchers. In recent years, MXenes, a family of two-dimen-
and energy density as well as long cycle life are crucial for the sional (2D) transition metal carbides and nitrides, have been
development of innovative portable and wearable electronic widely studied for supercapacitors as negative electrode
devices.1,2 However, the currently commercialized super- materials due to their suitable potential range and excellent
capacitors cannot satisfy this requirement because they mainly properties such as metallic conductivity, hydrophilic surface
use low-capacitance carbon materials as the negative electrode, and good elastic properties.7–12 Nevertheless, the restacking of
which results in low energy density of the device.3,4 Thus, MXene nanosheets (NSs) arising from the strong van der
Waals interaction between adjacent MXene NSs largely com-
promises their available surface area and limits the electro-
chemical performance.13,14 Wedging spacers between the
sheets is a straightforward strategy to alleviate this phenom-
enon and simultaneously increase the electrochemical pro-
aCondensed Matter Science and Technology Institute, Department of Physics,
Harbin Institute of Technology, Harbin 150001, People’s Republic of China
bKey Laboratory for Photonic and Electronic Bandgap Materials, Ministry of
Education, School of Physics and Electronic Engineering, Harbin Normal University,
Harbin 150025, People’s Republic of China. E-mail: xtzhangzhang@hotmail.com
cDepartment of Chemistry and Chemical Biology, Cornell University, Ithaca, USA
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8dt04374d
+
perties. For example, metal cations (Li+, Na+, K+, NH4 and
Mg2+),15 organic molecules (hydrazine molecule),16 carbon
nanomaterials (onion-like carbon, carbon nanotubes and gra-
phene),17,18 and metal oxides/sulfides (TiO2 nanoparticles,19
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MnO2 nanosheets14 and MoS2 nanosheets20) are introduced
between MXene nanosheets as interlayer spacers, which result
in improved electrochemical performance via increase in the
Experimental
Synthesis of Ti3C2Tx colloidal solution
electrode/electrolyte interface areas and decrease in the ion One g Ti3AlC2 powder was added to a premixed solution of
diffusion length within the active materials. Beyond that, poly- 1.56 g LiF and 20 mL HCl with continuous stirring at 38 °C for
mers are a special class of interlayer spacer materials. It has 48 h.26,27 Afterwards, the mixture was washed with 1 M HCl
been proven that the combination of MXenes with polymers solution, 1 M LiCl solution and deionized water several times.
can enhance their electrochemical performance and render The supernatant of Ti3C2Tx sheets was collected after centrifu-
the hybrids with superior mechanical properties, which is ging at 8000 rpm for 5 min.
necessary for the electrode materials of next-generation wear-
able and flexible supercapacitors. For instance, Ling et al.
mixed Ti3C2Tx with electrically neutral polyvinyl alcohol (PVA)
Preparation of H2SO4-treated Ti3C2Tx/PEDOT:PSS
(Ti3C2Tx/P-H) hybrid film
to produce Ti3C2Tx/PVA composites.21 The intercalation and To synthesize the Ti3C2Tx/PEDOT:PSS hybrid film, Ti3C2Tx dis-
confinement of the polymer between MXene flakes enhanced persions (1.5 mg mL−1) and Clevios PH1000 were mixed and
the cationic intercalation, offering remarkable volumetric stirred for 1 h to achieve homogeneous inks. Then, the
capacitance of ∼530 F cm−3 for the MXene/PVA-KOH compo- mixture was filtered through a PTFE membrane (0.22 μm pore
site film at 2 mV s−1; moreover, this strategy also increased size). Hybrid films with different ratios of Ti3C2Tx and PH1000
flexibility, exhibiting improved strength that was about four solutions were prepared and marked as Ti3C2Tx, Ti3C2Tx/P-50,
times larger than that of a pure Ti3C2Tx film by increasing the Ti3C2Tx/P-100, and Ti3C2Tx/P-200; the detailed experimental
PVA loading to 60 wt%. Boota et al. demonstrated a facile strat- parameters are listed in Table S1.† After soaking in concen-
egy for the in situ polymerization of pyrrole confined between trated H2SO4 for 24 h, these hybrid films were marked as
Ti3C2Tx layers, and the PPy/Ti3C2Tx composite yielded volu- Ti3C2Tx-H, Ti3C2Tx/P-50-H, Ti3C2Tx/P-100-H, and Ti3C2Tx/
metric capacitance of ≈1000 F cm−3 22
From a variety of poly- P-200-H. The PEDOT:PSS film was also prepared by filtering
.
mers, poly(3,4-ethylenedioxythiophene) (PEDOT) is preferred 2 ml PH1000 solution followed by soaking in H2SO4 for 24 h,
due to its mechanical flexibility, low cost, and commercial and it was named as P-H.
availability.23,24 Compared with the in situ polymerization of a
Preparation of reduced graphene oxide (rGO) film
monomer on the surface of the Ti3C2Tx sheet, the method of
directly mixing a PEDOT aqueous solution and Ti3C2Tx super- Graphite oxide was synthesized by a modified Hummers’
natant is much easier to operate and cheaper in industrial pro- method.28 The GO film was obtained by filtering 5 ml GO solu-
duction. Clevios PH1000 is a commercially available aqueous tion (2 mg mL−1), which was then immersed in a Teflon-lined
dispersion of PEDOT with poly(styrenesulfonate) (PSS) as a dis- stainless steel autoclave that contained 10 mL deionized water
persant. However, PSS is an insulator that impedes charge and 400 μL hydrazine hydrate (80%). Finally, the obtained rGO
transport. Based on a previously reported method, we inferred film was collected and dried after sealing the autoclave at
that a strong acid treatment is the most effective and simple 90 °C for 6 h in an electric oven.
way to remove the insulating PSS and improve the conductivity
Characterization
of the film dramatically.25 To the best of our knowledge, there
are no reports on an H2SO4-treated Ti3C2Tx/PEDOT:PSS compo- The morphology and chemical composition of samples were
site film electrode for supercapacitors, and the hybrid film is characterized by field-emission scanning electron microscopy
expected to be a high-performance negative electrode material. (FE-SEM; SU70, Hitachi, Japan) and transmission electron
In this study, a flexible H2SO4-treated Ti3C2Tx/PEDOT:PSS microscopy (TEM; FEI, Tecnai TF20). The X-ray powder diffrac-
(Ti3C2Tx/P-H) hybrid film electrode was prepared by filtering tion (XRD) spectra were collected on a Rigaku D/max2600 X-ray
the compound inks of Ti3C2Tx suspension and Clevios diffractometer. Raman spectra were characterized by a Micro-
PH1000, followed by immersion in a concentrated H2SO4 solu- Raman spectrometer (J-Y; HR800, France) under an excitation
tion. The optimal ratio of the Ti3C2Tx/P-100-H hybrid film wavelength of 488 nm. Fourier transform infrared (FT-IR)
electrode delivered ultrahigh volumetric capacitance of spectra were obtained using a Magna Model 560 FT-IR spectro-
1065 F cm−3 at 2 mV s−1, good rate performance of 62% after meter with a resolution of 1 cm−1. The nitrogen absorption/
increasing the scan rate by 50 times, and long-term cycling desorption isotherms were obtained on an ASAP 2010 acceler-
stability with an approximate decay rate of 0.002% per cycle ated surface area and porosimetry instrument (Micromeritics).
over 10 000 cycles. Furthermore, we fabricated an asymmetric Zeta potential was measured using Malvern Zetasizer Nano
supercapacitor (ASC) based on a Ti3C2Tx/P-100-H hybrid film ZS90new (USA).
negative electrode and rGO film positive electrode using 1 M
H2SO4 as the electrolyte. The device exhibited high energy
Electrochemical measurements
density and power density (the maximum energy density Electrochemical measurements were conducted in 1 M H2SO4
of 23 mW
h
cm−3 and maximum power density of electrolyte with a standard three-electrode system using a
7659 mW cm−3). A simple luminous band was lit by the ASC, VMP3 electrochemical workstation (BioLogic, France). Samples
suggesting its practicality for applications.
with size of 1.0 × 0.5 cm2, carbon rod and Ag/AgCl served as
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the working, counter and reference electrodes, respectively.
Electrochemical impedance spectroscopy (EIS) was conducted
with an AC perturbation of 10 mV over a frequency range from
0.01 Hz to 100 kHz. In the case of the asymmetric super-
capacitor device, we used the Ti3C2Tx/P-100-H hybrid film as
the negative electrode, a separator (PTFE, Shanghai Minglie
Chemical Technology Co., Ltd), rGO film as the positive elec-
trode and 1 M H2SO4 electrolyte. The energy density and power
density of the ASC were calculated based on the volume of the
active materials including the two working electrodes.
Results and discussion
To suppress the restacking of 2D Ti3C2Tx NSs and further
improve the electrochemical performance of the Ti3C2Tx elec-
trode, the conductive polymer PEDOT was introduced as a
spacer between Ti3C2Tx NSs. Fig. 1 schematically illustrates the
procedure used to prepare H2SO4-treated Ti3C2Tx/PEDOT:PSS
(Ti3C2Tx/P-H) hybrid film electrodes. The Ti3C2Tx suspension
and Clevios PH1000 aqueous solution are represented by
green-black and dark blue colors, respectively. The surfaces of
Ti3C2Tx and PH1000 were both negatively charged with
ζ-potentials of −20.9 mV and −73.5 mV (Fig. S1†), respectively.
After mixing the two solutions, the electrostatic repulsion
between Ti3C2Tx and PH1000 helped maintain the stability of
the colloidal mixture without apparent agglomeration. As
reported previously, when treating PEDOT:PSS with concen-
trated H2SO4, H+ from H2SO4 can associate with PSS− to form
hydrophilic PSSH, which can be removed by washing.25 After
treatment, Ti3C2Tx/P-H hybrid films were achieved.
Interestingly, the hybrid film Ti3C2Tx/P-100-H was found to
exhibit the best electrochemical performance; therefore, charac-
terizations of the morphology and structure for the optimum
composition of Ti3C2Tx/P-100-H were mainly addressed.
Fig. 2 (a) Digital images showing flexibility of the Ti3C2Tx/P-100-H film;
cross-section SEM images of (b) Ti3C2Tx, (c) Ti3C2Tx/P-100, and (d)
Ti3C2Tx/P-100-H films; HRTEM images of (e) Ti3C2Tx and (f) Ti3C2Tx/
P-100-H films (inset is a profile plot of the calibration for measuring the
spacing of Ti3C2Tx NSs).
excellent mechanical flexibility of the Ti3C2Tx/P-100-H hybrid
film, it is suitable for use as the electrode material of next-
generation wearable and flexible supercapacitors. Fig. 2b, c,
and d show the cross-section SEM images of pure Ti3C2Tx,
Ti3C2Tx/P-100, and Ti3C2Tx/P-100-H films, respectively. The
pristine Ti3C2Tx film (Fig. 2b) showed a well-aligned layered
structure and maintained the orderly stacked layers after con-
centrated H2SO4 treatment (Fig. S2†). When PH1000 was incor-
porated to form the Ti3C2Tx/P-100 film (Fig. 2c), it was found
that there were less clear layer structures with closed edges
over the entire film, which can be attributed to the alternating
arrangement and interlocking of Ti3C2Tx and PEDOT in the
presence of PSS (Fig. S2b†). However, the Ti3C2Tx/P-100-H film
(Fig. 2d) was restored to an open-edged and much sparser
layer structure after concentrated H2SO4 treatment, which indi-
cated that the remaining PEDOT, left after partially removing
PSS, as a spacer can effectively prevent the restacking of
Ti3C2Tx NSs and provide a higher accessible surface area. This
speculation was further confirmed by the N2 adsorption/de-
sorption measurement (Fig. S3†). The Ti3C2Tx/P-100-H film
exhibited a specific surface area of 73.7 m2 g−1, which was 4.5
times that of the restacked Ti3C2Tx film (16.2 m2 g−1). The
Ti3C2Tx/P-100-H film with a more open accessible surface area
provided excess active sites for electrochemical reactions.
Thus, we deduce that the Ti3C2Tx/P-100-H hybrid films can
exhibit improved electrochemical performance.
The as-prepared Ti3C2Tx/P-100-H film exhibited excellent
flexibility and could be folded or twisted many times without
any structural degradation, as shown in Fig. 2a. Due to the
To fully demonstrate the microstructure, Fig. 2e shows the
high-resolution transmission electron microscopy (HRTEM)
image of Ti3C2Tx; the single atom interlayer spacing of 12.8 Å
Fig. 1 Schematic illustration for the formation mechanism of Ti3C2Tx/
P-H hybrid film electrodes.
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(the inset of Fig. 2e) corresponds to the (002) plane of Ti3C2Tx. tion, the (002) peak position of Ti3C2Tx exhibited a clear down-
Compared with the observation for pure Ti3C2Tx NSs, the inter- shift from 6.9° to 6.2°, as shown in Fig. 3b, suggesting that
calation of PEDOT:PSS layers can be clearly identified from the PEDOT:PSS is intercalated between Ti3C2Tx layers. After treat-
HRTEM image of the Ti3C2Tx/P-100 hybrid film (Fig. S2b†). In ment by concentrated H2SO4, the (002) peak position contin-
the profile plot of the calibration for measuring the spacing of ued to downshift to 5.8°, and the corresponding interlayer
Ti3C2Tx layers, as shown in the inset of Fig. S2b,† there is a spacing was ≈15.2 Å, which was consistent with the TEM
new periodic structure, which is reflected by a combination of observation (Fig. 2f). This shift may be attributed to water or
a high peak and two low peaks, certifying the formation of proton intercalation. However, no diffraction peaks related to
alternating Ti3C2Tx and PEDOT:PSS layers (Fig. S2b†). the PEDOT polymer backbone were distinguished in the XRD
However, the periodic structure changes after concentrated patterns of Ti3C2Tx/P-100 and Ti3C2Tx/P-100-H hybrid films
H2SO4 treatment, which is reflected by a combination of a due to the amorphous crystal structure. The chemical structure
high peak and a low peak in the profile plot of the calibration, of the synthesized Ti3C2Tx/P-100-H hybrid film was further
as shown in the inset of Fig. 2f. Thus, we speculated that most characterized using Raman and FT-IR spectra. In the Raman
of the PSS is removed from the composite film after H2SO4 spectrum shown in Fig. 3c, the peaks at 1375 and 1576 cm−1
treatment. Besides, in the HRTEM image of the Ti3C2Tx/P-100-H for the Ti3C2Tx-H sample are characteristic of the D-mode and
hybrid film shown in Fig. 2f, the interlayer spacing between G-mode of graphitic carbon, which are attributed to exposed C
Ti3C2Tx layers of Ti3C2Tx/P-100-H hybrid film is 15.1 Å, which on the surface of Ti3C2Tx. In the Raman spectrum of H2SO4-
is much larger than that of pure Ti3C2Tx, confirming the sup- treated PEDOT:PSS film (P-H), Raman bands at 994, 1260,
pressive restacking of Ti3C2Tx NSs. The diffusion and transport 1370, 1429 and 1520 cm−1 correspond to oxyethylene ring
of electrolyte ions could be greatly promoted by the increased deformation, Cα–Cα inter-ring stretching vibrations, antisym-
interlayer spacing, and the ion transport pathways for fast metric Cβ–Cβ stretching deformations, symmetric stretching
charge storage could be significantly improved by ordered mode of the aromatic CvC band, and antisymmetric Cα–Cα
morphologies of alternating Ti3C2Tx and polymer layers.
stretching deformations.29,30 In addition, the Raman bands
To further illustrate the change in interlayer spacing between 1400 and 1600 cm−1 become narrower and the inten-
observed by TEM, the XRD patterns of the as-synthesized films sities of these peaks decrease after concentrated H2SO4 treat-
were obtained, as shown in Fig. 3a; Fig. 3b shows the corres- ment, as shown in Fig. S4,† which confirms that most of the
ponding enlarged XRD patterns. For the pure Ti3C2Tx film, the PSS is removed.31,32 The signature peaks for PEDOT and
existence of a clear characteristic (002) peak of Ti3C2Tx at 2θ = Ti3C2Tx were also found for the Ti3C2Tx/P-100-H hybrid film,
6.9° indicated the formation of a layered structure and the demonstrating that PEDOT was successfully intercalated
corresponding interlayer spacing value was calculated to be between Ti3C2Tx NSs. This result was further corroborated by
12.8 Å. After mixing the Ti3C2Tx suspension with PH1000 solu- the FT-IR spectra, as displayed in Fig. 3d. It can be seen that
Fig. 3 (a) XRD patterns of Ti3C2Tx, Ti3C2Tx/P-100 and Ti3C2Tx/P-100-H and (b) the corresponding enlarged XRD patterns, (c) Raman spectra and (d)
FTIR spectra of Ti3C2Tx-H, Ti3C2Tx/P-100-H and P-H films.
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C–O (∼1051 cm−1), C–F (∼1090 cm−1) and –OH (∼1600 cm−1
)
electrode materials were controlled with the same mass
groups exist in Ti3C2Tx-H;33 the peak at 665 cm−1 is attributed loading of Ti3C2Tx per unit area. CV curves for Ti3C2Tx/P-100-H
to the vibration of the Ti–O bond.34 For H2SO4-treated PEDOT: hybrid film electrode are shown in Fig. 4a; slight variations in
PSS film (P-H), the FT-IR bands at 1530, 1420 cm−1 and the shapes of the CV curves indicate high electron and electro-
1370 cm−1 were mainly due to the C–C and CvC stretchings lyte ion mobility in the hybrid film. Continuous change in the
of the quinoidal structure of the thiophene rings.35,36 The titanium oxidation state during the charge/discharge process
bands at 1258 and 1063 cm−1 were assigned to the C–O–C in H2SO4 electrolyte led to no distinct redox peaks in the CV
bond stretching in the ethylene dioxide units.24,36 The C–S curves.37 GCD curves in Fig. 4b exhibit a linear symmetrical
stretching mode exhibited IR bands at 1006, 922, 856 and charge–discharge profile with no noticeable voltage drop (iR
706 cm−1 35,36
The hybrid Ti3C2Tx/P-100-H film exhibited a drop), implying the high reversibility of the redox reactions
.
combined character of the PEDOT and Ti3C2Tx spectra, and low equivalent series resistance in the Ti3C2Tx/P-100-H
suggesting the intercalation of PEDOT between Ti3C2Tx layers. hybrid film electrode. As control groups, the electrochemical
The above SEM, TEM, XRD, Raman and FT-IR results suggest performances of Ti3C2Tx and Ti3C2Tx/P-100 film electrodes are
that PEDOT has been successfully intercalated between shown in Fig. S5.† The relationships of the volumetric capaci-
Ti3C2Tx NSs. The interlayered polymer can expand the space tances and specific capacitance as a function of scan rates for
between Ti3C2Tx NSs and effectively hinder the re-stacking the as-prepared samples are shown in Fig. 4c and Fig. S6.† In
phenomenon, which is beneficial to expose more active sites contrast, the pure Ti3C2Tx film electrode showed volumetric
and increase electron transfer and ion diffusion into the elec- capacitance of 952 F cm−3 (241 F g−1) at 2 mV s−1 and rate per-
trode materials more efficiently.
formance of 58% as the scan rate increased to 100 mV s−1. The
The electrochemical behavior of the Ti3C2Tx/P-100-H hybrid effect of H2SO4 treatment on the electrochemical performance
film was studied, as shown in Fig. 4. For comparison, all the of the Ti3C2Tx film electrode was almost negligible (Fig. S7†).
Fig. 4 (a) CV curves and (b) GCD curves of Ti3C2Tx/P-100-H hybrid film electrode at different scan rates and current densities. (c) Volumetric
capacitance of the as-prepared hybrid film electrodes as a function of scan rate. (d) Nyquist plots of the as-prepared hybrid film electrodes; inset
shows the magnified high-frequency region. (e) Cycling stability of pure Ti3C2Tx and Ti3C2Tx/P-100-H hybrid film electrodes at a current density of
5 mA cm−2
.
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Besides, the P-H electrode exhibited poor electrochemical per- the electrochemical durability beyond thousands of cycles. The
formance of only 106 F cm−3 at 2 mV s−1 (Fig. S8†). After influence of the amount of PEDOT in the hybrid film electrode
mixing PH1000 with Ti3C2Tx to form the Ti3C2Tx/P-100 film, the was also examined (Fig. S9 and S10†). It was found that
volumetric capacitance decreased dramatically to 712 F cm−3 decreasing or increasing the mass loading of PEDOT resulted
(216 F g−1) at 2 mV s−1 (Fig. 4c), which can be attributed in worse electrochemical properties; this could be because
to the closed surface of the Ti3C2Tx/P-100 film, as shown by decreasing the content of PEDOT decreases the interlayer
the SEM results (Fig. 2c). However, after concentrated H2SO4 spacing (Fig. S11†), whereas increasing the content of PEDOT
treatment, the volumetric capacitance of Ti3C2Tx/P-100-H film with poor electrochemical performance results in reduced
electrode significantly improved to 1065 F cm−3 (286 F g−1) at overall electrochemical properties of the hybrid electrode.
2 mV s−1, but it exhibited lower density (3.72 g cm−3) com- Combining the above analyses, the superior performance of
pared with the pure Ti3C2Tx film (3.95 g cm−3) (Table S1†). The the hybrid Ti3C2Tx/P-100-H electrode can be attributed to the
volumetric capacitance was higher than those of most pre- following reasons: (i) the effective suppression of restacking of
viously reported Ti3C2-based materials such as the few-layer Ti3C2Tx NSs and the increase in interlayer spacing improve
Ti3C2Tx film (350 F cm−3, 1 M KOH),15 PPy/Ti3C2Tx film (1000 accessibility of the ions due to pillaring of PEDOT; (ii) the
F cm−3, 1 M H2SO4),22 sandwich-like Ti3C2Tx/SWCNT film (390 enhancement in the exposed electroactive surface and the
F cm−3, 1 M H2SO4),17 Ti3C2Tx/PVA film (528 F cm−3, 1 M reduction in the ion diffusion pathway facilitate electron trans-
KOH),21 Ti3C2Tx/rGO film (1040 F cm−3, 3 M H2SO4),13 and fer and ion diffusion to promote the electrochemical reaction
Ti3C2Tx hydrogels (1500 F cm−3, 3 M H2SO4).38 More detailed of Ti3C2Tx; (iii) the conductive polymer intercalated between
comparison results are shown in Table S2.† Simultaneously, Ti3C2Tx NSs can form multidimensional pathways for electron
the Ti3C2Tx/P-100-H film electrode exhibited increased rate transport to accelerate the electrochemical reaction process.
performance of 62%, which may be due to PEDOT intercalated Therefore, the hybrid Ti3C2Tx/P-100-H electrode shows promis-
between Ti3C2Tx NSs, providing multidimensional conductive ing application prospects for a supercapacitor.
channels for charge transfer. This speculation can be further
Based on previously reported theoretical analysis and
confirmed by EIS shown in Fig. 4d. The Nyquist curves are experimental results, the total electrochemical stored charge of
composed of a high-frequency arc region and a low-frequency Ti3C2Tx can be separated into three fractions: (1) the faradaic
line region. The low-frequency lines of the Ti3C2Tx and contribution from the slow H+ ion insertion process limited by
Ti3C2Tx/P-100-H film electrodes are almost parallel to the ion diffusion, (2) the faradaic contribution from the fast
y-axis, indicating an ideal capacitive behavior. The equivalent charge-transfer process at the surfaces originating from con-
series resistance (ESR) values, intercept on Re (Z) in the high- tinuous change in the titanium oxidation state, which is also
frequency region, for Ti3C2Tx and Ti3C2Tx/P-100-H were 1.3 Ω referred to as pseudocapacitance, and (3) the nonfaradaic con-
and 1.0 Ω, respectively, as shown in the enlarged view inset of tribution from the fast electric double layer effect at the sur-
Fig. 2e, indicating improvement in conductivity. Furthermore, faces.40 Electrochemical analysis of different kinetic responses
Rct (charge transfer resistance) of Ti3C2Tx/P-100-H film elec- promotes a better understanding of the charge/discharge
trode reflected by the arc radius in the high-frequency region mechanism of the Ti3C2Tx/P-100-H hybrid film electrode.
was much smaller than that of Ti3C2Tx, suggesting an increase
The fast surface-reaction process and the slower diffusion-
in ion accessibility, reduction of ion diffusion distances and controlled process can be distinguished with the Trasatti’s
improvement in the electron transfer rate. Long-term cycling method.41 To estimate the surface capacitance, the charge is
stability is an important fundamental property for super- extrapolated to an infinitely fast scan rate by plotting charge
capacitors. The cycling life of the Ti3C2Tx/P-100-H film elec- vs. v−1/2. As shown in Fig. 5a, when v → ∞, the contribution
trode was evaluated through GCD measurements at a current from the diffusion-controlled process is eliminated, and the
density of 5 mA cm−2, as shown in Fig. 4e. The capacitance of y-intercept is the surface charge contribution (qs).
the Ti3C2Tx electrode maintained 96% of its initial value after Furthermore, the maximum total charge (qt) can be obtained
10 000 cycles; however, interestingly, we found that the capaci- at an infinitely slow scan rate. In Fig. 5b, when v → 0, the
tance retention of the Ti3C2Tx/P-100-H film electrode first y-intercept is qt−1 and can be acquired by plotting q−1 vs. v1/2
.
increased to 150% of the initial value at the 1300th cycle due As a consequence, the surface capacitance and total capaci-
to the self-activation process, then reached the initial value at tance of Ti3C2Tx/P-100-H hybrid film electrode were calculated
the 6500th cycle, and finally decayed to 80% after 10 000 to be 864.8 and 1216.5 F cm−3, which were higher than those
cycles, corresponding to an approximate decay rate of 0.002% of Ti3C2Tx (705.0 and 1096.5 F cm−3). The increased surface
per cycle. Generally, PEDOT can usually accommodate large capacitance of the Ti3C2Tx/P-100-H electrode could be attribu-
volumetric expansion and shrinkage during the charge/dis- ted to PEDOT acting as a spacer, which prevented the restack-
charge process.39 We speculated that for the Ti3C2Tx/P-100-H ing of Ti3C2Tx NSs effectively, increased interlayer spacing, and
film electrode, the PEDOT acted as a spacer to hinder the provided a higher accessible surface area, as demonstrated by
restacking of Ti3C2Tx NSs, and Ti3C2Tx NSs could effectively the results of SEM, XRD and BET analyses. Importantly, the
buffer the volume change of PEDOT. In agreement with this, surface capacitance for the Ti3C2Tx/P-100-H electrode was pre-
the activation of this electrode required a much longer time up dicted to be 71.1% of the total capacitance, which was 6.8%
to 1300 cycles with increase in capacitance, thus increasing more than that of Ti3C2Tx (64.3%) (Fig. 5c). This result
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Fig. 5 (a) Stored charge (q) versus inverse of square root of the scan rate (v), (b) inverse of the stored charge versus the square root of the scan rate,
(c) Trasatti’s method of deconvoluting charge storage contributions, (d) b values plotted against voltages (inset: current response plotted against
scan rate at different voltages), (e) capacitive contribution to charge storage at scan rates of 5 and 100 mV s−1, (f) percentage of capacitance contri-
bution at different scan rates.
explains the reason for the improvement in the rate perform- controlled. To determine the ratios of the capacitive contri-
ance of the Ti3C2Tx/P-100-H electrode from the kinetics. The bution quantitatively, Dunn’s method was introduced. This
Trasatti’s method indicated that Ti3C2Tx/P-100-H facilitated a method deconvolutes the surface-reaction and diffusion-con-
faster electrochemical reaction, resulting in both higher overall trolled contributions to current at a given voltage, i (V), based
capacitance and higher contribution from the v-dependent on the following equation:
charge, which included the contribution of the electrical
i ðVÞ ¼ k1v þ k2v1=2
ð3Þ
double layer and all fast faradaic reactions that are not
diffusion-limited. During the charging and discharging
process, the current response rate to the surface-reaction was
much faster than that to the H+ ion insertion process limited
by ion diffusion. Therefore, the higher the surface capacitance
contribution of the electrode, the faster the current response
speed, and the better the rate performance. This result pro-
vides a new solution for designing electrode materials having
ultra-high rate performance.
To deeply investigate the charge storage and ion transport
mechanism of Ti3C2Tx/P-100-H, the surface-reaction contri-
bution and diffusion-controlled contribution of the total
capacitance were further analyzed.41 Because the linear
relationships between the response current and scan are is → v
and id → v1/2 corresponding to the surface-reaction and
diffusion-controlled processes, a general expression can be
defined as eqn (1):
i ðVÞ
¼ k1v1=2 þ k2
ð4Þ
v1=2
Here, i (V), k1v, k2v1/2 and v are the current at a fixed poten-
tial, surface-reaction and diffusion-controlled currents, and
scan rate. Thus, by determining k1 and k2, we can quantify the
fraction of the current due to each of these contributions.
Furthermore, the ratios of capacitive contribution can be quan-
titatively determined by comparing the shaded area with the
total stored charge (Fig. 5e). It was found that the surface-reac-
tion capacitive contribution of the Ti3C2Tx/P-100-H electrode
increased from 70.3% to 99.6%, as can be seen in Fig. 5f.
Moreover, the surface-reaction capacitive contribution of the
Ti3C2Tx/P-100-H electrode was larger than that of Ti3C2Tx,
which was consistent with the result calculated using the
Trasatti’s method. The increase in surface-reaction capacitive
contribution can be attributed to the larger exposed active
surface area, faster electron transfer and shorter ion diffusion
length, which can explain why Ti3C2Tx/P-100-H has high rate
performance.
i ¼ is þ id ¼ avb
ð1Þ
ð2Þ
log i ¼ log a þ b log v
Here, i is the current, a and b are variable parameters, and
ASC and two ASC units are connected in series or in paral-
b-values can be determined from the slope of the plot of log i lel, (i) a simple luminous band powered by two Ti3C2Tx/P-100-
vs. log v. Ideally, the exponent b will have values of b = 0.5 for H//rGO ASCs in series.
the diffusion-controlled process and b = 1 for a surface reac-
To extend the working voltage and increase the energy
tion. In Fig. 5d, the b-values of Ti3C2Tx/P-100-H dependent on density, we assembled asymmetric supercapacitors (ASC) by
different potentials are in the range of 0.85–1, indicating that using the Ti3C2Tx/P-100-H hybrid film electrode as the negative
the kinetics of Ti3C2Tx/P-100-H was predominantly surface- electrode and the rGO film as the positive electrode (Fig. 6a).
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Fig. 6 (a) Schematic illustration for the assembled Ti3C2Tx/P-100-H//rGO ASC, (b) CV curves obtained for Ti3C2Tx/P-100-H and rGO electrodes at a
scan rate of 10 mV s−1, (c) CV curves and (d) GCD curves of ASC at different scan rates and current densities, (e) Ragone plots of ASC. The values
reported for other ASCs are added for comparison, (f) pictures and (g) CV curves of the ASC measured at different bent conditions, (h) GCD curves
obtained at a current density of 1.5 mA cm−2 for a single.
The rGO film (positive electrode) was selected due to its flexi- 15 mA cm−2. In Fig. 6e, we compare the volumetric power and
bility, good conductivity, high chemical stability and a match- energy densities of Ti3C2Tx/P-100-H//rGO ASC to those of other
ing potential application range with Ti3C2Tx/P-100-H. The as- supercapacitors reported previously. Ti3C2Tx/P-100-H//rGO ASC
prepared rGO film was characterized, as shown in Fig. S12.† In delivered high energy density of 23 mW h cm−3 and high
Fig. 6a, the working potential of Ti3C2Tx/P-100-H is from −0.3 power density of 7659 mW cm−3, which were closely compar-
to 0.3 V, whereas the rGO film operates in a potential window able with or in some cases much higher than those of
from 0.3 to 0.9 V, which can enable effective cell voltage of up previously reported Ti3C2-based supercapacitors and graphe-
to 1.2 V. Fig. 6c presents the CV curves of flexible Ti3C2Tx/ ne-based supercapacitors: Ti3C2//rGO (8.6 mW
h
cm−3
,
,
,
P-100-H//rGO ASC at various scan rates. Clearly, the assembled 200 mW cm−3),44 PPy/l-Ti3C2 symmetric SC (10 mW h cm−3
ASC exhibits rectangular CV curves even at a high scan rate of 500 mW cm−3),49 GO film//3D-DG@MnO2 (28.2 mW h cm−3
100 mV s−1, indicating an ideal capacitive behavior and fast 30 000 mW cm−3),52 etc.,13,42,43,45–48,50,51,53 as listed in
charge–discharge properties. In addition, the GCD curves of Table S3.† These results imply that the Ti3C2Tx/P-100-H film
ASC (Fig. 6d) are linear slopes and the triangle shape corro- electrode is expected to be a highly promising candidate for
borates the fast charge–discharge properties; the GCD curves application as the negative electrode in high-performance
are consistent with the CV curves. The ASC exhibited high energy storage systems.
volumetric capacitance of 117 F cm−3 calculated from GCD
curve at 1.5 mA cm−2 and capacitance retention to 69.9% at storage devices in flexible and wearable electronics, in addition
To evaluate the potential use of the developed energy
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to the good performance in the relaxed state, the performances
under different bending angles were characterized, as shown
in Fig. 6f. According to the CV curves in Fig. 6g, no significant
change in the integrated area of the CV curves is observed
under bending, indicating the excellent flexibility of Ti3C2Tx/
P-100-H//rGO ASC. To build integration patterns, the same
Ti3C2Tx/P-100-H//rGO ASCs were connected in series or in par-
allel for practical applications. The GCD curves of two ASCs
connected in parallel exhibit a discharge time about twice that
of each single ASC at the same current density, as shown in
Fig. 6h. As expected, the voltage window of two ASCs
connected in series could reach 2.4 V, maintaining the same
charging/discharging time; a simple luminous band that can
be worn by fans at a concert or worn by traffic police to direct
traffic using gesture signals at night was designed and
powered by our two Ti3C2Tx/P-100-H//rGO ASCs in series. After
being charged, the serial-connected ASCs could drive the lumi-
nous band for about five minutes, as shown in Fig. 6i and
Fig. S13,† which indicated that Ti3C2Tx/P-100-H//rGO ASC has
excellent application prospects in real life.
Notes and references
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