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A novel MnO2/MXene composite prepared by

Cite this: Inorg. Chem. Front., 2019,
electrostatic self-assembly and its use as an
6, 199 electrode for enhanced supercapacitive
performance†
Published on 19 November 2018. Downloaded on 1/20/2019 7:36:26 PM.

Shu Chen,a Yuanfang Xiang,a Weijian Xu *a and Chang Peng*a,b

MXene is a new 2D transition metal carbide possessing metallic conductivity and hydrophilic surfaces. It
has drawn widespread attention as a potential material for electrode use. However, the applications of
MXene are limited by its property of low electrical capacity. In this paper, a novel MnO2/MXene composite
is prepared by electrostatic self-assembly. Firstly, delaminated MnO2 nanosheets are obtained through the
intercalation delamination of multilayered H-MnO2 in a cationic Gemini surfactant (Gem) solution, leading
to a positively charged surface. Then, the positive MnO2 nanosheets are assembled on negative MXene
nanosheets through electrostatic self-assembly to form a MnO2/MXene composite. The characterization
results show that the MnO2 nanosheets are intimately assembled on the MXene nanosheets. As an elec-
trode material, the MnO2/MXene composite displays a specific capacitance of 340 F g−1 at 1 A g−1, which
Received 7th September 2018, is three times the performance of an MXene electrode. In addition, the MnO2/MXene electrode shows a
Accepted 16th November 2018
high retention rate (90.3% retention at 10 A g−1) and good cycling life (87.6% of the initial specific capaci-
DOI: 10.1039/c8qi00957k tance is retained after 2000 cycles at 4 A g−1). The properties of the proposed composite are attributed to
rsc.li/frontiers-inorganic the excellent conductivity of MXene and the high specific capacitance of MnO2.

1. Introduction carbon materials exhibit a low specific capacitance and

limited voltage window. Thus, transition metal oxides have
With the rapidly growing energy demand, the development of attracted more attention and become the most important
energy storage systems with high-efficiency and low-cost has electrode materials because of their high-performance in
become a critical issue.1–4 The exploration of new electrode pseudocapacitors.10–16 Recently, a two-dimensional titanium
materials is an essential topic to improve power storage. An carbide material labeled MXene was reported, which shows
ideal electrode material is expected to deliver the electrical both metallic conductivity and hydrophilic features.17 This
charges promptly and store the charges at high density. novel material is prepared by etching aluminium from the
However, it is not easy to find a material with the above two corresponding ‘MAX’ phase carbides in hydrofluoric acid.
requirements at the same. In recent years, supercapacitors MXene is classified as an inorganic graphene oxide, since it is
have been attracting more and more attention due to their highly conductive, like the ‘MAX’ phase before etching.18–20
good electrochemical properties, such as excellent cycling Thus, MXene has the potential to be used as an effective
stability, ultrafast charge-discharge, and long-term cycling.5–7 material for electrochemical supercapacitors, to manufacture
Therefore, researchers have investigated new electrode powerful energy storage devices.21,22 During the HF etching
materials, such as transition metal oxides and carbon process, the surfaces of MXene are terminated by –F, –O, and
materials, with outstanding electrochemical properties for –OH groups, and then the MXene can be named as Mn+1XnTx,
energy storage applications. Some of the most widely used where M is the transition metal, X is the carbon, and T is the
electrode materials are carbon materials, with active sites terminating group.23,24 Thus, MXene can be made to express
for redox reactions and high conductivity.8,9 However, different properties by modifying these terminating groups
with various addends.
MXenes exhibit high volumetric capacitance, stable cycling
a
College of Chemistry and Chemical Engineering, Hunan University, Hunan 410082,
performance, and long-term cycling.17 However, compared to
P.R. China. E-mail: weijianxu_59@sina.com, pch1026@126.com
b
College of Science, Hunan Agricultural University, Hunan 410128, P.R. China
graphene, the MXenes still have lower gravimetric capacitance,
† Electronic supplementary information (ESI) available. See DOI: 10.1039/ which could be improved in future studies. In general, the
c8qi00957k pseudocapacitive transition metal oxides provide a large

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capacity of charge storage, and then enhance the electro- 2. Materials and characterization
chemical performance of carbon materials. Meanwhile, the
high theoretical pseudocapacitance of the transition metal Synthesis of multilayered H-birnessite MnO2 (H-MnO2)
oxides can be effectively utilized by carbon materials with The birnessite MnO2 was synthesized according to the litera-
good mechanical properties and conductivity.25,26 For ture.38 The obtained birnessite MnO2 (1 g) was added to
example, MnO2 nanowhiskers have been used to fabricate 500 mL of HCl (0.1 M) under stirring for 48 h. After the
MnO2 nanowhisker/MXene composites by direct chemical syn- mixture was filtered and dried, multilayered H-MnO2 was
thesis at 60 °C, and the specific capacitance of the MXene can obtained.
be improved due to the additional contribution of the pseudo-
capacitance provided by the MnO2 nanowhiskers.26 Thus, Synthesis of the delaminated MnO2 nanosheets
MnO2 is an ideal candidate for enhancing the supercapacitive Gem (C18N22+Br2−) was synthesized according to the litera-
performance of MXene owing to its low cost, high theoretical ture.39 The 1H nuclear magnetic resonance (1H NMR) of Gem
value, and electron transfer/storage ability.27,28 is shown in Fig. S1 (ESI†). The synthesis of the MnO2
Recently, birnessite-type MnO2 nanosheets29–33 have been nanosheets was carried out in Gem solution. First, 0.5 g of
Published on 19 November 2018. Downloaded on 1/20/2019 7:36:26 PM.

reported as a new class of electrode materials due to their long H-MnO2 was added to 500 mL of Gem solution (0.5 M) under
cycling life and high specific capacitance. Thus, surface dec- stirring for 6 h and bath-sonicated for 2 h. The obtained dis-
oration using birnessite-type MnO2 nanosheets may also persion solution was centrifuged for 30 min at 8000 rpm. The
improve the electrochemical properties of MXene. However, brown supernatant was collected to obtain the delaminated
the simple physical and mechanical mixing of MnO2 MnO2 suspension. Finally, after washing the unbound Gem in
nanosheets and MXene nanosheets may not efficiently prevent the colloidal suspension and filtering the mixture, MnO2
the self-stacking of these two types of nanosheets due to van nanosheets were obtained.
der Waals interactions. A suitable integration strategy using
the MnO2 nanosheets for the preparation of MnO2/MXene Synthesis of multilayered Ti3C2Tx
composites remains a scientific problem. Electrostatic self- Multilayered Ti3C2Tx was prepared by etching Ti3AlC2. 1 g of
assembly is one of the simplest and most effective ways to fab- LiF (98.5%) was added to 10 mL of HCl solution (9 M), and 1 g
ricate advanced composites for power source applications.34–36 of Ti3AlC2 was added to the above solution under stirring for
MXene is a negatively charged material due to its surface func- 24 h at 35 °C. Then, the mixture was washed with water using
tional groups.24 When the two types of nanosheets have oppo- centrifugation until the pH of the supernatant was above
site surface charges to each other, the electrostatic interactions 5. Multilayered Ti3C2Tx was then obtained by freeze drying.
between the MnO2 nanosheets and MXene nanosheets induce
a self-assembly process that results in the formation of a Synthesis of delaminated Ti3C2Tx (MXene)
MnO2/MXene composite. The contact between the two types of Multilayered Ti3C2Tx (1 g) was added to 250 mL of deionized
nanosheets via this self-assembly synthesis can greatly water and sonicated for 1 h under Ar flow. Then, the dis-
promote the conductivity of the composite. However, to the persion solution was centrifuged for 1 h at 3500 rpm. The dark
best of our knowledge, the use of electrostatic self-assembly to green supernatant was collected to obtain the delaminated
fabricate a layered MnO2/MXene composite has not been MXene suspension. This MXene suspension was filtered and
reported before. freeze-dried to measure the concentration of the delaminated
Gemini surfactants (Gem) are a new type of surfactant with MXene.
two cationic head groups, and two hydrophobic tail chains,
linked by a spacer at the heads, which can alter the electro- Synthesis of MnO2/MXene composite
static charges to hybridize the 2D materials and provide many The MnO2/MXene composite was prepared by very slowly
attractive properties such as good aqueous dispersion and mixing two dispersion solutions of positive MnO2 nanosheets
modifiable ability.37 In our work, MnO2 nanosheets were (50 mL, 0.2 mg mL−1) and negative MXene nanosheets (50 mL,
obtained through Gem intercalation and exfoliation of multi- 0.2 mg mL−1). The composite quickly transforms into a pre-
layered H-MnO2 to change the charges on the surface of the cipitate and settles to the bottom due to electrostatic self-
MnO2 nanosheets to positive and form a stable dispersion. assembly. Finally, the MnO2/MXene composite was obtained
Since the surface charges of the MnO2 nanosheets and MXene by filtering the mixture.
nanosheets are opposite, the two types of nanosheets can
attract each other via electrostatic interactions. As a result, we Characterization
have fabricated a novel layered MnO2/MXene composite for the A Varian Inova-400 spectrometer was used to record the
1
first time through electrostatic self-assembly. The architecture H NMR of the Gem. A Malvern Zetasizer Nano ZS90 (Malvern
of the composite enabled excellent contact, thereby enhancing Instruments, UK) was used to measure all zeta potentials of
the interfacial electron transfer. Furthermore, we carefully the samples. Scanning electron microscopy (SEM) (JSM-6700F,
studied the electrochemical performance of the MnO2/MXene Hitachi Ltd) and transmission electron microscopy (TEM)
electrode using a three-electrode system, which led to excellent (JEM-2100F, JEOL) were used to inspect the morphology of the
results of high specific capacitance and good stability. materials. The element mapping analysis and energy disper-

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sive spectroscopy (EDS) results were acquired by using TEM. potential window in CV (V), and mass of electroactive material
An X-ray diffraction (XRD) instrument (Rigaku, Ltd, JP) was in the electrode (g) respectively.
used to measure the crystallinity of the materials. A PHI 5000c
IΔt
ESCA photoelectron spectrometer was used to record the X-ray C¼ ð2Þ
mΔV
photoelectron spectroscopy (XPS) spectra of the materials.
A CHI 760C workstation (CH Instruments Inc.) with a three- where C, m, I, ΔV, and Δt are the gravimetric capacitance
electrode system was used to measure the cycle voltammetry (F g−1), mass of electroactive material in the electrode (g), dis-
(CV) and galvanostatic charge-discharge (GCD) curves. The charge current (A), actual potential window in GCD (V), and
working electrode was prepared by mixing the active electrode discharge time (s), respectively. Electrochemical impedance
material, acetylene black, and polyvinylidene fluoride with a spectroscopy (EIS) was performed at open-circuit voltage using
mass ratio of 85 : 10 : 5 to form a slurry, and then the slurry a sinusoidal signal of 5 mV from 0.01 Hz to 100 kHz.
was coated onto stainless steel foil. The active material loading
on the stainless steel foil, which served as the working elec-
trode, was 5 mg cm−2. The stainless steel foil was ultrasoni- 3. Results and discussion
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cated in Na2SO4 (1 M) for 10 minutes before use, and rinsed

with acetone and deionized water, followed by vacuum drying. The fabrication of the MnO2/MXene composite, which was
All electrochemical experiments were carried out in 1 M obtained by electrostatic self-assembly between positive the
Na2SO4. Platinum wire and a calomel electrode were used as MnO2 nanosheets and negative MXene nanosheets, is schema-
the counter and reference electrodes, respectively. The gravi- tically illustrated in Fig. 1a. In our work, the Gem (1) makes
metric capacitances can be calculated using eqn (1) and (2): the MnO2 surface positively charged and forms a stable
aqueous dispersion due to electrostatic repulsion between the
Ð different positive MnO2 nanosheets, and (2) improves the
IdV surface reaction between the MnO2 nanosheets and the MXene
C¼ ð1Þ
mvΔV nanosheets. The delaminated MnO2 nanosheets were obtained
through the Gem intercalation and exfoliation of H-MnO2 in
where C, I, v, ΔV, and m are the gravimetric capacitance Gem solution. For the purpose of confirming this modifi-
(F g−1), response current (A), potential scan rate (V s−1), actual cation, the zeta potentials of the H-MnO2 and MnO2

Fig. 1 (a) Illustration of the process for preparing the MnO2/MXene composite; (b) the dispersion of the MXene nanosheets, MnO2 nanosheets, and
MnO2/MXene composite in water; (c) the zeta potentials of the composites at different wt ratios between the MnO2 nanosheets and MXene
nanosheets; (d) the zeta potential of the composite at MnO2/MXene (wt/wt) = 1/1.

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nanosheets were measured to be −23.5 and +26.1 mV, respect- promote the conductivity of the composite. Furthermore, XRD
ively (Fig. S2a and b in the ESI†). Here, the zeta potential of was used to analyze the chemical structure of the MnO2/
the MnO2 turned from a negative value to a positive value. MXene composite (Fig. 2e). The typical diffraction peak of
Since the Gem has two hydrophilic cation heads, the charge MXene (002) at 7.1°,41 with an interlayer spacing of 1.24 nm,
density and surface activity of the MnO2 nanosheets can be represents the pure MXene. The MnO2 nanosheets have a
increased by Gem. Then, the zeta potential result of MnO2 veri- layered structure, and strong diffraction peaks are observed at
fies that the Gem is firmly attached. Besides, the thin layered about 12.5° (001) and 25.0° (002), suggesting the formation of
structure of the MnO2 nanosheets is observed (Fig. S2c and d birnessite MnO2.34,40 The MnO2/MXene shows almost all of
in the ESI†). The formation of thin layers is a result of ion- the characteristic peaks of both the MnO2 and MXene.
intercalated functionalization by the Gem with cationic However, the (002) peak of MXene and the (001) peak of MnO2
groups. Owing to the electrostatic repulsion, the positive MnO2 are both broadened, suggesting that the MnO2 nanosheets
nanosheets disperse well in water (Fig. 1b). On the other hand, have been deposited on the surface of MXene. From this ana-
MXene was prepared by etching Ti3AlC2 and exfoliating the lysis, we can conclude that the structure of the MnO2/MXene
multilayered Ti3C2Tx, yielding a well dispersible suspension composite is analogous to a layered framework, which consists
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(Fig. 1b). The zeta potential of MXene is −26.6 mV, and the of layered MnO2 and layered MXene.
thin sheets of MXene clearly layered on top of each other The surface chemical composition and chemical valence
(Fig. S3a–c in the ESI†). When the positive MnO2 nanosheets states of the MnO2/MXene composite were analyzed by XPS. As
were added into the negative MXene nanosheet solution, the shown in Fig. 3a, N-atoms are found in the survey spectrum of
MnO2 nanosheets were adsorbed on the MXene nanosheets the MnO2 nanosheets and MnO2/MXene, while no N-atoms are
through electrostatic self-assembly. An MnO2/MXene compo- found in the MXene. Furthermore, after grafting the MnO2
site is formed and transforms into a precipitate at the bottom nanosheets onto the MXene, Mn 3s (84.5 eV) and Mn 2p
of the container (Fig. 1b). Fig. 1c shows the zeta potentials of (641.9 eV)42 are detected in the MnO2/MXene, which verified
the composites with different weight (wt) ratios between the that the N-atoms in MnO2/MXene come from Gem and the
MnO2 nanosheets and MXene nanosheets. The zeta potential Mn-atoms in MnO2/MXene come from MnO2. Table S1 in the
value increases with increasing wt ratio of MnO2/MXene. ESI† provides the element atomic ratio data from the XPS ana-
Notably, when the wt ratio equals 1/1, the zeta potential is lysis results. Fig. 3b–e are the high-resolution XPS spectra of
measured to be about 0 mV (Fig. 1d). Furthermore, when the MnO2/MXene. In Fig. 3b, the Ti 2p peak of MnO2/MXene can
wt ratios are more than 1/1, the zeta potentials pass from nega- be fitted to five constituent peaks: Ti–C at 455.4 eV, Ti–OH at
tive to positive due to the unbound MnO2 nanosheets with 457.1 eV, Ti–O (2p3/2) at 459.2 eV, Ti–F at 461.7 eV, and Ti–O
positive charges, resulting in the zeta potential changing to a (2p1/2) at 464.7 eV.43 Compared with the C 1s of MXene (Fig. S4
positive value in the composite solution. These results indicate in the ESI†), a new C–N group can be observed at 285.5 eV44 in
that the positive MnO2 nanosheets and negative MXene the C 1s of MnO2/MXene, which originates from the nitrogen
nanosheets fully self-assemble when the wt ratio between the in Gem (Fig. 3c). Fig. 3d and e show the Mn 3s and Mn 2p in
two types of nanosheets equals 1/1. From the analyses above, the high-resolution XPS of MnO2/MXene, respectively. These
we can conclude that electrostatic self-assembly is a mild and XPS results indicate that a large number of MnO2 nanosheets
efficient approach for MnO2/MXene composite preparation. are present in the MnO2/MXene composite due to electrostatic
The morphology of the MnO2/MXene composite (wt ratio = self-assembly.
1/1) was studied using SEM and TEM. The SEM of the compo- The electrochemical properties of the MnO2/MXene compo-
site presents a well-aligned multilayer microstructure (Fig. 2a). site were examined using a three-electrode system in 1 M
As shown in Fig. 2b, the TEM shows two characteristic mor- Na2SO4. The MXene and MnO2 nanosheet samples were also
phologies including the large plate-like MXene and small tested in the same system as control samples. In Fig. 4a, the
plate-like MnO2, in which the first type of nanosheet exhibits MnO2 nanosheets and MnO2/MXene (wt ratio = 1/1) electrodes
lattice fringes with a d-spacing of 0.31 nm reflecting the (006) exhibit an oxidation peak appearing at about 0.7 V due to the
plane of MXene (i),26 and the other type of nanosheet exhibits pseudocapacitive effect of MnO2, whereas no obvious redox
lattice fringes with a d-spacing of 0.25 nm indexed to the (100) peak is observed for the pure MXene. Compared to the MXene
plane of the MnO2 nanosheets (ii).40 This result indicates that and MnO2 nanosheets, the MnO2/MXene sample has a larger
the MnO2 nanosheets are intimately reassembled on the area in the closed-loop CV curve, which indicates that the
MXene nanosheets. The formation of the as-prepared MnO2/ MnO2/MXene electrode has better capacitive performance than
MXene composite is also verified by the elemental mapping the others. This demonstrates that electrostatic self-assembly
analysis (Fig. 2c). The signals of Mn and Ti are well distributed improves the surface reaction between the MnO2 nanosheets
according to the sites of the MnO2 nanosheets and MXene and MXene nanosheets. In Fig. 4b, symmetric oxidation and
nanosheets on the surface of the MnO2/MXene composite, reduction peaks can be observed at scan rates as high as
respectively. Besides, the EDS result verifies the elementary 125 mV s−1 for the MnO2/MXene, indicating the excellent
composition of the composite (Fig. 2d). It is believed that the reversibility of this electrode. In Fig. 4c, the oxidation peak cur-
self-assembly can prevent the MXene sheets from restacking, rents and scan rate are linearly correlated, which suggests that
and the effective contact between the MnO2 and MXene could the MnO2/MXene electrode has good kinetic performance and

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Fig. 2 (a) SEM images of MnO2/MXene; (b) TEM and HRTEM images of MnO2/MXene; (c) MnO2/MXene composites with the corresponding elemen-
tal mapping of C, O, N, Mn and Ti by TEM; (d) EDS of MnO2/MXene by TEM; (e) XRD of the MXene, MnO2 nanosheets, and MnO2/MXene.

rate capability. The MnO2 nanosheets that are immobilized on initial capacitance, respectively, when the constant current
the MXene surface can prompt ion dispersion at the electrode, density of GCD is increased to 10 A g−1 (Fig. 4e and f ). These
and thus improve these electrochemical characteristics. results show that the MnO2/MXene composite has a good
Moreover, the wt ratio between the MnO2 nanosheets and capacitance retention rate and higher specific capacitance.
MXene nanosheets could influence the capacitive behavior of Compared with the previously reported MnO2 nanowhisker/
the MnO2/MXene electrode. Thus, we also evaluated the MXene composite prepared by direct chemical synthesis,26 our
specific capacitances of the MnO2/MXene composites with layered MnO2/MXene composite prepared by electrostatic self-
different wt ratios of the two types of nanosheets using GCD assembly shows higher specific capacitance and better stabi-
(Fig. S5a and b in the ESI†). From this data, the optimized wt lity. The attractive electrochemical performance of the compo-
ratio in the MnO2/MXene composite was chosen to be 1/1. The site is mainly attributable to the following important reasons:
specific capacitance of the MnO2/MXene (wt ratio = 1/1) elec- (1) The moderate immobilized MnO2 nanosheets in the
trode (340 F g−1) calculated from GCD is about two and a half MXene material effectively improve the electron conductivity
times, and three times higher than that of the MnO2 and enhance the specific capacitance.
nanosheets (137 F g−1) and MXene (109 F g−1) at 1 A g−1, (2) The electrostatic self-assembly is mild and efficient and
respectively (Fig. 4d), which basically corresponds to the effectively inhibits the self-stacking of the MnO2 layers and
results calculated from CV (Table S2 in the ESI†). Besides, the MXene layers, which could significantly facilitate ion diffusion
specific capacitance values of the MnO2/MXene, MXene, and of the active material and enhance the surface accessibility to
MnO2 nanosheets remain at 90.3%, 78.0%, and 88.3% of the the electrolyte.

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Fig. 3 (a) Survey scan XPS of the MXene, MnO2 nanosheets, and MnO2/MXene; high-resolution XPS of (b) Ti 2p, (c) C 1s, (d) Mn 3s, and (e) Mn 2p of
MnO2/MXene.

(3) The Gem can increase the stability of the MnO2/MXene The cycling stability is an essential parameter to evaluate
composite. Hence, the enhanced high capability of the MnO2/ the performance of electrode materials. The GCD was tested
MXene electrode can be understood by the layered framework with 2000 galvanostatic cycles at 4 A g−1 to examine the MnO2/
of this composite. MXene electrode cycling life. The specific capacitance values

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Fig. 4 (a) CV curves of MXene, MnO2 nanosheets, and MnO2/MXene at 50 mV s−1 in 1 M Na2SO4 electrolyte; (b) CV curves of MnO2/MXene at
different scan rates in 1 M Na2SO4 electrolyte; (c) plot of the oxidation peak current vs. the scan rate of CV for the MnO2/MXene electrode; (d) GCD
curves of the MXene, MnO2 nanosheet, and MnO2/MXene electrodes at a current density of 1 A g−1; (e) GCD curves of the MnO2/MXene electrode at
different current densities (1, 2, 4, 6, 8, and 10 A g−1); (f) specific capacitance of the MXene, MnO2 nanosheet, and MnO2/MXene electrodes at
different current densities.

of the MnO2/MXene, MXene, and MnO2 nanosheets remain at peaks of the composite could still be indexed to the MnO2
87.6%, 78.7%, and 82.6% of the initial value, respectively nanosheets and MXene after 2000 GCD cycles, indicating the
(Fig. 5a). In addition, the inset in Fig. 5a shows that the MnO2/ outstanding reversible behavior during the GCD process.
MXene electrode has excellent cycling reversibility during the Therefore, both a high capacitance value and good cycling
2000 GCD cycle test, indicating its good cycling performance. stability can be achieved due to electrostatic self-assembly to
To further testify the cycling stability of the MnO2/MXene com- form the MnO2/MXene composite. EIS analysis was conducted
posite in the GCD process, XRD was employed before and after to further investigate the electrochemical performance of the
the 2000 GCD cycles (Fig. S6 in the ESI†). The main diffraction MnO2/MXene composite. Fig. 5b shows the Nyquist plots of

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Fig. 5 (a) Cycling stability at a current density of 4 A g−1 of the MXene, MnO2 nanosheet, and MnO2/MXene electrodes (inset: GCD curves of the
MnO2/MXene electrode for the indicated cycles); (b) EIS of the MXene, MnO2 nanosheet, and MnO2/MXene electrodes (inset: high frequency region
after magnification).

the MXene, MnO2 nanosheet, and MnO2/MXene electrodes, Acknowledgements

which consist of two regions including a low frequency region
and a high frequency region. The curve at the juncture of the This work was supported by the research funds for the
axis at high frequency represents the resistance of active National Natural Science Foundation of China (21606081).
material-based electrodes. The resistance of MnO2/MXene is
lower than that of the MnO2 nanosheets, demonstrating that
MnO2/MXene has better conductivity. Moreover, in the low fre-
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