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A review on MXenes: new-generation 2D materials

Cite this: DOI: 10.1039/d1se00918d
for supercapacitors
G. Murali, †a Jishu Rawal,†b Jeevan Kumar Reddy Modigunta, *a Young Ho Park,a
Jong-Hoon Lee,b Seul-Yi Lee,bc Soo-Jin Park *b and Insik In *a

MXenes are rapidly emerging two-dimensional (2D) materials in the present era of materials science, and
they are finding increasing applications in energy storage fields. They are among the most suitable
electrode materials for futuristic energy storage devices. The roles of 2D MXenes and their composite
materials are crucial in improving the performance of energy storage devices. They can be used as
electrodes, conductive additives, and current collectors in energy storage devices such as
Received 20th June 2021
Accepted 10th September 2021
supercapacitors. In this review, we present the recent advances of 2D MXenes and their composites in
supercapacitor applications and discuss factors influencing their performance in supercapacitors,
DOI: 10.1039/d1se00918d
mechanisms involved in capacity improvement, and synthesis and fabrication methods of electrode
rsc.li/sustainable-energy materials.

capacitors are a combination of EDLC and pseudocapacitor

1. Introduction properties with the use of metal ion-containing electrolytes.34,35
The increasing demand for highly efficient energy storage has Most of the metal-ion electrolytes used in hybrid capacitors
resulted in advanced and sustainable alternatives for the contain Li+, Na+, K+, Zn2+, Mg2+, Ca2+, and Al3+ metal ions,
current materials and improvements in electrochemical tech- which are used in naming the respective metal-ion hybrid
nologies.1,2 There are several electrochemical energy storage capacitors.36
devices such as secondary batteries,3–7 supercapacitors,8–13 fuel In the past few decades, layered 2D materials have been
cells,14–16 ow batteries,17–19 and photovoltaics.20–22 Among the dominant in materials science due to their extraordinary elec-
available energy storage devices, supercapacitors are widely trical, electrochemical, mechanical, and optoelectronic prop-
applied in the electrochemical interdisciplinary elds.23 Super- erties.37–39 The inclusion of new-generation 2D materials in
capacitors have high power density, as compared to all the energy storage and conversion devices has drastically improved
storage devices, for instant energy requirements within a short the performance as compared with conventional methods.40–42
period. Supercapacitors are classied based on their electrode In the 2D materials reported thus far, graphene and its deriva-
combinations, such as electrochemical double-layer capacitors tives are well-known for their layered morphologies, with
(EDLCs), hybrid capacitors, and pseudocapacitors.24 Perfor- dominant roles in research over the past few decades.43–47 In the
mance enhancement of the supercapacitors majorly depends process of inventing new 2D materials, a large family of 2D
on the materials used, such as electrodes and electrolytes.25 transition metal carbides and/or nitrides named “MXenes,”
Several kinds of electrode materials have been reported thus far, have been discovered, which are rapidly advancing.48 Naguib
such as metal oxides,26 metal suldes,27 and carbon-based et al. rst reported the exfoliation of the titanium aluminum
materials.28,29 Most electrolytes used in supercapacitors are carbide (Ti3AlC2) MAX phase to form MXene (Ti3C2) via the
aqueous or non-aqueous solutions30,31 and solid or gel types.32,33 hydrouoric acid (HF) etching method.49 Thereaer, MXene-
To achieve high energy and power density in supercapacitors, based compounds have been studied extensively to develop
hybrid capacitors have been introduced. These hybrid more varieties of MXenes.50 The scalable synthesis51 of MXenes
can be achieved along with suitable electronic,52 thermal,53,54
and electromagnetic shielding properties,55 and ease in the
a
Department of Polymer Science and Engineering, Department of IT-Energy preparation of aqueous inks for printable electronics.56,57
Convergence (BK21 FOUR), Chemical Industry Institute, Korea National University
MXenes have advantages over other 2D materials such as gra-
of Transportation, Chungju 23769, Republic of Korea. E-mail: in1@ut.ac.kr;
tojeevan.sss@gmail.com phene and metal chalcogenides, and further investigations of
b
Department of Chemistry, Inha University, Incheon 22212, Republic of Korea. E-mail: MXenes with respect to fundamental properties and in terms of
sjpark@inha.ac.kr synthesis for high-end applications are in progress.58 Currently,
c
Department of Mechanical Engineering and Institute for Critical Technology and only a few synthesis processes and applications have been re-
Applied Science, Virginia Tech, Blacksburg, VA 24061, USA ported;59 however, there are numerous unknown properties of
† Both are the rst author.

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MXenes that still need to be determined, in addition to the deposition,71–73 high-temperature self-propagating
commercial production, long-term storage, and stability. synthesis,74–76 spark-plasma sintering method,77–79 microwave
MXenes and their composites are considered as new-generation synthesis,80–82 sol–gel synthesis,83 physical vapor deposition,84,85
materials due to their high electrical conductivity that is nearly and solid-state reaction synthesis.86,87 In the MAX phase
equal to that of metals, and the ease of applicability in inter- (Fig. 1(a)), M is a transition metal atom, such as Ti, Nb, Ta, V, Cr,
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disciplinary elds.60 and Mn; A is a predominantly group III A or IV A element; and X

In recent years, the utilization of MXene-based materials as is carbon or nitrogen.65–67 The crystal structures of M2AX,
electrodes for supercapacitors has been extensively investigated M3AX2, and M4AX3 phases are shown in Fig. 1(b).48 The transi-
due to their high surface area, electrical conductivity, and tion metal (M) layers exhibit a close-packed structure with an
excellent volumetric capacitances of 300 F cm
3.61,62 However, interleaved A layer, and the octahedral interstitial sites of the
their intrinsic low specic capacitance restricts their applica- structure are occupied by the X element atoms in the MAX
tions, which can be further improved by enhancing the inter- phase. The M–X bond nature is very strong with mixed covalent
layer distance.63,64 In this review, we discuss in detail the and ionic or metallic characteristics. The bonding between M–
available synthesis methods for MXenes with respect to Al is metallic and weak; therefore, Al gets easily etched during
different factors, electrode preparation, device fabrication, and the synthesis of MXenes from the MAX phases. The SEM images
provide a summary of the applications of MXenes in super- show that the thick multilayer morphologies of Ti2AlC, Cr2Al,
capacitors as electrodes and their inuence on the performance Ti3SiC2, and Ti3AlC2 MAX phases in Fig. 1(c) contain some kink
of the electrodes, cycling ability, and compatibility with other bands (yellow arrows).66 The synthesis of MAX phase precursors
components of the device. is a crucial step in obtaining high-quality MXene sheets.48 The
size and thickness of the MAX phase play a vital role in deter-
2. MXene synthesis and mining the size, shape, and thickness of the nal product.

characterization 2.1. MXene synthesis

MXenes are synthesized from the MAX phase using the top- The transition metal carbide and nitride (Mn+1Xn) MXenes are
down approach by the chemical etching method.68 The MAX obtained through the removal of the group elements (A) stacked
phase is a layered compound with a wide range of compositions at the interlayers of the corresponding MAX phases. The weak
and combinations of elements. The MAX phase synthesis metallic bonds between M and A in the MAX phase are broken
methods include high-temperature powder combustion to form MXene nanosheets; this is possible due to the high
synthesis,49,51 molten metal synthesis,69,70 chemical vapor chemical activity of the metallic bonds that can be easily

Fig. 1 (a) Elements of the periodic table involved in different kinds of MAX phases (Mn+1AXn, where n ¼ 1, 2, or 3) and MXenes (this figure has been
adapted/reproduced from the following: ref. 65 with permission from Springer Nature, Copyright 2020; ref. 66 with permission from John Wiley
and Sons, copyright 2020; ref. 67 with permission from Royal Society of Chemistry, copyright 2020). (b) Crystal structures of M2AX, M3AX2, and
M4AX3 phases (this figure has been adapted/reproduced from ref. 48 with permission from Royal Society of Chemistry, copyright 2020). (c) SEM
images of Ti2AlC, Cr2AlC, Ti3SiC2, and Ti3AlC2 MAX phases (this figure has been adapted/reproduced from ref. 66 with permission from John
Wiley and Sons, copyright 2020).

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separated as compared to other bonds in the MAX phase. The safety during the synthesis and disposal aer the synthesis. The
synthesis of MXenes is highly dependent on the elements' (like concentration of HF, reaction time, and temperature main-
Al, Si, Ga, and Ge) etching agents such as HF, which is a very tained during the synthesis are some of the deciding factors for
strong etching agent for the removal of the same. Fig. 2(a) shows the size, thickness, and defects in the MXenes prepared. Most of
the crystal structure of the MXene synthesized from the MAX the synthesis process using pure HF used 5–50 wt% HF at
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phase using the chemical etching process.48 Thus far, two well- different reaction times and temperatures. Table 1 shows that
established synthesis methods have been reported that use HF the etching process is kinetically controlled and with the
and a combination of hydrochloric acid (HCl) and lithium increase of n value in Mn+1XnTx, higher acid strength and/or
uoride (LiF). The method that uses only HF further requires prolonged etching time are required.92 The possible chemical
a delamination step using delaminating agents such as lithium reactions involved in the MAX phase etching process are as
chloride (LiCl), dimethyl sulfoxide (DMSO), n-butylamine, iso- follows:.93,94
propylamine, hypochlorites, and other organic molecules such
as tetrabutylammonium hydroxide (TBAOH) and tetramethy- Mn+1AlXn + 3HF / AlF3 + Mn+1Xn + 1.5H2
lammonium hydroxide (TMAOH).88,89 The combination of HCl
and LiF generates in situ HF, which simultaneously delaminate Mn+1Xn + 2H2O / Mn+1Xn(OH)2 + H2
the MXene sheets aer the etching process compared to when
Mn+1Xn + 2HF /Mn+1XnF2 + H2
using pure HF. This method of using a combination of LiF and
HCl as the etching agent is called the minimally intensive layer Aer the nal step, the surface of the MXene has surface
delamination (MILD) etching method.92 In this method, the termination groups such as –O, –OH, and –F on the outermost
lithium ions available from LiF simultaneously proceed with metal layer. The change in the etching agents can result in
delamination right aer etching A from the MAX phase, thereby different surface functional groups with a specic functional
providing an efficient single-step process for obtaining high- group as predominant, e.g., the HF etchant results in F-
quality MXenes with minimal damage as compared to that in terminations as dominant, whereas LiF/HCl results in O-
the pure HF etching method. Different kinds of MXenes terminations as dominant in MXenes (MILD method). In the
(M1.33X, M2X, M3X2, and M4X3) are available, as shown in MILD process, the reactions between the precursor chemicals
Fig. 2(b).90 The detailed step-by-step synthesis of Ti3C2Tx MXene such as LiF and HCl to produce in situ HF are a good alternative
from the Ti3AlC2 MAX phase through the MILD etching process to avoid handling strong HF acid as in the case of the direct
is shown in Fig. 2(c).91 method discussed above. The end products of this in situ
In the direct HF etching process, the selectivity toward the synthesis process trap the solvents inside the etched MXenes,
metallic elements is substantial; however, utmost care should which requires a high temperature for drying aer the nal
be taken while handling highly hazardous HF with respect to

Fig. 2 (a) MXenes synthesized from the MAX phase using the chemical etching processes (this figure has been adapted/reproduced from ref. 48
with permission from Royal Society of Chemistry, copyright 2020). (b) Different kinds of MXenes (M1.33X, M2X, M3X2, and M4X3) and their general
crystal structures (this figure has been adapted/reproduced from ref. 90 with permission from De Gruyter, copyright 2020). (c) A detailed step-
by-step acid etching process of the MAX phase Ti3AlC2 to obtain Ti3C2Tx MXene, which was further processed to enhance MXene sheet
exfoliation by centrifugation and sonication (this figure has been adapted/reproduced from ref. 91 with permission from Springer Nature,
copyright 2021).

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Table 1 Conditions and precursors used for the synthesis of MXenes by different approaches

Conditions

MXene Precursor Etching agent(s) T ( C) Time (h) Ref.

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Conventional etching
Cr2TiC2Tx Cr2TiAlC2 5 M LiF + 6 M HCl 55 42 13
Hf3C2Tx Hf3[Al(Si)]4C6 35% HF RT 60 95
Mo2CTx Mo2Ga2C 3 M LiF + 12 M HCl 35 384 96
Mo4/3CTx (Mo2/3Sc1/3)2AlC 2 g LiF + 12 M HCl 35 48 97
Mo4/3CTx (Mo2/3Sc1/3)2AlC 48% HF RT 24 97
Mo2TiC2Tx Mo2TiAlC2 50% HF RT 48 13
Mo2Ti2C3Tx Mo2Ti2AlC3 50% HF 55 90 13
(Nb0.8Zr0.2)4C3Tx (Nb0.8Zr0.2)4AlC3 LiF + 12 M HCl 50 168 98
Nb2CTx Nb2AlC 50% HF RT 90 99
Nb4C3Tx Nb4AlC3 49% HF RT 140 100
Nb4/3CTx (Nb2/3Sc1/3)2AlC 48% HF RT 30 101
Ti3C2Tx Ti3AlC2 50% HF RT 2 49
Ti3C2Tx Ti3AlC2 1.6 g LiF + 9 M HCl 35 24 57
Ti2NTx Ti2AlN KF + HCl RT 3 102
(Ti0.5Nb0.5)2CTx (Ti0.5Nb0.5)2AlC 50% HF RT 28 103
Ta4C3Tx Ta4AlC3 50% HF RT 72 103
Ti3CNTx Ti3AlCN 30% HF RT 18 103
Ti3C2Tx Ti3AlC2 5 M LiF + 6 M HCl 40 45 104
Ti3CNTx Ti3AlCN 0.66 g LiF + 6 M HCl 30 12 105
Ti3C2Tx Ti3AlC2 1 M NH4HF2 RT 11 106
Ti3C2Tx Ti3AlC2 NH4F 150 24 107
V2CTx V2AlC 2 g NaF + 40 mL HCl 90 48 108
V4C3Tx V4AlC3 40% HF RT 165 81
V2CTx V2AlC 50% HF RT 92 109
(V0.5Cr0.5)3C2Tx (V0.5Cr0.5)3AlC2 50% HF RT 69 103
W1.33CTx (W2/3Sc1/3)2AlC 48% HF RT 30 110
W1.33CTx (W2/3Sc1/3)2AlC 4 g LiF + 12 M HCl 35 48 110
Zr3C2Tx Zr3Al3C5 50% HF RT 72 111

F-free etching
Ti3C2Tx Ti3AlC2 27.5 M aq. NaOH 270 12 112
Ti3C2Tx Ti3AlC2 0.2 M TMAOH RT 24 113

Non-aqueous etching
Ti3C2Tx Ti3AlC2 1 g NH4HF2, propylene carbonate as solvent 35 196 114

Ionic liquid-based etching

Ti3C2Tx Ti3AlC2 1-Ethyl-3-methylimidazolium tetrauoroborate (EMIMBF4) 80 20–44 115
Ti2CTx Ti2AlC 1-Butyl-3-methylimidazolium hexauorophosphate (BMIMPF6) 80 20 115

Molten salt etching

Ti4N3 Ti4AlN3 KF, NaF, and LiF 550 0.5 116
Ti3C2Tx Ti3AlC2 ZnCl2 550 5 117
Ti3C2Tx Ti3SiC2 CuCl2 750 24 118
Ti3C2Tx Ti3AlC2 CdCl2 or CdBr2 610 6 119
Nb2CTx Nb2AlC CdCl2 710 36 119

Electrochemical etching
Ti3C2Tx Ti3AlC2 1 M LiTFSI in TEGDME RT — 120
Ti3C2Tx Ti3AlC2 1 M NH4Cl & 0.2 M tetramethylammonium hydroxide (TMAOH) RT 5 121

step. The bonding between the M atoms and Tx is strong, which yield of single akes with irregular concentrations and more
results in the negative energy of MXenes. In the pure HF etching defects.92
process, the intercalation step is crucial to increase the spacing MXene synthesis includes several different experimental
between the MXene sheets. The delamination process is conditions to obtain the appropriate product. Some MXenes,
required for MXenes aer etching with HF, by means of soni- their synthesis conditions and precursors are listed in Table 1.
cation, mechanical exfoliation, or centrifugation results in a low MXenes such as Mo2CTx, Hf3C2Tx, and Zr3C2Tx are synthesized
from non-MAX phase precursors Mo2Ga2C, Hf3[Al(Si)]4C6, and

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Zr3Al3C5, respectively. Apart from the room-temperature acid nanosheets with smooth surfaces (Fig. 3(d and e)). A clear dot
etching process, etching was also performed at higher temper- selected area electron diffraction (SAED) pattern of a nanosheet
atures, such as 35  C, 50  C, 90  C, 150  C, 610  C, 710  C, and shown in the inset of (Fig. 3(e)) indicates the single-crystalline
750  C, to obtain MXenes. Bottom-up approaches such as state of the nanosheets. The atomic force microscopy (AFM)
chemical vapor deposition (CVD) were also utilized to synthe- height prole of these nanosheets indicated a thickness of
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size high-quality MXenes with fewer defects and large lateral 2 nm (Fig. 3(f)).
size.122 Although the CVD method for the synthesis of Mo2C is Fig. 4(a) shows the UV-vis absorption spectra of the Ti3AlC2
highly effective, it is not applicable for all kinds of MXene MAX phase, Ti3C2Tx MXene lm, and Ti3C2 quantum dots
syntheses. Among all the available methods, the selective (QDs). In the UV-vis absorption spectra of Ti3AlC2, Ti3C2Tx lm,
etching of MAX phases is considered as an effective top-down and Ti3C2 QDs, a typical absorption peak at 310 nm was
approach for commercial production. Signicant progress has observed.128 In the Ti3C2 QDs, besides the strong peak at
been achieved in the etching process by the development of new 310 nm, an absorption band centering at 350 nm was also
approaches like the uorine-free synthesis process, non- observed. According to rst-principles density functional theory
aqueous synthesis process, ionic liquid-based etching (DFT) calculations, these absorption peaks originated from
synthesis process, molten salt etching synthesis process, and inter-band transitions.130,131 The Raman spectrum of Ti3C2 QDs
electrochemical etching synthesis process, which are summa- in Fig. 4(b) shows two peaks (u3, u4) located at 580 cm
1 and
rized in Table 1. 800 cm
1, respectively, which originated from the Ti–C vibra-
tions.132 In Ti3AlC2, two more peaks (u1, u2) were observed,
2.2. MXene characterization which were assigned to the Al atom-related vibration modes,
indicating that aer the LiF/HCl treatment, the Al atoms were
The participation and behavior of free electrons on the surface
removed completely.128
reactive sites are responsible for the phenomenal catalytic and
Fig. 4(c and d) show the thermogravimetric analysis (TGA)
energy storage performance of MXenes. Electron function
curves of the Ti3C2Tx MXene obtained with temperatures
depends on the structural, electronic, optical, and electro-
varying from room temperature to 950  C under nitrogen and
chemical properties of MXenes. To understand the role of
oxygen atmospheres, respectively. The weight of the MXene was
MXenes in several applications and their design factors,
observed to change sequentially in three steps in the case of the
comprehensive knowledge of the properties is necessary. Aer
nitrogen atmosphere due to the loss of adsorbed water, func-
the HF etching, several new X-ray diffraction (XRD) peaks cor-
tional groups and bonded water, and decomposition of MXene
responding to the (00l) family of peaks of Ti3C2Tx MXene
(Fig. 4(c)). The decomposition temperature of Ti3C2Tx MXene
emerged while the characteristic Ti3AlC2 peaks within the 33–
was 785  C. In contrast, the TGA curves measured under the
43 range completely disappeared, which is a clear indication of
oxygen atmosphere indicated a four-step weight change of
the formation of phase-pure Ti3C2Tx MXene (Fig. 3(a)). The
MXenes. The oxidation of Ti3C2Tx MXene in the oxygen atmo-
removal of Al in the etching process transformed the stone-like
sphere occurred with the increase in the temperature from
morphology of Ti3AlC2 grains into an accordion-like multilay-
322  C to 729  C, which resulted in an increased mass of 20%.
ered structure (Fig. 3(b and c)). The delamination process
The decomposition temperature of MXene in the oxygen
assisted the formation of micrometer-sized monolayer
atmosphere was approximately 729  C (Fig. 4(d)). A full-scan
XPS spectrum of Ti3C2Tx and the corresponding high-
resolution XPS spectra in the Ti2p, O1s, and C1s regions are
shown in Fig. 4(e and f).129 The deconvolution of the Ti2p peaks
facilitated the identication of Ti–C, Ti(II), Ti(III), TiO2, and
TiO2
xFx species, while the O1s spectrum was tted to C–Ti–Ox
and C–Ti–(OH)x bonds. The C1s region is comprised of the
contributions from carbide (C–Ti–Tx) and graphitic (C–C)
components due to the unwanted elimination of the Ti atoms
during the etching process, and C–O bonds arising from surface
contamination.133

3. MXenes for supercapacitors

Fig. 3 Structural properties and morphology: (a) XRD of MAX phase
MXene lms are gaining considerable attention due to their
and MXene (adapted/reproduced from ref. 123 with permission from high electric conductivity, excellent mechanical properties, and
Royal Society of Chemistry, copyright 2018). (b–d) SEM images of MAX Faraday pseudocapacitive charge storage mechanism toward
phase and MXene (adapted/reproduced from ref. 124 with permission supercapacitor applications.134 Theoretically, the presence of
from Springer Nature, copyright 2020; from ref. 125 with permission
functional groups such as –F and –OH on the surface of MXenes
from IOP Publishing, copyright 2014; from ref. 126 with permission
from Elsevier, copyright 2020). (e) TEM and (f) AFM images of MXene may contribute to the mechanical tensile stress of the MXene
(adapted/reproduced from ref. 127 with permission from John Wiley lms, resulting in better tolerance to both biaxial and uniaxial
and Sons, copyright 2020). tensions as compared to other 2D materials such as graphene.

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Fig. 4 Properties of MXenes: (a) UV-visible and (b) Raman spectra of the Ti3AlC2 MAX phase, Ti3C2Tx MXene film, and Ti3C2 quantum dots (this
figure has been adapted/reproduced from ref. 128 with permission from Royal Society of Chemistry, copyright 2018). TGA curves of Ti3C2Tx
under (c) nitrogen and (d) oxygen atmospheres (this figure has been adapted/reproduced from ref. 53 with permission from American Chemical
Society, copyright 2018). XPS (e) full scan spectrum, and high-resolution deconvoluted spectra of (f) Ti2p, (g) O1s, and (h) C1s elements of Ti3C2Tx
MXene (this figure has been adapted/reproduced from ref. 129 with permission from Elsevier, copyright 2019).

This tolerance to both axials can suppress the breakdown of shown in Fig. 5(a). A supercapacitor with good performance
atomic layers and results in excellent mechanical elasticity.122 In requires highly porous electrodes, electrodes with high specic
contrast, the Faraday pseudocapacitance is obtained from the surface area, chemical stability, and low electrical resistivity. A
oxidation states of the Ti atom surface due to the reversible chemical reaction occurs when ions present in the liquid elec-
intercalation and de-intercalation of protons or charged ions, trolyte begin to transfer between the electrodes and separator,
and it is governed by diffusion.104 resulting in double-layer capacitance, as shown in Fig. 5(a).32 A
compact layer of closely packed ions on the electrode surface
causes different potential gradients due to which the diffusion
3.1. Basic principles and mechanism
layer and concentration of ion experience non-linearity and
The basic construction of a supercapacitor involves three ends in the bulk electrolyte solution, as shown in Fig. 5(b and c).
elements, namely, the separator, electrolyte, and electrodes, as

Fig. 5 (a) Electrochemical cell constructions for the two-electrode and three-electrode configurations. (b) Based on the mechanisms involved,
supercapacitors are classified as electrical double-layer capacitors (EDLC), pseudocapacitors, and hybrid capacitors. (c) Charge–discharge
mechanism of EDLC supercapacitors (this figure has been adapted/reproduced from ref. 32 with permission from Royal Society of Chemistry,
copyright 2019). (d) Suitable properties of 2D MXenes for application in supercapacitors (this figure has been adapted/reproduced from ref. 135
with permission from John Wiley and Sons, copyright 2020).

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Adsorption of ions from the electrolyte on the electrode surface 3.3. Electrochemical properties of MXenes
occurs through electrostatic attraction; this allows the deposi-
The electrochemical properties of MXenes are most important
tion of an opposite-charge layer at the interfaces representing
for energy storage applications. Three possible mechanisms
the charge storage mechanism of the capacitors. In general,
exist for supercapacitors, as discussed in previous sections.143
a supercapacitor can be classied into two types based on
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Based on the ionic population in the system, the ions rst

faradaic and non-faradaic reactions. (1) EDLC, which is
balance the charge of the device by counter-ion adsorption,
a commercial supercapacitor based on the non-faradaic charge
followed by the ion-exchange process in which counter-ion
storage mechanism using double layers (Fig. 5(c)). (2) Pseudo- adsorption and counter-ion desorption occur simultaneously.
capacitors, which are based on faradaic reactions where charge In the last stage, charging based on desorption of counter-ions
transfer occurs across the interface due to redox reactions, the
takes place. In general, MXene-based materials display pseu-
intercalation process, and electrosorption, as shown in Fig. 5(b).
docapacitance due to the layered morphology. Ion intercalation
The applicability of MXenes in supercapacitors is schematically
denes the capacitive behavior of MXenes. For instance, fully
depicted in Fig. 5(d).
hydrated ions lead to the formation of electric double layers that
eliminate charge transfer and directly adsorb partially dehy-
drated ions on the MXene surface, resulting in the depletion of
3.2. Role of MXenes in supercapacitors electrostatic potential that renders pseudocapacitance
Energy storage and conversion have been key scientic chal- dominance.
lenges for the past few decades due to specic requirements Cyclic voltammetry (CV) curves and gravimetric capacitances
such as lightweight, exible, conductive, and high surface-to- versus potential of Ti3C2Tx MXenes using different electrolytes
mass ratio, i.e., specic surface area.136 Carbon-based mate- with identical anions and different cations suggest that the
rials have a specic surface area of approximately 900–2500 Ti3C2Tx MXene electrodes exhibit excellent performance. The
m2 g
1.137,138 In 2D materials other than carbon-based mate- curves changed with each electrolyte due to the change in the
rials, transition metal dichalcogenides, such as WS2, in cations in the system, which indicates that the cations are the
combination with graphene are more popular for energy major deciding factor for the energy storage systems. The
storage applications.139 MXenes are a new class of 2D mate- surface functional groups and solvents used in the storage
rials that are considered as suitable electrodes for energy device also inuence the performance.146 Hui Shao et al.
applications. Compared to materials such as graphene, demonstrated the electrochemical redox mechanism of Ti3C2Tx
MXenes have a much smaller specic surface area. For MXene in aqueous-based electrolytes.147
example, experimentally produced Ti3C2Tx has a specic Among the aqueous-type electrolytes, sulfuric acid (H2SO4)
surface area of 66 m2 g
1 and V2CTx 19 m2 g
1;128 however, has two protons (hydronium form), which are terminated with
theoretically, Ti3C2Tx MXenes have a specic surface area of the oxygen groups during the discharge cycle, and during the
496 m2 g
1. Ren et al. reported the specic surface area for charge cycle, the bond dissociates on the negative MXene
porous Ti3C2Tx MXenes to be 93.6 m2 g
1.140,141 The volu- electrode. During these bonding and de-bonding cycles, the
metric capacity of functionalized graphene is close to 200 F valency of Ti changes and results in pseudocapacitance in acid
g
1, whereas MXenes exhibit approximately 320 F g
1 in the electrolytes. In contrast to acid electrolytes, in salt-based elec-
case of Ti3C2 MXene.142 Apart from these properties, MXenes trolytes such as (NH4)2SO4 or MgSO4, an EDLC was observed.
also have advantages such as high electrical conductivity, Therefore, it is evident that acid electrolytes are favored for ion
exibility, high mechanical strength, and good ionic exchange, and surface-capacitive effect-based redox reactions
conductivity, as shown in Fig. 6. occur. In the case of salt-based electrolytes, counter-ion
adsorption is favored. Therefore, acid-based electrolytes are
more favorable for MXene materials, which follow the process
of bonding/de-bonding cycle-based pseudocapacitance and
charge storage via the ion-exchange process, as shown in
Fig. 7(b).146 From the theoretical model studies of Ti2C2O2, the
role of redox surface chemistry and double layer charging have
been studied using DFT calculations.148 From the DFT analysis,
it was evident that H coverage had a signicant effect on the
electrode potential. The H coverage was approximately 0.68 at

1.0 V potential, which indicates incomplete protonation of the
oxygen surface group.
Fig. 7(a) suggests that the ion transportation in the MXene
electrodes is faster when it is vertical and slower when it is
horizontal to the plane of the current collector.149 This study
provides a new strategy for the fabrication of electrodes based
on the structural alignment of the materials used. Fig. 7(c)
shows the CV curve for MXene and galvanostatic charge–
Fig. 6 Advantages of MXenes for applications in supercapacitors.

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gravimetric capacitance as compared to pristine MXene.150

Similarly, MXene/SWCNT, MXene/rGO, MXene/rGO-PDA, and
MXene/bacterial cellulose-based electrode materials were also
reported with higher performances as compared to their cor-
responding MXenes. The surface functionalization of MXene
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with polyvinyl alcohol (PVA) or polydiallyldimethylammonium

chloride (PDDA) dramatically changed the properties of mate-
rials and their electrochemical performances. The synthesis of
a composite with PDDA improved the exibility of the MXene
lm and a dielectric property was observed. In contrast, the
synthesis of the composite with PVA resulted in the formation
of a exible and strong MXene lm (Fig. 9(a)). The CV curves of
the obtained Ti3C2Tx/PVA-KOH showed the highest volumetric
capacity, as compared to pristine Ti3C2Tx MXene and Ti3C2Tx/
PDDA, as shown in Fig. 9(a). In the case of GCD, it also supports
the properties as displayed in CV with a higher volumetric
capacity of 509 F cm
3 at lower scan rates of 2 mV s
1.151 The
volumetric capacity decreased with the increase in the scan rate
due to the increase in the speed of ion exchange. The mobility
and availability of ions were restricted at the electrode surface,
resulting in decreased capacity.
The composite of MXene with bacterial cellulose (BC)
resulted in good adhesion and alignment of the sheets due to
the self-assembly process and the attraction between the BC on
the surface of the MXenes. The ionic mobility was low in the
case of pristine MXene. The electrochemical impedance spectra
Fig. 7 (a) Ion transport in the MXene electrode is faster when it is
vertical and slower when it is horizontal to the plane of the current (EIS) of the MXene/BC composites with different weight load-
collector (this figure has been adapted/reproduced from ref. 144 with ings of MXene were tested for their ionic diffusion perfor-
permission from IOP Publishing, copyright 2020). (b) The pseudoca- mances. As shown in Fig. 9(b), with the increase in the MXene
pacitance of MXene in H2SO4 is enhanced with the rate of intercalation loading in the MXene/BC composite, the diffusion increased
and de-intercalation of ions (this figure has been adapted/reproduced
from ref. 145 with permission from Royal Society of Chemistry,
until 5 mg cm
2 and then decreased slowly at higher weight
copyright 2020). (c) The ideal CV curve for MXene, and galvanostatic loadings. In addition to the EIS analysis, MXene/BC with 5 mg
charge–discharge curve with respect to ionic diffusion (this figure has cm
2 composite was further investigated for GCD analysis at
been adapted/reproduced from ref. 146 with permission from Amer- different currents, as shown in Fig. 9(b). The GCD time was
ican Chemical Society, copyright 2018). decreased with the increase of current from 3 to 50 mA cm
2.
The redox curve peaks slowly disappeared with the increase in
the current density. The decrease in capacitance was further
discharge (GCD) curve with respect to ionic diffusion.146 The supported by the graph; for MXene with a weight load of 1.8 and
intercalation with the partial redox process did not show any 5 mg cm
2 is showing a rapid decrease in the capacity as
change in the phase of the materials; however, it provided compared to the MXene/BC samples with the same amount of
information regarding the intercalation and de-intercalation of MXene. For further understanding, the CV analysis was per-
the ions. The GCD curves showed a gradual decay with time, formed for both MXene and MXene/BC-5 samples. The redox
which is a typical indication of changes in the transition metal peaks for the MXene/BC-5 at a scan rate of 1 mV s
1 were visible
oxidation state. Further enhancement in the performance of as compared to that for pristine MXene; this is due to the BC
MXenes can be achieved by different modication methods inuence over MXene during the electrode fabrication process
such as material surface modication, morphology modica- and the intercalation and de-intercalation process. The
tion, and surface functionalization with other functional composites prepared and used instead of pure MXene eventu-
materials. ally improved the supercapacitor performance of the MXene
electrodes.
In general, for the research and development of super-
3.4. MXene and its composites for supercapacitors capacitor materials, a three-electrode system is preferred due to
As shown in Fig. 8, MXene performance can be enhanced by the wide information obtained from the study. In the three-
modifying the surface groups, interlayer structures, electrode electrode system the material under study was used as the
morphology, or by synthesizing a composite with an additional working electrode, Pt-based materials, such as Pt wires, or Pt
functional material such as PEDOT:PSS. strips, or Pt electrodes, were used as the counter electrode, and
Wen et al. reported N-doped MXene electrodes for high- Ag/AgCl, or Hg/HgO, or Hg/Hg2SO4, or standard calomel elec-
performance supercapacitors with a 460% increase in trode (SCE), was used as the reference electrode. In the case of

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Fig. 8 MXene modification methods with respect to interlayer structures, surface modification, electrode structure, and composites (this figure
has been adapted/reproduced from ref. 145 with permission from Royal Society of Chemistry, copyright 2020).

MXene-based electrode materials, there have been several aqueous-based electrolytes, MXene supercapacitors perform
electrolytes tested for the improvement of electrochemical well in terms of high capacitance; however, due to the low redox
performance such as aqueous electrolytes, non-aqueous elec- potential of water molecules, the voltage window is limited to
trolytes, and polymer-based gel electrolytes.152–154 In the case of below 2 V. In the case of non-aqueous electrolyte-based MXene

Fig. 9 (a) Schematic illustration of MXene-based functional films with adjustable properties (top) using PDDA and PVA, and (bottom) the cor-
responding Ti3C2Tx/PDDA and Ti3C2Tx/PVA-KOH films. The volumetric capacitance and cyclic voltammetry (CV) curves are shown with
a potential from
1.0 to
0.4 V vs. the Ag/AgCl electrode using 1 M KOH electrolyte. Inset: digital images showing the 3 mm thick Ti3C2Tx films
(diameter of 40 mm) that demonstrate the flexibility (this figure has been adapted/reproduced from ref. 151 with permission from National
Academy of Sciences, copyright 2014). (b) Nyquist plots (EIS), galvanostatic charge–discharge curves (GCD), capacitance with respect to current
density and CV of the porous MXene/BC electrode and planar MXene electrodes from left to right in a clockwise direction. The electrochemical
performance of MXene/bacterial cellulose (BC) electrodes with MXene mass loading varied from 1.2 to 8 mg cm
2 in 3 M H2SO4 electrolyte (this
figure has been adapted/reproduced from ref. 163 with permission from John Wiley and Sons, copyright 2019).

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supercapacitors, higher potential windows (>2 V) can be ach- nanowhiskers increased the surface area of the composite
ieved due to the presence of organic solvents in the system.155 electrode and enhanced the specic capacitance by nearly three
The electrochemical performance of the non-aqueous-based orders of magnitude as compared to that of pure MXene-based
MXene supercapacitors is low due to the bulky size of organic symmetric supercapacitors. Combined with MXene enhances
molecules in the solvent system, which restricts the diffusion of pseudocapacitance, the fabricated 3-MnO2/MXene super-
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ions in the electrodes. In the case of polymer-based gel elec- capacitors exhibited excellent cycling stability with 88% of the
trolytes, the device fabrication has the great advantages of the initial specic capacitance retention aer 10 000 cycles, which
exibility, safety, and long-life of the devices.156 Although each is much higher than that observed in pure 3-MnO2-based
type of electrolyte has advantages, eventually, the assembly of supercapacitors (74%). Fig. 10(b) shows the fabrication of
a high-performing device with most of the features will have the MXene/carbon bers by electrospinning the MXene/PAN solu-
edge over the conventional devices. Most reported MXene tion to obtain MXene carbonized bers.189
supercapacitors are studied using acid-based aqueous or gel- For the preparation of the electrospinning solution, a 2 : 1
type electrolytes for better electrochemical performance due ratio of MXene : PAN was mixed in N,N-dimethylformamide
to the mobility of the smaller H+ ions in the system during the (DMF) solvent and stirred for 4 h to obtain a uniform solution.
cycling process. In MXene supercapacitors, the acid-based The solution was electrospun and carbonized at different
electrolytes with higher concentrations give higher volumetric temperatures up to 800  C. All the samples carbonized at 800  C
capacitance than the electrolytes with lower concentrations; showed a good morphology with a good areal capacity of
this could be due to the higher concentration of H+ ions, which approximately 244 mF cm
2. A MXene/CNF freestanding lm
in turn show the pseudocapacitance mechanism depending on was obtained by the template peel-off method, as shown in
the redox peaks of the titanium chemical valence state. There- Fig. 10(c).190 The electrodes (MXene/CNF) showed a high
fore, H2SO4-based electrolytes are better candidates than non- performance of 29 mF cm
2, and similarly, by the freeze-dry
aqueous electrolytes for achieving better performances of the method, a MXene aerogel was prepared (Fig. 10(d)), which is
MXene-based supercapacitors. However, the poor oxidation lighter than the ower petals.191 The electrochemical perfor-
stability of the MXene in water and in the presence of oxygen is mance of the MXene with a high weight load showed a good
still of concern for future commercial applications. The capac- performance of 900 mF cm
2 with a 2 mV s
1 scan rate. As
itance values, electrolytes used, potential windows, and other shown in Fig. 10 all the materials were made as composites
information with respect to electrolyte type are summarized for using a semiconductor or a carbon-based material, and the
the MXenes and their composite materials in Table 2. supercapacitor performance of the MXene was enhanced.
Several other methods for the fabrication of MXene electrode
materials for supercapacitors have been reported, such as the
3.5. Electrode fabrication methods and electrochemical doctor blade method for coating MXene as a thin lm, spin
performance coating, and dip coating onto substrates like Ni foam and
The fabrication of the MXene electrode for testing and analysis carbon cloth. MXene yarn is prepared by fabricating a knittable
is a key step in obtaining a supercapacitor with good perfor- energy-storing ber with a high volumetric performance ob-
mance. Choosing a good combination of materials to accom- tained predominantly from MXene nanosheets.192 Further,
pany MXene is also a crucial step. MnO2 is one of the best MXene has been converted into inks using different solvents,
electrochemically active species188 and the combination of such as ethanol, methanol, tetrahydrofuran (THF), and DMF,
MXene with MnO2 improves the performance of the super- for applications in printable electronics and printable devices.
capacitor. The physical mixing or blending of separately The advancement in the technologies will further mobilize the
prepared MXene and MnO2 may not be a good choice as they supercapacitors to enhance the performance.
both have a certain phase difference and poor interactions. The MXene has been dispersed in aqueous and organic solvents
in situ synthesis of MnO2 on the surface of MXene can be a good in order to prepare the MXene inks for printable electronic and
choice (Fig. 10(a)) as it will grow and stay in good contact with energy storage applications.57,193 Advanced techniques like 3D
the surface of MXene through the support of the surface func- printing, extrusion printing, inkjet printing, stamping, and yarn
tional groups on MXene. spinning methods have been employed for the fabrication of
The synthesized MXene electrode materials of 3-MnO2/ electrodes for supercapacitors as shown in Fig. 11. The sche-
Ti2CTx and 3-MnO2/Ti3C2Tx were used to test the supercapacitor matic illustration of Ti3C2Tx MXene electrodes preparation by
through symmetric cell fabrication.166 In the symmetric cell using MXene inks obtained by the dispersion of MXene into
system, electrodes composed of identical materials with the aqueous and organic solvents (like ethanol) is shown in
same weight were taken as the cathode and anode. The elec- Fig. 11(a). The Ti3C2Tx organic ink is made using ethanol and
trodes were fabricated by mixing the sample with a PTFE binder used for inkjet printing of various patterns, such as micro-
and acetylene black as the conductive additive, using ethanol as supercapacitors (MSCs). The Ti3C2Tx aqueous inks are inten-
the solvent. The material mixture was coated on a carbon cloth ded for the extrusion printing and electrohydrodynamic
and dried in a vacuum oven for 12 h at 80  C. The MXene printing of MSCs and optoelectronics on different exible
nanosheet surfaces (3-MnO2/Ti2CTx and 3-MnO2/Ti3C2Tx) were substrates such as A4 papers and PET lms. Fig. 11(b) shows the
completely covered with MnO2, and the nanocomposite elec- GCD curves of a typical extrusion-printed MSCs at different
trodes were tested using an aqueous electrolyte. The 3-MnO2 current rates (20, 30, 40, 50, 100, and 200 mA cm
2) in the

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Table 2 Summary of MXene-based materials used in supercapacitors: the electrode, electrolyte, potential window, and capacitance

Electrode Electrolyte Morphology Potential window (V) Capacitance Ref.

Review

Aqueous system
Mo2TiC2Tx 1 M H2SO4 Paper
0.1 to 0.4 (vs. Ag/AgCl) 413 F cm
3 at 2 mV s
1 13
Mo2CTx 1 M H2SO4 Paper
0.3 to 0.3 (vs. Ag/AgCl) 196 F g
1 at 2 mV s
1 96
Ti3C2Tx 1 M H2SO4 Clay
0.3 to 0.25 (vs. Ag/AgCl) 900 F cm
3 at 2 mV s
1 104
Ti3C2Tx 3 M KOH Film 0 to 0.9 (vs. Ag/AgCl) 141 F cm
3 at 2 A g
1 107
Ti3C2Tx 1 M H2SO4 Disc
0.35 to 0.2 (vs. Ag/AgCl) 520 F cm
3 at 2 mV s
1 142
Ti3C2Tx/PVA 1 M KOH Film
1 to
0.4 (vs. Ag/AgCl) 528 F cm
3 at 2 mV s
1 151
Ti3C2Tx/PDDA 1 M KOH Film
1 to
0.4 (vs. Ag/AgCl) 296 F cm
3 at 2 mV s
1 151
Ti3C2Tx/SWCNT 1 M MgSO4 Film
0.8 to 0.1 (vs. Ag/AgCl) 390 F cm
2 at 2 mV s
1 157
Ti3C2Tx/rGO 1 M MgSO4 Film
0.8 to 0.1 (vs. Ag/AgCl) 435 F cm
2 at 2 mV s
1 157
Ti3C2Tx 1 M KOH Paper
0.9 to
0.4 (vs. Ag/AgCl) 340 F cm
3 at 1 A g
1 158
Ti3C2Tx 1 M H2SO4 Film
0.2 to 0.4 (vs. Ag/AgCl) 517 F g
1 at 1 A g
1 159
Ti3C2Tx 1 M KOH On Ni foam
0.75 to
0.25 (vs. SCE) 140 F g
1 at 5 mV s
1 160
Ti3C2Tx/PPy 1 M H2SO4 Paper
0.2 to 0.35 (vs. Ag/AgCl) 416 F g
1 at 5 mV s
1 161
Ti3C2Tx/PPy 1 M H2SO4 Paper
0.2 to 0.35 (vs. Ag/AgCl) 1000 F cm
3 at 5 mV s
1 161
Ti3C2Tx/antimonene 1 M H2SO4 Film
0.2 to 0.2 (vs. Ag/AgCl) 4255 F cm
3 at 0.5 mA cm
2 162

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Ti3C2Tx/bacterial cellulose 3 M H2SO4 Film 0.65 to 0.3 (vs. Ag/AgCl) 2084 mF cm
2 at 3 mA cm
2 163
Ti3C2Tx/rGO-PDDA 3 M H2SO4 Film
0.7 to 0.3 (vs. Ag/AgCl) 1040 F cm
3 at 2 mV s
1 164
Ti3C2Tx/PDA 1 M H2SO4 Film
0.35 to 0.2 (vs. Ag/AgCl) 715 mF cm
2 at 2 mV s
1 165
Ti3C2Tx/3-MnO2 30 wt% KOH On carbon cloth 0 to 0.7 symmetric 212.1 F g
1 at 2 A g
1 166
Ti2CTx/3-MnO2 30 wt% KOH On carbon cloth 0 to 0.7 symmetric 113.1 F g
1 at 2 A g
1 166
Ti3C2Tx 3 M H2SO4 Hydrogel lm
1 to
0.1 (vs. Hg/Hg2SO4) 210 F g
1 at 10 mV s
1 167
Ti3C2Tx/carbon fabric 1 M H2SO4 On carbon fabric
0.7 to 0.2 (vs. Ag/AgCl) 401 F g
1 at 10 mV s
1 168
Ti3C2Tx 3 M H2SO4 3D-aerogel
0.6 to 0.2 (vs. Ag/AgCl) 349 F g
1 at 2 V s
1 169
Ti3C2Tx (bi-layer) 1 M KCl Film
0.7 to 0.3 (vs. Ag/AgCl) 225 F cm
3 at 5 mV s
1 170
Ti3C2Tx (bi-layer) 1 M NaCl Film
0.7 to 0.3 (vs. Ag/AgCl) 300 F cm
3 at 5 mV s
1 170
Ti3C2Tx (bi-layer) 1 M MgCl2 Film
0.7 to 0.3 (vs. Ag/AgCl) 270 F cm
3 at 5 mV s
1 170
Ti3C2Tx/Fe(OH)3 3 M H2SO4 Film
0.5 to 0.3 (vs. Ag/AgCl) 351 F g
1 at 0.5 A g
1 171
N doped Ti3C2Tx 3 M H2SO4 Film
0.6 to 0.1 symmetric 927 F g
1 at 5 mV s
1 172
Ti3C2Tx 3 M KOH Disc 0 to 0.9 symmetric 119.8 F cm
3 at 2.5 A g
1 173
Ti3C2Tx/ionic-liquid 3 M H2SO4-0.8 M [Emim]HSO4 On glassy carbon
1.1 to
0.1 (vs. Hg/ 603 F g
1 at 2 mV s
1 174
electrode Hg2SO4)
Ti3C2Tx aerogel 3 M H2SO4 Film
1.2 to
0.2 (vs. Hg/ 393 F g
1 at 5 mV s
1 175
Hg2SO4)
TiO2/C–Ti3C2Tx/NiO 1 M KOH On Ni foam 0 to 0.8 asymmetric 4.7 mA h g
1 at 0.1 A g
1 176
Carbon-lled Ti3C2Tx 6 M KOH On Ni foam
1 to 0 (vs. Hg/HgO) 226 F g
1 at 1 A g
1 177

Non-aqueous system
Ti3C2Tx 1 M LiPF6 in EC/DMC (1 : 1) Film 0.2 to 2.2 (vs. Li/Li+) 323 F g
1 at 0.5 mV s
1 118
V doped Ti3C2Tx 2 M KCl On stainless steel mesh
0.3 to 0.3 (vs. SCE) 365.9 F g
1 at 10 mV s
1 178
2 M LiCl On stainless steel mesh
0.3 to 0.3 (vs. SCE) 404.9 F g
1 at 10 mV s
1 178
2 M NaCl On stainless steel mesh
0.3 to 0.3 (vs. SCE) 321.7 F g
1 at 10 mV s
1 178
Ti3C2Tx Lithium bis-trifuoromethylsulfonyl amine (LiTFSI)/DMSO Film
2.4 to 0 (vs. Ag wire) 130 F g
1 at 2 mV s
1 179
Ti3C2Tx LiTFSI/ACN Film
2.4 to 0 (vs. Ag wire) 110 F g
1 at 2 mV s
1 179
Ti3C2Tx LiTFSI/PC Film
2.4 to 0 (vs. Ag wire) 195 F g
1 at 2 mV s
1 179
Na–Ti3C2 On copper foil 1 to 3.8 asymmetric 168 F g
1 at 0.1 A g
1 180
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potential range of 0 to 0.5 V using a three-electrode system. As

Ref.

121
175
725 mF cm
2 at 1 mA cm
2 181

182
183
184

185
186

187
the current rate increased, the capacitance of the samples

61 mF cm
2 at 25 mA cm
2
decreased. The areal capacitances of the MXene electrodes

34.6 mF cm
2 at 1 mV s
1

2 F cm
2 at 1.2 mA cm
2
439 F cm
3 at 10 mV s
1

35.6 mF cm
2 at 0.3 mA

69.5 mF cm
2 at 0.5 mA

720.7 F cm
3 at 1 A g
1

fabricated by inkjet- and extrusion-printing were compared with

500 mF cm
2 at 1 V s
1

respect to the number of paths (hNi) used during the printing
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process.193 The extrusion-printing of the MXene was tested for

the high current densities as compared to inkjet-printed
Capacitance

MXenes as shown in Fig. 11(c), depending on the hNi value

used during the printing process. At the lower values of hNi ¼ 2

cm
2

cm
2
in inkjet-printing, the increase in the current density resulted in
the slow decay of the capacitance, whereas the decrease in
capacitance was higher in the case of hNi ¼ 25. The extrusion-
printing of the MXene with different hNi values resulted in

0.8 to 0 asymmetric
Potential window (V)

0 to 1.6 asymmetric
On Ti/Au coated PET lm 0 to 0.6 symmetric
0 to 0.6 symmetric

0.6 to 1 symmetric
0 to 0.6 symmetric
0 to 0.5 symmetric

0 to 0.6 symmetric
almost similar capacitance decay patterns with the increase of

0 to 1 symmetric
capacitance with respect to current densities.
Aqueous MXene inks were used for the fabrication of MXene
supercapacitors by using different types of stamping templates
as shown in Fig. 11(d–g). Aer the stamping process, the elec-
trodes were connected by means of Ag wires for analysis
purposes using a conductive Ag slurry. The connection junc-
tions were coated with nail polish in order to protect the
Micro-pattern lm
Micro-pattern lm
On PET (3D print)

connection from damage. Ti3C2Tx and Ti3CNTx MXene inks

On carbon cloth

On photo paper

were used as electrodes for symmetric MSCs fabrication using

Morphology

PVA/H2SO4 gel polymer electrolyte as shown in Fig. 11(g). The

3D-aerogel

stamped MSCs fabricated using a roll printing method showed

Film

Film

an internal resistance (IR) drop in the GCD curves at different

current rates, as shown in Fig. 11(h). The comparative Ragone
plot of Ti3C2Tx MSCs power density and energy densities is
1 M NaClO4 in EC/PC (v/v ¼ 1 : 1) with 5 wt% uoroethylene carbonate

shown in Fig. 11(i). The scale-up production of MSCs is shown

using these stamp pad-type and roll-type printing methods.183
The stamping and printing methods of MSCs resulted in
complete solid-state devices that showed a good resistant
structure even aer roll pressing between the rolls for good
adhesion. The MSC device fabricated using Ti3C2Tx through
rolling exhibited good capacitance of 56.8 mF cm
2 at 10 mV
s
1. In contrast, the areal capacitance decreased to 24.5 mF
cm
2 at 100 mV s
1 due to the relatively high resistance of the
fabricated solid-state device. Fig. 11(j) shows the spinning
method for the yarn-type MXene. The yarn was formulated
using the mixture of graphene oxide (GO) and Ti3C2Tx solution.
The yarns were prepared by injecting the GO/MXene solution
into a glacial acetic acid bath with a ow rate of 10 mL h
1. The
prepared yarn-type electrodes were studied using a three-
electrode system in acidic electrolyte. The GCD curves of the
1 M H2SO4/PVA

fabricated supercapacitor were studied with different yarns

PVA/H3PO4
PVA/H2SO4
PVA/H2SO4

PVA/H2SO4
PVA/H2SO4

PVA/H2SO4

containing weight ratios of 3, 15, 38, 64, and 88 wt% at a current

Electrolyte

PVA/LiCl

PVA/LiCl

density of 0.5 A cm
3 (Fig. 11(k)). The increase in the MXene

(FEC)

weight ratio in the yarn improved the volumetric capacitance of

the electrodes but the increase in the current density resulted in
a decrease in the volumetric capacitance as shown in Fig. 11(l).
Ti3C2Tx@Fe2O3/Carbon

MXene-based materials have achieved superior capacitance

Ti3C2Tx/polypyrrole
Polymer gel system

performance in neutral and aqueous sulfuric acid electrolytes.

(Contd. )

Apart from lithium, sodium, and other larger ions were inter-
Ti3C2Tx aerogel

Ti3C2Tx/rGO
Mo1.33C/PPy

calated into the MXene interlayers, showing the promising

Electrode

potential of MXenes for battery–capacitor hybrid devices.

Ti3C2Tx

Ti3C2Tx
Ti3C2Tx

Ti3C2Tx
Table 2

Although MXenes have many advantages over other materials,

cloth

they are still lagging in certain aspects such as the development

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Fig. 10 MXene electrode materials synthesis and fabrication of supercapacitor electrodes using (a) the modified MXene to prepare MnO2/MXene
composites by chemical reduction (this figure has been adapted/reproduced from ref. 166 with permission from American Chemical Society,
copyright 2016). (b) Electrospinning carbonized MXene from MXene/PAN fibers (this figure has been adapted/reproduced from ref. 189 with
permission from Royal Society of Chemistry, copyright 2019). (c) The template method (MXene/CNF) (this figure has been adapted/reproduced
from ref. 190 with permission from John Wiley and Sons, copyright 2019), and (d) freeze dry method (MXene aerogel) (this figure has been
adapted/reproduced from ref. 191 with permission from Elsevier, copyright 2017). On the right side of each method, their corresponding specific/
areal capacitance values are shown with respect to different current densities/scan rates.

of bulk-scale production in industries for commercial utiliza- delamination process gave an improved performance of 900 F
tion, stability at room temperature, and utilization of strong g
1 with H2SO4 as the electrolyte. If the sheet size decreases or is
and hazardous acids for the synthesis process. Most impor- small, the probability of oxidation increases at the sheet edges
tantly, the fabrication of MXenes with uoride-free strategies is with exposed metallic active sites.196 MXene nanosheets with
another key challenge for the research community. The fabri- a size of <2 mm were also studied for supercapacitor and other
cation of hierarchical MXene nanostructures with highly applications.197 However, the applicability of MXene quantum
exposed active metal components is another interesting area of dots are yet to be explored completely in the energy storage
development, and it presents a new challenge to be addressed applications due to the instability of quantum dots at room
in future studies. temperature since it is a known fact that the decrease in the
sheet size (nm scale) can increase the oxidation of
4. MXene influence on MXenes.198,199 All these major issues need to be addressed to
expand the application of the MXene family in interdisciplinary
supercapacitor performance elds.
MXenes have attractive and unique advantages such as excellent
electronic conductivity, tunable layer structure, and control- 4.2. Surface functionalization
lable interfacial chemistry. However, the practical applications
The surfaces of freshly prepared MXenes contain –O, –F, and
of MXenes in supercapacitors and other energy storage devices
–OH functional groups that always increase the chances of
are severely limited due to rapid reaction kinetics, poor stability
oxidation or interaction with the other materials, leading to
at room temperature, self-restacking, and limited active
contamination.200 Once the dried surfaces of the MXene
sites.135,194
samples are exposed to open air, the functional groups present
on the surface, particularly at the edges where oxidation occurs
4.1. Sheet size rapidly, open the pathway for the degradation of the MXene
As shown in Fig. 12, the MXene sheet size, surface functional sheets.199,201 Many studies have reported that more O-functional
groups aer the etching process, thickness, and electrolytes groups are present than F-functional groups, which are involved
used in the supercapacitor devices are some of the factors in the capacity enhancement of the supercapacitors, especially
inuencing the performance of the supercapacitors. The large while using acid electrolytes, as they can form hydrogen bonds
size of the MXene sheets is more suitable for a good electrode; with protons in the acids.202,203 The presence of –Cl functional
however, at the same time, the synthesis of large-sized MXene groups on the edges and surface of MXene sheets expands the
sheets is a challenging process. The prolonged etching time for interlayer spacing due to the large ionic size of chlorine, which
the synthesis of MXene is not effective as it improves the aids fast ionic diffusion during the charge–discharge process.204
capacitance only by 10–20 F g
1. Ball milling of the MXene As the size of the intercalating ions (such as Li+, Na+, K+, and so
sheets can enhance the performance due to the decrease in the on from respective salts or solutions) or molecules (such as
sheet size and increase in the electrochemically active surface hydrazine, polyvinylalcohol (PVA), and poly-
area that is available during the charge–discharge process.195 diallyldimethylammonium chloride (PDDA), trimethylalky-
The MILD etching method combined with the Li+-based lammonium salts, and so on) increases, the interlayer distance

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Fig. 11 (a) Schematic illustration of Ti3C2Tx MXene electrode preparation by using MXene inks (aqueous and organic inks). The Ti3C2Tx organic
ink is made using ethanol and used for inkjet printing of various patterns, such as micro-supercapacitors (MSCs), MXene letters, ohmic resistors,
etc. The Ti3C2Tx aqueous inks are used for the extrusion printing of MSCs and in optoelectronics on different flexible substrates. (b) Galvanostatic
charge–discharge (GCD) curves of a typical extrusion-printed MSCs at different current rates in the potential range of 0 to 0.5 V using the three-
electrode system. (c) The areal capacitance of inkjet- and extrusion-printed MSCs with the number of paths (hNi) used in the electrodes printing
process (this figure has been adapted/reproduced from ref. 193 with permission from Springer Nature, copyright 2019). (d–g) Different methods
for the fabrication of MSCs using different kinds of MXene inks (Ti3C2Tx and Ti3CNTx) using gel-type polymer electrolytes such as PVA/H2SO4. (h)
GCD profiles of the roll-printed Ti3C2Tx MSCs at different current rates and the corresponding comparative (i) Ragone plot with other reported
works (this figure has been adapted/reproduced from ref. 183 with permission from John Wiley and Sons, copyright 2018). (j) The schematic
representation of the spinning formulation using Ti3C2Tx and GO materials for the fiber spinning. (k) The GCD curves of MXene/rGO fibers
containing different amounts of MXene loading (3, 15, 38, 64, and 88 wt%) at a current density of 0.5 A cm
3, and (l) the effect of MXene weight
loading on the specific volumetric capacitance of the fiber electrodes obtained by the three-electrode system analysis using 1 M H2SO4 as the
electrolyte (this figure has been adapted/reproduced from ref. 192 with permission from Royal Society of Chemistry, copyright 2017).

between the sheets of multi-layer MXene increases, leading to Therefore the etching process and intercalating molecules also
faster ion diffusion pathways.151,157,205,206 Although the interlayer majorly inuence the interlayer distance, which is a crucial
distance is increased by the intercalation of larger molecules aspect for the electrochemical performance of MXenes.
some molecules restrict the ionic diffusion due to their large
size such as trimethylalkylammonium salts. The intercalation 4.3. Electrode morphology
of organic or polymer molecules results in good interlayer
The thickness of the prepared MXene sheets decides the
distances but it is not up to the mark for the enhancement of
performance of the supercapacitors, which is directly depen-
electrochemical performances in comparison to MXenes
dent on the intercalating ions or molecules used during the
prepared by HF or LiF/HCl etching methods. Similarly, expo-
synthesis process. Most of the MXene-based electrodes are
sure to oxidizing agents and UV-vis irradiation also results in
fabricated using the MXene (slurry) solution coating on current
the degradation of MXenes.207 The presence of organic func-
collectors such as Ni foam, or by vacuum ltration with
tional groups on the surface of MXenes will diminish the
different thicknesses depending on the concentration of the
available active sites for the interaction of the electrolyte or
solutions used.209–211 The electrode coating thickness on the
incoming ions, resulting in the decay in capacitance.208
current collector or the lm electrode thickness are important

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used acid electrolytes to achieve high performance from the

MXene supercapacitors.146,167 The high volumetric capacitance
of MXene is due to factors such as high conductivity, which
provides the fast electron transport, layer structures for ionic
intercalation and transport, the variable oxidation states of
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transition metals aid good charge transfer and O-terminations

in MXenes provide good redox-active sites during the charge–
discharge process; all these aspects indicate the pseudocapa-
citive nature of MXenes in acidic electrolyte environments.146
On the other hand, in the presence of electrolytes such as
MgSO4, Na2SO4, and (NH4)2SO4 solutions, MXenes showed
EDLC properties. However, for practical applications, acidic or
basic-type electrolytes are not a preferred choice due to the
corrosive effects on the other parts of the system such as current
collectors (Ni foam, copper foil, and aluminum foils). Although
Fig. 12 Factors influencing MXene performance in supercapacitors. some gel-type electrolytes such as polyvinyl alcohol (PVA)/H2SO4
and PVA/H3PO4 mitigate these problems, their conventional
applicability has been compromised due to limiting factors like
factors that change the electrochemical performance of super- the electrochemical window, compatibility, and performance
capacitors.212 The uniform thickness of the coated electrodes decay.168,169,219 Apart from the aqueous systems, non-aqueous
(like Ni foam-coated) is made by pressing the electrodes using electrolytes are also used in MXene supercapacitors such as
a calendering machine.213 Electrode thickness in the range of organic electrolytes and ionic-liquid gel electrolytes. Organic
a few nanometers to 0.4 mm was reported for the better and ionic-liquid-based electrolytes can extend the electro-
performance of MXene supercapacitors by using lms and chemical window of the MXene systems making them more
coated electrodes.145,149,213 Although multilayer MXenes will have advanced for the applications. Some of the surface functional
higher mobility of ions, single-layer MXenes will provide a good groups like –F are inactive towards the electrochemical activity;
surface area for the redox reactions, adsorption, and desorption in order to make them useful sites, the ionic–electronic
reactions.214 Apart from the single and/or multilayer morphol- coupling strategy has been studied.174 For instance, 1-ethyl-3-
ogies of MXenes, the porous structure also aids the electro- methylimidazolium hexauorophosphate ([Emim]PF6) ionic-
chemical performance by increasing the electrolyte access to the liquid has been used as an etching agent along with HCl (IL-
active sites on MXene and also good ionic diffusion for the HCl). The [Emim]+ ion simultaneously acts as both an inter-
betterment of the electrochemical performance.215 The porosity calating and delaminating agent, similar to Li+-ions in the
of the electrodes can be altered by the combination of different MILD etching method. These kinds of [Emim]+ ion interactions
dimensional structures like 0D–2D, 1D–2D, 2D–2D, and 2D–3D. with MXene interfaces and edges form a coupled conductor
The thickness and morphologies of MXene sheets not only with an added advantage of oxidative resistance.174 The coupled
inuence the performance in energy storage devices but also in interactions have been studied using density functional theory
other elds such as photovoltaics, water splitting reactions, and (DFT) in support of experimental results as shown in Fig. 13(a
dye degradation processes.216–218 and b).174 Similar cations with different anion combinations like
1-ethly-3-methylimidazolium bis-(triuoromethylsulfonyl)-
imide (EMITFSI) in acetonitrile solvent as electrolyte for the
4.4. Electrolytes Ti3C2Tx MXene have been studied in the electrochemical
The electrolytes are considered to be the vital part of the window of
0.2 to 0.6 V vs. Ag/AgCl for CV, resulting in broadly
supercapacitor, which creates a free path for the ions to diffuse distributed peaks.220 Recently, some researchers reported the
between the electrodes. The potential window of the electro- use of 1-ethyl-3-methylimidazolium-tetrauoroborate
chemical supercapacitor is decided by the electrolyte used in (EMIMBF4) and its combination with PVDF-HFP as the ion-
the system. There are different kinds of electrolytes used such as ogel electrolyte for the fabrication of micro-supercapacitors
aqueous-based electrolytes, non-aqueous-based electrolytes, (MSCs) upon prior interaction with MXene electrodes.221 The
and solid-state electrolytes. For instance, Lukatskaya et al. pre-interaction of MXene sheets with ionic liquid is an advan-
demonstrated, for the rst time, the spontaneous intercalation tage for the improvement in the electrochemical window of 0–
of cations, including Na+, K+, NH4+, Mg2+, and Al3+ obtained 3 V without restacking of the MXene sheets.221 Fig. 13(c) shows
from the aqueous salt solutions between the Ti3C2Tx MXene the schematic preparation method for the ionic-liquid pre-
layers.167 Ti3C2Tx MXene showed a high volumetric capacitance intercalated MXene lm by immersing the undried aqueous-
of 1500 F cm
3 with a 90 nm thick electrode in an acid elec- based lms. The difference between the dried, undried, and
trolyte (1 M H2SO4). In the basic solutions, such as KOH, ionic-liquid pre-intercalated lms is clearly shown by the (002)
a binder-free Ti3C2Tx paper can achieve a volumetric capaci- peak in XRD in Fig. 13(d). In Fig. 13(e), the difference in the
tance of 340 F cm
3 at 2 mV s
1 and exhibit almost no degra- redox peaks are seen in CV curves of the lms between 2.0–2.5 V
dation aer 10 000 cycles at 1 A g
1.167 Therefore, most studies due to the pseudocapacitive intercalation/deintercalation

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Fig. 13 The ionic–electronic coupling between the [Emim]+ ion and MXenes (a) Ti3C2F2 and (b) Ti3C2O2 calculated by density functional theory
(DFT). The yellow and green colour represent the electron accumulation and depletion at the interlayers of MXenes. The blue, brown, magenta,
red, and dark grey balls represent Ti, C, H, O, and F atoms, respectively (this figure has been adapted/reproduced from ref. 174 with permission
from Royal Society of Chemistry, copyright 2021). The comparison of EMIMBF4 pre-intercalated and dried MXene films for MXene-based micro-
supercapacitors (MSCs) using EMIMBF4 as the electrolyte. (c) Schematic representation of the fabrication of ionic liquid pre-intercalated MXene
and dried MXene electrode films. (d) XRD patterns of ionic liquid pre-intercalated and dried MXene films in comparison with the freshly prepared
undried film. (e) CV curves tested in the electrochemical window of 0–3 V at a scan rate of 50 mV s
1. (f) The volumetric capacitance graph, and
(g) EIS spectra (this figure has been adapted/reproduced from ref. 221 with permission from Royal Society of Chemistry, copyright 2019).

behaviour of EMIM+ cations in the interlayers of MXene lms.222 a major deciding factor towards the electrochemical perfor-
The pre-intercalation with EMIMBF4 showed a clear increase in mance of electrodes.
the volumetric capacitances and change in impedance values of These novel and exciting 2D MXene materials undoubtedly
the MXene lm (Fig. 13(f and g)). For the futuristic studies, show great potential as future electrode materials for energy
more ionic-liquid-based exfoliation and utilization as the elec- applications inuencing the energy market. The facile, low-cost,
trolyte have to be explored, which also gives good safety prop- rapid, and efficient production of MXene-based supercapacitors
erties for supercapacitors.The utilization of ionic-liquids and/or or energy storage devices by using simple printing or stamping
metal salts in combination with gel-type electrolytes might be technologies will be a critical advancement for the commer-
a good choice for future applications in solid-state devices, cialization of devices, which is mostly possible by means of
which is yet to be completely explored. All these factors dis- MXene-based inks and slurries in different solvents. MXene-
cussed in the Section 4 such as MXene sheet size, thickness, based composites are opening a new path to achieve higher-
surface functional groups, and delaminating agents highly performing new-generation materials. Still, some issues need
inuence the electrical conductivity of the MXenes, which is to be addressed, such as the prevention of agglomeration,
improved electronic conductivity for composites,

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electrochemical stability, good contact with the electrolyte, and 20201G1A1014959). This work was also supported by the Techno-
easy ion/electron transfer in these materials. Moreover, there is logical Innovation R&D Program (S2848103) funded by the Small
a need for a further understanding of the mechanisms involved and Medium Business Administration (SMBA, Korea) and sup-
in energy storage and conversion devices. We believe that the ported by the Korea Electric Power Corporation (Grant number:
present review on the recent advances in the synthesis and R21XO01-5). This work is supported by the R&D Program of
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applications of MXene materials for supercapacitors will serve Ministry of Culture, Sports and Tourism and Korea Creative
as a useful platform for understanding and gaining more Content Agency of Korea (Development of K-Black Ink containing
information for future endeavors in energy storage the Traditional Muk Material and its use in Digital Printing Tech-
applications. nology, Project Number: R2020040047).

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