Prospective Advances in MXene Inks: Screen Printable Sediments for Flexible Micro-Supercapacitor Applications

Abstract

Additive manufacturing industries have been focusing on the development of novel ink formulation strategies, that can incorporate functional materials for printing high-efficient electronic patterns for flexible devices. As such printed...

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Journal of
Materials Chemistry A
Materials for energy and sustainability
rsc.li/materials-a
ISSN 2050-7488
COMMUNICATION
Zhenhai Wen et al.
An electrochemically neutralized energy-assisted low-cost
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hydrogen generation
Volume 6
Number 12
28 March 2018
Pages 4883-5230
Journal of
Materials Chemistry A
Materials for energy and sustainability
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Mater. Chem. A, 2022, DOI: 10.1039/D1TA09334G.

1
Prospective Advances in MXene Inks: Screen Printable Sediments for
Flexible Micro-Supercapacitor Applications
Jalal Azadmanjiri1,*,†, Thuniki Naveen Reddy2,†, Bahareh Khezri1, Lukáš Děkanovský1, Abhilash
Karuthedath Parameswaran1, Bhupender Pal1, Saeed Ashtiani3, Shuangying Wei1, and Zdeněk
Sofer1,*
1Department of Inorganic Chemistry, University of Chemistry and Technology Prague,
Technická 5, 166 28 Prague 6, Czech Republic
2Department of Freshman Engineering, Geethanjali College of Engineering and Technology,
Cheeryal, Keesara, Hyderabad, Telangana, India, 501301
3Department of Physical Chemistry, University of Chemistry and Technology Prague, Technická
5, 166 28 Prague 6, Czech Republic
*Corresponding authors: jalal_azad2000@yahoo.com, jalal.azadmanjiri@vscht.cz (Jalal
Azadmanjiri) and zdenek.sofer@vscht.cz (Zdeněk Sofer)
†J.A. and T.N.R. contributed equally
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Abstract
Additive manufacturing industries have been focusing on the development of novel ink
formulation strategies that can incorporate functional materials to print high-efficient electronic
patterns for flexible devices. As such printed micropatterns were found to miniaturize the device
components, and their mechanical deformability offers wearability. In this regard, key issues like
the selection of functional materials, the choice of proper sediment ink, its effective formulation,
and the selection of appropriate printing technology provide a rational trajectory towards the
fabrication of high-performance devices. Recently, the MXene based screen printable inks are
gaining attention due to their unique mechanical, electronic, rheological properties and their
printed architectures have found the potential viability as charge storage components in flexible
devices. Herein, we report the recent advancements in screen printable transition metal carbides
and nitrides (MXenes) ink formulations and their challenges for flexible micro-supercapacitors
(MSCs) applications. This review work focuses on (i) efficient MXene based ink formulation
strategies for screen-printing applications, (ii) the strategies to improve the substrate and ink
interactions, (iii) methods to address the issues like oxidative and deformation stability of screen
printed MXene MSCs, and (iv) challenges in the integration of these MXene MSCs as energy
storage components in device architectures.
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1 Introduction
1.1 Overview of screen-printed electronic devices
The micro-architecture development of electronic components on flexible substrates is increasing
the demand for wearable electronics.1-3 However, the key components required for the fabrication
of wearable electronic devices are high-performance, flexible, and micro-sized energy-storage
system, in terms of either generating required power characteristics (i.e., high-energy-density, long
cyclic life, high-rate capability) or in mechanical performances (i.e., folding, bending, twisting).
Therefore, screen-printing of micro-architectures for flexible micro-sized energy-storage devices
like micro-supercapacitors (MSCs) have garnered lots of considerations in recent years.2-5
Formulation of functional ink for the screen-printing of MSCs with high-performance
nanomaterials in fabrication of next-generation flexible electronic devices is necessary. In this
context, the conductive and semi-conductive two-dimensional (2D) nanomaterials with high
surface area and unique physicochemical properties have been more focused in the last decade as
a functional material ink for this purpose.6-10 2D nanomaterials (2DNMs) such as graphene,6, 11
MoS2,12, 13 MoSe2,9, 12, 14 g-C3N4,15 h-BN,16 etcetera are well studied as active ingredients in ink
formulation for the printing of high-quality electronic devices in varieties of applications such as
MSCs,2 micro-batteries (MBs),17 sensors,18 photodetectors,19 electromagnetic interference
shielding,20, 21 and solar cells.22 However, the involvement of costly synthetic and processing
strategies, a significant amount of active components wastage, utilization of large quantity of
fillers, the poor performance of screen-printed components, and 2DNMs stability issues are the
major drawbacks that hinder the practical viability of these inks for printing of such micron-sized
devices on flexible substrates.
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Thus, new ink formulations containing MXene sediments are recently getting attracted due to the
MXene’s excellent viscoelastic, mechanical, and conductive properties.23 Unlike the 2D carbon
allotrope graphene, the MXenes are found good rheological properties, and some of their other
characteristics like negative surface zeta potential, low sheet resistance, and high mechanical
strength have made to choose these MXenes as potential candidates for making micro-electronic
components on flexible substrates,24 (Table 1).
Table 1. Overview of the obtained diverse Ti3C2Tx inks and their performances.
Ink type
MXene
size
MXene
concentration
Rheological
properties
Zeta potential
Conductivity/
resistivity
Areal
capacitance
energy
density
Application
Ref.
Organic
(PEDOT
: PSS)
~ 142
nm
30 mg mL-1
ρ = 1.067 g cm-3, γ =
55.64 mN m-1, η =
19.71 cP,
Z = 1.81
-40.5 mV
615 S cm-1
22.6 mF cm-
2
9.4 mW
h cm-3
MSCs
25
Organic
(Ethanol)
2.4 µm
0.7 mg mL-1
ρ = 0.791 g cm-3, γ =
22.1 mN m-1, η = 7.3
mPa s,
Z = 2.6
- 40 mV
2770 S cm−1
(510 S cm−1
after 6
months)
12 mF cm-2
0.32 μW
h cm-2
MSCs
26
Aqueous
(Water)
1.1 µm
20 and 30 mg
mL-1
η = 1-10 Pa s
- 30 mV
0.15 Ω cm-1
5 mF cm-2
-
MSCs
27
Aqueous
(Water)
350 nm
22.4 mg mL-1
γ = 68.8 mN m-1, η =
1.5 cP, Z = 18.9
- 40 mV
450 Ω cm-1
60 mF cm-2
12.36
μW h
cm-2
MSc
28
Aqueous
(Water)
2 µm
18 mg mL-1
γ = 76.5 mN m-1, η =
1.5 cP, Z = 19.9
- 40 mV
9.8 Ω cm-1
32 mF cm-2
-
MSc
28
Aqueous
(Water)
-
4.5 mg mL-1
γ = 80.3 mN m-1, η =
1.4 mP s, Z = 30
-
5.9 Ω sq-1
9.8 mF cm-2
0.49 μW
h cm-2
MSc
29
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Aqueous
(Water)
200 nm
5 mg mL-1,
ρ = ~ 1100 kg m-3, γ
= 73 mN m-1, η = 1
mP s, Z = 19.4
- 42.3 ± 4.1
mV
119 S cm-1
108.1 mF
cm-2
100.2
mW h
cm-3
MSc
30
Organic
(dimethy
lsulfoxid
e,
DMSO)
3-6 µm
2.25 mg mL-1
γ = 40 mN m-1, η =
3.4 cP, Z = 21.3
-
1080 ± 175 S
cm−1
-
-
Electromag
netic
shielding
20
Organic
(DMSO)
10-
30 μm
~ 2 mg mL-1
ρ = 0.8 g cm-3, γ = 23
mN m-1, η = 2.4 mPa
s,
Z = 8.3
-
-
-
-
Photonics
31
The printable MXene sediment inks for MSCs are gaining constant attention for their capability to
store and deliver high power densities.32 Moreover, the utilization of atomic size thin MXenes 2D
layer in ink formulation for MSCs offers the maximum areal capacitance compared with their bulk
states. However, the fabrication of these MXene based MSCs, involves the thin layers printing of
conductive electrodes on substrates as current collectors and sandwiching these electrodes with
the insulating material and electrolyte. The performance of such fabricated MSCs is dependent on
the charge accumulation around the electrical double layer (electrode and insulating layer
interfaces), the conductivity of the active materials, and the quality of the print. The improvement
in printing quality has challenges in ink preparations such as (i) scalable production of exfoliated
2D MXenes,33 (ii) efficient ink formulation techniques,33 and (iii) adoption of efficient printing
techniques. Besides, the pattering micro-electronic architectures on flexible substrates for
mechanically deformable MSCs sets other challenges like (iv) low-temperature processing,33 (v)
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deposition of a high amount of active material per unit area with good surface adhesion,26 and (vi)
stability under the deformations.34
The fabrication of MXene based micro-architectures on flexible substrates is carried out by various
methodologies like spray coating, spin coating, lithography, etc.5, 31 However, these methodologies
offer poor performance with high material wastage and poor control over the thickness of the
printing area. In this regard, screen-printing of MXenes is found as an impressive route to fabricate
low-cost and large-scale structures on any flexible substrate.5, 31 Moreover, screen-printing allows
the layer-by-layer printing of complex heterostructures for multiple functionalities, with improved
performance and high precision. Albeit, the ink formulation reported, using MXene through liquid-
phase exfoliation is still far from ideal for inkjet or screen-printing applications. Additionally, the
ink formulation methodologies reported till now involve the harsh synthetic strategies, time-
consuming, and utilization of expensive materials. Moreover, the composition of printable ink with
good rheological properties requires a large quantity of additives and expansive solvents.26 The
formulation of novel MXene printable ink by considering these challenges requires the MXene
dispersion in an aqueous medium with specific physical properties (surface tension, and density)
that are satisfying the requirements of screen-printing applications. On the other hand, the
materials like MXenes show moderate dispersibility in an aqueous medium and get agglomerated
at higher concentrations leading to obstacles for achieving these physical properties.35 The
theoretical and experimental investigations have demonstrated that the addition of a large quantity
of surfactants would resolve a partial part of these issues. Although the high percentage of additive
surfactants reduces the active components per unit area after printing and this requires additional
steps to remove such components.
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The surface interaction of screen-printing ink with flexible substrate is another important
consideration for getting flexible MSCs that can withstand deformations and the ink with good
wetting behavior. The selection of suitable binders is reported as an effective strategy to improve
these interactions.36 However, the addition of large quantity of binders again led to the decreases
print resolution, which eventually reduces the efficiency of devices. On the other hand, improper
wetting behavior printed patterns may lead to non-uniform distribution (coffee ring effect, CRE)
of active components and this leads to cracks in patterns. Hence, a new alternative solution for
high-performance MXene based jetting ink with good rheological properties and with good
wetting character is required for potentially viable flexible MSCs via screen-printing method.
1.2 Chemical characteristics of MXenes
Transition metal nitrides, carbides, and carbonitrides are well-known to MXenes. These are
synthesized from the MAX phases materials with hexagonal crystal structure and P63/mmc
symmetry, using wet chemical etching of A-layers. The parent MAX phase materials possess
intrinsically ternary nanolaminate arrangement atomic structures consisting of alternating layers
of Mn+1Xn (M stands for transition metal and X denotes either nitrogen or carbon, n = 1 to 4), and
mono-layers of A-atoms (p-element of III-A or IV-A groups in the periodic table) (Figure 1a). The
chemical etching process offers relatively strong chemical bonding (metallic bonding) between
MX layers after the removal of A elements from the MAX phase.37 The strong bonding helps in
etching of A-sheets from the stack layers of MAX phase and retains the sheet resistance of the
resultant MXene phase by lowering the surface defects. However, the exfoliation of these MX
layers after chemical etching of A-layers from its MAX phase, requires mechanical agitation that
again leads to generation of surface defects.38 To date, the MXene nanosheets are available in four
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molecular structures of M2X, M3X2, M4X3, and M5C4 with hydrophilic surfaces and with tunable
properties which can be used in diverse applications (Figure 1a). The A-layers selective etching
from the initial MAX phase results in the substitution of A atoms with surface termination species
(Tx) or intercalated compounds (IC). After terminal group substitution of intercalation, the
chemical formula of resultant MXene can be written as Mn+1XnTx or Mn+1XnTx-IC. The nature of
Tx and IC are depending on the etchant used and etching process parameters (i.e., etching time,
etching temperature, etc.), and generally the Tx and IC are could be -F, -OH or =O, -Cl, and NH3
or NH+4 groups.39, 40 These functional groups of Tx and IC are found to responsible for
hydrophilicity of MXenes sheets. The exchanged Tx functionalities weaken the electrostatic
interactions between the interlayers and develop the exfoliation process. For instance, the Ti-Al
bonds of the Ti3AlC2 are substituted by Ti-F, Ti-OH, and Ti-O bonds upon etching of the MAX
phase, subsequently the -F, -OH, and =O surface functional groups terminate the MXene surface.
Additionally, the Tx or IC functionalities are capable to change the nature of a MXene. For
example, it has been illustrated that the full functionalization of Ti2C MXene nanosheets with =O
can convert the metallic nature of the Ti2C to a semiconductor state with a bandgap of 0.34 eV and
a great Seebeck coefficient of ~ 1140 µV K-1 at 100 K.41 This phenomenon is due to the strong
hybridization of Ti-d and O-p orbitals.41
Among the different MXene families, the nitride-based ones are found to be difficult to synthesize
because of the generation of higher formation energy from its MAX phase parents.42 This causes
poor stability of the nitride MXene layers during the etching process. Hence, a mixture of
appropriate etching substances usually is utilized instead of a typical common hydrofluoric acid.
Similarly, the silicon-based MAX phase has significant resistance to such common acids and
difficult in etching due to the strong chemical bonding between the transition metal and silicon
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elements. Therefore, an oxidant-assisted selective etching strategy has been employed to facilitate
the etching of silica as silicon from its MAX phase via a mixture of appropriate acid with an
oxidant (i.e., H2O2, HNO3, persulfate, etc.).43 In this oxidant assisted method, a two-step method
is followed as (i) oxidation of the silicon by a selective oxidant in step-1, and (ii) dissolving the
generated silicon oxides by an appropriate acid in step-2. The delamination of such produced
MXene stacks (after chemical etching) were carried out using mechanical agitation. However, the
extraction of high quality MXene layers with mechanical agitation depends on the affinity of
interlayer interactions and the adopted mechanical process. In order to reduce affinity of stack
layers intercalation, a typical ion between the layers usually is chosen as affective strategy, and in
some cases simultaneous etching and intercalation strategy is used as applicable method. For
example, the diverse intercalated alkylammonium [(NRxH4-x)+] and tetramethylammonium
hydroxide (C4H13NO) have been traced by X-ray diffraction (XRD) characterization in Ti3C2
MXene powders which were extracted from Ti3AlC2 and Ti3SiC2, respectively (Figure1b).43
However, the experimental investigations demonstrate that the strong interlayer interaction
between the synthesized MXene nanosheets using these methods hinders the complete separation
of 2D nanolayers, leading to a larger amount of wastage or requiring an additional method for
redispersion of unexfoliated sediments and creating unwanted surface defects. These unwanted
surface defects may reduce the quality of 2D MXene layers. Hence, the development of alternate
exfoliation and etching strategies that can lead to high-quality 2D-MXene dispersion are necessary
for full exploitation of MXene properties in functional ink formulation.
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Figure1. (a) Schematic representation of the elements used to make MXenes and four typical
structures of MXenes. The M layers in a MAX phase are approximately close-packed and
octahedral sites have been occupied by X (C and N) atoms. The adjacent M-X layers also
interleaved with A layers. The MXene nanosheets obtained after etching and delamination
processes are in four atomic structures of M2X, M3X2, M4X3, and M5X4 with hydrophilic
surfaces. The MXene phase structures are showing the polar surface terminated ends that can be
interacted with the other polar molecules like DMSO and water. Transmission electron
microscope (TEM), and selected area electron diffraction (SAED) are displaying layer structures
of the MAX (top) and MXene (bottom) phases. Reproduced with permission from data published
in refs.,44, 45 Copyright 2021, American Chemical Society and Elsevier. (b) XRD
characterizations of Ti3C2 MXenes obtained after HF/H2O2 treatment of (i) Ti3SiC2 (MAX phase)
before HF treatment, (ii) after 50 wt% HF treatments, (iii) after complete etching of Si from
MAX phase with HF/H2O2 solution and (iv) after intercalation with tetramethylammonium
hydroxide (TMAOH). The shift of the (002) peak [MAX (Ti3SiC2) to MXene-TMAOH]
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observed from 6.1º to 8.9º, which is corresponding to a d-spacing enhancement from ~ 9.95 Å to
14.43 Å due to the intercalation of TMA+ ions. A stable colloidal solution of Ti3C2 can be
collected after liquid delamination of Ti3C2 powder with and without pre-intercalation of
TMAOH. Reproduced with permission from data published in ref.,43 Copyright 2018, John
Wiley and Sons.
1.3 The scope of this review
The micro-components that could be integrated in fabrication of wearable electronics require the
printing of efficient charge transport channels on flexible substrates. In this regard, the printing of
functional micro-devices such as MBs and MSCs as charge storage component in the wearable
device architectures is been focused in recent years. The MXene based MSCs are rising the hope
for direct printing as efficient charge transport channels with low temperature processability.
However, the performance of the printed component with MXenes, was found much lower
compared to their theoretical expectations and failed to achieve the desired outcome. This is may
be due to utilization of a high amount of resistive materials as additives and binders; it eventually
leads to low conductivity of printed area and the low quality of MXene ink. On the other hand, the
low surface interactions of MXene layer with polymeric flexible substrate could lowering
mechanical stability and more importantly, the lower oxidation stability of MXene sheets were
leading to the unsatisfied cyclic stability of printed electronic components. Hence, it is very
important to concentrate on efficient formulation strategies that can address these issues. The
present review is enwrapped with the discussions on recent advancements and challenges in
MXene based ink formulations that are suitable for screen printable MSCs on flexible substrates.
The progress of additive-free printable ink formulations and the effect of surface modification on
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MXene properties like dispersity, stability, and rheological properties are discussed. Following
these, challenges and some future scopes related to printing high-performance MSCs using cost-
effective MXene ink formulation strategies have been demonstrated.
2 Functionalization of MXenes for efficient purposes in screen printable sediment
inks
MXenes and their family have demonstrated moderate rheological properties at high volume
concentrations in water. The surface zeta potential (ξ) values of MXenes are between - 30 mV to
-80 mV.46, 47 A clay-like behavior at high volume concentrations has made the MXenes self-
resilience for formulating additive-free ink in spray coating, spin coating, and electrophoretic
deposition with low-resolution. However, the current MXenes rheological properties are not
satisfying the desired requirements for ultrafast printing technologies like inkjet printing, where
the fabrication of desirable electronic micro-components on flexible substrates practically requires,
high quality ink, with high-resolution printing.36 Therefore, it is urgent to understand the screen
printable ink qualities.
A desirable inkjet printable ink also requires a stable MXene solution. The quality of screen
printable ink is measured on inverse Ohnesorge number (Z) value parameter. The Z value with the
equation of Z = √𝛾𝜌𝐷/𝜂, depends on the viscosity (γ), the density of solution (ρ), nozzle diameter
(D), and surface tension (η). The Z value is expected to be in between 1-14 for a suitable printable
ink. An ink with a lower Z value can be favorable to avoid the nozzle blocking and developing
stable droplets. However, this value for an aqueous solution of MXene is higher than 14, hence,
the direct utilization of it may leads to blocking of printing nozzles.28 To reduce the Z value, some
costly procedures like the addition of additive, surfactants and binders in ink formulation are
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required. The removals such components, after printing on flexible substrates are make the process
hectic. Hence, some innovations in the preparation of an efficient aqueous solution with additive-
free ink formulation are essential. Uzun’s group has reported an additive-free aqueous Ti3C2Tx ink
for thermal inkjet printing on textiles.28 To do so, two types of inks were formulated with the
MXene flakes particle size of ~ 4 μm (called L-MXene ink) and < 500 nm (called S-MXene ink).
The reason for formulating two solutions is to achieve the dynamic viscosity at a high shear rate
with high filler loading. The MXenes flake size distributions were measured using dynamic light
scattering (DLS). The DLS results revealed that the average particle sizes of 2 μm and ~ 350 nm
for L-MXene and S-MXene inks. The change in dynamic viscosity of dispersion with these
modifications found to be 5.1 cP and 1.5 cP at 18 mg mL-1 active components loading, respectively
(Figure 2b). The Z value also is found as 19.9 for L-MXene and 18.9 for S-MXene formulated
inks. These nearly close Z values to inkjet printable ink range are indicating the viability of
aqueous Ti3C2Tx suspension as functional ink. Albeit, the large flake size may cause the blocking
of the nozzle. Although the nozzle size could be adjusted, such that the ink can produce stable
droplets without blocking.28 Hence, it is clear that the Z value can be tuned by changing the
rheology by MXene dispersion via playing with concentrations of fillers, resizing and surface
modifications.
MXenes ink rheological behavior and their rheological response varies with quality, size, and
concentration of MXene; and also, with type of solvents utilized. It is observed that the rheological
properties of single- and multi-layer MXene are different. For instance, a Ti3C2Tx with the particle
size of ~ 300 nm and 5 μm in aqueous dispersions showed variation in viscosity as a function of
shear rate, and also viscosity can be varied with concentration of MXene sheets in ink (Figure
2a).48, 49 The diagrams of viscosity versus shear rate in Figure 2a.i and 2a.ii of single- and multi-
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layer dispersion in water shows the behaviors of MXene ink. The dispersion with single-layer
MXene, initially, at the lowest concentration (0.18 mg mL-1) displayed the viscosity of 0.005 Pa
s. However, the viscosity increased to 7.5 Pa s once the amount of single- layer MXene in
suspension rose to 3.6 mg mL-1 (Figure 2a.i). These results suggest that the single-layer Ti3C2Tx
solution in water has displayed versatile rheological properties and it can be tuned by varying the
concentration and shear rate. However, at higher concentration viscosity changes of such solutions
is negligible. This proposes that the rheology of MXene ink prepared by single layers can only
varied at lower concentrations. Whereas, the multi-layer MXene dispersion has showed the
significant change in viscosity at higher concentrations, however, restacking of layers at higher
concentrations leading to blocking of printing nozzles. Moreover, the viscosity of such as-
synthesized MXene solutions even is not enough as a desirable printable ink in inkjet printing.48
In addition to good rheological behavior and optimal Z value, a high-demand inkjet printable ink
also should have adjustable characteristics for advanced screen printers with low material wastage.
Besides, the additional phenomena for consideration in the formulation of a favorable ink, the
interaction of ink with the substrate at post-deposition like spreading, drying, stabilizing, and
assembling are also important. The main requirement for this effective post printing process is to
formulate precise MXene ink with high filler concentrations (MXene Sheets) in achieving high-
resolution printing patterns with good efficacy, and with improved the solvent evaporation kinetics
and also improved adhesion character with flexible substrates. The viscosity of ink for inkjet
printing must be within the range of 1-20 mPa s, to generate suitable droplets that can deposit on
the flexible substrate based on the pre-programmed patterns. Besides, the surface tension of
printable ink has to be in accordance with the surface energy and texture of the substrate to control
solvent evaporation kinetics and reduce CRE is necessary. However, the MXene dispersion
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typically suffers from quick precipitation in low-boiling-point solvents; this phenomenon directly
affects the printing process by leading to clogging and non-uniform deposition. After jetting on a
suitable substrate, wetting and drying are important for effective interfacial interactions between
the MXene layers. It is suggested that ink with surface tension between 7 mN m-1 to 10 mN m-1 is
preferred and the surface energy of ink droplets must be less than the surface energy of substrates.26
Although an MXene in aqueous solvents with a small concentration can show good viscosity at a
higher shear rate,48 the surface tension of the solution is not suitable for printing on flexible
substrates. This obstacle leads to the compromise in the utilization of direct printing method or
either changing the substrate’s surface. Hence, ink with suitable surface tension and viscosity is
preferred.
The low stability of MXene ink in presence of air is another issue for long run cycles. Most of the
MXenes found to react with atmospheric oxygen and tend to form their oxidative products. For
instance, the quick reaction of Ti3C2Tx in air resulted formation of TiO2, thus formed TiO2 reduces
the conductance of printed patterns (due to relatively, low conductivity of TiO2 than Ti3C2Tx at
room temperature) and moreover these oxides tend to variation of rheological behavior of
formulated ink. In this context, the formulation of MXene ink with high oxidative stability is
necessary.
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Figure 2. (a) Rheological properties of Ti3C2Tx MXene with diverse concentrations; the
measured viscosity against the shear rate of (i) single-layer MXene flake suspensions with
concentrations ranging from 0.18 mg mL-1 to 3.6 mg mL-1 and (ii) multi-layer MXene
dispersions with concentrations ranging from 10 wt.% to 70 wt.%, (iii and iv) relative viscosity
versus volume fraction and concentration plots for single- and multi-layer Ti3C2Tx dispersions.
Also, the experimental data from selected publications have been superimposed for better
comparison between Ti3C2Tx, Kaolin clay, and polystyrene. Reproduced with permission from
data published in ref.,48 Copyright 2018, American Chemical Society. (b) Digital photograph (~
50 mL of an additive-free aqueous Ti3C2Tx ink, 22.4 mg mL-1), TEM, atomic force microscopy
(AFM), and the line profile images of L-Ti3C2Txflakes. The viscosity, Ti3C2Tx flake size, ink
concentration, the flake intensity distribution-diameter, and ink concentration-viscosity of L-
Ti3C2Tx and S-Ti3C2Tx inks during inkjet printing also illustrated. Reproduced with permission
from data published in ref.,28 Copyright 2020, John Wiley and Sons. (c) TEM and their
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corresponding HR-TEM images of (i) fresh delaminated Ti3C2Tx, and after exposure to air for
(ii) 7 days and (iii) 30 days [the insets are corresponding to SAED and fast Fourier transform
(FFT) patterns], (iv) Raman spectra of fresh and air-exposed MXenes, and XPS results of (v)
fresh and (vi) air-exposed MXenes after 25 days. Reproduced with permission from data
published in ref.,50 Copyright 2017, American Chemical Society.
We believe that the surface modification with suitable organic and inorganic functionalities is
efficient strategies for improving the cohesiveness in the dispersed MXene layers. This possesses
a direct influence on the physical parameter of ink such as surface tension, density, and viscosity
of the solution. For instance, Liu et al.,51 developed an aqueous Ti3C2 MXene ink functionalized
with poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). The rich surface
functional groups on MXene tend to interact with PEDOT:PSS and facilitate the re-dispersity and
improvement in ink properties such as viscosity, moduli, and shear yield stress. Moreover, the
surface functional groups are found to capable of reducing the corrosion (surface oxidative
reaction) of many inorganic materials. Thus, this section focuses on the recent advances in
improving the chemical and dispersion stability of ink based on the surface modification of MXene
layers and to assess the effect of surface modification of MXenes for improving their ink qualities.
2.1 Functionalization of MXenes as efficient delamination strategy
The bare MXene sheets with the surface terminal (Tx-resulted during chemical etching processes)
groups have shown negative surface zeta potential charges which can interact with positively
charged ions and molecules and lead to increases in the interlayer spacing. As a result, ion
intercalation and surface functionalization are found effective strategies for separating the MXene
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layers from its stack layers. The chemical etching process of A-layer atoms from MAX phases
generally helps in the separation of elementary MXenes by forming the terminal groups. However,
this separation of layers is not enough to overcome the weak van der Waals (vdW) interlayer
attractions. Hence, the mechanical methods are utilized to increase the separation. Usually, c-
lattice parameters in the X-ray diffraction analysis of MAX phases are compared with the
intercalated MXene layer to study the effects of surface modification. Therefore, the c-lattice
variation value (∆d = dMX-dMAX; dMX = the position of the c-lattice parameters in XRD of
functionalized MXene and dMAX = the position of the c-lattice parameter of MAX phase) is a direct
measurement for the change in interlayer spacing with the surface modifications. For instance, the
variation of c-lattice parameter for the produced Ti3C2Tx with hydrochloric acid (HCl) and lithium
fluoride (LiF) solution was found to be 27 Å to 28 Å which is reported as the interlayer separation.
Similar to HCl and LiF the MXene interlayer spacing is improved by HF solution (c = 20 Å) during
the etching process.52 In addition to this, a MXene produced with HCl and LiF in water was found
to show clay like behavior due to the hydration of Ti3C2Tx sheets which indicates the interlayer
spacing has direct effect on rheological behavior of MXene. The shift in the c-lattice parameter (~
40 Å) was observed for this hydrated Ti3C2Tx sediments. This increment in interlayer spacing was
attributed to the intercalation of the water molecule in MXene layers.52 As intercalated water
molecules was found to act as a lubricant and allow facial sharing; and thereby imparts the clay
like behavior. In another case, during the etching of A-layer from the MAX phase by using HCl
and LiF was found to improve the swelling of MXene layers by intercalation of Li+ and H+ ions.52
The interlayer spacing of such Li+ intercalated ions can also be further improved by the exchange
of Li+ by bigger size cations like Na+, K+, Cs+, and Mg2+. Additionally, these types of cations can
also improve the energy storage capacity (Figure 3a).53
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Alike to inorganic materials (i.e., H2O, HCl, LiF), the organic molecules (i.e., hydrazine, urea,
dimethyl sulfoxide, amino acids) also are found to increase the interlayer space of MXenes.54 For
instance, the organic base tetramethylammonium hydroxide (C4H13NO, TMAOH) could be found
to be reacted with the Al-layer of Ti3AlC2 and cleavage the Ti-Al metallic bonds through the
hydrolysis of Al.55 Figure 3b shows the structural characterization of the Ti3AlC2 sample before
and after TMAOH intercalation. It is demonstrating a basal spacing expansion from 0.92 nm to
1.50 nm due to the intercalation of TMAOH..55 Similarly, the intercalation and delamination
process with organic molecules is also found as an efficient method for synthesizing MXene
nanosheets (i.e., Ti3C2) from the MAX phase parent with Si-layer atoms (i.e., Ti3SiC2). In a
research work by Alhabeb et al.,43 the delaminated Ti3C2 nanosheets were confirmed by the
observation of the c-lattice and the corresponding d-spacing distance variations of MXene before
and after TMA+ ions interaction. The same as what has been observed for the inorganic molecules
intercalated with MXenes, molecular dynamic simulation results of Ti3C2 with -OH termination
and intercalated hydrazine (N2H4) illustrate an increase in the c-lattice parameter and interlayer
space with the number of intercalated species (Figure 3c). The improvement in interlayer spacing
found to weaken the attraction forces in between the layers and it helps in easy delamination of
stacked layers of MXene into mono- or bi-layers upon a weak mechanical agitation like sonication,
mechanical stirring, which leads to high quality MXene dispersion (low surface defects).
Additionally, this simple separation of MXene nanosheets by intercalated spices, assisted to
acquire a stable MXene dispersion in aqueous solutions.
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Figure 3. (a) Schematic illustration of cation (Li+, Na+, K+, Cs+, and Mg2+) distributions in
MXene interlayers and their intercalations with water (dark grey stands for C, light blue for Ti,
red for O, light pink for H. Note: data have been obtained by ab initio molecular dynamics
simulations). The cation-oxygen in water (cation-Ow) distances corresponding to the positions of
the maxima of radial distribution functions of Ow, demonstrate a clear correlation with values in
bulk solutions. Reproduced with permission from data published in ref.,53 Copyright 2020,The
Royal Society of Chemistry. b) XRD patterns (i), structural illustration (ii), XPS survey (iii), and
high-resolution spectra (iv) for the Ti 2p and C 1s regions of the Ti3AlC2 samples before and
after reaction in aqueous TMAOH. The schematic is showing an expansion of the basal spacing
from the origin of 0.92 nm to 1.50 nm. Reproduced with permission from data published in
ref.,55 Copyright 2016, John Wiley and Sons. (c) Molecular dynamic simulations of Ti3C2 with
OH termination intercalated with N2H4: (i) Variation in the c-lattice parameter as a function of
the number of N2H4 intercalated molecules. (ii) Molecular dynamic scheme of N2H4 intercalated
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Ti3C2 for N/C ratio 0.375 (6 hydrazine per a 4×2×1 Ti3C2 supercell) displaying a nearly complete
mono-layer of N2H4. (iii) Comparison of the XRD patterns of the Ti3C2 with OH termination
intercalated with N2H4 (black color stands for simulated and blue color for experimental results).
Reproduced with permission from data published in ref.,54 Copyright 2013, Nature Publishing
Group.
2.2 Functionalization of MXenes to improve their chemical stability
The stability of MXene colloidal ink is an equally important parameter in formulating stable
aqueous ink and fabricating long-lasting devices. In an examined work by Zhang et al.,50 the
degradation mechanism of delaminated Ti3C2Tx in an aqueous solution was explored (Figure 2c).
It was observed that the freshly prepared Ti3C2Tx samples in aqueous solution exhibited single
crystalline and having a size distribution of ~ 600 nm. Albeit, the formation of anatase TiO2
particles around 2 nm to 3 nm size along with the disordered carbon peaks were observed in the
same solution after exposure to air for one week. The solution on further aging for 30 days led to
the further dissociation of the MXene. As a result, the characterizations indicative of the low
chemical stability of Ti3C2Tx in an aqueous solution. To improve the stability of ink the air-tight
sealing of solution and storing at low temperature are preferred. However, this strategy for
improving the stability of printed devices for practical application is not possible. Moreover, the
oxidative reaction of MXene sheets is faster in aqueous solvents and humidity environment. For
instance, Ti3C2Tx reacts with water and forms the anatase TiO2 phase as a stable product and TixCy
(OH)z (hydroxyl functionalized MXene) as an intermediate compound (Figure 4a). This oxidation
reaction could proceed in a few days and drastic changes in the electrochemical behavior of MXene
are observed during the progress of the reaction. The reactivity of the Ti3C2Tx under aqueous
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environment can be explained by the bond energies of Ti with C and Tx, the bond energy of Ti-O
is found higher than Ti-F, Ti-I, Ti-N and Ti-Cl, hence the formation of TiO2 from Ti3C2Tx (Tx= -
F, -Cl, -I, -OH,) is spontaneous and can be activated at ambient conditions.58 Zhang et al.,50 studied
the oxidative degradation profile of delaminated Ti3C2Tx in water. The freshly synthesized Ti3C2Tx
MXene after exposure to the ambient environment has shown 42%, 85%, and 100% degradation
from its starting composition after 5, 10, and 15 days, respectively. This degradation caused the
formation of cloudy-white precipitation at the edges of the MXene sheets containing primarily
anatase TiO2 phase. The characterizations clearly demonstrated that they started degradation at the
edges initially, and its kinetics pursues the single exponential decay. Hence, the degradation was
progressed gradually inside the MXene flakes.50 The degradation rate also indicated an inverse
relationship to the size of MXene sheets so that the smaller sizes attained a quick rate of
degradation. The factors like light intensity and temperature were found to enhance the degradation
process of the MXene. These rapid destabilizations make the constriction of MXene-based
components with foreign materials which is hectic and inhibits full exploitation of properties of
MXenes. Therefore, the integration of MXenes with an effective protection layer represents a
promise to overcome this significant drawback. In this context, Chen et al.,56 investigated an
efficient method to enhance oxidative stability of Ti3C2Tx MXene by its chemical surface
functionalization strategy with a super hydrophobic protection layer of 1H,1H,2H,2H-
perfluorooctyltriethoxysilane (FOTS, denoted Ti3C2Tx-F) (Figure 4b). The stability of the surface
modified pristine Ti3C2Tx and Ti3C2Tx-F films using these protective agents under 5 and 100%
relative humidity conditions were characterized by observing the electrical conductance variation
of dispersion, with time (Figure 4c). Both of the samples (i.e., Ti3C2Tx and Ti3C2Tx-F) films
showed a very small loss in electrical conductivity under 5% humidity conditions, indicating the
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retention of stability after surface modification. However, a major variation is seen in 100%
humidity conditions. After two weeks, the pristine Ti3C2Tx film demonstrated a dramatic decrease
to only 2.7% from its original value. On the other hand, the functionalized Ti3C2Tx-F maintained
86.2% of its initial conductance in the same period. Moreover, the Ti3C2Tx-F film showed
relatively stable performance after 30 days, this shows the effective protection of -F ion after
surface modification of MXene layers. In the other research work, Ti3C2Tx MXene was
functionalized with (3-Aminopropyl)triethoxysilane (APTES) to discover its adjustable
hydrophilicity and oxidative stability. The pristine Ti3C2Tx flakes and the flakes modified by
APTES MXene showed the formation of TiO2 after 3 and 21 days of exposure to water,
respectively. However, the pristine MXene showed large defects and formation of maximum TiO2
nanoparticles on MXene basal planes after 15 days and was found to show low conductivity in
comparison with APTES modified MXenes.57 Additionally, the hydrophobicity characteristic of
the functionalized MXenes with APTES, FOTS, and hexadecyltrimethoxysilane were examined
using the contact angle measurement (Figure 4d) and this hydrophilicity was found to be the
protecting agent for MXene layers from oxidation. The contact angles of 136° and 57.9° were
obtained Ti3C2Tx-F and Ti3C2Tx respectively after functionalization with FOTS. Hence, this
typical superhydrophobic chemicals are employing as a robust route of functionalization that
propose high dispersity and stability to MXenes over the long term in an aqueous water solution.
As a result, the functionalized MXenes could be used as a stable ink when printed on a flexible
substrate to form planer interdigitated electrodes which are substantial in energy storage devices.30
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Figure 4. (a) Oxidative reaction of Ti3C2Tx in water solution and humidity conditions. (b)
Schematic, XRD patterns, SEM, and high-angle annular dark-field scanning transmission
electron microscopy (HAADF-STEM) images of the Ti3AlC2 MAX phase, Ti3C2Tx MXene, and
functionalized Ti3C2Tx-F, along with the corresponding element mappings of the functionalized
Ti3C2Tx-F nanosheets displaying a uniform distribution of Ti, C, and F. Reproduced with
permission from data published in ref.,56 Copyright 2020, American Chemical Society. (c) Water
contact angle, water contact angle variations (for 60 days storage in the air), and electrical
conductance changes (for 14 days in the humidity of 5 and 100%) measurements of Ti3C2Tx and
Ti3C2Tx-F. Reproduced with permission from data published in ref.,56 Copyright 2020, American
Chemical Society. (d) Water contact angle measurements of the pristine Ti3C2TxMXene, and
functionalized Ti3C2Tx with APTES, hexadecyltrimethoxysilane, and FOTS. Reproduced with
permission from data published in ref.,57 Copyright 2019, Elsevier.
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2.3 Functionalization of MXenes to develop their colloidal dispersibility
A high-quality screen-printed pattern requires colloidal dispersion stability in its ink formulation.
Therefore, the colloidal stability of MXene suspensions and their environmental stabilities should
be systematically investigated. The issues like clogging and the blockage of nozzle due to low
colloidal stability have been encountered with low quality dispersion, as well. Hence, to make the
MXenes effectively applicable, one substantial aim is to fabricate nanoscale building blocks to a
stable colloidal dispersion. In opposite to the usual pigment inks, the colloidal dispersion stability
procedure of MXene nanosheets for ink formulation is not straight. Here, we demonstrate the
recent strategies for enhancing the colloidal dispersibility of MXene nanosheets in both polar and
non-polar solvents.
It has been perceived that the surfaces of as-synthesized MXene flakes are intrinsically
functionalized with various functionalized groups (i.e., -OH, -O, and -F) which in turn opt for a
negative surface zeta potential at neutral pH. Generally, the stability of any colloidal solution such
as MXenes is found sensitive to pH. Under a neutral pH, the part of -OH groups on the surface of
a MXene get de-protonated to -O- which results in an repulsive interaction between the sheets with
negative zeta potential, and thereby improves the stability of the colloid suspension.58 Thus, a
strategy for the formation of oxygen-rich terminal groups could develop the colloidal dispersion
stability of a MXene in water. In this context, Shi et al.,59 employed a novel iodine (I2) assisted
etching method to synthesize Ti3C2Tx MXene resulted plentiful -O, and -OH terminals and pristine
lattice structure (Figure 5a). The characterization results of this functionalized Ti3C2Tx showed
colloidal stability for two weeks (Figure 5b), which is most probably due to the high density of
oxygen functional groups on top of Ti3C2Tx (Tx = -O and -OH).60, 61 It should be noted that the
study of MXenes colloidal dispersion stability is not limited to only polar solvents. Because polar
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aqueous solutions possess different drawbacks like (i) degradation of MXenes in the long-term
(reaction with air), (ii) difficulty in dewetting on low surface energy substrates, and (iii)
incompatibility with water-insoluble polymers.62 On the other hand, the colloidal stability of
MXene layers in non-polar solvents is poor because of polar terminal groups. Hence, the surface
functionalization of the MXenes with appropriate materials is another strategy to tune their
colloidal dispersion stability in nonpolar solvents. For instance, the precise chemically grafting of
Ti3C2Tx MXene nanosheets with organic lipophilic octyltriethoxysilane(OTS) molecules found to
developed colloidal dispersion stability of the MXene in hexane (C6H14) (Figure 5c).62 The OTS
molecules could react with Ti atoms of Ti3C2Tx, through Ti-O-Si covalent bonding and create
stable charge-transfer complexes that improve the colloidal stability. The grafted MXene with OTS
exhibited a remarkable hydrophobicity in comparison with the pristine MXene, whilst the surface
charges were negligible (Figure 5c). The grafted MXene sample in nonpolar hexane illustrated an
excellent long-term colloidal stability over four weeks. This developed colloidal dispersion
stability in non-polar solvents could be due to the steric effects of the linked octyl groups. Hence,
the surface modification of MXenes and developing their colloidal stability in non-polar dispersion
could be useful in diverse applications where a low surface energy and great volatility solvent is
essential.
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Figure 5. (a) Schematic illustration of the Ti3AlC2 I2-assisted etching and delamination to
obtained MXene nanosheets. (b) The optical and SEM images of the Ti3C2Tx MXenes etched
with hydrofluoric acid (left side, with stability < 1 week in water), and I2-assisted (right side,
with stability 2 weeks in water). Reproduced with permission from data published in ref.,59
Copyright 2021, John Wiley and Sons. (c) Grafted Ti3C2Tx MXene with OTS in 2 mg mL-1
hexane dispersion deposited on the water surface. TEM image and EDS mapping for the
elements of grafted Ti3C2Tx MXene with OTS. Four-week colloidal dispersion stability of the
grafted Ti3C2Tx MXene with OTS dispersed in 2 mg mL-1 hexane. The water contact angle
droplet dispensed on the macroscopic grafted Ti3C2Tx MXene with OTS surface (measured every
~ 10 minutes for 1 hour). Reproduced with permission from data published in ref.,62 Copyright
2019, Elsevier.
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3 Formulation of MXene sediment inks
The wafer-scale micro-electronics on soft and flexible substrates demand for the screen printable
ink, that must produce patterns with high-quality resolution (~ 150 μm), also with an ultra-speed
process (few centimeters per second). To prepare a high-quality MXene ink, the significant focus
should be on (i) ink composition, (ii) ink rheology, and (iii) dying and particle assembly
mechanisms. This section is typically limited to consider such parameters to achieve a high-quality
screen-printed pattern.
3.1 Screen printable MXene ink configuration
The configuration of screen printable inks commonly involves the active component (pigment),
additives(surfactants) to improve the surface wetting character,26 binder(polymers) to improve the
adhesion property of the print,63 and the solvents to keep the ink in a fluidic condition. After
synthesis and delamination of MXenes, their colloidal solution can be formulated into a functional
ink by the addition of appropriate amounts of these supplemental materials. The critical
components of a screen printable ink required 12-20 wt% pigment, 45-65 wt% binders, 20-30 wt%
solvent, and 1-5 wt% additives. Whilst at the same time, the viscosity of ink must be between
1000-10,000 mPa s.3, 64 Therefore, in order to formulate the ink desirable to screen-printing one
should not only focus on the composition of materials, the performance of each individual printed
component and tuning of ink in achieving the high performance must also be considered. For
instance MXene sheets with high concentrations found to form agglomeration and eventually
result in clogging of the nozzle in printers,58 and at low MXene concentration the printing patter
performance get reduced due low resolution. Likewise, large quantity of additives will reduce the
resolution as well as the removal of these additives on flexible substrates after printing postulates
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additional complicated steps.64 More importantly, MXene nanosheets (i.e., Ti3C2Tx) usually suffer
from agglomeration and rapid sedimentation in low boiling point solvents. These characteristics
restrict the formulation of MXene ink with high concentrations and utilization in low boiling point
solvents.63 On the other hand, MXenes with hydrophilic characteristics and surface termination
groups also possess poor adhesion on polymer-based soft substrates.28 Thus, improving the ink
adhesion properties by adding suitable binders and retaining the MXene nanosheets colloidal
stability in ink composition are certainly major challenges.3
The post-printing process of screen-printing, involves spreading, drying, and particle assembly is
important for getting uniform deposition and reproducible results. For instance, MXene nanosheets
in fast volatile solvents like isopropyl alcohol (C3H8O, IPA) that generally select for high-speed
printing technology possess low colloidal stability. Therefore, utilization of such rapid volatile
solvents may cause clogging of the printing nozzle. On the other side, the poor binding properties
of MXene nanosheets on polymer substrates may display the coffee ring effect (CRE). The
complications have raised for the solvent selection, for example, at one side the organic solvents
are good for dissolving the binders, additive; retain the oxidative stability of MXenes and can
evaporate easily after post treatments; and also, can interact efficiently with polymeric flexible
substrates. At the same time the high processing cost, low MXene colloidal stability and the
generation CRE effect due to fast evaporation rate are the drawbacks of using these organic
solvents. In this regard, the aqueous MXene inks are found to possess some advantages like (i)
high dispersibility, (ii) slow evaporation rate to mitigate the CRE, and (iii) low formulation cost.
Hence, the selection of solvents plays important role in getting the high-quality electrode patterns.
In some cases, the direct application of MXene in water medium as printable ink is noticed. It has
been investigated that the MXene sediments in aqueous medium demonstrate excellent intrinsic
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viscosity without additives.26, 28 The interaction of this MXene sediments with substrates surface
is monitored by dropping a liquid droplet of the MXene dispersion on the glass substrate and
positioning the substrate at an incline angle of 30° for 30 minutes. The MXene sediments exhibited
the stability for 30 minutes, this indicates the self-reliance of MXene nanosheets for ink
formulation without additives.35 However, the high surface tension and poor adhesion of aqueous
MXene dispersion may effects the spreading, drying and wetting behavior with hydrophobic
substrate surfaces. Accordingly, the development of high-quality aqueous-based sediment ink for
the fabrication of micro-components on polymer substrates has been set as a challenge. Moreover,
the high oxidation rates of MXene in aqueous solvents have found another obstacle in the
fabrication of reliable and reproducible electronic properties in aqueous solvents. Thus, the surface
modification of MXene nanosheets with organic and inorganic components is one of the suitable
approaches for tuning the liquid properties of MXene ink and controlling ink rheology and
oxidative stability.
3.2 The rheology and surface interaction of screen printable ink
The rheological properties of screen printable ink can be explained by the capability of stable ink
droplets formation and its interfacial interaction mechanism. The ink droplet formation during
printing with a given nozzle diameter can be influenced by its viscosity. Viscosity (Pa s) is the
ratio of shear stress (Pa) and shear strain rate (s) of liquids. The fluidity of liquid can be explained
by shear stress and shear rate graph, the viscosity change in the graph helpful for understanding
the liquid behavior under applied stress (Figure 6a).3 An ideal ink must show shear thinning
behavior, with low viscosity to generate stable ink droplets under applied stress. Intermolecular
interaction and relative fluid nature of active 2DNMs in the dispersion can be observed by the
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elastic moduli (G’) and viscous moduli (G”) of its dispersion as a function of frequency at constant
strain amplitude. The G’ provides information about the rigidity of 2DNMs in dispersion which
eventually direct to the nature of the ink. For instance, a relatively high G’ value and constant
elasticity modulus under the applied strain frequencies suggest that the liquid has gel-like nature.
This type of paste- or solid ink causes low-speed printing and clogging.64 On the other side, the
G” decides to relax and dissipated the nature of the ink under the applied stress. Thus, ink with a
high G” value shows a liquid-like nature which may lead to bleeding of ink from the nozzle (Figure
6b).48 Hence, the ratio of elastic modulus to viscous modulus (G’/G”) can be utilized for
determining the rheological properties of the formulated ink. In this case, the ink with G’/G” ratio
of less than one (<1) indicates the domination of G”. This condition shows high intermolecular
interaction and ink tends to be liquid-like nature with an easily spreadable that could be useful for
spray coating. However, the ink with G’/G” ratio of greater than one (>1) displays the dominate
of the G’ with plastic-like nature so that such inks are suitable for extrusion printing requires.48
Hence, the formulation of ink with optimal G’ and G” values are essential to have high-quality
printing. In this context, the screen-printing technique requires moderate viscous ink with a high
solidification rate and an easily movable trough nozzle to avoid the coffee ring and clogging effect,
respectively. The solvent and binder’s composition in formulation generally allows the ink to tune
its viscosity and elasticity. However, the high amounts of binders in ink formulation reduce the
concentration of active components and may directly influence the quality of print (reduction
resolution as we discussed earlier sections). Moreover, the deposition of formulated ink with
binders and additives on soft substrates requires complicated post-printing steps for their removal.
As discussed earlier, the Z value is the function of viscosity, surface tension, and elasticity, the
density, and the ideal screen printable ink must have an optimal Z value (between 1-14) to produces
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stable ink droplets. The surface tension of liquid decides the capillary action of the ink on
substrates. To achieve reasonable spreading and drying behavior of ink after printing on substrates,
surface tension of screen printable ink must be in the range of 7-10 mN m-1. An ink with high
surface tension shows the low capillary action and is found unable to penetrate from the ultra-
small-sized nozzles. Moreover, it exhibits slow-spreading behavior and leads to coffee ring effect.
Thus, the printable ink must have lower surface tension values than the substrate surface energy
the rate of ink spreading can be enhanced.64 To do so, the surfactants [i.e., Triton X-100, t-Oct-
C6H4-(OCH2CH2)xOH, x = 9-10] could be added to alter the surface tension during the formulation
of ink.65
Figure 6. (a) Graphical represents of diverse fluids is showing the shear stress versus shear rate.
Reproduced with permission from data published in ref.,64 Copyright 2018, The Royal Society of
Chemistry. (b) Schematics and plots represent the G’ and G’’ for single-layer MXene with
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different concentrations (0.90 mg mL-1, 2.70 mg mL-1, and 3.60 mg mL-1) on constant strain
amplitude at different frequencies. Reproduced with permission from data published in ref.,48
Copyright 2018, American Chemical Society. (c) Optical photos of 2DNMs (2D MnO2) ink
droplet formation and deposition at different times of printing. Reproduced with permission from
data published in ref.,66 Copyright 2018, Elsevier. (d)Schematic representation of contact angel
of ink with substrates according to Young’s equation. (e) Schematic representation of Marangoni
effect of solute. (f) Inverted optical micrographs of dried inkjet printed droplets on clean glass
for formulated inks using solvent exchange in IPA, IPA-ethanol, IPA-t-butanol, IPA-2-butanol,
IPA-2-butanol (20 v%), also a schematic demonstration of the drying process for indicating
coffee ring effect formation. Panels of e and f reproduced with permission from data published in
ref.,67 Copyright 2020, American Association for the Advancement of Science.
The post-printing process which involves the deposition of droplets, spreading, drying and
assembly of active components also determine the quality and morphology of the printed patterns
(Figure 6c). The concentration (density) of active components in the ink droplets is the deciding
factor for the thickness and quality of the printed pattern (active component per unit area). The
contact angle of ink droplets (tuned by altering surface tension by addition of surfactant) with the
interacted surfaces is the measurement factor for the ink spreading behavior (Figure 6d). It is noted
that the perfect spreading of ink can be seen below the contact angle of 90° and the non-wetting
behavior of ink can be seen above the contact angle of 90°. Drying involves the evaporation of the
solvent. The solvents with a low boiling point can lead to a faster drying rate so that it does not
give enough time for the ink to flow the spreading mechanism involves Marangoni flow,
nucleation, and self-assembly of solute.67 On the other hand, the high boiling point solvent may
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take a longer time for drying and may lead to the settling of particles within the droplet. Hence,
solvents with moderate boiling are generally preferred for ink formulation. The solvents and their
mixture are utilized to reduce the coffee ring effect during the drying process.67 Therefore, as it
can be observed in Figure 6f, the selection solvents/mixture and their concentrations play
important role in reducing the CRE. The solvents with lower surface tension change the drying
mechanism of ink by deriving the mechanism to Marangoni-enhanced spreading in the ink
formulation.
3.3 Additive-free MXene inks
A large quantity of additive portion in an ink formulation not only could reduce print resolution,
but it also complicates the post-printing removal process which is an expensive approach.
Recently, the additive-free inks are found to possess advantages for high-quality and affordable
device fabrications. MXenes with good rheological and mechanical properties could show a
remarkable ink in an aqueous solution without the addition of external additives to control their
rheological properties. For instance, Ti3C2Tx flakes have illustrated a high dispersibility in water
where the zeta potential of these flakes is ~ - 40 mV.28 The clay-like behavior of Ti3C2Tx flakes
also indicates the dilatant’s nature without any additives. As a result, combinations of these
properties of MXenes have made them easy in formulating additive-free ink. The previous studies
have demonstrated the possibilities of direct utilization of MXene dispersion in water and other
aqueous solvents in various applications using spraycoating,5, 68, 69 spin coating,70-72 and extrusion-
based 3D printing,73-76 as well as in screen-printing.28 However, the fabrication of flexible micro-
electronic devices requires a most reliable and high-quality ink that could be printable on soft
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substrates. Moreover, the ink rheology must be suitable for inkjet printing which offers ultrafast
printing with low materials wastage.
It has been found that the rheological properties of bare MXenes dispersed in an aqueous solution
could be tuned by its adopting proper exfoliation methodology, tuning the particle size, and
varying the concentrations.48 For instance, a single-layer dispersion of MXene with low
concentration shows a noticeable G’/G” ratio which is suitable for spray coating and spin coating
(Figure 7b). Whilst, the sample at moderate concentration range exhibits soft solid-like rheology
and a power-law scaling at low and high applied frequencies, respectively. This indicates that the
single-layer MXenes at moderate concentrations are suitable for ink-jet printing. However, the
post-printing process requires regaining the viscosity and providing rigidity against gravity. In
addition, the moderate concentration of MXenes in an aqueous medium is failed to provide such
rigidity.48 On the other hand, these single layers of MXenes at higher concentrations exhibiting a
gel-like nature that this structure found relatively weak and fails to stand with gravitational force.
The rheology of multi-layer MXenes have illustrated similar behaviors like the single-layers
dispersion at lower concentrations. However, the stacking of layers reduces the zeta potential
values and this causes lower dispersion stability. At a moderate concentration of multi-layer
MXenes, the G’/G” value was found to be high (gel like nature) (Figure 7c).48 The multi-layer
MXenes at higher concentrations show high G’/G” values at lower frequencies that lead to the
relaxation of suspension and may be clogging effect.28, 48, 77 In overall, these parameters indicates
that the MXene found self-resilience to print directly (i.e., without additives) using screen-printing.
The surface tension of ink as a crucial parameter for the post-printing process must be lower (7-10
mN m-1) than the surface energy of the substrates (Note: MXene dispersion with a noticeable
concentration of 12.5 mg mL-1 in the organic solvents has high surface tension values).26 However,
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each substrate has a specific surface energy and might be higher than the expected value (i.e., glass
and polyethylene terephthalate have a surface energy value of ~ 36 and ~ 48 mN m-1,
respectively).26 Hence, the substrate selection and tuning of the ink characteristics according to the
desired substrate surface energy are the key steps for achieving high performance patterns.
Interestingly, the surface modification of the flexible substrates is required to be considered to
improve their surface energy and make them suitable for the additive-free MXene inks as found
as another way of addressing this issue. In this context, inorganic AlOx coating on polyethylene
terephthalate surface has been found to increase the surface tension value of the substrate to ~ 66
mN m-1 (Figure 7d) and led to decent coating with additive free ink.26 Albeit, the substrate wetting
issues like CRE still have not been resolved that certainly need to be considered. Likewise, tuning
the surface tension by selecting the polar solvent with low boiling points that is also been proposed.
For instance, a mixture of organic N-Methyl-2-pyrrolidone and ethanol has been employed in
Ti3C2Tx MXene ink formulation to optimize the surface tension and improve the printing
resolution. Another way of reducing the surface tension is by changing the particle size. The
surface tension of the ink was found to be decreased to 68.8 mN m-1 for MXene with a particle
size of 350 nm from the initial 76.5 mN m-1 with a particle size of 2 μm.28 Nevertheless, this change
in surface area is still found higher than the surface energy of the most of the flexible substrates
substrate.28
The aqueous MXene inks have displayed a shear thinning behavior for a wide range of
concentrations (1.5-150 mg mL-1) in between 0.01 and 103 s-1. However, this range of shear value
is not suitable for inkjet printing where it should be 106 s-1. Therefore, viscosity which is another
tunable parameter could help to develop the shear value of additive-free MXene inks, and this
could be happened by changing the particle size of MXenes. In a research work, the reduced flake
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size of Ti3C2Tx MXene from 2 µm to ~ 300 nm has indicated a change in viscosity from 5.1 CP to
1.5 CP. This is a benignant method where such low viscosity of ink is preferred for inkjet
printing.28 The rheological properties are still unsatisfactory for large-scale printing of high
performance electronic components using inkjet using this additive free method. Therefore, either
the compromise in the ink resolution with the MXene ink, the usage of a additives, or with the
substrate surface modification is been still followed. Hence, exploration of alternative solutions
for ink formulation strategies is required to be discovered.26, 28
Figure 7. (a) Schematic comparison of the required layer thickness and printing resolution for
different printing methods. Reproduced with permission from data published in refs.,1 Copyright
2020, John Wiley and Sons. (b) and (c) Dependency of frequency to the G’/G” for single-layer
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and multi-layer Ti3C2Tx nanosheets dispersed in water (the bolded stars on the plot display the
estimated processing parameters utilized in supercapacitor (yellow and purple), thin-film (green),
and surface-enhanced Raman spectroscopy (light yellow). Reproduced with permission from
data published in refs.,48 Copyright 2018, American Chemical Society. (d) Optical images are
displaying MXene patterns on AlOx-PET substrate. AFM image displays the inkjet-printed line
with a homogenous surface including interconnected nanosheets. The plots illustration the height
versus distance and thickness versus the number of the printed paths. Reproduced with
permission from data published in refs.,26 Copyright 2019, Nature Publishing Group.
3.4 Advents in MXene ink formulations
Inkjet printing have been reported to have good compatibility with various flexible substrates at
low operating temperatures.78, 79 It also can print a pattern in a large-scale area with low material
wastage and high resolution. Moreover, robustness in controlling the material deposition offers
micron-size patterns with high precision and the jetting ink formulation is the key step for the full
exploitation of this technology. As discussed in above section, the low viscosity of water-based
MXene inks can hamper the stability of the jetting process leading to the formation of satellite
droplets and jetting deviation. Furthermore, the high surface tension of water-based MXene ink
will not form the sufficient droplets that can be generated by jetting nozzles less than 50 μm.5, 26,
33As a result, the MXene nanosheets in such droplets tend to be accumulated at the edges during
deposition on the substrates which causes CRE. Thus, the addition of binder content in water-
based ink formulation can help to increase the viscosity of the ink and facilitating particular
interactions with the substrate by reducing the surface tension. For instance, incorporation of
synthetic protein molecules (polypeptides) as binder found to be assembled with MXene
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nanosheets and facilitate to control of the aggregation of the matrix (Figure 8a).20 Besides, the
generated high viscosity allows the uniform distribution of MXene nanosheets in ink droplets
which eventually reduces the CRE (Figure 8b). These MXene/protein inks were successfully
employed for printing on various flexible substrates such as cellulose paper, polyethylene
terephthalate, and polymethylmethacrylate. Tandem repeat units of these protein molecules consist
of hydrophobic and polar amino acid functional groups which can significantly reduce the surface
tension and develop the viscosity of the ink. The tandem repeat protein with molecular weights of
40.5 kDa,80 and concentration of 0.95 mg mL-1,20 found to change the surface tension, viscosity,
and Z values to 40 mN m-1 (initial value of 51.5 mN m-1), 3.4 cP (initial value of 5.1 cP), and 21.3
(initial value of 27.5), respectively.20 Despite the other chemical binders, it has been found that the
protein functional groups on MXene nanosheets could form strong hydrogen bonding. Although,
there are several explorations about the interaction of protein and 2DNMs,81 the assembly
dynamics and interaction of MXenes with proteins are still unexamined.20 Also, the MXene ink
with protein (2.25 mg mL-1) functionalization has shown great stability of ~ 6 months, whereas the
bare MXene has illustrated stability of 2 months in DMSO. It should be noted that an increase in
the composition of non-conductive protein (binders) ink formulations causes some detriments like
(i) reducing the conductivity of MXene, and (ii) complicated degradation methodology after
printing patterns on soft substrates for some specific applications like MBs and MSCs.20 In this
context, Yu et al.,82 examined the crumpled N-doped MXene and employed it as aqueous ink for
screen-printing and extrusion 3D printing with effective formulations strategy (Figure 8c). To do
so, two different types of inks with and without binders are formulated for screen-printing and
extrusion 3D printing, respectively.82 The formulated MXene ink for screen-printing exhibited a
significant shear-thinning non-Newtonian fluid behavior, implying the viscosity decreasing with
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increasing shear rates. It was interesting that the incorporation of conductive carbon to the aqueous
MXene ink not only improved the rheological properties but also developed the conductivity of
MXene layers after printing (Figure 8d).82 This is whilst that the binder-free ink sample with
suitable rheological properties for inkjet printing is more advanced for large-scale production of
micro-device components. Similarly, a partially oxidized MXene solution (i.e., with TiO2),83 found
to offer good rheological properties in water than the pristine MXene nanosheets. The only
drawback of the oxidized MXene sample was an enhancement in sheet electrical resistance.
Figure 8. (a) Schematic illustration of Ti3C2Tx MXene, the tandem proteins with repetitive amino
acid units and the fluidic nature of corresponding ink (n=11 Tr42), schematic of protein-
mediated MXene assembly within inkjet printing, and photos of pristine and protein-based
Ti3C2Tx/DMSO dispersions with a diverse concentration of protein. (b) Optical photos of printed
samples on cellulose paper by diverse ink solutions during inkjet printing, and high-resolution
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images of inkjet-printed of (i) pristine and (ii) protein-based MXenes on cellulose paper
substrates. Panels of (a) and (b) are reproduced with permission from data published in ref.,20
Copyright 2018, John Wiley and Sons. (c) Schematic representation of synthesis crumpled and
direct ink printing of MXene-N ink by screen and extrusion printing. (d) Rheological behaviors
of the activated carbon/CNT/MXene-N/GO and activated carbon/CNT/GO inks. Panels of (c)
and (d) are reproduced with permission from data published in ref.,82 Copyright 2019, John
Wiley and Sons.
4 MXenes sediment inks for screen-printed micro-supercapacitors
In-plane MSCs as an advantage for the fabrication of flexible micro-devices, it can be utilized in
artificial intelligence and the internet of things.2, 84 MSCs could storage energy by creating an
electrical double layer in few nanometers on the surface of electrodes, which are either separated
by a solid insulator or electrolyte solution. Fabrication of in-plane MSCs requires the printing of
these electrodes on flexible substrates and sandwiching electrodes with an insulator/electrolyte.4
However, the viability of such printed MSCs depends on the performance; the performance of
device depends on the electrode materials selection. In this context, MXene nanosheets have been
found as efficient materials for the fabrication of conductive flexible electrodes.5, 85 MXene-based
MSCs are found to possess a capacity of high energy density with a long lifetime, and great
mechanical flexibility (Table 2). However, the processing of MXene nanosheets and printing them
on soft substrates is the major issue.5 The methodologies like laser scribing, spray masking, and
lithography is found inefficient in controlling the thickness of 2D nanolayers without damaging
the surface of soft substrates. Moreover, these methodologies are found not capable of production
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of MXene based MSCs on a large scale. In this context, the fabrication of highly efficient MSCs
on flexible substrates using low-cost routes is in demand and has advantages.
Table 2. Represents the screen printed MXene and modified MXene-based MSCs and their
performances
MXene type
Electrolyte
Areal (CA) or volumetric
capacity (CV) or Capacitance (C)
Stability
Ref.
Ascorbic
acid
modifed-
Ti3C2Tx
H2SO4/PVA
gel
electrolyte
CA= 108.1 mF cm-2 and CV =
720.7 F cm-3
Retention of 94.7%
specific capacitance
after 4000 cycles
30
Carbon
nanotube-
Ti3C2Tx
H2SO4/PVA
gel
electrolyte
CA= 317 mF cm-2
Retention of 32.8%
(at scan rate of 100 V
s-1)
86
RuO2.xH2O-
AgNW
incorporated
Ti3C2Tx
PVA/KOH
electrolyte
CV = 864.2 F cm-3
90% retention after
10000 cycles.
87
N-doped
Ti3C2Tx
H2SO4/PVA
gel
electrolyte
CA= 70.1 mF cm-2
92 % capacitance
retention after 7000
cycles
82
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Phenylsulfon
ic acid
functionalize
d Ti3C2
KOH
C = 160 F g-1
91 % capacitance
retention after 10000
cycles
82
Mn ion-
intercalated
Ti3C2Tx
H2SO4/PVA
gel
electrolyte
CA= 87 mF cm-2
82 % retention after
5000 cycles
88
Direct
printing of
Ti3C2Tx (on
textile
substrate)
H2SO4/PVA
gel
electrolyte
CA = 294 mF cm-2
98 % retention after
10000 Cycles
28
Ti3C2Tx
H3PO4/PVA
gel
electrolyte
CA = 887.5 μF cm-2
85% Retention of
capacitance after
10000 cycles
89
Ti3C2Tx
H2SO4/PVA
gel
electrolyte
CA = 158 mF cm-2
95.8% of Retention
of capacitance after
17000 cycles
35
Ti3C2Tx
KOH/PVA
CA= 28.5 mF cm-2
92% retention
capacitance after
10000 cycles
90
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Ti3C2Tx
H2SO4/PVA
gel
electrolyte
CA = 25 mF cm-2
92% retention of
Capacitance after
10,000 cycles
90
Ti3C2Tx
H2SO4/PVA
gel
electrolyte
CA = 43 mF cm-2, and CV = 562
F cm-3
-
26
In pursuit of alternate efficient methodologies, screen-printing,90 spin coating,91 and spray
coating92 of 2MXene as electrode patterns have found effective in the production of large-scale
flexible thin-film MSCs. However, the thickness of patterns produced and the wastage of materials
are considerably high and uncontrollable in the case of spin and spray coating methodologies. On
the other hand, the screen-printing was found much more efficient in controlling the thickness with
relatively low material wastage. Patterning of MXene electrodes on flexible substrates with high
efficacy is the key issue for practical application of MXene-based MSCs. More importantly, the
MSCs with high areal capacitance requires high-quality screen printable ink, development of
printing strategies, and scaling the production into industrially viable. Ti3C2Tx is the most studied
MXene that known to have high conductivity and demonstrated as an active area for flexible
MSCs. However, (i) its low areal capacitance, (ii) high production cost, and (iii) low stability of
this 2DNM under natural environmental conditions are the major issues for flexible MSCs.
Moreover, the MXene-based MSCs are printed on flexible substrates with additive inks leaving
the residual surfactant and binders on the substrate surface, which need to be removed after printing
using high-temperature annealing or by chemical treatment. Either of the above-mentioned
methodologies is not compatible with most soft substrates and may oxidize the MXene nanosheets
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in air. In this regard, it is believed that the production of industrially viable flexible MSCs relies
on the direct printing of MXene with high active component per unit area that could improve
interfacial interaction of MXene nanolayer and its composites. This direct printing strategy
(additive and binder free) has been found as an effective solution for improving the areal
capacitance. In a research work, direct printing of Ti3C2Tx MXene dispersed in N-Methyl-2-
pyrrolidone (NMP, C5H9NO) solvent on AlOx-PET substrates was investigated for the purpose of
their using in MSCs.26 The line thicknesses during printing were carefully adjusted by the number
of coatings and for instance the number of coatings (<N>) = 10 and 100, the thickness was resulted
as 100 ± 21.5 and 530 ± 120 nm, respectively. An increase in the number of coating layers from 1
to 5 demonstrated an exponential reduction in sheet resistance (Rs) value. This phenomenon
indicates effective charge transportation with the increase in conductive layers. In addition, the
areal capacitance of the MSC enhanced from 3.5 to 43 mF cm-2, with the number of coatings.
However, this areal capacitance was not enough for the practical applications. Moreover, in
symmetric MSCs made using by direct screen-printing method of MXenes suffer from the
generation of limited voltage between just 0 - 0.5 V.26 Hence, the expansion of operation potential
and the high charge storage capacity are necessary to be developed. Fabrication of asymmetric
MSCs is found to be an alternative solution to expand the voltage and the improved charge storage
capacity, it can be achieved by improving the interfacial interactions between MXene layers and
concentrating the ink with active components. In this context, the utilization of polymer
electrolytes and liquid metals or their gel solutions in ink formulations found to solve the problem
partially. The creation interfacial junctions and expansion of the operational voltage is achieved
by the addition of these materials. Albeit, the compatibility of these electrolytes in planner MSCs
sets other challenges like less life time and complicated fabrication strategies. To address this,
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Huang et al.,92 reported, MXene based MSCs all solid-state components as an alternative approach
to improve the life cycles with gel electrolytes. In this work flexible patterns on paper is designed
and a solid polymer electrolyte is used in fabrication of MSCs.92 Typical fabrication involves the
preparation of electrodes by spray coating of dispersed Ti3C2Tx on paper, followed by UV laser
cutting into interdigitated structures. The PVA/H2SO4 gel electrolyte was then drop-casted on top
of these electrodes and the MSCs were sealed after electrolyte casting by poly(dimethylsiloxane)
(PDMS, (C2H6OSi)n) glue. Thus, fabricated MSCs showed high areal capacitance of 23.4 mF cm-
2 with an excellent cycling stability of 92.4% capacitance retention after 5000 cycles. The
maximum volumetric energy densities, and a power density is observed (189.9 mW cm-3, attained
to 1.48 mWh cm-3) (Figure 9a).92 Though the life cycles are increased by utilization of polymer
electrolyte, the capacitance is not reached up to the mark.
Recently, despite delamination of MXene layers, the improvement of charge storage capacity is
also observed with intercalation. In this regard, the construction of intercalated layers with a
cation/polymer interface is demonstrated as a favorable methodology for improving the charge
storage capacity of MSCs. For instance, intercalation of metal ions like Li+, Na+, K+, NH4+, Cs+,
TEA+, Mg2+, Ca2+, Al3+ in MXenes has been found to be spontaneous in a reversible way. These
metal ions after intercalation can improve the energy storage capacity of MSCs by occupying
electrochemically active sites on the MXene nanosheets and participate in energy storage. In this
context, a flexible Ti3C2Tx film with high volumetric capacitance has been fabricated and
displayed a volumetric capacitance of more than the graphene-based electrodes with the
intercalation of H+ (Figure 9b).93 The simple cation H+ from 1M H2SO4 in combination with
Ti3C2Tx has shown an improved volumetric capacitance 900 F cm-3 from bare Ti3C2Tx (300 - 400
F cm-3). This enhancement in charge storage mechanism in sulfuric acid intercalated Ti3C2Tx was
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found to be due to a change in the oxidation state of the Ti atom during the charging and
discharging cycles in presence of H+. It was found that the change is purely pseudocapacitive and
has not diffusion-limited for at least scan rates of 20 mV s-1.93 The rapid protonation and
deprotonation process of the terminal oxygen atom of MXene layers with H+ ions was found to
boost the pseudocapacitance in MXene based MSCs after H+ intercalation. The smaller effect is
also shown by some other ions like Li+, NH4+ and Mg2+.93 The improved charge storage capacity
and expansion of voltage have been demonstrated with this type of cation/polymer electrolytes
interface with the printed electrode patterns.
Figure 9. (a) Schematic of the fabrication process of flexible all-solid-state MSC with MXene via
spray printing, and its measured areal capacitance, energy density, and capacitance retention.
Reproduced with permission from data published in ref.,92 Copyright 2019, Elsevier. (b)
Schematic illustration of the fabrication of MXene-based (Ti3C2Tx) flexible solid-state
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supercapacitor, and its volumetric capacitance comparison with the other diverse thin-film
system, as well as its measured areal capacitance compared with other various flexible
supercapacitors. Reproduced with permission from data published in ref.,93 Copyright 2017, John
Wiley and Sons.
Surface modification of the MXene element in its sediment ink is another approach for improving
the capacitance of MSCs. For instance, doping of N-atom in the MXene lattice has been found to
improve the electrochemical performance by increasing the conductivity and redox reactivity.82
Fabrication of N-dopped MXene MSC involves the printing of electrodes with N-MXene ink using
the screen-printing and the application of polyvinyl alcohol/H2SO4 as electrolyte is been
demonstrated. Thus, fabricated quasi-solid-state has showed an improve areal capacitance of ~ 70-
62 mF cm-2 under the applied potentials 10-100 mV S-1, from the undoped MXene sample with ~
25-20 mF cm-2 (Figure 10a).82 It is also believed that the concentration of the printing medium can
also improve the areal capacitance by increasing the resolution. To increase the concentration, the
direct/additive-free printing of MXene over soft substrates are preferred. For instance, the direct
printing of clay-like Ti3C2 as electrode materials for MSCs by stamping the patterns was reported
to have a concentration of 30 mg mL-1.27 The electrical resistance of such printed Ti3C2 patterns
with 1 mm width was found to be varied linearly across the lines. The slope resulted from the
graph is 0.16 kΩ cm-1 which is found to be higher than the conventional patterns of Ag and Cu and
lower than the carbon inks (~ 2.26 kΩ cm-1) (Figure 10b). MSCs fabricated using this ink without
any additional current collectors and in presence of PVA/H2SO4 gel electrolyte have showed much
higher areal capacitance (5 mF cm-1) than the carbon-based MSCs. These results show the potential
viability of additive-free/concentrated MXenes ink for high-quality MSC applications.27 In a
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research effort, direct printing of MXene sediments on flexible substrates with high concentrations
has been reported.35 The screen-printed MSCs using these sediments of MXene and without
additional current collectors and binders were used; and the PVA/H2SO4 gel electrolyte employed
for charge mobility. Hence, fabricated MSC exhibited a pseudocapacitive behavior when its
performance is tested. The normalized cyclic voltammograms (CVs) and galvanostatic analysis
showed the asymmetric and linear galvanostatic charge discharge (GCD) curve for this assembly.
Typical printed MSC performance varied with the number of depositions of MXene ink, at five
consecutive depositions of <N> = 5 the areal capacitances was found to maximum (Figure 10c)
(158 mF cm−2 at 0.08 mA cm-2, and 127 mF cm-2 at 2.4 mA cm-2 ).35
Though the additive free MXene based MSCs are showing the practically viable performance, the
stability and lower life cycles have found to hinder the development of MSCs on a large scale. To
overcome this limitation chemical fictionalization and utilization of non-aqueous solvents have
been reported as efficient strategies. For instance, surface modification of Ti3C2Tx nanosheets with
sodium ascorbate (SA) in printable ink found to remarkably enhance the oxidation resistance.30 In
another study the organic molecules such as glucose, glutathione, oxalic acid, sodium borohydride,
and hydrazine have found to similar affect as of SA. Thus, the modified SA-MXene ink was
utilized to fabricate active electrodes for in-plane MSCs using inkjet printing on paper and
integrated with PVA/H2SO4 electrolyte. The double-layer formed at electrode/electrolyte interface
has shown the volumetric capacitance of 720.7 F cm-3 with improved stability in its fabricated
MSC.30 However, The improvement of stability and the performance of these MXene based MSCs
prepared using the additive and additive free are not enough for the scalable production and
integration with other components that could be equipped in wearable devices such as displays,
sensors, solar cell. Hence, the challenges of these MXene based MSCs in fabrication of wearable
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devices should be considered. Moreover, the selection materials, ink preparation and deposition
process are found as important in realizing the long-lasting high performance wearable devices.
Figure 10. (a) The areal capacitance of the N-doped MXene ink MSC in comparison with the
other samples studied in different works under various scan rates. Reproduced with permission
from data published in ref.,82 Copyright 2019, John Wiley and Sons. (b) Slope line of resistance
dependence on the length of printed MXene ink with additive-free in water, and its electrical
resistivity comparison with silver, copper, and graphite/carbon nanotubes inks. Reproduced with
permission from data published in ref.,27 Copyright 2018, John Wiley and Sons. (c) Diverse
screen-printed of additive-free MXene inks for MSC, along with their line thickness, sheet
resistance, and plots as a function of the number of printed, and areal capacitance of MSCs with
a various number of the printed passes. Reproduced with permission from data published in
ref.,35 Copyright 2020, John Wiley and Sons.
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5 Future scope
Reproducibility, scalability, and compatibility are important issues for the printable MXene for
MSCs and integrating them with flexible and stretchable devices as charge storage components.
In this context, the printing and integration of high electrical performance MSCs with other active
components in device engineering are the key approaches for the development of miniaturized
devices. For instance, an interesting approach for development of stretchable and flexible self-
charging power device of MXene-based MSCs has been reported and it is successfully integrated
in triboelectric nanogenerators (Figure 11a).94-98 A triboelectric nanogenerator (TENG) is found
an efficient technology in energy harvesting by converting ambient mechanical energy (i.e., hand
tapping motion) to electrical energy for its straight usage, or accumulation by an electrochemical
storage device (Figure 11b).97 Generally, micro-batteries are used as energy storage units in
combination with the TENGs. However, low power density, limited life cycle and their non-
compatibility with TENGs are the major drawbacks of the current micro-batteries. Hence, MSCs
have been considered as this proposes to utilize them in TENGs because of their high-power
density and good life cycle. MXene sediment inks with superior fluidity and electronegativity are
one of the great candidates for TENGs and they are given long-term reliability and stable electrical
output to the fabricated TENGs. For instance, in a recent research work, cellulose nanofibrils and
Ti3C2Tx MXene (as CNFs/MXene liquid electrode) have been used in a stretchable TENG (CM-
TENG).4 The working mechanism and performance of the CM-TENG have been shown in Figure
11c. The output performance of this device demonstrated an open-circuit voltage, a short-circuit
current, and a short-circuit transferred charge of 300 V, 5.5 µA, and 120 nC, respectively.94
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Figure 11. (a) Schematic illustration of the stretchable and flexible CNFs/Ti3C2Tx liquid
electrode-based TENG. Reproduced with permission from data published in ref.,94 Copyright
2020, John Wiley and Sons. (b) Schematic illustration of a flexible self-charging system.
Reproduced with permission from data published in ref.,97 Copyright 2018, Elsevier. (c)
Schematic illustration of the charge distribution and working mechanism of the CM-TENG.
Reproduced with permission from data published in ref.,94 Copyright 2020, John Wiley and
Sons. (d) Short-circuit transferred charge, open-circuit voltage, and short-circuit current of the
generated TENG. Reproduced with permission from data published in ref.,97 Copyright 2018,
Elsevier.
In another research work, a TENG was fabricated with Ti3C2Tx MXene-based solid-state MSC
with PVA/H3PO4 gel as an electrolyte in a two-electrode configuration. The device was solidified
by cross-linking the hydrogel electrolyte by incorporating glutaraldehyde (C5H8O2). The
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solidification added the structural integrity and mechanical stability to fabricate the MSC
components. The characterization results exhibited excellent cycling stability with charge storage
capacitance retention of ~ 76% and a coulombic efficiency of 95% for 10000 cycles.97 The power
TENG system could be charged continuously by common human motion with around 5 Hz without
having a remarkable current leakage. The capacitance of the MCS and maximum output power of
the fabricated TENG were found to be 23 mF cm-2 and 7.8 µW cm-2, respectively.97 Figure 11d is
showing open-circuit voltage, short-circuit current, and short-circuit transferred charge of the
generated TENG.
Similarly, researchers have tried to fabricate an integrated triple mode sensor equipped with
Ti3C2Tx MXene MSC.99 The sensor is fabricated by sandwiching the three active layers in between
Cu foil, a composite made by mixing graphene, cellulose nanofibrils, and MXene components
acted as an active component that can give response for pressure, temperature, and light (Figure
12). In combination with this sensor, all-solid-state Ti3C2Tx MSC was equipped as a flexible and
portable power source. Typical Ti3C2Tx fabrication in that research work involves the utilization
of the same composite made using graphene/CNFs/MXene and the PVA/KOH as an electrolyte
solution. As-fabricated MSC was found to have efficient in performing the electrochemical cyclic
and demonstrated the volumetric capacitance of 148.25 F cm-3.99
The efficiency of such integrating devices relies on the fabrication approach for individual
components. Hence, the screen-printed MXene MSCs are found to have a significant effect on
providing and delivering such electrical energy performance. However, integrating these MSCs
with active components as power storage devices is still under prototype study. Likewise, scaling
these devices into commercial measures will open new challenges like reduction of production
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cost, highly functional, and ultra-sensitivity. More importantly, the high performance and
combatable MSCs are in high demand for such integrating approaches.
Screen-printing could offer a low material wastage and scalable production of MSCs. Besides, the
performance of the screen-printed MSCs can be further improved by chemical modification and
making the composite of MXene before the formulation of printable ink.100, 101 However, the
comparability of the MXene composites with screen printable ink rheology and substrates surface
interactions is an important consideration. The additive-free MXene ink on the other hand helps in
improving the areal capacitance, but the stability of bare MXene under the moister conditions is
to be more focused for MXene based MSCs with high reproducibility.
Figure 12. Illustration of synthesizing a stretchable MSC as an energy system, and a sensor by
graphene, cellulose nanofibrils, and Ti3C2Tx composite papers. Reproduced with permission
from data published in ref.,99 Copyright 2020, The Royal Society of Chemistry
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6 Conclusions
The present review elaborates the scope of scalable production of integrative MXene based MSC
as a power source for micro-electronic components in the fabrication of flexible miniaturized
devices, that can have promising applications in the health, environment, and energy sectors.
MXene based MSCs with high theoretical areal capacitance and excellent mechanical properties
can be embedded as power sources for flexible miniaturized devices. The issues like (i) chemical
and colloidal stability, (ii) low efficiency, (iii) utilization of a high quantity of binders, (iv)
utilization of aqueous, and (v) lack of methodologies for scalable production limits the potential
viability in flexible electronics fabrication. This review focuses on the screen printable MXene
MSCs by understanding and developing high-quality inks via looking into novel formulation
strategizes like additive-free, aqueous solvent-based, and ink with rheological properties that are
adaptable to screen-printing. More importantly, since flexible electronics are concerned the
discussion of ink formulation is extended to ink interactions with soft substrates like PET and A4
paper. The substrate interactions and the wetting conditions have been discussed for the efficient
printing of high-quality MXene patterns. The low oxidative stability is another important issue
with MXene based screen printable ink. The mechanistic discussion for the causes of high-rate
oxidation in aqueous conditions and the effect of surface modification of MXene nanosheet and
utilization of co-solvents to improve the stability by decreasing the rate of oxidations is discussed.
Also, the approaches of MXene patterns for the effective fabrication of MXene printed devices,
and the effect of utilization of liquid electrolyte with the purpose of utilization of solid electrolyte
and asymmetric MSCs are been elaborated. The integration of the MXene MSCs with sensors and
other components as prototype miniaturized devices on flexible substrates is shown and the
challenges in developing such devices require large scale and highly efficient MSCs production.
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The review discussed the hurdles and the large-scale production of MSCs and their integration as
electrical components in flexible device fabrication, where it is required to be considered for the
next-generation electronic devices.
Acknowledgments
The research leading to these results was supported by the European Structural and Investment
Funds, OP RDE-funded project 'CHEMFELLS IV' (No. CZ.02.2.69/0.0/0.0/20_079/0017899).
Z.S. was supported by Czech Science Foundation (GACR No. 20-16124J).
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