**4.3 MXenes (Ti3C2Tx)**

*Nanofibers - Synthesis, Properties and Applications*

SnS2 and insulators such as HfS2. Suitable electrode materials for MSCs are among

MoS2 can effectively store charges over a single atomic layer utilizing an inter and intrasheet double layers. Here the central atom Mo shows an oxidation state ranging from +2 to +6 and shows a pseudocapacitive behavior with a theoretical capacitance of about 1000 F/g. But aggregation and low electrical conductivity between the atomic layers of MoS2 hinder their extensive use in MSCs.

Hybridization of TMDs with carbon material, which provides quick-electron transport and more active-sites, is one approach to solve these problems. Hence Yang *et al.* reported a solid-state MSC using MoS2@rGO– photoresist-derived carbonnanotube (CNT) hybrid composite by spin coating followed by photoetching and pyrolysis similar to the MoS2@sulfonated rGO hybrid prepared by Xiao *et al.* shown in **Figure 4** [42]. This hybrid prepared by Yang *et al.* was then embedded in carbon microelectrodes, which synergistically increase the performance of the MSCs and exhibited high energy density (5.6 mWh cm−3) as well as areal capacitance (13.7 mF cm−2) with good capacitance retention [43]. Similarly, Haider *et al.* have reported other carbon microelectrodes based on TMD using the advantages of metallic VS2. The high energy density (15.6 mWh cm−3) and specific capacitance (86.4 Fcm−3) result from the synergistic combination of VS2 and carbon. This system exhibited excellent energy density and power density compared with the energy storage system developed by Xiao *et al.* and Yang *et al.* [44]. Besides constructing MoS2 hybrids with conductive carbonaceous material, the phase modification in which the Mo coordination changes from the trigonal prismatic (2H) phase to the octahedral (1 T) phase is another practical approach to improve the electrochemical performance of MoS2 [45]. Very recently, Xu *et al.* reported a femtosecond laser direct writing technique to fabricate an MSC based on 1 T MoS2. This is the first time an MSC with excellent performance based on 1 T MoS2 has been reported. These femtosecond lasers can help achieve a submicron resolution(~800 nm), which is nearly 40 times more accurate than that achieved with traditional nanosecond lasers with a resolution of around 10-200 μM. This approach is green, facile, maskless, flexible and high vacuum environments are not required. The electrochemical performance and

*(a) Schematic picture of fabrication of MoS2@SrGO MSC using gravure printing techniques (b) photograph of prepared MSC after Ag paste painting (c) KOH-PVA gel electrolyte coated on gravure printed electrode* 

*(d) MoS2@S r GO printed electrode in PI substrate. (Source: Reprinted from [42]).*

these metallic TMDs with large surface area and high conductivity [2].

**218**

**Figure 4.**

The overall performance of MSCs is based on the intrinsic properties of electrode materials. In many cases, carbonaceous materials such as graphene [49, 50], graphene oxide [51], CNTs [52, 53], carbide-derived carbon [54, 55] and their hybrids [56, 57] with charge storage via electric double layer, were reported in MSCs. Later, high capacity MSCs based on pseudocapacitive materials such as conductive polymers [58]**,** transition metal oxides/hydroxides [59, 60] and sulfides (VS2, MoS2) [61, 62] with surface redox reactions were reported. Nevertheless, the poor electrical conductivity and lower packing density of electrode materials in these MSCs restrict the accessible volumetric and areal capacitances, the two important parameters used to indicate the performance of MSCs [63]. Recently, MoS2 with high packing density served as a good electrode material to fabricate energy storage devices characterized by high power densities and volumetric energy [64]. A new group of layered 2D materials called MXenes, which includes transition metal nitrides, carbides, and carbonitrides, was recently reported.

**Figure 5.**

*(a) Steps involved in the fabrication of flexible MSC using MoS2 inks by inkjet printing (b) SEM image of printed MSC in PI substrate (c) relationship between areal capacitance vs. number of printed layers (d) capacitance retention under different bending radius of the fabricated supercapacitor [48].*

MXenes are promising layered materials derived from the precursors with the general formulae Mn + 1AXn (M refers to Ti, Sc, Nb *etc.*; A = Al, Sn, Si, *etc*.; *n* = 1, 2, 3). In the MAX phase, the M layers are hexagonally close-packed and MXenes can be synthesized by selective etching of A element from the MAX phase using acidic-fluoride-containing aqueous solutions [65]. The presence of an aqueous medium during the synthesis can create MXene flakes with various surface functional groups such as O, F, or OH. MXenes have characteristic properties such as rich surface chemistry, hydrophilicity, layered structure, high packing densities and intrinsic electronic conductivity. The first member Ti3C2Tx was reported in 2011, opened up an exciting research field, as revealed by the increasing number of publications on MXenes [66]. The distinctive properties and simplicity of processing have contributed to various applications such as energy storage for supercapacitors and the maximum theoretical capacity was reported to be at 615Cg−1 [65]. The higher pseudocapacitance and simplicity of solution processing of MXenes are highly advantageous for the designing of MSCs that are used to power wearable electronics, sensors, and micromechanical systems with low power consumption. Various methods have been adopted to fabricate MXene based MSC patterns on the submillimeter scale. These are known to be potential candidate for the design of MSCs due to the following factors: 1) MXenes have an electrical conductivity up to 6500 S cm−1 [67], which permit fast electron transfer and excludes the requirement for current collectors like noble metals. 2) MXenes exhibit higher gravimetric capacitances relatable to graphene, with higher packing density up to 4 g cm−3 [68, 69]**,** which needs to enhance the volumetric

**221**

*Recent Developments in All-Solid-State Micro-Supercapacitors Based on Two-Dimensional…*

characteristics (specific volumetric capacitance of 1000 F cm−3 for conventional three-electrode configuration [70]**,** which is more than that of supercapacitors

Flexible MSCs are highly demanding to manufacture portable and on-chip energy storage devices because of their high security, lightweight and miniaturization [72]. Direct printing of functional inks is crucial for various applications such as smart electronic devices, healthcare, and energy storage. Nevertheless, currently available inks are distant from ideality. A low concentration of ink or the additives/ surfactants are contained, which put on complexity to the fabrication and affects the printing resolution. Based on these facts, Zhang *et al.* demonstrated a direct printing method using two types of 2D MXene inks (aqueous for extrusion printing and organic for inkjet printing) to fabricate all-MXene MSCs with high resolution. The fabricated flexible MSCs displayed outstanding volumetric capacitance of 562 F cm−3 and a high energy density of 0.32 μW h cm−2, over all other printed MSCs reported yet. The approach of direct ink printing technique plays a major role beyond energy harvesting and storage applications, including sensors, circuits, and electronics, where simple, easily integrable and cost-effective components are required. This additive-free and low-temperature ink printing technique provides many applications in sensors, antennas, smart electronics and shielding [72]. Recently, Peng *et al.* adopted a solution spray coating and laser cutting to fabricate solid-state MSC based on interdigital L-s-Ti3C2Tx film on a glass substrate in which two layers of MXene (Ti3C2Tx) with different flake sizes were obtained. The larger MXene flakes (L-Ti3C2Tx, 3–6 μm) were stacked to form a bottom layer which act as current collectors. The top layer contains smaller MXene flakes (s-Ti3C2Tx, 1 μm) with numerous edges and defects to form an electroactive layer for energy storage. The excellent electrochemical characteristics and homogeneity in structures could

offer better cyclic performance and showed excellent areal capacitance of <sup>~</sup>

MSC showed excellent cyclic stability after 10,000 cycles without any decay of capacitance at 50 mV s−1. L-s-Ti3C2Tx film on a glass substrate was transferred onto the scotch tape substrate shows good flexibility without prominent cracks after

to its original rigid structure on a glass substrate. This approach opens up different designs for the fabrication of on-chip devices using different morphologies of MXenes and their composites, flake sizes, and chemistries [73]. The integration of flexible MSCs for on-chip energy storage applications still faces some challenges due to short cycling stability, complicated manufacturing processes, and low areal energy storage. To address this, Huang *et al.* utilized spray coating of MXene (Ti3C2Tx) conductive inks for the massive preparation of paper-based flexible MSCs by using a gel-like solid-state electrode (polyvinyl alcohol and H2SO4) and encapsulated layer of polydimethylsiloxane. As discussed above, a highly conductive and sprayable Ti3C2Tx interdigitated electrode served the dual role of the current collector and active materials. This flexible MSC delivers a large areal capacitance of 23.4 mF cm−2 and an excellent cycling capability with a capacitance retention up to 92.4% over 5000 cycles, together with exceptional flexibility [74]. The crucial obstacles in MSC applications are short discharge time, low voltage output, and low current. MSC array is known to be a solution to avoid the obstacles mentioned above. Based on the capacitors' theory, the parallel connection will increase the capacitance while the series connection will decrease the output voltage corresponding to the capacitance decrease. It means a MSC array (combination of some MSCs) can increase both capacitance and output voltage [75]. Recently, a lightweight and freestanding MXene/bacteria cellulose composite paper with

357 F cm−3 at 20 mV s−1. The L-s-Ti3C2Tx

, the areal capacitances were comparable

27 mF

*DOI: http://dx.doi.org/10.5772/intechopen.94535*

based on carbonaceous materials) [71].

cm−2 and volumetric capacitance of ~

bending up to 100 times at an angle of 60o

*Recent Developments in All-Solid-State Micro-Supercapacitors Based on Two-Dimensional… DOI: http://dx.doi.org/10.5772/intechopen.94535*

characteristics (specific volumetric capacitance of 1000 F cm−3 for conventional three-electrode configuration [70]**,** which is more than that of supercapacitors based on carbonaceous materials) [71].

Flexible MSCs are highly demanding to manufacture portable and on-chip energy storage devices because of their high security, lightweight and miniaturization [72]. Direct printing of functional inks is crucial for various applications such as smart electronic devices, healthcare, and energy storage. Nevertheless, currently available inks are distant from ideality. A low concentration of ink or the additives/ surfactants are contained, which put on complexity to the fabrication and affects the printing resolution. Based on these facts, Zhang *et al.* demonstrated a direct printing method using two types of 2D MXene inks (aqueous for extrusion printing and organic for inkjet printing) to fabricate all-MXene MSCs with high resolution. The fabricated flexible MSCs displayed outstanding volumetric capacitance of 562 F cm−3 and a high energy density of 0.32 μW h cm−2, over all other printed MSCs reported yet. The approach of direct ink printing technique plays a major role beyond energy harvesting and storage applications, including sensors, circuits, and electronics, where simple, easily integrable and cost-effective components are required. This additive-free and low-temperature ink printing technique provides many applications in sensors, antennas, smart electronics and shielding [72]. Recently, Peng *et al.* adopted a solution spray coating and laser cutting to fabricate solid-state MSC based on interdigital L-s-Ti3C2Tx film on a glass substrate in which two layers of MXene (Ti3C2Tx) with different flake sizes were obtained. The larger MXene flakes (L-Ti3C2Tx, 3–6 μm) were stacked to form a bottom layer which act as current collectors. The top layer contains smaller MXene flakes (s-Ti3C2Tx, 1 μm) with numerous edges and defects to form an electroactive layer for energy storage. The excellent electrochemical characteristics and homogeneity in structures could offer better cyclic performance and showed excellent areal capacitance of <sup>~</sup> 27 mF cm−2 and volumetric capacitance of ~ 357 F cm−3 at 20 mV s−1. The L-s-Ti3C2Tx MSC showed excellent cyclic stability after 10,000 cycles without any decay of capacitance at 50 mV s−1. L-s-Ti3C2Tx film on a glass substrate was transferred onto the scotch tape substrate shows good flexibility without prominent cracks after bending up to 100 times at an angle of 60o , the areal capacitances were comparable to its original rigid structure on a glass substrate. This approach opens up different designs for the fabrication of on-chip devices using different morphologies of MXenes and their composites, flake sizes, and chemistries [73]. The integration of flexible MSCs for on-chip energy storage applications still faces some challenges due to short cycling stability, complicated manufacturing processes, and low areal energy storage. To address this, Huang *et al.* utilized spray coating of MXene (Ti3C2Tx) conductive inks for the massive preparation of paper-based flexible MSCs by using a gel-like solid-state electrode (polyvinyl alcohol and H2SO4) and encapsulated layer of polydimethylsiloxane. As discussed above, a highly conductive and sprayable Ti3C2Tx interdigitated electrode served the dual role of the current collector and active materials. This flexible MSC delivers a large areal capacitance of 23.4 mF cm−2 and an excellent cycling capability with a capacitance retention up to 92.4% over 5000 cycles, together with exceptional flexibility [74]. The crucial obstacles in MSC applications are short discharge time, low voltage output, and low current. MSC array is known to be a solution to avoid the obstacles mentioned above. Based on the capacitors' theory, the parallel connection will increase the capacitance while the series connection will decrease the output voltage corresponding to the capacitance decrease. It means a MSC array (combination of some MSCs) can increase both capacitance and output voltage [75]. Recently, a lightweight and freestanding MXene/bacteria cellulose composite paper with

*Nanofibers - Synthesis, Properties and Applications*

MXenes are promising layered materials derived from the precursors with the general formulae Mn + 1AXn (M refers to Ti, Sc, Nb *etc.*; A = Al, Sn, Si, *etc*.; *n* = 1, 2, 3). In the MAX phase, the M layers are hexagonally close-packed and MXenes can be synthesized by selective etching of A element from the MAX phase using acidic-fluoride-containing aqueous solutions [65]. The presence of an aqueous medium during the synthesis can create MXene flakes with various surface functional groups such as O, F, or OH. MXenes have characteristic properties such as rich surface chemistry, hydrophilicity, layered structure, high packing densities and intrinsic electronic conductivity. The first member Ti3C2Tx was reported in 2011, opened up an exciting research field, as revealed by the increasing number of publications on MXenes [66]. The distinctive properties and simplicity of processing have contributed to various applications such as energy storage for supercapacitors and the maximum theoretical capacity was reported to be at 615Cg−1 [65]. The higher pseudocapacitance and simplicity of solution processing of MXenes are highly advantageous for the designing of MSCs that are used to power wearable electronics, sensors, and micromechanical systems with low power consumption. Various methods have been adopted to fabricate MXene based MSC patterns on the submillimeter scale. These are known to be potential candidate for the design of MSCs due to the following factors: 1) MXenes have an electrical conductivity up to 6500 S cm−1 [67], which permit fast electron transfer and excludes the requirement for current collectors like noble metals. 2) MXenes exhibit higher gravimetric capacitances relatable to graphene, with higher packing density up to 4 g cm−3 [68, 69]**,** which needs to enhance the volumetric

*(a) Steps involved in the fabrication of flexible MSC using MoS2 inks by inkjet printing (b) SEM image of printed MSC in PI substrate (c) relationship between areal capacitance vs. number of printed layers (d)* 

*capacitance retention under different bending radius of the fabricated supercapacitor [48].*

**220**

**Figure 5.**

outstanding electrochemical performance and mechanical stability through a facile all-solution based paper making method was fabricated by Jiao *et al*. Further, they adopted a laser-cutting kirigami patterning process for the fabrication of bendable, stretchable and twistable all-solid-state MSC arrays (**Figure 6a**). The structural design and excellent performance of MSC arrays could offer outstanding areal capacitance of 111.5 mF cm−2 and areal density of 0.0052 mWh cm−2 with electrochemical stability under mechanical deformation. The photograph of a paper crane made from MXene/BC composite paper is shown in **Figure 6b**, it is used as a conductor for lighting LED (**Figure 6c**). This technique presented a promising method for designing and manufacturing excellent mechanically deformable MSC arrays based on MXenes [76]. For practical applications, the electrodes with a 3D structure can be easily destroyed via mechanical deformation. It is possible to improve MSC's energy storage ability by fabricating them in a 3D structure [77]. In this context, Yue *et al.* developed a self-healable 3D MSC comprised of r-GO and MXene (Ti3C2Tx) composite aerogel electrode with an outer shell of self-healable

#### **Figure 6.**

*(a) Schematic illustration of the manufacturing process of Mxene/bacterial composite papers and a lasercutting kirigami patterning process for the fabrication of bendable, stretchable and twistable all-solid-state MSC arrays, (b) photograph of paper crane made from as-synthesized Mxene/BC, and (c) photograph of LED using paper crane working as conductor for lighting. (Source: Reprinted from [76]).*

**223**

**Figure 7.**

*Recent Developments in All-Solid-State Micro-Supercapacitors Based on Two-Dimensional…*

polyurethane through the ice-template method (**Figure 7a**). The composite aerogel electrode could resist the oxidation of MXene and prohibit the lamellar structure's restacking by combining the properties of components such as high conductivity of MXene and high surface area of r-GO. The fabricated MSc exhibited better performance with a large area-specific capacitance of 34.5 mF cm−2 at1mV

−1, as shown in **Figure 7c-e**. The 3D-MXene-r-GO composite aerogel electrode displayed excellent cycling performance with 91% retention of capacitance after 15,000 cycles (**Figure 7f**). The 3D MSC maintained outstanding self-healing capacity (**Figure 7b**) with capacitance retention up to 81.7% after the 5th healing. The synthesis of self-healable 3D-MXene-r-GO MSC dispensed an approach for fabricating next-generation durable electronic devices with multi-functionality to meet sustainable development [77]. The performance of recently reported all-

Transition metal oxides/hydroxides (TMOs/TMHs) are electrochemical pseudocapacitor materials and widely used as electrode materials in supercapacitor applications due to their high energy density, abundance and high capacitance [87]. But their performance as supercapacitor electrode materials limited because of low intrinsic conductivity. So, 2D TMOs/TMHs have been explored in supercapacitors owing to their enhanced electronic conductivity and high specific surface area [88].

*(a) Schematic illustration for the fabrication process of 3DMSC based on MXene-rGO composite aerogel, (b) photographs of the self-healable carboxylated polyurethane: Left (intial), right (after the healing) and middle (after damage). Electrochemical performance of MSCs based on the MXene-rGO composite aerogel (c) CV at various scan rates, (d) GCD at different current densities, (e) the variation in areal capacitances* vs *scan rate of MSC and (f) cycling stability of MSC based on MXene-rGO composite at the current density of* 

*2 mA cm−2. (Source: Reprinted from [77], with permission from, copyright@2018 ACS).*

solid-state MSCs based on 2D materials are summarized in **Table 3**.

*DOI: http://dx.doi.org/10.5772/intechopen.94535*

**4.4 Other important 2D materials**

s

*Recent Developments in All-Solid-State Micro-Supercapacitors Based on Two-Dimensional… DOI: http://dx.doi.org/10.5772/intechopen.94535*

polyurethane through the ice-template method (**Figure 7a**). The composite aerogel electrode could resist the oxidation of MXene and prohibit the lamellar structure's restacking by combining the properties of components such as high conductivity of MXene and high surface area of r-GO. The fabricated MSc exhibited better performance with a large area-specific capacitance of 34.5 mF cm−2 at1mV s −1, as shown in **Figure 7c-e**. The 3D-MXene-r-GO composite aerogel electrode displayed excellent cycling performance with 91% retention of capacitance after 15,000 cycles (**Figure 7f**). The 3D MSC maintained outstanding self-healing capacity (**Figure 7b**) with capacitance retention up to 81.7% after the 5th healing. The synthesis of self-healable 3D-MXene-r-GO MSC dispensed an approach for fabricating next-generation durable electronic devices with multi-functionality to meet sustainable development [77]. The performance of recently reported allsolid-state MSCs based on 2D materials are summarized in **Table 3**.
