**3. Performance metrics of MCSs**

The parameters used generally to assess supercapacitors' performance against volume and weight units are gravimetric capacitance, energy, and power. It is important to note that the supercapacitors gravimetric capacitance varies according to total

**213**

below

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

the areal performance are more adapted for electrochemical performance [17]. Since equipment need to be integrated with miniaturized electronic devices with limited area, a performance assessment against the footprint area of MSCs is essential. Therefore areal capacitance, power density and energy density are the more reliable parameters for MSC performance monitoring. These parameters can

density, mass and thickness of the electrode, and other components' weight. So it's hard to compare the various MSC based on gravimetric capacitance [16]. But this parameter is not suitable for planar MSC where electrode material's weight is insignificant and the device's volume and surface area are always limited. Since the overall mass load of active materials in MSCs is small, the volumetric performance and, more significantly,

> <sup>=</sup> <sup>∆</sup> *<sup>S</sup> <sup>Q</sup> <sup>C</sup>*

. <sup>∆</sup> <sup>=</sup> **<sup>2</sup> 0 5 3600** *<sup>S</sup>*

<sup>∆</sup> <sup>=</sup>

**4** *<sup>S</sup> <sup>V</sup> <sup>P</sup>*

array and ∆*V* is the voltage range. Ps and Es is the maximum power and energy density. The essential parameter to detect the electrode's areal performance is to

A promising material for the production of MCSs is Planar 2D molecules of atomic thickness with a large specific surface area. The reduced dimension of these materials also satisfies the miniaturization requirements of device size, offering new possibilities for high-performance MSC development [18, 19]. The material properties of this rich family consisting of graphene, transition metal oxides (TMOs), transition metal chalcogenides (TMDs), metal carbides and nitrides (MXene), black phosphorus (BP), etc., range from superconducting, metallic, semiconducting and insulating behavior due to its diverse electronic structure, offering a wide range of material solutions to achieve high-performance. The essential reasons why 2D material based solid-state MSC are essentials is enlisted

The essential qualification for high-performance MCS electrode materials is the excellent electrical conductivity, which can accelerate the adsorption and desorption of the charges and increase the diffusion rate of ions. This electron transport behavior is very closely linked to the electronic crystal structure, resulting in three typical insulating, semiconducting, and metallic transport behaviors. In general, metallic 2D materials, such as TMDs, have good electrical conductivity and several other 2D semiconductors like graphenes and BPs also offer favorable electron

*E*

*C V*

**2**

*s*

*s V* (1)

*s ESR* (3)

, 's' is the total area of microelectrode

(2)

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

be calculated using the equations given below [17]**.**

Where Cs is the areal capacitance in F/cm<sup>2</sup>

**4. Two- dimensional materials for MCSs**

• Excellent electrical conductivity

• Excellent-electrochemical activities

transport characteristics [19].

measure the total area accurately.

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

density, mass and thickness of the electrode, and other components' weight. So it's hard to compare the various MSC based on gravimetric capacitance [16]. But this parameter is not suitable for planar MSC where electrode material's weight is insignificant and the device's volume and surface area are always limited. Since the overall mass load of active materials in MSCs is small, the volumetric performance and, more significantly, the areal performance are more adapted for electrochemical performance [17].

Since equipment need to be integrated with miniaturized electronic devices with limited area, a performance assessment against the footprint area of MSCs is essential. Therefore areal capacitance, power density and energy density are the more reliable parameters for MSC performance monitoring. These parameters can be calculated using the equations given below [17]**.**

$$\mathbf{C}\_{\rm s} = \frac{\mathbf{Q}}{s \Delta \mathbf{V}} \tag{1}$$

$$E\_s = \frac{\mathbf{0.5 C} \Delta V^2}{\mathbf{3600 s}} \tag{2}$$

$$P\_s = \frac{\Delta \mathbf{V}^2}{\mathbf{4}s \, \mathbf{ESR}} \tag{3}$$

Where Cs is the areal capacitance in F/cm<sup>2</sup> , 's' is the total area of microelectrode array and ∆*V* is the voltage range. Ps and Es is the maximum power and energy density. The essential parameter to detect the electrode's areal performance is to measure the total area accurately.

### **4. Two- dimensional materials for MCSs**

A promising material for the production of MCSs is Planar 2D molecules of atomic thickness with a large specific surface area. The reduced dimension of these materials also satisfies the miniaturization requirements of device size, offering new possibilities for high-performance MSC development [18, 19]. The material properties of this rich family consisting of graphene, transition metal oxides (TMOs), transition metal chalcogenides (TMDs), metal carbides and nitrides (MXene), black phosphorus (BP), etc., range from superconducting, metallic, semiconducting and insulating behavior due to its diverse electronic structure, offering a wide range of material solutions to achieve high-performance. The essential reasons why 2D material based solid-state MSC are essentials is enlisted below

• Excellent electrical conductivity

The essential qualification for high-performance MCS electrode materials is the excellent electrical conductivity, which can accelerate the adsorption and desorption of the charges and increase the diffusion rate of ions. This electron transport behavior is very closely linked to the electronic crystal structure, resulting in three typical insulating, semiconducting, and metallic transport behaviors. In general, metallic 2D materials, such as TMDs, have good electrical conductivity and several other 2D semiconductors like graphenes and BPs also offer favorable electron transport characteristics [19].

• Excellent-electrochemical activities

*Nanofibers - Synthesis, Properties and Applications*

Electrolytic deposition Simple, efficient, cost-effective,

Electrophoretic deposition Cost-effective, simple, thickness can be controlled [2, 11].

production [2, 11]

Inkjet printing Cost-effective, fast process, low mass loading,

samples [13].

Vacuum filtration Low- cost, simple, convenient, thickness can be controlled [2].

Laser scribing Cost-effective, scalable, simple, gives high throughput [9].

Layer-by layer assembly Multilayer films can be easily prepared, cost-

resolution [14].

Chemical vapor deposition (CVD)

Photolithography (UV lithography)

Drop, spin and spray

coating

**Methods Advantage Disadvantage**

environmentally friendly and large scale-

precise thickness control, direct patterning, large scale production, fair resolution (around 50 μm) enhance the resolution and scalability because no manual assembly is required during device manufacturing [2, 12].

Screen printing Low-cost, scalable and fast process Low resolution

Cost-effective with high control precision [10], simple fabrication process [9], can produce uniform and accurate large-area

Facile, simple, thickness-control, large-scale fabrication, time and energy saving [2].

effective and straightforward method, high

Pyrolysis Single-step synthesis [2]. Complex and high-

Controlled design and structure [2] Expensive, time-

consuming process, low mass loading and vigorous reaction condition **(2)**.

Uncontrolled lateral direction growth [2].

Restricted by the species

Ink preparation is a complicated process, limited by resolution, jam of nozzle [2, 10].

A sacrificial template is required; hence it's a complicated process, long preparation time [9].

Low-production efficiency and heterogeneous [2].

Shape and size is limited

Time-consuming process

temperature process [2].

Non-universal [2].

charged [2].

The advantages and disadvantages of various techniques developed are explained

electrolytes, and interface between electrolytes and micro-electrodes [15].

The parameters used generally to assess supercapacitors' performance against volume and weight units are gravimetric capacitance, energy, and power. It is important to note that the supercapacitors gravimetric capacitance varies according to total

No strategy in the fabrication of MSC microelectrodes is yet dominant over the others. Therefore, improving existing assembly strategies and exploring new manufacturing methods to overcome those limitations has become essential. In the meantime, to select appropriate assembly strategies to achieve high-performance MSCs, consideration should be given to overall factors such as active electrode materials,

**212**

below in **Table 2.**

**Table 2.**

**3. Performance metrics of MCSs**

*Microfabrication techniques for fabrication of MSCs.*

The electrochemical activity of some 2D material would be useful for the redox reaction to further increase the pseudocapacitance. The promising electrode material of microsized pseudocapacitors, which generally display high capacitance performance, is proven to be 2D MXenes, layered double hydroxides (LDHs), metal oxides and hydroxides with excellent electrochemical properties [19].

• Large surface area

The extra-large surface area offers an energy storage platform with huge active sites to increase the electrochemical activity and charge adsorption, thus making 2D metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) with inherent porosity a promising MSC functional electrode material [19].

• Mechanical flexibility

Superior mechanical flexibility at the atomic level and a diverse array of various 2D nanosheets provide desirable flexibility and multiple functionalities [15].

#### **4.1 Graphene**

Graphene is the most widely studied electrode material for MSCs due to its excellent electrical conductivity, large specific area, chemical stability, excellent intrinsic double-layer capacitance of approximately 21 μF/cm2 and theoretical capacitance of around 550 F/g [20–22]. Several reviews of graphene-based MSCs have been published. Zhang *et al., Xiong* et al. tried to summarize the recent developments in graphene-based MCSs and the methods employed to produce highperformance MSCs [20, 21]. Similarly, Wu *et al.* classified graphene-based MSCs and provided a complete overview of on-chip graphene-based planar interdigital MSCs [21]. Gao et al. recently provided an overview of the MCS system's application, based on 1D, 2D and 3D graphene [23].

Several strategies were employed to enhance the electrochemical performance of graphene-based MSCs. The first approach was to improve the charge storage capacity of electrode materials by preparing graphene composites with other pseudocapacitive materials [11, 24, 25] or doping graphene with heteroatoms like boron [26] and fluorine [27]. The second approach consists of constructing an asymmetric structure [11, 25] and the third approach was to increase the loading quantity of active electrode materials by 3D electrode construction on the confined area of MSC [17]. To realize this 3D electrode construction, Wang *et al.* used a 3D printing technique to fabricate an all-solid-state flexible MSC using nitrogen (N)/oxygen (O)-doped graphene ink. This device shows a high power density of 0.23 mW cm−2 and an areal energy density of 2.59 μWh cm−2, excellent cycling stability, and good mechanical flexibility. This increase in electrochemical properties observed due to three reasons (i) increased surface area due to the doping with N and O (ii) enhanced hydrophilic property of N/O doped graphene ink (iii) increased electrical conductivity as well as the pseudocapacitive effect of O and N doping [28]. Like this study, Szymon and his group developed a new type of polyaniline (PANI) anchored pseudocapacitive MnOX passivated graphene microflake inks and manufactured a solid-state MCS using the inkjet printing technique in a 3D configuration given in **Figure 2**. This ink shows excellent stability and jetting performance that may be due to the dual-passivation process employed here. For the first time, this is to report such a fully inkjet-printed MSC with 3D electrode configurations

**215**

**Figure 2.**

*GMP MSC (d and e) 3D heterogeneous GMP MSC [29].*

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

metrical arrangement and produced an in-plane asymmetric interdigited MSC using mask-assisted vacuum filtration techniques, among the various strategies discussed to improve graphene performance. They develop this asymmetric MSC based on an all-graphene system, where both the working electrodes are graphene derivatives. Using the same chemical composition and microstructure electrodes enhances performance and helps avoid power imbalance that conventional hybrid and asymmetric systems usually encounter [32]. Therefore, the anode of this MSC is made of functional graphene oxide (FGO), while the cathode consists of functional reduced graphene oxide (FrGO). This FGO, which is electrochemically exfoliated from graphite papers, has enhanced hydrophilic properties compared to pristine graphene, difficult to disperse in water [33]. FGO has abundant functional groups responsible for the hydrophilic properties of the FGO, leading to the formation of wrinkles on its surfaces. However, this asymmetric MSC has the highest surface capacitance of approximately 7.3 mF cm−2 in PVA/Na2SO4 electrolyte. The high performance of this MSC is attributable to abundant functional group doping

*Inkjet-printed GMP (doped graphene passivated flakes) MSC developed by Delekta et al. (a) Photograph of inkjet-printed MSC (b) micrographs of printed GMP MSC on glass substrate (c) SEM images of patterned* 

and a power density of about 0.78 mW/cm<sup>2</sup>

with excellent rate performance and stability. However, these microflake inks have been developed based on the self-assembly behavior of 2D materials following Rebani et al.'s [30] 3D coarse-grained lattice gas model [29]. Toan *et al.* built an on-chip MSC where the electrode consists of silicon nanowire-graphene nanowall-PANI ternary composite. Here the silicon nanowire template with a high aspect ratio is fabricated using the metal-assisted chemical etching (MACE) technique. On this silicon nanowire, 3D hierarchical graphene nanowalls were produced using microwave plasma-enhanced chemical vapor deposition (PECVD), which significantly improves the electrochemical performance of the manufactured MSC. However, this solid-state 3D MSC delivers an areal energy density of about 10.8

[31]**.** Lu *et al.* chose asym-

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

μWh/cm<sup>2</sup>

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

with excellent rate performance and stability. However, these microflake inks have been developed based on the self-assembly behavior of 2D materials following Rebani et al.'s [30] 3D coarse-grained lattice gas model [29]. Toan *et al.* built an on-chip MSC where the electrode consists of silicon nanowire-graphene nanowall-PANI ternary composite. Here the silicon nanowire template with a high aspect ratio is fabricated using the metal-assisted chemical etching (MACE) technique. On this silicon nanowire, 3D hierarchical graphene nanowalls were produced using microwave plasma-enhanced chemical vapor deposition (PECVD), which significantly improves the electrochemical performance of the manufactured MSC. However, this solid-state 3D MSC delivers an areal energy density of about 10.8 μWh/cm<sup>2</sup> and a power density of about 0.78 mW/cm<sup>2</sup> [31]**.** Lu *et al.* chose asymmetrical arrangement and produced an in-plane asymmetric interdigited MSC using mask-assisted vacuum filtration techniques, among the various strategies discussed to improve graphene performance. They develop this asymmetric MSC based on an all-graphene system, where both the working electrodes are graphene derivatives. Using the same chemical composition and microstructure electrodes enhances performance and helps avoid power imbalance that conventional hybrid and asymmetric systems usually encounter [32]. Therefore, the anode of this MSC is made of functional graphene oxide (FGO), while the cathode consists of functional reduced graphene oxide (FrGO). This FGO, which is electrochemically exfoliated from graphite papers, has enhanced hydrophilic properties compared to pristine graphene, difficult to disperse in water [33]. FGO has abundant functional groups responsible for the hydrophilic properties of the FGO, leading to the formation of wrinkles on its surfaces. However, this asymmetric MSC has the highest surface capacitance of approximately 7.3 mF cm−2 in PVA/Na2SO4 electrolyte. The high performance of this MSC is attributable to abundant functional group doping

#### **Figure 2.**

*Nanofibers - Synthesis, Properties and Applications*

• Large surface area

• Mechanical flexibility

**4.1 Graphene**

The electrochemical activity of some 2D material would be useful for the redox reaction to further increase the pseudocapacitance. The promising electrode material of microsized pseudocapacitors, which generally display high capacitance performance, is proven to be 2D MXenes, layered double hydroxides (LDHs), metal

The extra-large surface area offers an energy storage platform with huge active sites to increase the electrochemical activity and charge adsorption, thus making 2D metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) with

Superior mechanical flexibility at the atomic level and a diverse array of various

2D nanosheets provide desirable flexibility and multiple functionalities [15].

Graphene is the most widely studied electrode material for MSCs due to its excellent electrical conductivity, large specific area, chemical stability, excellent

capacitance of around 550 F/g [20–22]. Several reviews of graphene-based MSCs have been published. Zhang *et al., Xiong* et al. tried to summarize the recent developments in graphene-based MCSs and the methods employed to produce highperformance MSCs [20, 21]. Similarly, Wu *et al.* classified graphene-based MSCs and provided a complete overview of on-chip graphene-based planar interdigital MSCs [21]. Gao et al. recently provided an overview of the MCS system's applica-

Several strategies were employed to enhance the electrochemical performance of graphene-based MSCs. The first approach was to improve the charge storage capacity of electrode materials by preparing graphene composites with other pseudocapacitive materials [11, 24, 25] or doping graphene with heteroatoms like boron [26] and fluorine [27]. The second approach consists of constructing an asymmetric structure [11, 25] and the third approach was to increase the loading quantity of active electrode materials by 3D electrode construction on the confined area of MSC [17]. To realize this 3D electrode construction, Wang *et al.* used a 3D printing technique to fabricate an all-solid-state flexible MSC using nitrogen (N)/oxygen (O)-doped graphene ink. This device shows a high power density of 0.23 mW cm−2 and an areal energy density of 2.59 μWh cm−2, excellent cycling stability, and good mechanical flexibility. This increase in electrochemical properties observed due to three reasons (i) increased surface area due to the doping with N and O (ii) enhanced hydrophilic property of N/O doped graphene ink (iii) increased electrical conductivity as well as the pseudocapacitive effect of O and N doping [28]. Like this study, Szymon and his group developed a new type of polyaniline (PANI) anchored pseudocapacitive MnOX passivated graphene microflake inks and manufactured a solid-state MCS using the inkjet printing technique in a 3D configuration given in **Figure 2**. This ink shows excellent stability and jetting performance that may be due to the dual-passivation process employed here. For the first time, this is to report such a fully inkjet-printed MSC with 3D electrode configurations

and theoretical

oxides and hydroxides with excellent electrochemical properties [19].

inherent porosity a promising MSC functional electrode material [19].

intrinsic double-layer capacitance of approximately 21 μF/cm2

tion, based on 1D, 2D and 3D graphene [23].

**214**

*Inkjet-printed GMP (doped graphene passivated flakes) MSC developed by Delekta et al. (a) Photograph of inkjet-printed MSC (b) micrographs of printed GMP MSC on glass substrate (c) SEM images of patterned GMP MSC (d and e) 3D heterogeneous GMP MSC [29].*

and heteroatom substitution with graphene elements N, O, P, S, and the in-plane interdigitated architecture. This MSC has demonstrated exceptional flexibility, showing the feasibility of the wearable application of the MSCs [34].

Lochmann *et al.* reported a stamping approach combined with soft lithography for MSC production for the first time, but this approach is time-consuming and not appropriate for producing PET and paper-based MSC [35]. Zhang *et al.* subsequently made a flexible MSC based on MXene in a paper substrate using a stamping method, but even this MSC suffered from lower areal and volumetric capacitances [36]. To this end, Esfahani and Khosravi reported the manufacture of a graphenebased flexible MSC using a single-step stamping method on a PET-coated parafilm using a pre-designed pattern. This imprinted parafilm-coated PET pattern was filled with graphene oxide (GO)/MnO2/carbon aerogel hybrid paste, which acts as a binder and additive-free active material and finally, with the help of nascent hydrogen GO reduction is made. This type of GO reduction improves stability and reduces the ohmic resistance of the prepared electrode. The active material, i.e., G/ CA/MnO2 used here, exhibits a unique morphology that enhances the electrochemical surface area and facilitates the diffusion of electrolytes ion. The edged structure of graphene also helps to improve the electrochemical activity and capacitance of the SC. Based on the Hota *et al.* [37] inferences, they also used fractal design of large and small finger widths and compared their performance with ID electrodes. The small finger width fractal design (SFWF) shows high areal and volumetric capacitance of around 14.2 mF cm-<sup>2</sup> and 71.3 Fcm−3 among these different electrode designs. This method proves feasible in designing various low-cost architectures and improves the flexible-graphene performance based on MSC, which has been reported to date. The manufacturing process of this flexible MSC provides a new direction in the modification of the substrate and the current collector and the manufacturing method and the method of graphene reduction. This MSC fabricated by this stamping method also demonstrates excellent flexibility, reflecting its potential in future flexible electronic devices [38]. The presence and regulation of the functional group are essential for improving the performance of the graphenebased MSCs. These functional groups not only provide active pseudocapacitance sites, but they also prevent the aggregation of graphene without affecting both wettability and conductivity of graphene. To date, several strategies have been reported for regulating graphene functional groups, such as the treatment of O2 plasma, laser power, etc. However, it is worth working on regulating the functional groups of graphene to balance the active site, electrical conductivity, and wettability, which have plenty of room for further performance improvement in MSCs. Consequently, to improve the electrochemical efficiency of reduced graphene oxide (RGO), an appropriate method of controlling the functional group is required. Based on these facts, Wu et *al.* fabricated a free planar MSC using symmetric graphene-based metal current collector, where the functional groups were regulated using both air-plasma treatment and exposure to blue violet-laser (BV-laser) as shown in **Figure 3a**. The XRD pattern of the prepared composite is shown in **Figure 3c** and it can understand from the figure that this BV-laser treatment and air-plasma treatment do not change the phase structure of RGO. The interlayer spacing calculated for this composite is higher than that of graphite structure, which indicates a π-π stacking between graphene sheets in these composites. These combined techniques balance the pseudocapacitance active sites, conductivity and wettability by tuning the functional groups on the graphene surface and the possible transformation pathway is shown in **Figure 3b**. The CV curves of this material are shown in **Figure 3d** exhibit a symmetric-quasi rectangular state that demonstrates that this material's capacitive behavior is due to the simultaneous pseudocapacitive and electrical double layer behavior (EDL). This symmetrical solid-state MSC exhibit excellent energy density

**217**

**Figure 3.**

*RGO) [39].*

electronic components [40]**.**

**4.2 Transition metal dichalcogenids (TMDs)**

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

and power density around 2.49 μWh cm−2 and 5 mW cm−2 and superior long term stability after 10,000 cycles with 99% retention, which exceeds most reported all-solid-state graphene-based MSCs. This manufacturing method is suitable for the process of micro-integrated circuit machining and has an enormous potential in the production of on-chip devices [39]. Liu *et al.* adopted a new series of interdigital setups without any internal connection by combining techniques such as photolithography and liquid-air interface self-assembly methods. This solid-state planar on-chip MSC without internal connection shows excellent cyclic stability and outstanding energy and power density. This work demonstrates that graphenebased planar on-chip MSCs with no internal connection can integrate better with

*(a) Fabrication steps involved in the manufacturing of in-planar PBV-RGO MSC developed by Wu et al. (b) the suggested pathway transformation of RGO on exposure with BV-laser exposing and air-plasma treatment to produce PBV-RGO electrode material (c) XRD pattern of PBV-RGO electrode material (d) CV curves of RGO (M RGO) after BV-laser treatment (M BV-RGO) followed by air-plasma treatment (M-PBV-*

Single or few layers TMDs have attracted considerable attention because of their tunable band gaps and extensive natural reserves [2]. These compounds show a typical MX2 formula, where M is an element in Group IV-VI metal, and X is a chalcogen (S, Se, or Te). In this case, Stoichiometry relies on the process and the strategy of producing a compound made up of transition metal and chalcogen elements. The layered TMDs are typically 6 to 7 Å thick and consist of an X-M-X hexagonal sandwich with a metal-atomic layer separated by both layers of chalcogen [41]. The physical properties of bulk TMD's vary from true metals such as VSe2 and TaS2, semi-metals such as WTe2 and TiS2, semiconductors such as MoS2 and

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

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

#### **Figure 3.**

*Nanofibers - Synthesis, Properties and Applications*

capacitance of around 14.2 mF cm-<sup>2</sup>

and heteroatom substitution with graphene elements N, O, P, S, and the in-plane interdigitated architecture. This MSC has demonstrated exceptional flexibility,

designs. This method proves feasible in designing various low-cost architectures and improves the flexible-graphene performance based on MSC, which has been reported to date. The manufacturing process of this flexible MSC provides a new direction in the modification of the substrate and the current collector and the manufacturing method and the method of graphene reduction. This MSC fabricated by this stamping method also demonstrates excellent flexibility, reflecting its potential in future flexible electronic devices [38]. The presence and regulation of the functional group are essential for improving the performance of the graphenebased MSCs. These functional groups not only provide active pseudocapacitance sites, but they also prevent the aggregation of graphene without affecting both wettability and conductivity of graphene. To date, several strategies have been reported for regulating graphene functional groups, such as the treatment of O2 plasma, laser power, etc. However, it is worth working on regulating the functional groups of graphene to balance the active site, electrical conductivity, and wettability, which have plenty of room for further performance improvement in MSCs. Consequently, to improve the electrochemical efficiency of reduced graphene oxide (RGO), an appropriate method of controlling the functional group is required. Based on these facts, Wu et *al.* fabricated a free planar MSC using symmetric graphene-based metal current collector, where the functional groups were regulated using both air-plasma treatment and exposure to blue violet-laser (BV-laser) as shown in **Figure 3a**. The XRD pattern of the prepared composite is shown in **Figure 3c** and it can understand from the figure that this BV-laser treatment and air-plasma treatment do not change the phase structure of RGO. The interlayer spacing calculated for this composite is higher than that of graphite structure, which indicates a π-π stacking between graphene sheets in these composites. These combined techniques balance the pseudocapacitance active sites, conductivity and wettability by tuning the functional groups on the graphene surface and the possible transformation pathway is shown in **Figure 3b**. The CV curves of this material are shown in **Figure 3d** exhibit a symmetric-quasi rectangular state that demonstrates that this material's capacitive behavior is due to the simultaneous pseudocapacitive and electrical double layer behavior (EDL). This symmetrical solid-state MSC exhibit excellent energy density

Lochmann *et al.* reported a stamping approach combined with soft lithography for MSC production for the first time, but this approach is time-consuming and not appropriate for producing PET and paper-based MSC [35]. Zhang *et al.* subsequently made a flexible MSC based on MXene in a paper substrate using a stamping method, but even this MSC suffered from lower areal and volumetric capacitances [36]. To this end, Esfahani and Khosravi reported the manufacture of a graphenebased flexible MSC using a single-step stamping method on a PET-coated parafilm using a pre-designed pattern. This imprinted parafilm-coated PET pattern was filled with graphene oxide (GO)/MnO2/carbon aerogel hybrid paste, which acts as a binder and additive-free active material and finally, with the help of nascent hydrogen GO reduction is made. This type of GO reduction improves stability and reduces the ohmic resistance of the prepared electrode. The active material, i.e., G/ CA/MnO2 used here, exhibits a unique morphology that enhances the electrochemical surface area and facilitates the diffusion of electrolytes ion. The edged structure of graphene also helps to improve the electrochemical activity and capacitance of the SC. Based on the Hota *et al.* [37] inferences, they also used fractal design of large and small finger widths and compared their performance with ID electrodes. The small finger width fractal design (SFWF) shows high areal and volumetric

and 71.3 Fcm−3 among these different electrode

showing the feasibility of the wearable application of the MSCs [34].

**216**

*(a) Fabrication steps involved in the manufacturing of in-planar PBV-RGO MSC developed by Wu et al. (b) the suggested pathway transformation of RGO on exposure with BV-laser exposing and air-plasma treatment to produce PBV-RGO electrode material (c) XRD pattern of PBV-RGO electrode material (d) CV curves of RGO (M RGO) after BV-laser treatment (M BV-RGO) followed by air-plasma treatment (M-PBV-RGO) [39].*

and power density around 2.49 μWh cm−2 and 5 mW cm−2 and superior long term stability after 10,000 cycles with 99% retention, which exceeds most reported all-solid-state graphene-based MSCs. This manufacturing method is suitable for the process of micro-integrated circuit machining and has an enormous potential in the production of on-chip devices [39]. Liu *et al.* adopted a new series of interdigital setups without any internal connection by combining techniques such as photolithography and liquid-air interface self-assembly methods. This solid-state planar on-chip MSC without internal connection shows excellent cyclic stability and outstanding energy and power density. This work demonstrates that graphenebased planar on-chip MSCs with no internal connection can integrate better with electronic components [40]**.**

#### **4.2 Transition metal dichalcogenids (TMDs)**

Single or few layers TMDs have attracted considerable attention because of their tunable band gaps and extensive natural reserves [2]. These compounds show a typical MX2 formula, where M is an element in Group IV-VI metal, and X is a chalcogen (S, Se, or Te). In this case, Stoichiometry relies on the process and the strategy of producing a compound made up of transition metal and chalcogen elements. The layered TMDs are typically 6 to 7 Å thick and consist of an X-M-X hexagonal sandwich with a metal-atomic layer separated by both layers of chalcogen [41]. The physical properties of bulk TMD's vary from true metals such as VSe2 and TaS2, semi-metals such as WTe2 and TiS2, semiconductors such as MoS2 and

SnS2 and insulators such as HfS2. Suitable electrode materials for MSCs are among these metallic TMDs with large surface area and high conductivity [2].

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

#### **Figure 4.**

*(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]).*

**219**

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

the MSC resolution are enhanced by lowering the thermal effect to regulate the phase transition of these 1 T MoS2 based electrode material. This MSC with in-plane configuration results in high-frequency response with ultrafast ion-diffusion rate, high specific capacitance, good cycle-life, low equivalent series resistance (ESR) value, unprecedented power density (nearly 14 kW cm−3) and high energy density (15.6 mWh cm−3) in PVA/H2 SO4 electrolyte. However, this MSC with a surface area

for AC line filters and other electronic devices demanding high power require-

The fabrication of supercapacitors with excellent energy storage capacity and flexibility in wearable smart electronics has recently attracted significant attention. Thus, a fabric supercapacitor using low-cost textile fabrics with good mechanical properties and biocompatibility coated with ternary composite poly(3,4ethylenedioxythiophene): poly(styrenesulfonate)/MoS2/ poly3,4ethylenedioxythiophene) (PEDOT: PSS/MoS2/PEDOT) is manufactured by Chen and the group. This all-solid-state fabric MCS was fabricated by vapor phase polymerization (VPP) and the vapor phase deposition method exhibits an

The fabric coated with this ternary composite has a 3D configuration with interconnected structure and exhibits a large surface area that enables fast electrolyte transport and provides active electrolyte accessibility. This MSC assembled in a belt-shaped device was also used by the group as transient power sources to operate the light-emitting diodes [47]. Very recently, Li *et al.* printed a MoS2 based allsolid-state in-plane MSC using inkjet printing. This MSC printed with MoS2 based inks has high loadings of active materials per unit area resulting in a thinner and more flexible supercapacitors than the conventional sandwich structure. PEDOT: PSS inks were first printed on PI substrate to improve the conductivity, followed by printing of MoS2 based inks subsequently to fabricate the MSC. This scalable synthesis technique is demonstrated in **Figure 5a**. The SEM image (**Figure 5b**) shows that the layered MoS2 formed a uniform 2D conductive network above the pre-printed PEDOT: PSS electrode, which differs from the morphology observed in the ternary composite prepared by Chen and group. They also demonstrated the relationship with the increase of electrode thickness vs. conductivity in **Figure 5c** and its practical application by powering an LED bulb by connecting MSC in series

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,

and a high-frequency response and time constant are suitable

and power density of around 0.82 W/cm3

.

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

energy density of around 1.81 mWh/cm3

of 100 × 100 μm<sup>2</sup>

combination [48].

**4.3 MXenes (Ti3C2Tx)**

was recently reported.

ments [46].

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

the MSC resolution are enhanced by lowering the thermal effect to regulate the phase transition of these 1 T MoS2 based electrode material. This MSC with in-plane configuration results in high-frequency response with ultrafast ion-diffusion rate, high specific capacitance, good cycle-life, low equivalent series resistance (ESR) value, unprecedented power density (nearly 14 kW cm−3) and high energy density (15.6 mWh cm−3) in PVA/H2 SO4 electrolyte. However, this MSC with a surface area of 100 × 100 μm<sup>2</sup> and a high-frequency response and time constant are suitable for AC line filters and other electronic devices demanding high power requirements [46].

The fabrication of supercapacitors with excellent energy storage capacity and flexibility in wearable smart electronics has recently attracted significant attention. Thus, a fabric supercapacitor using low-cost textile fabrics with good mechanical properties and biocompatibility coated with ternary composite poly(3,4ethylenedioxythiophene): poly(styrenesulfonate)/MoS2/ poly3,4ethylenedioxythiophene) (PEDOT: PSS/MoS2/PEDOT) is manufactured by Chen and the group. This all-solid-state fabric MCS was fabricated by vapor phase polymerization (VPP) and the vapor phase deposition method exhibits an energy density of around 1.81 mWh/cm3 and power density of around 0.82 W/cm3 . The fabric coated with this ternary composite has a 3D configuration with interconnected structure and exhibits a large surface area that enables fast electrolyte transport and provides active electrolyte accessibility. This MSC assembled in a belt-shaped device was also used by the group as transient power sources to operate the light-emitting diodes [47]. Very recently, Li *et al.* printed a MoS2 based allsolid-state in-plane MSC using inkjet printing. This MSC printed with MoS2 based inks has high loadings of active materials per unit area resulting in a thinner and more flexible supercapacitors than the conventional sandwich structure. PEDOT: PSS inks were first printed on PI substrate to improve the conductivity, followed by printing of MoS2 based inks subsequently to fabricate the MSC. This scalable synthesis technique is demonstrated in **Figure 5a**. The SEM image (**Figure 5b**) shows that the layered MoS2 formed a uniform 2D conductive network above the pre-printed PEDOT: PSS electrode, which differs from the morphology observed in the ternary composite prepared by Chen and group. They also demonstrated the relationship with the increase of electrode thickness vs. conductivity in **Figure 5c** and its practical application by powering an LED bulb by connecting MSC in series combination [48].
