**4.4 Other important 2D materials**

*Nanofibers - Synthesis, Properties and Applications*

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

*(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]).*

**222**

**Figure 6.**

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].

#### **Figure 7.**

*(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).*


**225**

**MSC** 2D Ti C3 T2

PANI/EG/ Ti C3 T2 x

Ti C3 T2 x-PET

Ti C3 T2

x/polymer

PVA-H PO3 4

0 to 0.8

8 mW cm−2

28 μWh cm−2

276

—

electrolyte (PE)

Extrusion printed MXene

PVA-H2SO4

0 to 0.5

11.4 *μ*W cm−2

0.32μWh cm−2

43


562

12



0 to 0.5

PVA-H2SO4

Inkjet printed MXene

Ti C3 T2 x

3DMXene-r-GO

PVA-H2SO4

0 to 0.6

180 *μ*W cm−2

1.33 *μ*Wh cm−2

34.6

—

2.18 *μ*Wh cm−2

0.00552 mWh

111.5

—

5000 (72.2%)

[76]

cm−2

60 *μ*W cm−2

composite aerogel

MXene/BC composite

PVA-H2SO4

0 to 0.6

—

paper electrodes

PVA-H2SO4

0 to 0.6

0.7–15 W cm−3

11–18 mWh cm−3

27

357

PVA-H PO3 4

0 to 0.6

225 mW cm−3

2.8 mWhh cm−3

23

—

25.5

2.3 mWh cm−3

744 mW cm−3

PVA-H PO3 4

0 to 0.7

159.6 mW cm−3

2.3 mWh cm−3

—

36.4

1.3 mWh cm−3

2015 mW cm−3

x/PDMS

PVA-H2SO4

0 to 0.6

189.9 mW cm−3

1.48 mW h cm−3

23.4

—

92.4%

[68]

After 5000 cycles

90%

[78]

After

8000

cycles

76% after

[79]

10,000 cycles

95%

[80]

After

1000

cycles

97%

[72]

After 10,000

cycles

100%

After

10,000

cycles

100%

[66]

After

10,000

cycles

15,000(91%)

[77]

**Electrolyte**

**Voltage window (V)**

**Power density**

**Energy density**

**Areal/mF cm−2**

**Volumetric/ F cm−3**

**Device performance**

**Specific capacitance**

**Cycling stability**

**Ref**

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

*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*

*Nanofibers - Synthesis, Properties and Applications*

[31]

**224**

**MSC** NOG-X-Y GMP microflakes based

PVA/H2SO4 gel

0 to 1

10 mW cm−2

1 μWh cm − 2

11

—

3D MSC

Silicon nanowire-Graphene- PANI

PVA/H2SO4 gel

−0.2 to 1

0.78 mW cm−2

10.8 μWh cm −2

77.7

—

G/CA/MnO2 based ID

PVA/H2SO4 gel

0 to 1

43.2 μW cm −2

1.2 μWh cm− 2

8.7

—

patterned MSC

M-PBV-RGO Graphene based

PVA/H2SO4 gel

0 to 2

68.268 mW cm−

**3**

3.792 mWh cm−3

—

27.30

integrated planar on-chip

MCS

MoS

2

MoS

2 C/VS2 1 T MoS2 (t-lf laser)

PEDOT: PSS / MoS2 /

PVA/H PO3 4 gel

−0.2 to 1

0.82 W/cm3

1.81 mWh/cm3

1.43

—

175 μF/cm3

PEDOT

Inkjet printed MSC based

PVA/H2SO4 gel

0 to 0.6

0.079 W cm−3

0.215 mWh cm−3

on MoS2

PVA/H2SO4 gel

0–0.5

14 kW cm−3

15.6 mWh cm−3

36

@rGO/CNT

PVA/H2SO4 gel

0–1

0–1.2

2.88 Wcm−3

15.6 mWh cm−3

—

86.4

—

5.6 mWh cm−3

13.7

—

@S/rGO

KOH-PVA gel

13.4 mWcm−3

0.58 mWh cm−3

6.56

—

91% after

[42]

1000 cycles

96.6% after

[43]

10,000 cycles

97.7% after

[44]

10,000 cycles

93% after

[46]

5000 cycles

93.6% after

[47]

5000 cycles

85.6% after

[48]

10,000 cycles

PVA/H2SO4 gel

0 to 1

5 mW cm−2

2.49 μWh cm−2

21.86

—

PVA-H PO3 4

0 to 1

0.23 mW cm−2

2.59 μWh cm − 2

18.70

—

93% after

[28]

10,000 cycles

80% after

[29]

2000 cycles

75% after

2000 cycles

85% after

[38]

5000 cycles

99% after

[39]

10,000 cycles

89% after

[40]

10,000 cycles

**Electrolyte**

**Voltage** 

**Device performance**

**Specific capacitance**

**Cycling stability**

**Ref**

**window** 

**(V)**

**Power density**

**Energy density**

**Areal/mF cm−2**

**Volumetric/ F cm−3**


**Table 3.**

**227**

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

Recently, research has been put into the fabrication of 2D TMOs/TMHs for MSC electrodes, limitations remain when using electrode based on a single material. The major disadvantages mainly rely on poor rate capacity caused by low electrical conductivity, restricted enhancement of energy density, and low capacitance, limiting their practical implementations [87]. To surpass the challenges of using single electrode materials, it is appropriate to fabricate nanoarchitectures based on composites of TMOs/TMHs. This can hone the configuration to avoid the agglomeration of 2D nanosheets and raise the performance level of various electrode materials to execute effective enhancement of supercapacitor performance [89]. Inspired from these findings, Wang *et al.* developed all-solid-state planar asymmetric MSCs based on Co(OH)2/EG and porous VN nanosheets/EG as positive and negative electrodes, respectively, together with an interdigital mask placed on the Nylon membrane. The developed electrodes showed high electrical conductivity, high flexibility, and homogeneity over a large area and were acted as flexible electrodes without any need for binder, additives and metal-constituted current collectors for VN//Co(OH)2 -PHMSs. The outstanding performance of the electrode was benefited from planar device geometry, synergy of Co(OH)2 nanoflower (charge storing like battery) and VN nanosheets (charging storing like capacitor) based hybrid structure, and highly conducting EG nanosheets, which served as both additives and current collectors. The enhancement of capacitance in PHMss (planar hybrid MSCs) was occurred due to the porous structure of VN and nanoflower morphology of Co(OH)2, these factors are suitable to enhance electrolyte ions and lessen their diffusion paths. The interdigital planar geometry of VN//Co(OH)2 –PHMSs permits the ultra-fast flow of electrolyte ions between the adjacent finger electrodes with a concise diffusion pathway. This improved charge storage via the effective exploration of highly active surface area of 2D nanosheets. Consequently, the fabricated PHMss exhibited areal capacitance of 21 mF cm−2 and volumetric capacitance of 39.7 F cm−3 with a notable energy density of 12.4 mWh cm−3 and 84% capacitance retention over 10,000 cycles [83]. Recently, Lee *et al.* fabricated an in-plane MSCs comprised of Co(OH)2 and r-GO through a cost-effective two-step fabrication method (**Figure 8a**). This method contained the fabrication of Co(OH)2 and r-GO on Au electrode using photolithography and electrodeposition method. The Au metal situated at the bottom of the electrode act as a current collector and effectively transfers electrons to the active material because of its high conductivity. The electrode's large surface area promotes the reaction between the electrolyte and the active material. This electrode structure maximizes the volumetric and areal capacitance of fabricated Co(OH)2//r-GO ASC (**Figure 8b**), shows a power density of 100.38 μW cm−2 and energy density of 0.35 μWh cm-2 for practical devices. and an excellent cycling capability with a capacitance retention up to 89% over 10,000 cycles, together with exceptional flexibility [86]. 2D MOFs are important 2D materials with tunable functionality and a designable porous structure with periodicity [90–92]. The high porosity of organic framework materials is suitable for producing electric double-layer capacitance and the heteroatoms like B, N, O and S located on the organic frameworks may show redox behavior for the pseudocapacitance [93]. The shortage of feasible microfabrication methods limits the practical implementation of MOF based electrode materials in MSCs. For the first time, a flexible symmetric MSC based on conductive Ni-catecholate-MOF possessing redox chemistry and high conductivity in the negative and positive windows was grown on 3D laser scribed graphene by Wu *et al*. The developed LSG/Ni-MOFbased MSCs showed outstanding areal capacitance of 15.2 mF cm−2 at 0.2 mA cm−2. The *π* conjugation of tricatecholate ligands resulted in decent electrical conductivity. The flow of electrolytes is enhanced due to the porous 1D open channels formed by the alternative stacking of 2D layers. The fabricated MSCs displayed the

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

*Summary of recently reported all-solid-state MSCs based on 2D materials.*

**226**

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

Recently, research has been put into the fabrication of 2D TMOs/TMHs for MSC electrodes, limitations remain when using electrode based on a single material. The major disadvantages mainly rely on poor rate capacity caused by low electrical conductivity, restricted enhancement of energy density, and low capacitance, limiting their practical implementations [87]. To surpass the challenges of using single electrode materials, it is appropriate to fabricate nanoarchitectures based on composites of TMOs/TMHs. This can hone the configuration to avoid the agglomeration of 2D nanosheets and raise the performance level of various electrode materials to execute effective enhancement of supercapacitor performance [89]. Inspired from these findings, Wang *et al.* developed all-solid-state planar asymmetric MSCs based on Co(OH)2/EG and porous VN nanosheets/EG as positive and negative electrodes, respectively, together with an interdigital mask placed on the Nylon membrane. The developed electrodes showed high electrical conductivity, high flexibility, and homogeneity over a large area and were acted as flexible electrodes without any need for binder, additives and metal-constituted current collectors for VN//Co(OH)2 -PHMSs. The outstanding performance of the electrode was benefited from planar device geometry, synergy of Co(OH)2 nanoflower (charge storing like battery) and VN nanosheets (charging storing like capacitor) based hybrid structure, and highly conducting EG nanosheets, which served as both additives and current collectors. The enhancement of capacitance in PHMss (planar hybrid MSCs) was occurred due to the porous structure of VN and nanoflower morphology of Co(OH)2, these factors are suitable to enhance electrolyte ions and lessen their diffusion paths. The interdigital planar geometry of VN//Co(OH)2 –PHMSs permits the ultra-fast flow of electrolyte ions between the adjacent finger electrodes with a concise diffusion pathway. This improved charge storage via the effective exploration of highly active surface area of 2D nanosheets. Consequently, the fabricated PHMss exhibited areal capacitance of 21 mF cm−2 and volumetric capacitance of 39.7 F cm−3 with a notable energy density of 12.4 mWh cm−3 and 84% capacitance retention over 10,000 cycles [83]. Recently, Lee *et al.* fabricated an in-plane MSCs comprised of Co(OH)2 and r-GO through a cost-effective two-step fabrication method (**Figure 8a**). This method contained the fabrication of Co(OH)2 and r-GO on Au electrode using photolithography and electrodeposition method. The Au metal situated at the bottom of the electrode act as a current collector and effectively transfers electrons to the active material because of its high conductivity. The electrode's large surface area promotes the reaction between the electrolyte and the active material. This electrode structure maximizes the volumetric and areal capacitance of fabricated Co(OH)2//r-GO ASC (**Figure 8b**), shows a power density of 100.38 μW cm−2 and energy density of 0.35 μWh cm-2 for practical devices. and an excellent cycling capability with a capacitance retention up to 89% over 10,000 cycles, together with exceptional flexibility [86]. 2D MOFs are important 2D materials with tunable functionality and a designable porous structure with periodicity [90–92]. The high porosity of organic framework materials is suitable for producing electric double-layer capacitance and the heteroatoms like B, N, O and S located on the organic frameworks may show redox behavior for the pseudocapacitance [93]. The shortage of feasible microfabrication methods limits the practical implementation of MOF based electrode materials in MSCs. For the first time, a flexible symmetric MSC based on conductive Ni-catecholate-MOF possessing redox chemistry and high conductivity in the negative and positive windows was grown on 3D laser scribed graphene by Wu *et al*. The developed LSG/Ni-MOFbased MSCs showed outstanding areal capacitance of 15.2 mF cm−2 at 0.2 mA cm−2. The *π* conjugation of tricatecholate ligands resulted in decent electrical conductivity. The flow of electrolytes is enhanced due to the porous 1D open channels formed by the alternative stacking of 2D layers. The fabricated MSCs displayed the

*Nanofibers - Synthesis, Properties and Applications*

**226**

**MSC** Ti C3 T2

@Silver-plated x

PVA-H2SO4

0 to 0.4

132 μW cm−2

7.3 μWh cm−2

328

—

10,000(80%)

[81]

Nylon Fiber Electrodes

r-GO/MnO2/Ag NW-PET

SiO2–1-butyl-3-

0 to 2.5

162.0 mW. cm−3

2.3 mWh. cm−3

—

2.72

90.3%

[82]

After 6000

cycles

methylimidazoli

umbis(trifluoro

methylsulfonyl)

imide

KOH/PVA

0 to 1.6

1750 mW cm−3

12.4 mWh cm−3

21

39.7

84%

[83]

After

10,000

cycles

87%

[84]

After

5000

cycles

VN// Co(OH)2

LSG/Ni-Catecholate-MOF

**Asymmetric system**

FGO//FrGO

MXene//

PVA-H PO3 4

0 to 1.2

0.8 W cm−3

9.7 mWh cm−3

19

63.3

MXene-MoO2-AMSCs

Co(OH)2//erGO

**Table 3.**

*Summary of recently reported all-solid-state MSCs based on 2D materials.*

PVA-KOH-KI

0 to 1.4

100.38μWh cm−2

0.35μWh cm−2

2.28

—

PVA/Na2SO4

28.3 μW cm−2

2.52 μWh cm−2

7.3

—

100% over

[34]

500 cycles

88%

[85]

After

10,000

cycles

89%

[86]

After

10,000 cycles

LiCl/PVA

0 to 1.6

7 mWcm−2

4.1 Wh cm−2

15.2

—

**Electrolyte**

**Voltage** 

**Device performance**

**Specific capacitance**

**Cycling stability**

**Ref**

**window** 

**(V)**

**Power density**

**Energy density**

**Areal/mF cm−2**

**Volumetric/ F cm−3**

#### **Figure 8.**

*(a) Schematic representation of the fabrication of Co(OH)2-e-rGO, (b) the variation in areal capacitances v/s current density of Co(OH)2//e-rGO MSC (Source: Reprinted from [13], with permission from RSC) and (c) schematic diagram of the fabrication of ternary hybrid film and displaying ternary hybrid film supported on cellulose acetate via vaccum filtration and fabrication process of RGO/MnO2/Ag NW-MSCs on alumina (Source: Reprinted from [82], with permission from, copyright@2015, ACS).*

highest power density of 7 mW cm−2 and a high energy density of 4.1 μW h cm−2. As illustrated in fig, MOF based MSC retained 87% of its initial capacitance in gel electrolyte even after 5000 cycles. This approach may shed light on fabricating novel MOF-based MSCs and electrochemical devices [84]. Recently, Liu *et al.* presented all-solid-state MSCs based on a flexible ternary hybrid film (RGMA) of RGO/MnO2/AgNW (silver nanowire) via facile vacuum filtration and thermal reduction (**Figure 8c**). It provided a great advantage to include metal oxide or metal nanoarchitectures into graphene film with strong potential for various thin-film energy storage devices. They adopted an efficient strategy to design graphenebased nanoarchitctures by incorporating the high electrical conductivity, interface integrity of the components and energy storage mechanisms (pseudocapacitance and electric double layer capacitance). Graphene is good material for the flexible energy storage devices due to its mechanical stability and MnO2 served to enhance the capacitive performances and inhibited the aggregation of graphene nanosheets. The ternary hybrid film's mechanical flexibility and electrical conductivity could be enhanced by the 1D AgNW, which functioned as a conducting bridge between Needle-like MnO2 and graphene nanosheets. This flexible MSC delivers a specific capacitance of 2.72 F cm−3 and an excellent cycling capability with a capacitance retention up to 90.3% after 6000 cycles, together with exceptional flexibility and volumetric energy density of 2.3 mWh cm−3 (power density of 162.0 mW cm−3) in ionic liquid gel electrolyte [82]. **Figure 9** illustrates the schematic representation of fabrication and characterization of all-solid-state MSCs based on 2D materials.

**229**

**5. Conclusion**

*Synthesis/characterization (Source: Reprinted from [76]).*

**Figure 9.**

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

MSCs as an energy storage devices attract considerable attention due to their notable characteristics such as smaller volume and high electrochemical performance. This chapter provides a brief overview of the recent developments in the field of 2D material-based all-solid-state MSCs. A brief note on the MSC device configuration and microfabrication methods for the microelectrodes has been illustrated. 2D materials based MSCs open up new avenues for the technologically relevant real-world applications. 2D materials such as MXenes, graphene, TMDs, and 2D metal–organic framework, TMOs/TMHs materials, have been described with regard to their electrochemical properties for MSCs. It is reported that the one issue faced by 2D materials is their unavoidable aggregation or restacking owing to their intense van der Waals interactions. To overcome this, there are approaches available like expansion of interlayer space with regard to enhanced storage ability or intercalation of guest molecules to increase the active sites. Moreover, for MSCs, 2D materials with vertical orientation grown on interdigitated current collectors is favorable to attain enhanced charge transport and low interfacial resistance. Additionally, to achieve higher conductivity and large specific surface area,

*Schematic of fabrication and characterization of MSC based on 2D materials. Graphene structure (Mary, 2020), TMD structure (Wikipedia contributors. (2020, October 12). Transition metal dichalcogenide monolayers. In Wikipedia, The Free Encyclopedia. Retrieved 07:20, October 17, 2020, from https://en.wikipedia. org/w/index.php?title=Transition\_metal\_dichalcogenide\_monolayers&oldid=983161701) MXene structure (Yujuan Zhang, Ningning Zhang and Changchun Ge- First-Principles Studies of Adsorptive Remediation of Water and Air Pollutants Using Two-Dimensional MXene Material, materials* 2018, *11 [11], 2281. Three electrode setup (Benjamin Hsia- Materials Synthesis and Characterization for Micro-supercapacitor Applications, Doctoral dissertion, University of California, Berkeley (2013)). Fabrication technologies 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 9.**

*Nanofibers - Synthesis, Properties and Applications*

highest power density of 7 mW cm−2 and a high energy density of 4.1 μW h cm−2. As illustrated in fig, MOF based MSC retained 87% of its initial capacitance in gel electrolyte even after 5000 cycles. This approach may shed light on fabricating novel MOF-based MSCs and electrochemical devices [84]. Recently, Liu *et al.* presented all-solid-state MSCs based on a flexible ternary hybrid film (RGMA) of RGO/MnO2/AgNW (silver nanowire) via facile vacuum filtration and thermal reduction (**Figure 8c**). It provided a great advantage to include metal oxide or metal nanoarchitectures into graphene film with strong potential for various thin-film energy storage devices. They adopted an efficient strategy to design graphenebased nanoarchitctures by incorporating the high electrical conductivity, interface integrity of the components and energy storage mechanisms (pseudocapacitance and electric double layer capacitance). Graphene is good material for the flexible energy storage devices due to its mechanical stability and MnO2 served to enhance the capacitive performances and inhibited the aggregation of graphene nanosheets. The ternary hybrid film's mechanical flexibility and electrical conductivity could be enhanced by the 1D AgNW, which functioned as a conducting bridge between Needle-like MnO2 and graphene nanosheets. This flexible MSC delivers a specific capacitance of 2.72 F cm−3 and an excellent cycling capability with a capacitance retention up to 90.3% after 6000 cycles, together with exceptional flexibility and volumetric energy density of 2.3 mWh cm−3 (power density of 162.0 mW cm−3) in ionic liquid gel electrolyte [82]. **Figure 9** illustrates the schematic representation of fabrication and characterization of all-solid-state MSCs based on 2D materials.

*(Source: Reprinted from [82], with permission from, copyright@2015, ACS).*

*(a) Schematic representation of the fabrication of Co(OH)2-e-rGO, (b) the variation in areal capacitances v/s current density of Co(OH)2//e-rGO MSC (Source: Reprinted from [13], with permission from RSC) and (c) schematic diagram of the fabrication of ternary hybrid film and displaying ternary hybrid film supported on cellulose acetate via vaccum filtration and fabrication process of RGO/MnO2/Ag NW-MSCs on alumina* 

**228**

**Figure 8.**

*Schematic of fabrication and characterization of MSC based on 2D materials. Graphene structure (Mary, 2020), TMD structure (Wikipedia contributors. (2020, October 12). Transition metal dichalcogenide monolayers. In Wikipedia, The Free Encyclopedia. Retrieved 07:20, October 17, 2020, from https://en.wikipedia. org/w/index.php?title=Transition\_metal\_dichalcogenide\_monolayers&oldid=983161701) MXene structure (Yujuan Zhang, Ningning Zhang and Changchun Ge- First-Principles Studies of Adsorptive Remediation of Water and Air Pollutants Using Two-Dimensional MXene Material, materials* 2018, *11 [11], 2281. Three electrode setup (Benjamin Hsia- Materials Synthesis and Characterization for Micro-supercapacitor Applications, Doctoral dissertion, University of California, Berkeley (2013)). Fabrication technologies and Synthesis/characterization (Source: Reprinted from [76]).*
