**1. Introduction**

The popularization of portable electronic equipment has concentrated heavily on miniaturization and convergence of different technologies. While technologies such as wearable sensors and flexible displays has progressed, advances in energy storage are still lagging behind innovations in other electronic devices. Miniaturization of energy sources is also essential for environmental, medical, biological and other applications. Consequently, the reduction in size and integration of micro-power systems such as micro-batteries, micro-fuel cells, microsupercapacitors (MSCs) and piezoelectric power harvesters are essential for the

future growth of portable electronic devices [1]. MSCs have gained considerable attention among these micro-power systems due to its high power densities, fast rate capabilities, ultra-long – cycle life and simple integration into the micro-nano electronic system as energy sources [2, 3]. Three main types of device configurations for MSCs have been developed to date: in-plane architecture, fiber shape and three-dimensional (3D) type (**Figure 1**). The advantages and disadvantages of these device configurations are given in **Table 1**. The performance of MSCs is determined not only by components but also by the combination of each individual element for the development of a device**.**

The choice of electrode materials and electrolytes is the two critical parameters influencing the electrochemical performance of the MSCs. Two-dimensional (2D) materials with unusual properties such as ultra-thin thickness, large lateral size, excellent flexibility and tunable physicochemical properties are currently the perfect choice for MCS as an electrode material. A large number of 2D materials have been developed to date, including graphene and analog nanosheets such as transition metal dichalcogenides (TMDs), transition metal oxides/hydroxides (TMOs/TMHs), metal carbides and nitrides (MXens), boron nitride (BN), phosphorene, and so on. In addition to electrode material selection, the electrolyte selection also plays a crucial role in the performance of MSC and these electrolytes can be classified into two types (i) conventional liquid electrolytes and (ii)solidstate electrolytes [9]. Conventional liquid electrolytes have a common disadvantage in terms of their liquid nature; therefore, a strict encapsulation process is required to avoid electrolyte leakage. However, when the device is damaged, electrolyte leakage remains unavoidable. Accordingly, to overcome this disadvantage, a solid-state electrolyte was developed by blending the acids, ionic liquids and salts into a polymer matrix. Several polymer matrixes have been used in solid-state electrolytes, including poly-(vinylidene-fluoride) (PVP), polyacrylonitrile (PAN)

#### **Figure 1.**

*Schematics of three major MSC architecture (a) stacked architecture (b) Interdigital finger electrode architecture (c) cross-section of electrodes [4].*

**211**

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

• They consist of interdigital electrodes with many dense micro-fingers, where the counter electrode interspaces are small enough for the transport of ions so that the devices have little impedance, high capacity and quick frequency responses [5].

• Ease of integration with other microelectronics.

batteries and supercapacitors. • This design contributes to the separator

• Thanks to their unique wire-shaped structure, they are highly flexible and can be woven or knitted with excellent

• Excellent versatility and can be produced in different shapes and different locations

• This design maximize the energy density

• The volume of electrolyte utilized is further reduced with this design, leading to enhanced volumetric/areal energy

elimination [6].

Fiber-shaped • Generally small in size and lightweight.

wearability [7].

[8].

of MSCs

density [4].

*Advantages and disadvantages of various MSC device configurations.*

• This design can make the active electrodes more accessible because they are exposed to electrolytes on the edges. Therefore high power density can be achieved with

**Advantages Disadvantages**

• The geometric parameters of the electrodes still have to be

• It cannot compete in terms of energy density with micro-batteries as energy storage equipment; large scale application is a big

• Require a series of complex micro-fabrication techniques. • Designing of 3D type MSCs with leakage-free electrolyte is still a challenge [4].

challenge [8]

optimized.

and poly-(vinyl-alcohol) (PVA) [9]. These electrolytes can provide long cyclelife, low leakage current, high ionic conductivity and high mechanical flexibility. For example, the ionic conductivity of PVA/H3PO4 is about 10−3 Scm−1, while the ionic conductivity PVA/H2SO4 can be even higher, about 7 x 10−3 Scm−1. However, aqueous solid-state electrolytes suffer from a low voltage window at about 1 V due to the electrolysis voltage of water similar to aqueous electrolytes. A high operating voltage of 2.5 V can be achieved for micro-supercapacitors through ionic liquid solid-state electrolytes, resulting in a high energy density in sequence [10]. They also allow additional functionality, such as flexibility and stretchability, in addition to easy encapsulation. Considering these advantages, the choice of solid-state

electrolytes in micro-supercapacitors is more reasonable.

**2. Microfabrication technologies for microelectrodes of MCSs**

This technology used for the fabrication of microelectrodes of MCSs can be grouped into two categories. The first categories include direct electrode material synthesis on the patterned current collectors using laser scribing, CVD, electrolytic deposition and pyrolysis. The second category consists of indirect manufacturing using existing electrode materials in powder or solution form.

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

**Device configuration**

In-plane architecture

Threedimensional type

**Table 1.**

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


#### **Table 1.**

*Nanofibers - Synthesis, Properties and Applications*

the development of a device**.**

future growth of portable electronic devices [1]. MSCs have gained considerable attention among these micro-power systems due to its high power densities, fast rate capabilities, ultra-long – cycle life and simple integration into the micro-nano electronic system as energy sources [2, 3]. Three main types of device configurations for MSCs have been developed to date: in-plane architecture, fiber shape and three-dimensional (3D) type (**Figure 1**). The advantages and disadvantages of these device configurations are given in **Table 1**. The performance of MSCs is determined not only by components but also by the combination of each individual element for

The choice of electrode materials and electrolytes is the two critical parameters

influencing the electrochemical performance of the MSCs. Two-dimensional (2D) materials with unusual properties such as ultra-thin thickness, large lateral size, excellent flexibility and tunable physicochemical properties are currently the perfect choice for MCS as an electrode material. A large number of 2D materials have been developed to date, including graphene and analog nanosheets such as transition metal dichalcogenides (TMDs), transition metal oxides/hydroxides (TMOs/TMHs), metal carbides and nitrides (MXens), boron nitride (BN), phosphorene, and so on. In addition to electrode material selection, the electrolyte selection also plays a crucial role in the performance of MSC and these electrolytes can be classified into two types (i) conventional liquid electrolytes and (ii)solidstate electrolytes [9]. Conventional liquid electrolytes have a common disadvantage in terms of their liquid nature; therefore, a strict encapsulation process is required to avoid electrolyte leakage. However, when the device is damaged, electrolyte leakage remains unavoidable. Accordingly, to overcome this disadvantage, a solid-state electrolyte was developed by blending the acids, ionic liquids and salts into a polymer matrix. Several polymer matrixes have been used in solid-state electrolytes, including poly-(vinylidene-fluoride) (PVP), polyacrylonitrile (PAN)

*Schematics of three major MSC architecture (a) stacked architecture (b) Interdigital finger electrode* 

**210**

**Figure 1.**

*architecture (c) cross-section of electrodes [4].*

*Advantages and disadvantages of various MSC device configurations.*

and poly-(vinyl-alcohol) (PVA) [9]. These electrolytes can provide long cyclelife, low leakage current, high ionic conductivity and high mechanical flexibility. For example, the ionic conductivity of PVA/H3PO4 is about 10−3 Scm−1, while the ionic conductivity PVA/H2SO4 can be even higher, about 7 x 10−3 Scm−1. However, aqueous solid-state electrolytes suffer from a low voltage window at about 1 V due to the electrolysis voltage of water similar to aqueous electrolytes. A high operating voltage of 2.5 V can be achieved for micro-supercapacitors through ionic liquid solid-state electrolytes, resulting in a high energy density in sequence [10]. They also allow additional functionality, such as flexibility and stretchability, in addition to easy encapsulation. Considering these advantages, the choice of solid-state electrolytes in micro-supercapacitors is more reasonable.
