**4. Challenges and perspectives**

#### **4.1. Electrode designs**

The current energy storage devices are usually too heavy, rigid and bulky to match the requirements of flexible electronics [1]. Therefore, there is keen interest in the development of the light, elastic and mechanical properties with shape conformability in the next generation of energy storages. However, the bottleneck issues faced by the current MSCs are the lesser surface area of the electrode material and the more substantial mean ionic path of the electrolyte ions due to the diffraction-limited spatial resolution of the laser beam used for the preparation of electrode materials [24].

The first generation of MSCs is mainly involved in the 2D planar designs. However, these planar 2D electrodes are limited in the amount of energy storage capacities; on the other hand, increasing the thickness of the electrodes results in the low transportation of the electrolyte ions, which limits the rate transfer capabilities. The issue is initially addressed by Gao et al. where a variety of onchip designs are tested to obtain an optimized design concept for the self-powering on-chip applications, and the interdigital electrode designs seem to be out-performing the other designs [6].

However, the fast growth of technologies like the flexible MEMS or self-reliant buildings demands the high-performance on-chip energy storages.

#### *4.1.1. Fractal designs*

*Cdevice*<sup>=</sup> *i*/{−*dV*/*dt*} (1)

where *i* is the applied current (in amps, A) and *dV/dt* is the slope of the discharge curve (in

*Cvol* = *Cdevice*/*V* (2)

) and volume (cm3

The power density of the device is calculated from galvanostatic curves at different charge/

*P* = (ΔE)2 /4 *RESR V* (3)

*E* = *Cv*<sup>∗</sup> (Δ*E*)2 /(2<sup>∗</sup> 3600) (4)

The current energy storage devices are usually too heavy, rigid and bulky to match the requirements of flexible electronics [1]. Therefore, there is keen interest in the development of the light, elastic and mechanical properties with shape conformability in the next generation of energy storages. However, the bottleneck issues faced by the current MSCs are the lesser surface area of the electrode material and the more substantial mean ionic path of the electrolyte ions due to the diffraction-limited spatial resolution of the laser beam used for the preparation

The first generation of MSCs is mainly involved in the 2D planar designs. However, these planar 2D electrodes are limited in the amount of energy storage capacities; on the other hand, increasing the thickness of the electrodes results in the low transportation of the electrolyte ions, which limits the rate transfer capabilities. The issue is initially addressed by Gao et al. where a variety of onchip designs are tested to obtain an optimized design concept for the self-powering on-chip applications, and the interdigital electrode designs seem to be out-performing the other designs [6].

), respectively.

, ΔE is the operating voltage window and *RESR* is the internal

, *Cv* is the volumetric capacitance and Δ*E* is the oper-

volts per second, V/s).

Volumetric capacitance was given by

108 Supercapacitors - Theoretical and Practical Solutions

where A and V refer to the area (cm<sup>2</sup>

where P is the power in *W/cm3*

ating voltage window, V.

**4.1. Electrode designs**

of electrode materials [24].

where E is the energy density in *Wh/cm3*

**4. Challenges and perspectives**

discharge densities and given by the formula:

resistance of the device and can be given by the formula:

The energy density of the device can be calculated by the formula:

New electrode designs can be a possible solution to improve the energy density of MSCs, and an example is the origami designs utilized in the flexible energy storages and electronics [25]. In the case of integrable energy storages using DLW, a better choice can be biomimetic designs [26]. The *Fern Leafs* is an efficient platform for energy storage in biological processes such as photosynthesis enabled by water transport on its vein density [27] as well as information compression [28]. The biomimetic structures inspired by the internal structure of *American Fern* (*Polystichum munitum*), which is also known as the *Barnsley fractals* [29], are a possible design for the enhancement of the active electrode material loading per unit area using DLW [30]. American fern leafs with self-repeating patterns of internal structure resemble the fractal design family known as *space-filling curves* [31].

From the comparison between the various designs of the space-filling fractal family, it is found that the Hilbert space-filling designs have the highest active surface area comparing to the other designs. The resulting MSCs have an energy density of 10−1 Whcm−3 without compensating the rate of charge transfer (power density) and have a flexibility up to 60° [30]. The fractal designs offer a further chance to improve the performance of MSCs and still need a detailed investigation of various fractal families (**Figure 5**).

#### *4.1.2. Three-dimensional micro-supercapacitors*

Printing of high-performance MSCs in lesser footprints is possible with the development of three-dimensional (3D) MSCs. In 2015, Tour's group demonstrated the concept of the layer by layer stacking of individual laser-induced graphene (LIG) MSCs obtained from the PET sheets, which result in an areal capacitance of >9 mF/cm<sup>2</sup> as shown in **Figure 6** [32] and can be considered for the ultra-portable and flexible application levels. In addition, reports of using multilayer structures made of rGO/Au particles [33] and LIG from polyamides using femtosecond laser reduction with improved spatial resolution are promising directions for the 3D energy storages [34] (**Table 1**).

**Figure 5.** Fractal MSC [30]. Copyright 2017, Nature Publishing Group.

1–2 V and ionic liquids (ILs) with a voltage range of 3.5–4.0 V. The solid or quasi-solid state electrolytes can be classified as organic and inorganic electrolytes with a voltage range of 2.5–2.7 V. Among different electrolytes, aqueous ones possess high conductivity and capacitance but are limited by low cell voltage window, whereas organic and IL electrolytes can operate at higher cell voltage windows [36]. ILs are widely used in the energy storages owing to their attractive properties like the non-flammability, low vapor pressure and large operating potential window. Solid state electrolytes are devoid of leakage issues but are limited by the low conductivity.

Direct Laser Writing of Supercapacitors http://dx.doi.org/10.5772/intechopen.73000 111

The considerable advantage of the DLW-MSC is the possibility of large-scale production with the industrial grade. However, the scale-up process might introduce the difficulties of heat, enhancement of effective series resistance (ESR) and overcharge [1]. In addition, the cost of the process needs to be compatible to the existing battery technology. DLW technique due to its straight single-step process, which combines with the efficient tuning of nanomaterials, can provide an efficient solution to the issue in long term. The successful generation of high-performance MSCs will enable a step closer to the biocompatible light-weight portable and wearable devices as well

The self-powered autonomous systems will be the future direction with impact in large areas of technologies by the inclusion of additional features like portability, flexibility and stretchability. One of the integrated DLW-MSC with existing renewable technology is discussed below.

Self-powered electronics and buildings, which utilize the renewable energy resources like solar energy, provide a green platform for the next-generation technology and find applications in skyscrapers, flexible, wearable, consumable and portable devices [37]. The primary issue faced by the renewable energies to be considered as the major electricity source such as the intermittent nature which limits the use of those energies during certain climatic conditions or in the remote areas which are isolated from the grid line electricity [38]. The current solar modules used to be accompanied by the energy storages in the commercial market are the traditional batteries, which intake almost 30% of the total cost of the solar module. In addition, the protective storage space for the energy storages becomes a significant issue when it comes to large-scale applications.

An integrated on-chip solar energy storages, which can be simultaneously charged using the solar electricity, can be a possible solution for the problem and energy stored can be used during the required times irrespective of the intermittent solar energy. However, the primary challenge for the integrated solar energy storage is to design the cell structure by incorporating both photoelectric and storage functions. The initial efforts are oriented around co-operating

**6. Scale-up process of micro-supercapacitors**

**7. Integration with applications**

**7.1. Solar energy storages**

as the replacement of large area spaces required for the energy storages.

**Figure 6.** Stackable laser-scribed supercapacitors obtained using the direct laser writing in PET substrate [32]. Copyright 2015, ACS Publishing Group.


**Table 1.** Summary of the graphene supercapacitors fabricated using direct laser writing.

### **5. Electrolytes**

The electrolyte has a significant role in determining other essential properties such as the energy density, power density, internal resistance, rate performance, operating temperature range, cycling lifetime, self-discharge, non-volatile nature and toxicity of the energy storage. The electrochemical range of an electrolyte decides the cell voltage window of the energy storages like the batteries and supercapacitors [9] as shown in the equation,

$$\mathbf{E} = \mathbf{1}/2\,\mathbf{C}\mathbf{V} \tag{5}$$

where E is the energy density, C is the specific capacitance and V is the cell voltage.

So far, the electrolytes used in an energy storage can be classified as liquid electrolytes and solid/quasi-solid state electrolytes [35]. Liquid electrolytes can be further grouped as aqueous electrolytes with a voltage range of 1.0–1.3 V, organic electrolytes within the voltage range of 1–2 V and ionic liquids (ILs) with a voltage range of 3.5–4.0 V. The solid or quasi-solid state electrolytes can be classified as organic and inorganic electrolytes with a voltage range of 2.5–2.7 V.

Among different electrolytes, aqueous ones possess high conductivity and capacitance but are limited by low cell voltage window, whereas organic and IL electrolytes can operate at higher cell voltage windows [36]. ILs are widely used in the energy storages owing to their attractive properties like the non-flammability, low vapor pressure and large operating potential window. Solid state electrolytes are devoid of leakage issues but are limited by the low conductivity.
