**3. Direct laser writing of micro-supercapacitors**

In the following sections, a detailed understanding of the DLW process on various electrode materials for MSCs is discussed.

#### **3.1. Electrode materials**

Micro-supercapacitors (MSCs) are a recent addition to the environmentally friendly energy storages with higher charge-discharge transfer rates on the contrary to traditional batteries [1]. The batteries whose lifetime is restricted due to the involvement of electrochemical redox reaction create the issues of additional storage and disposal space where MSCs which can be fabricated

The performance of MSC is determined by the available active electrode material and the voltage window of the electrolyte [2]. Based on the electrode-electrolyte interactions, the supercapacitors are divided into three major types: (i) electrochemical double layer capacitor (EDLC), (ii) pseudocapacitors and (iii) hybrid supercapacitors. EDLC works based on the electrostatic interaction between electrode and electrolyte ions where pseudocapacitors involve the redox reaction between electrodes and electrolyte ions similar to the batteries. A strong pseudocapacitance is not desirable in many applications due to the slow response time and high capacitance decay rate [3]. Hybrid supercapacitors combine the EDLC and pseudocapacitance effects in the performance which can be used to compensate the drawbacks of current

Several methods are used for the fabrication of various kind of supercapacitors. The main categories are chemical techniques based on the nanomaterials [4] and electron beam lithography (EBL) [5]. The lesser integrability for flexible applications and cost involved in the fabrication of these techniques make them less desirable for industrial scale production of commercial applications. The recent reports on the use of direct laser writing (DLW) technique for the supercapacitor fabrication [6] is a fast and reliable single-step method with the

The DLW method mainly uses an ultrafast laser (femtosecond, fs or picosecond, ps lasers) [7] but recently continuous wave (cw) lasers are also used for specific materials [8]. The laser

on any substrates utilize the electrostatic interactions of electrode-electrolyte materials.

commercial supercapacitors to become a replacement for the batteries.

possible integration of all substrates.

104 Supercapacitors - Theoretical and Practical Solutions

**Figure 1.** Schematic of direct laser writing setup.

**2. Direct laser writing**

The electrodes of MSC require high active surface area, long-term stability, resistance to electrochemical oxidation or reduction, the capability of multiple cycling materials, optimum pore size distribution, minimized ohmic resistance with the contacts, sufficient electrodeelectrolyte solution contact interface, mechanical integrity and less self-discharge [1].

#### *3.1.1. Laser-scribed graphene and its derivatives*

Materials such as carbon and its derivatives like porous activated carbon, carbon nanotubes, carbon aerogels or carbon-metal composites have a higher surface area of 100–220 m2 g−1 and they exhibit excellent stability but limited capacitance [4]. For activated carbons, only about 10–20% of the theoretical capacitance can be achieved due to the micropores that are inaccessible by the electrolyte [13]. The carbon nanotubes do not exhibit satisfactory capacitance unless a conducting polymer [14] is used to form a pseudocapacitance.

Graphene is a form of carbon with the high surface area up to 2675 m2 /g and the intrinsic capacitance of 21 μF/cm<sup>2</sup> , which sets the upper limit of EDLC capacitance of all carbon-based materials [15]. In addition, both faces of graphene sheets are readily accessible by the electrolyte. However, in practical applications, the surface area of graphene will be much reduced due to agglomeration.

Laser-scribed graphene (LSG), obtained from the DLW in graphene oxide (GO) material, is a cost-effective tunable alternative to graphene. The LSG films are used to fabricate MSC and other integrable applications and first reported from Ajayan's group with the use of carbon dioxide (CO<sup>2</sup> ) laser beam by adopting the design concept from capacitors [6] in 2011. This work is followed by the Kaner's group in 2012 through the production of high-performance LSG sandwich energy storages using a DVD burner [16].

Furthermore, two famous works came in the following years: The first work demonstrates the fabrication of all solid state MSCs using ionic gel electrolyte with interdigitated electrodes. This kind of electrodes improves electrolyte ion transport that effectively improves the energy storage density and power density up to 10−3 Wh/cm<sup>3</sup> and 101 W/cm<sup>3</sup> (**Figure 2**) [17]. The next reported energy storage used the pre-patterned CO<sup>2</sup> laser irradiation on polyethylene terephthalate (PET) substrate to generate the high-quality graphene for the energy storage [18].

## *3.1.2. Oxides and polymers*

The DLW method can be used for the fabrication of pseudocapacitors using different materials like ruthenium oxide (RuO<sup>2</sup> ) and manganese dioxide (MnO<sup>2</sup> ) and conductive polymers like polyaniline (PANI) with the LSG material becomes a direction of interest to achieve the highperformance MSCs in the given area [19]. For example, the hybrid of ultrathin MSCs made of MnO<sup>2</sup> sheets and graphene sheets using DLW offers an electrochemically active surface for fast absorption/desorption of electrolyte ions [20]. The contributions of additional interfaces at the hybridized interlayer areas to accelerate charge transport during charge/discharge process result in an energy density and power density of 2.4 mWh/cm<sup>3</sup> and 298 mW/cm<sup>3</sup> , respectively.

**3.2. Matrix-assisted pulsed laser evaporation direct write (MAPLE-DW)**

**Figure 3.** Direct laser writing for the fabrication of nanoporous gold film electrodes coated with MnO<sup>2</sup>

H2

**3.3. Parameter calculation**

2016, Royal Society of Chemistry.

nificantly low volumes.

current densities by the formula:

prior to laser machining [23]. Copyright 2002, SPIE Publishing Group.

MAPLE-DW involves the laser forwarded transfer of liquid transfer matrix (transfer vehicle), which is composed of material to be deposited on the substrate below. A relatively flat and uniform film is achieved with the presence of liquid without the need for high temperature or post-processing involved in the lithographic techniques [22]. Pseudocapacitors from RuO<sup>2</sup>

O with the sulfuric acid as transfer vehicle result in an ideal capacitor behavior instead of the contamination generated from the transfer vehicles [23]. Moreover, the obtained specific (volumetric) capacitance of 720 F/g is comparable to the other laser-printed MSCs (**Figure 4**).

Generally, DLW-MSC performance is calculated in specific (volumetric) and areal capacitance in metric quantity rather than the mass of the obtained electrodes due to the presence of sig-

In brief, the specific capacitance was calculated from galvanostatic (CC) curves at different

**Figure 4.** MAPLE-DW method for the fabrication of MSC. (a) Schematic of MAPLE-DW apparatus showing the method of forward laser transfer of an "ink" layer (b) Sample geometry: the two pseudocapacitors cells are 1 mm × 2 mm × 10 μm

·0.5

107

[21]. Copyright

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

#### *3.1.3. Porous gold*

The recent development of the well-connected nanoporous gold film using DLW is used in the fabrication of interdigital electrode materials for MSCs with high mechanical flexibility [21]. These MSCs exhibit a capacitance of 127 F cm−3 and energy density of 0.045 Wh cm−3. The gold metal is known for its high electrical conductivity and the concept adopted can be used efficiently to integrate with devices in lesser areal footprints (**Figure 3**).

The high-performance integrable MSCs fabricated using DLW which can be integrated with all platforms can be the future of energy storages.

**Figure 2.** Direct laser writing of MSC. Reproduced with permission [6]. Copyright 2011, Nature Publishing Group.

**Figure 3.** Direct laser writing for the fabrication of nanoporous gold film electrodes coated with MnO<sup>2</sup> [21]. Copyright 2016, Royal Society of Chemistry.

#### **3.2. Matrix-assisted pulsed laser evaporation direct write (MAPLE-DW)**

MAPLE-DW involves the laser forwarded transfer of liquid transfer matrix (transfer vehicle), which is composed of material to be deposited on the substrate below. A relatively flat and uniform film is achieved with the presence of liquid without the need for high temperature or post-processing involved in the lithographic techniques [22]. Pseudocapacitors from RuO<sup>2</sup> ·0.5 H2 O with the sulfuric acid as transfer vehicle result in an ideal capacitor behavior instead of the contamination generated from the transfer vehicles [23]. Moreover, the obtained specific (volumetric) capacitance of 720 F/g is comparable to the other laser-printed MSCs (**Figure 4**).

#### **3.3. Parameter calculation**

This kind of electrodes improves electrolyte ion transport that effectively improves the

ylene terephthalate (PET) substrate to generate the high-quality graphene for the energy

The DLW method can be used for the fabrication of pseudocapacitors using different materials

polyaniline (PANI) with the LSG material becomes a direction of interest to achieve the highperformance MSCs in the given area [19]. For example, the hybrid of ultrathin MSCs made of

The recent development of the well-connected nanoporous gold film using DLW is used in the fabrication of interdigital electrode materials for MSCs with high mechanical flexibility [21]. These MSCs exhibit a capacitance of 127 F cm−3 and energy density of 0.045 Wh cm−3. The gold metal is known for its high electrical conductivity and the concept adopted can be used

The high-performance integrable MSCs fabricated using DLW which can be integrated with

**Figure 2.** Direct laser writing of MSC. Reproduced with permission [6]. Copyright 2011, Nature Publishing Group.

 sheets and graphene sheets using DLW offers an electrochemically active surface for fast absorption/desorption of electrolyte ions [20]. The contributions of additional interfaces at the hybridized interlayer areas to accelerate charge transport during charge/discharge

) and manganese dioxide (MnO<sup>2</sup>

process result in an energy density and power density of 2.4 mWh/cm<sup>3</sup>

efficiently to integrate with devices in lesser areal footprints (**Figure 3**).

all platforms can be the future of energy storages.

and 101 W/cm<sup>3</sup>

(**Figure 2**) [17].

laser irradiation on polyeth-

) and conductive polymers like

and 298 mW/cm<sup>3</sup>

,

energy storage density and power density up to 10−3 Wh/cm<sup>3</sup>

storage [18].

MnO<sup>2</sup>

respectively.

*3.1.3. Porous gold*

*3.1.2. Oxides and polymers*

106 Supercapacitors - Theoretical and Practical Solutions

like ruthenium oxide (RuO<sup>2</sup>

The next reported energy storage used the pre-patterned CO<sup>2</sup>

Generally, DLW-MSC performance is calculated in specific (volumetric) and areal capacitance in metric quantity rather than the mass of the obtained electrodes due to the presence of significantly low volumes.

In brief, the specific capacitance was calculated from galvanostatic (CC) curves at different current densities by the formula:

**Figure 4.** MAPLE-DW method for the fabrication of MSC. (a) Schematic of MAPLE-DW apparatus showing the method of forward laser transfer of an "ink" layer (b) Sample geometry: the two pseudocapacitors cells are 1 mm × 2 mm × 10 μm prior to laser machining [23]. Copyright 2002, SPIE Publishing Group.

$$\mathbf{C}\_{denou} \text{ i/} \{-dV/dt\} \tag{1}$$

However, the fast growth of technologies like the flexible MEMS or self-reliant buildings

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

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

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

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

as shown in **Figure 6** [32] and can be

Direct Laser Writing of Supercapacitors http://dx.doi.org/10.5772/intechopen.73000 109

demands the high-performance on-chip energy storages.

design family known as *space-filling curves* [31].

*4.1.2. Three-dimensional micro-supercapacitors*

energy storages [34] (**Table 1**).

a detailed investigation of various fractal families (**Figure 5**).

sheets, which result in an areal capacitance of >9 mF/cm<sup>2</sup>

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

*4.1.1. Fractal designs*

where *i* is the applied current (in amps, A) and *dV/dt* is the slope of the discharge curve (in volts per second, V/s).

Volumetric capacitance was given by

$$\mathbf{C}\_{vol} = \mathbf{C}\_{drotac} / \mathbf{V} \tag{2}$$

where A and V refer to the area (cm<sup>2</sup> ) and volume (cm3 ), respectively.

The power density of the device is calculated from galvanostatic curves at different charge/ discharge densities and given by the formula:

$$P = \langle \Lambda \mathcal{E} \rangle^2 / 4 \, R\_{\mathbb{E}\mathcal{R}} \, V \tag{3}$$

where P is the power in *W/cm3* , ΔE is the operating voltage window and *RESR* is the internal resistance of the device and can be given by the formula:

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

$$E = C v^\* \langle \Delta E \rangle^2 / (2^\* \ 3600) \tag{4}$$

where E is the energy density in *Wh/cm3* , *Cv* is the volumetric capacitance and Δ*E* is the operating voltage window, V.
