**7. Integration with applications**

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.

#### **7.1. Solar energy storages**

**5. Electrolytes**

Ajayan [6] Hydrated graphene oxide films

2015, ACS Publishing Group.

110 Supercapacitors - Theoretical and Practical Solutions

Hu [33] Graphene oxide/gold nanoparticle

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

Direct laser writing (cw) Specific capacitance of 5 mF/cm<sup>2</sup>

sulfuric acid

0.77 mFcm−2

Direct laser writing (fs) PVA/ H2SO4 electrolyte with specific capacitance of

Kaner [17] Graphene oxide Direct laser writing (cw) Power density of 200 W cm−3 at 10,000 cycles in PVA-

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

in water

electrolyte with a specific capacitance of

>4 mF cm−2 and power densities ~ 9 mWcm−2

E = 1/2 CV (5)

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

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

storages like the batteries and supercapacitors [9] as shown in the equation,

**Material Methods Properties**

Kaner [16] Graphene oxide Direct laser writing (cw) Energy density- 10−2 Whcm−3

Gu [30] Graphene oxide Direct laser writing (cw) with specific capacitance of 350 mFcm−2 Hu [34] Multilayer polymer Direct laser writing (fs) With a specific capacitance of 42 mFcm−2

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

Tour [18] Polymer Direct laser writing (μs) BMIM-BF<sup>4</sup>

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

The improvement of further energy density obtained in laser-scribed supercapacitors using advanced techniques like super-resolution fabrication [41] along with the implementation of multifocal parallel array fabrication [42] can offer the fabrication in short period of time and deployment of self-powered autonomous systems in the next-generation technology like

Direct Laser Writing of Supercapacitors http://dx.doi.org/10.5772/intechopen.73000 113

The author acknowledges the RMIT University for the financial support.

buildings, sensors, imaging, etc.

**Acknowledgements**

**Conflict of interest**

**Nomenclature**

**Author details**

Litty V. Thekkekara

**References**

The author declares no conflict of interest.

DLW Direct laser writing

fs Femtosecond ps Picosecond

cw Continuous wave

NA Numerical aperture

EDLC Electrochemical double layer capacitance

School of Science, RMIT University, Melbourne, Australia

Address all correspondence to: littvarghese.thekkekara@rmit.edu.au

[1] Conway BE. Electrochemical supercapacitors: Scientific fundamentals and technological applications. Springer Science & Business Media. 2013. DOI: 10.1007/978-1-4757-3058-6

MSC Micro-supercapacitors

**Figure 7.** Solar energy storages using fractal electrodes on the reverse side of thin-film silicon solar cells [30]. Copyright 2017, Nature Publishing Group.

both photoelectric and storage functions in a single cell structure [39]. Due to the repeated oxidations and reductions shorten the lifetime of these batteries [1] and chemical storage functions in a single cell structure. The repeated oxidations and reductions shorten the lifetime of these batteries. The energy storages with storage mechanism free from electrochemical reactions as in batteries like capacitors can be a competent way of overcoming the problem.

In order to solve the issues, we developed solar energy storages using LSG electrodes in the interdigital form [40] as well as the fractal electrode designs (**Figure 7**) [30] and integrated on the reverse side of the silicon solar cells. The performance of the obtained solar energy storages is influenced by the efficiency of the solar cells and the discharge time for the output voltage is up to 22 days with excellent charge-discharge cycles of around 800.
