**1. Introduction**

#### **1.1. The energy problem**

The energy crisis during the 1970s sparked the development of renewable energy sources and energy conservation measures. As supply eventually met demand, these programs were scaled back. Ten years later, the hazards of pollution led to work on minimisation and reversal of the environmental impact of fossil fuel extraction, transport and consumption [1]. The United States Department of Energy predicts that 20 years from now, the world's energy consumption will increase by 20% (Figure 1). The growing concerns over the con‐ stant use of fossil fuels and its effect on climate change [2], has once again spurred re‐ search on sustainable energy development and on enhancement in renewable energy systems. Advances in energy storage and conversion systems that will make our energy usage more efficient are essential if we are to meet the challenge of global warming and the finite nature of fossil fuels [2, 3].

The need for the development of efficient energy storage systems is paramount in meet‐ ing the world's future energy targets, especially when energy costs are on the increase and more people need access to electricity [4, 5]. Energy storage technologies can improve effi‐ ciencies in supply systems by storing the energy when it is in excess, and then release it at a time of high demand [4]. Further material progression in research and development fundamentals, as well as engineering improvements need to be continued in order to cre‐ ate energy storage systems that will help alleviate humanities energy storage and conver‐ sion dilemmas.

© 2013 Antiohos et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Antiohos et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Low grade heat (around 130o C) is a by-product of almost all human activity, especially when energy conversion is involved. It is also known as "waste heat" because the dissipated heat into the environment is unutilised. Progress in the field of thermal energy conversion can lead to effective use of limited fossil fuels and provide supplemental power to current energy conversion systems [6].

carbon nanotubes (CNTs) has increased over the last 20 years, the cost of these materials has

Carbon Nanotubes for Energy Applications http://dx.doi.org/10.5772/51784 497

The advantage of incorporating carbon materials and specifically CNTs as part of the elec‐ trode material is the excellent mechanical and electrical properties. They provide mechanical support to the substrate while enhancing the conductive and electrochemical properties. The low cost of the carbon precursor material used to synthesise CNTs makes device fabrication scalable and economically viable [14]. CNT assemblies can have extremely high specific sur‐ face areas, which are extremely important in capacitor design. CNT electrode materials can be confined to a smaller area increasing the electrode-electrolyte contact and decreasing the weight of the device therefore maximising the overall gravimetric performance of the device [15]. CNTs are also chemically stable, which enhances the resistance to degradation of the

Electrical energy can be stored in two different forms and can best be described when con‐ sidering a battery and a capacitor. In a battery, it is the available chemical energy through the release of charges that performs work when two electroactive species undergo oxidation and reduction [17]; this is termed a Faradaic reaction. In a capacitor, electrostatic forces be‐ tween two oppositely charged plates will separate charge. The generated potential is due to an excess and deficiency of electron charges between the two plates without charge transfer taking place [17]. The current that is observed can be considered as a displacement current due to the rearrangement of charges [2]; this effect is termed as non-Faradaic in nature.

There are two types of electrochemical capacitors that are referred to as 1) electric double layer capacitors (EDLC) and 2) pseudocapacitors. The construction of these devices can vary, with electrodes being fabricated from porous carbon materials including activated car‐ bons, graphene, carbon nanotubes, templated carbons, metal oxides and conducting poly‐ mers [18, 19]. EDLC or supercapacitors have two electrodes immersed in an electrolyte solution, separated by a semi-permeable dielectric that allows the movement of ions to com‐ plete the circuit but prevents a short circuit from being formed. EDLCs are advantageous as they are able to provide relatively large power densities and larger energy densities than conventional capacitors, and long life cycles compared to that of a battery and ordinary ca‐ pacitor [20]. The performance of supercapacitors is affected by the power density require‐

significantly reduced alongside improvements in processability and scalability [13].

electrode surface [16].

**2. Supercapacitors**

**2.1. Background information**

*2.1.1. Supercapacitor operation and types*

**Figure 1.** The United States Department of Energy values and forecasts for energy utilisation in the period from 1980 to 2030 [5]. Reproduced with permission from Elsevier.

#### **1.2. How and why carbon nanotubes can address the issues of energy storage and conversion**

Nanostructured materials are of great interest in the energy storage and conversion field due to their favourable mechanical, and electrical properties [3, 7]. Carbon nanotubes (CNTs) are one type of nanostructured material that possess these favourable electrical and mechanical properties due to the confinement of one dimension, combined with the surface properties that contribute to the enhanced overall behaviour. The potential of nanostruc‐ tured materials is not only limited to energy storage and conversion devices; but also to nanotransistors [8, 9], actuators [8, 9], electron field emission [8, 9], and biological sensing devices [10, 11].

The use of carbon-based nanomaterials as electrode materials is practical and economically viable because cheap carbon pre-cursor materials are abundant [12]. As the research into carbon nanotubes (CNTs) has increased over the last 20 years, the cost of these materials has significantly reduced alongside improvements in processability and scalability [13].

The advantage of incorporating carbon materials and specifically CNTs as part of the elec‐ trode material is the excellent mechanical and electrical properties. They provide mechanical support to the substrate while enhancing the conductive and electrochemical properties. The low cost of the carbon precursor material used to synthesise CNTs makes device fabrication scalable and economically viable [14]. CNT assemblies can have extremely high specific sur‐ face areas, which are extremely important in capacitor design. CNT electrode materials can be confined to a smaller area increasing the electrode-electrolyte contact and decreasing the weight of the device therefore maximising the overall gravimetric performance of the device [15]. CNTs are also chemically stable, which enhances the resistance to degradation of the electrode surface [16].
