|Fig. 1: Carbon Nanotube Cylindrical Structure (Source: Wikimedia Commons)|
Nanotechnology is the study of man-made matter that has dimensions less than 100 nanometers. This area of science is relatively new; in fact this exploration is in the developmental stages, with extensive research being conducted on the many possible applications of this novel technology. The considerable investigations on the applications of nanotechnology are possible because of the National Nanotechnology Initiative (NNI), which is a federal initiative in support of the scientific priorities of the President of the United States.  The federal government is interested in innovation and has therefore committed $1.5 billion in the 2016 budget for cutting-edge research on nanotechnology. Since 2001, this initiative has received more than $22 billion because of the promise it shows in the development of new and revolutionary products. The list of products developed as applications of nanotechnology is rapidly growing and revolutionizing many industry and technology sectors; one of the most important results of nanotechnology research are carbon nanotubes.
Carbon nanotubes are alternate forms or "allotropes" of carbon, just like diamond and graphene (pencil lead), which are shaped like cylinders(Figure 1). The importance of carbon nanotubes is their unique and powerful properties. They have extremely high thermal conductivity, which allows them to conduct heat very efficiently. They also have incredible mechanical, electrical, and structural properties. They are the strongest materials discovered in terms of tensile strength, the energy needed to break the material by pulling it apart, and elastic modulus, a measure of a materials resistance to having its form being manipulated elastically. 
Since the discovery of carbon nanotubes and their unique properties, specifically being lighter, more flexible and more inexpensive than current solar cell materials, and the potential to deliver high performance, a photoactive solar cell composed of only carbon has been constructed. Carbon nanotubes have extraordinary electrical conductivity and light absorption properties, particularly in the near infra-red region, therefore they are used to construct a thin film solar cell in which graphene sheets that are one atom thick and single walled carbon nanotubes that are 10,000 times narrower than a human hair are used to replace the expensive, scarce conventional electrodes made of indium tin oxide (ITO) and silver. [3,4] The active layer in the solar cell is made of carbon nanotubes and buckyballs that are just one nanometer in diameter. [3,4]
Electrochemical capacitors are energy storage devices that currently find applications in electric vehicles, uninterrupted power supplies, renewable energy and mobile devices. Carbon nanotubes have been found to be effective in electrochemical capacitors because they have good chemical stability, good conductivity, large surface area and provide a strongly entangled network that allows for the deposition of metal oxides like MnO2, manganese (IV) oxide, which enhance their performance as capacitors. 
Carbon nanotubes could help us store and use solar energy even after the sun has set. Researchers at MIT and Harvard have designed photo switching molecules that can store solar energy, which can later be used in homes for cooking or heating purposes. An example of a photo switching molecule would be a photo switching organic compound called azobenzene, which is attached to substrates of carbon nanotubes. These molecules can absorb the sun as energy, store it stably and indefinitely, and then release on demand. [6,7]
With the advance of medical technology and the design of implantable biomedical devices like cardiac pacemakers, bone growth generators, drug delivery systems, cardiac defibrillators, neurostimulators and cochlear implants and cardiac resynchronization therapy devices, that require an efficient, long-lasting power supply source for their maintenance within the body, the need for safe, efficient biofuel cells is escalating. The common batteries are unsuitable inside the human body because of their chemical nature and their size; biofuel cells house themselves in the body and use only inserted electrodes. One of the latest biocathodes consists of an enzyme surrounded by carbon nanotubes enclosed in a porous silicate matrix deposited on an oxygen permeable membrane. The carbon nanotubes facilitate transport of electrons from the surface of the semipermeable membrane.
There is no doubt that carbon nanotubes are playing a significant role in helping us meet current and future energy needs. Carbon nanotubes have been used to build a solar cell of carbon, to develop a cathode for use in fuel cells and batteries to power medical devices, to function as a catalyst in fuel cells avoiding the use of expensive platinum and in ultracapacitors. Researchers continue to search for more ways to use carbon nanotubes for the storage and enhanced utilization of existing products and designing and developing novel creations that can be used to meet energy needs.
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