Carbon Nanotube-Conducting Polymer Composites in Supercapacitors
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Supercapacitors, also referred to as electrochemical capacitors or ultracapacitors, are devices used to store and deliver electrical energy in high-power pulses. With the advent of electric vehicles, digital communication, and other electronic devices that require significant bursts of electrical energy, the need for supercapacitors has expanded rapidly. At present, the most promising materials on which supercapacitors are based can be divided into two categories—those that make use of a double-layer charge storage mechanism (e.g., carbon nanotubes, carbon aerogels, and activated carbon black) and those employing a redox pseudo-capacitive charge storage mechanism (e.g., conducting polymers and transition metal oxides). Already, the electrical charge that can be stored in each of these materials is typically several orders of magnitude larger than that of most commercially available conventional capacitors. However, it has been shown in recent times that even greater charge storage capacitances can be achieved in composites made by combining carbon nanotubes (a double-layer capacitive material) with a conducting polymer (a redox pseudo-capacitive material). The superior charge storage performance of carbon nanotube-conducting polymer composite supercapacitors arises from their ability to merge the properties that separately make carbon nanotubes and conducting polymers so suited to their respective charge storage mechanisms. That is to say, the composites are able to combine the high surface area and electrical conductivity of carbon nanotubes with the redox electrochemistry of conducting polymers.