|
|
| Fig. 1: World energy consumption, 1990-2040. [1] (Courtesy of the U.S. Department of Energy) |
Rapid increase in global energy demand coupled with limited conventional energy resources (like coal, oil and gas) has motivated exploration of alternate energy resources. According to the International Energy Outlook 2013 (published by U.S. Energy Information Administration, EIA), it has been projected that world energy consumption will grow by 56 percent between 2010 and 2040 (Fig. 1). [1]
As a result, there has been an impetus on increased exploration and usage of alternative energy resources such as solar, wind, hydro etc. Technological advancements made in the field of alternative energy resources are evidenced by the fast increasing number of solar panels, battery powered hybrid vehicles etc. To enable these advancements, a key factor is effective and efficient energy storage using batteries and capacitors.
A capacitor, one of the building blocks of an electric circuit, is a two-terminal electric energy storage device made up of at least two electric conductor components separated by insulating material (dielectric). This basic nature of a capacitor is used for a wide variety of applications, ranging from energy storage to signal processing. With the recent technological advancements, the need for more efficient energy storage has implied a renewed interest in exploring more energy-dense capacitors (energy density refers to the energy-stored-to-volume ratio).
In particular, supercapacitors are a family of electrochemical capacitors being actively researched and used in the industry. Supercapacitors are also known as ultracapacitors or electric double layer capacitors (EDLC). In general, supercapacitors comprise very porous carbon impregnated with liquid electrolyte. [2]
While supercapacitors can recharge almost instantaneously, and have an extremely long lifespan, they have a rather low energy density. This implies that to store a large amount of energy, the supercapacitor needs to have a very large size. Thus, it has been a topic of research for a long time to come up with light and compact supercapacitors to meet the requirements of the rapidly progressing industry in various fields.
Graphene is a two-dimensional material made of carbon atoms arranged in a honeycomb structure that has generated unprecendented interest in the scientific community in the past decade, ever since its "accidental experimental discovery" in 2004. The properties of graphene, which is essentially a single sheet of graphite, have been under theoretical study since at least mid of twentieth century, but the material itself was believed not to be stable in its free form. Following its experimental isolation using a scotch tape and graphite, many other techniques have been reported for its synthesis.
Graphene is one of the most hotly researched topics in the field of solid state physics today. The unique chemical structure of graphene imparts this two-dimensional material many interesting properties, including very high conductivity and mechanical strength.
Various samples of graphene have been classified based on, including other factors, the number of carbon-atom layers in the sample. While strictly, graphene is defined as a single layer of carbon atoms, the scientific community has termed a sample with two layers of graphene as bilayer graphene (BLG). [3]
As a two-dimensional material, the surface area to mass ratio of graphene is exceptionally high. This, combined with the high conductivity of this material makes it very attractive for creating the conducting plates of supercapacitors in order to achieve a greater energy storage density in the supercapacitors. [2]
Recently, a graphene-based supercapacitor with energy density of 60 Watt-hours per liter has been demonstrated. [4] This number is comparable to that offered by lead-acid batteries. In this supercapacitor, porous carbon has been replaced by an adaptive graphene gel film. The liquid electrolyte used in the supercapacitor serves the additional purpose of maintaining less than a nanometer of spacing between the graphene sheets in the film.
This result is a significant breakthrough due to multiple reasons:
Firstly, the energy density of the supercapacitor has been improved almost twelve-fold.
Secondly, graphene sheet provides porosity competitive with the porous carbon that it has replaced in the supercapacitor, while maximizing the "density" of energy stored in the device.
Thirdly, the adaptive graphene gel film used in this supercapacitor can be prepared using a method similar to paper manufacture. Therefore, this supercapacitor can be manufactured on an industrial scale using a cost-effective process, making this technique very attractive for commercialization very soon.
Graphene, a two dimensional material is being actively researched for potential applications due to its intriguing physical and chemical properties. In particular, graphene has been demonstrated to be useful for fabricating high energy density supercapacitors for wide-ranging applications.
© Sumeet Trehan. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "International Energy Outlook 2013," U.S. Energy Information Administration, DOE/EIA-0484(2013), July 2013.
[2] A. Yu, V. Chabot and J. Zhang, Electrochemical Supercapacitors For Energy Storage and Delivery: Fundamentals and Applications (CRC Press, 2013).
[3] M. I. Katsnelson, Graphene: Carbon in Two Dimensions (Cambridge U. Press, 2012).
[4] X. Yang et al., "Liquid-Mediated Dense Integration of Graphene Materials for Compact Capacitive Energy Storage," Science 341 534 (2013).