Unlike prehistoric times, to harness energy is a primary survival need of human civilization in current times. However, the energy crisis is looming at us, with depleting fossil fuels. Environments are damaged world over in the process of storing and transporting energy. Clean energy that is sustainable and renewable is the goal of energy technologies.
Electrochemical Energy Storage Systems
Electrochemical energy storage systems are in place to meet these needs. These electrochemical energy storage devices consist of an electrolyte and electrodes where the charge transfer takes place. Using nano-structured materials in these systems provide enormous advantages in terms of better function and efficiency. Owing to their nanostructures, these materials open up large possibilities and unique characterizations in the energy storage technologies.
Properties and Advantages of Graphene
Graphene is a two-dimensional nanostructure that is one atom in thickness. The carbon atoms in graphene are sp2-bonded in a densely packed honey-comb structure that renders its unique properties. The wonder material of the century, graphene, has revolutionized the material sciences and its technological applications. Graphene’s unique properties include ‘high intrinsic carrier mobility (200000 cm2 V-1 s-1), exceptional thermal conductivity (5000 W m-1 K-1, 10 times better than copper), excellent optical transmittance of 97.7%, high Young’s modulus (1.0 TPa), good mechanical strength – high tensile strength with excellent flexibility, and ultrahigh surface area(theoretically, 2630 m2 g-1).’ These outstanding and unique properties make graphene a most suitable material for an assortment of applications – one of which is in the field of energy storage technology.
An additional advantage of graphene over other nanostructures is controlled fabrication and production ability in large-scale for use in industries and in practical fields. Also, graphene incurs low-cost in manufacturing.
“…Large-area graphene films are usually polycrystalline…properties of polycrystalline graphene are sensitively dependent on grain boundaries and other defects”, explain researchers Oleg Yazyev and Yong Chen.
Graphene in Energy Storage Applications
Graphene in large quantities as stacked graphene sheets contain defects and vacancies. Due to the method of synthesis, they are also called as reduced graphene oxide or functionalized graphene sheets. The required key features of energy storage applications are high energy and/or power density, long cycle life, and environmental benignity – as possible in graphene materials. Owing to all these, graphene has captured the imagination of researchers for energy storage applications.
In an electrochemical capacitor (or supercapacitors or ultracapacitors) energy is stored as ions in-between the carbon surface and the electrolyte, enabling a rapid charge and discharge. With graphene, the required combination of high surface area and excellent electrical conductivity is available. This is coupled with ion adsorption on the surface and edges of the sheets enabling continuously changing potentials - making it a great material for electrochemical capacitors.
Graphene is also used for functionalizing electrodes to enhance the electrochemical conductivity. Lithium-ion batteries, supercapacitors, and other rechargeable batteries (e.g., based upon lithium-sulfur (Li-S), lithium-oxygen (Li-O2), and sodium-ion (Na-ion) chemistries) are the prevalent energy storage devices today. Hybridizing graphene with lithium-ion batteries and supercapacitors has enhanced the performance of these energy storage systems. It improves charge/discharge efficiencies – one of the major hurdles in the energy applications.
With such high promising properties and possibilities, one question is raised: Are graphene technologies ready for commercialization? Indeed, safety concerns are, in particular, important issues to address before the common man uses these products on a daily basis. However, using graphene materials for energy applications has few challenges. Producing low-cost, large-scale and high-quality is not yet well-controlled and standardized. Graphene production is still expensive compared with other commonly used materials in commercial batteries and capacitors. Often graphene tends to stack into sheets that reduce the effective surface area. This may lead to the formation of layers, hindering ion and electron transport.
Graphene capacitors will be attractive for grid applications that require fast response time (milliseconds), high efficiency (over 95%), high power, long cycling life (more than thousands of cycles), and variation in the frequency caused by the power mismatch of supply and demand."
Jun Liu, 2014
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