REPLACING LITHIUM-ION BATTERIES: ADVANCING AUTOMOBILES WITH GRAPHENE SUPERCAPACITORS By Dan Passarello and Toby Sun The image above is an example of a hybrid racecar that contains a supercapacitor. This model is the Toyota Hybrid Racecar (TS040). Supercapacitors are starting to be seen in all different types of most modern day vehicles including cars, buses, and several other model racecars. For instance, Honda’s Civic hybrid, the European “micro-hybrid” cars, and hybrid buses in China all turn themselves off when they would normally be idling. That “start-stop” technology normally runs off batteries alone, but French carmaker PSA uses Maxwell supercapacitors in some of its Citroen and Peugeot cars. It is notable that supercapacitors outperform lithium-ion batteries in having a shorter charge time, longer life cycle, lower cell voltage, higher power density, longer life service in a vehicle, and lower charge and discharge temperatures. Whereas lithium-ion batteries outperform supercapacitors in energy density and cost. A graphene coating on the electrodes will help boost the energy density of the supercapacitor. Costs of graphene are high but as mass production methods develop, prices will presumably drop. The image below is a two-dimensional, one-atom thick single sheet of graphene. Graphene’s unique properties and structure make it ideal for the supercapacitor. Graphene is a very porous material, meaning that it allows electricity to flow through it without much resistance, more commonly known as being a great conductor of electricity. Carbon atoms have a total of 6 electrons; 2 in the inner shell and 4 in the outer shell. The image in the bottom right illustrates the fabrication of the graphene film to be placed on the electrodes of the supercapacitor. The fabrication of the graphene based stacked films (G) can consist of an electrolyte layer being sandwiched by two rGO (reduced graphene oxide) electrodes. The rGO serves as a film to maintain the spacing of such minute layers and also slightly improves the energy density of a supercapacitor. Supercapacitors vs Lithium-Ion Batteries Properties of Graphene Why a “Graphene” Supercapacitor? Supercapacitors have two metal plates and store energy in an electric field, like normal capacitors. How they differ though is that supercapacitors are usually coated with a sponge-like, porous material known as activated carbon. The porous material creates a larger surface area and boosts the charge density. They are also immersed in an electrolyte made of positive and negative ions dissolved in a solvent. The distance between the oppositely charged electrode and the ions in a supercapacitor is much smaller than the distance between the two metal plates in a capacitor. This results in a larger electric field for the supercapacitor resulting in much more energy storage capacity. The 4 outer shell electrons in an individual carbon atom are available for chemical bonding, but in graphene, each atom is connected to 3 other carbon atoms on the two dimensional plane, leaving 1 electron freely available in the third dimension for electronic conduction. When compared to the hundreds of other coating materials, graphene has the highest thermal conductivity and electron mobility. Thousands of layers of graphene can also be stacked on top of one another, up to 891 square meters per gram. The rGO film is essentially serving as a glue to make the graphene films stay together. Along with the graphene film (G), GO is used as a dielectric spacer. D1 has the “sandwiching” exfoliated graphene layers and D2 is only comprised of a GO film. The results of testing both stacks showed that the capacitance of D1 is over two times that of D2 because of the added graphene films between GO and Au. This proves graphene film does have a positive, enhancing effect on the capacitance. As capacitance increases, so does energy density. Real World Applications