Graphene+and+Ionic+Liquid+Based+Supercapacitors

=Introduction=

toc Supercapacitors are an alternative power source that have recently began to be researched more upon. [|Capacitors] are known to have small energy densities, which is why their applications as primary power sources to technologies has not been achieved. Research has gone into increasing the energy density of capacitors, and as of right now, recent supercapacitors are demonstrating higher capacitances than before. Supercapacitors have the ability to store and release energy at high rates, as well as having a long life time [1]. These traits have brought attention to supercapacitors as a potential candidate for energy storage. Additionally, producing supercapacitors that are Graphene and [|ionic liquid] based have shown to significantly increase the capacitance of these capacitors. These supercapacitors are made using Graphene plates as [|electrodes], and coating those electrodes with an ionic liquid. The high capacitance of these supercapacitors has increased research in this area due to potential applications of the devices in future technologies, as well their practicality in current technologies. Research has gone into these types of supercapacitors; focusing on the advancement of its capacitance, Graphene electrodes, ionic liquids for coating, methods of production, and their applications.

= Advancements in Capacitance =

Supercapacitors are given their name because they are capacitors that have the capability of storing more energy than regular capacitors. In recent years, innovations to the supercapacitor have yielded capacitances that are greater than those of ordinary capacitors. Capacitors are usually made using metals such as aluminum and tantalum, as well as others. These metals are used as electrodes and are combined with a [|dielectric] to store energy. Ordinary capacitors such as these do not hold an abundance of energy. A tantalum based capacitor has shown to have a capacitance between 1 to 100 μF [2]. Most ordinary capacitors fall into this range. Supercapacitors with a Graphene based electrode in combination with an ionic liquid have shown to have capacitances up to 691 F/g [3]. Research suggests that using Faradaic materials in supercapacitors is what boosts the energy density of the capacitor [3]. This capacitance can potentially be increased with different types of materials, as well as the methods of which its produced.

= Graphene =

Research in Graphene has increased in recent years due to its characteristics and potential scientific applications. Graphene is a one atom thick material that is a remarkable conductor, making it a strong candidate to be used as an electrode [4]. Graphene has the ability to output a tremendous amount of power, which fits the vision for supercapacitors as a power source [4]. The application of Graphene in supercapacitors in the form of an electrode has been widely recognized to be effective in increasing the capacitance [5]. This is attributed to its conductance characteristics, as well as its surface area and pores. Graphene has a large surface area which is vital to increasing the capacitance of a supercapacitor since capacitance depends on the area of the electrodes [5]. Graphene has a constant pore size distribution which also increases the capacitance of the capacitor. A constant pore size distribution along its area means that more ions are able to pass through the pores, which in turn will increase the capacitance of the system [5]. It is known that the method in which Graphene electrodes are architecturally constructed can increase the capacitance. Layering the Graphene sheets in specific positions allows ions to access inner Graphene sheets’ pores, thus increasing the specific capacitance [5]. It is still unknown how much further capacitance in Graphene supercapacitors can be increased, however Graphene has shown to be an excellent electrode, capable of providing large capacitances.

= Ionic Liquids =

Coating an electrode in an ionic liquid has the potential to increase the energy storage in a capacitor as well as its charge [6]. It has been shown that different types of ionic liquids have the capability to increase capacitance depending on the ionic liquid used. The ionic liquid NItrogen was used in combination with a Graphene electrode and produced a capacitance of 258 F/g [7]. Strong [|electrolyte] liquids such as sulfonated polyaniline have demonstrated capacitances up to 368.53 F/g [8]. These ionic liquids play a role in increasing the electron transfer rate reaction through the pores of the electrode [8]. By increasing this electron transfer rate, more ions can access pores on the electrodes, which can lead to an increase in capacitance. Ionic liquids have also shown to increase the voltage and life cycles of capacitors [9]. Increasing the voltage of a capacitor will in turn increase the capacitance. Efforts have gone into researching more about the effects that ionic liquids have on capacitors, however it is important to note that the type of ionic liquid used in combination with an electrode is what produces a higher capacitance [9]. A suitable ionic liquid must be used with an appropriate electrode, as the ionic liquid must be able to attach to the electrode, which can also depend on the properties of the electrode material used itself [6].

= Methods of Production =

Graphene and ionic liquid based supercapacitors can be produced in a wide variety of ways. There are many methods used by researchers to obtain ionic liquid-doped Graphene electrodes of which produce different capacitances. A popular way of producing Graphene electrodes is through the modified [|Hummers method]. This is a chemical process in which Potassium Permanganate is used to make Graphene electrodes [7]. The Graphene plates are then washed with water and ethanol, and then dried [10]. The process is simple and can be easily replicated amongst scientists. Many scientists use this method due to the fact that it has produced Graphene supercapacitors with high capacit ances. Supercapacit ors with capacitances up to 175 F/g in one study [7], and up to 775 F/g in a different study have been achieved [10]. This method assists in producing high conductive electrodes that increase the electron transfer rate reaction [10]. Graphene electrodes are also produced taking into account the architectural positioning of the Graphene sheets. One method uses spacers in between Graphene sheets in order to allow for the pores of the sheets to be accessible to ions, as stacking these sheets can sometimes lead to aggregation of the pores [11]. The spacers in this method have also shown to increase the capacitance. A different method for stacking Graphene sheets is to use curved Graphene electrodes [12]. Instead of using spacers this method provides accessibility to the Graphene’s pores through curved positioning of the sheets, thus leaving small gaps for pores and increasing the capacitance of the system.

= Applications =

Graphene supercapacitors are extremely flexible and lightweight, which when applied to technologies as a source of power, can dynamically change the way technology functions. Graphene supercapacitors can be applied to areas such as wearable and nanotechnology [4]. Flexible smartphones that go on your wrist can be powered through Graphene supercapacitors. Applications in vehicles such electric cars like [|Tesla] will function better with the use of Graphene supercapacitors. Electric cars can be potentially charged quickly, as well as having a lightweight and long lasting power source [4]. In hybrid and electric cars, these supercapacitors can be implemented into the car and can store energy from regenerative braking [13]. This will greatly increase the efficiency of the car, and will introduce into the car industry an eco-friendly alternative to combustion engine cars. It is still unclear if these supercapacitors can definitively replace batteries as a power source, but it is known that they can definitely act as a supportive energy source in applications such as these.

=Conclusion=

Scientists have clearly demonstrated that changing small components and production methods of the supercapacitor have yielded capacitances higher than those of ordinary capacitors. This is why scientists continue to explore more materials and methods of production to innovate the supercapacitor. It is important that scientists continue to research on the combination of certain ionic liquids with Graphene electrodes. Some combinations such as the Graphene and sulfonated polyaniline combination yielded better results than that of the Graphene and Nitrogen combination. Learning which ionic liquids complement the electrode’s pores and create easy access for ions will improve the capacitor. Combining that with the way that Graphene sheets are produced and positioned in an electrode, knowing that having a sufficient pore distribution is essential, will lead scientist to creating supercapacitors with a high energy density as studies have shown. Evidently, the method of which Graphene plates are produced plays a significant role in the capacitances demonstrated. Therefore, scientists have experimented with different techniques for producing them, and have shown that the modified Hummers method has worked well. However, researchers should explore other options for creating Graphene electrodes in order to observe if the capacitance and can be raised. We know that an improved supercapacitor can allow for flexible and lightweight technology to be possible, given the properties of the material, and therefore improve the functioning of technologies.

= References  =

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[4] ( 2017). The Home of Graphene [online]. Available: http://www.graphene.manchester.ac.uk/

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[11] B. Skinner, “ Model of large volumetric capacitance in graphene supercapacitors based on ion clustering,” Physical review. B, Condensed matter and materials physics, vol. 84, no. 23, pp. 235133, 2011, doi: 10.1103.

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