Aung1 Making Graphene-­‐Based Supercapacitors by LightScribe Theint Aung, Advisor: Dr. Derin Sherman Cornell College, Department of Physics, Mount Vernon, IA I. Introduction Supercapacitors, also known as electrochemical capacitors, fundamentally differ from conventional capacitors in that a standard capacitor comprises two metal plates and a dielectric material whereas a supercapacitor makes use of two electrodes separated by an ion porous medium in an electrolyte [1]. Given the added advantages of short charging period, flexibility, being lightweight and environmentally safe compared to batteries, supercapacitors are considered to be the solutions to our energy storage issues [2]. A recent study done by researchers at UCLA introduces a novel way of making graphene, which then can be used to build high performance supercapacitors [1]. For my project, I aim to repeat the experiments and study the electrical properties of graphene-­‐based supercapacitors. II. Project Overview Although batteries are widely used for their high energy densities (20-­‐150Wh/kg), their low power density, i.e., long charging time, raises practical issues when used in certain devices such as electric vehicles. Capacitors, on the other hand, exhibit high power densities despite their low energy densities (4-­‐5Wh/kg) [2]. Without a doubt, an innovation that combines the best attributes of batteries and capacitors will be a revolutionary advancement in energy storage technology. Currently, graphene-­‐based supercapacitors have fast become the most promising options since in theory they can provide both high power densities and ultrahigh energy densities given their high capacitance. Similar to that of standard capacitors, the capacitance of supercapacitors depend on the surface area of the electrodes accessible to the electrolyte since the capacitance comes from the charge accumulated at the electrode/electrolyte interface [2]. Therefore, to achieve high capacitance levels without increasing the size of the electrodes, the electrode material should have exceptionally high surface area to volume ratio. One such candidate that has long been considered is graphene. Graphene is a single atom-­‐layered sheet of graphite where sp2 carbon atoms are covalently bonded to form a 2D hexagonal lattice. Since, in a graphene sheet, a carbon atom with four valence electrons is only bonded to three other carbon atoms, one electron is left unbounded per atom. Such abundance of free electrons provides Aung2 graphene with excellent electrical conductivity [2]. Additionally, it has been shown that graphene sheets have high mechanical flexibility with only ~1% change in the electrical resistance after 1000 bending cycles. Most importantly, the specific surface area of graphene is a staggeringly high 2675m2/g. Given the fact that the intrinsic capacitance of a single-­‐layered graphene sheet is approximately 21μF/cm2, if we can make full use of the surface area available, a graphene-­‐based supercapacitor can achieve capacitance as high as 550F/g [1-­‐2]. However, up until recently, one major problem with using graphene in supercapacitors persisted. Due to strong Van Der Waals attractive forces between individual graphene sheets, the sheets in close proximity tend to restack during processing. Such restacking not only causes a lowering of accessible surface area to electrolytes but also decreases the charge/discharge rates of the supercapacitors. With restacking posing as a problem, the supercapacitors built only attained capacitance values only on the order of 100F/g, which is only ~20% of the theoretical upper limit [1]. The major breakthrough for graphene-­‐based supercapacitors came when the researchers at UCLA discovered an all-­‐solid approach of making graphene that avoided the restacking issue [1]. As shown in the schematic diagram below (Figure 1), a thin film of graphite oxide, an oxidized form of graphite, dispersion in water was drop-­‐casted onto a flexible substrate supported on a disc. The graphite oxide film was then burned with infrared laser irradiation using a commercially available LightScribe drive to produce graphene. Figure 1: Schematic Diagram of Making Graphene-­‐Based Supercapacitors Aung3 III. Experimental Procedure Equipment I used Fisher Scientific Ultrasonic Cleaner (100W, 42kHz) for ultrasonication and Thermal Spa UV Gel Nail Polish Dryer (45W, 375nm) as a UV lamp device. The LightScribe drive was LG Super Multi Drive GE24 complete with CyberLink Media Suite Labeling Software. Verbatim DVD-­‐R 16x LightScribe discs were used. III. A Preparation of Graphite Oxide from Graphite The following two-­‐step synthesis is referenced from the modified Hummer’s method reported previously [3]. Procedure: Pre-­‐oxidation step: Pour 7.5mL of concentrated sulfuric acid into a 100mL beaker with a thermometer in place. With stirring, add potassium persulfate (2.5g) and phosphorus pentoxide (2.5g) to the acid solution and gently heat the mixture. Care should be taken during this step since the reaction is highly exothermic and the temperature will go as high as 130oC. Once the solution turns clear and colorless, remove the solution from heat. When the temperature of the solution drops to 80oC, add graphite crystals (5g) slowly to the beaker. Frothing and volume expansion will be observed upon graphite addition. Stir the mixture until the mixture has turned into a thick, silvery dark blue paste. Wrap the beaker in aluminum foil and allow the paste to cool to room temperature over 3 hours. After the cooling period, add enough distilled water to the beaker (~70mL) to thin the paste. Afterwards, filter the solution and wash the solids with additional distilled water until the pH of the washes tests neutral. Dry the solids overnight at room temperature. The dried graphite obtained should be silvery, grey flakes. Hummer’s method: Pour 23mL of concentrated sulfuric acid in a 250mL beaker with a thermometer in place. Refrigerate the beaker in an ice bath for about 20 minutes to cool the acid solution down to about 0-­‐4oC. Add 1g of graphite crystals obtained previously to the beaker with stirring. Slowly add potassium permanganate (3g) to the mixture over 5-­‐10 minutes while carefully maintaining the temperature below 20oC at all times. By the end of the addition, the mixture should have thickened slightly and is very dark green. Stir the mixture for 2 hours at 45-­‐50oC. You will notice that the reaction itself produces heat and you will only need to heat intermittently. During this time, the mixture should completely lose its green color and thicken into a deep purplish grey paste. At the end of stirring, slowly add 46mL of distilled water into the paste. The solution is now a slight reddish brown color. Stir the solution for 15 more minutes before adding 140mL of distilled water and 2.5mL of 30% hydrogen peroxide solution. Upon the addition of hydrogen peroxide, the solution should immediately turn into a bright shade of yellow. Since the filtration process takes a long time (on the order of 2-­‐3 hours), I suggest centrifuging the solution and decanting the supernatant in order to Aung4 speed up the process if such equipment is available. After the filtration, disperse the solids in 250mL of hydrochloric acid solution (220 mL of distilled water and 30mL of hydrochloric acid) to get rid of the metal ions (Figure 2). Filter the solution in small portions to obtain brownish yellow cake of graphite oxide. Dry graphite oxide in a desiccate chamber overnight. Figure 2: Graphite Oxide Solids III. B Preparation of Graphite Oxide Sample for LightScribe Procedure: Cut a polyethylene terephthalate (PET) film into the shape of a disc. Care should be taken so that the cut film is perfectly flat and does not cover up the silver ring of barcode on the LightScribe disc. Also since the LightScribe drive is very sensitive, make sure that the film has no stray ends that can potentially interfere with the disc rotation. Dissolve 100mg of graphite oxide in 27mL of distilled water (3.7gmg/ml) and ultrasonicate for 1 hour until the solution is a homogenous, light brown viscous mixture [4]. Draw approximately 15mL of the dispersion in a syringe and drop-­‐cast onto the pre-­‐
cut PET film. Let the film dry at room temperature for 24 hours (Figure 3) [1]. Prior to burning the graphite oxide film in LightScribe drive, expose the film to ultraviolet radiation for 4 hours in a UV lamp device, which, in this experiment, is a UV nail polish dryer. After the treatment with UV, the film should be darkened (Figure 4) with the resistance markedly decreased from >20MΩ to <0.5MΩ. Aung5 Figure 3: Dried Graphite Oxide Film on PET Substrate Figure 4: Graphite Oxide Film After 4Hrs UV Exposure III. C Preparation of Laser Scribed Graphene (LSG) Procedure: Load the graphite oxide film onto the Light Scribe disc using Scotch Spray Mount. The film should be adhered to the label side of the disc, i.e., the textured side of the disc. Place the disc in the LightScribe drive with the film side down. With the contrast set to the best, burn the disc for at least 6 cycles to get consistently reduced areas of graphene on the film (Figure 5) [1]. To burn the entire surface of the film, each cycle of LightScribe may take up to 2 hours depending on the contrast settings. Aung6 Figure 5: Graphene Produced After 6 Cycles of LightScribe III. D Preparation of All Solid Gel-­‐Electrolyte Supercapacitor Procedure: Mix 1g of polyvinyl alcohol powder (PVA) with 10mL of distilled water in a beaker equipped with a stir bar and a thermometer. Stir and heat the mixture at ~90oC until the mixture thickens and turns clear. Take the mixture off the heat and allow it to cool down under ambient conditions. Add 0.8g of concentrated phosphoric acid (85.3%) to the mixture and stir thoroughly [4]. Cut out two identical pieces of LightScribed graphene to serve as electrodes. Put on a piece of copper tape on the handle of each electrode to ensure firm contact with the alligator clips. Coat the surface of the electrodes with PVA/phosphoric acid electrolyte (~100μL/cm2) and let the electrodes sit for 5 hours before assembling the two electrodes (Figure 6). Allow the electrolyte to solidify overnight [1]. Figure 6: Solid Electrolyte Graphene Supercapacitor (Area: 4cm2) III. E Preparation of Aqueous Electrolyte Supercapacitor Procedure: Cut out two identical pieces of LightScribed graphene to serve as electrodes. Put on a piece of copper tape on the handle of each electrode to ensure firm contact Aung7 with the alligator clips. Soak a piece of lens cleaning tissue in 1M phosphoric acid and wedge the tissue in between the two electrodes to serve as the separator. Place the entire ensemble between two microscope slides held together by paper clips. III. F Measuring the Capacitance of Supercapacitors Procedure: Construct a simple RC circuit by placing a known resistor in series with the supercapacitor of which the capacitance is unknown. With a function generator outputting square waves with amplitudes of 3V for gel-­‐electrolyte supercapacitors and 5V for aqueous electrolyte supercapacitors at a frequency of 1kHz, record the change in voltage across the capacitor with an oscilloscope. Determine the time taken by the capacitor to drop to half of its initial voltage (t). Using the following equation that relates resistance (R), capacitance (C) and half life (t), 𝑡
𝑅𝐶 = ln 2
the capacitance of the supercapacitor can be determined. IV. Results & Discussion IV. A Preparation of Graphite Oxide from Graphite The first part of this project is synthesizing graphite oxide, the starting material, from graphite. Although there are multiple variations of Hummer’s method used for graphite oxide synthesis, a pre-­‐oxidation step is shown to be necessary before graphite is subjected to Hummer’s method. Since the primary goal of Hummer’s method is to oxidize graphite, the pre-­‐oxidation step minimizes the occurrence of incompletely oxidized graphite-­‐core in the final product [3]. If unoxidized graphite is mixed with graphite oxide, it will be apparent in the color of the mixture, which, instead of bright yellow, will take after a shade of brownish orange after the addition of hydrogen peroxide at the end of the synthesis. IV. B Preparation of Graphene from Graphite Oxide Although the published procedure does not call for irradiation of graphite oxide film with UV prior to the LightScribe process, such intermediate step was included to maximize the yield of graphene on film. When this step was omitted, only sparse and segmented rings of graphene were observed. Varying the reaction parameters such as the concentration of graphite oxide dispersion and the amount used to coat the substrate also proved to have very little to no effect on improving the results. Although UV radiation has been shown to induce reduction of graphite oxide to graphene in solution [5], there is no previous data on solid-­‐state reduction. However, since UV exposure darkened the graphite oxide film and significantly increased its conductivity, Aung8 the reduction of graphite oxide to graphene must have occurred to a considerable extent. Such darkening of the film increases the absorbance of the irradiation during LightScribe, which may be one of the reasons UV exposure helps with the subsequent LightScribe process. IV. C Graphene-­‐Based Supercapacitors As for the supercapacitors, regardless of the electrolyte (PVA gel vs phosphoric acid), their capacitance values were measured to be approximately 2.5nF/cm2. Although the capacitance values were a fraction of previously reported value (~1.84mF/cm2), it is worth noting that at 2.5nF/cm2, the graphene capacitors exhibited capacitances much higher than what a standard parallel-­‐plate capacitor can offer (~9pF/cm2). The results may be improved by ensuring a complete reduction of graphite oxide to graphene through more cycles of LightScribe. It is possible that partially reduced graphite oxide present in the electrodes lowers the effective capacitance by acting as a small capacitor in series with a large one. V. Conclusion Although the capacitors produced were at a much lower capacitance than is experimentally feasible, I was able to make graphene-­‐based supercapacitors using LightScribe technique and address problems as they rose throughout the process. Future work includes, but is not limited to, optimizing the performance of supercapacitors, determining the mechanism behind UV irradiation on graphite oxide and characterizing the graphene produced after UV treatment and LightScribe. VI. References [1] M. F. El-­‐Kady et al., Laser Scribing of High-­‐Performance and Flexible Graphene-­‐Based Electrochemical Capacitors. Science. 335, 1326 (2012). doi:10.1126/science.1216744 [2] C. Liu et al., Graphene-­‐Based Supercapacitor with an Utrahigh Energy Density. Nano Letters. 10, 4863-­‐4868 (2010). doi:10.1021/nl102661q [3] N. I. Kovtyukhova et al., Layer-­‐by-­‐Layer assembly of ultrathin composite films from micronsized graphite oxide sheets and polycations. Chem. Mater. 11, 771 (1999). doi:10.1021/cm981085u [4] V. Strong et al., Patterning and Electronic Tuning of Laser Scribed Graphene for Flexible All-­‐Carbon Devices. ACS Nano. 6, 2 (2012). doi:10.1021/nn204200w [5] P. Kumar, K.S. Subrahmanyam, C.N. Rao, Graphene Produced by Radiation-­‐Induced Reduction of Graphene Oxide.