http://www.chem.ucla.edu/dept/Faculty/kaner/ Supporting Online Material for Laser Scribing of High-Performance and Flexible Graphene-Based Electrochemical Capacitors Maher F. El-Kady, Veronica Strong, Sergey Dubin, Richard B. Kaner*
- To whom correspondence should be sent. E-mail: email@example.com
Published 16 March 2012, Science 335, 1326 (2012) DOI: 10.1126/science.1216744 This PDF file includes: Materials and Methods Figs. S1 to S16 References (33–39) Other Supporting Online Material for this manuscript includes the following: (available at www.sciencemag.org/cgi/content/full/335/6074/1326/DC1) Movie S1 2 Materials and Methods 1. Synthesis of Laser Scribed Graphene (LSG) electrodes: GO was synthesized from high purity graphite powder using a modified Hummer’s method as reported previously (33). Dispersions of GO in water (3.7 mg/mL) were then used to make GO films on various substrates, including polyethylene terephthalate (PET), nitrocellulose membrane (with 0.4 μm pore size), aluminum foil, and regular Xerox paper, among others. GO films were made by either drop-casting or vacuum filtering GO dispersions onto substrates that were previously cut to the size of a CD/DVD media disc. The films were then allowed to dry for 24 hours under ambient conditions (Fig. S1). These films were affixed on top of a LightScribe enabled DVD media disc and moved into the DVD optical drive for laser treatment. LightScribe is a direct labeling technology that patterns text and graphics onto the surface of a CD/DVD disc. LightScribe DVD drives are commercially available for ~ $20 and the LightScribing process is completely controlled by a standard desktop computer. The drive uses a laser (optimum power output = 5 mW, wavelength = 788 nm) to pattern a computer-generated image onto a light sensitive dye that changes color when hit with the laser; here, we use the GO film instead. The images are patterned in concentric circles, moving outward from the center of the disc as shown schematically in Fig. 1. The laser irradiation process results in the removal of oxygen species and the reestablishment of the sp2 carbons. This causes a change in the conductivity of the film from the insulating graphite oxide, with a typical resistance of >20 MΩ/sq to highly conducting laser scribed graphene. We have shown that the number of times a film is laser-treated results in a significant and controllable change in conductivity (17). The electrodes used in the fabrication of the electrochemical capacitors are laser irradiated 6 times reaching an excellent conductivity of 1738 S/m. The laser irradiation process takes about 20 min per cycle. The produced laser scribed graphene (LSG) possesses very low oxygen content (only 3.5%) (17) that contributes to the very high cycling stability of the electrochemical capacitor. The thickness of the LSG layer, as measured from crosssectional SEM and profilometry, was found to be ~7.6 μm (Fig. S2). In the actual device, the area made accessible to the electrolyte was 1 cm2, which corresponds to a mass of 36.3 μg of the active material (LSG) per electrode or 72.8 μg for the device. 2. Electromechanical properties of laser scribed graphene electrodes: Flexible electronics have impacted our life in several ways, including flexible transistors, sensors, displays, etc. They are made possible by using flexible, mechanically robust and highly conducting electrodes. Nowadays, there is a great interest in using graphene films as flexible electrodes in these devices because of its excellent electrical, mechanical and optical properties (34). Here, we have investigated the electromechanical properties of the LSG electrodes to explore their potential for flexible devices. Fig. S3A shows the change in the resistance of an LSG electrode as a function of its bending radius. The electrical resistance shows a small decrease (2.8%) upon bending and this change is completely reversible by straightening the electrode back to its original flat state. Interestingly, the electrical resistance of the electrodes fluctuated up and down with only ~1% change after 1000 repeated bending cycles, Fig. S3B. Thus, LSG electrodes hold promise for other flexible electronics besides the electrochemical capacitors discussed in this paper. 3 3. Measurement of the specific surface area of laser scribed graphene: The surface area for the LSG was calculated to be ~1520 m2/g using the methylene blue adsorption method. Methylene blue is a common dye probe used to determine the surface area of graphitic materials, with each molecule of adsorbed methylene blue representing 1.35 nm2 of surface area (35-38). The surface area was calculated by adding a known mass of LSG into a standard concentration of methylene blue in DI water. LSG was stirred in the methylene blue solution continuously at a rate of 300 rpm for a total of 24 hr to reach maximum adsorption. The mixture was then allowed to settle and further centrifuged to remove any suspended material. The methylene blue concentration was determined by analyzing the supernatant through UV-vis spectroscopy at a wavelength of 665 nm and compared to the initial standard concentration of methylene blue prior to interacting with LSG. 4. Fabrication of LSG electrochemical capacitors: A simple device architecture has been employed to make the LSG electrochemical capacitors. Two LSG films serve as both electrodes and current collectors in a symmetric two-electrode cell configuration. LSG films made on various substrates are flexible and mechanically robust and thus can be directly used as electrochemical capacitor electrodes without any additional binders or conductive additives. Additionally, the high conductivity of the LSG films enables the construction of devices without the need for metallic current collector electrodes used in commercial electrochemical capacitors. In a typical device, an ion-porous separator [Celgard® 3501, (Celgard, Charlotte, NC)] was sandwiched between two identical pieces of LGC films (area, ~2 cm2) in a layered structure (see Fig. 1). This layered assembly was wrapped with Kapton tape and then dipped in the electrolyte. To evaluate the electrochemical capacitor performance, the device was connected to an electrochemical workstation using alligator clips. Electrode edges were lightly painted with conducting silver paint to ensure good electrical contact between the LSG films and the alligator clips. The dimensions of the device and volume are shown in Fig. S4. Celgard® 3501 separators were generously supplied by Celgard, LLC Corporate Headquarters in Charlotte, NC, USA. 5. Fabrication of all-solid-state flexible LSG electrochemical capacitors: All solid-state devices were assembled by pouring the polymer gelled electrolyte (100 μL electrolyte/1 cm2 of the electrode) slowly onto the LSG electrodes. This assembly was left under ambient conditions for 5 hours to ensure that the electrolyte completely wets the electrode and to allow for evaporation of any excess water. Two electrodes were then assembled face-to-face and left overnight until the electrolyte solidified. This results in mechanically robust devices with the polymer electrolyte acting as both the electrolyte and the ion-porous separator. The solidified polymer electrolyte simplifies the device architecture and helps maintain the electrodes in closer proximity. 5. Preparation of the polymer gelled electrolyte: The gelled electrolyte was prepared according to the method described in reference (39). Briefly, polyvinyl alcohol (PVA) (molecular weight 89,000-98,000, 99% hydrolyzed, Sigma-Aldrich) powder was mixed with water (1 g PVA/10 g H2O). The mixture was heated at ~90 °C under constant stirring until the solution turned clear. After cooling under ambient conditions, 0.8 g of concentrated phosphoric acid solution (85% solution in water, Fisher Scientific) was added and the solution was stirred thoroughly. 4 6. Characterization of the electrochemical capacitors: The performance of our prototype electrochemical capacitors were evaluated by cyclic voltammetry (CV), galvanostatic charge/discharge (CC) curves using a VersaSTAT3 electrochemical workstation (Princeton Applied Research, USA). The electrochemical capacitor performances were evaluated in various electrolytes including aqueous and organic electrolytes as well as ionic liquids. Two different aqueous electrolytes were used, 1.0 M H3PO4 and 1.0 M H2SO4 (Sigma-Aldrich). The organic electrolyte consisted of 1.0 M tetraethylammonium tetrafluoroborate (Simga-Aldrich) in dry acetonitrile (Sigma-Aldrich). The ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) was obtained from Sigma- Aldrich and was used without further treatment. Electrochemical impedance spectroscopy (EIS) was carried out on a Solartron electrochemical workstation (Solartron Analytical, Oak Ridge, TN). EIS experiments were carried out at open circuit potential with a sinusoidal signal with amplitude of 10 mV over a frequency range of 100 kHz to 10 mHz. The sheet resistance and conductivity values were calculated from two-point probe measurements and the film thicknesses. The morphologies of GO and LSG were imaged using scanning electron microscopy (JEOL 6700, Philips XL 30). The film thicknesses of each layer in the device (e.g., the penetration depth of the laser into the GO layer, LSG layer, gelled polymer layer, etc.) were measured on a Dektak 6 profilometer and using cross-sectional scanning electron microscopy. The device dimensions are summarized in Fig. S4. 7. Characterization of commercial capacitors: A commercially available activated carbon electrochemical capacitor, PANASONIC EECS0HD223H Supercap 5.5 V, 22 mF was used. It consists of two electrochemical capacitors connected in series. A single device was separated from this serial assembly for characterization. An aluminum electrolytic capacitor, Mouser Electronics, 75-TE1066-E3, 3V/300μF was also tested. All the devices were characterized under the same dynamic conditions as the LSG-EC. 8. Tandem electrochemical capacitors: To be useful as energy storage devices in vehicles and peak-load applications, tandem electrochemical capacitors are usually assembled in series, in parallel or in combinations of the two. By choosing different tandem electrochemical capacitor assemblies, it is possible to control and improve the output potential and current (capacitance) so that the energy and power requirements can be met. Tandem LSG electrochemical capacitors were made simply by wiring four devices together in series, in parallel and in series-parallel combinations and characterized as shown in Figs. S12-S14. 9. Calculations: The capacitance of each device was calculated from the galvanostatic (CC) curves at different current densities using the formula: Cdevice = i/(-dV/dt) (1) Where i is the current applied (in amps, A), and dV/dt is the slope of the discharge curve (in volts per second, V/s). Specific capacitances were calculated based on the area or the volume of the device stack according to the following formulae: Areal capacitance = Cdevice/A (2) Volumetric stack capacitance = Cdevice/V (3) 5 Where A and V refer to the area (cm2) and the volume (cm3) of the device, respectively. It is worth mentioning that the volumetric capacitances were calculated taking into account the volume of the device stack. This includes the active material, the flexible substrate (or the current collector) and the separator with electrolyte. For the device testing, we did not include the packaging. The area and volume of the device used in the calculations can be found in Fig. S4. The electrochemical performance of each device reported in Fig. 5 is based on the volume of the stack and measured under the same dynamic conditions from galvanostatic curves. The power of each device was calculated from the galvanostatic curves at different charge/discharge current densities using the formula given in Equation (4): P = (ΔE)2/4RESRV (4) where P is the power (W/cm3), ΔE is the operating voltage window (measured in volts and obtained from the discharge curve excluding the IR drop) and V is the volume of the stack as indicated earlier (in cm3). RESR is the internal resistance of the device which can be estimated from the voltage drop at the beginning of the discharge, Vdrop, at a constant current density (i) using the formula RESR = Vdrop/2i. The energy density of the device was obtained from the formula given in Equation (5): E = CV × (ΔE)2/(2 × 3600) (5) where E is the energy density in Wh/cm3, CV is the volumetric stack capacitance obtained from Equation (3) in F/cm3 and ΔE is the operating voltage window in volts. 6 Fig. S1. Schematic illustration of the preparation of GO films: (A) The GO precursor is made by oxidation of graphite using a modified Hummer’s method and then dispersed in water by ultrasonication. GO films are coated onto various flexible substrates such as polyethylene terephthalate, aluminum foil, a porous nitrocellulose membrane (shown in the photograph) or regular photocopy paper. 7 Fig. S2: Laser scribing of graphene from graphite oxide film on a polyethylene terephthalate (PET) flexible support. (A) A cross-sectional SEM image of the film at the interface between GO and LSG. As the LightScribe laser converts GO into LSG, a substantial expansion of the film is observed. (B) A zoomed-in view of the interface region. (C) and (D) show expanded views of the locations 1 and 2, respectively. The laser penetrates all the way into the film producing the expanded LSG network. The LSG electrode possesses a significant thickness and shows potential for electrochemical capacitors with high power and energy densities. 8 Fig. S3. Electromechanical properties of LSG electrodes made on polyethylene terephthalate (PET) flexible supports. (A) The electrical resistance change of an LSG electrode as a function of the bending radius. The electrode shows a decrease in the electrical resistance of ~2.8% and this change is completely reversible by flattening back the electrode to its original flat state. (B) The resistance change of an LSG electrode under repeated bending cycles for a bend radius of 5 mm. Only a slight change in resistance of ~1% can be observed after 1000 repeated bending cycles. 9 Fig. S4: Schematic diagrams showing cross-sections of the electrochemical capacitors with their device dimensions. (A) Two LSG electrodes are supported on a flexible substrate or an Al foil current collector and separated with an organic ion-porous separator [Celgard® 3501 (Celgard, Charlotte, NC)]. The electrolyte wets the LSG layer and the separator. This design of the electrochemical capacitor was tested in a variety of electrolytes including aqueous, organic and ionic liquid electrolytes. The device effective thickness is 82.2 μm, with an active area of 1 cm2 and a volume of 8.22 × 10–3 cm3. (B) In an all solid-state device, the organic separator is replaced with a gelled electrolyte that has a thickness of 11 μm and thus the device thickness becomes 68.2 μm with a volume of 6.82 × 10–3 cm3. 10 Fig. S5: Performance of an LSG electrochemical capacitor over a wide range of scan rates: cyclic voltammetry profiles of an LSG electrochemical capacitor in aqueous 1.0 M H3PO4 at different scan rates: (A) 100 mV/s to 1000 mV/s; and (B) 2000 mV/s to 10,000 mV/s. (C) The dependence of the capacitive current (extracted from the CV profiles at 0.5 V for the charge and discharge curves) on the applied scan rate. A linear relationship is observed with R2 = 0.997 and 0.996 for the charge and discharge curves, respectively. (D) The specific capacitance of the device calculated from the galvanostatic curves as a function of the charge/discharge current density. The specific capacitance data are presented on the basis of the area of the device or the volume of the entire device stack (Fig. S4). Note that the current density shown is based on the volume of the whole stack or the mass of the LSG in one electrode. The areal capacitance of the LSG-EC was calculated to be 3.67 mF/cm2 at 1 A/gLSG/electrode. This translates to a gravimetric capacitance of 202 F/gLSG/electrode. 11 Fig. S6: Capability of an LSG electrochemical capacitor for ultrafast charging and discharging in the sub-second time scale as tested in aqueous 1.0 M H3PO4: galvanostatic charge/discharge curves obtained at ultrahigh current densities of (A) 100 A/g; and (B) 500 A/g (based on the mass of the LSG per electrode). As can be seen in B, the device can be efficiently charged/discharged multiple times in less than a second. 12 Fig. S7: Performance of an LSG electrochemical capacitor in aqueous 1.0 M H2SO4: (A) Nearly rectangular CV profiles are observed at increasing potential scan rates from 100 to 1000 mV/s. (B) A linear relationship is obtained between the capacitive current (extracted from the CV profiles at 0.5 V for the charge and discharge segments) and the scan rate with R2 = 0.9998 for both curves. This trend confirms the formation of a pure electrostatic double layer. (C) Galvanostatic curves obtained at 10 A/gLSG/electrode; the device exhibits a voltage drop of only 0.014 V at the beginning of the discharge curve. This corresponds with a low internal resistance for the device. (D) Specific capacitance (areal and volumetric) as a function of the charge/discharge current density. The device shows an areal capacitance of 4.04 mF/cm2 at 1 A/gLSG/electrode which translates to a gravimetric capacitance of 222 F/gLSG/electrode. 13 Fig. S8: Performance of an LSG electrochemical capacitor when supported on a nitrocellulose membrane in aqueous 1.0 M H3PO4: (A) CV profiles at different scan rates (A) from 100 to 1000 mV/s and (B) from 2,000 to 10,000 mV/s. Nearly rectangular CV profiles are obtained at all the scan rates; (C) A linear relationship is observed between the capacitive current (extracted from the CV profiles at 0.5 V for the charge and discharge segments) and the scan rate with R2 = 0.9985 and 0.9970 for the charge and discharge curves, respectively. (D) Galvanostatic curves obtained at 10 A/gLSG/electrode; the device exhibits a voltage drop of only 0.019 V at the beginning of the discharge curve. This corresponds to a very low internal resistance for the device. These data show that an LSG can be made on different substrates and still maintain its high performance. 14 Fig. S9: Electrochemical characterization of the performance of a commercially available activated carbon electrochemical capacitor: CV profiles of the device shown in a potential window of 2.75 V at different scan rates of (A) 10 mV/s (B) 100 mV/s and (C) 1000 mV/s. In contrast to the LSG electrochemical capacitors, this commercial device exhibits the desired rectangular CV shape only at slow scan rates, 10 to 100 mV/s; (D) Dependence of the capacitive current (extracted from the CV profiles at 1.375 V) on the potential scan rate. A linear relationship is observed only between 10-100 mV/s, indicating the limited rate performance of this activated carbon electrochemical capacitor. 15 Fig. S10: Characterization of an all-solid state LSG electrochemical capacitor: (A) Areal and volumetric capacitance of the device as a function of the current density. (B) Cycling stability of an all solid-state LSG electrochemical capacitor: the device loses only about 3% of its capacitance after 10,000 cycles indicating very good cycling stability. 16 Fig. S11: Capability of an all solid-state LSG electrochemical capacitor for charge/discharge under bending conditions. (A) A schematic diagram showing the device under stress and defining the bend angle. (B) The device can be bent arbitrarily without any obvious decrease in performance compared to its planar state. The device retains about 95% of the initial capacitance after 1000 cycles when tested under the bent state (applying a bend angle = 150°). 17 Fig. S12: Increasing the output voltage via tandem serial electrochemical capacitors: portable equipment needing higher voltages use device packs with two or more devices connected in series. Adding electrochemical capacitors together in a serial string increases the voltage, while the current remains the same. Here, the performance of an all solid-state electrochemical capacitor pack with 4 devices connected in series was tested. (A) Cyclic voltammetry at 1000 mV/s shows that the voltage is extended to 4 volts (vs. 1 volt for a single device), while the produced current (represented by the area under the curve) is essentially the same for the single device and for the cell pack; (B) Galvanostatic charge/discharge curves for a single device and for the cell pack were all operated under the same constant current conditions. As can be seen, at the same charge/discharge current, the voltage is extended from 1 volt for a single device to 4 volts for a cell pack. Interestingly, a very low voltage drop is observed in the CC curves of the device pack indicating low internal resistance. This helps optimize the useful energy that can be drawn from the device. 18 Fig. S13: Increasing the output current via tandem parallel electrochemical capacitors: If higher operating currents are needed, two or more cells can be connected in parallel. Putting electrochemical capacitors together in parallel enhances the output current, while the operating voltage remains the same. Here, the performance of an all solid-state electrochemical capacitor pack with 4 devices connected in parallel was tested; (A) Cyclic voltammetry at 1000 mV/s shows that the capacitive current increases by a factor of 4; (B) Galvanostatic charge/discharge curves for the single device and for the cell pack were all operated under the same constant current conditions. Clearly, the runtime of the electrochemical capacitor pack (and thus the ampere-hour rating) increases by a factor of 4 under the same charge/discharge current. 19 Fig. S14: Increasing the output voltage and current via tandem serial/parallel electrochemical capacitors: In a serial/parallel design, the electrochemical capacitors are connected in a way that produces both the required voltage and current. Here, the performance of an all solid-state electrochemical capacitor pack with 4 devices connected in serial/parallel as shown in C was tested: (A) Cyclic voltammetry at 1000 mV/s shows that both the output voltage and current increase by a factor of 2; (B) Galvanostatic charge/discharge curves for a single device and for the cell pack were all operated under the same constant current conditions. Clearly, both the output voltage and the runtime of the electrochemical capacitor pack increases by a factor of two under the same charge/discharge current. 20 Fig. S15: The performance of an LSG electrochemical capacitor in an organic electrolyte of 1.0 M tetraethylammonium tetrafluoroborate (TEA-BF4) in acetonitrile: The capacitance was calculated from the galvanostatic curves and represented in terms of the area or the volume of the device stack. 21 Fig. S16: Performance of an LSG electrochemical capacitor in the ionic liquid 1- ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4). 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