Richard KANER & Maher EL-KADY
Graphene Micro-SuperCapacitor

Lightscibe DVD Burner Production of Flexible Thermoelectric Converters ...

Nature Communications, Vol.4, Article number: 1475
12 February 2013

Scalable fabrication of high-power graphene micro-supercapacitors
 for flexible and on-chip energy storage

Maher F. El-Kady // Richard B. Kaner

Maher F El-Kady        Richard B. Kaner

The rapid development of miniaturized electronic devices has increased the demand for compact on-chip energy storage. Microscale supercapacitors have great potential to complement or replace batteries and electrolytic capacitors in a variety of applications. However, conventional micro-fabrication techniques have proven to be cumbersome in building cost-effective micro-devices, thus limiting their widespread application. Here we demonstrate a scalable fabrication of graphene micro-supercapacitors over large areas by direct laser writing on graphite oxide films using a standard LightScribe DVD burner. More than 100 micro-supercapacitors can be produced on a single disc in 30?min or less. The devices are built on flexible substrates for flexible electronics and on-chip uses that can be integrated with MEMS or CMOS in a single chip. Remarkably, miniaturizing the devices to the microscale results in enhanced charge-storage capacity and rate capability. These micro-supercapacitors demonstrate a power density of ~200uW/cm-3, which is among the highest values achieved for any supercapacitor.

Figure 1: Fabrication of LSG-MSC

(ac) Schematic diagram showing the fabrication process for an LSG micro-supercapacitor. A GO film supported on a PET sheet is placed on a DVD media disc. The disc is inserted into a LightScribe DVD drive and a computer-designed circuit is etched onto the film. The laser inside the drive converts the golden-brown GO into black LSG at precise locations to produce interdigitated graphene circuits (a). Copper tape is applied along the edges to improve the electrical contacts, and the interdigitated area is defined by polyimide (Kapton) tape (b). An electrolyte overcoat is then added to create a planar micro-supercapacitor (c). (d,e) This technique has the potential for the direct writing of micro-devices with high areal density. More than 100 micro-devices can be produced on a single run. The micro-devices are completely flexible and can be produced on virtually any substrate.

[ Ed. Note -- Combine this with Palmer CRAIG's bismuth power device ]

Characterization of LSG micro-devices
Figure 2: Characterization of LSG micro-devices

(a) A digital photograph of the laser-scribed micro-devices with 4 (LSG-MSC4), 8 (LSG-MSC8) and 16 interdigitated electrodes (LSG-MSC16).
(b) An optical microscope image of LSG-MSC16 shows interdigitated fingers with 150-m spacings. The dark area corresponds to LSG and the light area is GO. Scale bar, 200?m. (c) A tilted-view (45) SEM image shows the direct reduction and expansion of the GO film after exposure to the laser beam. Scale bar, 10?m. (d) and (e) show the IV curves of GO and LSG, respectively. LSG exhibits a current enhanced by about 6 orders of magnitude, confirming the change from nearly insulating GO to conducting LSG. (f) A comparison of electrical conductivity values for GO and LSG.

Electrochemical performance of the LSG-MSC in PVA-H2SO4 gelled electrolyte
Figure 3: Electrochemical performance of the LSG-MSC in PVA-H2SO4 gelled electrolyte

CV profiles of LSG-MSC in sandwich and interdigitated structures with 4, 8 and 16 electrodes at scan rates of (a) 1,000?mV?s-1, (b) 5,000?mV?s-1 and (c) 10,000?mV?s-1. (d) Evolution of the specific capacitance of the different supercapacitors as a function of the scan rate. Symbol key for ad: sandwich (black), MSC(4) (red), MSC(8) (green) and MSC(16) (blue). (e) Galvanostatic charge/discharge curves of micro-supercapacitors based on interdigitated structures with 4, 8 and 16 electrodes, all operated at an ultrahigh current density of 1.68 104?mA?cm-3. (f) Volumetric stack capacitance of LSG-MSC in the sandwich and interdigitated structures as calculated from the charge/discharge curves at different current densities. Data for a commercial AC-SC are shown for comparison. (g) Complex plane plot of the impedance of a LSG-MSC(16) with a magnification of the high-frequency region is provided in the inset. (h) Impedance phase angle versus frequency for LSG-MSC(16) compared with commercial AC-SC and aluminium electrolytic capacitors. (i) The LSG-MSC(16) shows excellent stability, losing only about 4% of its initial capacitance over 10,000 cycles.

Behaviour of LSG-MSC under mechanical stress and in series/parallel combinations
Figure 4: Behaviour of LSG-MSC under mechanical stress and in series/parallel combinations

(a) A photograph of LSG-MSC(16) bent with a tweezers demonstrates the flexibility of the micro-device. (b) Bending/twisting the device has almost no effect on its performance, as can be seen from these CVs collected under different bending and twisting conditions at 1,000?mV?s-1. (c) Performance durability of the micro-device when tested under bending and twisting conditions. The device retains ~97% of its initial capacitance after 1,000 cycles under the bent state, followed by another 1,000 cycles under the twisted state. Galvanostatic charge/discharge curves for four tandem micro-supercapacitors connected (d) in series, (e) in parallel and (f) in a combination of series and parallel. A single device is shown for comparison. Both the tandem devices and the single device were operated at the same charge/discharge current. (Insets) the tandem micro-supercapacitor can be used to power a light-emitting diode (LED).

Fabrication and characterization of LSG-MSC on a chip
Figure 5: Fabrication and characterization of LSG-MSC on a chip

LightScribe can be used to produce LSG-MSC directly on a chip that contains integrated circuits, which they can then power. (a) An ionogel electrolyte was used in the assembly of the device. It is prepared by mixing together the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide with fumed silica nanopowder. (b) Schematic of the device; (c) Photograph of the micro-devices. (d) CV profile of LSG-MSC(16) at various scan rates, from low to high: 1000 (black), 2000 (red), 5000 (green) and 10000 (blue)?mV?s-1. (e) Galvanostatic charge/discharge curves of LSG-MSC(16) collected at different current densities: 1.06 104 (black), 5.05 103 (red), 2.42 103 (green) and 1.38 103 (blue)?mA?cm-3.

Testing the self-discharge rate of LSG-MSC
Figure 6: Testing the self-discharge rate of LSG-MSC

(a) Leakage current measurement of an LSG micro-supercapacitor (with 16 interdigitated electrodes) and two commercially available supercapacitors. A DC voltage (the voltage at which the supercapacitor is operated, Vmax) was applied across the capacitor; the current required to retain that voltage was measured over a period of 12?h. (b) Self-discharge curves of the respective supercapacitors obtained immediately after precharging to Vmax in the previous test. This involves measuring the open-circuit voltage across the supercapacitors between Vmax and Vmax versus the course of time. This involves 3.5?V/25?mF commercial supercapacitor (black), 2.75?V/44?mF commercial supercapacitor (red) and LSG micro-supercapacitor assembled using ionogel electrolyte (green).

Energy and power densities of LSG-MSCs compared with commercially available energy-storage systems
Figure 7: Energy and power densities of LSG-MSCs compared with commercially available energy-storage systems

LSG-MSCs exhibit ultrahigh power and energy densities compared with a commercially available AC-SC, an aluminium electrolytic capacitor and a lithium thin-film battery. LSG micro-devices can deliver ultrahigh power density comparable to those of an aluminium electrolytic capacitor, while providing three orders of magnitude higher energy density. Data for the Li battery are reproduced from ref. 2.
March 2012

Self-Charged Graphene Battery Harvests Electricity from Thermal Energy of the Environment

Authors: Zihan Xu, Guoan Tai, Yungang Zhou, Fei Gao, Kin Hung Wong

Abstract : The energy of ionic thermal motion presents universally, which is as high as 4 kJ\bullet kg-1\bullet K-1 in aqueous solution, where thermal velocity of ions is in the order of hundreds of meters per second at room temperature1,2. Moreover, the thermal velocity of ions can be maintained by the external environment, which means it is unlimited. However, little study has been reported on converting the ionic thermal energy into electricity. Here we present a graphene device with asymmetric electrodes configuration to capture such ionic thermal energy and convert it into electricity. An output voltage around 0.35 V was generated when the device was dipped into saturated CuCl2 solution, in which this value lasted over twenty days. A positive correlation between the open-circuit voltage and the temperature, as well as the cation concentration, was observed. Furthermore, we demonstrated that this finding is of practical value by lighting a commercial light-emitting diode up with six of such graphene devices connected in series. This finding provides a new way to understand the behavior of graphene at molecular scale and represents a huge breakthrough for the research of self-powered technology. Moreover, the finding will benefit quite a few applications, such as artificial organs, clean renewable energy and portable electronics.
February 19, 2013

UCLA researchers develop new technique to scale up production of graphene micro-supercapacitors


Davin Malasarn

While the demand for ever-smaller electronic devices has spurred the miniaturization of a variety of technologies, one area has lagged behind in this downsizing revolution: energy-storage units, such as batteries and capacitors.
Now, Richard Kaner, a member of the California NanoSystems Institute at UCLA and a professor of chemistry and biochemistry, and Maher El-Kady, a graduate student in Kaner's laboratory, may have changed the game.
The UCLA researchers have developed a groundbreaking technique that uses a DVD burner to fabricate micro-scale graphene-based supercapacitors   devices that can charge and discharge a hundred to a thousand times faster than standard batteries. These micro-supercapacitors, made from a one-atomthick layer of graphitic carbon, can be easily manufactured and readily integrated into small devices such as next-generation pacemakers.
The new cost-effective fabrication method, described in a study published this week in the journal Nature Communications, holds promise for the mass production of these supercapacitors, which have the potential to transform electronics and other fields.
"The integration of energy-storage units with electronic circuits is challenging and often limits the miniaturization of the entire system," said Kaner, who is also a professor of materials science and engineering at UCLA's Henry Samueli School of Engineering and Applied Science. "This is because the necessary energy-storage components scale down poorly in size and are not well suited to the planar geometries of most integrated fabrication processes."
"Traditional methods for the fabrication of micro-supercapacitors involve labor-intensive lithographic techniques that have proven difficult for building cost-effective devices, thus limiting their commercial application," El-Kady said. "Instead, we used a consumer-grade LightScribe DVD burner to produce graphene micro-supercapacitors over large areas at a fraction of the cost of traditional devices. Using this technique, we have been able to produce more than 100 micro-supercapacitors on a single disc in less than 30 minutes, using inexpensive materials."
The process of miniaturization often relies on flattening technology, making devices thinner and more like a geometric plane that has only two dimensions. In developing their new micro-supercapacitor, Kaner and El-Kady used a two-dimensional sheet of carbon, known as graphene, which only has the thickness of a single atom in the third dimension.
Kaner and El-Kady took advantage of a new structural design during the fabrication. For any supercapacitor to be effective, two separated electrodes have to be positioned so that the available surface area between them is maximized. This allows the supercapacitor to store a greater charge. A previous design stacked the layers of graphene serving as electrodes, like the slices of bread on a sandwich. While this design was functional, however, it was not compatible with integrated circuits.
In their new design, the researchers placed the electrodes side by side using an interdigitated pattern, akin to interwoven fingers. This helped to maximize the accessible surface area available for each of the two electrodes while also reducing the path over which ions in the electrolyte would need to diffuse. As a result, the new supercapacitors have more charge capacity and rate capability than their stacked counterparts.
Interestingly, the researchers found that by placing more electrodes per unit area, they boosted the micro-supercapacitor's ability to store even more charge.
Kaner and El-Kady were able to fabricate these intricate supercapacitors using an affordable and scalable technique that they had developed earlier. They glued a layer of plastic onto the surface of a DVD and then coated the plastic with a layer of graphite oxide. Then, they simply inserted the coated disc into a commercially available LightScribe optical drive traditionally used to label DVDs and took advantage of the drive's own laser to create the interdigitated pattern. The laser scribing is so precise that none of the "interwoven fingers" touch each other, which would short-circuit the supercapacitor.
"To label discs using LightScribe, the surface of the disc is coated with a reactive dye that changes color on exposure to the laser light. Instead of printing on this specialized coating, our approach is to coat the disc with a film of graphite oxide, which then can be directly printed on," Kaner said. "We previously found an unusual photo-thermal effect in which graphite oxide absorbs the laser light and is converted into graphene in a similar fashion to the commercial LightScribe process. With the precision of the laser, the drive renders the computer-designed pattern onto the graphite oxide film to produce the desired graphene circuits."
"The process is straightforward, cost-effective and can be done at home," El-Kady said. "One only needs a DVD burner and graphite oxide dispersion in water, which is commercially available at a moderate cost."
The new micro-supercapacitors are also highly bendable and twistable, making them potentially useful as energy-storage devices in flexible electronics like roll-up displays and TVs, e-paper, and even wearable electronics.
The researchers showed the utility of their new laser-scribed graphene micro-supercapacitor in an all-solid form, which would enable any new device incorporating them to be more easily shaped and flexible. The micro-supercapacitors can also be fabricated directly on a chip using the same technique, making them highly useful for integration into micro-electromechanical systems (MEMS) or complementary metal-oxide-semiconductors (CMOS).
These micro-supercapacitors show excellent cycling stability, an important advantage over micro-batteries, which have shorter lifespans and which could pose a major problem when embedded in permanent structures such as biomedical implants, active radio-frequency identification tags and embedded micro-sensors for which no maintenance or replacement is possible.
As they can be directly integrated on-chip, these micro-supercapacitors may help to better extract energy from solar, mechanical and thermal sources and thus make more efficient self-powered systems. They could also be fabricated on the backside of solar cells in both portable devices and rooftop installations to store power generated during the day for use after sundown, helping to provide electricity around the clock when connection to the grid is not possible.
"We are now looking for industry partners to help us mass-produce our graphene micro-supercapacitors," Kaner said.
Kaner's micro-scale graphene-based supercapacitor research is supported by Maxwell Technologies Inc., a global leader in manufacturing carbon-based supercapacitors and other energy storage devices.

Self-Charged Graphene Battery Harvests Electricity from Thermal Energy of the Environment
Zihan Xu, et al.
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