• Maher El-Kady

Rexresearch: Graphene Micro-Supercapacitor

::Source: Rexresearch::

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

(a–c) 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-µm spacings. The dark area corresponds to LSG and the light area is GO. Scale bar, 200?µm. (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?µm. (d) and (e) show the I–V 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 a–d: 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 ½Vmax 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.

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