WO2015126464A2

WO2015126464A2 - Stacked multilayers of alternating reduced graphene oxide and carbon nanotubes for ultrathin planar supercapacitors

Info

Publication number: WO2015126464A2
Application number: PCT/US2014/060232
Authority: WO
Inventors: Yadong Yin; Geon Dae MOON
Original assignee: The Regents Of The University Of California
Priority date: 2013-10-11
Filing date: 2014-10-13
Publication date: 2015-08-27
Also published as: WO2015126464A3

Abstract

A layer-by-layer approach for the construction of 2D (two dimensional) planar ultrathin supercapacitors of multi-stacked rGO (reduced grapheme oxide) and CNT (carbon nanotube) is disclosed. The 2D in-plane architecture can allow for the formation of an efficient electrical double layer and the utilization of the maximum active surface area of rGO nanosheets by using a CNT layer as a porous physical spacer to enhance the permeation of gel electrolyte inside the structure and reduce the agglomeration of rGO nanosheets along the out-of-plane axis. The electrochemical performance can be increased by controlling the number of layers and constructing serial/parallel tandem architectures. Furthermore, the LBL process can be extended to the fabrication of a large-sized ultrathin supercapacitor without diminishing its electrochemical properties, which can be important in a practical energy storage device.

Description

STACKED MULTILAYERS OF ALTERNATING REDUCED GRAPHENE OXIDE AND CARBON NANOTUBES FOR ULTRATHIN PLANAR

SUPERCAPACITORS

RELATED APPLICATION(S)

[0001] The present application claims priority under 35 U.S.C. 1 19 to U.S. Provisional Patent Application No. 61/889,845, filed on October 1 1 , 2013, the entire content of which is hereby incorporated by reference.

FIELD

[0002] The present disclosure relates to supercapacitors, for example, stacked multilayers of alternating reduced graphene oxide and carbon nanotubes for ultrathin planar supercapacitors.

BACKGROUND

[0003] Supercapacitors (for example, electrochemical capacitors, ECs) represent one of the most promising types of energy storage devices due to their high power density, long lifetime, and safe operation conditions.

Recent progress in large-scale production of graphene through derivation from graphene oxide (GO) has stimulated a variety of research on

developing cost-effective graphene-based energy storage materials with enhanced energy and power density as well as long life cycle. For example, as a two-dimensional carbon nanostructure, graphene is a suitable electrode material due to its large surface area (theoretically 2630 m²g^"1 for a single graphene sheet) and high in-plane electrical conductivity. However, specific capacitance can improve, considering the theoretical value of 550 F/g for single-layer graphene while currently achievable values can be below 300 F/g. The relatively low specific capacitance can be first attributed to restacking of the graphene sheets during processing due to sheet-to-sheet van der Waals interactions, leading to reduction of the specific surface area.

[0004] In an effort to tackle the restacking issue of graphene sheets, utilization of vertically oriented graphene nanosheets to minimize electronic and ionic resistances and produce high capacitance have been proposed. However, the properties of vertically aligned graphene nanosheets can be limited to a fixed dimension on a specific substrate, making it difficult to fabricate large-sized electrochemical cells, which is important to scalability for current energy storage needs.

[0005] Layer-by-layer (LBL) processes have also been attempted to assemble graphene nanosheets or alternating layers of graphene and other carbon materials such as carbon nanotubes (CNTs) and polyaniline (PANi) through electrostatic attraction. For example, a hybrid film supercapacitor composed of poly(ethyleneimine)-modified graphene nanosheets and acid- treated multiwalled CNT via sequential self-assembly in an aqueous electrolyte have been studied. The results obtained in this study

demonstrate that the incorporation of interconnected CNT networks with fine nanopores may enable fast ionic diffusion in graphene-based electrodes. The reduction of active surface area may also be attributed to the

conventional fabrication methods for supercapacitors, which can involve mixing carbon-based materials with organic binders to give good mechanical adhesion to the current collector. However, such fabrication scheme makes it impossible to fully utilize the active surface area of atomically-thick graphene sheets without agglomeration, which lead to reduction of the electrochemically active surface area.

[0006] For example, to maximize the merits of graphene nanosheets, the following criteria can be desirable: minimum restacking and agglomeration of graphene sheets, porous matrix structure for easy path of electrolyte ions, and good mechanical adhesion of active materials to the current collector.

SUMMARY

[0007] In accordance with an exemplary embodiment, a method of stacking multilayers of alternating graphene and carbon nanotubes (CNT) for a supercapacitor structure is disclosed, the method comprising: synthesizing graphene oxide (GO) from high purity graphite flake powder using a modified Hummer's method with an improved purification and then chemical reduction to form exfoliated graphene oxide; dispersing the reduced graphene oxide (rGO) nanosheets and CNTs in ethanol and 1-butanol, respectively;

dropping the suspensions of reduced graphene oxide and CNTs onto a water surface in separate dishes to form a graphene oxide film and a CNT film; and forming alternating layers of graphene oxide and CNTs from the separate graphene oxide and CNT films to form a stack of rGO and CNT films.

[0008] In accordance with an exemplary embodiment, an ultrathin supercapacitor structure is disclosed, comprising: a plurality of alternating layers of graphene and carbon nanotubes (CNT), wherein the layers of graphene are reduced graphene oxide nanosheets.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Exemplary embodiments will be disclosed more closely with reference to the accompanying drawings in which:

[0010] Figure 1 shows (a) a schematic illustration showing the formation of alternately stacked rGO/CNT films through repeated transfer of assembled monolayers of rGO and CNTs from water surfaces; (b,c) TEM images of rGO-CNT layer (b) and rGO-CNT-rGO layer (c), respectively, and wherein the inset in (b) shows an SEM image of the rGO-CNT layer, with a scale bar of 200 nm. (d,e) Cross-section SEM image (d) and AFM image (e) of alternately stacked rGO/CNT consisting of 16 rGO and 15 CNT layers. The thickness of the representative stacked film is about 200 nm.

[0011] Figure 2 shows (a) CV curves of alternately stacked rGO/CNT consisting of 16 rGO and 15 CNT layers at various scan rates in the range of 0 to 1V, (b) Galvanostatic charge-discharge curves of four samples at a current density of 2 μΑ/cm², (c) Nyquist plots for different samples over the frequency range from 1 MHz to 10 mHz. Inset: Enlarged plots in the high frequency region, (d) Cycling performance of representative samples at a current density of 5 juA/cm² for 3000 cycles, (e) The volumetric capacitance of the representative samples calculated from Galvanostatic

charge/discharge curves at different current densities, and (f) Ragone plots of the samples showing their energy and power densities.

[0012] Figure 3 shows (a) galvanostatic charge/discharge curves of layer- by-layer stacked rGO/CNT (L-rGO/CNT) supercapacitor with different number of layers at a current density of 2 μΑ/cm², (b) Areal capacitances calculated for L-rGO/CNT with different number of layers and unit areal capacitances by dividing them by the number of layers (N), and (c, d) Increase in the output voltage and current via tandem serial and parallel electrochemical capacitors. Galvanostatic charge/discharge curves for a single cell and two/four cells connected in series (c) and in parallel (d) under the same constant current conditions.

[0013] Figure 4 shows (a) a digital image of the large-sized planar stacked rGO/CNT supercapacitor and the stacked geometry, (b) CV curves of alternately stacked rGO/CNT consisting of 16 rGO and 15 CNT layers at various scan rates, (c) Galvanostatic charge-discharge curves of the sample at a current of 70 μΑ, and (d) Nyquist plots over the frequency range from 1 MHz to 10 mHz. Inset: Enlarged plots in the high frequency region.

DETAILED DESCRIPTION

[0014] An ultrathin planar supercapacitor electrode by alternatively stacking multilayers of reduced graphene oxide (rGO) and multi-walled CNTs without the need of any binding materials is disclosed. In accordance with an exemplary embodiment, two-dimensional monolayer assemblies of rGO and multi-walled CNTs were separately formed at the air/water interface and these films were readily transferred onto a solid substrate, repeatedly, to fabricate an alternating multi-stacked electrode. This in-plane architecture embedded with CNTs between rGO layers can enhance the interaction of electrolyte ions with all graphene layers leading to the full utilization of the high surface area of the graphene sheets, which can be important for thin film-based supercapacitors. As a result, the stacked multilayers of rGO and CNTs can be capable of offering higher output voltage and current production via serial and parallel connections. Owing to the method for preparing layer-by-layer assemblies on any sized substrate, a large-scale (5x7 cm²) stacked supercapacitor has been demonstrated without sacrificing the electrochemical performance.

[0015] Figure 1A shows the overall process of fabricating the stacked multilayers of rGO/CNT (L-rGO/CNT) for supercapacitor electrodes. The rGO was firstly prepared by a modified Hummer's method, followed by reduction in an aqueous solution with N₂H₄. Multi-walled CNTs were treated with a mixture of H₂SO and HNO₃ to render them dispersible in polar solvents. To form a 2D assembly, rGO and CNTs were firstly dispersed in alcohol (for example, ethanol, IPA, 1 -butanol) and then dropped on water surface with a pipette. The following mechanism is involved in the formation of 2D arrays: rGO sheets or CNTs spread on the water surface, then become trapped at the air/water interface, and finally produce closely- packed assembly. After the formation of 2D assemblies of rGO and CNTs in separate processes, the 2D monolayer of rGO was first collected on a current collector and then a CNT layer was gathered on the same substrate. The reduced graphene oxide nanosheets can be atomically-thin, leading to overlapped or crumpled areas due to the limited available air/water interface. Likewise, CNTs also overlap to appear as a fabric. This process was repeated, sequentially, to make an alternating multi-stacked rGO/CNT electrode. This layer-by-layer (LBL) electrochemical device structure can alleviate agglomeration of the 1 D and 2D carbon nanomaterials and can maximize the utilization of the surface of the graphene nanosheets to guarantee the permeation of electrolyte ions and reliable mechanical adhesion of the active materials to the current collector.

[0016] Figures 1 b and 1 c show TEM images of stacked rGO/CNT films exposing CNT and rGO layers on the top, respectively. The inset in Figure 1 b shows the rGO/CNT film in which the CNT fabric is well adhered to the bottom of the rGO layer. This layer-by-layer deposition of rGO and CNT in a sequential manner ensures full coverage of each nanomaterial, which can be important in planar supercapacitor devices in terms of uniform orientation with respect to the current collector. For comparison, a mixture solution of rGO and CNT in ethanol (0.5 mg/mL for both) was prepared and followed the same process to make 2D arrays on a water surface. However, the resultant film after transferring to a substrate shows partial coverage or phase separation due to a lack of strong interactions between the two components. It is known that graphene oxide nanosheets possess amphiphilic properties corresponding to their hydrophilic edges from ionizable carbonyl/carboxyl groups and the more hydrophobic basal plane. However, the π-π attraction between the hydrophobic plane of GO and CNTs can be considerably weak and non-uniform as the amphiphilicity of GO varies by the degree of reduction and the ττ-conjugated domains in CNTs have been significantly disrupted after acid treatment. As a result, simply mixing of rGO and CNTs in ethanol made phase-separated 2D films on the water surface with only a small contact area between the two components. In the LBL scheme, each CNT layer can be sandwiched between two rGO layers, producing uniform films with thicknesses

increasing from, for example, about 50, 100, 200, and 400 nm corresponding to 4, 8, 16, and 32 rGO layers (Figures 1 d and 1 e). The CNT layers can serve as well-defined porous spacers that not only prevent restacking of rGO sheets but also provide sufficient separation between the wrinkled rGO nanosheets to ensure efficient permeation of electrolyte ions.

[0017] In accordance with an exemplary embodiment, a demonstration or proof-of-concept demonstration for ultrathin solid-state planar

supercapacitors based on the alternating multi-stacked rGO/CNT (L- rGO/CNT) 2D in-plane architecture was performed. A polymer gel

(PVA/H3PO4) electrolyte was spread across the active electrode surface after the formation of the multilayered rGO/CNT film. Then, a supercapacitor device was made by stacking two identical L-rGO/CNT electrodes. In this study, electrodes consisting of 16 repeating layers with dimensions of, for example, 60.4 pm (thickness) χ 1 cm² (area) were used. The polymer gel electrolyte showed stable electrochemical performance in the voltage range of 1 V due to the use of water as solvent for electrolyte preparation.

Furthermore, the polymer gel electrolyte can serve as an ionic electrolyte as well as a separator, enabling a new supercapacitor design.

[0018] The electrochemical performance of the ultrathin planar L-rGO/CNT supercapacitor was investigated using cyclic voltammetry (CV) and galvanostatic charge/discharge (CD). The CV curves of L-rGO/CNT device were measured with various scan rates in the range of about 2 to 100 mV/s (Figure 2a). All the CV curves exhibit similar box-type symmetric shapes along the center, revealing the ideal capacitive behavior and good rate capability. Galvanostatic CD curves of the L-rGO/CNT device were recorded at various current densities in the voltage range of 0 to 1V. For comparison with other devices containing 16 layers, galvanostatic CD curves for four different devices are displayed at the same current density of 2 μΑ/cm² (Figure 2b). The calculated areal capacitances of L-rGO/CNT, randomly stacked rGO/CNT (R-rGO/CNT), CNT, and rGO devices were 2.63, 1.42, 0.97, and 0.66 mF/cm², respectively. The well-defined structure in the L- rGO/CNT device can result in a higher areal capacitance than the sum of rGO and CNT devices, indicating the full utilization of the active surface of the rGO nanosheets by incorporating CNT layers between rGO layers. This same trend can also be observed in CV curves of the four different devices measured at 10 mV/s, indicating a large increase in the current value for the L-rGO/CNT electrode. Furthermore, the R-rGO/CNT device shows a lower specific capacitance than L-rGO/CNT due to poorly defined morphology. Electrochemical impedance spectroscopy (EIS) confirmed the faster ion transport in the L-rGO/CNT electrode versus the pure rGO electrode, as shown in Figure 2c. The presence of a CNT layer between rGO layers can improve the inner resistance value by reducing the agglomeration of rGO nanosheets along the out-of-plane axis. This trend was also corroborated by comparing Bode plots, which indicate a lower resistance for the rGO device even at low frequency.

[0019] Figure 2d presents the cyclic performance of each device, indicating a better stability of the CNT-incorporated rGO devices (L- rGO/CNT and R-rGO/CNT) than the rGO device. The capacitance value calculated from the discharge curves after 3000 cycles improved from 34% (rGO) to 10% (L-rGO/CNT).

[0020] As demonstrated in Figure 2e, calculation of the volumetric capacitance of each solid-state supercapacitor by galvanostatic

charge/discharge curve based on the overall device volume indicates that the L-rGO/CNT device produces the highest values at every current density. The L-rGO/CNT electrode can effectively absorb the gel electrolyte and act as an efficient electrolyte reservoir because the porous CNT layer can facilitate ion transport and minimize the diffusion length to the interior surface of the rGO nanosheets. This synergistic effect originates from the large active surface area of the rGO nanosheets, which can be maintained due to the prevention of agglomeration through the intercalation of CNT layers, which in turn improves the conductivity of the device. Furthermore, this layer-by-layer process requires no organic binder, and thus enables a reduction in interfacial resistance and enhances the chemical reaction rate.

[0021] Figure 2f shows the Ragone plots of the four types of devices indicating the overall performance based on the specific volume of the whole device. In accordance with an exemplary embodiment, the same area (1 x1 cm²) of electrodes was used for calculation. The thickness of each device was in the range of 60.1 pm (rGO), 60.3 pm (CNT), 60.4 pm (R-rGO/CNT), and 60.4 pm (L-rGO/CNT) because the active layers are too thin compared to the electrolyte (about 10 pm) and current collector (2x25 pm). As expected based on previous electrochemical data, the L-rGO/CNT electrode shows better energy density compared to the other devices. In accordance with an exemplary embodiment, one advantage of the L-rGO/CNT supercapacitor can lie in the feasibility of controlling its electrochemical performance by changing the thickness of the active layer.

[0022] Figure 3a shows galvanostatic charge/discharge curves of four L- rGO/CNT electrodes comprising different numbers of layers. For example, the runtime of the charging/discharging increases with the number of layers, indicating efficient control over the capacitance value. The areal

capacitance values were plotted in Figure 3b, which shows a linear relationship between areal capacitance and the number of layers.

Furthermore, the unit capacitances, as calculated by dividing each areal capacitance by the number of layers (blue dots), was found to be in the range of 0.181 ± 0.042 mF/cm². In accordance with an exemplary

embodiment, it can be concluded that the incorporation of porous CNT buffer layers between rGO layers can enable a simple layer-by-layer alternating stacking scheme for constructing efficient electrochemical energy storage devices whose performance can be conveniently controlled by the number of layers.

[0023] Current electronics needs high output voltage and current to meet the minimum requirements for portable electronic equipment. Aqueous electrolyte can operate only in the voltage range of 0 to 1V, which can be limited to a small number of applications. For example, portable devices often require cell packaging either in series, in parallel, or in a combination of the two methods to meet energy and power requirements. Thus, it can be important to develop an electrochemical energy storage device with control over the operating voltage and current by constructing tandem serial and parallel assemblies while minimizing energy loss. The electrochemical performances of a set of serial and parallel tandem L-rGO/CNT

configurations were evaluated by connecting two and four devices with the same electrodes. Galvanostatic charge/discharge curves for serial tandem cell structures were obtained at the same current conditions. The serial tandem architectures connecting two and four devices exhibited a 2 and 4-V charge/discharge voltage range, respectively, compared to a single L- rGO/CNT device that operates at 1 V (Figure 3c). A very low voltage drop was observed in the curves of the packed devices, indicating low internal resistance, which can optimize the useful energy from each device.

[0024] Figure 3d shows galvanostatic charge/discharge curves of parallel tandem structures under the same constant current conditions. For example, the charging and discharging time of the L-rGO/CNT supercapacitor increases by a factor of two and four when connecting two and four devices, respectively. As with the single L-rGO/CNT device, the tandem devices (in series and in parallel) show nearly perfect triangular charge/discharge curves with a small voltage drop, which indicates good capacitive properties with minimal internal resistance. This leads to minimal energy losses of L- rGO/CNT device when using tandem architectures.

[0025] In 2D (two-dimensional) planar electrochemical capacitors, the scalability of the size of electrode is essential to carry high power and energy for real applications. As a demonstration of a large-sized planar

supercapacitor, a large substrate (5x7 cm²) was introduced as a current collector to stack rGO nanosheets and CNT layers in a layer-by-layer manner.

[0026] Figure 4a shows a digital image of the large-sized L-rGO/CNT electrodes after covering polymer gel electrolyte. The device architecture is the same as that of the small L-rGO/CNT electrodes (1 x1 cm²) as depicted in the scheme. The electrochemical properties were measured under the same conditions. Cyclic voltammography (CV) curves of the large-sized L- rGO/CNT device show a nearly rectangular shape at every scan rate (2 to 100 mV/s), indicating an efficient establishment of an electric double layer (Figure 4b). Galvanostatic charge/discharge curves in also exhibit the same trend as the regular-sized L-rGO/CNT device except for a large increase of capacitance according to the increased size of the active layers.

[0027] As seen in Figure 4c, the CD curves are close to a triangular shape, signifying the formation of an efficient EDL and good charge transport across the planar electrodes. Importantly, the large-sized L-rGO/CNT electrode exhibits stable cycling performance at five different current densities. The Nyquist plots from electrochemical impedance spectra display a good capacitive behavior with small internal resistance as seen in the inset in Figure 4d. These results evidently suggest the great promise of the 2D planar L-rGO/CNT electrode for high-performance electrochemical capacitors characterized by control over the thickness, tandem cell architectures, and scalability in size.

[0028] In accordance with an exemplary embodiment, a layer-by-layer approach for the construction of 2D planar ultrathin supercapacitors of multi- stacked rGO and CNTs has been disclosed. This 2D in-plane architecture allows for the formation of an efficient electrical double layer and the utilization of the maximum active surface area of rGO nanosheets by using a CNT layer as a porous physical spacer to enhance the permeation of gel electrolyte inside the structure and reduce the agglomeration of rGO nanosheets along the out-of-plane axis. The electrochemical performance can be increased simply by controlling the number of layers and constructing serial/parallel tandem architectures. Furthermore, the proposed LBL process can be extended to the fabrication of a large-sized ultrathin supercapacitor without diminishing its electrochemical properties, which is critically important in a practical energy storage device.

[0029] Experimental Section

[0030] Synthesis of reduced graphene oxide (rGO).

[0031] Graphene oxide (GO) was synthesized from high purity graphite flake powder using a modified Hummer's method with an improved purification method and then reduced by chemical reduction. The exfoliated graphene oxide was redispersed in 20 mL of water (0.5 mg/mL) after sonication for h (one hour), then mixed with water (20 mL), N2H4 (60 pL), and NH₄OH (420 pL). After heating the reaction at 70 °C for 1 h, the sample was washed with H₂O three times. [0032] Fabrication of the solid-state stacked supercapacitor.

[0033] Reduced graphene oxide nanosheets and CNTs were redispersed in ethanol and 1-butanol, respectively. These suspensions were dropped on a water surface in separate Petri dishes until a robust film was formed. The film was scooped up by a substrate, repeatedly, to make a multi-layer film. All solid-state devices were assembled by pouring polymer gel electrolyte (100 pL/cm²) onto the stacked rGO/CNT electrodes. The device was left overnight in a desiccator for complete drying of the gel electrolyte. The polymer gel electrolyte used in this study is a mixture of poly (vinyl alcohol) (PVA, M_w = 88,000) and phosphoric acid (H₃P0₄), prepared as described elsewhere. Two electrodes were assembled face-to-face and left overnight until the electrolyte solidified.

[0034] Characterization

[0035] The morphology of the graphene oxide, reduced graphene oxide and CNTs were investigated with transmission electron microscopy (TEM, Philips Tecnai 12, 200 kV). The cross-section images of the stacked rGO/CNT film were taken by a scanning electron microscope (SEM, JSM- 6700F, JEOL). The height profile was obtained with an AFM (Model dimension 3100, Digital Instrument Co.).

[0036] Electrochemical measurements of the stacked supercapacitor

[0037] The electrochemical properties of the samples were measured with cyclic voltammetry (CV) and galvanostatic charge-discharge measurements in a conventional three-electrode system by using a potentiostat

(VersaSTAT 4, Princeton Applied Research). The area of the electrodes was confined to 1 x1 cm² (5 x 7 cm² for the large-sized electrodes).

Electrochemical impedance spectra (EIS) experiments were carried out at an open circuit potential with a sinusoidal signal with an amplitude of 10 mV in a frequency range of 1 MHz to 10 mHz. [0038] When the word "about" is used in this specification in connection with a numerical value, it is intended that the associated numerical value include a tolerance of ±10% around the stated numerical value.

[0039] Moreover, when the words "generally", "relatively", and

"substantially" are used in connection with geometric shapes, it is intended that precision of the geometric shape is not required but that latitude for the shape is within the scope of the disclosure. When used with geometric terms, the words "generally", "relatively", and "substantially" are intended to encompass not only features which meet the strict definitions but also features which fairly approximate the strict definitions.

[0040] It is to be understood that the form of this invention as shown is merely a preferred embodiment. Various changes may be made in the function and arrangement of parts; equivalent means may be substituted for those illustrated and described; and certain features may be used

independently from others without departing from the spirit and scope of the invention as defined in the following claims.

Claims

What is claimed is:

1. A method of stacking multilayers of alternating graphene and carbon nanotubes (CNT) for a supercapacitor structure, the method comprising:

synthesizing graphene oxide (GO) from high purity graphite flake powder using a modified Hummer's method with an improved purification and then by chemical reduction to form exfoliated graphene oxide;

dispersing the reduced graphene oxide (rGO) nanosheets and CNTs in ethanol and 1-butanol, respectively;

dropping the suspensions of reduced graphene oxide and CNTs onto a water surface in separate dishes to form a graphene oxide film and a CNT film; and

forming alternating layers of graphene oxide and CNTs from the separate graphene oxide and CNT films to form a stack of rGO and CNT films.

2. The method of Claim 1 , wherein the CNT layer acts as a porous physical spacer, which enhances the permeation of a gel electrolyte inside the structure and reduces the agglomeration of rGO nanosheets along the out-of-plane axis.

3. The method of Claim 1 , comprising:

increasing an electrochemical performance of the structure by controlling a number of layers and constructing serial/parallel tandem architectures.

4. The method of Claim 1 , wherein the chemical reduction of the graphene oxide comprises:

redispersing the exfoliated graphene oxide in water after sonication; mixing the redispersed exfoliated graphene oxide with water N2H₄, and NH₄OH; and

heating the redispersed exfoliated graphene oxide at 70 °C for about

1 hour.

5. The method of Claim 1 , comprising:

assembling a solid-state device by pouring a gel electrolyte onto the stacked rGO/CNT films.

6. The method of Claim 5, further comprising:

placing the solid-state device in a desiccator for drying of the gel electrolyte.

7. The method of Claim 6, wherein the gel electrolyte is a polymer gel electrolyte.

8. The method of Claim 7, further comprising:

assembling two electrodes face-to-face; and

solidifying the electrolyte.

9. The method of Claim 7, wherein the polymer gel is a mixture of poly (vinyl alcohol) and phosphoric acid (HsP0₄).

10. An ultrathin supercapacitor structure, comprising:

a plurality of alternating layers of graphene and carbon nanotubes (CNT), wherein the layers of graphene are reduced graphene oxide nanosheets.

1 1. The structure of Claim 10, wherein the plurality of alternating layers of graphene and carbon nanotubes (CNT) are formed by:

synthesizing graphene oxide (GO) from high purity graphite flake powder using a modified Hummer's method with an improved purification and then chemical reduction to form exfoliated graphene oxide;

dispersing the reduced graphene oxide (rGO) nanosheets and CNTs in ethanol and 1 -butanol, respectively;

12. The structure of Claim 10, wherein the CNT layer acts as a porous physical spacer, which enhances the permeation of a gel electrolyte inside the structure and reduces the agglomeration of rGO nanosheets along the out-of-plane axis.

13. The structure of Claim 10, comprising:

14. The structure of Claim 1 1 , wherein the chemical reduction of the graphene oxide comprises:

redispersing the exfoliated graphene oxide in water after sonication; mixing the redispersed exfoliated graphene oxide with water N2H , and NH₄OH; and

heating the redispersed exfoliated graphene oxide at 70 °C for about

1 hour.

15. The structure of Claim 10, comprising:

a solid-state device formed by pouring a gel electrolyte onto the stacked rGO/CNT films.

16. The structure of Claim 15, wherein the solid-state device is placed in a desiccator for drying of the gel electrolyte.

17. The structure of Claim 16, wherein the gel electrolyte is a polymer gel electrolyte.

18. The structure of Claim 17, comprising:

assembling two electrodes face-to-face; and

solidifying the electrolyte.

19. The structure of Claim 17, wherein the polymer gel is a mixture of poly (vinyl alcohol) and phosphoric acid (H₃P0₄).