Claims:1. A method for preparing graphene-metal foam composite, comprising-
coating metal foam with solution of shellac; and
heating the shellac coated metal foam, followed by cooling to obtain the graphene-metal foam composite.

2. The method as claimed in the claim 1, wherein the solution of shellac is prepared by dissolving shellac in a solvent selected from a group comprising isopropanol, ethanol, methanol at a temperature ranging from about 80°C to 90°C, followed by cooling and filtering; wherein concentration of the shellac is ranging from about 10wt% to 15wt%.

3. The method as claimed in claim 1, wherein the coating is by technique selected from a group comprising dip coating, drop casting and spray coating.

4. The method as claimed in claim 1, wherein the metal foam is selected from a group comprising nickel foam, silver foam, iron foam, copper foam, aluminum foam and steel foam.

5. The method as claimed in claim 1, wherein the metal foam, prior to the coating is treated with solvent selected from a group comprising acetone, ethanol, isopropanol, at a temperature ranging from about 20°C to 40°C.

6. The method as claimed in claim 1, wherein the heating is carried out at a temperature ranging from about 800 °C to 830 °C for a period ranging from about 10 minutes to 15 minutes, at a pressure ranging from about 1×10-2 mbar to 9×10-2 mbar, under vacuum; and cooling is carried to a temperature ranging from about 20°C to 40°C.

7. A graphene-metal foam composite prepared by the method as claimed in claim 1.

8. The graphene-metal foam composite as claimed in claim 7, comprises at most 10 graphene layers; density of the graphene-metal foam composite is ranging from about 0.50 g/cc to 0.56 g/cc; the graphene in the graphene-metal foam composite is porous with average pore diameter ranging from about 100 microns to 300 microns.

9. The graphene-metal foam composite as claimed in claim 7, wherein intensity ratio of D-band to G-band (ID/IG) of the graphene is ranging from about 0.13 to0.48 range, under Raman Spectroscopy; and wherein crystal grain size of the graphene is ranging from about 148nm to40 nm.

10. The graphene-metal foam composite as claimed in claim 7, wherein the composite has an areal capacitance ranging from about 0.40 F/cm2 to 1.7F/cm2, and the composite has capacitance retention of about 89% for about 1000 charge-discharge cycles.

11. An apparatus comprising the graphene-metal foam composite as claimed in claim 7.
, Description:TECHNICAL FIELD
The present disclosure relates to field of material science and nano science. The disclosure particularly relates to a method of preparing a graphene-metal foam composite, wherein the said method is a rapid method for preparation of porous graphene-metal foam composite employing low-cost carbon source, such as shellac. The said method is simple, cost-effective, eco-friendly and industrial compatible. The disclosure further relates to a graphene-metal foam composite prepared by the said method. The disclosure furthermore relates to an apparatus comprising the said graphene-metal foam composite.

BACKGROUND OF THE DISCLOSURE
Due to the three-dimensional structure and excellent conductivity nickel (Ni) foam is extensively used as the current collector in supercapacitors and graphene is known to be grown on Ni foam to extend its applications. Methods such as Chemical vapour deposition (CVD) and Hummers’ solution followed by hydrothermal process or heating are known to grow or deposit graphene on Ni foam. In CVD method, carbon containing gases such as methane (CH4) or ethylene (C2H4) is used as a carbon source mixed with H2/Ar gases. Using this method, good quality graphene on Ni foam can be prepared. However, relatively at high manufacturing cost, with a threat of toxicity from methane and use of highly flammable H2 gas adds to the disadvantages of the method.

The Hummers’ method, which is a chemical solution method that involves oxidation of graphite powder in solution to produce graphene oxide (GO) that is further reduced to obtain reduced graphene oxide (rGO). Using hydrothermal method or heating, GO or rGO is fabricated on Ni foam. This method undergoes rigorous multiple steps and obtained GO or rGO structure on Ni foam suffers from low conductivity and inferior crystal quality due to the presence of oxygen-related functional groups. Furthermore, graphite powder employed in the method is relatively expensive, indicating that production cost of Hummers’ method is higher, and the product obtained by this method is associated with low quality graphene.

Thus, there is a need for a manufacturing method which is rapid, simple and cost-effective, wherein the graphene formed on the substrate is of highest quality. The present disclosure intends to address all the above mentioned limitations.

SUMMARY OF THE DISCLOSURE
The object of the present disclosure is to provide a simple, cost-effective and rapid method, which is environmentally friendly for preparation of graphene-metal foam composite comprising graphene of highest quality with superior properties.

Accordingly, the present disclosure relates to a method of preparing graphene-metal foam composite comprising-coating metal foam with solution of shellac and heating the shellac coated metal foam, followed by cooling to obtain a graphene-metal foam composite.

The present disclosure relates to a graphene-metal foam composite prepared by- coating metal foam with solution of shellac and heating the shellac coated metal foam, followed by cooling, wherein the intensity ratio of D-band to G-band (ID/IG) of the graphene is ranging from about 0.13 to 0.48 under Raman spectroscopy, indicating that the graphene formed on the metal foam has very low defect.

The disclosure also relates to an apparatus comprising the said graphene-metal foam composite.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
In order that the disclosure may be readily understood and put into practical effect, reference will now be made to exemplary embodiments as illustrated with reference to the accompanying figures. The figures together with detailed description below, are incorporated in and form part of the specification, and serve to further illustrate the embodiments and explain various principles and advantages, in accordance with the present disclosure where:

FIGURE 1a illustrates chemical structure of shellac biopolymer (Molecular formula: C30H50O11), showing long aliphatic carbon chains and cyclic carbon rings attached to hydroxyl and carboxyl functional groups.
FIGURE 1b illustrates Thermogravimetric analysis (TGA) of shellac in nitrogen environment.

FIGURE 2a illustrates Raman spectrum of the graphene of the graphene-metal (Ni) foam composite.
FIGURE 2b illustrates X-ray powder diffraction (XRD) pattern of the graphene of the graphene- metal (Ni) foam composite.
FIGURE 2c illustrates digital photographs of bare Ni foam (upper panel) and graphene-metal (Ni) foam (lower panel).
FIGURE 2d illustrates scanning electron microscope (SEM) image of graphene-metal (Ni) foam composite.

FIGURE 3 illustrates Energy-dispersive X-ray spectroscopy (EDS) of graphene of the graphene-metal (Ni) foam composite.

FIGURE 4a illustrates CV curves of graphene of the graphene-metal (Ni) foam composite at scan rates of 1mV/s, 2mV/s, 10mV/s, 30mV/s, 50mV/s and 100 mV/s, respectively.
FIGURE 4b illustrates a plot comprising the calculated areal capacitances derived from figure 4a against scan rates at 1mV/s, 2mV/s, 10mV/s, 30mV/s, 50mV/s and 100 mV/s, respectively.
FIGURE 4C illustrates charge-discharge curves of graphene-metal (Ni) foam composite at current densities of 1 mA/cm2, 4 mA/cm2 and 10 mA/cm2, respectively.
FIGURE 4D illustrates a plot comprising calculated areal capacitances derived Figure 4c against current densities at 1 mA/cm2, 4 mA/cm2 and 10 mA/cm2, respectively.

FIGURE 5illustrates CV curves of bare Ni foam (black curve) and graphene-metal (Ni) foam composite (red curve) with the scan rate of 10 mV/s in 6 M KOH solution.

FIGURE 6a illustrates capacitance retention (%) of graphene-metal foam over 1000 cycles.
FIGURE 6B illustrates charge-discharge curves during different cycles. The inset shows the charge-discharge cycles approaching 1000th cycle.

FIGURE 7a illustrates EIS (Nyquist plot) of graphene-metal (Ni) foam composite before charge-discharge cycle.
FIGURE 7b illustrates EIS (Nyquist plot) of graphene-metal (Ni) foam composite after 1000 cycles of charge-discharge.

DETAILED DESCRIPTION OF THE DISCLOSURE
The present disclosure relates to a method of preparing graphene-metal foam composite.

In an embodiment, the said method of preparing graphene-metal foam composite is simple, economical and environmentally friendly.

In an embodiment, the carbon source that is employed in the method of the present disclosure is an inexpensive carbon source, such as shellac.

In an embodiment, the method provides for a graphene-metal foam composite, wherein the graphene is of highest quality with superior properties.

In an embodiment, the method of preparing graphene-metal foam composite, comprises-
coating metal foam with solution of shellac; and
heating the shellac coated metal foam, followed by cooling to obtain a graphene-metal foam composite.

In an embodiment, the solution of shellac is prepared by dissolving shellac fakes in a solvent at a temperature ranging from about 80 °C to 90 °C, followed by cooling to a temperature ranging from about 20°C to 40°C and filtering the solution using Whatman filter paper, such as Whatman 41 filter paper.

In an embodiment, the solution of shellac is prepared by dissolving shellac flakes in a solvent selected from a group comprising isopropanol, ethanol, methanol at a temperature of about 80 °C, about 81°C, about 82°C, about 83°C, about 84°C, about 85°C, about 86°C, about 87°C, about 88°C, about 89°C or about 90 °C, followed by cooling to a temperature of about 20°C, about 22°C, about 24°C, about 26°C, about 28°C, about 30°C, about 32°C, about 34°C, about 36°C, about 38°C or about 40°C and filtering the solution using Whatman filter paper, such as Whatman 41 filter paper.

In an embodiment, the shellac in the shellac solution is in an amount ranging from about 10 wt% to 15 wt%.

In another embodiment, the shellac in the shellac solution is in an amount of about 10wt%, about 10.5wt%, about 11wt%, about 11.5wt%, about 12wt%, about 12.5wt%, about 13wt%, about 13.5wt%, about 14wt%, about 14.5wt% or about 15wt%.

In an embodiment, the metal foam is selected from a group comprising nickel foam, silver foam, iron foam, copper foam, aluminum foam and steel foam.

In an embodiment, the shellac solution is coated to the metal foam by technique including but it is not limited to dip coating, drop casting, spin coating and spray coating.

In another embodiment, the shellac solution is coated to the metal foam by technique such as, dip coating.

In an embodiment, the metal foam prior to coating with the shellac solution is treated with solvent including but it is not limited to acetone and it is subjected to drying at a temperature ranging from about 27 °C to 100 °C.

In another embodiment, the metal foam prior to coating with the shellac solution is treated with solvent including but it is no limited to acetone and it is subjected to drying at a temperature of about 27°C, about 30°C, about 35°C, about 40°C, about 45°C, about 50°C, about 55°C, about 60°C, about 65°C, about 70°C, about 75°C, about 80°C, about 85°C, about 90°C, about 95°C or about 100°C.

In an embodiment, the metal foam is dip coated with the shellac solution at least two times, wherein after each dip coating, the metal foam is dried and subjected to subsequent dip coating in order to achieve efficient coating of the shellac solution on the metal foam.

In another embodiment, the metal foam is dip coated with the shellac solution at least three times, wherein after each dip coating, the metal foam is dried and subjected to subsequent dip coating in order to achieve efficient coating of the shellac solution on the metal foam.

In an embodiment, shellac coated metal foam is heated to a temperature ranging from about 800 °C to 830 °C at a pressure ranging from about 1 × 10-2 mbar to 9 × 10-2 mbar for a period ranging from about 10 minutes to 15 minutes, followed by cooling to a temperature ranging from about 20°C to 40°C, under vacuum.

In another embodiment, shellac coated metal foam is heated to a temperature of about 800°C, about 805°C, about 810°C, about 815°C, about 820°C, about 825°C or about 830°C at a pressure of about 1 × 10-2 mbar, about 2 × 10-2 mbar, about 3 × 10-2 mbar, about 4× 10-2 mbar, about 5× 10-2 mbar, about 6× 10-2 mbar, about 7× 10-2 mbar, about 8× 10-2 mbar or about 9× 10-2 mbar for a period of about 10minutes, about 11 minutes, about 12 minutes, about 13 minutes, about 14 minutes or about 15 minutes, followed by cooling to a temperature of about 20°C, about 22°C, about 24°C, about 26°C, about 28°C, about 30°C, about 32°C, about 34°C, about 36°C, about 38°C or about 40°C, under vacuum.
In an embodiment, the method causes formation of porous graphene on the metal foam, wherein the average pore diameter is ranging from about 100 microns to 300 microns.

In another embodiment, the average pore diameter of the porous graphene on the graphene-metal foam is about 100microns, about 120 microns, about 140 microns, about 160 microns, about 180 microns, about 200 microns, about 220 microns, about 240 microns, about 260 microns, about 280 microns or about 300 microns.

In an embodiment, in the said method the conditions are optimal such that the shellac undergoes removal of oxygen functional groups with bond rearrangement, resulting in the formation of graphene structure on the metal foam with minimal oxygen concentration. Figure 3 illustrates the EDS of the graphene in the graphene-metal (Ni) foam composite, depicting lowest oxygen concentration of less than 0.2% in the graphene.

In another embodiment, the quality of graphene obtained by the said method possess low defect density (ID/IG) ratio and negligible oxygen related impurity.

In an embodiment, the said method does not require any expensive instrumentation and employs non-expensive material, including the carbon source, as a result the method is economical.

In an embodiment, the method does not employ any gaseous sources, such as hydrogen or methane, as a result the said method is environmentally friendly.

In an embodiment, the method can be scaled-up for bulk synthesis of graphene-metal foam composite and the method is industrially compatible.

In an embodiment, the method is a rapid method with one-step heating of metal foam coated with shellac.

The present disclosure further relates to a graphene-metal foam composite prepared by the above defined method.

In an embodiment, the graphene-metal foam composite comprises graphene formed of shellac.

In an embodiment, the graphene-metal foam composite comprises at most 10 graphene layers.

In another embodiment, the graphene-metal foam composite comprises graphene layers ranging from about 1 to 10.

In another embodiment, the graphene-metal foam composite comprises about 2, about 3, about 4, about 5, about 6, about 7, about 8, about 9 or about 10 graphene layers.

In an embodiment, the density of the metal foam is ranging from about 0.50 g/cc to 0.55 g/cc.

In another embodiment, the density of the metal foam is about 0.50 g/cc, about 0.51 g/cc, about 0.52 g/cc, about 0.53 g/cc, about 0.54 g/cc or about 0.55 g/cc.

In an embodiment, the density of the graphene-metal foam composite is ranging from about 0.50 g/cc to 0.56 g/cc.

In another embodiment, the density of the graphene metal foam is about 0.50 g/cc, about 0.51 g/cc, about 0.52 g/cc, about 0.53 g/cc, about 0.54 g/cc, about 0.55 g/cc or about 0.56 g/cc.

In another embodiment, the density of the graphene-nickel foam composite is ranging from about 0.50 g/cc to 0.56 g/cc.

In another embodiment, the density of the graphene nickel foam is about 0.50 g/cc, about 0.51 g/cc, about 0.52 g/cc, about 0.53 g/cc, about 0.54 g/cc, about 0.55 g/cc or about 0.56 g/cc.
In an embodiment, the intensity ratio of D-band and G-band (ID/IG) of the graphene in the graphene-metal foam is ranging from about 0.13 to 0.48, and the full width at half maximum of the 2D band is about 81cm-1, under Raman spectroscopy, indicating that the graphene is of superior quality with very low defect.

In an embodiment, the intensity ratio of D-band and G-band (ID/IG) of the graphene in the graphene-metal foam is about 0.13, about 0.15, about 0.20, about 0.25, about 0.30, about 0.35, about 0.40, about 0.45 or about 0.48.

In an embodiment, the graphene-metal foam composite possesses areal capacitance ranging from about 0.40 F/cm2 to 1.7 F/cm2.

In another embodiment, the graphene-metal foam composite possesses areal capacitance of about 0.40 F/cm2, about 0.45 F/cm2, about 0.50 F/cm2, about 0.55 F/cm2, about 0.60 F/cm2, about 0.65 F/cm2, about 0.70 F/cm2, about 0.75 F/cm2, about 0.80 F/cm2, about 0.85 F/cm2, about 0.90 F/cm2, about 0.95 F/cm2, about 1.0 F/cm2, about 1.05 F/cm2, about 1.1 F/cm2, about 1.15 F/cm2, about 1.2 F/cm2, about 1.25 F/cm2, about 1.30 F/cm2, about 1.35 F/cm2, about 1.40 F/cm2, about 1.45 F/cm2, about 1.50 F/cm2, about 1.55 F/cm2, about 1.60 F/cm2, about 1.65 F/cm2 or about 1.7 F/cm2.

In an embodiment, the graphene-metal foam composite has capacitance retention of about 89% relative to 1000 charge-discharge cycles.

In an embodiment, the grain size of the graphene in the graphene-metal foam is ranging from about 148 nm to 40 nm.

The present disclosure further relates to an apparatus comprising the graphene-metal foam composite.

In an embodiment, the apparatus includes but it is not limiting to energy storage device, fuel cells, gas sensors and biomedical devices.

In an embodiment, the apparatus is a supercapacitor.

The advantages of the present disclosure are-
• the present disclosure provides a novel and inventive method of producing a graphene-metal foam composite, wherein the graphene is porous graphene on three-dimensional (3D) metal foam using low-cost shellac as a carbon source.
• the graphene-metal foam exhibits high areal capacitance and prolonged cyclic stability.
• the porous nanostructure of the graphene in the graphene-metal foam composite provides larger specific surface area, low internal resistance and shorter channel length for the diffusion of electrolyte ions, as a result contributes to the higher areal capacitance.
Additional embodiments and features of the present disclosure will be apparent to one of ordinary skill in art based upon the description provided. The embodiments provide various features and advantageous details thereof in the description. Descriptions of well-known/conventional methods and techniques are omitted so as to not unnecessarily obscure the embodiments. The examples provided herein are intended merely to facilitate an understanding of ways in which the embodiments provided may be practiced and to further enable those of skill in the art to practice the embodiments provided. Accordingly, the following examples should not be construed as limiting the scope of the embodiments.

EXAMPLES

EXAMPLE 1: Preparation of graphene-Ni foam
Shellac flakes are dissolved in isopropanol at a temperature of about 80°C, followed by cooling to a temperature ranging from about 20°Cto 40°C and filtered using Whatman 41 filter paper to obtain a shellac solution having shellac of about 15wt%.
Nickel (Ni) foam was cut into predetermined rectangular pieces and subjected to pretreated, wherein the Ni foam is washed with acetone and was dried at a temperature of about 80°C. The washed Ni is dip coated with shellac soliton three times, wherein after each dip coating, the Ni foam was subjected to drying such the coating is effectively formed on the Ni foam.
The shellac coated Ni foam is heated to a temperature of about 830°C, at pressure of about 10-2mbar for a period of about 15 minutes, followed by cooling to a temperature of about 20°C to 40°C, under vacuum to obtain the graphene-Ni foam composite.

EXAMPLE 2: Characterization of the graphene-Ni foam composite.
The graphene-Ni foam composite prepared in the Example 1 is characterized using Raman Spectroscopy, X-ray diffraction (XRD) technique, Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive spectroscopy (EDS). The characterization of the graphene-Ni foam composite was carried out in a three-electrode configuration, wherein the graphene-Ni foam composite is a working electrode, platinum is a counter electrode and Ag/AgCl is a reference electrode and the electrolyte was about 6M potassium hydroxide.

Under Raman spectroscopy G-band was observed at 1585 cm-1, D-band was observed at 1360 cm-1 and 2D-band was observed at 2714 cm-1 (illustrated in figure 1a). The G-band in Raman spectra of graphene is associated with phonon vibrations in sp2-bonded carbon, and D-band is originated due to disorder induced by phonon scattering at impurity and defect sites. The 2D-band evolves from a second-order double resonance between non-equivalent K points in the Brillouin Zone of graphene. From the plot in figure 1a it is apparent that there is negligible D band, implying that there is negligible defect in the graphene prepared by the method of the present disclosure.
The graphene with a low value of ID/IG of about 0.13 and shaper 2D-band (full width at half maximum of about 81cm-1) are an indication that the graphene in the graphene-Ni foam composite is of superior quality.
Subsequently, the crystalline grain size of the graphene in the graphene-Ni foam composite was estimated using the equation La (nm)= (2.4x10-10x?4)/(ID/IG), wherein ? is the wavelength of about 532nm used in the Raman measurement. The calculated crystalline grain size is about 148nm, indicating that the graphene formed is in the highly order crystalline form.
The EDS measurement of the graphene in the graphene-Ni foam composite shows that there is negligible oxygen present in the graphene (illustrated in figure 3), indicating that graphene in the graphene-Ni foam is of highest quality without any oxygen contamination.

The XRD pattern of the graphene in the graphene-Ni foam composites shows (illustrates in figure 3) a broad and low-intensity peak centered at 23.5°, that is indexed at (002) plane of few-layer graphene. The peaks at 44.90°, 52.22° and 76.83° are indexed to (111), (200) and (220) planes of nickel (illustrated in figure 2b). Thus, the XRD pattern demonstrates that the Ni foam is effectively coated with graphene in the graphene-Ni foam composite.

EXAMPLE 3: illustration of Supercapacitive property of the graphene-Ni foam composite
The supercapacitive performance of graphene coated Ni foam is evaluated using electrochemical measurements in 6M KOH solution in the potential window of about -0.1V to 0.6V.
Figure 4a illustrates the CV curves of the sample at difference scan rates such as 1mV/s, 2mV/s,10mV/s, 30mV/s, 50mV/s and 100mV/s, respectively. The areal capacitance of the graphene-Ni foam composite was calculated based on the CV curves at the said different scan rates and it was found to be about 1.663F/cm2, about 1.610F/cm2, about 1.405F/cm2, about 0.974F/cm2, about 0.775 F/cm2, and about 0.405F/cm2, respectively (illustrated in figure 4b).
The CV curves show the redox peaks that are sustained at different scan rates, indicating that the capacitive behavior of the graphene-Ni foam composite is maintained over the entire range of the scan rates.

The performance of the supercapacitive property of the graphene-Ni foam composite is further examined by galvanostatic charge-discharge measurements (GCD tests) in the potential window of about 0V to 0.45V. The GCD curves at different current densities are illustrated in figure 4c. The GCD curves show symmetrical behavior at various current densities, implying excellent electrochemical reversibility and charge-discharge properties of the graphene-Ni foam composite. Furthermore, the well-retained shape of the GCD curves with low IR drop (electrical potential difference at two ends) at various current densities indicates a reversible faradic redox transition.
From the GCD curves, the calculated areal capacitance are about 1.706F/cm2, 1.484F/cm2 and 1.288F/cm2 at current densities of 1mA/cm2, 4mA/cm2 and 10mA/cm2, respectively (illustrated in figure 4d).
Further, the CV measurements of bare Ni foam (uncoated Ni foam) are compared with the graphene-Ni foam (illustrated in figure 5). The comparison shows that the graphene-Ni foam composite exhibits higher integrated area in CV curves compared to that of the bare Ni foam, suggesting that the graphene is playing a significant role in improving the capacitance of the graphene-metal foam.

EXAMPLE 4: illustration of cyclic stability of the graphene-Ni foam composite
The cyclic stability of the graphene-Ni foam composite was assessed by charging-discharging cycles at the current density of 10mA/cm2 and the results are illustrated in figure 6.
Figure 6a illustrates the capacitance retention of graphene-Ni foam composite, from which it is observed that the capacitance retention of graphene-Ni foam composite is about 89% for about 1000 charge-discharge cycles. The behavior of charge-discharge curves with different cycles is illustrated in figure 6b, the inset of figure 6b exhibits charge-discharge cycles that are approaching 1000th cycle. It is observed that the charge-discharge cycles retain their shapes and amplitude during charging-discharging for 1000 cycles, thus demonstrating good stability of the capacitive performance of the graphene-Ni foam composite.

The graphene-Ni foam composite was further subjected to electrochemical impedance spectroscopy (EIS). The EIS measurements were performed in the frequency ranging from about 0.01Hz to 1M Hz by applying an AC voltage with about 5mV perturbation.
Figure 7a illustrates the EIS (Nyquist plot) of graphene-Ni foam composite before charge-discharge cycle, wherein combined resistance (Rs) is about 1.49? and charge transfer resistance (Rct) is about 2.32?.
Figure 7b illustrates the EIS (Nyquist plot) of graphene-Ni foam composite after 1000 cycles of charge-discharge, wherein combined resistance (Rs) is about 4.1? and charge transfer resistance (Rct) is about 2.72?.
It is observed that there is no significant difference in the values of Rct before and after charge-discharge cycles, suggesting that electrochemical performance of the graphene-Ni foam composite is stable even after 1000 charge-discharge cycles.
Generally, in low-frequency region, an ideal capacitance would give rise to a straight line parallel to the imaginary axis (Z" axis). However, for the graphene-Ni foam composite, as per the plots of figure 7, the straight line parts lean closure towards the Z" axis, indicating that the graphene-Ni foam composite has good capacitive behavior.
Also, the deviation from the ideal capacitor behavior before and after charge-discharge cycles observed for graphene-Ni foam composite indicates that a resistive element is associated with Warburg impedance that is attributed to the diffusion of electrolyte ions into the porous graphene in the intermediate frequency region.