WO2021022337A1

WO2021022337A1 - A solid-state supercapacitor and a process for producing a solid-state supercapacitor

Info

Publication number: WO2021022337A1
Application number: PCT/AU2020/050812
Authority: WO
Inventors: Francesca Iacopi; Mojtaba AMJADI POOR; Dawei Su
Original assignee: University Of Technology Sydney
Priority date: 2019-08-06
Filing date: 2020-08-06
Publication date: 2021-02-11

Abstract

A process for producing a solid-state supercapacitor, including the steps of: forming at least one pair of mutually spaced electrodes, each said electrode including a graphene surface layer; and introducing a gel electrolyte between the mutually spaced electrodes and onto the graphene surface layer of each electrode to form a double layer supercapacitor, wherein the gel electrolyte is water-based and does not include a redox agent; and applying a voltage across each pair of electrodes to generate an electric field strong enough to decompose water between the mutually spaced electrodes by hydrolysis and functionalise a surface of the electrodes to substantially increase capacitance of the supercapacitor in the absence of a redox agent, the increased capacitance arising from at least one of electrical double layer capacitance of the electrodes and redox reaction capacitance.

Description

A SOLID-STATE SUPERCAPACITOR AND A PROCESS FOR PRODUCING A SOLID-STATE SUPERCAPACITOR TECHNICAL FIELD

The present invention relates to solid-state microelectronics, and in particular to a solid- state supercapacitor and a process for producing a solid-state supercapacitor for microelectronic integration.

BACKGROUND

The rapid development of portable and wireless electronics has significantly increased the demand for energy storage devices. Additionally, the pressing need for clean energy sources such as solar and wind energies further increases the demand for reliable and maintenance-free batteries and supercapacitors. Electrochemical supercapacitors have attracted a lot of attention due to their superior energy storage capabilities such as high power density and fast charging-discharging. In addition, electrochemical supercapacitors are considered environmentally friendly, and have a long lifetime which is extremely desirable for future integrated devices.

Charge storage in electrochemical supercapacitors is mainly based on electrical double layer capacitance (EDLC) and redox reactions. EDLC charge storage relies on electrolyte ion absorption at the interface between the supercapacitor electrodes and an electrolyte in the presence of an electric field, whereas redox reactions are effected by adding a redox 'agent' to the electrolyte, enabling charge storage via a fast and repeatable charge transfer reaction between the electrolyte ions and the electrodes.

Most supercapacitors are fabricated using liquid electrolytes and often suffer from leakage, making them undesirable for industry. The robust packaging required to avoid leakage makes the devices too large and less desirable for many applications. Consequently, gel-based electrolytes have been suggested to address this difficulty. The most commonly used gel electrolyte is a mixture of a polymer (typically polyvinyl alcohol, or "PVA") and an acid (typically H₂SO₄ or H₃PO₄). The use of gel electrolytes also helps to address other challenges such as corrosion, self-discharge, solvent evaporation, and electrolyte loss at elevated temperatures.

Solid-state supercapacitors have great promise for possible integration with integrated circuits, although challenges remain. It is desired to provide a solid-state supercapacitor and a process for producing a solid-state supercapacitor that alleviate one or more difficulties of the prior art, or to at least provide a useful alternative.

SUMMARY

In accordance with some embodiments of the present invention, there is provided a process for producing a solid-state supercapacitor, including the steps of: forming at least one pair of mutually spaced electrodes, each said electrode including a graphene surface layer; and introducing a gel electrolyte between the mutually spaced electrodes and onto the graphene surface layer of each electrode to form a double layer supercapacitor, wherein the gel electrolyte is water-based and does not include a redox agent; and applying a voltage across each pair of electrodes to generate an electric field strong enough to decompose water between the mutually spaced electrodes by hydrolysis and functionalise a surface of the electrodes to substantially increase capacitance of the supercapacitor in the absence of a redox agent, the increased capacitance arising from at least one of electrical double layer capacitance of the electrodes and redox reaction capacitance.

In some embodiments, the step of applying the voltage enables redox charge transfer operation of the supercapacitor in the absence of a redox agent.

In some embodiments, the step of applying the voltage increases the electrical double layer capacitance of the supercapacitor.

The generated electric field may have a strength of at least 4 kV/m. In some embodiments, the generated electric field has a strength of at least 7 kV/m and is applied for at least 2 minutes. In some embodiments, the water-based gel electrolyte is a polyvinyl alcohol and acid gel electrolyte.

In some embodiments, the process includes forming the graphene surface layer on a silicon carbide on silicon substrate by a metal catalysis process.

In some embodiments, the metal catalysis process includes the steps of:

(i) depositing nickel metal or nickel and copper metals on the silicon carbide on silicon substrate; and

(ii) heating the deposited metal or metals and the silicon carbide on silicon substrate to react at least the nickel with the silicon carbide and form a graphene layer disposed between a remaining metal residue layer and the silicon carbide on silicon substrate; and

(iii) removing the metal residue layer to expose the graphene layer.

In some embodiments, the process includes a step of oxidising the deposited metal prior to the heating step (ii).

In some embodiments, the process includes a step of repeating steps (i) to (iii) at least once to form a porous surface of the silicon carbide substrate coated with graphene.

In accordance with some embodiments of the present invention, there is provided a solid-state supercapacitor produced by any one of the above processes.

In accordance with some embodiments of the present invention, there is provided a solid-state supercapacitor, including: mutually spaced electrodes, each said electrode including a graphene layer on a silicon carbide on silicon substrate; a gel electrolyte between the mutually spaced electrodes and on the graphene layer of each electrode, wherein the gel electrolyte is water-based and does not include a redox mediator agent; and the electrodes having functionalised surfaces whose capacitance arises from electrical double layer capacitance of the electrodes and redox reaction capacitance. In some embodiments, the gel electrolyte is a polyvinyl alcohol (PVA) and acid gel electrolyte, and graphene layers of the graphene electrodes have an increased spacing due to the intercalation of PVA molecules therebetween.

Also described herein is a solid-state supercapacitor, including: mutually spaced electrodes, each said electrode including a graphene layer on a silicon carbide on silicon substrate; a gel electrolyte on the graphene layer of each electrode, wherein the gel electrolyte does not include a redox mediator agent; and the electrodes having functionalised surfaces whose capacitance arises from electrical double layer capacitance of the electrodes and redox reaction capacitance.

Also described herein is a process for producing a solid-state supercapacitor, including the steps of: forming at least one pair of mutually spaced electrodes, each said electrode including a graphene surface layer; and introducing a gel electrolyte onto the graphene surface layer of each electrode to form a double layer supercapacitor, wherein the gel electrolyte does not include a redox agent; and applying a voltage greater than 1.2 volts across each pair of electrodes to functionalise a surface of the electrodes and substantially increase capacitance of the supercapacitor, the capacitance arising from at least one of electrical double layer capacitance of the electrodes and redox reaction capacitance.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the present invention are hereinafter described, by way of example only, with reference to the accompanying drawings, wherein:

Figures 1 (a) to (c) are SEM images and Figures 1 (d) to (f) are AFM images showing the surface morphology of: (a),(d) low-defect graphene fabricated via 10 nm Ni and 20 nm Cu deposition; (b),(e) high defect density (oxidised) graphene synthesized by depositing 3 nm Ni only; and (c),(f) porous graphene fabricated using two rounds of 8 nm Ni deposition; Figure 2 is a set of three AFM line scans taken along the lines indicated in Figures 1 (d) to (f), showing the height profiles of low-defect graphene electrodes, oxidized electrodes, and porous electrodes, respectively;

Figure 3 includes Raman spectra of: (a) low-defect graphene electrodes, (b) oxidized electrodes, and (c) porous electrodes;

Figure 4 is a schematic diagram illustrating the sandwich configuration of all-solid- state supercapacitors fabricated using graphitized 3C-SiC/Si(100) electrodes and a PVA + H2SO₄ gel electrolyte;

Figure 5 is a set of cyclic voltammetry curves, as follows: (a) initial results from a supercapacitor with high quality graphenic electrodes; (b) after functionalization; (c) initial results of a supercapacitor with oxidized electrodes; (d) after functionalization; (e) initial results of a supercapacitor with porous electrodes; (f) after functionalization;

Figure 6 shows AC Impedance spectroscopy results for the electrochemical cell with oxidized electrodes: (a) full spectra and (b) the low impedance region indicated in (a); (c) long cycling charge/discharge test results of the same cell, and (d) CV curves comparing the cell capacitance with as-prepared electrodes, after in situ functionalization, and the latter after 6000 charge/discharge cycles, showing still substantially higher capacitance than the cell with as-prepared electrodes;

Figure 7 is a schematic illustration of the bonding resulting from an in situ functionalization process step, leading to significant capacitance improvement; (a) initial condition and (b) after the functionalization;

Figure 8 shows AC Impedance spectroscopy results (a) to (c) and respective Long cycling charge / discharge test results (d) to (f) for the three types of supercapacitors with, respectively: (a),(d) low-defect electrodes, (b),(e) oxidized electrodes, and (c),(f) porous electrodes; and

Figure 9 includes (a) Raman spectra and (b) FTIR spectra of the oxidized electrodes before and after the agent-free in situ functionalization process. DETAILED DESCRIPTION

Embodiments of the present invention include a hybrid type of supercapacitor whose capacitance arises from both electrical double layer capacitance (EDLC) and redox reactions. As described above, prior art redox-based supercapacitors require the addition of a redox agent to an electrolyte in order to initiate the required redox reactions. The most commonly used redox agent additives are P-phenylenediamine, hydroquinone and indigo carmine. Unfortunately, these additives are highly toxic and are also not generally compatible with semiconductor manufacturing.

However, embodiments of the present invention include a solid-state supercapacitor and a process for producing a solid-state supercapacitor that surprisingly but advantageously provide high capacitance without using redox agents to initiate or otherwise promote redox reactions. In particular, the inventors have devised an electrode conditioning or functionalising process step to initiate redox reactions in the absence of any redox agent. Additionally, the inventors believe that the electrode conditioning step also increases the capacitance by intercalation, as described below.

Embodiments of the present invention involve a supercapacitor having electrodes with a graphene surface layer. Carbon based electrodes formed from materials such as graphite, graphene oxide, carbon nanotubes, and graphene have been widely explored for energy storage applications, and graphene based electrodes are the most promising choice due to graphene's advantageous properties such as high surface area and high electron mobility. While employing graphene oxide based electrodes is the most desired option for flexible devices, epitaxial graphene formed on a silicon substrate has significant potential for developing on-chip supercapacitors in integrated circuits. Accordingly, in the described embodiments, the supercapacitor electrodes include a graphene surface layer on a silicon carbide on silicon substrate.

Among various processes for forming epitaxial graphene on silicon substrates, the inventors consider that a metal catalytic approach is the most versatile and compatible with current semiconductor industry technologies, being binder-free and able to form graphene directly on silicon wafers with high adhesion and tunable characteristics. Accordingly, in the described embodiments, the graphene surface layer on silicon carbide is formed by a metal catalytic process as described below. Briefly, the process includes the deposition of a thin layer of nickel or nickel and copper onto a silicon carbide on silicon substrate, followed by thermal processing to react at least the nickel with the silicon carbide to form a silicide layer and a graphene layer at the interface between the silicide layer and the remaining silicon carbide, and the removal of the resulting silicide layer and any unreacted metal (e.g., by chemical etching) to expose the graphene layer.

In the described embodiments, unintentionally- doped (~ 10¹⁴ carriers cm ²) 3C-SiC (cubic polytype) epitaxially grown on lowly-doped (~ 10¹⁴ carriers cm ²) Si <100> substrates are purchased from NOVASIC; these substrates have been chemically and mechanically polished (StepSiC® by NOVASIC (France)). The SiC layer is ~500 nm thick. Graphitization of the 3C-SiC/Si(100) substrates is carried out using the catalytic alloy process described in F. Iacopi, N. Mishra, B.V. Cunning, D. Goding, S. Dimitrijev, R. Brock, R.H. Dauskardt, B. Wood, and J. Boeckl, A catalytic alloy approach for graphene on epitaxial SiC on silicon wafers, Journal of Materials Research, 2015 30(05): p. 609-616 and N. Mishra, J.J. Boeckl, A. Tadich, R.T. Jones, P.J. Pigram, M. Edmonds, M.S. Fuhrer, B.M. Nichols, and F. Iacopi, Solid source growth of graphene with Ni-Cu catalysts: towards high quality in situ graphene on silicon, Journal of Physics D: Applied Physics, 2017. 50(9): p. 095302. In this process, a metal catalysis layer is deposited on the 3C-SiC/Si(100) substrate. The catalysis layer typically consists of only a Ni layer or an aggregate of Ni and Cu layers (or a single layer of co-deposited Ni and Cu).

In the described embodiments, a Cryopumped deposition chamber is used to deposit Ni and Cu metal layers by sputtering. The metal coated substrates are annealed at a temperature of approximately 1100 °C for approximately an hour under vacuum (~10 ⁵ mbar). High temperature annealing in the presence of the metallic layer results in breaking the C and Si bonds of the SiC, allowing the Ni atoms to bond with the Si atoms, and freeing the remaining C atoms to form graphene. The remaining metal residues are removed using chemical wet etching (Freckle solution).

This catalytic approach provides control over the porosity and defect density of the graphitized electrodes, as described in M. Ahmed, B. Wang, B. Gupta, J.J. Boeckl, N. Motta, and F. Iacopi, On-silicon supercapacitors with enhanced storage performance, Journal of The Electrochemical Society, 2017 164(4): p. A638-A644. In one embodiment, low-defect graphene is synthesized by depositing a Ni layer (~10 nm) onto the SiC surface, and a Cu layer (~ 20 nm) onto the Ni layer, followed by high temperature annealing. Figure la is a scanning electron microscope (SEM) image of a representative sample, showing an irregular surface morphology. In a second embodiment, depositing only a thin Ni layer (~ 3 nm) onto the SiC surface leads to a higher defect density, as shown in the SEM image of Figure lb. The higher defect density leads to a higher oxidization level due to the higher density of active sites (edges and vacancy defects) for bonding with oxygen. Exposing the sample to air for 24 hours after the Ni deposition further enhances the oxidization level. In order to make a porous-graphitized electrode, the metal catalytic procedure can be repeated one or more times. For example, in a third embodiment, two rounds of Ni deposition (~8nm), annealing and Freckle etching are performed, leading to a porous surface as shown in the SEM image of Figure 1C compared to other electrodes (compare Figures la and b).

Open pores with micron dimensions are clearly visible on the porous electrode surfaces, resulting from the repeated Ni diffusion, intrusion and removal of nickel silicides. In contrast, the oxidized (Figure lb, le) and the low-defect electrodes (Figure la, Id) do not show significant porosity on the surface, but they reveal a finer granular pattern that seems to be common across all of the electrode types. This pattern corresponds to particle sizes in the range of 80-150 nm, which is also confirmed by the power spectral densities (PSD) extracted from the corresponding AFM images of Figures Id, le, and If. Although the PSDs for the different electrodes show very different distributions, all of them show evident peaks at spatial frequencies in the 8-12 pm range (Figure S3), corresponding to the granularity discussed above. The RMS roughness estimated for the porous electrodes is ~ 72 nm, which is significantly higher than the low-defect (~ 25 nm) and the oxidized electrodes (~ 20 nm). The corresponding AFM height profiles in Figures 2a-c provide a further comparison of the different surface roughnesses of the three sample types.

Raman spectroscopy has been used to characterise the fabricated graphitized electrodes, using a Witec Raman microscope with green (583 nm) or blue (488 nm) lasers. As known by those skilled in the art, graphene has three distinct peaks in its Raman spectrum, known as the D, G, and 2D peaks. The D peak represents the defect density within the graphene lattice, whereas the G and 2D peaks are related to the sp2 bonded carbon lattice. The other peaks in the Raman spectra (Figure 3) are the TO peak at ~850 cm ¹, and the LO peak at ~972 cm ¹, related to the 3C-SiC/Si substrate.

The ratio of the D peak and G peak intensities (referred to as the "ID/IG" ratio) is commonly used as a metric of the graphene defect density: the higher the ID/IG ratio, the higher the defect density. As can be noted from the Raman spectra of Figure 3, electrodes graphitized by depositing both Ni and Cu layers have the lowest defect density (Figure 3a), and samples fabricated by depositing only a thin layer of Ni (3nm) have the highest defect density (Figure 3b). The intensity ratio of the LO peak to the 2D peak (ILO/I2D) represents the graphene coverage over the 3C-SiC/Si surface, wherein a lower ILO/I2D ratio means higher graphene coverage. Electrodes fabricated by deposition of ~10 nm Ni and ~ 20 nm Cu have the lowest ILO/I2D and ID/IG ratios, and consequently these electrodes are referred to herein as the electrodes with 'low- defect' graphene. All-solid-state supercapacitors with a sandwich configuration, as shown in Figure 4, are fabricated by forming two mutually spaced planar electrodes with a water-based gel electrolyte filling the gap between the electrodes. In the described embodiments, the thickness of the gel electrolyte and the gap width is about 300 pm. However, it will be apparent to those skilled in the art that the gap width may be any practical value in other embodiments. For example, in some embodiments the gap is only a few microns.

The electrochemical performance of the supercapacitors has been evaluated using an electrochemical workstation (CH Instruments, 660 E Model) in a two-probe configuration. Cyclic voltammetry ("CV") test results of supercapacitors fabricated from the different electrodes described above are shown in Figure 5. The CV curves have a quasi-rectangular shape which is known to be characteristic of EDLC-based charge storage. Comparing the CV results of a supercapacitor with oxidized electrodes (Figure 5c) to the other ones (Figures 5a and e), it is apparent that the supercapacitors with oxidized electrodes have significantly better EDLC charge storage performance than the supercapacitors with other electrode types.

In accordance with the present invention, an electric field strong enough to decompose water by hydrolysis is applied to the surface of each electrode to functionalise the surface of each electrode. In the described embodiments where the electrodes are separated by a gap of about 300 pm and the water-based gel electrolyte is a polyvinyl alcohol ("PVA") + acid gel electrolyte, this corresponds to a voltage of at least 1.2 volts applied across the electrochemical cell, corresponding to an electric field of about 4 kV/m, and in the described embodiments a voltage of 2 volts (corresponding to an electric field of about 7 kV/m) was applied across the cell at a current of 1 to 5 pA/cm² for a duration of at least 2 and up to 15 minutes. In general, an electric field strength in the range of 4-10 kV/m is suitable. As the gel electrolyte contains a significant amount of water (in this case the composition of the electrolyte is lg of PVA : 1 g of H₂SO₄ : 10 mL of DI water), the inventors believe that applying a voltage greater than 1.2 V between the adjacent electrodes causes the water in the gel between these electrodes to partially decompose into H⁺ and OH which are chemically active and mobile, and are able to react with the graphene surface of each electrode, particularly at defective sites. It will be apparent to those skilled in the art that other water-based gel electrolytes may be used in other embodiments, any may produce other functional groups.

This in situ functionalization process step leads to a significant capacitance enhancement. As shown below, the capacitance performance improvement is due to a combination of EDLC enhancement and promoting redox reactions. Figures 5b, d, and f are graphs showing the CV curves corresponding to Figures a, c and e, respectively, but after the functionalising step. Comparison with Figures 5a, c, and e shows a significant improvement in EDLC charge storage as represented by the increased rectangular area of the CV curves. The inventors attribute this increase to the bonding of PVA molecules to the graphene layers, forming functionalized graphene with increased layer spacing, as described below and as illustrated in Figure 7. Additionally, the two Faradaic peaks in the CV curves appearing only after the functionalization process are characteristic of redox charge transfer reactions. Surprisingly, the conditioning/functionalising process step promotes the redox reactions in the absence of any redox agent.

Figure 6 is a set of graphs showing the galvanostatic charge / discharge of supercapacitors with the three types of electrodes, and both before and after the conditioning process step. It is clear that the capacitance of all three supercapacitor types increases substantially after functionalization: more than three times for the supercapacitors with oxidized electrodes (see Table 1). In addition to the capacitance enhancement, there is an improvement in Coulombic efficiency, again particularly for the supercapacitors with oxidized electrodes (see Table 1). The charge / discharge results further confirm the significant effect of the functionalization process step in enhancing the capacitance performance of the all-solid-state supercapacitors fabricated with graphitized SiC/Si electrodes.

Without wanting to be bound by theory, the inventors believe that charging the supercapacitors with potentials over the water decomposition voltage splits some of the water molecules within the electrolyte, creating free hydroxyl groups which can then react with the graphitic electrodes' surface, forming oxygen-containing groups, including epoxy. This mechanism leads to a simple, in situ preparation of functionalized electrodes, which is the key to triggering redox reactions and enhancing the EDLC performance. The functionalized-graphitized electrodes are then capable of promoting the redox reactions without requiring the addition of any redox agent to the electrolyte. Additionally, the inventors believe that the intercalation of PVA molecules between the graphene layers increases the graphene layer spacing, potentially by up to 0.45 nm. As illustrated in Figure 7, this factor, as well as an anticipated improved wettability of the graphitic electrodes due to the functionalization, enables the PVA molecules to diffuse more easily between the electrode layers and intercalate the graphene, greatly enhancing the total extent of electrode/electrolyte interface in the cell, and consequently leading to a significant enhancement in EDLC performance. Note that the PVA does not necessarily need to become chemically bonded to the graphitic electrodes for this mechanism to occur. Similarly, the Coulombic efficiency improvement is likely a result of the enhanced electrode/electrolyte intercalation, leading to shorter ionic transport paths.

The data described herein also show the importance of chemically-active graphitic electrodes in triggering redox reactions using a PVA + H2S04 gel electrolyte. The inventors suggest that the redox reaction at the electrode/electrolyte interface originates from the elevated presence of oxygen functional groups.

Although additional surface mechanisms, we believe that the most likely redox reaction is the repetitive transformation of the hydroxyl groups to the epoxy groups at the interface (hydroxyl <® epoxy + 2H⁺+2e^~). Also, some redox contribution may arise from the reaction suggested by Aljafari et al. through reacting part of the OH branches in the PVA with the H2S04 anions within the volume of the gel electrolyte.

Table 1. All-solid-state supercapacitor cells performance calculated from galvanostatic charge/discharge tests.

AC impedance spectroscopy measurements of the supercapacitors were made before and after the functionalization process step. Figures 8(a) and 8(b) are Nyquist impedance plots for the cell fabricated with oxidized electrodes, showing that the cell resistance slightly increases after the functionalization step. This resistance increase at the interface most likely stems from the functionalising of the graphitized electrodes. Although the shapes of the curves are different, both other cell types exhibit a similar increase in transfer impedance after the in situ functionalization process. Figure 8(c) shows a representative prolonged charge/discharge test of a supercapacitor with oxidised electrodes, demonstrating long life performance, with capacitance retention of approximately 90% of the original value after 6,000 cycles. As shown in Figure 8(d), cyclic voltammetry results before and after the long charge/discharge cycling tests still show the presence of the redox contribution, which appears thus to be a consistent contribution to the overall cell capacitance after functionalization.

In order to further investigate these phenomena, supercapacitors have been opened and their electrodes characterised using Raman and FTIR spectroscopy, as shown in Figure 8. Figure 8a shows Raman spectra of the graphene electrodes before and after functionalization process step for the (best performing) supercapacitors with oxidized electrodes. The G peak remains intact, but the D peak's line shape and the position of the 2D peak have changed, indicating modification of graphene. The FTIR spectra show the modification of graphene specifically in the wavenumber region above 1200 cm ¹ (Figure 8b). The inventors attribute this change to the presence of the carboxyl species, consistent with the bonding of PVA molecules to graphene.

The electrode conditioning/functionalisation process described herein provides a significant capacitance performance enhancement for all-solid-state supercapacitors fabricated using graphene electrodes and PVA + acid electrolyte. Moreover, the in situ functionalization process described herein enables redox reactions without employing any redox agent. The conditioning process step provides enough energy for graphenic electrodes and PVA to synthesize functionalized electrodes. This also further improves EDLC charge storage by increasing the available surface area of the graphene electrodes. Accordingly, the described supercapacitors and processes facilitate the development of high performance all-solid-state supercapacitors for use in integrated devices. Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention.

Claims

CLAIMS:

1. A process for producing a solid-state supercapacitor, including the steps of: forming at least one pair of mutually spaced electrodes, each said electrode including a graphene surface layer; and introducing a gel electrolyte between the mutually spaced electrodes and onto the graphene surface layer of each electrode to form a double layer supercapacitor, wherein the gel electrolyte is water-based and does not include a redox agent; and applying a voltage across each pair of electrodes to generate an electric field strong enough to decompose water between the mutually spaced electrodes by hydrolysis and functionalise a surface of the electrodes to substantially increase capacitance of the supercapacitor in the absence of a redox agent, the increased capacitance arising from at least one of electrical double layer capacitance of the electrodes and redox reaction capacitance.

2. The process of claim 1, wherein the step of applying the voltage enables redox charge transfer operation of the supercapacitor in the absence of a redox agent.

3. The process of claim 1 or 2, wherein the step of applying the voltage increases the electrical double layer capacitance of the supercapacitor.

4. The process of any one of claims 1 to 3, wherein the generated electric field has a strength of at least 4 kV/m.

5. The process of any one of claims 1 to 4, wherein the generated electric field has a strength of at least 7 kV/m and is applied for at least 2 minutes.

6. The process of any one of claims 1 to 5, wherein the gel electrolyte is a polyvinyl alcohol and acid gel electrolyte.

7. The process of any one of claims 1 to 6, including forming the graphene surface layer on a silicon carbide on silicon substrate by a metal catalysis process.

8. The process of claim 7, wherein the metal catalysis process includes the steps of:

(iii) removing the metal residue layer to expose the graphene layer.

9. The process of claim 8, including a step of oxidising the deposited metal prior to the heating step (ii).

10. The process of claim 8 or 9, including the step of repeating steps (i) to (iii) at least once to form a porous surface of the silicon carbide substrate coated with graphene.

11. A solid-state supercapacitor produced by a process according to any one of claims 1 to 10.

12. A solid-state supercapacitor, including: mutually spaced electrodes, each said electrode including a graphene layer on a silicon carbide on silicon substrate; a gel electrolyte between the mutually spaced electrodes and on the graphene layer of each electrode, wherein the gel electrolyte is water-based and does not include a redox mediator agent; and the electrodes having functionalised surfaces whose capacitance arises from electrical double layer capacitance of the electrodes and redox reaction capacitance.

13. The solid-state supercapacitor of claim 12, wherein the gel electrolyte is a polyvinyl alcohol (PVA) and acid gel electrolyte, and graphene layers of the graphene electrodes have an increased spacing due to the intercalation of PVA molecules therebetween.