WO2014097015A1

WO2014097015A1 - Graphene-based in-plane supercapacitors

Info

Publication number: WO2014097015A1
Application number: PCT/IB2013/060269
Authority: WO
Inventors: Matthias Georg SCHWAB; Klaus MÜLLEN; Xinliang Feng; Zhong-Shuai WU
Original assignee: Basf Se; MAX-PLANCK-Gesellschaft zur Förderung der Wissenschaften e.V.; Basf Schweiz Ag; Basf (China) Company Limited
Priority date: 2012-12-19
Filing date: 2013-11-20
Publication date: 2014-06-26
Also published as: TW201428787A

Abstract

The present invention relates to a process for preparing a supercapacitor, comprising the following steps: (a) preparing a graphene film on a substrate S1, (b) optionally transferring the graphene film to a substrate S2, which is different from the substrate S1, (c) preparing in-plane graphene electrodes of interdigital structure and at least one current collector of interdigital structure by (c1) providing a mask on the graphene film, wherein the mask has a mask pattern which leaves at least one graphene area of interdigital structure uncovered, (c2) applying the current collector onto the graphene area of interdigital structure and removing the mask, (c3) removing the parts of the graphene film which are not covered by the current collector, (d)adding an electrolyte such that the electrolyte is in contact with the in-plane graphene electrodes of interdigital structure.

Description

Graphene-based in-plane supercapacitors

Electric double layer capacitors, also known supercapacitors or ultracapacitors, store charges only at electrolyte-electrode interface of active materials electrode through the fast and reversible absorption/desorption of ions to form electric double layers. Typically, they can simultaneously maintain an order of magnitude larger power density (-10000 W/kg) than batteries, which possess high energy but work slowly, and two order of magnitude more energy density (~10 Wh/kg) than electrolytic capacitors, which offer fast power delivery at high rates.

Currently, carbon materials based on activated carbon are the most commercially- applied electrodes for supercapacitors since they are of importance to be generally maintenance free and unlimited cycle life that are favored in many electronic applications. So far, a notable improvement in performance has been significantly achieved through the understanding of charge storage mechanisms and the development of advanced nanostructured materials, however, further increasing the energy delivery and power discharge of supercapacitors especially at ultrafast rates of <3.6 ms is still a great challenge. Besides storing electrical energy supercapacitors could in principle be applied to filtering voltage ripple. Filtering refers to the conversion of alternating current (ac) to direct current (dc) power to run electronic equipment The conversion process involves diode rectification followed by removal of voltage fluctuations followed by filtering with a capacitor device. Filtering today is performed primarily by means of aluminum electrolytic capacitors, which usually are rather bulky electrical components. Today's supercapacitor technology that relies on the use of activated carbon active materials is not suitable for performing the filtering due to the resistor- capacitor (RC) time constants on the order of τ = ~1 s.

Development of miniaturized electronic devices, such as micro-electromechanical systems, microrobots and implantable medical devices, has extremely stimulated the increasing need for micro-/nano-scale and high power density power sources that can be integrated with such devices. Although thin film micro-batteries store the energy by either the redox or expansion-contraction reactions similar to classical batteries are the most dominantly used micro-power source among the power generation systems, but they have certain drawbacks of finite life time (hundreds or thousands cycles) and low power densities (~10 ² W/cm³) that limits their applications that require high power in a short time range.

Micro-supercapacitors are one newly-developed miniaturized electrochemical energy storage devices that can offer several order of magnitude higher power density than conventional batteries and supercapacitors assignable to the shorted diffusion length. Reference can be made e.g. to J. Chmiola et al., Science 328, 480-483 (2010); D. Pech et al, Nature Nanotech. 5, 651-654 (2010); and W. Gao et al, Nature

Nanotech. 6, 496-500 (2011). Importantly, such on-chip microdevices could be directly integrated into other miniaturized electronic devices, such as micro-batteries or energy harvesting micro-systems, to provide peak power in a number of applications. However, designing such a micro-supercapacitor that combines the ultrahigh power density of electrolytic capacitor (~10³ W/cm³) with the high energy delivery of thin-film battery (~10^~2 Wh/cm³) is currently regarded as an

insurmountable obstacle that requires the thin- film electrode with highly accessible electrochemically active surface area and high intrinsic conductivity, superior interfacial integrity of main components (electrode, separator, electrolyte, substrate) as well as elaborate design of device geometry with a short diffusion pathway.

Graphene-based materials are emerging as highly robust and attractive materials for real applications in supercapacitors because of their excellent intrinsic conductivity, high surface-to-volume ratio, exceptionally intrinsic double-layer capacitance (~21 μΡ/cm²), high theoretical capacitance (550 F/g), flexibility and overall mechanical robustness. On the one hand, the conventional supercapacitor sandwich devices have demonstrated improved performance in terms of energy density. On the other hand, in-plane supercapacitor devices based on single/multi- layer graphene (Yoo, J.J. et al, Nano Lett. 11, 1423-1427 (2011)) and laser-patterned hydrated graphene oxide (GO) film (Gao, W. et al, Nature Nanotech. 6, 496-500 (2011)) as electrodes that can fully take advantage of its atomic layer thickness and flat morphology to increase the interacting ability of electrolyte ions with all the graphene layers are superior to the conventional sandwich devices. The performance of in-plane supercapacitors in term of specific area capacitance reported has been greatly improved, for instance, nearly twice (for laser-patterned hydrated GO film) and three times (for multi-layer reduced graphene) that of the sandwich geometry. However, their discharge rates performed are not more than 100 mV s^"1 and the power delivery are not efficiently utilized especially at high rates, possibly assignable to the low conductivity of graphene film or limitation of device architectures.

It was also recently shown that graphene as active material may lead to the application of supercapacitors to filtering applications:

Miller, J. R. et al. describe the fabrication of special vertically aligned graphene sheets (Science 329, 1637-1639 (2010)). Sheng, K- et al. report ultrahigh-rate supercapacitors based on electrochemically reduced graphene oxide (Sci. Rep. 2, 247 (2012)). Maher F. El-Kady, et al. discuss the use of a laser scribing method for the fabrication of graphene-based supercapacitors (Science 335, 1326-1330 (2012)). Thus, it is an object of the present invention to provide a supercapacitor which has high power density but still results in high energy delivery and achieves high discharge rates, and to provide a manufacturing method for such a supercapacitor. It is a further object of the present invention to provide a supercapacitor suitable for filtering applications. It is also an object of the present invention to provide a process for preparing high-performance graphene layers which can be applied to a number of different substrates including flexible substrates.

According to a first aspect, the present invention provides a process for preparing a supercapacitor, comprising the following steps:

(a) preparing a graphene film on a substrate S 1 ,

(b) optionally transferring the graphene film to a substrate S2, which is different from the substrate SI,

(c) preparing in-plane graphene electrodes of interdigital structure and at least one current collector of interdigital structure by

(cl) providing a mask on the graphene film, wherein the mask has a mask pattern which leaves at least one graphene area of interdigital structure uncovered,

(c2) applying the current collector onto the graphene area of interdigital structure and removing the mask,

(c3) removing the parts of the graphene film which are not covered by the current collector,

(d) adding an electrolyte such that the electrolyte is in contact with the in-plane graphene electrodes of interdigital structure. The supercapacitor obtained with the manufacturing method as defined above contains in-plane graphene electrodes (i.e. the graphene electrodes are located on the same substrate) of interdigital structure. The term "electrodes of interdigital structure" is used according to its commonly accepted meaning and therefore relates to a structure in which the length of the region between two electrodes is increased by an interlocking-finger design of the electrodes. Such an electrode configuration is also referred to as an "interdigitated structure". The interdigital graphene electrodes prepared in step (c) have a lower surface which is in contact with the substrate S 1 (alternatively with the substrate S2 if transfer step (b) was carried out), and an upper surface which is in contact with a current collector of interdigital structur. As will be discussed below in further detail, it has been realized in the present invention that the process as defined above very efficiently provides a supercapacitor which has high energy density and high power density and can be operated at high scan rates. In step (a), a graphene film is prepared on a substrate SI .

Thickness of the graphene film may vary. As graphene is a monolayer material, the graphene film can be a monolayer film. However, it is also possible that several graphene layers are stacked upon each other on the substrate surface. Preferably, the graphene film has a maximum thickness which is less than 1000 nm, more preferably less than 100 nm, even more preferably less than 50 nm or even less than 20 nm. In a preferred embodiment, the graphene film is a single layer graphene film. Thickness means the height of the graphene electrode, i.e. the distance between its lower surface which is in contact with the substrate and its upper surface.

Film thickness is measured by a surface profiler (KLA Tencor P-16+) as follows: First, injector pinhead is used to scratch the film on the substrate, and one linear gap is obtained. Thus, the tip of surface profiler is performed to cross over the gap from one side to the other side, and simultaneously the computer is recording the depth of the gap, that is, the thickness of the film. The graphene film can be prepared by methods commonly know to the skilled person. The graphene film can be directly prepared on the substrate S 1 by chemical vapor deposition, epitaxial growth, or surface-assisted bottom-up organic synthesis. It is also possible to prepare graphene in an external medium first, followed by applying the graphene onto the substrate SI so as to form the graphene film. The "externally" prepared graphene can be obtained by e.g. exfoliation of graphite, micromechanical cleavage, or bottom-up organic synthesis. These methods are known to the skilled person.

According to a further alternative, the graphene film can be prepared in step (a) by: (al) preparing a graphene oxide,

(a2) coating the substrate SI with the graphene oxide so as to obtain a graphene oxide film,

(a3) reducing the graphene oxide film so as to obtain the graphene film.

The graphene oxide film can be reduced e.g. by thermal treatment or by treatment with a chemical reducing agent such as hydrazine N₂H₄, hydrogen iodide HI, hydrogen gas H₂, etc.

In a preferred embodiment of the present invention, the graphene oxide film is reduced by treatment with a plasma. As will be discussed in further detail below, it has been realized that the graphene film performance (as e.g. indicated by high conductivity values) can be further improved if obtained via plasma treatment of graphene oxide films.

Methods for preparing graphene oxide are known to the skilled person. Typically, graphite is oxidized to graphite oxide by treatment with an appropriate oxidizing agent such as a mixture of sulfuric acid and potassium permanganate. If dispersed e.g. in a basic medium, monolayers of graphene oxide are obtained. Further details about the preparation of graphene oxide are described e.g. in Chem. Soc. Rev., Ch.W. Bielawski et al, 2010, 39, pp. 228-240, , and Adv. Mater. 21, 2009, pp. 1679- 1783, K. Mullen, X. Feng, D. Wu, Y. Liang. In such dispersions, the graphene oxide is typically present in the form of "monolayer flakes".

For coating the substrate S 1 with the graphene oxide or graphene, commonly known methods can be applied, such as spin coating, drop coating, layer by layer self- assembly, electrochemical deposition, filtration, or combinations thereof.

The graphene oxide can be provided in the form of a dispersion (e.g. an aqueous dispersion) which is then coated (e.g. spin-coated) onto the substrate SI . Preferably, the graphene oxide dispersion is sonicated prior to the step (b).

As mentioned above, in such dispersions, the graphene oxide is typically present in the form of "monolayer flakes". Accordingly, the graphene oxide film obtained from the coating step (a2) is typically not a film which is continuous over a large area but is rather a film made of discrete graphene oxide flakes.

Any substrate SI which is compatible with graphene oxide/graphene can be used. Preferably, the substrate S 1 should also be a material which is compatible with the intended final use such as supercapacitor applications. However, as will be discussed below in further detail with regard to a preferred embodiment, it is also possible to provide the graphene film (e.g. the graphene oxide film) on a substrate SI first, and transferring later on the graphene film (e.g. in the form of the reduced graphene oxide film) to a different substrate S2, which may then become part of the final device. The substrate can be chosen from a broad variety of different materials. The substrate can be rigid but may also be flexible (e.g. in the form of a foil). Appropriate substrates include e.g. metals (such as copper, platinum, nickel, titanium, and alloys thereof), semiconductors (such as silicon, in particular silicon wafers), inorganic substrates (such as oxides, e.g. Si0₂, glass, HOPG, mica, or any combination thereof), flexible substrates that may be made of e.g. polymers such as polyethylene terephthalate, polyethylene naphthalate, polymethyl methacrylate, polypropylene adipate, polyimide or combinations or blends thereof. The substrate can be subjected to a pretreatment (such as plasma treatment) so as to improve adhesion of the graphene oxide or graphene film on the substrate surface.

Depending on the coating conditions chosen in step (a2), thickness of the graphene oxide film may vary. As graphene oxide is a monolayer material, the graphene oxide film can be a monolayer film. However, it is also possible that several graphene oxide layers are stacked upon each other on the substrate surface. Preferably, the graphene oxide film obtained in step (a2) has a maximum thickness which is less than 1000 nm, more preferably less than 100 nm, even more preferably less than 50 nm or even less than 20 nm. In a preferred embodiment, the graphene oxide film is a single layer graphene oxide film.

Film thickness is measured by a surface profiler as described above.

Preferably, the plasma is selected from a hydrocarbon plasma, a non-carbon gas plasma, or a mixture thereof.

With regard to hydrocarbon plasma, the following ones can be mentioned: Ci_8 alkane plasma (like methane), alkene plasma (like ethylene), alkyne plasma (like acetylene), and arene plasma (like benzene), and combinations thereof. With regard to non-carbon gas plasma, the following ones can be mentioned:

Hydrogen plasma, argon plasma, nitrogen plasma, NH₃ gas plasma, and

combinations thereof. Methods for preparing a plasma are generally known to the skilled person and include e.g. direct current plasma, radio frequency, and microwave.

So as to efficiently reduce the graphene oxide to graphene, it is preferred that the plasma treatment step is at least partly carried out at increased temperature, e.g. of at least 200°C, or at least 500°C, or at least 600°C. On the other hand, the plasma treatment temperature should not be too high and is preferably less than 2000°C, more preferably less than 1000°C.

The time period of the plasma treatment in step (a3) may vary and can be from 1 second to 3600 seconds, more preferably from 10 seconds to 600 seconds.

In a preferred embodiment, the graphene oxide film is treated with the plasma (e.g. methane plasma) at a temperature of from 200°C to 2000°C, more preferably 500°C to 1000 °C, for a time period of from 1 second to 3600 seconds, more preferably 10 seconds to 600 seconds.

As already mentioned above, the process of the present invention also offers the opportunity to prepare the graphene film on the substrate SI first, and then transfer the graphene film to another substrate S2. Just as an example, the substrate SI can be made of a material which is more convenient for preparing the graphene film (e.g. high thermal stability, compatible with plasma treatment at high temperature, etc.), whereas the substrate S2 is adapted to the intended use of the final device. Accordingly, in a preferred embodiment, the process of the present invention comprises a step (b) wherein the graphene film is transferred to a substrate S2, which is different from the substrate SI. In principle, any of those materials mentioned above with regard to the substrate SI can be used for the substrate S2 as well. Of course, a transfer of the graphene film from substrate S 1 to substrate S2 is in particular of interest if S2 is different from S 1.

For some end applications, it might be of interest to provide the graphene film on a flexible and/or transparent substrate such as flexible and transparent polymer substrates. High flexibility can be achieved by using a very thin substrate which may have a thickness of about 10 to 1000 μιη. Furthermore, by selecting appropriate materials such as polymers, a transparent substrate can be provided. A transparent substrate is preferably having a transmittance of at least 50%, more preferably at least 70%, even more at least 90% with regard to a wave length of from 200 to 2000 nm, more preferably 300 to 1000 nm, or 400 to 700 nm.

Thus, in a preferred embodiment, the substrate S2 is a flexible and/or transparent substrate, such as a flexible and/or transparent polymer foil (e.g. a foil made of polyethylene terephthalate, polyethylene naphthalate, polymethyl methacrylate, polypropylene adipate, polyimide or combinations or blends thereof).

The graphene film on the substrate S 1 has a lower surface which is in contact with the substrate SI and an uncovered upper surface. The transfer of the graphene film from the substrate SI to the substrate S2 can be accomplished by applying the substrate S2 onto the upper surface of the graphene film, followed by removal of the substrate SI (e.g. by dissolution of the substrate SI or peeling off the substrate SI).

Alternatively, the transfer can be accomplished by providing a temporary material on the upper surface of the graphene film, followed by removal of the substrate SI (e.g. by dissolution of the substrate SI or peeling off the substrate SI) so as to obtain a graphene film now having an uncovered lower surface and an upper surface which is in contact with the temporary material, subsequently applying the substrate S2 onto the lower surface of the graphene film, followed by removal of the temporary material (e.g. by dissolution of the temporary material or peeling off the temporary material) from the upper surface of the graphene film.

Applying the substrate S2 onto the lower surface of the graphene film may include a thermal treatment, so as to improve the adhesion between the substrate S2 and the graphene film.

With the term "temporary material", it is indicated that the material is provided on the graphene film only temporarily and is removed after the graphene film has been attached to the substrate S2.

The temporary material can be a polymer. In the process of the present invention, it is possible that the temporary material such as a polymer is prepared on the upper surface of the graphene film, e.g. by providing a precursor material (such as monomer compounds or an uncured polymer resin) on the upper surface of the graphene film, followed by converting the precursor material into the temporary material (e.g. by polymerization of the monomer compounds or a curing step).

Alternatively, it is also possible that the temporary material is prepared externally (i.e. not on the graphene surface) and then provided on the uncovered upper surface of the graphene film. For example a thermal release tape can be applied to the upper surface of the graphene film under. Preferably, such a thermal release tape is applied at mild pressure. A thermal release tape as such is known, e.g. Bae et al, Nature Nanotechnology, 5, 574-578, 2010.

In a preferred embodiment, the temporary material is provided on the upper surface of the graphene film by coating the upper surface with a precursor material, followed by a treatment step (such as polymerization, curing, etc.) so as to convert the precursor material to the temporary material.

In a preferred embodiment which includes such a transfer, a metal (such as copper) is used as substrate SI and a curable polymer such as polymethyl methacrylate PMMA (i.e. the precursor material) is applied onto the uncovered upper surface of the graphene film, followed by curing the curable polymer so as to provide the temporary material. Subsequently, the metal substrate SI is removed, e.g. by dissolution in an appropriate etching liquid, from the lower surface of the graphene film. Then, a flexible and optionally transparent polymer foil (e.g. a polyethylene terephthalate foil) is provided on the lower surface of the graphene film, followed by removal of the temporary material, e.g. by dissolution in an appropriate solvent.

After etching, the lower surface of the graphene film can be targeted on the flexible substrate S2. Subsequently, spin-coating can be used to remove the residual water between the graphene film and substrate and increase the interfacial contact. Then, the temporary material (such as PMMA) on the upper surface of the graphene film can be removed and a thermal treatment at about 60-100°C can be carried out.

In step (c), in-plane graphene electrodes of interdigital structure and at least one current collector of interdigital structure are prepared by:

(c3) removing the parts of the graphene film which are not covered by the current collector.

As already mentioned above, the term "electrodes of interdigital structure" is used according to its commonly accepted meaning and therefore relates to a structure in which the length of the region between two electrodes is increased by an

interlocking-finger design of the electrodes. Such an electrode configuration is also referred to as an "interdigitated structure". So, each electrode preferably has a comblike structure with two or more fingers and the electrodes are positioned relative to each other such that the fingers of different electrodes are parallel to each other.

The number of fingers of each electrode can vary over a broad range. Each of the in- plane graphene electrodes can have e.g. at least two fingers or at least 4 fingers, more preferably at least 8 fingers, even more preferably at least 15 fingers or at least 16 fingers. So, if there are two electrodes of interdigital structure, the total number of fingers can be e.g. at least 4 fingers or at least 8 fingers, more preferably at least 16 fingers, even more preferably at least 30 fingers or at least 32 fingers.

With the process of the present invention, supercapacitors in varying sizes can be manufactured. If a micro supercapacitor is to be prepared, it is preferred that the interdigital electrode structure is exclusively made of structural elements having a maximum width (in particular an in-plane width) of less than ΙΟΟΟΟμιη, more preferably less than 1000 μιη. This can be accomplished by using a mask with an appropriate mask pattern. Preferably, the fingers of the electrodes of interdigital structure have a maximum width of less than 1 ΟΟΟΟμιη, more preferably less than 1000 μιη. The width can be measured by known methods such as scanning electron microscopy.

Masks that can be used in the present invention are commonly known to the skilled person, e.g. masks commonly used in photolithography.

The mask having an interdigital pattern can be prepared externally and then be applied onto the graphene film, thereby leaving a graphene area of interdigital structure uncovered. Methods for preparing such masks are commonly known to the skilled person (e.g. lithographical methods). A number of different materials such as metals or polymers can be used for preparing such a mask.

Alternatively, the mask can be prepared directly on the graphene film, e.g. by lithography, in particular photolithography (as known e.g. from semiconductor technology). Such methods are commonly known to the skilled person. Preparing the mask directly on the graphene film can be accomplished e.g. by applying a film of a photoresist material (such as a light-sensitive polymer) on the graphene film, exposing a selected area of the photoresist material (e.g. to light such as UV light or an electron beam) and subsequently removing a part of the photoresist material so that a mask pattern is obtained which leaves an area of interdigital structure of the underlying graphene film uncovered. The photoresist can be a positive photoresist or a negative photoresist. As known to the skilled person, a positive photoresist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes soluble to the photoresist developer and the portion of the photoresist that is unexposed remains insoluble to the photoresist developer; while a negative photoresist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes insoluble to the photoresist developer and the unexposed portion of the photoresist is dissolved by the photoresist developer. Exposing a selective area of the photoresist can be accomplished by using a photomask.

As mentioned above, step (c2) comprises applying the current collector onto the graphene area of interdigital structure and removing the mask, Preferably, the current collector is a metal. Appropriate metals are known to the skilled person and include e.g. Au, Ag, Cu, Al, W, Zn, Fe, Co, Ni, Pt, Sn, Pb, Pd, Ti, or any alloy or combination thereof.

The current collector can be applied onto the graphene film by methods which are commonly known to the skilled person, such as thermal evaporation under vaccum. Removal of those parts of the graphene film which are not covered by the current collector can be accomplished by known methods such as plasma etching (e.g.

oxygen plasma etching).

In step (d), an electrolyte is added such that the electrolyte is in contact with the in- plane graphene electrodes of interdigital structure.

Appropriate electrolytes for use in supercapacitors are commonly known to the skilled person.

The electrolyte used in the present invention can be liquid or non-liquid. Electrolytes which are non-liquid include gel electrolytes (such as polymer gel electrolytes). In can be preferred that no liquid electrolyte is added, thereby obtaining an all- so lid supercapacitor.

Preferably, the electrolyte is selected from a polyvinyl alcohol/H₂S0₄ gel (a gel made of polyvinyl alcohol (PVA) and H₂S0₄), a PVA/H₃P0₄ gel, a PVA/KOH gel, a PVA/NaOH gel, a PVA/Na₂S0₄ gel, an ionic liquid polymer gel, or any mixture thereof. In principle, these gels are known to the skilled person. Ionic liquid polymer gels as such and preparation methods thereof are described e.g. in Adv. Funct.

Mater., M. Gratzel et al, 2009, 19, pp. 2187-2202.

In case an ionic liquid polymer gel is used, it is preferably l-alkyl-3- methylimidazolium halide, wherein alkyl is preferably C₃_9 alkyl, and/or halide is preferably iodide. As polymer or gelator within said ionic liquid polymer gel, a polymer of low molecular weight is preferred. A preferred gelator is

poly(vinylidenefluoride-co-hexafluoropropylene). With the process of the present invention, it is possible to provide a supercapacitor (preferably a micro supercapacitor) of improved electrochemical performance, as described below in further detail. No addition of any organic binder and/or any additional conductive additive is necessary. So, the process of the present invention can be carried out without adding any organic binder and/or any additional conductive additive such as carbon-based conductive additives different from graphenen (e.g. carbon black, carbon nanotubes etc.), or silver nanoparticles.

According to a further aspect, the present invention provides a supercapacitor comprising:

(a) a substrate,

(b) in-plane graphene electrodes of interdigital structure, the graphene electrodes having a lower surface, which is in contact with the substrate, and an upper surface, and having a maximum thickness of less than 1000 nm,

(c) at least one current collector of interdigital structure, which is at least partially covering the upper surface of the graphene electrodes,

(d) an electrolyte which is in contact with the in-plane graphene electrodes of interdigital structure. With regard to the substrate, reference can be made to the above statements.

Accordingly, the substrate can be chosen from a broad variety of different materials.

The substrate can be rigid but may also be flexible (e.g. in the form of a foil).

Appropriate substrates include e.g. metals (such as copper, platinum, nickel, titanium), semiconductors (such as silicon, in particular silicon wafers), inorganic substrates (such as oxides, e.g. Si0₂, all kinds of glass, HOPG, mica, polymer substrates such as polyethylene terephthalate substrates, polyethylene naphthalate substrates, polymethyl methacrylate substrates, polypropylene adipate substrates, polyimide substrates or combinations or blends thereof). For some end applications, it might be of interest to provide the graphene film on a flexible and/or transparent substrate such as flexible and transparent polymer substrates. High flexibility can be achieved by using a very thin substrate which may have a thickness of about 10 to 1000 μιη. Furthermore, by selecting appropriate materials such as polymers, a transparent substrate can be provided. A transparent substrate is preferably having a transmittance of at least 50%, more preferably at least 70%, even more at least 90% with regard to a wave length of from 200 to 2000 nm, more preferably 300 to 1000 nm, or 400 to 700 nm. In a preferred embodiment, the substrate S2 is a flexible and/or transparent substrate, such as a flexible and/or transparent polymer foil (e.g. a polyethylene terephthalate foil).

As indicated above, the in-plane interdigital graphene electrodes have a maximum thickness of less than 1000 nm. Electrode thickness is measured by a surface profiler as described above.

Preferably, the in-plane interdigital graphene electrodes have a maximum thickness of less than 100 nm, more preferably less than 50 nm, even more preferably less than 30 nm or even less than 20 nm. In a preferred embodiment, the graphene electrodes are made of a single-layer graphene.

The graphene forming the in-plane interdigital graphene electrodes is obtainable by those methods as already mentioned above when describing the preparation of the supercapacitor (such as from graphene oxide by treatment with a plasma). Preferably, the graphene of the interdigital graphene electrodes has a ratio of carbon atoms to oxygen atoms of at least 6, more preferably at least 7, even more preferably at least 8 or even at least 9, as determined by X-ray photoelectron spectroscopy.

Preferably, the supercapacitor has a stack capacitance of at least 1 F/cm³, more preferably at least 40 F/cm³, even more preferably at least 50 F/cm³. As known to the skilled person, the capacitance value is calculated from the CV data according to the following e uation (1):

wherein v is the scan rate, V and V, are the integration potential limits of the voltammetric curve, and /(V) is the voltammetric discharge current (A).

Stack capacitance (sometimes also referred to as volumetric capacitance) is calculated based on the volume of the device stack according to the following formula (2):

C_stack = C_deviJV (2)

wherein C_stack refers to the volumetric stack capacitance of the device. Vis the corresponding total volume of the positive and negative electrodes of graphene in the device. In the present invention, it is possible that the interdigital graphene electrodes do not contain any binder and/or any additional conductive material, in particular any carbon-based conductive materials different from graphene (such as carbon black and carbon nanotubes) or silver nanoparticles. The interdigital graphene electrodes may consist of graphene.

Supercapacitors in varying sizes can be provided by the present invention. In a preferred embodiment, the supercapacitor is a micro supercapacitor. For the micro supercapacitor, it is preferred that the interdigital electrode structure is exclusively made of structural elements having a width (in particular an in-plane width) of less than 10000 μιη, more preferably less than 1000 μιη.

With regard to the electrolyte, reference can be made to the above statements.

Preferably, a non-liquid electrolyte, in particular a gel electrolyte (such as a polymer gel electrolyte) is used. In a preferred embodiment, the supercapacitor of the present invention does not contain a liquid electrolyte.

Mater., M. Gratzel et al, 2009, 19, pp. 2187-2202.

poly(vinylidenefluoride-co-hexafluoropropylene).

The graphene electrodes of interdigital structure have a lower surface which is in contact with the substrate and an upper surface. A current collector is present on the upper surface of the graphene electrodes of interdigitated structure and has an interdigital structure as well. More preferably, the interdigital structure of the graphene electrodes corresponds to the interdigital structure of the current collector. Appropriate current collector materials are commonly known to the skilled person and include metals such as Au, Ag, Cu, Al, W, Zn, Fe, Co, Ni, Pt, Sn, Pb, Pd, Ti, or any alloy or combination thereof.

According to a further aspect, the present invention provides a supercapacitor, more preferably a microsupercapacitor which is obtainable by the process as described above. The present invention also relates to arrangements of two or more of the

supercapacitors described above, wherein at least two of the supercapacitors are connected in parallel or in series.

As mentioned above, it was also realized in the present invention that a graphene film provided on a first substrate can be efficiently transferred to a second substrate (e.g. a flexible substrate) while preserving its beneficial properties.

Thus, according to a further aspect, the present invention provides a process for preparing a layered assembly, which comprises:

(a) preparing on a first substrate SI a graphene film which has a lower surface being in contact with the substrate S 1 and an uncovered upper surface,

(b) transferring the graphene film from the substrate SI to a substrate S2, which is different from the substrate S 1 , by providing a temporary material on the upper surface of the graphene film, followed by removal of the substrate SI so that the graphene film now has an uncovered lower surface and an upper surface which is in contact with the temporary material, subsequently applying the substrate S2 onto the lower surface of the graphene film, followed by removal of the temporary material from the upper surface of the graphene film.

With regard to the preparation of the graphene film in step (a), reference can be made to the statements provided above when describing the preparation of the

supercapacitor.

Preferably, the graphene film is obtained by:

(al) preparing a graphene oxide,

(a3) reducing the graphene oxide film so as to obtain the graphene film. In a preferred embodiment, the graphene oxide film is reduced by plasma treatment.

With regard to the details of the transfer step (b), reference can be made to the statements provided above when describing the preparation of the supercapacitor. Preferably, the substrate S2 is a flexible substrate (e.g. a polymer foil etc.).

As also mentioned above, it has been realized in the present invention that a continuous graphene film of high quality can be prepared over an extended area by plasma treatment of a graphene oxide film.

Thus, according to a further aspect, the present invention provides a process for preparing a layered assembly, comprising the following steps:

(a) preparing a graphene oxide,

(b) coating a substrate S 1 with the graphene oxide so as to obtain a graphene oxide film,

(c) reducing the graphene oxide film by treatment with a plasma so as to obtain a graphene film.

(d) optionally transferring the graphene film from the substrate S 1 to a substrate S2, which is different from the substrate SI .

With regard to further details of steps (a) to (d), reference can be made to the statements provided above when describing the preparation of the supercapacitor.

According to a further aspect, the present invention provides a layered assembly comprising a substrate (preferably a flexible substrate) and a graphene film on the substrate, the layered assembly being obtainable by the process as described above.

Preferably, the graphene film has a maximum thickness t_max of less than 1000 nm, more preferably of less than 100 nm, even more preferably less than 30 nm or even less than 20 nm; and a conductivity σ of at least 100 S/cm, more preferably at least 200 S/cm, even more preferably at least 250 S/cm.

Electrical conductivity is measured by a common four-probe system with a Keithley 2700 Multimeter.

Preferably, the graphene film has a ratio of carbon atoms to oxygen atoms of at least 6, more preferably at least 7, even more preferably at least 8 or even at least 9, as determined by X-ray photoelectron spectroscopy.

The graphene film can be continuous over an area of at leastl x lO⁹ μιη², more preferably at least 3x 10⁸ μιη², as determined by optical microscopy at a

magnification of 10. With the term "continuous", it is meant that the substrate surface is completely covered by the graphene film over the area indicated and no substrate surface is detectable within this area by optical microscopy.

According to a further aspect, the present invention relates to the use of the layered assembly described above for manufacturing energy storage devices, such as supercapacitors (e.g. micro supercapacitors). A preferred supercapacitor, in particular micro supercapacitor, is the one described above having interdigital graphene electrodes and an interdigital current collector. Such supercapacitors may, besides energy storage, also advantageously be used for current filtering.

The present invention will now be described in further detail by the following Examples. Examples

In the Examples, the following graphene films and supercapacitors were prepared:

Example 1 : A supercapacitor obtained via plasma treatment of a graphene oxide film and containing in-plane interdigital graphene electrodes on a Si substrate (in the following referred to as MPG MSC (methane pjasma graphene micro

supercapacitor)

Example 2: A supercapacitor obtained via plasma treatment of a graphene oxide film and graphene film transfer, containing in-plane interdigital graphene electrodes on a flexible substrate (in the following referred to as "Flexible MPG MSC')

Comparative Example 1 : A supercapacitor obtained by plasma treatment of a graphene oxide film, containing two graphene films as electrodes, each graphene film being present on a separate Si substrate opposed to each other and thereby resulting in a stack geometry (in the following referred to as MPG SSC (methane plasma graphene stacked supercapacitor)

Example 3 : Supercapacitors (two samples) obtained via plasma treatment of a graphene oxide film and containing in-plane interdigital graphene electrodes (first sample: 2 x 8 = 16 fingers; second sample: 2 x 16 = 32 fingers) which have been prepared via a mask provided by photolithography (in the following referred to as MPG-PL MSC (methane plasma grapheme p_hoto-lithography micro supercapacitor), wherein the mask was prepared directly on the graphene film.

Materials characterization: Materials characterization were conducted by SEM (Gemini 1530 LEO), optical microscopy, AFM (Veeco Dimension 3100), surface profiler (KLA Tencor P-16+), XRD (SEIFERT XRD 3000 TT Bragg-Brentano diffractometer with Cu Ka radiation between 10° and 60° and an incident wavelength of 0.15418 nm), Raman spectra (Bruker, 532 nm) and XPS (Omicron Multiprobe equipped with the monochromatic Al Ka source, electron analyzer resolution of 0.9 eV).

Electrochemical characterization: Cyclic voltammerty (CV) tested at the scan rates of 0.01-1000 V s^"1 and electrochemical impedance spectroscopy (EIS) recorded in the frequency range of 1-100 kHz with a 5 mV ac amplitude were carried out to characterize the supercapacitor performance using an CHI 760D electrochemical workstation. The H₂SO₄/PVA gel electrolyte was prepared by mixing 6 g H₂SO₄ and 6 g PVA in 60 ml deionized water and thus heated up to 80 °C for 1 h under vigorous stirring.

Example 1

Preparation of a graphene film by plasma treatment and its use for

manufacturing the MPG MSC

First, graphene oxide (GO) was prepared from natural graphite by Hummers methods as reported by Liang, Y., Wu, D., Feng, X., & Mullen, K., Adv. Mater. 21, 1679- 1783 (2009). Next, GO dispersion (1.0-3.0 mg mL^"1) was sonicated for 2-10 h and then spin-coated (3000 rpm) on the oxygen plasma-treated silicon wafer using 300- 500 W rf power for 10 min (Plasma System 200-G, Technics Plamsa GmbH), and if needed the spin-coating steps were repeated to fabricate the thick GO film until the desirable film thickness was achieved. The produced GO films were then reduced at 700 °C for 20 s with the methane (C¾) plasma with a heating rate of 50 °C/min. Subsequently, 20-50 nm of gold (Au) was thermally evaporated under vacuum onto the MPG film through a 30-interdigital finger mask. The patterns of MPG micro- electrodes on wafer were then created by oxidative etching of the exposed graphene in an 0₂-plasma cleaner for 1-10 min (dependent on the thickness) with 0₂ flow and 100-200 W rf power. The thickness of MPG film as electrodes had a thickness of 15 nm. After that, H₂SO₄/PVA gel electrolyte was drop-cast onto the surface of interdigital electrode by syringes and solidified overnight. Finally, one on-chip all solid-state MPG-MSCs with an in-plane geometry can be fabricated.

The steps of the process are illustrated in Figure 1 which shows:

(a) Providing a Si wafer;

(b) Oxygen plasma treatment of the Si wafer and providing a graphene oxide (GO) film on the Si wafer via spin coating;

(c) Reducing the GO film to a graphene film ("reduced GO" or "RGO" film) via CH₄ plasma reduction;

(d) Providing a mask on the graphene film, wherein the mask has a mask pattern which leaves at least one graphene area of interdigital structure uncovered, providing an Au film on the mask and the uncovered graphene film by Au sputtering, and removing the mask Au collector of interdigital structure on graphene film;

(e) Removing the parts of the graphene film which are not covered by the Au collector by oxygen plasma etching, thereby obtaining graphene electrodes of interdigital structure (graphene area of interdigital structure covered by the Au collector) on the Si wafer;

(f) Applying a gel as an electrolyte.

Figure 2 shows an AFM image of the graphene oxide spin-coated onto the Si wafer substrate. As can be seen from Figure 2, the graphene oxide is present in the form of irregularly shaped "flakes" but not as a continuous, closed film.

Figures 3 a and 3b show SEM images of the graphene film obtained by plasma treatment of the graphene oxide film. The SEM images clearly demonstrate that a continuous, closed graphene film can be obtained over a large area.

As determined by XPS spectra, the graphene film has a ratio of carbon atoms to oxygen atoms of 9.2, wherein said ratio is 2.3 in the graphene oxide film. Thus, plasma treatment is very effectively reducing the graphene oxide to graphene, and is additionally transforming the individual graphene oxide flakes to a continuous, closed graphene film. Figure 4 shows electrical conductivity of the graphene film as a function of thickness. The results indicate that a graphene film of high conductivity is obtained by plasma treatment.

Figure 5 shows a SEM image of several electrode fingers of the interdigital electrode structure. As can be seen from Figure 5, the structural elements of the interdigital electrodes have a very uniform width.

In order to confirm the excellent performance of the plasma reduced graphene-based microsupercapacitors (MPG-MSCs), we carried out the cyclic voltammetry (CV) experiments at scan rates from 0.01 to 1000 V s^"1 to evaluate the power capability. The results are shown in Figures 6a- f. The MPG-MSC shows an exceptionally enhanced electrochemical performance with a nearly rectangular CV shape even at higher scan rate of 1 to 100 V s^"1 (Fig. 6a-c), suggesting of typical double-layer capacitive behavior. A linear dependence of the discharge current on the scan rate is observed at least up to 200 V s^"1 (Figure 6f). Remarkably, the MPG-MSC can be ultrafast charged and discharged over a wide range of scan rates up to 1000 V s^"1 and even still remain the impressive capacitance contribution (Figure 6a-e), which is suggestive of a high instantaneous power. The cycling performance of the device was tested up to 100,000 times at ultrafast scan rate of 50 V s^"1. The results are shown in Figures 7a-b. The CV shapes remain almost unchanged after 100,000 cycles, representative of a superior stable capacitive behavior.

Example 2 Preparation of a Flexible MPG MSC via transfer of a plasma-reduced graphene film from a first substrate to a second substrate A flexible MPG-MSC was fabricated on a flexible polyethylene terephthalate (PET) substrate, which replaces the above silicon wafer with Cu foil (-25 μηι thick) as a sacrificial support and PET substrate as a flexible support. Except for the transfer process, the other steps were kept the same as for the MPG-MSC on silicon wafer as described above. Specifically, the transfer of the plasma-reduced graphene film from Cu foil to PET substrate was made as follows: First, a polymethyl methacrylate

(PMMA) solution was spin coated on the top surface of the graphene film on Cu foil with a 2000 rpm for 1 min and heated at 80 °C to achieve the cure of the polymer. PMMA acts as the temporary material. Then, the Cu foil was etched away overnight on a solution of aqueous Fe(NOs)3. After Cu etching, the plasma-reduced graphene film with the temporary PMMA coating was transferred to a de-ionized water bath, rinsed several times by de-ionized water and isopropanol to fully wash off residual Cu etchant, and then carefully extracted the water in bath to transfer the film on the target PET substrate. Finally, the PMMA/graphene/PET film was hung on the above surface of the boiled acetone at 75 °C to remove PMMA, and then rinsed by isopropanol for several times. After this, the transfer process was complete.

The graphene film had a thickness of 15 nm. After its transfer from the Cu substrate to the flexible PET substrate, conductivity of the graphene film was determined. A value of 297 S/cm was obtained.

CV curves of the flexible micro supercapacitor obtained at different scan rates from 1, 10, and 100, 500, 1000 V s^"1 are shown in Figures 8a-b. The results indicate a typical electric double-layer capacitive behavior even at the ultrahigh scan rates, demonstrating its ultrahigh power ability. The Flexible MPG MSC had an area capacitance of up to 102 μΡ/cm² and stack capacitance of -68 F/cm³. Up to -99.1% of capacitance was maintained after 100,000 times at ultrafast scan rate of 200 V s^"1, thereby demonstrating the high cycling stability of the flexible supercapacitor.

Comparative Example 1

Preparation of a micro supercapacitor with graphene electrodes attached to opposed substrates (MPG-SSC)

In this Comparative Example, graphene film electrodes were prepared on two substrates opposed to each other, and an electrolyte was provided in between these two substrates. The process is illustrated in Figure 9. Figures lOa-b show a comparison of the area capacitance and stack capacitance of MPG-MSC with MPG-SSC. The area capacitance and stack capacitance of MPG- MSC were calculated to be -105 μΡ/cm² (-420 μΡ/cm² in electrode) and -70 F/cm³ (-280 F/cm³ in electrode). The MPG-MSC also exhibits a very high rate capability while still remaining a capacitance of more than -1 μΡ/cm² and -10 F/cm³ even at an ultrafast scan rate of 400 V s^"1. Apparently, these values in term of capacitance and rate capability in MPG-MSC are much higher than those of the MPG-SSC, which has 50-100 times longer ion diffusion length.

Figure 11 shows the impedance phase angle as a function of frequency for MPG- MSC and MPG-SSC. It can be seen that both devices exhibit capacitive behavior at low frequency and inductive behavior at high frequency. The characteristic frequency f₀ for a phase angle of - 45 ° is 3579 Hz for MPG-MSC and 16 Hz for MPG-SSC. Corresponding resistor-capacitor (RC) time constants of τ = 0.28 ms for MPG-MSC and τ = 62.5 ms for MPG-SSC can be calculated by the formula τ = 1/fo. Example 3

Preparation of the MPG-PL MSC using a mask prepared directly on the graphene film by photolithography

First, graphene oxide (GO) was synthesized from natural flake graphite with

Hummers method, as reported by Liang, Y., Wu, D., Feng, X., & Mullen, K., Adv. Mater. 21, 1679-1783 (2009). Then, a stable GO dispersion (2.5 mg mL^"1) obtained after 2 h by sonication was spin-coated several times at 2000 rpm for 60 s to achieve a desirable uniform GO film on the oxygen plasma-treated silicon wafer (300 nm Si0₂ layer) using 300 W rf power for 10 min (Plasma System 200-G, Technics Plamsa GmbH). Subsequently, the fabricated GO films were fast reduced at 700 °C for 20 s with a methane (C¾) plasma (AIXTRON, Nano instruments Black Magic) with a heating rate of 50 °C/min to form the graphene film. The designed flow rate of C¾ gas into plasma chamber was -100 seem. The plasma was operated with a 15 kHz waveform drive, and ignited with a high-voltage of 800 V. The chamber pressure during plasma treatment was -6.20 Torr. The graphene film had a thickness of about 15 nm, a high carbon-to-oxygen ratio of 9.2 and an electrical conductivity of 345 S/cm.

After that, standard photolithography techniques were used for preparing directly on the graphene film a mask having an interdigital mask pattern. A positive photoresist G1805 was spin coated on the surface of the graphene film at a speed of 4000 rpm for 30 s. Then the resulting photoresist film was soft baked for 60 s at 115 °C on a hot plate. Afterwards, the baked photoresist film was exposed to UV light using a photomask (Karl Suss MJB3 Mask Aligner, vacuum contact.) for 4 s. For preparing the 16- finger and 32-fmger supercapacitor samples, two different photomasks were used. Depending on the photomask, two different areas of the photoresist were exposed to UV light, i.e. either an area having a 16-fmger interdigital structure or an area having a 32-fmger interdigital structure. Next, hard bake was conducted for 60 s at 115 °C on a hot plate. After this, the photoresist was developed (i.e. removing the parts of the photoresist that have been exposed to UV light) for 30 s in the ma-D330 developer, thereby obtaining a mask (made of the remaining photoresist material) having a mask pattern which leaves an area of interdigital structure of the underlying graphene film uncovered. Two micro-supercapacitor samples were prepared. In the first sample, each of the two graphene electrodes had 8 fingers (i.e. interdigital structure having in total 16 fingers). In the second sample, each of the two graphene electrodes had 16 fingers (i.e. interdigital structure having in total 32 fingers).

After photoresist rinsing in DI water and drying, thin Au layer (30 nm, Premion, 99.9985% metals basis, Alfa Aesar) was deposited on the mask and the uncovered graphene area. The thermal evaporation rate of Au was controlled at -2.0 A/s and the chamber pressure is -3.75 x 10^"6 Torr (EDWARDS FL400). Later, the photoresist was lifted off in acetone with assistance of sonication for several minutes. The resulting Au micro -patterns served as a protection mask against oxygen plasma etching (Plasma System 200-G, Technics Plamsa GmbH, with 20 seem 0₂ flow and 100-200 W rf power under the vacuum of less than 0.05 mbar) of the graphene to create the patterns of graphene micro-electrodes on Si0₂/Si wafer. Afterwards, 5 μΐ, H₂SO₄/PVA gel electrolyte was drop-casted onto the surface of interdigital electrodes and solidified overnight. Finally, one on-chip all solid-state graphene- based MSCs was completely achieved.

The process is illustrated in Figure 12 which shows:

Step 1 : Providing an oxygen-plasma-treated Si wafer;

Step 2: Providing a graphene oxide film on the Si wafer via spin coating;

Step 3 : Reducing the GO film to a graphene film (reduced GO film) via CH₄ plasma reduction;

Step 4: Depositing a photoresist film on the grapheme film;

Step 5: Exposing the photoresist film to UV light using a photo Step 6: Development of the exposed photoresist film, thereby obtaining the mask (made of the remaining photoresist material) having a mask pattern of interdigital structure;

Step 7: Providing an Au film on the mask and the uncovered graphene film by

Au sputtering;

Step 8: Lifting off the mask, Au collector of interdigital structure on graphene film;

Step 9: Removing the parts of the graphene film which are not covered by the

Au collector by oxidative etching, thereby obtaining graphene electrodes of interdigital structure on the Si wafer;

Step 10: Applying a gel as an electrolyte.

The electrochemical properties of supercapacitor having the 32-fmger interdigital structure were first examined by cyclic voltammetry (CV) measurements at scan rates ranging from 0.01 to 2000 V s^_1. The results are shown in Figures 13a-g.

Notably, the supercapacitor exhibited a typical electric doubly-layer capacitive behaviour with a nearly rectangular CV shape, even at an ultrahigh scan rate of 1000 V s^"1 (Figure 2f), indicative of the ultrahigh power capability. Remarkably, the supercapacitor allowed the operation at ultrahigh discharge rate up to 2000 V s^"1 while maintaining excellent capacitance (Figure 2g), characteristic of a high instantaneous power. This value of 2000 V s^"1 is at least three orders of magnitude higher than that of conventional supercapacitors.

Figure 14 is a graph showing discharge current as a function of scan rate for both the 16-finger-supercapacitor and the 32-finger-supercapacitor.

Figure 15 shows cycling stability of the 32-finger-supercapacitor measured at a scan rate of 100 V/s. After 50,000 cycles, -98.5% of the initial capacitance was kept, revealing excellent cycling stability. The stack capacitance of the 32-finger-supercapacitor is shown in Figure 16. For comparison, a graphene-based sandwich-supercapacitor (denoted as sandwich-SC) is also included in the plot. At a low scan rate of 10 mV/s, the stack volumetric capacitance of the inventive supercapacitor is calculated to be -77.7 F/cm³ which is significantly higher than stack volumetric capacitance of the sandwich-type supercapacitor. Upon increasing scan rates, the capacitance of the inventive supercapacitor dropped very slowly. In this case, a stack capacitance of -13.7 F/cm³ was maintained at 100 V s^"1. Even at an ultrafast scan rate of 2000 Vs^"1, the inventive supercapacitor still retained a stack capacitance of -3.8 F/cm³.

Furthermore, the sandwich-SC, assembled with H₂SO₄/PVA gel electrolyte in between two same MPG film (-15 nm) electrodes, was examined to probe the influence of device geometry on the performance. Apparently, the obtained stack capacitance (-42.8 F/cm³) and rate capability (-1.3 F/cm³ at 100 V s^"1) for sandwich- SC were much lower than those of the inventive supercapacitor, demonstrative of the superiority of the in-plane geometry over sandwich geometry on the enhancement of electrochemical performance.

To evaluate the energy and power densities of the 32-finger-supercapacitor prepared in Example 3, a Ragone plot is shown in Figure 17. The data from the sandwich-SCs are included for comparison. The inventive supercapacitor delivered a volumetric energy density of -3.6 mWh/cm³, which is significantly higher than volumetric energy density of the sandwich-type supercapacitor. Furthermore, the inventive supercapacitor offered an ultrahigh power density of 1270 W/cm³ discharged within an extremely short discharge time of -0.5 ms, superior to the sandwich-type supercapacitor. It should be highlighted that this value (-1270 W/cm³) is at least three orders of magnitude higher than that of the conventional supercapacitors (typically <10 W/cm³), and higher than that of typical high-power electrolytic capacitors (typically 10²~10³ W/cm³). The phase angle as a function of the frequency for the 32-finger-supercapacitor prepared in Example 3 is presented in Figure 18. The data from the sandwich-SC are included for comparison. Apparently, it can be seen that the characteristic frequency o at the phase angle of -45° was -1572 Hz for MSC(32), which is much higher than that of sandwich-SC (-13 Hz). On the basis of the following equation, τ₀=1/ ο (Time constant το is defined as the minimum time that discharges all the energy from the device with an efficiency of more than 50%), the corresponding time constant το is calculated to be -0.64 ms for MSC(32), suggesting the fast accessibility of the ions within MPG-MSCs. In sharp contrast, sandwich-SC shows a much larger το of 76.9 ms at -45° phase angle.

The data of Figure 18 demonstrate that the supercapacitor of the present invention can be advantageously used for current filtering.

Claims

A process for preparing a supercapacitor, comprising the following steps:

(a) preparing a graphene film on a substrate S 1 ,

(c2) applying the current collector onto the graphene area of

interdigital structure and removing the mask,

(d) adding an electrolyte such that the electrolyte is in contact with the in- plane graphene electrodes of interdigital structure.

The process according to claim 1 , wherein the graphene film is prepared in step (a) by:

(al) preparing a graphene oxide,

(a3) reducing the graphene oxide film by treatment with a plasma so as to obtain the graphene film.

The process according to claim 2, wherein the graphene oxide film obtained in (a2) has a maximum thickness of less than 1000 nm.

The process according to claim 2 or 3, wherein the graphene oxide is treated with the plasma at a temperature of at least 200°C, preferably for a time period of from 1 second to 3600 seconds. The process according to one of the preceding claims, wherein in step (b), if carried out, the transfer of the graphene film, which has a lower surface being in contact with the substrate S 1 and an upper surface being at least partially uncovered, is accomplished by providing a temporary material on the upper surface of the graphene film, followed by removal of the substrate SI so that the graphene film now has an uncovered lower surface and an upper surface which is in contact with the temporary material, subsequently applying the substrate S2 onto the lower surface of the graphene film, followed by removal of the temporary material from the upper surface of the graphene film; and wherein the substrate S2 is preferably a flexible substrate.

The process according to one of the preceding claims, wherein the

supercapacitor is a micro supercapacitor.

The process according to one of the preceding claims, wherein the parts of the graphene film which are not covered by the current collector are removed by plasma etching; and/or wherein the electrolyte is a non-liquid electrolyte, preferably a polymer gel electrolyte.

A supercapacitor comprising:

(a) a substrate,

(d) an electrolyte which is in contact with the in-plane graphene

electrodes of interdigital structure. The supercapacitor according to claim 8, obtainable by the process according to one of the claims 1 to 7.

The supercapacitor according to claim 8 or 9, wherein the graphene of the graphene electrodes has a ratio of carbon atoms to oxygen atoms of at least 6, determined by X-ray photoelectron spectroscopy; and/or the supercapacitor has a stack capacitance of at least 1 F/cm³.

The supercapacitor according to one of the claims 8 to 10, wherein the graphene electrodes do not contain any organic binder; and/or do not contain any conductive additive different from graphene.

A process for preparing a layered assembly, which comprises:

(a) preparing on a first substrate SI a graphene film which has a lower surface being in contact with the substrate SI and an upper surface being at least partially uncovered,

The process according to claim 12, wherein in step (a) the graphene film is prepared according to one of the claims 2 to 4; and/or wherein the substrate S2 is a flexible and optionally transparent substrate. A process for preparing a layered assembly, comprising the following steps:

(a) preparing a graphene oxide,

(b) coating a substrate S 1 with the graphene oxide so as to obtain a

graphene oxide film,

(c) reducing the graphene oxide film by treatment with a plasma so as to obtain a graphene film,

A layered assembly comprising a substrate and a graphene film on the substrate, the layered assembly being obtainable by the process according to one of the claims 12 to 14.

The layered assembly according to claim 15, wherein the graphene film has a maximum thickness t_max of less than 1000 nm, and a conductivity σ of at least 100 S/cm.

Use of the layered assembly according to claim 15 or 16 for manufacturing an energy storage device, preferably a supercapacitor.