WO2019122386A1 - Production of graphene materials - Google Patents

Abstract

Methods for the production in an electrochemical cell of graphene and/or graphite nanopiatelet structures having a thickness of less than 100 nm, in a cell having a positive electrode which is graphitic and an electrolyte comprising an intercalating anion and a cobalt cation. The methods comprise the step of passing a current through the cell to intercalate anions into the graphitic positive electrode so as to exfoliate the graphitic positive electrode to produce the graphene and/or graphite nanopiatelet structures.

Description

PRODUCTION OF GRAPHENE MATERIALS

This application claims priority from GB1721816.5 filed 22 December 2017, the contents and elements of which are herein incorporated by reference for all purposes.

Field of the Invention

The present invention relates to methods for the production of graphene and graphite nanoplatelet structures by anodic electrochemical exfoliation.

Background

Graphene was discovered, isolated and characterised in 2004 by Andre Geim and Konstantin Novoselov at the University of Manchester. In its pristine form, graphene is a single layer of carbon atoms arranged in a hexagonal lattice.

Graphene is the strongest material ever tested. It conducts both heat and electricity, and is transparent. Owing to these unusual properties, graphene is incredibly interesting both scientifically and

technologically, and graphene and related materials are already finding applications in a wide variety of innovative technologies.

The discovery of graphene gave birth to myriad research programmes investigating ways of making and using graphene, as well as functionalised graphene materials and numerous 2-dimensional

heterostructures.

Methods for the production of graphene include both bottom-up and top-down synthetic approaches, with each method having its own benefits and drawbacks. For example, chemical vapour deposition produces relatively high quality graphene but in low quantity, which chemical exfoliation of graphite produces large quantities near electrically insulating monolayer graphene oxide (GO). Solution exfoliation of graphite produces pristine graphene platelets with yields of typically less than a percent.

The production of graphene via electrochemical exfoliation in aqueous solution is considered very attractive in terms of scalability, reproducibility and affordability, but controlling the quality and properties of the product is often challenging, not least because of the tendency for oxidation of the graphene, especially during anodic exfoliation processes, leading to materials having comparatively low electrical conductivity.

The oxidation of water at the graphite surface during electrolysis of water generates the hydroxyl radical (HO’) and other oxygen radicals. These radicals rapidly react with graphite surface, resulting in an exfoliated product with oxygen containing functional groups and also introducing defects in the form of holes or strains to the graphene sheets. Typically, the oxygen content for graphene obtained by anodic electrochemical exfoliation methods is in range of 10% to 20%. This is comparable to the oxygen content of reduced graphene oxide.¹¹·²'³·⁴¹

This may limit the use of electrochemically exfoliated graphene in electrical applications, such as battery technology.

WO2015/158711 describes anodic electrochemical exfoliation of graphite using sulfate salts and sulfonate esters as intercalating species, both in aqueous solution and as ionic liquids.¹⁵¹ Where the oxygen content of the product is reported, it is described as being about 8%, which is reported as a carbon to oxygen atomic ration of about 11.5.

Attempts to reduce the effects of HO^* have been described in the art. Miillen et al. demonstrated the use of a series of reducing agent additives. In particular, using (2,2,6,6-tetramethylpiperidin-1-yl)oxyl

[TEMPO] the authors showed that it was possible to minimize significantly the oxygen content in the final product, achieving a carbon to oxygen ratio of 25.3.^[6] It has been suggested that the HO^* preferentially reacts with TEMPO rather than the graphite surface, resulting in exfoliated graphene sheets with low oxygen content (3.8%) and comparatively few defects. The use of radical scavengers such as TEMPO in electrochemical exfoliation methods is also described in WO2017/050689.1⁷¹

The addition of melamine to the exfoliation solution H₂S04(aq) also increased the C/O from ~16 (without melamine) to 26. The suggested mechanism attributed protection of the graphite surface against oxidation to adsorption of the aromatic compound onto the graphene surface.¹⁸¹ In an alternative approach, Munuera et al. proposed the use of bulky sulfonated aromatic hydrocarbons both as intercalating and reducing agent to combat the oxygen functionalisation. They achieved an O/C ratio of between 0.04 and 0.12.¹⁹¹

Despite these advances, there remains a need in the art for methods for the production of high quality graphene via anodic electrochemical exfoliation.

In Carbon, 2016, 107, p379-387, Yan R et al. describe methods of synthesis and in-situ functionalisation of graphene films through graphite charging in aqueous Fe2(S04)3. The authors describe the preparation of both graphene and Fe₃0₄/graphene films via electrochemical exfoliation of graphite paper in aqueous Fe2(S0₄)3.

Summary of the Invention

The present invention relates to methods for the production of graphene and graphite nanoplatelets by anodic electrochemical exfoliation. The methods make use of a metal cation which prevents or reduces oxidation of the product, leading to high quality, low oxygen content material with comparatively few defects.

In a first aspect, the invention provides a method for the production in an electrochemical cell of graphene and/or graphite nanoplatelet structures having a thickness of less than 100 nm, wherein the cell comprises:

(a) a positive electrode which is graphitic; (b) a negative electrode; and

(c) an electrolyte comprising an intercalating anion and a cobalt cation;

and wherein the method comprises the step of passing a current through the cell to intercalate anions into the graphitic positive electrode so as to exfoliate the graphitic positive electrode to produce the graphene and/or graphite nanoplatelet structures.

Suitably, the negative electrode is also graphitic. For example, it may comprise graphite rod.

Suitably, the electrolyte is an aqueous solution. In other words, suitably the electrolyte is a salt solution in water.

The methods may include the step of isolating the produced graphene and/or graphite nanoplatelets. These may collected by any suitable process. For example, the electrolyte solution may be filtered to retrieve the product, which may be washed several times with, for example, water. The produce may be re-dispersed for use or further processing, for example in DMF. Sonication may aid re-dispersal.

As described herein, the material produced has relatively low graphene oxide character. In other words, the material may be substantially free of graphene oxide. “Substantially free” means less than 10% by weight, preferably less than 5% by weight, more preferably less than 1 % by weight of graphene oxide.

The metal cation

The electrolyte includes a cobalt cationin the inventors have also observed reduced oxidation when iron is used. Accordingly, also described here in is a method in which the electrolyte comprises iron cations, for example, Fe(lll) rather than cobalt cations. The inventors have demonstrated that the presence of either cation reduces the oxygen content of the product. As described herein, Co(ll) was more effective.

Accordingly, Co(ll) is preferred.

The metal cation is suitably provided as a salt. Suitable counter ions include sulfate, nitrate and acetate. As described herein, sulfate may be preferred. The inventors have found that use of cobalt sulfate results in a lower oxygen content than use of nitrate and acetate, although in all cases the oxygen content is low.

As described herein, sulfate is a suitable intercalating ion for anodic electrochemical exfoliation of graphite. Accordingly, the counter ion to the metal cation may also serve as intercalating anion.

In some cases, the concentration of the metal in the electrolyte is greater than 3 mM, for example greater than 5 mM, for example greater than 10 mM, for example greater than 15 mM, for example greater than 25 mM. The inventors have found that increasing the concentration of cobalt above 25 mM does not appear to lead to improvement in the quality of material. Accordingly, the concentration of the metal may be less than 30 mM.

In some cases, the concentration of the metal (cobalt or, if used instead, iron) in the electrolyte is between 3 and 30 mM, for example between 5 and 30 mM, for example between 10 and 30 mM, for example between 15 and 30 mM, for example between 20 and 30 mM. It may be about 25 mM.

It will be appreciated that the electrolyte may include one or more additional metal cations, for example as counter ion to the intercalating anion. The intercalating anion

The electrolyte further comprises anions suitable for intercalating into the graphitic working electrode so as to exfoliate the material. Suitable anions are known in the art and include those referred to in and in WO 2015/158711 ^ which is herein incorporated by reference in its entirety for all purposes.

In some embodiments, the electrolyte comprises sulfate anions. In other words, the electrolyte may include a sulfate salt.

The anion may be provided in the form of counterion to the cation of the metal species to be deposited as an oxide, or as a separate salt. Providing the intercalating anion as a separate salt permits the concentrations of the metal cation for oxide electrodeposition and the intercalating anion to be independently varied. Suitable counterions include both metal and non-metal counterions. Some are described in Feng et al. ^[9] the entire contents of which, and in particular the disclosure of the exemplified salt forms, are incorporated by reference. In some cases, the counterion is sodium.

For example, the electrolyte may include a sulfate salt, for example sodium sulfate, and a further metal salt, the sulfate ions acting as an intercalating species during the process.

Suitable concentrations for the intercalating anion may vary with cell set up and operation, and selecting a suitable concentration is within the remit of the skilled person. The concentration may be less than 2.5 M, for example less than 2 M, for example less than 1 M. For example, it may be 0.1 to 2.5 M, 0.1 to 1.5 M or 0.1 to 1 M. In some cases is it 0.1 to 1 M, for example 0.3 to 1 M. In the examples described herein, and accordingly in some embodiments, the concentration is about 0.5 M.

Certain exemplary electrolytes

In some cases, the electrolyte is an aqueous solution of a cobalt(ll) salt and sodium sulfate. Exemplary cobalt(ll) salts include cobalt sulfate, cobalt nitrate and cobalt acetate.

In some cases, the electrolyte is an aqueous solution of cobalt sulfate and sodium sulfate.

In some cases, the concentration of cobalt in the electrolyte is greater than 3 mM, for example greater than 5 mM, for example greater than 10 mM, for example greater than 15 mM, for example greater than 25 mM. The inventors have found that increasing the concentration above 25 mM does not appear to lead to improvement in the quality of material. Accordingly, the concentration of cobalt may be less than 30 mM.

In some cases, the concentration of cobalt in the electrolyte is between 3 and 30 mM, for example between 5 and 30 mM, for example between 10 and 30 mM, for example between 15 and 30 mM, for example between 20 and 30 mM. It may be about 25 mM.

In some cases, the concentration of sodium sulfate in the electrolyte is be 0.1 to 2.5 M, 0.1 to 1.5 M or 0.1 to 1 M. In some cases is it 0.1 to 1 M, for example 0.3 to 1 M. In the examples described herein, and accordingly in some embodiments, the concentration is about 0.5 M.

The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided. Definitions and Further Details

Graphene

The term graphene is conventionally used in the art to refer to both monolayer graphene, sometimes called pristine graphene, and few layer graphene. In the present application, the term“graphene” is used to describe materials consisting of ideally one to ten graphene layers, preferably where the distribution of the number of layers in the product is controlled.

In some cases, electrochemical methods as described herein usefully product thicker material (i.e.

material having more than 10 carbon layers). The“graphene-like” properties of materials may be a continuum, and products having greater than 10 layers may be produced and have the same or similar properties to graphene having one to ten graphene layers. These materials are referred to herein as graphite nanoplatelets and graphite nanoplatelet structures. In other words, the method may also be used to make graphite nanoplatelet structures under 100 nm in thickness, more preferably under 50nm in thickness, more preferably under 20 nm in thickness, and more preferably under 10 nm in thickness. The size of the graphene flakes produced can vary from nanometres across to millimetres, depending on the morphology desired.

The corresponding“bulk” material of graphene is graphite. This typically consists of thousands of layers of graphene.

In some embodiments, the material produced is graphene having up to ten layers. The graphene produced may have one, two, three, four, five, six, seven, eight, nine or ten layers.

Electrode

In the methods of the present invention, a graphitic anode is exfoliated. Accordingly, the methods of the present invention use a positive electrode which is graphitic. In other words, the positive electrode comprises graphite. The graphite may be provided in any suitable form. For example, it may be provided a rod, as graphite foil, or a powder, which may be provided as a composite in a polymeric support, or in a mesh. In some cases, the positive electrode comprises graphite which has been pre-expanded, although this is not essential. In other words, the positive electrode may comprise graphite having at least some inter-layer distances of greater than 0.335 nm. In some cases, at least 5% of the graphite layers have a greater than 0.335 nm spacing. In some cases, at least 10% of the graphite layers have a greater than 0.335 nm spacing. In some cases, at least 15% of the graphite layers have a greater than 0.335 nm spacing. In some cases, at least 20% of the graphite layers have a greater than 0.335 nm spacing. In some cases, at least 25% of the graphite layers have a greater than 0.335 nm spacing. In some cases, at least 30% of the graphite layers have a greater than 0.335 n spacing. In some cases, at least 40% of the graphite layers have a greater than 0.335 nm spacing. In some cases, at least 50% of the graphite layers have a greater than 0.335 nm spacing. In some cases, at least 60% of the graphite layers have a greater than 0.335 nm spacing. In some cases, at least 70% of the graphite layers have a greater than 0.335 nm spacing. In some cases, at least 80% of the graphite layers have a greater than 0.335 nm spacing. Inter-layer distances of more than 0.335 nm may, for example, be greater than 0.35 nm, for example greater than 0.37 nm, for example greater than 0.40 nm, for example greater than 0.45 nm.

Methods to pre-expand graphite are known in the art and include, for example, immersing the graphite in very low temperature liquids (less than -100 °C) followed by a solvent, for example an alcohol such as ethanol. In the examples described herein, the graphite foil working electrode was pre-expanded by immersion in liquid nitrogen for 30 s following by transferring into absolute ethanol.

The inventors have used both pre-expanded and non-expanded graphite in methods described herein. In each case, exfoliation occurred.

The cathode (negative electrode) may be graphitic or another material. For example, the negative electrode may be a graphite rod.

A reference electrode may be used.

Exfoliation Cell

The cell contains a graphitic positive electrode for exfoliation, a negative electrode which may be graphitic or another material, and an electrolyte. An H-type electrochemical cell may be used. Suitably, the electrochemical cell is configured to provide separate anode and cathode compartments. For example, the anode and cathode may be separated by a glass frit. Such an electrochemical cell potentially prevents the contamination of the exfoliated graphene samples by metal hydroxides since the hydroxyl ion that continuously generates at the counter electrode (cathode) from reduction of water may react with the metal.

In summary, the inventors have shown that certain metal cations act as effective barrier against surface oxidation of graphite during anodic electrochemical exfoliation, producing high quality graphene with exceptionally high carbon to oxygen ratios. In other words, the metal cations may act as antioxidants during the electrochemical process. The inventors believe that the use of these simple and cheap salts paves the way for the scalable production of high quality graphene.

Summary of the Figures

Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

Figure 1 shows survey-scan (A) and high-resolution XP spectra in the C1s region (B) of electrochemically exfoliated graphene in presence and absence of 50 mM CoSCu, and of graphite. All peak positions were charge-corrected by setting the binding energy of the C 1s signal to 285 eV. The inset in (B) shows the picture of the electrochemical cell used during electrolysis in 50 mM C0SO4. In each case, the electrolysis was carried out using 0.5 M Na2S0₄ (aq) electrolyte.

Figure 2 shows (A) Raman spectra of graphene obtained by electrochemical exfoliation of graphite with and without C0SO4 and the samples for Raman analysis were prepared by drop-coating the dispersion of graphene on to Si/SiC>2 wafer, (B) and (C) Survey-scan XP spectra for indicated graphene samples. All peak positions were charge-corrected by setting the binding energy of the C 1 s signal to 285 eV. (D) High resolution TEM images of graphene flakes obtained by electrochemical exfoliation of graphite in presence of 50 mM C0SO4 and absence of C0SO4 (inset picture).

Detailed Description of the Invention

Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

The inventors have found that the inclusion of certain metal cations in the exfoliation solution during the electrochemical exfoliation of graphene leads to exfoliated graphene sheets having reduced oxygen content and/or fewer defects when compared to exfoliation is the absence of those cations.

In particular, the inventors have demonstrated that a Co(ll) salt in the exfoliation solution acts as an efficient hydroxyl radical scavenger, without functionalising the exfoliated graphene with cobalt oxides. X-ray photoelectron spectroscopy (XPS) showed that the inclusion of cobalt lowered the oxygen content of graphene by 80% when compared to the control (exfoliated in the absence of Co salt) sample. High resolution transmission electron microscopy and Raman spectroscopy also revealed the substantial reduction of defects and/or holes across the graphene sheets.

Without wishing to be bound by any particular theory, the inventors propose the following mechanisms to describe the role of the Co(ll) salt during the electrochemical exfoliation of graphite.

The application of positive biased voltage causes the oxidation of Co²⁺ to Coⁿ⁺ along with water oxidation at graphite surface. It is postulated that the oxidation product of Co²⁺ deposits on graphite surface and is metastable. It is thought that this film may oxidise water to oxygen without significant formation of the OH-^* intermediate, thereby preventing or reducing oxidation at the graphite surface. Mixed cobalt oxides, such as C03O4, may function as an electrocatalyst for water oxidation!^10] and the inventors speculate that the deposit may be a mixed cobalt oxide. The deposits could potentially also act as an insulating surface to prevent or reduce oxidation of the graphite by OH-^* radicals while permitting the anion intercalation for exfoliation. This mechanism find some support in the observations of Cronin et al ^ In any event, the inventors believe that any metastable electrodeposits dissolve from the exfoliated surface, leaving behind a near pristine few layer graphene substantially free of metal deposits.

Indeed, the dissolution of cobalt deposits from the exfoliated materials can be seen by solution colour change from pink to brown near the vicinity of the electrode in the anodic compartment as exfoliation proceeds. Analysis of the pH before and after electrolysis indicated that the electrolytic media changed from neutral (pH=6.4) to acidic (pH=1.5) respectively. The inventors speculate that this could be due to the effective water oxidation to oxygen at Co deposits which adds H⁺ to the solution. Moreover, UV- visible spectroscopy data showed substantial reduction of Co(ll) after electrolysis. Spectroscopy data also confirm the metastability of the Co deposits in the solution as the signal due to Co (II) re-emerges gradually at longer electrolysis time. The inventors have further observed that the inclusion of an Fe(lll) salt reduced the oxygen content of the exfoliated graphene materials by half (to about 7%).

Unless stated otherwise, all references to percentage element (e.g. percentage oxygen) content refer to atom percentages (a measure of the number of atoms of that type relative to the total number of atoms).

Materials and reagents

Anhydrous sodium sulfate, cobalt (II) sulfate (99.9%), cobalt (II) nitrate (98%), and cobalt (II) acetate (99.9%) were obtained from Sigma-Aldrich. All electrochemical measurements were performed either using an Autolab potentiostat model (PGSTAT302N, Metrohm Autolab, The Netherlands) or power source. Graphite foil (>99%) was obtained from Gee Graphite Ltd (UK). Omnipore membrane filters made of poly(tetrafluoroethylene) (JVWP01300) were used, pore size of 0.1 pm. Ultra-pure water (18.2 MW cm resistivity) was obtained from a Milli-Q water purification system.

Characterisation of the Exfoliated Product

Raman spectra were obtained using a Renishaw inVia microscope with a 532 nm excitation laser operated at a power of 3.32 mW with a grating of 1800 lines/mm and 100x objective. The samples for Raman measurement were prepared by drop coating the dispersion (in DMF) onto a Si/Si0₂ wafer and then dried on a hot plate at 150 °C to evaporate the solvent. For AFM analysis, the composite dispersion was spray-coated onto a Si/SiC>2 substrate which was dried in a vacuum oven at 80 °C. SEM analysis was carried out using an FEI Quanta 650 FEG environmental scanning electron microscope. (S)TEM was carried out by FEI Talos F200X operated at 200 kV and FEI Titan³ G2 60-300 operated at 80 kV. A sample for (S)TEM was prepared by dispersing dried composite sample into DMF solution for few sec and then drop casted over TEM grid. The concentration of the graphene dispersion was measured with UV-vis spectroscopy using a model DH-2000-BAL (Ocean Optics). X-ray photoelectron spectroscopy (XPS) was performed using a Kratos Axis Ultra DLD spectrometer with a monochromated Al Ka X-ray source (E = 1486.6 eV, 10 mA emission). X-ray Diffraction (XRD) was performed on a Philips X’pert PRO diffractometer with Cu Ka radiation (l = 0.154 nm) operating at 40 kV and 30 mA.

Electrochemical Exfoliation of Graphite in the Presence of Metal Cations

An H-type electrochemical cell consisting of a graphite foil working electrode (pre-expanded by immersing in liquid nitrogen for 30 s followed by transferring into absolute ethanol) in the anode compartment and a graphite rod counter electrode in the cathode compartment was used. The electrochemical cell was separated by porous glass frit and the compartment was separated by 7 cm (inset of Figure 1B).

A combination of 50 mM C0SO4 (aq) and 0.5 M Na2SC>4 (aq) was used as electrolyte where the sulfate anion act as an intercalating species into graphite to cause exfoliation. Electrochemical exfoliation and functionalisation of graphite carried out by applying +20 V to graphite foil (anode compartment) using power source. The exfoliated product was washed several times with water and then re-dispersed in DMF by sonicating for 20 min. During electrolysis, a green precipitate was formed at the cathode (graphite rod) compartment due to the formation of Co(OH)2, whilst at the anode compartment an instantaneous exfoliation of graphite was observed. This demonstrates that electrooxidation of Co²⁺ at the graphite surface did not prevent the exfoliation via sulfate ion intercalation.

The exfoliated samples were characterised by X-ray photoelectron spectroscopy (XPS) and X-ray energy dispersive spectroscopy (EDX). Figure 1 A shows the survey scan obtained for the exfoliated graphene product in the presence and absence of cobalt salt. In each case the survey scan showed only the signal due to C1 s and 01 s, with no signals for cobalt species. This confirms that the exfoliated graphene was not functionalised with any form of cobalt species.

The inventors also overserved that the oxygen content of the exfoliated product in the presence of C0SO4 decreased by 80% when compared to the one exfoliated in the absence of C0SO4 (used as control). The addition of C0SO4 in the exfoliation solution led to a graphene product with a carbon to oxygen ratio of 36. The increase in oxygen content for the graphene sample relative to the starting graphite foil was only 1.3%, demonstrating that the electrooxidation product of Co²⁺ plays a crucial role in reducing the extent of graphite surface oxidation. The atomic concentrations of the bulk sample were also analysed by EDX and the data showed that it contained 98 % of C and 2 % of O, in an agreement with XPS analysis.

These results compare very well with previously reported carbon/oxygen ratios for anodically exfoliated graphene. Ambrosi and Pumera reported C/O rations of 8.8, 8.1 and 4.4 respectively for

electrochemically exfoliated graphite samples in Na2S0₄(aq), H2S0₄(aq) and LiCI0₄(aq)J¹²i Parvez et al. reported a C/O ratio of 17 for anodically exfoliated graphene samples in 0.1 M (NH₄)₂S0₄ and 12 in 0.1 M

H₂S0₄.'¹³'¹⁴1

High resolution C1 s spectra were collected to get more insight into the extent of graphene surface oxidation. The control (0 mM C0SO4) sample shows a shoulder peak (Figure 1 B) at the characteristic binding energy position of carbon functionalisation by oxygen-containing groups such as epoxy, hydroxyl, carbonyl and carboxylic acid.!¹²! In contrast, the C1 s spectra of graphene sample obtained in 50 mM C0SO4 shows the absence of any surface functionalisation and the spectrum shows an identical shape to that of the parent graphite. This suggests that the residual oxygen content (1 .3 %) was most likely a result of non-covalently bonded adsorbed oxygen and that cobalt efficiently protect the graphene surface against oxidation.

A comparison of the intensity ratio of the D-band to the G-band (IDHG) for the graphene sample obtained in presence of 50 mM C0SO4 and control also supports the conclusion drawn from XPS analysis (Figure 2A). The IDHG of the control (0 mM C0SO4) sample was 0.98, and this value decreased to 0.2 for the graphene sample obtained in 50 mM C0SO4 demonstrating that the later sample was less defective than the former. High resolution TEM images also show substantial reduction of defect and holes when compared to graphene exfoliated in the absence of cobalt salt (the inset image of Figure 2D).

Furthermore, the blue shift of the 2D position by 20 cnr¹ relative to the starting graphite confirms the formation of a few layer graphene after exfoliation.

The reduction efficiency of various cobalt (II) salt anions as well as the optimum Co(ll) salt concentrations was studied and analysed by XPS. Three counter anions were used: sulfate, nitrate and acetate, all at 50 mM concentrations. The inventors observed that S0₄ ² resulted in the lowest oxygen content compared to the other two anions (see Figure 2B), albeit with low oxygen content in each case. The inventors attribute this to the lower electrochemical stability of acetate and nitrate anions compared to the sulfate ion, which may lead to some graphene functionalisation as a result of electrooxidation during electrolysis. Consequently, a sulfate counter ion may be preferred.

Following this, the effect of changing the concentration of CoS0₄ was studied. The inventors have showed that the oxygen content decreases by 50% in comparison to the control sample when only 3 mM of C0SO4 is used (Figure 2C). In other words, the effect is observed even at low concentrations.

The oxygen content further deceased from 7 % to 4 % as the concentration of C0SO4 increased from 3 mM to 10 mM. The optimum C0SO4 concentration was determined to be 25 mM (2.6 % oxygen) as increasing above this concentration did not significantly influence the oxygen content.

The impact of oxygen content on the conductivity of the graphene film was assessed using four-point probe resistivity measuring device. The graphene film was made on PTFE membrane using syringe pump and in each case 0.1 mg mL·¹ graphene concentration was used to control the thickness of each film. The sheet resistance of the control sample was 845 W/sq and the resistance decreased to 285, 132 and 67 W/sq for the graphene exfoliated in 50 mM Co(NC>3)2, 50 mM Co(Ac)2 and 50 mM C0SO4 respectively. The corresponding conductivity value was 2370 S rrr¹ for the control sample and was 29,833 S nr¹ for 50 mM C0SO4. This demonstrates that the presence of Co²⁺ preserves the sp² hybridised carbon from functionalisation in agreement with analysis of XPS data.

The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

Throughout this specification, including the claims which follow, unless the context requires otherwise, the word“comprise” and“include”, and variations such as“comprises",“comprising”, and“including" will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. It must be noted that, as used in the specification and the appended claims, the singular forms“a,”“an,” and“the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from“about” one particular value, and/or to“about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term“about” in relation to a numerical value is optional and means for example +/- 10%.

References

A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. The entirety of each of these references is incorporated herein.

[1] Z.Y. Liu, Z.S. Wu, S. Yang, R.H. Dong, X.L. Feng, K. Mullen, Ultraflexible In-Plane Micro- Supercapacitors by Direct Printing of Solution-Processable Electrochemically Exfoliated

Graphene, Adv. Mater. 28(11 ) (2016) 2217-2222.

[2] J.Z. Liu, M. Notarianni, G. Will, V.T. Tiong, H.X. Wang, N. Motta, Electrochemically Exfoliated Graphene for Electrode Films: Effect of Graphene Flake Thickness on the Sheet Resistance and Capacitive Properties, Langmuir 29(43) (2013) 13307-13314.

[3] J.l. Paredes, J.M. Munuera, Recent advances and energy-related applications of high

quality/chemically doped graphenes obtained by electrochemical exfoliation methods, Journal of Materials Chemistry A 5(16) (2017) 7228-7242.

[4] A. Ambrosi, M. Pumera, Electrochemically Exfoliated Graphene and Graphene Oxide for Energy Storage and Electrochemistry Applications, Chem.-Eur. J. 22(1 ) (2016) 153-159.

[5] WO2015/158711

[6] S. Yang, S. BrOller, Z.-S. Wu, Z. Liu, K. Parvez, R. Dong, F. Richard, P. Sarnori, X. Feng, K.

Mullen, Organic Radical-Assisted Electrochemical Exfoliation for the Scalable Production of High- Quality Graphene, J. Am. Chem. Soc. 137(43) (2015) 13927-13932.

[7] WO2017/050689

[8] C.H. Chen, S.W. Yang, M.C. Chuang, W.Y. Woon, C.Y. Su, Towards the continuous production of high crystallinity graphene via electrochemical exfoliation with molecular in situ encapsulation, Nanoscale 7(37) (2015) 15362-15373.

[9] J.M. Munuera, J.l. Paredes, S. Villar-Rodil, M. Ayan-Varela, A. Martinez-Alonso, J.M.D. Tascon, Electrolytic exfoliation of graphite in water with multifunctional electrolytes: en route towards high quality, oxide-free graphene flakes, Nanoscale 8(5) (2016) 2982-2998. [10] H. Tuysuz, Y.J. Hwang, S.B. Khan, A.M. Asiri, P.D. Yang, Mesoporous Co304 as an electrocatalyst for water oxidation, Nano Res. 6(1 ) (2013) 47-54.

[11] L.G. Bloor, P.l. Molina, M.D. Symes, L. Cronin, Low pH Electrolytic Water Splitting Using Earth- Abundant Metastable Catalysts That Self-Assemble in Situ, J. Am. Chem. Soc. 136(8) (2014) 3304-3311.

[12] A. Ambrosi, M. Pumera, Electrochemically Exfoliated Graphene and Graphene Oxide for Energy Storage and Electrochemistry Applications, Chem.-Eur. J. 22(1 ) (2016) 153-159.

[13] K. Parvez, Z.S. Wu, R.J. Li, X.J. Liu, R. Graf, X.L. Feng, K. Mullen, Exfoliation of Graphite into Graphene in Aqueous Solutions of Inorganic Salts, J. Am. Chem. Soc. 136(16) (2014) 6083- 6091.

[14] K. Parvez, R.J. Li, S.R. Puniredd, Y. Hernandez, F. Hinkel, S.H. Wang, X.L. Feng, K. Mullen, Electrochemically Exfoliated Graphene as Solution-Processable, Highly Conductive Electrodes for Organic Electronics, Acs Nano 7(4) (2013) 3598-3606.

Carbon, 2016, 107, p379-387, Yan R et al.

Claims

Claims:

1. A method for the production in an electrochemical cell of graphene and/or graphite nanoplatelet structures having a thickness of less than 100 nm, wherein the cell comprises:

(a) a positive electrode which is graphitic;

(b) a negative electrode; and

(c) an electrolyte comprising an intercalating anion and a cobalt cation;

and wherein the method comprises the step of passing a current through the cell to intercalate anions into the graphitic positive electrode so as to exfoliate the graphitic positive electrode to produce the graphene and/or graphite nanoplatelet structures.

2. The method of claim 1 , wherein the intercalating anion is sulfate.

3. The method of claim 1 or claim 2, wherein the electrolyte is an aqueous solution of a cobalt(ll) salt and sodium sulfate.

4. The method of claim 3, wherein the electrolyte is an aqueous solution of cobalt(ll) sulfate and sodium sulfate.

5. The method of any preceding claim, wherein the concentration of cobalt cations in the electrolyte is between 3 and 30 mM.

6. The method of any preceding claim, wherein the concentration of cobalt cations in the electrolyte is between 10 and 30 mM.

7. The method of any preceding claim, wherein the concentration of cobalt cations in the electrolyte is about 25 mM.