Production of graphene materials

Abstract

Disclosed is a method for the production of metal oxide deposited graphene and / or graphite nanoplatelet structures having a thickness of less than 100 nm, the method comprising providing an electrochemical cell, wherein the cell comprises a positive electrode which is graphitic, a negative electrode, and an electrolyte comprising an intercalating anion and a metal cation; passing a current through the cell to intercalate anions into the graphitic positive electrode so as to exfoliate the graphitic positive electrode; and increasing the pH of the electrolyte to at least pH 3, such that the metal ion is deposited in the form of the corresponding metal oxide to produce the metal oxide deposited graphene and / or graphite nanoplatelet structures. Also disclosed are metal oxide deposited graphene and / or graphite nanoplatelet structures, and uses of them.

Description

[0001] PRODUCTION OF GRAPHENE MATERIALS

[0002] TECHNICAL FIELD

[0003] This invention relates to metal oxide deposited graphene materials, and use of these materials as electrodes and catalysts. The invention further relates to supercapacitors having an electrode comprising a metal oxide deposited graphene material.

[0004] BACKGROUND

[0005] 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.

[0006] 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.

[0007] 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.

[0008] 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, while chemical exfoliation of graphite produces large quantities of near electrically insulating monolayer graphene oxide (GO). Solution exfoliation of graphite produces pristine graphene platelets with yields of typically less than a percent.

[0009] 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. This may limit the use of electrochemically exfoliated graphene in electrical applications, such as battery technology.

[0010] There is also growing interest in so-called functionalised graphene materials. These are 2D carbon- based materials that include chemical functionality in the form of, for example, substituent groups. These may be inert or reactive, and can be useful for the altering or fine-tuning of the electrical and / or physical properties of graphene to suit applications. In a conference paper attributed to the 6th International Conference on Nanostructures, Dizaji A K ef al. describe an apparent one-step electrochemical method of graphene / copper particle nanocomposite preparation using a graphite rod cathode. The electrolyte contained 1000 ppm CuO and 1000 ppm NH4CI in deionized water. The authors note that the mechanism is not understood, but speculate that cations of the electrolyte intercalate between the interlayer space of the graphite cathode, and that reduction of those cations leads to reduction / deposition. The described exfoliation is attributed to internal stress attributed by intercalation.

[0011] WO2019 / 122379 describes methods for the production in an electrochemical cell of metal oxide deposited graphene and / or graphite nanoplatelet 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 metal cation, wherein the metal is selected from ruthenium, manganese, iridium, tin, and silver. The methods comprising 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 and such that the metal ion undergoes electrodeposition in the form of the corresponding metal oxide to produce the metal oxide deposited graphene and / or graphite nanoplatelet structures. The method was found to work only for certain metal ions, some of which are relatively expensive or rare.

[0012] WO2019 / 122386 describes methods for the production in an electrochemical cell of graphene and / or graphite nanoplatelet 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 nanoplatelet structures. This method exploits the fact that the cobalt was found to act as a hydroxyl radical scavenger without functionalising the exfoliated graphene with cobalt oxides, in order to provide exfoliated graphene sheets with reduced oxygen content and free of metal oxide deposits.

[0013] Improved methods for the production of metal oxide-deposited graphene materials are therefore desirable.

[0014] SUMMARY OF THE INVENTION

[0015] The invention provides a method for the production of metal oxide deposited graphene and / or graphite nanoplatelet structures having a thickness of less than 100 nm, the method comprising: i. providing an electrochemical cell, 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 metal cation; ii. passing a current through the cell to intercalate anions into the graphitic positive electrode so as to exfoliate the graphitic positive electrode; and iii. increasing the pH of the electrolyte to at least pH 3, such that the metal ion is deposited in the form of the corresponding metal oxide to produce the metal oxide deposited graphene and / or graphite nanoplatelet structures.

[0016] The invention also provides metal oxide deposited graphene and / or graphite nanoplatelet structures obtainable by the method of the invention.

[0017] The invention also provides a composition comprising graphene and / or graphite nanoplatelet structures having a thickness of less than 100 nm, wherein the graphene and / or graphite nanoplatelet structures have metal oxide deposited on the basal surface, wherein the metal oxide comprises cobalt oxide, iron oxide, nickel oxide, zinc oxide, aluminium oxide, vanadium oxide, copper oxide or mixtures thereof.

[0018] The invention also provides an electrocatalyst comprising the metal oxide deposited graphene and / or graphite nanoplatelet structures of the invention, or the composition of any of the invention.

[0019] The invention also provides use of the metal oxide deposited graphene and / or graphite nanoplatelet structures of the invention, or the composition of the invention, in electrocatalysis.

[0020] Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", mean "including but not limited to", and do not exclude other components, integers or steps. Moreover the singular encompasses the plural unless the context otherwise requires: in particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0021] Preferred features of each aspect of the invention may be as described in connection with any of the other aspects. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0023] Figure 1A is a graph of X-ray photoelectron spectroscopy (XPS) scans in the full binding energy range for relevant samples from examples 1-3; Figure IB is a table of atomic percentages detected for relevant samples; Figure 1C is a graph of high resolution XPS in the Co 2p binding region for relevant samples.

[0024] Figures 2A-D are a set of images taken using scanning electron microscopy (SEM) of a relevant sample from example 1 with increasing magnification taken at 2000 (Figure 2A), 10000 (Figure 2B), 20000(Figure 2C) and 40000 (Figure 2D) x magnification.

[0025] Figures 3A-D are a set of images showing elemental maps from energy dispersive X- ray spectroscopy (EDX) of a relevant sample from example 1. Figure 3A shows carbon mapping, Figure 3B shows cobalt mapping, Figure 3C shows oxygen mapping and Figure 3D shows C / Co ratios.

[0026] Figure 4A is a graph from thermal gravimetric analysis (TGA) of relevant samples from examples 1-3; Figure 4B is a table of residual mass percentages of CoOxfor the samples.

[0027] Figure 5A is a graph of XPS scans in the full binding energy range for a relevant sample from example 5; Figure 5B is a table of atomic percentages detected for the example; Figure 5C is a graph of high resolution XPS in the Co 2p binding region for the example Figure 5D is a graph of high resolution XPS in the Mn 2p binding region for the example.

[0028] Figure 6A is an image taken using SEM of a relevant sample from example 5, along with a table of elemental percentages taken using EDX; Figures 6B-E are a set of images showing elemental maps from EDX relating to the sample (Figure 6B - Co, C and Mn; Figure 6C - C; Figure 6D - Mn; Figure 6E - Co).

[0029] Figure 7 is a graph from TGA of a relevant sample from example 5.

[0030] DETAILED DESCRIPTION

[0031] The invention relates to metal oxide deposited graphene materials. The invention provides electrochemical methods for the production of these materials by anodic exfoliation of graphite, and electrocatalysts comprising such materials. The invention further provides uses and applications of metal oxide deposited graphene materials. The inventors have found that graphene sheets decorated with certain metal oxides, including mono- and mixed-metal oxides, have desirable electrochemical properties, making them potentially useful in battery technology and catalysis, to name two applications.

[0032] The methods of the invention provide in situ electrochemical exfoliation and decoration of graphene in an anodic process through inclusion of metal salt in the exfoliation process and control of pH.

[0033] Previous electrochemical exfoliation methods have only been successful with certain metal salts, whilst other metals (e.g. cobalt and iron) cannot be successfully deposited. The inventors have developed an inventive method which allows successful deposition of oxides of these, and other, metals, onto graphene materials. This opens up the range of mono- and mixed-metal oxides that can be deposited to include less expensive and more abundant metals, as well as those that provide superior properties for certain applications such as electrocatalysis.

[0034] Suitable salts include salts which are readily available, inexpensive, and easy to handle. The inventors have found that the resultant products have comparatively low levels of graphene oxide character, which improves properties for many applications, and have surfaces decorated with metal oxide products which alter the electrochemical properties of the products.

[0035] The invention provides a method for the production of metal oxide deposited graphene and / or graphite nanoplatelet structures having a thickness of less than 100 nm, the method comprising: i. providing an electrochemical cell, 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 metal cation; ii. passing a current through the cell to intercalate anions into the graphitic positive electrode so as to exfoliate the graphitic positive electrode; and iii. increasing the pH of the electrolyte, such that the metal ion is deposited in the form of the corresponding metal oxide to produce the metal oxide deposited graphene and / or graphite nanoplatelet structures.

[0036] Suitably, the electrolyte is an aqueous solution. In other words, suitably the electrolyte is a salt solution in water. The electrolyte comprises a metal cation for electrodeposition onto the nascent graphene surface as the corresponding metal oxide and an intercalating anion for intercalating into the graphitic anode so as to cause exfoliation.

[0037] The metal cation

[0038] The electrolyte comprises a metal cation. The metal cation is generally provided in the form of a salt. In some cases, more than one salt is used, for example to produce the mixed- metal oxide-decorated products of some embodiments of the present invention. Suitably, the metal salt is provided as an aqueous solution, which is used as the electrolyte.

[0039] The cation component of the salt is electrodeposited on the nascent graphene surface during or after the electrochemical process, in the form of a metal oxide. Advantageously, the present invention allows for the successful deposition of metal cations that have not been possible in previous electrochemical deposition processes. These include cobalt, iron, nickel, zinc, aluminium, vanadium and copper, preferably cobalt, iron, nickel, zinc, aluminium and copper. More preferably, the metal cation is cobalt or iron, still more preferably cobalt. Cobalt is preferred as it is relatively abundant and inexpensive, and is an excellent candidate for electrocatalysis, either alone or as part of a mixed oxide.

[0040] Without wishing to be bound by any particular theory, the inventors speculate that the positive biased voltage at the anode causes oxidation of the metal cation species alongside water oxidation. The higher oxidation state metal cation is thought to react with the water, forming the corresponding metal oxide and electrodepositing onto the nascent graphene layers and or the surface of the electrode. This reaction is thought to prevent or reduce reaction between the reactive products of water oxidation and the graphene or graphite at or in the vicinity of the anode, thereby reducing oxidation of the carbon structure of the product.

[0041] In previous processes, certain metal oxides (such as cobalt), once formed in such a process, have not been retained on the surface of the graphene / graphite product. Rather, they have been dissolved from the exfoliated surface, leading to non-functionalised products.

[0042] In the case of cobalt, it is believed that 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 the 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 Co3O4, may function as an electrocatalyst for water oxidation and the deposit may be a metastable mixed cobalt oxide. Any metastable electrodeposits have previously been found to dissolve from the exfoliated surface, leaving behind a near pristine few layer graphene substantially free of metal deposits. Analogous mechanisms are likely to apply to other metal cations (for example iron) for which electrodeposition onto graphene has been unsuccessful.

[0043] The method of the present invention unexpectedly prevents and / or reverses the dissolution of the oxide deposits, thereby leading to a metal oxide-deposited graphene product.

[0044] The method would thus be expected to work for other metals whose oxide deposits ordinarily are not retained on the graphene surface due to similar mechanistic considerations. Iron is one such metal.

[0045] In preferred methods, the electrolyte includes a cation of a metal selected from cobalt, iron, nickel, zinc, aluminium, vanadium and copper, preferably cobalt, iron, nickel, zinc, aluminium and copper. Oxidation states for stable salts of these metals are known in the art. For example, the electrolyte may include a cation selected from Co (II), Fe (III), Fe (II), Ni (II), Zn (II), Al (III), V (III) or Cu (II)..

[0046] Mixtures of metal cations may also be used, and in some cases may be preferred, for example for the superior properties of mixed-metal oxide-deposited graphene / graphite products as electrocatalysts. The inventors have demonstrated that mixed-metal oxide products are possible.

[0047] Thus, in some methods of the invention, the electrolyte preferably further comprises a second metal cation so as to produce mixed-metal oxide deposited graphene and / or graphite nanoplatelet structures. In this case the metal cation hereinbefore described may be referred to as the first metal cation. The second metal cation is different from the first metal cation and may preferably be selected from cobalt, iron, nickel, zinc, aluminium, vanadium, copper, manganese, ruthenium, iridium, tin, silver, platinum, palladium and gold. More preferably, the second metal cation is selected from cobalt, manganese, ruthenium and iridium. Preferably, the first metal cation is cobalt or iron (e.g. cobalt) and the second metal cation is manganese, ruthenium or iridium. The second metal cation may preferably be manganese. Alternatively, the second metal cation may preferably be iridium.

[0048] For example, the electrolyte may include two cations independently selected from Co (II), Fe (II), Fe (III), Ni (II), Al (III), V (III), Cu (II), Ru (III), Mn (II), Ir (III), Sn (II), Ag(l), Pt (IV), Pd (II) and Au (III). Preferably, the electrolyte includes a Co (II) cation and a Mn (II) cation.

[0049] In some methods of the invention, the electrolyte may additionally comprise further metal cations, e.g. a third metal cation, that are different from the first and second metal cations. In this case, suitable further metal cations are as described above with respect to the first and second metal cations. Suitable counterions for the first and (where used) second (third etc) metal cations will be apparent to the skilled person and include, for example, sulfate, chloride and nitrate. Nitrates and sulfates, particularly sulfates, are preferred. A preferred first metal salt is cobalt sufate. A preferred second metal salt is manganese sulfate.

[0050] The extent of decoration on the graphene surface may be controlled by controlling the concentration of the metal ion(s) to be deposited as metal oxide. Accordingly, it will be appreciated that the concentration of metal ion(s) to be deposited as metal oxide may be varied to suit the desired product of the process.

[0051] In some embodiments, the concentration of metal ion(s) to be deposited as metal oxide (i.e. the concentration of metal ion(s) in the electrolyte) is between 5 and 50 mM, for example between 5 and 40 mM, for example between 5 and 35 mM, for example between 10 and 30 mM, for example between 15 and 25 mM. In some cases, it is about 20 mM. In some cases, it is higher, for example, about 25 mM or even about 30 mM. The inventors have found that about 20 mM is an especially useful concentration. Accordingly, a range of 15 mM to 25 mM may be preferred.

[0052] In some cases, more than one metal ion is used to produce a mixed-metal oxide structure. For example, cobalt and manganese may be used as exemplified herein. In those cases, it will be understood that the values in the preceding paragraphs refer to the combined concentration of metal ion(s) to be deposited as metal oxide. Where two metals are used, the ratio may be about 1 : 1, or the ratio may be varied. For example, in some embodiments, the concentration of cobalt ions is about 10 mM and the concentration of manganese ions is about 10 mM.

[0053] The intercalating anion

[0054] 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 WO 2015 / 158711 which is herein incorporated by reference in its entirety for all purposes. Anions may include anions derived from sulfonic acid and sulfuric acid, including sulfuric acid monoesters.

[0055] In some embodiments, the electrolyte comprises sulfate anions. In other words, the electrolyte may include a sulfate salt.

[0056] The anion may be provided in the form of a 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. 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.

[0057] 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.

[0058] 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. pH adjustment

[0059] The method of the invention includes the step of increasing the pH of the electrolyte, such that the metal ion is deposited in the form of the corresponding metal oxide to produce the metal oxide deposited graphene and / or graphite nanoplatelet structures.

[0060] The inventors have surprisingly found that by increasing the pH of the electrolyte the lasting deposition of metal oxides on the graphene / graphite surface can be successfully achieved. This allows for the production of metal oxide-deposited products in the case of metals such as cobalt that previously have not been successfully produced via similar methods.

[0061] Without wishing to be bound by theory, it is thought that the initial electrolytic graphite exfoliation results in a low pH environment which favours transformation of certain metal oxides back to their cationic form. This effect may be most pronounced in the case of metastable metal oxides, such as cobalt oxides (e.g. CO3O4). This leads to the dissolution of metal oxides formed during the exfoliation back into solution, and thus the removal of the metal oxide from the graphene / graphite surface. For example, the electrolytic media may change from neutral (e.g. pH 6.4) prior to exfoliation to acidic (e.g. pH 1.5) following exfoliation.

[0062] Increasing the pH of the electrolyte may shift the balance in favour of the metal oxides, reducing or reversing their transformation back to the cationic form and subsequent dissolution from the surface. Thus, the metal remains in its oxide form on the surface of the graphene / graphite, so enabling production of functionalised graphene products.

[0063] The pH may be increased to any value that achieves the lasting deposition of the metal oxide on the graphene / graphite surface to provide the metal oxide-deposited product. This may vary depending on the nature of the metal. Preferably, the pH is increased to at least pH 3, at least pH 4, at least pH 5 or at least pH 6. In some preferred methods, the pH is increased to from pH 3 to pH 14, from pH 4 to pH 13, from pH 5 to pH 12 or from pH 6 to pH 10, e.g. from pH 6 to pH 8. Adjustment to too low a pH may prove ineffective to prevent or reverse the transformation of metal oxides back to cationic form and their subsequent dissolution. Use of higher pHs may promote the formation of hydroxyl species away from the graphene / graphite surface. In each case this would result in products with less metal oxide functionalisation than may be desired.

[0064] The pH of the electrolyte may be measured by standard techniques known in the art, for example using pH paper or a pH probe. For ease of measurement, it may be expedient to remove an aliquot of electrolyte from the electrochemical cell prior and measure the pH of the aliquot.

[0065] The pH may advantageously be increased following the initial electrolytic exfoliation of the positive electrode, i.e. step (iii) of the method of the invention takes place after step (ii). It has been found that if the pH is increased before this, e.g. by utilising a basic electrolyte from the start, formation of metal hydroxide is promoted which then precipitates out of the solution and so does not lead to the desired functionalised products.

[0066] Step (iii) make take place after the entirety of step (ii) or after a part thereof, for example after an initial electrolytic exfoliation in step (ii). Advantageously, the method may comprise awaiting a drop in pH resulting from step (ii) before step (iii) takes place.

[0067] Indeed, in alternative embodiments, step (iii) may even take place before step (ii). While this may lead to the formation of metal hydroxide, metal can subsequently be solvated again following a drop in pH resulting from step (ii).

[0068] The pH may be adjusted in any suitable manner. Preferably, the pH is increased by addition of a base to the electrolyte. Exemplary bases include hydroxides such as hydroxides of alkali metals (e.g. sodium hydroxide, potassium hydroxide), particularly sodium hydroxide, but the skilled person will appreciate that any suitable base may be used. The base may be added to the electrolyte in the form of an aqueous solution.

[0069] Preferably, a current is passed through the cell whilst the pH of the electrolyte is increased. In preferred methods of the invention, increasing the pH of the electrolyte comprises adding a base to the electrolyte in the electrochemical cell, the method further comprising passing a current through the cell whilst the base is added to the electrolyte.

[0070] Where current is passed in both step (ii) and step (iii), intercalation of anions into the graphitic positive electrode to exfoliate the graphitic positive electrode may of course occur at the same time as production of metal oxide deposited graphene and / or graphite nanoplatelet structures at increased pH. Thus, expressed in another way, step (ii) and step (iii) may overlap.

[0071] Optionally, increasing the pH may comprise a plurality of additions of base to the electrolyte. Thus, the pH may be increased a plurality of times. The method may comprise monitoring the pH of the electrolyte and adding further base if needed to ensure the electrolyte is maintained at a threshold.

[0072] Suitably, the pH of the electrolyte may be increased and maintained above a threshold for a time-period. The threshold may be the increased pH.

[0073] Suitably, the threshold may be at least pH 3, at least pH 4, at least pH 5 or at least pH 6.

[0074] For example, the pH may be maintained for at least 1 minute, at least 10 minutes or at least 30 minutes or at least 1 hour.

[0075] Suitably, a current may be passed through the cell while the pH is maintained.

[0076] Advantageously, the pH of the electrolyte may be increased and maintained during electrolytic exfoliation of the positive electrode, i.e. step (iii) of the method overlaps with step (ii) of the method.

[0077] Optionally, the pH may be maintained substantially throughout electrolytic exfoliation of the positive electrode, i.e. step (ii).

[0078] It has been found that increasing the pH of the electrolyte whilst under electrochemical bias advantageously results in functionalised graphene / graphite products having a greater atomic % of metal deposited thereon. If the pH is increased without the simultaneous application of charge then some functionalisation is still observed in the product, which may still be useful, but this is to a lesser degree. Without wishing to be bound by theory, it is believed that the application of charge during the addition of base to the electrolyte favours the transformation of the cationic metal species in solution back to its metal oxide form for deposition on the graphene / graphite surface. In the absence of charge, it is thought that increasing the pH promotes deposition of existing metal oxide species on the graphene / graphite surface but may not promote the formation of additional metal oxides from cationic species in solution, or not to the same extent.

[0079] Alternatively, therefore, no charge is applied whilst the pH of the electrolyte is increased, although this is not preferred. In such a method, the step of increasing the pH may, but does not have to, take place within the electrochemical cell. For example, the electrolyte, or part of it (e.g. the anolyte) may be removed from the electrochemical cell and then a base added to it to increase its pH. Isolation and processing of product

[0080] The method of the invention 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 entire electrolyte solution may be treated in this way, or just part of it, for example the anolyte.

[0081] Preferred methods of the invention further comprise heating the metal oxide deposited graphene and / or graphite nanoplatelet structures to a temperature of from 100 °C to 800 °C, preferably from 150 °C to 500 °C, e.g. from 200 °C to 400 °C. Heating the product advantageously drives off any water, promoting formation of desired metal oxides over hydroxides on the surface of the graphene / graphite product. The heating may be carried out in air or in an inert atmosphere such as nitrogen or argon. An inert atmosphere may allow for higher temperatures (e.g. up to 800 °C) to be used. In air, temperatures of 100 °C to 500 °C or 200 °C to 400 °C are preferred.

[0082] The product may be re-dispersed for use or further processing, for example in DMF. Sonication may aid re-dispersal.

[0083] Further preferred features of the product formed by the method of the present invention are as described below with respect to the product and composition of the invention.

[0084] Product

[0085] The present invention also provides metal oxide deposited graphene and / or graphite nanoplatelet structures obtainable by the method of the invention as hereinbefore described. Further preferred features of this aspect are as described above with respect to the method of the invention, and as described below with respect to the composition of the invention.

[0086] The present invention also provides a composition comprising graphene and / or graphite nanoplatelet structures having a thickness of less than 100 nm, wherein the graphene and / or graphite nanoplatelet structures have metal oxide (e.g. metal oxide nanostructures) deposited on the basal surface, wherein the metal oxide comprises cobalt oxide, iron oxide, nickel oxide, zinc oxide, aluminium oxide, vanadium oxide, copper oxide or mixtures thereof.

[0087] Preferably, the metal oxide comprises cobalt oxide, iron oxide, nickel oxide, zinc oxide, aluminium oxide, copper oxide or mixtures thereof, more preferably cobalt oxide and / or iron oxide, still more preferably cobalt oxide. The metal oxide may comprise one or more oxides of a single metal (e.g. cobalt) only, i.e. it may be a mono-metal oxide, or may comprise oxides of two or more metals, i.e. it may be a mixed-metal oxide. Graphene / graphite products functionalised with oxides of more than one metal may be preferred for their high catalytic activities.

[0088] The metal oxide may, therefore, further comprise cobalt oxide, iron oxide, nickel oxide, zinc oxide, aluminium oxide, vanadium oxide, copper oxide, manganese oxide, ruthenium oxide, iridium oxide, tin oxide, silver oxide, platinum oxide, palladium oxide and gold oxide.

[0089] The metal oxide may preferably comprise oxides of at least two metals selected from cobalt, iron, nickel, zinc, aluminium, vanadium, copper, manganese, ruthenium, iridium, tin, silver, platinum, palladium and gold, wherein at least one of the metals is cobalt, iron, nickel, zinc, aluminium, vanadium or copper. Preferably at least one of the metals is cobalt, iron, nickel, zinc, aluminium or copper, more preferably cobalt or iron, still more preferably cobalt. Preferably at least one of the metals is manganese, ruthenium or iridium, e.g. manganese. The oxides may comprise more than two metals, e.g. three metals, in which case each metal is preferably selected from the aforementioned list.

[0090] In one preferred product of the invention, the metal oxide comprises cobalt oxide and manganese oxide.

[0091] It will be appreciated that when the graphene and / or graphite nanoplatelet structures have mixed-metal oxides deposited thereon, this may include multiple separate oxides each containing a single type of metal (for example, CowOxplus MnyOz) and / or oxides containing multiple metal species (for example CoaMnbOc). The metal oxides may also contain different oxides of a given metal species, for example with the metal in different oxidation states. An example of a mixed valence mono-metal oxide is Co3O4.

[0092] The metal oxide deposited graphene and / or graphite nanoplatelet structures may preferably 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. In other words, as compared to conventional anodic exfoliation methods, the inclusion of the metal leads to exfoliation of a product with reduced graphene oxide character.

[0093] 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.

[0094] The graphene and / or graphite nanoplatelet structures may comprise at least 0.5 atm%, preferably at least 0.75 atm%, more preferably at least 1 atm%, still more preferably at least 1.5 atm%, e.g. at least 2 atm% of the metal (e.g. cobalt). The graphene and / or graphite nanoplatelet structures may comprise up to 20 atm%, preferably up to 15 atm%, or up to 10 atm% of the metal (e.g. cobalt). The graphene and / or graphite nanoplatelet structures may comprise from 0.5 atm% to 20 atm%, preferably from 0.75 atm% to 15 atm%, more preferably from 1 atm% to 10 atm%, still more preferably from 1.5 atm% to 5 atm% of the metal (e.g. cobalt). Where the graphene and / or graphite nanoplatelet structures have more than one metal deposited thereon, these atomic percentages (atm%) may refer to the atomic percentage of each individual metal, or to the combined total of the metals.

[0095] The graphene and / or graphite nanoplatelet structures may comprise up to 96 atm%, preferably up to 95 atm%, more preferably up to 92.5 atm%, still more preferably up to 91 atm% of carbon. The graphene and / or graphite nanoplatelet structures may comprise from 30 atm% to 96 atm%, preferably from 50 atm% to 95 atm%, more preferably from 70 atm% to 92.5 atm%, still more preferably from 80 atm% to 91 atm% of carbon.

[0096] The graphene and / or graphite nanoplatelet structures may comprise at least 2 atm%, preferably at least 3 atm%, more preferably at least 4 atm%, still more preferably at least 6 atm%, e.g. at least 7 atm% of oxygen. The graphene and / or graphite nanoplatelet structures may comprise up to 60 atm%, preferably up to 40 atm%, more preferably up to 30 atm%, still more preferably up to 20 atm%, e.g. up to 15 atm% of oxygen. The graphene and / or graphite nanoplatelet structures may comprise from 2 atm% to 60 atm%, preferably from 3 atm% to 40 atm%, more preferably from 4 atm% to 20 atm%, still more preferably from 6 atm% to 15 atm% of oxygen.

[0097] The atomic percentages of metals, carbon and oxygen in the product may refer to the composition at the surface of the graphene and / or graphite nanoplatelet structures, and may be measured using X-ray photoelectron spectroscopy (XPS), e.g. as described in the examples below.

[0098] The graphene and / or graphite nanoplatelet structures may comprise at least 10 mass% of metal oxide, at least 20 mass% of metal oxide or at least 30 mass% of metal oxide, e.g. at least 40 mass% of metal oxide. The graphene and / or graphite nanoplatelet structures may comprise up to 80 mass% of metal oxide, or up to 70 mass% of metal oxide, or up to 60 mass% of metal oxide, e.g. up to 50 mass% of metal oxide. The graphene and / or graphite nanoplatelet structures may comprise from 10 mass% to 80 mass%, or from 20 mass% to 70 mass%, or from 30 mass% to 60 mass%, e.g. from 40 mass% to 50 mass% of metal oxide.

[0099] The mass% of metal oxide may refer to the residual mass% of metal oxide and may be measured by thermal gravimetric analysis (TGA), e.g. as described in the examples below. In the case of mixed-metal oxides the mass% may refer to the combined total mass% of all metal oxides.

[0100] Applications and uses of product

[0101] The metal oxide deposited graphene and / or graphite nanoplatelet structures and compositions of the invention have the potential for utility in widespread applications.

[0102] The metal oxide deposited graphene and / or graphite nanoplatelet structures and compositions of the invention are particularly well suited for electrocatalysis applications. The graphene structure advantageously provides thermal and electrical conductivity and a base to efficiently distribute the metal oxides. These include electrocatalysis for overall water splitting in alkaline, proton exchange membrane (PEM), and solid-oxide electrolysers.

[0103] The invention thus also provides an electrocatalyst (e.g. an oxygen evolution electrocatalyst) comprising the metal oxide deposited graphene and / or graphite nanoplatelet structures or the composition of the invention. The electrocatalyst may comprise an electrode comprising a metal substrate coated with the metal oxide deposited graphene and / or graphite nanoplatelet structures or the composition of the invention.

[0104] The invention also provides use of the metal oxide deposited graphene and / or graphite nanoplatelet structures or the composition of the invention in electrocatalysis (e.g. in oxygen evolution electrocatalysis).

[0105] The materials may be useful as electrode material in supercapacitor devices.

[0106] Accordingly, in a further aspect, the invention may provide a supercapacitor having an electrode comprising a composition as described herein.

[0107] Additional applications include combustion catalysts, use of the materials as industrial catalysts (for example in peroxide formation), photocatalysts, lithium battery cathodes, and fuel cell catalysts.

[0108] Definitions and Further Details

[0109] Graphene

[0110] 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.

[0111] In some cases, electrochemical methods as described herein usefully produce 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 50 nm 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.

[0112] The corresponding "bulk" material of graphene is graphite. This typically consists of thousands of layers of graphene.

[0113] Decorated graphene

[0114] The term decorated graphene, and the associated term decorated graphite nanoplatelets, is used herein to described graphene (and / or nanoplatelets) having a metal oxide deposited on the layer surface. The metal oxide is electrodeposited during the electrochemical exfoliation process. It will be appreciated that, except where context clearly dictates otherwise, the term metal oxide refers to both mono-metal oxides and mixed-metal oxides.

[0115] As described herein, the amount of deposited metal oxide may be controlled by adjusting the concentration of metal ion in the electrolyte. It will be appreciated that the morphology of the deposited metal oxide may vary with the degree of deposition (which may be controlled by the concentration of the metal ion) and the nature of the oxide or metal oxides. Morphologies obtained may include nanoparticles, nanowhiskers, lamellar honeycombs and petal structures. Deposited metal oxide structures may be spread uniformly across the graphene surface, or may be arranged in aggregates on the surface.

[0116] In some embodiments, the deposited metal oxide structure is a honeycomb lamellar structure. The thickness may be between 1.5 nm and 2.5 nm. In some embodiments, it is about 2 nm.

[0117] In some embodiments, the deposited metal oxide structure is metal oxide nanoparticles. The average size of the metal oxide nanoparticles may be between 1.5 nm and 2.5 nm. In some embodiments, it is about 2 nm.

[0118] In the case of mixed-metal oxide deposited graphene / graphite nanoplatelet structures, combinations of these, or other, structures may occur.

[0119] The metal oxide deposited on the graphene / graphite nanoplatelet structures may contain a given metal in a single oxidation state or in more than one oxidation state (i.e. a mixed oxide). Electrode

[0120] 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 as 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 interlayer 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 nm 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.

[0121] 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.

[0122] 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.

[0123] The cathode (negative electrode) may be graphitic or another material. For example, the negative electrode may be a graphite rod. Alternatively, the negative electrode may be a material other than graphite, for example a metal such as platinum or titanium. A metal negative electrode, such as titanium, may be preferred for the increased stability it provides in the cell.

[0124] A reference electrode may be used. Exfoliation Cell

[0125] 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 or by another suitable material such as microporous polypropylene felt. 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. It also ensures that any functionalisation of the nascent exfoliated graphene is solely due to the codeposition of metal oxides at the anode rather than due to physical mixtures from the metal hydroxides generated at the cathode.

[0126] EXAMPLES

[0127] Materials and Reagents

[0128] Cobalt (II) sulfate heptahydrate (>99%), anhydrous sodium sulfate (>98%), manganese (II) sulfate tetrahydrate (>99%), sodium hydroxide (>98%), and iso-propyl alcohol (99%) were purchased from Sigma-Aldrich and used with no further purification. Graphite foil was obtained from Gee Graphite. Vein graphite was obtained from First Graphene Ltd. Nylon filters (45 pm pore size, 90 mm diameter) were obtained from Cole-Parmer.

[0129] Characterisation

[0130] Thermal gravimetric analysis was carried out using a TGA 5500 (TA Instruments), in air, at 20 °C min L Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were carried out using an SU5000 SEM (Hitachi) and a QUANTAX (Bruker). X-ray photoelectron spectroscopy (XPS) was performed by the University of Manchester Photon Science Institute using Al Ko X-rays (1.486 keV photon energy, 150 W) in a in a ESCA2SR spectrometer (ScientaOmicron GmbH).

[0131] For analysis using SEM / EDX, the sample was dispersed in iso-propyl alcohol at a concentration of 1 mg mL-1and drop-cast onto an Si / SiO2wafer for imaging of isolated flakes. The wafer was left to dry in air. TGA and XPS were measured using powder samples after processing and purification.

[0132] Example 1 - graphite foil anode - alkaline addition under electrochemical bias

[0133] Graphite foil was expanded before use as an anode material. First, a piece of graphite foil (3 cm x 15 cm) was immersed in liquid nitrogen for 60 seconds, then placed immediately into a beaker of water. Rapid gas evolution caused the surface of the foil to expand significantly, increasing the available surface area for exfoliation and functionalisation.

[0134] Electrochemical exfoliation was carried out in a bespoke H-cell, with compartments separated by microporous polypropylene felt. The power source was a programmable power supply (GW Instek, PSP-405). Expanded graphite foil was used as the anode, and a titanium plate was used as the cathode. 200 mL of an aqueous electrolytic solution of cobalt (II) sulfate heptahydrate (20 mM) and anhydrous sodium sulfate (0.5 M) was used, which was pink-red in colour. The initial pH was neutral. A potential of 20 V was applied for 1 hour. During this time, a blue-green precipitate (likely cobalt (II) hydroxide) formed at the cathodic side, which turned grey-pink and sedimented. Graphene was exfoliated at the anode to form a black dispersion.

[0135] After an hour, while the cell was still under electrochemical bias, sodium hydroxide solution (2M, 100 mL) was added via pipette over the course of a minute. Prior to addition of NaOH, the pH was 1. After addition of NaOH, the solution was neutralised. When all of the NaOH was added, the potential was kept at 20 V for a further 10 minutes before processing and purification. During this time, the anodic graphene dispersion turned brown-black.

[0136] The anolyte and catholyte were collected separately from the cell using a syringe. The anolyte was filtered under reduced pressure through a 45 pm Nylon filter in a glass / sintered glass filtration unit. The filtrate was clear and colourless, with the pink colour (attributed to Co2+) completely gone during exfoliation / functionalisation. The resulting powder sample was washed with 4 litres of deionised water and dried at 100 °C in an oven. Finally, the powder was calcinated at 250 °C overnight.

[0137] Example 2- graphite foil anode - alkaline addition as a post-treatment

[0138] Graphite foil was expanded before use as an anode. First, a piece of graphite foil (3 cm x 15 cm) was immersed in liquid nitrogen for 60 seconds, then placed immediately into a beaker of water. Rapid gas evolution caused the surface of the foil to expand significantly, increasing the available surface area for exfoliation and functionalisation.

[0139] Electrochemical exfoliation was carried out in a bespoke H-cell, with compartments separated by microporous polypropylene felt. The power source was a programmable power supply (GW Instek, PSP-405). Expanded graphite foil was used as the anode, and a titanium plate was used as the cathode. 200 mL of an aqueous electrolytic solution of cobalt (II) sulfate heptahydrate (20 mM) and anhydrous sodium sulfate (0.5 M) was used, which was pink-red in colour. The initial pH was neutral. A potential of 20 V was applied for 1 hour. During this time, a blue-green precipitate (likely cobalt (II) hydroxide) formed at the cathodic side, which turned grey-pink and sedimented. Graphene was exfoliated at the anode to form a black dispersion. After an hour, the anolyte and catholyte were collected separately from the cell using a syringe. The anolyte (pH =1) was mixed with aqueous sodium hydroxide solution (2M, 100 mL) to produce a pH neutral solution and left overnight before processing and purification.

[0140] The anolyte was filtered under reduced pressure through a 45 pm Nylon filter in a glass / sintered glass filtration unit. The filtrate was clear and colourless, with the pink colour (attributed to Co2+) completely gone during exfoliation / functionalisation. The resulting powder sample was washed with 4 litres of deionised water and dried at 100 °C in an oven. Finally, the powder was calcinated at 250 °C overnight.

[0141] Example 3 - graphite foil anode - exfoliation in Co2+without alkaline treatment

[0142] Electrochemical exfoliation was carried out as described in the work by Ejigu et al., in a bespoke H-cell, with compartments separated by microporous polypropylene felt. The power source was a programmable power supply (GW Instek, PSP-405). Expanded graphite foil was used as the anode, and a titanium plate was used as the cathode. 200 mL of an aqueous electrolytic solution of cobalt (II) sulfate heptahydrate (20 mM), and anhydrous sodium sulfate (0.5 M) was used for exfoliation. The initial pH was neutral. A potential of 20 V was applied for 1 hour. The final pH was 1.

[0143] The anolyte was filtered under reduced pressure through a 45 pm Nylon filter in a glass / sintered glass filtration unit. The filtrate was transparent and pink-red, visually unchanged from the electrolyte before exfoliation. The resulting powder sample was washed with 4 litres of deionised water and dried at 100 °C in an oven. Finally, the powder was calcinated at 250 °C overnight.

[0144] Examples 1-3: Results and discussion

[0145] Figure 1 A shows XPS overall scans for samples from examples 1, 2 and 3. Figure IB shows a table of atomic percentages of Co, O and C found in samples from examples 1, 2 and 3; Figure 1C shows high resolution XPS in the Co 2p binding region for samples from examples 1 and 2.

[0146] XPS was performed for samples produced by the methods defined in examples 1, 2 and 3. Overall scans (Figure 1A) were taken within the binding energy range of 0 - 1300 eV, with high resolution spectra recorded in the Co 2p, O Is and C Is regions. All binding energy values were calibrated to an adventitious carbon binding energy value of 284.5 eV. Examples 1 and 2 both show significant Co 2p peak presence (Figure 1C), with atomic percentages of 2% and 0.85% respectively (Figure IB). In both cases, peak shapes and positioning were comparable, and mixed cobalt oxide phases (expected to include CoO and CO3O4) were present. The lower atomic percentage found in example 2 suggests that the post-exfoliation alkaline treatment is less effective for deposition of CoOxspecies. Example 3 is provided as proof of concept to show that alkaline treatment is essential for significant surface functionalisation with CoOx. With no pH neutralisation, electrochemical exfoliation proceeds with minimal quantities (0.13%) of CoOxretained on the surface of the graphene.

[0147] Figures 2A-D show SEM images of an isolated flake from example 1, with increasing magnification. Figures 3A-D show EDX elemental mapping of C (Figure 3A), Co (Figure 3B), and O (Figure 3C) for the isolated flake from example 1.

[0148] The SEM images and EDX elemental maps were taken of isolated flakes of graphene functionalised with CoOx, produced via the methodology defined in example 1. Figure 2 shows SEM images, taken at 2000 (Figure 2A), 10000 (Figure 2B), 20000(Figure 2C) and 40000 (Figure 2D) x magnification. They show an example of a graphene flake (approximately 25 pm lateral size), with a highly uniform, crystalline coverage of nanoparticles across the surface.

[0149] Figure 3 shows EDX elemental mapping of the same flake and shows cobalt, localised to the flake and uniform across the surface. Oxygen and carbon are both expected in an EDX map from the background, but both are shown as localised and more densely concentrated at the points expected from the imaged flake. Atomic and mass ratios of carbon to cobalt are expected to vary when imaging isolated flakes.

[0150] Figure 4A shows TGA plots for samples from examples 1, 2 and 3. Figure 4B is a table of residual mass percentage of CoOxfor samples from examples 1, 2 and 3.

[0151] TGA was carried out in air, at a heat ramping rate of 20 °C min_1, for samples produced by the methodology defined in examples 1, 2, and 3. The resulting plots show the rate of change to the samples' mass as the temperature increased (figure 4A). The final readings at 1000 °C (figure 4B) show the residual mass of CoOxspecies, after all graphitic carbon had combusted. Examples 1 and 2 are shown to consist of 47% and 31% CoOxspecies by mass, respectively. This suggests that the post-exfoliation alkaline treatment is less effective for deposition of CoOxspecies than the addition of OH- under an electrochemical bias. Example 3 is provided as proof of concept to show that alkaline treatment is essential for surface functionalisation with CoOx. With no pH neutralisation, electrochemical exfoliation proceeds with minimal quantities (1%) of CoOxretained on the surface of the graphene.

[0152] Example 4 - vein graphite anode - alkaline addition under electrochemical bias

[0153] Electrochemical exfoliation was carried out in a bespoke H-cell, with compartments separated by microporous polypropylene felt. The power source was a programmable power supply (GW Instek, PSP-405). Vein graphite was used as the anode, and a titanium plate was used as the cathode. 200 mL of an aqueous electrolytic solution of cobalt (II) sulfate heptahydrate (20 mM) and anhydrous sodium sulfate (0.5 M) was used for simultaneous exfoliation and surface functionalisation. The initial pH was neutral. A potential of 20 V was applied for 1 hour. During this time, a blue-green precipitate (likely cobalt (II) hydroxide) formed at the cathodic side, which turned grey-pink and sedimented. Graphene was exfoliated at the anode to form a black dispersion.

[0154] After an hour, while the cell was still under electrochemical bias, sodium hydroxide solution (2M, 100 mL) was added via pipette over the course of a minute. Prior to NaOH addition, the pH was 1. When all of the NaOH was added, to form a pH neutral solution, the potential was kept at 20 V for a further 10 minutes.

[0155] Samples produced via the methodology in example 4 are analogous to those produced in example 1 - the primary difference is the graphite source used as the anode material.

[0156] Example 5 - graphite foil anode - to create a mixed metal oxide

[0157] Electrochemical exfoliation was carried out in a bespoke H-cell, with compartments separated by microporous polypropylene felt. The power source was a programmable power supply (GW Instek, PSP-405). Expanded graphite foil was used as the anode, and a titanium plate was used as the cathode. 200 mL of an aqueous electrolytic solution of cobalt (II) sulfate heptahydrate (10 mM), manganese (II) sulfate tetrahydrate (10 mM) and anhydrous sodium sulfate (0.5 M) was used for simultaneous exfoliation and surface functionalisation. The initial pH was neutral. A potential of 20 V was applied for 1 hour. During this time, a mixture of blue-green and brown precipitates (likely cobalt (II) hydroxide and manganese (II) hydroxide respectively) formed at the cathodic side, which turned grey-pink / brown and sedimented. Graphene was exfoliated at the anode to form a black dispersion.

[0158] After an hour, while the cell was still under electrochemical bias, sodium hydroxide solution (2M, 100 mL) was added via pipette over the course of a minute. Prior to NaOH addition, the pH was 1.5. When all of the NaOH was added to form a neutral solution, the potential was kept at 20V for a further 10 minutes.

[0159] Example 5: Results and discussion

[0160] Figure 5 shows results for a sample from example 5: Figure 5A shows an XPS overall scan across the full binding energy range; Figure 5B is a table of atomic percentages of C, O, Mn, and Co; Figure 5C shows high resolution Co 2p spectra; Figure 5D shows high resolution Mn 2p spectra.

[0161] XPS was performed for the sample produced by the method defined in 5. Overall scans were taken within the binding energy range of 0 - 1300 eV (figure 5A), with high resolution spectra recorded in the Co 2p, Mn 2p, O Is, and C Is regions. All binding energy values were calibrated to an adventitious carbon binding energy value of 284.5 eV. Co and Mn were detected at an atomic ratio of roughly 1 : 1 (figure 5B). Mixed cobalt oxide phases (expected to include CoO and Co3O4) were present (figure 5C). Mn is expected to be present in the form of MnO2(figure 5D).

[0162] Figure 6 shows results for a sample from example 5; Figure 6A shows SEM imaging of nanoparticles found on an isolated flake along with an EDX elemental mapping table for the image; Figures 6B-E show EDX elemental maps with C (Fig 6C), Mn (Fig 6D) Co (Fig 6E) and all three (Fig 6B) for the image in Fig 6A.

[0163] The SEM image and EDX elemental map was taken of an isolated flake of graphene functionalised with CoOxand MnOy, produced via the methodology defined in example 5. The image shows a highly uniform, crystalline coverage of nanoparticles across the surface (figure 6A). The nanoparticles are confirmed as consisting of manganese and cobalt oxide species from EDX elemental mapping (figure 6B-E). Both elements are uniform in coverage across the surface of the graphene. The detected atomic percentage of manganese is present is approximately 5 times that of cobalt, but this varies across different imaging regions (as can be seen from the table in figure 6A). Oxygen and carbon are both expected in an EDX map from the background, but both are shown as localised and more densely concentrated at the points expected from the imaged flake. Figure 7 shows a TGA plot for a sample from example 5. TGA was carried out in air, at a heat ramping rate of 20 °C min-1, for samples produced by the methodology defined in example 5. The resulting plots show the rate of change to the samples' mass as the temperature increased. The final reading at 1000 °C shows the residual mass of CoOxand MnOyspecies, after all graphitic carbon had combusted, which totals 57%.

[0164] Example 6 - increased overlap of electrochemical exfoliation and functionalisation: The effectiveness of increasing pH shortly after the start of exfoliation was explored with a further experiment by comparing two methods.

[0165] Method A

[0166] Electrochemical exfoliation was carried out in a bespoke H-cell, with compartments separated by microporous polypropylene felt. The power source was a programmable power supply (GW Instek, PSP-405). Either vein graphite or expanded commercial graphite foil was used as an anode, and a titanium plate was used as the cathode. 200 mL of an aqueous electrolytic solution of sodium sulfate (0.5 M) was used, with a total of 20 mM metal salt or salts. A potential of 20 V was applied for 1 hour. Graphene was exfoliated at the anode to form a black dispersion, and for all metal salts a precipitate formed at the cathode.

[0167] After an hour, while the cell was still under electrochemical bias, sodium hydroxide solution (2M, 100 mL) was added via pipette over the course of a minute. When all of the NaOH was added, the potential was kept at 20 V for a further 10 minutes before processing and purification.

[0168] The anolyte and catholyte were collected separately from the cell using a syringe. The anolyte was filtered under reduced pressure through a 45 pm Nylon filter in a glass / sintered glass filtration unit. For exfoliations in each metal salt, filtrate was clear and colourless, with any associated colour (e.g. red-pink, attributed to Co2+) completely gone during exfoliation / functionalisation. The resulting powder sample was washed with 4 litres of deionised water and dried at 100 °C in an oven. Finally, the powder was calcinated at 250 °C overnight.

[0169] Method B

[0170] Electrochemical exfoliation was carried out in a bespoke H-cell, with compartments separated by microporous polypropylene felt. The power source was a programmable power supply (GW Instek, PSP-405). Either vein graphite or expanded commercial graphite foil was used as an anode, and a titanium plate was used as the cathode. 200 mL of an aqueous electrolytic solution of sodium sulfate (0.5 M) was used, with a total of 20 mM metal salt or salts. The cell was equipped with a magnetic stirrer bar in the anodic chamber, and an in situ pH probe. A potential of 20 V was applied for 1 hour. Graphene was exfoliated at the anode to form a black dispersion, and for all metal salts a precipitate formed at the cathode. Sodium hydroxide solution (2M) was added under stirring over the course of the hour, to maintain a neutral pH throughout.

[0171] The anolyte and catholyte were collected separately from the cell using a syringe. The anolyte was filtered under reduced pressure through a 45 pm Nylon filter in a glass / sintered glass filtration unit. For exfoliations in each metal salt, filtrate was clear and colourless, with any associated colour (e.g. red-pink, attributed to Co2+) completely gone during exfoliation / functionalisation. The resulting powder sample was washed with 4 litres of deionised water and dried at 100 °C in an oven. Finally, the powder was calcinated at 250 °C overnight.

[0172] Example 6 - Results & discussion

[0173] The as-produced, functionalised graphene obtained from each of methods A and B was characterised using TGA. The weight percentage of residual cobalt oxide species was 58.38% in the product obtained from Method A, 10% higher than in the product obtained from method B.

[0174] References

[0175] 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.

[0176] Z.Y. Liu, Z.S. Wu, S. Yang, R.H. Dong, X.L. Feng, K. Mullen, Ultraflexible In-Plane Micro¬

[0177] Supercapacitors by Direct Printing of Solution-Processable Electrochemically Exfoliated Graphene, Adv. Mater. 28(11 ) (2016) 2217-2222.

[0178] A. Ejigu, K. Fujisawa, B. F. Spencer, B. Wang, M. Terrones, I. A. Kinloch, and R.A. W. Dryfe, Adv. Fund. Mater., 2018, 28, 1804357

Claims

CLAIMS1. A method for the production of metal oxide deposited graphene and / or graphite nanoplatelet structures having a thickness of less than 100 nm, the method comprising: i. providing an electrochemical cell, 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 metal cation; ii. passing a current through the cell to intercalate anions into the graphitic positive electrode so as to exfoliate the graphitic positive electrode; and iii. increasing the pH of the electrolyte to at least pH 3, such that the metal ion is deposited in the form of the corresponding metal oxide to produce the metal oxide deposited graphene and / or graphite nanoplatelet structures.

2. The method of claim 1, wherein, in step (iii), the pH of the electrolyte is increased to from pH 5 to pH 12.

3. The method of claim 1 or claim 2, wherein step (iii) takes place after the entirety of step (ii) or after a part thereof.

4. The method of any preceding claim, comprising awaiting a drop in pH resulting from step (ii) before step (iii) takes place.

5. The method of any preceding claim, wherein increasing the pH of the electrolyte comprises adding a base to the electrolyte in the electrochemical cell, the method further comprising passing a current through the cell whilst the base is added to the electrolyte.

6. The method of any preceding claim, wherein increasing the pH comprises a plurality of additions of base to the electrolyte.

7. The method of any preceding claim, comprising maintaining the pH of the electrolyte above a threshold for a time-period, optionally substantially throughout step (ii), the threshold optionally being at least pH 3.

8. The method of any preceding claim, wherein the metal cation is selected from cobalt, iron, nickel, zinc, aluminium, vanadium and copper.

9. The method of any preceding claim, wherein the metal cation is cobalt.

10. The method of any preceding claim, wherein the electrolyte further comprises a second metal cation so as to produce mixed-metal oxide deposited graphene and / or graphite nanoplatelet structures.

11. The method of claim 10, wherein the second metal cation is selected from cobalt, iron, nickel, zinc, aluminium, vanadium, copper, manganese, ruthenium, iridium, tin, silver, platinum, palladium and gold.

12. The method of claim 10, wherein the second metal cation is manganese.

13. The method of any preceding claim, wherein the concentration of metal cation(s) in the electrolyte is between 5 mM and 35 mM.

14. The method of any preceding claim, wherein the intercalating anion is sulfate.

15. The method of any preceding claim, wherein the concentration of intercalating anion in the electrolyte is 0.1 to 2.5 M.

16. The method of any preceding claim, further comprising heating the metal oxide deposited graphene and / or graphite nanoplatelet structures to a temperature of from 100 °C to 500 °C.

17. Metal oxide deposited graphene and / or graphite nanoplatelet structures obtainable by the method of any preceding claim.

18. A composition comprising graphene and / or graphite nanoplatelet structures having a thickness of less than 100 nm, wherein said graphene and / or graphite nanoplatelet structures have metal oxide deposited on the basal surface, wherein the metal oxide comprises cobalt oxide, iron oxide, nickel oxide, zinc oxide, aluminium oxide, vanadium oxide, copper oxide or mixtures thereof.

19. The composition of claim 18, wherein the metal oxide comprises cobalt oxide, iron oxide, nickel oxide, zinc oxide, aluminium oxide, copper oxide or mixtures thereof, preferably cobalt oxide.

20. The composition of claim 18 or claim 19, wherein the metal oxide comprises oxides of at least two metals selected from cobalt, iron, nickel, zinc, aluminium, vanadium, copper, manganese, ruthenium, iridium, tin, silver, platinum, palladium and gold, wherein at least one of the metals is cobalt, iron, nickel, zinc, aluminium, vanadium or copper.

21. The composition or method of any preceding claim, wherein the metal oxide deposited graphene and / or graphite nanoplatelet structures are substantially free of graphene oxide.

22. The composition or method of any preceding claim, wherein the graphene and / or graphite nanoplatelet structures comprise from 0.5 atm% to 10 atm% of metal.

23. The composition or method of any preceding claim, wherein the graphene and / or graphite nanoplatelet structures comprise from 70 atm% to 96 atm% of carbon.

24. The composition or method of any preceding claim, wherein the graphene and / or graphite nanoplatelet structures comprise from 3 atm% to 20 atm% of oxygen.

25. An electrocatalyst comprising the metal oxide deposited graphene and / or graphite nanoplatelet structures of claim 17, or the composition of any of claims 18 to 24.

26. Use of the metal oxide deposited graphene and / or graphite nanoplatelet structures of claim 17, or the composition of any of claims 18 to 24, in electrocatalysis.

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