This application is the U.S. National Stage of International Application No. PCT/EP2018/086698, filed on Dec. 21, 2018, which designates the U.S., is published in English, and claims priority under 35 U.S.C. § 119 or 365(c) to Great Britain Application No. 1721817.3, filed on Dec. 22, 2017 22. The entire teachings of the above applications are incorporated herein by reference.

FIELD OF THE INVENTION

The present 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. The invention further provides a protic ionic liquid and its use in supercapacitors.

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, 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.

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.

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 et 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 NH4Cl 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.

SUMMARY OF THE INVENTION

The invention relates to metal oxide deposited graphene materials, and use of these materials as electrodes and electrocatalysts. The invention provides straightforward electrochemical methods for the production of these materials by anodic exfoliation of graphite, novel metal oxide-decorated graphene materials, and electrodes and cells comprising such materials. The invention further provides uses 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.

Importantly, the methods of the invention provide in situ electrochemical exfoliation and decoration of graphene in a single stage anodic process through inclusion of metal salt in the exfoliation process. 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, and in some cases dramatically improve capacitance.

The inventors have shown that controlling the concentration of the salt can be used to control the extent of decoration, which may be useful for tuning the properties of the product.

Using the methods of the invention, the inventors have produced metal oxide decorated sheets having capacitance values significantly higher than those of conventional electrochemically exfoliated graphene.

The present invention relates to a method 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, wherein the cell comprises:

(a) a positive electrode which is graphitic;

(b) a negative electrode; and

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

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 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.

Suitably, the cation is the cation of a metal, the oxide of which is suitable for use as an electrocatalyst for oxygen evolution. Suitably, the cation is not a cation of a metal selected from cobalt, iron and nickel.

The cation may be the cation of a metal selected from ruthenium, manganese, iridium, vanadium, titanium, tin, and silver.

In a first aspect, the invention may provide a method 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, 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, wherein the metal is selected from ruthenium, manganese, iridium, tin, and silver;

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 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.

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 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.

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.

The Metal Cation

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

The cation component of the salt is electrodeposited on the nascent graphene surface during the electrochemical process, in the form of a metal oxide. A representative reaction is shown below.
Mn2++2H2O→MnO2+4H++2e−

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.

In some cases, the electrolyte includes a cation of a metal selected from ruthenium, manganese, iridium, tin, and silver. Oxidation states for stable salts of these metals are known in the art. For example, the electrolyte may include a cation selected from Ru (III), Mn (II), Ir (III), Sn (II) and Ag(I). As described elsewhere, vanadium and titanium may also be used. Oxidation states for stable salts of these metals are known in the art. Examples may be selected from Ti (II), Ti (IV), V (II), V (III), V (IV) and V (V). Suitable counterions will be apparent to the skilled person and include, for example, chloride and nitrate.

In some cases, the electrolyte includes a cation of a metal selected from ruthenium, manganese and iridium; for example Ru (III), Mn (II) or Ir (III). In some cases, the metal is ruthenium. In some cases, the metal is manganese. In some cases, the metal is iridium.

Mixtures of metal cations may also be used, and in some cases may be preferred. The inventors have demonstrated that mixed-oxide products are possible, and in at least some cases have surprising good electronic properties. In some cases, the electrolyte includes at least two cationic species which are cations of metals selected from ruthenium, manganese, iridium, tin, and silver. For example, the electrolyte may include two cations independently selected from Ru (III), Mn (II), Ir (III), Sn (II) and Ag(I). In some cases, the electrolyte includes two cationic species selected from cations of ruthenium, manganese and iridium; for example Ru (III), Mn (II) or Ir (III). In some cases, at least one of the metals is ruthenium. In some cases, at least one of the metals is manganese.

For example, the electrolyte may include a cation of ruthenium and a cation of manganese, for example, Ru (III) and Mn(II).

As described herein, the inventors have observed that 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.

In some embodiments, the concentration of metal ion(s) to be deposited as metal oxide is between 5 and 50 mM, for example between 5 and 35 mM, for example between 5 and 25 mM, for example between 10 and 25 mM, for example between 15 and 25 mM. In some cases, it is about 7 mM. In some cases, it is about 10 mM. In some cases, it is about 15 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 usefully concentration. Accordingly, a range of 15 mM to 25 mM may be preferred.

In some cases, more than one metal ion is used to produce a mixed metal oxide structure. For example, ruthenium 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 ruthenium ions is about 10 mM and the concentration of manganese ions is about 10 mM.

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 WO 2015/158711[1]which is herein incorporated by reference in its entirety for all purposes. Anions may include and anions derived from sulfonic acid and sulfuric acid, including sulfuric acid monoesters.

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.[2]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.

In a further aspect, the invention may provide a composition comprising graphene and/or graphite nanoplatelet structures, wherein said graphene and/or graphite nanoplatelet structures have metal oxide nanostructures deposited on the basal surface.

In some cases, the metal oxide is a mono-metal oxide selected from ruthenium oxide, manganese oxide, iridium oxide, tin oxide, and silver oxide. For example, the metal oxide may be selected from ruthenium oxide, manganese oxide and iridium oxide.

In a further aspect, more than one metal oxide is deposited. This may be referred to as a mixed-metal oxide. The metal oxides may be, independently, any metal oxide as described herein. In some cases, the metal oxides are ruthenium oxide and a manganese oxide (manganese (II, Ill) oxide).

In some methods and compositions of the present invention, metal oxide deposited graphene and/or graphite nanoplatelet structures having a thickness of less than 100 nm is substantially free 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.

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

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

In some cases, the electrode comprises metal oxide deposited graphene and/or graphite nanoplatelet structures having a thickness of less than 100 nm, wherein the metal oxide comprises ruthenium oxide.

In some cases, the supercapacitor has an electrolyte comprising diethylmethylammonium bisulfate. This is suitably provided in an organic solvent, for example, in acetonitrile. Suitable concentrations may depend on the set up and/or intended use of the cell. In some cases, the concentration of diethylmethylammonium bisulfate is 0.1 to 1.5 M, for example 0.5 to 1.5 M. It may be about 1 M.

In a further aspect, the invention provides diethylmethylammonium bisulfate, optionally wherein the diethylmethylammonium bisulfate is provided as a solution in an organic solvent, suitably an aprotic solvent such as acetonitrile. In some cases, the concentrations are as described in the preceding paragraph. The invention further provides use of diethylmethylammonium bisulfate in an electrolyte in an electrochemical cell; optionally wherein the electrochemical cell is a supercapacitor.

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 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.

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. It may be preferred that the material produced is 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.

Decorated Graphene

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.

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.

In some embodiments, the metal oxide is MnO2and the deposited 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.

In some embodiments, the metal oxide is RuO2and the deposited structure is metal oxide nanoparticles. The average size of the RuO2nanoparticles may be between 1.5 nm and 2.5 nm. In some embodiments, it is about 2 nm.

In embodiments, the graphene is decorated with combination of RuO2and manganese oxide, understood to be manganese (II,III) oxide (the combination is referred to herein as a mixed oxide) and the deposited structure is a manganese oxide honeycomb structure with rod and plate-shaped RuO2nanostructures growing out of the honeycomb structure.

At the priority date, the inventors believed the “mixed oxide” graphene to be decorated with a combination of RuO2and MnO2. On the basis of further work (as discussed below) it is now understood that the “mixed oxide” graphene is decorated with a combination of RuO2and Mn3O4, that is, manganese (II,III) oxide.

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 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 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 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.

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 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. It also ensures that any functionalisation of the nascent exfoliated graphene is solely due to the co-deposition of metal oxides at anode rather than due to the physical mixtures from the metal hydroxides generated at cathode.

In summary, the inventors have shown that the use of certain metal cations during electrochemical exfoliation of graphite can be used to produce metal-oxide decorated materials which may be useful for use as electrodes and as electrocatalysts. Advantageously, in some cases, the graphene oxide character of the product is low. A bi-metal oxide decorated graphene product, decorated with a uniformly grown RuO2and Mn3O4(originally thought to be RuO2—MnO2) structure was found to be an efficient electrode for a supercapacitor (500 F g−1) and an efficient electrocatalyst for water splitting (1.4 V).

