US20160115601A1

US20160115601A1 - Electrochemical process for production of graphene

Info

Publication number: US20160115601A1
Application number: US14/894,924
Authority: US
Inventors: Robert Angus William Dryfe; Amr Mohamed Abdelkader; Matej Velicky; Briony Megan Setterfield-Price; Alexander Rakowski
Original assignee: University of Manchester
Current assignee: University of Manchester
Priority date: 2013-05-30
Filing date: 2014-05-30
Publication date: 2016-04-28
Also published as: EP3004423A1; CN105452533A; CN105452533B; WO2014191765A1; GB201309639D0

Abstract

A process of producing graphene and/or graphite nanoplatelet structures by the electrochemical reduction of carbon oxide in an electrochemical cell is provided, wherein the cell includes (a) a negative electrode including a transition metal, transition metal-containing alloy, transition metal-containing oxide, transition metal containing ceramic, or a combination thereof; (b) a positive electrode; and (c) an electrolyte; wherein the process includes the step of passing a current between the electrodes in the presence of the carbon oxide.

Description

FIELD OF INVENTION

The present invention relates to a method for the production of graphene and related graphite nanoplatelet structures.

BACKGROUND

Graphene is an atomically thick, two dimensional sheet composed of sp²carbons in a honeycomb structure. It can be viewed as the building block for all the other graphitic carbon allotropes. Graphite (3-D) for example includes many layers of graphene stacked on top of each other, with an interlayer spacing of ˜3.4 Å and carbon nanotubes are graphene tubes.
Single-layer graphene is one of the strongest materials ever measured, with a tensile strength of ˜130 GPa and possesses a modulus of ˜1 TPa. Graphene's theoretical surface area is ˜2630 m²/g and the layers are gas impermeable. It has very high thermal (5000 W/mK) and electrical conductivities (up to 6000 S/cm).
There many potential applications for graphene, including but not limited to:

- (a) an additive for mechanical, electrical, thermal, barrier and fire resistant properties of a polymer;
- (b) an active component of an electrode for enhancing surface area and conductivity for applications such as fuel cells, super-capacitors and lithium ion batteries;
- (c) a conductive, transparent coating for the replacement of indium tin oxide (such as used in touch-screen technologies); and
- (d) a component in electronics.

Graphene was first reported in 2004, following its isolation by Professor Geim's group at the University of Manchester. Graphene research since then has increased rapidly. Much of the “graphene” literature is not on true monolayer graphene, however, but rather two closely related structures:

- (i) “few layer graphene”, which is typically 2 to 10 graphene layers thick. The properties of the material, particularly the electronic properties, are a function of the number of graphene layers, and at greater than 10 layers the material becomes effectively bulk graphite; and
- (ii) functionalised graphene (such as graphene oxide (GO) which is a graphene layer that has been heavily oxidised and has typically 30 at % oxygen content). Research into the utility of functionalised graphene is a growing field, although such functionalised material typically does not exhibit the excellent electrical and mechanical properties exhibited by pristine graphene.

Novoselov et al. initially produced graphene flakes by the mechanical exfoliation of graphite using an adhesive tape to isolate individual layers [Novoselov 2004]. However, a variety of methods for the production of graphene have been reported since then [Ruoff 2009].

Ultrasonic Exfoliation (USE)

These methods operate under ambient/near ambient conditions and involve the exfoliation of monolayer materials from the parent structure using power ultrasound, applied in an appropriate solution. It has been shown that graphite can be exfoliated by the application of ultrasonic energy to separate the graphitic layers when in an appropriate solvent, such as NMP (N-methyl pyrrolidone) [Coleman 2008 & 2009].
The disadvantages are that these methods produce a mixture of materials dispersed in solution (centrifugation is needed for separation). Furthermore, desirable yields of monolayer samples can only be achieved with prolonged application of USE meaning that the lateral flake dimensions are very small (<1 micron), thus precluding many applications in electronic devices. Furthermore, the large-scale use of power ultrasound has raised safety concerns amongst industrial parties.
Wang et al. have shown that ionic liquids are also appropriate solvents for ultrasonic exfoliation. In this case, graphite powder was mixed with an ionic liquid such as 1-butyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide ([Bmim][Tf₂N]) and then the mixture was subjected to tip ultrasonication for a total of 60 minutes using 5-10 minute cycles. The resultant mixture was then centrifuged [Wang 2010]. Ionic liquids are used to stabilise the graphene produced by the ultrasonication.
WO 2011/162727 discloses the formation of graphene using lithium ion exfoliation of graphite, the exfoliation being aided by the insertion of solvent between the layers and sonication. This work is also discussed in a related paper [Wang 2011].
The incorporation of intercalating species in graphite to form intercalation compounds has also been investigated. Intercalation compounds can be produced by introducing metal in the vapour phase and then reacting these metals with the graphite. The layers of the intercalation compound can then be separated more readily than graphite, such as by stirring in an appropriate solvent, such as NMP [Valles 2008]. An intercalation approach has also been taken to separate graphene oxide aggregates by electrostatically attracting tetrabutylammonium cations in between the layers of the graphene oxide [Ang 2009]. This technique relies on the charges present in graphene oxide to attract the tetrabutylammonium cations.

Oxidative Exfoliation

Oxidation of graphite, to graphite oxide, allows for more ready exfoliation in aqueous solution to form graphene oxide compared to analogous methods which provide graphene by exfoliating graphite. The profound disadvantage with this method is that graphene oxide, rather than graphene, is produced. Various methods have been advocated (electrochemical, thermal, chemical, photochemical) to reduce the graphene oxide produced to graphene [see, e.g. Li 2008], but so far it has not proven to be possible to reduce graphene oxide completely, and so the reduced material is not of a desirable quality.

Chemical Vapour Deposition (CVD)

The preparation of graphene using CVD techniques is known in the art. For example, reported techniques use methane as a carbon source and copper as a receiving surface [Bae 2010]. Similar methods are also used to form carbon nanotubes [Simate 2010]. However, these methods are typically procedurally complex, require extremely high temperatures (e.g. up to 1000° C.) and usually require elaborate isolation techniques to obtain the material formed.

Thermal Decomposition of Carbides

Alternatively silicon carbide can be decomposed to make a graphene film.

Electrochemical Exfoliation of Graphite

Electrochemical approaches can also be taken to access graphene by exfoliating graphite. Liu et al. [Liu 2008] reported the exfoliation of graphite using an ionic liquid-water mixture electrolyte to form “kind of ionic-liquid-functionalized” graphene nanosheets. Lu et al. showed subsequently that the graphene nanosheet production is exclusively at the anode and is due to an interaction of decomposed water species and the anions from the ionic liquid, such as BF₄ ⁻[Lu 2009].
A cationic electrochemical exfoliation technique that produces graphene and related graphite nanoplatelet structures by exfoliation driven by the electrochemical insertion of alkylammonium ions into a negative graphitic electrode is disclosed in WO 2012/120264 A1.
Electrochemical exfoliation is unlikely to lead to graphene sheets with a large surface area, however, such as is required for touch screens or similar applications because such electrochemical exfoliation methods are inherently limited by the grain size of the starting material being exfoliated.

Reduction of Carbon Dioxide

There has been growing interest in recent years in the reduction of carbon dioxide to form a variety of carbon-based materials. Such processes are particularly attractive from an environmental perspective as they convert the greenhouse pollutant gas carbon dioxide into potentially useful materials. Reported methods include the electrochemical reduction of carbon dioxide to form carboxylic acids such as formates and oxalates (e.g. U.S. Pat. No. 4,608,133A & GB2171115A), formaldehyde (e.g. U.S. Pat. No. 4,608,133A), and hydrocarbons such as methane and ethylene (e.g. [DeWulf 1989], JP2004143488A & JP2001089887A).
Methods directed to the reduction of carbon dioxide to elemental carbon have seldom been reported, however, and such methods tend to be complex and expensive. For instance, the thermal reduction of carbon dioxide to carbon using the reducing metal lithium or calcium has been reported [Suzuki 2012]. In the reported process, elemental lithium and calcium (formed at high temperature by electrochemical reduction of molten Li₂O or CaO respectively) reduces the carbon dioxide thermally to produce amorphous carbon deposits and rod-like graphite crystals. The process reported in WO2011/010109 requires the use of diamond-based electrodes and further requires the irradiation of the electrodes during the reaction. Furthermore, WO2008/019154 postulates that pre-prepared seed materials may be used to promote carbon growth. However, such methods have not proven to be successful in producing graphene.
Thus, there is a need to provide new methods for producing graphene/graphite nanoplatelet structures that mitigate or obviate the problems identified above. For instance, there is a desire to provide new methods that produce graphene selectively over other carbon allotropes, which are amenable to scale-up to an industrial platform, which are more efficient, reliable, environmentally friendly, provide higher quality material, provide increased yields of material, provide larger sheets or material, provide easier isolation of material and/or which are procedurally simpler and or cheaper than the methods of the prior art.

SUMMARY OF INVENTION

The present inventors have conceived a new process for producing graphene and related graphene nanoplatelet structures by electrochemical reduction of carbon oxide.

DESCRIPTION OF THE INVENTION

In a first aspect, the present invention provides a process for producing graphene and/or graphite nanoplatelet structures having a thickness of less than 100 nm, the process including electrochemical reduction of carbon oxide (e.g. carbon dioxide) in an electrochemical cell, wherein the cell includes:

- (a) a negative electrode including a transition metal, transition metal-containing alloy, transition metal-containing oxide, transition metal-containing ceramic or a combination thereof;
- (b) a positive electrode; and
- (c) an electrolyte;
  wherein the process includes passing a current between the electrodes in the presence of carbon oxide.

Advantageously, the present electrochemical process does not require high temperatures and/or pressures. For example, the process may be conducted at room temperature and atmospheric pressure, if desired. Furthermore, the process is cost effective and procedurally simple and so may be particularly amenable to industrial scale production. For instance, elaborate electrode materials or reaction conditions are not required and by producing the material in an electrochemical cell, isolation of the product is straightforward: material in the electrolyte may be isolated by filtration, and material formed on an electrode may be liberated, for example, by decomposition of the electrode (such as by oxidation) in a subsequent electrochemical or etching step.
In addition to the advantages described above, as indicated in the examples and figures, the process of the invention is able to produce material of high quality.

Graphene and Graphite Nanoplatelet Structures

The process of the present invention produces graphene and/or graphite nanoplatelet structures having a thickness of less than 100 nm. In embodiments, the process produces graphene or graphite nanoplatelet structures having a thickness of less than 100 nm. In embodiments, the process produces graphene and graphite nanoplatelet structures having a thickness of less than 100 nm. In embodiments, the process of the present invention produces graphene. In embodiments, the process produces graphite nanoplatelet structures having a thickness of less than 100 nm. The process of the present invention may for example produce graphene or a combination of graphene and graphite nanoplatelet structures having a thickness of less than 100 nm.
In embodiments, the process produces more graphene by surface area than graphite nanoplatelet structures having a thickness of less than 100 nm, preferably wherein substantially all material produced by the process is graphene by surface area (wherein at least 90%, preferably at least 95%, more preferably at least 98%, e.g. at least 99% of the material produced by the process is graphene by surface area), such as wherein all material produced by the process is graphene. In embodiments, the process produces more graphene by weight than graphite nanoplatelet structures having a thickness of less than 100 nm, preferably wherein substantially all material produced by the process is graphene by weight (wherein at least 90%, preferably at least 95%, more preferably at least 98%, e.g. at least 99% of the material produced by the process is graphene by weight), such as wherein all material produced by the process is graphene.
For instance in some embodiments, the present invention provides a process for producing graphene, the process including electrochemical reduction of carbon oxide (e.g. carbon dioxide) in an electrochemical cell, wherein the cell includes:

- (a) a negative electrode including a transition metal, transition metal-containing alloy, transition metal-containing oxide, transition-metal containing ceramic, or a combination thereof;
- (b) a positive electrode; and
- (c) an electrolyte;
  wherein the process includes passing a current between the electrodes in the presence of the carbon oxide.

In the present application, the term “graphene” is used to describe material consisting of from one to ten layers of graphene, preferably where the distribution of the number of layers in the product is controlled. Thus, in some embodiments, the graphene consists of one to five graphene layers, preferably one to four graphene layers, more preferably one to three graphene layers, for instance one to two graphene layers, e.g. one layer. The graphene produced may therefore have one, two, three, four, five, six, seven, eight, nine or ten layers.
The honeycomb of carbon atoms in graphene is typically a uniform polyhexagonal structure. The skilled person will understand however that graphene may include one or more amorphous regions (i.e. regions of amorphous graphene), such as wherein the carbon atoms do not form uniform hexagons. For instance, graphene may include Stone-Wales defects wherein one or more rings of carbon atoms contains other than six carbons in number (i.e. where the number of carbons in a single ring is other than six carbons in number), such as rings of carbon atoms independently selected from five (e.g. pentagonal), seven (heptagonal), eight (octagonal), nine (nonagonal) and ten (decagonal) carbon atoms, particularly one or more of five (e.g. pentagonal), seven (heptagonal), and eight (octagonal) carbon atoms, more particularly one or more of five (e.g. pentagonal) and seven (heptagonal) carbon atoms. Typically, the material produced by the present process is substantially free of amorphous graphene. For instance, the material may contain less than 10% by weight, for example less than 5% by weight, preferably less than 2% by weight, more preferably less than 1% by weight of amorphous graphene. In embodiments, the material produced by the present process does not include amorphous graphene.
The graphene and/or graphite nanoplatelet structures produced by the present process may contain one or more functionalised regions. “Functionalised” and “functionalisation” in this context refers to the covalent bonding of an atom to the surface of graphene and/or graphite nanoplatelet structures, such as the bonding of one or more hydrogen atoms (such as in graphane) or one or more oxygen atoms (such as in graphene oxide), etc. Typically, the material produced by the present process is substantially free of functionalisation, for instance, wherein less than 10% by weight, such as less than 5% by weight, preferably less than 2% by weight, more preferably less than 1% by weight of the relevant product is functionalised. For instance, in the above aspect and embodiments it may be preferred that the material produced is substantially free of graphene oxide (i.e. wherein less than 10% by weight, such as less than 5% by weight, preferably less than 2%, more preferably less than 1% by weight of the material produced is graphene oxide). The functionalisation, where present, may occur on the material surface and/or near or at the grain boundary. Typically, the functionalisation, where present, occurs at the grain boundary but not on the material surface. In preferred embodiments, the graphene produced by the present process is not functionalised.
The atomic composition of material produced by the present process may be quantified by X-ray photoelectron spectroscopy (XPS). Raman spectroscopy (as described in the Examples) may be used to determine the level of defects in the material.
In some embodiments, the material produced by the present process includes at least 10% by weight of graphene having up to ten layers, preferably at least 25% by weight more preferably at least 50% by weight of graphene having up to ten layers, preferably at least 60% by weight, at least 70% by weight, at least 80% by weight, at least 90% by weight, at least 95% by weight, at least 98% by weight, more preferably at least 99% by weight. In embodiments, graphene is produced in the absence of graphite nanoplatelet structures.
The graphite nanoplatelet structures have a thickness of less than 100 nm. In embodiments, the graphite nanoplatelet structures are ≦90 nm thick, such as ≦80, ≦70, ≦60, ≦50, ≦40, ≦30 or ≦20 nm thick, preferably ≦10 nm thick and more preferably ≦1 nm thick.
The graphene and/or graphite nanoplatelet structures typically form on an electrode in the electrochemical cell, preferably on the negative electrode. For instance, the graphene and/or graphite nanoplatelet structures may completely or partially coat the electrode (e.g. as a film or as a deposit of flakes). The graphene and/or graphite nanoplatelet structures may accumulate in the electrolyte (e.g. as a suspension and/or deposit at the bottom of the cell). In embodiments, the graphene and/or graphite nanoplatelet structures form on the electrode (typically the negative electrode) and accumulate in the electrolyte (e.g. as a suspension and/or deposit at the bottom of the cell).
Typically, the process of the present invention produces flakes of graphene on the electrode and/or in the electrolyte. The size of the graphene flakes produced can vary from nanometres across to millimetres, depending on the morphology desired. The flakes produced are desirably at least 90 μm in length, such as at least 80 μm, 70 μm, 60 μm, 50 μm, 40 μm, 30 μm, 20 μm, 10 μm, or 5 μm, for example at least 1 μm. In embodiments, the flakes produced are 1 to 100 μm in length, such as 1 to 90 μm, 1 to 80 μm, 1 to 70 μm, 1 to 60 μm, 1 to 50 μm, 1 to 40 μm, 1 to 30 μm, 1 to 20 μm, 1 to 10 μm, or 1 to 5 μm in length.

Carbon Oxide

In the present application, the term “carbon oxide” refers to carbon monoxide, carbon dioxide or a combination thereof. In embodiments, the carbon oxide is a combination of carbon monoxide and carbon dioxide. Preferably, the carbon oxide is carbon monoxide or carbon dioxide. For instance, in embodiments the carbon oxide is carbon monoxide. More preferably, the carbon oxide is carbon dioxide.
Accordingly, in embodiments, the present invention provides a process for producing graphene and/or graphite nanoplatelet structures having a thickness of less than 100 nm, the process including electrochemical reduction of carbon dioxide in an electrochemical cell, wherein the cell includes:

- (a) a negative electrode including a transition metal, transition metal-containing alloy, transition metal-containing oxide, transition metal-containing ceramic or a combination thereof;
- (b) a positive electrode; and
- (c) an electrolyte;
  wherein the process includes passing a current between the electrodes in the presence of carbon dioxide.

For instance, in embodiments, the present invention provides a process for producing graphene, the process including electrochemical reduction of carbon dioxide in an electrochemical cell, wherein the cell includes:

Without wishing to be bound by theory, it is envisaged that reduction of carbon dioxide at the electrode to form graphene and/or graphite nanoplatelet involves production of carbon monoxide as an intermediate.
The carbon oxide may be supplied to the electrochemical cell in any desired form. Carbon monoxide may for example be supplied to the electrochemical cell in gaseous or solvated form, preferably in gaseous form. Carbon dioxide for instance may be provided in solid, liquid, supercritical, gaseous and/or solvated forms. In embodiments, the carbon dioxide is supplied to the electrochemical cell in a form selected from solid, liquid and gaseous, preferably selected from liquid and gaseous, more preferably the carbon dioxide is supplied to the electrochemical cell in gaseous form. In embodiments, the carbon oxide is supplied to the electrochemical cell pre-solvated or pre-dissolved in an electrolyte.
Typically, gaseous carbon oxide (e.g. carbon dioxide) is used for reasons of practicality. The use of gaseous carbon oxide desirably permits operation of the process at ambient temperatures and pressures. Gaseous carbon oxide may be supplied in a mixture of gases, such as in air, or argon, etc. or it may be provided as pure carbon oxide gas.
The carbon oxide is reduced by the negative electrode. In typical embodiments, carbon oxide is supplied to the cell prior to passing a current between the electrodes, for instance to pre-saturate the electrolyte with carbon oxide. For example, the carbon oxide may be bubbled through the electrolyte (e.g. for up to 1 hour to 2 hours, such as between 1 and 2 hours) prior to passing a current between the electrodes. Alternatively, the carbon oxide may not be supplied to the cell prior to the step of passing a current between the electrodes (i.e. it is supplied to the cell only after the current has begun passing between the electrodes).
In the above embodiments, the carbon oxide may be supplied to the cell by any suitable charging method, such as a single charge (e.g. by bolus or continuous charge) or by multiple intermittent charges. Typically the carbon oxide is supplied to the electrochemical cell continuously during the electrochemical reaction, such as by bubbling it through the electrolyte.
The rate that the carbon oxide is provided to the cell will depend on its solubility and mobility in the electrolyte as well as the reaction conditions (e.g. temperature, electrolyte viscosity, scale, surface area of the negative electrode, etc.). The flow rate may for example be from 10-1000 cm³/min, such as from 50-500 cm³/min, e.g. about 80-120 cm³/min. These flow rates may be particularly suitable when the surface area of the negative electrode is around 1-10 cm², such as around 5 cm².
The reaction yield and/or rate may be improved by electrolyte mixing (by mechanical means and/or as a consequence of gas flow) and/or electrode movement in the vessel, e.g. electrode rotation.

Negative Electrode

The negative electrode is the electrode held at the more negative potential out of the negative and positive electrodes. As discussed below, an additional reference electrode may also be used (which may be any suitable material, such as Ag/AgBF₄).
The negative electrode includes a transition metal, transition metal-containing alloy, transition metal-containing oxide, transition metal-containing ceramic or a combination thereof. In embodiments, the electrode includes one said transition metal, transition metal-containing alloy, transition metal-containing oxide, transition metal-containing ceramic or combination thereof. In alternative embodiments, the electrode includes more than one transition metal, transition metal-containing alloy, transition metal-containing oxide, transition metal-containing ceramic or combination thereof. In embodiments, the negative electrode includes a transition metal, transition metal-containing alloy, transition metal-containing oxide or a combination thereof, preferably a transition metal, transition metal-containing alloy or a combination thereof, more preferably a transition metal or transition metal-containing alloy, even more preferably a transition metal.
In embodiments, the negative electrode consists substantially of said transition metal(s), transition metal-containing alloy(s), transition metal-containing oxide(s), transition metal-containing ceramic(s) or combination(s) thereof (i.e. wherein at least 90% by weight of the electrode consists of said transition metal(s), transition metal-containing alloy(s), transition metal-containing oxide(s), transition metal-containing ceramic(s) or combination thereof, for instance at least 95% by weight, 98% by weight or 99% by weight).
Suitably, said transition metal(s), transition metal-containing alloy(s), transition metal-containing oxide(s), transition metal-containing ceramic(s) or combination(s) thereof is/are included at the electrode surface configured to contact the electrolyte. In embodiments, at least 10% by area of the electrode surface configured to contact the electrolyte consists of said transition metal(s), transition metal-containing alloy(s), transition metal-containing oxide(s), transition metal-containing ceramic(s) or combination thereof. In typical embodiments, at least 20% by area of the electrode surface configured to contact the electrolyte consists of said transition metal(s), transition metal-containing alloy(s), transition metal-containing oxide(s), transition metal-containing ceramic(s) or combination thereof, preferably at least 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95%, 98%, 99% or more, preferably 100% of the surface area of the electrode surface configured to contact the electrolyte consists of said transition metal(s), transition metal-containing alloy(s), transition metal-containing oxide(s), transition metal-containing ceramic(s) or combination(s) thereof.
In embodiments, the negative electrode consists of said transition metal(s), transition metal-containing alloy(s), transition metal-containing oxide(s), transition metal-containing ceramic(s) or combination(s) thereof.
Suitably, the negative electrode (i.e. the part in contact with the electrolyte) catalyses the carbon oxide reduction but does not react with the graphene or graphite nanoplatelet structures formed. Desirably, the transition metal, transition metal-containing alloy, transition metal-containing oxide and/or transition metal-containing ceramic used at the negative electrode exhibits a low propensity to evolve hydrogen under the electrochemical conditions. This property is desirable because significant levels of hydrogen evolution can decrease the efficiency of the graphene-forming reaction and disrupt the formation of graphene deposits on the electrode surface.
Any suitable transition metal may be used. In the above aspect and embodiments, each transition metal in said transition metal(s), transition metal-containing alloy(s), transition metal-containing oxide(s), transition metal-containing ceramic(s) or combination(s) thereof described above is preferably selected independently from the group consisting of copper, nickel, molybdenum, cobalt, iron, titanium and zinc, such as from the group consisting of copper, nickel, titanium, molybdenum, iron and zinc, more preferably from the group consisting of copper, titanium, nickel and molybdenum, more preferably copper. In the above embodiments, the transition metal-containing alloy may be selected from the group consisting of nickel-molybdenum alloy (Ni—Mo) and molybdenum-titanium alloy (Mo—Ti).
In embodiments, the negative electrode includes copper, nickel, molybdenum, cobalt, iron, titanium, zinc, nickel-molybdenum alloy, molybdenum-titanium alloy or a combination thereof, such as copper, nickel, molybdenum, titanium, zinc, nickel-molybdenum alloy, molybdenum-titanium alloy or a combination thereof, preferably copper, nickel, molybdenum, titanium, nickel-molybdenum alloy, molybdenum-titanium alloy or a combination thereof, more preferably copper, nickel, molybdenum, cobalt, iron, titanium, zinc, nickel-molybdenum alloy or molybdenum-titanium alloy. In other embodiments, the negative electrode includes copper, nickel-molybdenum alloy and/or molybdenum-titanium alloy, such as copper, nickel-molybdenum alloy or molybdenum-titanium alloy. In other embodiments, the negative electrode includes nickel-molybdenum alloy or molybdenum-titanium alloy. For instance, the negative electrode may include nickel-molybdenum alloy. In some embodiments the negative electrode includes molybdenum-titanium alloy. Preferably, the negative electrode includes copper, such as copper foil. Most preferably, the electrode consists of copper, such as copper foil.
In an embodiment of any of the above aspects and embodiments, the negative electrode does not include a carbon-based material at the surface configured to contact the electrolyte before electrochemical reduction of carbon oxide has begun (the skilled person will understand however that carbonaceous deposits may form on the electrode after the electrochemical reduction of carbon oxide has begun, i.e. as the reaction progresses). Thus, in an embodiment the negative electrode does not include graphene, graphite, intercalated graphite, diamond or diamond which has been doped (e.g. boron-doped diamond) at the surface configured to contact the electrolyte before electrochemical reduction of carbon oxide has begun. In a further embodiment, the negative electrode does not include graphite, intercalated graphite, diamond or diamond which has been doped (e.g. boron-doped diamond) at the surface configured to contact the electrolyte before electrochemical reduction of carbon oxide has begun. In further embodiments, the negative electrode does not include diamond or diamond which has been doped (for instance boron-doped diamond) at the surface configured to contact the electrolyte before electrochemical reduction of carbon oxide has begun. In embodiments, the negative electrode does not include a carbon-based material. In some embodiments, the negative electrode does not include a material selected from the group consisting of graphite, intercalated graphite, diamond and diamond which has been doped, for instance boron-doped diamond.
The negative electrode may be treated prior to use in order to improve its electrochemical properties. In some embodiments, the negative electrode may be surrounded by a membrane. The use of a membrane may help retain any graphene or graphitic nanoplatelet structures which break away from the electrode surface during the reaction. In some embodiments, the pore size of the membrane may vary from 10 nm to 500 nm. Suitable membranes include cellulose dialysis membrane (e.g., Spectra Por 7, 25 nm pores) and polycarbonate membranes (e.g. 450 nm pores).
The negative electrode may be of any suitable shape. However, arrangements which provide high surface areas are preferred in order to maximise the exposure of the reducing surface of the electrode to the carbon oxide and to provide larger graphene coatings. For instance, highly folded, lattice, sheet-like and/or highly corrugated structures may be preferred.
The process of the present invention as described in the above aspect and embodiments usually results in the formation of a deposit (such as a coating) of graphene and/or graphite nanoplatelet structures of 100 nm or less on the surface of the negative electrode (i.e. on the surface of the transition metal, transition metal-containing alloy transition metal-containing oxide and/or transition metal-containing ceramic). Advantageously, such deposits will typically conform to the general surface profile of the negative electrode. The present processes therefore offer the advantage that the shape of the electrode may be specifically adapted to reflect the desired shape of the resulting graphene/graphite nanoplatelet material in its desired end use application.

Positive Electrode

The positive electrode is the electrode held at the more positive potential of the negative and positive two electrodes. The positive electrode may consist of any suitable electrode material known to those skilled in the art.
The positive electrode may thus be selected independently from any of the embodiments described herein for the negative electrode. The positive electrode may be identical or different in substance to the negative electrode, typically different. Where the positive electrode is identical in substance to the negative electrode, the electrodes will differ only in terms of their relative electrical potential. For instance, the positive electrode may include a material selected from the groups consisting of transition metals, transition metal-containing alloys, transition metal-containing oxides, transition metal-containing ceramics and combinations thereof. Preferably, the positive electrode is made from an inert material. In embodiments, the positive electrode includes gold, silver, platinum or carbon, preferably gold, silver or platinum, more preferably platinum. Platinum mesh is particularly suitable. In embodiments, the positive electrode consists substantially of said gold, silver, platinum or carbon (i.e. wherein at least 90% by weight of the electrode consists of said gold, silver, platinum or carbon, for instance at least 95% by weight, 98% by weight or 99% by weight). In embodiments, the positive electrode consists of said gold, silver, platinum or carbon. Suitably, said gold, silver, platinum or carbon is included at the surface of the electrode configured to contact the electrolyte, preferably wherein at least 10% by area of said electrode surface consists of said gold, silver, platinum or carbon, more preferably at least 20% by area, such as 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95%, 98%, 99%, or more preferably 100% by area.
In embodiments the positive electrode includes platinum (e.g. platinum mesh) and the negative electrode includes copper (e.g. copper foil) and/or nickel-molybdenum alloy. For example, in some embodiments the positive electrode includes platinum mesh and the negative electrode includes copper foil. In other embodiments, the positive electrode includes platinum mesh and the negative electrode includes nickel-molybdenum alloy.
If the reaction at the positive electrode generates a gas, the positive electrode surface area is ideally kept as large as possible to prevent gas bubbles wetting it and/or disrupting the process at the negative electrode. The positive and/or reference electrode may also be placed in a membrane to prevent undesired reactions in the electrolyte or at either electrode.

Reference Electrode

A reference electrode may also be used, in addition to the negative and positive electrodes. The reference electrode may be any suitable material, such as Ag/AgBF₄. Indeed, in embodiments, the use of a reference electrode has been found to provide particularly effective control of the potential distribution of the system. In turn this can lead to improved reproducibility.
In embodiments, the use of a reference electrode results in the graphitic reduction product being p[referentially formed on the electrode surface. That is, more of the reduction product, suitably a majority, suitably substantially all, of the reduction product is formed on and recoverable from, the electrode surface, rather than, for example, the electrolyte.

Electrolyte

Any suitable electrolyte may be used in the process of the present invention.
The electrolyte may include a solid electrolyte, such as a dry polymer electrolyte or a solid ceramic electrolyte. In some embodiments, the electrolyte includes an ion-containing fluid, such as an ion-containing gas, liquid and/or gel. Typically, the electrolyte includes an ion-containing liquid. Suitable liquids may for example dissolve the relevant carbon oxide and/or form complexes with the solvated carbon oxide to be reduced in the electrochemical process.
In some embodiments, the ion-containing liquid is an ionic liquid, eutectic solvent, ionic solution or combination thereof, such as an ionic liquid, eutectic solvent or ionic solution. Preferably, the ion-containing liquid is an ionic liquid or eutectic solvent, such as an ionic liquid. Alternatively, the ion-containing liquid may be a eutectic solvent.
Any suitable ionic liquid known in the art may be used in the present processes. The choice of the ionic liquid will depend on the properties of the material and the desired reaction conditions. For instance, molten salts may be used provided the reaction is conducted at suitably high temperature. The term “molten salts” herein refers to salts that typically have a very high melting point, such as at least two hundred degrees above room temperature. Molten salts may include for example alkali-metal halides, alkali-metal carbonates, metal hydroxides, or metal oxides, preferably selected from CaCl₂, Cryolite, Na₂CO₃, K₂CO₃and KCl.
On the other hand, if lower temperature reaction conditions are desired, an ionic liquid having a low melting point will be required, such as a room temperature ionic liquid. Suitable ionic liquids having a low melting point can be provided by combining a cation selected from the group consisting of 1-alkyl-3-methylimidazolium, 1-alkylpyridinium, N-methyl-N-alkylpyrrolidinium and various ammonium ions (such as choline salts) and phosphonium cations with an anion selected from the group consisting of halides (e.g. F, Cl, Br and I), tetrafluoroborate, hexafluorophosphate, bistriflimide, triflate, tosylate formate, alkylsulfate, alkylphosphate and glycolate.
In the above embodiments the ionic liquid may for example be selected from the group consisting of 1-butyl-3-methylimidazolium tetrafluoroborate (i.e. [bmim][BF₄]), 1-butyl-3-methylimidazolium hexafluorophosphate (i.e. [bmim][PF₆]) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (i.e. [bmim][NTf₂]). For example, the ionic liquid may be [bmim][BF₄] or [bmim][PF₆], such as [bmim][PF₆].
Particularly suitable ion-containing liquids include eutectic solvents, which can be formed between one or more salts, as well as between salts/salt hydrates and hydrogen bond-donors. Eutectic solvents have an advantage over conventional ionic liquids in that they are typically cheaper to make and generally less toxic. Eutectic solvents exhibit melting points much lower than the melting points of their constituent components and thus represent useful electrolyte materials, particularly where ambient temperatures and pressures are desired. However, molten eutectic mixtures, such as KOH—NaOH or CaO—CaCl₂may also be used as the electrolyte if the reaction temperature is suitably high. Eutectic mixtures are preferred where molten salts are used as they form a molten liquid at a lower temperature than if the constituent molten salts were used as the electrolyte alone.
In the above embodiments the eutectic solvent may for example be selected from the group consisting of a mixture of ZnCl₂+choline chloride, a mixture of CoCl₂*6H₂O+choline chloride, a mixture of choline chloride+urea (typically in a ratio of 1:2), a mixture of ZnCl₂+urea, a mixture of choline chloride+malonic acid, a mixture of choline chloride+phenol, and a mixture of choline chloride+glycerol. For example, the eutectic solvent may be a mixture of choline chloride+urea, for instance in a mole ratio of 1:2.
Suitable ionic solutions include solutions (such as aqueous solutions) containing an ammonium salt (such as an ammonium halide, e.g. choline chloride), an alkali metal salt, suitably selected from an alkali metal bicarbonate (e.g. such as LiHCO₃, NaHCO₃or KHCO₃), alkali metal carbonate (such as Li₂CO₃, Na₂CO₃or K₂CO₃) and an alkali metal halide (e.g. such as Li, Na or K halide, such as LiF or LiCl). In some embodiments, the ionic solution contains an ammonium salt (such as a halide, e.g. choline chloride), LiHCO₃, NaHCO₃, Na₂CO₃, K₂CO₃, or a sodium or potassium halides, or combinations thereof. In embodiments, LiBF₄is used.
Typical ammonium salts for use in the ionic solutions include tetraalkyl ammonium salts, (including tetrabutyl ammonium (TBA, [(C₄H₉]₄N⁺), tetraethyl ammonium (TEA, (C₂H₅)₄N⁺) and tetramethyl ammonium (TMA, (CH₃)₄N⁺) salts), trialkylammonium salts (such as tributyl ammonium ([(C₄H₉]₃NH⁺), triethyl ammonium ((C₂H₅)₃NH⁺), trimethyl ammonium ((CH₃)₃NH⁺) salts) and dialkylammonium salts (such as dibutyl ammonium ([(C₄H₉]₂NH₂ ⁺), diethyl ammonium ((C₂H₅)₂NH₂ ⁺) and dimethyl ammonium ((CH₃)₂NH₂ ⁺) salts). In such ammonium salts, the alkyl chains may contain up to 100 carbon atoms, more preferably up to 20 carbon atoms and most preferably up to 5 carbon atoms long. The alkyl chains may contain only a single carbon atom, but preferably contain at least two carbon atoms. The alkyl chains may all be the same, or may be different. Furthermore, a mixture of different ammonium ions may be used including a mixture of dialkylammonium cations, trialkylammonium cations and tetraalkyl ammonium cations. In such ammonium salts, and indeed for ionic solutions where the cation is other than an ammonium cation, e.g. an alkali metal cation, the counter-ions may be relatively lipophilic ions, e.g. tetrafluoroborate (BF₄ ⁻), perchlorate (ClO₄ ⁻) or hexafluorophosphate (PF₆ ⁻). Other soluble, inorganic ions may be used, such as tetraphenyl borate. In embodiments, TBABF₄is used.
Suitable solvents for use in the ionic solutions include water, propylene carbonate (PC), ethylene carbonate (EC), chloroethylenene carbonate (Cl-EC), vinyl carbonate (VC), dimethyl carbonate (DMC), NMP, DMSO (dimethyl sulfoxide), DMF (N,N′-dimethyl formamide) and mixtures thereof. For instance, in some embodiments, the solvent is selected from water, NMP, DMSO (dimethyl sulfoxide), DMF (N,N′-dimethyl formamide) and mixtures thereof, preferably NMP, DMSO (dimethyl sulfoxide), DMF (N,N′-dimethyl formamide) and mixtures thereof. NMP, DMSO and DMF are particularly preferred where the process of the invention further involves sonication of the graphene/graphite nanoplatelet structures after/during the electrochemical reaction.
In some embodiments, the solvent has an affinity for graphene or graphite nanoplatelet structures so that the material produced at the electrode is taken away by the solvent. In another embodiment, the solvent has little or no affinity for graphene or graphite nanoplatelet structures, so that the material produced is more likely to coat the electrode or fall to the bottom of the electrochemical cell.
Accordingly, the electrolyte may be selected from the group consisting of 1-butyl-3-methylimidazolium tetrafluoroborate (i.e. [bmim][BF₄]), 1-butyl-3-methylimidazolium hexafluorophosphate (i.e. [bmim][PF₆]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (i.e. [bmim][NTf₂]), a mixture of ZnCl₂+choline chloride, mixtures of CoCl₂*6H₂O+choline chloride, a mixture of choline chloride+urea (typically in a ratio of 1:2), a mixture of ZnCl₂+urea, a mixture of choline chloride+malonic acid, a mixture of choline chloride+phenol, a mixture of choline chloride+glycerol and solutions of ammonium salts (such as halides, e.g. choline chloride), alkali metal bicarbonates (e.g. LiHCO₃, NaHCO₃and/or KHCO₃), alkali metal carbonates (e.g. Li₂CO₃, Na₂CO₃and/or K₂CO₃) and alkali metal halides (e.g. Li, Na and/or K halides, such as LiF and/or LiCl).
In further embodiments, the electrolyte is selected from the group consisting of 1-butyl-3-methylimidazolium tetrafluoroborate (i.e. [bmim][BF₄]), 1-butyl-3-methylimidazolium hexafluorophosphate (i.e. [bmim][PF₆]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (i.e. [bmim][NTf₂]), a mixture of ZnCl₂+choline chloride, a mixture of CoCl₂*6H₂O+choline chloride, a mixture of choline chloride+urea (typically in a ratio of 1:2), a mixture of ZnCl₂+urea, choline chloride+malonic acid, a mixture of choline chloride+phenol, a mixture of choline chloride+glycerol, and a solution including an ionic salt selected from an ammonium salt (such as an ammonium halide, e.g. choline chloride), LiHCO₃, NaHCO₃, Na₂CO₃, K₂CO₃, and a sodium or potassium halide.
In still further embodiments, the electrolyte is selected from the group consisting of 1-butyl-3-methylimidazolium tetrafluoroborate (i.e. [bmim][BF₄]), 1-butyl-3-methylimidazolium hexafluorophosphate (i.e. [bmim][PF₆]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (i.e. [bmim][NTf₂]), a mixture of ZnCl₂+choline chloride, a mixture of CoCl₂*6H₂O+choline chloride, a mixture of choline chloride+urea (typically in a ratio of 1:2), a mixture of ZnCl₂+urea, a mixture of choline chloride+malonic acid, a mixture of choline chloride+phenol, and a mixture of choline chloride+glycerol.
In typical embodiments, the electrolyte is selected from the group consisting of 1-butyl-3-methylimidazolium tetrafluoroborate (i.e. [bmim][BF₄]), 1-butyl-3-methylimidazolium hexafluorophosphate (i.e. [bmim][PF₆]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (i.e. [bmim][NTf₂]), and a mixture of choline chloride+urea (typically in a ratio of 2:1), for example 1-butyl-3-methylimidazolium hexafluorophosphate (i.e. [bmim][PF₆]) or a mixture of choline chloride+urea (typically in a ratio of 1:2).

The Process

Cell Potential and Current Density

The working potential of the cell will be at least that of the standard potential for reduction of the carbon oxide.
An overpotential may be used in order to increase the reaction rate. Preferably an overpotential of 1 mV to 10 V is used against a suitable reference as known by those skilled in the art, more preferably 1 mV to 5 V. In cells with only two terminals and no reference, a larger potential may be applied across the electrodes but a significant amount of the potential drop will occur over the cell resistance, rather than act as an overpotential at the electrodes. In these cases the potential applied may be up to 20V or 30V.
Typically the potential difference across the electrodes is held constant. In other embodiments, the potential may be cycled or swept. In one embodiment, both positive and negative electrodes include a transition metal, transition metal-containing alloy, transition metal-containing oxide, transition-metal containing ceramic, or combination thereof and the potential is swept so that electrodes change from positive to negative and optionally vice versa. In the present process, the graphene/graphite nanoplatelet material is typically formed at the negative electrode. In this embodiment, the formation of graphene/graphite nanoplatelet structures may therefore occur at either electrode, depending on the polarity of the electrode during the voltage cycle (e.g. depending on which electrode is the negative electrode at any time in the cycle).
The current density at the negative electrode can be controlled through a combination of the electrode's surface area and overpotential used.
Typically, current is allowed to pass between the electrodes at a potential difference of from 1 to 10 V, such as from 2 to 8 V, for example 2 to 5 V, e.g. 3 to 5 V. For instance, the current allowed to pass between the electrodes may be at a potential difference of about 1 V, about 2 V, about 3 V, about 4 V, about 5 V, about 6 V, about 7 V, about 8 V, about 9 V or about 10 V. Typically, the current is allowed to pass between the electrodes at a potential difference of about 3 V.

Operating Temperature

The electrochemical cell may be operated at any suitable temperature that allows for production of the desired graphene/graphite nanoplatelet structures. The optimum operating temperature will depend on the nature of the electrolyte and/or the form of carbon oxide used (as well as its solubility in the electrolyte medium).
The temperature within the electrochemical cell may thus be at least 10° C., preferably at least 20° C. For instance, the temperature within the electrochemical cell may be cell room temperature. In some embodiments, the temperature within the electrochemical cell is at least 30° C., 40° C., 50° C., 60° C., 70° C., 80° C., 90° C. or 100° C. In the case of molten salts, the temperature within the cell may for example be up to 1500° C. In some embodiments, the temperature within the cell does not exceed 1000° C., 900° C., 800° C. or 700° C., preferably the cell operating temperature does not exceed 650° C., 600° C., 550° C., 500° C., 450° C., 400° C., 350° C., 300° C., 250° C., 200° C., 150° C. or more preferably 120° C. In other embodiments, the temperature within the cell does not exceed 110° C., more preferably the temperature within the cell does not exceed 100° C., 90° C., 80° C., 70° C., 60° C. or more preferably 50° C.

Operating Pressure

The electrochemical cell may be operated at any suitable pressure that allows for production of the desired graphene/graphite nanoplatelet structures. The optimum operating pressure will depend on the nature of the electrolyte and/or the form of carbon oxide used. Typically, the cell is operated at or above atmospheric pressure. The cell may for example be operated at pressures greater than atmospheric pressure, which would have the advantage of increasing carbon oxide gas solubility in the electrolyte. High pressures are also desirable where liquid or supercritical carbon dioxide is used.

Operating Atmosphere

The electrochemical cell may be operated under any suitable gaseous atmosphere. For example, the electrochemical cell in processes of the invention may be operated under an anhydrous atmosphere, such as under nitrogen, and/or argon. In alternative embodiments, the electrochemical cell is operated under air or preferably under pure carbon oxide, e.g. pure carbon dioxide.

Duration of Reaction

The electrochemical process may be operated for a length of time adequate to provide a desirable yield of graphene and/or graphite nanoplatelet structures. The duration of the process typically refers to the length of time that a current is passed between the electrodes in the presence of carbon oxide prior to isolation of the graphene/graphite nanoplatelet structures. The current may be passed between the electrodes continuously or intermittently, typically continuously.
In some embodiments, the length of time that a current is passed between the electrodes in the presence of carbon oxide is greater than one minute, preferably greater than 5 min, 10 min, 20 min, 30 min, 40 min, 50 min preferably greater than one hour. Typically, the reaction duration from 1 h to 72 h, such as from 1 h to 48 h, for instance 1 h to 24 h. In further embodiments, the length of time that a current is passed between the electrodes in the presence of carbon oxide is from 1 h to 10 h, 1 h to 5 h or 1 h to 4 h. Typically the length of time that a current is passed between the electrodes in the presence of carbon oxide is about 3 h. In alternative embodiments, the reaction is continuous.

Additional Process Steps

In some embodiments, the process includes an initial step of forming a deposit of graphene on an electrode, typically the negative electrode, to provide a seed for further deposition of graphene.
In some embodiments, the process further includes the step of isolating the graphene/graphite nanoplatelet structures.
For instance, in some embodiments the present invention provides a process for producing graphene and/or graphite nanoplatelet structures having a thickness of less than 100 nm, the process including electrochemical reduction of carbon oxide (e.g. carbon dioxide) in an electrochemical cell, wherein the cell includes:

- (a) a negative electrode including a transition metal, transition metal-containing alloy, transition metal-containing oxide, transition metal-containing ceramic or combination thereof;
- (b) a positive electrode; and
- (c) an electrolyte;
  wherein the process further includes the steps of i) passing a current between the electrodes in the presence of the carbon oxide; and ii) isolating the graphene and/or graphite nanoplatelet structures produced.

Where the graphene/graphite nanoplatelet structures are suspended in the electrolyte or have fallen to the floor of the electrochemical cell, isolation of the graphene/graphite nanoplatelet structures can be achieved by separation from the electrolyte according to a number of separation techniques, including:
(a) filtering;
(b) using centrifugal forces to precipitate/accumulate the graphene or graphite nanoplatelet structures; and
(c) collecting the graphene or graphite nanoplatelet structures at the interface of two immiscible solvents.
In some embodiments the graphene/graphite nanoplatelet structures are isolated by filtration. Typically, the graphene/graphite nanoplatelet structures are isolated by filtration using a fine membrane material, such as Anopore™ inorganic membrane (i.e. Anodisc™ which is commercially available from GE Healthcare).
Where the graphene and/or graphite nanoplatelet structures having a thickness of less than 100 nm are obtained as a deposit (e.g. coat) on the negative electrode, isolation may be performed by mechanical removal of the deposit from the electrode surface, such as by mechanical abrasion or by ultrasonication. In embodiments where the negative electrode includes a transition metal, transition metal alloy or combination thereof, the coating may alternatively be released from the electrode by chemical removal of the transition metal/transition metal alloy from the coat. For instance, chemical removal may include subjecting the electrode to a further electrochemical step to dissolve/erode the electrode. The remaining graphene/graphite nanoplatelet structures can then be isolated in the usual way. This further electrochemical step may be performed in practice by modifying the electrolysis conditions within the same electrochemical cell used to form the graphene/graphite nanoplatelet structures, or by introducing the electrode to an alternative electrochemical apparatus. Thus, the present electrochemical process enables the straightforward isolation of the graphene/graphite nanoplatelet structures, which offers a distinct advantage over alternative prior art processes, such as CVD, where elaborate methods are typically required to obtain the product from the surface on which it was initially formed. For instance, copper may be removed from the graphene/graphite nanoplatelet structures by electrochemical reaction with ammonium persulphate solution ((NH₄)₂S₂O₈), for instance as a 0.1 M solution.
Alternatively, etching solutions may be used to remove the transition metal/transition metal alloy from the graphene/graphite nanoplatelet structures. Copper electrodes in particular would be amenable to either of the above electrochemical/etching isolation steps. Suitable etching solutions will be known to the skilled person. For instance, ferric chloride is particularly suitable as a copper etchant.
The process may include the further step of manipulating the graphene/graphite nanoplatelet structures either prior to isolation (such as in the electrochemical cell), or after isolation from the electrochemical cell. For example, the graphene/graphite nanoplatelet structures may be washed to remove contaminants prior to or following isolation, for instance to remove residual electrolyte from the product surface. In embodiments, the process includes the step of forming and/or shaping the graphene/graphite nanoplatelet structures prior to, or following, isolation, such as forming and/or shaping the graphene into an article. In embodiments, the process includes the step of incorporating the graphene and or graphite nanoplatelet structures into an article.
In some embodiments, the graphene/graphite nanoplatelet structures are subject to exfoliation, such as by using ultrasonic energy and/or other techniques known to those skilled in the art to decrease the flake size and number of graphene layers. Exfoliation by sonication for instance may be performed after the electrochemical reaction has completed and/or during the electrochemical reaction.
Suitably, the present processes include a pre-electrolysis step to purify the electrolyte prior to passing a current between the electrodes. Typically the pre-electrolysis step includes passing a current through the electrolyte between two additional electrodes before the electrochemical reduction of carbon oxide has commenced. The additional electrodes may be formed of any suitable conducting material, such as platinum.
Typically, the process of the present invention does not include irradiating the cathode surface. Preferably the process of the present invention does not include irradiating an electrode surface.
In other embodiments, the process of the present invention does not include the step of providing a carbon molecule seed (for growth of the graphene/graphite nanoplatelet structures in the cell) prior to applying a potential difference between the electrodes.
In a further aspect of the invention is provided graphene and/or graphene nanoplatelet structures prepared according to a process as described in any of the above aspects and embodiments. In a further aspect, the invention provides a composition including graphene and/or graphite nanoplatelet structures prepared according to a process as described in any of the above aspects and embodiments. In a still further aspect is provided an article including said composition or said graphene and/or graphite nanoplatelet structures prepared according to a process as described in any of the above aspects and embodiments, or, optionally, a derivative of said composition or graphene and/or graphite nanoplatelet structures.
The skilled person will understand that the above embodiments are described by way of example only. Other embodiments falling within the scope of the claims will be apparent to the skilled reader. It will be appreciated that the features specified in each aspect and embodiment may be combined with other specified features in other embodiments, to provide further embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a cross section of an electrochemical cell for use in the processes of the present invention.

FIGS. 2 and 3 provide Raman spectra for samples taken from different batches of material prepared according to the Example 1.

FIG. 4 provides the Raman spectrum for material prepared according to Example 2. Although the vertical y coordinates are omitted from the graph, the graph plots Raman shift against Counts as per FIGS. 2, 3, 5 and 6.

FIG. 5 provides the Raman spectrum for material prepared according to Example 3.

FIG. 6 provides the Raman spectrum for material prepared according to Example 4.

FIG. 7 provides an SEM image of a few-layer graphene flake prepared according to Example 5 clearly showing the copper surface features running underneath.

FIG. 8 provides a zoom image of a flake prepared according to the process of the invention which comprises graphene that is thin (bottom in FIG. 8a ) to a few layers thick (top in FIG. 8a ) and FIGS. 8b and 8c show an indication of the layered structure.

FIG. 9 provides the Raman spectrum for material prepared according to Example 6

The present invention is described in more detail by way of example only with reference to the following Examples.

Experimental Analysis

Raman spectroscopy was conducted using a Renishaw RM Mkl system 1000 with 633 nm HeNe laser (at power <mW) as the excitation source; and Renishaw inVia Raman microscope equipped with 532 nm and 633 nm excitation sources. Graphene flakes were deposited on an oxide-covered silicon wafer. Scanning electron microscopy (using Philips XL30 FEG-SEM HKL EBSD) and optical microscopy (using Olympus BH-2 microscope with 50× objective) were also used to locate and further characterise the carbonaceous electrodeposits. SEM images were obtained using E-SEM FEI Quanta 200.

Analysis of Graphene by Raman Spectroscopy

It is well established in the literature that Raman spectroscopy can be used to determine the number of layers that a carbon flake possesses through the shape, intensity and position of the D (˜1350 cm⁻¹), G (˜1580 cm⁻¹) and 2D (˜2700 cm⁻¹) peaks (the 2D peak may be alternatively referred to as the G′ peak).
The exact positions of the Raman peaks depend on the excitation wavelength used and the level of doping in the sample [Ferrari 2006]. In general, the Raman spectrum for single layer graphene comprises a 2D peak which can be fitted with a single component and is similar or higher in intensity than the G peak. The 2D peak for monolayer graphene occurs at approximately 2637 cm⁻¹when measured using a 633 nm excitation laser. As the number of layers increase, the 2D peak decreases in relative intensity to the G peak. The 2D peak also widens and its position increases in wavenumber [Hao 2010]. For example, the 2D peak for two layers is well described by four components. Significantly as the number of layers increase, the spectrum becomes less symmetrical and approaches a peak with two components, i.e. having a main peak with a less intense shoulder at a lower wavenumber.
The 2D peak would be expected to be centred at approximately 2637, 2663, 2665, 2675 and 2688 cm⁻¹for 1-layer, 2-layer, 3-layer, many-layer and graphite respectively using a 633 nm laser to measure graphene flakes deposited on an oxide-covered silicon wafer. For other laser excitation energies such as 532 nm, the 2D peak position is slightly shifted but is similarly well defined for 1-layer and few layer graphene.
The intensity of the D peak relative to the G peak also provides an indication of the number of structural defects such as graphene edges and sub-domain boundaries in the material produced. A D peak to G peak ratio (I_D/I_G) of around 0.2 may be expected for pristine graphene and the lower the ratio the better the quality material produced [Malard 2009].

Example 1

An electrolysis cell was provided with platinum mesh anode and copper foil (total surface area of 5 cm²) cathode, with the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆]) functioning as electrolyte (Aldrich). The cell was hosted in a glass container similar to that illustrated in FIG. 1. The cell assemblage allowed CO₂gas to enter at the bottom of the cell. All the electrolysis and electrolyte handling was conducted under an Argon atmosphere inside a glovebox. Before injecting any CO₂, the electrolyte was treated through a pre-electrolysis step in which 1.5 V was applied between two Pt wires. The CO₂was injected into the cell with a flow rate of 100 cm³/min for 1 hour before electrolysis. Current was allowed to pass through the cell between the platinum mesh and copper foil electrodes at constant voltage of 3 V for 3 hours. After electrolysis, the electrolyte was taken out of the glove box and filtered using Anopore™ inorganic membrane (Anodisc™). The particles on the membrane surface were then washed in situ with water and acetone, and then subjected to Raman analysis. Examples of the Raman spectra for material isolated using this method are provided in FIGS. 2 and 3.
As seen in FIGS. 2 and 3, the Raman spectra show 2D peaks at 2662 and 2647 cm⁻¹indicating that 1- to 3-layer graphene had been formed.

Example 2

This reaction was conducted in an identical way to Example 1 except that the cathode was Ni—Mo alloy. Similar material to Example 1 was produced.
As seen in FIG. 4, the Raman spectrum shows a 2D peak at 2658 cm⁻¹indicating that 1- to 2-layer graphene had been formed.

Example 3

This reaction was conducted in an identical way to Example 1 except that the cathode was Mo—Ti alloy. Similar material to Example 1 was produced.
As seen in FIG. 5, the Raman spectrum shows a 2D peak at 2655 cm⁻¹indicating that 1- to 2-layer graphene had been formed.

Example 4

An electrolysis cell was provided with platinum mesh anode and copper foil cathode, similar to Example 1. All the electrolysis and electrolyte handling was conducted under an Argon atmosphere inside a glove box. The electrolyte, however, was prepared by mixing small portions of choline chloride and urea in a mole ratio of 1:2 in an inert atmosphere. The mixture was then heated to 50° C. and CO₂was injected into the cell with a flow rate of 100 cm³/min for 1 hour before electrolysis. Current was then allowed to pass through the cell between the platinum mesh and copper foil electrodes at a constant voltage of 3 V for 3 hours with the CO₂continually bubbling at the same flow rate. After electrolysis, the electrolyte was filtered. The copper electrode was also rinsed with water and acetone and the material coated on its surface subjected to Raman analysis. The Raman spectrum for material deposited on the copper surface is provided in FIG. 6.
As seen in FIG. 6, the Raman spectrum shows a 2D peak at 2658 cm⁻¹indicating that 1- to 2-layer graphene had been formed.

Example 5

A three-electrode electrolysis cell was used (copper foil cathode, platinum mesh anode, silver/silver tetrafluoroborate reference electrode) with Autolab Potentiostat (PGSTAT 100, Eco-Chemie) using GPES and NOVA software. The room temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]) [Sigma Aldrich] was used as electrolyte. The cell was prepared in a similar fashion to Example 1, although the use of a three-electrode configuration meant that a lower potential was applied to the working electrode (copper foil) as this potential relates to the copper/electrolyte interface rather than the overall cell potential between the anode and cathode. Consequently a potential of −1.3 V, with respect to the reference electrode, was applied for 1 hour in this Example.
An SEM image of the resulting graphene flakes (FIG. 7) clearly shows copper surface features running underneath, thus indicating that the deposit formed is only a few layers thick.

Example 6

The same electrolysis cell as Example 5 was used. Similarly, the same room temperature ionic liquid was used as electrolyte. CO₂reduction was carried out at room temperature and at atmospheric pressure at a potential of −1.3 V with respect to the reference electrode, for 1 hour.
Raman analysis was carried out in the same way as for the other examples but in this case an excitation wavelength of 532 nm was used. FIG. 9 shows the spectrum obtained from the reduced material after transfer via PMMA spin coating to a Si/SiO₂wafer. The location and shape of the peaks as well as the relative intensities of the D, G and 2D bands confirms 5-layer graphene, turbostratically stacked, with a thickness of approximately 2 nm has been produced.
Furthermore, Raman analysis of the copper electrode prior to transfer to the silicon wafer shows direct evidence of graphitic features. SEM analysis shows that the majority of the electrode surface is covered in CO₂reduction products.
Furthermore, AFM imaging confirms that the flake-like deposits comprise few layer graphene.

Example 7

An electrolysis cell similar to that used in Examples 5 and 6 was used to form elemental carbon from CO₂. The electrolyte was N-methyl pyrrolidone (NMP) with tetrabutylammonium tetrafluoroborate (TBABF₄). The TBABF₄was used at a concentration of 0.1 M. The reduction was carried out at −2.2V vs Ag/AgCl.
Optical microscopy and AFM imaging shows that graphite nanoplatelet structures having a thickness of less than 100 nm were formed.

Example 8

An electrolysis cell similar to that used in Examples 5 to 7 was used with a copper foil working electrode and a gold anode to achieve effective reduction of CO₂. The electrolyte was N-methyl pyrrolidone (NMP) with 0.1M lithium tetrafluoroborate (LiBF₄).
The applied potential corresponded to that of the lithium underpotential deposition for the electrode.
Raman analysis confirms the formation of graphite nanoplatelet structures having a thickness of less than 100 nm. In addition, XPS analysis of the electrode surface shows evidence of a 55-65% carbon coverage based on surface area with a majority of this in the sp²hybridised state and having a low oxygen content, this being consistent with deposition of graphene/graphite nanoplatelet structures having a thickness of less than 100 nm. XPS measurements were made using the NEXUS service on an AXIS Nova XPS Spectrometer (Kratos Analytical). The following experimental parameters were used; XPS spectrum lens mode with field of view 1, with a survey resolution of pass energy 160 and acquisition time of 362 s for 3 sweeps. An aluminium anode (225 W) was employed with a step size of 1000 meV and dwell time 100 ms with the charge neutraliser off. Analysis and fitting was done using CasaXPS software Version 2.3.17dev6.2a.

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Claims

1. A process for producing graphene and/or graphite nanoplatelet structures having a thickness of less than 100 nm, the process comprising electrochemical reduction of carbon oxide in an electrochemical cell, wherein the cell includes:

(a) a negative electrode including a transition metal, transition metal-containing alloy, transition metal-containing oxide, transition metal containing ceramic or combination thereof;

(b) a positive electrode; and

(c) an electrolyte;

wherein the process further includes the step of passing a current between the electrodes in the presence of the carbon oxide.

2. A process according to claim 1 wherein the process is for producing graphene, preferably wherein the graphene consists of from one to five graphene layers.

3. The process of claim 1 wherein the carbon oxide is carbon dioxide.

4. The process of claim 1 wherein the negative electrode includes copper, nickel-molybdenum alloy, molybdenum-titanium alloy or a combination thereof.

5. A process according to claim 4 wherein the negative electrode includes copper.

6. The process of claim 1 wherein the electrolyte is an ion-containing liquid, preferably selected from an ionic liquid, eutectic solvent, ionic solution or combination thereof, preferably wherein the ion-containing liquid is an ionic liquid or eutectic solvent.

7. A process according to claim 6 wherein the ion-containing liquid is an ionic liquid and wherein the ionic liquid includes a cation selected from the group consisting of 1-alkyl-3-methylimidazolium, 1-alkylpyridinium, N-methyl-N-alkylpyrrolidinium and ammonium and phosphonium ions; and an anion selected from the group consisting of a halide, tetrafluoroborate, hexafluorophosphate, bistriflimide, triflate, tosylate formate, alkylsulfate, alkylphosphate and glycolate.

8. A process according to claim 7 wherein the ionic liquid is selected from the group consisting of 1-butyl-3-methylimidazolium tetrafluoroborate (i.e. [bmim][BF₄]), 1-butyl-3-methylimidazolium hexafluorophosphate (i.e. [bmim][PF₆]) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (i.e. [bmim][NTf₂]).

9. A process according to claim 6 wherein the ion-containing liquid is an eutectic solvent and wherein the eutectic solvent is selected from the group consisting of a mixture of ZnCl₂and choline chloride, a mixture of CoCl₂*6H₂O and choline chloride, a mixture of choline chloride and urea, a mixture of ZnCl₂and urea, a mixture of choline chloride and malonic acid, a mixture of choline chloride and phenol, and a mixture of choline chloride and glycerol.

10. A process according to claim 9 wherein the eutectic solvent is a mixture of choline chloride and urea.

11. A process according to claim 6 wherein the ion-containing liquid is an ionic solution and wherein the ionic solution includes an ammonium salt, an alkali metal bicarbonate, an alkali metal carbonate, an alkali metal halide or a combination thereof.

12. A process according to claim 11 wherein the ionic solution includes an ammonium salt, LiHCO₃, NaHCO₃, Na₂CO₃, K₂CO₃, a sodium halide, a potassium halide or a combination thereof.

13. The process of claim 1 wherein the temperature within the electrochemical cell is at least 10° C. and wherein the temperature within the electrochemical cell does not exceed 120° C.

14. The process of claim 1 further comprising the step of isolating the graphene and/or graphite nanoplatelet structures.

15. The process of claim 1 wherein the process further includes the step of exfoliating the graphene and/or graphite nanoplatelet structures.