Patent Application
20200330961
The present invention relates to composite materials having catalytic activity in the electrolytic oxidation of water.
Hydrogen as Fuel.
Hydrogen is “the fuel of the future” because it is abundant, as a component of water, and benign in terms of emissions. It holds tremendous promise as an indefinitely renewable liquid fuel. Hydrogen can be used in fuel cells or directly combusted for transportation or power generation. Portable applications include consumer electronics or auxiliary power units. In transportation, hydrogen can power fuel cells or be burned directly in appropriately adapted internal combustion engines. Finally, hydrogen fuel cells can be used for power generation at remote locations, in backup power units for conventional power plants or as stand-alone, stationary power plants. Global hydrogen production is currently derived from natural gas (48%), oil (30%), coal (18%) and electrolysis of water (4%). As hydrogen is currently sourced mostly from fossil fuels, a sustainable “hydrogen economy” must shift away from these inputs and extract hydrogen from water. This can be achieved by electrolysis.
Photosynthesis.
Plants use sunlight to split water to form hydrogen (as protons) and oxygen within photosynthesis. The step of converting two water molecules to an oxygen molecule and four protons occurs within the Oxygen Evolving Complex (OEC) in Photosystem II (PS-II), a catalytic centre comprising of an oxo-bridged, cubical CaMn4O4 cluster (see
Bio-Inspiration in Water Oxidation Catalysis.
Numerous studies have been performed, aiming to mimic, in some measure, water oxidation catalysis by the PSII-OEC. Many have involved the use of sacrificial chemical oxidants, such as Ce4+, to cycle and turnover the catalysts. Others have sought to achieve turnover by direct electrochemical electron transfer. While interesting and important advances have been made in this field, the key principles needed to guide bio-inspiration in respect of water oxidation catalysis, have generally remained unclear.
In view of the above, a synthetic mimic of PSII-OEC which can be utilised for the efficient electrolysis of water would be highly desirable.
An object of the present invention is to provide a material having catalytic activity in the electrolytic oxidation of water. Another object of the present invention is to provide a material which is suitable for facilitating the electrolytic oxidation of water, when used as the anode in an electrolytic cell.
In a first aspect of the invention, there is provided a composite material comprising a graphene-based material, manganese oxide, and group II metal ions.
The following options may be used in conjunction with the first aspect, either individually or in any suitable combination.
The group II metal ions may be Ca2+ ions, Sr2+ ions, or a mixture thereof.
The manganese oxide may be amorphous.
The graphene-based material may comprise any one or more of graphene, reduced graphene oxide, liquid crystalline reduced graphene oxide, basal plane pyrolytic graphite and carbon nanotubes. The graphene-based material may have a thickness of about 0.1 to about 1000 μm. The graphene-based material may have a conductance of about 100 to about 500 S/cm
The graphene-based material may be functionalised with an organic moiety comprising an acidic functional group. The graphene-based material may be functionalised with an organic moiety comprising an acidic functional group at an edge of the graphene-based material, to provide edge functionalised graphene-based material. The organic moiety may be an amino acid. The amino acid may be tyrosine or glutamate. The amino acid may be an aminoalkanoic acid. The organic moiety may be an aminophenol or an aminobenzoic acid.
The composite material may be disposed as a layer on a substrate. The substrate may comprise an electrically conductive material which is in contact with the graphene-based material. In one embodiment, the electrically conductive material may be a film. The substrate may further comprise an electrically non-conductive material and the film is disposed thereon. The electrically non-conductive material may be poly(ethylene terephthalate). The film may comprise copper, silver, aluminium, nickel, stainless steel, or a mixture thereof. In an alternative embodiment, the electrically conductive material may be a metal mesh. The metal may be nickel or stainless steel.
In one embodiment there is provided a composite material comprising liquid crystalline reduced graphene oxide, amorphous manganese oxide, and group II metal ions, for example Ca2+ ions or Sr2+ ions.
In another embodiment there is provided a composite material comprising liquid crystalline reduced graphene oxide, amorphous manganese oxide, and group II metal ions, for example Ca2+ ions or Sr2+ ions, wherein said composite material is disposed as a layer on a substrate. The substrate may comprise an electrically non-conductive material. The substrate may additionally or alternatively comprise an electrically conductive material.
In another embodiment there is provided a composite material comprising liquid crystalline reduced graphene oxide, amorphous manganese oxide, and group II metal ions, for example Ca2+ ions or Sr2+ ions, wherein said composite material is disposed as a layer on a substrate, the substrate comprising an electrically conductive material in the form of a film. The film may be disposed between an electrically non-conductive material and the composite material layer and may be in physical contact with the liquid crystalline reduced graphene oxide.
In another embodiment there is provided a composite material comprising liquid crystalline reduced graphene oxide, amorphous manganese oxide, and group II metal ions, for example Ca2+ ions or Sr2+ ions, wherein said composite material is disposed as a layer on a substrate, the substrate comprising an electrically conductive material which is a metal mesh. The metal mesh may in physical contact with the liquid crystalline reduced graphene oxide.
In another embodiment there is provided a composite material comprising liquid crystalline reduced graphene oxide, amorphous manganese oxide, and group II metal ions, for example Ca2+ ions or Sr2+ ions, wherein said composite material is disposed as a layer on a substrate, the substrate comprising an electrically conductive material, and wherein the liquid crystalline reduced graphene oxide is functionalised with an organic moiety comprising an acidic functional group.
In another embodiment there is provided a composite material comprising liquid crystalline reduced graphene oxide, amorphous manganese oxide, group II metal ions, for example Ca2+ ions or Sr2+ ions, and a second graphene-based material which is functionalised with an organic moiety comprising an acidic functional group, wherein said composite material is disposed as a layer on a substrate, the substrate comprising an electrically conductive material. The second graphene-based material may be disposed on an upper (i.e. away from the substrate) surface of the liquid crystalline reduced graphene oxide.
In another embodiment there is provided a composite material comprising liquid crystalline reduced graphene oxide, amorphous manganese oxide, and group II metal ions, for example Ca2+ ions or Sr2+ ions, wherein said composite material is disposed as a layer on a substrate, the substrate comprising an electrically conductive material which is a film. The film may be disposed between an electrically non-conductive material and the layer. The liquid crystalline reduced graphene oxide may be functionalised with an organic moiety comprising an acidic functional group. The film may be in contact with the liquid crystalline reduced graphene oxide.
In another embodiment there is provided a composite material comprising liquid crystalline reduced graphene oxide, amorphous manganese oxide, and group II metal ions, for example Ca2+ ions or Sr2+ ions, wherein said composite material is disposed as a layer on a substrate, the substrate comprising an electrically conductive material which is a metal mesh, and wherein the liquid crystalline reduced graphene oxide is functionalised with an organic moiety comprising an acidic functional group. The mesh may be in contact with the liquid crystalline reduced graphene oxide.
In another embodiment there is provided a composite material comprising liquid crystalline reduced graphene oxide, amorphous manganese oxide, group II metal ions, for example Ca2+ ions or Sr2+ ions, and a second graphene-based material which is functionalised with an organic moiety comprising an acidic functional group, wherein said composite material is disposed as a layer on a substrate, the substrate comprising an electrically conductive material which is a film. The film may be disposed between an electrically non-conductive material and the composite material layer and be in physical contact with the liquid crystalline reduced graphene oxide.
In another embodiment there is provided a composite material comprising liquid crystalline reduced graphene oxide, amorphous manganese oxide, group II metal ions, for example Ca2+ ions or Sr2+ ions, and a second graphene-based material which is functionalised with an organic moiety comprising an acidic functional group, wherein said composite material is disposed as a layer on a substrate, the substrate comprising a metal mesh. The metal mesh may be in physical contact with the liquid crystalline reduced graphene oxide.
In another embodiment there is provided a composite material comprising edge functionalised graphene functionalised with an organic moiety comprising an acidic functional group, amorphous manganese oxide, and group II metal ions, for example Ca2+ ions or Sr2+ ions, wherein said composite material is disposed as a layer on a substrate, the substrate comprising an electrically conductive material which is a metal mesh. The organic moiety may be an aminobenzoic acid. The organic moiety may be tyrosine. The organic moiety may be an aminoalkanoic acid. The mesh may be in contact with the edge functionalised graphene.
In a second aspect of the invention, there is provided a method of producing a composite material, the method comprising:
The following options may be used in conjunction with the first aspect, either individually or in any suitable combination.
The group II metal ions may be Ca2+ ions, Sr2+ ions, or a mixture thereof.
Step c. may be such that the applied manganese oxide is amorphous.
The graphene-based material may comprise any one or more of graphene, reduced graphene oxide, liquid crystalline reduced graphene oxide, basal plane pyrolytic graphite or carbon nanotubes.
Step a. may comprise dip-coating said substrate. Step a. may be such that the applied graphene-based material has a thickness of about 0.1 to about 1000 μm. Step a. may be such that the applied graphene-based material has a conductance of about 100 to about 500 S/cm
Step b. may comprise immersing the product of step a. in an aqueous solution comprising group II metal ions.
Step c. may comprise electrodeposition of manganese oxide onto said surface.
The method may further comprise functionalising the graphene-based material with an organic moiety comprising an acidic functional group. Said functionalising with an organic moiety comprising an acidic functional group may comprise covalently attaching an organic molecule comprising an acidic functional group to the graphene-based material. In one embodiment, said covalently attaching may occur at an edge of the graphene-based material to provide edge functionalised graphene-based material. In an alternative embodiment, said covalently attaching the organic molecule may comprise attaching an amine linker molecule to the graphene-based material through a diazonium coupling, followed by attachment of the organic molecule comprising an acidic functional group to said linker molecule by an amide coupling. The organic moiety or molecule may be an amino acid. The amino acid may be tyrosine or glutamate. The amino acid may be an aminoalkanoic acid. The organic moiety or molecule may be an aminophenol or an aminobenzoic acid.
The substrate may comprise an electrically conductive material which is in contact with the graphene-based material. In one embodiment, the electrically conductive material may be a film. The substrate may further comprise an electrically non-conductive material and the film is disposed thereon. The electrically non-conductive material may be poly(ethylene terephthalate). The film may comprise copper, silver, aluminium, nickel, stainless steel, or a mixture thereof. In an alternative embodiment, the electrically conductive material may be a metal mesh. The metal may be nickel or stainless steel.
In one embodiment there is provided a method of producing a composite material, the method comprising applying liquid crystalline reduced graphene oxide to a substrate comprising poly(ethylene terephthalate), treating the liquid crystalline reduced graphene oxide with group II metal ions, for example Ca2+ ions or Sr2+ ions, and applying manganese oxide to the graphene-based material.
In another embodiment there is provided a method of producing a composite material, the method comprising applying liquid crystalline reduced graphene oxide to a substrate comprising poly(ethylene terephthalate) and an electrically conductive film on a surface thereof, treating the liquid crystalline reduced graphene oxide with group II metal ions, for example Ca2+ ions or Sr2+ ions, and applying manganese oxide to the graphene-based material.
In another embodiment there is provided a method of producing a composite material, the method comprising applying liquid crystalline reduced graphene oxide to a substrate comprising poly(ethylene terephthalate), treating the resulting material with group II metal ions, for example Ca2+ ions or Sr2+ ions, and applying manganese oxide to the resulting material, wherein the liquid crystalline reduced graphene oxide is functionalised with an organic moiety comprising an acidic functional group.
In another embodiment there is provided a method of producing a composite material, the method comprising applying liquid crystalline reduced graphene oxide to a substrate comprising a metal mesh, treating the resulting material with group II metal ions, for example Ca2+ ions or Sr2+ ions, and applying manganese oxide to the resulting material, wherein the liquid crystalline reduced graphene oxide is functionalised with an organic moiety comprising an acidic functional group.
In another embodiment there is provided a method of producing a composite material, the method comprising applying liquid crystalline reduced graphene oxide to a substrate comprising poly(ethylene terephthalate) and an electrically conductive film on a surface thereof; applying a second graphene-based material to the resulting material, wherein the second graphene-based material is functionalised with an organic moiety comprising an acidic functional group; treating the resulting material with group II metal ions, for example Ca2+ ions or Sr2+ ions; and applying manganese oxide to the resulting material.
In another embodiment there is provided a method of producing a composite material, the method comprising applying liquid crystalline reduced graphene oxide to a substrate comprising a metal mesh; applying a second graphene-based material to the resulting material, wherein the second graphene-based material is functionalised with an organic moiety comprising an acidic functional group; treating the resulting material with group II metal ions, for example Ca2+ ions or Sr2+ ions; and applying manganese oxide to the resulting material.
In another embodiment there is provided a method of producing a composite material, the method comprising knife-coating a mixture of liquid crystalline graphene oxide and hypophosphorous acid onto a substrate comprising poly(ethylene terephthalate), immersing the resulting material in an aqueous calcium chloride solution, and electrodepositing manganese oxide onto the resulting material.
In another embodiment there is provided a method of producing a composite material, the method comprising knife-coating a mixture of liquid crystalline graphene oxide and hypophosphorous acid onto a substrate comprising poly(ethylene terephthalate) and a copper film on a surface thereof, immersing the resulting material in an aqueous calcium chloride solution, and electrodepositing manganese oxide onto the resulting material.
In another embodiment there is provided a method of producing a composite material, the method comprising dip-coating a mixture of liquid crystalline graphene oxide onto a substrate comprising a metal mesh followed by reduction, then immersing the resulting material in an aqueous calcium chloride solution, and electrodepositing manganese oxide onto the resulting material.
In another embodiment there is provided a method of producing a composite material, the method comprising applying a mixture of edge functionalised graphene, wherein the edge-functionalised graphene is functionalised with an organic moiety comprising an acidic functional group, onto a substrate comprising a metal mesh, then immersing the resulting material in an aqueous calcium chloride solution, and electrodepositing manganese oxide onto the resulting material.
In a third aspect of the invention, there is provided a composite material produced by the method of the second aspect of the invention.
In a fourth aspect of the invention, there is provided an electrode comprising the composite material of any one of the first or third aspects of the invention.
In a fifth aspect of the invention, there is provided a method of electrolysing water, the method comprising at least partially immersing the electrode of the fourth aspect of the invention and a counter electrode in an aqueous solution and applying a voltage between said electrodes.
In a sixth aspect of the invention, there is provided a use of the composite material of the first or third aspects of the invention in the fabrication of an electrode.
In a seventh aspect of the invention, there is provided a use of the electrode of the fourth aspect of the invention for the electrolysis of water.
In an eighth aspect of the invention, there is provided a method of generating H2, the method comprising at least partially immersing the electrode of the fourth aspect of the invention and a counter electrode in an aqueous solution and applying a voltage between said electrodes.
RGO: reduced graphene oxide
RLCGO: reduced liquid crystalline graphene oxide
LGCO: liquid crystalline graphene oxide
EFG: edge functionalised graphene
PET: poly(ethylene terephthalate)
MnOx: manganese oxide
PSII-OEC: photosystem II—oxygen evolving complex
FRGO: functionalised reduced graphene oxide
The present specification relates to a composite material which may be used to facilitate the electrolytic oxidation of water when used as the anode in an electrolytic cell.
The inventors have unexpectedly found that important components in the PSII-OEC are the Ca cap and the Mn(4) ion of the CaMn4O4 cluster. These species appear to facilitate the step of O—O bond formation, which may further rely on coupled e−/H+ transfer to the phenoxyl O of an oxidised tyrosine group. The inventors have proposed that to mimic the action of the PSII-OEC, non-biological man-made systems should contain closely proximate, co-located, Ca and Mn oxide species, with a nearby conducting, organic material capable of facilitating ready electron transfer. Ca may also be replaced by Sr in the composite material of the invention.
Surprisingly, the inventors have found that an electrode fabricated from a composite material designed to mimic PSII-OEC according to the above principles exceeds the activity of an industrial standard platinum electrode in the electrolytic oxidation of water. It is suggested that the catalytic efficiency of the PSII-OEC may be replicated by co-locating Ca2+ ions or Sr2+ ions in close proximity to a manganese oxide layer, which is, in turn, proximate to a conducting organic species capable of facilitating electron transfer.
The composite material of the invention comprises a graphene-based material, Ca2+ ions or Sr2+ ions and manganese oxide. The graphene-based material may be functionalised with an organic moiety, which is thought may aid in electron transport. It is thought that when in close proximity with one another, Ca2+ or Sr2+ ions and manganese oxide may function as a catalyst for water oxidation. The function of the graphene-based material is thought to be to conduct electrons between the “catalyst” and the electrolytic circuit. The composite material may be disposed on a substrate, which may be electrically conductive. The substrate may also be electrically non-conductive.
In the context of this specification, the term “comprising” is taken to require the presence of the recited integer(s) but does not preclude the presence of others and does not imply any particular concentration or proportion of the recited integer(s).
In the context of this specification, a graphene-based material is any material comprising sheets of carbon-based extended aromatic, commonly benzenoid, systems. The graphene-based material may comprise other atoms. It may comprise regions which are not aromatic. The sheets of extended aromatic systems may, or may not, be covalently linked to one another. Examples of graphene-based materials include, but are not limited to, graphene, reduced graphene oxide, graphene oxide, partially exfoliated graphite, liquid crystalline reduced graphene oxide (RLCGO), basal plane pyrolytic graphite and carbon nanotubes (single or multi-walled).
In the context of this specification, “coating” may refer to total or partial coverage of a surface. Where the coverage is partial, the coating may be distributed approximately evenly across the entirety of the surface.
The graphene-based material may have a thickness of about 0.1 to about 1000 μm, or about 0.1-5, 5-10, 0.1-1, 1-2.5, 2.5-5, 5-7.5, 7.5-10, 10-20, 20-40, 40-60, 60-100, 100-200, 200-300, 400-500, 500-600, 700-800, 900-1000, 1-100, 200-500, or 500-1000 μm. The thickness may be about 0.5, 1.0, 1.5, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10, 20, 40, 60, 100, 200, 300, 400, 500, 600, 700, 800, 900 or 1000 μm. The graphene-based material may have a conductance of about 100 to about 500 S/cm, or about 100-250, 250-500, 100-200, 200-300, 300-400 or 400-500 S/cm. The conductance may be about 100, 200, 225, 250, 300, 400 or 500 μm.
In the context of this specification the term “about” is taken to mean±10% of the stated value, unless signified otherwise by the context.
The manganese oxide may be a particular oxide of manganese or it may refer to a mixture of different manganese oxides. Manganese oxides which may be present include, but are not limited to, manganese(II) oxide, manganese(II,III) oxide, manganese(III) oxide and manganese(IV) oxide. Manganese(III) oxy species may also be present. The manganese oxide may be amorphous. In particular, manganese oxide prepared by electrodeposition is likely be amorphous, especially when electrodeposition is performed in the presence of group II metal ions such as Ca2+ and/or Sr2+. It may be at least about 80% amorphous, or at least about 85, 90 or 95% amorphous, or may be about 100% amorphous. It may have no detectable crystallinity. Amorphicity may be determined by X-ray powder diffraction.
The graphene-based material may be deposited in the form of a partial or complete coating directly on the substrate. Ca2+ ions or Sr2+ ions may be present on a surface of the graphene-based material. The surface may be the upper surface (i.e. away from the substrate) or the lower surface (i.e. towards the substrate). The Ca2+ ions or Sr2+ ions may be present on both surfaces. They may be present throughout the graphene-based material. Manganese oxide may be present on an upper of lower surface of the graphene-based material. It is hypothesised that the Ca2+ ions or Sr2+ ions may engage in intermolecular interactions with the graphene-based material, and/or with the manganese oxide, or with both simultaneously.
The composite material may be disposed as a layer on a substrate. The substrate may comprise any solid material. In some embodiments, the substrate may comprise or consist of an electrically non-conductive material and the graphene-based material may be in contact with the electrically non-conductive material. In other embodiments, the substrate may comprise or consist of an electrically conductive material, which is in contact with the graphene-based material. The substrate may also comprise both an electrically non-conductive material and an electrically conductive material.
The electrically non-conductive material may have a conductivity of less than about 1×10−10 S/m. The conductivity may be between about 0 to about 1×10−10 S/m, or about 0-1×10−20, 0-1×10−30, 0-1×10−40, or between about 0-1×10−50 S/m. The conductivity may be about 1×10−10 S/m, or about 1×10−20, 1×10−30, 1×10−40, or about 0-1×10−50 S/m. The substrate may be comprised of a material which may be flexible or it may be rigid. Non-limiting examples of electrically non-conductive materials suitable for use in the substrate in the present invention are plastics such as polyolefins, polyesters, polyamides or polyacrylates, glass, paper, and ceramic materials such as silica or alumina. A suitable material is poly(ethylene terephthalate).
The electrically conductive material may be any material, such as a metal, which is a good electrical conductor. Good electrical conductors may have a conductivity of about 1×105 S/m to about 1×1010 S/m. The electrically conductive material may have a conductivity of at least about 1×107, 2.5×107, 5×107, 1×108, 1×109, or about 1×1010 S/m. Examples of suitable metals include, but are not limited to, copper, gold, silver, aluminium, titanium, or nickel. The electrically conductive material may comprise a mixture or alloy of more than one metal, such as stainless steel. The electrically conductive material may comprise a conductive non-metal such as indium tin oxide, lanthanum-doped strontium titanate, yttrium-doped strontium titanate, doped polyacetylenes, polypyrroles, polyanilines, poly(p-phenylene vinylene), polythiophenes, or mixtures and/or co-polymers thereof.
Where the substrate comprises or consists of an electrically conductive material, the substrate may be formed in a range of possible morphologies and structures. For example, when the substrate is an electrically conductive material it may be a foil, mesh, foam or a customised 3D structure. Another suitable electrically conductive material may be a titanium/expanded graphite electrode. Such structures may be made by additive manufacturing. The electrically conductive material may be a mesh, in particular a metal mesh. A suitable material for a mesh is nickel, or stainless steel. The mesh should be of sufficient thread density to ensure adequate conductivity. A suitable thread density is at least 25 strands/cm. The thread density may be 25 strands/cm, or 30, 40, 50, 60, 70, 80, 90, 100 strands/cm. The electrically conductive material may be a 3D metal foam, for example a nickel or titanium metal foam. Where the substrate is a mesh, the composite material may be disposed on the mesh in the sense that it coats the surface of the fibers of the mesh, or, it may alternatively or additionally be disposed on the mesh in the sense that it coats an upper or a lower surface of the mesh as a whole.
The substrate may comprise an electrically conductive material which is a film (or a foil). A film and a foil are understood to refer to the same structure, with the distinction that a foil is freestanding, in contrast to a film, which is usually disposed on a surface. In the context of this specification, a film (or a foil) is a very thin layer of thickness at most about 1000 μm. A film (or a foil) may have a thickness of at most about 500, 200, 100, 50, 25 or 10 μm. The thickness of the film may be about 0.1 to about 1000 μm, or about 0.1-5, 5-10, 0.1-1, 1-2.5, 2.5-5, 5-7.7, 7.5-10, 10-20, 20-40, 40-60, 60-100, 100-200, 200-300, 400-500, 500-600, 700-800, 900-1000, 1-100, 200-500, 500-1000 μm. The thickness may be about 0.5, 1.0, 1.5, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900, or 1000 μm.
Where the substrate comprises an electrically conductive material which is a film, the substrate may in some embodiments further comprise an electrically non-conductive material onto which the film is disposed. The electrically conductive film may be applied to the non-conductive material by any suitable method. For example, the electrically conductive film may be applied by sputter coating, vapour deposition or chemical reductive deposition from a solution of soluble metal salt. Commercially available materials such as indium tin oxide-coated plastic may also be suitable as the substrate of the invention.
In its simplest form, the composite material comprises a graphene-based material, group II metal ions, such as Ca2+ ions or Sr2+ ions, and manganese oxide. In some embodiments, the composite material may be disposed as a layer on a substrate. The substrate is typically an electrically conductive structure such as a mesh or foil, or comprises an electrically conductive film coated on an electrically non-conductive material. The graphene-based material may be in direct physical contact with the substrate, in particular the electrically conductive structure (such as a mesh or film). The group II metal ions such as Ca2+ ions or Sr2+ ions may be on a surface of the graphene-based material. The group II metal ions such as Ca2+ ions or Sr2+ ions may be on a surface of the manganese oxide. In some embodiments both the group II metal ions, such as Ca2+ ions or Sr2+ ions, and the manganese oxide are on a surface of the graphene-based material.
The graphene-based material may be functionalised with an organic moiety comprising an acidic functional group. The composite material may further comprise a second graphene-based material which may be functionalised with an organic moiety comprising an acidic functional group. The composite material may comprise both a functionalised graphene-based material and a functionalised second graphene-based material. Typically, the second graphene-based material will be disposed on an upper (i.e. away from the substrate) surface of the graphene-based material, or it may be disposed on a surface of the group II metal ions such as Ca2+ ions or Sr2+ ions, or on a surface of the manganese oxide. All the above options share the common feature that all comprise a composite material comprising a graphene-based material, group II metal ions, and manganese oxide. The composite material is optionally disposed as a layer on a substrate, the substrate optionally comprising an electrically conductive material which is in contact with the graphene-based material.
The graphene-based material (as well as the second graphene-based material) may be functionalised with an organic moiety comprising an acidic functional group. The organic moiety is derived from an organic molecule comprising at least one acidic functional group. Acidic functional groups may include carboxylic acids, thiocarboxylic acids, phenols, sulfonic acids, sulfinic acids, nitric acids, phosphinic acids or phosphonic acids. Any functional group which is capable of acting as a proton donor and/or is a Brønsted acid, may be suitable as an acidic functional group of the invention. The acidic functional group may be protonated, deprotonated or partially protonated. The extent of the protonation of the acidic functional group will be dependent on pH. If the acidic functional group is deprotonated the counter ion may be any suitable cation such as sodium, calcium, potassium, lithium or ammonium. The organic molecule may also comprise other functional groups, such as a functional group through which it is attached to the graphene-based material. Examples of suitable organic molecules include suitably functionalised amino acids such as tyrosine or glutamate, or benzoic acids such as terephthalic acid or aminobenzoic acid, aminoalkanoic acids such as aminobutyric acid, aminohexanoic acid, aminooctanoic acid, aminodecanoic acid, aminododecanoic acid (and isomers thereof) or phenols such as 4-hydroxybenzoic acid. ‘Aminoalkanoic acid’ refers to straight or branched saturated carbon chains bearing an amine group and a carboxylate. The graphene-based material may also be directly functionalised with an acidic functional group, for example it may be nitrated or sulfated.
In the context of this specification, “functionalisation” may refer to attachment through covalent or ionic bonds or intermolecular forces. Intermolecular forces may include dipole-dipole interactions, hydrogen bonding or Van der Waals forces. Many approaches are known to the skilled person for functionalising surfaces, including surfaces of graphene-based materials, with acidic functional groups and/or organic molecules. Any suitable method may be used as described below with respect to a method of the invention.
In one form, therefore, there is provided a composite material comprising:
In another form, therefore, there is provided a composite material comprising:
The present invention also relates to a method of producing a composite material, the method comprising first, a. applying a graphene-based material to a substrate, b. treating the product of step a. with Ca2+ ions or Sr2+ ions and c. applying manganese oxide to the product of step b., wherein the substrate comprises an electrically conductive material. The steps of b. treating with Ca2+ ions or Sr2+ ions and c. applying manganese oxide may be performed in any suitable order e.g. step b. followed by step c.; or step c. followed by step b. Steps of the method may be performed simultaneously, for example, steps b. and c., or a. and b., may be performed simultaneously.
The graphene-based material may be applied to the substrate by knife coating. Knife coating refers to a known method of fabrication of films, and involves spreading a material over a surface using a blade at a fixed height from the surface of the substrate. By controlling the fixed height, a desired film thickness may be obtained. Liquid crystalline reduced graphene oxide (RLCGO) may be applied onto the substrate by knife coating the substrate with a slurry comprising a mixture of liquid crystalline graphene oxide and hypophosphorous acid. In this process, the graphene oxide is reduced by the hypophosphorous acid, resulting in a coating of liquid crystalline reduced graphene oxide.
The graphene-based material may also be applied to the substrate by dipping the substrate into a solution of the graphene-based material, or a precursor of the graphene-based material (referred to as dip-coating). Reduced liquid crystalline reduced graphene oxide (RLCGO) may be applied onto the substrate by dipping the substrate into aqueous dispersion of liquid crystalline graphene oxide, leaving the coated substrate to dry overnight, then reducing the liquid crystalline graphene oxide at first by heat annealing in an oven at 120° C., then later by chemical reduction with Na2S2O4 in water.
The graphene-based material may also be applied to the substrate by drop coating (also known as drop casting) a solution of the graphene-based material, or a precursor thereof, onto the substrate.
A surface of the material may be treated with group II metal ions such as Ca2+ ions or Sr2+ ions by immersing the material into an aqueous solution of said metal ions. The group II metal ions may be provided as any suitable, water soluble, salt of the metal. For example, Ca2+ may be provided as any suitable water-soluble salt of calcium, or Sr2+ may be provided as any suitable water-soluble salt of strontium. In the context of this specification, “soluble” may refer to a material having a solubility of about 1 to about 30 mL/g (in water at 25° C.). The material may have a solubility of less than 1 mL/g. Examples of suitable calcium salts include, but are not limited to, calcium chloride, calcium bromide, calcium acetate, or calcium nitrate. Examples of suitable strontium salts include, but are not limited to, strontium chloride, strontium acetate, or strontium nitrate. The aqueous solution may comprise non-aqueous co-solvents or other organic or inorganic components. The concentration of the group II metal ion in the solution may be about 0.1 M to about 10M, or about 0.1-5, 5-10, 0.1-1, 1-2.5, 2.5-5, 5-7.5 or 7.5-10M. The concentration may be about 0.2, 0.5, 1, 2, 5 or 10M. The immersing may be for a period of about 10 to about 24 h. It may be for about 10-17, 17-24, 10-15, 15-20 or 20-24 h. It may be for about 10, 15, 20 or 24 h. Without wishing to be bound by any particular theory, it is hypothesised that group II metal ions such as Ca2+ ions and Sr2+ ions are able to bind to and complex with graphene-based material, and may complex with the benzenoid ring system thereof.
Manganese oxide may be coated onto a surface of the graphene-based material by electrodeposition. Electrodeposition refers to a known method of coating materials onto an electrically conductive surface, in which a charged species from a conductive solution is deposited onto an active electrode. This is process is disclosed in detail in, for example, Zhou, F., A. Izgorodin, R. K. Hocking, V. Armel, L. Spiccia, and D. R. MacFarlane, ChernSusChern, 2013, 6(4): p 643-651, which is incorporated herein by cross-reference. As an example, manganese oxide may be deposited from an aqueous solution of sodium nitrate and manganese acetate tetrahydrate, or alternatively from a solution of manganese dichloride. The electrodeposition may be carried out galvanostatically. The electrodeposition may be carried out at a current density of about 100 to about 500 μA/cm2, or about 100-250, 250-500, 100-200, 300-400 or 400-500 μA/cm2. The current density may be about 100, 200 or 500 μA/cm2. The electrodeposition may be carried out for a period of about 1 to about 15 minutes, or about 1-5, 5-10, 10-15, 1-2.5, 2.5-5, 5-7.5, 7.5-10, or 10-15 minutes. The electrodeposition may be carried out for about 1, 2, 5, 10, or 15 min.
The method may further comprise functionalising the graphene-based material with an acidic functional group. The acidic functional group may be part of an organic moiety. The step of functionalising may be carried out prior to step a., such that functionalised graphene-based material is applied to the substrate. Alternatively, the graphene-based material may be applied to the substrate in step a., followed by the step of functionalising.
Where the acidic functional group is part of an organic moiety, the step of functionalising with the organic moiety comprising the acidic functional group may comprise covalent attachment of an organic molecule comprising the acidic functional group. The organic moiety is derived from the organic molecule. Covalent attachment of the organic molecule comprising an acidic functional group may be achieved by any suitable method. One possible procedure is as follows. A linker molecule may be attached to the graphene-based material, followed by attachment of the organic molecule comprising an acidic functional group. For example, the linker molecule could be attached to the surface by radical chemistry e.g. diazonium chemistry, carbene chemistry e.g. using a diazirene, via complexation with a metal or nucleophilic addition e.g. Bingel reaction. The linker molecule may be suitably functionalised in order to facilitate the subsequent attachment of the organic molecule comprising an acidic functional group. Suitable strategies for attaching the organic molecule comprising an acidic functional group may include amide coupling, azide-alkyne Huisgen cycloaddition or Staudinger ligation.
An example of a suitable method for covalent functionalisation is diazonium coupling followed by amide coupling, which may proceed as follows. First, a linker molecule is attached to the graphene-based material (steps (i)-(iii) in
Another example of a suitable method for covalent functionalisation involves the use of a graphene-based material that is functionalised about an edge thereof. An example of such a material is edge-functionalised graphene (EFG). EFG comprises sheets of graphene having functional groups, typically carboxyl groups, disposed at their edges. These functional groups may further be covalently attached to other molecules, such as an organic molecule comprising an acidic functional group, such that the functional group is part of an organic moiety. The functional group may be an acidic functional group. Said covalently attaching may occur at an edge of the graphene-based material to provide edge functionalised graphene-based material. Edge functionalised graphene based materials may be prepared by literature methods (see, for example, Xiang et al., Adv. Mater. 2016, 28, p. 6253-6261 and references therein). Various graphene-based materials may be edge functionalised, including graphene and graphene oxide. Known methods for preparing EFG and edge-functionalised graphene oxide include covalently grafting 4-aminibenzoic acid (ABA) molecules, as organic molecular wedges, directly onto graphite particles in the presence of poly(phosphoric acid) in N-methyl-2-pyrrolidone, leading to high-yield exfoliation of the 3D graphite into 2D graphene-like sheets, oxidation-exfoliation of graphite with strong oxidising reagents (e.g., H2SO4, KMnO4) followed by reduction, and ball milling graphite with dry ice. The edge functionalisation may be with a range of functional groups or organic moieties including carboxylate or 4-aminobenzoic acid. For example, the graphene-based material may be edge-functionalised with carboxylate groups, which may be prepared by literature methods. This may be followed by amide coupling between the carboxylate groups and an organic molecule comprising both an acidic functional group and an amine. Examples of suitable coupling partners include aminoalkanoic acids, for example aminobutyric acid, aminohexanoic acid, aminooctanoic acid, aminodecanoic acid, aminododecanoic acid (and isomers thereof), amino acids such as tyrosine or glutamate, or aminobenzoic acids. Other suitable coupling partners include molecules comprising an amine and a functional group that may be converted into an acidic functional group. Alternatively, where the graphene-based material is edge functionalised with acidic functional groups, the EFG may itself be a functionalised graphene-based material.
In some embodiments the step of functionalising the graphene-based material comprises functionalising a second graphene-based material followed by applying the functionalised second graphene-based material onto a surface of the composite material. In such embodiments the application of the functionalised second graphene-based material may be carried out between steps a. and b., between steps b. and c. or after step c.
The functionalised second graphene-based material may be applied by drop coating. Drop coating (also referred to as drop casting) refers to a known method for the application of a thin coating to a sample by depositing consecutive drops of a solution on its surface, and allowing the solvent to evaporate.
The composite material of the invention may be used in the fabrication of an electrode, resulting in an electrode comprising the composite material. In general, the fabrication of an electrode will involve attaching a metal wire to the material to enable current to flow between the material and the circuit. For example, the fabrication of an electrode from the composite material may include the following: cutting the composite material to a desired size, attaching a copper wire along an edge of the composite material using silver paste and epoxy glue and coating said wire and paste with epoxy. Other steps may be required.
Deposition of Conductive Reduced Liquid Crystalline Graphene Oxide (RLCGO) on Flexible PET Films (RLCGO-PET).
Liquid crystalline graphene oxide (LCGO) dispersed in water (1.1 wt %, 5-10 μm sheet size) was obtained from the Australian National Fabrication Facility (ANNF) Materials Node at the University of Wollongong. Hypophosphorous acid (50 wt %, 1 mL) was added and mixed into the LCGO solution (10 mL) as the reducing agent. The mixture was then coated onto Multapex PET sheets (75 micron thickness) of dimensions ca. 10 cm×20 cm at room temperature using a doctor blade (0.6 mm blade height). The resulting, wet, brown-black films were dried in air at room temperature for 2 h and then left to dry in an oven at 80° C. overnight. They were thereafter dipped in water to wash away excess reducing agent and allowed to dry thoroughly in air. The resulting flexible, metallic, black graphene films on PET (RLCGO-PET) exhibited an average thickness of 1.5±0.3 μm, with conductances of 225±56 S/cm and resistivities of 40±10 Ω/square. The latter compared well with commercially supplied ITO glass, which displays resistivities of 7-50 Ω/square. The RLCGO-PET were carefully cut into sections having dimensions ca. 2.0×2.5 cm.
Fabrication of Ca-Doped Conductive Reduced Liquid Crystalline Graphene Oxide (RLCGO) on Flexible PET Films (Ca-RLCGO-PET).
The above sheets of RLCGO-PET were prepared by submerging RLCGO-PET samples overnight in a 1 M CaCl2 solution and thereafter dipping the films into water, followed by air-drying. The Ca-RLCGO-PET sheets were carefully cut into sections having dimensions ca. 2.0×2.5 cm.
Fabrication of RLCGO-PET and Ca-RLCGO-PET Electrodes. Electrodeposition of MnOx Films on RLCGO-PET (MnOx—RLCGO-PET and Ca-RLCGO-PET (MnOx-Ca-RLCGO-PET)—Shown in
A stripped copper wire was attached to the edge of each 2.0×2.5 cm RLCGO-PET sheet/Ca-RLCGO-PET sample with silver paste and epoxy glue. The wire and paste were coated with epoxy to prevent contact with electrolyte in later experiments. The electrodes, thus fabricated, were now ready to be electrocoated with MnOx. MnOx films were electrodeposited on the sheets of RLCGO-PET/Ca-RLCGO-PET. A MnOx electro-coating solution was made as follows: A NaNO3 solution in water (20 mL; 1M) was prepared, to which manganese acetate tetrahydrate (10 mM) was added. Using custom-built, small-volume cells (ca. 1-3 mL), a layer of MnOx was thereafter electrodeposited on the RLCGO-PET and Ca-RLCGO-PET electrodes at a constant current density of 200 μA/cm2 for 5 min. A three-electrode cell arrangement was used for the electrodeposition, involving a miniature Ag/AgCl reference electrode and a Pt mesh counter electrode (ca. 1.3×1.3 cm). After coating, each electrode was dipped in water to rinse it; it was thereafter gently dried with N2 gas. The resulting MnOx-RLCGO-PET and MnOx—Ca-RLCGO-PET, s electrodes were then heat treated at 90° C. for 30 min in air on a hotplate, after which they were stored in a desiccator.
Fabrication of Control Pt-PET Electrodes.
A PET sheet was sputter coated with platinum (100 nm thick). The Pt-coated sheet was then carefully cut into sections having dimensions ca. 2.0×2.5 cm. Stripped copper wire was thereafter attached to the edge of each 2.0×2.5 cm Pt-PET sheet with silver paste and epoxy glue. The wire and paste were coated with epoxy to prevent contact with electrolyte in later experiments.
Electrochemical Experiments.
All electrochemical measurements were carried out using an electrolyte of 0.1 M Na2SO4 on an EDAQ466 potentiostat. Linear sweep voltammograms were carried out at a scan rate of 5 mV/s over a range of 0.0-1.6 V (vs. Ag/AgCl). Chronoamperometry was performed at 1.15 V and 1.6 V (vs. Ag/AgCl). The electrochemical cell employed for the experiments comprised a rectangular chamber with fixed positions for the working electrode (MnOx—RLCGO-PET and MnOx—Ca-RLCGO-PET), reference electrode (Ag/AgCl) and counter electrode (Pt mesh; ca. 1.0×1.5 cm). The distance between the working and counter-electrode was 25 mm.
Results of Electrochemical Experiments.
To test the MnOx-Ca-RLCGO-PET and MnOx—RLCGO-PET electrodes, their electrocatalytic properties as the working anode in a cell containing 0.1 M Na2SO4 electrolyte, with an Ag/AgCl reference electrode and Pt mesh counter electrode, were examined. The best performing of these composites were those electro-coated for 5 min. These data are depicted in
The MnOx-Ca-RLCGO-PET and MnOx—RLCGO-PET electrodes displayed currents under linear sweep conditions that exceeded those of control platinum (
Thus, the early onset potential of the RLCGO composites (ca. 0.7 V vs. Ag/AgCl) resulted in the production of currents at voltages well below the onset potential of platinum, which was ca. 1.20 V. Moreover, the internal resistance-corrected currents exceeded platinum at all of the measured voltages.
To establish whether the currents produced by the RLCGO composites below 1.2 V were transient or sustained, the best performing MnOx-Ca-RLCGO-PET and MnOx—RLCGO-PET electrodes were poised at a fixed voltage of 1.15 V and their performance over time relative to the control Pt-PET was observed. The resulting chronoamperograms are shown in
As can be seen from
Chronoamperograms of the above samples at a fixed, applied voltage of 1.6 V vs. Ag/AgCl were also measured. As can be seen in
During the experiments depicted in
Results with Non-Conductive Substrate Coated with Electrically Conductive Film.
A significant ohmic potential drop occurs in the plane of the electrode, when operating, since the current is collected at the edge. If Sheet Resistance is Rs (Ω/□), then total linear resistance RL, of sheet area L×W (along L direction) is:
R
L
=R
s
×L/W(Ω)
Here L=W (about 2 cm) and RL equals Rs=40Ω. The average resistance of the active area along the L direction is half this, ie. RL/2=20Ω. See
The true, sheet resistance corrected, electrochemical voltage, V (corr'd), corresponding to an operating current density of I (mA/cm2) is therefore:
V(corr'd)=V(measured)−20ILW=V(measured)−80I×10−3(V)
This is shown in
This indicates that results with a substrate coated with an electrically conductive film, (e.g. Cu film on plastic/glass etc.), so that the current pathway length through the graphene-based material is minimised (e.g. film thickness of about 1.5 μm), are likely to match the internal resistance-corrected results of
Fabrication of MnOx Electrodes on Metal Substrates.
In a typical experiment the substrate (nickel foil, nickel mesh, or stainless steel mesh) was cleaned by sonication in acetone or isopropyl alcohol for 30 min. These were then plasma cleaned for 15 min and coated by dipping the material into aqueous dispersion of LCGO (approx. 2 mg/g for the nickel substrates and 3.6 mg/g for the stainless steel substrate, mg of LCGO per g of total dispersion). The coated substrates were left to dry overnight. Where applicable, the LCGO was reduced at first by heat annealing in an oven at 120° C., later by chemical reduction with Na2S2O4 in water (50 mg/mL) at 95° C. for one hour. The coated substrate was rinsed twice in milli-Q water and air dried. Afterwards the substrate material was fashioned into electrodes by lamination and the active area confined to 1 cm2.
The electrodes were then again plasma cleaned for 15 min before MnOx deposition by the established method (Zhou, F., A. Izgorodin, R. K. Hocking, V. Armel, L. Spiccia, and D. R. MacFarlane, ChemSusChem, 2013, 6(4): p. 643-651, incorporated herein by cross-reference). An aqueous solution of Mn (CH3COO)2.4H2O (10 mM) and NaNO3 (1M) was prepared fresh for each deposition experiment. This was then transferred to a simple 3 electrode cell prepared earlier (a smaller version of the large test box) where the sample substrate was the working electrode set to a galvanostatic current of 200 μA/cm2, with a Ag/AgCl reference and platinum mesh counter electrode. The standard protocol was 10 min deposition time, after which the electrode was taken out of solution and submerged in milli-Q water to rinse it. The electrode was air dried and then heated in an oven to 90° C. in air for 30 min.
Fabrication and Testing of Ca-MnOx Electrodes on Steel Mesh Substrate.
A MnOx-Ca-RLCGO electrode on steel mesh substrate was fabricated using the methods described above, with 10 min MnOx deposition time.
Testing of Electrodes Fabricated on Metal Substrates.
The finished electrode samples were tested in an electrochemical cell using aqueous Na2SO4 (0.1M) with the sample as working electrode, an Ag/AgCl reference and platinum counter electrode (see
A performance comparison of electrodes on Ni substrates is shown in
Functionalised Composite Material.
The performance decline with time seen in the current MnOx-Ca-RLCGO-PET system, which uses unfunctionalised liquid crystalline reduced graphene oxide, may be due to the progressive conversion of the initially deposited, more active hydrated Mn(III-IV) oxy/hydroxy species, into MnO2 nanoparticles. The latter are substantially less active in water oxidation, but more thermodynamically stable. The inventors hypothesise that the natural, OEC catalytic site uses carboxylate binding of the MnOx centre to the protein cavity, as it prevents this irreversible conversion to MnO2 like forms.
Fabrication of Functionalised Reduced Graphene Oxide (FRGO) Electrode.
Reduced graphene oxide (RGO) produced by heat annealing of graphene oxide was functionalised using a literature diazonium formation method (Wang, A., W. Yu, Z. Huang, F. Zhou, J. Song, Y. Song, L. Long, M. Cifuentes, M. Humphrey, L. Zhang, J. Shao, and C. Zhang, Covalent functionalization of reduced graphene oxide with porphyrin by means of diazonium chemistry for nonlinear optical performance. Vol. 6. 2016. 23325, incorporated herein by cross-reference). A typical experimental procedure for coupling 4-aminophenol is as follows. An aryldiazonium ion solution was prepared according to the following method. Sodium nitrite (0.117 g), 4-aminophenol (0.154 g) and sodium hydroxide (0.0398 g) were dissolved in deionised water (9 mL). The solution was then added dropwise to dilute hydrochloric acid solution (0.1 mol L−1, 6 mL) in an ice bath with stirring. The pH of the resultant mixture was adjusted to acidic by adding further hydrochloric acid solution. The aryldiazonium ion solution was added dropwise to a previously-prepared suspension of purified RGO (16 mg) in deionised water (14 mL, dispersed by ultra-sonication) with stirring. The resultant mixture was kept in the ice bath for 7 h and then at room temperature for a further 8 h. The contents were filtered through a 0.45 μm nylon membrane, and the collected solid was washed repeatedly with deionised water, ethanol and acetone to remove excess diazonium salts. This afforded the phenol-functionalized RGO hybrid as a black solid, which was vacuum-dried at room temperature for 24 h.
Diazo-derivatives of 4-aminophenol, or 4-aminobenzoic acid, or 3-aminobenzoic acid were formed according to the method of Wang et al, referenced above, and coupled to RGO dispersed in water by ultra-sonication. The mixture was stirred in an ice bath over several days. These functionalisation groups are of the type expected to provide OEC like ligation (carboxylate, phenolic) to Mn. The dispersed, functionalised RGO was then dip-coated onto Ni mesh electrodes and MnOx electro-deposited, where appropriate, as described above. The performance of electrodes comprising FRGO is shown in
Fabrication of Electrode with Edge Functionalised Graphene (EFG).
EFG which was functionalised with carboxylate groups was obtained from the Australian National Fabrication Facility (ANNF) Materials Node at the University of Wollongong and aminohexanoic acid, tyrosine, and para-aminobenzoic acid were covalently attached by amide coupling. Dispersions of the functionalised graphene species in water were dip coated onto nickel mesh, then MnOx electro-deposited. If necessary, the dispersion concentration was modified, to improve graphene dispersal in water (particularly in the case of the benzoic acid functionalised species). The performance of electrodes comprising FRGO is shown in
A summary of samples prepared by the above methods is shown in the following table. Recited solvents and concentrations refer to conditions for depositing graphene based material on to substrate e.g. ‘Ni foil tyrosine EFG in ethanol 3 mg/mL’ means a 3 mg/mL solution of tyrosine EFG in ethanol was prepared and deposited onto Ni foil. ‘EFG’ refers to edge functionalised graphene without further modification, i.e. bearing only carboxy groups on the edges of the graphene sheet. ‘EFG-PEG’ refers to a mixture of EFG and PEG, typically 1:1.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2017905074 | Dec 2017 | AU | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/AU2018/051365 | 12/19/2018 | WO | 00 |
