Electrode material

Abstract

The present invention relates to electrode materials which comprise quaternary metal oxides and their use in energy storage applications. The use of hydrothermal methods to prepare the quaternary metal oxides provides particulate materials having high crystallinity, high porosity and high specific surface area. The morphology of the materials leads to high electrical conductivity making them particularly suitable for use as electrode materials in supercapacitors. In one aspect, the invention relates to an electrode for a supercapacitor, the electrode comprising a quaternary metal oxide of the formula CuXSnO4, wherein X is selected from the group consisting of nickel (Ni), cobalt (Co), iron (Fe) and manganese (Mn).

Description

[0001]ELECTRODE MATERIAL Technical fieldThe present invention relates generally to electrode materials, to processes for theirpreparation and to their use. More particularly, it relates to electrode materialswhich comprise quaternary metal oxides and their use in energy storageapplications. The use of hydrothermal methods to prepare the quaternary metaloxides provides particulate materials having high crystallinity, high porosity and ahigh specific surface area. The morphology of the materials leads to high electricalconductivity making them particularly suitable for use as electrode materials insupercapacitors. Background of the invention The need to develop alternative energy conversion and storage systems is a majorchallenge as environmental impacts, such as limited levels of fossil fuels and globalwarming, have adverse effects. Although wind, geothermal and hydropower areless polluting and more environmentally friendly, these are hindered by the capabilities of current energy storage systems. Electrochemical technologies such as rechargeable batteries, solar cells, supercapacitors, and fuel cells provide sources of energy storage. Fuel cells arehigh energy systems and a clean energy source, but the long-term stability of theseenergy storage devices curbs their use in transportation. Rechargeable batteriescan deliver intermediate power and high energy density. Supercapacitors (alsocalled ultracapacitors) are emerging as energy storage devices with high powerdensity, good cycle life with high coulombic efficiency, and a fast charge-dischargerate. Supercapacitors have the potential to fill the energy / power density gap between the high specific energy of conventional batteries and the high specific power of conventional electrostatic capacitors.In a supercapacitor, a pair of highly porous electrodes is manufactured by providingelectrode materials on electrode current collectors (typically thin metal foils orporous metal substrates, e.g. metal foams) that are coupled to each other. Theelectrodes are impregnated with a liquid electrolyte and separated by anelectrically-insulating and ion-permeable membrane (separator). When a voltage is applied between the electrodes, negative ions from the electrolyte flow to thepositive electrode (cathode) and positive ions from the electrolyte flow to thenegative electrode (anode). The result is an electric double layer formed at each electrode / electrolyte interface due to the accumulation of ionic charge. Energy is stored by the separation of positive and negative charge at each interface. The separator prevents electrical contact between the conductive electrodes but permits the exchange of ions. Discharge of the supercapacitor results in the flow of currentas the ions discharge from the electrode surfaces. Supercapacitors can be chargedand discharged over multiple cycles and are particularly suited to applications thatrequire frequent and rapid delivery of power, such as hybrid vehicles. Supercapacitors are classified based on the mechanisms by which they store charge. Electrostatic double-layer capacitors store charge in an electrochemical double layer at the electrode / electrolyte interface. These supercapacitors are typically based on a symmetric configuration of two highly porous, high surface area carbon electrodes, for example activated carbon (AC) electrodes. Electrochemicalcapacitors (also called electrochemical pseudocapacitors) store charge viaFaradaic electron charge-transfer reactions in addition to the electrical double-layer capacitance. Pseudocapacitance is achieved via redox reactions involving, forexample, metal oxides, metal nitrides or conducting polymer electrodes. The use ofsuch electrode materials provides an asymmetric configuration. The cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) profiles of asupercapacitor reveal the charge storage mechanisms. The redox peak in CV andthe non-linear curves in GCD distinguish the asymmetric (also termed “hybrid”)supercapacitor. In an asymmetric AC / metal oxide supercapacitor, one electrodestores charge via a non-faradaic reaction of ion adsorption / desorption on thesurface of the active carbon electrode, and the metal oxide electrode uses a redoxreaction to store charge. Recent research has focused on the development of electrode materials (bothanode and cathode materials) for enhancing the electrochemical properties ofsupercapacitors. These should have advanced properties such as good electricalconductivity, high-temperature and chemical stability, high-level corrosionresistance, and a high surface area (porosity), in addition to being economical andenvironmentally friendly. Various materials have been investigated based on thetransition metal oxides and hydroxides, such as RuO2, NiO, MnO2, Co3O4, Ni(OH)2and Co(OH)2. Ternary metal oxides, containing two different metal cations, such asNiCo2O4, MnCo2O4and MgCo2O4, are considered to be potential candidates for use in energy storage devices. Mixed metal oxides have benefits such as attainableoxidation states, high energy performance and higher electrical conductivities,whilst being eco-friendly and cost-effective. The micro / nano-structures of themixed metal oxides also significantly improves the specific capacitance ofelectrodes that incorporate these materials. Despite these improvements, there still remains a need for alternative electrodematerials for use in energy storage devices such as supercapacitors. In particular,there remains a need for such materials that have high performance and a long life- cycle, and which can be readily prepared on a large scale using eco-friendly and cost-effective methods. The inventors now propose the use of certain quaternary transition metal oxides ofthe general formula CuXSnO4 (X = Ni, Co, Fe or Mn) as active electrode materials.Preparation of the metal oxides using a hydrothermal method as herein described provides crystalline materials that exhibit high porosity, a high specific surface area and high electrical conductivity. The morphology of the materials makes them particularly suitable for use as active electrode materials in supercapacitors.Certain crystalline forms of CuNiSnO4 and CuCoSnO4 have previously beendescribed by Sampath et al. (see Bull. Mater. Sci.17(5): 487-492, 1994 and J.Phys. Chem. Solids 50(9): 921-924, 1989). The crystalline materials are formedusing a standard ceramic method in which equimolar proportions of the constituentoxides are mixed in acetone. Using polyvinyl acetate as a binder, the resultingmixture is formed into pellets that are fired in steps of 400, 600 and 800°C for 15-20hours, and in a final phase at 1100°C for 40-45 hours. The CuNiSnO4 compositioncrystallises in orthorhombic symmetry and is found to be semi-conducting in thetemperature range of 400-800K. The CuCoSnO4 composition similarly crystallisesin orthorhombic symmetry and is found to be semi-conducting in the temperaturerange of 450-750K. The authors of these earlier papers do not investigate theporosity or morphology of the materials or their electrochemical performance aspotential electrode materials. Summary of the inventionThe present invention provides quaternary transition metal oxides for use aselectrode materials and a hydrothermal route for their preparation. A majoradvantage of these quaternary materials is that they have three distinct metalcations. As evidenced herein, when used as electrode materials, these have goodelectrochemical results which are obtained due to their multiple metal cations,improving redox activity, conductivity, active charge storage sites, rate capabilityand longevity. This makes them particularly suitable for use in supercapacitorapplications. The hydrothermal route used for their synthesis provides large, good quality crystals, while maintaining structural superiority in their composition. Whencompared to known binary and ternary materials, the quaternary metal oxidematerials show an improved charge storage capacity due to their high electricalconductivity, surface area and multiple metal combinations. These thereforerepresent materials with the potential for large-scale production of cheap, easy-to-operate, hybrid supercapacitors.In one aspect the invention provides an electrode for a supercapacitor, theelectrode comprising a quaternary metal oxide having the formula CuXSnO4,wherein X is selected from the group consisting of nickel (Ni), cobalt (Co), iron (Fe) and manganese (Mn). In another aspect the invention provides a method of forming an electrode for asupercapacitor, said method comprising contacting a composition comprising aquaternary metal oxide having the formula CuXSnO4, wherein X is selected fromthe group consisting of nickel (Ni), cobalt (Co), iron (Fe) and manganese (Mn), withan electrode current collector to form said electrode.In another aspect the invention provides a supercapacitor which comprises at least one capacitor cell comprising a first porous electrode, a second porous electrode, an electrolyte in contact with said first and second porous electrodes, and a separator separating the first porous electrode from the second porous electrode,wherein the first porous electrode comprises a quaternary metal oxide having theformula CuXSnO4, wherein X is selected from the group consisting of nickel (Ni), cobalt (Co), iron (Fe) and manganese (Mn).In another aspect the invention provides a quaternary metal oxide having theformula CuXSnO4, wherein X is iron (Fe) or manganese (Mn).In another aspect the invention provides a method of preparing a quaternary metaloxide having the formula CuXSnO4, wherein X is selected from the group consisting of nickel (Ni), cobalt (Co), iron (Fe) and manganese (Mn), said method comprisingthe step of subjecting an aqueous solution which comprises the constituent metalions of the metal oxide to a hydrothermal reaction.In another aspect, the invention provides a quaternary metal oxide having the formula CuXSnO4, wherein X is selected from the group consisting of nickel (Ni), cobalt (Co), iron (Fe) and manganese (Mn), wherein said quaternary metal oxide is obtainable, obtained or directly obtained by a method as herein described. In another aspect, the invention provides the use of a quaternary metal oxide having the formula CuXSnO4, wherein X is selected from the group consisting ofnickel (Ni), cobalt (Co), iron (Fe) and manganese (Mn), as an active electrodematerial, for example as an active electrode material in a supercapacitor. Detailed description of the invention The present invention is directed to an electrode material comprising a quaternarymetal oxide as herein defined and its use in forming an electrode suitable for use ina supercapacitor. This material allows the production of a cost-effectivesupercapacitor having improved capacitance and a prolonged cycle life.According to one aspect, the invention provides an electrode for a supercapacitor,the electrode comprising a quaternary metal oxide having the formula CuXSnO4,wherein X is selected from the group consisting of nickel (Ni), cobalt (Co), iron (Fe), and manganese (Mn). In some embodiments, the electrode may comprise acombination of two or more of the quaternary metal oxides. In other embodiments,the electrode may comprise a single quaternary metal oxide as herein described.The quaternary metal oxide functions as the active material in the electrode. As used herein, the term “active material” refers to a material that is directly involved inthe electrochemical reaction which results in energy storage or release. This is incontrast to non-active (or “passive”) materials, such as binders or conductive additives, which function to maintain the structural (i.e. mechanical) or electrical integrity of the electrodes, respectively.The quaternary metal oxide for use in the invention is selected from CuNiSnO4,CuCoSnO4, CuFeSnO4 and CuMnSnO4. In one embodiment, the quaternary metaloxide is CuCoSnO4, CuFeSnO4 or CuMnSnO4. In a preferred embodiment, thequaternary metal oxide is CuCoSnO4. The quaternary metal oxides for use in the invention have high porosity and high specific surface area making these particularly suitable for use as electrodematerials. The Brunauer-Emmett-Teller (BET) method may be used to determinethe specific surface area and pore sizes.In some embodiments, the quaternary metal oxides are characterised by aBrunauer-Emmett-Teller (BET) specific surface area of from 30 to 200 m2 / g. The Brunauer-Emmett-Teller (BET) specific surface area is preferably from 35 to 180 m2 / g, more preferably 40 to 170 m2 / g, for example 50 to 160 m2 / g.In one embodiment, the quaternary metal oxide is CuNiSnO4 having a BET specificsurface area of from about 25 to about 45 m2 / g, preferably from about 30 to about40 m2 / g, e.g. about 35 m2 / g. In one embodiment, the quaternary metal oxide is CuCoSnO4 having a BET specific surface area of from about 40 to about 60 m2 / g, preferably from about 45 to about 55 m2 / g, e.g. about 50 m2 / g. In one embodiment, the quaternary metal oxide is CuFeSnO4 having a BET specific surface area of fromabout 150 to about 170 m2 / g, preferably from about 155 to about 165 m2 / g, e.g.about 160 m2 / g. In one embodiment, the quaternary metal oxide is CuMnSnO4 having a BET specific surface area of from about 90 to about 110 m2 / g, preferably from about 95 to about 105 m2 / g, e.g. about 100 m2 / g. In some embodiments, the quaternary metal oxides are characterised by a Barrett-Joyner-Halenda (BJH) average pore diameter of from 2 nm to 50 nm and may thusbe considered “mesoporous” materials. The Barrett-Joyner-Halenda (BJH) averagepore diameter of the metal oxide materials may, for example, be in the range from 1nm to 20 nm, e.g. in the range from 2 nm to 15 nm.In one embodiment, the quaternary metal oxide is CuNiSnO4 having a BJH averagepore diameter in the range from 2 nm to 5 nm, preferably from 2 nm to 3 nm, e.g. about 2 nm. In one embodiment, the quaternary metal oxide is CuCoSnO4having a BJH average pore diameter in the range from 5 nm to 15 nm, preferably from 8 nm to 12 nm, e.g. about 10 nm. In one embodiment, the quaternary metal oxide is CuFeSnO4having a BJH average pore diameter in the range from 2 nm to 10 nm, preferably from 4 nm to 8 nm, e.g. about 5 nm. In one embodiment, the quaternary metal oxide is CuMnSnO4having a BJH average pore diameter in the range from 2nm to 15 nm, preferably from 5 nm to 10 nm, e.g. about 8 nm.Typically, the electrode material herein described will be provided on at least onesurface of a current collector, for example it may be provided in the form of acoating on a current collector. Where this is coated on a current collector, it may becoated completely or in part. It may, for example, be coated on one or both sides of the current collector. Where the current collector is porous, it will be understood that provision of the electrode material on a surface of the current collector willinclude deposition of the material within the pores of the porous current collector.As used herein, the term “current collector” refers to a material that acts as an electron transfer channel for electrons formed as a result of the electrochemical reactions in the electrical energy storage device to an external circuit. A current collector may also be referred to herein as a “substrate”.The current collector may be a metal or a metal-based material such as copper,titanium, brass, silver, platinum, aluminium, nickel or alloys thereof. The currentcollector may be a carbon-based material, such as graphite. In someembodiments, the current collector is porous. For example, it may be provided inthe form of a porous metal, porous metal alloy, metal foam or metal alloy foam. Insome embodiments, the current collector is a nickel foam.In some embodiments, the electrode material additionally comprises materials toimprove the porosity and / or conductivity of the electrode. For example, it maycomprise a porous material and / or a conductive additive. In some embodiments, it may additionally comprise a binder (i.e. an adhesive).In some embodiments, the electrode material comprises from 30 to 95 wt.% of thequaternary metal oxide. For example, the electrode material may comprise from 40wt.% to 95 wt.% of the quaternary metal oxide, e.g. from 50 wt.% to 95 wt.% of thequaternary metal oxide, e.g. from 60 wt.% to 90 wt.% of the quaternary metal oxide,e.g. from 70 wt.% to 85 wt.% of the quaternary metal oxide, e.g. from 75 wt.% to 85wt.% of the quaternary metal oxide, e.g. from 80 wt.% to 85 wt.% of the quaternarymetal oxide. As used herein, the term “weight percent” or “wt.%” refers to the concentration as apercentage by mass of the different components of the electrode material based onthe total dry mass of the electrode material. The term “total dry mass” refers to thetotal mass of the solid components of the electrode material, i.e. it does not includethe mass of any solvent that may be employed to aid in deposition of the electrode material onto the current collector.In some embodiments, the porous material present in the electrode material isselected from the group consisting of a carbon-based filler, a ceramic filler, a metalfiller or a mixture thereof. For example, a carbon-based filler may be graphene, graphite, carbon nanotubes, carbon black, activated carbon or charcoal. Forexample, a ceramic filler may be selected from boron nitride, titanium carbide,silicon carbide, titanium diboride, titanium nitride, titanium monoboride and boronmonofluoride. For example, a metal filler may be silver, gold, nickel or copper. In apreferred embodiment, the porous material may be a carbon-based material, such as activated carbon, graphite, carbon black, or a mixture thereof.The porous material may be provided in any suitable form. For example, it may beprovided as particles, nanoparticles, microparticles, flakes, shavings or powder. Where present, the porous material may be present in an amount of up to 25 wt.%,for example in the range of from 1 to 20 wt.%, preferably 5 to 15 wt.%, e.g. about10 wt.% (based on the total dry weight of the electrode material).In some embodiments, the electrode material additionally comprises a conductiveadditive. Such materials include carbon-based materials such as acetylene carbon black, graphite, activated carbon, carbon black, carbon black Super P, and carbon nanotubes (CNTs). Where present, the conductive additive may be present in anamount of up to 10 wt.%, preferably from 1 to 10 wt.%, e.g. about 5 wt.% (based onthe total dry weight of the electrode material).In some embodiments, the electrode material additionally comprises a binder whichmay be used to hold together the various components of the electrode material andto retain the electrode material in contact with the current collector when this is in contact with the electrolyte. Suitable binder materials are well known for use insupercapacitors and include, but are not limited to, polymeric materials such asNafion (tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acidcopolymer), poly(vinylidene fluoride) (PVDF), polyvinylalcohol (PVA), poly(acrylicacid) (PAA), polyacrylonitrile (PAN), carboxymethylcellulose (CMC), polytetrafluoroethylene (PTFE), polyethylene (e.g. HDPE or LDPE), polypropylene,polyvinyl chloride (PVC), polystyrene, nylon, teflon, polyethylene terephthalate(PET), poly(methyl methacrylate) (PMMA), and mixtures thereof. In oneembodiment, the binder material may be selected from Nafion, poly(vinylidenefluoride) and polytetrafluoroethylene. In one embodiment, the binder may bepoly(vinylidene fluoride).Where present, the binder may be present in an amount in the range of up to 10wt.%, preferably from 1 to 10 wt.%, e.g. about 5 wt.% (based on the total dry weightof the electrode material).In some embodiments, the electrode material consists essentially of the quaternarymetal oxide as herein defined, a porous material, a conductive additive and abinder. In some embodiments, the electrode material consists of the quaternary metal oxide as herein defined, a porous material, a conductive additive and a binder. The electrode may be prepared by methods known in the art and will generally involve mixing of the quaternary metal oxide (i.e. the active electrode material), the porous material, conductive additive and the binder in a suitable solvent. Suitablesolvents are well known in the art. Examples of solvents that may be used include,but are not limited to, N-methyl-2-pyrrolidone and ethylene glycol due to theirexcellent solubilising properties, high boiling point, low freezing point, ease ofhandling, low toxicity and low cost. The resulting slurry or paste containing theelectrode material may then be processed to form an electrode. Typically, theslurry or paste may be coated onto or impregnated into a suitable current collector,for example it may be coated onto a metal foil or deposited or impregnated into aporous metal substrate. The electrode is the final product after the electrode material comprising the active quaternary metal oxide has been applied to thecurrent collector and dried, and is ready for assembly into an energy storage devicesuch as a supercapacitor. The resulting electrode may be flat or planar or may bewrapped or wound in a spiral configuration. In a further aspect, the invention also provides a method of forming an electrode comprising the step of contacting an electrode material as herein described with a current collector to form an electrode. An appropriate amount of electrode material to be applied to the current collectormay readily be determined by those skilled in the art and may vary depending onthe choice of active electrode material. The invention further provides a supercapacitor comprising at least one capacitor cell comprising a first porous electrode, a second porous electrode, an electrolyte in contact with said first and second porous electrodes, and a separator separating the first porous electrode from the second porous electrode, wherein the firstporous electrode comprises a quaternary metal oxide having the formula CuXSnO4,wherein X is an element selected from the group consisting of nickel (Ni), cobalt(Co), iron (Fe) and manganese (Mn). The term “porous”, as used herein, refers to a structure of interconnected pores or voids that provides a continuous pathway through a material. In some embodiments, the porosity of the electrodes is at least 80%, preferably at least 90%, more preferably at least 95%. In some embodiments, the porosity of the electrodes is from 95 to 98%.Typically, the second porous electrode will comprise a high surface area carbonmaterial selected from the group consisting of amorphous carbon, graphite, carbon nanotubes (CNTs), graphene or any combination thereof. In one embodiment, thesecond porous electrode comprises amorphous carbon. In a preferredembodiment, the amorphous carbon is activated carbon (AC). In someembodiments, the activated carbon has a surface area in the range from about 950to 2000 m2 / g.An electrolyte is provided between the first and second electrodes to provide asuitable medium through which ions can travel. The electrolyte comprises a solvent and dissolved materials that provide positive and negative ions that make the electrolyte electrically conductive and which provide the electrical conduction between the first and second electrodes. The electrolyte additionally provides the ions for the formation of the double-layer and the ions for pseudocapacitance. Suitable electrolytes include aqueous solutions, organic liquids and ionic liquids.Aqueous electrolytes can include acidic solutions (for example, sulfuric acid),alkaline solutions (for example, sodium hydroxide or potassium hydroxide), andneutral solutions (for example, lithium sulfate, sodium sulfate or potassiumchloride). Organic electrolytes include acetonitrile and propylene carbonate. Ionic liquids suitable for use in supercapacitors are well known in the art and include saltsthat are molten at ambient temperature.In some embodiments, the electrolyte comprises a dissolved salt selected from the group consisting of potassium hydroxide, sodium hydroxide, lithium oxide, potassium sulfate, sodium perchlorate, sulfuric acid, hydrochloric acid, nitric acid, methanesulfonic acid, and tetrafluoroboric acid. In some embodiments, the electrolyte comprises potassium hydroxide.A suitable separator may be provided between the first and second electrodes toprevent their direct contact with one another. The separator comprises an electrically-insulating, ion-permeable material, i.e. it prevents the transfer ofelectrons but allows the exchange of ions. In some embodiments, the separator willbe porous. The choice of separator is dependent on the choice of electrolyte material. Suitable separators are well known in the art and include porous polymer materials such as cellulose, polyethylene terephthalate (PET), polypropylene (PP) (e.g. Celgard), poly(vinylidene fluoride) (PVDF) and poly(vinyl alcohol) (PVA). Some of the quaternary metal oxides herein described are in themselves novel and form part of the invention. In a further aspect, the invention thus provides a quaternary metal oxide having the formula CuXSnO4, wherein X is iron (Fe) or manganese (Mn). The quaternary metal oxides for use in the invention may be produced by methods known in the art, for example using standard ceramic techniques such as those described in Sampath et al., Bull. Mater. Sci.17(5): 487-492, 1994 and Sampath et al., J. Phys. Chem. Solids 50(9): 921-924, 1989, the entire contents of which are incorporated herein by reference. However, the invention also provides a new synthetic route to the metal oxide materials involving a hydrothermal method.In another aspect the invention provides methods of making the quaternary metaloxides as herein described in which an aqueous solution comprising the constituentmetal ions is subjected to a hydrothermal reaction. Specifically, such methods arebased on hydrothermal reactions of an aqueous solution containing a mixture ofmetal salts, a precipitating agent, a reducing agent, and at least one structure-directing agent such as a surfactant or polymer. The metal salts are water solubleunder the hydrothermal reaction conditions and provide a source of Cu2+ ions, Xn+ions (where X is Ni, Co, Fe or Mn, n is 2 when X is Ni, Co or Mn, and n is 3 when Xis Fe) and Sn4+ ions. These metal salts are also generally referred to herein as“precursors”, i.e. they are precursors to the desired metal cations.In a further aspect, the invention thus provides a method of preparing a quaternarymetal oxide having the formula CuXSnO4, wherein X is selected from the group consisting of nickel (Ni), cobalt (Co), iron (Fe) and manganese (Mn), said method comprising the step of subjecting an aqueous solution which comprises the constituent metal ions of the metal oxide to a hydrothermal reaction. In one embodiment, the method for preparing the quaternary metal oxide comprises the following steps: (a) providing an aqueous reaction mixture containing: Cu2+ions, Xn+ ions (wherein X is Ni, Co, Fe or Mn, n is 2 when X is Ni, Co orMn, and n is 3 when X is Fe), Sn4+ions, a precipitating agent, a reducing agent, and one or more structure-directing agents; (b) subjecting the aqueous reaction mixture to a hydrothermal reaction; (c) allowing the reaction mixture to cool to ambient temperature; (d) isolating a solid material containing the quaternary metal oxide; and (e) optionally washing and drying the solid material. In a preferred embodiment, the method may additionally comprise the step ofsubjecting the solid material to calcination in order to eliminate impurities and / orvolatile substances thereby enhancing crystallisation. Following calcination, theresulting quaternary metal oxide may be provided in particulate form, for example inthe form of nanoparticles. Optionally, the particulate material may be subjected to further grinding or milling,for example to increase its homogeneity and to further reduce its particle size.Additional sintering steps may also be performed to enhance the activity of the particulate material. The method described herein to prepare the quaternary metal oxides involves a hydrothermal reaction, i.e. a reaction that is carried out in the presence of water at high temperature and high pressure. Such reaction methods are well known in the art and are particularly suitable for use in preparing highly crystalline materials. The method will generally be performed in a sealed pressure vessel, such as a sealed autoclave which consists of a steel pressure vessel, and is conducted under conditions effective to produce a crystalline mesoporous solid. During the hydrothermal crystallisation, the crystalline mesoporous solid nucleates spontaneously from the reaction mixture. Suitable temperatures, pressures and reaction times for the hydrothermal reaction may readily be determined by those skilled in the art. A temperature in the range offrom 120°C to 250°C, preferably from 160°C to 200°C, e.g. about 180°C, isconsidered to be suitable. A pressure in the range of from 0.3 to 4 MPa may be appropriate. The duration of the hydrothermal reaction may range from 10 to 24 hours, for example from 15 to 20 hours, e.g. about 16 hours. Following completion of the hydrothermal reaction, the reaction mixture can be cooled to ambient temperature, for example to a temperature of from 18 to 25°C.The reaction mixture may be allowed to cool to ambient temperature without activecooling. Alternatively, it may be subjected to controlled cooling. The resulting solid material can be separated from the reaction mixture using conventional separation techniques such as filtration or centrifugation. The solidmaterial may be washed to remove contaminants such as any unreacted ions andany residual precipitating agent, reducing agent or structure-directing agents. Washing may be carried out in one or more washing steps using water (e.g. deionised or distilled water) and / or organic solvents, such as methanol or ethanol. In one embodiment, washing may involve a plurality of water washing steps followed by one or more washing steps involving the use of organic solvents. Following washing the solid material may be dried. Typically drying will be carriedout at elevated temperature, for example in the range of 60°C to 100°C. For thispurpose, a hot-air oven may be used. Drying may be carried out for a period of upto 24 hours, for example a period of from 8 to 24 hours. The drying step may be carried out at atmospheric pressure or under a vacuum.Following drying, the solid material may be subjected to calcination. Suitablecalcination temperatures may be in the range of from 300°C to 800°C, preferablyfrom 300°C to 700°C. For example, the calcination temperature may be in therange of from 300°C to 600°C, preferably 300°C to 400°C, for example about350°C. Duration of the step of calcination will generally be at least 1 hour,preferably from 2 to 6 hours, e.g. about 3 hours or about 5 hours.In step (a) the aqueous reaction mixture may be provided by step-wise mixing ofseparately prepared aqueous precursor solutions containing Cu2+ ions, Xn+ ions(wherein X and n are as herein defined), Sn4+ions, and an aqueous solutioncontaining a precipitating agent. For example, a precursor solution containing Xn+ions, a precursor solution containing Sn4+ ions, and an aqueous solution containinga precipitating agent may each be added step-wise to a precursor solutioncontaining Cu2+ions. Mixing of the resulting aqueous solution ensures homogeneity. To the mixed solution, the reducing agent and structure-directingagent(s) may then be added, preferably with continuous mixing to maintain thehomogeneity of the solution.Suitable Cu2+-containing precursor solutions include, but are not limited to, copper(II) nitrate, copper (II) chloride, copper (II) acetate, copper (II) sulfate, and copper(II) carbonate. In one embodiment, the Cu2+-containing precursor solution may beselected from copper (II) nitrate trihydrate (Cu(NO3)2·3H2O), copper (II) nitratedihydrate (Cu(NO3)2·2H2O), copper (II) chloride dihydrate (CuCl2·2H2O), copper (II)acetate monohydrate (Cu(CO2CH3)2·H2O), copper (II) acetate pentahydrate(Cu(CO2CH3)2·5H2O), copper (II) acetate hexahydrate (Cu(CO2CH3)2·6H2O), copper(II) sulfate pentahydrate (CuSO4·5H2O), copper (II) sulfate hexahydrate(CuSO4.6H2O), copper sulfate monohydrate (CuSO4.H2O), copper carbonate monohydrate (CuCO3.H2O).Appropriate concentrations of Cu2+ ions in the Cu2+- containing precursor solutionmay be selected by those skilled in the art. In some embodiments, theconcentration of Cu2+ ions may be in the range of from 1 mM to 10 mM, preferablyabout 5 mM. For large scale synthesis of the quaternary metal oxides, theconcentration of Cu2+ ions may be higher, for example up to 0.5 M, e.g. about 0.3M. Xn+-containing precursor solutions that may be used in the method include aqueoussolutions of the metal nitrates, metal chlorides, metal sulfates, metal carbonatesand metal acetates.Suitable Ni2+-containing precursor solutions include, but are not limited to, nickel (II)nitrate hexahydrate (Ni(NO3)2·6H2O), nickel (II) chloride hexahydrate (NiCl2·6H2O),nickel (II) acetate hydrate (Ni(CH3COO)2.xH2O), nickel (II) acetate tetrahydrate(Ni(OCOCH3)2·4H2O), nickel(II) acetate hexahydrate (Ni(OCOCH3)2·6H2O), nickel(II) sulfate (NiSO4), nickel (II) sulfate heptahydrate (NiSO4·7H2O), nickel (II) sulfatehexahydrate (NiSO4·6H2O), and nickel (II) carbonate hydrate (NiCO3.H2O).Suitable Co2+-containing precursor solutions include, but are not limited to, cobalt(II) chloride hexahydrate (CoCl2·6H2O), cobalt (II) acetate tetrahydrate((CH3COO)2Co·4H2O), cobalt (II) acetate hexahydrate ((CH3COO)2Co·6H2O),cobalt (II) sulfate hydrate (CoSO4·xH2O), cobalt (II) sulfate heptahydrate(CoSO4·7H2O), and cobalt (II) carbonate hydrate (CoCO3·xH2O).Suitable Fe3+-containing precursor solutions include, but are not limited to, iron (III)nitrate nonahydrate (Fe(NO3)3·9H2O), anhydrous iron (III) chloride (FeCl3), iron (III)chloride hexahydrate (FeCl3·6H2O), iron (III) acetylacetonate (Fe(C5H7O2)3), iron(III)phosphate dihydrate (FePO4·2H2O), iron (III) citrate tribasic monohydrate(C6H5FeO7·H2O), ammonium iron (III) citrate (C6H8O7·xFe·xNH3), iron (III) tartrate(Fe2(C4H4O6)3), and iron(III) perchlorate hydrate (Fe(ClO4)3·xH2O).Suitable Mn2+-containing precursor solutions include, but are not limited to,manganese (II) nitrate hexahydrate (Mn(NO3)2·6H2O), manganese (II) chloridetetrahydrate (MnCl2·4H2O), manganese (II) acetate tetrahydrate((CH3COO)2Mn·4H2O), manganese (II) sulfate hexahydrate (MnSO4.6H2O), and manganese (II) carbonate (MnCO3).Appropriate concentrations of Xn+ ions in the Xn+-containing precursor solution maybe selected by those skilled in the art. In some embodiments, the concentration ofXn+ ions is in the range of from 1 mM to 5 mM, preferably about 3 mM. For largescale synthesis of the quaternary metal oxides, the concentration of Xn+ions may be higher, for example up to 0.5 M, e.g. about 0.3 M. Suitable Sn4+-containing precursor solutions include tin (IV) chloride, for example tin (IV) chloride pentahydrate, i.e. SnCl4·5H2O. Appropriate concentrations of Sn4+ions in the Sn4+-containing precursor solutionmay be selected by those skilled in the art. In some embodiments, theconcentration of Sn4+ ions is in the range of from 1 mM to 5 mM, preferably about 3mM. For large scale synthesis of the quaternary metal oxides, the concentration ofSn4+ions may be higher, for example up to 0.5 M, e.g. about 0.3 M.The precipitating agent for use in the methods herein described aids in precipitationof the metal oxide nanoparticles during the synthesis process. Suitable precipitating agents are well known for use in hydrothermal crystallisation processes and include urea, ammonia (NH3), tetramethylammonium hydroxide (TMAH), ammonium carbonate (NH4)2CO3, sodium carbonate (Na2CO3), potassium carbonate (K2CO3), potassium hydroxide (KOH), sodium hydroxide (NaOH), sulfuric acid (H2SO4), hydrochloric acid (HCl), sodium borohydride (NaBH4), ammonium fluoride (NH4F), and combinations thereof. In one embodiment, the precipitating agent is urea, ammonia (NH3), ammonium carbonate (NH4)2CO3, sodium carbonate (Na2CO3), potassium carbonate (K2CO3), potassium hydroxide (KOH), sodiumhydroxide (NaOH), sulfuric acid (H2SO4), hydrochloric acid (HCl) or sodiumborohydride (NaBH4). In one embodiment, the precipitating agent is urea. The precipitating agent may be present in the reaction mixture at a concentration inthe range from 5 to 20 wt.%, preferably 5 to 10 wt.%.Suitable reducing agents for use in the methods herein described are well known inthe art. Examples of suitable reducing agents include, for example, citric acid, ascorbic acid, acetic acid, oxalic acid, sulphur dioxide, hydrogen, phosphites, phosphorous acid, and hypophosphites. The reducing agent can be a combinationof such agents. In one embodiment, the reducing agent is citric acid, acetic acid oroxalic acid. In one embodiment, the reducing agent is citric acid. The reducing agent may be present in the reaction mixture at a concentration in the range from 5 to 20 wt.%, preferably 10 to 20 wt.%.One or more structure-directing agents may be employed in the preparationmethods herein described. As used herein, the term “structure-directing agent”refers to an agent that aids in formation of the desired crystalline metal oxidenanoparticles and influences the final morphology of the product. The structure-directing agent may be present in the reaction mixture at a concentration in therange from 5 to 20 wt.%, preferably 10 to 20 wt.%.Suitable structure-directing agents include surfactants and polymers that arecapable of stabilising the nanoparticles. Examples of suitable surfactants include,but are not limited to, potassium laurate, sodium octant sulfonate, sodium decanesulfonate, sodium dodecane sulfonate, sodium lauryl sulfate, ammonium laurylsulfate, sodium myreth sulfate, sodium di-2-ethylhexyl sulfosuccinate,decyltrimethylammonium bromide, dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, tetradecyltrimethylammonium chloride, dodecylammonium chloride, hexamethylenetetramine (HMTA), cetyltrimethylammonium bromide (CTAB), cetyltrimethylammonium chloride (CTAC), and combinations thereof.Examples of suitable polymers for use as structure-directing agents include water-soluble polymers such as polyethylene glycols and derivatives thereof (e.g.polyethylene glycol (PEG) ester), polyacrylamides, polyvinyl acetates, polyvinylalcohols and polyvinylpyrrolidone (PVP).In one embodiment, the structure-directing agent for use in the invention is polyvinylpyrrolidone. Certain compounds can be both a reducing agent and a structure-directing agent.For example, citric acid, ascorbic acid, and oxalic acid as herein described arereducing agents that can also act as structure-directing agents. Appropriate molar ratios of the components of the reaction mixture may readily be determined by those skilled in the art.The ratio of the number of moles of Cu2+ ions : Xn+ ions : Sn4+ ions in the reactionmixture may be in the range from about 1:1:1 to about 2:1:1, for example about1.6:1:1 to about 2:1:1. In certain embodiments, the ratio of the number of moles ofCu2+ions : Xn+ions : Sn4+ions may be 1.6:1:1, 1.7:1:1, 1.8:1:1, 1.9:1:1 or 2:1:1. In some embodiments, the ratio of the number of moles of Cu2+ions : Xn+ions : Sn4+ions may be about 1:1:1. The ratio of the total number of moles of metal ions (i.e. Cu2+ions, Xn+ions and Sn4+ions) to the number of moles of precipitating agent may be in the range fromabout 1:1 to about 1:2.5, preferably from about 1:1 to about 1:2.2, or morepreferably from about 1:1.5 to about 1:2. In certain embodiments, the ratio of thetotal number of moles of metal ions to the number of moles of precipitating agentmay be 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, 1:2, 1:2.1 or1:2.2. In one embodiment, the precipitating agent is urea. The ratio of the total number of moles of metal ions (i.e. Cu2+ions, Xn+ions and Sn4+ions) to the number of moles of urea may be in the range from about 1:1 to about 1:2.5, preferably from about 1:1 to about 1:2.2, or more preferably from about 1:1.5 to about 1:2. In certainembodiments, the ratio of the total number of moles of metal ions to the number ofmoles of urea may be 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, 1:2, 1:2.1 or 1:2.2. In one embodiment, the reaction mixture comprises the following components: Cu2+ions: 1.6 to 2.0; Xn+ ions: 1.0; Sn4+ ions: 1.0; precipitating agent: 4 to 8, structure-directing agent: 0.025 to 0.05; and reducing agent: 0.5 to 1.0, based on relativemolar ratios. In one embodiment, the reaction mixture comprises the following components: Cu2+ions: 1.0 to 2.0; Xn+ ions: 1.0; Sn4+ ions: 1.0; precipitating agent: 3 to 8, structure-directing agent: 0.05 to 0.1; and reducing agent: 0.5 to 3.0, based on relative molar ratios. The quaternary metal oxide materials produced according to the methods hereindescribed are highly porous solids, for example they are mesoporous solids. Asused herein, the term “mesoporous” is used to define materials having a pore size(i.e. diameter) in the range of from 2 nm to 50 nm.The resulting quaternary metal oxide materials are crystalline in structure. In someembodiments, the quaternary metal oxide has diffraction peaks at a diffraction angle of 2θ of 26.27°± 0.5°, 33.88°± 0.5°, 37.97°± 0.5°, 51.95°± 0.5°, 62.06°± 0.5°and 65.23± 0.5° in a powder X-ray diffraction obtained using CuKα radiation havinga wavelength of 1.5406Å. In some embodiments, the quaternary metal oxide has diffraction peaks at adiffraction angle of 2θ of 26.48° °± 0.5°, 33.90° ± 0.5°, 37.22° ± 0.5°, 43.24° ± 0.5°and 51.76° °± 0.5° in a powder X-ray diffraction obtained using CuKα radiationhaving a wavelength of 1.5406Å.The quaternary metal oxides that are produced via a hydrothermal method asherein described are novel and also form part of the invention. In a further aspect,the invention thus provides a quaternary metal oxide obtainable, obtained or directlyobtained by a hydrothermal method as herein described. Such materials may havea porosity and / or crystallinity as herein described.Use of a quaternary metal oxide as herein described as an active electrodematerial, e.g. as a positive electrode material (cathode), in a supercapacitor alsoforms part of the invention. The invention is illustrated further by way of the following non-limiting Examples and the accompanying figures, in which: Figure 1: Schematic illustration of the formation of CuXSnO4 (X = Ni, Co, Fe, Mn)via a hydrothermal method.Figure 2: X-ray diffraction patterns for the CuNiSnO4, CuCoSnO4, CuFeSnO4 and CuMnSnO4 materials produced in Example 1. Figure 3: Crystallographic images of the CuNiSnO4, CuCoSnO4, CuFeSnO4 andCuMnSnO4 materials produced in Example 1.Figure 4: (a) UV-Visible absorption spectra and (b-e) Tauc plots of the CuNiSnO4,CuCoSnO4, CuFeSnO4 and CuMnSnO4 materials produced in Example 1.Figure 5: Raman spectra of the CuNiSnO4, CuCoSnO4, CuFeSnO4 and CuMnSnO4materials produced in Example 1.Figure 6: FTIR spectra of the CuNiSnO4, CuCoSnO4, CuFeSnO4 and CuMnSnO4materials produced in Example 1.Figure 7: FE-SEM analysis of (a,b) CuNiSnO4, (c,d) CuCoSnO4, (e,f) CuFeSnO4,and (g,h) CuMnSnO4materials produced in Example 1.Figure 8: EDX analysis of (a,b) CuNiSnO4, (c,d) CuCoSnO4, (e,f) CuFeSnO4, and(g,h) CuMnSnO4materials produced in Example 1.Figure 9: TEM analysis of the CuNiSnO4, CuCoSnO4, CuFeSnO4 and CuMnSnO4materials produced in Example 1.Figure 10: XPS analysis: Survey spectrum of (a) CuNiSnO4 (CNTO), (b) CuCoSnO4(CCTO), (c) CuFeSnO4 (CFTO), (d) CuMnSnO4 (CMTO), and high-resolutionelemental spectra of (e) Cu2p, (f) Ni2p, (g) Co2p, (h) Fe2p, (i) Mn2p, (j) Sn3d, (k) O1s.Figure 11: BET analysis of (a-d) N2 adsorption-desorption isotherm and (e-h)Barrett-Joyner-Halenda (BJH) pore size distribution analysis.Figure 12: CV profiles of (a) CNTO, (b) CCTO, (c) CFTO, (d) CMTO and (e) Conediagram of specific capacity vs. scan rate.Figure 13: Capacitive, diffusive contribution from Trasatti’s method: (a-d) Linear fitsof C-1 vs. ʋ1 / 2, (e-h) Linear fits of C vs. ʋ-1 / 2 and (i) percentage of capacitive anddiffusive contribution for CNTO, CCTO, CFTO, CMTO respectively.Figure 14: GCD profiles of (a) CNTO, (b) CCTO, (c) CFTO, (d) CMTO and (e) Conediagram of specific capacity vs. current density.Figure 15: (a) EIS and (b) stability analysis of CNTO, CCTO, CFTO and CMTO. Figure 16: CV and GCD analysis of (a,e) CNTO, (b,f) CCTO, (c,g) CFTO and (f,h) CMTO. Figure 17: Capacitive retention and Coulombic efficiency of (a) CNTO, (b) CCTO, (c) CFTO and (f) CMTO.Figure 18: Ragone plots of the plots of the as-assembled CuNiSnO4 / / AC,CuCoSnO4 / / AC, CuFeSnO4 / / AC and CuMnSnO4 / / AC asymmetric supercapacitors compared with other reported results. Figure 19: X-ray diffraction patterns for the CuNiSnO4, CuCoSnO4, CuFeSnO4and CuMnSnO4 materials produced in Example 5. Examples The following chemicals were obtained in reagent grade from Sigma Aldrich andused without further purification or treatment: copper (II) nitrate trihydrate(Cu(NO3)2·3H2O) (≥ 99 %), nickel (II) nitrate hexahydrate (Ni(NO3)2·6H2O) (≥ 99 %),cobalt (II) nitrate hexahydrate (Co(NO3)2·6H2O) (≥ 99 %), manganese (II) nitratehexahydrate (Mn(NO3)2·6H2O) (≥ 99 %), iron (III) nitrate nonahydrate(Fe(NO3)3·9H2O), (≥ 99 %), tin (IV) chloride pentahydrate (SnCl4·5H2O) (≥98%),urea (NH2CONH2) (≥ 99.5 %), poly(vinylpyrrolidone) (PVP) (C6H9NO)n (99 %), andcitric acid (CA) (HOC(COOH)(CH2COOH)2) (≥ 99.5 %). Deionised water was obtained from a Millipore water purification plant.Example 1 – Synthesis of quaternary metal oxides: CuNiSnO4, CuCoSnO4,CuFeSnO4 and CuMnSnO4 Precursor solutions were prepared by separately mixing each of 5 mM copper (II)nitrate trihydrate, 3 mM nickel (II) nitrate hexahydrate, 3 mM tin (IV) chloridepentahydrate and 20 mM urea with 20 mL deionised water. The precursor solutionswere each stirred for 30 minutes. Each precursor solution was then mixed one byone with the copper precursor solution using high quality airbrush 3 mL disposable plastic eye dropper transfer graduated pipettes. The mixed solution was thenagitated for at least one hour to achieve homogeneity. During agitation, 0.1 g ofPVP followed by 0.1 g of citric acid was added to the mixed solution. The resultinghomogenous solution was then poured into a 100 mL hydrothermal autoclave stainless steel reactor for high pressure and temperature treatment. The temperature was maintained at 180°C for 16 hours in the muffle furnace. Following the reaction, the reactor was allowed to cool naturally to room temperature. Theresulting sediment material was washed three times using deionised water, thenwashed twice with methanol and ethanol to remove contaminants before beingdried overnight at 80°C in a hot-air oven. The obtained product was calcined at350°C for 3 hours to obtain CuNiSnO4nanoparticles. The same synthetic route was used to synthesise CuCoSnO4, CuFeSnO4and CuMnSnO4using their respective precursor solutions, i.e. cobalt (II) nitrate hexahydrate, iron (III) nitrate nonahydrate, and manganese (II) nitrate. The sameconcentrations of each precursor solution and the same amounts of PVP and citricacid were employed. The hydrothermal synthesis process is shown schematically in Figure 1.Example 2 – Characterisation of quaternary metal oxides: CuNiSnO4,CuCoSnO4, CuFeSnO4 and CuMnSnO4Various characterisation analysis was used to determine the structural, optical,morphological, and electrochemical supercapacitor performance of the materials prepared in Example 1. X-ray diffraction (XRD) analysis of the material was studiedusing the PANalytical - X'Pert3 Powder (X-ray Diffractometer with CuKα radiationhaving a wavelength of 1.5406 Å). Absorption spectra were measured using a UV-Vis-NIR spectrophotometer. Raman analysis was carried out using a micro-LaserRaman instrument (Imaging Spectrograph STR 500 mm Focal Length LaserRaman spectrometer; Flat field: 27 mm (W) × 14 mm (H); Resolution:1 / 0.6 cm-1 / pixel; Seiki, Japan). A Thermo Nicolet 380 FTIR spectrophotometer wasused to measure vibrational absorption spectra of the materials recorded in thewavelength range of 4000 to 400 cm-1 with an accuracy of 0.01 cm-1. A ZEISSSUPRA 55VP Field emission scanning electron microscope (at KV range: 100 V to30 kV) was used to observe the surface morphology and the elements of the ‘as prepared’ materials were examined via energy-dispersive X-ray spectroscopy. AJEOL-2100+ (Accelerating Voltage: 200kV; Resolution: Point: 0.194 nm) HighResolution Transmission Electron Microscope was used to examine themorphological structure of the materials. The chemical state and elementcomposition were studied by X-ray photoelectron spectrometry (PHI -VERSAPROBE III–XPS); monochromatic X-ray Beam (15 µm). A QuantachromeNovaWin ©1994-2018, Quantachrome Instrument v11.05 was used to quantify BETsurface area measurements. The electrochemical measurements were performedusing a Bio-logic SP-150 Potentiostat at ambient temperature.X-ray diffraction (XRD) analysis:X-ray diffraction characterization was used to determine the crystallographicstructure of the synthesised materials. Figure 2 shows the X-ray diffraction patternof the quaternary mixed metal oxide materials. The standard JCPDS card no. 49-0212 clearly evidenced the presence of CuNiSnO4formation of diffraction peaks at2θ = 26.27°, 33.88°, 37.97°, 51.95°, 62.06° and 65.23° corresponding to the (003),(211), (220), (043), (053) and (340) planes. It can be seen that the replacement ofNi with Co, Fe or Mn does not alter the crystallographic structure although there is asmall shift in the diffraction peaks and the intensity changes which may be due tothe different ionic radius of the Ni, Co, Fe and Mn elements. Figure 2 shows thatthe diffraction peaks of the 2θ values at 26.54, 26.58, 26.64 and 26.71 are highintensity peaks. Figure 3 shows crystallographic images obtained from the VSTAsoftware. UV-Visible spectral analysis: Figure 4(a) shows the optical properties of the metal oxide materials using the UV– Visible absorption spectra of the CuNiSnO4, CuCoSnO4, CuFeSnO4 andCuMnSnO4 samples in the range of 200-800 nm at ambient temperature. Themaximum absorption peaks were observed at 278 nm, 295 nm, 308 nm and 300 nm for the CuNiSnO4, CuCoSnO4, CuFeSnO4 and CuMnSnO4 materials,respectively. The high intensity absorption peak slightly shifted in the higherwavelength side (red shift). The bandgap energies of all materials were calculated by the Tauc plots (Figure 4(b-e)) using the following relation: ^ℎ^ = ^(ℎ^ − ^^)^where α is the absorption coefficient, ^ is the frequency, h is Plank’s constant, A isa constant known as the band tailing parameter, Eg is the Energy of the optical band gap and n is the power factor of the transition mode where n = ½, n = 2 for thedirect and indirect allowed transition of semiconductors (see Asaithambi et al.,Advanced Powder Technology, 33, (2022) 103442). The plotting of (αhν)2vs photon energy (h^) gives a straight line in a certain region. The extrapolation of thislinear straight line with intercept of the h^ axis gives the value of the indirect opticalband gap. The calculated energy band gap values are 2.68 eV,1.96 eV, 1.70 eV and 1.88 eV for the CuNiSnO4, CuCoSnO4, CuFeSnO4and CuMnSnO4materials, respectively. Raman analysis:Figure 5 shows the Raman spectra of the CuNiSnO4, CuCoSnO4, CuFeSnO4 andCuMnSnO4materials. As the particle size of the nanoparticles varies in the synthesised samples, their optical phonon confinement in nanoparticles changestheir position and band shape of the Raman spectra. All of the peaks wereobserved with very good intensities. The appearance of the Raman spectra of all materials replicates the optical phonon modes of the rutile SnO2nanoparticles. The spectrum of SnO2shows the four fundamental Raman active modes (A1g, B1g, B2g,and Eg) in the region of 400-800 cm-1. The Raman peak observed at 293.72 cm-1and 638.35 cm-1is responsible for Raman active optical-phonon Ag and B2g mode of CuO. The small peak at ~ 400-500 cm−1may be due to the metal cation vibration in the octahedral site. The Raman band illustrates that the broad peaks at lower wavenumbers shift less than 1000 cm–1, which may be due to Ni-O, Co-O, Fe-O, Mn-O and Sn-O stretching vibrational modes due to the metal oxides first-order longitudinal optical (LO) phonon modes. The peak around ~1100 cm-1may be the reason of 2Bg mode and the C-O stretching vibrations. The Raman band at 563.92 cm-1is associated with vibrations of Cu2+ions and oxygen vacancies. The peak positioned at ~ 293.72 cm−1could be ascribed to the E1g mode of phonon vibrations. The CH3 symmetric (1313.33 cm−1) and asymmetric (1403.05 cm−1) deformation bands were also detected. Due to the symmetric and antisymmetric intramolecular stretching vibrational modes of the water molecules, some Raman bands are found in longer wavenumber regions. FTIR Analysis:Information about the material identification was obtained by FTIR interferometryusing the IR beam over a wide spectrum of 4000 to 400 cm-1in wavelength of mid-IR region. The functional group analysis of the materials was studied using theFourier transformed infra-red spectrum (shown in Figure 6). The FT-IR spectrumdisplays different characteristic absorption peaks at 3448, 2920, 2845, 2350, 1623,1422, 1311, 1085, 617, 471 cm-1. Metal oxide vibration peaks were observed at thefingerprint region and the strong bond centred at 471 cm-1 was denoted themolecular vibrations of Cu-O, Ni-O, Co-O, Fe-O, and Mn-O. The Sn-O-Snstretching vibrations of the Sn-O mode was observed at 617cm-1. The peak 1311cm-1can be attributed to the presence of C-O bonds. The bending vibrations of water molecules band was observed at 1623 cm−1. The wide and less intense peak observed at 3448 cm-1corresponds to O-H stretching vibrations. Scanning Electron Microscope (SEM) analysis:Morphological behaviour is an important parameter to enhance the electrochemicalperformance of a supercapacitor. The morphological behaviour of the materialswas examined by scanning electron microscopy (SEM) analysis as shown in Figure 7. The hierarchical CuNiSnO4 microspheres (Figure 7(a,b)) are roughly distributedwith a mean diameter of ~0.5 to 1 μm, and all microspheres are bounded by asponge-like morphology covering small hierarchical nanostructures leading to the formation of cluster microsphere formation. The CuCoSnO4 material (Figure 7(c,d))exhibited porous hollow microspheres which are building blocks in an energystorage application. Highly dispersed and spherical spheres with uneven, porous structure without the agglomeration distributing the anchoring sites due to the cobaltions were detected. The particle diameters were roughly calculated and estimatedto be about ~3.0 μm. The porous nature of the CuCoSnO4 material (Figure 7(c,d))provides a high surface area for achieving a high specific capacitance. TheCuFeSnO4 material (Figure 7(e,f)) shows a sponge-like morphology which is helpfulin conducting the ion transport at an electrode-electrolyte interface. This cancontrol the formation of highly scattered perfect fluffy structures for CuFeSnO4 that can provide fastening electroactive sites by the replaced ferrite crystals. TheCuMnSnO4 material (Figure 7(g,h)) shows a perfect uniform micro-cubical shapewith a smooth surface which could provide a larger specific surface area andtherefore enable the electrolytic passage of ions and access by shortening the ion- diffusion paths for improving energy storage capability. The composite hierarchicalmicrostructures are expected to provide significantly improved electrochemicalperformance. The hollow and porous structures of the nanospheres offers asuperior porosity and admirable performance of molecular sieve, enabling it tointeract with atoms, molecules on their interior and exterior surface which can assistance its electrochemical performance as an electroactive material. Energy Dispersive X-Ray Analysis (EDX):The elemental composition of the materials was determined by Energy DispersiveX-Ray Analysis (EDX) analysis. The EDX spectra (shown in Figure 8) validate the composition of Cu, Ni, Co, Mn, Fe, O. No peaks of additional elements are detected, which is reliable with XRD and XPS patterns. When coupled with the X- ray diffraction analysis and with reference to the JCPDS card no.49-0212 for CuNiSnO4, this supports the conclusion that the elements Cu, X, Sn and O are present in the materials in the expected atomic ratio of 1:1:1:4. Transmission electron microscopy (TEM) analysis:Figure 9 shows the TEM images of as synthesised CuNiSnO4, CuCoSnO4,CuFeSnO4 and CuMnSnO4. The TEM images of CuNiSnO4 (Figure 9 (a,b)) reveala spherical shape with a high uniformity of monodisperse spheres having anaverage diameter of about 174.57 nm. This can provide more active sites on the surface, not only effectively avoiding the occurrence of hole clogging but also highlyfavourable for rapid adsorption / desorption of electrolyte ions. Figure 9 (d,e) showsthe TEM images of the CuCoSnO4 microspheres. The average particle size ofCuCoSnO4 was estimated to be 300 to 400 nm. Figure 9 (g,h) displays the TEMimages of CuFeSnO4 which show that the material is composed of highly uniform and nearly spherical nanoparticles. The size of the particles is 10-20 nm. Thesurface of the nanocomposite is rough with an even distribution over the surfacewhich undergo stacking and interlacing with the nanocomposites. This uniquestructure not only prevents the agglomeration of CuFeSnO4 nanoparticles, but also facilitates electrolyte penetration and faster ion / electron transfer, which enhances the electrochemical performance. The TEM images of CuMnSnO4 also support itsconsistent cubical shape, as depicted by its SEM images (shown in Figure 7 (g,h)),and the enlarged TEM image of CuMnSnO4 (Figure 9 (j,k)) illustrates that the cubesare solid and dense. These cubes are densely distributed such that the available captivating porous microstructure and vacuum gaps provide ample space for effective volume variation during continuous electrochemical cycling. The average thickness of CuMnSnO4cubes measured from the TEM images is ~50 to 60 nm. X-ray photoelectron spectroscopy (XPS) analysis: XPS analysis further confirmed the chemical composition of the nanoparticulate materials. Figure 10 (a-d) displays the full survey spectrum of the synthesised CuNiSnO4, CuCoSnO4, CuFeSnO4and CuMnSnO4samples which confirms thecomponents of Cu, Ni, Co, Mn, Fe, O in the respective high-resolution elementalspectra. The binding energy of 933.85 eV, 942.55 eV, and 953.34 eV are fitted withthe peaks of Cu 2p3 / 2, Cu 2p3 / 2, Sat and Cu 2p1 / 2 in the spectrum of Cu2p whichensures the valance state of Cu2+present in all samples as shown in Figure 10 (e).The Ni2+ state (Figure 10 (f)) is confirmed by the existing peak of spectral lines at Ni2p3 / 2 (856.23 eV), Ni 2p3 / 2, Sat (861.35 eV), and Ni 2p1 / 2 (873.66 eV), Ni 2p1 / 2, Sat(880.05 eV). Co 2p levels are positioned at the binding energies of Co 2p3 / 2(780.47 eV), Co 2p3 / 2, Sat (786.08 eV), and Co 2p1 / 2 (796.01 eV), Co 2p1 / 2,Sat(803.59 eV) with the valance state of Co2+(Figure 10 (g)). In Figure 10(h), thebinding energy peak centred at 711.45 eV (Fe 2p3 / 2), 715.80 eV (Fe 2p3 / 2) and723.78 eV (Fe 2p1 / 2) signify the Fe3+ oxidation state. The peaks of Mn 2p3 / 2, Mn2p1 / 2 are situated at 641.89 eV and 652.13 eV respectively which shows the formation of Mn2+valance state (Figure 10 (i)). The Sn 3d5 / 2 and Sn 3d3 / 2 are observed at the binding energy of 486.69, 495.05 respectively for Sn (IV) configuration (Figure 10 (j)). The lattice oxygen of the synthesised materials wasfound to be 531.04 eV for O1s state (Figure 10 (k) (Morozov et al., Solid StateSciences, 126, (2022) 106854). Measurement of surface area and pore size distribution: The Brunauer-Emmett-Teller (BET) method is a powerful technique generally used to compute the specific surface area and pore sizes under high-vacuum conditions based on nitrogen adsorption isotherm measurements at 77 K. Typically, data are measured in the low-pressure range of 0.05 to 0.3. The BET model is used to consider the multi-layer adsorption of gases on the surface of the adsorber. Therefore, attentiveness should be paid to the selection of data for surface analysis of synthesised materials, in which capillary compression occurs specifically in small pores (2-3 nm in diameter), as an alternative to multi-layer adsorption in the higher- pressure region. All the N2adsorption-desorption isotherm curves in Figure 11 (a-d) revealed type IV isotherm with a hysteresis loop signifying the occurrence ofmesopores behaviour during the capillary condensation occurs. Figure 11 (e-h)displays the Barrett-Joyner-Halenda (BJH) pore size distribution analysis whichshows the surface area characteristic of mesoporous materials, which are usuallypore diameter between 2-50 nm, gives this type of type IV isotherm. Small pores are found in solids at pressures below the saturation pressure of gases. In such low-pressure regions first, monolayer formation occurs followed by multilayer formations. The measured BET specific surface area (m2 / g), cumulative porevolume (cc / g) and average pore diameter (nm) are summarised in Table 1 below:Table 1: BET analysis: BJH desorption summary Sample BET specificCumulative pore Average pore surface area volume diameter (m2 / g) (cc / g) (nm) CuNiSnO4 35.754 0.054 2.214CuCoSnO4 51.322 0.136 10.650CuFeSnO4 159.522 0.187 4.814CuMnSnO4 101.319 0.190 8.279The CuFeSnO4sample has a much larger specific surface area distribution owing to its superior potential for gas adsorption than the other CuNiSnO4, CuCoSnO4 and CuMnSnO4 samples, resulting in improved electrochemical performances of the CuFeSnO4 samples. Therefore, mesoporous structure and small pore surface of the composite materials may be helpful to provide a conductive network for ion diffusion in the electrolyte to enhance the electrochemical performance of the electrode materials.Example 3 – Assembly of an asymmetric supercapacitor device andelectrochemical measurements The quaternary metal oxide nanoparticles prepared in Example 1 were used toproduce electrode materials. Active material (80 wt.%), activated carbon (10 wt.%),acetylene carbon black (5 wt.%) and poly(vinylidene fluoride) (5 wt.%) (binder) weremixed with a mortar using N-methyl-2-pyrrolidinone as a solvent to make a semi-solid gel slurry. A minimum amount of the resulting slurry (approx.2-3 mg) wasuniformly coated onto a porous Ni foam substrate (current collector) having adimension of 2 x 1 cm using a simple paint coating method under ambientconditions. The prepared substrates were then dried at 80°C in a vacuum air oven for 12 hours. The working electrode area (about 1 cm2) of the coated Ni foamsurface was immersed in an electrolyte solution.A biologic SP-150 instrument was used to study the electrochemical performancesof both three electrode and two electrode combinations measurements. Threeelectrode performances were studied with active material (CuXSnO4: X = Ni, Co,Mn, Fe), a standard Ag / AgCl, Pt wire as working, reference and counter electrode, respectively. The specific capacity of the prepared electrodes was evaluated by calculating the integral area of the cyclic voltammetry (CV) curves. The galvanostatic charge- discharge technique (GCD) was used to estimate specific capacitance using discharge time at charge discharge curves. The capacitive and resistive electrical parameters were studied by Electrochemical impedance spectroscopy (EIS). The electrochemical impedance spectroscopy (EIS) measurements were taken in the frequency range from 0.01 Hz to 100 kHz at 5 mV amplitude. Since the CV and GCD profiles of the prepared electrodes show battery-like curves, it is imperative to express their specific capacity in C / g or mAh / g. The specific capacity from CV and GCD were calculated as stated by the following equations. An asymmetric supercapacitor (ASC) device was assembled with a positiveelectrode containing CuXSnO4: X = Ni, Co, Mn, Fe and a negative electrode withactivated carbon in a 2 M KOH electrolyte. A Whatman filter paper acted as aseparator soaked in electrolyte for 30 minutes before assembling the ASC.Additionally, mass balance of the cathode and anode can be altered based on the following formula, ^^^^ × Δ^^= (3) ^^^^ × Δ^^Where m+and m−indicates the active substances at the positive and negative electrodes, Q+and Q−signifies the specific capacity of the electrodes, and ΔV+and ΔV−is the potential window of the electrodes. The energy density and power density of the ASC device were calculated by the following equations. ^× Δ^^ ^= ^7.2(^ℎ / ^ ) (4)^ = 3600 × ^Δ^(^ / ^^) (5)Where I (mA), ^ ((mV / s)), ΔV (V), Δt (s), m (mg), E (Wh / g), P (W / kg) and Cs (F / g)have their conventional meanings. Cyclic Voltammetry:Cyclic voltammetry was used to study the oxidation and reduction processes of theelectrode materials by measuring the current response of a redox active solution to a linearly cycled potential sweep between two potential ranges from 0-0.6 V as shown in Figure 12 (a-d). The electron transfer rate is not defined in the electrochemically reversible processes, which produces stable redox pairs. Thesymmetric nature of the CV curves in all the samples (Figure 12) shows thereversible behaviour. The shape of the CV curves for all materials displays the strong boundaries of the cathodic and anodic redox peaks associated with the Nernst reactions. As the scan rate increases, CV curves of all the samples occur at approximately identical peaks with an increase in current density. However, thearea of the curves increases as the scan rate increases, referring the large amountof charge storage. Also, it exhibits high-rate performance and exceptional electrochemical kinetic reversibility as the current density being directly proportional to scan rate. In the samples studied, the CuFeSnO4 electrode exhibited a relatively high current density compared to the CuNiSnO4, CuCoSnO4 and CuMnSnO4electrode materials which may be due to its porous morphology, high surface areaand virtuous conductivity. The shifting of redox peaks towards cathodic and anodic potentials indicates the formation of a greater number of oxygen ion vacancies and the polarization effect and irreversible reactions due to the internal resistance of theelectrode and electrolyte ions. The CV curves revealed a non-rectangular shapewith wide redox peaks, which confirms the nature of the reversible faradic reactions and the combination both the battery type and the characteristic of the electricdouble-layer capacitor electrodes. The specific capacity for all scan rates wascalculated using eqn.1 and the values are shown in the cone diagram in Figure 12 (e).The Trasatti method is a means to provide additional insight into the total chargestored by the prepared electrode materials and the charge stored under fastkinetics. The Trasatti method is described in Isacfranklin et al., NanoscaleAdvances, 2021, 3, 486-498. For this method, the cyclic voltametric curves areconsidered to be the function of different scan rates and the total voltammetriccharge (qtotal) stored by the electrode material is separated by the surface capacitive charge (qsurface) and the diffusion controlled (qdiffusion) charge: qtotal = qsurface + qdiffusion.Detailed charge storage methods and formulas are discussed in NanoscaleAdvances, 3, (2021) 486-489. From the Trasatti method, the CuFeSnO4 electrode exhibited 65.23% of qtotal due to diffusion process, thus showing more pseudocapacitive nature and remaining 34.77% of qtotal due to capacitive process.The diffusion process of the other materials is illustrated in Figure 13 (i). Hence,the CuFeSnO4 electrode shows high diffusive nature even at high sweep rates, makes it a favourable active cathode material for the high power-high energy supercapacitors. Galvanostatic charge discharge (GCD) analysis:GCD measurements were used to study the specific capacity performance and ratecapability of the electrode materials. Figure 14 (a-d) displays the GCD profile of theprepared samples in the potential range of 0-0.5 V. All GCD curves haverepresentative voltage bases and slopes, which demonstrates that the charge storage mechanism is based on Faraday processes. The specific capacity for allthe current density was calculated using equation 2 and the corresponding valuesare displayed in cone diagram in Figure 14 (e). Among the as-prepared quaternary metal oxides electrodes, CuFeSnO4exposes the highest specific capacity of 45.76 mAh / g than the CuNiSnO4(20.62 mAh / g), CuCoSnO4(23.47 mAh / g), andCuMnSnO4 (27.72 mAh / g) at 1 A / g. The highest specific capacity of CuFeSnO4electrode is due to the porous behaviour, large surface area, low particle size, and good conductivity of the material which helps easy penetration of electrolyte ionsbetween the electrode and electrolyte interface and plentiful electroactive sites forFaraday reaction. Electrochemical Impedance Spectroscopy (EIS) and stability analysis:Electrochemical impedance spectroscopy (EIS) was used to investigate the chargetransport and ion diffusion characteristics of the electrode materials. EISmeasurements were taken in the frequency range from 0.01 Hz to 100 kHz at 5 mVamplitude. The Nyquist plot consists of partial semicircles followed by inclinedlines, which denoted the presence of charge transfer resistance (Rct) and constantphase element (CPE), of the synthesised product as shown in Figure 15 (a). Theequivalent circuit model is fitted using Z-fit analysis and this corresponds exactly to the equation R1+Q2 / R2+Q2. The CPE is used as an alternative of the capacitance to ensure the frequency dispersion of EDL generated from the surface defect, porosity, or inhomogeneity of the electrode material. All the EIS parameters aresummarised in Table 2 below:Table 2: EIS parameters The Rct values of CuNiSnO4, CuCoSnO4, CuFeSnO4, CuMnSnO4 are measured to be 13.08, 10.34, 4.66, 8.52 Ω, respectively. The CuFeSnO4 electrode exhibits comparatively lower Rct value than other electrodes, indicating the occurrence of rapid charge transfer kinetics between their electrode / electrolyte interfaces. In addition, the same CuFeSnO4 electrode offers smaller intrinsic solution resistance (0.1632 Ω) than the other electrodes. The Rct and Rs values for the four types of electrodes are identical and partial, signifying their ideal electrical conductivity properties. In the low frequency region, CuFeSnO4electrode provides large linear gradients, which exhibit low values of diffusion resistance with effective ion diffusion activity during redox reactions. Finally, the capacitive retention of stability measurements of all the CuNiSnO4, CuCoSnO4, CuFeSnO4, CuFeSnO4electrodes are obtained to be 92.17%, 92.40%, 95.74% and 92.43%, respectively for 5000continuous charge discharge cycles. The better cyclic performances were obtainedfor CuFeSnO4 which may be due to its small particle size distribution. Almost all thesamples display the capacitive retention above 90% which might be attributed tothe high OH- diffusion rate and redox reaction kinetics.Example 4 – Asymmetric supercapacitor (ASC) device performanceThe ASC device was designed by combining the CuNiSnO4, CuCoSnO4, CuFeSnO4, CuMnSnO4electrode materials as positive electrode, activated carbon (AC) as a negative electrode, and Whatman filter paper as a separator to measure the potential utility of the synthesised materials. Prior to that, the electrochemical properties of AC were studied by CV and GCD analysis in the three-electrode systems and the results related to the mass balancing equations were confirmed. The rectangular CV and triangular GCD shapes in the negative potential range at -1 to 0 V clearly showed its electric double layer behaviours. The mass loading for ASC was determined based on the charge balance theory, which estimated thecalculated mass ratio of CuNiSnO4, CuCoSnO4, CuFeSnO4, CuMnSnO4 and AC tobe approximately 1: 3. In the three-electrode system the working potential is optimised for both positive and negative electrodes and the operating potential of ASC is estimated to be 0–1.5 V. Figure 16 shows the CV curves of the as fabricated CuNiSnO4 / / AC, CuCoSnO4 / / AC, CuFeSnO4 / / AC and CuMnSnO4 / / AC ASC at constant cell voltages of 0-1.5 V in 2 M KOH aqueous electrolyte at 10-60mV / s scan rate. The CV curves reveal the combine features of the electric doublelayer and the Faradic reaction without any noticeable distortion, indicates its excellent charge transport capacity and rate efficiency. Figure 16 shows the GCD curves of the ACS device at a constant potential range of 0-1.5 V. The combined properties of its double layer capacity and the faradaic reaction was further confirmed by GCD studies with triangular GCD profiles with plateaus. The ASC device performances such as specific capacitances, energy and power densities of all the samples were calculated from GCD profiles at different current density of 1-10 A / g according to the equations 5 and 6.The calculated specific capacitances, energy and power densities of theasymmetric cell with constant cell voltages are summarised in Table 3 below:Table 3 – Electrochemical results (two electrode performance)Current Potential V2Discharge Specific Energy Power density (V) time capacitance density density (A / g) (s) (F / g) (Wh / kg) (W / kg) 11.5 2.25 140.43 93.62 29.25 749.8342 1.5 2.25 61.64 82.18 25.68 1499.80On3 1.5 2.25 32.51 65.02 20.31 2249.03SiNu4 1.5 2.25 22.47 59.92 18.72 2999.19C5 1.5 2.25 16.61 55.36 17.30 3749.5410 1.5 2.25 5.95 39.66 12.39 7496.471 1.5 2.25 177.64 118.42 37.00 749.8342 1.5 2.25 82.16 109.54 34.23 1499.85On S3 1.5 2.25 49.74 99.48 31.08 2249.45o C4 1.5 2.25 33.12 88.32 27.60 3000.00u C5 1.5 2.25 22.56 75.20 23.50 3750.0010 1.5 2.25 7.9 52.66 16.45 7496.201 1.5 2.25 278.39 185.59 57.99 749.8942 1.5 2.25 127.72 170.29 53.21 1499.81On3 1.5 2.25 76.63 153.26 47.89 2249.82SeF4 1.5 2.25 48.26 128.69 40.21 2999.50u C5 1.5 2.25 34.61 115.36 36.05 3749.7810 1.5 2.25 15.92 106.13 33.16 7498.491 1.5 2.25 206.67 137.78 43.05 749.8942 1.5 2.25 95.67 126.90 39.65 1499.68On S3 1.5 2.25 59.19 118.38 36.99 2249.77n M4 1.5 2.25 41.15 109.73 34.29 2999.85u C5 1.5 2.25 24.95 83.16 25.98 3748.6110 1.5 2.25 10.35 69.00 21.56 7499.13It can be seen that the specific capacitance gradually decreases with increasing current density in all GCD curves, which may be due to the lower utilization of electroactive materials at higher current densities because the ions in theelectrolyte do not have the ability to enter into the inner structure of the activematerials, only the outer surface is utilized for charge storage. As mentionedabove, the specific capacitance of the CuFeSnO4 / / AC is much higher than that of the CuNiSnO4 / / AC, CuCoSnO4 / / AC and CuMnSnO4 / / AC ASC device due to its low internal resistance, high surface area and good conductivity. The capacitive retention (CR) (cycling stability) and coulombic efficiency (CE) of the as-prepared CuNiSnO4 / / AC, CuCoSnO4 / / AC, CuFeSnO4 / / AC and CuMnSnO4 / / AC ASC was assessed by galvanostatic charge / discharge test at current density of 10A / g for continuous 20000 charge discharge cycles (Figure 17). As shown in Figure17, the stability of CuFeSnO4 / / AC exhibits 97.47% capacitive retention with 99.57% coulombic efficiency than the CuNiSnO4 / / AC (CR:81.85%, CE:98.65%),CuCoSnO4 / / AC (CR:84.24%, CE:98.68%), and CuMnSnO4 / / AC (CR: 94.24%,CE:98.72), signifying its excellent sustainability and reversibility. The variousreported energy and power density values are summarised in Table 4 andcompared in the Ragone plot (Figure 18).Table 4 – Energy and power density values: comparison with known electrode materialsElectrode Electrolyte Morphology WorkingEnergy Power Capacitance potential density density retention (V) (Wh / Kg) (W / kg) (%) @ cycle @ current density (A / g) High 0.5 M Irregular1.6 14.22 218.8 67%graphitic Li2SO4 tubular @0.25 @10000 porous shape biomass carbon (HGPBC) Fe-Co6 M KOH Leaf-like 1.65 20.20 830.0 87%binary structure @1 @3000 oxides / and nano- graphene particles Co-Zn-1 M KOH Nano-arrays 1.5 25.71 404.00 80%S@CuO- @0.52 @4500 CF / / Fe-S / GO-NF NiCo2O4 / / AC 2 M KOH Micro- 1.7 26.20 920.80 99.4%spheres @1 @5000 CoNi- PVA-KOH Nanowire1.6 28.88 800.00 80.52%LDH / NiCo2S4 gel array @1 @10000 / RGO / / AC electrolyte CuCo2O4 / 2 M KOH Nano-wires 1.6 33.00 200.00 83.00%CuO / / RGO @0.25 @5000 / Fe2O3 Ni3V2O8 / Ni PVA-KOH-Nano-sheets 1.2 33.20 2400.00 73.20% / / AC KI polymer @4 @10000 Fe–Ni–Co–1 M KOH Nano-flake 1.5 33.60 750.00 87.40%O / / rGO array @1 @5000 Zn–Ni–Co–6 M KOH Nano-wire 1.5 35.6 187.60 71.20%O / / AC array @1 @10000 RGO-6 M KOH Nano- 1.4 35.6 699.90 77.20%MnNiCoO particles @1 @10000 / / RGO anchored with nano- sheets CuNiSnO4 2 M KOH Micro- 1.5 29.50@1 749.83 81.85% / / AC sphere @20000 CuCoSnO42 M KOH Porous 1.5 37.00 749.83 84.24% / / AC hollow @1 @20000 micro- spheres CuFeSnO4 2 M KOH Sponge like 1.5 57.99@1 749.89 97.47%@20000 / / AC nano- particles CuMnSnO4 2 M KOH Micro 1.5 43.05@1 749.89 94.24% / / AC cubical @20000 shapeExample 5 – Scalable synthesis of quaternary metal oxides: CuNiSnO4,CuCoSnO4, CuFeSnO4and CuMnSnO4100 g samples of CuNiSnO₄, CuCoSnO₄, CuMnSnO₄, and CuFeSnO₄ wereprepared in a 500 mL hydrothermal autoclave using a precursor solution preparedby mixing higher concentrations of metal salts, urea, citric acid andpoly(vinylpyrrolidone) with deionised water. For each quaternary metal oxide, thesample was produced by preparing four batches in which each batch yielded approx.25 g of product.For the preparation of CuNiSnO4, a solution was prepared by mixing precursorsolutions of 0.3 M copper (II) nitrate trihydrate, 0.3 M nickel (II) nitrate hexahydrate,0.3 M tin (IV) chloride pentahydrate, 0.9 M urea, and 0.9 M citric acid, and 0.2 g ofpoly(vinylpyrrolidone) in 400 mL deionised water. In this large scale synthesis,equal molarities of the metal precursors (Cu:Ni:Sn) were used. This change in themolarity of the metal salts did not affect the crystal structure of the resultingquaternary metal oxide materials. The same synthesis procedure was followed forpreparing solutions to be used in the synthesis of CuCoSnO4, CuMnSnO4 andCuFeSnO4 using the corresponding metal salts and the same concentrations: 0.3 Mcobalt (II) nitrate hexahydrate, 0.3 M manganese (II) nitrate hexahydrate, and 0.3 Miron (III) nitrate nonahydrate.For the hydrothermal synthesis of each of CuNiSnO₄, CuCoSnO₄, CuMnSnO₄, andCuFeSnO₄, the prepared solutions were transferred to a 500 mL autoclave. Themolar ratio of Cu:X:Sn (where X = Ni, Co, Mn or Fe) was 1:1:1. 0.2 g ofpoly(vinylpyrrolidone) was used as a structure directing agent. The precipitatingagent (urea) and reducing agent (citric acid) were employed at a molar concentration 3 times that of the metal salts.The hydrothermal reaction was carried out at 180°C for 16 hours. After naturalcooling, the precipitate was centrifuged, washed four times with deionized water,once with ethanol, and dried at 80°C for 12 hours. The prepared materials werethen calcined at 700°C for 5 hours to obtain the required phase-purity of thematerials.Example 6 – Characterisation of quaternary metal oxides: CuNiSnO4,CuCoSnO4, CuFeSnO4 and CuMnSnO4 X-ray diffraction (XRD) analysis of the materials prepared in Example 5 was conducted. X-ray diffraction measurements were taken at 10-70° 2θ with a step size of 0.0110. Figure 19 shows the X-ray diffraction pattern of the quaternary metal oxide materials. XRD peak positions and intensities of the as-prepared materials were consistentwith the powder diffraction file (PDF) database for CuNiSnO₄ and thus verified thephase purity and structure of the as-prepared sample. According to a study published by Sampath et al. in Bull. Mater. Sci.7(5): 487-492,1994, the compoundCu²⁺Ni²⁺SnO₄ has the lattice parameters a0 = 5.773 ± 0.01 Å, b0 = 8.377 ± 0.01 Å,and c0 = 10.094 ± 0.01 Å, which has been verified to exhibit orthorhombicsymmetry. The major 2θ peaks observed in the experimental data for CuNiSnO4, i.e.26.4756°,33.8953°, 37.2171°, 43.2402° and 51.7638°, is well matched with the JCPDS 49-0212 major peaks 26.498°, 33.916°, 37.137°, 43.124° and 51.596°. The d-spacing values are also nearly identical, indicating a good phase match. Therefore, the experimental XRD pattern shows a strong match with JCPDS 49-0212, confirmingthe successful synthesis of the CuNiSnO₄ phase in a scalable process.The remaining CuCoSnO₄, CuMnSnO₄, and CuFeSnO₄ materials exhibited thesame XRD pattern as that of the materials prepared in the small scale synthesis ofExample 1. Thus, these results also confirm an effective large scale synthesis forthese other quaternary metal oxide materials.

Claims

Claims1. An electrode for a supercapacitor, the electrode comprising a quaternarymetal oxide of the formula CuXSnO4, wherein X is selected from the group consisting of nickel (Ni), cobalt (Co), iron (Fe) and manganese (Mn).

2. An electrode as claimed in claim 1, wherein said quaternary metal oxide isCuCoSnO4, CuFeSnO4 or CuMnSnO4, preferably CuCoSnO4.

3. An electrode as claimed in claim 1 or claim 2, wherein said quaternary metaloxide is crystalline and has diffraction peaks at a diffraction angle of 2θ of 26.27°±0.5°, 33.88°± 0.5°, 37.97°± 0.5°, 51.95°± 0.5°, 62.06°± 0.5° and 65.23± 0.5° in apowder X-ray diffraction obtained using CuKα radiation having a wavelength of1.5406Å.

4. An electrode as claimed in any one of the preceding claims, wherein saidquaternary metal oxide is characterised by: -a Brunauer-Emmett-Teller (BET) specific surface area of from 30 to 200m2 / g, preferably from 35 to 180 m2 / g, more preferably from 40 to 170 m2 / g, for example from 50 to 160 m2 / g; and / or -a Barrett-Joyner-Halenda (BJH) average pore diameter of from 1 nm to 50nm, preferably from 1 nm to 20 nm, for example from 2 nm to 15 nm.

5. An electrode as claimed in any one of the preceding claims, wherein saidquaternary metal oxide is provided in the form of nanoparticles.

6. An electrode as claimed in any one of the preceding claims whichadditionally comprises one or more of the following components: a porous material,a conductive additive and a binder.

7. An electrode as claimed in claim 6, wherein said porous material is selectedfrom the group consisting of a carbon-based filler, a ceramic filler, a metal filler or a mixture thereof.

8. An electrode as claimed in claim 7, wherein said carbon-based filler isgraphene, graphite, carbon nanotubes, carbon black, activated carbon or charcoal.

9. An electrode as claimed in any one of claims 6 to 8, wherein said conductiveadditive is a carbon-based material, for example acetylene carbon black.

10. An electrode as claimed in any one of claims 6 to 9, wherein said binder ispoly(vinylidene fluoride) (PVDF), polyvinylalcohol (PVA), poly(acrylic acid) (PAA),polyacrylonitrile (PAN), carboxymethylcellulose (CMC), polytetrafluoroethylene (PTFE), polyethylene (e.g. HDPE or LDPE), polypropylene, polyvinyl chloride(PVC), polystyrene, nylon, teflon, polyethylene terephthalate (PET), poly(methylmethacrylate) (PMMA), or a mixture thereof.

11. A supercapacitor which comprises at least one capacitor cell comprising afirst porous electrode, a second porous electrode, an electrolyte in contact with said first and second porous electrodes, and a separator separating the first porous electrode from the second porous electrode, wherein the first porous electrode is an electrode as claimed in any one of claims 1 to 10.

12. A supercapacitor as claimed in claim 11, wherein the second porouselectrode is a high surface area carbon material selected from the group consisting of amorphous carbon, graphite, carbon nanotubes (CNTs), activated carbon (AC), graphene or any combination thereof, preferably activated carbon (AC).

13. A quaternary metal oxide having the formula CuXSnO4, wherein X is iron(Fe) or manganese (Mn).

14. A quaternary metal oxide as claimed in claim 13 having diffraction peaks ata diffraction angle of 2θ of 26.27°± 0.5°, 33.88°± 0.5°, 37.97°± 0.5°, 51.95°± 0.5°,62.06°± 0.5° and 65.23± 0.5° in a powder X-ray diffraction obtained using CuKαradiation having a wavelength of 1.5406Å.

15. A quaternary metal oxide as claimed in claim 13 or claim 14 characterisedby:- a Brunauer-Emmett-Teller (BET) specific surface area of from 30 to 200m2 / g, preferably from 35 to 180 m2 / g, more preferably from 40 to 170 m2 / g, for example from 50 to 160 m2 / g; and / or -a Barrett-Joyner-Halenda (BJH) average pore diameter of from 1 nm to 50nm, preferably from 1 nm to 20 nm, for example from 2 nm to 15 nm.

16. A quaternary metal oxide as claimed in any one of claims 13 to 15, whereinsaid metal oxide is provided in the form of nanoparticles.

17. A method of preparing a quaternary metal oxide of the formula CuXSnO4,wherein X is selected from the group consisting of nickel (Ni), cobalt (Co), iron (Fe)and manganese (Mn), said method comprising the step of subjecting an aqueoussolution which comprises the constituent metal ions of the metal oxide to a hydrothermal reaction.

18. A method as claimed in claim 17 which comprises the following steps:(a) providing an aqueous reaction mixture containing: Cu2+ions, Xn+ ions (wherein X is Ni, Co, Fe or Mn, n is 2 when X is Ni, Co orMn, and n is 3 when X is Fe), Sn4+ions, a precipitating agent, a reducing agent, and one or more structure-directing agents; (b) subjecting the aqueous reaction mixture to a hydrothermal reaction; (c) allowing the reaction mixture to cool to ambient temperature; (d) isolating a solid material containing the quaternary metal oxide; and (e) optionally washing and drying the solid material.

19. A method as claimed in claim 18 which further comprises the step ofsubjecting the solid material to calcination, preferably wherein calcination isperformed at a temperature in the range of from 300°C to 800°C, more preferably300°C to 400°C, yet more preferably about 350°C.

20. A method as claimed in claim 18 or claim 19, wherein said precipitatingagent is urea, ammonia (NH3), tetramethylammonium hydroxide (TMAH),ammonium carbonate ((NH4)2CO3), sodium carbonate (Na₂CO₃), potassiumcarbonate (K2CO3), potassium hydroxide (KOH), sodium hydroxide (NaOH), sulfuric acid (H2SO4), hydrochloric acid (HCl), sodium borohydride (NaBH4), ammonium fluoride (NH4F), or any combination thereof.

21. A method as claimed in claim 20, wherein said precipitating agent is urea.

22. A method as claimed in any one of claims 18 to 21, wherein said reducingagent is citric acid, ascorbic acid, acetic acid, oxalic acid, sulphur dioxide, hydrogen, a phosphite, phosphorous acid, a hypophosphite, or a combination thereof.

23. A method as claimed in claim 22, wherein said reducing agent is citric acid.

24. A method as claimed in any one of claims 18 to 23, wherein said structure-directing agent is a surfactant or a polymer.

25. A method as claimed in claim 24, wherein said surfactant is potassiumlaurate, sodium octant sulfonate, sodium decane sulfonate, sodium dodecane sulfonate, sodium lauryl sulfate, ammonium lauryl sulfate, sodium myreth sulfate, sodium di-2-ethylhexyl sulfosuccinate, decyltrimethylammonium bromide, dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, tetradecyltrimethylammonium chloride, dodecylammonium chloride, hexamethylenetetramine (HMTA), cetyltrimethylammonium bromide (CTAB), cetyltrimethylammonium chloride (CTAC), or wherein said polymer is polyethyleneglycol or a derivative thereof (e.g. polyethylene glycol (PEG) ester), apolyacrylamide, a polyvinyl acetate, a polyvinyl alcohol or poly(vinylpyrrolidone).

26. A method as claimed in claim 24, wherein said structure-directing agent ispoly(vinylpyrrolidone).

27. A method as claimed in any one of claims 18 to 26, wherein thehydrothermal reaction in step (b) is carried out at a temperature in the range of from120°C to 250°C, preferably from 160°C to 200°C, e.g. about 180°C, under apressure in the range of from 0.3 to 4 MPa.

28. A quaternary metal oxide obtainable, obtained or directly obtained by amethod as claimed in any one of claims 17 to 27.

29. Use of a quaternary metal oxide of the formula CuXSnO4, wherein X isselected from the group consisting of nickel (Ni), cobalt (Co), iron (Fe) andmanganese (Mn) as an active electrode material, for example as an activeelectrode material in a supercapacitor.

30. Use as claimed in claim 29, wherein said quaternary metal oxide is asdefined in claim 28.