WO2012155196A1

WO2012155196A1 - Spray pyrolysis method for in situ production of graphene oxide based composites

Info

Publication number: WO2012155196A1
Application number: PCT/AU2012/000530
Authority: WO
Inventors: Konstantin Krasimirov KONSTANTINOV
Original assignee: University Of Wollongong
Priority date: 2011-05-13
Filing date: 2012-05-11
Publication date: 2012-11-22

Abstract

There is provided a spray pyrolysis method for producing a graphene oxide based composite. The method includes spraying graphene oxide in suspension or solution into a reaction chamber. Simultaneously or separately spraying at least one precursor in suspension or solution into the reaction chamber. Heating the reaction chamber to produce an in situ chemical reaction between the graphene oxide and the at least one precursor, and retrieving a graphene oxide based composite from the reaction chamber. GO-Mn₂O₃/Mn₃O₄ composites were synthesized using the spray pyrolysis method. The results show unique morphology, afforded by the modified spray solution used including a hydroxide suspension, and enhanced capacitance. An exceptionally high capacitance value of about 606 Fg^-1 at 5 mVs^-1 was observed for an example 20% GO-Mn₂O₃/Mn₃O₄ material.

Description

SPRAY PYROLYSIS METHOD FOR IN SITU PRODUCTION

OF GRAPHENE OXIDE BASED COMPOSITES

Technical Field

[001] The present invention generally relates to nanostructured composites, preferably graphene oxide based, and a method of production or synthesis thereof. More particularly, in one form the present invention relates to a spray pyrolysis method for in situ production of graphene oxide based composites. The graphene oxide based composites can be used as improved electrode materials in supercapacitors, batteries or other energy storage devices.

Background

[002] Electrochemical capacitors, often referred to as supercapacitors, can be distinguished mainly by the type of electrode material used in their manufacture. While the type of electrode material has a significant bearing on the cycle life, energy density and power density of a supercapacitor, a large surface area is almost compulsory for achieving relatively high capacitance. Generally, two types of supercapacitors exist, namely: Electrical Double Layer Capacitors (EDLCs) and pseudocapacitors. EDLCs make use of a double layer on the electrode surface for charge storage, while pseudocapacitors rely on Faradaic reactions occurring at the electrode-electrolyte interface.

[003] Carbonaceous materials, such as carbon nanotubes, graphene and activated carbon have been previously studied as electrode materials for use in EDLCs. Alternatively, metal oxides and conducting polymers have also been previously studied as electrode materials for use in pseudocapacitors due to their ease of preparation and low cost. Furthermore, metal oxides and conducting polymers are known to have a relatively high capacitance, although the cycle life of these materials is generally not as long as compared with carbonaceous materials that generally have lower capacitance.

[004] Graphene is a two dimensional allotrope of carbon, made up of one atom thick planar sheets of carbon atoms bonded together. Recently, graphene has been investigated as a potential supercapacitor electrode material. Such attention can be attributed to graphene's extraordinary conductivity, high mechanical strength, low cost and large surface area. Unlike carbon nanotubes and activated carbon, where a large surface area is provided by a porous structure, graphene's large surface area is mainly due to the layered structure and presence of edge sites on the edge of the planes, as well as defects on the basal plane. [005] By chemically reducing graphene to graphene oxide (GO), more edge sites and oxygenated groups are added into the structure resulting in higher capacitance. Recently, Ruoff et al. (Stoller, M.D., et al, Graphene-Based Ultracapacitors, Nano Letters, 2008. 8(10): p. 3498-3502) reported that chemically modifying graphene sheets by using Hummer's method significantly increased the capacitance to 135 Fg^"1 compared to 117 Fg^"1 reported by Rao et al. (Vivekchand, S., et al, Graphene-based electrochemical supercapacitors, Journal of Chemical Sciences, 2008. 120(1): p. 9-13).

[006] It is known that metal oxides such as Ru0₂, Mn0₂, C03O4 and NiO are potential candidates for supercapacitor electrodes, with ruthenium oxide being particularly promising. However, the high cost and toxicity of ruthenium renders it unfavourable for supercapacitor applications. Although other metal oxides have been studied as alternatives, they do not provide capacitances as high as 720 Fg^"1, which has been reported for Ru0₂.

[007] Manganese dioxide provides a promising substitute to ruthenium oxide as it is cheaper and more environmentally friendly. This has led to extensive research being carried out on manganese dioxide (Mn0₂) and manganese (III) oxide (Mn₃0₄) over recent years. Prasad et al. potentiodynamically deposited manganese dioxide onto stainless steel substrates and tested the electrode in 0.1 M Na₂S0₄ to obtain capacitance values as high as 482 Fg^"1 using cyclic voltammetry at 10 mVs^"1 (Prasad, K.R. and N. Miura, Potentiodynamically deposited nanostructured manganese dioxide as electrode material or electrochemical redox supercapacitors, Journal of Power Sources, 2004. 135(1-2): p. 354-360).

[008] Recently, Jiang et al. synthesized Mn₃0₄ using an ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) assisted hydrothermal route to fabricate supercapacitors with capacitance of up to 322 Fg^"1 (Jiang, H., et al, Hydrothermal synthesis of novel Μη$θ4 nano-octahedrons with enhanced supercapacitors performances, Nanoscale, 2010. 2(10): p. 2195-2198). [009] Studies on manganese oxides have been carried out in order to attempt to exploit the favourable electrical double layer capacitance properties of carbonaceous material and pseudocapacitance of metal oxides. Recently, Cui et al. reported homogeneously dispersed Mn₃0₄ with carbon nanotube arrays (CNTA) produced using a dip casting method to record capacitance values as high as 292 Fg^"1 (Cui, X., et al, Dense and long carbon nanotube arrays decorated with Mns0₄ nanoparticles for electrodes of electrochemical supercapacitors, Carbon, 201 1. 49(4): p. 1225-1234). [010] Wang et al. reported a capacitance of 256 Fg^"1 for Mn₃0₄ nanoparticles embedded into graphene sheets based on the preparation of a Mn0₂ organosol using 6M NaOH as an electrolyte (Wang, B., et al,

nanoparticles embedded into graphene nanosheets: Preparation, characterization, and electrochemical properties for supercapacitors, Electrochimica Acta, 2010. 55(22): p. 6812-6817). Although these manganese-graphene composites have been promising, the capacitance levels have not been exceptional.

[011] In a related field, lithium-ion batteries are currently the dominant power sources for portable electronic devices and are also considered as promising power sources in electric vehicles (EV) and hybrid electric vehicles (HEV). However, current lithium-ion batteries are approaching limits set by the electrode materials. To improve their energy density, cycling life, and especially, their high-rate capability is a major challenge in next- generation lithium-ion batteries. Investigations are ongoing in seeking to produce new and improved electrode materials for batteries, such as lithium-ion batteries. [012] One particular aspect of these varied research efforts that has not as yet been addressed, or suitably addressed, is obtaining a satisfactory method or process for the production of homogeneous nanocomposite materials, such as materials that could be commercially used as electrode materials in supercapacitors or batteries. There remains a need to provide a suitable method or process for commercially producing or fabricating satisfactory homogeneous nanocomposite materials.

[013] For example, there are significant challenges remaining in producing homogeneous nanocomposite materials on a large or industrial scale. So far, in relation to graphene, such composites have been prepared mainly by co-precipitation based methods in which the oxide nanoparticles are precipitated on the surface of graphene product sheets. This is time consuming, needs filtering and washing of the obtained products, lacks good homogeneity and is suitable only for low concentration ratios of oxide/graphene products.

[014] The reference in this specification to any prior publication (or information derived from the prior publication), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that the prior publication (or information derived from the prior publication) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Brief Summary

[015] According to a first broad form there is provided a homogeneous nanocomposite material and/or a method of production thereof. The Applicant has developed a spray pyrolysis based method where chemical reactions in a reaction chamber occur in situ to produce homogeneous nanocomposite materials. Reference to homogeneous should be read as including substantially homogeneous. The method or process is scalable for cost effective high volume industrial production and good homogeneity of produced materials is obtained. In one form, the method makes use of a spray pyrolysis system to produce, in situ, a graphene oxide based nanocomposite material.

[016] In one aspect, there is provided a spray pyrolysis method for producing a graphene oxide based composite, including the steps of: spraying graphene oxide and at least one precursor, in the form of at least one suspension or solution, into a reaction chamber; heating the reaction chamber to produce an in situ reaction between the graphene oxide and the at least one precursor; and, retrieving a graphene oxide based composite from the reaction chamber. [017] According to various non-limiting examples: the graphene oxide based composite is a nanocomposite; the graphene oxide based composite is a solid powder; the graphene oxide and the at least one precursor are in the same suspension or solution prior to spraying; and/or the graphene oxide and the at least one precursor are in different suspensions or solutions prior to spraying. Preferably, the spraying and heating is in the presence of one or more carrier and/or reaction gases, for example air or inert gas. The carrier gas in one embodiment is not oxygen, although this is not necessarily required. Preferably, heating is performed at about 600°C.

[018] In various other example forms: the at least one precursor includes manganese hydroxide; the at least one precursor includes manganese (II) nitrate hydrate; the at least one precursor includes cobalt oxide; the graphene oxide based composite is a GO- manganese-based nanocomposite; and/or the graphene oxide based composite is a GO- (Mn₂0₃ and/or Mn₃0₄) nanocomposite.

[019] In various example aspects: the nanocomposite is about 20% to about 30% GO by weight; the nanocomposite has a specific capacitance of greater than 300 Fg^"1; the nanocomposite has a specific capacitance of greater than 500 Fg^"1; the nanocomposite includes manganese oxide nanoneedles encapsulated in graphene oxide sheets; at least one nanoneedle includes a plurality of joined nanorods; and/or the nanoneedles or nanorods are entangled. In another example the graphene oxide based composite is a GO-Co₃0₄ nanocomposite. Brief Description of Figures

[020] Example embodiments should become apparent from the following description, which, is given by way of example only, of at least one preferred but non-limiting embodiment, described in connection with the accompanying figures. [021 ] Figure 1 illustrates a schematic of an example spray pyrolysis system;

[022] Figure 2 illustrates a flow chart of an example spray pyrolysis method for producing a graphene oxide based composite material;

[023] Figure 3 shows XRD patterns of example produced samples, including a pure manganese based material and hybrid (composite) materials spray pyrolysed at 600°C;

[024] Figure 4 shows SEM micrographs of (a) and (b) pure manganese based material; (c) and (d) 20% GO-Mn₂0₃/Mn₃0₄; and (e) and (f) 30% GO-Mn₂0₃/Mn₃0₄;

[025] Figure 5 shows TEM micrographs of (a) and (b) pure manganese based material; and (c) and (d) 20% GO- Mn₂0₃ Mn₃0 ; [026] Figure 6 shows (a) comparative CV's of precursor manganese oxide material, 20% GO-Mn₂0₃ Mn₃0₄ and 30% GO-Mn₂0₃/Mn₃0₄ at SmVs^"1; (b) CV's for the best performing electrode (20% GO-Mn₂0₃/Mn₃0₄) at different scan rates; (c) variation of capacitance with increase in scan rate; (d) variation of current density with increase in scan rate (all tests performed in 1 M NaOH);

[027] Figure 7 shows comparative charge/discharge profiles of precursor manganese oxide, 20% GO-Mn₂0₃/Mn₃0₄ and 30% GO-Mn₂0₃/Mn₃0₄ at 0.35 Ag^"1 in 1 M NaOH at 25°C;

[028] Figure 8 shows Nyquist plots for the three example electrodes studied at 0.1V in 1 M NaOH at 25°C;

[029] Figure 9 shows stability tests over 2000 cycles for precursor manganese oxide and the best performing electrode (20 % GO-Mn₂0₃/Mn₃0₄) performed at 50 mVs^"1 in 1M NaOH at 25°C;

[030] Figure 10 shows SEM photographs of the obtained GO-Co₃0₄ nanocomposite;

[031] Figure 1 1 shows an EDS mapping of the obtained GO-Co₃0₄ nanocomposite;

[032] Figure 12 shows CV for GO-Co₃0₄ at 50 mVs^"1 in 1M NaOH at room temperature;

[033] Figure 13 shows CV for GO-Co₃0₄ at 5 mVs^"1 in 1M NaOH;

[034] Figure 14 shows FESEM micrographs of a pure sample (NiO) at a) low magnification, b) higher magnification, and c) still higher magnification;

[035] Figure 15 shows FESEM micrographs of an example sample containing 20% graphene oxide (20% GO-NiO) at a) and b) low magnification, and c) and d) high magnification;

[036] Figure 16 shows comparative CV of the 20% GO - NiO example electrode at 5 mVs^"1 and 100 mVs^"1 in 1M NaOH.

Preferred Embodiments

[037] The following modes, given by way of example only, are described in order to provide a more precise understanding of the subject matter of a preferred embodiment or embodiments.

Spray pyrolysis method and system

[038] The Applicant has used a spray pyrolysis method for producing composites (i.e. composite materials), preferably nanostructured composites (i.e. nanocomposites), with relatively high specific surface area. Additionally in situ synthesis of the composites was obtained. The method combines great flexibility regarding the type and use of precursors, i.e. initial solutions and/or suspensions, excellent homogeneity of the produced composites, high performance and productivity, and industrial up-scaling capabilities.

[039] The Applicant realised that an advantage of reducing graphene to graphene oxide is an increased hydrophilicity, making it possible to have a uniform dispersion of graphene oxide in water or other liquid. The Applicant moreover realised that this would enable graphene oxide to be introduced as a spray into a reaction chamber, i.e. vapourised for improved reaction kinetics. The Applicant is not aware of any reported studies involving the ability to spray graphene oxide.

[040] The novel method involves the introduction as a spray, or one or more sprays, of a suspension or solution of precursor compounds and a suspension or solution of graphene oxide (GO) into a furnace or reaction chamber, where drying, decomposition and chemical reactions are performed in situ. The obtained composite material(s), e.g. a powder, is collected in a downstream collector vessel. The spraying can be performed by a variety of means, for example conventional two fluid nozzle, using a carrier gas, high speed rotation nozzle for better productivity, or by ultrasonically assisted spray nozzles if smaller particles are required.

[041] In one particular example, the Applicant used a spray pyrolysis method to produce, in situ, homogenous nanocomposites of graphene oxide (GO) and manganese based materials, such as manganese oxides (GO-(Mn₂0₃ and/or Mn₃0₄)) with unique nanostructures. The obtained nanocomposites have application as an electrode material, for example in electrochemical capacitors. Moreover, the spray pyrolysis method can be used to produce a variety of homogenous composites of graphene oxide (GO) and metal oxides, such as Ru0₂, Mn0₂, C03O₄ and NiO. [042] The metal oxides are formed in situ in a spray pyrolysis system due to the high temperature used, and are formed as composite particles containing a homogeneous dispersion of graphene oxide, which can be considered a one-step process. [043] The methodology has at least the following commercially useful advantages:

Flexibility to use either a solution or a suspension precursor;

Customisable, it is possible to prepare either high density materials or powders with a highly developed specific surface area;

Applying ultra-short sintering time, it is possible to prepare extremely small nano- crystallites;

It is possible to obtain nanostructured ceramics or composites, which require a conventional high-temperature sintering process;

The method/system can be scaled to an industrial scale;

The nanostructured powders are relatively soft agglomerated, and are therefore more easily dispersable for end-use;

There is excellent homogeneity of components in the composites at the nanoscale.

[044] Referring to Figure I, there is shown a schematic of a spray pyrolysis system 100 having a large operating temperature range (for example about 100°C to about 1000°C). Produced composite materials are synthesized in situ. A graphene oxide based composite is produced by forming a vapour (i.e. a spray or mist) from one or more solutions or suspensions containing at least graphene oxide and another precursor compound, heating the vapour, and then collecting a resulting powder. Heating of the vapour is at a selected temperature, and is preferably within a range of between about 500°C to about 1000°C. In a particular preferred example, heating of the vapour is at a temperature of about 600°C. In other examples, heating can be performed at about 500°C, 700°C, 800°C, 900°C or 1000°C. The produced graphene oxide based composite can be mixed with a binder to form at least part of an electrode, such as for use in a lithium-ion battery or a supercapacitor.

[045] One or more precursor solutions or suspensions 1 10, which include at least graphene oxide (GO) in solution or suspension, is drawn by pump 120 and introduced, optionally with a carrier and/or reaction gas or gases 130, into at least one nozzle 140 to be vapourised to fine vapour droplets and sprayed into furnace 150, which in this example has three heating zones Tl , T2 and T3 provided by heaters or heating elements. It is possible to use one, or a plurality of different heating zones. It is also possible to use one or more nozzles 140, which may each spray different precursor solutions or suspensions, or the same mixture of precursor solutions or suspensions. The precursor solutions or suspensions 110 may be pre-mixed in a single container or introduced into the furnace 150 from different containers. The solutions or suspensions 110 are sprayed into furnace 150 having variable operating temperatures, preferably, but not necessarily, in the following examples about 600 °C, and for example using nitrogen as the carrier gas. Other carrier and/or reaction gases or mixtures of gases can be used such as helium, neon, argon, xenon, oxygen, etc.. The resultant powder is separated from the hot gas stream via collector 170. Vertically oriented furnace 150 includes a cylindrical glass tube 160 in which in situ drying, decomposition and chemical reactions occur. An extraction system includes sample collector 170 to collect the produced graphene based composite material 190, with suction system 180 drawing produced materials out of tube 160. The temperature zones Tl, T2, and T3 can be independently varied depending on desired reaction conditions. The system is modular and able to use various types, locations and numbers of spray nozzles, including ultrasonic nozzles, vertical temperature profiles and variable reaction times to achieve in situ synthesis of the composites. This illustrates an example spray pyrolysis system, but it should be noted that a variety of other spray pyrolysis systems could be utilised.

[046] Referring to Figure 2, there is illustrated a spray pyrolysis method 200 that can be used to produce graphene oxide based composites, preferably nanocomposites. The produced composites can have differing morphologies depending on reaction conditions, thus allowing optimisation of reaction conditions for producing a composite with an increased surface area. For example, composites might be substantially spherical, "broken spheroids", rod-like, sheet-like, etc., or combinations thereof. Spray pyrolysis method 200 produces very good homogeneity of the produced composites. A variety of precursor materials can be used together with a graphene oxide suspension, such as various oxides of ruthenium, manganese, cobalt or nickel, by way of example.

[047] Precursor materials, initially in solution or suspension, are sprayed into a reaction chamber. This involves, at step 210, spraying graphene oxide, in suspension or solution, into the reaction chamber. Preferably simultaneously from a different suspension or solution, or concurrently from the same suspension or solution, at step 220, spraying at least one precursor into the reaction chamber also occurs. At step 230, the reaction chamber is heated to produce an in situ chemical reaction between the graphene oxide and the at least one precursor. At step 240, a graphene oxide based composite is retrieved from the reaction chamber as a powder.

[048] The following examples provide a more detailed discussion of particular embodiments. The examples are intended to be merely illustrative and not limiting to the scope of the present invention.

Preparation of graphene oxide (GO)

[049] A variety of procedures for the synthesis of graphene oxide are known. The method of producing a graphene oxide composite can be adapted to use any available source of graphene. In one specific non-limiting example, 1 g of natural graphite flakes (Asbury Graphite Mills, US) was thermally expanded at 1050°C for 15 sec. The final expanded graphite (EG) was then used for the production of graphene oxide. 1 g of EG and 200 ml of sulphuric acid (H₂SO , 98%, Merck) were mixed and stirred in a three neck flask. Next, 5 g of KMn0₄ was added to the mixture while stirring. The mixture was then stirred at 30°C for about 24 h. Next, 200 ml of de-ionized water and 50 ml of H₂0₂ were poured slowly into the mixture changing the colour of the suspension to light brown. Having stirred for another 30 min, the graphene oxide particles were then washed and centrifuged with HC1 solution (9:1 by volume watenHCl) three times, then centrifuged again and washed with de-ionized water until the pH of the solution was about 5 to 6. The obtained graphene oxide particles were then diluted using DI water (~1.8 mg/ml) and delaminated by gentle shaking.

Example 1: graphene oxide - manganese oxide composite

[050] In one specific example, high surface area graphene oxide - manganese oxide/hydroxide needle shaped composites were synthesized in situ using the spray pyrolysis method. In this example, a suspension of hydroxide is sprayed instead of a dissolved salt, which changes the reaction pathway. That is, instead of drying and decomposing the formed particles from clear solution and further sintering of oxides, the formed hydroxide particles are decomposed to appropriate oxides.

[051] The materials were tested for supercapacitive behaviour in 1 M NaOH solution using cyclic voltammetry (CV), chronopotentiometry and electrochemical impedance spectroscopy (EIS). The unique nano-morphologies of the manganese oxide composites resulted in high specific surface area values (up to about 139m²/g), exceptionally high capacitance and remarkable stability over 2000 cycles. Specific capacitances as high as 606, 388 and 336 Fg ¹ were observed for a 20% GO-Mn₂0₃/Mn₃04, 30% GO- Mn₂0₃/Mn₃0 and manganese oxide composite, respectively, using CV. Stability tests over 2000 cycles showed good capacitance retention of about 77% for the best performing 20% GO-Mn₂0₃/Mn₃0₄ sample and about 85% for a precursor manganese oxide composite electrode. Synthesis of pure manganese based composite material

[052] In an example synthesis, 27.39 g anhydrous based manganese (II) nitrate hydrate (Μη(Ν0₃)₂·χΗ20, 98%, Sigma) powder was added into water. In the obtained solution, dissolved oxygen oxidizes manganese(II) ions to the tetravalent state, the same as the Winkler test, as presented in equation (1):

2 Mn(N0₃)₂(s) + 0₂(aq)→ 2 MnO(OH)₂(s) (1 )

[053] The red-brownish precipitates were then suspended in the water and were stirred for 30 min using a conventional magnetic stirrer. The suspension could then be spray- pyrolysed at 600°C into a vertical-type spray pyrolysis reactor.

Synthesis of GO-manganese oxides hybrid materials

[054] In an example synthesis, anhydrous based manganese (II) nitrate hydrate (Mn (Ν0₃)₂·χΗ₂0, 98%, Sigma) powder was added into a diluted GO dispersion in water with mass ratios of 1 :7 and 1 : 12 (GO/Mn(N0₃)₂-xH₂0). Then the dispersion was stirred for 30 min using a conventional magnetic stirrer. The hybrid material was then obtained in situ by spray-pyrolyzing the suspensions at about 600^° C into a vertical-type spray-pyrolysis reactor to obtain 20% GO-Mn₂0₃/Mn₃0₄ and 30% GO-Mn₂0₃/Mn₃0₄ composites (where the % indicated is the % weight of GO relative to the total weight of GO-Mn₂0₃/Mn₃0₄).

[055] It should be appreciated that a variety of reaction conditions can be used and optimised for various outcomes. For example, the reaction temperature can be changed dependant on the metal oxide composite, the rate of fluid flow into the spray pyrolysis system and/or the extraction speed. A typical range of reaction temperatures is about 500°C to about l OOO'C, with about 600°C being preferable for a fluid flow rate of between about 4 ml/min and about 20 ml/rnin. The gas flow rate of the extraction unit is between about 50 1/s and about 130 1/s and the rate of the carrier gas flow is between about 1 1/s and about 10 1/s. Additionally, when a noble carrier gas is used, a positive partial pressure for the noble carrier gas can be introduced within the reaction chamber which influences the produced graphene oxide based composite. An example positive partial pressure that can be used is between about 3 kPa and about 10 kPa. Materials characterization

[056] Referring to Figure 3, there is shown X-ray diffraction (XRD) patterns of an example sample of the pure manganese based composite material, as discussed above, and hybrid materials spray pyrolysed at 600°C. The X-ray spectrum of the "starting" material represents a composite material containing Mn(OH)₂, MnOOH, Mn0₂ and Mn₂0₃.

[057] The XRD pattern clearly shows that the spray pyrolysis of MnO(OH)₂ promotes the conversion of MnO(OH)₂ to Mn(OH)₂, MnO(OH), Mn0₂ and Mn₂0₃. A whole range of manganese oxides and hydroxides can be fabricated upon spray-pyrolysis of MnO(OH)₂. However, the addition of graphene oxide (GO) to the suspension results in the conversion of MnO(OH)₂ to Mn₂0₃ and Mn₃0₄. The conversion of Mn₂0₃ to Mn₃0₄ usually needs heat-treatment at temperature in excess of 800°C. However, the kinetics involved in spray pyrolysis allows the formation of such high-temperature phases. In addition, no trace of hydroxide compounds can be observed in both hybrid materials suggesting that the existence of graphene oxide has promoted the decomposition of manganese hydroxide compounds.

[058] Graphene oxide sheets exhibit oxygen functional groups in the form of carboxyl, hydroxyl or epoxy groups on their basal planes and edges. These functional groups, which also contain hydrogen, might act as reducing agents and consequently alleviate the decomposition of manganese hydroxide compounds and the reduction of Mn₂0₃ to Mn₃0₄. Therefore, the existence of the higher percentage of Mn₃0₄ in 30% GO-Mn₂0₃/Mn₃0₄ hybrid material, compared to 20% GO-Mn₂0₃/ n₃0₄, can be attributed to the higher percentage of GO and consequently higher percentage of hydrogen containing functional groups.

[059] Scanning electron microscopy (SEM) was employed to investigate the morphology of the obtained samples. Referring to Figure 4, there is shown SEM micrographs of three example obtained samples used for materials testing purposes. Interestingly, the pure sample (of manganese only based composites) consists of spherically shaped particles as well as nanorods. Both manganese oxide and hydroxide have the tendency to form rods or needle like structures. It is proposed that intercalation and adsorption of manganese ions on the surface of primary MnO(OH)₂ particles, followed by the nucleation and growth of the crystals via dissolution-crystallization and oriented attachment mechanisms are responsible for the formation of nanorods. This hypothesis is further supported by TEM results.

[060] However, in the case of the sample containing 20% GO, the attachment of both primary particles and manganese ions onto the surface of GO via oxygen multi- functionalities might be responsible. The higher number of defects and functional groups on the surface of GO sheets might provide additional nucleation sites and therefore facilitate the nucleation and growth of nanorods. The increase in the number of nucleation sites also results in shorter needle like structures. SEM micrographs clearly suggest that GO sheets encapsulate nanorods and inhibit further growth of nanorods. However, the addition of 30% GO which provides a huge increase in the surface area and promotes the reduction of manganese oxide and hydroxide compounds hinders the further growth of manganese oxide particles upon keeping manganese oxide particles spaced apart from each other.

[061] To investigate the formation process, HRTEM was conducted on the samples (refer to Figure 5). Figure 5(a) clearly shows an aggregation of different particles suggesting that the sample contains irregular shaped particles. Figure 5(c) shows the encapsulation of manganese oxide nanoneedles within graphene oxide sheets. Figure 5(d) shows the tip of one of the nanoneedles where it can be clearly seen that the nanoneedle is actually composed of a few primary nanorods aggregated along the lateral faces. The nanorods of the centre portions are longer than others, giving an indication that the oriented attachment mechanism played an important role in the formation of the nanoneedles. Electrochemical characterization

[062] Electrochemical experiments were performed at room temperature on a CHI660C (CH Instruments, Inc) electrochemical workstation using a three electrode system in a beaker type cell. An electrolyte solution of 1M NaOH, a silver|silver chloride reference electrode and a platinum foil as a counter electrode were used. The working electrode was made from a stainless steel sheet with a surface area of 1cm². Cyclic voltammetry was performed over a voltage range of -0.2 V to 0.5 V at various scan rates (5 mVs^"1 to 100 mVs^*1). Electrochemical Impedance Spectroscopy (EIS) measurements were carried out between 10 kHz and 0.01 Hz using a 5 mV rms sinusoidal modulation. Chronopotentiometry tests were performed at varying current densities over a potential window range from -0.2 V to 0.5 V. Specific capacitances were calculated at 5 mVs^"1 for CV and 0.35 Ag^"1 for charge-discharge (CD). Working electrode preparation

[063] An example working electrode was prepared by coating the produced materials onto stainless steel sheets (1 cm x 1 cm) previously polished with sand paper and ultrasonicated in ethanol for an hour. 7 mg of the produced electro-active material (GO- Mn₂0₃/Mn₃04) was mixed with 2 mg of carbon black and 1 mg of PVDF binder in an argate mortar in (N-methyl pyrrolidinone) NMP solvent and ground using a pestle. The resulting slurry was then spread on to the polished stainless steel surface to achieve mass loadings between 0.85 and 1 mg and allowed to dry in a vacuum oven for 24 hours.

Cyclic voltammetry

[064] Cyclic voltammetry (CV) was used as a diagnostic tool for the electrochemical characterisation of the Mn₃0₄ and GO-Mn₂0₃/Mn₃04 composites. Figure 6(a) shows CV's for the three example electrodes in 1 M NaOH solution. At a scan rate as low as 5 mVs^"1, the cyclic voltammograms significantly deviate from the ideal rectangular shape expected for EDLCs. This is due to the pseudocapacitance contribution by the Mn₂0₃/Mn₃0₄ and oxide groups on the graphene oxide. A large current separation was also observed for the GO-Mn₂0₃/Mn₃0₄ electrode suggesting higher capacitance compared to GO- Μη₂θ₃/Μη₃θ4 and manganese oxide composite electrodes. The specific capacitance was calculated from the CV data by integrating the area under the current-potential curve over the potential window.

[065] The highest capacitance at 607 Fg^"1 was recorded for the 20% GO-Mn₂03 Mn30₄ sample, followed by 388 Fg^"1 for the 30% GO-M^Cb/M^C sample and 336 Fg^"1 for the precursor Μη₃θ₄ sample. This trend could be attributed to the distinctive morphologies of the three electrodes as shown in Figure 4. The capacitance of the produced materials appears linked to the morphology of the materials. [066] Although the lowest capacitance recorded for the three example electrodes is 336 Fg^"1, for the initial manganese composite, it is slightly higher than that previously reported for hydrothermally synthesised Mn₃0₄ nano-octahedrons and also for Mn0₂ nanorods. This is due to the porous nature of the electrode material and unique morphology unexpectedly provided by the spray pyrolysis method. Mn₃0₄ nanorods were observed to have grown on the spherical metal oxide grains during the actual spraying process apparently enhancing the surface area and consequently the capacitance.

[067] A combination of graphene oxide and manganese oxides with 20% graphene oxide resulted in a very interesting structure with thicker nanorods and as a result, an unstacked network of graphene oxide sheets. This may have allowed spaces between the graphene sheets to be used for energy storage as the ions responsible for this have better access to storage sites on the basal and edge planes of the graphene sheets. Additionally, the roughness of the metal oxide due to the nanowire formation observed for the precursor metal oxide could have enhanced the overall capacitance.

[068] However, a further increase in the amount of graphene oxide to 30% showed a detrimental effect on the capacitance. The graphene oxide is believed to have inhibited the growth of the nanorods (as can be seen from the SEM images in Figure 4) resulting in lower capacitance. Some of the graphene oxide nanosheets seem to be stacked on top of each other due to Van der Waals forces, reducing the surface area available for charge storage. [069] The presence of some Μη₂0₃/Μη₃θ₄ nanoparticles on the surface of the graphene oxide may have contributed to higher capacitance when compared to the precursor material. Figure 6(b) shows scan rate studies for the 20% GO-M ₂0₃/Mn304 electrode from 5 mVs^"1 to 100 mVs !. The charge separation increases with increase in scan rate with no significant change in the shape of the CVs. The specific capacitances calculated for the different scan rates are shown in Figure 6(c). Figure 6(d) shows the variation of current density with scan rate, showing a linear relationship, in agreement with the linear relationship obeyed by an ideal capacitor, thus reflecting good power capability and reversibility.

Charge- Discharge

[070] To further clarify the results obtained by CV, charge-discharge tests were performed at different current densities. Figure 7 shows the charge-discharge profile for the three example electrodes at 0.35 Ag^"1. The three profiles presented in Figure 7 show linear charging and discharging slopes implying good reversibility and capacitive properties of the three example electrodes. The absence of any IR drop from the profiles suggest highly conducting materials which are very good for supercapacitor electrode applications with low ESR. The specific capacitance for the precursor material and the produced composites were again calculated. The capacitance values obtained from the charge-discharge method are in agreement with the CV results, with the 20% GO- Mn₂0₃/Mn₃0₄ achieving a specific capacitance of 509 Fg^"1 followed by 30% GO- Mn₂0₃/Mn₃0₄ with a specific capacitance of 355 Fg^"1, and 254 Fg^"1 for the precursor manganese oxide. [071] Although these capacitance results are slightly lower than those obtained from CV, the same trend is observed and the unusually higher capacitance maintained, as is seen in Table ! . Specific capacitance (Fg^" )

Technique Precursor 20 % 30 %

manganese oxide GO-Mn₂0₃/Mn₃04 GO-Mn₂0₃/Mn₃0₄

CV 388 607 336

CD 355 509 254

Table 1. Comparative specific capacitance values from CV and CD results.

Electrochemical impedance Spectroscopy

[072] Electrochemical impedance spectroscopy was used to investigate the mechanistic effects on the active electrode area such as ion transfer, conduction and capacitive behaviour. Figure 8 shows comparative Nyquist plots for the three example electrodes. The impedance plot in Figure 8 is made up of two regions: the high frequency and low frequency regions with each point on the Nyquist plot representing a particular frequency were a measurement was taken. From the inset in Figure 8, the small semicircle observed in the high frequency region is due to charge transfer resistance (RCT) on the electrode|electrolyte interface. The intercept between the plots and the Z' axis represent the ohmic resistance (Rs). From the plot, the precursor manganese oxide composite material is observed to have a small RCT value, possibly due to the large number of nanorods that enhance the surface area and conductivity across the electrode surface. An addition of graphene oxide clearly enhances the conductivity of the composite, as can be observed from the reduction in the RCT with an increase in graphene oxide content. The impedance of an electrode varies from purely resistive behaviour at high frequencies to a purely capacitive behaviour in the low frequency region. In the low frequency region, an almost vertical line is observed on the Nyquist plots implying purely capacitive behaviour due to Faradaic reactions taking place on the electrode surface.

Stability studies

[073] Cyclic voltammetry was used to test the stability between the precursor material and the best performing example electrode (20% GO-Mn₂0₃/Mn₃0₄). Figure 9 shows the comparative stability plots of the two example electrodes performed at 50 mVs^"1. The precursor material exhibited remarkable capacitance retention of 85% while the produced composite material retained 77% over 2000 cycles. The slight difference between the capacitance retention could be due to an extra pseudocapacitance contribution from the graphene oxide sheets. Pseudocapacitance from the graphene oxide is due to the presence of functional groups such as carbonyl, epoxy, carbonyl and hydroxyl groups, it is well known that Faradaic reactions on the electrode surface increase capacitance but have an opposite effect on cycle life. However, the overall capacitance retention in the produced composite material is high enough for the material to be used for supercapacitor electrodes. Conclusions

[074] GO-Mn₂0₃/Mn₃0₄ composites were successfully synthesized using a simple and low-cost spray pyrolysis method from a precursor manganese hydroxide suspension. The results show that the unexpected and unique morphology afforded by the spray pyrolysis method enhances capacitance, even for the Mn₃0₄ electrodes, by forming nanorods which become entangled, creating a network on the electrode. When combined with GO, a further unexpected increase in capacitance was observed and a dependence on the amount of graphene oxide on capacitance was deduced. An exceptionally high capacitance value of about 606 Fg^"1 at 5 mVs^"1 was observed for an example 20% GO-Mn₂0₃/Mn₃0₄ material. [075] The produced materials are ideal for application in supercapacitor devices due to their high stability even after 2000 cycles. A 23% reduction in capacitance for the best performing electrode was observed while that of the precursor material was lower at 12%.

Example 2: graphene oxide - cobalt oxide composite

[076] In another non-limiting example, the spray pyrolysis method was used to produce a series of GO-Co₃0₄ nanocomposites. In a typical synthesis, 9.784 g cobalt hydroxide (Co(OH)₂, 95%, Sigma) powder was solved in 100 ml of one molar nitric acid solution. To obtain the composites, cobalt hydroxide (Co(OH)₂, 95%, Aldrich) powder dissolved in one molar nitric acid solution was added into a diluted GO dispersion in water with mass ratios of 1 :4.57 and 3:8 (GO/Co(OH)₂). Then the solution was stirred for 30 min using a conventional magnetic stirrer. The hybrid material was then obtained in situ by spray- pyrolyzing the suspensions into a vertical type spray pyrolysis reactor to obtain 20% GO- Co₃0₄ and 30% GO- Co₃0₄ composites. A typical range of reaction temperatures is about 500^°C to about 1000°C, with about 600°C being preferable for a fluid flow rate of between about 4 ml/min and about 20 ml/min. The gas flow rate of the extraction unit is between about 50 1/s and about 130 1/s, and the rate of the carrier gas flow is between about 1 1/s and about 10 1/s. Additionally, when a noble carrier gas is used, a positive partial pressure for the noble carrier gas can be introduced within the reaction chamber which influences the produced graphene oxide based composite. An example positive partial pressure that can be used is between about 3 kPa and about 10 kPa.

[077] An excellent level of homogeneity is obtained in the GO-C03O₄ nanocomposites. Electrochemical tests of the produced material as an electrochemical capacitor show good performance (about 233 Fg^"1) in low concentration sodium hydroxide, compared to conventional EDL capacitors (having less than 200 Fg^"1).

[078] A series of GO-C03O4 nanocomposites were produced with mass ratios of Co₃0₄ / GO of 90/10, 80/20 and 70/30 %. The materials were characterized by XRD, SEM, EDX, BET, and electrochemical tests to evaluate their capacitance in electrochemical supercapacitors. The results from XRD, SEM, EDX and BET tests demonstrate a very good homogeneity of the obtained GO-Co₃0₄ nanocomposites, having a specific surface area of about 60 m /g.

[079] The average crystallite size of C03O₄ single phase was calculated from XRD results to be about 2 nm. The GO sheets are incorporated into the composite nanostructured aggregates as evident from the SEM photographs presented in Figures 10(a) to (c). The high magnification reveals (see Figure 10(d) and (e)) the edges of incorporated GO sheets inside the composite nanoaggregated particles. Figures 10(f) and (g) show a stand alone GO sheet coated with ceramic nano Co₃0₄ particles at various magnifications. The EDS mapping shown in Figure 1 1 shows the good homogeneity of the obtained nanocomposite particles, demonstrated by the substantially even distribution of Co and carbon across the sample. A small stand alone GO sheet is visible in the top right corner where a weak carbon spot indicates the presence of only carbon material. The electrochemical tests of this material as an electrochemical capacitor show good performance (about 233 F/g) in low concentration 1 M NaOH, compared to conventional EDL capacitors (less than 200 F/g). Figure 12 shows a CV plot for the GO-Co₃0₄ composite at 50 mV/s in 1M NaOH at room temperature.

Example 3: grapheme oxide - nickel oxide composites

[080] In another non-limiting example, the spray pyrolysis method was used to additionally produce GO-NiO nanocomposites. In a typical synthesis, 29.079 g crystalline Nickel(II) nitrate hexahydrate (Ni(N0₃)₂x 6H₂0, Sigma) powder was solved in 100 ml of water. The solution was then stirred for 30 min using a conventional magnetic stirrer. Nickel(II) nitrate hexahydrate (Ni(N0₃)₂x 6H₂0, Sigma) powder dissolved in water was added into a diluted GO dispersion in water with mass ratios of 1 : 15.57 and 1 :9 (GO/Ni(N0₃)₂x 6H₂0). Then the solution was stirred for 30 min using a conventional magnetic stirrer. The hybrid material was then obtained in situ by spray pyrolyzing the suspensions into a vertical type spray pyrolysis reactor to obtain 20% GO-NiO and 30% GO-NiO composites. A typical range of reaction temperatures is about 500°C to about 1000°C, with about 600°C being preferable for a fluid flow rate of between about 4 ml/min and about 20 ml/min. The gas flow rate of the extraction unit is between about 50 1/s and about 130 1/s, and the rate of the carrier gas flow is between about 1 1/s and about 10 1/s. Additionally, when a noble carrier gas is used, a positive partial pressure for the noble carrier gas can be introduced within the reaction chamber which influences the produced graphene oxide based composite. An example positive partial pressure that can be used is between about 3 kPa and about 10 kPa.

[081] Example working electrodes were then prepared by coating the produced active material on to stainless steel sheets (1cm x 1cm) previously polished with sand paper and ultrasonicated in ethanol for an hour. 7mg of the produced active material was mixed with 2mg of carbon black and 1 mg of PVDF binder in an argate mortar in (N-methyl pyrrolidinone) NMP solvent and ground using a pestle. The resulting slurry was then spread on to the polished stainless steel surface to achieve mass loadings between 0.85 and 1 mg and allowed to dry in a vacuum oven for 24 hours.

[082] The electrochemical behaviour of the GO-NiO and GO-C03O₄ electrodes were evaluated using CV over a potential window from -0.2 V to 0.5 V in 1 M NaOH. Referring to Figure 13, the GO-C03O₄ electrode showed redox activity with the reversible peaks responsible for pseudo capacitance. The chemical reactions in this range are given as:

Co₃0₄ + H₂0 + OH^"1 <→ 3CoOOH + e (1)

CoOOH + OH^"1 *→ Co0₂ + H₂0 + e (2)

[083] The large current separation indicates capacitive behaviour which was calculated to yield values as high as 687.4 Fg^"1 at 5 raVs^"1 in alkaline electrolyte. This value is significantly larger than that of the precursor C03O₄ which recorded a capacitance of 419.7 Fg^"1. The unusually high capacitance of the precursor material was largely due to the unique morphology provided by the spray pyrolysis method compared to what has previously been obtained. Furthermore, the nanostructured Co₃0₄ results in a larger electroactive surface area resulting in enhanced capacitance. [084] NiO adopts the NaCl structure, with octahedral Ni (II) and 0^2~ sites. The conceptually simple structure is commonly known as the rock salt structure. Like many other binary metal oxides, NiO is often non-stoichiometric, meaning that the Ni:0 ratio deviates from 1 :1. In nickel oxide this non-stoichiometry is accompanied by a colour change, with the stoichiometrically correct NiO being green and the non-stoichiometric NiO being black. The powders obtained from the present spray pyrolysis method are black powders, which demonstrate that the so-prepared NiO powders are nbn-stoichiometric powders.

[085] Scanning electron microscopy (SEM) was employed to investigate the morphology of the obtained samples. Figure 14 shows SEM micrographs of a pure NiO sample. Interestingly, the pure NiO sample consists of hierarchical spherical shaped particles. Figure 15 shows SEM micrographs of a NiO sample containing 20% graphene oxide. The same hierarchical structure is kept intact. However, wing like graphene oxide sheets can be observed within the structure.

[086] The capacitance of the example GO-NiO electrodes was calculated from CV graphs to obtain a capacitance of about 650 Fg^"1 in 1M NaOH at SmVs^"1. Figure 16 shows comparative CV's of the GO-NiO electrodes at different scan rates. The unique structure with wing like features due to the presence of graphene oxide results in an enhanced surface area and consequently high capacitance compared to other composites earlier reported using carbon nanotubes. [087] The CV graphs of 20% GO-NiO shown in Figure 16 indicates anodic/cathodic redox peaks at 0.45 and 0.22 V vs the Ag|AgCl electrode, which are responsible for the Ni(OH)₂/NiOOH redox reaction. The bulk of the capacitance is believed to be mainly derived from the pseudo capacitance of the NiO as graphene oxide has a capacity of about 10.9 Fg^"1.

[088] Optional embodiments of the present invention may also be said to broadly consist in the parts, elements and features referred to or indicated herein, individually or collectively, in any or all combinations of two or more of the parts, elements or features, and wherein specific integers are mentioned herein which have known equivalents in the art to which the invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

[089] Although a preferred embodiment has been described in detail, it should be ^* understood that various changes, substitutions, and alterations can be made by one of ordinary skill in the art without departing from the scope of the present invention.

Claims

The claims:

1. A spray pyrolysis method for producing a graphene oxide based composite, comprising the steps of:

spraying graphene oxide and at least one precursor, in the form of at least one suspension or solution, into a reaction chamber;

heating the reaction chamber to produce an in situ reaction between the graphene oxide and the at least one precursor; and,

retrieving a graphene oxide based composite from the reaction chamber.

2. The method of claim 1, wherein the graphene oxide based composite is a nanocomposite.

3. The method of claim 1, wherein the graphene oxide based composite is a solid powder.

4. The method of any one of claims 1 to 3, wherein the graphene oxide and the at least one precursor are in the same suspension or solution prior to spraying.

5. The method of any one of claims 1 to 3, wherein the graphene oxide and the at least one precursor are in different suspensions or solutions prior to spraying.

6. The method of any one of claims 1 to 5, wherein the spraying and the heating is in the presence of a carrier gas.

7. The method of any one of claims i to 6, wherein the heating is performed at about 600T.

8. The method of any one of claims 1 to 7, wherein the at least one precursor includes manganese hydroxide.

9. The method of any one of claims 1 to 8, wherein the at least one precursor includes manganese (II) nitrate hydrate. .

10. The method of any one of claims 1 to 9, wherein the at least one precursor includes cobalt oxide.

1 1. The method of claim 2, wherein the graphene oxide based composite is a GO- manganese-based nanocomposite or a GO-(Mn₂0₃ and/or Mn₃0₄) nanocomposite.

12. The method of claim 11, wherein the nanocomposite is about 20% to about 30% GO by weight.

13. The method of either claim 11 or 12, wherein the nanocomposite has a specific capacitance of greater than 300 Fg^"1.

14. The method of either claim 11 or 12, wherein the nanocomposite has a specific capacitance of greater than 500 Fg^"1.

15. The method of any one of claims 11 to 14_f wherein the nanocomposite includes manganese oxide nanoneedles encapsulated in graphene oxide sheets.

16. The method of claim 15, wherein at least one nanoneedle includes a plurality of joined nanorods.

17. The method of claim 15, wherein the nanoneedles are entangled.

18. The method of claim 13, wherein the specific capacitance is adjusted by adjusting an amount of the graphene oxide.

19. The method of claim 2, wherein the graphene oxide based composite is a GO- Co₃0₄ nanocomposite.

20. The method of claim 1, wherein the graphene oxide based composite is a GO- metal oxide composite, and wherein the metal oxide is selected from the group consisting of Ru0₂, Mn0₂, Co₃0₄ and NiO.

21. The method of any one of claims 1 to 20, wherein the graphene oxide based composite is produced by the in situ reaction as a one step reaction process in the reaction chamber.

22. A spray pyrolysis system, comprising:

at least one nozzle for spraying graphene oxide and at least one precursor into a reaction chamber;

at least one heater to heat at least part of the reaction chamber; and

an extraction system;

wherein, in use, the graphene oxide and the at least one precursor are sprayed into■ the reaction chamber, and the reaction chamber is at least partially heated using the at least one heater to produce an in situ reaction between the graphene oxide and the at least one precursor, and a graphene oxide based composite is retrieved from the reaction chamber using the extraction system.

23. A graphene oxide - manganese-based composite including between about 25% to about 30% graphene oxide by weight, and having a specific capacitance of greater than 300 Fg^"1.

24. The composite of claim 23, wherein the graphene oxide - manganese-based composite is a GO-(Mn₂0₃ and/or Mn₃0₄) nanocomposite.

25. The composite of either claim 23 or 24, wherein the composite includes manganese oxide nanoneedles encapsulated in graphene oxide sheets.