WO2013132259A1

WO2013132259A1 - Graphene and graphene oxide aerogels/xerogels for co2 capture

Info

Publication number: WO2013132259A1
Application number: PCT/GB2013/050570
Authority: WO
Inventors: Ainara Garcia GALLASTEGUI; Milo Shaffer; Abdulrahman O ALYOUBI; Sulaiman BASAHEL
Original assignee: Bio Nano Consulting; King Abdulaziz University
Priority date: 2012-03-09
Filing date: 2013-03-07
Publication date: 2013-09-12
Also published as: GB201204169D0; GB201417256D0; GB2515425A; GB2515425B

Abstract

The present invention relates to graphene materials, particularly to aerogels and xerogels which comprise graphene or graphene oxide, and also contain layered double hydroxides (LDHs). The invention is also concerned with the method of preparing such graphene or graphene oxide aerogels and xerogels and use of such materials for sorption and gas storage.

Description

GRAPHENE AND GRAPHENE OXIDE AEROGELS/XEROGELS FOR CO, CAPTURE Field of the Invention

Background of the Invention

Over the years, there has been a dramatic increase in the concentration of C0₂ in the atmosphere, which in turn has made a significant contribution to global warming. The main source of C0₂ associated with human activities is fossil fuel combustion used for power, transport and heat generation, which in all produces almost 70% of the world emissions of C0₂ (Greenhouse Gas Carbon Dioxide Mitigation: Science and Technology, Halmann, M. M.. Steinberg, M., Boca Raton, Lewis Publishers: Florida, 1999). Unfortunately, an immediate C0₂-emission halt is not possible since energy demands are expected to increase significantly during the next decades and the so-called green energies are not sufficiently developed to replace fossil fuels on a large scale (Yu, K. M. K., Curcic, I., Gabriel, J., Tsang, S. C. E., Chem. Sus. Chem., 2008, 1, 893.). As a consequence major technological efforts continue to be devoted to finding suitable processes for carbon dioxide capture and storage (CCS). Gas-solid adsorption is one of the most promising strategies for both post- combustion and pre-combustion capture applications. Unlike liquid sorbents, solid sorbents can be used over a wide temperature range.

An ideal C0₂ adsorbent must have high selectivity and adsorption capacity for C0₂, fast adsorption-desorption kinetics, adequate multicycle stability and good performance in the presence of competing species, such as water. In response to these demanding requirements, a range of potential C0₂ adsorbents have previously been proposed. Zeolites, activated carbons, organic-inorganic hybrids (e.g. amines covalently bound to silica) and metal-organic frameworks have been reported to be competitive at temperatures below -393 K. On the other hand, chemisorbents, such as layered double hydroxides (-473 K-723 K), lithium zirconates (-673 K-873 K) and calcium oxides (-723 K-973 K) are promising C0₂ adsorbents for high temperature CCS and other applications involving C0₂ equilibria (Yong, Z., Mata, V., Rodrigues, A. E., Sep. Purif. Technol., 2002, 26, 195; Choi, S., Drese, J. H., Jones, C. W., Chem. Sus. Chem., 2009, 2, 796; Wang, Q., Luo, J., Zhong, Z., Borgna, A., Energy Environ. Sci., 2011, 4, 42).

Layered double hydroxides (LDHs), also known as hydrotalcite-like compounds, belong to a large class of synthetic two-dimensional (2D) nanostructured basic, anionic clays. Their structure is composed of positively charged brucite-like Mg(OH)₂ layers in which a fraction of divalent cations, octahedrally coordinated by hydroxyls, are partially substituted by trivalent cations. The excess of positive charge is balanced by intercalated anions. Loosely bound water molecules may occupy the remaining free space in the interlayer regions. The charge-neutral LDH structure can be represented by the general formula [M²⁺i_

where M²⁺, M³⁺ and A^m" commonly represent Mg²⁺, Al³⁺ and C0₃ ^2" respectively, and x is usually between 0.17 and 0.33. LDHs require less energy to be regenerated and show better multicycle stability than other potential C0₂ solid adsorbents (e.g. calcium oxides) (Choi, S., Drese, J. H., Jones, C. W., Chem. Sus. Chem., 2009, 2, 796).

In addition, they exhibit fast adsorption-desorption kinetics and benefit from the presence of water, making them very attractive not only for pre-combustion capture applications but also for sorption-enhanced hydrogen production (Hufton, J. R., Mayorga, S., Sircar, S., AlChE Journal., 1999, 45, 248; Ding, Y., Alpay, E., Chem. Eng. Sci., 2000, 55, 3461; Ding, Y., Alpay E., Chem. Eng. Sci., 2000, 55, 3929; van Selow, E. R., Cobden, P. D., Verbraeken, P. A., Hufton, J. R., van den Brink, R. W., Ind. Eng. Chem. Res., 2009, 48, 4184).

Despite these positive adsorption properties, LDHs exhibit relatively low C0₂ adsorption capacities which limit their commercial use. In order to alleviate this weakness and to improve overall adsorption performance, several studies have focused primarily on altering the chemistry of LDHs by exchanging their structural cations and anions and/or by incorporating alkali dopants such as potassium and caesium (U.S. Patent 0144227A1; Hutson, N. D., Attwood, B. C, Adsorption, 2008, 14, 781; Reijers, H. T. J., Valster- Schiermeier, S. E. A., Cobden, P. D., van den Brick . W., Ind. Eng. Chem. Res., 2006, 45, 2522; Oliveira, E. L. G., Grande, C. A., Rodrigues, A. E., Sep. Purif. Techno!., 2008, 62, 137).

In addition, it has been reported recently that the C0₂ adsorption performance of LDHs is considerably enhanced by supporting/combining them with high surface area materials. Meis, N. N. A. H., Bitter, J. H., de Jong K. P., Ind. Eng. Chem. Res., 2010, 49, 1229 found that the adsorption capacity of activated Mg-AI LDHs was increased by an order of magnitude when they were supported on carbon nanofibers (CNFs). These authors proposed that the active sites for C0₂ adsorption were associated with low-coordination 0^2~Mg²⁺ sites positioned at the edges and corners on the Mg(AI)O_x nanoparticles, and that there was a higher concentration of these sites on the supported adsorbents. Aschenbrenner, O., McGuire, P., Alsamaq, S., Wang, J., Supasitmongkol, S., Al-Duri, B., Styring, P., Wood, J., Chem. Eng. Res. Des., 2011, 89, 1711 improved the thermal stability and mechanical strength of Ni-Mg-AI and Ni-Mg-AI-Fe LDHs by mixing with a boehmite support.

Graphene, as an ideal atomic-thick 2D material provides an extremely large surface area (theoretical specific surface area is up to 2600 m² g ¹) (Chen Y., Zhang X., Yu P., Y. W. Ma Chem. Commun., 2009, 4527).

There are some examples in the literature describing the synthesis of LDH onto graphene, graphene oxide and exfoliated graphene (graphene nanosheets) for different applications.

Graphene oxide and Co-AI layered double hydroxide nanosheet (Co-AI LDH-NS) has been synthesised as electrode material for application as a pseudocapacitor where exfoliated host layers of LDHs (i.e., nanosheets) have been used as 2D building blocks (Wang L., Wang D., Dong X. Y., Zhang Z. J., Pei X. F., Chen X. J. Chem. Commun., 2011, 47, 3556-3558).

In another example, the exfoliated graphite oxide (GO) is reduced to graphene using glucose as the reductant, and then Ni/AI LDH platelets are formed in situ on the surfaces of the graphene nanosheets. The as-obtained graphene nanosheets/LDH composite exhibited a high specific capacitance (781.5 F/g at 5 mV.s^"1) (Gao Z., Wang J., Li Z., Yang W., Wang B., Hou M., He Y., Liu Q., Mann T., Yang P., Zhang M., Liu L. Chem. Mater.2011, 23, 3509- 3516).

In a further example, positively charged hydroxide nanosheets were produced, while negatively charged graphene oxide nanosheets were directly obtained from the exfoliation of GO in water. Subsequent thermal treatments resulted in the formation of the NiCo₂0₄- reduced graphene oxide composites. As a consequence, the NiCo₂0₄-reduced graphene oxide composite shows a very high specific capacitance of 1050 F.g^"1, thus showing great potential as an electrode material for high performance supercapacitors(Wang H. W., Hu Z. A., Chang Y. Q., Chen Y. L, Wu H. Y., Zhang Z. Y., Yang Y. Y. J. Mater. Chem., 2011, 21, 10504).

In another example, magnetite-graphene and Mg/AI LDHs were synthesised to remove arsenate from aqueous solutions (Wu X. L, Wang L, Chen C. L, Xu A. W., Wang X. K. J. Mater. Chem., 2011, 21, 17353-17359). Inorganic nanostructures (ZnO) grown directly on graphene layers can also been found in the literature (Won II Park W., Lee C. H., Lee J. M., Kimb N. J., Yi G. C, Nanoscale, 2011, 3, 3522).

The addition of graphene into LDHs has been used as a means of improving charge transport in LDHs for electrocatalytic applications (Wang Y., Peng W., Liu L., Tang M., Gao F., Li M. Microchim Acta, 2011, 174, 41-46; Li M., Zhu J. E., Zhang L, Chen X., Zhang H., Zhang F., Xu S., Evansa D. G. Nanoscale, 2011, 3, 4240).

In another example, the exfoliated graphite oxide (GO) is simultaneously reduced to graphene in company with the homogeneous precipitation of Ni²⁺-Fe³⁺ LDH (Li H., Zhu G., Liu Z. H., Yang Z., Wang Z. Carbon, 2010, 48, 4391-4396). Finally, multilayer hybrid films has been fabricated comprising polyvinyl alcohol)/graphene and LDH hybrid (Chen D., Wang X., Liu T., Wang X., Li J. Applied Materials & Interfaces, 2010, 2, 2005-2010).

Despite concerted efforts, there still exists a need for an improved system for gas sorption and storage, in particular carbon dioxide capture and storage. As yet, there are no examples in the literature comprising the synthesis of LDH/graphene oxide hybrids for C0₂ uptake applications. Therefore present invention seeks to provide such a system by supporting LDHs onto graphene containing materials.

A number of attempts have been made to prepare graphene based aerogels. However, the reported methods for the preparation of 3D graphene structures are still limited, and most of them are compositions of graphene and polymers: graphene/poly vinyl alcohol PVA composites (H. Bai, C. Li, X. L Wang, G. Q. Shi, Chem. Commun., 2010, 46, 2376-2378), chemical modification of exfoliated GO sheets with organic diisocyanates (S. Z. Zu and B. H. Han, J. Phys. Chem. C, 2009, 113, 13651-13657), graphene oxide/poly (allylamine) by chemical crosslinking using carbodiimide coupling (A. Satti, P. Larpent, Y. Gun'ko, Carbon, 2010, 48, 3376-3381) or graphene/carbon nanotubes composites (D. S. Yu, L. M. Dai, J. Phys. Chem. Lett., 2010, 1, 467-470; L. He, J. Q. Lu, H. Q. Jiang, Small, 2009, 5, 2802-2806; X. Cao, Q. He, W. Shi, B. Li, Z. Zeng, Y. Shi, Q. Yan, H. Zhang, Small, 2011, 7, 1199).

Graphene aerogel with high electrical conductivity (lxlO² S m^"1) has been synthesised by sol-gel polymerization of resorcinol ( ) and formaldehyde (F) with sodium carbonate as a catalyst (C) in an aqueous suspension of graphene oxide (GO) (M. A. Worsley, P. J. Pauzauskie, T. Y. Olson, J. Biener, J. H. Satcher, T. F. Baumann, J. Am. Chem. Soc, 2010, 132, 14067-14069).

Ion linkages have also been applied for the preparation of 3D architectures of graphene (Z. H. Tang, S. L. Shen, J. Zhuang and X. Wang, Angew. Chem., Int. Ed., 2010, 49, 4603-4607; X. Jiang, Y. Ma, J. Li, Q. Fan and W. Huang, J. Phys. Chem. C, 2010, 114, 22462).

Graphene oxide sponges were synthesised by vacuum centrifugal evaporating system (F. Liu, T. S. Seo, Adv. Funct. Mater., 2010, 20, 1930-1936).

Graphene hydrogel has been prepared by an hydrothermal process under high pressure, and the obtained hydrogel is electrically conductive, mechanically strong, and exhibits a high specific capacitance (Y. X. Xu, K. X. Sheng, C. Li, G. Q. Shi, ACS Nano, 2010, 4, 4324- 4330).

3D architectures of graphene have been fabricated via an in situ self-assembly of graphene obtained by mild chemical reduction of graphene oxide in water under atmospheric pressure (W. Chen, L.Yan, 2011, Nanoscale, 3, 3132-3137).

Summary of the Invention

According to a first aspect of the present invention, there is provided a material selected from graphene aerogel comprising a layered double hydroxide (LDH). graphene oxide aerogel comprising a layered double hydroxide, graphene xerogel comprising a layered double hydroxide, graphene oxide xerogel comprising a layered double hydroxide, and mixtures thereof.

By supporting LDH onto such graphene or graphene oxide aerogel and xerogel, the C0₂ adsorption capacity of the LDH is increased by enhancement of particle dispersion and gas accessibility. In addition, the regeneration and stability after continuous absorption- desorption cycles is increased by the supporting LDH onto a high surface area material that separates and stabilises the active particles. Moreover, the mechanical properties and high thermal stability of graphene-based porous materials are able to produce hybrids that can withstand harsher industrial conditions. The term "layered double hydroxide" will be hereinafter referred to as LDH.

According to a second aspect of the invention, there is provided a method of producing a material selected from graphene aerogel comprising LDH, graphene oxide aerogel comprising LDH, graphene xerogel comprising LDH, graphene oxide xerogel comprising LDH, and mixtures thereof. According to a further aspect of the invention, there is provided a catalytic system comprising a catalyst and a material selected from graphene aerogel comprising LDH, graphene oxide aerogel comprising LDH, graphene xerogel comprising LDH, graphene oxide xerogel comprising LDH, and mixtures thereof.

According to a further aspect of the invention, there is provided a gas adsorption medium comprising a material selected from graphene aerogel comprising LDH. graphene oxide aerogel comprising LDH, graphene xerogel comprising LDH, graphene oxide xerogel comprising LDH, graphene gel comprising LDH, graphene oxide gel comprising LDH, and mixtures thereof.

According to a further aspect of the invention, there is provided a gas adsorption medium comprising a material selected from graphene aerogel and xerogel where the aerogel or xerogel may comprise any other materials suitable for carrying out the stated objective. For example, the compositions may comprise resins, gelling agents, polymers, fillers and the like.

According to a further aspect of the invention, there is provided a gas adsorption medium comprising a material selected from graphene or graphene oxide comprising LDH. According to a further aspect of the invention, there is provided the use of a material selected from graphene aerogel comprising LDH, graphene oxide aerogel comprising LDH, graphene xerogel comprising LDH, graphene oxide xerogel comprising LDH, and mixtures thereof, for gas sorption, capture and/or storage, preferably for C0₂ sorption, capture and/or storage.

According to a further aspect of the invention, there is provided the use of a material selected from graphene aerogel and xerogel where the aerogel or xerogel may comprise any other materials suitable for carrying out the stated objective. For example, the compositions may comprise resins, gelling agents, polymers, fillers and the like. The material selected from graphene aerogel comprising LDH, graphene oxide aerogel comprising LDH, graphene xerogel comprising LDH, graphene oxide xerogel comprising LDH, and mixtures thereof, are used for gas sorption, capture and/or storage, preferably for C0₂ sorption, capture and/or storage.

According to a further aspect of the invention, there is provided the use of a material selected from graphene or graphene oxide comprising LDH for gas sorption, capture and/or storage, preferably for C0₂ sorption, capture and/or storage.

As used herein, "graphene aerogel" or "graphene oxide aerogel" means an aerogel comprising graphene or graphene oxide respectively. Further materials may be present in the aerogel. Alternatively, no further materials are present other than residual reagent materials, gas and/or solvents.

As used herein, "graphene xerogel" or "graphene oxide xerogel" means a xerogel comprising graphene or graphene oxide respectively. Further materials may be present in the xerogel. Alternatively, no further materials are present other than residual reagent materials, gas and/or solvents. Unless further specified herein, reference herein to an "aerogel" is deemed to refer to an aerogel comprising graphene or graphene oxide.

Unless further specified herein, reference herein to a "xerogel" is deemed to refer to a xerogel comprising graphene or graphene oxide. Graphene, as an ideal atomic-thick 2D material provides an extremely large surface area. Aerogels and xerogels containing graphene open a new interesting possibility: electrical swing adsorption. Specifically, the graphene or graphene oxide network which is selected from an aerogels/xerogels may be selected to be electrically conductive such that when a power supply is applied across the network, a current passes through the matrix, with a resulting desorption of the adsorbed component. The desorption may be due to a resistive heating of the matrix, or due to a direct electrical effect on the sorbate-sorbent interactions.

The present invention seeks to provide a method of obtaining cross-linked graphene or graphene oxide networks, which are in the form of aerogels or xerogels. The present invention also seeks to provide cross-linked graphene or graphene oxide networks which are in the form of aerogels or xerogels, and which allow more control over the density, shape, conductivity and internal surface of the graphene or, so that they display desirable electrical and mechanical properties. The present invention seeks to provide a method of obtaining graphene or graphene oxide aerogels or xerogels in the presence of a one or more gelling agents to provide greater support to the gel-forming process. These agents may include polymers. A preferred polymer according to the present invention is carboxymethyl cellulose (CMC). The present invention also seeks to provide graphene or graphene oxide aerogels or xerogels in the presence of a one or more gelling agents to provide greater support to the gel-forming process. These agents may include polymers. A preferred polymer according to the present invention is carboxymethyl cellulose (CMC).

The present invention seeks to provide LDH supported onto graphene or graphene oxide aerogels or xerogels as an improved system for gas sorption and storage, in particular carbon dioxide capture and storage.

In yet another embodiment the graphene or graphene oxide aerogels or xerogels of the invention can act as adsorbents for liquid ions and gas molecules in general.

In yet another embodiment, the graphene or graphene oxide aerogels or xerogels of the invention can act as solid base catalysts. The viability of such catalysts can be improved by supporting the LDH on the graphene or graphene oxide aerogels or xerogels to maximize the dispersion and stability of the catalyst (LDH) over which the active phase can be distributed. The resultant aerogels, and xerogels can be used as a heterogeneous catalyst for preparing compounds selected from the group consisting of aldols, α,β-unsaturated nitriles, α,β-unsaturated esters, transesterified products, β-nitroalkanols, Michael adducts and epoxides by reacting with corresponding aldehydes with acetone (Aldol condensation), aldehydes with activated nitriles or esters (Knoevenagel condensation), alcohols with β- keto or simple esters (transesterification), aldehydes with nitro alkanes (Henry reaction), activated methylenes with α,β-unsaturated compounds (Michael addition), and epoxidation of olefins. The obtained products are important intermediates for the preparations of materials selected from the group consisting of drugs, pharmaceuticals, perfumes, cosmetics, oils, paint and fine chemicals. For example the products of benzylidene derivatives prepared by Knoevenagel condensation are used to inhibit tyrosine proteinase kinase, fine chemicals such as styrene oxide, 1-decene oxide, 1-octene oxide, 1-hexene oxide, cyclohexene oxide, cyclopentene oxide, epoxy chalcones by epoxidation of olefins, can be obtained by this method.

As used herein, the term "graphene" refers preferably to non-functionalised graphene, and other functionalised graphene, such as carboxylated graphene. Such functionalised graphene may be used instead of graphene oxide to avoid the carboxylated polyaromatic hydrocarbons generated during the conventional oxidation process because it has been observed that these carboxylated polyaromatic hydrocarbons may inhibit some reactions as a results of contamination of LDH surface. Polar functionalised graphene may be prepared by adapting a variety of well-known chemical methods. Polar groups, such as carboxylates that can coordinate to LDH, can be introduced without the other oxidation debris associated with graphene oxide production by acid oxidation techniques.

In general, graphene and graphene oxide per se do not themselves retain C0₂.

In one embodiment, aerogels and xerogels synthesised from graphene oxide are preferable because, unlike graphene, graphene oxide is easy to disperse in water due to the hydrophilic groups on the surface. Graphene oxide (GO) nanosheets are stable in an aqueous solution as a result of ionization of the carboxylic acid and phenolic hydroxyl groups that are known to exist on the GO sheets.

By supporting LDH onto such graphene and graphene oxide aerogels or xerogels, the C0₂ adsorption capacity of the LDH is increased by enhancement of particle dispersion and gas accessibility. In addition there may be modification of the LDH particles, due to changes in dimensions, increases in surface area, defect concentration/edge sites, or even intrinsic properties, for example, due to the intercalation into the graphene or graphene oxide layers. Furthermore, the regeneration and stability after continuous absorption-desorption cycles is increased by the supporting LDH onto graphene or graphene oxide aerogel or xerogel. Moreover, the mechanical properties and high thermal stability of graphenes and graphene oxide are able to produce hybrids that can withstand harsher industrial conditions.

According to the present invention, in order of preference, the following materials are preferred: polar-functional graphene aerogel comprising LDH > graphene oxide aerogel comprising LDH > graphene aerogel comprising LDH > polar-functional graphene xerogel comprising LDH > graphene oxide xerogel comprising LDH > graphene xerogel comprising LDH.

Layered Double Hydroxide

Any Layered double hydroxide (LDH) is suitable for use in the present invention. Preferably, the LDH comprises a mixed metal oxide. The mixed metal oxide is formed by calcinations of the LDH at elevated temperature (typically 450 °C).

Mixtures of LDHs may be preferably used in the present invention.

LDH according to the present invention may be defined in various ways. For example, LDH based on a combination of divalent and trivalent metal cations may have the general formula [M"i_-X M'"x(OH)₂] [X^Q~ _X/Q nH₂0] or [M"-M'"-X] or [M"-M^im], where [M"i_-X M"'_X(OH)₂] ([M"-M^im]) represents the layer, and [X^Q" _X/_Q nH₂0] the interlayer composition.

Extension to multicomponent systems may be expressed as [M"-M'"-M^I"-M"^I"-X-Y]. Tetravalent cations such as Zr⁴⁺ and Sn⁴⁺ can also be incorporated. LDH according to the present invention preferably refers to those having the general formula [M^z+ ₁._xM³⁺ _x(OH)₂]^q+(A^n")_q/n-/iH₂0], wherein when z = 2, M²⁺ = Ca²⁺, Mg²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺ or Zn²⁺ and q = x; when z = 1, M⁺ = Li⁺ and q = 2x - 1; A = C0^2~ ₃, benzoate, succinate, halide, non-metal oxoanions (B0₃ ^3~, C0₃ ^2" N0₃ ^~,

Si₂05^2~, HP0₄ ^2", S0₄ ^2", CIO^" ₄, As0₄ ^3~, Se0₄ ^2~, Br0₄ ^~), oxometallate anions (V0₄ ^3~, Cr0₄ ^2", Mn0₄ ^", Vio0₂8^6", Cr₂0₇ ^2~, Mo₇0₂₄ ^6~, PWi₂O₄₀ ^3~), anionic complexes of transition metals (Fe(CN)₆ ^2~), volatile organic anions (CH₃COO^", C₆H₅COO^~, C₁₂H₂₅COO^", C₂0₄ ^2~, C₆H₅S0₃ ^~ ), anionic polymers (PSS, PVS, etc.) or NO^~ ₃; S0^2" ₄ , CI^", dicarboxylic acids, Fe(CN)^4~ ₆; carboxylated hydrocarbons, carboxylated polyaromatic hydrocarbons, graphene oxide fragments or other oxidation debris associated with graphene oxide production. x is preferably between 0.17 and 0.33; n is preferably between 0.1 and 4.0; and M³⁺ = AI³⁺, Fe³⁺, Cr³⁺ or Ga³⁺.

In a preferred embodiment of the present invention LDH is or comprises Mg-AI-LDH which has the general formula

wherein M²⁺, M³⁺ and A^m" are Mg²⁺, Al³⁺ and C0₃ ^2" respectively, n is between 0.5 and 4.0 and x is between 0.15 and 0.35.

In a preferred embodiment Mg-AI-LDH is prepared using the method according to Example 3.

In yet a further preferred embodiment the LDH is or comprises Mg₂AI(OH)₆(CO₃)₀.₅-0.15H₂O.

LDHs need to be calcined or thermally decomposed in an inert atmosphere in order to obtain the corresponding metal oxides that have the surface area/basic sites required for the C0₂ adsorption. The typical calcinations temperature is 673K. The ratio of LDH:graphene or graphene oxide aerogel or xerogel (w/w) is preferably between 100:0.01 and 0.01:100, more preferably between 100:0.01and 0.01:100, even more preferably between 20:1 and 0.1:1. Therefore, the ratio of LDH:graphene (w/w) may be 0.1/1, 1/1, 10/1, 20/1. Preferably, LDH is present within the graphene or graphene oxide aerogel or xerogel in an amount of between 0.1 to 99.99 wt%, preferably between 1 and 99 wt% and even more preferably between 10 and 95 wt% by total weight of the LDH supported graphene or graphene oxide aerogel or xerogel. The present invention further provides a composition comprising a material selected from graphene aerogel comprising LDH, graphene oxide aerogel comprising LDH, graphene xerogel comprising LDH, graphene oxide xerogel comprising LDH, further comprising at least one adjunct material, preferably a support material, for example, alumina.

The aerogel and xerogel compositions of the present invention may comprise any other materials suitable for carrying out the stated objective. For example, the compositions may comprise resins, gelling agents, polymers, fillers and the like.

Method of Preparing an Aerogel, Xerogel or Gel Comprising LDH according to the invention

According to a further aspect of the present invention, there is provided a method of preparing a graphene aerogel, graphene xerogel, graphene oxide aerogel or graphene oxide xerogel, comprising LDH, comprising the steps of: a) providing a material selected from the group consisting of graphene aerogel, graphene xerogel, , graphene oxide aerogel or graphene oxide xerogel; and b) depositing LDH onto said material.

The LDH may be deposited onto the graphene or graphene oxide aerogel or xerogel by any method known to those skilled in the art. For example, the deposition may be by a method selected from the group consisting of coprecipitation, the urea method, induced hydrolysis reconstruction, sol-gel technique, hydrothermal, microwave, ultrasounds treatments and anion exchange reactions.

In a preferred method, LDH is directly grown or precipitated on the graphene or graphene oxide aerogel or xerogel.

In a further preferred method, LDH is directly grown or precipitated on the graphene or graphene oxide. In a further preferred embodiment, the deposition of LDH is carried out by co-precipitation of Mg²⁺ and Al²⁺ ions under alkaline conditions.

In yet another preferred method, the LDH is prepared using the method according to Example 3. In a preferred method of the present invention, the graphene is oxidised prior to the deposition of LDH in order to produce graphene oxide.

In a preferred embodiment, the carboxylic acid concentration on the graphene oxide is in the range of 0.01 mmol.g^"1 to 100 mmol.g^"1, more preferably 1 mmol.g^"1 to 50 mmol.g^"1. Most preferably, the carboxylic acid concentration on the graphene oxide is about 1.23 mmol.g^"1.

Three experimental chemical routes have been developed for graphene oxidation. The first is a one-step process, and is achieved through direct oxidisation of graphite with strong oxidants such as concentrated sulfuric acid, concentrated nitric acid, or potassium permanganate. The second is a two-step process, in which graphite is oxidized through Hummers', Brodies', Staudenmaiers', or modified Hummer's methods, or (W. Hummers and . Offema, J. Am. Chem. Soc, 1958, 80, 1339; W. F. Chen, L. F. Yan and P. R. Bangal, Carbon, 2010, 48, 1146-1152) electrochemical oxidation, followed by exfoliating or thermally expanding the graphite oxide obtained. The third is a physicochemical process: graphene oxide nanoribbons are created through lengthwise cutting and unravelling the sidewalls of multiwalled carbon nanotube (MWCNTs) by oxidative processes (L. Yan, Y. B. Zheng, F. Zhao, S. Li, X. Gao, B. Xu, P. S. Weiss, Y. Zhao, Chem. Soc. Rev., 2012, 41, 97-114).

The acid oxidation of graphite generates oxygenated species like carboxyl, epoxy and hydroxyl on the material, generating graphene oxide (W. Gao, L. B. Alemany, L. J. Ci and P. M. Ajayan, Nat. Chem., 2009, 1, 403; W. Cai, R. D. Piner, F. J. Stadermann, S. Park, M. A. Shaibat, Y. Ishii, D. Yang, A. Velamakanni, S. J. An and M. Stoller,Science, 2008, 321, 1815).

Carboxylic acid groups and alcohol groups generated on the graphenes during the oxidation step make possible a further esterification between graphenes in order to obtain a covalently crosslinked network robust enough to achieve the gel conformation. LDH can be synthesised onto the crosslinked graphene network gel. The coprecipitation can be made at neutral pH to avoid the hydrolysis of the ester bonds between the graphenes and hence, to avoid the loss of the network structure. The synthesised LDH/graphene gel hybrid is purified by water solvent exchange. The resulting LDH/graphene gel hybrid is freeze dried or dried at ambient pressure after solvent exchange processes (water/acetone/hexane) to avoid the collapse of the graphene gel structure obtaining a graphene xerogel and finally graphene aerogel/LDH hybrid.

In a further preferred method, graphene oxide may be cross-linked by forming covalent bonds between the existing oxide groups on the graphene surface using a direct condensation reaction. Direct condensation between the existing surface oxides occurs only at the contact points between graphene oxide, leaving the remaining oxidised surface unchanged, or available for subsequent LDH deposition. Carboxylates are particularly preferred for their compatibility with LDH.

In a further preferred method, the graphene gel is formed by using linking molecules that directly bond to the graphene surface, without using oxidised groups, using chemistries described below. The advantage, here, that cross-links will form between graphene in close proximately which can be stable under a wide range of conditions (of pH and temperature) allowing greater flexibility in the synthesis of LDH or other secondary phases. Non- crosslinked regions will be surface functionalised by saturation with unsatisfied linking molecules; the remaining unreacted terminus can be converted to carboxylate, or other polar group, using standard chemistry, before LDH or other secondary phase deposition. In another aspect of the present invention, a method may include the use of a blend of graphene or graphene oxide and a carbon nanotube network which is selected from an aerogel and xerogel, crosslinked by a mutually compatible chemistry as described for (oxidised) nanotubes. LDH may then subsequently be deposited on the graphene-carbon nanotube blend to provide a carbon nanotube-graphene network which is selected from an aerogel and xerogel comprising LDH.

In a further aspect of the present invention, a method may include the use of a blend of graphene and a carbon nanotube network which is selected from an aerogel and xerogel, crosslinked using linking molecules that directly bond to the graphene and nanotube surface using chemistries described below. LDH may then subsequently be deposited on the graphene-carbon nanotube blend to provide a carbon nanotube-graphene network which is selected from an aerogel and xerogel comprising LDH. In yet another preferred method, LDH can be synthesised onto graphene oxide. The coprecipitation can be made at neutral or basic pH. These chemical routes provide sites to coordinate metal ions, and balance charge during LDH synthesis. Furthermore, it allows for homogeneous aqueous dispersions of graphenes for the deposition of LDH crystals by co- precipitation of Mg²⁺ and Al³⁺ ions, under alkaline conditions. The synthesised LDH/graphene oxide is preferably purified by vacuum filtration and water washing. Preferably, the resultant LDH/graphene oxide can be dried.

A further aspect of the present invention involves the use of the graphene or graphene oxide aerogel or xerogel comprising LDH for C0₂ sorption, capture and storage. A further aspect of the present invention involves the use of the graphene or graphene oxide comprising LDH for C0₂ sorption, capture and storage.

Heating Graphene or Graphene Oxide Aerogel/Xerogel Comprising LDH

According to a further aspect of the present invention, the graphene or graphene oxide aerogel or xerogel comprising LDH may be heated in an inert atmosphere to a temperature of up to 3273 K, preferably between 373 K and 1273 K, more preferably between 373 K and 737 K. If the aerogel or xerogel are heated in air, they may be heated to a temperature of up to 873 K, preferably between 373 K and 823 K, more preferably between 150 and 737 K, even more preferably between 673 K and 723 K.

Heating the aerogel or xerogel comprising LDH above approximately 773 K in inert atmosphere (preferably by electrical heating) would likely carbonise any polymer structure such as the CMC mentioned before, forming a more stable carbonaceous binding of the junctions. On heating above 673 K, thermal decomposition plays a crucial role in the C0₂ adsorption properties of LDHs. Generally, LDHs are dehydrated below 473 K, partially decarbonated between 473 K and 673 K, dehydroxylated between 673 K and 873 K and further decarbonated, becoming amorphous metastable mixed oxide solid solutions above 873 K. It has been found that a calcination temperature of about 673 K produces LDH derivatives with an optimum balance between surface area and basic sites, which maximizes their C0₂ adsorption capacities and favored reversible adsorption.

Above approximately 2073 K, the graphenes may graphitise and fuse together to form an inherently continuous and robust structure with a high degree of graphiticity. In a further aspect of the present invention, heating of the graphene or graphene oxide aerogel or xerogel comprising LDH may be carried out by cycling between two or more temperatures. Accordingly, in one aspect of the invention, cycling is carried out between 100°C and 1000°C, more preferably between 200°C and 600°C, even more preferably between 300°C and 400°C repeating the adsorption/desorption process up to 10000 cycles, preferably 5000 cycles, more preferably 2000 cycles even more preferably 1000 cycles. However, as would be understood by a skilled person, the number of cycles will be dependent upon a number of factors such as its application or scale. Although it would be favourable to have as many cycles as possible, this must be balanced against loss of performance. The rate of decline will depend on conditions and application. The desire is for a stable structure, wherein the graphene aerogel network is itself robust, and helps to support and stabilise the LDH through as many cycles as possible whilst avoiding typical losses in performance due to sintering, ripening, leaching or recrystallisation as other phases, etc. Graphene or graphene oxide aerogel or xerogel comprising LDH provide high electrical and thermal conductivity, to generate a robust, high surface area network that can be electrically-heated. By processing a graphene or graphene oxide aerogel or xerogel comprising LDH into a desired shape, a continuously-connected homogeneous network is provided, through which current can flow. Due to Joule heating within the branches of the network, the local temperature is raised throughout, by internal heating. The need for thermal diffusion is minimised and any local variations in temperature is reduced by the high thermal conductivity of the graphene and graphene oxide. Furthermore, graphenes and graphene oxides are stable to high temperatures, (at least 773 K even under oxidising conditions, and much higher temperatures in inert atmospheres). The temperature within the network can be rapidly adjusted by varying the current or applied voltage to immediately vary the local Joule heating effect. In addition, aerogel or xerogel comprising LDH according to the invention have very low heat capacity, helping to reduce response time.

The temperature may be monitored by one or more external thermocouple(s) placed in the aerogel or xerogel comprising LDH of the invention or embedded during fabrication, or by optical pyrometry. In a further embodiment according to the present invention, the measured resistance of the aerogel or xerogel comprising LDH of the invention itself may be used as an indication of temperature.

The shape of the aerogel or xerogel of the invention comprising LDH and type of electrical contacts can be varied widely to suit particular applications. Examples include a volume monolith within a reaction chamber, a disc shape (akin to a sintered glass frit), or a high aspect ratio filling for a pipe.

The aerogel or xerogel comprising LDH of the invention may be used in a flow-through geometry, in which gas or liquid passes through the pores in the structure, coming into contact with the graphene surface (or, optionally, additional material supported on the graphenes) and therefore also being heated to the desired temperature.

The resistance at each temperature may be calculated from the current and voltage values. The temperature can be measured using an optical pyrometer, an embedded thermocouple, or other methods known in the art. The result will be a calibration relation between aerogel or xerogel resistance and temperature; this dependence will vary with device geometry, aerogel or xerogel density, and graphene or graphene oxide type (both intrinsic structure and cross-linking chemistry). In operation, the resistance can then be used to determine the temperature, and hence provide appropriate feedback control. In this way, the requisite heating can be predetermined for a given aerogel or xerogel comprising LDH by adjusting the applied voltage/current accordingly.

Specifically, when a power supply is applied across the carbon nanomaterial graphene aerogel or graphene xerogel, a current passes through the network and the temperature increases by a local Joule heating effect, with a resulting desorption of the C0₂ adsorbed on the LDH. In yet another embodiment, the graphene or graphene oxide surface (or modifications) can act as a sorbent/filter either to purify a gas/liquid stream, or to store (a fraction of) it. Subsequently, heating of the graphene or graphene oxide aerogel or xerogel comprising LDH will regenerate the sorbent by decomposition or desorption of the trapped species. The aerogel or xerogel comprising LDH of the invention provides advantages such as rapid and homogeneous heating, fast cycling, rapid emission of stored species, and minimal thermal degradation. Furthermore, accurate temperature control allows fractionation of adsorbed species by type. In addition, the graphene aerogel or graphene xerogel comprising LDH provide enhanced C0₂ adsorption capacity and cycling stability.

Graphene or Graphene Oxide Aerogel/ Xerogel The graphene oxide aerogel, graphene oxide xerogel, graphene aerogel or graphene xerogel according to the present invention may be produced by any method known to those skilled in the art.

According to a further aspect of the present invention, there is provided a method of producing a graphene or graphene oxide network which is selected from an aerogel and a xerogel comprising the steps of: (a) dispersing graphene or graphene oxide in a solvent compatible therewith; (b) cross-linking said graphene or graphene oxide using functional groups already present thereon or with a linking molecule comprising at least two functional sites capable of reacting with the pure surface of said graphene or graphene oxide, to form a covalently cross-linked gel network; and (c) removing said solvent to give a cross-linked graphene or graphene oxide network which is selected from an aerogel and a xerogel with a solvent content of less than 10% by weight.

Preferably, the solvent content of the cross-linked graphene network which is selected from an aerogel and a xerogel is less than 2% by weight, more preferably less than 0.5% by weight, more preferably less than 0.1% by weight. Graphene according to the present invention is reacted using functional groups already present thereon or with a linking molecule comprising at least two functional sites capable of reacting with the surface of said graphene or graphene oxide, to form a covalently cross- linked gel network.

In terms of chemical reactions, acylation reactions are among the most common approaches used for linking molecular moieties onto oxygenated groups at the edges of graphene oxide. The acylation reaction between the carboxyl acid groups of graphene oxide and octadecylamine (after SOCI₂ activation of the COOH groups) can be used to modify graphene oxide by long alkyl chains. Besides small organic molecules, graphene nanosheets can be functionalized with polymers like polyvinyl alcohol) (PVA) through the carbodiimide-activated esterification reaction between the carboxylic acid moieties on the nanosheets and hydroxyl groups on PVA using N,N-dicyclohexylcarbodiimide (DCC), 4-(dimethylamino)-pyridine (DMAP), and N- hydroxybenzotriazole (HOBT) in DMSO.

According to one embodiment of the present invention, the linking molecule may have functionalities that can couple directly to the graphene or graphene oxide surface For example, bis(diazonium) salts, or multifunctional molecules suitable for 1,3-dipolar cycloadditions, or Bingel condensations using known graphene surface chemistry.

More generally, radical addition, electrophile addition or cycloaddition, or all the reactions involving the reduction of the graphenes followed by the reaction of the graphene with an electrophilic cross-linking molecule is of interest. Such reductions may be carried out by the addition of electrons (reduction) to the graphene layers. One example uses ternary potassium salt K(THF)xC₂4 (THF ) tetrahydrofurane, where x=l-3. Direct coupling to the sidewalls, avoids the necessity for damaging pre-oxidation steps. By avoiding unstable linking groups, a more thermally and chemically stable framework can be produced, of greater versatility in application, for example, by creating an aerogel linked by only carbon-carbon bonds. Direct coupling is particularly amenable to creating conjugated linking systems that aid electrical conductivity. In addition, some direct chemistries, such as alkylation following the reductive charging in Birch reactions, are particularly suitable for improving the dispersion of the graphenes and graphene oxide to form a good gel and hence a homogeneous aerogel.

Alternatively, according to a preferred embodiment, such a linking molecule may react with oxide groups of graphene oxide to produce, for example, ester, ether, or amide linkages. Suitable cross-linking agents include alkyl diamines, aromatic diamines, alkyl diols, aromatic diols, polyols, bis-sodium alkoxides, dicarboxylic acids, di acid chlorides, di siloxane halides, di siloxane alkoxides, preferably Ci_₂o alkyl diamines, C₅.₂4 aromatic diamines, Ci_₂o alkyl diols, C5.20 aromatic diols, C₂_ioo polyols, bis-sodium Ci_-2o alkoxides, C₂_₂₀ dicarboxylic acids, C₂_₂₀ di acid chlorides, more preferably Ci_i₀ alkyl diamines, C₆.i₈ aromatic diamines, C_2-io alkyl diols, C₆_i8 aromatic diols, C₂_₂₀ polyols, bis-sodium C_2-io alkoxides, C₂_i₀ dicarboxylic acids, C₂_i₀ di acid chlorides, and the like. Preferably, the two reactive groups are located on different atoms of the linking molecule, more preferably remote atoms (for example, at least 3 or more atoms apart), to maximise the chance of reacting with two different graphene or graphene oxide molecules. The use of small rigid molecules is thought to maximise the establishment of a cross-link. A preferred linker is 1,4-diamino benzene. Such a compound limits the possibility of reacting twice with the same graphene or graphene oxide molecule.

In an alternative, preferred embodiment, the graphene oxide according to the present invention are cross-linked using any linking groups which are capable of forming covalent bonds by direct reaction between the oxides on the graphene surface. In this case, there is no additional linking molecule interposed between the graphene; the covalent bond forms directly by condensation between the existing oxide groups. This approach has the advantage of bringing the graphenes into close contact, maximising the electrical conductivity of the junction, and minimising both the additional reagents required and subsequent parasitic mass added to the network. It is worth noting that, in the previous embodiment, the additional linking molecules will saturate the entire surface, although graphene cross-links will only occur relatively rarely. These molecules may be wasteful and may, undesirably, occlude the conductive surface that is desirable in certain applications such as electrochemical electrodes. Direct condensation between the existing surface oxides occurs only at the contact points between the graphenes, leaving the remaining surface unchanged, or available for subsequent differential functionalisation. In a further aspect of the present invention, the aerogel/xerogel, may be synthesised in the presence of a one or more gelling agents to provide greater support to the gel-forming process. These agents may include polymers. A preferred polymer according to the present invention is carboxymethyl cellulose (CMC).

The cross-linked aerogel or xerogel of the invention may be advantageous for some applications since they provide more physical stability and lower electrical resistance. The chemical structure of the cross-link can be tuned to adjust its resistance, and hence the rate of energy dissipation within the structure, for example by modulating the degree of conjugation and/or the molecular weight. As disclosed previously, the preferred embodiments according to the present invention may involve the use of graphene oxide which may be obtained commercially or, more usually, be those that have further been oxidised according to any standard method.

The term "graphene oxide" as used herein refers to any graphene with one or more oxide groups present on the surface of the graphene. A wide range of surface oxides are known in carbon chemistry. In the present invention, the "oxide groups" are selected from the group consisting of quinones, ketones, lactones, pyrones, carboxylic acids, carboxylates, hydroxides and hydroxyl groups, and groups derivable from these via oxidation, and mixtures of two or more thereof. In a particularly preferred embodiment, the surface oxides are carboxylic and/or hydroxide groups.

The level of oxidation of the graphenes will vary according to the desired mechanical and electrical properties required. Typically, the level of oxidation on the oxidised graphene is between 0.001 - 100 mmol/g, preferably 0.1 mmol/g or greater.

In a preferred method, the oxidised graphenes are cross-linked to form an ester or ether bond, most preferably an ester bond. The reaction is preferably a condensation reaction, one that releases a small molecule byproduct such as water, rather than introducing additional atoms into the resulting linkage. In yet another embodiment, the surface oxides may be converted to other simple functional groups for direct condensation. In such an embodiment, the surface alcohols on the graphenes may be converted to, for example, an amine functionality, which subsequently allows the cross-links to be formed via an amide bond. Other direct molecular condensations such as those to form imines, thioethers, thioesters, and ureas, also fall within the scope of the present invention.

In a preferred embodiment, the cross-links between the oxidised graphenes may be formed using a coupling agent. The term "coupling agent" as used herein does not have the conventional meaning often used in polymer resin chemistry but refers to any substance capable of facilitating the formation of a bonding link between two reagents, as in the field of organic chemistry. Such compounds include Ν,Ν'-dicyclohexylcarbodiimide (DCC), Ν,Ν'- diisopropylcarbodiimide (DIC), ethyl-(N',N'-dimethylamino)propylcarbodiimide hydrochloride (EDC) [adding an equivalent of 1-hydroxybenzotriazole (HOBt) to minimize the racemisation], 4-(N,N-dimethylamino) pyridine (DMAP), (benzotriazol-1- yloxy)tris(dimethylamino)phosphonium hexafluorophosphate (BOP), (benzotriazol-1- yloxy)tripyrrolidinophosphonium hexafluorophosphate, bromotripyrrolidinophosphonium hexafluorophosphate, 0-(benzotriazol-l-yl)-N,N,N',N'-tetramethyluronium hexafluorophosphate (HBTU), O-(benzotriazol-l-yl)- Ν,Ν,Ν',Ν'-tetramethyluronium tetrafluoroborate (TBTU), 0-(7-azabenzotriazol-l-yl)-N,N,N',N'-tetramethyluronium hexafluorophosphate (HATU), 0-(6-chlorobenzotriazol-l-yl)-N,N,N',N'-tetramethyluronium hexafluorophosphate (HCTU), 0-(3,4-dihydro-4-oxo-l,2,3-benzotriazine-3-yl)-N,N,N',N' tetramethyluronium tetrafluoroborate (TDBTU), 3-(diethylphosphoryloxy)-l,2,3 benzotriazin-4(3H)-one (DEPBT), carbonyldilmidazole (CDI) and mixtures thereof. In a preferred embodiment, a carbodiimide is used to couple a suitable functional group and a carbonyl group such as an ester or an acid. Preferred examples of carbodiimides include but are not limited to l-ethyl-3-(3-dimethylaminopropyl)carbodiimide, Ν,Ν'- dicyclohexyl carbodiimide, Ν,Ν'-diisopropyl carbodiimide, bis(trimethylsilyl)carbodiimide and N-cyclohexyl-N'-( -[N-methylmorpholino]ethyl)carbodiimide p-toluenesulfonate. In a particularly preferred embodiment, the coupling agent is selected from the group consisting of N,N'-dicyclohexylcarbodiimide, Ν,Ν'-diisopropylcarbodiimide and l-ethyl-3-(3- dimethylaminopropyl)carbodiimide.

The coupling agent may be supplemented by an additional agent such as those known to enhance extra selectivity or yield of such condensation reactions, such as N- hydroxybenzotriazole or N-hydroxysuccinimide.

The cross-linking process may be carried out at any reasonable temperature and left for any length of time necessary to complete the reaction, so long as the reaction is carried out at a temperature below the boiling point of the reaction solvent(s). In a preferred method, the cross-linking reaction is carried out at a temperature of between 288 K to 333 K, preferably 293 K to 303 K. The reaction time is preferably between 0.1 to 50 hours and more preferably between 1 and 12 hours.

Alternatively, the cross-linking process may be carried out by dehydration. The term dehydration as used herein refers to a chemical reaction which involves the loss of water from the reacting molecule(s). In a preferred embodiment, dehydration is carried out by using groups on the graphene oxide. Such groups include ether linkages which may be formed by dehydration at a temperature greater than 393 K, preferably greater than - 403 K and even more preferably greater than 423 K, preferably using an acid catalyst. Cross- linking process will require a solvent with a high boiling point, greater than the reaction temperature. In a preferred embodiment, the boiling point of the solvent is greater than 293 K, preferably greater than 403 K and even more preferably greater than 423 K.

During the course of the reaction, the graphenes or graphene oxides are cross-linked to form a gel phase. As used herein, the term "gel" refers to what those skilled in the art understand by the term, and preferably refers to a composition which retains its shape during the drying process. The term "gel" as used herein (in isolation) more preferably refers to a precursor of the aerogel/xerogel prior to the removal of the solvent or drying step. The term "gel" in itself is not intended to cover an aerogel or a xerogel. The gel phase is formed by a continuous network of covalently bound graphenes within the solvent. Under small shear deformations the response is predominantly elastic rather than viscous; in dynamic shear rheology experiments, at the gel point there is a characteristic crossover of G' and G"/tan(nn/2) given by the equation below:

G'M=G"M/tan(nn/2)=S_g ⁿr(l-n)cos(nn/2) where G' is the storage modulus, G" is the loss modulus, Γ is the gamma function, n is the relaxation exponent, S_g is the gel strength and ω is the frequency. Using the values of G' at crossover points and the equation described before the S_g value can be estimated, characteristic of the synthesised gel (Gelation in Graphene/Polymer Composites, Liu C. et al., Polymer, 44, 2003, 7529-7532). By carrying out the process of covalent cross-linking in a gel phase, the resultant graphenes can retain their structural integrity during the removal of the solvent. During subsequent steps, it is possible to minimise the effects of the meniscus associated with liquid-phase drying such that the mesopores within the gel structure can be prevented from collapsing, allowing for a cross-linked graphene or graphene oxide aerogel or xerogel with high porosity and large surface area.

In the case where cross-linking is carried out by direct reaction between the oxides on the graphene surface, only a small proportion (approximately 1-3%) of the surface oxides react to form the cross-links between the graphenes or graphene oxide. The cross-linked aerogel or xerogel thus obtained will have unreacted oxide groups on the surface of the graphene oxide. These groups impart hydrophilicity (i.e. tendency to interact with or be dissolved by water and other polar substances) to the resulting aerogel/xerogel. Also the ionic interaction between the negatively-charged graphene oxide and positively-charged LDH nanosheets plays an important role in the LDH growth process. The method can further comprise a step of capping residual surface oxides on the oxidised graphenes. The term "capping" according to the present invention refers to any step which alters or transforms the surface oxides into other functionalities. In this respect, it can be any functional group which is able to react with the surface oxide group such as a metal, haloalkanes, acid halides and the like. In a preferred embodiment, the surface oxides are capped using a hydrophobic functional group. I n a particularly preferred embodiment, the hydrophobic functional group is preferably selected from the group consisting of haloalkyl, alkyl and siloxane, more preferably Ci_i₂ haloalkyl and Ci_i₈ alkyl, most preferably a Ci_i₂ - haloalkyl and more preferably Ci_i₀ haloalkyl. Preferably the hydrophobic functional group is a haloalkyl containing more than 1 fluorine atom, preferably 3 to 20 fluorine atoms, preferably 8 to 16 fluorine atoms, more preferably 10 to 14 fluorine atoms, for example 13 fluorine atoms. Preferably the capping group reagent is a hydroxyhaloalkyl compound, preferably a Ci_i₂ hydroxyhaloalkyl compound, preferably trifluoroethanol. By hydrophobic, it is meant that the group imparts increased hydrophobic character to the graphene, thereby reducing the solid surface tension. Where a linking molecule is used to form the cross-links, although only a small proportion of the carbon surface is involved with cross-linking, unlike the direct condensation reactions, the remaining surface will already be saturated with excess linking molecules. Since, both sides are saturated, these molecules are unlikely to covalently cross-link during drying (depending on the reagent), but may well be relatively polar and form undesirable non-covalent interactions that encourage collapse. In this case a further reaction with a capping agent, as described above, could be used to lower the surface tension, where the hydrophobic end group is reacted with the remaining unreacted end of the excess linking molecules.

In the method according to the present invention, a solvent which is compatible with the graphene may be used. In this respect, the term "compatible" refers to any solvent in which the graphenes or graphene oxides form a substantially homogeneous solution or dispersion. Preferably, the solvent which is compatible with the graphenes or graphene oxide is miscible therewith. Preferably, the coupling agent is also substantially soluble in the solvent. In a preferred embodiment, the solvent is selected from dimethyl formamide, benzene, dichloromethane, chlorobenzene, chloroform, toluene, xylene, dioxane, dimethylsulfoxide, tetrahydrofuran, amide solvents and mixtures thereof, most preferably dimethyl formamide. As used herein amide solvents refers to any solvent which contains an amide group. Preferred amide solvents include N-methyl-2-pyrrolidone and cyclohexyl pyrrolidone.

The graphenes or graphene oxide may be present in the solvent at any given concentration. Preferably, the graphenes or graphene oxide must be sufficiently concentrated that they can form a continuous connected network across the whole composition. Preferably, this concentration is above the rheological percolation threshold for the chosen graphenes dispersion in the chosen solvent. In a preferred method, the graphenes or graphene oxide are present in the solvent at a concentration of between 0.01-30 vol.%, more preferably 0.1-20 vol.%, more preferably 1-5 vol.%.

In a further preferred method according to the present invention, the removal of solvent is carried out by solvent exchange with at least one solvent having lower surface tension than the initial solvent. The term "surface tension," as used herein, refers to the attractive force in any liquid exerted by the molecules below the surface upon those at the surface/air interface, which force tends to restrain a liquid from flowing. Preferably, the term "low surface tension," as used herein refers to liquids having a surface tension of less than or equal to about 30 mN/m as measured at 25 °C and atmospheric pressure. However, this value may be more or less, since the critical tolerable surface tension to avoid collapse during the drying step will depend on the network. In particular, as any one of the graphene thickness the cross-link density, or degree of hydrophobicity increases, the need for low surface tension decreases. Therefore in principle, some graphene gels may be dried without solvent exchange and others will need very low surface tensions. Whether a particular network requires such solvent exchange will depend on the individual properties of the gels. The lower density, higher surface area networks have more desirable properties but tend to be less robust so need solvent exchange or other controlled drying technique. In a particularly preferred embodiment, solvent exchange is carried out using acetone, followed by C₃-Ci₀ hydrocarbon, preferably hexane. In another embodiment, the aerogel/xerogel is synthesised without covalent crosslinking of the graphenes; instead relying on a non-covalent binder to create the stabilse the required monolith.

In a further aspect of the present invention, the aerogel/xerogel, may be synthesised in the presence of a one or more gelling agents to provide greater support to the gel-forming process. These agents may include polymers. A preferred polymer according to the present invention is carboxymethyl cellulose (CMC).

In one particularly preferred embodiment, the aerogel or xerogel may be synthesised by providing pristine graphene and CMC or other polymer in water or other solvent, and freeze drying. In a further aspect of the present invention the aerogel or xerogel of the invention may be synthesised by providing oxidised graphene and CMC or other polymer in water or the solvent, and freeze drying. In yet another aspect of the present invention the aerogel or xerogelof the invention may be synthesised by providing oxidised cross-linked graphene and CMC in water, and freeze drying. In a further aspect of the present invention the aerogel or xerogel of the invention may be synthesised by providing oxidised cross- linked graphene or graphene oxide in water, and freeze drying. In a further aspect of the present invention the aerogel or xerogel, of the invention may be synthesised by providing oxidised cross-linked graphene or graphene oxide in water, carrying out solvent exchange (preferably using water-acetone-hexane) and drying at ambient temperature and pressure. The graphene or graphene oxide network according to the present invention is preferably an aerogel or xerogel, most preferably an aerogel. Aerogels may be more advantageous for particular applications given their higher porosity and surface area.

As used herein, the term "aerogel" refers to a highly porous material of low density, which is prepared by forming a chemically-crosslinked gel and then removing liquid from the gel while substantially retaining the gel structure. In some embodiments of the current invention, the usual solvent removal step may optionally be omitted, if the system is to be used in a liquid-related application; if necessary, the gel fabrication solvent may be exchanged with the intended application solvent, by means of one or more solvent exchange steps; thus the cross-linked graphene or graphene oxide "gel" will be used, rather than the dried form. Preferably, an "aerogel" according to the present invention comprises a graphene or graphene oxide network wherein the volume change on drying of the gel is less than 30%, preferably less than 20%, preferably less than 10%, preferably less than 5%. Aerogels have open-celled microporous or mesoporous structures. Typically, they have pore sizes of less than 1000 nm and surface areas of greater than 100 m² per gram. Preferably they have pore sizes of less than 200 nm and surface areas of greater than 400 m² per gram. They often have low densities, e.g., from 500 mg/cm³ down to as little as 1 mg/cm³ preferably in the range of 15 to 300 mg/cm³. Exceptionally, unlike other existing aerogels, those produced from graphenes or graphene oxides, may have ultra-low densities, high surface areas, but large pore sizes; in principle, the pore size may approach the scale of the individual graphene lengths which can reach millimetres or even centimetres.

Preferably, aerogels are materials in which the liquid has been removed from the gel under supercritical conditions. In one method according to the present invention, removal of solvent may carried out by supercritical drying or lyophilisation (freezing-vacuum process) to form an aerogel. The most common method for supercritical drying involves the removal of the solvent with supercritical carbon dioxide, and this may be used in the present invention.

In a preferred method according to the present invention, the drying process is carried out at room temperature and/or ambient pressure. This method is a more versatile procedure to fabricate an aerogel since it does not require supercritical C0₂, or a lyophilisation (freezing-vacuum process). The aerogel can be obtained by simply drying the gel. The objective is to evaporate the solvent producing the minimum volume reduction when obtaining the aerogel from the gel. Where a method involves cross-linking between the graphenes and optional hydrophobic functionalisation of the graphene surface, this may help the process. Moreover, the method may further comprise a solvent exchange process to a solvent with lower surface tension. The functionalisation during the preparation of the gel permits simplification of the later drying step.

The term "xerogel" as used herein refers to a type of aerogel in which the volume change on drying of the gel is greater than approximately 30%. In this case, although the gel partially collapses during drying, the strong covalent network of graphenes limits the process, yielding a more useful, more porous, less dense structure, than obtained from drying physical gels or other graphene suspensions.

Preferably, each graphene or graphene oxide used in the present invention has high electric conductivity and allows a current flow at a current density of greater than 0.1 mA/cm², preferably greater than 500 A/cm² or more. A network of graphenes is therefore thought to display excellent electrical conductivity and current density, compared to existing carbon aerogels.

In addition, graphenes and graphene oxide have desirable intrinsic mechanical characteristics, including high strength, stiffness, and flexibility, at low density. These properties make them desirable for many industrial applications, and lend desirable properties to the resulting aerogel networks.

The shape of the aerogel or xerogel of the invention can be controlled by controlling the shape of the vessel used during the gelation step. The density of the final aerogel can be controlled by varying the volume fraction of graphenes or graphene oxides within the initial gel.

The present invention also provides catalysts, catalyst supports, fluid heaters and electrically-regenerable filters/sorbents comprising an graphene aerogel and or xerogel according to the present invention.

A further embodiment according to the present invention involves the use of an aerogel, or xerogel of the invention comprising LDH for sorption and/or gas storage.

Detailed Description of the Invention

General

Unless otherwise specified, the term "comprising" encompasses "including" as well as "consisting" e.g. a composition "comprising" X may consist exclusively of X or may include something additional e.g. X + Y.

The term "about" in relation to a numerical value x means, for example, x+10%. The word "substantially" does not exclude "completely" e.g. a composition which is "substantially free" from Y may be completely free from Y. Where necessary, the word "substantially" may be omitted from the definition of the invention.

As used herein, the term "alkyl" refers to a straight or branched saturated monovalent hydrocarbon radical, having the number of carbon atoms as indicated. By way of non limiting example, suitable alkyl groups include propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, and the like.

The term monolayer graphene stands for a single atomic sheet of graphene, or a sample composed substantially of such material. The term graphene is usually taken to include "few layer graphene", including graphite substantially composed of platelets of 5 atomic graphene monolayers or fewer. The "graphene oxide monolayer" stands for a single sheet of graphene with one or more oxide groups present on the surface of the graphene. The term "graphene oxide" as used herein refers to any graphene with one or more oxide groups present on the surface of the graphene. A wide range of surface oxides are known in carbon chemistry, including quinones, ketones, lactones, pyrones, carboxylic acids, carboxylates, hydroxides and hydroxyl groups etc., and groups derivable from these via oxidation. In a particularly preferred embodiment, the surface oxides are carboxylic and/or hydroxide groups.

Examples of the Present Invention The following examples of the present invention are merely exemplary and should not be viewed as limiting the scope of the invention.

It is worth noting that recent neutron data shows that the structure of the periclase maps rather closely to the original LDH, with much less rearrangement than seems to be implied by the X D data (Mourad, M. C. D.; Mokhtar, M.; Tucker, M. G.; Barney, E. R.; Smith, R. I.; Alyoubi, A. O.; Basahel, S. N.; Shaffer, M. S. P.; Skipper, N. T. J. Mater. Chem. 2011, 21, 15479).

EXAM PLE 1

The aerogels and xerogels according to the present invention may be produced by directly, covalently crosslinking graphene to form a gel network, followed by solvent removal. The gel structure is created by rigid graphenes directly bound to each other or with another gel- forming component. Direct binding between the graphene provides high strength, high electrical conductivity, high temperature conductivity and a large accessible surface area - all of which are beneficial across a number of applications. The gel phase allows casting into any desired moulded shape and controllable density.

The gel can be synthesised directly in a cuvette where the electrical power will be applied to generate an electrical current through the graphene network. The aerogel is heated due to the Joule effect.

An aerogel is introduced or synthesised in a cuvette that has two of its sides composed by aluminium foil. A DC power supply with controllable voltage, was used together with a portable ammeter to monitor the current while increasing the voltage. The temperature was monitored using an I camera as the voltage was increased. After applying each voltage step, the temperature increases and rapidly stabilised, whereupon the temperature was recorded. EXAM PLE 2

An aerogel according to the invention may be produced under the following reaction scheme 1:

Reaction scheme 1 Crosslinkinp of the praphene oxide (gelation)

400 mg of graphene oxide (Nanoinnova Technologies, S.L) was placed under nitrogen in a rectangular 10 ml flask sealed with a septum. 1 mL of anhydrous dimethylformamide (99.8 % Sigma-Aldrich) was injected using a syringe and the mixture was sonicated for 1 minute (ultrasonic bath Grant XUB5, 22.2 W/l) in order to obtain a homogeneous dimethylformamide graphene suspension. To this, 688 mg (10 % carbon mol) of 1, 3- dicyclohexylcarbodiimide (DCC, Fluka) was added to catalyse the esterification reaction between the alcohols and acid groups of graphene oxide. After 12 hours the black phase- presumably composed by crosslinked graphenegraphenes- was highly viscous and did not deform even when the flask was turned upside down. The volume fraction of the graphene in the gel is estimated to be 15%.

Preparation of hydrophobic graphenegraphene

In order to avoid the collapse of the gel during the drying of the solvent, the contact angle between the solvent and the graphenes was increased by introducing hydrophobic functional groups onto the graphene surface. This hydrophobisation was achieved through an additional esterification with a fluorinated alcohol; specifically, 0.8 ml (33 % carbon mol) of 2,2,2-trifluoroethanol ( eagentPlus, >99%, Sigma-Aldrich) was added to the gel. After 12 hours the supernatant was set aside. In order to wash the sample, 2 ml of dimethylformamide were added to the gel and after 5 minutes the supernatant was set aside. The washing step was repeated up to 3 times.

Exchange the pore fluid with a selected solvent

The objective in this case is to exchange the pore fluid with the more hydrophobic n-hexane to reduce the effective surface tension during the drying of the gel. Since dimethylformamide and n-hexane are immiscible, acetone is used as an intermediate exchange agent as it is completely soluble in both liquids. Solvent exchange of pore-filled dimethylformamide with acetone and subsequently, of acetone with hexane was carried out. For this purpose 2 ml of the solvent were added to the gel and after 5 minutes the supernatant was set aside. The same process was repeated 3 times with each solvent. The sample was dried at room temperature to obtain the resulting graphene oxide aerogel. Products The shape of the Aerogel can be modulated by controlling the shape of the vessel during the gelation step. The density of the final Aerogel can be modulated by varying the volume fraction of graphenes within the gel. For example, between at least the 15 vol % value described in the specific example and the percolation threshold of these specific crosslinked graphenes in dimethylformamide (estimated to be around 1 vol %).

EXAM PLE 3

2.1. Materials

Graphene oxide was purchased from (Nanoinnova Technologies, L. T. D.).Mg(N0₃)2.6H₂0 (99%) and AI(N0₃)3.9H₂0 (98%) were purchased from Sigma-Aldrich; NaOH, was purchased from AnalaR and Na₂C0₃ was purchased from Riedel-de Haen. Polycarbonate membranes were from Millipore (HTTP Isopore membrane).

2.2. Synthesis of Mg-AI LDHs

Unsupported Layered Double Hydroxides (LDHs) were prepared via co-precipitation. An Mg/AI ratio of 2 was selected as it has been reported to be optimal for C0₂ sorption. An aqueous solution (50 mL) of 0.1 mol Mg(N0₃)₂.6H₂0 and 0.05 mol AI(N0₃)₃.9H₂0 was added to an aqueous solution (75 mL) containing 0.35 mol of NaOH and 0.09 mol of Na₂C0₃. The resulting white suspension was heated at 333 K for 12 hours under stirring (300 rpm). The resulting precipitate was filtered using 0.4 μιη polycarbonate membranes and washed with 500 mL of water at 333 K. The sample was dried for 12 hours at 393 K. 2.3. Preparation of nanostructured LDH/graphene oxide hybrids

Graphene oxide was dispersed in an aqueous solution (2.06 mL) containing 9.9 mmol NaOH and 2.5 mmol Na₂C0₃. Subsequently, 1.39 mL of a salt solution of 2.8 mmol Mg(N0₃)₂.6H₂0 and 1.4 mmol AI(N0₃)₃.9H₂0 was added. The resulting black suspension was aged at 333 K for 12 hours under stirring (300 rpm). The sample was filtered and dried as explained above for the preparation of unsupported LDHs. Four different LDH/graphene oxide hybrids were prepared containing varying graphene oxide weight percentages [LDH/graphene oxide mass ratios] 9 wt% [0.1/1], 50 wt% [1/1], 90 wt% [10/1] and 95 wt% [20/1] obtained varying the volume of the base solution and nitrate based solution.

2.4. Preparation of graphene xerogel/aeroqel Following the methodology described in the EXAMPLE 2, graphene xerogel/aerogel were synthesised using cross-linked graphenes. Graphene xerogel/aerogel can be also synthesised using polymers to reinforce the aerogel structure.

2.5. Preparation of nanostructured LDH/grgphene gel/xerogel/gerogel hybrids Using graphene gel/xeroael/aerogels consisting of graphene and polymers

1) Graphene gel was impregnated with an aqueous solution (2.06 mL) containing 9.9 mmol NaOH and 2.5 mmol Na₂C0₃. Subsequently, 1.39 mL of a salt solution of 2.8 mmol Mg(N0₃)2.6H₂0 and 1.4 mmol AI(N0₃)3.9H₂0 was added. No stirring is used and the sample was aged at 333 K for 12 hours. The LDH is synthesised onto the graphene gel by precipitation of the cations in alkaline conditions.

2) Graphene gel was impregnated with 1.39 mL of a salt solution of 2.8 mmol Mg(N0₃)2.6H₂0 and 1.4 mmol AI(N0₃)3.9H₂0 and an aqueous solution (2.06 mL) containing 9.9 mmol NaOH and 2.5 mmol Na₂C0₃ was added. No stirring is used and the sample was aged at 333 K for 12 hours. The LDH is synthesised onto the graphene gel by in situ co- precipitation of the cations at variable pH conditions.

3) The LDH is synthesised (in alkaline conditions or at variable pH conditions) and once the co-precipitation takes place and the LDH is formed, the LDH can be added directly onto a graphene gel.

After the LDH precipitation at constant pH, variable pH or the direct addition of the LDH onto the graphene gel, the LDH/gel was washed by solvent exchange with water as explained in the EXAMPLE 2. The pore-filled water was eliminated by freeze drying or by solvent exchange/room temperature drying. For the later, solvent exchange of pore-filled water with acetone and subsequently, of acetone with hexane was carried out. For this purpose 2 ml of the solvent were added to the gel and after 5 minutes the supernatant was set aside. The same process was repeated 3 times with each solvent. As a result, LDH/Graphene gel/xerogel hybrid is dried at room temperature to obtain LDH/Graphene aerogel.

Grgphene gel/xerogel/gerogels by cross-linking grgphene 4) All the methodologies above (1, 2, 3) but using graphene aerogels consisting of covalent cross-linked graphene. The pH of the solution has to be kept neutral (without NaOH) to avoid the hydrolysis of the ester functional groups bonding the graphene.

5) All the methodologies above (1, 2, 3) but using covalent cross-linked graphene aerogels reinforced by polymers. NaOH can be used depending on the stability of the polymer at alkaline pH.

6) All the methodologies above (1, 2, 3) but using graphene aerogels consisting of covalent cross-linked graphene and modified with hydrophobic functional groups such as fluorine to avoid the collapse of the graphene aerogel. The pH of the solution has to be kept neutral (without NaOH) to avoid the hydrolysis of the ester functional groups bonding the graphene.

7) All the methodologies described above (1, 2, 3) but using graphene aerogels consisting of covalent cross-linked graphene where the linkages are alkyl groups. The methodology for the synthesis of the graphene aerogel is described in the PCT GB2011/052224 and is based on carbanionic graphenes that react with electrophilic linking molecules. The collapse of graphene aerogels synthesised using this methodology during the LDH synthesis is less likely than with aerogels consisting of graphenes cross-linked by ester bonds.

2.6. Calcination of nanostructured LD / graphene hybrids

All the materials were calcined prior to C0₂ adsorption measurements at 673 K during 4 hours flowing 100 mL.min^"1 of nitrogen using a tubular quartz reactor (ID = 5 cm) placed in a horizontal Carbolite furnace.

Claims

1. A material selected from graphene aerogel comprising a layered double hydroxide (LDH), graphene oxide aerogel comprising LDH, graphene xerogel comprising LDH, graphene oxide xerogel comprising LDH, and mixtures thereof.

2. The material according to claim 1, wherein said LDH has the general formula [M^z+i_ _xM³⁺ _x(OH)₂]^q+(Aⁿ )_q/n-nH₂0], wherein when z = 2, M²⁺ = Ca²⁺, Mg²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺ or Zn²⁺ and q = x; when z = 1, M⁺ = Li⁺ and q = 2x - 1;

A = C0 benzoate, succinate, halide, non-metal oxoanions (B0₃ ^3~, C0₃ ^2" N0₃ ^~, Si₂05^2~, HP0₄ ^2", S0₄ ^2", CIO^" ₄, As0₄ ^3~, Se0₄ ^2~, Br0₄ ^~), oxometallate anions (V0₄ ^3~, Cr0₄ ^2",

Mn0₄ ^", Vio0 Cr₂0₇ ^2~, Mo₇0₂₄ ^6~, PWi₂O₄₀ ^3~), anionic complexes of transition metals (Fe(CN)₆ ^2~), volatile organic anions (CH₃COO^", C₆H₅COO^", C₁₂H₂₅COO^", C₂0₄ ^2", C₆H₅S0₃ ^~ ), anionic polymers (PSS, PVS, etc.) or NO^~ ₃; S0^2" ₄ , CI^", dicarboxylic acids Fe(CN)^4~ ₆; carboxylated hydrocarbons, carboxylated polyaromatic hydrocarbons, graphene oxide fragments or other oxidation debris associated with graphene oxide production; x is between 0.1 and 5.0; n is between 0.1 and 4.0; and

M = Al , Fe , Cr or Ga .

3. The material according to claim 2, wherein said LDH has a general formula [M²⁺i_ _xM³⁺ _x(OH)₂]^x+[A^m" _)</m/iH₂0]^x", wherein M²⁺, M³⁺ and A^m" are Mg²⁺, Al³⁺ and C0₃ ^2" respectively, n is between 0.5 and 4.0 and x is between 0.15 and 0.35.

4. The material according to any preceding claim, wherein the ratio of LDH:graphene or LDH:graphene oxide (w/w) is between 100:0.01 and 0.01:100, preferably between 20:0.1 and 0.01:1.

5. The material according to any preceding claim wherein said LDH is present within the aerogel, xerogel or wherein the ratio of LDH:graphene (w/w) is between 100:0.01 and 0.01:100, preferably between 20:0.1 and 0.5:1.

6. The material according to any preceding claim wherein said material comprises graphene oxide(s).

7. The material according to claim 6, wherein oxides are selected from the group consisting of hydroxide, carboxylate, lactone, hydroxyl, quinone and carboxylic acid groups, and mixtures thereof, preferably a combination of hydroxide and carboxyl acid.

8. The material according to claim 6 or claim 7, wherein the level of oxidation on the oxidised graphenes is between 0.001 - 100 mmol/g, preferably 0.1 mmol/g or greater.

9. The material according to any preceding claim, wherein said graphene or graphene oxide is cross-linked.

10. The material according to claim 9, wherein said graphene or graphene oxide is cross-linked via an ester bond.

11. The material according to any preceding claim wherein the LDH has been calcined between 373K and 1273K, preferably between 573K and 873K.

12. The material according any preceding claim, wherein said aerogel or xerogel has a density of from 1 to 1500 mg/cm³.

13. The material according any preceding claim, wherein said aerogel or xerogel is in the form of a continuously-connected homogeneous network.

14. A method of preparing a material according to any preceding claim, comprising the steps of: a) providing a graphene aerogel, graphene oxide aerogel, graphene xerogel, graphene oxide xerogel, or a mixture of two or more thereof; and b) depositing LDH thereon.

15. The method according to claim 14, wherein said LDH is deposited by direct growth or precipitation onto the graphene aerogel, graphene oxide aerogel, graphene xerogel, graphene oxide xerogel, and mixtures thereof.

16. The method according to claim 15, wherein said deposition of LDH is carried out by co-precipitation of Mg²⁺ and Al³⁺ ions under alkaline conditions.

17. The method according to any of claims 14 to 16, wherein said graphene aerogel, graphene oxide aerogel, graphene xerogel, graphene oxide xerogel, or a mixture of two or more thereof is prepared by a) dispersing graphene or graphene oxide or a mixture thereof in a solvent compatible therewith; b) cross-linking said graphene or graphene oxide using functional groups already present thereon or with a linking molecule comprising at least two functional sites capable of reacting with the surface thereof, to form a covalently cross-linked gel network; and c) removing said solvent to give an aerogel or xerogel with a solvent content of less than 10 %.

18. The method according to claim 17, wherein said graphenes are oxidised graphenes and wherein said cross-linking is carried out by direct reaction between the surface oxides on the graphene surface.

19. The method according to claim 17, wherein said graphenes are oxidised graphenes and wherein said cross-linking is carried out with a linking molecule selected from the group consisting of alkyl diamines, aromatic diamines, alkyl diols, aromatic diols, polyols, bis- sodium alkoxides, dicarboxylic acids, di acid chlorides, di siloxane halides and di siloxane alkoxides.

20. The method according to claim any of claims 17 to 19, wherein said cross-linking is formed using a coupling agent or by dehydration.

21. The method according claim 20, wherein said cross-linking is formed using a coupling agent and is selected from the group consisting of N,N'-dicyclohexylcarbodiimide (DCC), Ν,Ν'-diisopropylcarbodiimide (DIC), ethyl-(N',N'-dimethylamino)propylcarbodiimide hydrochloride (EDC), 4-(N,N-dimethylamino) pyridine (DMAP), (benzotriazol-1- yloxy)tris(dimethylamino)phosphonium hexafluorophosphate (BOP), (benzotriazol-1- yloxy)tripyrrolidinophosphonium hexafluorophosphate, bromotripyrrolidinophosphonium hexafluorophosphate, 0-(benzotriazol-l-yl)-N,N,N',N'-tetramethyluronium hexafluorophosphate (HBTU), O-(benzotriazol-l-yl)- Ν,Ν,Ν',Ν'-tetramethyluronium tetrafluoroborate (TBTU), 0-(7-azabenzotriazol-l-yl)-N,N,N',N'-tetramethyluronium hexafluorophosphate (HATU), 0-(6-chlorobenzotriazol-l-yl)-N,N,N',N'-tetramethyluronium hexafluorophosphate (HCTU), 0-(3,4-Dihydro-4-oxo-l,2,3-benzotriazine-3-yl)-N,N,N',N' tetramethyluronium tetrafluoroborate (TDBTU), 3-(diethylphosphoryloxy)-l,2,3 benzotriazin-4(3H)-one (DEPBT) and carbonyldilmidazole (CDI), and mixtures thereof.

22. The method according to claim 21, wherein said cross-linking is formed by dehydration using an acid catalyst at a temperature greater than 120 °C.

23. The method according to claim 17, wherein said cross-linking is carried out with a linking molecule selected from the group consisting of bis(diazonium) salts and multifunctional molecules suitable for 1,3-dipolar cycloadditions or Bingel condensations, or by the reduction of the graphenes followed by the reaction of the graphenes with an electrophilic cross-linking molecule.

24. The method according to any of claims 17 to 23, further comprising the step of capping residual functional groups on the graphene or graphene oxide prior to the removal of the solvent.

25. The method according to claim 24, wherein said the residual functional groups are capped using a hydrophobic functional group.

26. The method according to claim 25, wherein said hydrophobic functional group is selected from the group consisting of alkyl, haloalkyl, siloxane and mixtures thereof.

27. The method according to any of claims 17 to 26, wherein said solvent is selected from the group consisting of dimethyl formamide, benzene, dichloromethane, chlorobenzene, dichlorobenzene, chloroform, toluene, xylene, dioxane, dimethylsulfoxide, tetrahydrofuran, amide solvents and mixtures thereof.

28. The method according to any of claims 17 to 27, wherein said removal of solvent is carried out by solvent exchange with at least one solvent having lower surface tension than the initial solvent.

29. The method according to claim 28, wherein said solvent exchange is carried out using acetone, followed by C₃-Ci₀ hydrocarbon, siloxane or fluorinated C₃-Ci₀ hydrocarbon.

30. The method according to any of claims 17 to 29, wherein said removal of solvent is carried out by a technique selected from the group consisting of supercritical drying, lyophilisation, room temperature and ambient pressure drying, to form an aerogel.

31. The method according to any of claims 17 to 30, wherein said graphene or graphene oxide are present in the solvent at a concentration of between 0.01-30 vol.%, preferably 0.1-20 vol.%, preferably 1-5 vol.%.

32. The method according to any of claims 14 to 16, wherein said aerogel or xerogel is prepared by providing graphene and carboxymethylcellulose in water, and freeze drying.

33. The method according to any of claims 14 to 16, wherein said aerogel or xerogel is prepared by providing oxidised graphene or graphene and carboxymethylcellulose in water, and freeze drying.

34. The method according to any of claims 14 to 16, wherein said aerogel or xerogel is prepared by providing oxidised cross-linked graphene and carboxymethylcellulose in water, and freeze drying.

35. The method according to any of claims 14 to 16, wherein said aerogel or xerogel is prepared by providing oxidised cross-linked graphene in water, and freeze drying.

36. The method according to any of claims 14 to 35, further comprising the step of heating the aerogel or xerogel.

37. The method according to claim 36, further comprising the step of measuring the resistance of the aerogel or xerogel to provide an indication of temperature.

38. The method according to claim 36 or 37, wherein said heating is achieved by connecting metallic electrodes to the aerogel or xerogel

39. The method according to any of claims 14 to 38, further comprising the step of depositing one or more additional materials onto the aerogel, or xerogel.

40. The method according to claim 39, wherein said additional material comprises a catalytic material.

41. The method according to any of claims 14 to 40 further comprising an additional heating step which is carried out by cycling between two or more temperatures for the adsorption/desorption of species.

42. The method according to any of claims 14 to 41 further comprising a system for the fractionation of adsorbed species by adsorbate/adsorbent interaction using an accurate temperature control by varying the electrical current passing through the graphene or graphene oxide networks which are selected from aerogels and xerogels,

43. A catalytic system comprising a catalyst and a material according to any of claims 1 to 13.

44. A gas adsorption medium comprising a material according to any of claims 1 to 13.

45. The use of the material according to any of claims 1 to 13 for gas sorption, capture and/or storage, preferably for C0₂ sorption, capture and/or storage.

46. A fluid heater comprising an aerogel or xerogel according to any of claims 1 to 13.

47. An electrically-regenerable filter or sorbent comprising an aerogel or xerogel according to any of claims 1 to 13.

48. A composition comprising a material according to any of claims 1 to 13, further comprising at least one adjunct material, preferably a support material.

49. A material selected from graphene aerogel comprising a layered double hydroxide (LDH), graphene oxide aerogel comprising LDH, graphene xerogel comprising LDH, graphene oxide xerogel comprising LDH, and mixtures thereof, obtainable by a method according to any of claims 14 to 42.

50. A method of heating an aerogel or xerogel according to any of claims 1 to 13, comprising the steps of: a) providing a graphene aerogel, graphene oxide aerogel, graphene xerogel, graphene oxide xerogel, or a mixture of two or more thereof, according to any of claims 1 to

13; and b) applying an electrical current thereto.

51. The method according to claim 50, wherein said electrical current is provided by applying a current of up to 1 A, preferably between 3 and 500 mA, more preferably between 5 and 100 mA, more preferably between 6 and 18 mA.

52. The method according to claim 50 or claim 51, wherein the applied voltage is up to 240 V, preferably between 0.5 and 150 V, more preferably between 5 and 100 V, more preferably between 10 and 30 V.

53. The method according to any of claims 50 to 52, wherein said electrical current is applied to achieve a current density of up to 500 A/cm², preferably between 0.1 mA/cm² and 100 A/cm², more preferably between 1 mA/cm² and 100 mA/cm², more preferably between 5 mA/cm² and 20 mA/cm².

54. The method according to any of claims 50 to 53, wherein said electrical current is applied to achieve an electric field of up to 100 V/cm, preferably between 0.001 and 20 V/cm, more preferably between 0.005 and 10 V/cm, more preferably between 0.1 and 1 V/cm.

55. The method according to any of claims 50 to 54, wherein said graphene aerogel, graphene oxide aerogel, graphene xerogel, graphene oxide xerogel, or a mixture of two or more thereof, may be heated in an inert atmosphere to a temperature of up to 3000 °C, preferably between 100 to 1000 °C, more preferably between 200 and 500 °C.

56. The method according to any of claims 50 to 55, wherein said graphene aerogel, graphene oxide aerogel, graphene xerogel, graphene oxide xerogel, or a mixture of two or more thereof, may be heated in air to a temperature of up to 600 °C, preferably between 100 and 550 °C, more preferably between 150 and 500 °C, even more preferably between 200 and 450 °C.