WO2021118459A1

WO2021118459A1 - Porous composites, scaffolds, foams, methods of fabrication and uses thereof

Info

Publication number: WO2021118459A1
Application number: PCT/SG2020/050718
Authority: WO
Inventors: Jun Ding; Xi Xu; Cao Guan
Original assignee: National University Of Singapore
Priority date: 2019-12-12
Filing date: 2020-12-04
Publication date: 2021-06-17

Abstract

The present invention relates, in general terms, to a method of fabricating a porous graphene composite and foam comprising forming a sacrificial scaffold from a ceramic slurry, depositing graphene on the scaffold using chemical vapour deposition and removing the scaffold. The present invention also relates to a porous graphene composite and foam, ceramic scaffold and its applications thereof.

Description

POROUS COMPOSITES, SCAFFOLDS, FOAMS, METHODS OF FABRICATION AND USES THEREOF

Technical Field

The present invention relates, in general terms, to a method of fabricating a porous ceramic composite and scaffold, a method of fabricating a porous graphene composite and foam. The present invention also relates to a porous ceramic composite and scaffold, a porous graphene composite and foam, and applications thereof.

Background

The introduction of graphene since it was first prepared in 2004 heralded a new epoch in research. The fascinating properties of graphene, such as large surface area, high thermal conductivity, and enhanced electrical conductivity, have led to applications in many engineering-, energy-, and environmental-related areas.

However, it is found that graphene cannot be easily incorporated "as is" into materials while still retaining its desired properties. To this end, methods have been developed to facilitate the incorporation of graphene. However, much improvement is still required. For example, incorporating a binder into the fabrication of a 3D graphene architecture may significantly block the active surface area and cause degradation of electrical behaviour. Further, flat 2D graphene sheets tend to agglomerate due to their strong n-n interlayer reactions, which prevents the realization of high specific area and fast carrier mobility. As a work around, it is thought that agglomeration can be avoided by combining individual graphene sheets into an object of macroscopic scale. However, it was found that the mechanical properties of 2D graphene do not translate easily into a robust 3D graphene assembly. Current 3D graphene assemblies, fabricated by solution-based gelation of graphene oxide sheets followed by a chemical reduction process or pyrolysis routes (template- free or template-assisted), are restricted to laboratory scale. Moreover, for reassembly of exfoliated graphene oxide sheets, preset laminar order and inferior quality of end products restrict its practical applications to a certain degree.

Accordingly, there is a demand for new processes of fabricating 3D structures that can preserve graphene's innate properties and achieve the promised functionalities.

To form a 3D structure, the choice of sacrificial template determines the interconnected graphene network and macroscopic architecture. Existing templates range from metallic foam/foil/powder products to various salts and oxides in the form of powders and aerogels. However, for foam/foil and powder templates having relatively large pore size in the range of hundreds of micrometres, as-fabricated graphene monoliths are not given enough bonding density unless enhanced by a polymer matrix. Efforts to fabricate templates with a finer interconnected network have resulted in the development of nanoporous templates (seashell, aerogel). However, classical acid-catalysed sol-gel processes generally produce a predetermined microstructure. Also, aerogels such as silica aerogels usually contain many unreacted hydrophilic silanol (Si-OH) groups, which can cause volume changes. Issues of shrinkage and cracking lead to difficulties in the use of large-volume or high-aspect-ratio templates. Despite advances made previously, several challenges still remained unsolved.

It would be desirable to overcome or ameliorate at least one of the above-described problems, or at least to provide a useful alternative. Summary

The present invention relates to a method of fabricating a geometrically-complex porous three dimensional free-standing graphene foam via a ceramic precursor. The method of forming the graphene foam can include the use of an addictive manufacturing technology. The present invention also relates to a method of fabricating a graphene composite and foam from a ceramic scaffold. The graphene materials can be used for various energy and environmental applications.

The present invention provides a method of fabricating a porous graphene foam, comprising: a) forming a printed structure from a ceramic slurry using an additive manufacturing technique; b) at least partially debinding the printed structure to form a calcined structure; c) at least partially sintering the calcined structure to form a porous ceramic scaffold; d) depositing graphene on the porous ceramic scaffold using chemical vapour deposition; and e) at least partially etching the porous ceramic scaffold in order to form the graphene foam; wherein the ceramic slurry comprises a ceramic with a ceramic loading of about 60 wt% to about 75 wt%.

Advantageously, the ceramic loading of the ceramic slurry allows for inter-particulate interactions such that during the steps of debinding and sintering, the pore size distribution of the ceramic composite can be controlled. When further deposited with graphene, this allows for the properties of the graphene composite and graphene foam to be fine tuned. The additive manufacturing technique allows for complex shapes to be created. In some embodiments, the scaffold forming step (step (a)) is selected from robocasting, stereolithography (SLA), digital light processing (DLP) or materials ink jetting.

In some embodiments, the ceramic slurry comprises a ceramic selected from silicon dioxide (silica or SiO₂), aluminium oxide (AI₂O₃), zirconium dioxide (ZrO₂), ZnO, Fe₂O₃, Fe₃O₄, MFe₂O₄ (where M = Ni, Co, Mn, Cu), MFe₁₂O₁₉ (where M = Sr, Ba), titanates (TiO₂, BaTiO₃, ZrTiO₃), nitrides (Si₃N₄), carbides (SiC), or a combination thereof. In some embodiments, the ceramic slurry comprises a porous ceramic.

In some embodiments, the ceramic is ceramic particles having an average particle size of about 1 pm to about 5 pm.

In some embodiments, the ceramic slurry further comprises a quaternary ammonium compound. In some embodiments, the quaternary ammonium compound has a loading of about 10 wt% to about 15 wt%.

In some embodiments, the ceramic slurry further comprises at least a polymer monomer selected from ethoxylate 1,6-hexanediol diacrylate (E-HDDA), ethoxylated trimethylolpropane triacrylate (E-TMPTA), ethoxylate pentaerythritol tetraacrylate, 1,6- hexanediol diacrylate, 1,1,1-trimethylolpropane triacrylate, or a combination thereof.

In some embodiments, a volume ratio of a diacrylate monomer to a triacrylate monomer or a tetraacrylate monomer is about 20:4 to about 30:3.

In some embodiments, the ceramic slurry comprises a mixture of polymer monomers selected from E-HDDA and E-TMPTA, E-REGA and HD DA, and TMPTA and HDDA.

In some embodiments, the polymer monomer or mixtures thereof is present at about 25 wt% to about 40 wt% of the ceramic slurry.

In some embodiments, the ceramic slurry has a viscosity of about 5000 cP (5 Pa·s) to about 1000 cP (1 Pa·s). In some embodiments, the step of debinding the scaffold (step (b)) is performed at a temperature of about 100 °C to about 600 °C.

In some embodiments, the step of debinding the printed structure (step (b)) further comprises holding or maintain the temperature for a certain length of time at about 200 °C, about 400 °C, about 500 °C, or a combination thereof.

In some embodiments, the step of sintering the calcined scaffold (step (c)) is performed under at temperature of about 1100 °C to about 1600 °C.

In some embodiments, the step of depositing graphene on the porous ceramic scaffold (step (d)) is performed at a flow rate of about 10 standard cubic centimeters per minute (seem) to about 20 seem. In some embodiments, the step of etching the porous ceramic scaffold (step (e)) is performed in a chemical bath. In some embodiments, the step of etching the porous ceramic scaffold (step (e)) is performed in HF solution, sulfuric acid, nitric acid hydrochloric acid, or a combination thereof.

In some embodiments, the method further comprises a step of functionalising the graphene foam (step (f)).

In some embodiments, the graphene foam is functionalised with transition metal oxide, zinc, NiS, or a combination thereof.

The present invention also provide a porous graphene foam comprising an interconnected bicontinuous network of graphene and air pockets, having a ceramic content of less than about 5 atomic percentage (at. %).

In some embodiments, the graphene foam has an oxygen content of less than about 2 at. %.

In some embodiments, the graphene foam has a density of about 18 mg/cm³.

In some embodiments, the graphene foam has a porosity of more than about 99%.

In some embodiments, the graphene foam has an I_2D/I_G ratio of about 0.5. In some embodiments, the graphene foam has a specific surface area of about 990 m²/g to about 1000 m²/g.

In some embodiments, the graphene foam has a Young's modulus of about 200 kPa to about 300 kPa.

In some embodiments, the graphene foam has a conductivity of about 2 Scm ^-1 to about 3 Scm ^-1 at room temperature.

In some embodiments, the graphene foam has a resistance change (AR/Ro) of about 90% at a compression strain of 25%.

The present invention also provides an electrode, actuator or heat absorbing material comprising the graphene foam as disclosed herein. Brief description of the drawings

Embodiments of the present invention will now be described, by way of non-limiting example, with reference to the drawings in which:

Figures la-c illustrate design consideration of templates: (a) macroscopic porous foam template; (b) solid template with microscopic nanopores; (c) template with stratified pores; Figure Id shows multiple generic steps for fabricating 3D graphene assembly of the present invention and the corresponding scanning electron microscopic (SEM), micro- CT scanning, transmission electron microscopic (TEM) images;

Figure le shows an embodiment of the method for fabricating graphene foam; Figure 2 shows optical images of (a) printed, (b) sintered, (c) CVD-grown silica, (d) 3D freestanding graphene; (e-g) SEM images of (e) as-printed silica, (f) sintered silica, (g) CVD-grown, (h) 3D freestanding graphene gyroid slab; (i) micro-CT scanning of CVD- grown silica to show the cross-sectional structures;

Figure 3 shows (a) an SEM image of the colloidal silica resin used; (b -c) sintering of the spin-coated resin and 3D printed fine structure with same feature size; (d) conventional casting and (e) 3D-printed template lead to different product features after wet etching;

Figure 4 shows (a) an SEM image of 3D graphene foam; (b) corresponding EDX image; (c) TEM images of 3D graphene foam; (d) XPS summary spectra of 3D graphene foam; (e) XRD pattern of 3D graphene foam; (f) comparison of Raman spectra between carbon-based materials and 3D graphene foam;

Figure 5 shows (a) BET surface area N2 adsorption-desorption isotherm plots measured over a silica template and 3D graphene foam; (b) compressive stress at 10%, 20%, and 40% and compressive strain plotted versus cycle number; (c) p versus E of the first cycle of 3D graphene foam compared to other comparators; (d) electrical conductivities of 3D graphene foam compared to other comparators; (e) resistance changes with bending strain exhibiting similar recovery behavior at 32° bending motion (structure: ribbon; length: 3 cm; width: 0.2 mm; thickness: 500 pm); (f) resistance changes with compression strain up to 60% compression (structure: gyroid slab; length: 2.7 cm; width: 1 cm; thickness: 0.4 cm);

Figure 6 shows a plot of strength against density of the graphene foam compared to comparators in the material property chart;

Figure 7 shows (a-f) SEM images of NiFe LDH/CC, NiFe LDH/GP, and NiFe LDH/GF; (g, h) TEM images of NiFe LDFI; (i-n) electrochemical performance test of samples for (i-k) OER; (l-n) HER;

Figure 8 shows (a) iR-corrected LSV curves of samples; (b) overpotential required to drive 10 mA/cm² for samples before and after iR correction;

Figure 9 shows Tafel slope of samples for OER reaction;

Figure 10 shows (a-c) Solar steam generation of as-fabricated samples: GF and treated GF. (d) Efficiency toward solar steam generation for all samples, (e) UV-vis spectra for light absorption of treated GF. (f) Water contact angle measurements of GF and treated GF. (g-i) Highresolution XPS spectra of (g, h) C Is and (i) O Is. (j) Absorption and removal of hexane: (I) mixture of red-stained hexane and blue-stained DI water, (II) dropping the as-fabricated graphene foam into the absorption mixture, (III) absorption of hexane, (IV) taking out the graphene foam, (V) burning away of hexane (inset: after the burning); and

Figure 11 shows mass specific current density for NiFeLDH/CC, NiFeLDH/GP, and NiFeLDH/GF at 400 mV;

Figure 12 shows photographic and SEM images of graphene foams with different structures;

Figure 13 shows images of a variety of silica sintered structures and three-dimensional free-standing graphene foam;

Figure 14 shows SEM images of three-dimensional free-standing graphene foam;

Figure 15 shows (a) pore distribution of the sacrificial scaffold (silica template) and three-dimensional graphene foam; (b) excellent flexibility demonstration; (c) EDX signal showing close to 100% C after chemical bath etching; and

Figure 16 shows SEM images of ceramic scaffolds formed in different conditions.

Detailed description This invention concerns method and compositions related to the fabrication of geometrically-complex and porous graphene composites and foams by various ceramic additive manufacturing and chemical vapour deposition, and their products thereof. The invention also relates to methods of fabricating porous ceramic composites and scaffolds and their products thereof.

The present invention is predicated on the understanding that in order to form a graphene foam with desirable properties, a sacrificial scaffold can be used as a template for the deposition or growth of graphene. The grownth of graphene can be controlled in a layer by layer fashion, such that aggregation of graphene is minimised. The removal of the sacrificial scaffold results in a graphene foam that substantially comprises graphene, and thus can allows the graphene foam to retain substantially its pure graphene properties.

The inventors have found a method of synthesizing porous ceramic scaffolds that can be used to form graphene composites, and hence graphene foams. This is predicated on the understanding that ceramic materials such as SiO₂ (silica) powder and its aerogel can be an appropriate template for graphene nucleation and growth. Further, compared to fused silica or quartz and glass, porous silica can be effectively removed under mild conditions. By synergistically (or at least additively) combining at least a silica templating method with 3D printing and graphene assembly approaches, using porous silica, for example, as a sacrificial scaffold can also significantly reduce processing time and complications.

As used herein, "slurry" refers to a semi-liquid mixture, and can comprise a dispersion of particles suspended in a liquid medium. The particles can be ceramic particles. The medium can be an aqueous solvent or mixture, for example water, or can be an organic solvent or mixture.

As used herein, "composite" refers to a material produced from two or more constituent materials with dissimilar chemical or physical properties that, when merged, create a material with properties, unlike the individual elements. The individual components remain separate and distinct within the finished structure, distinguishing composites from mixtures and solid solutions. In this regard, a ceramic composite comprises ceramic as one of the components. Graphene composite comprises graphene as one of the components. As used herein, "structure" refers to an object constructed using 3D printing. A graphene structure refers to a 3D printed object which comprises graphene.

As used herein, "green body" is an object whose main constituent is the material of the ceramic slurry. The green body can be held together by bonded ceramic and polymerized monomer residues before it has been calcined and/or sintered. The green body can optionally further comprise other organic or inorganic additives, Additives can be solvents, dispersants (deflocculants), binders, plasticizers, lubricants, or wetting agents.

As used herein, "scaffold" refers to a framework that can be used to support the deposition or growth of other entities. For example, a ceramic scaffold can be used to provide a surface for growth and/or deposition of graphene. In this regard, the scaffold is used as a template for graphene deposition and the resultant graphene composite substantially conforms to the structure of the scaffold.

As used herein, "foam" refers to object formed by trapping pockets of gas in a solid structure. Solid foams can be closed-cell or open-cell. In closed-cell foam, the gas forms discrete pockets, each completely surrounded by the solid material. In open-cell foam, gas pockets (or pores) connect to each other (i.e. bicontinuous). A graphene foam is thus a 3D printed object which comprises graphene and gas pockets.

To tie these terminologies together, an embodiment of the present invention is briefly discussed (Figure le). A ceramic slurry can be printed 110 into a green body which is a ceramic composite using a 3D printing method. The green body has a printed 3D structure. The printed structure can be debinded (step 120) and sintered (step 130) to form a porous ceramic scaffold, the process of which can cause only the ceramic to be retained, albeit in a different physical form. The ceramic scaffold can then be coated with graphene to form a graphene composite (step 140), following which by removing the ceramic component via etching, a graphene foam is formed (step 150).

Accordingly, in a first aspect, the present invention provides a 3D-printed sacrificial template for forming a bicontinuous porous 3D graphene foam. The method of fabricating a porous ceramic scaffold comprises: a) forming a printed structure from a ceramic slurry using an additive manufacturing technique; b) at least partially debinding the printed structure to form a calcined structure; and c) at least partially sintering the calcined structure to form the porous ceramic scaffold; wherein the ceramic slurry comprises a ceramic with a ceramic loading of about 60 wt% to about 75 wt%. When the printed structure is completely or substantially debinded to form a calcined structure, and the calcined structure is completely or substantially sintered, the porous ceramic scaffold comprises only or substantially ceramics.

Previous works on 3D printing of carbon-based materials uses a direct ink writing technique due to its wide range of materials selection. For example, while digital light processing (DLP) can print out structures with a smooth surface finish and higher resolution (tens of micrometers), it cannot work for making 3D graphene foam. This is due to the limitations of the starting precursor, of which the inventors have overcome.

Advantageously, the ceramic loading of the ceramic slurry allows for inter-particulate interactions such that during the steps of debinding and sintering, the pore size distribution of the ceramic composite can be controlled. When further deposited with graphene, this allows for the properties of the graphene composite and hence the graphene foam to be fine-tuned. The additive manufacturing technique allows for complex shapes to be created.

Further advantageously, by using an additive manufacturing technique, the need for having the ceramic precursor in a dry powder form is avoided. This removes (or at least minimizes) the problems of aggregation and/or agglomeration which can result from the spray drying step, and which can cause defects due to the particle size distribution. Further, as the ceramic slurry is maintained in a liquid form which helps disperse the powder more uniformly, a wider range of binders and/or polymers can be used.

In some embodiments, the printed structure forming step (step (a)) is selected from robocasting, stereolithography (SLA), digital light processing (DLP) or materials ink jetting. In other embodiments, digital light processing (DLP) is selected. Digital Light Processing (DLP) is a process in additive manufacturing, in which a vat of liquid polymer or slurry is exposed to light from a DLP projector. The DLP projector displays the image of the 3D model onto the liquid polymer in slices or layers. The exposed liquid polymer or slurry hardens and the build plate moves down and the liquid polymer is once more exposed to light. The process is repeated until the 3D model is complete and the vat is drained of liquid to give the solidified 3D model. In the context of the present invention, the solidified 3D model is the printed structure.

In some embodiments, the printed structure and/or ceramic slurry comprises silicon dioxide (silica or SiO₂), aluminium oxide (AI₂O₃), zirconium dioxide (ZrO₂), or a combination thereof. In some embodiments, the porous ceramic scaffold comprises various oxide and non-oxide ceramic materials selected from silica, alumina, zirconia, rare-earth oxide, metal oxides (ZnO, Fe₂O₃, Fe3O₄, MFe₂O₄ where M = Ni, Co, Mn, Cu, MFe₁₂O₁₉ where M = Sr, Ba), titanates (TiO₂, BaTiO₃, ZrTiO₃), nitrides (Si₃N₄), carbides (SiC), or a combination thereof.

While the ceramic can be of a porous or a non-porous nature, the inventors have found that porous ceramics are further advantageous in that the porosity in porous ceramics can translate to an increased porosity in the ceramic composite, which not only helps the growth of graphene, but also promotes the chemical etching rate. In some embodiments, the ceramic slurry comprises ceramic and other additives. The ceramic can be in the form of ceramic particles.

In some embodiments, the ceramic particles have an average particle size of about 1 pm to about 5 pm. In other embodiments, about 80% of the ceramic particles fall within this size range. It was further found that a smaller average particle size can be further advantageous as it can cause aggregation and can be beneficial for slurry stability.

In some embodiments, the printed structure and/or the ceramic slurry has a ceramic loading of about 60 wt% to about 75 wt%. In some embodiments, when the ceramic is silica, the printed structure and/or the ceramic slurry has a silica loading of about 60 wt% to about 75 wt%. In some embodiments, the silica loading is about 60 wt% to about 70 wt%, about 62 wt% to about 70 wt%, about 64 wt% to about 70 wt%, or about 66 wt% to about 70 wt%. In some embodiments, the printed structure and/or the ceramic slurry has a silica loading of about 60 wt%, about 62 wt%, about 64 wt%, about 66 wt%, about 68 wt%, about 70 wt%, about 72 wt%, or about 75 wt%.

Advantageously, the high loading of ceramics provides for a scaffold which is porous after the sintering step. It also provides a platform for graphene to assemble, grow and/or deposit.

In some embodiments, the printed structure and/or the ceramic slurry further comprises an additive. For example, a quaternary ammonium compound can be added as an additive. The additive can act as an emulsifier or dispersant for the ceramic. An example of an additive is VARIQUAT® CC 42 NS, which is a polypropoxy quaternary ammonium chloride compound.

In some embodiments, the additive has a loading of about 10 wt% to about 15 wt%. In some embodiments, when DLP is used, an acrylate-based resin is mixed with filler materials and is polymerized via photopolymerization in a layer by layer manner.

In some embodiments, ceramic slurry comprises polymer monomers. In some embodiments, the ceramic slurry comprises at least a polymer monomer. The polymer monomer can consist of hydroxyl moieties. The polymer monomer can also consist or acrylate moieties. For example, ethoxylate 1,6-hexanediol diacrylate (E-FIDDA), ethoxylated trimethylolpropane triacrylate (E-TMPTA), ethoxylate pentaerythritol tetraacrylate, 1,6-hexanediol diacrylate, 1,1,1-trimethylolpropane triacrylate, or a combination thereof can be used. A combination of polyacrylate and diacrylate can be used to control the rate of free radical polymerisation under UV light activation, and can be used to reduce viscosity and to improve curing properties. For example, the volume ratio of a diacrylate monomer to a polyacrylate monomer can be about 20:4 to about 30:3. The volume ratio of a triacrylate monomer to a polyacrylate monomer can be about 20:4 to about 30:3. In some embodiments, the volume ratio of a diacrylate monomer to a triacrylate monomer or a tetraacrylate monomer is about 20:4 to about 30:3. In some embodiments, a volume ratio of E-HDDA and E-TMPTA is about 22:3. In some embodiments, a volume ratio of ethoxylate pentaerythritol tetraacrylate (E-PETA) to 1,6-hexanediol diacrylate (HDDA) is about 3:24. In some embodiments, a volume ratio of trimethylolpropane triacrylate (TMPTA) to 1,6-hexanediol diacrylate (HDDA) is about 4:23.

Further advantageously, it was found that a higher ratio of the diacrylate to the polyacrylate (triacrylate or tetraacrylate) helps to reduce the viscosity of the slurry. In some embodiments, diacrylate is about 6 to 10 times more (based on volume %) compared to the polyacrylate (triacrylate or tetraacrylate), or about 7 times more.

In some embodiments, the polymer monomer is present at about 25 wt% to about 40 wt% of the ceramic slurry.

The printed structure is formed from a ceramic slurry using an additive manufacturing technique. For example, under UV irradiation, polymer monomers in the ceramic slurry can be polymerised and/or cross-linked to form polymerised monomer residues. The polymerised monomer residues form a polymer matrix which provides the ceramic composite with a printed structure.

In some embodiments, the printed structure and/or the ceramic slurry comprises a photoinitiator. The photoinitiator can be diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO). Other Type 1 photoinitiators (for example hydroxyacetophenone (HAP)) can be used. Type 1 photoinitiators are characterized by a cleavage reaction into two radical fragments of the original photoinitiator. The irradiation with UV-light leads to a homolytic bondage cleavage and generation of two highly reactive radical species. The photoinitiator can be added at a loading of about 0.2 wt% to about 3 wt%. Alternatively, the photoinitiator can be added at about 0.2 wt% to about 2.5 wt%, about 0.2 wt% to about 2 wt%, about 0.5 wt% to about 2 wt%, or about 1 wt% to about 2 wt%.

In some embodiments, the printed structure and/or the ceramic slurry further comprises a binder. A binder or binding agent is a material or substance that holds or draws other materials together to form a cohesive whole mechanically, chemically, by adhesion or cohesion. For example, binders can be liquid or dough-like substances that harden by a chemical or physical process and bind fibres, filler powder and other particles mixed with it. In the context of the present invention, the binder can act to improve the dispersibility of the ceramic particles in the ceramic slurry. In some embodiments, the polymer monomer as disclosed above can function as a binder. In other embodiments, the binder is selected from epoxy, polyester and phenolic polymers. Other UV-curable systems can also be applicable in this invention.

Further advantageously, the binder can further increase the viscosity of the ceramic slurry such that it is suitable for use in additive manufacturing. The increase in viscosity also ensures that the ceramics is homogenously dispersed over an extended period of time; i.e. a concentration gradient is not formed due to the settling of the ceramics, which has a higher density.

In some embodiments, the binder is added at a loading of about 0.1 wt% to about 35 wt% .

In some embodiments, the ceramic slurry has a viscosity of about 5000 cP (5 Pa·s), about 4000 cP, about 3000 cP, about 2000 cP, about 1000 cP, about 800 cP, about 600 cP, or about 400 cP. In some embodiments, the ceramic slurry has a viscosity of about 5000 cP (5 Pa'S) to about 1000 cP (1 Pa·s), about 4000 cP to about 1000 cP, about 3000 cP to about 1000 cP, or about 2000 cP to about 1000 cP.

It was found that the combination of components as disclosed above provides for a slurry that is particularly advantageous for use in additive manufacturing. In this regard, the slurry has a viscosity that is easy to handle, yet is suitable for use in additive manufacturing. In this regard, the scaffold can be printed accurately and precisely without defects such as bumps forming, which can be due to a "run-away" radical polymerisation reaction when activated by UV laser. When layers of the structure is printed and pulled out from the slurry bath, the slurry was also found not to stick or adhere to the printed structure, allowing for less wastage.

In some embodiments, when DLP is used, the printed structure is printed with a printing layer thickness of about 100 pm. In other embodiments, the layer thickness is about 90 pm, about 80 pm, about 70 pm, about 60 pm, or about 50 pm. As used herein, 'debinding' refers to calcining. Calcination is a thermal treatment of solids which aims to heat the material at a high temperature in the absence of air or oxygen (or at least under no gas flow). The purpose of calcination may also be to eliminate undesirable impurities or carbon based material. Accordingly, the debinding step functions to remove undesirable organic components or impurities from the scaffold, for example by decomposing the organic components. Alternatively, the phase of the material may be changed; for example a ceramic hydroxide can be calcined to a ceramic oxide.

The printed structure is at least partially debinded. In this regard, the polymerized monomer residues formed from the polymer monomers are at least partially removed. The removal can be by a decomposition process, in which the polymer is burnt off. In some embodiments, at least about 50% of the polymerized monomer residues present in the printed scaffold is removed. In other embodiments, at least about 60%, about 70%, about 80%, about 90%, about 92%, about 94%, about 96%, about 98%, or about 99% is removed. If the printed structure is completely debinded, 100% of the polymerized monomer residues is removed. Other components present in the ceramic slurry, such as the photoinitiator, binder and additive can also be removed in this step.

In some embodiments, the step of debinding the printed structure (step (b)) is performed at a temperature of about 100 °C to about 600 °C. In some embodiments, the step of debinding the printed structure (step (b)) is performed at a temperature of about 200 °C, about 400 °C, or about 500 °C. In some embodiments, the step of debinding the printed structure (step (b)) further comprises holding or maintain the temperature for a certain length of time (dwelling temperature step) at about 200 °C, about 400 °C, about 500 °C, or a combination thereof. The holding temperature can be for about 10 min to about 5 h.

Further advantageously, it was found that the dwelling steps allows for a more complete calcination process. By gradually increasing the temperature and holding the temperature at these specific points, specific carbon bonds can be more efficiently broken without excessive formation of soot.

In some embodiments, the step of debinding the printed structure (step (b)) is performed under an absence of oxygen.

Following the steps of the above disclosed method, the ceramic composite that forms the printed structure is converted from a dense solid to a porous structure with substantially only ceramics after calcination. As used herein, 'sintering' is a process of compacting material using pressure and/or relatively high temperature which is usually below the melting temperature of the material. The atoms in the materials diffuse across the boundaries of the particles, starting from the formation of necks between particles to final elimination of small pores at the end of the process, thus fusing the particles together and creating one solid piece. The driving force for densification is the change in free energy from the decrease in surface area and lowering of the surface free energy by the replacement of solid-vapor interfaces. It forms new but lower-energy solid-solid interfaces with a total decrease in free energy occurrence. On a microscopic scale, material transfer is affected by the change in pressure and differences in free energy across the curved surface. If the size of the particle is small (and its curvature is high), these effects become very large in magnitude. The change in energy is much higher when the radius of curvature is less than a few micrometres. In this regard, sintering generally refers to densification, where a green body is heated to produce a dense, monolithic component. Further advantageously, the sintering step strengthens the calcined structure, such that the ceramic scaffold is easier to manipulate for graphene assembly, growth and/or deposition.

The calcined structure is at least partially sintered. In this regard, the ceramic particles in the calcined structure are at least partially fused together. In this step, the shape of the particles, due to the high temperature, changes as atoms diffuse across the boundaries of the particles and form necks between particles. When the sintering step is taken further, small pores can be eliminated, thus improving the porosity and pore size distribution of the end product. In some embodiments, at least about 50% of the ceramic present in the calcined structure is sintered. In other embodiments, at least about 60%, about 70%, about 80%, about 90%, about 92%, about 94%, about 96%, about 98%, or about 99% is sintered.

Grain-boundary diffusion and volume diffusion (and hence the sintering process) rely upon factors such as temperature, the size and distribution of particles of the material, the materials composition, and often the sintering environment to be controlled.

Accordingly, control of temperature can influence the porosity of the calcined structure. In some embodiments, the step of sintering the calcined structure (step (c)) is performed at temperature of about 1100 °C to about 1600 °C. In some embodiments, the step of sintering the calcined structure (step (c)) is performed with temperature steps, the temperature steps selected at a temperature of about 1250 °C, about 1350 °C or about 1450 °C. In some embodiments, the step of sintering the calcined structure (step (c)) is performed and for a duration of about 4 h to about 12 h. In other embodiments, the duration is about 5 h to about 12 h, about 6 h to about 12 h, about 7 h to about 12 h, or about 8 h to about 12 h. In some embodiments, the step of sintering the calcined structure (step (c)) is performed under a temperature ramping rate at about 1 °C/min. In other embodiments, the ramping rate is about 1.5 °C/min, about 2 °C/min, about 2.5 °C/min, or about 5 °C/min. Figure 16 shows SEM images of ceramic scaffold (silica) under different sintering condition: (a) dwelling time: 5 hours, sintering temperature: 1250 ^°C ; (b) dwelling time: 5 hours, sintering temperature: 1350 ^°C ; (c) dwelling time: 5 hours, sintering temperature: 1450 ^°C (Inserted picture on the bottom right showed sintering cracks under this condition); (d) dwelling time: 10 hours, sintering temperature: 1350 ^°C . (Inserted picture on the bottom left showed uniform sintering under this condition).

In some embodiments, the step of sintering the calcined structure (step (c)) is performed under atmospheric pressure. This is also known as "pressureless sintering". In other embodiments, a second and/or third external force is used during the sintering process. For example, a pressure can further be applied, or an electrical current can be applied.

In some embodiments, a ceramic slurry or ceramic paste precursors was developed for fabrication of geometrically-complex and porous metal oxide ceramic materials by ceramic digital light processing/DLP (a subset of additive manufacturing/3D printing technologies). The ceramic paste precursor can further be combined with other components. Such ceramic slurry or ceramic paste precursors are UV curable and printable. As used herein, the ceramic paste precursor can have a viscosity of about 2,000 cps to about 20,000 cps, preferably within a range from 2,000 to 10,000 cps as measured at 22°C. The ceramic slurry can have a viscosity of about 500 cps to about 2,000 cps. According to some examples, the present disclosure describes various silicon dioxide paste precursors that are able to be printed out with designed structures.

In some embodiments, porous ceramic scaffold is obtained after debinding and sintering. The ceramic materials can be used for the methods of this invention including but not limited to silicon dioxide (Silica or SiO₂), aluminium oxide (AI₂O₃), zirconium dioxide (ZrO₂), or a combination thereof. These oxides can also be removed by chemical etching.

The present invention also provide a printed structure for forming a porous ceramic scaffold as disclosed herein, comprising: a) a ceramic with a ceramic loading of about 60 wt% to about 75 wt%; and b) polymerised monomer residues, the polymer monomer selected from ethoxylate 1,6-hexanediol diacrylate (E-HDDA), ethoxylated trimethylolpropane triacrylate (E- TMPTA), ethoxylate pentaerythritol tetraacrylate, 1,6-hexanediol diacrylate, 1,1,1- trimethylolpropane triacrylate, or a combination thereof.

In some embodiments, the printed structure further comprises an additive. For example, the additive can be a quaternary ammonium compound. The additive can be added at a loading of about 10 wt% to about 15 wt%.

Monomers are the building blocks of a polymer chain, and polymers are commonly named according to the type of the constituent monomer residues or repeating units. For example, the generic name for cellulose is poly-(l,4-β-d-glucose), based on the fact that it is derived from d-glucose units linked through β (1→4)-glycosidic bonds.

In some embodiments, the printed structure further comprises polymerised monomer residues. The polymerised monomer residues is formed from the polymer monomers of the ceramic slurry as disclosed herein and can, for example, be an acrylate polymer. The polymer can be formed in the presence of a photoinitiator under UV light. The polymer can be at a loading of about 25 wt% to about 40 wt%.

In some embodiments, the polymer has a mole ratio of diacrylate to (triacrylate or tetracrylate) of about 6: 1 to about 9: 1, or about 7: 1. The polymer can comprise E- HDDA and E-TMPTA, 1,6-hexanediol diacrylate (FIDDA) and ethoxylate pentaerythritol tetraacrylate (E-PETA), or 1,6-hexanediol diacrylate (HDDA) and trimethylolpropane triacrylate (TMPTA).

In some embodiments, the printed structure further comprises a binder. The binder can be at a loading of about 1 wt% to about 10 wt%.

The present invention also provides a porous ceramic scaffold formed from the method as disclosed herein.

The porous ceramic scaffold can comprise an interconnected bicontinuous network of ceramic, having a ceramic loading of more than about 60 wt%. The porous ceramic scaffold can have a porosity of more than about 90%. The skilled person would understand that a ceramic loading of more than about 60 wt% is a result of the debinding step in which the polymerized monomer residues are removed, and the porosity of more than about 90% is a result of the sintering step in which the ceramic particles fuse together.

The porous ceramic scaffold can have a porosity of more than about 90%, about 95%, about 96%, about 97%, about 98%, or about 99%. Alternatively, the porosity can be more than about 99.1%, about 99.2%, about 99.3%, about 99.4%, or about 99.5%. The porous ceramic scaffold can have a BET surface area of about 20 m²/g to about 50 m²/g. Alternatively, the BET surface area can be about 20 m²/g to about 40 m²/g, or about 20 m²/g, about 25 m²/g, about 30 m²/g, about 35 m²/g, or about 40 m²/g.

In some embodiments, the porous ceramic scaffold has a pore radius of less than about 1.5 pm. In other embodiments, the pore radius is less than about 1.4 pm, about 1.3 pm, about 1.2 pm, about 1.1 pm or about 1 pm.

In order to form a graphene composite and graphene foam from the porous ceramic scaffold, chemical vapour deposition (CVD) is advantageous over other techniques as it allows for growth of a highly crystalline 2D graphene film and can directly form a covalently bonded 3D framework onto the ceramic scaffold. CVD can produce single- or few-layer graphene with centimetre scale. However, the gradient distribution of precursor concentration and temperature along the gas flow and heating source can influence the uniformity and quality of graphene grown on the substrate, thus hampering the scalability of end products and yields. This is especially so when graphene is grown on a bulky scaffold. Accordingly, the production capacity is reduced due to the tedious removal process, which can further increase the cost.

Particularly advantageously, the present invention can overcome the drawback of a lack of complexity in macroscopic design and/or lack of microscopic porosity optimization to increase the exposed surface area. Such graphene monoliths grown from macroscopic templates can later emerge as superior multifunctional supports for various applications. For example, the graphene monoliths can be used as catalyst support and/or current collector for multiple electrochemical energy storage devices. It was found that additive manufacturing can be used for the industrial production of graphene monoliths and also for customized design of graphene-based electronics, devices and filter membranes.

In another aspect, the present invention provides a method of fabricating a porous graphene composite, comprising: a) forming a ceramic scaffold as disclosed herein; and b) depositing graphene on the ceramic scaffold using chemical vapour deposition in order to form a porous graphene composite.

As the graphene composite is formed on the ceramic scaffold, the graphene composite exhibited a similar if not a same microstructure as the ceramic scaffold. In another aspect, the present invention provides a method of fabricating a porous graphene foam, comprising: a) forming a ceramic scaffold as disclosed herein; and b) depositing graphene on the ceramic scaffold using chemical vapour deposition to form a porous graphene composite; and c) at least partially etching the ceramic scaffold in order to form the graphene foam.

As the graphene foam is formed from the ceramic scaffold, the graphene foam exhibited a similar if not a same microstructure.

As mentioned above, the ceramic slurry can comprise a ceramic with a ceramic loading of about 60 wt% to about 75 wt%.

The porous ceramic scaffold is at least partially etched away to form the porous graphene foam. In some embodiments, the porous ceramic scaffold is at least about 50% etched. In other embodiments, at least about 60%, about 70%, about 80%, about 90%, about 92%, about 94%, about 96%, about 98%, or about 99% is etched.

In some embodiments, the step of depositing graphene on the porous ceramic scaffold (step (d)) is performed using CFU as a carbon precursor.

In some embodiments, the step of depositing graphene on the porous ceramic scaffold (step (d)) is performed at a flow rate of about 10 standard cubic centimeters per minute (seem) to about 20 seem.

In some embodiments, the step of depositing graphene on the porous ceramic scaffold (step (d)) is performed at a temperature of about 1100 °C.

In some embodiments, the step of depositing graphene on the porous ceramic scaffold (step (d)) is performed for about 1 h to about 2 h.

In some embodiments, the step of etching the porous ceramic scaffold (step (e)) is a wet etching step. In some embodiments, the step of etching the porous ceramic scaffold (step (e)) is performed in a chemical bath. In some embodiments, the step of etching the porous ceramic scaffold (step (e)) is performed using an aqueous HF solution. The HF solution can be a 15% H F solution.

In some embodiments, the step of etching the porous ceramic scaffold (step (e)) is performed for about 1 h, about 2 h, about 3 h, about 4 h, about 5 h, about 6h or about 7 h.

Further advantageously, the etching step at least partially removes the ceramic scaffold. For example, silica can be etched by H F, the rate of which can be determined by the concentration of H F. In doing so, the graphene deposited on the ceramic scaffold is unaffected, and when the ceramic scaffold is at least partially removed, a graphene foam results. Other etchants such as sulfuric acid, nitric acid and hydrochloric acid can also be used.

In some embodiments, the etching step completely (or to a substantial degree) removes the ceramic scaffold. The total removal of the ceramic can be confirmed using various characterization methods such as Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy.

In some embodiments, the method further comprises a step of washing the graphene composite and/or foam. The washing step removes the impurities such as etchant within the graphene composite and/or foam, and importantly, within the pores of the graphene composite and/or foam.

In some embodiments, the method further comprises a step of freeze drying the graphene composite and/or foam. The freeze drying step can be performed using liquid nitrogen. In some embodiments, the freeze drying step is performed for about 8 h to about 12 h.

In some embodiments, the graphene foam is functionalized with metal, oxide/hydroxide, sulfide, polymer, or inorganic/organic materials. For example, the graphene foam can be functionalised with transition metal oxide, zinc, and/or NiS. In some embodiments, the step of functionalising the graphene foam (step (f)) comprises treating the graphene foam with HNO3. The nitric acid can be in a concentration range of about 20% to about 40%. The graphene foam can be treated for about 15min at room temperature. In some embodiments, the method of fabrication of a three-dimensional graphene foam, comprises providing a three-dimensional ceramic porous scaffold, for fabricating a three-dimensional graphene foam, depositing a layer of graphene onto the ceramic scaffold by chemical vapor deposition, removing the three-dimensional ceramic scaffold via wet chemical etching and freeze-drying, such that the graphene foam retains the three-dimensional configuration.

In general, the method is based on the following strategies:

1) Fabrication of porous ceramic scaffold using ceramic additive manufacturing techniques (e.g., robocasting, stereolithography (SLA), digital light processing (DLP) or materials ink jetting).

2) Due to its intrinsic behaviour of ceramics, the as sintered porous ceramic scaffold can be used as a sacrificed template. Carbon source can be introduced via chemical vapour deposition (CVD) followed by removing the porous ceramics template in a chemical bath. Three-Dimensional free-standing graphene foam can be obtained. 3) The ceramic scaffold can be changed and tuned to obtain suitable structures or intrinsic behaviour. As such, various oxide and non-oxide ceramic materials can be chosen with a customized choice of the chemical bath.

4) The post-sintering coating or surface functionalization can be changed and tuned to obtained proper surface behaviour or surface functionality. Different inorganic, organic and hybrid coating can be employed for full cell water splitting, seawater desalination and so on.

The 3D graphene foam has a high surface area, excellent conductivity, superior mechanical properties, and can be custom designed for various applications. Optimal solid loading and appropriate rheology behavior were evaluated. The compatibility of the silica filler with the polymer resin was tested to ensure good layer adhesion, reasonable curing time, and stability during long-term printing. The optimal debinding process leading to a rational control over porosity was determined after sintering study. A class of complex structures that cannot be fabricated by conventional methods was demonstrated. 3D graphene foam formed from such a template followed the macroscopic design and was proven to facilitate better dissolution as compared to many state-of-the-art monolith templates. Four separate applications toward electrical (strain-induced resistance change), energy (overall water splitting), and environmental (seawater desalination/ steam generation and oil adsorption) related issues were investigated to demonstrate the wide-ranging possibilities of the invention.

The present invention can be varied in (but is not limited to) the following ways:

1) A standalone ceramic sintered body with either simple or complex geometry or with either porous or dense structures can be fabricated from ceramic additive manufacturing which is difficult to be obtained from conventional ceramic manufacturing techniques;

2) Porous ceramic materials refer to the presence of the micro-porosity within the ceramic body can be realized with ceramic additive manufacturing in combination with suitable ceramic slurry or paste precursors. Pore distribution can be further optimized by manipulating debinding and sintering process as compared to conventional solid state sintering of ceramics;

3) Various three-dimensional free-standing graphene foams with different geometries can be fabricated with chemical vapour deposition. The ceramic template mentioned in the embodiments is not limited only to silica, alumina, zirconia and other rare-earth oxide materials. Other oxide and non-oxide ceramics may include but not limited to functional electroceramics such as other metal oxides (ZnO, Fe203, Fe304, MFe204 where M = Ni, Co, Mn, Cu, MFe₁₂O₁₉ where M = Sr, Ba), titanates (TiO₂, BaTi03, ZrTi03), nitrides (Si₃N₄) and carbides (SiC). Chemical bath etching can be modified according to the choice of template materials. High resolution and finer feature size of printed structures ensure better etching processes and improved final products; and

4) To impart more functionality, various surface functionalization and post-etching processes to impart active species coating or different wetting behaviour can be selected. The active species coating can be chosen from a wide range of metal, oxide/hydroxide, sulfide, and polymer materials. Surface modification treatment may include high performance inorganic/organic materials. A hybrid inorganic/organic coating may be included as well. The invention is useful for making geometrically-complex and porous graphene foam structures using various ceramic additive manufacturing techniques for a wide range of applications that require conductive and freestanding carbon skeleton.

Figure 12 shows graphene foams with different structures. For example, the graphene foam can have a pillar and/or column structure. The column can have a void along its longitudinal axis. For example, graphene foam with porous gyroid cube, circular mesh, filter mesh, and solid cube shape can be easily fabricated following the steps. Additional functionality can be imparted onto the porous ceramic structures beyond its original intrinsic characteristic simply by allowing secondary or post-sintering chemical surface functionalization.

The present invention provides a graphene composite fabricated by the method as disclosed herein.

The present invention provides a graphene foam fabricated by the method as disclosed herein. The present invention also provides a graphene foam, comprising an interconnected bicontinuous network of graphene. The graphene foam also comprises gas and/or air pockets.

As used herein, a bicontinuous structure is a bicontinuous partitioning in which each sub-volume is filled with a distinct, but not necessarily uniform composition or state of matter. For example, one sub-volume can be a solid or semi-solid, for example sandstone or sponge or foam. An interspersion of two phases is bicontinuous only if each phase is connected across the specimen. The interconnected bicontinuous network can be beneficial to improve the conductivity and therefore can promote the performance towards energy related application.

When the graphene composite is substantially or completely etched to remove the ceramic scaffold, only graphene remains as the end product. The graphene is formed as a foam and can have properties as discussed herein. Towards this end, in some embodiments, the graphene foam has a ceramic content of less than about 5 atomic percentage (at. %), less than about 4 at.%, less than about 3 at.%, less than about 2 at.%, or less than about 1 at.%.

The graphene foam can have a density of about 18 mg/cm³.

The graphene foam can have a porosity of more than about 90%, about 95%, about 96%, about 97%, about 98%, or about 99%. Alternatively, the porosity can be more than about 99.1%, about 99.2%, about 99.3%, about 99.4%, or about 99.5%. The porosity of the graphene foam can be modified by controlling the CVD conditions. In this way, the porosity of the graphene foam can be tuned. In some embodiments, the graphene foam has a pore radius of less than about 1.5 pm. In other embodiments, the pore radius is less than about 1.4 pm, about 1.3 pm, about 1.2 pm, about 1.1 pm or about 1 pm.

The graphene foam can have an IID/IG ratio of about 0.5.

In some embodiments, the graphene foam has a specific surface area of more than about 990 m²/g. In other embodiments, specific surface area is about 990 m²/g, about 992 m²/g, about 994 m²/g, about 996 m²/g, about 998 m²/g, or about 1000 m²/g. In other embodiments, specific surface area is about 990 m²/g to about 1000 m²/g.

In some embodiments, the graphene foam has an oxygen content of about 1 atomic percentage (at. %) to about 2 at. %. In other embodiments, the impurity is about 1 at. %, about 1.2 at. %, about 1.3 at. %, about 1.4 at. %, about 1.5 at. %, about 1.6 at. %, about 1.7 at. %, about 1.8 at. %, about 1.9 at. %, or about 2 at. %. In other embodiments, the oxygen content is less than about 2 at. % or less than about 1 at.%.

In some embodiments, the graphene foam has a Young's modulus of about 200 kPa to about 300 kPa, about 200 kPa to about 280 kPa, about 200 kPa to about 260 kPa, or about 220 kPa to about 260 kPa. In some embodiments, the graphene composite has a Young's modulus of about 200 kPa, about 220 kPa, about 240 kPa, about 260 kPa, about 280 kPa, or about 300 kPa. In some embodiments, the graphene foam has a stress of about 0.10 MPa to about 0.09 MPa at a compression strain of about 55%. In other embodiments, the stress is about 0.095 MPa at a compression strain of about 55%.

In some embodiments, the graphene foam has a strength to density ratio of about 5. In some embodiments, the graphene composite has a strength of about 0.1 MPa at a density of about 0.02 Mg/m³.

In some embodiments, the graphene foam has a gyroid slab structure. In some embodiments, the graphene foam has a conductivity of about 2 Scm ^-1 to about 3 Scm ^-1 at room temperature. In some embodiments, the graphene foam has a conductivity of about 2 Scm ^-1 to about 2.8 Scm ^-1, about 2 Scm ^-1 to about 2.6 Scm ^-1, or about 2.2 Scm ^-1 to about 2.6 Scm ^-1. In other embodiments, the graphene foam has a conductivity of about 2 Scm ^-1' about 2.2 Scm ^-1, about 2.4 Scm ^-1, about 2.6 Scm ^-1, about 2.7 Scm ^-1, or about 3 Scm ^-1.

In some embodiments, the graphene foam has a resistance variation of less than about 10% when subjected to a bending of 32°. In some embodiments, the graphene foam has a resistance variation of about 9% when subjected to a bending of 32°. In some embodiments, the graphene foam has a resistance change (AR/Ro) of about 90% at a compression strain of 25%.

The present invention also provides a sensor, actuator and/or electrical resistor comprising the graphene foam as disclosed herein.

In some embodiments, the invention relates to a freestanding graphene based-electrode providing a catalytic assembly comprising a porous electrically conductive graphene foam as a substrate, a porous active species coating the substrate. According to other examples, active species can exhibit catalytic activity towards, for example, Oxygen Evolution Reaction (OER) and/or Hydrogen Evolution Reaction (HER). As a demonstration, nickel/iron bimetallic double layered hydroxide coated graphene foams have been successfully demonstrated for its water desalination. The present invention also provides an electrode or energy storage apparatus comprising the graphene foam as disclosed herein.

In some embodiments, the electrode further comprises a double layer hydroxides (LDH) disposed on the graphene foam. In some embodiments, the electrode further comprises a nickel/iron double layer hydroxides (NiFe LDH) disposed on the graphene foam. The NiFe LDH can have a mass loading on the graphene foam of about 15 mg/cm² to about 20 mg/cm². Alternatively, the loading can be about 15 mg/cm², about 16 mg/cm², about 17 mg/cm², about 18 mg/cm², about 19 mg/cm², or about 20 mg/cm². In some embodiments, the NiFe LDH are disposed as spheres.

In some embodiments, the electrode has an oxygen evolution reaction performance of about 180 mV to about 500 mV at 10 mA/cm², or about 300 mV to about 400 mV at 10 mA/cm². In some embodiments, the electrode has an oxygen evolution reaction performance of about 190 mV, about 200 mV, about 380 mV, about 390 mV, or about 400 mV.

In some embodiments, the electrode has an electrochemical active surface area of about 70 mF/cm².

In some embodiments, the electrode has a Tafel slope of about 70 mV/dec to about 80 mV/dec. In some embodiments, the electrode has a Tafel slope of about 72 mV/dec, about 74 mV/dec, about 75 mV/dec, about 78 mV/dec, or about 80 mV/dec. In some embodiments, the electrode has O2 turnover frequency (TOF) of about 0.06 s^_1 when a 300 mV overpotential is applied.

In some embodiments, the electrode has a hydrogen evolution reaction performance of about 200 mV to about 500 mV at 10 mA/cm², or about 400 mV to about 500 mV at 10 mA/cm². In some embodiments, the electrode has a hydrogen evolution reaction performance of about 200 mV, about 220 mV, about 240 mV, about 260 mV, about 270 mV, about 300 mV, about 400 mV, about 410 mV, about 420 mV, about 430 mV, or about 440 mV.

In some embodiments, the electrode has an electrochemical surface area (ECSA) of about 140 mF/cm² to about 160 mF/cm². In some embodiments, the electrode has an electrochemical surface area (ECSA) of about 150 mF/cm², 155 mF/cm², or about 160 mF/cm².

The present invention also provides a heat absorbing material comprising the graphene foam as disclosed herein.

In some embodiments, the invention relates to a freestanding graphene foam and fabrication thereof as photo-thermal transformation material for seawater desalination. The as-fabricated graphene foam has certain materials affinity. For instance, graphene has a large surface area and excellent photothermal transformation ability, compromising a promising candidate for desalination. To further enhance, improve or even alter the surface behavior, secondary or post-sintering chemical surface functionalization can be imparted onto the porous graphene structures. These include inorganic, organic or hybrid inorganic/organic etching and coating. For instance, oxidant agents can be applied on the surface of graphene foam to impart hydrophilicity and enable mass transportation. As a demonstration, nitric acids etched graphene foams have been successfully demonstrated for its water desalination.

In some embodiments, the graphene foam is capable of a temperature rise of about 10 °C to about 20 °C when exposed to one sun illumination for 30 min. In some embodiments, the increase in temperature is about 12 °C, about 14 °C, about 15 °C, about 16 °C, about 18 °C, or about 20 °C.

In some embodiments, the graphene foam has an evaporation rate of water of about 1.2 kg/m²h to about 1.6 kg/m²h. In some embodiments, the graphene foam has an evaporation rate of water of about 1.2 kg/m²h, about 1.3 kg/m²h, about 1.4 kg/m²h, about 1.5 kg/m²h, or about 1.6 kg/m²h.

In some embodiments, the graphene foam has a solar energy adsorption efficiency of more than about 80%. In some embodiments, the graphene foam has a solar energy adsorption efficiency of more than about 82%, about 84%, about 86%, about 88%, about 90%, about 92%, about 94%, about 96%, or about 98%. In other embodiments, the adsorption is in the near-infrared, visible, and near-ultraviolet region. Advantageously, the ceramic additive manufacturing technique allows the fabrication of porous and geometrically-complex ceramic structures with high precision and resolutions. For SLA/DLP, the feature size is highly dependable on the light source resolution as well as the light scattering factors of the photopolymerizable slurry. Further, the as-sintered ceramic scaffold can be used as a sacrificed template which when combined with chemical vapour deposition method allow geometrically design with precise control and scale-up fabrication of three-dimensional free-standing graphene foam.

The present invention can be applied to the fabrication of assembled catalytic system (surface coating of active species onto the three-dimensional free-standing graphene foam), which allows the uniform distribution and high loading of active species that may be altered or designed to promote different overall water splitting behaviour or other energy storage reactions in the application of flexible electronics or wearable devices. The present invention can also be applied to the fabrication of three-dimensional graphene foams based on porous ceramic template enables large surface area, gives excellent performance on desalination or separation process (e.g., oil/water separation).

In general, the porous ceramic scaffolds and chemical vapor deposition method allow the as-fabricated three dimensional graphene foam directly can be used for separation, desalination process where large surface area is needed. Further surface modification can be used to adjust for various process. For example, fabrication of assembled catalytic system based on three-dimensional free-standing graphene foam can serve as a proper conductive and mechanical support for energy storage and conversion reaction. The combination of both free-standing graphene structures and suitable surface functionality allows the graphene to be used in various chemical, petroleum, pharmaceuticals, flexible electronics, and wearable device industries.

A detailed description of the workings of the invention is laid out below. In the embodiments that follow, the invention is described in relation to some conditions for consistency to showcase the present invention. Flowever, the skilled person would understand that the invention is not limited to such. As used herein, "gyroid" refers to a material with an infinitely connected periodic minimal surface containing no straight lines.

Ink Formulation and 3D Graphene Monolith

The fabrication process for 3D graphene foam is summarized in Figure Id, containing the 3D printing of a sacrificial silica template and the growth of high-quality graphene. In Figure Id, the SEM, micro-CT scanning and TEM images shows (I) DLP printing of silica green body (macroscopic structure design), (II) debinding of polymer additives (or ligand removal) and sintering procedure for porosity formation (microscopic porosity formation), and (III— IV) chemical vapor deposition and wet etching of a sacrificial silica template to form 3D graphene assemblies. Porous silica templates with complex designed shapes were first prepared by a digital light process method with photopolymerization of UV-curable resin containing ceramic particles in suspension. The resin was found to require a high loading to produce reliable ceramic parts. Flowever, it must also have a low viscosity (<5000 cP or <5 Pa·s) for successful recoating and selfleveling following the requirements of many commercial products. Therefore, prior to the printing, the rheology behavior and resin stability were studied. Resin stability is important to ensure good interlayer adhesion, resulting in better load transfer from the resin to the filler, hence giving superior strengthening effects. The viscosity at shear rates of 0.1 s ¹ and 180 s ¹ showed minor changes over 50 cycles within 2 days, indicating excellent stability. High loading of solid silica particles did not affect the resin flowability much, as illustrated by the low viscosity value (~1 Pa·s). Solid loading of the resin was determined by thermogravimetric analysis (TGA) to be 67.8 wt % silica. Different computer-aided design files were then sent to the printer, sliced, and printed in a layer-by-layer manner. For efficient printing, the layer thickness was set to be 100 pm.

After obtaining the printed structures, these objects were subjected to further debinding and sintering processes. From TGA, polymer decomposition started slowly at 100 °C and rapidly from 400 to 500 °C. Therefore, extra dwelling steps at temperatures of 200, 400, and 500 °C were added to ensure complete polymer burnout. A rational polymer burnoff step can be therefore established according to this result. To study sintering conditions, the as-prepared structures were then sintered under atmospheric conditions at various temperatures (1250, 1350, and 1450 °C) and soaking times (5, 10 h).A slow ramping rate at 1 °C/min was selected to maintain uniform pore distribution. The formation of 3D graphene required the CVD method. CVD was performed using CH4 at a flow rate of 10 seem as the precursor and elevate to 1100 °C for 1 h. Changing the flow rate to 20 seem or increasing the time range to 2 h will distort or block, respectively, some of the original features, thus providing a means to control the graphene assembly. A gyroid slab structure (2.7 x 1 x 0.4 cm³) was adopted for the following demonstration unless additional annotation is added, as presented in Figure 2. The observed layers stacking in the vertical direction illustrate that the adjacent two layers were cured and fused cleanly after printing and thermal hydrolysis. On top of this, Figure 2i demonstrates a graphene coated silica gyroid cube with no internal cracks via micro-CT cross-section scans. After CVD, silica was etched away with an aqueous HF solution, obtaining the freestanding graphene foam. Fabrication of crack-free and phase-pure bicontinuous complex graphene foam cannot be obtained from conventional ceramic shaping methods, as shown in Figure 3b, d. Figure 3a shows the scanning electron microscopy (SEM) of the colloidal silica resin used in this study. Sintering of the spin- coated resin film results in completely different cracking behavior as compared with 3D- printed fine structures with the same feature size as demonstrated in the same figure. Limited structures can be provided that are not prone to sintering cracks via conventional casting. Additive manufacturing instead rationally applies shape design to enable a crack-free sintered body. Moreover, as for removal of the sacrificial template, the 3D graphene assembly from conventional casting leads to the anisotropic etching behavior of the coated silica template that cracks severely after the wet etching step. Effective etching took place at a few hundred micrometers from the contact surface, as indicated in the elemental line-scan analysis. The same process conducted on a 3D- printed template with finer feature size led to a uniform and mild reaction procedure that instead resists volume shrinkage or cracking. The SEM image and EDX mapping of the microstructure of the fully processed 3D graphene foam showed a homogeneous grain size and shape. As shown in Figure 4, final products showed an interconnected network of foam-like graphene and a density of about 18 mg/cm³. Figure 4a, b suggests that a similar microstructure and pore distribution can be maintained after chemical etching of a sacrificial template. The microscopic features such as step edges were still preserved (Figure 2h). The foam-like graphene network is built up by few-layered/ multilayered graphene (Figure 4c-f). EDX elemental mapping was conducted on the surface of 3D graphene foam. From the results, elemental C fully covered the original microstructures (Figure 4b). Elemental quantification exhibited 100% C, highlighting the excellent purity of the as-fabricated sample. Similar extrapolation echoed well in the XPS full pattern spectra, where only 1.27 at. % oxygen impurity can be detected. Fligh-resolution analysis of the 0 Is region suggested the surface adsorbent (-OFI, 533.6 eV) mainly contributed to such minimal impurity (Figure lOi). To further investigate the quality of the graphene, Raman spectroscopy was presented as shown in Figure 4f. A characteristic 2D band peak shift was detected in the Raman spectra as a proof of the existence of few-layer/multilayered graphene. Comparison between 3D graphene foam and other carbon-based materials in the intensity ratio of the D band and G band further highlights the formation of high-quality graphene. The position of the 2D bands located as around 2690 cm^-1 as well as the WIG ratio (about 0.5) indicated that few- layer/multilayered graphene should be formed. Such inference matched well with high- resolution XPS C Is spectra (Figure lOg). A large specific surface area of 994.2 m²/g was determined by the BET analysis. A well-matched pore distribution in the silica template with the as-processed graphene foam was observed, showing the coexistence of mesopores and micropores. The porosity of 3D-printed graphene foam was 99.2%, by considering the density of graphite (which is 2.23 g/cm³). Porosity is taken as ((Total Volume - Volume of the Solid) / Total Volume) x 100%. The highly porous graphene foam can potentially be of great value to energy storage and mass transport related applications. From a practical point of view, the structural robustness of the synthesized porous graphene foam can be important. The interconnected graphene network was tested to be also robust and durable under simulated working conditions. Young's modulus, given by the slope of the tangent of the stress-strain curve, is about 239.7 kPa at p = 18 mg/cm³ for as-fabricated 3D assembly and stands out from many other carbon-based monoliths (Figure 5c). The stress remained almost constant at 55% compression strain for different loops, suggesting elasticity and flexibility (Figure 5b). Moreover, the strength-density chart (Figure 6) indicated such a foam possesses better strength-to-weight ratio than flexible polymer foams and is comparable to rigid polymer foam. The conductivity of 3D graphene foam is determined as 2.39 Scm ^-1 at room temperature by four-probe measurements, showing a superior conductivity as compared to other carbon-based materials at the same density level (Figure 5d). A well- constructed bicontinuous network and low impurity level may contribute to the excellent electrical behavior. Strain-Induced Resistance Change

To demonstrate the possibilities of this freestanding graphene foam, several well-known property tests were herein presented. First, foam-like graphene structures with flexibility and robustness under mechanical strain and stress can be crucial in application of sensors, wearable devices, and actuators. When subjected to compressive strain, it should function as an electrical resistor with a drop in resistance. Figure 5e,f demonstrates a setup of compression-assisted resistivity testing and the resistance change with strain using a gyroid slab graphene. Bending- induced signals were collected based on a determined value of 32° during the downward movement. The resistance variation (~9%) was observed to be minimal during the repeating cycles. Under another compression strain-induced test, the sensitivity first increased significantly (AR/Ro ~ 90% at a strain of 25%) and showed less sensitivity to higher compressive strain. The starting resistance (-300 ohm) can be higher than that of the original 3D graphene foam (measured by bulk plate) due to the contact resistance and higher porosity of the gyroid design. The results again confirmed the formation of durable and robust graphene foam with no degradation of its characteristic properties.

Catalytic Behavior toward Oxyaen Evolution Reaction (PER) and Hydrogen Evolution

Reaction (HER)

Second, a cellular carbon-based monolith has been regarded as the most promising electrode to support catalysts due to its extraordinary specific surface area (SSA). A robust graphene foam with excellent conductivity should be explored for wider applications provided it inherited a high SSA. As compared to powder-based research regarding the same materials, fabrication of a high-quality graphene monolith can benefit such applications. Herein, the as-prepared graphene foam was confirmed to have a surface area as high as 994.2 m²/g by BET. A large surface area and hierarchically constructed porous structure can be essential to retain an outstanding electro-chemical performance with a high mass loading for energy conversion and storage reactions. Recently, catalyst materials such as transition metal hydroxides have long been well studied and proven to exhibit sound activity toward overall water splitting. Therefore, nickel/iron double layer hydroxides (NiFe LDH) were grown on a thin 3D graphene gyroid slab (2.7 x 1 x 0.1 cm³, denoted as NiFe LDH/GF) via a typical urea hydrolysis of a mixture of nickel nitrate and iron nitrate. In a control experiment, other commonly used carbon substrates such as carbon cloth (denoted as NiFe LDH/CC) and graphite paper (denoted as NiFe LDH/GP) were adopted. The deposition of NiFe LDH was uniformly coated over the gyroid slab (Figure 7c, g), while coating took place only at the very top layer of carbon cloth (Figure 7a, e) and inconsistency existed on the surface of graphite paper (Figure 7b, f). Regarding the loading mass, 17 mg/cm² can be obtained, which is almost 10 times higher than carbon cloth (1.7 mg) and graphite paper (2.3 mg). Electrochemical properties of the three substrates and coated substrates were investigated using 1 M KOH in a three-electrode cell for overall water-splitting behavior (Figure 7). Oxygen evolution reaction performance was recorded in Figure 7 i — k. From the polarization curves in Figure 7 i, the graphene foam (392 mV @ 10 mA/cm²) and NiFe LDH/GF (198 mV @ 10 mA/cm²) exhibited the lowest overpotential values as compared to other substrates selected, indicating a large increase in loading mass did not retard charge transfer efficiency. After iR correction, the overpotential decreased further to 192 mV for NiFe LDH/GF and 384 mV for as-fabricated graphene as presented in Figure 8. Cyclic voltammetry (CV) scanning was also provided to show the electrochemical double-layer capacitance (Cdl). Notably, the electrochemical active surface area was significantly improved by coating of NiFe LDH on gyroid graphene foam, changing from 30.7 to 69.9 mF/cm². The drastic increase surpassed that of NiFe LDH/CC (1.57 times higher than blank substrate) and NiFe LDH/GP (1.54 times higher than blank substrate). The Tafel slope was also extracted from voltammetry at a small scan rate of 5 mV/s and plotted over the region that is not close to the equilibrium potential. A higher scan rate will add a significantly high capacitive current to the current of the catalytic reactions. Results are given in Figure 9, and NiFe LDFI/GF displayed the lowest value of 74 mV/dec. From the OER equation, a good OER electrocatalyst system should have a favorable electron transfer performance, excellent conductivity, and large ECAS property. All these factors will influence the electron transfer performance. The electro- catalytic OER activity of NiFe LDFI @ GF is further evaluated by comparing the O2 turnover frequency (TOF) calculated by the total number of electrochemically active sites. When a 300 mV overpotential is applied, NiFe LDFI @ GF has a comparably high TOF of 0.059 s^_1. The FIER was also conducted to get the activity of the same sample class. The NiFe LDFI/GF yet again provided the best FIER performance with an overpotential of 230 mV @ 10 mA/cm² and extremely high ECSA of 155.6 mF/cm². All the results are summarized in Figure 7j-m and Figure 9 for a better comparison. These results supported the proposal of adopting such graphene foam for various electrochemical energy storage devices where a conductive electrode with high specific/electrochemical active surface areas is desired. Moreover, the graphene foam also exhibited good catalytic performance as compared to other substrates. Surface oxygen incorporation and defects can contribute to this. Thermal Management and Surface Chemistry

Third, apart from the excellent electrical conductivity and high surface area, graphene materials have a versatile surface chemistry and thermal management ability. Solar energy is regarded as the largest source of renewable energy that can be used for daily life. Thermal management is one of the most direct paths of utilizing solar energy. Solar technologies such as solar steam generation has been well developed and has great potential in global water supply. Numerous works have described carbon-based heat absorbers for seawater desalination or solar steam generation. The atomically smooth and thin nature of graphene facilitates the efficient transport of water through nanochannels, microscopic porosity, and defects. In Figure 10, the 3D gyroid graphene foam (2.7 x 1 x 0.4 cm³) is applied as a floating heat absorber system (denoted as GF). The gyroid macroscopic structures were also believed to improve water uptake behavior. As hydrophilic contact with open porosity can serve well for water transportation ability, surface treatment (immersing into an aqueous solution of 50% FINO3, denoted as treated GF) was further applied on the graphene surface, modifying the surface from originally hydrophobic to hydrophilic. The wettability of GF and treated GF was recorded by water contact angle measurements. Treated GF showed transformation to a hydrophilic behavior, while untreated GF exhibited a contact angle of 126.7° (Figure lOf). Changes in surface chemistry originated from the increase of hydrophilic functional groups such as -OFI. As indicated in Figure lOi, the surface oxygen level increased recognizably, confirmed by the increase in peak intensity of high-resolution O Is spectra. The signal assigned to the -OFI group was amplified as well. Another observation was the decrease of total C Is signal intensity and increase of C-O, C=0, and O-C-O (285.6, 286.7, and 288.6 eV, respectively) peaks. The hydrophilic surface channelled the water through the porous structure and favored the water desalination process. To monitor the optical adsorption ability, the UV-vis spectrum was also measured. It showed 98% adsorption efficiency in the near- infra red, visible, and near-ultraviolet region (Figure 10e). An IR camera was used to record the temperature distribution. The maximum temperature under one sun illumination can rise to 43 °C within a few minutes and remained constant afterward for treated GF, while water under the same illumination only experienced a slight temperature increase (~3 °C), as indicated in Figure 7c. Such improvement in thermal adsorption can be attributed to (a) hierarchically constructed porosity form a heat localization layer to confine the radiation; (b) a rough surface and surface modification promoted lower light reflectance and thermal management. The evaporation rate was plotted according to the weight loss as a function of time under 1 sun irradiation. The cumulative weight loss is linear with illumination time, demonstrating a steady evaporation rate as in Figure 10a, b. The largest weight loss and evaporation rate over the same illumination time period were obtained by treated GF, manifesting the best steam generation performance. The evaporation rate of water facilitated by treated GF is about 1.436 kg/m²h, which is 3.55 times higher than that of pure water. For GF, the evaporation rate showed a lower value of 1.321 kg/m²h. The solar energy conversion efficiency therefore can be calculated. While used for solar steam generation, such a floating system displayed an efficiency of 84.5% and 93% for GF and treated GF, respectively. As a control experiment, pure water was also tested for its conversion efficiency, which showed an efficiency of 25.5%. The dark field performance is also presented in the tables below together with a comparison with other reported solar steam generation systems. Similar surface modification strategies can also be adjusted case by case when being used for other applications such as oil-water separation and filtration. Moreover, GF itself is a hydrophobic system and can also be used for oil/solvent absorption, as demonstrated in Figure lOj. The solvent absorption of red-stained hexane by the as-fabricated graphene foam was demonstrated (Figure lOj). Such a material hence showed extensive applicability and high efficiency for oil/solvent contaminant adsorption and recyclability.

The tables below compare the graphene foam of the present invention with some comparators.

Table 1. Representative works on fabricating of 3D graphene monolith

Table 2. Representative works on carbon-materials for desalination

Table 3. Comparison of electrolysis behaviour with commercial catalysts in alkaline solution after activation

In summary, a sacrificial silica template with complex-designed structures prepared by 3D-printing techniques enables the fabrication of crack-free and phase-pure bicontinuous complex graphene foam. As-fabricated hierarchically porous graphene foam inherited a large surface area (994.2 m²/g), excellent conductivity (2.39 S/cm), reliable mechanical properties (E = 239.7 kPa), and tunable surface chemistry. Such 3D graphene foam was demonstrated as a strain sensor, catalyst support, and solar steam generator. This work represents a step toward fulfilling the grand promise of 2D graphene in a 3D world. A major constraint to the utilization of graphene up to now has been the limited form of the material due to conventional fabrication methods. The fabrication of high- quality 3D graphene enables greater adoption in wide-ranging fields such as wearable sensors, energy storage, and water supply. Given the vast opportunity of using graphene in various energy and environment sectors together with the ease of fabrication, the future of three-dimensionally free-formable graphene foam can be bright. Materials. Silica powder, photoinitiator and viscosity modifier such as diphenyl(2,4,6 trimethyl benzoyljphosphine oxide (TPO), ethoxylate 1,6-hexanediol diacrylate (E- HDDA), and ethoxylated trimethylolpropane triacrylate (E-TMPTA) were purchased from Sigma-Aldrich. Variquat CC 42 NS was kindly provided by Evonik and served as the dispersant in this experiment. Sigma-Aldrich also provided isopropyl alcohol (IPA) and hydrofluoric acid (HF, 45 wt%) used for washing and etching.

Preparation of DLP Silica Resin. In an amber-colored bottle, 53 g of silica powder, 3 mL of Variquat CC 42 NS, and 25 mL of photocurable resin (consisting of E-HDDA and E- TMPTA at a ratio of 3:22 with 2 wt % TPO photoinitiator) were weighed. The photocurable slurry was then homogenized and printed using Asiga Max at 50 to 100 pm layer height setting. E-HDDA and E-TMPTA were added to reduce viscosity and to improve curing properties.

Alternative method 2: Weigh 53 grams Silica powder, 3 mL of Variquat CC 42 NS and 27 mL of Photocurable Resin (consisting of 3 mL ethoxylate pentaerythritol tetraacrylate,

24 mL 1,6-hexanediol diacrylate with 0.48 grams diphenyl(2,4,6- trimethylbenzoyl)phosphine oxide photoinitiator). The photocurable slurry is then homogenized with roller mixer and kept in an amber-coloured bottle. Alternative method 3: Weigh 53 grams SiC>2. In amber colour bottle, pre-dissolve 3 mL of Variquat CC 9 NS and 27 mL of Photocurable Resin (consisting of 4 mL trimethylolpropane triacrylate, 23 mL 1,6-hexanediol diacrylate with 0.46 grams diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide photoinitiator.) The photocurable slurry is then homogenized with roller mixer and kept in an amber-coloured bottle.

Alternative method 4:

1. Weigh 53 grams SiO₂.

2. Solution A: In amber colour bottle, pre-dissolve 4 grams of photoinitiator diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) in 200mL of 1,6-hexanediol diacrylate (HDDA).

3. Using mortar and pestle, add 26.5 grams SiO₂ with 4 mL of trimethylolpropane triacrylate (TMPTA), 3 mL of Variquat CC 9 NS and 23 mL of Solution A. Grind to homogeneity.

4. Add the remaining 26.5 grams SiO₂ gradually and grind to homogeneity.

Alternative method 5: 1. Weigh 53 grams SiO₂.

2. Solution A: In amber colour bottle, pre-dissolve 4 grams of photoinitiator diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) in 200mL of 1,6-hexanediol diacrylate (HDDA). 3. Using mortar and pestle, add 26.5 grams SiO₂ with 3 mL of ethoxylate pentaerythritol tetraacrylate (E-PETA), 3 mL of Variquat CC 9 NS and 24 mL of Solution A. Grind to homogeneity.

4. Add the remaining 26.5 grams SiO₂ gradually and grind to homogeneity. 24 mL of Solution A is used because of higher E-PETA viscosity than TMPTA.

Alternative method 6:

1. Weigh 53 grams SiO₂.

2. Solution A: In amber colour bottle, pre-dissolve 2 wt% of photoinitiator diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) in lOOmL of ethoxylate 1,6- hexanediol diacrylate (E-HDDA).

3. Solution B: In amber colour bottle, pre-dissolve 2wt% of photoinitiator diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) in lOOmL of ethoxylate trimethylolpropane triacrylate (E-TMPTA) 4. Using mortar and pestle, add 26.5 grams SiO₂ with 4 mL of Variquat CC 42 NS,

3 mL of Solution B and 22 mL of Solution A. Grind to homogeneity.

5. Add the remaining 26.5 grams SiO₂ gradually and grind to homogeneity.

E-HDDA and E-TMPTA can be added to reduce viscosity and to improve curing properties. 22 mL of Solution A is used because E-HDDA helps to reduce viscosity.

The ceramic slurry can be printed using Asiga Max at 50 to 100 pm layer height setting. The as-obtained Silica green body was then debinded at 200°C, 400°C, 600°C for 3 - 8 hours respectively and subsequently sintered at 1200°C - 1450°C for 2 -10 hours.

Stereolithography and Heat Treatment of Silica. For stereolithography, the resin was processed with Asiga MAX (commercial printer with a UV source of 385 nm). The built structure can have dimensions of up to 119 x 67 x 75 mm³. Before printing, the STL file was sliced by the software that comes with the printer. The layer thickness was set to be 100 pm for each print. The burn-in layer time was 7 s, and the exposure time for each layer was 1-1.5 s. The printed structure was removed gently from the building platform with transparency film purchased from Suremark. The structure was directly moved to a 100 mL beaker and immersed in IPA. An ultrasonic bath was applied to remove any unpolymerized residue and to reveal the designed shape. This step was repeated five times before subjected to drying at room temperature. A debinding condition (soak at 200, 400, 500 °C for 2 h, respectively) was applied before the sintering of silica (Carbolite AAF 1100 furnace). After that, the debinded silica structures were later transferred to the high-temperature furnace (Carbolite HTF 1800 furnace) and annealed at 1250-1450 °C for 5-10 h. Fabrication of Graphene Foam. The 2D graphene was grown on the gyroid silica template under a gas flow containing 10-20 seem CFU for 1-2 h. The heating and cooling were done in the same tube under a carrier gas flow of Fh (50 seem) and Ar (300 seem) using an SHW-300C hot-wall CVD system. After cooling to room temperature, the whole product was immersed in 15% aqueous HF solution overnight to remove the silica template. After etching, the HF solution was drawn out and changed to DI water to wash away the residual solution. Freeze-drying with liquid I h was applied to obtain the final 3D graphene sample. Alternative method 1 : The as-obtained silica sintered body was then cleaned in 99% ethanol to clean the surface, washed and then graphene was grown at 1100 °C by CVD onto a porous silica template in a gas flow of CH (10 seem - 20 seem) + H (50 seem) + Ar (300 seem), for various time (10 seem - 40 seem) for various time (30min- 120min). Heating and cooling are carried out in carrier gas: H (50 seem) + Ar (300 seem). After cooling to room temperature, the product was dipped into aqueous HF solution (chemical bath choice can be altered according to template materials) to remove the template overnight and deionized water to removal HF residual overnight. Three-Dimensional free-standing graphene foam can be obtained after freeze-drying for 8-12 hours.

NiFe LDH @ 3D Graphene Foam, Carbon Cloth, and Graphite Paper Electrode. Nii.sFeo.s LDHs spheres were grown at various carbon-based substrates using the well- documented hydrothermal method. All the substrates were cut into 1 cm² pieces and put in the hydrothermal vessel after washing with deionized water and ethanol. For every 40 mL of aqueous solution added into the 50 mL Teflon-lined autoclave, 0.1745 g of Nί(Nq3)2·6H2q, 0.0808 g of Fe(NC>3)3'9H20, 0.24 g of urea (4 mmol), and 0.05925 g of NH F were included. The hydrothermal process was maintained at 120 °C for 12 h and subsequently cooled to room temperature. Afterward, the coated samples were collected and rinsed with deionized water and ethanol several times, followed by drying in a fume hood overnight, yielding NiFeLDH @ GF, NiFeLDH @ CC, and NiFeLDH @ GP.

Alternative method 1: To prepare the assembled catalytic electrode (NiFe LDH/ Freestanding graphene foam), a simple hydrothermal method by immersing the three- dimensional graphene foam in an aqueous solution is adopted. For a typical run, Nί(Nq3)2·6H2q, Fe(N03)3'9H20, urea (4 mmol), and NH F (1.6 mmol) were dissolved in

40 mL of distilled water and stirred for 5 min to form a homogenous solution. After that, the solution was transferred into 50 mL Teflon-lined autoclave, maintained at 120 °C for 12 h, then naturally cooled to room temperature (n Nί(Nq3)2·6H2q + n Fe(NC>3)3'9H20 = 0.8 mmol). Other active species can also be imparted onto surface via other methods. For example, Huang et al . , Nano Energy, 2017, 34, 472-480 can be referred to and is incorporated by reference herein.

Alternative method 2: To prepare superhydrophilicity three-dimensional free-standing graphene foam, the as-fabricated graphene foam after free-drying is dipped inside nitric acid (20% - 40%) for 15min at room temperature. Other surface modification methods can be applied and adjusted according to the final application. Characterization. Ceramic resin stability measurements were performed on a TA Instruments DHR-2 rheometer using 40 mm diameter parallel plates at 500 pm measurement gap over 48 h. The apparent viscosity of the suspensions was measured at 25 °C in a shear rate sweep mode with shear rate ranging from 0.01 to 180 s^_1 over 48 h. The surface morphology of as-fabricated electrodes was observed using a scanning electron microscope with an acceleration voltage of 10 kV with EDX mapping using the same voltage (Zeiss; FESEM Supra 40). X-ray powder diffraction (XRD) patterns were obtained by a Bruker D8 Advanced diffractometer system with a Cu Ka radiation source. An Axis Ultra DLD X-ray photoelectron spectrophotometer (XPS) equipped with an Al Ka excitation source (1486.69 eV) was used to record compositional information on all the samples. The energy step size of the XPS was 1 eV for the survey scans and 0.1 eV for the fine scans. Raman spectra were conducted on a Horiba Micro Raman HR evolution system. High-resolution transmission electron microscopy (HR-TEM) of the samples was characterized using a field-emission transmission electron microscope (FE-TEM, JEM- 2010F, JEOL, Japan), which was operated at an accelerating voltage of 200 kV. Water contact angle measurement was done using a VCA Optima series. The density was given by measuring the mass per volume and tested for five different batches. Compression testing was done by a Lloyd ezlO. The four-point probe measurement method was carried out in the 2638A Hydra Series III data acquisition unit, with the precision of 0.0001 ohm. The catalytic behavior of the as-prepared integrated electrode was recorded by a VMP3 electrochemical workstation (Biologic Inc.). All the measurements were conducted based on a three-electrode system consisting of a self-fabricated electrode as a working electrode, a platinum plate as the counter electrode, and a Hg/HgO electrode (1 M KOH) as a reference electrode. Before actual data recording, the working electrode was scanned 20 cycles for linear sweep voltammetry (LSV) to obtain steady graphs in 1 M KOH with a scan speed of 5 mV/s at room temperature. The electrochemical tests were also /R-corrected, according to the equation E_COrr = E - iR. AC impedance was measured with a frequency range and amplitude of 5 mV from 200 kHz to 100 mHz. In order to calculate ECSA, the electrochemical double-layer capacitance (EDLC) of the working electrodes was measured by CV scans at different scan rates (5, 10, 20, 40, 80 mV/s, voltage window from 0.1 to 0.2 V for OER and -0.05 V to -0.15 V). EDLC was therefore calculated by plotting graphs of scan rate versus current density at a particular potential against the reference electrode. Tafel slopes of the experiments were all derived from LSV curves. Stability measurements were tested over 10 h at a constant current density (10 mA/cm²). TOF calculation was given according to the following equation:

where J is the current density at h = 300 mV, A is the surface area of the electrode, F is the Faraday constant, and m is the number of moles of metal on the electrode, which was obtained from inductively coupled plasma mass spectrometry.

The as-fabricated and surface-modified (immersed in 50% HNO3 aqueous solution) graphene foams were placed in a punched polystyrene foam (thermal conductivity ~ 0.04 Wm/k) with the graphene foams inserted in the opening. The entire structure floats on the surface of the water with only the bottom side in direct contact with water. The experiments were typically conducted at an ambient T of -25 °C and a humidity of -41% in a Petri dish. The solvent absorption capability of the as-fabricated graphene foam was tested by absorbing the red-stained hexane from water. Oil absorption performance was investigated by absorbing cooking oil and soybean oil from water.

It will be appreciated that many further modifications and permutations of various aspects of the described embodiments are possible. Accordingly, the described aspects are intended to embrace all such alterations, modifications, and variations that fall within the spirit and scope of the appended claims.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

Throughout this specification and the claims which follow, unless the context requires otherwise, the phrase "consisting essentially of", and variations such as "consists essentially of" will be understood to indicate that the recited element(s) is/are essential i.e. necessary elements of the invention. The phrase allows for the presence of other non-recited elements which do not materially affect the characteristics of the invention but excludes additional unspecified elements which would affect the basic and novel characteristics of the method defined. The reference in this specification to any prior publication (or information derived from it), 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 that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Claims

1. A method of fabricating a porous graphene foam, comprising: a) forming a printed structure from a ceramic slurry using an additive manufacturing technique; b) at least partially debinding the printed structure to form a calcined structure; c) at least partially sintering the calcined structure to form a porous ceramic scaffold; d) depositing graphene on the porous ceramic scaffold using chemical vapour deposition; and e) at least partially etching the porous ceramic scaffold in order to form the graphene foam; wherein the ceramic slurry comprises a ceramic with a ceramic loading of about 60 wt% to about 75 wt%.

2. The method according to claim 1, wherein the additive manufacturing technique of step (a) is selected from robocasting, stereolithography (SLA), digital light processing (DLP) and materials ink jetting.

3. The method according to claim 1 or 2, wherein the ceramic slurry comprises a ceramic selected from silicon dioxide (silica or SiO₂), aluminium oxide (AI₂O₃), zirconium dioxide (ZrO₂), ZnO, Fe₂O₃, Fe₃O₄, MFe₂O₄ (where M = Ni, Co, Mn, Cu), MFe₁₂O₁₉ (where M = Sr, Ba), titanates (TiO₂, BaTiO₃, ZrTiO₃), nitrides (Si₃N₄), carbides (SiC), or a combination thereof.

4. The method according to any one of claims 1 to 3, wherein the ceramic slurry further comprises a quaternary ammonium compound.

5. The method according to claim 4, wherein the quaternary ammonium compound has a loading of about 10 wt% to about 15 wt%.

6. The method according to any one of claims 1 to 5, wherein the ceramic slurry further comprises at least a polymer monomer selected from ethoxylate 1,6-hexanediol diacrylate (E-FIDDA), ethoxylated trimethylolpropane triacrylate (E-TMPTA), ethoxylate pentaerythritol tetraacrylate (E-PETA), 1,6-hexanediol diacrylate (HDDA), 1,1,1- trimethylolpropane triacrylate, or a combination thereof.

7. The method according to claim 6, wherein a volume ratio of a diacrylate monomer to a triacrylate monomer or a tetraacrylate monomer is about 20:4 to about 30:3.

8. The method according to claims 6 or 7, wherein the polymer monomer or mixtures thereof is present at about 25 wt% to about 40 wt% of the ceramic slurry.

9. The method according to any one of claims 1 to 8, wherein the ceramic slurry has a viscosity of about 5000 cP (5 Pa·s) to about 1000 cP (1 Pa·s).

10. The method according to any one of claims 1 to 9, wherein the step of etching the porous ceramic scaffold (step (e)) is performed in HF solution, sulfuric acid, nitric acid hydrochloric acid, or a combination thereof.

11. The method according to any one of claims 1 to 10, further comprising a step of functionalising the graphene foam (step (f)).

12. A porous graphene foam comprising an interconnected bicontinuous network of graphene and air pockets, having a ceramic content of less than about 5 atomic percentage (at. %).

13. The porous graphene foam according to claim 12, having an oxygen content of less than about 2 at. %.

14. The porous graphene foam according to claim 12 or 13, having a density of about 18 mg/cm³.

15. The porous graphene foam according to any one of claims 12 to 14, having an I2D/IG ratio of about 0.5.

16. The porous graphene foam according to any one of claims 12 to 15, having a specific surface area of about 990 m²/g to about 1000 m²/g.

17. The porous graphene foam according to any one of claims 12 to 16, having a

Young's modulus of about 200 kPa to about 300 kPa.

18. The porous graphene foam according to any one of claims 12 to 17, having a conductivity of about 2 Scm ^-1 to about 3 Scm ^-1 at room temperature.

19. The porous graphene foam according to any one of claims 12 to 18, having a resistance change (AR/Ro) of about 90% at a compression strain of 25%.

20. An electrode, actuator or heat absorbing material comprising the graphene foam according to any one of claims 12 to 19.