EP3994096A1

EP3994096A1 - Hydrogen storage material

Info

Publication number: EP3994096A1
Application number: EP20836489.3A
Authority: EP
Inventors: Yu-Sheng Tseng; Sammy Lap Ip CHAN
Original assignee: NewSouth Innovations Pty Ltd
Current assignee: NewSouth Innovations Pty Ltd
Priority date: 2019-07-05
Filing date: 2020-07-02
Publication date: 2022-05-11
Also published as: CN113874319B; EP3994096A4; AU2020310943A1; CN113874319A; WO2021003520A1

Abstract

The present invention relates to hydrogen storage. In particular, the present invention relates to a method of preparing pore-containing carbon nitride tubes, the method comprising hydrothermally treating melamine by subjecting melamine in contact with water to a temperature of from about 100 °C to about 300 °C; and calcining the hydrothermally treated melamine in a sealed vessel at a temperature of from about 500 °C to about 600 °C. Pore containing carbon nitride tubes prepared by the method and uses of the carbon nitride tubes to absorb and desorb hydrogen and as a hydrogen storage material are also described.

Description

Hydrogen Storage Material

Field of the Invention

[0001] The present invention relates to hydrogen storage. In particular, the present invention relates to a solid-state hydrogen storage material and a method of preparing the solid-state hydrogen storage material.

Background of the Invention

[0002] Hydrogen is an attractive energy carrier (i.e. fuel source) for many reasons. For example, hydrogen has a high gravimetric energy density and may be used in processes, such a combustion, or apparatus, such as fuel cells, to provide power or electricity, respectively, without forming carbon dioxide.

[0003] The storage of hydrogen, however, is considered by many to be a bottleneck to the realization of a hydrogen economy. At present, hydrogen is typically stored in highly pressurized tanks (e.g. ~700 bar) for use in transportable fuel cells (e.g. fuel cells in electric vehicles). Tanks that are strong enough to withstand such high pressures are typically heavy, which offsets hydrogen’s high gravimetric energy density. The high pressure also raises safety concerns. These limitations are amplified in refueling stations, which typically store much larger amounts of hydrogen at higher pressures.

[0004] Solid-state hydrogen storage systems have been used to counter some of these limitations. However, some limitations still exist for solid-state hydrogen storage systems. For example, alloys based on Li and Mg have a high hydrogen storage capability but the temperature for hydrogen release is too high to be used in an on-board system (e.g. for use in transportable fuel cells). AB₅-type alloys have moderate hydrogen storage properties with easy initial activation, low plateau pressure, low ambient working temperature, but the hydrogen capacity is typically too low (~1.4 wt%) to be useful in transportable fuel cells. Chemical hydride systems have very high gravimetric hydrogen capacities, but effective means of re-hydrogenation remain challenging. Metal Organic Frameworks typically require very low temperatures for hydrogenation and/or exhibit slow dehydrogenation rates.

[0005] There is a need for alternative materials suitable for use as a solid-state hydrogen storage material. There is also a need for material suitable for use as a transportable solid-state hydrogen storage material.

Summary of the invention

[0006] In a first aspect, the present invention provides a method of preparing pore- containing carbon nitride tubes, the method comprising:

(a) hydrothermally treating melamine by subjecting melamine in contact with water to a temperature of from about 100 °C to about 300 °C; and

(b) calcining the hydrothermally treated melamine in a sealed vessel at a temperature of from about 500 °C to about 600 °C.

[0007] Advantageously, pore-containing carbon nitride (CN) tubes prepared by the method of the present invention may have properties that are desirable for hydrogen storage. For example, in at least some embodiments, the pore-containing carbon nitride tubes prepared by the method are porous and/or have a large surface area. Furthermore, melamine is typically readily available and inexpensive. Further still, the method of the present invention does not require exotic and/or expensive catalysts, which are presently typically used to prepare carbon nitride materials.

[0008] In some embodiments, the hydrothermally treated melamine is in contact with a bulking agent for at least a portion of the calcination step (step (b)). In some embodiments the bulking agent is ceramic AI₂O₃ particles (e g. ceramic AI₂O₃ balls). [0009] In some embodiments, the temperature in step (a) is from about 180 °C to about 220 °C.

[0010] In some embodiments, the temperature in step (b) is from about 540 °C to about 580 °C.

[0011] In some embodiments, the pores of the carbon nitride tubes have a diameter of from about 1 nm to about 15 nm, especially from about 5 nm to about 7 nm.

[0012] In some embodiments, the tubes have a diameter of from about 0.5 mm to about 20 mm. In some embodiments, the tubes have a length of from about 15 mm to about 150 mm.

[0013] In a second aspect, the present invention provides a pore-containing carbon nitride tube obtained by the method according to the first aspect.

[0014] In a third aspect, the present invention provides the use of a pore-containing carbon nitride tube obtained by the method according to the first aspect in the absorption, adsorption or desorption of hydrogen. In another aspect, the present invention provides the use of a pore-containing carbon nitride tube obtained by the method according to the first aspect in the absorption, adsorption or desorption of a gas. In some embodiments, the gas is CO₂, CH₄, N₂O, SO₂, O₃, H₂O, fluorinated gases, biofuel gas, synthesis gas or a mixture thereof. In some embodiments, the pore- containing carbon nitride tube is used as an absorbent to remove a gas from a gas- containing atmosphere, for example, CO₂ from a container or greenhouse gases, such as CO₂, CH₄, N₂O, SO₂, O₃, H₂O and fluorinated gas, from manufacturing effluent or the earth’s atmosphere.

[0015] In a fourth aspect, the present invention provides a solid-state hydrogen storage material comprising pore-containing carbon nitride tubes obtained by the method according to the first aspect. [0016] In another aspect, the present invention provides a method of preparing pore- containing carbon nitride tubes analogous to that described in the first aspect, wherein a melamine precursor, such as cyanamide or dicyandiamide, is used in place of melamine. As will be appreciated, the melamine precursor forms melamine in situ under the conditions of the hydrothermal treatment step.

Brief Description of the Figures

[0017] Particular embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows Scanning Electron Microscope (SEM) images of (a) pure melamine and (b-r) hydrothermally treated melamine under different synthesis conditions, as described in the Examples.

Figure 2 shows SEM images of melamine cyanurate treated hydrothermally with the autoclave which has (a) 100 mL of volume, (b) the reduced volume from 100 mL to 70 mL and (c) 100 mL of volume but horizontally placed in an oven, as described in the Examples.

Figure 3 shows X-Ray Diffraction (XRD) patterns of (a) pure melamine and (b-r) hydrothermally treated melamine, as described in the Examples.

Figure 4 shows Fourier Transform-InfraRed (FT-IR) spectrum of pure melamine and hydrothermally treated melamine, as described in the Examples.

Figure 5 shows SEM images (a) to (k) of CN materials prepared at different pyrolysis conditions corresponding to Table 5, as described in the Examples.

Figure 6 shows SEM images of selected CN materials: (a) Bulk g-CN; (b) H₂₀₀-C₅₆₀- CN; (c) H₂₀₅-C₅₆₀-CN; (d) H₂₀₅-C₆₀₀-CN, as described in the Examples.

Figure 7 shows Transmission Electron Microscope (TEM) images of H₂₀₅-C₅₆₀-CN: (a) microtube; (b) nanotube, as described in the Examples. The outer tube diameter of nanotubes is around 200 nm and the thickness of tube wall is about 30 nm. In addition to the individual nanotubes (Figure 7b), nanotubes were also observed along the interior wall of microtubes (Figure 7a).

Figure 8 shows Energy Dispersive X-Ray Spectrometer (EDS) images of selected CN materials: (a) Bulk g-CN; (b) H₂₀₀-C₅₆₀-CN; (c) H₂₀₅-C₅₆₀-CN; (d) H₂₀₅-C₆₀₀-CN, as described in the Examples.

Figure 9 shows nitrogen adsorption-desorption isotherms of selected CN materials at 77 K, as described in the Examples.

Figure 10 shows pore size distribution of selected CN materials, as described in the Examples.

Figure 11 shows XRD of selected CN materials: (a) Bulk g-CN; (b) H₂₀₀-C₅₆₀-CN; (c) H₂₀₅-C₅₆₀-CN; (d) H₂₀₅-C₆₀₀-CN, as described in the Examples.

Figure 12 shows FT-IR of selected CN materials: (a) Bulk g-CN; (b) H₂₀₀-C₅₆₀-CN; (c) H₂₀₅-C₅₆₀-CN; (d) H₂₀₅-C₆₀₀-CN, as described in the Examples.

Figure 13 shows pressure-composition-isotherms for the hydrogen absorption capacities of selected CN materials at 20 ^°C, as described in the Examples.

Figure 14 shows correlation between surface area and H₂ uptake of selected CN materials, as described in the Examples.

Figure 15 shows pressure-composition-isotherms of H₂₀₅-C₅₆₀-CN at 20, 50 and 100 ^°C, as described in the Examples.

Figure 16 shows chemisorption measurements of H₂₀₅-C₅₆₀-CN from 20 ^°C to 500 ^°C, where the sample was pre-charged with 10% H₂+Ar gas at 20, 50 and 100 ^°C, as described in the Examples.

Figure 17 shows absorption and desorption kinetics of H₂₀₅-C₅₆₀-CN, as described in the Examples. Figure 18 shows extrapolation of hydrogen storage capacity of H₂₀₅-C₅₆₀-CN up to 10 MPa, as described in the Examples.

Figure 19 is a schematic representation depicting an inverted crucible (4) placed over a reaction vessel (3) containing hydrothermally treated melamine (1) and a bulking agent (2), sealed to a floor (6) by means of a sealant (5), as described in the Examples at paragraph [0052]

Detailed description of the Invention

[0018] In a first aspect, the present invention provides a method of preparing pore- containing carbon nitride tubes, the method comprising:

[0019] Carbon nitride is a nitrogen-rich carbonaceous material comprising carbon and nitrogen. The method of the present invention may be used to prepare pore- containing carbon nitride tubes. In some embodiments, the method of the present invention produces pore-containing carbon nitride tubes having a high nitrogen content. It is believed that the hydrogen storage properties improve as the nitrogen content increases. It is believed that as more carbon atoms are replaced by nitrogen atoms, the hydrogen storage properties are improved (compared to pure carbon tubes) because the tube wall has more defects caused by nitrogen substitution. That is, pore-containing carbon nitride tubes absorb hydrogen, and the hydrogen uptake amount is generally improved at higher nitrogen content. The terminology“C₃N₄” is used in the art to describe carbon nitride with a carbon to nitrogen ratio of approximately 3:4. As a person skilled in the art will appreciate,“C₃N₄” may be used to describe carbon nitride with a C:N ratio similar to 3:4, but not exactly 3:4 (e.g. from about 3 :3.5 to about 3:4.5). As used herein, except where the context requires otherwise due to express language or necessary implication, the term“C₃N₄” is used to refer to carbon nitride having a carbon to nitrogen ratio close to 3:4 and can include carbon nitride having a carbon to nitrogen ratio of from about 3 :3.5 to about 3:4.5. The C:N ratio of the pore-containing carbon nitride tubes (sometimes referred to herein as just“CN tubes”) obtained by the method of the present invention can vary and may depend on, for example, the properties of the starting materials and the conditions used in the method. The CN tubes obtained by the method of the present invention typically have a C:N ratio of greater than 3:2.4 (e.g. from about 3 :2.4 to about 3:4 or from about 3:2.4 to about 3 :3.86). In some

embodiments, the pore-containing carbon nitride tubes have ratio of carbon to nitrogen (C:N) of from about 3 :2 to about 3 :4, for example, from about 3 :2.5 to about 3:4, from about 3:3 to about 3 :4, from about 3 :3.5 to about 3:4 or from about 3 :3.7 to about 3:4. In some embodiments, the method of the present invention produces pore-containing carbon nitride tubes having ratio of carbon to nitrogen of about 3 :4. In some

embodiments, the carbon nitride tubes obtained by the method of the present invention may be described as“C₃N₄” tubes.

[0020] The method of the present invention forms carbon nitride in the form of pore-containing tubes. As used herein,“tube” or“tubes” refers to an elongated structure having an outer wall which encloses an internal space, which may or may not be vacant. The ends of the elongated structure may be open or closed (i.e. the tubes may be open at one or both ends or may be closed at one or both ends). The tubes are not restricted to being cylindrical in shape; the tubes may have cross-sectional geometries other than circular (e.g. regular shapes such as a square, a pentagon, a hexagon etc. or irregular shapes). Typically, however, the tubes produced by the method of the present invention are approximately cylindrical shaped and are typically hollow. The method of the present invention may also form carbon nitride in other forms, however, pore- containing carbon nitride tubes are the most notable of the carbon nitride forms produced by the method as it is believed that it is the pore-containing carbon nitride tubes that are responsible for the advantageous hydrogen storage properties of the material produced by the method.

[0021] Pore-containing carbon nitride tubes produced by the method of the present invention are not particularly limited by dimension. The pore-containing carbon nitride tubes produced by the method of the present invention typically have an outer diameter (sometimes referred to herein as just“diameter”) of between about 1 nm and about 1 mm, more typically from about 0.1 mm to about 20 mm or from about 0.2 mm to about 20 mm, and a length of from about 1 mm to about 300 mm, more typically from about 10 mm to about 200 mm. The thickness of the tube walls is typically about 5 nm to about 100 nm. In some embodiments, the tubes are microtubes. Microtubes have a diameter in the microscale (i.e. have an outer diameter ranging from about 1 mm to about 1000 mm). In some embodiments, the tubes are nanotubes. Nanotubes have a diameter in the nanoscale (i.e. have an outer diameter ranging from about 1 nm to about 100 nm). In some embodiments, the tubes have an outer diameter of from about 1 nm to about 1 mm, for example, from about 10 nm to about 100 mm, from about 10 nm to about 10 mm, from about 10 nm to about 1 mm, from about 100 nm to about 500 nm or from about 100 nm to about 300 nm. In some embodiments, tubes having a smaller outer diameter may be preferred (e.g. from about 0.1 mm to about 20 mm or from about 0.2 mm to about 5 mm) as it is believed that better properties, such as hydrogen storage, may be obtained, likely as a result of there being a larger surface area to weight ratio of the material. In some embodiments, the tubes have a length of from about 1 mm to about 300 mm, for example, from about 5 mm to about 200 mm, from about 10 mm to about 100 mm, from about 20 mm to about 80 mm or from about 30 mm to about 70 mm. In some embodiments, tubes having a shorter length may be preferred as it is believed that better properties, such as hydrogen storage, may be obtained, likely as a result of there being a lower energy barrier to the absorption, adsorption and/or desorption of hydrogen. In some embodiments, the thickness of the tube walls is about 5 nm to about 100 nm, for example, from about 5 nm to about 70 nm, from about 10 nm to about 50 nm or from about 20 nm to about 40 nm.

[0022] The present invention provides a method of preparing pore-containing carbon nitride tubes (i.e. carbon nitride tubes comprising pores). As used herein,

“pores” refers to holes located in the walls of tubes (i.e. not the“ends” of the tubes). Without wishing to be bound by theory, it is believed that the presence of pores increases the ability of the tubes to incorporate hydrogen by increasing the surface area of the carbon nitride tube material. In some embodiments, the pores have a diameter of from about 1 nm to about 50 nm, for example, from about 1 nm to about 30 nm, from about 1 nm to about 15 nm, from about 2 nm to about 10 nm, from about 3 nm to about 7 nm or from about 5 nm to about 7 nm. It will be appreciated that the pores may be circular or irregular shaped. For irregular shaped pores,“diameter” refers to the diameter of a circular hole having approximately the same area as the cross-sectional area of the irregular shaped pore. In some embodiments, the pore area represents from about 10% to about 60%, preferably from about 20% to about 50% of the surface of the carbon nitride tubes

[0023] As used herein,“hydrothermally treating” (and variations such as “hydrothermal treatment”) refers to a process in which melamine is heated in the presence of water. The hydrothermal treatment of melamine yields hydrothermally treated melamine. The hydrothermally treated melamine is believed to be melamine cyanurate, or at least predominantly melamine cyanurate. [0024] In the method of the present invention, the hydrothermal treatment is performed at a temperature of from about 100 °C to about 300 °C. In other words, the melamine is brought into contact with water and heated to a temperature of from about 100 °C to about 300 °C. The melamine may be brought into contact with water by adding water to the melamine or by adding melamine to water, so long as the melamine and water are in contact at a temperature of from about 100 °C to about 300 °C. In some embodiments, the water is in the form of a liquid. In some embodiments, the water is in the form of a gas. In some embodiments, the water is in an equilibrium state between liquid and gas. In some embodiments, the water is in the form of an aerosol. The water may contain other components (e.g. the water may be part of an aqueous solution). In some embodiments, the hydrothermal treatment step is performed at a temperature of from about 150 °C to about 250 °C, for example, from about 180 °C to about 220 °C, from about 190 °C to about 210 °C, from about 200 °C to about 210 °C or about 205 °C. In some embodiments, the hydrothermal treatment step is performed at increased pressure. In some embodiments, the increased pressure is between about 5 and about 30 bar. In some embodiments, the hydrothermal treatment step is performed at autogenic pressure (typically 10 to 28 bar). In some embodiments, the hydrothermal treatment step is performed in an autoclave. In some embodiments, the weight ratio of melamine in water (%w/w) is from about 1% to about 90%, for example, from about 2% to about 50%, from about 5% to about 30% or about 10%. A greater the amount of“bar-like” material tends to form when there is a lower weight ratio of melamine to water. Without wishing to be bound by theory, it is believed that the more“bar-like” material entering the calcination step, the more pore-containing carbon nitride tubes are produced in the calcination step. The amount of“bar-like” material, as well as the dimensions/size of the“bars”, produced in the hydrothermal step can also be affected by other parameters, such as temperature, melamine : water ratio, reaction vessel filling ratio and surface area of the reaction mixture. There is typically a correlation between the dimensions of the pore-containing carbon nitride tubes and the dimensions of the“bars” of the“bar-like” material that they were derived from. In some embodiments, the length and/or diameter of the pore-containing carbon nitride tubes is the same, or approximately the same, and the length and/or diameter, respectively, of the“bars” of the“bar-like” material from which they were derived.

[0025] In some embodiments, the melamine and water are mixed prior to thermal treatment so that the melamine is dispersed in the water. For example, the melamine and water may be sonicated and/or mixed by stirring to give a dispersion. In particular embodiments, the melamine is evenly dispersed in the water.

[0026] In the hydrothermal treatment step, the orientation of the vessel containing the melamine and water may be manipulated to increase the surface area of the reaction mixture (comprising melamine and water). Accordingly, in some embodiments, the surface area of the melamine and/or water is increased by orienting the reaction vessel in a particular orientation, for example, laying a tall vessel on its side. In some embodiments, the increased surface area during hydrothermal treatment leads to a higher formation of bar-like hydrothermally treated melamine.

[0027] In the method of the present invention, the melamine is hydrothermally treated in a reaction vessel that is typically sealed and typically under pressure (e.g. an autoclave). In the context of the hydrothermal treatment step, the“filling ratio” is the proportion of the volume occupied by the melamine and water (and any other non- gaseous reagents that may be present) relative to the internal volume of the vessel. As an example of how the“filling ratio” is calculated, if the internal volume of the vessel were 100 cm³, the melamine occupied 10 cm³ and the water occupied 40 cm³, the filling ratio would be 50%. As another example, if the internal volume of the vessel were 100 cm³, the melamine occupied 10 cm³, the water occupied 40 cm³, and a solid reagent occupied 10 cm³, the filling ratio would be 60%. Lower filling ratios tend to produce a greater proportion bar-like material, whereas higher filling ratios tend to produce a greater proportion of bulk material.

[0028] As used herein,“calcining” (and variations such as“calcination” and “calcined”) refers to a thermal treatment process in the absence, or limited supply, of oxygen to bring about a thermal decomposition or reaction. Absence of oxygen may be achieved by calcining under an inert atmosphere, such as under an atmosphere of argon or nitrogen. Accordingly, in some embodiments, the hydrothermally treated melamine is calcined under an inert atmosphere, such as argon or nitrogen. In other words, in some embodiments, the hydrothermally treated melamine is calcined in a sealed vessel wherein the atmosphere within the sealed vessel is an inert atmosphere, such as argon or nitrogen. A limited supply of oxygen may be achieved by calcining under an atmosphere having limited air or oxygen, for example, in a vessel that is sealed containing an atmosphere of air, wherein the amount of oxygen may be depleted during the process. Accordingly, in some embodiments, the hydrothermally treated melamine is calcined in a sealed vessel wherein the atmosphere within the sealed vessel (at the start of the calcination step) is or comprises air. In some embodiments, the gas generated from the calcination of the hydrothermally treated melamine (e.g. gas generated by the thermal decomposition of the hydrothermally treated melamine) may form a substantial portion of the atmosphere within the sealed vessel. In some embodiments, the gas generated from the initial stages of the calcination of the hydrothermally treated melamine may act as a protective gas for the rest of the calcination step.

[0029] In the method of the present invention, the calcination step (step (b)) is performed at a temperature of from about 500 °C and about 600 °C. In some embodiments, the calcination step (step (b)) is performed at a temperature of from about 520 °C and about 590 °C, for example, from about 530 °C and about 580 °C, from about 540 °C and about 570 °C, from about 550 °C and about 570 °C, from about 555 °C and about 565 °C or about 560 °C. Temperatures below about 500 °C generally give poorer results (e.g. lower surface area), presumably due to incomplete calcination. Temperatures above about 600 °C generally give poorer results (e.g. lower surface area), presumably due to decomposition. In some embodiments, the heating rate is from about 1 °C/min to about 50 °C/min, for example, from about 2 °C/min to about 20 °C/min, from about 3 °C/min to about 15 °C/min, from about 5 °C/min to about 10 °C/min, about 5 °C/min or about 10 °C/min. If the heating rate is too low (e.g. below about 1 °C/min), the time required to attain the desired temperature may result in the exposure of the hydrothermally treated melamine to elevated temperatures for extended periods of time, leading to material being“burnt away”. High heating rates (e.g. above about 50 °C/min), may lead to uneven heating, leaving some material to be“burnt away” whilst the remaining material is not completely calcined. This may be offset by leaving the material at the set calcining temperature for a period of time to sufficient to allow equilibration of the temperature in the reaction mixture. For this reason, faster heating rates (e.g. 5 °C/min or 10 °C/min) may be preferred in some embodiments. As a person skilled in the art will appreciate, the optimal heating rate may depend on other factors, such as scale or specific heat capacity of the components (e.g. hydrothermally treated melamine, vessel, bulking agent etc.). In some embodiments, the calcination step is performed in a sealed vessel in a tube furnace. In some embodiments, the calcination step is performed in a sealed vessel in a muffle furnace. In some embodiments, the calcination step is performed in a sealed vessel in a high temperature oven or the like.

[0030] As used herein,“sealed vessel” refers to a vessel that is sealed against the ingress or egress of gas, or at least the substantial ingress of egress of gas. Sealing the vessel was observed to improve the yield of the pore-containing carbon nitride tubes, presumably a result of there being less material“burnt off’ or sublimed during the calcination step. In addition, sealing the vessel improved the surface area of the pore- containing carbon nitride tubes that were obtained, giving better hydrogen storage properties. A“sealed vessel” may be formed by various configurations of elements (e.g. a vessel and a lid). For example, a vessel may be sealed by sealing a lid on the vessel (e.g. a crucible) by means of a sealant or glue or other such similar substance. As will be appreciated, the sealant or glue should be capable of withstanding temperatures of over 500 °C for the duration (or at least substantial duration) of the calcination process.

There are many high temperature sealants that are commercially available that may be suitable, for example, inorganics or ceramics, such as oxides (AI₂O₃, S1O2, Na20, MgO, ZrCh), nitride, boride and carbide. Further examples of high temperature sealants that may be used to seal the vessel include aluminium phosphate monobasic. The vessel may be formed of any suitable material. To be suitable, the material should be capable of withstanding the high temperatures of the calcination, should be non-porous to the gas and hydrothermally treated melamine and should be capable of being sealed. Such materials include ceramics, steels, metals (e.g. Al, Pt, W, Ti, Rh, Au, Ag, Zr) and alloys with high melting points (e.g. above 600 °C) and carbon-based materials such as graphite. In some embodiments, the sealed vessel is formed of a ceramic. In some embodiments, a crucible (or other similar shaped vessel) is inverted and placed over the reaction vessel. The crucible may then be sealed against the floor on which the reaction vessel sits, to thereby form a sealed vessel which encloses the reaction vessel. A sealed vessel may also be sealed by physical/mechanical means, for example, a threaded lid which is“screwed in” to a threaded neck of a vessel, such that it seals the neck of the vessel to form a sealed vessel. Other configurations include lids that are held on the opening of the vessel by external means (e.g. bolted on). Various forms of sealed vessel may be contemplated by a person skilled in the art. [0031] In the method of the present invention, the hydrothermally treated melamine is calcined in a sealed vessel. In the context of the calcination step, the“filling ratio” is the proportion of the volume occupied by the hydrothermally treated melamine (and optional bulking agent, as described below) relative to the internal volume of the sealed vessel. As an example of how the“filling ratio” is calculated, if the internal volume of the sealed vessel were 100 cm³ and the hydrothermally treated melamine occupied 50 cm³, the filling ratio would be 50%. As another example, if the internal volume of the sealed vessel were 100 cm³, the hydrothermally treated melamine occupied 50 cm³ and the bulking agent occupied 25 cm³, the filling ratio would be 75%. In yet another example, if the calcination step were performed in a reaction vessel positioned within a sealed vessel and the internal volume of the sealed vessel were 100 cm³, the hydrothermally treated melamine occupied 50 cm³, the bulking agent occupied 25 cm³ and the reaction vessel (positioned entirely within the sealed vessel) occupied 10 cm³, the filling ratio would be 85%. Lower filling ratios tend to give lower yields, presumably due to more material being“burnt off’ or sublimed during the course of the calcination step. Without wishing to be bound by theory, it is believed that when material is“burnt off’ or sublimed, the gaseous material resulting from the“burning off’ or sublimation may form a protective gas over the remaining material for the calcination process, hence, a greater filling ratio may lead to less material being consumed to form the protective gas, and therefore give a greater yield of pore- containing carbon nitride tubes. In some embodiments, the filling ratio is greater than about 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or 90%. In some embodiments, the filling ratio is up to about 99%, 95%, 90%, 80%, 70%, 60%, 50%, 40%, 30%, 20% or 10%. In some embodiments, the filling ratio is from about 10% to about 99%, from about 20% to about 95%, from about 30% to about 90%, from about 40% to about 80% or from about 50% to about 70%. [0032] In some embodiments, the calcination step (step (b)) is conducted with a bulking agent in contact with the hydrothermally treated melamine for at least a portion of the calcination step, typically the entire calcination step. As used herein, a“bulking agent” refers to an inert particulate material that does not react in the calcination process, but may be included to increase the filling ratio in the sealed vessel. It is believed that the bulking agent may perform more than one function. For example, the bulking agent may also increase the surface area of the hydrothermally treated melamine. Without wishing to be bound by theory, it is believed that the increase in surface area achieved by adding a bulking agent can provide more space for gas diffusion and enhance the pyrolysis, increasing the surface area of CN tubes. This effect may be more pronounced in large vessels containing a relatively small amount of material, such as, for example, calcining a small amount of material in an apparatus which has a relatively large sized vessel. Examples of bulking agents include ceramic particles, such as AhCb particles, particles composed of or comprising steels, metals (e.g. Al, Pt, W, Ti, Rh, Au, Ag, Zr) and alloys with high melting points (e.g. above 600 °C), particles composed of or comprising carbon-based materials such as graphite. In some embodiments, the inert particulate material is in the form of balls. In some embodiments, the diameter of the balls is from about 1 mm to about 50 mm, for example, from about 5 mm to about 20 mm, from about 5 mm to about 15 mm or about 10 mm. As will be appreciated, the optimal size may depend on factors such as the amount of material to be calcined and the size of the reaction vessel. A person skilled in the art will be able to determine suitable amounts and sizes of bulking agents. In some embodiments, the bulking agent is or comprises AI₂O₃ balls. As will be appreciated, the addition of a bulking agent increases the“filling ratio”, which may affect the properties of the pore-containing carbon nitride tubes produced by the method. In some embodiments, addition of a bulking agent (e.g. alumina balls) to achieve predetermined filling ratio produces pore-containing carbon nitride tubes having a lower density and/or higher surface area than pore-containing carbon nitride tubes produced from an analogous process wherein the same predetermined filling ratio was achieved by simply adding more hydrothermally treated melamine. In other words, the addition of a bulking agent may, in some embodiments, provide pore-containing carbon nitride tubes having a lower density and higher surface area. It is believed that the addition of the bulking agent allows the reaction (from the hydrothermally treated melamine to the pore- containing carbon nitride tubes) to progress further towards completion.

[0033] Also provided herein is a pore-containing carbon nitride tube obtained by the method of the present invention.

[0034] Pore-containing carbon nitride tubes obtained by the method of the present invention may be useful for the absorption, adsorption and/or desorption of hydrogen, and/or other gasses (such as CO₂, CH₄, N₂O, SO₂, O₃, H₂O, fluorinated gases, biofuel gas, synthesis gas or a mixture thereof), especially hydrogen. Accordingly, in a further aspect the present invention provides the use of a pore-containing carbon nitride tube obtained by the method of the present invention in the absorption, adsorption or desorption of hydrogen, and/or other gasses (such as CO₂, CH₄, N₂O, SO₂, O₃, H₂O, fluorinated gases, biofuel gas, synthesis gas or a mixture thereof), especially hydrogen. As will be appreciated, pore-containing carbon nitride tubes obtained by the method of the present invention may therefore be useful for the uptake, storage and/or delivery of hydrogen. Accordingly, the present invention also provides a solid-state hydrogen storage material comprising pore-containing carbon nitride tubes obtained by the method of the present invention. In some embodiments, the solid-state hydrogen storage material is or comprises pore-containing carbon nitride tubes in the form of a powder. In some embodiments, the solid-state hydrogen storage material is or comprises pore- containing carbon nitride tubes dispersed (e.g. embedded) in a matrix. In some embodiments, the matrix is a flexible matrix. In some embodiments, the matrix is a porous polymer. In some embodiments, the solid-state hydrogen storage material is or comprises pore-containing carbon nitride tubes in the form of a solid. In some embodiments, the solid-state hydrogen storage material is or comprises pore-containing carbon nitride tubes in the form of a solid obtained by compressing pore-containing carbon nitride tubes that were in the form of a powder. In some embodiments, the solid- state hydrogen storage material is or comprises pore-containing carbon nitride tubes in the form of a porous sponge. In some embodiments, the solid-state hydrogen storage material is reusable. In some embodiments, the solid-state hydrogen storage material is suitable for use as a transportable solid-state hydrogen storage material.

Examples

[0035] The present invention is further described below by reference to the following non-limiting Examples.

Preparation of Pore-Containing Carbon Nitride Tubes from Melamine

[0036] Pore-containing carbon nitride tubes were prepared using a two-step process. The first step comprised the hydrothermal treatment of melamine in an airtight autoclave. The second step comprised the calcination of the hydrothermally treated melamine. Carbon nitride materials prepared by this process may be referred to herein using the notation Hx-Cy-CN, where x = the temperature at which the hydrothermal treatment was conducted, and y = the temperature at which the calcination was conducted. Preparation of Pore-Containing Carbon Nitride Tubes - Hydrothermal Treatment Studies

[0037] In a typical procedure, 5 g of melamine was loaded into a polyphenylene (PPL) vessel (50 mL tank volume). Then 50 mL of deionized water was added into the container. The solution was sonicated for 15 minutes and stirred for 30 minutes to evenly disperse the melamine powder. Following the dispersion, the polyphenylene vessel containing the homogenized solution was transferred to a stainless-steel autoclave and the polyphenylene vessel sealed by the mechanical force provided by the stainless-steel autoclave. The autoclave was heated at 200 ^°C for 24 hours and cooled down naturally. During the hydrothermal treatment, the melamine sample was condensed at an autogenic pressure. The white product was collected by filtration and was washed for five times with deionized water. Then the obtained product was dried at 40 ^°C for 12 hours.

[0038] A series of trials investigating the conditions for the hydrothermal treatment step were carried out to examine the effect on the morphology of melamine and to increase the production of pore-containing carbon nitride tubes in the subsequent calcination step. In these trials, a polyphenylene-lined autoclave with 70 mL of inner volume was used. The melamine/deionized water solution was prepared in different proportions, and the solutions were added into polyphenylene-lined autoclave with different filling ratios. The samples were dispersed by sonication for 15 minutes and stirring for 30 minutes. Then the samples were hydrothermally treated with different temperatures ranging from 180 ^°C to 220 ^°C for 24 hours and cooled down naturally. The hydrothermally treated melamine was filtered, washed and dried before calcination. The shapes of these melamine samples were observed by SEM, which showed that the condensed samples possessed the optimum bar-like melamine cyanurate formation and highest yield when the weight ratio of melamine/deionized water was 10 %, the filling ratio was 85 % and the heating temperature was 205 ^°C.

[0039] The morphologies of melamine and hydrothermally treated melamine samples observed by SEM are shown in Figure 1. Pure melamine had an irregular structure (Figure la) and it transformed to different configurations after hydrothermal treatment (Figure lb-lr). A crystal-like structure intermingled with branch-like melamine cyanurate was formed by means of a typical hydrothermal treatment, where 5 g of pure melamine plus 50 mL of deionized water were heated in an autoclave having 50 mL of volume at 200 ^°C for 24 hours (Figure lb). The size of branch-like melamine cyanurate is generally larger than bar-like melamine cyanurate. The tubes produced from the calcined branch-like melamine cyanurate may have larger tube diameter, leading to a lower surface area per unit weight of the material.

[0040] A series of experiments looking at the hydrothermal treatment were designed and performed to increase the amount of bar-like material, as tabulated in Table 1.

Three dominant factors, including heating temperatures (top row), weight ratio of melamine/deionized water (left column) and filling ratio (right column), were altered to observe the effect on the shape of the hydrothermally treated melamine samples. A polyphenylene-lined autoclave with 70 mL of volume was used in this study.

[0041] To investigate the impact of these factors on the morphology of

hydrothermally treated melamine, the microstructures of different samples were compared. As can be seen from the samples synthesized at various heating temperatures with fixed solution concentration and filling ratio, the formation of bar-like melamine cyanurate was enhanced as the temperature increased, while the yield of product was largely decreased. This could be attributed to the increased solubility of melamine in water with the elevated temperature, leading to a more concentrated alkaline solution caused by the generation of ammonia from melamine condensation. The alkaline environment facilitates the hydrolysis of melamine and converts it to cyanuric acid by the substitution of hydroxyl functional groups for amino groups. Melamine and cyanuric acid can bind together via hydrogen bonding to form melamine cyanurate, a large molecule prone to precipitation due to reduced solubility. The reduced amount of product could be related to the boosted autogenous pressure in hydrothermal process with the growing temperature, which may induce a more serious vessel deformation and force the dissolved melamine cyanurate to escape through the pressurized vapor. This hypothesis may be supported by the observation of a white solid residue on the interior wall of stainless steel autoclave.

[0042] By comparing Figure If and lp, Figure lk and lq, Figure In and lo, and Figure lm and lr, respectively, one can find that the lower the ratio of melamine to deionized water, the greater the amount of bar-like melamine cyanurate. It is believed that a more completed hydrolysis and cross-binding reaction to form melamine cyanurate can be achieved when less melamine reactant was used at a low solution concentration.

[0043] The effect of“filling ratio” was explored by checking the sample morphologies at 10 % of melamine/deionized water at 200 ^°C. A low filling ratio was found to have better gain in bar-like melamine cyanurate. This is because less melamine was loaded at a low filling ratio under a fixed solution concentration. Hence a more completed hydrothermal reaction was expected. Another reason could be the lower autogenic pressure generated under a low filling ratio, which resulted in a spontaneous reaction to produce more NH₃ gas so as to improve the formation of cyanuric acid as well as the yield of melamine cyanurate.

[0044] The samples with good conversion to bar-like melamine cyanurate are shown in the enlarged images: Figure 1dl, 1fl, 1il, 111 and 1ml . The length and diameter of the melamine cyanurate bars are listed in Table 2. The results show that the samples have reasonable dimensions, several tens of mm in length and 1-3 mm in diameter, when they were synthesized under higher temperatures and lower filling ratios. However, the yield of the product was relatively low. Considering that a larger sample amount is required for calcination process and hydrogen storage measurement, the parameters applied to preparing the sample shown in Figure 11 (sample 11) were chosen as the optimum method for the hydrothermal treatment in this study.

Table 1: Experimental parameters and yields of product in hydrothermal treatment

Table 2: Dimension of bar-like melamine cyanurate with different synthesis conditions [0045] To assess the reproducibility in a larger vessel, a PPL-lined autoclave with a 100 mL volume was used in an additional experiment. The parameters from the preparation of sample 11 were implemented to synthesize bar-like melamine cyanurate. However, a large proportion of bulk melamine cyanurate with the dimension of several tens of microns was found in the product, indicating a poor transformation, as shown in Figure 2a. A Teflon-made solid cylinder with a 30 mL volume was then placed in the vessel to occupy the inner volume so as to simulate the same volume as used in the preparation of sample 11. As illustrated in Figure 2b, the treated sample still contained considerable bulk melamine cyanurate. The 100 mL autoclave was then placed horizontally in the oven in order to increase the reaction surface of melamine with water. The SEM image (Figure 2c) exhibited a high transformation into bar-like material with around 3 mm diameter, suggesting the reaction mixture surface area is important for bar-like sample formation. This correlation may be beneficial for scaling up the process.

[0046] The crystal structures of as-purchased melamine and melamine cyanurate condensed under different hydrothermal treatment conditions are reflected in XRD patterns (Figure 3). The samples denoted as (a) to (r) in Figure 3 correspond to the samples shown in Table 1 and Figure 1. A change of peak distribution suggested a new arrangement in melamine structure after hydrothermal reaction. The appearance of three peaks at around 10.9°, 18.4° and 21.96°, indexed as (100), (110), and (200), is evidence of in-planar packing in melamine-cyanuric acid complex. Moreover, an intense peak observed at around 28°, which can be indexed as (002), is an indicator of the lamellar stacking of melamine cyanurate. Owing to the variations of sample morphologies under different synthesis conditions, the intensity of the peaks is changed according to distinct growth directions and preferred orientations. For example, the samples predominantly containing bulk or sheet-like melamine cyanurate have very high intensity in (002) peak, such as the samples shown in Figure 3(b), (c), (e), (g), (n), (o), (p) and (r). On the contrary, the samples with high transformation of bar-like melamine cyanurate reveal a large reduction of the (002) peak. The absence of the above-mentioned peaks in Figure 3(q) suggests an incomplete formation of melamine cyanurate.

[0047] In addition to XRD, the formation of melamine cyanurate was further substantiated by FT-IR, as shown in Figure 4. The samples chosen for FT-IR analysis were pure melamine, typical HT melamine (hydrothermally treated by the typical method) and optimized HT melamine (hydrothermally treated with the optimized parameters: 11 in Table 1). The appearance of two new peaks at 1729 and 1780 cm^-1 indicates the existence of C=0 stretching vibration of melamine cyanurate.

Furthermore, due to the hydrogen bonding between cyanuric acid and melamine, the triazine ring vibration of melamine moved to a lower frequency; from 810 to 764 cm^-1.

[0048] The hydrothermally treated samples in this study displayed XRD and FT-IR spectra that were consistent with melamine cyanurate (and are referred to herein as either hydrothermally treated melamine or melamine cyanurate).

Preparation of Pore-Containing Carbon Nitride Tubes - Calcination Studies

[0049] In a typical process, hydrothermally treated melamine was calcined in a sealed crucible with alumina balls as bulking agent at 560 ^°C in air for 4 hours. For comparison, another batch of this sample was loaded into a boat alumina crucible which was placed in a semi-closed quartz tube and calcined at 600 ^°C under Ar atmosphere for 4 hours. These two pore-containing carbon nitride tube materials were denoted as H₂₀₅- C₅₆₀-CN and H₂₀₅-C₆₀₀-CN, respectively. Also, for comparison, bulk graphitic carbon nitride (g-CN) was synthesized by the calcination of pure melamine powder at 600 ^°C for 4 hours in a semi-closed quartz tube under an Ar gas atmosphere. [0050] Various parameters, such as sample amount, sample grinding, crucible airtightness, furnace type, protective gas, bulking agent, heating rate, heating duration and heating temperature, were also assessed to investigate the influence of calcination conditions on the properties of the final product. Conditions that gave the best yield and the highest surface area, as measured by Brunauer-Emmett-Teller (BET) analysis, were: 1.8 g of as-made hydrothermally treated melamine contained in a sealed crucible;

alumina balls as bulking agent; calcination in air at 560 ^°C for 4 hours with 5 ^°C/minute of heating rate. After calcination, a light yellowish material was collected and it was ascertained by XRD to be CN. A sample prepared according to these conditions, denoted as H₂₀₀-C₅₆₀-CN (Hx-Cy-CN: x: hydrothermal temperature; y: calcination temperature), was chosen as one of the candidates for hydrogen storage tests.

[0051] The calcination parameters and results for the preparation of carbon nitride materials are shown in Table 3 and 4. Pure CN was synthesized by loading pure melamine powder in a AI₂O₃ boat crucible, which was placed in the middle of a quartz tube, and heated at 600 ^°C under Ar for 4 hours. To investigate the optimum calcination conditions for pore-containing carbon nitride tubes, the melamine cyanurate samples used in this pyrolysis study were prepared following the typical method (Figure lb). Several factors were observed to influence the calcination outcome. For example, if the quantity of hydrothermally treated melamine is low, the yield is typically low, presumably due to a severe sublimation.

[0052] The limited chamber size of the tube furnace makes it less suitable for mass production. Hence, a method was developed to increase the product yield using a muffle furnace. Advantageously, this method can be extended to other heating apparatuses. In this particular method, a cup crucible holding melamine was placed on a AI₂O₃ plate and covered with a larger inverted cup crucible. The gap between the plate and the fringe of the bigger crucible was sealed by a high-temperature sealant, aluminium phosphate monobasic. In this case, the sublimation of carbon nitride was significantly reduced, presumably by the pressure generated from the condensation process.

[0053] A temperature below 600 ^°C for the calcination process was observed to be better for the synthesis of pore-containing carbon nitride tubes. Higher temperatures and long heating times brought about a substantial reduction of the product yield. However, a low temperature may lead to an incomplete pyrolysis. For instance, the surface area of pore-containing carbon nitride tubes obtained at 500 ^°C is lower than that obtained at 560 ^°C, which is discussed later under the heading“Surface Area Analysis”. On the other hand, the effect of heating rate on the result may not be overly significant.

[0054] AI₂O₃ balls, used as a bulking agent, can provide more space for gas diffusion and enhance the pyrolysis, increasing the surface area of pore-containing carbon nitride tubes. This is described further in the BET surface area section below.

The grinding of hydrothermally treated melamine samples appeared to damage the tubular structure of pore-containing carbon nitride tubes. Thus, grinding the bar-like melamine cyanurate should be avoided. The best results were obtained using larger quantities of as-made hydrothermally treated melamine in a sealed crucible in air at 560 ^°C for 4 hours with alumina balls as the bulking agent.

Table 3: Calcination conditions for pure melamine and as-made melamine cyanurate

Table 4: Calcination conditions for ground melamine cyanurate

K)

a Two different types of melamine samples.“Pure” is the as-received melamine and“HT” is the hydrothermally treated melamine (i.e. sample shown in Figure 1b).

b“Yes” indicates the melamine samples were ground by mortar before calcination and“No” means the as-prepared samples.

c The weight of hydrothermally treated melamine samples loaded for calcination.

d Two different types of alumina crucibles, cup and boat.

e“Semi” refers to the cup crucible being covered by an alumina lid or the quartz tube where a boat crucible was loaded and filled with heat insulation materials at the open end.“Yes” indicates the cup crucible was placed on an alumina plate and covered with a larger cup crucible, and then the gap between the lid and the larger crucible was sealed by aluminium phosphate monobasic, which is a high-temperature sealant.

f The hydrothermally treated melamine samples were directly loaded in the crucibles or separated by alumina balls (i.e. bulking agent).

g Two different types of furnaces were used for calcination. Muffle furnace was used for cup crucibles and tube furnace was used for the quartz tube with a boat crucible

loaded.

^h The hydrothermally treated melamine samples were calcined in air or in protective gas, such as Ar and N₂.

1 Ramping rate for calcination.

J Heating duration for calcination.

k Heating temperature for calcination.

l The weight of obtained carbon nitride samples after calcination.

m The weight percentage of carbon nitride sample remaining after calcination.

to

Morphological Studies

[0055] To study the effects of calcination conditions on the morphology of pore- containing carbon nitride tubes, some selected samples (prepared under various pyrolysis states (Table 5)) were observed by SEM, as shown in Figure 5. The heating rate for the calcination is not shown in the table as it was fixed at 5 ^°C/min.

[0056] Inspection of Table 5 and Figure 5 gives the following observations. First, the pyrolysis using a boat crucible and tube furnace was able to convert the as-made bar-like melamine cyanurate into tubular carbon nitride under air or inert gas (Figure 5g-5j). When the heating period was doubled, carbon nitride tubes could still form (Figure 5h). In contrast to the tube furnace, the muffle furnace coupled with semi-closed cup crucible burned away CN tubes and only sheet-like CN remained (Figure 5a). However, if the cup crucible was sealed, similar tube formation as given by tube furnace could be achieved by muffle furnace (Figure 5b-5f). An advantage of this method over using a tube furnace is that muffle furnaces are generally more common and cheaper than tube furnaces and this method can be used to produce a larger quantity of the pore-containing carbon nitride tubes. The influence of bulking agent on the morphology is hard to tell from SEM images (Figure 5b and 5d), but differences are revealed in surface area (discussed under the heading“Surface Area Analysis”). It is noted that the bar-like structure of melamine cyanurate was destroyed by grinding and only some porous frameworks remained after calcination (Figure 5k).

Table 5: Pyrolysis conditions for CN tubes preparation

[0057] Four carbon nitride samples were selected for further characterization and hydrogen storage measurements, namely: (a) Bulk g-CN: pure carbon nitride synthesized by the calcination of pure melamine powder at 600 ^°C for 4 hours in a semi-closed quartz tube under Ar gas protected; (b) H₂₀₀-C₅₆₀-CN: typical HT (hydrothermally treated) melamine was loaded in a sealed crucible, with alumina balls as bulking agent, and calcined in air at 560 ^°C for 4 hours with 5 ^°C/minute of heating rate; (c) H₂₀₅-C₅₆₀-CN: optimum HT melamine was calcined in a sealed crucible with alumina balls as bulking agent under 560 ^°C in air for 4 hours; (d) H₂₀₅-C₆₀₀-CN: optimum HT melamine was contained in a boat crucible, which was placed in a semi-closed quartz tube and calcined at 600 ^°C under Ar atmosphere for 4 hours. Figure 6 shows the SEM images of these four samples.

[0058] Bulk g-CN shows irregular structure which is quite dissimilar from other three samples. H₂₀₀-C₅₆₀-CN has a tubular structure as well as cotton-like frameworks due to its relatively lower content of bar-like material before calcination. The cotton-like frameworks may be converted from bulk melamine cyanurate in the pristine samples. Both H₂₀₅ -C₅₆₀- CN and H₂₀₅-C₆₀₀-CN were prepared from HT melamine having a high proportion of bar like structure. However, with the different calcination methods, the morphologies of the carbon nitride products varied. H₂₀₅-C₅₆₀-CN possessed a high ratio of CN tubes lengths in the range of several tens of microns, whereas H₂₀₅-C₆₀₀-CN showed shorter CN tube length. This indicated that excessively high calcination temperature may have induced some destruction of the tubular structure in the H₂₀₅-C₆₀₀-CN sample, even though it was calcined under an argon atmosphere.

Characterization and Properties

Characterization

[0059] Morphology was observed using a Hitachi 3400X scanning electron microscope (SEM) equipped with a secondary electron (SE) detector at an operating voltage of 20 kV, and the elemental distribution was determined by an energy dispersive X-ray spectrometer (EDS) at 15 kV. Detailed morphology was characterized by transmission electron microscope (TEM, Phillips CM200). The TEM sample were prepared via dispersing the powder in ethanol and the dispersed samples were collected by copper grids. The sample were then dried at 40 ^°C for a day prior to TEM characterization. The crystal structure and phase identification were investigated with a PANalytical Xpert Pro MPD X-ray diffractometer with Cu Ka radiation (l= 1.5406 A) with an operating voltage at 45 kV and current at 40 mA. The analysis of the diffraction pattern was completed by using X'pert HighScore Plus software. The surface area and internal pore characteristics were measured at 77 K by Micromeritics TriStar 3000 Analyzer applying Brunauer-Emmett-Teller (BET) manner and Barrett- Joy ner-Halenda (BJH) desorption method, after degassing at 150 ^°C for 3 hours to remove moisture. Fourier transform infrared spectrometer (Spectrum 100, PerkinElmer) was employed to study the functional group vibrations, scanning from 600- 4000 cm^-1 in UATR mode. Composition Analysis

[0060] The composition analysis of carbon nitride materials was performed using energy dispersive X-ray spectrometer (EDS). Figure 8 shows there were three dominant elements: carbon, nitrogen and oxygen in all the samples and the quantitative results are shown in Table 6. As shown in the results, the samples were high in nitrogen content; the nitrogen content was between 41 at% and 55 at%.

Table 6: Composition distribution of selected CN materials

Surface Area Analysis

[0061] Investigation of the pore texture of the selected carbon nitride materials was conducted by nitrogen sorption measured at 77 K. The pore characteristics, such as surface area, pore volume and average pore size, are shown in Table 7 and the nitrogen adsorption- desorption isotherm is shown in Figure 9. In addition to the four selected samples assessed above for hydrogen storage, two additional samples were also examined by BET surface area analysis to investigate the effects of bulking agent on the pore structure and the effects of calcination temperature on the pore structure. The two samples were: (1)“H₂₀₀ -C₅₆₀-CN without bulking agent” (Figure 5b), which was prepared using an analogous method as H₂₀₀ -C₅₆₀ -CN, but no bulking agent was loaded in the crucible, and (2)“H₂₀₀ -C₅₀₀-CN” (shown in Figure 5c), where the same HT melamine that was used for preparing H₂₀₀ -C₅₆₀ - CN, was instead calcined at 500 ^°C.

[0062] Based on IUPAC classification, all of the N2 adsorption-desorption isotherms could be classified as type IV isotherms and type H3 hysteresis loops, indicating the existence of mesopores and the occurrence of capillary condensation in the pores. As it can be seen in Table 7, among the selected samples, H₂₀₅ -C₅₆₀ -CN has the highest surface area, 148.69 m²/g, which is about 7 times higher than that of Bulk g-CN (21.38 m²/g). The high surface area may be ascribed to the high proportion of tube-like CN and its porous structure. It is of interest that H₂₀₅ -C₆₀₀ -CN has relatively low surface area (51.54 m²/g) and large average pore size (34.85 nm) despite the employment of the same HT melamine used in the H₂₀₅ -C₅₆₀-CN synthesis. This may suggest that higher calcination temperatures (e.g. >600 ^°C) could result in the loss of CN tubes, possibly due to the large surface area of the tubes being vulnerable to sublimation, which is consistent with the SEM image displayed in Figure 6d.

[0063] H₂₀₀-C₅₆₀-CN has a reasonable surface area (105.98 m²/g) and small pore size

(5.88 nm). However, if there was no bulking agent (e.g. AI₂O₃ balls) loaded in the crucible during calcination, the surface area was significantly reduced by 24 % to 80.33 m²/g (H₂₀₀- C₅₆₀-CN without bulking agent). If the pyrolysis temperature was lowered to 500 ^°C, the surface area was considerably decreased by 49 % to 54.27 m²/g (H₂₀₀-C₅₀₀-CN) and the pore volume (0.098 cm³/g) is even lower than that of Bulk g-CN (0.176 cm³/g), indicating poor conversion into porous texture.

[0064] The pore size distribution of the selected CN materials is shown in Figure 10. As can be observed, two samples, Bulk g-CN and H₂₀₅-C₆₀₀-CN which were both calcined at 600 ^°C under Ar, possess broad pore size regions above 30 nm, likely due to the deficiency of tubular CN. On the contrary, other samples, which were calcined at 560 ^°C in an isolated environment (i.e. sealed vessel), have a narrow pore size range centred at 4 nm.

[0065] In summary, calcination temperatures above about 600 °C or below about 500 °C tended to decrease the surface area of the pore-containing carbon nitride tubes obtained, likely as a result of increased decomposition of CN tubes above about 600 °C (even under Ar(g) atmosphere) or incomplete calcination below about 500 °C. A moderate temperature of about 560 ^°C was observed to be suitable for the preparation of pore-containing carbon nitride tubes with high surface area. In addition, conducting the calcination in a sealed vessel (e.g. a sealed crucible in a muffle furnace), was observed to be an advantageous way to produce a large quantity of good quality CN samples.

Table 7: BET surface area characteristics of carbon nitride materials

a BJH Desorption cumulative pore volume.

b BJH Desorption average pore diameter.

c The sample was prepared by the approach for synthesizing H₂₀₀-C₅₆₀-CN, but there was no bulking agent present during pyrolysis (Figure 5b).

d The sample was prepared by the approach for synthesizing H₂₀₀-C₅₆₀-CN, but the calcination temperature was 500 ^°C (Figure 5c).

* The samples in the thick frame are to compare the pore features of the CN materials prepared under three different pyrolysis conditions.

XRD Studies

[0066] The crystal structures of the selected carbon nitride materials were studied by XRD, as plotted in Figure 11. All of the treated samples have two major diffraction peaks, which is in good accordance with bulk g-CN. This indicates the crystal structure of pore- containing carbon nitride tubes remained unchanged as that of bulk g-CN after subjected to different thermal condensation treatments. The peak at 12.76° which has been indexed as the (210) plane in an orthorhombic geometry corresponds to the intralayer of tri-s-triazine units. The intensive peak at 27.7°, identified as the (002) plane, reflects the inter-planar distance between the graphitic layers of the stacking of the conjugated aromatic system. As observed in Figure 11, the low-angle (210) peak intensity of the samples containing tubular CN (Figure 1 lb-1 Id) is less pronounced compared with the intensity given by bulk g-CN. This could be due to the reduced planar size in pore-containing carbon nitride tubes.

Furthermore, the weaker intensity of high-angle (002) peak provided by tube-like samples could be attributed to the smaller size and loose stacking of graphitic CN layers. FTIR Studies

[0067] The structures of functional groups in four selected carbon nitride materials were further investigated by FTIR spectroscopy, as shown in Figure 12. The features of IR spectrum for these four samples are nearly identical, indicating the chemical structure of pore-containing carbon nitride tubes is similar to Bulk g-CN. That is, the hydrothermal treatment merely altered the physical structure of g-CN, rather than the chemical structure. The band at 806 cm^-1 represents the breathing mode of the tri-s-triazine ring units. The peaks at 1230, 1313 and 1396 cm^-1 correspond to aromatic C-N stretching, and the peaks at around 1568 and 1627 cm^-1 could be assigned to the existence of CN double bonds. The broad absorption band in the region of 3000-3500 cm^-1 could be related to the N-H stretching and O-H bonds of the adsorbed water molecules.

Hydrogen Storage Properties

[0068] Hydrogen storage and kinetics measurements were conducted using volumetric method by Sievert instrument where ultra-high purity hydrogen gas (99.999%) was utilized to avoid undesired toxic gas. The dead volumes of the system chambers were calibrated by introducing 0.1-0.5 MPa of hydrogen gas at different temperatures, which is on the basis of Japanese Industrial Standard H 7201 (JIS H 7201). Prior to the hydrogen storage investigation, the carbon nitride samples were degassed at 150 ^°C for 4 hours under vacuum (10 ³ torr) to eliminate moisture and residual gases, followed by a cooling down to room temperature.

[0069] The activated samples were stepwisely charged with pure hydrogen gas at room temperature and then the equilibrium values at different pressures were recorded. The desorption studies were established by reducing the hydrogen pressure in the system chamber isothermally. The operations were repeated to measure the hydrogen storage capacity of all the samples at 50 ^°C and 100 ^°C, respectively. Before each measurement, the samples were evacuated at 150 ^°C under vacuum for 4 hours. Based on the calculation of pressure drops, the hydrogen storage capacities can be obtained in weight percentage and be plotted in the form of pressure-composition-isotherm (PCI).

[0070] Because the samples were found to possess a low hydrogen absorption rate and very high desorption rate at or above 50 ^°C, the hydrogen absorption/desorption kinetics of the samples were studied at room temperature only. The absorption rate was measured under a sudden exposure to 3.6 MPa of pure hydrogen gas; while the desorption rate was measured under vacuum. The pressure decrease as a function of time was recorded.

[0071] The chemisorption of hydrogen was also measured by ChemiSorb 2750 (Micromeritics) from room temperature to 500 ^°C. Before chemisorption studies, the samples were charged with 10 %v/v H₂(g) in Ar(g) flowing gas at 20 ^°C, 50 ^°C and 100 ^°C, respectively.

[0072] The hydrogen storage capacities of four selected CN materials were

investigated in a volumetric manner under room temperature and up to 3.6 MPa of H₂ pressure, as depicted in Figure 13. The maximum hydrogen uptake of Bulk g-CN, H₂₀₀ -C₅₆₀ -CN, H₂₀₅ -C₅₆₀-CN and H₂₀₅ -C₆₀₀ -CN were 0.19, 0.31, 0.62 and 0.28 wt%,

respectively. Among these samples, H₂₀₅ -C₅₆₀ -CN provided the greatest hydrogen capacity of 0.62 wt%, which may be due to its highest surface area. As observed in Figure 14, the hydrogen storage capacities of the CN samples show good agreement with the variation of surface area. Since the results of EDS, XRD and FTIR did not show much contrast between the samples in elemental distribution, crystal structure and chemical structure, the difference in the BET surface area could be a critical determinant for the hydrogen storage performance of these CN materials.

[0073] In addition to the measurement at 20 ^°C, the hydrogen storage capacity of H₂₀₅- C₅₆₀-CN was further characterized at 50 and 100 ^°C, as displayed in Figure 15. The absorption/desorption capacities of H₂₀₅-C₅₆₀-CN were quite limited at 50 ^°C and 100 ^°C, indicating that at higher temperatures it is hard to physisorb hydrogen molecules on CN surface and chemisorb hydrogen atoms to form chemical bonds. Moreover, most of the hydrogen absorbed at 20 ^°C was able to be released at room temperature, which reveals advantageous storage reversibility. This was further supported by chemisorption test (Figure 16), where H₂₀₅-C₅₆₀-CN was first charged with 10% H₂+Ar at 20 ^°C, 50 ^°C and 100 ^°C and then dynamically discharged by heating from room temperature to 500 ^°C. The sample charged at 20 ^°C showed a significant desorption peak at around 25 ^°C and a lower peak at 60 ^°C, suggesting that the major desorption took place at room temperature and the complete desorption occurred at 60 ^°C. The samples charged at 50 ^°C and 100 ^°C both gave a desorption peak at about 40 ^°C, which was lower than their absorption temperatures. This desorption may originate from the residual hydrogen adsorbed on the material surface and the hydrogen is relatively easy to escape from the surface at near room temperature. The outcome of chemisorption is consistent with the result of hydrogen storage capacity measured by volumetric method in Figure 15, demonstrating that H₂₀₅ -C₅₆₀-CN is a room- temperature operational material.

[0074] The absorption and desorption kinetics of H₂₀₅ -C₅₆₀-CN was performed by a sudden exposure of the sample to 3.6 MPa of pure hydrogen and a vacuum condition, respectively. As illustrated in Figure 17, H₂₀₅ -C₅₆₀-CN was able to absorb and desorb hydrogen completely within 1 minute and 2.5 minutes, respectively, implying a rapid kinetics. This may be ascribed to the porous structure of CN tubes, where the reaction sites were saturated by H₂ or vacated promptly as hydrogen molecules can easily diffuse through the CN tube walls.

[0075] Based on the absorption capacity characterized at 20 °C, the H₂ capacity of H₂₀₅ -C₅₆₀ -CN up to 10 MPa was extrapolated, as shown in Figure 18, which shows that tubular carbon nitride material is expected to absorb 2.62 wt% of hydrogen at 10 MPa. As a person skilled in the art will appreciate, the pore-containing carbon nitride tubes may also find utility in the absorption, adsorption and/or desorption of gases other than hydrogen because of its highly porous structure.

[0076] It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

[0077] In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word“comprise” or variations such as“comprises” or“comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Claims

Claims:

1. A method of preparing pore-containing carbon nitride tubes, the method comprising:

2. The method according to claim 1, wherein the hydrothermally treated melamine is in contact with a bulking agent for at least a portion of the calcination step (step (b)).

3. The method according to claim 2, wherein the bulking agent is ceramic AI₂O₃ particles.

4. The method according to any one of claims 1 to 3, wherein the temperature in step (a) is from about 180 °C to about 220 °C.

5. The method according to any one of claims 1 to 4, wherein the temperature in step (b) is from about 540 °C to about 580 °C.

6. The method according to any one of claims 1 to 5, wherein the pores have a diameter of from about 1 nm to about 15 nm.

7. The method according to claim 6, wherein the pores have a diameter of from about

3 nm to about 7 nm.

8. The method according to any one of claims 1 to 7, wherein the tubes have a diameter of from about 0.2 mm to about 20 mm.

9. The method according to any one of claims 1 to 8, wherein the tubes have a length of from about 15 mm to about 150 mm.

10. A pore-containing carbon nitride tube obtained by the method according to any one of claims 1 to 9.

11. Use of a pore-containing carbon nitride tube obtained by the method according to any one of claims 1 to 9 in the adsorption or desorption of hydrogen.

12. A solid-state hydrogen storage material comprising pore-containing carbon nitride tubes obtained by the method according to any one of claims 1 to 9.