Hydrogen storage medium

Abstract

The present disclosure relates to hydrogen storage mediums comprising sodium borohydride, magnesium oxide, and magnesium which release hydrogen when contacted with a proton source. The present disclosure also relates to methods of producing hydrogen storage mediums.

Description

HYDROGEN STORAGE MEDIUM

[0001] This application claims priority from Australian Provisional Application No. 2024902636 filed on 23 August 2024, the contents of which are to be taken as incorporated herein by this reference.Technical Field

[0002] The present disclosure generally relates to systems and processes for hydrogen storage, more specifically to sodium borohydride-based hydrogen storage mediums that provide controlled release of hydrogen.Background of Invention

[0003] Hydrogen is a promising source of fuel due to its high energy per mass. Moreover, hydrogen is considered a clean source of energy because when it is used as a fuel, the only by-products are water and heat. However, hydrogen in its natural gaseous state, has a low energy density per unit volume and is challenging to store and transport. Hence, hydrogen storage is a key technology that will enable the use of hydrogen as a fuel. Solid-state hydrogen storage is one of the promising hydrogen storage technologies, which involves storing hydrogen in solid materials, from which the hydrogen can be released when needed.

[0004] Sodium borohydride (NaBH4) shows promise as solid-state hydrogen storage medium. In addition to having a high hydrogen storage capacity (10.6 wt% theoretically), being in solid state also means that it can store a large amount of hydrogen per unit volume, making it critical in energy storage applications. When sodium borohydride is contacted with a proton source, such as water, it can undergo a dehydrogenation reaction, releasing hydrogen:NaBH4+ (2 + X)H2O -> NaBO2-xH2O + 4H2(1)

[0005] The released hydrogen can be used as fuel. Moreover, the dehydrogenation reaction is exothermic, and the heat released can be harnessed for energy production.However, several challenges exist in the use of sodium borohydride as a hydrogen storage medium.

[0006] Sodium borohydride has a high activation energy and generally requires heat to initiate the hydrogen release process. This translates to slow hydrogen release rates at lowtemperatures which is impractical for many applications. In order to improve the hydrogen release rate, the use of a catalyst like nickel, cobalt, copper and ruthenium has been previously described. However, the hydrogen release efficiency and cost-effectiveness of using catalysts remains a challenge. Moreover, the reaction to release hydrogen from sodium borohydride produces normally hydrated sodium metaborate (NaBO2-xH2O) as a by-product, which is less soluble than sodium borohydride. As a result, more water is needed to fully dissolve the sodium metaborate, otherwise it can clog up the system. Having to add more water compromises the hydrogen capacity of the sodium borohydride hydrogen storage system.

[0007] Further, the reaction of sodium borohydride to release hydrogen is not easily reversible. Once the hydrogen is released, the process cannot be easily reversed to store hydrogen again. This limits the reuse of the sodium borohydride as a hydrogen storage medium. It is previously described that sodium metaborate can be ball-milled with a reducing agent (such as magnesium) to regenerate sodium borohydride.NaBO2-xH2O + 4Mg NaBH4+ 4MgO (2)

[0008] A previously described improvement on this regeneration process demonstrated that sodium metaborate in aqueous solution reacts with carbon dioxide, forming borax (Na2B4O7-10H2O) and sodium carbonate.4NaBO2-xH2O + CO2+ (10 - 4x)H2O -> Na2B4O7-10H2O + Na2CO3(3)

[0009] Both of these can then be ball-milled with a reducing agent (such as magnesium) under ambient conditions to form sodium borohydride.Na2B4O7-10H2O + Na2CO3+ 20Mg -> 4NaBH4+ 20MgO + CH4(4)

[0010] This previously published ball-milling process still has several drawbacks that would hinder the widespread adoption of sodium borohydride as a solid-state hydrogen storage medium. For example, separation of the sodium borohydride from the magnesium oxide is difficult, leading to loss of yield as well as hazardous conditions due to pyrophoric intermediates present. This separation step is also costly, both in terms of capital expenditure relating to equipment for separation and drying as well as operational expenditure such as labour, power and solvent. The costs and hazards associated with thisseparation step mean that use of sodium borohydride as a solid-state hydrogen storage medium becomes non-viable.

[0011] There is therefore an ongoing need for a high efficiency solid-state hydrogen storage medium, which at least partially addresses one or more of the above-mentioned short-comings or provides a useful alternative.

[0012] The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.Summary of Invention

[0013] The present inventors found that grinding a combination comprising borax, magnesium, sodium carbonate and optionally an additive, results in a mechanochemical reaction which surprisingly provides a solid-state hydrogen storage medium and obviates the need for costly and hazardous isolation of sodium borohydride. Moreover, hydrogen evolution from this medium does not require catalysts and can result in a higher hydrogen yield. This solid-state hydrogen storage medium comprises sodium borohydride, magnesium oxide, and magnesium and releases hydrogen in a controlled manner via a dehydrogenation reaction, either thermally or on contact with a proton source. This innovation provides a practical and efficient solution for hydrogen storage and transport.

[0014] In a first aspect, therefore, the invention provides a hydrogen storage medium comprising sodium borohydride, magnesium oxide, and magnesium produced by mechanochemical reaction of borax, magnesium, sodium carbonate, wherein the hydrogen storage medium releases hydrogen via a dehydrogenation mechanism.

[0015] In a further aspect, the mechanochemical reaction further comprises an additive, which is preferably selected from the group consisting of nickel, iron chloride, sodium chloride, stearic acid, methanol, tetrahydrofuran, and mixtures thereof. In another aspect, the dehydrogenation reaction occurs when the hydrogen storage medium is contacted with a proton source. Preferably, the proton source is carbonic acid. In another aspect, thedehydrogenation reaction is a thermal reaction which occurs when the hydrogen storage medium is heated.

[0016] In a further aspect, the invention provides a hydrogen storage medium according to the first aspect, wherein the hydrogen storage medium further produces one or more byproduct^) when contacted with the proton source, wherein the by-product(s) are soluble in water.

[0017] In a second aspect, the invention provides a process for producing a hydrogen storage medium, said process comprising: (i) combining borax, magnesium, sodium carbonate; and (ii) grinding the combination of borax, magnesium, sodium carbonate, resulting in a mechanochemical reaction to provide the hydrogen storage medium; wherein the hydrogen storage medium comprises sodium borohydride. In another aspect, the hydrogen storage medium further comprises magnesium and magnesium oxide. In another further aspect, the mechanochemical reaction further comprises an additive. In another aspect, the mechanochemical reaction further produces methane.

[0018] In a further aspect, the invention provides a process according to the second aspect, wherein the process further comprises (iii) contacting the hydrogen storage medium with water or other proton source, to initiate a dehydrogenation reaction to release hydrogen; and (iv) producing a regeneration feedstock comprising borax, magnesium salts, and sodium carbonate. Preferably, the proton source is carbonic acid.

[0019] The present disclosure is not to be limited in scope by the specific examples described herein, which are intended for the purpose of exemplification only. Functionally equivalent products, compositions and methods are clearly within the scope of the present disclosure.

[0020] Any embodiment of the present disclosure herein shall be taken to apply mutatis mutandis to any other embodiment of the disclosure unless specifically stated otherwise.Brief Description of Drawings

[0021] Embodiments of the invention will herein be illustrated by way of example only with reference to the accompanying drawings in which:

[0022] Figure 1 schematically depicts the process to produce a hydrogen storage medium and an isolated sodium borohydride according to some embodiments of the invention.

[0023] Figure 2 schematically depicts the release of hydrogen gas from the hydrogen storage medium according to some embodiments of the invention.

[0024] Figure 3 schematically depicts the release of hydrogen and regeneration of borax and sodium carbonate from the isolated sodium borohydride according to some embodiments of the invention.

[0025] Figure 4 illustrates ion current signals for Ar (m / z=40), H2 (m / z = 2) and CO2 (m / z = 44) for the residue gas analysis when the BMP is heated in a TGA furnace stepwise from room temperature to 600 °C, with 1 hour dwelling at 100 °C, 200 °C, 300 °C, 400 °C, 500 °C, 600 °C, respectively, (d) shows the sample temperature profile. The stepwise thermal dehydrogenation is demonstrated by the separate peaks of ion current signal of m / z = 2.

[0026] Figure 5 illustrates the hydrogen evolution rate for NaBFU self-hydrolysis (0 standard cubic centimetres per minute or seem), CO2 assisted NaBF hydrolysis (1 seem, 2 seem and 4 seem); and mass spectroscopy analysis of the gas collected for the 4 seem according to some embodiments of the invention.

[0027] Figure 6 illustrates the gas analysis for CO2 assisted NaBF hydrolysis at a flow rate of 1 seem and 2 seem according to some embodiments of the invention.

[0028] Figure 7 illustrates the FTIR spectra for CO2 assisted NaBF hydrolysis at a flow rate of 1 seem; 10 seem; 2 seem and 20 seem according to some embodiments of the invention.

[0029] Figure 8 illustrates the by-product characterisation for CO2 assisted NaBF hydrolysis at a flow rate of 10 seem: XRD and FTIR of the filtered solid residue showing the main by-product is borax decahydrate; XRD and FTIR of the dried products from the filtrate which consists of borax pentahydrate, Na2COs and NaCOOH according to some embodiments of the invention.

[0030] Figure 9 illustrates the thermogravimetric analysis (TGA) of air-dried filtered solid residue and commercial borax decahydrate, wherein the highlighted mass loss correspondsto the initial 5 structural water loss of borax decahydrate when heated according to some embodiments of the invention.

[0031] Figure 10 illustrates the characterisation of regenerated NaBFU with a comparison of the commercial product: FTIR spectra and XRD patterns according to some embodiments of the invention.

[0032] Figure 11 illustrates the pH value measurement for the CO2 assisted NaBH4 hydrolysis at a flow rate 1 seem and 2 seem and their hydrogen evolution performance according to some embodiments of the invention.

[0033] Figure 12 illustrates different by-products' effects on the hydrogen evolution performance with the pH value of the corresponding solution; a comparison between hydrogen evolution between B005, 0 seem and 1 seem according to some embodiments of the invention.

[0034] Figure 13 illustrates the FTIR for the CO2 assisted hydrolysis of NaBH4 over 120 minutes for 1 seem and 0 seem. Measurements are taken every 10 minutes.nB NMR at 0 min for both 0 seem and 1 seem;nB NMR at 120 min for both 0 seem and 1 seem, according to some embodiments of the invention.

[0035] Figure 14 illustrates the hydrogen evolution profile for the reactions of different borate solution (20 mL) added to 300 mg NaBH4 according to some embodiments of the invention.Detailed Description

[0036] Before describing the present invention in detail, it is to be understood that the terminology used herein is for the purpose of describing embodiments only and is not intended to be limiting.

[0037] Unless specifically defined otherwise, all technical and scientific terms used herein shall be taken to have the same meaning as commonly understood by one of ordinary skill in the art.

[0038] As used in this specification and the appended claims, the singular forms "a", "an" and "the" include plural referents unless the content clearly dictates otherwise.

[0039] Throughout the description and claims of the specification the word "comprise" and variations of the word, such as "comprising" and "comprises", is not intended to exclude other components, integers or steps. As used herein, "comprises" means "includes". Thus, "comprising A or B," means "including A, B, or A and B," without excluding additional elements.

[0040] The term "and / or" as used herein is to be taken as specific disclosure of each of the two specified features or components with or without the other. For example "A and / or B" is to be taken as specific disclosure of each of (i) A, (ii) B and (iii) A and B, just as if each is set out individually herein.

[0041] Those skilled in the art will appreciate that terms such as "hydrogen release", "hydrogen generation" and "hydrogen evolution" may be used interchangeably, and where these terms are used herein, they are to be taken as having the same meaning.

[0042] The present invention relates to a hydrogen storage medium comprising sodium borohydride, magnesium oxide, and magnesium produced by mechanochemical reaction of borax, magnesium, sodium carbonate, wherein the hydrogen storage medium releases hydrogen via a dehydrogenation mechanism.

[0043] The hydrogen storage medium is unpurified following the mechanochemical reaction. This obviates the need for hazardous and costly isolation of sodium borohydride. Additionally, the unpurified hydrogen storage medium more effectively stores hydrogen resulting in a higher hydrogen yield.

[0044] In one embodiment, the mechanochemical reaction further comprises an additive. Those skilled in the art will appreciate that the term "additive" as used herein refers to an added substance which contributes to the mechanochemical reaction to provide the hydrogen storage medium. Any additive suitable for the use in the present invention may be used. In a preferred embodiment, the additive is selected from the group consisting of nickel, iron chloride, sodium chloride, stearic acid, methanol, tetrahydrofuran, and mixtures thereof.

[0045] In one embodiment, the dehydrogenation reaction occurs when the hydrogen storage medium is contacted with a proton source. Those skilled in the art will appreciate that the term "proton source" as used herein is understood as a source of protons, whereinthe proton is preferably a hydrogen ion. Preferably, the proton source is selected from the group consisting of isopropyl alcohol, methanol, water, carbonic acid, formic acid, acetic acid, hydrochloric acid, and mixtures thereof. More preferably, the proton source is carbonic acid. A skilled person will appreciate that carbonic acid can be generated in a variety of ways, such as, for example, by introducing carbon dioxide into an aqueous solution (e.g., by bubbling gaseous CO2or by adding carbonated water), by acidifying a bicarbonate salt, by thermal decomposition of bicarbonates, by electrochemical conversion of carbon dioxide in aqueous media, by enzymatic hydration of carbon dioxide using carbonic anhydrase, or by increasing carbon dioxide solubility under pressure. In some embodiments, carbonic acid is generated in situ by bubbling carbon dioxide gas into the reaction solution or by adding sparkling water as a source of dissolved carbon dioxide. In some embodiments, carbonic acid is generated in situ by bubbling carbon dioxide gas into the reaction solution.

[0046] In another embodiment, the dehydrogenation reaction is a thermal reaction which occurs when the hydrogen storage medium is heated. For example, when the hydrogen storage medium is heated to 600 deg at a rate of 10 deg / C and held at a temperature of around 1002C, 2002C, 3002C, 4002C, 5002C and 6002C for about 30 -60 min, stepwise hydrogen release occurs at the temperature of around 1002C, 2002C, 3002C, 400 ?C, 500 ?C, and 600 ?C.

[0047] According to a preferred embodiment, the ratio of sodium borohydride to magnesium oxide of the hydrogen storage medium ranges from 1:1 to 1:6. For example, 1:1 to 1:2, 1:1 to 1:3, 1:1 to 1:4, 1:1 to 1:5, 1:2 to 1:6, 1:3 to 1:6, 1:4 to 1:6, 1:5 to 1:6, 1:2 to 1:4, or 1:3 to 1:5.

[0048] In another preferred embodiment, the ratio of sodium borohydride to magnesium ranges from 1:3 to 1:6. For example, 1:3 to 1:4, 1:3 to 1:5, 1:3 to 1:6, 1:4 to 1:5, 1:4 to 1:6, or 1:5 to 1:6.

[0049] According to another embodiment, the dehydrogenation reaction further produces one or more by-products, wherein the by-products comprise magnesium oxide, or magnesium salts, or mixtures thereof, wherein the by-products are soluble in water. Those skilled in the art will appreciate that the term "by-products" is understood as one or moresecondary products made as a result of the hydrogen storage medium coming in contact with a proton source.

[0050] The hydrogen storage medium described in the present disclosure provides efficient hydrogen release. In a further embodiment, the hydrogen release is more than 1.4 wt%. For example, more than 1.4, more than 1.6, more than 1.8, more than 2.0, more than 2.2, more than 2.4, more than 2.6, more than 2.8, more than 3.0, more than 3.2, more than 3.4, more than 3.6, more than 3.8, or more than 4.0. Preferably, the hydrogen release is more than 3.7 wt%.

[0051] The hydrogen storage medium described in the present disclosure provides for hydrogen release at a controlled rate. According to another embodiment, the hydrogen release occurs at a rate of 0.5-400 mL / min / g. For example, 0.5-50, 0.5-100, 5-150, 0.5-200, 0.5-250, 0.5-300, 0.5-350, 0.5-400, 5-50, 5-100, 5-150, 5-200, 5-250, 5-300, 5-350, 5-400, 50- 100, 50-150, 50-200, 50-250, 50-300, 50-350, 50-400, 100-150, 100-200, 100-250, 100-300, 100-350, 100-400, 150-200, 150-250, 150-300, 150-350, 150-400, 200-250, 200-300, 200-350, 200-400, 250-300, 250-350, 250-400, 300-350, 300-400, or 350-400 mL / min / g. Preferably, the hydrogen release occurs at a rate of 200-300 mL / min / g.

[0052] Those skilled in the art will appreciate that hydrogen release can be measured in a variety of ways such as, for example, by water displacement or mass flow controller. The units for hydrogen release might include, for example, wt%, and volume per unit time per unit mass.

[0053] Advantageously, the present disclosure allows hydrogen release to take place under ambient conditions. In a preferred embodiment, the hydrogen release takes place at 10-50 °C. For example, the hydrogen release may take place at 10-20, 10-30, 10-40, 10-50, 20-30, 20-40, 20-50, 30-40, 30-50, or 40-50 °C. In another preferred embodiment, the hydrogen release takes place at 1-5 atm. For example, the hydrogen release may take place at 1-2, 1-3, 1-4, 1-5, 2-3, 2-4, 2-5, 3-4, 3-5, or 4-5 atm. In some embodiments, the hydrogen release may occur at significantly higher pressures, such as up to 700 atm, depending on the system configuration and reaction conditions. In another preferred embodiment, the hydrogen release takes place at 1-700 atm. For example, the hydrogen release may take place at 5-50, 50-100, 100-200, 200-300, 300-400, 400-500, 500-600, or 600-700 atm.

[0054] The present disclosure provides a hydrogen storage medium with an efficient hydrogen storage capacity. In preferred embodiment, the hydrogen storage capacity is up to 2 wt%, preferably 3 wt%, more preferably 4 wt%. In some embodiments, the hydrogen storage capacity will include hydrogen from the proton source. Those skilled in the art will appreciate that hydrogen storage capacity can be expressed as wt% and in practical applications, the hydrogen storage capacity might be lower than the theoretical value due to various factors such as release mechanism and reaction mechanism.

[0055] Advantageously, the hydrogen storage medium demonstrates some air stability meaning that handling and storage are more straightforward. According to one embodiment, the hydrogen storage medium is air-stable for at least 30 minutes. For example, at least 5 min, at least 10 min, at least 15 min, at least 20 min, at least 25 min, or at least 30 min. Those skilled in the art will appreciate that the term "air-stable" as used herein refers to no substantial degradation in hydrogen storage capacity under normal temperature and pressure for at least the time period specified.

[0056] The present disclosure further relates to a process for producing a hydrogen storage medium, said process comprising: (i) combining borax, magnesium, sodium carbonate; and (ii) grinding the combination of borax, magnesium, sodium carbonate, resulting in a mechanochemical reaction to provide the hydrogen storage medium; wherein the hydrogen storage medium comprises sodium borohydride. In an embodiment, the hydrogen storage medium further comprises magnesium and magnesium oxide.

[0057] In one embodiment, the process in (i) further comprises combining an additive, and the combination in (ii) further comprises the additive. Any additive suitable for the use in the present invention may be used in the process. In a preferred embodiment, the additive is selected from the group consisting of nickel, iron chloride, sodium chloride, stearic acid, methanol, tetrahydrofuran, and mixtures thereof.

[0058] According to another preferred embodiment, the hydrogen storage medium releases hydrogen when in contact with a proton source or upon heating. Preferably, the proton source is selected from the group consisting of isopropyl alcohol, methanol, water, carbonic acid, formic acid, acetic acid, hydrochloric acid, and mixtures thereof. More preferably, the proton source is carbonic acid. A skilled person will appreciate that carbonicacid can be generated in a variety of ways, such as, for example, by introducing carbon dioxide into an aqueous solution (e.g., by bubbling gaseous CO2or by adding carbonated water), by acidifying a bicarbonate salt, by thermal decomposition of bicarbonates, by electrochemical conversion of carbon dioxide in aqueous media, by enzymatic hydration of carbon dioxide using carbonic anhydrase, or by increasing carbon dioxide solubility under pressure. In some embodiments, carbonic acid is generated in situ by bubbling carbon dioxide gas into the reaction solution or by adding sparkling water as a source of dissolved carbon dioxide. In some embodiments, carbonic acid is generated in situ by bubbling carbon dioxide gas into the reaction solution.

[0059] In another embodiment, the mechanochemical reaction takes place for 40-80 hours. For example, 40-50, 40-60, 40-70, 40-80, 50-60, 50-70, 50-80, 60-70, 60-80, or 70-80 hours.

[0060] In a further embodiment, the mechanochemical reaction takes place at 10-70 °C. For example, 10-20, 10-30, 10-40, 10-50, 10-60, 10-70, 20-30, 20-40, 20-50, 20-60, 20-70, 30- 40, 30-50, 30-60, 30-70, 40-50, 40-60, 40-70, 50-60, 50-70, or 60-70 °C.

[0061] According to another embodiment, the mechanochemical reaction takes place at1-10 bar in the ball mill. For example, 1-2, 1-3, 1-4, 1-5, 1-6, 1-7, 1-8, 1-9, 1-10, 2-3, 2-4, 2-5,2-6, 2-7, 2-8, 2-9, 2-10, 3-4, 3-5, 3-6, 3-7, 3-8, 3-9, 3-10, 4-5, 4-6, 4-7, 4-8, 4-9, 4-10, 5-6, 5-7, 5-8, 5-9, 5-10, 6-7, 6-8, 6-9, 6-10, 7-8, 7-9, 7-10, 8-9, 8-10, or 9-10 bar, preferably 1-2 bar.

[0062] According to a preferred embodiment, the ratio of borax to magnesium ranges from 1:1 to 1:3. For example, 1:1 to 1:1.5, 1:1 to 1:2, 1:1 to 1:2.5, 1:1 to 1:3, 1:1.5 to 1:2, 1:1.5 to 1:2.5, 1:1.5 to 1:3, 1:2 to 1:2.5, 1:2 to 1:3, or 1:2.5 to 1:3.

[0063] According to a preferred embodiment, the ratio of borax to sodium carbonate ranges from 1:1 to 1:2. For example, 1:1 to 1:1.5, 1:1 to 1:2, or 1:1.5 to 1:2.

[0064] According to a preferred embodiment, the ratio of magnesium to sodium carbonate is 15:1 to 25:1, For example, 15:1 to 16:1, 15:1 to 17:1, 15:1 to 18:1, 15:1 to 19:1, 15:1 to 20:1, 15:1 to 21:1, 15:1 to 22:1, 15:1 to 23:1, 15:1 to 24:1, 15:1 to 25:1, 16:1 to 17:1, 16:1 to 18:1, 16:1 to 19:1, 16:1 to 20:1, 16:1 to 21:1, 16:1 to 22:1, 16:1 to 23:1, 16:1 to 24:1, 16:1 to 25:1, 17:1 to 18:1, 17:1 to 19:1, 17:1 to 20:1, 17:1 to 21:1, 17:1 to 22:1, 17:1 to 23:1, 17:1 to 24:1, 17:1 to 25:1, 18:1 to 19:1, 18:1 to 20:1, 18:1 to 21:1, 18:1 to 22:1, 18:1 to 23:1,18:1 to 24:1, 18:1 to 25:1, 19:1 to 20:1, 19:1 to 21:1, 19:1 to 22:1, 19:1 to 23:1, 19:1 to 24:1,19:1 to 25:1, 20:1 to 21:1, 20:1 to 22:1, 20:1 to 23:1, 20:1 to 24:1, 20:1 to 25:1, 21:1 to 22:1,21:1 to 23:1, 21:1 to 24:1, 21:1 to 25:1, 22:1 to 23:1, 22:1 to 24:1, 22:1 to 25:1, 23:1 to 24:1,23:1 to 25:1, or 24:1 to 25:1.

[0065] According to a further embodiment, the ball mill is a vibration ball mill or a planetary ball mill. In a preferred embodiment, the mechanochemical reaction takes place in the vibrational ball mill at 850 - 1000 cycles per minute (CPM). For example, 850-900, 850- 950, 850-1000, 900-950, 900-1000, or 950-1000 CPM. In an alternate preferred embodiment, the mechanochemical reaction takes place in the planetary ball mill at 300 - 1800 rotations per minute (RPM). For example, 00 to 1800 include 300-600, 300-900, 300-1200, 300-1500, 300-1800, 600-900, 600-1200, 600-1500, 600-1800, 900-1200, 900-1500, 900-1800, 1200- 1500, 1200-1800, or 1500-1800 RPM.

[0066] The present invention further relates to the process for producing a hydrogen storage medium, wherein the process further comprises: (iii) contacting the hydrogen storage medium with a proton source or with heat, to initiate a dehydrogenation reaction to release hydrogen; and (iv) producing a regeneration feedstock comprising borax, magnesium carbonate, and sodium carbonate. Preferably, the proton source in step (iii) is selected from the group consisting of isopropyl alcohol, methanol, carbonic acid, formic acid, acetic acid, hydrochloric acid, and mixtures thereof. More preferably, the proton source is carbonic acid. A skilled person will appreciate that carbonic acid can be generated in a variety of ways, such as, for example, by introducing carbon dioxide into an aqueous solution (e.g., by bubbling gaseous CO2or by adding carbonated water), by acidifying a bicarbonate salt, by thermal decomposition of bicarbonates, by electrochemical conversion of carbon dioxide in aqueous media, by enzymatic hydration of carbon dioxide using carbonic anhydrase, or by increasing carbon dioxide solubility under pressure. In some embodiments, carbonic acid is generated in situ by bubbling carbon dioxide gas into the reaction solution or by adding sparkling water as a source of dissolved carbon dioxide. In some embodiments, carbonic acid is generated in situ by bubbling carbon dioxide gas into the reaction solution.

[0067] According to a preferred embodiment, the process further comprises recovering borax, magnesium salts, and sodium carbonate from the hydrogen storage medium.

[0068] In some further embodiments, the mechanochemical reaction further produces methane, which may be detected via gas analysis.Examples

[0069] The invention will now be further explained and illustrated by reference to the following non-limiting examples.Example 1: Ball Mill Procedures

[0070] 3-5 grams of borax, Mg, NazCCh, and additives were loaded into grinding jars (SO-125 mL) in an argon-filled glovebox. The grinding jars were sealed under argon and then inserted into the grinding stations of the ball mill. Ball milling was commenced and proceeded for a certain amount of time (40 - 60 hours for the vibration ball mill, 80 hours for the planetary ball mill). Upon completion of the ball milling, the grinding jars were transferred back to the glovebox, opened and the sample was collected. The yield of each sample was tested by quantitative NMR and iodometry, and the results are shown in Table 1 and Table 2. Table 1: Ball milling experiments carried out on planetary ball millTable 2: Ball milling experiments carried out on high energy ball millExample 2: Hydrogen Release via CO2 Assisted Hydrolysis Experimental Setup

[0071] A three-neck round bottom flask (100 mL) was used for CO2 assisted hydrogen release reaction. One neck was connected to a pressure-equalising addition funnel for adding water. One neck was connected to the CO2 supply for injecting CO2. One neck was connected to an adaptor used as the gas outlet. CO2 supply was connected to a mass flow controller for precise control of the flow rate. CO2 was bubbled into the solution via a long needle. The gas outlet was connected to a gas washing bottle filled with 60 mL IM NaOH as a CO2 trap. The CO2 trap was then connected to the burette for collecting the hydrogen.Hydrogen Release

[0072] 30 mg NaBF or 200 mg hydrogen storage medium (BMP) and 10 mL THF was loaded into the two-neck flask with a stirrer bar. The flask was connected to syringe pump, gas outlet, the CO2 supply and placed on a stirrer plate. 20 mL of quenching agent was addedusing a syringe pump. The stirrer plate was set the rpm at 300. For hydrogen release using water + CO2, the CO2 supply was switched on with a flow rate of 1 standard cubic centimetres per minute (seem). A CO2 trap of 1 M NaOH was used to trap excess CO2. The volume of hydrogen gas collected in the burette was noted every 10 minutes for 120 minutes. The results are shown in Table 3. For some proton sources, notably water (entries 1,2) and water plus carbon dioxide (entries 7,8), the ball mill product(BMP) outperforms sodium borohydride based on the yield of hydrogen.Table 3: Results from hydrogen release experimentstheoretical hydrogen volume is based on the amount of magnesium added during the ball milling step not the theoretical amount of sodium borohydride formed during the ball millingExample 3: Thermal Dehydrogenation

[0073] 9.3 mg of BMP was loaded into a TGA furnace which was connected with a Mass spectrometer for residue gas analysis. The TGA furnace was purged with argon gas for 30 minutes before heating up. The heating rate was 10 °C / min, the temperature was held constant for 1 hour at 100 °C, 200 °C, 300 °C, 400 °C, 500 °C, 600 °C. The ion currents for Ar (m / z=40), H2 (m / z = 2) and CO2 (m / z = 44) were recorded, and the results are shown in Figure 4.Example 4: CO2 Assisted Hydrogen Release via NaBH Hydrolysis

[0074] 300 mg NaBH4was loaded into a three-neck 50 mL round bottom flask. At the start of the reaction, 20 mL of DI water was injected into the flask via a syringe. CO2 was introduced into the solution via a long needle and the flowrate was controlled by a mass flow controller. The flowrate ranges from 1 seem to 4 seem. The reaction was stirred, and the gas outlet of the flask was connected to a CO2 trap filled with 60 mL 2M NaOH. A graduated burette was used to collect and measure hydrogen volume every 5 minutes for 120 minutes.Hydrogen Generation Performance

[0075] Figure 5 shows that CO2injection significantly increased hydrogen evolution rates. At 1 seem, the rate was approximately 8x higher than self-hydrolysis. Increasing the flow rate to 2 seem improved yield by 22%, while 4 seem gave a further 4.5% increase. Mass spectroscopy confirmed minimal CO2in the collected gas (Figure 6), validating the effectiveness of the NaOH trap.By-products Characterisation and Reaction Pathways

[0076] Upscaled reactions (3 g NaBH4, 50 mL DI water, 10-40 seem CO2) were performed to facilitate by-product recovery. After the CO2-assisted hydrolysis reactions were completed, the by-product solutions were cooled down to 0 °C for 30 min and the solid by-products (white precipitation) and liquid by-products were separated by filtration. Characterisation was carried out for both the filtered solid residue and the dried by-products from the filtrate. The filtered solid residue was identified as borax decahydrate, confirmed by XRD and FTIRanalyses (Figure 8a and 8b). This residue was air-dried, and TGA (Figure 9) showed that drying was sufficient to remove moisture.

[0077] The filtrate was dried via rotary evaporation, and the dried residue from the filtrate contained carbonate and formate species, specifically Na2CO3and NaCOOH. These were confirmed by XRD and FTIR (Figure 8c and 8d), and further supported by ion chromatography and NMR data (Figure 13). These results show that the by-products of CO2- assisted NaBH4 hydrolysis reaction contain borax, NazCCh and NaCOOH.Mechanistic Insights

[0078] The carbon element in the by-products exists as carbonate and formate. Without wishing to be bound by theory, the inventors suggest this implies that the CO2-assisted hydrolysis of NaBH4proceeds via two distinct pathways, both contributing to hydrogen evolution but with different chemical outcomes:• Pathway P#1 involves the reaction of NaBH4with water in the presence of CO2, leading to the formation of borax (Na2B4O7-10H2O) and sodium carbonate (Na2CO3). This pathway does not involve redox chemistry between CO2and BH4“. The role of CO2here is primarily to acidify the solution via carbonic acid formation, which shifts the pH to a range (9.5-9.8) that favours borate condensation into borax. The hydrogen yield from this pathway is equivalent to that of standard NaBH4hydrolysis (4 mol H2per mol NaBH4). This is the dominant pathway, as confirmed by stoichiometric analysis and ion chromatography (see Table 5).• Pathway P#2 involves a redox reaction between BH4“ and CO2, resulting in the formation of sodium formate (NaCOOH) alongside borax. This pathway consumes two equivalents of hydride per mole of CO2reduced, leading to a slightly lower hydrogen yield (14 mol H2per 4 mol NaBH4). The presence of NaCOOH in the dried filtrate (Figure 8d) and its quantification by ion chromatography (7.43 wt%) confirm this secondary route. The estimated contribution of P#2 is minor, with a P#1:P#2 ratio of approximately 11.3:1, corresponding to only ~1% hydrogen loss.

[0079] The formation of borax, particularly its three-coordinate boron species (B(3 )— O), appears to play a catalytic role in enhancing the hydrolysis kinetics. FTIR spectra (Figure 7) show the emergence of B(3)-0 and B(4)-0 bands within 10 minutes of CO2injection,coinciding with the onset of accelerated hydrogen evolution. NMR data (Figure 13) further support this, with B(OH)3signals appearing only in the CO2-assisted reactions.

[0080] Control experiments (Figure 12) show that adding borax to NaBH4in the absence of CO2significantly boosts hydrogen evolution (up to 503 mL in 120 min), while Na2CO3slows the reaction and NaCOOH has negligible effect. These results suggest that the enhanced performance of CO2-assisted hydrolysis is due to both increased proton availability and the in-situ formation of borax, which may facilitate hydrolysis through a catalytic or buffering mechanism.By-products Content Calculation

[0081] In a typical by-product recovery experiment, 3 g of NaBH4(98% purity) was hydrolysed under a CO2flow rate of 10 seem. The resulting solid and filtrate were dried and analysed. Based on the total boron and sodium content, the molar quantities of the byproducts were calculated to be 16.54 mmol of Na2B4O7-10H2O, 2.63 mmol of Na2B4O7-5H2O, 17.86 mmol of Na2CO3, and 3.17 mmol of NaCOOH. These results are consistent with the byproduct contents calculated according to the solubility of borax and ion chromatography as shown in Table 5. The ratio of NaCOOH to Na2CO3was approximately 1:5.6, indicating that the majority of NaBH4reacted via the carbonate-forming pathway.Table 5. The by-product content for the 10 seem.Regeneration of Na BH4 Experimental Setup

[0082] The regeneration of NaBH4was carried out by ball milling a mixture of 1.12 g borax decahydrate from the filtered solid residue by-product, 1.57 g of Mg and 0.31 g Na2COs in a Retsch PM 200 planetary ball mill at 400 cycles per minute for 80 hours. The loading of starting materials was done in an argon filled glovebox. The yield was determined from iodometry.Regeneration of Na BH4

[0083] The regeneration of NaBH4was carried out using borax decahydrate recovered from the CO2-assisted hydrolysis by-products, without further purification. A mixture of 1.12 g borax decahydrate, 1.57 g magnesium, and 0.31 g sodium carbonate (corresponding to a molar ratio of Mg:Na2CO3:borax = 22:1:1) was loaded into a planetary ball mill (Retsch PM 200) and milled at 400 rpm for 80 hours under an inert argon atmosphere. The yield of NaBH4was determined by iodometric titration and found to be 76.3%.

[0084] Characterisation of the regenerated product by FTIR and XRD confirmed the formation of high-purity NaBH4. The FTIR spectrum showed characteristic B-H stretching and bending vibrations at 2200-2400 cm-1and 1100 cm-1, respectively, consistent with commercial NaBH4(Figure 10). The XRD pattern matched the reference pattern for NaBH4, and was indistinguishable from that of the commercial standard (Figure 10).

[0085] Signals consistent with the presence of methane (CH4) were observed in the gas phase during milling, which may be indicative of the overall reaction equation (4). While not definitive, these observations lend support to the proposed mechanism and suggest the potential for carbon capture and utilisation via conversion of carbonate to methane under mechanochemical conditions.Mechanistic insights into Enhanced Hydrogen Evolution Performance

[0086] Without wishing to be bound by theory, the enhanced hydrogen evolution observed during CO2-assisted hydrolysis of NaBH4is attributed to both increased proton availability and the formation of borate species, particularly borax. CO2dissolves in water to form carbonic acid, which lowers the pH and facilitates hydrolysis. As shown in Figure 11, the pH remains relatively stable throughout the reaction, indicating that the increased hydrogen evolution rate is not solely due to acidification but also involves other chemical effects.

[0087] FTIR analysis (Figure 7) revealed the early appearance of bands corresponding to three- and four-coordinate boron species, whilenB NMR spectra (Figure 13) confirmed the formation of B(OH)3in CO2-assisted reactions but not in self-hydrolysis. These species are known to accelerate NaBH4hydrolysis, and their presence correlates with the observed increase in hydrogen evolution rate.

[0088] Comparative experiments (Figure 12) demonstrated that the addition of borax significantly enhances hydrogen evolution, whereas Na2CO3inhibits it and NaCOOH has negligible effect. Further experiments using different borate solutions (Figure 14) showed that hydrogen evolution increases with the concentration of three-coordinate boron species, regardless of pH. These results suggest that the formation of borax not only contributes to the reaction pathway but also plays a catalytic role in improving the kinetics of hydrogen release.* * *

[0089] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.

Claims

CLAIMS1. A hydrogen storage medium comprising sodium borohydride, magnesium oxide, and magnesium produced by mechanochemical reaction of borax, magnesium, sodium carbonate; wherein the hydrogen storage medium releases hydrogen via a dehydrogenation reaction.

2. The hydrogen storage medium according to claim 1, wherein the mechanochemical reaction further comprises an additive.

3. The hydrogen storage medium according to claim 2, wherein the additive is selected from the group consisting of nickel, iron chloride, sodium chloride, stearic acid, methanol, tetrahydrofuran, and mixtures thereof.

4. The hydrogen storage medium according to any one of claims 1 to 3, wherein the dehydrogenation reaction occurs when the hydrogen storage medium is contacted with a proton source.

5. The hydrogen storage medium according to claim 4, wherein the proton source is selected from the group consisting of isopropyl alcohol, methanol, water, carbonic acid, formic acid, acetic acid, hydrochloric acid, and mixtures thereof.

6. The hydrogen storage medium according to claim 5, wherein the proton source is carbonic acid.

7. The hydrogen storage medium according to claim 6, wherein the carbonic acid is generated in situ.

8. The hydrogen storage medium according to claim 7, wherein the carbonic acid is generated in situ by one or more of: introducing carbon dioxide into an aqueous solution; acidifying a bicarbonate salt; thermal decomposition of a bicarbonate salt; electrochemical conversion of carbon dioxide in aqueous media; enzymatic hydration of carbon dioxide; or increasing carbon dioxide solubility under pressure.

9. The hydrogen storage medium according to claim 8, wherein the carbonic acid is generated in situ by bubbling carbon dioxide gas into the reaction solution or by adding sparkling water containing dissolved carbon dioxide.

10. The hydrogen storage medium according to any one of claims 1 to 3, wherein the dehydrogenation reaction is a thermal reaction which occurs when the hydrogen storage medium is heated.

11. The hydrogen storage medium according to any one of claims 1 to 10, wherein the ratio of sodium borohydride to magnesium oxide is from 1:19 to 1:6.

12. The hydrogen storage medium according to claim 11, wherein the ratio of sodium borohydride to magnesium is from 1:3 to 1:6.

13. The hydrogen storage medium according to any one of claims 1 to 12, wherein the hydrogen storage medium further produces one or more by-products when contacted with the proton source.

14. The hydrogen storage medium according to claim 13, wherein the by-products comprise magnesium oxide, or magnesium salts, or mixtures thereof.

15. The hydrogen storage medium according to claim 13 or claim 14, wherein the byproducts are soluble in water.

16. The hydrogen storage medium according to any one of claims 1 to 15, wherein the hydrogen release is more than 2 wt%, preferably more than 3.7 wt%.

17. The hydrogen storage medium according to any one of claims 1 to 16, wherein the hydrogen release occurs at a rate of 0.5-400 mL / min / g.

18. The hydrogen storage medium according to any one of claims 1 to 17, wherein the hydrogen release takes place at 10-50 °C.

19. The hydrogen storage medium according to any one of claims 1 to 18, wherein the hydrogen release takes place at 1-700 atm.

20. The hydrogen storage medium according to any one of claims 1 to 19, wherein the hydrogen storage capacity is up to 2 wt%, preferably 3 wt%, more preferably 4 wt%.

21. The hydrogen storage medium according to any one of claims 1 to 20, wherein the hydrogen storage medium is air-stable for at least 30 min.

22. A process for producing a hydrogen storage medium, said process comprising:(i) combining borax, magnesium, sodium carbonate; and(ii) grinding the combination of borax, magnesium, sodium carbonate, resulting in a mechanochemical reaction to provide the hydrogen storage medium; wherein the hydrogen storage medium comprises sodium borohydride.

23. The process according to claim 22, wherein (i) further comprises combining an additive; and wherein the combination in (ii) further comprises the additive.

24. The process according to claim 23, wherein the additive is selected from the group consisting of nickel, iron chloride, sodium chloride, stearic acid, methanol, tetrahydrofuran, and mixtures thereof.

25. The process according to any one of claims 22 to 24, wherein the hydrogen storage medium further comprises magnesium and magnesium oxide.

26. A process according to any one of claims 22 to 25, wherein the hydrogen storage medium releases hydrogen when in contact with a proton source.

27. The process according to any one of claims 22 to 26, wherein the mechanochemical reaction takes place for 40 - 80 hours.

28. The process according to any one of claims 22 to 27, wherein the mechanochemical reaction takes place at 10-70 °C.

29. The process according to any one of claims 22 to 28, wherein the mechanochemical reaction takes place at 1-10 bar in the ball mill.

30. The process according to any one of claims 22 to 29, wherein the ratio of borax to magnesium ranges from 1: 18 to 1:24.75.

31. The process according to any one of claims 22 to 30, wherein the ratio of borax to sodium carbonate ranges from 1:1 to 1:1.5.

32. The process according to any one of claims 22 to 31, wherein the ratio of magnesium to sodium carbonate is 15:1 to 24.75:1.

33. The process according to any one of claims 22 to 32, wherein the mechanochemical reaction takes place in a ball mill.

34. The process according to claim 33, wherein the ball mill is a vibration ball mill or a planetary ball mill.

35. The process according to claim 24, wherein the mechanochemical reaction takes place in the vibrational ball mill at 850 - 1000 cycles per minute (CPM).

36. The process according to claim 24, wherein the mechanochemical reaction takes place in the planetary ball mill at 300 - 1800 rotations per minute (RPM).

37. The process according to any one of claims 22 to 36, wherein the process further comprises:(iii) contacting the hydrogen storage medium with a proton source or with heat, to initiate a dehydrogenation reaction to release hydrogen; and(iv) producing a regeneration feedstock comprising borax, magnesium salts, and sodium carbonate.

38. The process according to claim 37, wherein the proton source in step (iii) is selected from the group consisting of water, isopropyl alcohol, methanol, carbonic acid, formic acid, acetic acid, hydrochloric acid, and mixtures thereof.

39. The process according to claim 37 or claim 38, wherein the proton source is carbonic acid.

40. The process according to claim 39, wherein the carbonic acid is generated in situ.

41. The process according to claim 40, wherein the carbonic acid is generated in situ by one or more of: introducing carbon dioxide into an aqueous solution; acidifying a bicarbonate salt; thermal decomposition of a bicarbonate salt; electrochemical conversion of carbon dioxide in aqueous media; enzymatic hydration of carbon dioxide; or increasing carbon dioxide solubility under pressure.

42. The process according to claim 41, wherein the carbonic acid is generated in situ by bubbling carbon dioxide gas into the reaction solution or by adding sparkling water containing dissolved carbon dioxide.

43. The process according to claim 37, wherein the mechanochemical reaction further produces methane.

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