Introduction

Continuous use of fossil fuels has led to an annual increase in the average temperature of the Earth. To prevent the rise in global average temperature, the 21st United Nations Climate Change Conference, also known as the Paris Agreement, was adopted in December 2015 [1]. This agreement aims to gradually reduce greenhouse gas emissions. Consequently, a seismic shift is currently underway, transitioning from conventional carbon-based energy paradigms towards the sustainable and eco-friendly realm of hydrogen energy [2, 3]. Hydrogen, a fundamental element constituting more than 90% of the universe, is a clean energy source devoid of polluting byproducts. Unlike finite fossil fuels, hydrogen’s abundance is virtually inexhaustible. Traditional internal combustion engines that rely on fossil fuels operate with an efficiency of only 20–30%. In contrast, hydrogen-powered fuel cell systems can achieve an impressive 50–60% efficiency in electricity production and soar to 80–90% efficiency when harnessing waste heat, thus enabling remarkably efficient energy generation [4,5,6]. Due to these advantages, intensive research endeavors are underway to harness hydrogen for energy production, transportation, and storage.

Gas hydrates, crystalline compounds formed by capturing gas molecules within water-framework cages, have garnered much attention as a promising storage solution both environmentally and economically [7]. Due to their water-based composition, gas hydrates do not emit pollutants during gas storage and regeneration processes. Explorations into harnessing hydrates for hydrogen storage, particularly under conditions of low pressure and high temperature, are in progress. However, challenges remain, encompassing modification of formation requirements, enhancement of hydrogen storage capacity, and acceleration of formation rate of hydrates. Many researchers are actively exploring diverse strategies, employing thermodynamic and kinetic promoters to address these challenges.

Initially, research focused on capturing hydrogen within hydrates (i.e., formation of pure H2 hydrate), yielding substantial hydrogen storage capacity but demanding high-pressure, extremely low-temperature conditions. To overcome this issue, the use of thermodynamic promoters such as Tetrahydrofuran (THF) [8,9,10], Cyclopentane (CP) [11,12,13], and 1,3-dioxolane [14, 15] was proposed to achieve moderate hydrate-forming conditions. However, these thermodynamic promoters [16, 17], often called as “large guest molecules (LGMs)”, occupy a significant portion of the hydrate's large cages, diminishing space available for hydrogen storage. This trade-off relationship between hydrate-forming conditions and hydrogen storage capacities of binary (promoters + H2) hydrates poses a notable drawback for the utilization of promoters for hydrate-based hydrogen storage. Many researchers proposed a novel approach by using sub-stoichiometric concentrations of thermodynamic promoters to secure hydrogen capture space, offering a significant increase in hydrogen storage capacity.

Diverse research is actively being conducted to improve the formation rate of hydrates. The addition of kinetic promoters serves to expedite the hydrate formation process [16, 18]. Surfactants like Sodium Dodecyl Sulfate (SDS) [19, 20], alongside porous materials like silica gel [21,22,23,24], carbon nanotubes [25, 26], superabsorbent polymers [27,28,29], and porous carbon, are under exploration. These materials increase the interfacial area between water and gas, thereby promoting hydrate formation.

Moving forward, research and development in hydrogen technology must persist. The advancement of technologies that enable more stable and efficient production, storage, and transportation of hydrogen is a significant leap for our environment and the future. The collective thrust of research and development is directed at the hydrogen supply chain, including the progression of gas hydrate technologies. It is important to enhance the kinetics and storage capacity of gas hydrates for large-scale processes. Many researchers are making significant efforts to address this challenge. Future research directions also call for additional studies to capitalize on the environmentally friendly and economically advantageous features of hydrates while enabling their large-scale commercialization through viable processes.

Pure Hydrogen Hydrate

Many early studies focused on trapping pure hydrogen within hydrate structures. However, storing hydrogen within hydrate cages is challenging owing to its small molecular size. Typically, pure hydrogen hydrates are formed under harsh high-pressure and low-temperature conditions. According to Villard's rule, when each cage accommodates a guest molecule, the maximum H2:H2O ratio is 1:6. Vos et al. undertook the first attempt at capturing hydrogen within hydrates. Given the small size of hydrogen, exceptionally high-pressure conditions are required to trap it within the hydrate cages. In their study, hydrogen was stored in structure II (sII) hydrates at pressures ranging from 0.75 to 3.1 GPa and a temperature of 295 K. A hydrate form with an H2:H2O ratio of 1:1 was identified around 2.3 GPa [30]. Dyadin et al. used differential thermal analysis to examine the decomposition curve of sII hydrogen hydrates and asserted that an H2:H2O ratio of 1:6 could be achieved [31, 32].

The hydrogen–water system allows for the capture of hydrogen in two forms: sII hydrogen hydrates and the filled ice phase. Mao et al. captured 5.3 wt% hydrogen at 300 MPa and 249 K. The stability field of H2(H2O)-filled ice, which can store 11.2 wt% hydrogen, extends from 2300 MPa at 300 K to 600 MPa at 190 K [33]. Additionally, sII hydrogen hydrate formation with an H2/H2O molar ratio of 1:2 was observed at 220 MPa and 234 K. Raman spectroscopy revealed two hydrogen molecules in the small cage and four in the large cage, with the hydrate remaining stable at 145 K under a suitable pressure [34]. Zaghloul et al. also formed hydrogen hydrates and performed Raman spectroscopic analysis. All the samples were formed by pressurizing hydrogen below 2500 bar and then maintained under constant gas pressure for 1–2 days at 253 K. Raman analysis revealed an average of 1–3 hydrogen molecules captured in the large cage under the experimental temperature and pressure conditions [35].

Zhdanov et al. analyzed the phase diagram and composition of hydrogen–water system at pressure up to 2.5 GPa and temperatures from 200 to 330 K. They considered the phases of water: liquid, ice Ih filled with hydrogen, ice II filled with hydrogen, ice Ic filled with hydrogen and hydrogen hydrate of cubic structure. They calculated monovariant equilibrium lines of hydrogen—water system and the calculated data showed good agreement with known experimental data. Thus, it provides valuable insights into the behavior of hydrogen hydrates [36].

As mentioned above, capturing pure hydrogen within hydrates requires exceptionally high pressures; thus, this hinders hydrate-based hydrogen storage. Therefore, numerous studies have been conducted to lower the required pressure and raise the required temperature conditions for enabling hydrate-based hydrogen storage.

Binary Hydrogen Hydrate with Thermodynamic Promoters

Hydrates commonly form structure I (sI), sII, and structure H (sH) depending on the guest molecules trapped in the cages. In certain cases, semi-clathrate hydrates composed of ionically bound hydrate structures are formed. The structure of hydrates can be determined via X-ray diffraction (XRD) analysis. Pure hydrogen hydrates are generally known to have an sII structure. To lower the formation pressure and raise the formation temperature, thermodynamic promoters are adopted, and most promoters form sII hydrate by occupying the large cages. However, some promoters having a large molecular size are known to form structure H (sH) hydrates. This well-known structure of hydrates can be determined by the guest size: sI, which typically forms with small guest molecules (0.4–0.55 nm) and is the most common gas hydrate structure on Earth; sII, which generally forms with somewhat larger guest molecules (0.6–0.7 nm); and sH, which requires a combination of both small and large guest molecules for formation. The three types consist of a hydrogen-bonded water framework centered around a nearly spherical structural unit known as the pentagonal dodecahedron (small cage), which has 12 planar pentagonal faces (512). The preferred structure of clathrates is determined by the configuration that maximizes stability through the optimal matching of the size and composition of guest molecules and the cages. Matsumoto and Tanaka constructed a phase diagram to illustrate the hydrate formation. They plotted the chemical potential of water against the Lennard–Jones parameters for a single-component gas in various hydrate structures. This highlighted the conditions under which each structure becomes thermodynamically favorable [37].

In general, utilizing thermodynamic promoters can result in more favorable conditions for gas hydrate formation compared to those involving only pure hydrogen. One of the key indicators used to assess the conditions for gas hydrate formation is the phase boundary. When thermodynamic promoters are introduced, the phase boundary of hydrates shifts further to the right on the pressure–temperature diagram (indicating higher temperatures at any given pressure and lower pressures at any given temperature). This signifies that gas hydrates can be formed at elevated temperatures and reduced pressures compared to the conditions required for forming pure hydrogen hydrates. A diverse range of substances can function as thermodynamic promoters, and the choice of promoter can exert a significant impact on the phase equilibrium boundary of hydrates. Figures 1, 2, 3, and 4 illustrate the phase equilibrium boundaries of various hydrate systems having sI, sII, sH, and semi-clathrate hydrates, respectively.

Fig. 1
figure 1

Phase equilibria of sI hydrates with different hydrogen contents in gas mixtures: a 0–30 mol%, b 30–60 mol%, and c 60–90 mol%

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Fig. 2
figure 2

Phase equilibria of sII hydrates. Hydrogen hydrates using a THF, b other liquid promoters, and c gas promoters (* H2 (27.02%) + N2 (5.01%) + CH4 (61.87%) + C2H6 (3.22%) + nC4 (1.60%) + nC5 (0.64%); ** H2 (17.61%) + N2 (3.33%) + CH4 (57.73%) + C2H6 (2.05%) + nC4 (1.03%) + nC5 (0.41%) + CO2 (41.17%))

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Fig. 3
figure 3

Phase equilibria of sH hydrates

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Fig. 4
figure 4

Phase equilibria of semi-clathrate hydrates (*Concentration of promoters were recalculated)

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Zhang et al. investigated hydrate formation conditions using a mixture of hydrogen and hydrocarbon gases. This study employed 12 different gas mixture compositions within the temperature range of 274–280 K and the pressure range of 0.65–7.0 K. The reactor was pressurized to points higher than the estimated equilibrium values. Once hydrate formation was observed, they gradually reduced the pressure until the hydrate crystals disappeared. Subsequently, the pressure was slowly increased in steps of 0.05 MPa until hydrates reappeared. During a period of 4–6 h, temperature and pressure were maintained at constant values to attain equilibrium conditions. Through this approach, the equilibrium points exhibited strong agreement with previously reported data [38]. (Fig. 2c) Trueba et al. conducted experiments centered on the phase equilibrium of hydrates. They extensively investigated the phase equilibrium boundaries of hydrogen hydrates employing various thermodynamic promoters. The pressure range for phase equilibrium measurements spanned 2.0–14.0 MPa. They selected a binary system composed of cyclopentane (CP) (5.60 mol%), furan (5.61 mol%), 2,5-dihydrofuran (5.90 mol%), tetrahydropyran (6.02 mol%), and 1,3-dioxolane (5.62 mol%). These organic compounds acted as promoters for the hydrogen enclathration process. The stability sequence of the clathrate hydrates was ranked as follows: 1,3-dioxolane < 2,5-dihydrofuran < tetrahydropyran < furan < CP. Both the size and geometry of the large guest compounds influenced the stability of clathrate hydrates [39] (Fig. 2b).

The phase equilibrium studies of sI [40,41,42,43,44,45,46], sII [39,40,41, 44, 46,47,48,49,50,51,52,53,54,55,56,57], sH [58,59,60], and semi-clathrate hydrates [61,62,63,64,65,66,67,68] can provide valuable insights into gas hydrate research and the utilization of thermodynamic promoters, highlighting the potential to optimize gas hydrate formation conditions for various industrial applications, such as gas storage and transportation (Figs. 1, 2, 3, and 4).

sI Hydrogen Hydrate

The sI hydrate has a unit cell structure of 2(512)∙6(51262)∙46 H2O, comprised of 46 water molecules forming 8 cages within each unit [69, 70]. This particular structure is commonly formed in the presence of significantly small guest molecules (ranging from 0.4 to 0.55 nm). Notably, the sI hydrate has demonstrated the ability to accommodate various gas-phase co-guest molecules. Specifically, instances of CO2 and CH4 coexisting with hydrogen within the sI hydrate lattice have been reported. Grim et al. tried to store hydrogen within CO2 hydrate. Initially, they synthesized CO2 hydrate by pressurizing ice powder with CO2, subsequently subjecting it to hydrogen pressurization at 70 MPa and 258 K to generate the CO2 + H2 hydrate. Through Raman spectroscopy, the secure capture of single hydrogen molecules within the small cage and 1–4 hydrogen molecules within the large cage were confirmed. Similarly, CH4 hydrate was synthesized using an analogous methodology, followed by hydrogen pressurization at 10–130 MPa and 258 K to produce CH4 + H2 hydrate. In this case, the resultant CH4 + H2 hydrate exhibited the presence of a single hydrogen molecule within the small cage and 1–4 hydrogen molecules within the large cage [71]. Another investigation by Kim and Lee involved the synthesis of binary CO2 + H2 hydrates via a gas mixture of CO2 (20%) + H2 (80%) at 270 K and 120 bar. Remarkably, this process yielded a gas storage capacity of 7.5 mol%, a finding corroborated by spectroscopic analysis confirming the capture of two hydrogen molecules within the small cage [72].

Choi et al., conducted an analysis of spectroscopic patterns concerning mixed gas hydrates composed of H2 + CH4 and H2 + Xe. The experimental procedure involved grinding frozen deuterium oxide to a fine powder (approximately 200 μm) and subsequently pressurizing it with a feed gas. This study employed gas mixtures of H2 (165 bar) + CH4 (15 bar) and H2 (171 bar) + Xe (9 bar) at 203 K. Notably, the research revealed an intriguing phenomenon: at lower temperatures, hydrogen molecules were initially captured within the confines of the small cage within the sI hydrate structure. Subsequently, these molecules transitioned to the large cage through the shared pentagonal faces of the 51262 cages. It is worth highlighting that the hexagonal faces of the 51262 cage provided diffusion pathways for the hydrogen molecules. A pivotal factor in facilitating guest diffusion and enhancing their occupancy within intricate clathrate hydrate matrices was the interconnected vacant channels formed due to specific cage linkages. This intricate diffusion process and the role of unique cage arrangements in promoting guest molecule movement contribute significantly to our understanding of complex clathrate hydrate behaviors [73].

Kumar et al. utilized a gas mixture composed of CO2 (40 mol%) + H2 (60 mol%) to generate sI hydrate under conditions of 8.0 MPa and 253.15 K, yielding a hydrogen content of 8.2 mol%. Furthermore, through the pressurization of a mixture containing CO2 (38.2 mol%) + H2 (59.2 mol%) + C3H8 (2.6 mol%) at 3.8 MPa and 253.15 K, they achieved an elevated hydrogen storage capacity of 11.2 mol%. In this particular instance case, the presence of propane led to exclusive occupancy of the large cage, resulting in the formation of only sII hydrate. A sample analysis was conducted using powder PXRD, 1H MAS MR, 13C MAS NMR, mass spectroscopy, Raman spectroscopy, and FTIR. In sI hydrate preparations involving CO2 (40 mol%) + H2 (60 mol%) mixtures, the analysis revealed full occupancy of the large cage by CO2, while the small cage exhibited storage of H2 (9.3% of the small cage accommodated 2 H2 molecules, and 6.2% of the small cage accommodated 1 H2, as confirmed by NMR) [74].

sII Hydrogen Hydrate

The sII hydrates consist of a cubic unit cell containing 136 water molecules. This unit cell comprises 16 small 512 cages and eight large 51264 cages. The small cages exhibit a pentagonal dodecahedral shape, while the large cages are hexakaidecahedral structures, featuring 12 pentagonal faces and four hexagonal faces (51264) [75, 76].

The following studies were conducted using tetrahydrofuran (THF), which is the most common thermodynamic promoter for hydrate formation, to alleviate the formation conditions. Hester et al. analyzed THF + H2 hydrates using GPPD neutron diffraction to determine the average filling of hydrogen molecules. Pressurization resulted in hydrogen filling in the THF-d8 clathrate hydrate lattice. At 20 K, hydrogen molecules were observed to be occupied in sII small cages. The nuclear density was approximately spherically distributed and centered in the small cage. It is noteworthy that under a formation pressure of 70 MPa, hydrogen was observed to solely occupy the small cage of sII hydrate. This observation provides insights into the hydrogen occupation behavior in the THF binary hydrate system and helps resolve previous discrepancies [77]. Zhang et al. investigated the formation of hydrates by pressurizing a CH4 + H2 gas mixture with 6 mol% THF. The THF concentration was consistently maintained at 6 mol%, while the hydrogen ratio within the gas mixture was varied from 0 to 97.85 mol% in different experiments. Using the “pressure search method”, hydrate formation was verified within the temperature range of 277.7–288.4 K and a pressure range of 0.12–8.86 MPa [78]. Strobel et al. successfully stored approximately 1 wt% hydrogen in the sII large cage by introducing 5.6 mol% THF (according to chemical stoichiometry) at 13.8 MPa and 270 K. The process involved freezing a 5.6 mol% THF solution at 250 K for 3 days and then pressurizing the powdered THF hydrate with hydrogen. Additionally, at 270 K, they formed hydrogen hydrates using 0.5, 1.0, and 2.5 mol% THF solutions, resulting in a hydrogen storage capacity of 0.44 wt% [79]. Nagai et al. pressurized hydrogen into powdered THF hydrate with different particle sizes obtained using sieves of three ranges: 212–355 μm, 500–600 μm, and 1180–1400 μm. Smaller particle sizes resulted in an improved kinetic performance. Among these, the highest storage capacity of 0.24 wt% was obtained by grinding 5.56 mol% powdered THF hydrate (212–355 μm) at 269.5 K and 6.5 MPa [80]. Ogata et al. prepared powdered THF hydrates and pressurized them with hydrogen at 277.15 K, observing hydrogen storage capacities of 1.6 mol (H2)/mol (THF) (0.8 wt%) and 2.0 mol (H2)/mol (THF) at 70 and 85 MPa, respectively [81]. Sugahara et al. mixed powdered ice with solid THF (0.05 mol%) and pressurized it with 60 MPa hydrogen at 255 K. Hydrogen storage was verified by Raman spectroscopy, revealing storage in both small and large cages due to the lower THF concentration used. Volumetric gas release measurements showed a hydrogen storage capacity of 3.4 wt% [82]. Anderson et al. formed THF + H2 hydrate by pressurizing a 5.56 mol% THF solution with 30.3 MPa hydrogen at 283 K, observing a hydrogen storage capacity of 0.95 wt%. Also, 0.2 mol% THF + H2 hydrates showed 0.83 wt% of hydrogen storage capacity at 270 K, 30 MPa. phase equilibrium of THF + H2 hydrate was also presented [53]. (Fig. 2a) Grim et al. prepared a mixture of ground solid THF (~ 180 um) and solid ice, then pressurized it with hydrogen. The samples were quenched to liquid nitrogen (LN2), resulting in rapid growth of sII THF + H2 hydrate within 20 min. This growth occurred when preformed hydrate "seeds" of THF + H2 were present alongside unconverted ice during LN2 quenching. Confocal Raman spectroscopy was used to characterize the hydrate and post-quenched samples. The observations indicate that quenching to the LN2 temperature (a common preservation technique for ex-situ hydrate analysis) can unintentionally cause rapid hydrate growth, particularly due to increased kinetic effects facilitated by a preformed hydrate template [83]. Cai et al. used a THF aqueous solution with a concentration of 3.0 mol% to form THF + H2 hydrates for hydrogen storage at 273.15 K and 14.53 MPa. However, despite an extended growth period of 192 h, only a modest hydrogen density of 1.875 g/L water (equivalent to a gravimetric hydrogen storage capacity of 0.167 wt. %) was achieved for the formed hydrates [84]. Cai et al. investigated equilibrium points at 9.26, 10.94, and 12.67 °C and pressures of 18, 25, and 34 MPa, respectively, using a high-pressure micro-differential scanning calorimeter. The presence of memory water influenced only hydrate formation behavior and not dissociation behavior. The dissociation temperature of THF + H2 hydrate increased with higher operating pressures, and dissociation equilibrium data were obtained [85].

Tusda et al. reported that the hydrogen storage capacities of tetrahydrothiophene (THT) and furan hydrate are higher than that of THF hydrate, even though those hydrates have an identical sII structure. Powdered furan/THT hydrate was pressurized with hydrogen at 275.1 K, showing a storage capacity of 1.15 mol (hydrogen)/mol (THT hydrate) and 1.11 mol (hydrogen)/mol (furan hydrate). Furan hydrates with 5.6 mol% and THT hydrate with 5.6 mol% exhibited hydrogen storage of 0.6 mol% each at 41.8 MPa and 275.1 K [57]. Yoon et al. pressurized hydrogen with 12 MPa at 233 K into powdered 1,4-dioxane (with a concentration of 5.56 mol%) hydrate and the hydrogen was stored in the small cage of the sII hydrate. This yielded a hydrogen storage of 0.4 wt%. As the 1,4-dioxane concentration decreased, the hydrogen storage capacity increased up to a certain concentration. A maximum hydrogen storage capacity of 1.1 wt% was achieved at 0.2 mol% of 1,4-dioxane [86].

Seol et al. proposed epoxycyclopentane (ECP) as an innovative guest compound for energy storage in hydrates. ECP hydrates exhibited significantly improved properties in terms of thermodynamic stability, storage capacity, and formation conditions compared to hydrates containing THF and cyclopentane (CP). Furthermore, ECP was a powerful thermodynamic promoter that demonstrated a highly improved thermodynamic promotion effect compared to THF [87]. Chen et al. were also utilized ECP to form binary ECP + H2 hydrates: various approaches were evaluated to enhance the hydrogen storage capacity and formation rate in ECP hydrates at moderate pressures. This addressed the challenges of low gas-storage capacity and slow hydrate formation. The results demonstrated that the presence of ECP hydrate particles facilitated hydrate formation with smaller particle sizes. Therefore, the use of a three-layer stainless steel mesh combined with ECP hydrate particles yielded the highest gas storage capacity and rate. Dispersing ECP hydrate particles on a stainless-steel mesh caused significant enhancements in the gas storage capacity (0.64 wt%) and average gas storage rate (26.51 cm3/(cm3hydrate hour−1)). Other techniques, such as the addition of sodium dodecyl sulfate (SDS) or stainless steel fibers, have been adopted, but they showed limited effects on the hydrogen hydrate nucleation, growth, or storage capacity [88].

Ohgaki et al. used gas-phase co-guest species of propane and determined the cage occupancy of hydrogen at a temperature and pressure of 276.1 K and 5 MPa, respectively. A mixture of propane and hydrogen was pressurized with water, and equilibrium measurements were conducted. Raman spectroscopic analysis was performed to verify the cage occupancy. Hydrogen was trapped in the small cage of the sII structure. Gas chromatography equipped with a thermal conductivity detector was used to verify the presence of trapped hydrogen within the hydrates [89]. Ghaani et al. have also evaluated the utilization of gas-phase promoters for forming hydrogen hydrates. Propane was used as a gas promoter to store hydrogen at 25 MPa and 270 K. The storage process initially involved the generation of pure propane hydrate and subsequently, the loading of pure hydrogen. Molecular dynamics simulations were employed to assess the hydrogen storage capacity by assuming 100% occupancy of the large cage. This yielded a calculated hydrogen storage of 1.13 wt% (an H2O/C3H8/H2 ratio of 1360/80/160) However, the actual maximum hydrogen storage achieved experimentally was 1.04 wt% [90]. Skiba et al. investigated double clathrate hydrate of propane and hydrogen using Raman, XRD, and thermal analysis. The structure of double hydrates was the sII structure, which was pressurized to 2500 bar. The authors also formed hydrates with gas mixtures containing hydrogen from 40 to 80 mol% [91]. Yu et al. formed gas hydrates for CO2 recovery (CO2 (40.0 mol%) + H2 (60 mol%)) at 276.15 K and 6.0 MPa. Initially, a pure CP hydrate was formed. This was followed by pressurization with CO2 + H2 gas mixture to generate mixed hydrates. The presence of CP + gas (CO2, H2) mixed hydrates (sII) and pure carbon dioxide hydrates (sI) was verified by XRD and Raman spectroscopy [92].

sH Hydrogen Hydrate

The unit cell of sH hydrates having hexagonal symmetry contains 36 water molecules and include three 512 cages, two irregular dodecahedrons 435663 cages (medium sH cages), and a large icosahedron 51268 cage (large sH cage) [93, 94] that is isostructural with the hexagonal clathrate dodecasil 1H [95, 96]. Strobel et al. used cyclohexanone to encapsulate hydrogen within the hydrate structure. Typically, sH hydrate is formed with two types of guest molecules: a larger molecule with sizes between 7.5 and 9 Å occupies the large cages and smaller “help-gas” molecules occupy the small cages. If hydrogen can function as a help-gas molecule, hydrogen can be securely stored in small cages of binary cyclohexanone + H2 sH hydrate, and its hydrogen storage capacity could increase by approximately 40% compared to that of THF + H2 hydrate. The authors demonstrated the feasibility of utilizing the sH structure for storing hydrogen in hydrate media; however, the formation pressure required for these systems was considerably higher than that required for the sII structure achieved using THF as a promoter [97]. Strobel et al. also demonstrated the sH hydrate formed with other sH hydrate forming agents could achieve enhanced hydrogen storage capacity (40%) compared to sII hydrate. They chose sH hydrate-forming agents of methyl tert-butyl ether, methylcyclohexane, 2,2,3-trimethylbutane, and 1,1-dimethylcyclohexane. Crushed ice particles (< 250 µm) were saturated with the selected large guest molecules at a concentration of 2.9 mol%. The hydrates formed at 150 MPa and 275 K stored a hydrogen molecule in the small cage and the medium cage (1S, 1 M) with sH forming agents stored within large cages. Those sH hydrates exhibited a hydrogen storage capacity that was approximately 40% higher (by weight) than that of the sII hydrate [98].

Martin and Peters presented a calculation model for the phase equilibrium properties, which successfully predicted experimental results for methyl cyclohexane, methyl tert-butyl ether, 1,1-dimethyl cyclohexane. Under the experimental conditions for sH hydrate formation, the hydrogen storage capacity ranged from 0.85 wt% to 1.05 wt%. The model utilizes the van der Waals–Platteeuw statistical–thermodynamic model for hydrates and the cubic-plus-association equation of state for fluid phases, achieving good agreement between experimental and calculated data. Average absolute pressure deviations ranged from 1.2 to 2.1%, depending on the promoter. The model also estimated the cage occupancy of hydrogen and promoter molecules. Hydrogen occupied small and medium cages with promoters occupying large cages of sH hydrates. Under the conditions for sH hydrate formation (270–280 K and 60–10 MPa), the hydrogen storage capacity ranged between 0.85 and 1.05 wt% [99].

Babaee et al. developed a thermodynamic model to incorporate the promoting effects of various organic compounds, including alkanes, alkenes, alkynes, cycloalkanes, and cycloalkene. The model predicted that hydrogen would occupy 50–57% of the volume of the hydrate unit cell in all the cases. The study also predicted the phase equilibrium of the hydrogen–water system in the presence of organic promoters, using the van der Waals–Platteeuw solid solution theory and the Valerama–Patel–Teja equation of state (VPT-EoS) with non-density-dependent mixing rules. Although the experimental solubility data for hydrogen in the investigated promoters were insufficient, the VPT-EoS-GE method incorporating the UNIFAC activity model and modified Huron–Vidal mixing rules was used to model the phase behavior. The results showed a reasonable agreement with experimental data from the literature. This study also predicted the hydrogen storage capacity of the corresponding clathrate hydrates and the occupancies of the hydrate structures [100] (Fig. 3).

Semi-Clathrate Hydrate

Hydrates composed of quaternary ammonium salt or quaternary phosphonium salt ionic compounds featuring cations such as tetrabutylammonium (TBA+) or tetrabutylphosphonium (TBP+) exhibit a thermodynamic promoting effect; these are referred to as ionic or semi-clathrate hydrates. Commonly used quaternary ammonium salts include tetrabutylammonium bromide (TBAB) [101, 102], tetrabutylammonium chloride (TBAC) [103, 104], and tetrabutylammonium fluoride (TBAF) [105, 106]. Those ionic species cannot fit within the typical large cages of sI, sII, or sH hydrates; thus, the quaternary ammonium/phosphonium cations occupy four partially broken cages, forming semi-clathrate hydrate. Interestingly, unique structural transition phenomena have been reported in hydrates containing ionic species: tetrapropylammonium fluoride(TPAF) forms sII hydrate in the presence of THF at concentrations exceeding 149 mol% [107]. While tetramethylammonium hydroxide was known to form an orthorhombic Pnma structure with water molecules [108], a cubic Fd3m sII hydrate could be formed when small gaseous guest molecules coexist in the system [109]. Veluswamy et al. synthesized hydrogen clathrate hydrates using a combination of large guests, THF and TBAB: pressurizing 3.5 mol% TBAB with hydrogen at 279.2 K and 12 MPa resulted in a hydrogen storage capacity of 0.052 wt% [110]. Karimi et al. used 0.83 to 3.23 mol% TBAB to form hydrogen hydrates and analyze them by high-pressure differential scanning calorimetry. Higher pressures and tetrabutylammonium hydroxide (TBAOH) concentrations led to improved hydrogen storage capacity. The storage capacity of hydrates shows 0.35–0.47 wt%. The phase equilibrium of TBAOH hydrate at concentrations of 0.083–0.0323 mol% was determined in the temperature range of 273.15–303.15 K [111] (Fig. 4).

Deschamps and Dalmazzone formed hydrogen hydrates using tetrabutylphosphonium bromide (TBPB) at temperatures ranging from 285.0 to 287.2 K and pressures ranging from 12.1 to 23.3 MPa. At a TBPB concentration of 3.03 mol% and a pressure of 12.9 MPa, a hydrogen storage of 0.14 wt% was achieved at 285 K. This alternative approach using TBPB provides a potential solution for hydrogen hydrate formation with reduced environmental concerns, as TBPB is less hazardous than TBAF or TBAB [112]. Strobel et al. formed TBAB + H2 hydrate by pressurizing hydrogen on powdered TBAB hydrate at 13.8 MPa and 279.5 K. The TBAB semi-clathrate hydrate has a cage structure of 6(512)·4(51262)·4(51263)·76H2O, with six vacant dodecahedral cages per unit cell. The TBA+ occupied a large cage consisting of two tetrakaidecahedrons and two pentakaidecahedrons at stoichiometric concentration. Compared with THF + H2 hydrate, which stored 0.43 wt% hydrogen, the TBAB concentration of 2.71 mol% yielded a storage of 0.214 wt% hydrogen. The hydrogen stored in the semi-clathrate hydrate was trapped in the small cage at a hydrogen fugacity similar to that in the sII of THF + H2 hydrate [113]. Trueba et al. conducted experiments with different TBAB concentrations (2.6 and 3.7 mol%) and methods (the T-cycle and T-constant methods). The T-cycle method involves stabilizing the temperature at 289.15 K, pressurizing with hydrogen in the range of 5–16 MPa, and then gradually reducing the temperature to 281.15 K at a rate of 0.1 K/min. The T-constant method involves stabilizing the temperature at 281.15 K and subsequently pressurizing with hydrogen to the desired pressure. The higher pressures resulted in an increased hydrogen storage capacity and a decreased induction time. Hydrate formation was faster at a TBAB concentration of 3.7 mol% than at 2.6 mol%. At 16 MPa and 281.15 K, the hydrogen storage capacities were 0.031 and 0.046 wt% for TBAB 2.6 and 3.7 mol%, respectively. To achieve optimal hydrogen storage capacity, overcoming the mass transfer limitation is necessary, as hydrogen molecules do not reach the bottom of the sample. This can be addressed by increasing the contact area between the hydrate and the gas or liquid phases before or after hydrate formation [114]. The TBAF concentration had a negligible impact on the kinetics, whereas the pressure significantly influenced the formation rate. With 3.4 mol% TBAF, 0.024 wt% hydrogen was stored. Raman analysis verified that hydrogen dissolved better in the solution than in the TBAB + H2 semi-clathrate hydrate (at 13 MPa and 294.15 K). The higher solubility of hydrogen resulted in improved mass transfer and kinetics when TBAF was used [105].

Shin et al. achieved hydrogen storage of 0.5–1.35 wt% at pressures ranging from 70 to 253 MPa with a concentration of 2.54 mol%. In addition to the hydrogen released during hydrate dissociation, extra hydrogen was generated through the hydrolysis reaction between host water molecules and BH4− ions (NaBH4 + 2H2O → NaBO2 + 4H2). This yielded a 27% higher gravimetric hydrogen storage capacity compared to that of THF + H2 hydrate. The decomposition temperature of semi-clathrate hydrate of tetra-n-butylammonium borohydride (TBABh) (5.7 °C) is higher than the dissociation temperature of pure THF hydrate (4.4 °C). BH4− anions within substances forming semi-clathrate hydrates like TBAB or TBAF can stabilize tetraalkylammonium hydrates [115]. Prasad et al. formed double tert-butylamine + H2 hydrate (with structure VI) using 0.98–9.31 mol% t-BuNH2 at 13.58 MPa and 250 K. A structural transition from sVI to sII occurred under hydrogen pressure (13.8 MPa). Hydrogen molecules occupied a small cage, similar to THF + H2 hydrates, implying comparable occupancy. The hydrogen storage capacity reached up to 0.7 wt% at a molar concentration (5.56 mol%), which is identical to the stoichiometric concentration of sII hydrate [116]. Ogata et al. formed hydrates using 0.047 mol% trimethylamine (TMA) within pressure and temperature ranges of 0.33–156.9 MPa and 275.76–298.01 K, respectively. The high-pressure optical reactor used in this study was constructed from heat-treated stainless steel (SUS 630) and had a volume of 0.2 cm3. It featured a pair of sapphire windows, each 5.5 mm thick, positioned on the upper and lower parts of the reactor, allowing visual observation. The reactor was designed to withstand pressures of up to 500 MPa. Hydrogen molecules exclusively filled the small cages, while the mole fraction of TMA remained independent of the system pressure. The maximum hydrogen storage capacity was observed at 80 MPa. No structural or hydrogen occupancy variations were noted until the experimental pressure was increased to 170 MPa. With increasing pressure, hydrogen molecules progressively occupied vacant small cages until, at 80 MPa, all cages were filled [117].

A notable advantage of semi-structured ionic hydrates is their ability to maintain structure at high temperatures and low pressures compared to conventional gas hydrates, making them attractive for large-scale applications. However, due to the occupation of cages by thermodynamic promoters within hydrate structures, available space for hydrogen storage diminishes. Various research efforts have been undertaken with the aim of finding solutions to increase the hydrogen storage capacity achieved under moderate pressure and temperature conditions.

Enhancing H2 Storage Capacity Through Multiple H2 Occupancy

The utilization of thermodynamic promoters for forming binary hydrogen hydrates presents a notable advantage in alleviating formation conditions. However, it also carries the drawback of diminishing the hydrogen storage capacity due to the occupation of the large cages in the hydrates by the thermodynamic promoter. Several approaches have been explored to overcome these limitations. One well-known technique involves loading two to four hydrogen molecules into certain large cages of hydrates. Using a lower concentration of thermodynamic promoters creates empty large cages, allowing additional storage of hydrogen molecules (Table 1).

Table 1 Formation conditions, cage occupancies, and hydrogen storage capacities of hydrogen hydrates using various promoters (*1S and 2S: one and two hydrogen molecules in a single small cage; 1L, 2L, 3L and 4L: one, two, three, and four hydrogen molecules in a single large cage)
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Lee et al. formed THF hydrate by lowering the temperature of the THF solution, followed by crushing it into fine powders. The hydrogen was then pressurized to form a binary hydrogen hydrate with a THF concentration of 0.15 mol%. This enabled the creation of sufficient space to store hydrogen, with a hydrogen molecule stored in small cages and 1–2 hydrogen molecules stored in large cages. As a result, there was an increased storage capacity of 4.03 wt%. This increase is attributed to the tuning effect. Herein, the interaction between the thermodynamic promoter, water molecules, and cages causes the expansion of the cages, thereby allowing an increased number of hydrogen molecules (a maximum of three) to be accommodated in large cages. The non-stoichiometric concentration of 0.15 mol% yielded a higher storage capacity than the stoichiometric THF concentration of 5.6 mol% [55]. Sugahara et al. conducted an experiment to store hydrogen using acetone as a promoter at a concentration of 0.58 mol% under a pressure of 74 MPa and a temperature of 255 ± 2 K. The hydrates were formed by combining a frozen promoter with powdered ice. In this study, THF, acetone, cyclohexanone, and methylcyclohexane were used as promoters and Raman spectroscopy, volumetric gas release measurements, XRD were used to analyze them. At 70 MPa, H2 hydrates formed with THF, acetone, and cyclohexanone show hydrogen occupancy in large cages, while methylcyclohexane + H2 hydrates do not. The highest hydrogen storage capacity (3.6 wt%) occurs with 0.5 mol% acetone + H2 hydrates. Raman peaks of 0.5 mol% acetone + H2 hydrates reveal multiple hydrogen occupancies: 4 H2 in large cages, and 1 H2 in small cages. Additionally, they attempted to store hydrogen by varying the concentration. The experiments were performed at a temperature of 255 K and a pressure of 72 MPa [118]. Lu et al. employed a guest-exchange reaction to introduce hydrogen into pre-synthesized N2 hydrates. The authors hypothesized that larger N2 molecule could generate correspondingly larger cages, facilitating the capture of hydrogen clusters. The initial N2 hydrate particles were synthesized at 15 MPa and 243 K, and subsequently, they were subjected to a reaction with hydrogen at 243 K for a duration of 4 days. During the hydrogen charging process, temperature and pressure conditions were set at 77 K and 15 MPa, after which the temperature elevated to 243 K. While the pressure condition at 243 K was not specified, it is expected to have been significantly higher than 15 MPa due to the temperature increase. Although a minor presence of sII hydrates was detected in the sample, successful storage of hydrogen clusters within the hydrate matrix was achieved: two hydrogen molecules were in the small cages, and up to four hydrogen molecules were in the large cages. This guest-exchange reaction effectively lowered the pressure requirement for achieving multiple hydrogen occupancy within hydrate cages (traditionally necessitating several hundred MPa). Nonetheless, given the substantial remaining amount of N2 following the guest-exchange reaction, further research is imperative to enhance sample homogeneity [119]. Park et al. also utilized a guest-exchange reaction for loading hydrogen into binary (THF or pyrrolidine (PRD) + N2) hydrates. An aqueous solution containing water and 1 mol% of THF / PRD solution was frozen at 248 K and then ground into a fine powder. The powder was pressurized with N2 at 20 MPa and 243 K, forming binary THF / PRD + N2 hydrates. After 4 days, the gas phase was changed to H2, and the pressure was set at 15 and 35 MPa to initiate the guest-exchange reaction. The authors discovered that 1–2 hydrogen molecules were stored in the small cages at 15 and 35 MPa for THF + H2 hydrates. At 35 MPa, 1–4 hydrogen molecules were stored in the large cages, implying that the tuning phenomenon was successfully achieved. Similar results were obtained for the PRD + H2 hydrates, except for the large cage occupancy at 15 MPa, where 1–3 hydrogen molecules were found in the large cages of PRD + H2 hydrate. The preoccupied N2 could affect the H2 cage occupancy of binary (THF / PRD + H2) hydrates [120].

Some unique strategies have been suggested for achieving multiple hydrogen occupancies. Koh et al. employed a combination of structural transformation of sH hydrates to sII hydrates and the tuning effect. They employed water-soluble sH hydrate-forming agents: 1-methylpiperidine (1-MPD), 2-methylpiperidine (2-MPD), 3-methylpiperidine (3-MPD), and 4-methylpiperidine (4-MPD). Solutions of 1-MPD, 2-MPD, 3-MPD, and 4-MPD with a concentration of 0.5 mol% were exposed to hydrogen at 58 MPa and 240 K for over 2 days. Raman spectroscopy revealed that only 1-MPD hydrate exhibited a full tuning effect: 2–4 hydrogen molecules were stored in the large cages. The authors prepared 1-MPD hydrates varying the concentration of 1-MPD from 2.9 mol% (stoichiometric concentration of sH hydrate), to 2.0 mol%, 1.0 mol%, and 0.5 mol%. As expected, a sample with a 2.9 mol% showed the PXRD pattern of sH hydrates. For concentrations of 2.0 and 1.0 mol%, the PXRD patterns of both sII and sH hydrates were combined, while only sII PXRD patterns were present with the 0.5 mol% system. Considering the H2/H2O ratio of sII and sH hydrates, structurally transformed sII hydrate exhibited superior hydrogen storage capacity due to the tuning phenomenon, resulting in ratios of 1/2.8 to 1/4.25 compared to the sH hydrates (1/6.8) [121]. Moon et al. introduced an innovative approach involving proton irradiation for expanding the hydrate lattice. They prepared pure THF hydrate particles, which were then ground with a 150 μm sieve. These particles were irradiated by protons at a dose of 3 kGy, with a proton energy of 67 MeV, for 2 min. This study revealed that proton irradiation led to the generation of tetrahydrofuran-2-yl and tetrahydrofuran-3-yl radicals, as demonstrated through electron spin resonance analysis. Moreover, proton irradiation caused the polar-covalent bonds to shorten and the hydrogen bonds to elongate within the host water framework, resulting in an expanded hydrate lattice. The authors tried to load hydrogen into this expanded hydrate lattice at 10 MPa and 263 K, which was relatively higher than the conventional hydrogen storage temperature (typically around 243 K for achieving multiple hydrogen occupancies). Due to this expanded hydrate lattice structure, the small cages were able to securely capture up to two hydrogen molecules [122].

Some components of natural gas were employed to stabilize the hydrate structure enabling multiple hydrogen occupancies in hydrates. Koh et al. induced multiple hydrogen occupancies in hydrates using propane (at concentrations of 5.56, 1.0, and 0.5 mol%). They prepared both stoichiometric and nonstoichiometric hydrate samples by adjusting the concentration of liquefied propane. For the stoichiometric hydrate samples, pre-synthesized propane hydrates (0.8 MPa and 243 K for a week) were pressurized with H2 at 10–50 MPa and 243 K. Starting from 30 MPa, two hydrogen molecules in the small cages became apparent, and the population continued to increase up to 50 MPa. Regarding the nonstoichiometric hydrate samples, liquefied propane was mixed with ice powders with a concentration of 0.5 and 1.0 mol%. These powder samples were exposed to hydrogen at 50 MPa and 243 K for 3 days. These nonstoichiometric hydrate samples exhibited a tuning effect, enabling two to four hydrogen molecules to be stored in the large cages of hydrates with an expanded hydrate lattice (Fig. 5a). These experimental results were validated through grand canonical Monte Carlo (GCMC) simulations [123].

Fig. 5
figure 5

Schematics of multiple hydrogen occupations in small and large cages of hydrates and their spectroscopic evidence confirmed by Raman spectra: a stoichiometric (5.56 mol%) and nonstoichiometric concentrations (0.5 and 1.0 mol%) of propane + H2 hydrates [123], b synthetic pathway-dependent H2 enclathration behaviors within sI and sII hydrates [51] ((a) Reprinted (adapted) with permission from D. Y. Koh, H. Kang, J. Jeon, Y. H. Ahn, Y. Park, H. Kim, and H. Lee, The Journal of Physical Chemistry C, 2014, 118, 3324–3330. Copyright (2014) American Chemical Society; b Reprinted from Energy Storage Materials, 24, Y. H. Ahn, S. Moon, D. Y. Koh, S. Hong, H. Lee, J. W. Lee, and Y. Park, One-step formation of hydrogen clusters in clathrate hydrates stabilized via natural gas blending, 655–661, Copyright (2020), with permission from Elsevier)

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Ahn et al. utilized hydrogen-natural gas blend hydrates (consisting of CH4, C2H6, and H2) formed under moderate formation conditions (partial pressure of H2 at 3 MPa and a temperature of 263 K) without the involvement of organic promoter molecules. Since propane contributes exclusively to sII hydrate formation, the authors employed methane and ethane only to explore both sI and sII hydrates, varying the feed gas composition as follows: CH4 (70 mol%) + C2H6 (30 mol%) for sI hydrates, and CH4 (90 mol%) + C2H6 (10 mol%) for sII hydrates. Although certain gas-phase hydrocarbon promoters have been previously investigated, the guest-exchange method did not enable the multiple loading of H2 into a single cage of pre-synthesized hydrocarbon hydrate. A key discovery of this study lies in the inclusion of clustered molecular hydrogen (up to 3 molecules per cage), achieved by directly transforming ice particles into hydrates. This approach overcomes the storage capacity hurdle posed by hydrate synthesis using organic promoters. A direct evolution of mixed gas hydrates from ice powders with a feed gas composition of CH4 (60 mol%) + C2H6 (6.7 mol%) + H2 (33.3 mol%) achieved simultaneous multiple hydrogen occupancies in both large cages (~ 3 H2 molecules) and small cages (~ 2 H2 molecules), resulting in 22.4 mol% hydrogen within hydrate media [51] (Fig. 5b). Moon et al. tried to identify the critical hydrogen concentration in hydrogen-natural gas blends for maximizing hydrogen storage capacity in mixed gas hydrates. They prepared eight combinations of feed gas mixtures, varying methane, ethane, and hydrogen contents, and synthesized mixed gas hydrates through direct evolution at 9 MPa and 263.15 K. A CH4 to C2H6 ratio of 7–3 typically yielded sI hydrate. However, when hydrogen content became dominant (CH4 (14 mol%) + C2H6 (6 mol%) + H2 (80 mol%)) while maintaining the same CH4 to C2H6 ratio, a coexistence of sI and sII hydrates was observed. These findings emphasize the significance of both the CH4 to C2H6 ratio and hydrogen content in determining the structure of mixed gas hydrates. For case 3 with the feed gas composition of CH4 (27 mol%) + C2H6 (3 mol%) + H2 (70 mol%), the highest hydrogen storage capacity (30 mol% hydrogen) was achieved in sII hydrate, which exhibited multiple hydrogen occupancies in both large cages (2–4 H2 molecules) and small cages (2 H2 molecules); thus, this feed gas composition has a critical hydrogen concentration of 70 mol% [52].

Enhancing the Rate of H2 Enclathration

Gas hydrates are typically produced within a stirred reactor, as the bulk water phase necessitates agitation to facilitate the formation of these crystalline structures. However, the application of hydrate-based gas storage on a large scale introduces a challenge due to the energy-intensive nature of bulk water stirring, which consumes substantial amounts of energy [7, 125, 126]. Consequently, achieving an energy-efficient gas hydrate formation process calls for a reexamination of the fundamentals of the gas hydrate formation process. This formation process involves multiple stages, encompassing four distinct phases: (i) dissolution of gas, (ii) induction, (iii) nucleation, and (iv) growth of hydrate particles. Initially, the gas must be dissolved in water and subjected to an induction phase before nucleation can commence. During this stage, the foundational cages of the hydrate structure begin to take shape. After the induction phase, nucleation occurs, setting off the swift emergence of hydrate crystals that grow in size. The nucleation process involves the creation of small clusters of water molecules around the guest molecules, which then arrange themselves into nuclei forming a crystalline lattice structure. With nucleation initiated, the growth of hydrate particles accelerates; thus, the nucleation step governs the overall rate of gas hydrate formation (i.e., the nucleation step is the rate-determining step of gas hydrate formation) [123, 127,128,129,130].

Given the inherent complexity of hydrate formation reactions, the pursuit of enhanced kinetics assumes paramount significance, particularly within large-scale applications. In this endeavor, researchers have exhaustively explored various strategies to boost the reaction rate (Tables 2, 3). These strategies encompass reducing the induction period and enhancing the growth rate. To achieve these goals, researchers have harnessed porous materials and surfactants to amplify the interfacial interaction between water and gas molecules. Through these innovative approaches, the objective is to facilitate a more efficient mass transfer process, ultimately resulting in significantly elevated rates of hydrate formation [131,132,133,134] (Fig. 6a).

Table 2 Kinetic performance and storage capacity of hydrogen hydrates using porous materials (* H2 adsorption (wt%) at 90 min)
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Table 3 Kinetic parameter of hydrogen hydrates using surfactants and amino acids (* G1: CH4 (47.0%) + C2H6 (20.0%) + H2 (33.0%), ** G2: CH4 (27.0%) + C2H6 (3.0%) + H2 (70.0%), *** G3: CH4 (14.0%) + C2H6 (6.0%) + H2 (80.0%))
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Fig. 6
figure 6

Copyright © 2009 WILEY–VCH Verlag GmbH & Co. KGaA, Weinheim)

Schematics of accelerating hydrogen enclathration via various strategies: a Hydrate seeds with surfactants [156], b Amino acids [152], and c Superabsorbent polymers (lightly crosslinked poly(acrylic acid) sodium salt, PSA) [140] (a Reprinted (adapted) with permission from W. Lee, D. W. Kang, Y. H. Ahn, and J. W. Lee, ACS Sustainable Chemistry & Engineering, 2021, 9, 8414–8424. Copyright (2021) American Chemical Society; b Reprinted from Chemical Engineering Journal, 467, J. Zhang, Y. Li, Z. Yin, P. Linga, T. He, and X. Zheng, Coupling amino acid L-Val with THF for superior hydrogen hydrate kinetics: Implication for hydrate-based hydrogen storage, 143459, Copyright (2023), with permission from Elsevier, c Adapted with permission from Su et al.

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Porous Media

Saha and Deng loaded hydrogen into THF solution-bearing ordered mesoporous carbon. In contrast to the conventional hydrate formation process, they conducted sorption analysis to elucidate the hydrogen adsorption/desorption behavior within THF hydrate at 270 K and 163 bar. The authors aimed to increase hydrogen sorption capacity in ordered mesoporous carbons through THF + H2 hydrate formation; thus, they did not perform a systematic study to probe the impact of mesoporous carbon on the rate of hydrogen enclathration. They reported that hydrogen absorption capacities on the carbon loaded with 0.5 wt% and 0.75 wt% THF solution increased to 0.747 wt% and 0.646 wt%, respectively, compared to that of pure carbon (0.585 wt%) at 270 K and 163 bar. Although the authors did not explicitly mention whether the induction time decreased due to the dispersion of THF solution into the mesoporous carbon, it is reasonable to anticipate that the rate of hydrogen enclathration would be enhanced due to the increased contact area between the THF solution and hydrogen [135].

Saha and Deng performed a similar study and successfully achieved hydrogen storage at 1 wt% using porous media with THF (5.56 mol%) under conditions of 10 MPa and 270 K [136]. They prepared four types of porous materials with varying pore sizes: activated alumina synthesized by sol-gel technique (49 Å), 3'-DMT-da(Bz)-Suc-CPG (65 Å), silica gel (100 Å), and packing materials of an HPLC column separated from the binder (226 Å). Employing a shaker, they introduced a THF solution into the pores of the material for a 10-min duration. Subsequently, they introduced pure hydrogen into the reactor to form THF + H2 hydrate. The absorption/desorption behavior of hydrogen within THF hydrate was measured at 270 K and 100 bar, following the protocol reported in their previous study [135]. Through the application of a modified shrinking core kinetic model, they obtained the hydrogen diffusivity to be within the range of 10-18–10-19 m2/s. Notably, their findings indicated an inverse relationship between hydrogen diffusivity and the pore size of the matrix (corresponding to the particle size of the THF + H2 binary hydrate), consistent with a previous report [137, 138].

Su et al. employed emulsion-templated polymerized high internal phase emulsion (polyHIPE) as a supporting material for THF solution dispersion. Mercury porosimetry analysis revealed a narrow peak in the pore size distribution of polyHIPE at 9.1 μm. The dispersion of THF solution within porous polyHIPE created a substantial interfacial area between THF solution and hydrogen, along with a short diffusion path for hydrogen. Consequently, this led to a rapid pressure drop devoid of an induction time during the initial stage of hydrogen enclathration at conditions of 270 K and 11.6 MPa. The authors derived an H2 storage capacity of approximately 0.15 wt% from the pressure drop analysis and 0.4 wt% from the volumetric release experiments [139].

Su et al. conducted an additional study involving superabsorbent polymer networks to enhance the kinetics and recyclability of hydrogen enclathration (Fig. 6c). The superabsorbent polymer network was a crosslinked poly(acrylic acid) sodium salt (denoted as PSA in this study), having hydrophilic water-swellable characteristics. Their findings indicated that 20.0 g of THF solution could be absorbed by 1.0 g of dry PSA particles, resulting in the formation of transparent hydrogel particles. Following 5 cycles of hydrogen enclathration—regeneration, the THF solution-loaded PSA particles were able to retain their original structure and morphology. This suggests that this platform is reusable, demonstrating no phase separation during utilization. When compared to THF solution-bearing glass beads or emulsion-templated polystyrene, the PSA system exhibited an enhanced hydrogen enclathration rate, ultimately yielding a total hydrogen storage capacity of 0.15 wt%. NMR spectroscopy confirmed that hydrogen molecules were stored in the small cages of the hydrate structure [140]. In their conclusion, the authors stated that designing new hydrogels with distinct interconnected PSA networks could mitigate the loss of volatile THF promoters. However, Kang et al. reported that the hydrogen storage capacity within the THF solution-bearing superabsorbent polymer remained relatively constant, despite the loss of THF through evaporation over 20 cycles of hydrogen enclathration—regeneration. This phenomenon led to a tuning effect, where some methane molecules became trapped in the large cages of the sII hydrate structure formed by the evaporated THF molecules [27, 141].

A range of carbon materials has been utilized to accelerate hydrogen enclathration. Investigating the influence of activated carbon on the hydrogen storage attributes of THF hydrates, Fang et al. conducted experiments employing a high-pressure hydrate experimental apparatus. The findings revealed that while activated carbon did lead to a reduction in induction time, the impact was not notably substantial. On the other hand, it did manifest an observable effect on pressure drop, with activated carbon resulting in a more pronounced pressure drop compared to the absence of carbon. This differential suggested a higher hydrogen storage density. At an initial pressure of 8.4 MPa, the hydrogen storage density exhibited a considerable increase with the presence of activated carbon (0.0082 wt%) in contrast to its absence (0.0031 wt%). This investigation elucidated that activated carbon notably enhances the hydrogen storage density within THF hydrates, especially under elevated pressures [142]. Prasad and colleagues conducted a study to explore how various promoters could enhance the storage capabilities of clathrates under specific conditions: 263 K temperature and 10 MPa hydrogen pressure. They employed multiwall carbon nanotubes in their investigation. The focus was on hydrogen adsorption kinetics across four different clathrates, utilizing promoters such as THF, tetrahydropyran, 1,3 dioxolane, and 2,3 dihydrofuran, with multiwall carbon nanotubes serving as the substrates. The outcomes illustrated a notable uptake of about 1.5 wt% hydrogen within a 90-min timeframe when utilizing carbon nanotubes as the substrate. Notably, the highest hydrogen storage capacity (roughly 1.5 wt%) within 90 min was observed in clathrates formed with THF as the promoter. The findings from simulations underscore the significance of maintaining optimal temperatures to enhance hydrogen storage capacity within the reactor bed [143].

Although the primary objective of the study was CO2 capture, various porous materials such as polyurethane foam, cellulose foam [144], silica gel [21,22,23], and superabsorbent polymers [28] have been utilized to enhance the overall formation rate of gas hydrates. The main reason for utilizing porous materials is to increase the interfacial area between water and gas molecules: intensified interactions between water and gas molecules lead to a higher probability of gas hydrate nucleation and formation. Furthermore, given that water molecules are dispersed within the porous medium, gas molecules are readily absorbed into water droplets and efficiently diffused through the interconnected pore network. Due to these characteristics, the dispersion of water within porous materials does not require an energy-intensive stirring process during gas hydrate formation. As a result, porous materials offer a suitable solution for large-scale hydrate-based gas storage.

Surfactants and Amino Acids

Many studies have utilized kinetic promoters, such as surfactants or amino acids, to improve hydrate kinetics. A surfactant is composed of a hydrophilic head and a lipophilic tail, serving as a promoter for hydrate kinetics. Surfactants can be categorized into four primary types: (i) non-ionic, (ii) anionic, (iii) cationic, and (iv) zwitterionic surfactants. By enhancing gas solubility and reducing interfacial tension, surfactants contribute to rapid hydrate growth on reactor walls. Moreover, decreased interfacial tension can enhance water–gas contact, facilitating efficient gas diffusion within the bulk water [131, 146].

Amino acid molecules having hydrophilic groups exhibit a strong affinity for water, rendering them soluble in aqueous environments. However, their hydrophobic segments tend to repel water. The hydrophobic parts of amino acids tend to accumulate at the interface between water and other substances. This accumulation leads to a reduction in surface tension between the two phases, thus promoting hydrate formation. Additionally, the amino acids’ side chains are of sufficient length to hinder the formation of hydrogen bonds with water, thereby exerting a positive influence on the kinetics of hydrate formation. This particular property enables amino acids to disrupt the water structure surrounding the hydrate, prevent the aggregation of hydrate particles, enhance the local water structure, and ultimately increase the contact area between water and gas [132].

Veluswamy et al. prepared a THF solution, pressurizing it with hydrogen under driving forces of 2, 5, and 7 MPa. They performed kinetic experiments for hydrate formation varying THF concentrations ranging from 1 to 5 mol%. The increase in driving force and variations in THF concentration had a limited impact on reducing the induction time. However, at a concentration of 5 mol%, the hydrate growth rate increased compared to that at lower concentrations. The higher driving forces resulted in elevated overall growth rates. Interestingly, the addition of sodium dodecyl sulfate (SDS), an anionic surfactant, did not significantly affect the kinetics of THF + H2 hydrate. While the addition of SDS typically improves hydrate formation kinetics, this system did not show a significant enhancement in hydrogen enclathration rate. The authors suggested that a combination of molecular-level studies and macroscopic kinetic analyses is necessary to comprehend the effect of SDS for scaled-up applications. The highest gas consumption was observed at 5 mol% THF with a driving force of 7 MPa, resulting in a hydrogen uptake of 0.108 mol with a water-to-hydrate conversion of 10.8% [147]. Furthermore, Veluswamy et al. explored a mixed hydrate system composed of propane and hydrogen. To facilitate the formation of mixed gas hydrates, SDS was employed again using a stirring system (500 rpm) under conditions of 8.5 MPa and 274.2 K. In this system, the utilization of SDS effectively improved the overall rate of mixed gas hydrate formation. The time required for 90% completion of hydrate formation decreased from 334.2 minutes (without SDS) to 25.5 minutes (with over 100 ppm of SDS) in their reactor setup (internal volume of 142 ml with an additional gas chamber of 75 ml, and a loaded solution volume of 53 ml). Visual observation confirmed the growth of hydrates above the liquid-gas interface along the reactor walls, contributing to the rapid growth of mixed gas hydrates [148]. Comparing these two studies [147, 148], it is plausible to hypothesize that SDS might be effective when the system involves gas-phase co-guest species for accelerating hydrogen enclathration.

Utilizing 5.6 mol% of THF, CP, tetrahydropyran, dioxolane, tetrahydrothiophene, and 2,5-dihydrofuran as additives along with an anionic surfactant, Di Profio et al. engineered reverse micelles to enhance gas–water interface contact and improve hydrate formation rate at 10 MPa and temperatures ranging from 271.5 to 278.4 K. To establish reverse micelles and enhance the gas-water interface contact, isooctane (an organic solvent) and aerosol OT (AOT; an anionic surfactant) were employed, leading to an improved formation rate. Reverse micelles are characterized by enclosed microstructures wherein the hydrophilic part of an amphiphilic substance faces the interior while the hydrophobic part faces the exterior. In this study, tetrahydrothiophene exhibited the highest hydrogen storage capacity of 0.50 wt%, accompanied by a short induction time of 3 minutes. Conversely, CP and THF demonstrated lower capacities of 0.11 and 0.12 wt%, respectively. While the CP system showed an induction time of 11 minutes, hydrate formation promptly occurred in the THF system. Notably, the hydrogen storage capacity of these substances was independent of partition coefficient (log P) values, which typically relate to the dispersibility of polar and nonpolar substances. Instead, it was influenced by the topological polar surface area. This observation suggested that elevated values of this parameter corresponded to higher hydrogen storage capacities [149].

Veluswamy et al. conducted a comparative study on the effect of cationic surfactant (dodecyl trimethylammonium chloride; DTAC) and non-ionic surfactant (Tween-20) on the kinetics of hydrogen enclathration in the presence of 5.6 mol% THF solution. The authors examined hydrate formation rates while varying the concentrations of DTAC [0.01/0.05/0.5/1 wt%] and Tween-20 [0.01/0.05/0.1 wt%]. They found that 0.5 wt% DTAC and 0.1 wt% Tween-20 exhibited the swiftest hydrate formation kinetics, presenting an improvement of around 20% compared to the system without surfactant. Notably, the degree of enhancement in hydrate growth and the normalized rate of hydrate formation demonstrated no differentiation between the cationic and non-ionic surfactants. Additionally, the authors explored the decomposition kinetics of mixed THF + H2 hydrates; however, the type of surfactant did not significantly impact on the decomposition kinetics, similar to the findings in enclathration kinetics [150].

Pandey et al. performed a systematic study focusing on the formation of hydrogen-rich natural gas (HRNG) hydrates under various conditions. They manipulated the composition of gas mixtures containing methane, ethane, and hydrogen, and employed two types of thermodynamic promoters (THF and 1,3-dioxolane) along with two types of kinetic promoters (SDS and L-methionine). SDS concentrations were set at 500 and 3000 ppm, while L-methionine concentrations were 500, 3000, and 10,000 ppm. Through the rocking cell experiment, the authors confirmed that SDS performed better than L-methionine at the given concentrations of 500 and 3000 ppm. However, when altering the hydrogen contents in the feed gas mixtures, those kinetic promoters were ineffective in the case of higher hydrogen contents. This led to delayed formation kinetics and a diminished overall hydrogen storage capacity. Therefore, in hydrogen enclathration using methane and ethane as co-guest thermodynamic promoters, it becomes imperative to meticulously design the feed gas’s hydrogen content to attain a synergistic effect with the kinetic promoters [151].

Zhang et al., introduced a combined system involving a kinetic promoter (L-valine; L-Val) and a thermodynamic promoter (THF) to enhance the hydrogen enclathration rate at 12 MPa and 274.2 K (Fig. 6b). The induction time of THF + H2 hydrate was measured by varying the concentration of L-Val from 0.01 wt% to 1.0 wt%. The induction time monotonously decreased as the concentration of L-Val increased from 0 to 0.3 wt%, while a minor upturn was observed at an L-Val concentration of 1.0 wt%. The time-dependent volumetric hydrogen uptake exhibited a rapid increase within 2 hours after nucleation, with the increasing trends distinctly noticeable from 0.01 to 0.3 wt% of the L-Val system. The authors postulated that employing L-Val induced a porous hydrate structure, facilitating hydrogen diffusion into the solution’s lower section, consequently expediting hydrate formation and growth. However, the overall volumetric uptake decreased at an L-Val concentration of 1.0 wt%, attributed to the upper limit of amino acids in promoting gas hydrate formation [152].

The utilization of amino acids has undergone extensive investigation for methane enclathration [153] and carbon dioxide enclathration [154]. In particular, amino acids show promise as a viable substitute for SDS in the CO2 hydrate formation process. Dissolved bicarbonate ions (HCO3-) originating from carbon dioxide and DS- ions competitively adsorbs onto the hydrate surface [155]. Thus, the kinetic promotion capabilities of various amino acids such as L-Valine, L-Phenylalanine, L-Methionine, and others, along with their potential synergistic effects, have been explored to expedite CO2 hydrate formation [154]. However, the realm of hydrogen enclathration lacks sufficient research into the kinetic promotion effect of amino acids. Thus, further investigations are imperative at this stage to better understand their potential impact on hydrogen hydrate kinetics.

Future Prospects

Gas hydrates can serve as a gas storage media that store gas molecules within the cages constructed by water molecules. With water as their primary component, gas hydrates are regarded as environmentally friendly storage media, free from byproducts that could lead to pollution during the charging or discharging of gas species. Applying hydrates on a large scale presents challenges, primarily concerning kinetics and storage capacity improvement. Researchers are actively exploring strategies to optimize performance through the use of various thermodynamic and kinetic promoters.

Recent research endeavors have focused on elevating the formation rate of gas hydrates. Investigations have featured porous carbon and additives, such as surfactants and amino acids. While many of the previously investigated porous materials and additives have been geared toward methane or carbon dioxide storage applications, their application in hydrogen utilization systems would be promising. Furthermore, there is a need for research into hybrid systems that simultaneously employ multiple promoting agents, fostering synergistic effect among them. Studies should explore methods for enhancing storage capacity through the integration of non-stoichiometric concentrations of thermodynamic promoters, which would enable multiple hydrogen occupancies within the cages of hydrate.

Our review has encompassed the aforementioned studies; however, substantial breakthroughs are still necessary to achieve a sustainable, energy-efficient system that enables high hydrogen storage capacities. Very recently, Lee et al. proposed an innovative approach to enhance both hydrogen storage capacity and the formation rate of hydrates. They introduced a combination system involving pre-synthesized CP hydrate seed solution, which had an excess of liquid CP coexisting with SDS [156]. Hydrate seed particles serve as nuclei; thus, there was no induction time resulting in rapid growth of hydrate particles. With the help of SDS, CP + H2 hydrates grew vertically along the reactor wall, attaining a high hydrogen storage capacity (34.41 mmol H2/mol water) under moderate temperature (278 K) and pressure (12 MPa) conditions compared to previous studies [157]. Following their lead, innovative approaches should be explored to realize large-scale hydrate-based hydrogen storage techniques.