Hydrate-based H₂ storage with porous materials as heterogeneous promoters: state of the art and challenges

Abstract

Clathrate hydrates, which can store hydrogen inside crystalline, ice-like structures, have great potential for hydrogen storage. However, kinetic and thermodynamic promoters are often needed to improve the formation rates and stability ranges. Porous materials exhibit significant potential for hydrate-based hydrogen storage by modulating the kinetics, stability, and storage capacity, unlocking substantial application prospects. This review systematically elucidates the critical mechanisms through which porous materials influence hydrogen hydrate behavior, with a comprehensive analysis of the synergistic roles of material properties and engineering operation conditions. Material properties include the nano-confinement effect, which markedly enhances hydrate formation, optimized pore and particle sizes that increase contact area, functionalized surfaces and rough structures that improve nucleation and stability, and moderate hydrophobicity that enhances gas–water contact. Engineering operation conditions involve maintaining suitable temperatures and pressures to ensure stable hydrate formation, uniform spatial layouts to optimize gas diffusion, and water saturation control to boost reaction efficiency. The review further summarizes the application characteristics of various porous materials, including carbon-based materials (e.g. activated carbon), inorganic materials (e.g. silica), organic porous polymers (e.g. polyurethane foam), and hybrid materials (e.g. metal–organic frameworks), evaluating their respective strengths, limitations and suitability. Multiscale insights highlight the macroscopic focus on hydrate formation within high-pressure reactors, the mesoscopic emphasis on optimizing particle surface reactions, and the microscopic attention to confined hydrate growth within pore structures. Future research should prioritize the refinement of nanopore architectures, the development of advanced hydrophilic/hydrophobic materials, the enhancement of reactor designs, and the integration of thermal management and kinetic optimization to propel hydrogen hydrate storage technology toward practical implementation.

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Lijin Chen

Lijin Chen is a PhD candidate at the School of Engineering, Australian National University (ANU). She commenced her PhD in 2024 with support from the ANU HDR and Energy Research Supplementary Scholarships, following a Master's degree in Thermal Engineering from Tianjin University. Her research explores advanced hydrate-based hydrogen storage, with a particular interest in improving hydrogen storage capacity through nanoconfinement and thermodynamic–kinetic approaches. With a background spanning both materials and system-level engineering, her work takes an interdisciplinary perspective aimed at contributing to sustainable energy research.

Valeska P. Ting

Valeska Ting (CEng, FIOM3, FRACI) is a Professor in the Research School of Chemistry & Associate Dean (Engagement, Research and Impact) for the College of Systems & Society at the Australian National University, leading research into functional nanoporous materials for sustainable technologies. She spent >15 years working in hydrogen storage at the Universities of Bristol, Bath and Southampton (UK), before returning to Australia in 2022. Notable awards include the 2013 Sir Frederick Warner medal, the UK's Parliamentary and Scientific Committee's SET for Britain Gold Medal for Engineering and Westminster Medal in 2013 and being named in the Top 50 Women in Engineering (2020).

Yuxuan Zhang

Dr Yuxuan Zhang is a Research Fellow at the ANU School of Engineering. He received his PhD from Australian National University in 2025, following a joint Master's degree in Sustainable Energy from the Norwegian University of Science and Technology and Shanghai Jiao Tong University, and a dual Bachelor's degree in Renewable Energy Engineering and Economics from Shanghai Jiao Tong University. His research focuses on carbon capture and storage, thermal energy systems, and gas hydrate technologies, with applications in decarbonisation, cold-chain logistics, and sustainable energy.

Xiaolin Wang

A/Prof Xiaolin Wang is a distinguished researcher who has made significant contributions to science and society through research, communication, and outreach. Within two years of completing her PhD, she received the prestigious ARC DECRA. She has established collaborations with over 20 leading institutions across mechanical, materials, and chemical engineering, and secured ∼$3 million in research funding, mostly as lead CI. Xiaolin has published 110+ high-impact papers with 2000+ citations, and three of her patents have been commercialised. Her work focuses on energy efficiency and sustainability, translating research into practical solutions through academic and industry collaboration.

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1 Introduction

Hydrogen (H₂) has become a key energy carrier in the global shift toward sustainable and carbon-neutral energy systems. With its high gravimetric energy density (120 MJ kg⁻¹) and clean combustion and oxidation by-product (water), H₂ holds significant potential for applications across various sectors, including transportation, industrial processes, and renewable energy storage.^1–3 Despite these advantages, the widespread adoption of H₂ is constrained by the challenge of developing safe, efficient, and cost-effective storage methods.⁴ Conventional techniques, such as high-pressure gaseous storage and cryogenic liquid storage, present safety risks, and have substantial energy and infrastructure requirements, underscoring the need for alternative storage solutions to enable significant practical application.^5,6

Among these alternatives, H₂ clathrate hydrates, which were first identified by Vos et al.⁷ in 1993, have garnered increasing attention due to their intrinsic advantages. H₂ molecules can be confined within the water-formed cavities through physical encapsulation, where van der Waals interactions stabilize the clathrate structure.^8,9 Unlike chemical H₂ storage methods, clathrate hydrates do not involve covalent bonding, allowing reversible H₂ capture and release.¹⁰ H₂ hydrate systems offer promising attributes, such as the potential for parasitic energy consumption impacting roundtrip efficiency of storage compared to conventional H₂ storage methods, and minimal environmental impact due to their non-toxic and low-cost water-based composition.^11,12

Clathrate hydrates are crystalline, ice-like solids consisting of a three-dimensional H₂-bonded network of water molecules that encapsulate gas molecules within well-defined clathrate structures,¹³ including sI, sII, sH, as shown in Fig. 1(a). In addition to clathrate hydrates, semi-clathrates are a distinct class of inclusion structures where cations occupy hydrate cavities, and anions integrate into the lattice, enhancing stability.¹⁴ Generally, H₂ hydrates adopt the sII crystal structure, consisting of two distinct cage types: “small” dodecahedral (5¹²) cages with an average crystallographic radius of 3.95 Å (2.5 Å accessible to H₂) and “large” hexakaidecahedral (6⁴5¹²) cages with an average radius of 4.75 Å (3.3 Å accessible).¹⁵ The cubic unit cell (lattice parameter a = 17.047 Å) comprises 8 large and 16 small cages, allowing for a maximum composition of 48H₂·136H₂O, as shown in Fig. 1(b).¹⁶ The stability of this storage mechanism is governed by van der Waals forces, intermolecular interactions, and an extensive H₂-bonding network within the water framework.¹⁷ Typically, each water cage accommodates a single gas molecule; however, due to the small molecular size of H₂ (kinetic diameter 0.29 nm/2.9 Å),^18,19 multiple molecules could occupy a single cavity, as shown in Fig. 1 (b). Efficient storage of H₂ in hydrates currently still requires either high pressures (e.g. 200 MPa at 273 K) or extremely low temperatures (e.g. 0.1 MPa at 100 K) to maintain structural stability and enhance storage capacity.²⁰

Fig. 1 (a) Schematic of sI and sII polymorphs with host lattice shown in terms of 5¹², 5¹²6² and 5¹²6⁴ cages. The numbers represent the count of different polyhedral cage types (small pentagonal dodecahedron, medium hexagonal truncated trapezohedron, and large hexakaidecahedron) that constitute the unit cells of sI, sII, and sH hydrate structures. For sII, each unit cell contains 136 water molecules and 24 cages. Reproduced with permission from ref. 21. Copyright 2021, MDPI. (b) Structure of sII H₂ clathrates. H₂ molecule sites are indicated by the spheres, framework water molecules are at the vertices of the polyhedral shown by the lines. The clathrate of type sII is formed by 8 large hexakaidecahedral cages (6⁴5¹²) and 16 small dodecahedral cages (5¹²) with mean crystallographic radii of 4.73 Å and 3.95 Å, respectively. Reproduced with permission from ref. 22. Copyright 2017, Trans Tech Publications.

1.1 The history of H₂ hydrates

The study of H₂ hydrates dates back to the 1990s, when pioneering research began to explore their potential as a H₂ storage medium. Dyadin et al.²³ first discovered that H₂ could form ice-like hydrates under extremely high pressures (up to 1500 MPa), though practical applications were constrained by these high-pressure requirements. In 2002, Mao et al.¹⁶ experimentally confirmed the existence of pure H₂ sII hydrates using high-pressure Raman, infrared, X-ray, and neutron analysis, demonstrating that H₂ molecules could occupy both the large and small cages of the structure. Building on this, in 2004 Florusse et al.²⁴ identified mixed H₂ and tetrahydrofuran (THF) hydrates, showing that the inclusion of guest molecules like THF could significantly reduce the pressure needed for hydrate formation from 300 MPa, 280 K to 5 MPa, 279.6 K, opening new avenues for H₂ hydrate research. In 2005, Lee et al.²⁵ further advanced this field by optimizing the THF/H₂ ratio, leveraging the tuning effect (induced by different THF concentrations) to enhance H₂ storage capacity, achieving 4.0 wt% under conditions of 12 MPa and 270 K. Since 2005, various thermodynamic promoters, such as cyclopentane (CYC),²⁶ tetrahydrothiophene (THT),²⁷ propane (C₃H₈),²⁸ methane (CH₄),²⁹ tetrabutylammonium bromide (TBAB),³⁰ 1,3-dioxolane (1,3-DIOX),³¹ cyclopropane (CP),³² and ethylcyclopentane (ECP),³³ have been successively introduced to further facilitate milder hydrate formation conditions and improve the stability of H₂ storage in hydrates. In 2009, Wang et al.³⁴ proposed the theoretical basis for determining double occupancy in sII-5¹² cages. Subsequently, in 2011, Kawamura et al.³⁵ synthesized and characterized various H₂-containing structure II hydrates, expanding their potential applications to H₂ storage by demonstrating improved stability and lower pressure requirements compared to conventional sI hydrates. In 2013, Veluswamy and Linga³⁶ highlighted the persistent kinetic challenges in the formation of mixed H₂/THF hydrates, specifically the sluggish nucleation rate and long induction time due to the strong H₂ bonding network of water molecules. By 2015, Veluswamy et al.³⁷ shifted their focus to improving the kinetic efficiency of hydrate formation by introducing external materials such as sodium dodecyl sulfate (SDS) and other surfactants, which can promote hydrate formation by reducing interfacial tension, enhancing gas dispersion, and accelerating nucleation. In 2018, Di Profio et al.³⁸ introduced the use of reverse micelle technology to enhance the kinetics of H₂ hydrate formation, laying the foundation for further research on nanoscale confinement. Building on this, in 2002 Farrando-Perez et al.³⁹ applied nanoscale confinement to H₂ hydrates, demonstrating that H₂ hydrates could rapidly form within confined spaces, such as in the pores of activated carbon (AC) which is, a highly porous material with a large surface area and tunable pore size, facilitating gas adsorption and molecular confinement. This work highlighted the significant role of nanoscale confinement in accelerating hydrate formation and improving H₂ volumetric storage capacity such that international benchmarks could be met, such as the U.S. Department of Energy (DOE) target of 30 g L⁻¹.⁴⁰ Following that, in 2023 Firuznia et al.⁴¹ proposed a modified zeolite utilizing nanopore confinement and surface chemistry to enable efficient, low-pressure H₂ storage, overcoming challenges of high-pressure and slow kinetics in conventional methods. In recent years, various porous materials, including AC,^39,42 metal–organic frameworks (MOFs),⁴³ and silica,^44,45 have been incorporated into H₂ hydrate systems, leveraging nanoscale structures to enhance storage efficiency and stability. The development on the above-mentioned direction will be detailed in Section 2. The timeline of research into hydrate-based H₂ storage is shown in Fig. 2.

Fig. 2 An indicative timeline of key advancements of hydrate-based H₂ storage research.

1.2 The challenges and significance of studying H₂ hydrates

The formation of H₂ hydrates presents a series of formidable challenges that stem from both thermodynamic and kinetic limitations, significantly hindering their practical application in H₂ storage applications.⁹ From a thermodynamic standpoint, the stabilization of pure H₂ hydrates necessitates extremely high pressures, often exceeding 200 MPa, and sub-zero (273 K) temperatures.⁴⁶ These harsh conditions are not only energy-intensive but also demand sophisticated equipment, which dramatically increase operational and infrastructural costs. Such requirements pose severe constraints for real-world scalability and accessibility, particularly in industries aiming for sustainable and cost-effective energy solutions. Furthermore, the exceedingly high pressures (200 MPa at 280 K for pure H₂ hydrate) required for stability limit the feasibility of employing H₂ hydrates in large-scale applications,¹⁶ where safety concerns and energy efficiency are paramount.⁴⁷

On the kinetic front, the formation process of H₂ hydrates is impeded by lengthy induction times, slow crystal growth, and low overall formation rates.²¹ These factors collectively reduce the efficiency of H₂ capture and storage, making the process economically unviable. The intrinsic nature of H₂'s small molecular size and the weak van der Waals interactions between H₂ molecules and water cages further exacerbates these issues, as H₂ molecules struggle to nucleate and form stable hydrate cages.⁴⁸

Beyond these immediate technical barriers, the low storage capacity of H₂ hydrates presents a fundamental limitation. Even under optimal conditions, the H₂ storage capacity of hydrates rarely meets the thresholds required for practical applications, such as the DOE targets for vehicular or grid storage systems.⁴⁹ The inherent limitation of water's stoichiometry in forming clathrate cages results in a relatively low H₂-to-water ratio (1 : 6),^50,51 which restricts the energy density achievable by hydrate-based storage.⁵² This low capacity underscores the urgent need for strategies that can significantly enhance the storage efficiency of hydrate systems.

Despite these challenges, the scientific and technological significance of researching H₂ hydrates cannot be overstated. Compared to conventional high-pressure gaseous or cryogenic liquid storage, hydrate-based systems offer distinct advantages, including inherent safety due to the stable, solid-state nature of hydrates, as well as reduced energy requirements for compression or liquefaction.⁵³ Furthermore, the ability to store H₂ under more moderate conditions has the potential to drastically lower infrastructure costs and improve the accessibility of H₂ technologies in diverse settings.⁵⁴ From a broader scientific viewpoint, studying H₂ hydrates offers insights into complex phenomena such as molecular confinement, phase transitions, and nanoscale interactions within porous materials. These insights not only advance the understanding of hydrate systems but also contribute to the design of novel materials and processes in fields such as natural gas storage, CO₂ sequestration, and advanced catalysis.

Given these challenges and the compelling need for breakthroughs, thermodynamic and kinetic promoters emerge as transformative tools in addressing the limitations of H₂ hydrates. The following sections will explore in detail how these thermodynamic and kinetic promoters are being leveraged to overcome the challenges of H₂ hydrates, unlocking their potential as a cornerstone in the global shift toward sustainable and efficient energy systems.

1.3 The promoters for H₂ hydrates

In the development of H₂ hydrates, it was observed that the addition of additives, known as hydrate promoters, helped address issues such as long induction times, slow growth rates of hydrate particles, and low H₂ storage capacity. Generally, hydrate promoters were classified into thermodynamic hydrate promoters (THPs) and kinetic hydrate promoters (KHPs).

These promoters can also be broadly categorized based on their physical phase into homogeneous and heterogeneous types,⁵⁵ as shown in Table 1. Homogeneous promoters (e.g. THF, TBAB) are typically liquid additives uniformly mixed with water and are commonly associated with thermodynamic promotion, shifting the phase equilibrium to milder conditions and stabilizing the hydrate structure.⁵⁶ In contrast, heterogeneous promoters (e.g. nanoparticles, porous materials) are solid additives generally act as kinetic promoters. These materials offer access to interfacial nano-structural effects, including nanoconfinement, surface chemistry modulation, and enhanced gas–liquid contact.⁵⁷ Notably, heterogeneous hydrate nucleation is facilitated by structured water layers forming at the liquid–solid interface, which contribute to more favorable local environments for clathrate nucleation.⁵⁸ In addition, heterogeneous promoters can increase the density of nucleation sites, further accelerating hydrate formation.⁵⁹

Table 1 Comparison of homogeneous and heterogeneous promoters for H₂ hydrate

Property	Homogeneous promoters	Heterogeneous promoters
State	Soluble (liquid or miscible solid)	Insoluble solid
Distribution	Molecular level	Particle dispersion
Mode of Action	Alters phase equilibrium/gas solubility	Enhances nucleation via surface/interface effects
Examples	THF, TBAB, SDS	MOFs, AC, silica
Functions	• Modify the phase equilibrium: lower hydrate formation pressure or increase formation temperature	• Provide nucleation sites: due to high surface area and surface energy
		• Create nanoconfinement: pore spaces and interfaces facilitate hydrate formation at milder conditions
	• Improve kinetics: surfactants can reduce surface tension and enhance gas–liquid contact	• Modulate water mobility and gas diffusion
Characteristics	• Act uniformly throughout the solution	• Operate via interfacial effects, such as surface chemistry, wettability, and thermal conductivity
	• Often influence both nucleation and growth stages	• Can be designed or functionalized to promote specific interactions
	• Do not create physical interfaces or confinement effects	• Do not alter bulk thermodynamic equilibrium but enhance kinetic pathways

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1.3.1 Thermodynamic hydrate promoters. Thermodynamic promoters, generally are the homogeneous promoters shift the hydrate formation equilibrium curve to milder conditions, such as lower pressures and higher temperatures, while kinetic promoters reduce the induction time for hydrate formation, enhancing growth rates, and improving storage capacity.⁶⁰ THPs play a critical role in enabling the formation of H₂ hydrates with different structural types, such as sII clathrate, semi-clathrate, and sH hydrates, under more practical conditions. As indicated in Table 2, sII hydrates formed with THF demonstrate significantly reduced formation pressures, with moderate conditions and achieve H₂ storage capacities of up to 4.03 wt% at 12 MPa and 270 K. These promoters stabilize the hydrate lattice by co-occupying cavities, facilitating the inclusion of H₂ molecules. Semi-clathrate hydrates, formed using quaternary ammonium salts like TBAB, further enhance stability and allow for hydrate formation at moderate pressures and higher temperatures, such as 13.8 MPa at 279.5 K, though the H₂ storage capacities are generally lower (∼0.22 wt%).⁶¹ Additionally, sH hydrates, which consist of a unique combination of larger and smaller cages with methylcyclohexane (MCH), show potential for encapsulating multiple H₂ molecules, further expanding the scope of hydrate-based storage. In current research, the most effective THPs, such as THF, CP, 1,3-DIOX, certain hydrocarbons, and inert gases, have been widely studied and applied. However, the development of new THPs is limited by strict requirements on molecular size and interactions.⁶² For an integration into the H₂-bond network of water, THP molecules must be smaller than 1 nm in size.⁶² Furthermore, their interaction with water must be carefully balanced, as excessively strong H₂ bonding can disrupt the water cage structure, causing the THP to act as an inhibitor rather than a promoter.^63–65 These constraints pose significant challenges for advancing THP design.

Table 2 Summary of the different conditions of THPs for H₂ hydrate

H2 hydrate structure	THPs	THPs chemical structure	THPs concentration (mol%)	Operating temperature (K)	Operating pressure (MPa)	H2 storage capacity (wt%)	Ref.
sII	THF	C4H8O	0.15	270	12	4.03	66
	THF		0.2	270	30	0.83	67
	THF		0.2	283	30	0.95	68
	THF		0.5	272	8.8	0.12	69
	THF		0.5	255 ± 2	74	3.4	70
	THF		1	270	13.1	0.1	71
	THF		1	265–270	13.8	0.43	66
	THF		1	270	57	0.98	66
	THF		2	269.5	3.6	0.18	72
	THF		2	270	13.5	0.2	71
	THF		2	270	13.8	0.43	66
	THF		2.78	274	11	0.05	73
	THF		3	273.15	14.53	1.875	74
	THF		3	255	75	3.44	70
	THF (fresh)		3.5	279.2	12	0.155	75
	THF (memory)		3.5	279.2	12	0.169	75
	THF		5	279.6	5	0.0209	24
	THF		5	274	6.4	0.026	76
	THF		5	278	8.8	0.12	77
	THF		5	267.7	74	0.835	78
	THF		5.3	279.8	13	0.183	75
	THF		5.56	265.1	5	0.19	79
	THF		5.56	266.7	6.5	0.28	72
	THF		5.56	270	6.5	1	80
	THF		5.56	274	11	0.027	73
	THF		5.56	270	11.6	0.3	81
	THF		5.56	270	12	4.03	25
	THF		5.56	274.2	12	0.149	82
	THF		5.56	277.85	12.3	0.066	31
	THF		5.56	270	16.3	0.747	69
	THF		5.56	270	16.3	0.4–0.5	83
	THF		5.56	283	37	3.4	68
	THF		5.56	277.15	66.4	1.05	78
	THF		5.6	276.2	10	0.12	12
	THF		5.6	277.15	10.1	0.19	78
	THF		5.6	270	13.8	0.438	66
	THF		5.6	277.15	31.9	0.51	78
	THF		5.6	277.15	40.5	0.615	78
	THF		5.6	277.15	66.4	0.835	78
	THF		5.88	273	3.8	3.08	84
	THF		8.34	274	11	0.027	73
	1,3-DIOX	C4H8O2	5.56	271.15	12.3	0.216	31
	CP	C3H6	5.6	278.4	10	0.11	38
	CP		5.6	275.15	10–18	0.27	85
	CP		0.11 mol% CP seeds + 5.45 mol% liquid	275	12	0.32	59
	CP		0.11 mol% CP seeds + 5.45 mol% liquid	278	12	0.37	59
	THT	C4H8S	5	275.1	15.4	0.25	86
	THT		5	275.1	32	0.43	86
	THT		5	275.1	41.8	0.6	86
	THT		5.6	274.5	10	0.5	38
	THT		5.6	275.15	41.5	0.6	86
	THP	C5H10O	5.6	272.3	10	0.19	38
	Furan	C4H4O	5	275.1	15.5	0.23	86
	Furan		5	275.1	32	0.47	86
	Furan		5	275.1	41.8	0.59	86
	1,4 Dioxane	C4H8O2	0.2	233	12	1.1	87
	1,1-Dichloro-1-fluoroethane	C2H3Cl2F	5.6	273	6	0.24	88
	1,1-Dichloro-1-fluoroethane		5.6	273	8	0.32	88
	1,1-Dichloro-1-fluoroethane		5.6	273	10	0.36	88
	1,1-Dichloro-1-fluoroethane		5.6	273	12	0.4	88
Semi-clathrate	TBAB	C16H36BrN	1	279.5	13.8	0.1	14
	TBAB		2.6	281.15	16	0.031	89
	TBAB		2.71	279.5	13.8	0.214	14
	TBAB		3	279.5	13.8	0.22	14
	TBAB		3.5	279.2	12	0.052	75
	TBAB		3.7	281.15	16	0.046	89
	TBAB		4	287	16	0.6	90
	TBABh	C16H38BN	2.54	253	70	1.35	91
	TBABh		4	100	0.1	0.07	91
	TBAC	C16H36ClN	0.35	288.55	2.45	0.019	92
	TBAC		0.35	288.64	4.27	0.033	92
	TBAC		0.35	288.77	5.01	0.038	92
	TBAC		0.35	288.97	7.27	0.055	92
	TBAC		0.35	289.24	9.73	0.073	92
	TBAC		0.35	289.72	15.5	0.11	92
	TBAC		3.26	288.9	14.9	0.12	93
	TBAF	C16H36FN	1.8	294	10	0.34	94
	TBAF		1.8	294.15	13	0.009	94
	TBAF		3.4	294	10	0.45	94
	TBAF		3.4	294.15	13	0.024	94
	TBAOH	C16H37NO	0.0323	290	20	0.47	95
	Tetrabutylamine	C16H35N	5.56	250	13.8	0.7	96
sH	MCH	C7H14	0.4	273	149	1.38	97
	MCH		1.6	274	25	0.6	97
	DMCH	C8H16	3	275	60	0.85	98
	MTBE	C5H12O	5	273	70	0.94	98
	ECP	C7H14	5.56	273.25	12.2	0.31	99

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1.3.2 Kinetic hydrate promoters. KHPs, unlike THPs, do not occupy hydrate cages or alter equilibrium conditions but instead enhance formation kinetics by reducing induction time and accelerating growth rates.¹⁰⁰ At the same time, KHPs exhibit diversity in molecular weight, size, structure, and chemical properties, providing researchers with greater flexibility to select or design new molecules.¹⁰⁰ For H₂ hydrates, KHPs can be categorized into two groups according to the different mechanisms: homogeneous and heterogeneous promoters.

Homogeneous promoters such as surfactants, amino acids dissolve uniformly in the aqueous phase, enhancing hydrate formation primarily by improving gas–liquid interface properties.¹⁰¹ As shown in Table 3, surfactants such as sodium dodecyl sulfate (SDS) (100 ppm) reduce the time for 90% gas uptake by 13-fold at 8.5 MPa and 274.2 K. By lowering surface tension, these surfactants enhance the gas–liquid contact area and mass transfer efficiency between two phases.¹⁰² However, surfactants remain in hydrates as residual agents that can reduce the hydrate purity, and additionally SDS generates a lot of foam that covers the gas–liquid interface, and causes additional environmental pollution.¹⁰² Similarly, amino acids as biofriendly KHPs, with unique molecular structures, enhance H₂-bond networks and reduce mass transfer resistance at the gas–liquid interface.¹⁰³ For example, hydrates incorporating L-methionine have shown improved H₂ storage capacities, achieving up to 0.474 wt% under 12 MPa and 274.15 K.

Table 3 Summary of the different conditions of KHPs for H₂ hydrate

KHPs			KHPs concentration/mass	Operating temperature (K)	Operating pressure (MPa)	Effects on H2 hydrate	Ref.
Homogeneous promoters	Surfactants	SDS	100 ppm	274.2 K	8.5 MPa	The time required to achieve 90% of the gas uptake (t90) is reduced by 13 times	106
		SDS	250 ppm	274.5 K	11.3 Mpa	SDS surfactant has no effect in improving the kinetics of mixed H2/THF hydrates	77
		Cationic dodecyl trimethylammonium chloride (DTAC) + non-ionic Tween-20 (polysorbate 20)	0.5 wt% DTAC and 0.1 wt% Tween-20	278.2 K	7.13 Mpa	Reduction of hydrate formation rates by approximately 20%	107
	Amino acids	L-Valine	0.3 wt%	274.2 K	12.0 MPa	The maximum gravimetric H2 (GH2) reaches 0.26 ± 0.01 wt%	82
		L-Methionine	12 mmol L^−1	274.15 K	12.0 MPa	H2 storage capacity is 0.474 wt%	104
		L-Leucine	12 mmol L^−1	274.15 K	12.0 MPa	H2 storage capacity is 0.416 wt%	104
		D-Leucine	12 mmol L^−1	274.15 K	12.0 MPa	H2 storage capacity is 0.261 wt%	104
		Tryptophan	12 mmol L^−1	274.15 K	12.0 MPa	H2 storage capacity is 0.142 wt%	104
Heterogeneous promoters	Nano-particles	Reverse micelles	200 mL isooctane + proper amount of AOT	274 K	20 MPa	Gravimetric H2 storage gives up to about 0.5 wt% of H2	38
		Functional groups –SO3^−@PSNS of nano spheres	1 mmol L^−1	274.15 K	12.0 MPa	H2 storage capacity is 0.467 wt%	104
	Porous materials	AC	0.05 g	274.15 K	12.0 MPa	H2 storage capacity is 0.13 wt%	104
		ZIF-8	0.05 g	274.15 K	12.0 MPa	H2 storage capacity is 0.133 wt%	104
		CNT	0.05 g	274.15 K	12.0 MPa	H2 storage capacity is 0.301 wt%	104
		MOF-5	0.05 g	274.15 K	12.0 MPa	H2 storage capacity is 0.17 wt%	104

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In contrast, heterogeneous promoters such as porous materials, nanoparticles promote hydrate formation through surface-mediated effects without dissolving into the water phase. Porous materials such as AC, carbon nanotubes (CNTs), ZIF-8, and MOF-5,¹⁰⁴ with their intricate pore structures and high specific surface areas, provide support for hydrate particles and facilitate gas diffusion pathways. These properties significantly reduce the nucleation and growth time of hydrates, making porous materials an effective approach for enhancing the kinetic performance of hydrate-based H₂ storage.¹⁰⁵ As shown in Table 3, MOF-5 achieves a H₂ storage capacity of 0.17 wt% under 12 MPa and 274.15 K, highlighting the potential of these materials to improve both storage efficiency and kinetics. These promoters are essentially solid, making them easier to collect and recycle from the solvent. Additionally, they exhibit minimal reactivity with either the gas hydrate system or the surrounding environment. It is therefore essential to investigate the effect of porous materials on H₂ hydrate. While previous review articles have extensively explored the thermodynamic, kinetic, and structural aspects of H₂ hydrate formation, most of them focus on bulk hydrate systems or the effects of simple additives, such as surfactants and thermodynamic promoters. However, the integration of porous materials into H₂ hydrate systems introduces a novel approach that leverages nano-confinement effects, enhanced surface properties, and tailored pore structures to address the challenges of low storage capacity and slow kinetics. This perspective remains underexplored, particularly in terms of the interplay between pore size, surface functionality, and gas diffusion pathways. Therefore, this study offers a fresh and comprehensive insight into how porous materials influence H₂ hydrate formation, stability, and storage efficiency, bridging the gap between fundamental mechanisms and practical applications for advanced H₂ storage technologies.

2 Mechanism for promoting H₂ hydrate formation with porous materials

Porous materials are composed of a solid framework interspersed with densely packed microscopic voids. The key physical characteristics include extremely small pore sizes and a high specific surface area. The microscopic voids within porous media may be fully interconnected, partially connected, or entirely isolated.¹⁰⁸ According to the classification standards of the International Union of Pure and Applied Chemistry (IUPAC), porous materials are categorized into three types based on pore size: micropores (pore diameter <2 nm), mesopores (pore diameter between 2 and 50 nm), and macropores (pore diameter >50 nm).¹⁰⁹ The unique characteristics of porous materials as shown in Table 4, result in pronounced interfacial phenomena, often exhibiting strong surface tension and capillary condensation effects. These properties create significant differences in the formation of gas hydrates within porous materials compared to those in bulk systems,¹¹⁰ notably influencing the formation and distribution behavior of H₂ hydrates. Porous materials can enhance hydrate formation by reducing the activation energy for nucleation and providing additional nucleation sites, compared to homogeneous nucleation.¹¹¹ They are also expected to influence the distribution of formed nuclei seeds, facilitating their ability to bond more easily. Overall, they exert two primary effects on the hydrate formation process: first, porous materials significantly increase the gas–water contact area, promoting local supersaturation and creating more nucleation opportunities;¹¹² second, capillary forces within the pores influence water activity, further affecting hydrate nucleation and growth dynamics.¹¹³ The H₂ hydrate formation mechanism is shown in Fig. 3. It has been proven that porous AC, as well as other porous materials like MOFs and CNTs, have demonstrated potential in facilitating the formation and storage of H₂ hydrates.^114–117

Table 4 Classification of different porous materials and their various advantages and disadvantages

Classification	Porous materials	Morphology	Specific surface area (cm^2 g^−1)	Pore volume (cm^3 g^−1)	Formation	Advantages	Disadvantages	Ref.
Carbon-based porous materials	AC		500–3000	0.2–2.0	Produced from carbonization and activation (e.g. coal, wood, peat, lignite)	Large specific surface area (>1000 m^2 g^−1), pore volume, and chemical functional groups	Flammability, high permeability, pore resistance, and hygroscopicity	247 and 248
	CNTs		50–1300	0.1–1.5	Consist of a graphene sheet that is rolled into a cylindrical structure based on sp^2 hybridized carbon atoms	Good electrical conductivity, optical rotation and mechanical strength, larger surface area, natural hydrophobicity, and strong thermal stability	Aggregate and bundle together	249–251
	Graphene		100–3000	0.1–2.5	Two-dimensional (2D) graphene consists of hexagonally arranged sheets of carbon atoms that share sp^2 hybridized orbitals with three adjacent carbon atoms	Excellent thermal conductivity, significantly high physical specific surface area, and structural stability	Synthesis is complex and aggregates	252 and 253
Inorganic porous materials	Silica gel		200–800	0.4–1.5	Silica gel is an amorphous inorganic material with a three-dimensional tetrahedral structure and silicol groups on its surface	Excellent thermal, mechanical, and chemical stabilities, low density, high pore surface area, and numerous functional groups	Hygroscopicity	254
	Zeolite		300–1200	0.1–0.8	Zeolite has a crystalline aluminosilicate frame consisting of an infinite three-dimensional (3D) arrangement of TO4 tetrahedra (T is Al or Si)	Good hydrophobicity, large surface area, adjustable porosity, incombustibility, hydrothermal stability and chemical stability, good thermal stability	The synthetic process can be complex, time-consuming, and expensive	255 and 256
	Glass bead		<1–10	—	Glass bead is composed of silicon dioxide (SiO2) along with small amounts of other oxides forming an inert and stable glass structure	Chemical stability, good thermal conductivity, controllable particle size, industrially very well-established	Low mechanical stability, single pore structure, smooth surface	257
	Electrodeposited Cu2O nanoflower		—	—	Electrochemical deposition on Cu foam substrate	Superhydrophobicity, hierarchical roughness, mechanical adhesion, antibacterial activity	Limited thermal stability	258
	NiSe2 hollow nanotube foam		—	—	Hydrothermal synthesis and ion exchange on Ni foam substrate	High electrical conductivity, hierarchical porosity, 1D tubular channels, open gas diffusion paths	Applied mainly in electrochemistry	259
Organic porous polymers	Polyurethane foam		1–200	0.5–5.0	Foamed polyurethane is formed from a reaction between polyisocyanates and polyols, with catalysts and blowing agents	High porosity and excellent mechanical properties, good compressive, tensile, and shear strength across various densities, adaptation to extreme conditions	Sensitivity to temperature and humidity	260
	Emulsion-templated polymers		10–500	1.0–10.0	Emulsion-templated polymer is synthesized within surfactant-stabilized water-in-oil HIPEs by using free radical polymerization	High specific surface areas and well-defined porosities, high variability of macromolecular structures, allowing tailored designs	The synthetic process is complex, time-consuming, and expensive	261–263
	Super absorbent polymers		1–100	0.5–15.0	SAPs are composed of a hydrophilic, water-swellable polymer 3D network	Excellent water absorption, retention capabilities, low cost	Required specific synthesis or post-treatment	81 and 264
Hybrid porous materials	Metal organic frameworks		500–7000	0.2–4.0	Metal ions or clusters of coordination with organic ligands are constructed in an ordered 1, 2, 3D framework	Ultra-high surface area, excellent thermal stability, tunable porosity, and easy functionalization	Large void space, weak dispersing force, favorable for coordination and insufficient active metal catalyst sites, and high preparation costs, limited possibilities for scale-up, and limited stability	265–268

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Fig. 3 The H₂ hydrate formation mechanism at three stages: pre-nucleation, nucleation and growth.

Subsequent studies^118,119 have proposed five theoretical models to describe hydrate formation processes in porous media, based on the interactions between gas hydrates and the porous framework. These models, illustrated in Fig. 4(a–e),¹¹⁹ include: (a) pore-filling,¹²⁰ where hydrates form; (b) load-bearing,¹²¹ where hydrates form between grains, contributing to the particle skeleton; (c) grain-cementing,¹²² where hydrates grow at grain contact points; (d) grain-coating,¹¹⁸ where hydrates envelop the grains; and (e) patchy,¹¹⁸ where hydrates occupy interconnected pore spaces entirely. It is noted that the intricate relationship between hydrate occurrence patterns and growth habits is complicated. Li et al.¹²³ highlights the critical role of initial gas and water saturation in determining hydrate occurrence patterns and growth mechanisms. When water is the primary continuous phase, hydrates tend to grow towards the pore centers. Conversely, when gas dominates as the continuous phase, hydrates grow both along the pore walls and towards the center, with mass transfer occurring between water and gas phases, as shown in Fig. 4(f and g). These findings emphasize that the growth mechanism, influenced by mass transfer dynamics, ultimately dictates the hydrate occurrence pattern.¹²⁴ Therefore, this review focuses on the effects of various porous materials on H₂ hydrate, including the characteristics of various porous materials, as well as the formation, distribution and gas storage behavior of H₂ hydrates.

Fig. 4 Theoretical models of hydrate occurrence patterns and growth behavior: (a) pore-filling, (b) load-bearing, (c) grain-cementing, (d) grain-coating, and (e) patchy. Reproduced with permission from ref. 125. Copyright 2022, Elsevier. Hydrate occurrence patterns: (f) pore-filling, and (g) mixed growth habits. Reproduced with permission from ref. 123. Copyright 2022, Elsevier.

2.1 Effect of intrinsic material properties on H₂ hydrates within porous materials

The formation of H₂ hydrates within confined environments, especially in the nanopores of porous materials, is a highly intricate process influenced by various factors.⁶² While previous studies have confirmed that a higher surface-to-volume ratio promotes clathrate hydrate growth, this phenomenon is also significantly affected by properties such as surface chemistry, pore structure, and material wettability.^66,126 Specifically, the pore size and network configuration not only regulates gas–liquid interactions but also, in conjunction with the surface chemical properties, critically alters the spatial arrangement of water molecules,¹²⁷ thereby playing a pivotal role in initiating hydrate nucleation. According to Nguyen et al.,^100,128 a material must exhibit four essential properties to serve effectively as a “nanoreactor” for H₂ hydrate formation: moderate wettability, optimal pore size, suitable solid–water interactions, and having an ice-like molecular arrangement of the surface. Therefore, this review comprehensively examines how the pore structure of porous media, including pore size, particle size, surface area, and morphology, along with surface characteristics such as surface roughness and chemical composition, and wettability factors like hydrophilicity/hydrophobicity and water saturation within pores, collectively influence the nucleation and growth behavior of H₂ hydrates, as shown in Fig. 5.

Fig. 5 Key material factors relating to mechanisms for H₂ hydrate nucleation and growth.

2.1.1 Nano-confinement effect. Recently, studies reported generation of clathrate hydrates under milder conditions, where water is present in a nanoconfinement environment and not as the bulk.^129–133 Nanoconfinement refers to the influence of fluid–pore and fluid–fluid interactions within confined spaces.¹³⁴ Nanoconfinement effects could alter the thermodynamic, kinetic, and hydromechanical properties of the water phase, thereby impacting the hydrate formation process.^135,136 Specifically, when water molecules are confined within nanoscale spaces, their H₂-bonding network and physicochemical properties undergo significant changes, affecting the nucleation and growth behavior of gas hydrates.¹³⁷ Therefore, nanoconfinement plays a crucial role in regulating hydrate formation conditions and enhancing gas storage efficiency.^138–140

For H₂ hydrates, nanoconfinement within small pores (typically 1–50 nm) enhances hydrate formation by increasing the interfacial area between H₂ gas and water, thereby facilitating nucleation and growth.^138,141 Water molecules in nanopores exhibit distinct structural and dynamic properties due to interactions with pore surfaces, which modify the H₂ bonding network and influence thermodynamic and kinetic pathways.¹⁴² These effects accelerate hydrate formation, allowing rapid growth within minutes by providing abundant nucleation sites and increased surface area.¹⁴⁰ Additionally, nanoconfinement enables hydrate formation at lower pressures compared to bulk conditions, as the confined environment stabilizes hydrate structures under milder conditions.¹⁴³ During nucleation, confined spaces induce metastable crystal growth, influencing hydrate arrangement and stability, leading to unique growth patterns not observed in bulk phases.^144,145 Furthermore, nanoconfinement directs hydrate crystallization by exposing dominant crystal faces, optimizing growth orientation and morphology.^144,146,147 Collectively, these effects contribute to a more efficient and controlled hydrate formation process, advancing its potential for H₂ storage applications. While nanoconfinement within the entire 1–50 nm range offers significant advantages, pores larger than 2 nm, particularly mesopores between 2–10 nm, play a distinct and complementary role. Micropores (<2 nm) primarily facilitate hydrogen physisorption through strong van der Waals surface interactions but may restrict the spatial rearrangement of water molecules needed to form complete hydrate cages.^148–150 In contrast, mesopores provide sufficient volume for hydrate nucleation and cage development, while maintaining confinement strong enough to stabilize intermediate phases.¹⁴⁰ Moreover, mesopores enable a dual-mode storage mechanism, where hydrogen is first physisorbed onto pore walls and then enclathrated into growing hydrate cages.^139,151 This pre-concentration of gas molecules within the mesopore environment enhances the local driving force for hydrate formation. In addition, mesopores allow more efficient gas–water contact and molecular transport compared to micropores, reducing internal diffusion limitations and promoting sustained hydrate growth.¹⁵² Thus, by balancing spatial accessibility with confinement effects, mesoporous structures act as active microreactors that complement the sorptive advantages of micropores.¹⁵³

As shown in Fig. 6(a), within the pores, a water film initially forms, enhancing the gas–water interface and facilitating hydrate nucleation at the water-pore surface boundary. This interface supports gas diffusion into the water film, driving hydrate growth. The process continues until equilibrium is reached or mass transfer limitations inhibit further hydrate formation.^113,154 Liu et al.¹⁵⁵ reported that hydrate formation in activated carbon pores occurs in two primary stages. Initially, water molecules are replaced by gas molecules at the interface. This is followed by gas displacing water within the pore, leading to gas condensation and the subsequent growth of hydrate crystals. This effect is more pronounced on hydrophobic surfaces but is also influenced by the volume and geometry of the confined space, which play a crucial role in determining these altered properties.^127,156 The reduced dimensionality and strong interactions between the confined phase and porous materials create large interfacial areas and quasi-high pressures, which play a crucial role in influencing the nucleation and growth of H₂ hydrates.¹³⁷ Quasi-high pressure is not a traditional macroscopic high pressure, but a local pressure effect due to the unique physical and chemical environment at the nanoscale.¹⁵⁷ The formation of quasi-high pressure is primarily attributed to the following mechanisms: first, the confinement effect within nanopores restricts the movement of H₂ molecules, enhancing intermolecular interactions and creating a localized high-pressure environment.¹³⁵ Phenomena typically requiring extremely high pressures in bulk systems, such as high-pressure chemical reactions and solid phase transitions (∼10⁴ bar), have been observed at significantly lower pressures (∼1 bar) within confined spaces due to quasi-high pressure effects.^157,158 Second, capillary condensation reduces the vapor pressure of the gas, allowing H₂ molecules to achieve local densification at lower macroscopic pressures.^159–161 Additionally, the high surface energy of pore walls alters the thermodynamic state of H₂ molecules, further contributing to the localized pressure increase.¹⁶² Under quasi-high pressure conditions, H₂ molecules can more readily incorporate into hydrate lattices, significantly lowering the formation pressure of hydrates and accelerating their formation kinetics. Simultaneously, the nano-confinement environment suppresses the growth and aggregation of hydrate crystals, improving their thermodynamic stability and resistance to decomposition, particularly under multiple storage cycles.¹³⁴ Furthermore, quasi-high pressure enhances the storage density of H₂ per unit volume and promotes the formation of structure II hydrates, which exhibit higher H₂ storage capacity.^137,163 In short, these effects facilitate the formation and growth of gas hydrates under milder conditions compared to bulk fluids.¹⁶⁴ Factors such as pore size and the chemical properties of the solid interface play a crucial role in hydrate formation. Stronger water–pore wall interactions influence the distribution and structuring of interfacial water, which can facilitate hydrate nucleation by providing a confined environment.¹⁶⁵

Fig. 6 Schematic diagram of the (a) hydrate formation mechanism in pores of porous material. Reproduced with permission from ref. 166. Copyright 2021, American Chemical Society. (b) Formation process of H₂ adsorption and H₂ hydrates in a porous carbon under nano-confinement. Modified with permission from ref. 167. Copyright 2019, American Chemical Society.

Confinement effects also contribute to the increased thermal stability of H₂ hydrates beyond their normal stability range. This enhanced stability (≥240 K compared to the typical 145 K) represents progress toward utilizing these confined crystals as H₂ storage reservoirs.¹³¹ Furthermore, in nano-confinement environments, H₂ storage capacity is significantly enhanced due to the coexistence of two H₂ storage mechanisms: adsorption and hydrate formation, as shown in Fig. 6(b). On one hand, H₂ molecules are physically adsorbed onto the surfaces of nanoporous materials through van der Waals forces or surface interactions, a process that is highly dependent on pore structure and material surface area. On the other hand, in the presence of water and under suitable temperature and pressure conditions, H₂ molecules can form stable hydrates with water molecules within the confined spaces. The nano-confinement effect not only promotes the adsorption of H₂ on pore surfaces but also facilitates the nucleation and stabilization of hydrates by restricting hydrate crystal growth. This dual storage mechanism effectively enhances H₂ storage capacity while offering complementary release pathways, making nano-confinement an attractive approach for advanced H₂ storage systems.

2.1.2 Contact area. The contact area is critical for hydrate formation, with its influence closely linked to factors such as pore size, particle size, and shape.^62,168,169 To some extent, the contact area is affected by the pore size and particle size. Generally, the behavior of guest molecules within pores is closely related to pore size. In macroporous materials, molecular adsorption typically follows a layer-by-layer mode on the internal surfaces. In mesoporous materials, capillary condensation predominantly occurs. Meanwhile, in microporous materials, molecular behavior is primarily characterized by one-dimensional filling along the pore channels.¹⁷⁰Fig. 7(a) illustrates the formation of H₂ hydrates in porous model carbons, spanning from micropores to mesopores and macropores.¹⁷¹Fig. 7(b) presents the adaptation of pore sizes for H₂ hydrate formation modified from these papers.^172–174 Pan et al.¹⁷⁵ proposed that pore size influences both capillary forces and specific surface area, which exert opposing effects on hydrate formation. Pore size influences the specific surface area, which in turn dictates the relative significance of interfacial regions compared to the bulk region.¹⁷¹ In homogeneous media, specific surface area typically decreases as pore size increases. Systems with smaller pore sizes offer a larger specific surface area, providing more nucleation sites and accelerating reactions.^176,177 However, the smaller pores can impede gas migration, making it challenging to achieve conditions suitable for the formation of hydrates.^178–180 Additionally, the increased capillary forces and surface tension in these pores further hinder hydrate formation. Conversely, larger pore sizes facilitate mass and heat transfer, enhance the driving force for hydrate formation, and reduce capillary effects. However, these systems often lack sufficient reaction interfaces, and as pore size increases, the properties of water in the pores approach those of the bulk phase. This can lead to the formation of hydrate films that impede further reactions, limiting overall efficiency.¹⁸¹ Theoretically, there exists an optimal size range where the effects of porous materials on promoting hydrate formation by increasing nucleation sites and inhibiting it by reducing water activity reach a balance. Rothmund et al.¹⁵¹ explored the effect of pore size on H₂ storage in hydrates confined within porous activated carbon, as shown in Fig. 7(c). The results identified a critical pore size of approximately 2 nm, below which H₂ hydrates decompose. This critical size is mainly influenced by temperature and oxygen surface groups, with minimal impact from other activated carbon properties. Additionally, physisorbed H₂ gas was found to occupy the smallest pores (around 1 nm). A dual-storage mechanism was observed, where H₂ is stored both in micropores through physisorption and in larger meso- and macro-pores through enclathration. Building on this, pores slightly larger than 2 nm may serve as transitional domains, offering sufficient space for hydrate formation without spatial frustration. These mesopores enhance gas–liquid contact and promote better distribution of confined water, facilitating continuous cage growth. Moreover, in the 2–10 nm range, quasi-high-pressure conditions induced by capillary condensation and fluid–wall interactions allow H₂ molecules to reach high local densities, facilitating rapid nucleation and stable hydrate growth at pressures much lower than required in bulk systems. This is supported by molecular dynamics simulations from Rothmund et al.,¹⁵¹ which show that pores just above the 2 nm threshold allow for a transition from pure adsorption to enclathration behavior, resulting in enhanced hydrogen uptake through synergistic storage modes. However, research on the critical size remains limited and largely speculative, preventing the development of a comprehensive theoretical framework. The existing literature lacks a sufficiently broad range of pore sizes, which leads to a knowledge gap regarding critical pore size determination and its effect on hydrate stability, making it difficult to draw definitive conclusions. Others examine the ranges of sizes that are too large to provide precise and rigorous results.¹⁸¹

Fig. 7 (a) H₂ hydrate formation in porous model carbons covering the range from micro- to meso- and macropores. Modified with permission from ref. 171. Copyright 2016, Royal Society of Chemistry. (b) Adaptation of pore size (micropore and mesopore) to H₂ hydrates. Modified with permission from ref. 172. Copyright 2011, Elsevier. (c) Simulation (T = 220 K, P = 135 MPa) of H₂ molecule distribution in the H₂ hydrate in activated carbon systems, split into contributions from H₂ inside the hydrate structure (enclathrated) and H₂ outside the water phase (physisorbed). Approximately 13% of the H₂ is found in the intermediate water/ice layer between activated carbon and H₂ hydrate, which is included in the total H₂ histogram. The data for both plots was accumulated from 10 frames of 10 independent simulation trajectories with different randomized activated carbon structures. Only the atomic part of the ACs are considered as pore walls for the pore size distribution calculation, meaning that, for instance, a physisorbed H₂ molecule at 1 nm in the pore size distribution is in a 1 nm pore of the AC (edge to edge distance). Reproduced with permission from ref. 151. Copyright 2024, Royal Society of Chemistry.

For porous materials, in addition to pore size, particle size must also be considered. Previous studies have shown that particle size, as one of the main physical properties of porous materials, has a great influence on the whole hydrate formation process.^182,183 Particle size determines the arrangement and size of the interstitial pore spaces, influencing particle packing, porosity, and the fluid transport characteristics of the overall structure.¹⁸⁴ While particle size shapes the macroscopic pore network, pore size directly affects the nucleation and growth behavior of hydrates within the pores. Smaller particles with larger interstitial pore spaces enhance gas–water contact, increasing hydrate formation rates.^176,185 However, smaller than a certain particle size, transport resistance and reduced gas permeability slow hydrate growth.¹⁸⁶ Additionally, strong water-material interactions can limit the availability of free water, inhibiting hydrate formation.¹⁸⁷ Previous studies have shown that particle size and distribution within a sand bed play a critical role in determining interstitial pore spaces size and influencing hydrate formation rates.^188–190 A higher proportion of small particles fills the gaps between larger ones, reducing the system's specific surface area and connectivity, which hinders mass transfer and slows down hydrate formation. In addition, an optimal ratio of particle sizes can increase the surface area and improve formation efficiency. Under ideal conditions, where gas and water are evenly distributed, capillary forces enable a continuous supply of water to the gas–water interface, promoting the reaction. Smaller particles provide a larger specific surface area and reaction interface, enhancing water migration and accelerating hydrate growth. At this stage, the system's reaction becomes primarily controlled by diffusion, and smaller particle sizes amplify this limitation.^191,192 Su et al.⁸¹ suggested that poly (acrylic acid) sodium salt (PSA) with smaller particle sizes can significantly improve the kinetics of the H₂ enclathration process. This enhancement is contingent on the gel particles remaining discrete and avoiding agglomeration in their swollen state. Saha et al.⁸⁰ investigated the formation of mixed H₂/THF hydrates in porous media with sizes of 49, 65, 100, and 226 Å. They observed that the induction time for hydrate formation in 49 Å silica gel pores was only 27 minutes, significantly faster—by a factor of 6 to 22 (3–10 hours)—compared to bulk ice, as shown in Fig. 8(a). However, as the particle size increased, the induction time also became longer. Siangsai et al.¹⁸⁵ examined the effect of activated carbon particle sizes on methane hydrate formation under pre-adsorbed water conditions at 277 K, 8 MPa. Methane storage capacity increases as hydrates form in the interstitial spaces between particles, enhancing formation rates. Larger particle (841–1680 µm) AC, with greater interstitial spaces, showed the highest average water conversion to hydrate, 96.5%. Smaller particle size (250–420 µm) promotes methane hydrate formation by increasing interconnectivity in the crystallizer, as shown in Fig. 8(b), which enhances gas–water contact. Babu et al.¹⁹³ studied methane hydrate formation using silica sand and activated carbon of varying particle sizes. Stable hydrate formation and front movement occurred in silica sand but not in large-particle AC. Crushing the activated carbon into smaller particles enabled stable hydrate formation and front movement, demonstrating the importance of particle size and pore interconnectivity, as shown in Fig. 8(c).¹⁹³

Fig. 8 (a) Kinetic plots (fractional H₂ uptake versus time) for THF–H₂ binary clathrate hydrate formation in four porous materials. Reproduced with permission from ref. 80. Copyright 2010, American Chemical Society. (b) Typical methane consumption and temperature profiles during the methane hydrate formation experiments conducted with the activated carbon size of 841–1680 mm at 4 °C and 6 MPa. Reproduced with permission from ref. 185. Copyright 2015, Elsevier. (c) Sequential images of the hydrate formation and dissociation in activated carbon bed with large particle size. The ringed grain highlights the formation and subsequent dissociation of the methane hydrate crystal (left). Sequential images of the stable hydrate growth front in the bed of activated carbon crushed into small particle size (right). Reproduced with permission from ref. 193. Copyright 2013, American Chemical Society.

Beyond particle size, both the geometry and distribution of pores play a vital role in determining the connectivity and contact area of porous materials.¹⁸¹ Research by Li et al.¹⁹⁴ revealed that the spatial arrangement of pore structures directly impacts the growth rate, orientation, and morphology of water molecules, which are critical factors for hydrate formation. A well-structured H₂-bonding network within confined spaces accelerates nucleation and crystal growth, enhancing the growth rate of hydrates.¹⁹⁵ The orientation of water molecules, dictated by pore geometry and surface interactions, affects dipole alignment and H₂-bond structuring, which in turn governs nucleation efficiency.¹⁹⁶ Meanwhile, the morphology of water molecules, shaped by confinement effects and pore–wall interactions, determines water structuring and clustering behavior, influencing nucleation site availability and hydrate stability.¹⁹⁷ Regular pore structures tend to offer uniform and consistent pathways for gas–liquid interaction, whereas irregularly shaped pores create a diverse and complex network that can enhance nucleation due to increased surface heterogeneity.¹⁹⁸

Irregular and multi-sized pore structures also facilitate more effective gas–liquid contact and accelerating hydrate formation by introducing abundant nucleation sites. This irregularity improves heterogeneous nucleation by offering surface defects that serve as initiation points for early water structuring, which is essential for hydrate crystallization.¹⁹⁹ In contrast, materials with highly ordered and uniform pore structures, such as zeolites, are less effective in promoting hydrate formation due to limited irregularities and reduced active sites for nucleation.²⁰⁰

Moreover, the degree of diversity of pore shape regularity affects the internal connectivity and structural integrity of the material.¹⁸¹ Materials with irregular pore geometries exhibit enhanced surface heterogeneity, which can increase adsorption capacities by providing more active sites for gas storage. Additionally, variations in pore structure affect capillary forces, influencing fluid distribution and hydrate nucleation efficiency. However, these effects depend on the material's specific surface chemistry and porosity characteristics. Therefore, tailoring pore morphology including pore size, shape, and distribution, is essential for optimizing H₂ storage efficiency. This involves achieving a balance between structural integrity and surface heterogeneity to enhance gas–liquid interaction, nucleation, and mass transfer, ultimately improving hydrate formation and storage performance.

2.1.3 Surface characteristics. Solid surfaces, even porous materials that appear smooth, exhibit microscopic irregularities. In porous material, no wall surface is perfectly smooth, and surface roughness invariably affects liquid flow. The impact of roughness varies depending on the relative roughness size and its influence on flow dynamics.²⁰¹ Surface roughness influences gas adsorption by altering adsorbability and creating capillary pores with varying capacities.¹⁸¹ In addition, irregular solid surfaces can promote heterogeneous nucleation, with surface defects serving as nucleation sites that facilitate the formation of initial water structures favorable for hydrate formation.¹⁹⁹

The surface chemistry of porous materials is pivotal in influencing H₂ hydrate formation and stability. Surface chemical modifications, such as introducing functional groups like hydroxyl or carboxyl groups, can regulate nucleation rate and the stability of hydrates by enhancing interactions at the gas–liquid interface. Functional groups like these stabilize hydrate crystals via H₂ bonding.²⁰¹ For instance, graphene oxide (GO), containing oxygen-rich groups such as epoxy, hydroxyl, and carboxylic acids, exhibits amphiphilic properties and excellent water dispersibility, making it a promising material for promoting gas hydrate formation,²⁰² shown in Fig. 9(a). However, in certain cases, hydrophobic modifications have also been found to enhance hydrate formation by improving gas diffusion and lowering nucleation energy barriers. Hydrophobic functionalization of silica surfaces has also been found to enhance hydrate formation.²⁰³ For example, alkyl groups introduced through coupling agents like hexadecyltrimethoxysilane (HDTMS) or octyltrimethoxysilane (OTMS) reduce surface hydrophilicity. These agents undergo hydrolysis and condensation reactions with the hydroxy groups on silica surfaces, resulting in hydrophobic silica particles, as shown in Fig. 9(b) and (c). Hydrophobized silica materials, including SBA-15, have demonstrated superior clathrate-promoting effects by improving gas diffusion and lowering nucleation energy barriers.²⁰⁴ Kummamuru et al.²⁰⁵ demonstrated that functionalizing mesoporous cellular foam (MCF) surfaces with tetrahydrofurfuryloxypropyl triethoxysilane, a THF-like molecule, significantly improved the kinetics and H₂ storage capacity of binary H₂–THF hydrates by leveraging the molecule's mobility through its long ether chain. These surface chemical modifications highlight the importance of understanding structure–property relationships. The variety in surface ligands, their distribution, and the pore sizes of materials call for a systematic approach to material design. Such an approach reduces dependence on empirical methods and facilitates the rational optimization of hydrate formation processes. Surface roughness and chemical modifications significantly influence hydrate formation by altering gas adsorption, nucleation efficiency, and gas–liquid interactions. Practical applications should focus on optimizing surface functionalization by introducing specific chemical groups (e.g. hydroxyl, carboxyl, or alkyl groups, and so on) to modify surface chemistry, regulate intermolecular interactions, and enhance nucleation efficiency. Additionally, adjusting surface roughness can improve hydrate stability and promote efficient gas storage performance.

Fig. 9 (a) Diagram of the chemical structure of graphene oxide (GO). Reproduced with permission from ref. 201. Copyright 2022, American Chemical Society. (b) Schematic representation of the reaction mechanism for the surface modification of silica sand using coupling agents. Hydroxy groups on the surface of silica sand are replaced by hydrophobic moieties. (c) FESEM images and EDS analyses of the M (medium size), M–C₈ (modified with OTMS), and M–C₁₆ (modified with HDTMS) silica sand samples. Reproduced with permission from ref. 203. Copyright 2023, Elsevier.

2.1.4 Wettability. The influence of wettability on H₂ hydrate formation can be discussed from two aspects: external surface wettability and internal pore surface wettability. Surface wettability describes how solid surfaces interact with water molecules,²⁰⁶ whereas the wettability of inner pores in porous materials specifically affects the interfacial energy balance among water, ice, pore walls, and hydrate phases.²⁰¹

Experiments show that external surfaces with moderate wettability, balancing hydrophilic and hydrophobic properties, promote hydrate nucleation.^62,128 However, in terms of nucleation efficiency, hydrophobic surfaces exhibit superior performance compared to hydrophilic surfaces, as Wang et al.²⁰⁷ experimentally demonstrated, showing that the induction time for gas hydrate formation was more than eight times shorter on hydrophobic surfaces compared to hydrophilic ones. This improvement is attributed to the reorganization of water molecules into an ice-like tetrahedral structure on hydrophobic surfaces, which enhances nucleation efficiency.^{69,111,201,208,209} Additionally, molecular simulations by Bai et al.²¹⁰ revealed that clathrate hydrate nucleation occurs more readily on less hydrophilic surfaces, where water molecules near pore walls form more organized layers, further supporting hydrate growth. Hydrophobic surfaces also provide an advantage by adsorbing guest molecules, thereby increasing their local concentration at the gas–liquid interface.^62,211 In contrast, hydrophilic surfaces stabilize hydrate cages through H₂ bonding with hydroxyl groups but may hinder gas diffusion and reduce nucleation efficiency.^212–214 The driving force for hydrate nucleation, as described by Skovborg et al.,²¹⁵ is the chemical potential difference between liquid water and the hydrate phase, which directly influences the formation rate. Hydrophobic surfaces increase the chemical potential of nearby water molecules, amplifying this driving force and promoting faster hydrate formation, as shown in Fig. 10(a).²¹⁶

Fig. 10 (a) Mechanism of hydrate promotion for surface hydrophobic nano-SiO₂. Reproduced with permission from ref. 216. Copyright 2018, Taylor & Francis. (b–d) Proposed scenarios of water filling pores as dependent upon surface wettability. Labels are self-explanatory. Drawing scales are instructive but not necessarily precise. Gas hydrate formation in interior pore spaces is not feasible in (b) and (c) due to insufficient gas–water contact. System (d) favors gas–water contact and can promote gas hydrate formation. Reproduced with permission from ref. 62. Copyright 2020, American Chemical Society. (e) Proposed mechanism of THF solution and dissolved gases (CO₂, and H₂) filling pores as dependent upon wettability of silica gel pore. Reproduced with permission from ref. 221. Copyright 2022, Elsevier.

The wettability of internal pore surfaces in porous materials significantly influences the behavior of H₂O and gas molecules by affecting interfacial energies between ice, water, the pore wall, and hydrate phases.²¹⁷ Hydrophobic pores might not be easily wetted by water due to positive capillary pressures, as shown in Fig. 10(b). Hydrophobic pore surfaces reduce water activity, encouraging clustering of water molecules and enhancing gas–water interactions. This clustering minimizes free energy and amplifies water molecule mobility, facilitating hydrate nucleation and growth.⁶² Conversely, hydrophilic pores are fully filled with water, excluding gas molecules and impeding effective gas-hydrate formation, as shown in Fig. 10(c). Hydrophobic nano-confinement also plays a key role in hydrate formation. Studies show that hydrophobic pores may remain unwetted under certain pressures, creating conditions favorable for hydrate nucleation by reducing the capillary effects that hinder gas migration. In contrast, hydrophilic nano-confinements promote disordered water layers, which are less conducive to hydrate growth.^164,218 Balancing hydrophilic and hydrophobic properties is crucial for optimizing hydrate formation. As shown in Fig. 10(d), in moderately hydrophobic pores, condensed water molecules maintain their three-dimensional structure,²¹⁹ facilitating the formation of internal gas–water interfaces and enhancing gas diffusion pathways.²²⁰ Meanwhile, Lee et al.²²¹ suggested that the combination of concentrated THF hydrate systems with hydrophobic silica gels pores offers the most significant theoretical promotion effect, as shown in Fig. 10(e). From a broader perspective, the interaction between thermodynamic promoters and the hydrophobicity of silica gels can serve as a guiding principle for designing hydrate storage systems with enhanced thermodynamic stability. While hydrophobic pores favor hydrate nucleation and growth, excessive hydrophobicity may hinder gas diffusion and mass transfer efficiency. Therefore, optimizing wettability within a moderate range is essential to striking a balance between promoting nucleation and ensuring efficient gas permeability and favorable reaction kinetics.

2.2 Effect of external engineering operation on H₂ hydrate with porous materials

Porous materials in practical applications are often packed into beds,²²² where the external arrangement, interstitial pore connectivity, and packing density play critical roles in governing gas–liquid distribution, mass transfer efficiency, and heat dissipation.^134,223 While the internal material properties (as discussed in 2.1) governs formation interaction at the microscopic level, the external operation (as shown in 2.2) focuses on macroscopic factors that affect overall reaction performance. These external factors not only affect the accessibility and utilization of internal pores but also introduce dynamic interactions between the engineering, material and the hydrate formation process. Therefore, external engineering factors related to their spatial layout within the packed bed and operational conditions like temperature, pressure and water saturation must also be considered when evaluating their influence on H₂ hydrate formation, as shown in Fig. 11.

Fig. 11 Key engineering factors relating to mechanisms for H₂ hydrate nucleation and growth.

2.2.1 Temperature and pressure. The formation and dissociation of H₂ hydrates are significantly influenced by external temperature and pressure conditions, with porous materials playing a key role in enhancing nucleation, stability, and gas release efficiency. Fig. 12(a) illustrates that without porous support (Curve A), the P–T behavior follows the ideal gas law, indicating the absence of hydrate formation due to high nucleation barriers. In contrast, with poly high internal phase emulsion (polyHIPE) support (Curve B), a notable pressure drop (∼1 MPa) during cooling (277.1–275.2 K) confirms successful H₂ hydrate formation, demonstrating that the porous matrix provides nucleation sites. Upon heating, a sharp pressure increase (275.2–282.3 K) signifies hydrate dissociation and H₂ release, confirming the enhanced enclathration facilitated by the porous material.²²⁴ Similarly, Fig. 12(b) highlights the effect of different porous fillers on hydrate formation. Using 3 cm³ glass beads (Curve A), hydrate enclathration occurs at 270.0 K, accompanied by a moderate pressure drop, indicating some degree of hydrate formation. However, with 3 g PSA (Curve B), the pressure drop is more pronounced, signifying higher H₂ storage capacity. Furthermore, the faster pressure recovery upon heating suggests enhanced dissociation kinetics, allowing for more efficient H₂ release when needed.⁸¹ In Fig. 12(c), the addition of TBAB–Al₂O₃ and TBAB–CNT nanoporous materials further accelerates hydrate formation, as evidenced by steeper pressure drops over time. Notably, TBAB–CNT exhibits superior promotion effects compared to TBAB–Al₂O₃, indicating that carbon nanotubes provide more effective nucleation sites and enhance gas incorporation. Additionally, the reformation behavior in TBAB–Al₂O₃ suggests a two-step hydrate transition, where sI CO₂/H₂ hydrates form initially, before transforming into semi-clathrate structures, highlighting the complexity of hydrate phase behavior.²²⁵

Fig. 12 (a) P–T plots for H₂–THF–H₂O ternary system (20.0 g THF–H₂O solution, 5.56 mol% THF) during cooling and heating. (A) Without polyHIPE; (B) with 3.0 g polyHIPE support. Reproduced with permission from ref. 224. Copyright 2008, John Wiley and Sons. (b) P–T plot of enclathration and subsequent dissociation for H₂–THF–H₂O system under H₂ pressure. (A) 20.0 g of THF–H₂O solution and 3.0 cm³ of glass beads; (B) 20.0 g of THF–H₂O solution and 3.0 g of PSA. Reproduced with permission from ref. 81. Copyright 2009, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (c) Pressure drop and temperature profile with time for CO₂/H₂/H₂O hydrate formation process. (A) with 0.5 wt% nano Al₂O₃ and 11 wt% TBAB; (B) with 0.5 wt% nano CNT and 11 wt% TBAB. Reproduced with permission from ref. 225. Copyright 2019, Elsevier.

Temperature plays a crucial role in hydrate formation and dissociation. Cooling below a certain threshold promotes nucleation, while heating above a specific temperature induces dissociation.²²⁶ The presence of porous materials lowers the nucleation barrier, enabling hydrates to form at higher temperatures than in bulk systems.¹⁴⁰ Pressure is equally critical—higher pressures (>10 MPa) enhance enclathration, but the extent of H₂ uptake depends on the properties of the porous material. PSA and polyHIPE significantly enhance H₂ storage efficiency, leading to more pronounced pressure reductions, while nanoporous materials such as TBAB–CNT further accelerate hydrate formation and improve stability, outperforming TBAB–Al₂O₃ in promoting gas uptake and retention.

The synergy between external temperature–pressure conditions and porous material selection is fundamental to optimizing H₂ hydrate-based storage. By reducing nucleation barriers, increasing enclathration efficiency, and accelerating dissociation kinetics, porous supports significantly enhance the practicality and efficiency of hydrate-based H₂ storage, making them essential for advancing next-generation H₂ storage technologies.

2.2.2 Spatial layout. Interstitial spaces within packed beds are dictated by external engineering conditions, as porous materials can be arranged in different packing configurations, influencing void distribution, permeability, and mass transfer pathways. Variations in particle shape and size directly affect packing density and interstitial structure, as shown in Fig. 13(a).¹⁹³ The external particle arrangement within packed beds determines the overall pore connectivity, gas–liquid mass transfer efficiency, and hydrate nucleation sites. Li et al.¹⁹⁴ demonstrated that the geometric distribution of interstitial spaces, which is dependent on the engineered packing method, affects water molecule orientation, hydrate growth rate, and overall hydrate organization. Generally, ordered porous materials exhibit uniform and predictable pore structures, whereas irregularly packed materials result in heterogeneous pore networks with complex flow dynamics.¹⁸¹ Zhang et al.²²⁷ compared homogeneous and heterogeneous packing configurations, showing that flow velocity distribution is more intricate in irregularly packed systems, leading to variations in permeability, as illustrated in Fig. 13(b–e). Additionally, pore connectivity and alignment with flow direction are crucial, misalignment reduces effective porosity, directly affecting gas and water migration during hydrate formation. These external engineering conditions determine hydrate distribution patterns, impacting the efficiency of porous material-based systems.

Fig. 13 (a) Interstitial spaces are formed by porous materials of different shapes and sizes. Reproduced with permission from ref. 193. Copyright 2013, American Chemical Society. Comparison of velocity contours of methane hydrates in (b) homogeneous pore-filling model and (d) heterogeneous model, and velocity vectors in (c) homogeneous pore-filling model and (e) heterogeneous mode. Reproduced with permission from ref. 227. Copyright 2022, Elsevier. Distribution of hydrate modes. (f) Hydrate coats grains. (g–i) Hydrate occupies pore centers. (g) The shape of the hydrate is round. (h) The shape of the hydrate is square. (i) The shape of the hydrate is a star, and the star hydrate is formed by changing the shortest distance between particle and hydrate. Reproduced with permission from ref. 232. Copyright 2018, Elsevier.

Hydrate formation within porous materials modifies the internal structure, altering permeability, heat transfer, and mass transport mechanisms.²²⁸ This interaction depends not only on the intrinsic properties of the material but also on the engineered spatial arrangement and packing density of the particles within the bed.²²⁹ Prior research has identified three primary hydrate distribution patterns – grain-coating, pore-filling, and particle cementation – through numerical modeling.^230,231 Hou et al.²³² developed four models examining hydrate formation mechanisms and particle arrangements (e.g. round, square, and star distributions), as shown in Fig. 13(f–i). These models, based on engineered packing conditions, were used to evaluate permeability variations under different hydrate formation scenarios. The 2D modeling results showed that particle arrangement itself had minimal impact on permeability, but hydrate saturation levels and formation processes played a significant role. At low hydrate saturation, pore-filling hydrates obstructed flow pathways, while at higher saturation, the differences between pore-filling and grain-coating patterns became less pronounced. Furthermore, while the geometric shape of hydrates (round, square, or star) had little effect at lower saturation, the permeability gap widened as saturation increased. Experimental data further confirmed that hydrates primarily form in pore centers under controlled conditions.

Qin et al.²³³ highlighted the influence of saturated and supersaturated conditions on hydrate nucleation within engineered packed beds, where external water distribution and capillary forces regulate hydrate migration and distribution patterns. Under supersaturated conditions, as shown in Fig. 14(a–c), capillary forces and liquid bridge effects drive hydrate formation in cemented structures, where hydrate layers gradually bind porous material particles together and migrate upwards. Hydrates accumulate in interstitial spaces (pore-filling type), restricting liquid transport pathways.²³⁴ In saturated and unsaturated conditions, as depicted in Fig. 14(d–f), hydrates primarily form along pore walls, integrating with neighboring particles and progressively filling engineered interstitial spaces in the packed bed, while central regions remain largely unoccupied. Additionally, liquid migration into micropores results in thin hydrate films coating particle surfaces. These findings emphasize that hydrate distribution is strongly dictated by external engineering conditions, particularly the spatial packing configuration and saturation levels within the porous material bed.

Fig. 14 Hydrate growth model under supersaturated and saturated conditions: (a) initial nucleation, (b) interface growth, (c) cementation and pore filling, (d) homogeneous distribution, (e) capillary-driven migration, (f) cemented or hydrate-free states. Reproduced with permission from ref. 233. Copyright 2022, American Chemical Society.

This analysis highlights that the external arrangement of porous materials within packed beds is a key engineering control factor, shaping gas–liquid distribution, mass transfer efficiency, and hydrate formation pathways. By optimizing pore connectivity, adjusting particle packing arrangements, and regulating hydrate distribution patterns, hydrate-based storage systems can achieve greater efficiency and improved performance under practical operating conditions.

2.2.3 Water saturation. The water saturation level in porous materials is primarily determined by external operational conditions, which directly influence hydrate formation, distribution, and storage capacity. While intrinsic wettability (hydrophilicity or hydrophobicity) affects how materials retain water, it is externally controlled saturation levels that dictate the availability of nucleation sites, hydrate growth efficiency, and overall gas storage performance. The effects of water saturation span across multiple parameters, including hydrate nucleation, growth, morphology, and formation kinetics. However, the optimal water content for hydrate-based gas storage remains a subject of debate, as both insufficient and excessive saturation levels pose challenges.

At low water saturation, limited hydrate formation occurs due to pre-adsorbed water blocking micropores and restricted gas diffusion, reducing the water-to-hydrate conversion efficiency.^{115,235–238} For example, in materials such as Y-shp-MOF-5,¹¹⁶ very few hydrates form even under high pressures (6–7 MPa) due to inadequate water availability. Conversely, as water saturation increases under controlled conditions, pore spaces can become fully occupied, further inhibiting gas diffusion and decreasing adsorption capacity.¹¹⁶ This effect varies with porous structure, as shown in Fig. 15(b)—in microporous materials (e.g. AC), hydrates tend to form on external surfaces rather than within nanopores when excessive water saturates the material.²³⁷ In mesoporous systems (e.g. MIL-101), hydrate growth occurs outside pore cavities under oversaturation, reducing confinement limitations while enhancing gas storage, as observed in Fig. 15(a).²³⁹ Therefore, the presence of excess pre-absorbed water on the outer surfaces of pores or within intergranular spaces contributes to hydrate formation and enhances gas storage capacity.¹³⁹

Fig. 15 (a) Microscopic simulation of the adsorption-hydration in MIL-101 under different water contents (R_w = 0: dry MIL-101 without H₂O; R_w = 0.68: MIL-101 cavities saturated with 1105 and 636 H₂O molecules in large and small pores; R_w = 9.46: entire simulation box, including MIL-101 cavities and outer space, filled with H₂O). Reproduced with permission from ref. 239. Copyright 2019, American Chemical Society. (b) Variation in the water content in porous media with different particle sizes and methane hydrate saturation. Reproduced with permission from ref. 245. Copyright 2021, Elsevier. (c) Comparison of the pressure change for methane hydrate formation in ∼230 µm silica sand with different water contents (84.40, 75.35, 76.42, and 63.26% for experiments 12, 14, 17, and 19, respectively). Reproduced with permission from ref. 246. Copyright 2021, American Chemical Society.

External water saturation levels also influence hydrate formation pressure and kinetics, as depicted in Fig. 15(c). The spatial distribution of water within porous materials is heavily influenced by external saturation conditions, which in turn affect the preferred nucleation sites. In hydrophilic materials, micropores exhibit higher adsorption potential, leading to water molecules concentrating in smaller pores. As externally applied saturation increases, hydrate formation shifts toward larger pores or external surfaces, reducing the pressure required for hydrate formation. However, excessive saturation leads to restricted diffusion pathways, hindering gas transport and preventing hydrate formation inside the pore network. Under these conditions, hydrate growth becomes limited to particle voids and pore surfaces.^154,236,238 Notably, hydrate formation pressure does not decrease linearly with increasing water content. Beyond a critical threshold, further increases in water saturation can elevate the required pressure for hydrate formation while simultaneously reducing the water-to-hydrate conversion efficiency.²³⁵

The external water saturation level also affects nano-scale interactions in hydrophobic porous materials. Casco et al.¹¹⁵ observed that higher pre-adsorbed water levels led to smaller water nanodroplets, which provided a larger solid–liquid interface and a shorter induction period for hydrate nucleation. In contrast, Zhang et al.²⁴⁰ proposed an adsorption-induced nano-convection mechanism, where excessive water content clogs internal voids in activated carbon, weakening gas–water interactions and slowing nucleation and hydrate growth kinetics. This slowdown is attributed to reduced nano-convection and fewer available nucleation sites at high saturation levels. Notably, the effect of moderate hydrophobicity on hydrate formation is strongly scale-dependent and differs markedly between macro-packed beds and nanoconfined frameworks like MOFs. In macro-packed beds (e.g. polymer foams or emulsion-templated scaffolds), moderate hydrophobicity influences meso- to macro-scale capillary flow and fluid distribution.^209,241 It helps prevent complete pore flooding while maintaining gas-accessible domains, thereby supporting persistent three-phase contact essential for dynamic hydrate nucleation.^209,242 In contrast, hydrophobicity in MOFs arises from internal surface chemistry at the molecular level. Excessive hydrophobicity may hinder water entry into nanopores,^142,243 while overly hydrophilic frameworks risk over-saturation, reducing gas diffusion and confinement effects.^152,244 Thus, balanced internal wettability is key to enabling confined gas–water interactions. These differences highlight that hydrophobicity must be interpreted in relation to pore scale, geometry, and saturation behavior. In composite systems, such as MOF-infused macro-packed beds, multiscale wettability interactions jointly influence phase distribution and nucleation pathways, underscoring the need for integrated structural design.

The optimal water saturation for hydrate formation is highly dependent on external operational parameters, including pressure, temperature, and gas flow rates, as well as the physical properties of the porous material (e.g. pore size, connectivity, and surface chemistry) and packing density.¹³⁹ While some studies suggest full saturation as the ideal condition, others emphasize the benefits of supersaturation in maximizing storage efficiency. Therefore, precisely controlling water saturation through external operational adjustments is crucial to enhancing hydrate formation, improving gas storage capacity, and optimizing overall system efficiency. In practical applications, managing water saturation through external operational controls is crucial for regulating gas diffusion and optimizing H₂ hydrate formation conditions. The saturation level significantly affects nucleation site availability, gas–liquid interface interactions, and overall hydrate growth behavior. Maintaining an appropriate saturation balance is essential to support efficient nucleation while preventing limitations caused by either insufficient water, which restricts hydrate formation or excessive saturation, which obstructs gas transport and raises formation pressure thresholds. Thus, precisely adjusting water saturation levels based on external conditions is key to enhancing H₂ hydrate stability, improving storage capacity, and facilitating controlled dissociation when necessary.

3 Effect of different porous materials on H₂ hydrate

3.1 Classification of porous materials for H₂ hydrate promotion

The following porous materials are compiled from all available literature on H₂ hydrate-related porous materials, and can be broadly categorized into carbon-based porous materials, inorganic porous materials, organic porous polymers, and hybrid porous materials. Carbon-based materials, such as activated carbon, CNTs, and graphene, are recognized for their large specific surface areas, high electrical conductivity, and mechanical strength, although they often face challenges related to flammability, aggregation, and complex synthesis. Inorganic porous materials such as zeolites, silica gels, and glass beads, offer high thermal and chemical stability, adjustable porosity, and ease of functionalization but are often limited by complex and costly synthesis processes. Organic porous polymers, such as polyurethane foam, emulsion-templated polymers, and superabsorbent polymers (SAPs), exhibit excellent mechanical properties, high porosity, and water absorption capabilities but may suffer from sensitivity to environmental conditions. MOFs represent a unique class of hybrid porous materials, combining metal nodes with organic linkers, which allows for highly tunable porosity, large surface areas, and chemical versatility, making them promising candidates for gas storage and separation applications. The classification, formation methods, advantages, and disadvantages of these porous materials are summarized in detail in Table 4. Understanding the advantages and limitations of these diverse porous materials is essential for optimizing their performance in H₂ storage and other advanced energy applications.

3.2 Carbon-based porous materials

3.2.1 Activated carbon. AC exhibits a significant promoting effect on the formation and H₂ storage performance of H₂ hydrates. The unique porous structure of activated carbon, along with its pore size and surface chemistry, directly influences nucleation, formation kinetics, and storage capacity of H₂ hydrates.

Farrando-Perez et al.³⁹ demonstrated that AC derived from optimized Petroleum Pitch (PPAC) using KOH as activating agent at 1073 K, which features a tailored porous structure with suitable surface chemistry. The synthesized material is characterized by a large apparent surface area (S_BET ∼3690 m² g⁻¹), with extensively developed microporosity (V_micro ∼1.06 cm³ g⁻¹) and mesoporosity (V_meso ∼1.90 cm³ g⁻¹). The microporous and mesoporous structures of the optimized AC (PPAC) provided an ideal “nano-reactor” environment that facilitates the formation of H₂ hydrates at lower pressures. As shown in Fig. 16(a), the nanoconfinement within PPAC cavities facilitates the nearly complete transformation of D₂O into clathrate hydrates in the D₂O-PPAC system. A key observation is that confinement significantly lowers the formation pressure by over 65 MPa (∼30%) without requiring chemical additives, enabling H₂ hydrate formation at 135 MPa, compared to the 200–220 MPa needed in bulk conditions. Additionally, nanoconfinement enhances the thermal stability of H₂ hydrates well beyond the conventional stability range, with decomposition temperatures reaching ≥240 K, as opposed to 145 K in bulk systems,²⁴ as shown in Fig. 16(b) and (c). What's more, this paper also concluded that two key aspects for 3D carbon networks to facilitate H₂ clathrate formation are: (i) an optimized porous structure, characterized by high surface area and a balanced distribution of micropores and mesopores, and (ii) suitable wettability, ensuring D₂O adsorption while maintaining the hydrophobicity required for water–H₂ interactions. The AC surface facilitates tetrahedral ordering of interfacial water molecules, promoting nucleation, while nanoconfinement effects enhance H₂ solubility at the hydrophobic interface, increasing local gas density. Combined with an extended water–gas interface, these factors accelerate nucleation by lowering activation energy, enabling hydrate formation at much lower pressures than bulk systems without altering nucleation thermodynamics, as illustrated in Fig. 17.

Fig. 16 (a) Neutron diffraction patterns obtained at 5 K for the D₂O-PPAC pressurized with normal H₂ at (a) 100, (b) 135, and (c) 200 MPa. Theoretical patterns for hexagonal ice (Ih) and sII structure in gas hydrates are included for comparison. Thermal stability of confined (b) hexagonal ice and (c) H₂ clathrate hydrate up to 240 K followed by neutron diffraction. Before the experiment, sample D₂O-PPAC was pressurized at (b) 100 MPa and (c) 135 MPa. After pressurization, the samples were cooled to 5 K, the pressure cell was decreased down to 0.1 MPa, and the sample cell temperature was increased stepwise. Reproduced with permission from ref. 39. Copyright 2022, Springer Nature.

Fig. 17 Illustrative scheme of the H₂ clathrate formation in bulk water and confined environments created by AC. Reproduced with permission from ref. 39. Copyright 2022, Springer Nature.

Through molecular dynamics simulations, Rothmund et al.¹⁵¹ discovered that the optimal pore size of AC is approximately 2 nm. In pores exceeding the critical diameter of ∼2 nm, most H₂ is enclathrated within a hydrate structure, making activated carbon with meso- and macroporosity highly promising for H₂ storage. In contrast, physisorbed H₂ primarily accumulates around 1 nm pores, with contributions extending up to the micro-to-meso-pore transition (∼2 nm). However, physisorption efficiency declines with increasing pore size due to a lower surface area-to-volume ratio, consistent with experimental findings identifying micropores as optimal for H₂ storage. Notably, the pore size threshold for effective physisorption (<2 nm) aligns with the lower limit for hydrate-based storage (>2 nm), enabling a dual-storage mechanism where micropores store H₂via physisorption, while larger pores facilitate enclathration within a hydrate lattice, as shown in Fig. 18(a). The study also examined the influence of AC properties and external conditions on H₂ hydrate stability, as shown in Fig. 18(b). While structural parameters had minimal impact on the critical pore size, increased oxygen content (R_O) significantly enhanced hydrate stability by reducing surface hydrophobicity and lowering capillary pressure, allowing water retention in smaller pores. Further research is needed to determine the optimal oxygen concentration. Variations in AC density and curvature primarily affected pore distribution rather than hydrate formation. Lower defect density (R_defect) produced flatter graphene aggregates, while higher density (ρ) reduced pore size, though neither directly influenced hydrate stability. Smaller fragment sizes (S_fragment) slightly improved stability due to higher relative oxygen content, with a doubling of S_fragment reducing oxygen content by 24% and increasing the critical pore size by ∼1 Å.

Fig. 18 (a) Distribution of H₂ molecules in one representative H₂ hydrate in activated carbon system, categorized by different storage mechanisms using a-shapes. H₂ molecules are either enclathrated (inside a-shape of hydrate water molecules), in the intermediate layer (IML) (inside a-shape of non-hydrate water, representing a transitional state between free gas and hydrate encapsulation), or physisorbed (outside both a-shapes). (b) Fraction of water molecules in a hydrate phase as a function of pore diameter with varied conditions and AC properties. From left to right, the top row displays the effects of temperature, pressure, and AC oxygenation rate, here defined as the average fraction of OH terminal groups on carbon fragments relative to H terminal groups. Reproduced with permission from ref. 151. Copyright 2024, The Royal Society of Chemistry.

Bai et al.²⁶⁹ showed that the addition of AC (model XZ-40, particle diameter 4 mm) reduced the induction time of THF hydrate formation and significantly increased H₂ storage capacity. Under isothermal and constant volume experimental conditions, the average induction time without AC was 333 minutes, which decreased to 250 minutes with the addition of AC. Furthermore, across various pressure conditions, the addition of AC notably enhanced the H₂ storage capacity of THF hydrate. At a pressure of 8.4 MPa, the H₂ storage capacity increased from 0.0031 wt% to 0.0082 wt%, an improvement of 164.52%.

3.2.2 Carbon nanotubes. CNTs have garnered significant scientific interest due to their unique physical properties and broad application potential, particularly in gas hydrate formation.^270,271 Their tubular structure and molecular-scale pores, both on the surface and within interlayer spaces, provide a high specific surface area, enhancing gas adsorption and mass transfer.²⁷² The porosity of CNTs, which is dictated by their diameter (typically ranging from 0.5 nm for single-walled carbon nanotubes, SWCNTs to 100 nm for multi-walled carbon nanotubes, MWCNTs) and by interstitial spacing, can result in surface areas in excess of 1000 m² g⁻¹, making them particularly interesting in the context of gas clathrate formation for H₂ storage.²⁷³ Additionally, CNTs exhibit high thermal conductivity and frictionless surface properties, facilitating rapid gas and water transport, key factors in promoting hydrate nucleation and growth.^274–277

Prasad et al.²⁷⁸ demonstrated that using MWCNTs as a substrate for clathrate formation significantly accelerates the H₂ adsorption rate, achieving approximately 1.5 wt% H₂ capacity within 90 minutes at 10 MPa, 263 K. Such rapid adsorption kinetics highlight the high-efficiency adsorption properties of CNTs. Their nanoscale pore sizes (5–20 nm in this example) provides ideal channels for H₂ molecules to penetrate the internal tubes, increasing the contact area between H₂ and water molecules. Fig. 19(a) and (b) present the temporal evolution of clathrate hydrate concentration in the reactor bed, corresponding to single and double H₂ occupancy in the hydrate cages. The observed concentration profiles closely follow H₂ adsorption kinetics, exhibiting a steady increase until equilibrium is reached. Effective temperature regulation is essential for optimizing hydrate formation, ensuring uniform distribution and maximizing storage efficiency. Additionally, experimental findings by Zang et al.²⁷⁹ revealed that acid-treated CNTs exhibit enhanced surface activity due to the presence of –OH, –COOH, C–N, C–O groups, which promote hydrate nucleation and growth while increasing H₂ adsorption. In the presence of THF as the promoter, CNTs were shown to stabilize H₂ hydrate formation and maintain a high H₂ storage capacity. However, the choice of CNTs' diameter plays a crucial role in H₂ hydrate storage performance, as excessively large or small diameters can negatively affect the stability and efficiency of H₂ storage.

Fig. 19 Simulation results showing concentration values of clathrate hydrate formed in reactor bed with the passage of time for (a) single, (b) double H₂ occupancy in small clathrate cage at 10 MPa, 263 K. Reproduced with permission from ref. 278. Copyright 2023, Elsevier.

Zhao et al.²⁸⁰ revealed that the pore size of CNTs can influence the formation of hydrate structures at the nanoscale by molecular dynamics simulations. Within SWCNTs of nanometer-sized diameter (1–1.3 nm), one-dimensional (Q1D) H₂ hydrates were observed to form spontaneously near ambient temperature, as shown in Fig. 20(a). Unlike traditional three-dimensional hydrate cages, where H₂ molecules are encapsulated within discrete clathrate structures,²⁸¹ these Q1D hydrates feature H₂ molecules arranged in molecular lines embedded within the continuous channels of one-dimensional water nanotubes. The hydrate molecular structure in nanotube structures varied depending on the diameter of the CNTs, producing configurations such as hexagonal and heptagonal shapes, with each structural unit accommodating 1–2 H₂ molecules. The summaries with structures of Q1D H₂ Hydrates, H₂O/H₂ molecular ratio, occupancies of H₂ per polygonal prism, H₂ weight percentage (wt%), and the collapse temperature of hydrates are shown in Table 5. The study that in the (15, 0) zigzag SWCNT, H₂ molecules predominantly arrange into a hexagonal hydrate structure, where each hexagonal prism encapsulates a single H₂ molecule. Similarly, the (16, 0) variant favors a heptagonal hydrate configuration, maintaining a one-to-one ratio of H₂ molecules per heptagonal prism. In contrast, the (17, 0) zigzag SWCNT facilitates the formation of an octagonal H₂ hydrate, with each pentagonal prism capable of accommodating either one or two H₂ molecules, allowing for both single and double occupancy. These structural variations underscore the critical role of nanotube diameter in determining hydrate formation patterns and H₂ storage efficiency. Notably, in hexagonal and heptagonal ice nanotubes, H₂ exhibits a solid-like behavior, characterized by an extremely low axial diffusion constant (<5 × 10⁻¹⁰ cm² s⁻¹), as illustrated in Fig. 20(b–d). In contrast, within octagonal ice nanotubes, H₂ behaves more like a liquid, with an axial diffusion constant approaching 10⁻⁵ cm² s⁻¹, as shown in Fig. 20(e–g). This distinction highlights the influence of nanotube geometry on H₂ mobility, suggesting that structural confinement plays a crucial role in regulating H₂ diffusion dynamics. This unique Q1D H₂ hydrate structure offers new perspectives for the design and optimization of nanoporous materials in H₂ storage applications, where the exquisite tuneability of pore sizes in CNTs can be exploited. CNTs not only enhance H₂ storage capacity through physical adsorption but also enable more efficient and stable H₂ storage by regulating the hydrate structure within the nanoscale channels. Despite strong evidence from calculations that CNTs favor the formation of H₂ clathrate hydrates, experimental demonstration remains elusive. Further research is needed to bridge the gap between theoretical predictions and experimental validation, ensuring practical applications of CNTs-based materials in H₂ storage technologies.

Fig. 20 (a) H₂ enclathration in CNTs – transition from ice nanotube to quasi-1D H₂ hydrate. Calculated mean square displacement (MSD) in the axial direction for H₂ and water molecules in the center of Q1D H₂ hydrates (b) 5-gonal hydrate, (c) 6-gonal hydrate, (d) 7-gonal hydrate, (e) 8-gonal hydrate (single), (f) 8-gonal hydrate (single/double), (g) 8-gonal hydrate (double) (formed in SWCNTs). Reproduced with permission from ref. 280. Copyright 2014, American Chemical Society.

Table 5 Structures of Q1D H₂ hydrates, H₂O/H₂ molecular ratio, occupancies of H₂ per polygonal prism, H₂ weight percentage (wt%), and collapse temperature of hydrates²⁸⁰

SWCNTs diameter (nm)	H2 hydrate	H2O/H2 ratio	Occupancy	H2 (wt%)	T collapse (K)
a T collapse is the temperature at which 50% clathrate cages are collapsed, computed by heating the clathrates from 250 in 10 K temperature step (20 ns per temperature step). The initial configurations are the perfect hydrate structures to ignore effects of defects.
1.10	5-Gonal	5 : 01	Single	0.37	290
1.17	6-Gonal	6 : 01	Single	0.34	390
	Phase-separated	<6 : 1	Single	—	390
1.25	7-Gonal	7 : 01	Single	0.32	400
	Mixed 7-gonal/6-gonal	(6 : 1, 7 : 1)	Single	(0.32, 0.33)	400
1.33	6-Gonal	6 : 01	Single	0.33	300
	7-Gonal	7 : 01	Single	0.3	330
	Mixed 8-gonal/7-gonal	(7 : 1, 8 : 1)	Single	(0.29, 0.30)	330
	8-Gonal	8 : 01	Single	0.29	330
	8-Gonal	(8 : 1, 8 : 2)	Single/double	(0.29, 0.58)	330
	8-Gonal	8 : 2	Double	0.58	410

---

3.2.3 Graphene-based materials. Graphene, a two-dimensional monolayer composed of sp²-hybridized carbon atoms arranged in a honeycomb lattice, exhibits outstanding mechanical strength, electrical conductivity, and thermal stability.²⁸² Due to these exceptional properties, graphene-based materials are highly suitable for applications in nanocomposites.²⁸³ In the context of hydrate formation processes (HFP), graphene's superior thermal conductivity facilitates efficient heat dissipation, thereby enhancing heat transfer and preventing the thermal degradation of hydrate crystals caused by excessive heat accumulation.²⁸² Moreover, the large specific surface area of graphene-based materials offers numerous active sites for nucleation, significantly improving mass transfer and promoting hydrate formation.²⁸⁴ Additionally, the introduction of graphene materials increases system heterogeneity, favoring heterogeneous nucleation over homogeneous nucleation, which further accelerates the formation of hydrate crystals.²⁸⁵

Zhao et al.²⁸⁶ proposed that two-dimensional H₂ hydrates could form within graphene-confined environments through molecular dynamics simulations. Confined between two parallel graphene sheets, these hydrates exhibited unique bilayer structures, such as hexagonal crystals. Within graphene confinements of 9–11 Å, the formation efficiency of the bilayer hydrates was highest, demonstrating excellent thermodynamic stability. Each hexagonal cage in the bilayer structure accommodated one H₂ molecule, resulting in a significantly higher H₂ storage density compared to traditional three-dimensional hydrates.

Subsequently, Zhong et al.²⁸⁷ further analyzed the structure and thermodynamic stability of hydrates under graphene bilayer confinement using density functional theory (DFT) calculations. Fig. 21(a) presents the computed hydrate structures within graphene confinement, and results indicate that maintaining structural integrity becomes challenging below 8.0 Å due to spatial constraints. The study also proposed four types of bilayer H₂ hydrate crystals (BLHH-I to BLHH-IV), as illustrated in Fig. 21(b–e), and investigated the dynamic stability of these crystals under varying temperatures. The results showed that within the temperature range of 213 K to 273 K, BLHH-I and BLHH-II exhibited the highest stability under graphene confinement. BLHH-I demonstrated an H₂ storage capacity of up to 2.703 wt%, indicating its suitability for H₂ storage applications over a wide temperature range. Additionally, a confinement spacing of 9 Å was identified as the optimal condition for hydrate formation.

Fig. 21 (a) Basic structures of two-dimensional H₂ hydrates confined between two graphene sheets. Structures of two-dimensional H₂ hydrates: (b) BLHH-I (4.6²4⁶) unit cell; (c) BLHH-II (2.4⁶, 2.8²4⁸) unit cell; (d) BLHH-III (2.5²4⁵, 6.6²4⁶, 2.7²4⁷) unit cell; (e) BLHH-IV (4.5²4⁵, 4.6²4⁶, 2.8²4⁸). The water and H₂ molecules are shown as a stick model, and the C atoms are shown as a ball and stick model, where the O atom, H atom and C atom are shown in red, white and black respectively. Reproduced with permission from ref. 287. Copyright 2020, The Royal Society of Chemistry.

Using molecular dynamics calculations, Abbaspour et al.²⁸⁸ emphasized the role of graphene's high specific surface area and surface adsorption capability in enhancing the order and formation rate of hydrate crystals. In graphene-confined spaces, the adsorption energy was calculated to be −17.974 kcal mol⁻¹, indicating that the capture of H₂ molecules was a spontaneous process. Moreover, the interaction between water molecules and graphene was relatively weak, minimizing the risk of strong interactions disrupting the H₂-bond network of water molecules. The orderly arrangement of water and H₂ molecules, as illustrated in Fig. 22, increased the storage density of H₂ molecules. Simulation experiments revealed that the bilayer hydrates exhibited the highest H₂ storage density.

Fig. 22 (a) Ice, (b) H₂ clathrate structures formed and (c) Snapshots of the side view of the formed H₂-clathrate structures in graphene surface systems. The oxygen atoms are shown in red color, H₂ atoms in white, and H₂ molecules in blue. Reproduced with permission from ref. 288. Copyright 2024, The Royal Society of Chemistry.

3.3 Inorganic porous materials

Inorganic porous materials, such as mesoporous silica, zeolites, and glass beads exhibit advanced capabilities for H₂ storage and hydrate formation due to their unique physicochemical properties.²⁸⁹ These materials exhibit high thermal and chemical stability, ensuring durability under extreme conditions, and nearly infinite possibilities for surface functionalization.²⁹⁰ Their well-defined and tunable pore structures facilitate efficient molecular confinement, enhancing gas diffusion and increasing local H₂ concentration, which is crucial for promoting hydrate nucleation.^44,291 The rigid frameworks of these materials further support the structural integrity of H₂ hydrates, preventing collapse and enhancing long-term stability.²⁹²

3.3.1 Silica. Silica materials have become an important research direction for optimizing H₂ storage technology in clathrate hydrates due to their unique pore structure, high specific surface area, surface adjustability and excellent thermodynamic properties.^293–295 The high pore volume of the material plays a key role in its water uptake and the formation of H₂ clathrates. MCF silica features pore sizes of 150–500 Å, comprising spherical cells (200–500 Å) interconnected by windows (100–150 Å).^296,297 The structure forms a 3D porous network, with cells framed by silica struts, as shown in Fig. 23. These large mesopores provide favorable conditions for the binding of active sites of catalysts or enzymes, and enhance the diffusion of reactants or substrates.²⁹⁶ It further supports the growth and expansion of the crystal network of inclusion compounds. Moreover, the uniform pore size distribution provides an excellent model for investigating hydrate formation in confined spaces.⁴⁴

Fig. 23 SEM image of (a) spherical MCF silica particles; (b) a single spherical MCF particle at higher magnification; (c) schematic cross section of the structure exhibited by MCF silica; (d) silanol groups (≡Si–OH) on wall of MCF silica; (e) 3D cells and windows in MCF silica. Reproduced with permission from ref. 298. Copyright 2019, De Gruyter.

Ciocarlan et al.⁴⁴ and Kummamuru et al.⁴⁵ both underscored the critical role of MCF silica materials in promoting the nanoscale confinement effect for H₂ hydrate formation, leveraging their distinct pore structures and tunable surface properties, as summarized in Fig. 24. Despite their shared focus, the two studies employed different functionalization strategies. Ciocarlan et al. introduced phenethyl-functionalized surfaces to induce hydrophobicity on silica, reducing excessive water (D₂O was used in these experiments) adhesion to silica walls and thereby enhancing hydrate formation. The findings demonstrated that nanoscale confinement reduced the pressure required for rapid H₂ hydrate nucleation by at least 20% compared to bulk systems. Moreover, the material featured two H₂ storage sites: clathrate cages and unmodified silica walls. Stability tests at 280 K and 0.1 MPa revealed that confined D₂O–H₂ clathrate hydrates, once formed, remained stable at temperatures up to 280 K, exceeding the freezing point of D₂O, as shown in Fig. 25(a). In contrast, Kummamuru et al. focused on functionalizing MCF with THF-like groups to enhance hydrophobicity and optimize the local arrangement of water molecules. Their experiments showed that under conditions of 7 MPa and 262 K, functionalized MCF (f-1) achieved a water conversion rate of 39.78% and a H₂ storage capacity of 0.52 wt%, as illustrated in Fig. 25(b).

Fig. 24 N₂ sorption at 77 K and pore size distributions of MCF grafting (a) phenethyl groups (MCFPhene). Reproduced with permission from ref. 44. Copyright 2024, Springer Nature. (b) THF. Reproduced with permission from ref. 45. Copyright 2024, The Royal Society of Chemistry.

Fig. 25 (a) Neutron diffraction data at different initial pressures and temperatures. Neutron diffraction of the MCFPhene-D₂O–H₂ system after in situ synthesis at different pressures and measured at 0.1 MPa and 5 K (left). Stability experiments for 165 MPa experiment, measured at 0.1 MPa (right). Reproduced with permission from ref. 44. Copyright 2024, Springer Nature. (b) H₂ storage capacity in the THF-like functionalized MCF (f-1) porous material at three different temperatures with an initial pressure of 7 MPa. Reproduced with permission from ref. 45. Copyright 2024, The Royal Society of Chemistry.

Watson et al.²⁹⁹ demonstrated that functionalized hollow ring periodic mesoporous organosilica (HRPMO) materials with uniform mesoporous structure (1.29–1.89 nm) and surface area (620–850 m² g⁻¹) provide ideal nanoscale confinement and increased gas–liquid interfacial contact. Surface functionalization with THF-like groups optimizes hydrophobicity and promotes structured H₂ bonding, reducing Gibbs free energy barriers and accelerating hydrate nucleation, as shown in Fig. 26. The study demonstrated that while non-functionalized HRPMO facilitates binary H₂–THF clathrate formation, the incorporation of surface-bound promoter agents significantly enhances both formation kinetics and storage capacity. Optimal promoter loading at 0.14 mmol per g THF solution (as shown in Fig. 27 of HR₉₅-THF₅-PMO) increased H₂ storage capacity by 3% (to 0.26 wt%) at 7 MPa, 265 K and accelerated clathrate growth by 28%, highlighting the critical role of surface functionalization in optimizing solid-state H₂ storage performance.

Fig. 26 Depiction of differences in clathrate formation in HRPMO (left, non-functionalization) versus HR_x-THFy-PMO (right, functionalization with different THF solution loading) Reproduced with permission from ref. 299. Copyright 2023, The Royal Society of Chemistry.

Fig. 27 Solid-state tuning effect on H₂ storage capacity of THF-functionalized HRPMO materials at different temperatures, and different THF ratios. Reproduced with permission from ref. 299. Copyright 2024, The Authors.

3.3.2 Zeolites. Zeolites are crystalline aluminosilicates based on tetrahedral structural units (SiO₄ and AlO₄).³⁰⁰ Their high thermal and chemical stability make these materials ideal for use in many different applications, such as petrochemical, chemical, environment, and gas processing.³⁰¹ Due to their unique microporous structure and strong polar adsorption capability, zeolites are extensively utilized across various industrial applications.^302–304 Their fine particle size and abundant micropores offer a substantial surface area, effectively facilitating hydrate nucleation by providing numerous active contact sites.³⁰⁵

Firuznia et al.⁴¹ proposed a modified zeolite (Z3) based on four key characteristics: (1) it supports the formation of hydrates at the interface rather than in bulk; (2) optimized pore dimensions promote water molecule layering within the host material, increasing H₂ absorption by 2–3 times; (3) the curved pore structure enhances the nucleation efficiency of hydrate particles; and (4) the tailored pore surface chemistry facilitates water molecule reorganization, forming double donor–double acceptor (DDAA) bonds that drive higher nucleation rates and improved H₂ storage, making it an ideal host framework material for promoting H₂ hydrate formation, as shown in Fig. 28. Experiments demonstrated that at 10 bar, the H₂ storage capacity of H₂–THF hydrate with Z3 reached 2.1 wt% in the 10 mol% THF solution, approaching the theoretical maximum of sII hydrates and significantly outperforming bulk water systems, with an over 200-fold increase in storage capacity, as shown in Fig. 29. Additionally, the nucleation rate of H₂ hydrates improved by a factor of 10, achieving full saturation within 10 minutes. The hydrates also enabled rapid H₂ release at ambient temperature without the need for high-energy conditions.

Fig. 28 (a) Schematic of a material platform for high-capacity H₂ storage in which a powder of mesoporous zeolite Z3 with a pore diameter of 2.4 nm is placed at the bottom of the chamber. H₂ gas (green) is stored in the form of H₂ hydrate in the pores of Z3. (b) The role of pore diameter on H₂ solubility compared to the bulk material. The ordering of water molecules in pores spanning 2–3 nm leads to approximately 2-fold enhancement of H₂ solubility. (c) The role of concavity of pores with dimensions much smaller than critical nucleus size (∼6.82 nm) on shape function. A reduced value interfacial energy f(m, x) corresponds to a lower Gibbs energy barrier for hydrate nucleation (m = interfacial energy, x = interface geometry). (d) The role of pore size on the nucleation rate of H₂ hydrates compared to the bulk medium, for different values of m. Reproduced with permission from ref. 41. Copyright 2023, Elsevier.

Fig. 29 (a) H₂ storage capacity of hydrates with various pore dimensions are compared with Z3 offering 2.1% storage capacity, which is the maximum capacity of the cubic H₂ hydrate structure for 10 mol% THF solution. (b) H₂ storage capacity of Z3 vs. bulk water and THF, suggesting that storage capacity can be boosted by more than 200 times by Z3 structures. Reprinted with permission from ref. 41. Copyright 2023, Elsevier.

3.3.3 Glass beads. Glass beads serve as an inert particle support, offering increased surface-to-volume ratios that facilitate gas diffusion and hydrate nucleation. Although the glass beads themselves do not actively participate in the chemical process, their smooth surfaces and uniform pore distribution provide efficient gas–liquid interfaces, reducing diffusion barriers.³⁰⁶ Kumar et al.³⁰⁷ stated that glass beads can create a highly permeable porous medium, effectively minimizing mass transfer and fluid flow resistance. In Su et al.,⁸¹ glass beads were used as a baseline comparison against more advanced support materials like hydrogels and emulsion-templated polymers. The study found that using glass beads resulted in significantly slower H₂ uptake kinetics compared to hydrophilic, high-surface-area materials. Despite this, the glass beads offered better stability and reusability over multiple hydrate formation–dissociation cycles, highlighting their potential in systems prioritizing operational reliability over rapid formation. Yang et al.³⁰⁸ further highlighted that the highly ordered structure of glass beads allows for controlled nucleation and hydrate growth.

One major limitation of glass beads is their limited pore volume, which constrains the amount of H₂ that can be stored. The bulk density of glass beads reduces the system's gravimetric efficiency, leading to relatively low H₂ storage capacities. Furthermore, the inert nature and hydrophilic surface of glass beads limit their effectiveness in inducing heterogeneous nucleation, especially when compared to materials with tailored surface chemistry. It resulted in slower hydrate formation rates and lower storage efficiency under moderate pressure and temperature conditions.

Glass beads can moderately enhance the formation of H₂ hydrates by providing a stable and increased surface area for gas–liquid interaction, improving mass transfer. Future research could focus on modifying the surface chemistry or combining glass beads with other porous materials to balance stability and storage performance.

3.4 Organic porous polymers

3.4.1 Polyurethane foam. It is indicated that the enhanced hydrate formation observed in polyurethane (PU) porous materials can be attributed to their well-connected interstitial pore spaces, as well as the material's connectivity and water wettability, as illustrated in Fig. 30.³⁰⁹ The interconnected pore structure facilitates efficient water molecule migration and gas transport through the material's pathways, promoting favorable conditions for hydrate formation.

Fig. 30 (a) Scanning electron microscopy (SEM) and (b) optical microscope images of PU foam showing the interconnected pores. Reproduced with permission from ref. 309. Copyright 2013, American Chemical Society.

According to Talyzin et al.,³¹⁰ the unique structural properties of PU foam, including its large pore size (200–300 µm) and excellent pore interconnectivity, provided an ideal gas–liquid interface and efficient transport pathways for hydrate formation. These features significantly improved the hydrate formation rate and H₂ storage efficiency. For example, under 135 bar and 2 mol% THF conditions, the H₂ storage capacity of the PU foam-supported system reached 0.2 wt%, far exceeding that of conventional bulk samples. However, H₂ release from this system was relatively slow, attributed to the complex pore network structure, which caused gas bubble retention and hindered the rapid escape of H₂. Additionally, the local pressure within the pores under high-pressure conditions further delayed gas release. The pore size of PU foam is significantly larger than the nano-scale ideal size, suggesting that its mechanism for promoting H₂ hydrate formation may differ. Unlike the nano-confinement effect, which stabilizes hydrates and provides a quasi-high pressure environment, PU foam enhances nucleation and growth through its interconnected pore structure and excellent gas–liquid transport properties.

3.4.2 Emulsion-templated polymers. Studies have shown that utilizing ultralow-density, emulsion-templated polyHIPE materials as support can significantly enhance the kinetics and reusability of gas clathrate formation.^311–313

Su et al.²²⁴ were the first to apply this material for studying rapid and reversible H₂ storage in clathrate hydrates, as shown in Fig. 31. The polyHIPE material featured an open-cell structure interconnected by pore windows, with a narrow pore size distribution centered at 9.1 µm and a BET surface area of 230 m² g⁻¹. Despite the material's highly hydrophobic nature, an aqueous solution containing 5.56 mol% THF exhibited sufficient wettability to form a supporting film within the interconnected pore structure. This enabled the utilization of its large interfacial area and short diffusion paths (approximately a few microns) to facilitate gas clathrate formation. A key characteristic of this material was its interconnected pore structure and extremely low bulk density (0.056 g cm⁻³), making it highly suitable for hydrate-based H₂ storage applications. Results have shown that under conditions of 270 K and approximately 11.6 MPa, the time required for polyHIPE materials to encapsulate 90% of H₂ was reduced from over 11 days in traditional bulk THF–H₂O systems to just 60 minutes, achieving a more than 250-fold improvement in kinetics. Additionally, polyHIPE materials exhibited excellent stability across multiple freeze–thaw cycles, overcoming the significant performance degradation observed in conventional bulk or crushed hydrates after the first cycle. In terms of H₂ storage performance, hydrate systems supported by polyHIPE materials achieved a storage capacity of 0.15–0.18 wt%.

Fig. 31 (a) SEM image and pore size distribution (inset) for macroporous polyHIPE (50 mm scale bar). (b) Schematic illustration of clathrate hydrate dispersed on the polyHIPE support. Reproduced with permission from ref. 224. Copyright 2008, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

3.4.3 Superabsorbent polymers. Su et al.⁸¹ suggested the other use of superabsorbent polymers (such as PSA) as kinetic promoters for THF–H₂ hydrate formation in a static system. SAPs can quickly absorb large amounts of water and form stable hydrogels and can be used to reduce or completely eliminate the induction time for H₂ hydrate formation and increase the formation rate of hydrates,¹⁹⁵ providing a versatile, reversible matrix with a high surface area for the hydrate formation.^81,314,315 The clathration process using swollen PSA particles as a H₂-storage material is shown in Fig. 32. At 270 K and 11.6 MPa, PSA-supported systems reduced the H₂ encapsulation time from 11 days in bulk systems to 6.5 minutes, achieving a 600-fold kinetic improvement. This was due to PSA's high water absorption and uniformly distributed hydrogel, which increased gas–liquid contact and shortened diffusion paths, accelerating hydrate formation. Additionally, PSA systems retained over 90% H₂ storage capacity after three cycles, outperforming polyHIPE in promoting hydrate kinetics. Despite its promise, the long-term stability of SAPs under repeated hydrate formation–dissociation cycles requires further study. Potential issues include hydrogel degradation and reduced water retention over time. Additionally, the scalability, cost-effectiveness, and overall efficiency of SAP-based hydrate systems for large-scale H₂ storage remain to be evaluated.

Fig. 32 (a) Schematic illustration of clathrate hydrate dispersed within PSA gel particles. (b) Dry PSA, particle size <1000 µm. (c) Fresh THF–H₂O hydrogel particles before exposure to H₂. (d) THF–H₂O hydrogel after five cycles of H₂ enclathration and dissociation. Reproduced with permission from ref. 81. Copyright 2009, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Lee et al.³¹⁶ explored the effects of SAPs on H₂ hydrate formation via two synthesis pathways: solution-borne and hydrate-borne. In the solution-borne method, the THF solution stabilized in the liquid phase was exposed to precooled H₂ gas (12 MPa) to form hydrates, with THF acting as the reaction medium for direct contact with H₂. In the hydrate-borne method, pure THF hydrates were first formed below 263 K as precursors, followed by the introduction of precooled 12 MPa H₂ to form binary THF–H₂ hydrates, as illustrated in Fig. 33. Results showed that at 5.56 mol% THF, the H₂ storage capacity was higher for the hydrate-borne pathway (19.6 mmol H₂ per mol H₂O) compared to the solution-borne pathway (13.6 mmol H₂ per mol H₂O), indicating that optimizing the synthesis route significantly enhances SAP-supported H₂ hydrates. Raman spectroscopy and X-ray powder diffraction (XRD) analyses, as shown in Fig. 34, revealed that SAP systems at 1.0 mol% and 5.56 mol% THF exhibited a substantial increase in the proportion of large-cage structures. This was attributed to the localized enrichment and uneven distribution of THF, which created regions favorable for large-cage formation. These THF-enriched zones enhanced the local H₂-bond network among water molecules, significantly reducing the energy barrier for large-cage formation and preferentially capturing more H₂ molecules.

Fig. 33 Schematic of (a) solution dispersing in SAPs and (b) two synthetic pathways of forming binary THF–H₂ hydrates. Reproduced with permission from ref. 316. Copyright 2025, Elsevier.

Fig. 34 Raman spectra of hydrate-borne and solution-borne binary THF–H₂ hydrates with (a) 1.0 mol% of THF solutions and (b) 5.56 mol% of THF solutions. (c) Nuclear magnetic resonance (NMR) spectra of binary THF–H₂ hydrates. (d) Temperature-dependent Raman spectra of hydrate-borne binary THF–H₂ hydrates with 5.56 mol% of THF solutions. Reproduced with permission from ref. 316. Copyright 2025, Elsevier.

3.5 Hybrid porous materials

3.5.1 Metal–organic frameworks, MOFs. MOFs with nanometer-size control and/or functionalization of pores have long been considered among the most promising candidate materials for H₂ storage,^317,318 as their crystalline frameworks with open structures and internal cavities can potentially accommodate and store large amounts of molecular H₂.³¹⁹ Balderas-Xicohténcatl et al.³²⁰ identified a linear relationship between volumetric absolute H₂ uptake and volumetric surface area in MOFs, highlighting the effectiveness of high-surface-area MOFs for H₂ storage.

In 2024, Carrillo-Carrión et al.⁴³ reported the first example of the formation of H₂ clathrates inside the cavities of a MOF. They designed a zirconium-porphyrin metal–organic framework (PCN-222) with micropores (1.7 nm) and mesopores (3.7 nm), ideal for sII hydrate crystal growth, as shown in Fig. 35. Its high specific surface area (1885 m² g⁻¹) and moderate hydrophilicity enhanced the uniform distribution of water molecules, promoting hydrate formation. Results showed that PCN-222 mesopores lowered the nucleation pressure to 1.35 kbar (compared to 2.0 kbar in a bulk system) and reduced water-to-hydrate conversion time to 30 minutes, as shown in Fig. 36. In Fig. 36(a), showing inelastic neutron scattering spectra, the band at 7.5–12 meV was attributed to the entrapment of H₂ in clathrate cages, whereas on Fig. 36(b), the neutron diffraction pattern of hexagonal ice is replaced by that of H₂ clathrate hydrates when hydrogen is introduced in the system. The mesopores provided a highly ordered environment, enabling rapid and stable sII hydrate formation, with significantly higher H₂ storage capacity than bulk hydrate systems. Neutron scattering studies confirmed that mesopores were the primary sites for hydrate formation, while micropores played a minimal role due to spatial constraints, highlighting the size compatibility between PCN-222 mesopores and sII hydrates. A drawback highlighted in this study is the degradation of the structure of the MOF under the conditions of pressure and humidity required to induce the formation of hydrogen clathrate hydrates. Nevertheless, considering the wide variety of MOFs that can be designed, this demonstration paves the way to enabling their use for the formation of H₂ clathrate hydrates.

Fig. 35 (a) Structure of PCN-222 viewed along the c-axis to show the two types of 1D open channels. PCN-222 is formed by tetrakis(4-carboxyphenyl)porphyrin (TCPP) linkers attached to four 8-connected Zr6 clusters (building units shown in circles). (b) SEM image of the as-prepared PCN-222 particles under microwave irradiation. (Inset) Histogram of the number distribution of the length of the particles as determined from SEM images; average length L = 188 ± 10 nm. (c) High-resolution TEM image of a PCN-222 particle, revealing the existence of highly oriented mesopores. (d) Powder XRD pattern of the PCN-222 nanoparticles, showing magnification within the range 2θ = 3°–12° (inset). (e) Dynamic light scattering (DLS) size distributions by intensity of the particles dispersed in methanol (n = 3, mean hydrodynamic length of L_b = 195 ± 3 nm). (f) Colloidal stability over time of the particles dispersed in either methanol or water, as determined by DLS. (g) Thermogravimetric analysis (TGA) and dynamic scanning calorimetry (DSC) curves of the PCN-222 recorded in a dynamic air atmosphere. Reproduced with permission from ref. 43. Copyright 2023, The Authors.

Fig. 36 (a) Inelastic neutron scattering (INS) spectra, and (b) neutron diffraction (ND) patterns at 5 K for D₂O impregnated PCN-222 after pressurization with H₂ at three different pressures, 1.35 kbar, 1.65 kbar and 2.0 kbar. ND pattern in the absence of H₂ as control is also shown. Reproduced with permission from ref. 43. Copyright 2023, The Authors.

3.6 Summary and comparison of porous materials for H₂ hydrate

Table 6 presents a systematic comparison of representative porous materials for H₂ hydrate storage based on the studies discussed in Sections 3.2 to 3.5. The materials are evaluated across eight key performance dimensions, which are grouped into two categories: material properties (including nano-confinement effect, contact area, surface characteristics, and wettability) and engineering operation conditions (including temperature, pressure, spatial layout, and water saturation). Table 7 provides a standardized quantitative summary of key performance indicators, specifically, pressure, temperature, induction time, and H₂ storage capacity using porous promoter systems. Table 7 enables direct benchmarking under comparable hydrate formation conditions, thereby clarifying how different porous materials perform in terms of kinetics and storage efficiency.

Table 6 Comparative evaluation of porous materials for H₂ hydrate storage based on material properties and engineering operation conditions

Criteria		Porous materials (PMs)
		Carbon-based PMs	Inorganic PMs	Organic PMs	Hybrid PMs
Material properties	Nano-confinement	AC: micropores ∼2 nm with strong confinement	Zeolites: ∼2–3 nm with crystalline rigidity and shape selectivity	PU foam: large, tortuous structure (∼200–300 µm)	Precisely engineered micropores (1.7–3.7 nm)
		SWCNTs: 1–20 nm channels for 1D growth	Silica: mesopores (150–500 Å)	PolyHIPE: open-cell foam (∼9 µm)	Well-matched to H2 molecule size
		Graphene: ∼9–11 Å interlayer spacing supports 2D hydrate stabilization	Glass: weak/no confinement	SAPs: gel-like nanonetworks with strong confinement	Enable low-barrier nucleation
	Contact area	AC: micropores provide internal contact surface; high surface area supports hydrate nucleation	Zeolites: 200–850 m^2 g; small particle size increases dispersion	SAPs/polyHIPE: high contact area from porous gel networks	MOFs: extremely high surface area (e.g. PCN-222: 1885 m^2 g^−1). Uniform micropores provide consistent internal contact
		CNTs: 1–20 nm hollow cores + outer surfaces	Silica: moderate area	PU foam: moderate roughness and interconnectivity
		Graphene: high SA	Glass beads: low surface area; used more for structure
	Surface characteristics	CNTs: nanorough surfaces enhance nucleation	Zeolites: tunable acidity + cavity chemistry	SAPs: polar gel network enhances gas uptake	Functionalized frameworks (–COOH, –OH) enhance hydrate nucleation
		AC: oxygen-rich groups aid gas interaction	Silica: can introduce –OH or THF-like groups	PolyHIPE: supports THF coating	Highly designable interface chemistry
		Graphene: inert unless oxidized	Glass: unmodified glass is inert	PU foam: tortuous surface improves gas–solid interaction
	Wettability	AC/graphene: hydrophobic unless oxidized	Zeolites: hydrophilic, support structured water layers	SAPs: highly hydrophilic; great water retention	Polarity-tuned frameworks balance hydration and gas access; prevent overflooding while maintaining nucleation
		CNTs tunable through oxidation, plasma, or silanization	Silica: tunable	PU & polyHIPE: adjustable wettability via chemical groups
			Glass: poorly wettable unless coated
Engineering operation conditions	Temperature	AC: Stable hydrate formation at >240 K	Zeolites & silica allow formation up to 280 K	SAPs: rapid formation at 270 K (≤6.5 min)	MOFs: hydrate formation at 280 K within 30 min. Thermally stable under repeated cycling
		Graphene and CNTs also allow room T operation	Glass: requires higher T/P	PU: ∼270 K
				PolyHIPE: THF system supports higher T
	Pressure	AC & CNTs reduce pressure requirement via enhanced nucleation kinetics (∼20–30%)	Zeolites lower formation pressure by ∼20%	SAPs: effective at 11.6 MPa	MOFs: Reduce pressure from 2.0 kbar to ∼1.35 kbar
			Silica: mild P.	PU foam: 135 bar	Suitable for mild condition systems
				PolyHIPE: supports moderate pressure under THF presence
	Spatial layout	CNTs & graphene: directional transport pathways	Zeolites: need controlled particle size to avoid blockage	SAPs: gel-like uniform structure	Intrinsically ordered frameworks provide uniform diffusion
		AC: random porosity with good packing efficiency	Silica: open-pore structure	PU/polyHIPE: open-cell form enhances mass transport	Reduce local concentration gradients
			Glass: used mainly as filler
	Water saturation	CNTs/AC: sensitive to flooding or underfilling	Zeolites: retain water well; promote structured layers	SAPs: superior water retention	Water retention stabilized by pore polarity
		Graphene needs oxidation to improve hydration uniformity	Silica: controllable	PU/polyHIPE: sensitive to overhydration, needs optimization	Avoids overflooding while maintaining active nucleation layers
			Glass: poor unless hybridized

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Table 7 Quantitative comparison of H₂ hydrate storage performance using various porous materials as heterogeneous promoters

Porous materials		Thermodynamics promotors	Pressure (MPa)	Temperature (K)	Induction time (min)	H2 storage capacity (wt%)	Ref.
Carbon-based porous materials	PPAC	—	135	280 K	<10 min	4.1	39
	AC	THF	8.4	274.2	250	0.0082	269
	MWCNTs	THF	10	263	90	1.5	278
	SWNTs	THF	16.5	273.7	—	0.37	279
	SWCNTs	—	—	260	—	0.37	280
	Graphene BLHH-I	—	—	213	—	2.703	287
Inorganic porous materials	MCFPhene	—	135	280	—	3.2	44
	MCF (f-1)	THF	7	262	<10 min	0.52	45
		THF	7	265	<10 min	0.21	45
		THF	7	268	<10 min	0.16	45
	HRPMO	THF	7	265	8	0.26	130
		THF	7	269	9	0.24	130
		THF	7	273	16.5	0.22	130
	Zeolites	THF	1	263	—	2.1	117
	Glass beads	THF + SDS	8.8	290	27	—	321
		THF	—	270	—	0.8	81
Organic porous polymers	PU foam	THF	13.5	273	800	0.2	71
	PolyHIPE	THF	11.6	270	60	0.4	83
	PSA	THF	11.6	270	6.5	0.3	81
	SAPs	THF	12	274	—	0.219	316
Hybrid porous materials	MOFs PCN-222	—	135	280	30	—	43

---

Combined insights from Tables 6 and 7 indicate that MOFs offer well-balanced performance, combining tunable nanoconfinement, high surface area, and functionalized interfaces. They enable fast hydrate formation (∼30 min) under a moderate temperature. SAPs stand out for their rapid kinetics (<10 min) and competitive H₂ storage (∼0.3 wt%) under mild pressures (11.6–12 MPa). CNTs and polyHIPE materials provide interconnected pathways and tunable wettability, supporting moderate storage and facilitating gas–liquid contact. Zeolites and PU foams offer reliable hydration stability under practical conditions. AC remains attractive for scalable and cost-effective system deployment. Silica-based materials are scalable and rigid but generally require surface modification to enhance performance.

Together, these qualitative and quantitative comparisons establish a practical and evidence-based basis for material screening and system design. By integrating performance metrics with mechanistic understanding, this review supports the rational development of porous-material-enabled, efficient, and operable hydrate-based hydrogen storage platforms.

4 Key challenges in materials and engineering applications

4.1 Challenges in cross-comparing different heterogeneous promoters

4.1.1 Variability in material properties. Heterogeneous additives exhibit wide differences in intrinsic properties like, pore structures, surface chemistries, and hydrophobicity, which significantly influence how water and gas molecules interact at the interface, ultimately affecting hydrate nucleation and growth. For example, AC and MCF silica possess vastly different pore sizes (2 nm vs. 150–500 Å) and surface characteristics, leading to distinct nucleation behaviors. While nanoconfinement promotes formation in AC, silica relies on enhanced water accessibility. Without standardized reporting of these parameters, direct material comparison remains unreliable, making it difficult to establish a fair basis for comparison.

4.1.2 Different modes of action. Heterogeneous additives can enhance hydrate formation through multiple mechanisms, including nanoconfinement, improved gas solubility at interfaces, altered thermal conductivity, and surface-induced nucleation. These mechanisms may act in synergy or competition, and the dominant effect can vary significantly depending on the material and experimental setup. For example, CNTs enhance gas solubility and mass transfer, whereas SAPs facilitate localized water retention and rapid clathrate growth. This multiplicity of modes makes it difficult to isolate and evaluate the specific contribution of each additive, which complicates cross-study analysis and optimization.

4.1.3 Difficulties in visualization and characterization. Studying hydrate formation within heterogeneous systems presents significant technical challenges. It is often difficult to directly observe where hydrate nucleation occurs within porous structures or to quantify the extent of active surface area involved. Although neutron scattering provided evidence of hydrate growth confined within PCN-222 mesopores, distinguishing internal versus external nucleation sites is still limited. Real-time characterizations of water distribution, phase transitions, or interfacial processes remains limited by the current capabilities of imaging and spectroscopic tools. These limitations constrain our ability to understand the mechanistic underpinnings of hydrate promotion and hinder rational material design.

4.1.4 Unknown material reusability and long-term behaviors. Most studies focus on the initial performance of heterogeneous additives, with limited attention given to their long-term stability or reusability across multiple hydrate formation–dissociation cycles. For instance, PSA-supported hydrates retained >90% H₂ storage after three cycles, but comparable data are scarce for ACs, MOFs, and other porous materials. However, structural degradation, pore blocking, or loss of surface activity over time can significantly impact practical application. MOFs, in particular, have drawn attention for hydrate promotion due to their tunable pore structures, large surface areas, and rich chemical functionalities. Yet their long-term applicability is often constrained by limited hydrolytic stability. Under humid or aqueous environments, especially during repeated cycling, many MOFs degrade via framework collapse, ligand dissociation, or pore obstruction, ultimately compromising surface activity and structural integrity.^322,323 Although hydrolytically stable variants (e.g. Zr-based UiO-66) show improved water tolerance, challenges such as synthesis cost, scale-up feasibility, and mechanical robustness under cyclic thermodynamic stress remain underexplored.³²⁴ Therefore, beyond lab-scale performance, systematic evaluations of additive durability, including multicycle stability, aging, and water sensitivity, are urgently needed to assess real-world applicability. Without systematic evaluation of these factors, it is difficult to assess whether a promising additive in the lab will remain effective in real-world scenarios.

4.1.5 Interplay with promoters or co-additives. In many experimental systems, heterogeneous additives are used in combination with thermodynamic promoters (such as THF) or surfactants to enhance hydrate formation. While this can boost performance, it introduces confounding variables that make it difficult to isolate the effects of the heterogeneous additive itself. For example, polyHIPE/THF systems showed 250-fold faster formation than bulk THF–H₂O systems, but the extent attributable to polyHIPE alone remains unclear without rigorous control experiments. Consequently, the comparative effectiveness of different materials is often obscured by the influence of other additives, and the intrinsic merits of the heterogeneous component may be over- or under-estimated.

4.1.6 Scalability and economic feasibility constraints. While many porous materials have demonstrated excellent performance in promoting hydrate nucleation and growth, their practical deployment faces significant challenges related to large-scale fabrication and cost-effectiveness. Advanced materials such as MOFs, carbon aerogels, and designer nanocomposites often require multistep synthesis, costly precursors, or stringent processing conditions, which limit their scalability and economic appeal. There are also concerns over batch-to-batch consistency, structural reproducibility, and compatibility with industrial production methods. Furthermore, scaling up from lab-prepared samples to bulk quantities may lead to variability in pore structure, surface area, or mechanical stability, which in turn compromises performance. Although alternative materials such as polymer foams or silica-based frameworks offer improved scalability, they may suffer from lower promotion efficiency. Balancing material performance with manufacturability and cost remains a major hurdle in the translation of porous promoters to practical hydrate-based storage systems.

4.2 Challenges in real hydrate-based H₂ storage engineering applications

4.2.1 Lack of standardized experimental conditions. H₂ hydrate formation is sensitive to operating conditions such as pressure, temperature, water saturation level, and spatial layout. Across studies, variations such as using 135 MPa for PPAC versus 8.4 MPa for AC (both targeting H₂ hydrate formation) introduce significant uncertainties. Additionally, different water saturation levels affect nucleation location (surface vs. bulk pore), complicating the benchmarking of materials even under nominally similar conditions. What's more, different studies often employ varying experimental protocols, even when investigating similar materials. As a result, performance indicators such as induction time, gas uptake, and formation rate are not directly comparable across the literature, limiting our ability to identify the most effective materials.

4.2.2 Inconsistent evaluation metrics. Another barrier to comparison is the lack of a unified framework for assessing performance. Performance is reported through different indicators—such as storage capacity (e.g. PU foam ∼0.2 wt%), induction time (e.g. PSA: 6.5 minutes), or water conversion rate (e.g. MCF (f-1): 39.78%). The variation in performance metrics not only hinders objective benchmarking but also affects how conclusions are drawn regarding the effectiveness of different operation conditions and materials. A standardized set of criteria would help facilitate more rigorous comparisons.

4.2.3 Scale-up limitations and reactor engineering. Moving from laboratory-scale to pilot- or industrial-scale hydrate-based H₂ storage introduces significant challenges. Variations in spatial layout, water distribution, and gas flow pathways can severely impact hydrate nucleation and growth behavior. Laboratory setups often use idealized uniform beds, but scaling up to larger reactors leads to issues such as gas bypassing, uneven water saturation, and limited mass transfer. Reactor designs must therefore optimize pore connectivity, spatial layout, and multiphase flow management to maintain efficiency across scales. Additionally, the integration of porous materials into practical reactor systems adds further complexity. While materials such as MOFs, carbon foams, and composites offer superior performance under controlled conditions, their deployment in large-scale systems requires consideration of mechanical robustness, shaping feasibility, and long-term structural stability. High material cost, complex fabrication routes, and limited recyclability can significantly affect the economic viability of large-scale systems. Hence, bridging the gap between lab-scale functionality and real-world scalability requires holistic engineering strategies to ensure both technical effectiveness and commercial feasibility.

4.2.4 Thermal management. H₂ hydrate formation is an exothermic process, while hydrate dissociation is endothermic. Poor thermal regulation can lead to localized overheating during formation or overcooling during dissociation, as identified in porous systems where non-uniform heat transfer affects hydrate stability and growth. Especially in dense porous beds, inadequate heat removal can inhibit further hydrate nucleation or cause partial decomposition, compromising storage capacity and cycle efficiency. Designing reactor systems of porous materials with enhanced thermal conductivity remains a critical engineering need.

5 Conclusions and future work

The introduction of porous materials as heterogeneous promoters for H₂ hydrate formation brings several key advancements compared to bulk water formation, as summarized in Fig. 37. First, the porous materials provide nanoconfinement, which alters hydrate stability and formation conditions, potentially lowering pressure requirements. Second, unlike bulk water, where gas diffusion can be limited, the porous structure increases the gas–water interface, promoting more efficient hydrate formation process. Third, porous materials' surfaces tend to change homogenous nucleation into heterogeneous nucleation, which requires lower energy barriers. The physical and chemical properties of the porous medium influence hydrate nucleation, growth, and stability, allowing better control over the process. Lastly, in bulk water, hydrate layers can block further formation. The porous materials structure mitigates this issue by distributing hydrates throughout the medium.

Fig. 37 Summary of advancements brought by introducing porous materials in H₂ hydrate formation.

Porous materials as heterogeneous promoters have demonstrated significant potential in enhancing the performance of hydrate-based H₂ storage systems. As illustrated in Fig. 38, the influence of porous materials on hydrate formation spans across macroscopic, mesoscopic, and microscopic scales, each contributing uniquely to the efficiency and stability of H₂ storage. At the macroscopic level, H₂ hydrates primarily form in the interstitial voids between particles. These voids significantly impact gas–liquid distribution, mass transfer efficiency, and heat dissipation, which are critical for effective hydrate formation. At the mesoscopic level, H₂ hydrates tend to nucleate on the surface of porous particles. Factors such as particle size, surface area, surface roughness, surface functionalization, and surface hydrophilicity or hydrophobicity play pivotal roles in enhancing nucleation rates, expanding contact surfaces, and improving thermal conductivity. At the microscopic level, H₂ hydrates are confined within the pores of porous materials. Tailored pore size, pore shape, and inner surface wettability generate quasi-high-pressure conditions within confined spaces. This nano-confinement effect significantly promotes the nucleation and growth of hydrates under milder temperature and pressure conditions, thus stabilizing the hydrate structure and improving H₂ storage efficiency.

Fig. 38 Multiscale mechanisms of H₂ hydrate formation in porous materials.

Carbon-based materials such as AC, CNTs, and graphene offer high surface areas, superior conductivity, and enhanced mechanical properties, which collectively improve hydrate nucleation, growth kinetics, and H₂ storage capacity. Inorganic porous materials, including zeolites, and silica gels, provide tunable pore structures, high thermal and chemical stability, and customizable surface chemistries, facilitating more efficient hydrate formation under milder conditions. Organic porous polymers like polyurethane foam and emulsion-templated polymers offer high porosity and adaptable structural designs, further contributing to improved gas storage capabilities. Hybrid porous materials like MOFs combine the advantages of inorganic and organic frameworks, offering highly tunable pore structures, large surface areas, and functionalized sites that enhance H₂ storage, nucleation efficiency, and hydrate stability under controlled conditions.

The integration of these porous materials into hydrate systems effectively addresses the challenges of slow kinetics, high-pressure requirements, and limited storage capacities. The confinement effects within nanoporous structures, surface functionalization, and optimized wettability collectively enhance mass and heat transfer, lower induction times, and stabilize hydrate structures. Experimental and theoretical studies confirm that nanoscale confinement, tailored pore sizes, surface modifications, and wettability are key to promoting rapid and stable hydrate formation, leading to more efficient H₂ storage.

Despite promising advancements, several challenges remain in realizing the full potential of porous materials for hydrate-based H₂ storage. Future research should mainly focus on materials properties and engineering operations two aspects, as shown in Fig. 39: firstly, optimized nano-confinement by choosing nano scale porous materials with hierarchical porous structure can form quasi-high pressure and enhance hydrate stability. Optimize pore size and particle size to enlarge contact area and facilitate interaction and growth. Functionalized surfaces with oxygen-containing groups enhance nucleation and stability, while rough surfaces increase nucleation sites. Moderately hydrophobic materials ensure efficient gas–water contact while preventing excess water blockage. What's more, hybrid materials that integrate the advantages of carbon-based, inorganic, and polymeric materials could further improve performance by combining high surface area, stability, and flexibility. The selection and optimization of porous materials should be tailored to specific application needs, requiring precise structural, chemical, and wettability modifications for different material types to achieve optimal hydrate formation, stability, and H₂ storage efficiency. Secondly, to ensure efficient and scalable H₂ storage, external operational conditions must be carefully controlled, such as through optimizing formation conditions within practical ranges (∼240–270 K and <10 MPa) to enhance storage stability and efficiency. Selected materials should sustain hydrates under these conditions. Materials should be arranged to maximize pore connectivity and mass transfer efficiency. Structured configurations with well-interconnected pores minimizes mass transfer resistance and enhances gas–liquid interaction. An ideal optimal water saturation range is a trade-off between hydrate growth and pore accessibility and should be experimentally determined based on specific material properties and hydrate formation kinetics.

Fig. 39 Summary of key considerations for H₂ hydrate storage: material properties and engineering operations optimization.

For practical applications, the successful integration of optimized porous materials and engineered operating conditions into scalable H₂ hydrate storage systems requires further research. Key factors such as energy efficiency, safety, and cost-effectiveness must be considered to enable real-world deployment. Materials with high mechanical stability, recyclability, and tunable structural properties will be prioritized to ensure durability under repeated hydrate formation–dissociation cycles. Additionally, system-level assessments, including thermal management, hydrate stability under dynamic conditions, and large-scale process feasibility, must be conducted. Advancing reactor design, hydrate formation kinetics, and efficient gas release strategies will be crucial for implementing H₂ hydrate-based storage solutions in renewable energy grids, H₂ fuel cells, and transportation applications, ultimately contributing to a sustainable H₂ economy. Building upon these advances, hydrate-based hydrogen systems also show promise for integration into broader hydrogen infrastructures. By enabling low-energy, on-site storage under moderate conditions, they can buffer intermittencies in renewable H₂ generation (e.g. from electrolysis), decouple supply and demand, and enhance grid resilience.^325,326 Rather than replacing conventional storage methods, hydrate systems complement them, supporting a more flexible and scalable hydrogen supply chain. Furthermore, porous materials with catalytic functionalities (e.g. MOFs, functionalized carbons) open up multifunctional pathways,^327,328 enabling hydrate composites to serve as both storage media and catalytic platforms for H₂ release or downstream utilization such as hydrogenation or ammonia synthesis.^21,329 From a broader perspective, co-locating hydrate dissociation with catalytic ammonia synthesis, though still conceptual, presents a compelling direction for safer, modular, and space-efficient green ammonia production in distributed energy systems.^330,331 Realizing this vision will require deeper investigation into hydrate dissociation kinetics, catalytic integration, and system-level optimization. Recent developments in single-atom catalysts, multifunctional electrode architectures, and hydrate-compatible nanostructures provide a conceptual foundation for such integration,^332–334 where storage media also act as reactive platforms for in situ H₂ utilization, including electrochemical ammonia synthesis and fuel upgrading. While these system-level designs remain in early stages, they present promising directions toward compact, circular, and scalable energy platforms that unify solid-state storage with catalytic conversion. Ultimately, hydrate-based composites, synergistically combining tailored porosity, interfacial chemistry, and reactor-level engineering may contribute substantially to the flexibility, resilience, and sustainability of future clean energy systems.

Data availability

This article is a review and does not report any original experimental data. All data discussed are derived from previously published sources, which are appropriately cited throughout the manuscript. No new datasets, software, or code were generated or analysed as part of this work.

Author contributions

Lijin Chen: writing – original draft, methodology, formal analysis, conceptualization. Valeska P. Ting: writing – review & editing, supervision, methodology, formal analysis. Yuxuan Zhang: writing – review & editing, methodology, formal analysis. Joe Coventry: writing – review & editing. Alireza Rahbari: writing – review & editing, supervision. Zhenyuan Yin: writing – review & editing. Fei Wang: writing – review & editing. Mi Tian: writing – review & editing. Sebastien Rochat: writing – review & editing. Zhongbin Zhang: writing – review & editing. Shuai Deng: writing – review & editing. Melinda Krebsz: writing – review & editing. Parimal Bhomick: writing – review & editing. Xiaolin Wang: writing – review & editing, supervision, resources, methodology, funding acquisition, formal analysis, conceptualization.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This project is funded by the Australian Renewable Energy Agency (Project number: 2023/TRAC733 (PRO-1050)). Dr Xiaolin Wang is the recipient of an Australian Research Council Discovery Early Career Researcher Award (Project number: DE200100326) funded by the Australian Government.

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