Physical Hydrogen Storage Materials

This entry is adapted from the peer-reviewed paper 10.3390/ma17030666

Hydrogen is a future energy carrier in the global energy system and has the potential to produce zero carbon emissions. For the non-fossil energy sources, hydrogen and electricity are considered the dominant energy carriers for providing end-user services, because they can satisfy most of the consumer requirements. Hence, the development of both hydrogen production and storage is necessary to meet the standards of a “hydrogen economy”. The physical and chemical absorption of hydrogen in solid storage materials is a promising hydrogen storage method because of the high storage and transportation performance.

hydrogen physical hydrogen storage hollow spheres carbon-based materials

1. Compressed Hydrogen Storage Materials

Hydrogen compression is the most widely used technology to store and utilize hydrogen gas as an energy source, with several outstanding advantages ^[1]^[2]. First, the compression technique is well developed, with hydrogen filling and release occurring at high rates. Moreover, hydrogen release does not require energy ^[2]^[3]. Among the various storage materials, hollow spheres are most widely used for compressed hydrogen storage. Hollow spheres not only provide high surface area but also encapsulate hydrogen within pores. Moreover, the high surface area and porosity allow hybridization with other materials via binding sites on the surface or in the pores ^[4]. Hence, the absorption and catalytic activities of the hollow spheres can be increased significantly. In addition, the good mechanical strength and low specific weight of hollow spheres make them good hydrogen containers ^[5]. Controlling the different conditions, such as reaction time, pH, temperature, and ratio of reactants, can simplify the fabrication of hollow spheres of uniform sizes. In hollow nanospheres, hydrogen can be absorbed in both atomic and molecular forms ^[6].

Hollow spheres with average sizes ranging from micrometers to nanometers have attracted considerable research attention. Various types of hollow spheres of different sizes and from different materials, including carbon, glass, metal, and nonmetal, have been developed.

1.1. Hollow Carbon Spheres (HCSs)

HCSs are mainly fabricated by two methods: hard templating and soft templating, which use different template types to create a hollow sphere (HS). The hard-templating approach employs special hard particles as a “core template” to build a hollow structure. The core is removed after a carbon shell is formed on the surface of the core. Silica, polymers, and hard metal particles are often used as core templates in hard-templating procedures. In soft templating, a hollow structure is directly generated by the self-assembly of carbon and other organic compounds. Hence, the core templates are “soft” precursor molecules, which easily decompose during the final pyrolysis.

HCSs are excellent materials for hydrogen storage. Their mesoporous structure provides a high Brunauer–Emmett–Teller (BET) value, voids, and hollow spaces, which can be occupied by hydrogen; thus, a high hydrogen capacity can be achieved. However, the HCS structure has a significant influence on the hydrogen storage capacity. The shape and size of the HCS play important roles in hydrogen diffusion in the substrate. When the adsorption energy is higher than the release energy, hydrogen can easily occupy the active sites that strongly depend on the morphologies of the material. In addition, the deposition of hydrogen into the pores of the HCS can be facilitated by the defects and thin walls of the HCS; hence, the spaces between the layers can be freed, increasing the amount of hydrogen stored in the HCS ^[7]^[8]^[9]. Hydrogen storage based on HCS has been reported in several studies, as partly presented in Table 1.

Table 1. Various types of HCS used for hydrogen storage.

Materials	Temperature (°C)	Pressure (bar)	H₂ Storage Capacity (wt%)	Ave. Particle Size (nm)	Ave. Pore Size (nm)	Total Pore Volume (cm³/g)	Surface Area (m²/g)	Reference
1	Pd-hollow carbon spheres	40	24	0.36	~250	n/a	0.2514–0.9754	147–617	^[10]
2	Necklace-like hollow carbon nanospheres (CNS)			0.89	60	5	n/a	594.32	^[7]
3	Ni-decorated hollow carbon spheres	25	90	1.23	5100	n/a	0.31	28.6	^[11]
4	Hollow nitrogen-containing carbon spheres (N-HCS)	−196	80	1.03	~250	1.38–20	0.84	872	^[12]
4	Hollow nitrogen-containing carbon spheres (N-HCS)	25	80	2.21					^[12]
5	Fe-nanoparticle–loaded hollow carbon spheres	300	20	5.6	200–500	~90	n/a	160	^[13]
6	Metallic Mg ions diffused in hollow carbon nanospheres (HCNS)	270		6.78	440–9800	28	n/a	1810	^[14]
6		370		7.85	440–9800	28	n/a	1810	^[14]
7	Zeolite-like hollow carbon	−196	1	2.6	~5000	0.6–0.8	~2.41	3200	^[15]

Wu et al. successfully synthesized necklace-like hollow carbon nanospheres (CNSs) with narrow pore distributions using pentagon-containing reactants. The results of their electrochemical hydrogen storage experiments showed that CNSs afforded a capacity of 242 mAh/g at a current density of 200 mA/g, corresponding to a hydrogen storage of 0.89 wt%, which is considerably greater than the electrochemical capacities of solid chain carbon spheres and multiwalled carbon nanotubes ^[7]. This confirms that the structure and morphology of carbon materials strongly affect the electrochemical hydrogen storage.

Doping hollow carbon materials with metals for hydrogen storage is known as hydrogen spillover ^[16]. Metals doped onto carbon materials can increase the binding energy between hydrogen and the pore walls. In addition, binding can occur between the hydrogen molecules and doped metallic atoms ^[17]. Thus, metal doping can enhance the hydrogen storage of materials. Among the various metal-doped hollow carbons used for enhancing hydrogen storage, palladium (Pd)-doped carbon materials have been reported to improve the hydrogen storage performance ^[18]^[19]^[20]. Hydrogen molecules easily dissociate on the Pd surface, which limits the storage efficiency. Hence, to decrease the active surface area of Pd to achieve good catalytic performance and improve the hydrogen storage capacity, small particles of Pd are doped onto the porous surface of the material.

In addition to metal and nonmetal doping, the hydrogen storage capacity can be enhanced by improving the surface area of the HCS. Yang et al. used chemical vapor deposition to successfully synthesize zeolite-like HC materials with large surface areas. This material demonstrated the effect of surface area on hydrogen storage capacity, which was 2.6 wt% at a pressure of 1 bar and reached 8.33 wt% at a pressure of 20 bar. This capacity of the zeolite HC material was recorded as the highest value ever reported among HC materials.

1.2. Hollow Glass Microspheres (HGMs)

HGMs, also known as microballoons or microbubbles, are mainly composed of borosilicate and soda lime silica. The sizes of HGMs range from 100 nm to 5 µm with a wall thickness ranging from 1.5 to 3 µm and pore size from 100 to 500 nm ^[21]^[22]. The wall thickness of HGMs strongly determines their crush strength; the higher the sphere density, the higher the crush strength. Owing to their excellent chemical stability, chemical resistance, strong mechanics, low density, high-temperature operation, high water resistance, noncombustibility, nonexplosibility, nontoxicity, and particularly, low-cost production, HGMs are considered as high-potential hydrogen transporting and storing materials ^[9]^[10]^[23].

HGMs must be operated under high pressure and high temperature to enhance the hydrogen diffusion into the HGMs. This is because not only the sizes of the gas molecules but also the temperature can change the gas diffusivity. After hydrogen molecules are absorbed in the HGMs, the temperature must be decreased to room temperature to reduce the diffusivity rate and retain the loaded hydrogen molecules in the pore cavities. For the hydrogen-release process, it is necessary to heat the HGMs to high temperatures. The optimal temperature for both hydrogen absorption and desorption is over 300 °C ^[24]^[25].

HGMs can be fabricated using dry-gel or liquid-droplet approaches. The high-temperature furnace in which the initial particles are formed plays an important role in both methods. First, the blowing agent is broken down to release the gas within the dried gel or liquid. The fast growth of gaseous products leads to the appearance of bubbles and the formation of hollow droplets. Thereafter, the hollow droplets rapidly cool from the liquid state to form HGMs ^[26]. Regarding the synthesis of HGMs, Wang et al. presented a basic formula for a new type of HGM, based on the traditional fabrication approaches, experiences in this material industry, and interrelated businesses. According to their proposal, HGM comprised quartz sand, K₂CO₃, Na₂B₈O₁₃.4H₂O, Na₂CO₃, Ca(OH)₂, NaAlO₂, Li₂CO₃, and H₂O in weight percentages of 27, 14.5, 15, 10, 3.5, 3, 0.5, and 26.5%, respectively. Hence, the main chemical components of HGM were SiO₂ (68–75%), Na₂O (5–15%), CaO (8–15%), B₂O₃ (15–20%)_, and Al₂O₃ (2–3%) ^[27].

Dalai et al. presented a type of HGM for hydrogen storage, which was synthesized using an air–acetylene flame spheroidization method using urea as the blowing agent. With a microsphere diameter of 10–200 μm and wall thickness of 0.5–2 μm, they showed hydrogen storage capacity at ambient temperature and at 200 °C under a pressure of 10 bar. The results indicated that the adsorption capacity at ambient temperature was lower than that at 200 °C ^[28]. This confirmed that the higher the temperature, the better the gas diffusivity. At high temperatures, hydrogen molecules could pass through the HGM walls toward the hollow pores and were retained in them.

The poor thermal conductivity of HGMs restricts hydrogen molecule loading during the absorption and desorption processes, resulting in a poor hydrogen storage capacity ^[25]. It has been discovered that when HGMs are doped with photoactive agents such as Ti, Cr, V, Fe, Zn, Mg, and Co, the gas diffusion rate is strongly enhanced by light illumination compared with that of traditional heating furnace approaches ^[25]^[29]^[30]^[31].

Dalai et al. synthesized HGM samples loaded with Fe and Mg to improve their heat transfer properties, thereby enhancing their hydrogen storage capacity. For Fe doping, the feed glass powder was mixed with certain amounts of ferrous chloride tetrahydrate solution and magnesium nitrate hexahydrate salt solution to obtain 0.2–2 wt% Fe loading and 0.2–3.0 wt% Mg loading, respectively, in the HGMs. Hydrogen adsorption experiments on all HGMs doped with Fe and Mg were performed at 10 bar and 200 °C for 5 h. The results indicated that the hydrogen uptake capacity of Fe-doped HGMs was approximately 0.56 wt% at 0.5 wt% iron loading. Increasing the iron content did not enhance the hydrogen absorption because of spheroidization and the FeO/Fe block, which prevented the accessibility of some pores. This was confirmed by the results of 0.21 wt% uptake capacity with 2 wt% Fe doping in HGMs. Meanwhile, the Mg-doped sample exhibited a hydrogen adsorption increase from 1.23 to 2.0 wt% as the Mg loading percentage increased from 0 to 2.0 wt%. This is similar to the phenomenon of Fe doping; if the amount of Mg in the HGMs was over 2 wt%, nanocrystals of MgO/Mg that could close the pores appeared, thereby decreasing hydrogen storage capacity ^[32].

To sum up, the metal doping can increase the porosity of HGMs and then lead to an enhancement in hydrogen storage capacity. However, the too high concentrations of metal can result in the formations of metal oxide layers or the agglomeration metal particles, which can close the pores of HGMs and then decrease the hydrogen loading volume. Hence, doped-metal concentration optimization is extremely crucial to obtain the highest hydrogen storage capacity for HGMs. Overall, the researchers summarize the general characterization of the glass microspheres in Table 2.

Table 2. Summary of characterization of glass microspheres (GMs).

Preparation Method	Types	Advantages	Drawbacks
- Flame synthesis - Liquid droplet - Dried gel - Electrical arc plasma	- Solid GM - Hollow GM - Porous GM - Doped GM	- Excellent chemical stability - Chemical resistance - Strong mechanical strength - Low density - High thermal stability - High water resistance - Noncombustibility - Nonexplosibility - Nontoxicity - Fair cost	- Harsh synthetic condition - Fragility: weakness to impact damage - Not enough compressive strength - Poor thermal conductivity - Relatively difficult morphological control - Temperature sensitivity in hydrogen storage - Limited hydrogen storage capacity

Co-loaded HGMs have also been fabricated ^[33]^[34]. The results revealed that the number of pores in the HGMs increased significantly and that the hydrogen storage capacity reached a maximum of approximately 2 wt% at Co concentration ≤2 wt%. When the amount of Co exceeded 2 wt%, energy-dispersive spectroscopy (EDS) analysis confirmed the formation of cobalt oxides on the walls of HGMs, which occupied the pores and prevented hydrogen diffusion.

2. Physical Absorption Materials

The physical hydrogen absorption process involves loading hydrogen onto the material surface. The origin of this process is the resonant fluctuations in the charge distribution, which are known as dispersive forces or van der Waals interactions. The interaction forces between hydrogen molecules and other materials are quite weak; therefore, physisorption only occurs at temperatures lower than 0 °C ^[35]. However, because of its advantages of high energy efficiency ^[21], high rates of loading and unloading ^[21]^[36], and good refueling time ^[37], physisorption has been widely used in the recent years. Hence, absorbent materials have been developed and improved to meet the physisorption requirements. A good hydrogen absorbent depends on two factors: (1) the binding energy between hydrogen molecules and materials, which directly affects the operating temperature of the hydrogen storage system, and (2) the availability of a high average surface area per unit volume ^[38]. Recently, materials considered excellent candidates for hydrogen physisorption include carbon-based materials such as activated carbon (AC), carbon nanotubes (CNT), graphite nanofibers (GNF), graphene, zeolites, and MOFs, which are further discussed in the following sections.

2.1. Carbon-Based Materials

Carbon is one of the most abundant elements in both living and nonliving organisms. Carbon materials are easily prepared as powders with high porosity and have a propensity to interact with gas molecules. Therefore, carbon is a well-known medium capable of absorbing gases and has been used as a detoxifier and purifier.

Carbon-based materials have attracted much attention as promising materials for hydrogen storage because of their several advantages such as high surface area, high porosity with diverse pore structures, good chemical stability, low weight, and low cost ^[39]^[40]. Various carbon-based materials are used for hydrogen storage. The researchers focus on the use of materials such as AC, CNT, GNF, and graphene for hydrogen storage, as summarized in Table 3.

Table 3. Carbon-based materials used in hydrogen storage.

Material	Definition	Advantages	wt% H₂	T (K)	P (bar)	Ref.
Activated carbon (AC)	Modified synthetic carbon consisting of high-surface-area amorphous carbon and graphite; fabricated through thermal or chemical procedures ^[41]	High specific surface area Mechanical and chemical stabilities Microporous structure Relatively low cost Feasible commercial scaling ^[42]^[43]	0.1	298	10	^[44]
			2.02	77	10	^[44]
			0.6	298	120	^[45]
			4	77	120	^[45]
			0.85	77	100	^[46]
			1.2	298	200	^[47]
			2.7	298	500
			5.6	77	40
			1.09–2.05	298	0.00011	^[48]
			5	77	30–60	^[49]
Carbon nanotubes (CNTs)	CNTs are macromolecules containing a hexagonal arrangement of hybridized carbon atoms, which may be formed by rolling up a single sheet of graphene to form single-walled nanotubes (SWNTs) or multiple sheets of graphene to form multiwalled nanotubes (MWNTs) ^[50]	Unique structure Narrow size distribution and pore volume High surface area High strength Good electrical conductivity Good mechanical and thermal properties Low density Chemical stability Special functional properties ^[51]^[52]^[53]^[54]	SWNT
			0.8		30	^[55]
			1.73	77	100	^[56]
			4.77	323		^[57]
			4.77	323		^[58]
			MWNT
			0.2	298	100	^[59]
			0.54	77	10	^[60]
			1.7	298	120	^[61]
			2		0.05	^[62]
			3.46		127.9	^[63]
			3.8	425	30	^[64]
Graphite nanofibers (GNFs)	GNFs are produced from the dissociation of carbon-containing gases over a catalyst surface (e.g., metal or alloy) through chemical deposition. The solid consists of very small graphite platelets of width 30–500 Å, stacked in a perfectly arranged conformation ^[65]	Herringbone structure High degree of defects (exhibits the best performance for hydrogen storage) Several pretreatment procedures: oxidative, reductive, and inert environments ^[66]	1	300	20	^[67]
			1.2	77	20	^[68]
			3.3	298	48.3	^[69]
			4–6.5	298	121.6	^[70]
			1.3–7.5	298	100	^[71]
			10–15	300		^[72]
			7–10 (irreversible)			^[73]
			20–30 (reversible)			^[73]
Graphene-based materials	Graphene is a monolayer while graphite is a multilayer of carbon atoms strongly bound in a hexagonal crystal lattice. This is a carbon allotrope in the sp² hybridized form with a molecular bond length of 0.142 nm ^[74]	High specific surface area High mechanical strength High corrosive-environment resistance High thermal and electrical conductivities ^[75]^[76] Various oxygen-containing functional groups	0.055	293	1.06	^[77]
			0.13	293	1.06	^[77]
			0.14	298	1	^[78]
			1.18	298	60	^[78]
			2.2	298	100	^[79]
			3.3	323		^[80]
			4.3	298	40	^[81]
			1	293	120
			5	77	120
			6.28	298	1.01	^[82]
			10.5	77	10	^[83]

2.2. Zeolites

Zeolites are three-dimensional, microporous, and highly crystalline aluminosilicate materials. Zeolites have a tetrahedral framework structure in which Si and Al atoms are tetrahedrally coordinated through shared oxygen atoms. Zeolites are well-known materials with channels and cages in which cations, water, and small molecules can reside. In addition, their high thermal stabilities and good ion-exchange capacities render them a huge potential promising media for hydrogen storage ^[84]^[85].

Zeolites can be classified as natural or synthetic. Natural zeolites exhibit improved resistivity and thermal stability in different environments ^[86]. However, they contain impurities that are not uniform in crystal size; therefore, they cannot be utilized in industrial applications ^[87].

Zeolites can be fabricated using various raw materials. To meet the requirements of economic efficiency, the raw materials should be readily available, relatively pure, selective, and inexpensive. Raw materials such as kaolin, rice husk ash, paper sludge, fly ash, blast furnace slag, lithium slag, and municipal solid waste are mostly used for zeolite fabrication.

Zeolites can be synthesized though different approaches including solvothermal, hydrothermal, ionothermal, alkali fusion and leaching, microwave, and ultrasound energy methods, which have been presented in previous studies ^[87]. Among these approaches, the hydrothermal process is the most widely used, particularly for the synthesis of zeolite membranes ^[88].

Owing to their excellent properties and various synthetic processes, zeolites are considered as good hydrogen storage candidates. Dong et al. investigated the hydrogen storage capacities of different zeolites including Na-LEV, H-OFF, Na-MAZ, and Li-ABW. The results showed that at a pressure of 1.6 MPa and temperature of 77 K, the capacities of Na-LEV, H-OFF, Na-MAZ, and Li-ABW were 2.07, 1.75, 1.64, and 1.02 wt%, respectively ^[85]. This result confirms that micropores with a higher volume and diameter, approximating the kinetic diameters of the hydrogen molecules, play a vital role in enhancing the zeolite capacity.

Recently, Sun et al. developed Monte Carlo simulations to predict hydrogen loading under different pressure and temperature conditions for MOFs, hypercrosslinked polymers, and zeolites. Regarding the pressure value, meta-learning allowed the identification of the optimal temperature with the highest working capacity for hydrogen storage.

2.3. MOFs

MOFs are an extensive class of crystalline materials that contain metal ions, metal clusters, and organic linkers. Organic linkers such as azoles (1,2,3-triazole, pyrrodiazole, etc.), dicarboxylates (glutaric acid, oxalic acid, succinic acid, malonic acid, etc.), and tricarboxylates (citric acid, trimesic acid, etc.) are commonly used in MOF fabrication ^[89]^[90]. In addition to controlling the metal/ligand ratio, changing the reaction temperature and modifying the organic linker are strategies to afford MOFs with good pore structures and high-dimensional frameworks ^[91], which significantly affect the hydrogen storage capacity. The main hydrogen storage mechanism of MOFs is similar to that of other porous materials such as hollow spheres, carbon-based materials, or zeolites: hydrogen molecules occupy and are kept in their vacants (the pores). The schematic mechanism examples of zeolites and MOFs are described in Figure 1.

Figure 1. (A) Hydrogen adsorption of zeolites materials named (a) aAg0.29Li0.71-LSX, (b) bAg0.43Li0.57-LSX, and (c) cAg0.29Li0.71-LSX. Reprinted with permission from ^[92]. Copyright 2019, MDPI; (B) Scheme of MOFs capturing hydrogen via their adsorbent properties. Reprinted with permission from ^[93]. Copyright 2022, MDPI.

MOFs materials are excellent candidates for potential applications in clean energy such as media for storing gases such as hydrogen and methane because of their high crystallinity, large internal surface area that can reach up to 6000 m²/g, and ultrahigh porosity with up to 90% free volume ^[94]. In addition, the variability in the metal and organic components in their structures endows MOFs with good designability, tunable structures, and properties. However, the production of MOFs is quite complex, and the hydrogen uptake efficiency of MOFs at ambient temperature is low. Therefore, improvements in the MOF fabrication technology are necessary ^[95].

A wide range of MOFs synthesis processes have been developed, which are summarized in Table 4. After synthesis, the MOF material obtained is typically available in a powder form. The incorporation of powders into relevant devices is quite difficult; this can prevent the utilization of MOFs in energy or gas-storage applications ^[96]. For example, if the MOFs are introduced as a loose powder into a tank with pipe fittings, they may be easily blown off into the surroundings, causing difficulties in handling and contamination in pipe fittings. In addition, their low packing density can compromise the volumetric capacities of the pipe fittings. Scientists have made great efforts to shape MOFs using the techniques presented in Table 4. The choice of the shaping technique depends on the fabrication approach and expected textural properties of the MOF materials ^[97]^[98]. Shaped MOF materials with appropriate mechanical strength, low flow resistance, and intact or high secondary surface areas and pore volumes are desired for better hydrogen storage performance ^[96].

Table 4. Various synthesis and powder-shaping methods for metal–organic framework (MOF) materials.

MOF Synthesis Methods	MOF Powder-Shaping Methods
1. Microwave assisted method	1. Uniaxial pressing
2. Sonochemitry	2. Coating
3. Electrochemistry	3. Foaming
4. Mechanochemistry	4. Templating
5. Hydrothermal approach	5. Casting
6. Solvothermal approach	6. Granulation
	7. Extrusion
	8. Pulsed current processing

Hydrogen molecules have low polarizability. In addition, interactions between hydrogen and most MOFs are relatively weak. Previous research showed that the isosteric heat of hydrogen absorbed in the highly porous MOFs was approximately −5 kJ/mol because of the weak interactions of hydrogen molecules ^[95]. Hence, cryogenic temperatures are essential for obtaining reasonable hydrogen capacities in MOFs. Many research groups synthesized and investigated their MOF materials for hydrogen storage, such as MIL-53 (Cr) modified with Pd-loaded activated carbon (MIL-53 Cr) ^[99], MIL-101 (Cr) with zeolite-templated carbon ZTC (MIL-101 (Cr)/ZTC) ^[100], iron-based MOF (Fe-BTT) ^[101], and Co-based MOF (Co-MOF) ^[102]. They obtained hydrogen storage capacities of less than 5 wt%. With the improvements in the components and structures of materials, as well as using suitable pressures, the recent MOF materials including Zr-based MOFs (NU-1101, NU-1102, NU-1103), Al-based MOFs (NU-1501-Al) ^[103], and robust azolate-based MOFs (MFU-4/Li) ^[104] achieved remarkable increases of 9.1, 9.6, 12.6, 14, and 9.4 wt%, respectively, in hydrogen storage capacities.

However, storage at ambient temperatures remains a challenge. To overcome the limitations of the driving ranges in fuel-cell vehicles when using adsorption-based hydrogen storage technology, researchers have been developing different methods to increase the performance of MOF materials at higher temperatures. For example, open metal sites in the framework can act as strong adsorption sites for hydrogen loading at ambient temperatures. Lim et al. synthesized Be-based MOF (Be-BTB) with open metal sites. The Be-BTB material with a high BET surface area of 4400 m²/g presented a hydrogen adsorption capacity of 2.3 wt% at 298 K and 100 bar, which is much higher than that of the previous studies ^[105]. In addition, doping with metal ions or incorporating carbon-based materials may introduce a hydrogen spillover effect (HSPE) that can improve the hydrogen storage capacity of materials at room temperature. The HSPE is a surface phenomenon in which active hydrogen atoms generated by the dissociation of hydrogen molecules on metal surface migrate to the support surface and take part in the catalytic reaction of substance adsorbed on that site ^[106]^[107]. There are two conditions ensuring the HSPE can occur. First, there is the existence of metals capable of adsorbing dissociated hydrogen to convert hydrogen molecules into hydrogen atoms or hydrogen ions. Second, there is the presence of an acceptor for active hydrogen species and a channel and driver for the transfer of reactive hydrogen species ^[108]. The discovery of the HSPE has opened a new strategy in designing efficient catalysts. Especially, the HSPE has attracted much attention as one of the most potential technologies for enhancing the hydrogen storage performance of porous materials including MOFs at ambient temperature. The capacity of materials with induced hydrogen spillover was approximately five times higher than that of pristine MOFs ^[109]. Yang and co-workers doped 10 wt% Pt/AC catalysts into isoreticular MOF (IRMOF) materials to form a bridging spillover structure between carbon bridges and hydrogen spillovers, leading to a significant enhancement in the hydrogen storage performance. At 298 K and 100 bar, the hydrogen storage capacities of IRMOF-1 and IRMOF-8 increased from 0.4 to 3 wt% and from 0.5 to 4 wt%, respectively ^[110]^[111]. Table 5 summarizes the hydrogen storage capacities of some MOF materials.

Table 5. Summary of hydrogen storage capacities of various MOF materials.

MOF Materials Used for Hydrogen Storage at Low Temperature
Material	Temperature (K)	Pressure (bar)	H₂ Storage Capacity (wt%)	Reference
Co-MOF	77	1	1.62	^[102]
MIL-53	77	60	1.92	^[99]
MIL-101 (Cr)/ZTC	77	1	2.55	^[100]
CAU-1	70	1	4
UBMOF-31	77	60	4.9	^[101]
Fe-BTT	77	Low Pressure	2.3	^[112]
	87	Low Pressure	1.6
	77	95	4.1
NOTT-400	77	1	2.14	^[113]
		20	3.84
NOTT-401	77	1	2.31
		20	4.44
NU-1101	77–160	100–5	9.1	^[114]
NU-1102	77–160	100–5	9.6
NU-1103	77–160	100–5	12.6
NPF-200	77	100–5	8.7	^[115]
NU-1500	77–160	100–5	8.2	^[116]
NU-1501-Al	77–160	100–5	14	^[103]
MFU-4/Li	77–160	100–5	9.4	^[104]
MOF materials used for hydrogen storage at ambient temperature
Material	Temperature (K)	Pressure (bar)	H₂ storage capacity (wt%)	Reference
HKUST-1	298	65	0.35	^[117]
Cu-BTC	303	35	0.47	^[118]
HKUST-1	303	35	0.47	^[119]
CB/Pt/MOF-5	298	100	0.62	^[120]
Zn₂ (dobpdc)MOFs	298	100	1.3	^[121]
Mg₂ (dobpdc)MOFs	298	100	1.8	^[121]
Pd-CNMS/MOF-5	298	100	1.8	^[122]
		75	1.6
		50	1.4
Be-BTB	298	100	2.3	^[105]
V₂Cl₂.₈	298	100	1.64	^[123]
NU-150I-Al	296	100	2.9
IRMOF-1	298	100	3	^[110]
IRMOF-8	298	100	4	^[90]

3. Advantages and Disadvantages of Physical Hydrogen Storage Materials

Hydrogen storage technologies play an extremely important role in the “hydrogen economy”. It is necessary to continuously improve and develop storage materials for the hydrogen industry. As mentioned in the Introduction section, compared to chemical storage materials, the outstanding properties (Table 6) of the physical absorbents and the fact that they do not release greenhouse gases facilitate the global energy transition proceeding effectively as well as achieving the global “net zero emission” target sooner. However, each physical material has both its strengths and weaknesses. The limitations of these materials (hollow spheres^[124]^[125], carbon-based materials^[126]^[127]^[128], zeolites^[129]^[130], MOFs^[131]^[132]) presented in Table 6 are still challenges in practical hydrogen storage applications.

Table 6. Advantages and disadvantages of physical hydrogen storage materials.

Property	Prospects	Consequences
Material	Prospects	Consequences
Hollow spheres		- High surface areas - High binding sites - High gravimetric energy density - Inexpensive manufacturing process - Safety - Scalable possibility	- Cryogenic temperatures - Agglomeration - Low volumetric energy density - Heating requirements for loading and unloading - Energy cost
Carbon-based materials		- Chemical stability - Good thermal - Simple processibility - Non-expensive	- Pore size uniformity - Binding site deficiency - Pore structure control
Zeolites		- Good crystallinity - Hydrolytic and thermal stability - Non-expensive	- Binding site deficiency - Low gravimetric loading - Tunability - Structural diversity limitation
MOFs		- High surface area - High pore volume - Good crystallinity - Tunability - Design control possibility - Rich metal sites	- Low hydrogen loading at room temperature - Fabrication process

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Afzal, I.; Akhter, T.; Hassan, S.U.; Lee, H.K.; Razzaque, S.; Mahmood, A.; Al-Masry, W.; Lee, H.; Park, C.H. Logical Optimization of Metal–Organic Frameworks for Photocatalytic Degradation of Organic Pollutants in Water via Box–Behnken Design. ACS EST Water. 2024, 4, 648–660.
Siddiqa, A.; Akhter, T.; Faheem, M.; Razzaque, S.; Mahmood, A.; Al-Masry, W.; Nadeem, S.; Hassan, S.U.; Yang, H.; Park, C.H. Bismuth-Rich Co/Ni Bimetallic Metal–Organic Frameworks as Photocatalysts toward Efficient Removal of Organic Contaminants under Environmental Conditions. Micromachines. 2023, 14, 899.

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Version	Summary	Created by	Modification	Content Size	Created at	Operation
1		Chan Ho Park	--	4062	2024-04-17 04:08:30	\|
2	format correct	Catherine Yang	-3 word(s)	4059	2024-04-18 10:01:25	\|