Hydrogen Storage Technology, and Its Challenges: A Review

Abstract

This paper aims to present an overview of the current state of hydrogen storage methods, and materials, assess the potential benefits and challenges of various storage techniques, and outline future research directions towards achieving effective, economical, safe, and scalable storage solutions. Hydrogen is recognized as a clean, secure, and cost-effective green energy carrier with zero emissions at the point of use, offering significant contributions to reaching carbon neutrality goals by 2050. Hydrogen, as an energy vector, bridges the gap between fossil fuels, which produce greenhouse gas emissions, global climate change and negatively impact health, and renewable energy sources, which are often intermittent and lack sustainability. However, widespread acceptance of hydrogen as a fuel source is hindered by storage challenges. Crucially, the development of compact, lightweight, safe, and cost-effective storage solutions is vital for realizing a hydrogen economy. Various storage methods, including compressed gas, liquefied hydrogen, cryo-compressed storage, underground storage, and solid-state storage (material-based), each present unique advantages and challenges. Literature suggests that compressed hydrogen storage holds promise for mobile applications. However, further optimization is desired to resolve concerns such as low volumetric density, safety worries, and cost. Cryo-compressed hydrogen storage also is seen as optimal for storing hydrogen onboard and offers notable benefits for storage due to its combination of benefits from compressed gas and liquefied hydrogen storage, by tackling issues related to slow refueling, boil-off, and high energy consumption. Material-based storage methods offer advantages in terms of energy densities, safety, and weight reduction, but challenges remain in achieving optimal stability and capacities. Both physical and material-based storage approaches are being researched in parallel to meet diverse hydrogen application needs. Currently, no single storage method is universally efficient, robust, and economical for every sector especially for transportation to use hydrogen as a fuel, with each method having its own advantages and limitations. Moreover, future research should focus on developing novel materials and engineering approaches in order to overcome existing limitations, provide higher energy density than compressed hydrogen and cryo-compressed hydrogen storage at 70 MPa, enhance cost-effectiveness, and accelerate the deployment of hydrogen as a clean energy vector.

1. Introduction

In recent years, hydrogen energy vectors have gained substantial popularity as a truly auspicious source of clean energy with significant potential as a substitute for fossil fuels without carbon emissions, a plentiful and non-toxic energy source [1]. In pursuit of a carbon dioxide-free global economy, renewable energy sources such as wind energy, geothermal energy, solar power, and hydropower have been investigated as alternative energy solutions. However, these energy sources are affected by intermittency, seasonality, changes in wind patterns, local weather conditions, and sunlight intensity based on the weather and time of day and cannot consistently supply reliable energy to meet the required demands [2,3]. To align the intermittent nature of the power supply with the load demand, it is essential to integrate high-efficiency energy storage solutions and batteries with sources of renewable energy [4]. In this context, hydrogen storage and generation systems are regarded as the most promising solutions for generating clean energy. Thus, most recent efforts have been directed towards the replacement of carbon fuels with clean hydrogen energy carriers, owing to hydrogen’s high energy storage density (gravimetric density of 119.9 MJ/kg) [5,6,7]. Notably, the energy yield per unit mass of hydrogen surpasses that of gasoline by threefold.

Table 1. Comparison of the properties of hydrogen and other fuels [8,9].

Property	Hydrogen	Methane	Gasoline	Diesel
Molecular weight	2.016	16.043	110	170
Auto-ignition temperature (K)	853	813	623	523
Carbon content (mass%)	0	75	84	86
Boiling point (K)	20.3	111	298–488	453–633
HHV (MJ/kg)	141.9	55.5	47.3	44.8
LHV (MJ/kg)	119.9	50	44.5	42.5
Density (at 1 bar and 273 K; kg/m3)	0.089	0.72	730–780	830
Adiabatic flame temperature (at 1 bar and 298 K; at stoichiometry; K)	2480	2214	2580	2300
Volumetric energy content (at 1 bar and 273 K; MJ/m3)	10.7	33	33,000	35,000
Stoichiometry air/fuel mass ratio	34.4	17.2	14.7	14.5

provides a comparative analysis of the characteristics of hydrogen relative to other fuels [8,9]. The hydrogen economy (HE) offers an alternative to the current energy landscape, addressing its significant contributions to greenhouse gas emissions and associated environmental and health challenges [10]. Hence, as an energy carrier, hydrogen has the potential to bridge the gap with fossil fuels which accounts for 74% of total greenhouse gas emissions and the intermittent nature of renewable energy sources [11,12].

The global community is actively pursuing a transition toward a carbon-neutral economy, aiming to achieve net-zero carbon dioxide emissions by the year 2050. This ambitious goal necessitates transitioning to green hydrogen as a primary energy source, produced through the electrolysis of water using renewable energy sources, transforming it into a versatile, storable, and universally applicable carbon-free energy carrier, to substitute fossil fuels [13,14]. Hydrogen energy is crucial for future energy systems seeking to eliminate air pollution, water pollution, and CO₂ emissions [15,16,17]. The demand for hydrogen as an energy carrier is rising due to its affordability, diverse production, high gravimetric energy density (119.9 MJ/kg), eco-friendliness, renewability, and abundance. These characteristics make hydrogen a pivotal element in advancing sustainable energy systems [18]. However, hydrogen is impractical for long-distance energy carrier due to its low volumetric energy density (9.8 kJ/L) at standard temperature and pressure, which is significantly lower than that of gasoline (15.8 MJ/L) and methanol (31.7 MJ/L) [19,20,21,22].

Ongoing research continues to explore hydrogen’s applications across various sectors, including large-scale energy storage, aviation, military applications, and space exploration. Hydrogen plays a crucial role in powering vehicles, generating electricity, and supporting various industrial processes such as methanol and ammonia synthesis, steel manufacturing, fertilizer production, and metal treatment [23,24]. However, the use of hydrogen in vehicles requires advanced capabilities. These include gravimetric and volumetric efficiency, favorable temperature and pressure conditions, rapid kinetics, and minimal heat generation during storage and release. Table 2 compares hydrogen’s applications, emphasizing its role in clean energy and emission reduction, alongside challenges like cost, infrastructure, and storage [25,26].

The hydrogen economy infrastructure comprises four key components. These are production, storage, transportation, and utilization, as illustrated in Figure 1. Among these, hydrogen storage systems face substantial challenges that hinder the advancement of hydrogen-based energy systems [27].

A significant challenge in ensuring compact, reliable, robust, and safe hydrogen storage methods lies in the need to store large volumes of hydrogen to address the disparity between energy supply and demand [28]. These challenges arise from hydrogen’s unique physicochemical properties such as high flammability (requires special storage and transportation infrastructure) low molecular density, low volumetric energy density (9.8 kJ/L), high diffusivity, reactivity, and small molecular hydrogen which complicate safe and efficient storage methods [29].

Table 2. Overview of hydrogen applications, highlighting its role in clean energy and emission reduction, alongside key challenges such as cost, infrastructure, and storage limitations [30,31].

Application of Hydrogen Energy	Description	Advantages	Challenges
Transportation	Used in fuel cell vehicles (FCVs) for cars, buses, and trains trucks. Explored as a fuel for rockets and aircraft.	Zero emissions, fast refueling, high energy efficiency.	Storage challenges High production costs, limited refueling infrastructure
Industrial Processes	Utilized in refining, ammonia production, and methanol synthesis, steel manufacturing, fertilizer production.	Reduces carbon footprint in industries, essential for chemical production.	Dependency on fossil fuels for grey hydrogen, high energy requirements.
Energy Storage	Stores excess renewable energy as hydrogen via electrolysis.	Enables long-term storage, balances grid intermittency.	Low round-trip efficiency, high costs for electrolyzers and storage systems.
Heating	Blended with natural gas for residential and commercial heating.	Reduces carbon emissions in heating systems.	Infrastructure modifications needed.
Power Generation	Used in gas turbines or fuel cells for electricity generation.	Clean energy production, compatible with existing infrastructure.	High costs, requires pure hydrogen to avoid emissions.

Table 2. Overview of hydrogen applications, highlighting its role in clean energy and emission reduction, alongside key challenges such as cost, infrastructure, and storage limitations [30,31].

Application of Hydrogen Energy	Description	Advantages	Challenges
Transportation	Used in fuel cell vehicles (FCVs) for cars, buses, and trains trucks. Explored as a fuel for rockets and aircraft.	Zero emissions, fast refueling, high energy efficiency.	Storage challenges High production costs, limited refueling infrastructure
Industrial Processes	Utilized in refining, ammonia production, and methanol synthesis, steel manufacturing, fertilizer production.	Reduces carbon footprint in industries, essential for chemical production.	Dependency on fossil fuels for grey hydrogen, high energy requirements.
Energy Storage	Stores excess renewable energy as hydrogen via electrolysis.	Enables long-term storage, balances grid intermittency.	Low round-trip efficiency, high costs for electrolyzers and storage systems.
Heating	Blended with natural gas for residential and commercial heating.	Reduces carbon emissions in heating systems.	Infrastructure modifications needed.
Power Generation	Used in gas turbines or fuel cells for electricity generation.	Clean energy production, compatible with existing infrastructure.	High costs, requires pure hydrogen to avoid emissions.

Hydrogen production technologies, particularly those utilizing renewable configurations, hold significant potential to drive the global energy transition [32,33]. Among these, green hydrogen is regarded as the top environmentally sustainable choice and should be prioritized as the main production method [34,35,36]. Techniques for producing hydrogen vary based on the technology and energy sources used, including biomass, nuclear energy, natural gas, wind power, and solar energy [37,38]. To categorize these methods, different colors are assigned to represent hydrogen production methods and their associated impact on CO₂ emissions during production, as illustrated in Figure 2 [39]. The main categories include green hydrogen (signifying clean hydrogen production methods), white hydrogen occurs naturally in underground reservoirs through geological processes such as serpentinization and mantle degassing, blue (indicating a transitional approach toward more environmentally friendly hydrogen production), and grey (representing polluting production methods). Blue hydrogen is typically produced through steam methane reforming using natural gas or biomass, while grey hydrogen is produced via coal gasification [40].

Hydrogen storage is a critical area of development within the hydrogen energy sector, with growing recognition of its equal importance to hydrogen production processes in advancing the hydrogen economy. These operations are critical to ensuring the efficient distribution and utilization of hydrogen, thereby influencing its integration into various industries and applications [41]. A main factor in realizing a viable hydrogen economy is the development of hydrogen storage solutions that are efficient, compact, cost-effective, safe, and lightweight. Hydrogen storage methods encompass a range of approaches designed for different applications [42]. However, meeting the 2030 goals set by the United States Department of Energy (DOE) remains a main challenge, especially for onboard 5.5 wt% and 62 kg/m³ for gravimetric and volumetric hydrogen capacity, respectively [43]. Active methods for hydrogen storage include physical-based storage systems, such as compressed hydrogen storage at high pressure in specially designed cylinders, liquefied hydrogen storage at cryogenic temperatures, and cryo-compressed methods, which combine both high-pressure and low-temperature techniques. Another prominent method is solid-state storage by the principle of physisorption or chemisorption hydrogen storage, where hydrogen is chemically or physically bound to other materials, and underground storage solutions [5,16]. Each hydrogen storage method presents distinct challenges and advantages, tailored to specific applications and the diverse requirements of hydrogen as an energy carrier. An ideal hydrogen storage method should exhibit key characteristics, including economic feasibility for large-scale storage, operational safety, high volumetric density, seamless integration with renewable energy sources and existing energy infrastructure, system reliability, and an extended operational lifespan [44]. Extensive research is underway to identify space-efficient, secure, and economically viable materials and technologies for hydrogen storage. However, most studies focus on specific storage types, lacking comparative analyses of their relative advantages, challenges, application areas, and potential for future advancements [44,45,46,47]. This review addresses a critical gap by conducting a comprehensive comparative analysis of the most promising hydrogen storage methods, examining their key challenges, highlighting recent advancements in storage materials, and proposing future research directions to support the global transition to net-zero carbon emissions by 2050.

2. Physical-Based Hydrogen Storage

2.1. Compressed Hydrogen Storage

Compressed hydrogen storage represents the simplest and the most widely used method of hydrogen storage, finding application in both stationary systems, such as hydrogen-powered energy plants, and mobile applications. This method is favored for its operational simplicity, characterized by rapid hydrogen filling and release. Compressed hydrogen storage exhibits a volumetric energy density of 4.5 MJ/L, a volumetric capacity of 10–15 g/L, and a gravimetric capacity of 1–2%, with an approximate cost ranging from $500 to $1000 per kilogram of stored hydrogen [48]. This method relies on specialized cylinders designed to withstand high pressures while capable of withstanding the effects of hydrogen embrittlement, remaining lightweight, cost-effective, and resistant to hydrogen diffusion [49,50]. The principle of compressed hydrogen storage involves increasing the gas pressure by compression to enhance its volumetric energy density. However, this high-pressure requirement introduces safety risks and utilizes approximately 13–18% of hydrogen’s lower heating value which influences the overall cost-effectiveness of the approach [51,52]. Furthermore, its greatest drawback is the process is volumetrically and gravimetrically inefficient. The compression process also demands substantial energy input to compress hydrogen from ambient conditions to the required high-pressure state, thus diminishing the overall efficiency of hydrogen as an energy vector [50,53]. From a thermodynamic perspective, the minimum work required for compression can be achieved through isothermal reversible compression, which can be estimated under the assumption of ideal gas behavior [54]. Despite these challenges, compressed hydrogen storage remains a critical technology in the broader context of hydrogen energy systems.

$$\mathrm{W}=\mathrm{R}{\mathrm{T}}_1\mathrm{l}\mathrm{n}\left({\displaystyle \frac{{\mathrm{P}}_1}{{\mathrm{P}}_2}}\right)$$

(1)

where W is the specific compression work (kJ/kg), R is the hydrogen gas constant (4.157 kJ/(kg · K)), P₁ is the initial pressure in bar or MPa, P₂ is the final pressure in bar or MPa, T₁ is the initial temperature in Kelvin (K), and n is the polytropic constant (approximately 1.41) for an isentropic process for hydrogen.

In practical scenarios, despite the utilization of multi-stage inter-cooling technology, hydrogen experiences a noticeable temperature increase during compression. As a result, a polytropic process offers a more accurate depiction of the compression process. For a given process, the energy consumption can be calculated as follows:

$$\mathrm{W}={\displaystyle \frac{\mathrm{n}}{\mathrm{n}-1}} \mathrm{R}{\mathrm{T}}_1\left[\right({\displaystyle \frac{{\mathrm{P}}_2}{{\mathrm{P}}_1}}){\displaystyle \frac{\mathrm{n}-1}{\mathrm{n}}}-1]$$

(2)

Reducing the inlet hydrogen temperature proportionally decreases the compression work, as demonstrated by Equations (1) and (2).

The storage vessels for compressed hydrogen storage are typically constructed from steel, aluminum, or composite materials. Hydrogen is usually stored in steel tanks at pressures ranging from 15 to 70 MPa. However, using steel tanks, only 1.5 weight percent gravimetric density and 10–12 ${\displaystyle \frac{\mathrm{k}\mathrm{g}}{{\mathrm{m}}^3}}$ volumetric density can be achieved. The drawbacks of the compressed hydrogen storage method include poor volumetric and gravimetric densities, even at high pressures (70 MPa) [55]. Li et al. [56] analyzed an 82 MPa compressed hydrogen storage for fast-filling, studying temperature rise, hydrogen density, and safety considerations during the filling process using theoretical analysis, and numerical methods. During the rapid refueling of a high-pressure hydrogen tank, the temperature of the hydrogen can increase substantially, potentially causing tank failure. Moreover, this temperature rise reduces the density of hydrogen in the tank, causing a decrease in the mass of hydrogen stored. The storage vessels require lightweight, economical materials designed to endure high pressure. Additionally, the chosen material must demonstrate resistance to hydrogen permeation and reduce the risk of embrittlement caused by stored hydrogen [57]. Based on these criteria and the manufacturing process, five distinct types of vessels (Type I, II, III, IV, and V (under development)) presented in Figure 3, are employed for storing compressed hydrogen gas [3].

2.1.1. Type I Vessels

Type I vessels are fully seamless metallic steel pressure vessels. This type is the most conventional, least expensive, costing only 83 USD/kg [59], and heaviest compressed hydrogen storage tank (vessel). These tanks are primarily used for warehouse (for stationary) storage purposes and can withstand pressures of up to 20 MPa [49]. They are typically made of steel or aluminum and have a volumetric energy density of 1.4 MJ/L and a low gravimetric density of 4–5 MJ/kg, making them unsuitable for onboard applications [60].

2.1.2. Type II Vessels

Type II vessels (tanks) are pressure vessels with a glass fiber composite partially overwrap, utilizing either aluminum liners or steel. The structural load is comparably shared between the steel and composite materials. Although they are 30–40% lighter compared to type I tanks, their manufacturing costs are about 50% higher [61]. These tanks are primarily used for stationary applications requiring only high pressures up to 30 MPa. They have a gravimetric density of 2.1, a volumetric energy density of 2.9 MJ/L, and an approximate cost of 86 USD/kg. The primary challenge with metallic high-pressure tanks is corrosion, and hydrogen embrittlement, which happens when hydrogen diffuses into the metal, weakening its bonds. This can result in delayed fractures and a reduction in the metal’s ductility. However, to harness hydrogen as a next-generation energy resource, research focused on preventing hydrogen embrittlement in metals and enhancing their resistance to this phenomenon is gaining substantial attention [62]. According to a study by Akram et al. [63] the application of Poly tetrafluoroethylene (PTFE) coating to 316 L stainless steel through a spin coating technique improves its corrosion resistance. But authors do not discuss the PTFE coating’s effect on hydrogen embrittlement resistance. Also, Malik et al. [64] prove the PTFE coating on stainless steel-304 enhances resistance to corrosion, wear, and hydrogen embrittlement.

2.1.3. Type III Vessels

Type III high-pressure tanks incorporate a complete composite wrap surrounding a metal liner as shown in Figure 4. The carbon fiber composite primarily bears the structural load, while the aluminum liner serves as a sealing component and contributes to approximately 5% of the mechanical load. These tanks have proven reliable at working pressures up to 45 MPa but they can be used up to 70 MPa. They have a gravimetric density of 4.21 and cost 700 USD/kg.

Type III hydrogen storage systems experience significantly higher costs compared to Type I and Type II, despite only doubling the gravimetric density of hydrogen compared to Type II [65]. Fu et al. [66] investigate the fatigue life and strength of Type III vessels using grid theory, validated through simulation studies and hydrodynamic bursting tests. The simulation results show that the highest fiber stress is concentrated in the innermost layer of the ring winding, and the load-bearing capacity of the cylinder improves with specific layer arrangements. Although Type III cylinders weigh approximately half as much as Type II, they cost twice as much. They are primarily designed for portable applications where weight savings are crucial.

2.1.4. Type IV Vessels

Type IV tanks are fully composite utilizing high-density polyethylene (HDPE), and polyamide-based polymers as a common liner. Composites made of carbon fiber-reinforced polymer support the structural load at an operating pressure of 70 MPa [67]. Polyethylene, extensively used as the liner for type IV compressed hydrogen vessels, provides outstanding gas impermeability and mechanical properties at low temperatures and blocks hydrogen leakage, preventing microcracks in the composites. Type IV hydrogen storage cylinders are known for their lightweight nature, high hydrogen storage density gravimetric density of up to 5.7, and volumetric energy density of 4.9 MJ/L [68]. Type IV hydrogen storage cylinders are known for their lightweight nature, high hydrogen storage density gravimetric density of up to 5.7 wt%, and good fatigue performance. They represent the most recognized and commonly utilized option for onboard applications [69]. It reduces weight by 75% compared to type I which is fully metallic. It is used in current fuel cell electric vehicles (FCEVs) to store hydrogen [70]. Despite being the lightest type of pressure vessel, its cost remains relatively high at up to 633 euros, whereas Type I costs only 83 euros [59]. Figure 5 shows an image representation of a type IV tank.

2.1.5. Type V Vessels

Type V high-pressure liner-less tanks are currently under development in the production sector. The absence of a metal liner eliminates the risk of hydrogen embrittlement, a common issue with metal-lined tanks. This makes Type V tanks more durable and safer for long-term hydrogen storage. This design is an evolution of Type IV tanks, enhanced with reinforcing, space-filling structures [72]. According to a recent report [73] the patent for these tanks was submitted in 2014. Type V is designed to have even higher volumetric and gravimetric densities. These vessels are, however, not yet available commercially. These liner-less, fully composite vessels are aimed at operating at pressures exceeding 70 MPa and are expected to be 10% to 20% lighter than Type IV vessels (tanks). Constructed entirely from composite materials, they are resistant to corrosion. Thus, the composite layer serves dual purposes. It offers structural support and acts as a barrier against hydrogen, effectively preventing issues like hydrogen embrittlement and liner collapse [74,75]. Nonetheless, pressurized tanks used onboard encounter reduced public acceptance due to heightened safety concerns, especially the potential for explosions caused by sudden shocks. To mitigate these risks, advanced valve technology is necessary to regulate the pressure to the engine’s inlet requirements. The potential advantages, materials used for tank construction, challenges, and operating conditions of all types of tanks of compressed hydrogen storage are summarized in

Table 3. Vessels for storing compressed hydrogen at high pressure, and their main features.

Types	Material/Description	Advantages	Challenges	Ref.
Type I	Fully metallic pressure vessel (usually 4130 steels, stainless steel, high-strength carbon steel), aluminum. Used in industrial gas storage, low-pressure applications.	Cheapest option and widely available.	Heavy, hydrogen embrittlement, internal corrosion, limiting operating pressure, low gravimetric density, not applicable for onboard application.	[69]
Type II	Metallic pressure vessel hoop-wrapped with glass fiber composite, used in CNG storage and transport, moderate pressure.	Lighter weight compared to type I, highest pressure tolerance	Serious hydrogen embrittlement problem, more expensive than type I, short lifetime.	[76]
Type III	Full composite wrap with metal liners such as aluminum, stainless steel. Used in hydrogen refueling stations, heavy-duty vehicles, aerospace.	High strength-to-weight ratio, reducing weight no permeation.	Linear fatigue, high burst pressure, more expensive compared to steel tank	[61]
Type IV	Fully composite (high-density polyethylene (HDPE) inner with carbon glass or carbon fiber), used in FCEVs, portable hydrogen storage, high-pressure transport.	Lightweight, ideal for mobile applications, longer life, lower burst pressure, and permeation through liner	Permeation, cost is still comparatively high, linear collapse, embrittlement.	[77]
Type V	Fully composite materials, such as carbon fiber-reinforced polymer (CFRP), with no metal liner. Used in FCEVs, aerospace, and high-pressure storage.	Lighter than other types of tanks, designed to store hydrogen at very high pressures up to 100 MPa, eliminates the risk of hydrogen embrittlement.	High manufacturing cost due to the use of advanced composite materials.	[72]

. Generally, these different types of high-pressure compressed hydrogen storage vessels offer diverse options for various applications, ranging from stationary industrial use to mobile transportation and portable power generation. They cater to various needs and requirements within the hydrogen economy. Type V compressed hydrogen storage vessels are aimed to address the key challenges related to previous vessels [49].

2.2. Liquefied Hydrogen Storage

Liquefied hydrogen storage techniques involve cooling hydrogen to extremely low temperatures (to −253 °C) to liquefy it. Keeping these low temperatures needs a substantial amount of energy. Liquid hydrogen storage, like hydrogen compression, is a well-established technology. In liquid hydrogen storage, the low boiling point of hydrogen necessitates special containers equipped with insulation systems to prevent boil-off and maintain efficiency. The liquefied hydrogen is then stored in an insulated tank. High-vacuum adiabatic low-pressure tanks are commonly used for liquid hydrogen storage to minimize vaporization. Strict heat insulation is necessary to achieve this goal effectively [78].

The process of liquefaction is extremely energy-intensive, consuming over 30% of the overall energy contained in hydrogen. The liquid hydrogen storage tank is mainly composed of three components, as shown in Figure 6: the inner vessel, the insulation layer, and the outer shell [79]. The inner vessel, which comes into direct contact with the liquid hydrogen, must maintain its mechanical properties at low temperatures. The outer shell protects the inner vessel and serves to connect the tank to the external structure. To minimize radiative heat transfer to the inner vessel, a multilayer insulation is positioned between the inner vessel and the outer shell. Furthermore, the space between the inner vessel and the outer shell is maintained under vacuum, reducing convective and conductive heat transfer from any residual gas present within the insulation layers [79].

Hydrogen storage in its liquefied form is an encouraging option for long-term storage and large-scale transportation, offering a high gravimetric density of 70.8 kg/m³ and the potential for achieving significant volumetric density to 8.5 MJ/L [80], with an approximate cost ranging from $1500 to $3000 per kilogram of stored hydrogen [48]. This method is widely employed in the aerospace sector in the United States, including NASA’s Space Shuttle programs, where liquefied hydrogen serves as a primary fuel source. It is recognized as a viable hydrogen storage technology [81].

Compared to compressed hydrogen storage, liquid hydrogen storage operates at pressures below 1 MPa, which reduces the high costs associated with the compression of gas. However, the utilization of liquid hydrogen poses challenges such as the requirement for extremely low temperatures, and boil-off losses influenced by factors including thermal insulation quality, hydrogen volume, storage duration, environmental conditions, and tank geometry [82]. Additionally, liquid hydrogen storage suffers from low energy efficiency, high overall costs, and significant energy consumption during the liquefaction process, known as the liquefaction energy penalty. Consequently, this storage approach is primarily suitable for short-term applications due to the continuous boil-off risk. Despite being an established technology, liquid hydrogen storage systems require ongoing advancements at the material level to address challenges such as reducing tank weight, improving corrosion resistance, minimizing boil-off losses, and enhancing tank strength [78].

Materials commonly used for constructing liquid hydrogen storage tanks include nickel alloys (notable for their robustness but higher weight), aluminum alloys, and stainless steel. Among these, stainless steel is the most widely utilized because of its excellent performance at low temperatures and its resistance to hydrogen embrittlement [62]. Research efforts are focused on the development of advanced lightweight insulating materials such as aerogels and cryogenic materials, including composites and titanium alloys. Titanium alloys, in particular, are promising due to their exceptional mechanical properties at low temperatures and superior strength-to-weight ratio [83].

Future studies should prioritize optimizing materials for tank construction to address the economic and technical obstacles associated with liquid hydrogen storage, thereby facilitating the development of more efficient, lightweight, and sustainable energy storage solutions.

2.3. Cryo-Compressed Hydrogen Storage

The foundation of hydrogen storage through cryo-compression is the cryogenic temperature (−253 °C to −163 °C), and pressures (25 MPa to 35 MPa) to store hydrogen. This hybrid storage method combines principles from both compressed storage and liquefied hydrogen storage techniques, showing promise by storing hydrogen at extremely low temperatures and moderate to high pressures within specialized containers to achieve greater gravimetric and volumetric density. Cryo-compressed hydrogen storage provides higher densities, around 80 g/L, which is about 10 g/L more than liquefied hydrogen’s density, and helps to significantly minimize boil-off losses [84].

At the moment, compared to other storage options, cryo-compressed storage techniques represent the best-developed commercial technology in the transportation industry, especially in fuel cell electric vehicles. These storage tanks must withstand extremely low temperatures and high pressures. Compared to other storage methods, cryo-compressed storage is advantageous because it offers medium to high volumetric and gravimetric density, lower storage costs, and the potential for long-range vehicle applications [85].

The most significant shortcomings of storage of hydrogen in its liquefied form (boil-off loss) and compressed hydrogen technologies are addressed by cryo-compression since it combines the advantages of the previously mentioned alternatives and mitigates their drawbacks. Researchers revealed that cryo-compressed hydrogen storage has high-density and feasible costs. The exceptional design features of cryo-compressed vessels, such as double-shell insulation and pressure regulation systems, contribute to their safety and efficiency [86]. Currently, cryo-compressed hydrogen is stored in composite over-wrapped pressure vessels, primarily consisting of an inner liner and a fiber winding layer. While cryo-compressed hydrogen storage can achieve dormancy as well as high gravimetric and volumetric densities, it falls short in terms of well-to-wheel efficiency and manufacturing costs. In addition, a major problem with hydrogen storage through cryogenic compression is the substantial rise in temperature during the filling process which reduces hydrogen density and compromises storage tank safety. The filling standards for compressed gas do not fully apply to CcH₂ due to its cryogenic temperature range, limiting temperature increases during fast filling [87]. Research should focus on optimizing cryogenic hydrogen filling processes to manage the temperature rise during fast filling, ensuring safety. This includes developing advanced cooling systems, new materials, and dynamic filling protocols to enhance efficiency and regulatory compliance. In addition, further investigation is needed into the fatigue and strength of cryo-compressed hydrogen storage tanks to enhance their safety, storage efficiency, and life cycle assessment [88]. Figure 7 reveals the design schematic of cryo-compressed hydrogen storage [89].

3. Solid-State Hydrogen Storage (SSHS)

Hydrogen storage in solid state by the principle of physisorption or chemisorption is the most affordable, economical, reliable, secure, volumetrically efficient, and consumes less energy compared to physical-based storage methods. These advantages make it an attractive option for hydrogen storage and transportation. Hydrogen in a solid state can be stored at pressures between 1 and 10 MPa, due to its adsorbent properties. Solid-state storage utilizes metal hydrides or other chemical compounds to reversibly absorb and release hydrogen through chemical reactions [90]. It provides a high volumetric storage capacity, typically ranging from 100 to 130 g/L, with a gravimetric capacity of 1–1.5% and an approximate cost of $2000–$5000 per kilogram of stored H₂ [48].

Solid-state hydrogen storage encompasses storing hydrogen in a solid form within a small volume for various applications, making it superior to other storage methods due to its space efficiency. Different approaches have been proposed to enhance efficiency and safety in solid-state hydrogen storage devices. One method includes utilizing a reactor within a storage unit for reversible and irreversible solid-state hydrogen storage materials, enabling hydrolysis reactions and heat release [44,50,91]. Another innovation focuses on improving heat transfer efficiency by stacking hydrogen storage materials with different reaction temperatures around a heat exchange tube. Additionally, the design incorporates a steel cylinder with a ceramic sintered body for hydrogen storage, surrounded by fire-retarding stuffing material for protection [92]. Furthermore, a solid-state hydrogen storage apparatus with high heat exchange characteristics that feature multiple storage spaces for rapid hydrogen absorption and uniform heat distribution was recommended. These developments seek to improve the thermal performance and weight efficiency of hydrogen storage through solid-state technology [91].

Hydrogen storage systems utilizing solid-state methods utilize physisorption (which involves adsorption on porous materials and requires low temperatures for reasonable uptake) or chemisorption (which involves the formation of hydrides and has high energy barriers, poor reversibility, and slow kinetics). Storing hydrogen in solid form presents several advantages over storing it in a liquid state, or under pressure, particularly concerning volumetric density. Solid-state hydrogen storage, including metal-organic frameworks (MOFs), carbonaceous nanomaterials, metal hydrides, and complex hydrides, while promising hydrogen storage methods, has issues with poor reversibility and cost-effectiveness [93]. For instance, solid-state hydrogen storage, using complex hydrides, shows promise but faces challenges like poor kinetics and thermodynamic stability [94].

Metal hydrides and liquid organic hydrogen carriers are two hydrogen storage reliant on materials technologies that offer exciting qualities, making them suitable for certain applications, even in storage at a large scale. Many research teams have recently become interested in liquid organic hydrogen carriers as a potentially valuable technology for the storage of hydrogen and transportation. It has been demonstrated that liquid organic hydrogen carriers offer the most affordable choice for large-volume, long-distance transport. Hydrogen can be stored in various materials at different temperatures and pressure conditions or through chemical storage and physisorption processes [95,96]. This review does not aim to cover all solid-state hydrogen storage methods. Instead, it concentrates on the most promising and widely considered options, which have been explained in this section.

3.1. Chemical Storage

Chemical hydrogen storage involves technologies in which hydrogen is produced via chemical reactions, utilizing materials such as formic acid, ammonia (NH₃), synthetic hydrocarbons, liquid organic hydrogen carriers (LOHCs), and metal hydrides. The capacity of hydrogen uptake and the catalysts used for hydrogenation and dehydrogenation of Common LOHCs systems are summarized in Table 4. Ammonia offers a high hydrogen density and can release hydrogen through decomposition reactions. Metal hydrides store hydrogen through chemical bonds with metals or alloys and release it under specific conditions, though challenges with reversibility and high temperatures persist. Formic acid can release hydrogen through catalytic dehydrogenation, which makes it well-suited for mobile applications [97,98]. Synthetic hydrocarbons like methanol and methane store hydrogen through chemical bonds and release it via reforming or combustion. LOHCs absorb and release hydrogen through reversible hydrogenation and dehydrogenation reactions, offering stability and ease of handling under ambient conditions. Each of these materials presents unique challenges and advantages related to storage capacity, release kinetics, reversibility, and safety, making chemical hydrogen storage a promising yet complex solution for hydrogen storage and transportation [53].

3.1.1. Metal Hydrides Hydrogen Storage

Hydrogen chemically reacts with metals and metal alloys to form metal hydrides. The method of storing hydrogen using metal hydrides provides a practical solution for long-term storage without the need for continuous energy input, they only need thermal energy to release the stored hydrogen. Due to the strong bond between metal hydrides and hydrogen, temperatures between 120–200 °C are needed to release the hydrogen. Metal hydrides are hydrogen compounds formed through metal-hydrogen bonds, imparting unique properties. Additionally, the attributes and the characteristics of metal hydrides as hydrogen storage are outlined as follows [99,100].

• Excellent safety
• Good reversible cycling performance
• High hydrogen storage capacity (compared to physical-based storage)
• High hydrogen density
• High purity of stored hydrogen
• Low operational, maintenance, and energy costs.

The host metal in metal hydrides could be an alloy, an elemental form, or a metal complex. Consequently, these materials play crucial roles in hydrogen fuel tanks, hydrogen purification, and gas separation. Metal hydrides are crucial for hydrogen storage due to their safe reaction and potential for efficient storage, energy conversion, and high hydrogen storage density [101]. Using metal hydrides offers superior safety and higher volumetric density compared to both liquefied hydrogen and compressed hydrogen storage approaches [102]. There are also oxygen and moisture-sensitive hydrides that have to be handled in an inert environment. That is maintained, in most cases, through a glove box. This is needed because of the danger these types of materials carry since they spontaneously react violently when they encounter water or air. A glove box is essentially a closed atmosphere that allows the manipulation of such compounds in a manner that will not expose the compound to the presence of air. Simple glove box configurations can be achieved with readily available materials, e.g., medium-density fiberboard and PVC gloves, to produce an inert environment with consistency. Advanced systems have safety aspects, e.g., pressure regulators to supply sub-atmospheric conditions to exclude air entry in the case of inert gas supply malfunction. For instance, metal hydrides like 2LiBH₄·MgH₂ and NH₃BH₃ have been kept safe within an argon-filled glove box while transferring to test equipment in a move aimed at both safety and the aversion of undesirable reactions [103,104].

The metal hydrides can achieve storage capacities of about 5–7 wt% when metal hydrides are heated to temperatures of 2500 °C or higher. Hydrogen is chemically bonded to metal alloys or metals in the case of metal hydride storage [105]. Mg₂NiH₄ (contains 3.59 wt% hydrogen), LaNi₅H₆, NaAlH₄, FeTiH₂ (contains 1.89 wt% hydrogen) Mg(BH₄)₂, Mg₂FeH₆ (contains 5.5 wt% hydrogen), Li₂NH, NaBH₄, LiBH₄, AlH₃, LiAlH₄, ammonia borane (NH₃BH₃), amide/imide systems, MgH₂ (is a notable material due to its 7.6 wt% hydrogen content [106], and it has a relatively low hydrogen release temperature, ranging from 350–400 °C) [107], and Li₃NH are the most hydrides currently capable of achieving the 9 wt% the gravimetric hydrogen storage objective set by the U.S. Department of Energy [53,108]. MgH₂ has attracted significant attention due to its ability to store large amounts of hydrogen and high gravimetric energy density. Its volumetric energy density is nearly twice that of liquid hydrogen. However, the kinetics of the MgH₂ formation is relatively slow [106]. MgH₂ has been successfully utilized for stationary hydrogen storage applications [109]; however, its limitations restrict its use in onboard systems. Additional research is required to improve its properties for vehicle applications. The incorporation of transition metals such as Ti into sodium alanate (NaAlH₄), where Ti replaces Na on the surface, attracts a significant number of hydrogen atoms to its vicinity. Mg₂NiH₄ stands out as a promising hydrogen storage material owing to its high capacity, affordability, lightweight, low toxicity, and distinctive structural characteristics. Although LaNi₅ and FeTi can store hydrogen, their storage capacity remains below 2 wt%. These materials are particularly noteworthy due to their low desorption temperatures and pressures [110].

Metal hydride storage methods are interested in both stationary and transportation applications like marine transportation. However, challenges such as low desorption and sorption kinetics, high cost, low hydrogen storage capacity, sluggish kinetics, and high dehydrogenation temperatures persist hindering practical applications [111]. To address these limitations, techniques such as severe plastic deformation and nano structuring have been utilized to improve the properties of hydrogen sorption and cycling performance of metal hydrides. Furthermore, advancements in thermal management techniques, including reactor design optimization and the use of heat exchangers, nano oxide additives, and high thermal conductivity materials, aim to improve overall efficiency in metal hydride systems. These innovative approaches pave the way for tailored strategies and advancements in the field of hydrogen storage via metal hydrides. Hydride formation begins when a hydrogen molecule dissociates into atomic hydrogen at the surface, which subsequently diffuses into the bulk material and becomes chemisorbed within the metal or alloy structure [95], as illustrated in Figure 8 [53]. This chemisorption process can lead to a lattice expansion of approximately 20–30% of the original volume [109]. Hydride formation can take place either through the direct reaction of hydrogen with the metal or via the electrochemical dissociation of water molecules [50].

In addition, complex metal hydrides; such as metal borohydrides and alanates have received substantial interest for their suitability in hydrogen storage applications, primarily owing to their impressive gravimetric and volumetric capacities [112]. Complex metal hydrides are formed via the covalent bonding of hydrogen atoms to a central atom in coordination complexes such as aluminum tetrahydride, and aluminum hexahydride, with cations like magnesium, zinc, and lithium stabilizing the anion. These hydrides are grouped into three categories: alanates, borohydrides, and amides-imides [113]. Complex metal hydrides have attracted significant attention for hydrogen storage applications due to their impressive gravimetric and volumetric capacities. These materials offer a highly promising solution for effective hydrogen storage, which is essential for a wide range of energy systems, such as fuel cells and renewable energy storage [101]. However, they face challenges like operating at high temperatures and pressures, slow reaction rates, and material stability issues. Researchers are investigating different complex hydride systems, such as those containing Li, Na, Ca, borohydrides, and amides/imides, to enhance hydrogen storage capabilities [104]. Recent advancements in material design, synthesis, and modeling have provided valuable insights into improving the reversible hydrogen release and uptake mechanisms of complex hydrides, paving the way for broader applications in the future [111].

3.1.2. Ammonia (NH₃)

Ammonia is an effective hydrogen storage and transportation medium due to its high hydrogen storage capacity (17.6 wt%) and a high volumetric energy density (108 kg H₂/m³), making it a promising zero-carbon energy carrier for large-scale energy storage [114]. It offers stability, reliability, operation at mild temperatures, and low storage pressure [115]. Additionally, ammonia is a promising alternative to traditional fuels due to its lack of CO₂ emissions. Ammonia has a higher volumetric energy density (10.5 MJ/L) compared to compressed H₂ (4.5 MJ/L) and liquid H2 (8.5 MJ/L) [116], with an estimated cost of $700–$1500 per kg of H₂ [117], making it a suitable energy carrier for hydrogen storage and transport.

NH₃ can be produced from various energy sources and used directly or decomposed into H₂ and N₂ for energy applications, providing high-purity H₂ without a carbon footprint, unlike hydrocarbon-based carriers like methanol and methane, which release CO₂. As the second most produced chemical globally, ammonia benefits from a well-established infrastructure for synthesis, transportation, and distribution. Ammonia can be combined with water and stored as a liquid under ambient temperature and pressure conditions, taking advantage of advanced production technologies and an efficient distribution network. Additionally, ammonia can be easily decomposed catalytically, offering high densities and efficient release, making it a promising candidate for hydrogen storage in liquid form. Ammonia’s non-flammability and high hydrogen density further enhance its suitability as a hydrogen carrier. However, challenges remain, including its toxicity, the presence of trace ammonia in hydrogen after decomposition, and the high-temperature requirement for the release (dehydrogenation) process in larger storage plants. Furthermore, ammonia’s endothermic decomposition requires elevated temperatures and the use of noble catalysts, posing additional obstacles for practical applications and hydrogen storage systems for vehicles onboard [115,118].

3.1.3. Liquid Organic Hydrogen Carrier (LOHCs)

Liquid organic hydrogen carriers provide a novel approach to storing hydrogen with potential advantages such as stability, high hydrogen content, and superior hydrogen storage capacity. Methyl cyclohexane, N-ethylcarbazole (NEC), and toluene [119], are considered highly promising as LOHCs for hydrogen storage, with typical storage densities ranging from 5–6 wt%, and use organic compounds to safely store hydrogen through covalent bonds and enable efficient hydrogen release [120]. LOHC systems operate through a reversible chemical reaction in which hydrogen interacts with organic compounds under the influence of catalysts. Organic liquid hydrogen storage encompasses two main processes, the dehydrogenation of hydrogen-rich compounds and the hydrogenation of hydrogen-deficient compounds [121]. Figure 9, illustrates the core principle underlying the reactions in LOHC technology. Hydrogen-deficient molecules can be classified into two categories, those originating from atmospheric or exhaust gas mixtures, such as methanol, ammonia, and formic acid, known as “circular” hydrogen carriers, and LOHCs [122].

Various types of hydrogen carriers such as toluene, dibenzyl toluene [123], biphenyl/bicyclohexyl, and N-ethylcarbazole (NEC) can be used as LOHCs. The comparison between dibenzyltoluene and N-ethylcarbazole (NEC) highlights significant differences in their suitability for hydrogen storage, particularly in fuel cell vehicles. While dibenzyltoluene has a high dehydrogenation temperature, making it less favorable for onboard systems [124]. NEC exhibits a hydrogen storage capacity of 5.8 wt% which is advantageous for practical applications [125]. The presence of nitrogen in NEC also influences its hydrogenation process, potentially mitigating the effects of impurity gases. In addition, 9-ethylcarbazole shows effective hydrogen storage performance, as it can theoretically store 6 moles of hydrogen (5.7 wt%) [124]. Also, Biphenyl and diphenylmethane are popular hydrogen carriers due to their high capacity, stability, and cost-effectiveness. However, their solid state in ambient conditions limits their suitability for hydrogen storage applications. The hydrogenation of 9-ethylcarbazole (9-ECZ) is a critical process for hydrogen storage in fuel cells, typically facilitated by noble metal catalysts, with ruthenium being the most prominent. Efficient hydrogenation of molten 9-ECZ to produce 9-ethyl-perhydrocarbazole is essential for advancing reversible hydrogen storage systems based on 9-ECZ [124]. These carriers release it through dehydrogenation (an endothermic reaction), and capture hydrogen through hydrogenation (an exothermic process where the storage liquid is mixed with hydrogen in a reactor), with enthalpy changes playing a critical role in these reactions. The system is then heated to a specific temperature with the aid of a catalyst, enabling the formation of the saturated hydride [126]. Compounds such as dibenzyl toluene and N-ethylcarbazole have undergone extensive study for their hydrogen storage capabilities. Recent research has shifted towards electrochemical LOHCs, which are considered more efficient and practical, utilizing substances like isopropanol and organic acids for hydrogen storage. Optimization studies on N-ethylcarbazole have shown promising results for commercial applications, achieving high hydrogenation and dehydrogenation efficiencies under optimal conditions. These advancements highlight the potential of LOHCs to revolutionize hydrogen storage for energy applications. Therefore, Liquid Organic Hydrogen Carriers (LOHCs) with 5–8 wt% hydrogen content, reversible hydrogenation, moderate dehydrogenation temperatures, and alignment with the current gasoline system are promising for onboard and large-scale hydrogen transport, leveraging their practicality and commercial availability [127,128].

Liquid Organic Hydrogen Carriers are an advanced hydrogen storage technology that utilizes reversible chemical processes to store and release hydrogen. Through hydrogenation, hydrogen molecules are chemically bound to LOHC compounds, significantly enhancing gravimetric and volumetric storage capacities. When needed, hydrogen is released via a dehydrogenation process. Ideal LOHCs are liquid at ambient conditions, safe, cost-effective, and non-toxic, simplifying handling and transportation. Dehydrogenation of LOHCs is normally carried out at temperatures greater than hydrogenation with materials such as perhydro-dibenzyltoluene usually dehydrogenating around a temperature of about 300 °C, while hydrogenation occurs at lower temperatures (Decreasing the dehydrogenation temperature makes it feasible to integrate with low-temperature waste heat and makes energy efficiency better. Research suggests that approaches such as low-pressure dehydrogenation and reactive distillation can facilitate hydrogen release at reduced temperatures, improving the practicality of LOHC systems for sustainable hydrogen storage. For instance; N-Ethylcarbazole (NEC) demonstrated a potential liquid organic hydrogen carrier (LOHC) with characteristics of lowered hydrogen storage cycle temperature and acceptable hydrogen storage capacity as compared to benzene, toluene, etc. [129]. LOHCs offer a robust solution for long-term energy storage, eliminating challenges such as leakage, boil-off, or hydrogen losses, making them a promising technology for sustainable energy systems [130].

The primary advantage of LOHC-based hydrogen storage over solid carrier materials is their liquid state, which facilitates easier handling similar to fuel and its high volumetric density. The liquid nature allows for pumping and controlled hydrogen release by removing the catalyst, with superior kinetics compared to metal hydrides. The drawbacks of Liquid Organic Hydrogen Carriers include high energy consumption for dehydrogenation, as the process is endothermic and requires significant energy input. Additionally, developing efficient dehydrogenation catalysts remains challenging, as catalysts must balance activity, stability, and cost. Furthermore, LOHC systems often suffer from a short cycle life due to the degradation of carrier molecules and catalysts over repeated hydrogenation and dehydrogenation cycles, limiting their long-term efficiency and increasing maintenance costs. These issues necessitate ongoing research to enhance the performance and sustainability of LOHC systems [131]. In addition, for commercial applications of liquid organic hydrogen carriers to be viable, it is crucial to substitute expensive noble metal catalysts with cost-effective yet equally efficient alternatives.

3.2. Physisorption

Physisorption is a process in which hydrogen molecules are weakly adsorbed onto the surface of materials through van der Waals forces, without changing their molecular identity. This technique allows for the retention of hydrogen’s molecular form, which improves the kinetics of storage, offers high rates of adsorption, and refueling time is not an issue [132]. The most widely studied materials for physisorption-based hydrogen storage include porous structures such as carbon-based materials (e.g., graphene, fullerenes, nanotubes), zeolites, covalent organic frameworks, and metal-organic frameworks. These materials are favored for their tunable pore structures, high surface areas, and the ability to store hydrogen efficiently at relatively low pressures and temperatures [133]. For instance, Zhou et al. [134] demonstrated that activated carbons hold 5 wt% hydrogens at pressures of 3–6 MPa and a temperature of −196 °C. In zeolites, hydrogen molecules are absorbed into the pores of the molecular sieve at higher temperatures and pressures, which are influenced by the varying pore architecture and composition. Upon cooling the zeolites to room temperature, hydrogen is retained within these cavities, and its release can be induced by increasing the temperature of the system. The key to improving the performance of these materials lies in optimizing their surface properties to maximize hydrogen uptake while maintaining the integrity of the molecular hydrogen during adsorption and desorption processes.

3.2.1. Metal Organic Frameworks (MOFs)

A Metal-organic frameworks (MOFs) are considered a promising class of materials for hydrogen storage [135]. It represents a novel category of porous materials with a modular structure, consisting of inorganic clusters as nodes connected by multifunctional organic linkers. MOFs can be synthesized by combining a metal oxide core or ion with a linker. These structures have gained significant interest for hydrogen storage, owing to their large surface areas, affordability, adjustable pore size, and substantial pore volumes [136].

Metal-organic frameworks are highly effective for gas storage, separation, and catalysis due to their porous nature. They offer the largest surface area among all hydrogen storage materials [137]. For instance, MOF210 has demonstrated an experimentally measured surface area of 6240 m²/g, the highest recorded thus far, with a hydrogen capacity of 7.9 wt% at −196 °C and 8 MPa [138]. Also, Isoreticular (IR) MOF series, have shown promising hydrogen storage capabilities [139]. Langmi et al. [140] conducted an extensive study on the hydrogen storage capabilities of MOFs, noting that their hydrogen capacities at ambient temperature generally remain under 1 wt%. The adsorption of hydrogen onto MOFs occurs through physical adsorption, where hydrogen molecules are captured in the pores of the framework by weak van der Waals forces. The performance of MOFs is highly influenced by pressure and temperature, with reduced temperatures and higher pressures enhancing their hydrogen storage capacity [141].

Challenges associated with physical-based hydrogen storage, such as maintaining liquid hydrogen at low temperatures or storing it as a gas under high pressures both of which increase the risk of leakage can be mitigated by utilizing MOFs for hydrogen storage through chemisorption or physisorption. In physisorption, hydrogen molecules adhere to the surfaces of materials like activated carbon or MOFs, forming weak bonds with metal centers or unsaturated sites via van der Waals forces and dipole interactions. The high electron density in MOFs induces temporary dipoles in hydrogen, enabling these weak interactions. The effectiveness of hydrogen storage in MOFs is dependent on factors such as temperature, pressure, and the specific structure of the MOF, with reversible adsorption and desorption processes making them viable for hydrogen storage applications. In contrast, chemisorption involves stronger chemical bonds typically covalent or ionic between the adsorbate and the surface [142]. Figure 10 demonstrates the fundamental mechanism of hydrogen desorption and adsorption in MOFs [135].

These highly crystalline, porous hybrid materials have an extensive network structure, distinct from zeolites, molecular sieves, and activated carbons. They consist of organic bridging ligands that link rigid organic molecules, coordinated by metal ions or clusters, making them ideal for precise gas adsorption and developing effective adsorbents. The high porosity of MOFs makes them appropriate for several applications, such as hydrogen storage for fuel cell-powered vehicles and other sustainable processes [143].

In a metal-organic framework, a central metal atom and organic ligands form a three-dimensional network with well-defined pores through covalent bonding [144]. In MOFs, stability is determined by atomic-level bonding between the inorganic framework and organic ligands, as shown in Figure 11. The figure illustrates metal-organic framework (MOF) concepts. In (a), a layered MOF structure with an ordered arrangement of organic and inorganic components is depicted, while in (b), a three-dimensional MOF with an interconnected network showcases potential structural stability and porosity. (c) presents a dense and highly symmetric framework, suggesting strong intermolecular interactions and structural rigidity. (d) features a highly porous MOF with an open network, likely optimized for adsorption or gas storage applications. These variations demonstrate the diverse structural possibilities in MOF design, each tailored for specific experimental synthesis approaches and functional applications. MOFs are effective for removing hazardous gases like CO₂ from the atmosphere due to their efficient gas adsorption and storage capabilities [145,146]. Thus, it contributes to the hydrogen economy by enhancing the sustainability of hydrogen storage and reducing carbon emissions. Their integration into storage systems supports a cleaner energy transition, improving efficiency and environmental impact [147]. MOFs generally have lower gravimetric hydrogen storage capacities than metal hydrides. However, they offset this limitation with their large surface areas and customizable porosity, which significantly improve their volumetric storage capacity. Furthermore, MOFs function at lower temperatures and pressures during hydrogen adsorption and desorption, enhancing their practicality for specific applications [142].

Advantages and Challenges of Metal-Organic Frameworks (MOFs) for Hydrogen Storage

The key advantage of MOFs lies in their nanoscale porosity, large specific surface area, and structural diversity, making them highly efficient for gas storage applications with enhanced storage capacities [148]. MOFs exhibit promising gravimetric hydrogen capacities at extremely low temperatures near −196.15 °C, this requires costly refrigeration systems and well-insulated containers to maintain extremely low temperatures. However, their storage capacity diminishes at ambient temperatures due to low hydrogen enthalpy and the relatively high system weight compared to fossil fuels, operational challenges arise from high-pressure requirements, necessitating the use of advanced pressure vessels for hydrogen storage. While powder densification can increase surface area, it often compromises MOF stability, operational challenges arise from high-pressure requirements, necessitating the use of advanced pressure vessels for hydrogen storage. While powder densification can increase surface area, it often compromises MOF stability [149].

MOF-based materials are promising for hydrogen generation and storage, but their properties and storage capacities are significantly influenced by synthesis techniques. Furthermore, the hydrogen uptake capacity is directly influenced by the applied pressure, which in turn increases the costs of storage tanks. Heat dissipation during onboard hydrogen fueling also presents considerable challenges. The hydrogen adsorption capacity of existing materials remains insufficient to meet the standards set by the U.S. Department of Energy [135]. Future research should focus on improving the MOFs for hydrogen handling at ambient temperatures, as they currently demonstrate limited hydrogen uptake. Additionally, there is a need for comprehensive cost analysis and economic evaluations of various hydrogen storage methods. A key area of future research should involve enhancing the properties of MOFs and investigating novel materials for more efficient hydrogen storage.

3.2.2. Carbon Nanotubes (CNs)

Carbon nanomaterials (CNs) are highly effective gas adsorbents, characterized by their porous structure, fine powder form, and specific interactions between corresponding gas molecules and carbon atoms. This process is referred to as physisorption. These materials include microscopic carbon nanotubes with a thickness of approximately two nanometers, capable of storing hydrogen within their microscopic pores or tubular structures. Nanotubes feature single or multi-walled structures, a high packing density, and multiple adsorption sites with an estimated hydrogen storage capacity of approximately 6 wt% [150]. Storing hydrogen in carbon nanotubes offers a lightweight, compact solution for mobile applications. But faces challenges in terms of cost and volumetric efficiency [151]. Figure 12 reveals the schematic illustration of hydrogen storage within carbon nanotubes [53].

The application of carbon nanotubes for hydrogen storage offers significant advantages for portable and mobile energy applications due to their lightweight nature and compact storage potential. However, challenges remain in achieving practical volumetric storage densities and addressing economic concerns related to material synthesis and scalability [151]. Physical adsorbents for hydrogen exhibit low storage capacity at ambient temperature, making them viable only at cryogenic temperatures or elevated pressures. At cryogenic temperatures, desorption losses, akin to boil-off losses in liquid hydrogen storage can occur. Due to these limitations, physical adsorbents are considered unsuitable for hydrogen storage applications. Current research focuses on overcoming these limitations through advancements in nanomaterial engineering, such as creating hybrid carbo nano-based composites or integrating carbon nanotubes with other porous frameworks like MOFs [50].

4. Underground Hydrogen Storage

Underground hydrogen storage represents a promising technology with substantial potential for large-scale hydrogen energy storage due to its high storage capacity. Hydrogen gas, due to its substantial volume under standard atmospheric conditions, requires high-pressure or cryogenic storage methods to effectively address energy demands. In this context, underground hydrogen storage is regarded as an efficient and promising means of storing hydrogen energy in various geological formations and engineered facilities, as illustrated in Figure 13. The distinct attributes of hydrogen, such as its rapid diffusion, influence its behavior in geological formations [152].

Underground hydrogen storage provides significant advantages, it enhances safety by reducing fire and attack risks, has a relatively low risk of leaks, optimizes space use, is a cost-effective, simple technology, has rapid filling and discharging capabilities, and utilizes widely available geological formations [153]. Underground hydrogen storage aims to utilize natural subsurface formations, including porous rock structures such as depleted natural gas or oil reservoirs, aquifers, and salt caverns, for large-scale hydrogen containment [154]. Refer to Figure 13, which illustrates a range of underground hydrogen storage options, including depleted oil, salt caverns, gas reservoirs, and saline aquifers. However, successful UHS implementation requires suitable geological storage sites, specific geological structures, and ample space to address safety risks, economic considerations, legal requirements, and various technical challenges [155]. Some selected sites for hydrogen storage around the globe for all underground storage are found in the USA [156], Canada [157], Spain and Germany [158], Turkey, United Kingdom [159], Denmark [160], and China [161]. These sites have successfully implemented underground hydrogen storage geological investigations, assurance of long-term safety, evaluation of storage capacity, and leveraging their geological conditions and existing infrastructure to support hydrogen as a key energy carrier. One notable limitation is that the volume density of hydrogen gas does not rise proportionally with pressure, unlike physical-based storage methods. Additionally, studies suggest that hydrogen can react with organic-rich shale, potentially producing methane during storage [162]. Using hydrocarbon gas as cushion gas in UHS can reduce cost, however, numerical simulations study by Zhao et al. [163] indicate the importance of considering molecular diffusion within the reservoir for accurate predictions of hydrogen recovery and methane contamination. Factors like reservoir permeability, heterogeneity, and diffusion coefficients play vital roles in determining the purity of hydrogen production and the efficiency of UHS. Understanding these aspects is crucial for optimizing UHS operations and ensuring the cost-effectiveness of large-scale hydrogen storage schemes.

Over the past decade, underground hydrogen storage, either in its pure form or blended within gas mixtures, has gained recognition as a promising approach to decreasing dependence on fossil fuels, offering negligible or nearly zero greenhouse gas emissions [164]. This approach entails storing high-pressure hydrogen within underground geological formations, including depleted gas and oil reservoirs, mines, and caverns, capitalizing on the cost-efficiency of existing infrastructure and facilitating seamless integration with pipelines. Efficient underground hydrogen storage (UHS) technology is vital for the effective large-scale application of hydrogen energy. UHS allows the storage of megatons of hydrogen for lengthy periods, needs minimal surface space, and naturally isolates hydrogen from oxygen, making it a promising solution for energy storage. Underground hydrogen storage utilizes depleted gas fields and geological formations for hydrogen storage, where molecular diffusion is pivotal in enhancing storage efficiency and facilitating hydrogen recovery [165]. China envisions future hydrogen storage systems that combine solid-state and salt-cavern storage methods, emphasizing economic and geographical factors for deploying underground hydrogen storage.

Deep geological formations, such as depleted hydrocarbon reservoirs, aquifers, and salt caverns, offer significant potential for large-scale hydrogen storage. However, the availability of suitable geological sites limits the widespread use of these storage options [153]. The challenge with underground hydrogen storage lies in maintaining purity, as losses may result from reactions with minerals, microbial activity, or entrapment in pore-scale capillaries. Careful site investigation and selection, including avoiding elements like iron and sulfur, are essential to prevent structural compromise during injection and withdrawal, even though salt caverns provide a promising solution because of their chemical resistance to hydrogen and stability under varying pressures. Underground storage sites are categorized into conventional options, including aquifers, salt caverns, and depleted hydrocarbon reservoirs, and unconventional options, such as lined hard rock caverns, refrigerated mined caverns, and abandoned coal mines. Currently, the three types of subsurface formations suitable for large-scale gas storage are salt caverns, depleted oil and gas reservoirs, and deep saline aquifers [166,167].

4.1. Salt Caverns

Salt caverns are widely regarded as the most employed geological formations for underground hydrogen storage, thereby contributing to future renewable energy goals. These caverns, created by dissolving salt deposits with water, are ideal for the long-term storage of hydrogen. In comparison to other underground storage alternatives, such as aquifers or depleted oil and gas fields, salt caverns offer lower maintenance and operational expenses [168]. According to their geological structure, salt caverns can be classified into two types, bedded salt caverns and domal salt caverns. Domal salt caverns consist of a unified cavity within a thick rock salt layer. On the other hand, bedded salt caverns have discontinuous rock salt layers, so the cavities are formed solely within these layers, leading to a segmented cavity. The internal composition, especially in domes, is complicated. Therefore, it is crucial to thoroughly examine the internal structure of salt caverns to identify a salt massif that is adequately wide, thick, and uniform for the development of saline caverns [169].

Salt caverns are influenced by geological factors like the depth of the cavern, location, salt deposit structure, and mineralogy, leading to diverse shapes and dimensions globally. The estimation of storage capacity is a critical aspect of designing salt caverns, incorporating parameters such as cavern size, depth, shape, and the amount of working gas required to sustain supply-demand balance. Ensuring their integrity and stability is essential for the safe storage of hydrogen [170]. An average salt cavern can extend up to 2000 m deep, with a volume of 1,000,000 cubic meters, a height varying from 300 to 500 m, and a diameter ranging between 50 and 100 m, providing extensive storage capacity [171]. Salt caverns are crucial hydrogen storage methods due to their excellent sealing strength, low gas permeability (0.15 × 10⁻⁵ mD), large volumetric capacities, favorable creep properties, low porosity (in the order of 1%), as illustrated in Figure 14, good rheology, low cushion gas requirement, advantageous mechanical characteristics, and the beneficial properties of rock salt, which ensure efficient and safe storage even at elevated pressures, thus providing the most effective solution for hydrogen storage and preventing leakage [172].

Currently, many international projects (particularly four international projects) in the USA and the UK are actively storing pure hydrogen (with 95% purity) in underground structures, all of which utilize salt caverns as their preferred storage method [173]. This underscores the potential of salt caverns for underground hydrogen storage. Figure 15, highlights ongoing and prospective underground hydrogen storage initiatives globally, encompassing aquifers, depleted reservoirs, and salt caverns.

4.2. Saline Aquifer Storage

Storing hydrogen gas in subsurface saline aquifers presents significant potential by utilizing available pore spaces with minimal environmental impact. However, this method is not as well-established as salt cavern storage or depleted gas reservoirs. An aquifer, typically an underground layer of porous, permeable rock containing either fresh or saline water and often extending hundreds of feet in depth, is commonly found worldwide and is considered a feasible option for underground hydrogen storage. Hydrogen gas can be stored in aquifers by displacing the saline water within them. Saline aquifers are crucial for underground hydrogen storage due to their potential for large storage capacities and cost-effectiveness. Compared to depleted hydrocarbon reservoirs, saline aquifers offer substantial advantages in terms of geographical flexibility and storage volume [173,174].

Aquifers require a larger volume of cushion gas compared to depleted hydrocarbon reservoirs. One major challenge of hydrogen storage in aquifers is the extraction process, as the coexistence of liquid and gas phases can complicate production. Despite this, saline aquifers hold significant promise for large-scale energy storage. The research highlights the importance of considering regional conditions and site-specific factors to minimize economic and safety risks in hydrogen storage operations. Thorough site surveys are essential to identify and evaluate suitable geological storage sites. Key parameters for optimal storage include porosity, permeability, brine salinity, and pH [174].

4.3. Depleted Gas Reservoirs

Depleted gas reservoirs are crucial for underground hydrogen storage (UHS), especially when considering biogeochemical factors. Their suitability for hydrogen storage is due to features such as high porosity, permeability, and large storage capacities. However, the potential for mixing with residual fluids and their limited availability necessitates careful consideration of alternative options [175]. Depleted porous reservoirs, particularly depleted gas reservoirs, offer significantly higher storage capacity and operational efficiency compared to other UHS options like aquifers and salt caverns. Their proven storage integrity, established through previous natural gas storage operations, ensures long-term reliability for hydrogen storage.

Table 4. The capacity of hydrogen uptake and the catalysts used for hydrogenation and dehydrogenation of Common LOHCs systems.

Hydrogen Storage Agents	Hydrogen Carrier	Hydrogen Storage Capacity (wt%)/(kg/m3)	Dehydrogenation Temperature/°C
N-ethycarbazole [164,165,176]	Dodecahydro-N-ethylcarbazole	5.8/-	170–200
Toluene [177]	Methylcyclohexane	6.2/47.4	300–350
Dibenzytoluene [178,179]	Perhydro-dibenzytoluene	6.2/57	260/310
Benzene [173,180]	Cyclohexane	7.2/55.9	300–320
Biphenyl [180]	Bicyclohexyl	7.27/-	310–330
Carbazole [87]	Dodecahydro-carbazole	6.7/-	150–170

Table 4. The capacity of hydrogen uptake and the catalysts used for hydrogenation and dehydrogenation of Common LOHCs systems.

Hydrogen Storage Agents	Hydrogen Carrier	Hydrogen Storage Capacity (wt%)/(kg/m3)	Dehydrogenation Temperature/°C
N-ethycarbazole [164,165,176]	Dodecahydro-N-ethylcarbazole	5.8/-	170–200
Toluene [177]	Methylcyclohexane	6.2/47.4	300–350
Dibenzytoluene [178,179]	Perhydro-dibenzytoluene	6.2/57	260/310
Benzene [173,180]	Cyclohexane	7.2/55.9	300–320
Biphenyl [180]	Bicyclohexyl	7.27/-	310–330
Carbazole [87]	Dodecahydro-carbazole	6.7/-	150–170

One key advantage of depleted gas reservoirs is the presence of residual gas, which reduces the need for large amounts of cushion gas to maintain pressure during the injection and extraction processes. Typically, only 50–60% cushion gas is required for gas reservoirs, compared to 80% for aquifers, making gas reservoirs more cost-effective and efficient for large-scale hydrogen storage while optimizing space and reducing operational costs.

Aquifer and depleted hydrocarbon storage are ideal for seasonal gas storage due to their flexibility in cycling. Both types of reservoirs can accommodate multiple injection and extraction cycles, making them well-suited for hydrogen storage. The reduced need for cushion gas, particularly in depleted gas reservoirs, enhances their cost-effectiveness and reliability, providing an efficient solution for balancing seasonal energy supply and demand [175].

Figure 15. Schematic worldwide UHS sites. Reproduced with permission from ref. [175], Copyright (2024), Elsevier.

Figure 15. Schematic worldwide UHS sites. Reproduced with permission from ref. [175], Copyright (2024), Elsevier.

The comparison of different hydrogen storage technologies are listed in

Table 5. Comparison of different hydrogen storage technologies.

Hydrogen Storage Method (Source)	Advantages	Disadvantages/Challenges	Application Area
Compressed hydrogen storage [178]	Mature technology, fast and reliable refueling process, technology simplicity, low energy consumption compared to liquefied hydrogen storage methods, simple.	High-pressure requirements, volumetrically and gravimetrically inefficient, space inefficiency, gas leakage (safety risks), small quantity storage, energy consumption.	Common cylinders, stationary, and mobile applications, aircraft, lightweight, high pressure hydrogen storage tanks.
Liquefied hydrogen storage [179]	High volumetric densities, lower storage pressure, fast kinetics, fast refueling process, reliable, safe.	High energy consumption in the liquefaction process boil-off phenomena, tank cost, complex equipment, ultra-low temperatures, high vessel insulation requirements.	Space exploration, long-distance transport, rocket cryogenic propulsion.
Cryo-compressed hydrogen gas storage [87]	Fast filling, long dormancy, offer medium to high gravimetric and volumetric capacities, a long non-emission time, exhibit excellent heat resistance, high safety factor.	Temperature rise during the filling process causes risks such as diffusion, deflagration, cost, detonation.	Heavy-duty vehicles, onboard (fuel cell driven bus), stationary storage solutions backup power systems.
Solid-state hydrogen storage [180]	Simple and safe process, high reliability, Low pressure, volumetric efficiency, ideal storage density	Immature technology, high energy requirements during adsorption, slow hydrogen sorption and desorption kinetics, high operating temperatures, high materials requirement, small quantity storage, cost	Used in hydrogen fuel cell vehicles (FCVs), small-scale transportation, stationary power systems
Underground hydrogen storage [169]	Exhibited the lowest storage cost than any storage methods, large storage capacity, enables higher storage pressures, enhances safety protocols, stability	Keeping purity, site selection	Long-term storage

. The research by Chen et al. [176] showed that a depleted gas reservoir is the most cost-effective option among the three geological storage alternatives. The need for cushion gas can be minimized by using the residual gas within the reservoir as cushion gas. However, designing hydrogen geologic storage requires careful consideration of several issues. The entire storage process, including storage longevity, withdrawal rate, and injectivity is significantly influenced by the heterogeneity of reservoir rock mineralogy and the wettability of rocks [177]. Furthermore, the presence of microorganisms may cause hydrogen to be consumed and produce other gases, lowering the purity and quantity of stored hydrogen, as shown in Figure 16 [177].

5. Advantages and Challenges of Hydrogen as an Energy Vector

5.1. Advantages

Hydrogen is seen as an energy carrier that is both environmentally beneficial and promotes sustainability, viewed as a substitute for fossil fuels (oil, natural gas, coal). Many possible uses for hydrogen have led to its extensive use. According to the literature review, the identified main advantages of hydrogen energy vector and challenges related to hydrogen infrastructure, particularly storage, have been outlined. Below are the primary advantages:

• Hydrogen offers an alternative to fossil fuels and serves as a fuel in various vehicles, replacing conventional energy sources in automobiles trucks, ships, and rockets.
• Hydrogen energy vector bridges the gap with renewable energy sources, which are often intermittent and lack sustainability, and fossil fuels, which contribute to climate change on a global scale and have negative impacts on health.
• Hydrogen possesses significant potential to expedite the transition toward more environmentally friendly and sustainable energy solutions.
• Hydrogen finds utility in powering vehicles, generating electricity, and serving various industrial purposes. These include the production of methanol and ammonia, steel manufacturing, metal treatment process, and fertilizer production.
• Hydrogen generated from renewable energy sources provides a sustainable option for reducing greenhouse gas emissions due to its high energy content, surpassing other energy sources like ethanol, methanol, diesel, gasoline, and propane.

5.2. Challenges

In addition to the numerous benefits of hydrogen, several potential challenges necessitate attention and resolve:

• There is a significant challenge in ensuring safe, economical, robust, compact, and reliable hydrogen storage methods. This is primarily attributed to hydrogen’s distinctive physical properties and the requirement to store substantial quantities to manage energy demand and supply.
• The storage of hydrogen at its utilization site could potentially result in energy inefficiency due to the fact that hydrogen’s low volumetric energy density does not currently meet the required standards set by the United States Department of Energy (DOE) 2030.
• Currently, most hydrogen is produced from natural gas, coal, and other fossil fuels, contributing to increased carbon dioxide levels in the atmosphere. Therefore, it is essential to generate hydrogen using electricity from renewable sources to reduce environmental harm.
• Utilizing hydrogen for onboard vehicles poses substantial challenges, primarily because of the large volume, weight, extremely low temperature, high pressures, and cost of hydrogen (especially for storage methods providing high gravimetric and volumetric density, like liquefied, and cryo-compressed hydrogen storage). These factors severely limit the feasibility of hydrogen-powered vehicles. Addressing these issues requires the development of new and innovative materials capable of tackling these challenges.
• The main challenge in developing material-based hydrogen storage is to create cost-effective options that offer high hydrogen density both by volume and mass. This is due to the characteristic properties of hydrogen, such as its low molecular size, low volumetric energy density, high flammability, low molecular density, high diffusivity, and reactivity. Among the materials-based storage, liquid organic hydrogen carriers and metal hydrides are two hydrogen storage reliant on materials technologies that offer exciting qualities, making them suitable for certain applications, even in storage at a large scale. (LOHCs) provide the most promising means for long-duration and safe hydrogen storage using reversible chemical reactions driven by appropriate catalysts. It has been demonstrated that liquid organic hydrogen carriers offer the most affordable choice for large-volume, long-distance transport. However, LOHC systems face significant challenges, particularly their limited cycle life due to the degradation of carrier molecules and catalysts over repeated hydrogenation and dehydrogenation cycles. This degradation reduces long-term efficiency and increases maintenance costs. In addition, LOHCs need significant energy input during dehydrogenation since the reaction is endothermic, which further affects overall efficiency. These problems need to be addressed through continued research that can enhance the performance, sustainability, and economic feasibility of LOHC-based hydrogen storage systems.

6. Conclusions and Outlook

This paper provides an overview of the hydrogen energy vector, focusing specifically on hydrogen storage methods, their benefits, obstacles, and prospective paths for further research. The key insights from this review are summarized below.

• Hydrogen has become extensively recognized as a highly promising clean energy source due to its potential as an efficient energy carrier, renewable nature, environmental friendliness, ease of production, abundance, cleanliness, high utilization rate, and sustainability. It is viewed as a pivotal solution for securing future energy needs and fostering global economic stability. The increasing global demand for hydrogen as an energy carrier, driven by the vision of a robust hydrogen economy, is essential for expediting the transition to a carbon dioxide-free global economy and achieving net-zero carbon emissions by 2050. However, a significant challenge lies in hydrogen storage methods. Presently, there is a lack of effective and efficient techniques applicable across all sectors, including transportation and industries.
• Hydrogen has significant potential to expedite the transition to a carbon-neutral, cleaner, and greener economy with a goal to achieve net-zero carbon dioxide emissions by 2050. That can be made possible only if utmost priority is given to producing green hydrogen from water electrolysis by renewable sources, and that makes it an adaptable and versatile carbon-free energy carrier.
• Currently, compressed gaseous cylinders, particularly Type III and Type IV cylinders, are the most widely accepted hydrogen storage technology for onboard applications compared to other methods. It is also, used for stationary storage of hydrogen energy.
• Liquid hydrogen storage faces challenges in maintaining cryogenic temperatures, minimizing boil-off losses, and preventing heat transfer. Effective insulation and vacuum maintenance are difficult over long periods. Materials need to maintain their mechanical properties at low temperatures, resist corrosion, and handle pressure variations. Common materials like stainless steel, aluminum, and nickel alloys come with tradeoffs in terms of weight, cost, and strength. A significant challenge lies in developing cost-effective, high-performance materials. Active thermal protection systems for zero boil-off storage are expensive and primarily used in space missions. The development of large-scale liquid hydrogen (LH₂) infrastructure is still limited, especially in the civilian sector. However, innovations in insulation materials, materials science, and thermal protection technologies, such as cryogenic heat pipes and heat exchangers, offer promising solutions to enhance storage efficiency and feasibility.
• Advancements in liquefied hydrogen storage and cryo-compressed hydrogen storage are underway to facilitate global medium-scale hydrogen storage by addressing slow refueling, evaporation, and high energy consumption issues. Resolving technical challenges associated with storing liquefied hydrogen (which consumes a significant amount of energy) and cryo-compressed hydrogen storage (a tank with elevated pressure) could lead to the broad implementation of this technology in the future.
• Underground hydrogen storage has been found to be a promising solution for large-scale hydrogen storage. However, successful implementation and utilization of them require suitable geological storage sites, specific geological structures, and ample space to address safety risks, economic considerations, legal requirements, and various technical challenges. The selection of an appropriate site is critical for the successful operation of underground hydrogen storage (UHS) at geological sites. It is considered the foremost challenge that must be tackled at the outset of any UHS design project.
• All hydrogen storage methods come with their limitations, hindering the wider adoption of hydrogen energy as a fossil fuel alternative and impeding progress toward global carbon emission reduction goals. Therefore, considering the demands of modern society and the emerging challenges, developing a novel, environmentally friendly, and cost-effective hydrogen storage system is crucial for the future hydrogen economy.
• Furthermore, the study should focus on the development of new materials that can store hydrogen at high volumetric and gravimetric densities, resist microcracks, avoid volatile component loss, and retain both stiffness and ductility in every sector condition used for tank construction in order to overcome challenges and barriers with physical-based hydrogen storage systems and pave the way for more efficient and sustainable energy solutions.