Recent Progress Using Solid-State Materials for Hydrogen Storage: A Short Review

: With the rapid growth in demand for effective and renewable energy, the hydrogen era has begun. To meet commercial requirements, efﬁcient hydrogen storage techniques are required. So far, four techniques have been suggested for hydrogen storage: compressed storage, hydrogen liquefaction, chemical absorption, and physical adsorption. Currently, high-pressure compressed tanks are used in the industry; however, certain limitations such as high costs, safety concerns, undesirable amounts of occupied space, and low storage capacities are still challenges. Physical hydrogen adsorption is one of the most promising techniques; it uses porous adsorbents, which have material beneﬁts such as low costs, high storage densities, and fast charging–discharging kinetics. During adsorption on material surfaces, hydrogen molecules weakly adsorb at the surface of adsorbents via long-range dispersion forces. The largest challenge in the hydrogen era is the development of progressive materials for efﬁcient hydrogen storage. In designing efﬁcient adsorbents, understanding interfacial interactions between hydrogen molecules and porous material surfaces is important. In this review, we brieﬂy summarize a hydrogen storage technique based on US DOE classiﬁcations and examine hydrogen storage targets for feasible commercialization. We also address recent trends in the development of hydrogen storage materials. Lastly, we propose spillover mechanisms for efﬁcient hydrogen storage using solid-state adsorbents.


Introduction
Recently, global warming issues surrounding growing populations and increases in the use of fossil fuels compel the replacement of fossil fuels for alternative resources [1][2][3]. Hydrogen is one of the most promising alternative energy sources because of its high energy efficiency, environmental friendliness, and non-toxicity [4][5][6].
The US DOE has announced annual technical targets that it requires to be met for the realistic adoption and expansion of a hydrogen-based society as shown Figure 1 [7][8][9][10]. From the latest study of the annual plan in 2017, We summarize in Table 1 certain important technical targets from the latest study of the annual plan in 2017. Hydrogen storage applications, storage capacity levels, durability/operability, charging-discharging rate, dormancy, and safety are the primary measurements [11,12]. In the capacity section, targets are set as hydrogen uptake per absolute weight, volume, and cost, all of which are based on their conformity with commercialization. Ultimate targets aligned with the current storage system propose the capability of a minimum of 500 miles, which is the general distance capacity for the light-duty vehicle market in the USA. The resulting capacity target is 6.5 wt%, 2.2 kWh/kg, and 0.065 kg H 2 /kg system as the gravimetric capacity and 1.7 kWh/L and 0.050 kg H 2 /L system as the volumetric capacity. The cost Figure 1. The US DOE targets for hydrogen storage technology. Reproduced from ref. [2]; copyright (2019) with permission from Royal Society of Chemistry, Reproduced from ref. [7]; copyright (2017) with permission from US DOE. Table 1. Technical system targets for onboard hydrogen storage for light-duty fuel cell vehicles.
Reproduced with permission from ref. [7]; copyright (2017), the US DOE.  Reproduced from ref. [2]; copyright (2019) with permission from Royal Society of Chemistry, Reproduced from ref. [7]; copyright (2017) with permission from US DOE. This review provides a brief summary, with pros and cons, of the following practical hydrogen storage techniques: high-pressure gas storage, hydrogen liquefaction, chemical absorption, and physical adsorption. We address the recent progress of physical adsorption technologies using adsorbents that are divided into carbonaceous and non-carbonaceous materials. Furthermore, this review describes adsorption models for highly efficient hydrogen adsorption behaviors and specific characteristics of hydrogen molecules on spin isomers depending on their temperatures. Figure 2 shows the feasible techniques for hydrogen storage, which are classified into four types: high-pressure gas storage, hydrogen liquefaction, chemical absorption, and physical adsorption [13][14][15]. Currently, a method involving physically compressed storage with high-pressure tanks is the only commercialized hydrogen storage technique for mobile systems. The four types of compressed tanks are classified by their cylinder's materials: Type I comprises entirely metal, Type II comprises metal liner with hoop wrapping, Type III comprises metal liner with full composite wrapping, and Type IV comprises plastic liner with full composite wrapping [16,17]. Both steel and aluminum are used in conventional high-pressure hydrogen storage tanks; however, they are not sufficiently strong. Recently, the development of vessels made of composites, including carbon fiber and epoxy with high mechanical strength, is in progress [18][19][20][21]. However, the high cost and maintenance of high-pressure cylinders have been an impediment to expanding hydrogen fuel cell-based technology. This method requires a considerable amount of energy, and there is a risk of leaking hydrogen at the high pressure of 700 bar. Moreover, compressed hydrogen does not achieve the target value set by the US DOE in 2020 for a gravimetric density of 0.045 kg H 2 /kg system; the compressed method reached 0.042 kg H 2 /kg system at a pressure of 700 bar [22][23][24]. In the case of liquefied hydrogen, a large amount of hydrogen is stored in a confined volume compared to high-pressure compression-based storage [25,26]. Cryogenic hydrogen storage at 20 K requires a cooling system and additional energy compared to the high-pressure storage method [27][28][29]. This storage method has a remarkably high storage density compared to other methods. However, the energy cost required for liquefaction is high, and there are hydrogen gas losses because it vaporizes at room temperature. Moreover, it requires double-walled cylinders with good thermal insulation systems. A chemisorption technique is used for storing hydrogen gas as a form of metal hydrides in a solid-state [30][31][32]. This technique has been spotlighted due to its high storage density, remarkable stability, and small space occupancy [33][34][35][36]. A number of metal hydride vessels have been developed [37][38][39]. NaAlH 4 , LiAlH 4 , LiBH 4 , NaBH 4 , and AlH 3 are used as metal hydrogen storage materials; in particular, Mg 2 NiH 4 is in the limelight because of its advantages, such as its high storage capacity, low cost, and light weight [40]. However, the disadvantage is that high temperatures are required for desorption because of the lack of reversibility of hydrogen adsorption-desorption due to its strong bonding force and slow kinetics [41]. Physisorption storage is a process by which hydrogen molecules are weakly adsorbed at the material's surface via London dispersive forces [42][43][44][45]. Physisorption technology is the most competitive in price since it stores hydrogen molecules at room temperature and at relatively low pressures. In general, materials used for physical adsorption are porous because they require a large specific surface area. This strategy's advantages are its light weight, high storage density, superior reversibility and cycle stability, and fast charging-discharging speed. Compared to liquid storage at cryogenic temperatures, physisorption reduces hydrogen boil-off loss during storage and consumes relatively low amounts of energy during charging and discharging [23,46]. Porous materials such as zeolites, metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and carbon materials (fullerenes, nanotubes, and graphene) are the most extensively examined materials [47].

Hydrogen Storage Materials for Physisorption Methods
Porous materials are mostly used for the physical adsorption of hydrogen because of their high specific surface area, porosity, fluid permeability, regularity, and uniform pore structure [9,[48][49][50][51]. Materials such as ACs, porous silica, MOFs, COFs, and zeolites have various characteristics such as shape selectivity, adsorption, stability, and durability [5,52,53]. The high hydrogen storage capacity of the porous materials has been reported as per porosity and uniformity improvement [54,55]. Low energy is required for using hydrogen because of the physical adsorption and desorption of hydrogen molecules [56][57][58][59]. Therefore, to overcome the limitations of existing hydrogen storage methods and reach the target DOE value, physical adsorption storage using porous materials is important ( Figure 3c).

Hydrogen Storage Materials for Physisorption Methods
Porous materials are mostly used for the physical adsorption of hydrogen because of their high specific surface area, porosity, fluid permeability, regularity, and uniform pore structure [9,[48][49][50][51]. Materials such as ACs, porous silica, MOFs, COFs, and zeolites have various characteristics such as shape selectivity, adsorption, stability, and durability [5,52,53]. The high hydrogen storage capacity of the porous materials has been reported as per porosity and uniformity improvement [54,55]. Low energy is required for using hydrogen because of the physical adsorption and desorption of hydrogen molecules [56][57][58][59]. Therefore, to overcome the limitations of existing hydrogen storage methods and reach the target DOE value, physical adsorption storage using porous materials is important ( Figure 3c). organic frameworks (COFs), and carbon materials (fullerenes, nanotubes, and graphene) are the most extensively examined materials [47].

Hydrogen Storage Materials for Physisorption Methods
Porous materials are mostly used for the physical adsorption of hydrogen because of their high specific surface area, porosity, fluid permeability, regularity, and uniform pore structure [9,[48][49][50][51]. Materials such as ACs, porous silica, MOFs, COFs, and zeolites have various characteristics such as shape selectivity, adsorption, stability, and durability [5,52,53]. The high hydrogen storage capacity of the porous materials has been reported as per porosity and uniformity improvement [54,55]. Low energy is required for using hydrogen because of the physical adsorption and desorption of hydrogen molecules [56][57][58][59]. Therefore, to overcome the limitations of existing hydrogen storage methods and reach the target DOE value, physical adsorption storage using porous materials is important ( Figure 3c).

Non-Carbonaceous Materials for Hydrogen Storage
Zeolites are representative silica-based molecular sieves with micropores for catalysts and adsorbents [62,63]. In particular, in addition to cage and channel structures that have high thermal stability and large ion-exchange capacity, they have considerable potential for the physisorption of non-polar gases [64,65]. The maximum hydrogen capacity was up to 2.07 wt% for the zeolite Na-LEV (Figure 3a). In a recent study, an ideal zeolite structure for hydrogen adsorption was estimated from the meta-learning of a zeolites database with Monte Carlo simulations [47]. It reports that the RWY-type zeolite represents a hydrogen uptake of 35 g L −1 (around 7 wt%) at 100 bar and 77 K. The AWO-type zeolite had a hydrogen uptake of 10 g L −1 (around 7 wt%) at 100 bar and 77 K and 35 g L −1 (around 2 wt%) at 100 bar ( Figure 4). adsorbents. Reproduced from ref. [60]; copyright (2007) with permission from Elsevier., Reproduced from ref. [61]; copyright (2004) with permission from Nature Research.

Non-Carbonaceous Materials for Hydrogen Storage
Zeolites are representative silica-based molecular sieves with micropores for catalysts and adsorbents [62,63]. In particular, in addition to cage and channel structures that have high thermal stability and large ion-exchange capacity, they have considerable potential for the physisorption of non-polar gases [64,65]. The maximum hydrogen capacity was up to 2.07 wt% for the zeolite Na-LEV (Figure 3a). In a recent study, an ideal zeolite structure for hydrogen adsorption was estimated from the meta-learning of a zeolites database with Monte Carlo simulations [47]. It reports that the RWY-type zeolite represents a hydrogen uptake of 35 g L −1 (around 7 wt%) at 100 bar and 77 K. The AWO-type zeolite had a hydrogen uptake of 10 g L −1 (around 7 wt%) at 100 bar and 77 K and 35 g L −1 (around 2 wt%) at 100 bar ( Figure 4). Reproduced from ref. [47]; copyright (2021) with permission from AAAS.
Metal-organic frameworks (MOFs), which have a microporous crystalline structure comprising metal ions or clusters, are connected via molecular bridges [66][67][68]. MOFs have good stability, high void volumes, well-defined tailorable cavities of uniform size, high surface areas, and adjustable pore sizes [69][70][71]. In particular, the design flexibility for tuning the porosity of MOFs has attracted attention for their usage as hydrogen adsorbents [72]. Chae et al. synthesized one of the high-surface-area MOFs, known as MOF-177, in 2004 [61]. MOF-177 has a specific surface area of 4500 m 2 g −1 when measured at 77 K by N2. Generally, MOFs have high surface areas of >3000 m 2 g −1 . This is remarkably higher than that of disordered-structure carbons (2030 m 2 g −1 ) and zeolites (904 m 2 g −1 ). Ahmed et al. computationally screened 500,000 reported compounds and demonstrated the record holder of MOF adsorbents ( Figure 5). NU-100 shows amounts of 14 and 2 wt% at 100 bar and 1 bar, respectively [73]. Metal-organic frameworks (MOFs), which have a microporous crystalline structure comprising metal ions or clusters, are connected via molecular bridges [66][67][68]. MOFs have good stability, high void volumes, well-defined tailorable cavities of uniform size, high surface areas, and adjustable pore sizes [69][70][71]. In particular, the design flexibility for tuning the porosity of MOFs has attracted attention for their usage as hydrogen adsorbents [72]. Chae et al. synthesized one of the high-surface-area MOFs, known as MOF-177, in 2004 [61]. MOF-177 has a specific surface area of 4500 m 2 g −1 when measured at 77 K by N 2 . Generally, MOFs have high surface areas of >3000 m 2 g −1 . This is remarkably higher than that of disordered-structure carbons (2030 m 2 g −1 ) and zeolites (904 m 2 g −1 ). Ahmed et al. computationally screened 500,000 reported compounds and demonstrated the record holder of MOF adsorbents ( Figure 5). NU-100 shows amounts of 14 and 2 wt% at 100 bar and 1 bar, respectively [73].  [73]; copyright (2019) with permission from Nature R Zeolitic imidazolate frameworks (ZIFs) are a class of MOFs that has a simila ogy to zeolites [74][75][76][77]. Zhan et al. synthesized the core-shell structure of ZIFs for oshka-type ZIFs via a step-wise liquid-phase epitaxial growth to evaluate the s mechanism [78]. They demonstrated that hydrogen atoms migrate through the (100 of ZIFs. The dominant factors for the spillover effect are demonstrated also includi perature, hydrogen concentration, and pressure via multi-layer ZIF nanocubes as ( Figure 6).  Zeolitic imidazolate frameworks (ZIFs) are a class of MOFs that has a similar topology to zeolites [74][75][76][77]. Zhan et al. synthesized the core-shell structure of ZIFs for Matryoshkatype ZIFs via a step-wise liquid-phase epitaxial growth to evaluate the spillover mechanism [78]. They demonstrated that hydrogen atoms migrate through the (100) facets of ZIFs. The dominant factors for the spillover effect are demonstrated also including temperature, hydrogen concentration, and pressure via multi-layer ZIF nanocubes as probes ( Figure 6). Zeolitic imidazolate frameworks (ZIFs) are a class of MOFs that has a similar topol ogy to zeolites [74][75][76][77]. Zhan et al. synthesized the core-shell structure of ZIFs for Matry oshka-type ZIFs via a step-wise liquid-phase epitaxial growth to evaluate the spillove mechanism [78]. They demonstrated that hydrogen atoms migrate through the (100) facets of ZIFs. The dominant factors for the spillover effect are demonstrated also including tem perature, hydrogen concentration, and pressure via multi-layer ZIF nanocubes as probes ( Figure 6). Covalent organic frameworks (COFs), a family of crystalline microporous materials that are free of metals and entirely comprise strong covalent bonds, have been considered Covalent organic frameworks (COFs), a family of crystalline microporous materials that are free of metals and entirely comprise strong covalent bonds, have been considered promising candidates for hydrogen storage because of their high specific surface area and large pore volume [79][80][81]. However, COFs still suffer from high preparation costs, complex synthesis processes, and mechanical instability. Ramirez-Vidal et al. synthesized hypercrosslinked polymers (HCPs) by employing the Friedel-Crafts reaction using carbazole, anthracene, dibenzothiophene, and benzene as precursors and dimethoxymethane as a crosslinker [82]. The synthesized HCPs (B1FeM2) have a specific surface area of 1137 m 2 g −1 and a total pore volume of 0.87 cm 3 g −1 . The maximum hydrogen capacity was 2.1 wt% at 77 K and 40 bar. After hydrogen adsorption at 140 bar, an irreversible collapse of the texture of the HCPs was observed (Figure 7).
CNTs are carbon macromolecules that have narrow distributions of pore volumes for efficient gas adsorption [96][97][98][99]. Thus, CNTs were obtained in two different primary species characterized by their wall structures as single-walled nanotubes (SWNTs) or multi-walled nanotubes (MWNTs) [100][101][102][103][104]. In 1997, Dillon et al. examined the hydrogen adsorption of CNTs and concluded that they were promising materials for hydrogen storage (Figure 3b) [105]. Fullerene (C 60 ) was predicted in 1991 when it was determined that the hydrogen molecule could be trapped in a C 60 cage. Although it is usually unfavorable for hydrogen to be stored in such a space, the high energy barrier required to break the cage open stabilized the hydrogen molecule inside. When fullerene contained 1-29 hydrogen molecules, the C-C bond length increased by 9.3%. The bond was broken at a 14-15% change in length. Changes in the bond's length caused the fullerenes to depart from their spherical shape [106].
Activated carbons (ACs) are another one of the promising candidates for hydrogen storage materials because of their high specific surface areas, chemical and mechanical stabilities, microporous structures, and low costs [107][108][109][110][111][112][113]. Zhou et al. demonstrated that the hydrogen storage capacity is better for slit-type pores (ACs) than cylinder-type pores (CNTs) [114]. Moreover, they have the characteristic that their pore size can be controlled via conditions of chemical and physical activation [115][116][117][118][119]. One advantage of ACs is that agricultural waste, such as coffee bean dregs, coconut husks, and rice husks, have been used as raw materials for ACs [120][121][122][123]. These characteristics help not only to reduce environmental pollution but also to enable the production of ACs with various properties using various raw materials. Moreover, the advantages of ACs that are the most promising for hydrogen storage materials are that they can be mass-produced, require relatively low costs, and are mostly suitable for commercial use because of their lack of vulnerability to moisture. In one study, microporous carbon derived from melamine and isophthalaldehyde had a 4.0 wt% hydrogen uptake at 77 K and 1 bar [124]. This work recommended one-pot condensation and activation in a process involving a molten salt medium, which helps lower the melting point and leads to the condensation of monomers at a desired polymerization temperature. Molten salt media was selected from two different reagents of eutectic mixture including KOH and NaOH. The prepared melamine-and isophthalaldehyde-based ACs via the KOH-NaOH reagents (MIKN) achieved a specific surface area of 2984 m 2 /g and a total pore volume of 1.98 cm 3 /g (Figure 8). to moisture. In one study, microporous carbon derived from melamine and isophthalaldehyde had a 4.0 wt% hydrogen uptake at 77 K and 1 bar [124]. This work recommended one-pot condensation and activation in a process involving a molten salt medium, which helps lower the melting point and leads to the condensation of monomers at a desired polymerization temperature. Molten salt media was selected from two different reagents of eutectic mixture including KOH and NaOH. The prepared melamine-and isophthalaldehyde-based ACs via the KOH-NaOH reagents (MIKN) achieved a specific surface area of 2984 m 2 /g and a total pore volume of 1.98 cm 3 /g (Figure 8). Graphene derivatives such as graphene oxide (GO) and reduced graphene oxide (rGO) are other promising materials for the hydrogen storage system [125][126][127]. Functional groups such as hydroxyls, carbonyls, and carboxyls trap hydrogen molecules via hydrogen bonds that are stronger than the London dispersive force [128]. Moreover, oxygen-containing functional groups not only form gaps between graphene layers but also creates suitable conditions for loading metal nanoparticles in order to create higher hy- Graphene derivatives such as graphene oxide (GO) and reduced graphene oxide (rGO) are other promising materials for the hydrogen storage system [125][126][127]. Functional groups such as hydroxyls, carbonyls, and carboxyls trap hydrogen molecules via hydrogen bonds that are stronger than the London dispersive force [128]. Moreover, oxygen-containing functional groups not only form gaps between graphene layers but also creates suitable conditions for loading metal nanoparticles in order to create higher hydrogen capacities [10,129]. Graphitic carbons, which are sp 2 hybridized and have a sheet-like structure, interact using van der Waals forces. Singh et al. synthesized exfoliated graphite oxide (EGO) with a thermal exfoliation method under various gas conditions [130]. Air-exfoliated EGO represents a specific surface area of 268 m 2 g −1 , average pore size of 2.9 nm, and total pore volumes of 1.2 cm 3 g. The maximum hydrogen uptake of EGOs was 3.34 wt% at 77K and 30 bar. After five cycles, the hydrogen's capacity decreased by only 9%, indicating a storage amount of 3.02 wt% (Figure 9). Porous graphite hybridized with nickel (Ni/PG) has been reported as a potential hydrogen storage material [131,132]. Ni/PG provides a specific surface area of 145 m 2 /g and a total pore volume of 0.056 cm 3 /g. The optimized samples achieved 4.48 wt% of hydrogen uptake at 298 K and 100 bar (Figure 10). Porous graphite hybridized with nickel (Ni/PG) has been reported as a potential hydrogen storage material [131,132]. Ni/PG provides a specific surface area of 145 m 2 /g and a total pore volume of 0.056 cm 3 /g. The optimized samples achieved 4.48 wt% of hydrogen uptake at 298 K and 100 bar (Figure 10). Because of a large specific surface and high surface activity, MXenes have attracte attention for use as hydrogen storage adsorbents [133][134][135]. MXenes are 2D transitio metal carbides or nitrides that have chemical formulations such as Mn+1XnTx (M is an ear transition metal, X is a member of group 13 or 14 of the periodic table, and T is a function group) [136]. MXenes presented the Kubas interaction during hydrogen adsorptio which provides binding energy between chemisorption and physisorption from transitio metals in their structures [137][138][139]. Therefore, these unique properties provide superio hydrogen uptakes with adsorption-desorption reversibility. Recently, a partially etche MXene was suggested for high hydrogen uptake performance [140]. The bell-mouth stru ture from an incompletely etched multilayer provides a fast H2 transport channel for i creases in hydrogen uptake. Moreover, additional weak chemisorption via a nanopum effect from a narrow interlayer structure of 7 Å helps to maximize the hydrogen capacit This work achieved 8.8 wt% H2 uptake at 298 K and 60 bar (Figure 11). Additionally, was found that the hydrogen storage capacitance of this MXene at the first cycle is 8.0 Because of a large specific surface and high surface activity, MXenes have attracted attention for use as hydrogen storage adsorbents [133][134][135]. MXenes are 2D transition metal carbides or nitrides that have chemical formulations such as M n+1 X n T x (M is an early transition metal, X is a member of group 13 or 14 of the periodic table, and T is a functional group) [136]. MXenes presented the Kubas interaction during hydrogen adsorption, which provides binding energy between chemisorption and physisorption from transition metals in their structures [137][138][139]. Therefore, these unique properties provide superior hydrogen uptakes with adsorption-desorption reversibility. Recently, a partially etched MXene was suggested for high hydrogen uptake performance [140]. The bell-mouth structure from an incompletely etched multilayer provides a fast H 2 transport channel for increases in hydrogen uptake. Moreover, additional weak chemisorption via a nanopump effect from a narrow interlayer structure of 7 Å helps to maximize the hydrogen capacity. This work achieved 8.8 wt% H 2 uptake at 298 K and 60 bar ( Figure 11). Additionally, it was found that the hydrogen storage capacitance of this MXene at the first cycle is 8.07 wt%. After five cycles, the storage amount was 7.56 wt% which represented a retention of 94%, compared to the initial uptake.

The Adsorption Models for Hydrogen Storage
Understanding the behaviors of hydrogen adsorption is important for improving hydrogen storage capacity. Moreover, the sorption density on the physisorption of carbonaceous materials is extremely low and has a low hydrogen capacity [141][142][143][144]. Therefore, a progressive strategy is required to create stronger attraction between hydrogen and adsorbents. It is well-known that hydrogen spillover helps obtain highly efficient hydrogen storage capacity by introducing a transition metal component to the surface of porous materials [145][146][147][148][149][150].
The phenomenon of hydrogen spillover can be explained as the dissociative chemisorption of hydrogen molecules to hydrogen atoms on the metal (primary spillover) and migration of atomic hydrogen on the surface of supports (secondary spillover) [151,152]. However, the spillover of hydrogen atoms directly to supports is energetically an unfavorable phenomenon [153]. Therefore, a bridge between metal particles and supports was proposed to allow the surface diffusion of hydrogen atoms from the metals to the supports [154,155]. In this strategy, the energy barriers for the surface diffusion of hydrogen atoms from the metal to the support were overcome via physical bridges with intimate contacts for secondary spillover. However, the secondary spillover from the proposed model is Figure 11. (a,b) TEM images of an MXene for hydrogen storage with a bell-mouth structure, (c) the hydrogen storage mechanism on an incompletely etched MXene, (d,e) hydrogen uptakes of MXene samples at 298 K and 60 bar, (f) XRD patterns of pristine, hydrogenated, and dehydrogenated MXene, and (g) hydrogen uptake cycles. Reproduced from ref. [140]; copyright (2021) with permission from Nature Research.

The Adsorption Models for Hydrogen Storage
Understanding the behaviors of hydrogen adsorption is important for improving hydrogen storage capacity. Moreover, the sorption density on the physisorption of carbonaceous materials is extremely low and has a low hydrogen capacity [141][142][143][144]. Therefore, a progressive strategy is required to create stronger attraction between hydrogen and adsorbents. It is well-known that hydrogen spillover helps obtain highly efficient hydrogen storage capacity by introducing a transition metal component to the surface of porous materials [145][146][147][148][149][150].
The phenomenon of hydrogen spillover can be explained as the dissociative chemisorption of hydrogen molecules to hydrogen atoms on the metal (primary spillover) and migration of atomic hydrogen on the surface of supports (secondary spillover) [151,152]. However, the spillover of hydrogen atoms directly to supports is energetically an unfavorable phenomenon [153]. Therefore, a bridge between metal particles and supports was proposed to allow the surface diffusion of hydrogen atoms from the metals to the supports [154,155]. In this strategy, the energy barriers for the surface diffusion of hydrogen atoms from the metal to the support were overcome via physical bridges with intimate con-tacts for secondary spillover. However, the secondary spillover from the proposed model is arguable because of a significant energy barrier for the surface migration of hydrogen atoms from metals to supports and the lack of a driving force.
In recently reported studies [131,156], a modified spillover model was proposed to redeem the gap between the real improvement of hydrogen capacities and expected hydrogen spillover effects. The hydrogen molecules adjacent to the adsorbents are classified via three layers in this model ( Figure 12). The hydrogen molecules of the first layer dissociatively adsorbed on the metal surfaces via the Kubas interaction from the high oxidative power of the metal [157]. The second layer of hydrogen, physically adsorbed on an adsorbent, was partially dipole-induced because of the strong electron-acceptor characteristics of the metal. The other hydrogen molecules were close to the physically adsorbed adsorbent; however, the equilibrium of the third layer was broken by the attraction of partially acidic carbon surfaces. The carbon surface surrounding the metal lost its electrons (electron acceptor properties) because of the strong electron-acceptor characteristics of the metallic catalysts [158,159]. This environment acts as a driving force for hydrogen affinity properties on carbon surfaces via electron-acceptor-donor interaction behaviors.

04
12 of 19 In recently reported studies [131,156], a modified spillover model was proposed to redeem the gap between the real improvement of hydrogen capacities and expected hydrogen spillover effects. The hydrogen molecules adjacent to the adsorbents are classified via three layers in this model ( Figure 12). The hydrogen molecules of the first layer dissociatively adsorbed on the metal surfaces via the Kubas interaction from the high oxidative power of the metal [157]. The second layer of hydrogen, physically adsorbed on an adsorbent, was partially dipole-induced because of the strong electron-acceptor characteristics of the metal. The other hydrogen molecules were close to the physically adsorbed adsorbent; however, the equilibrium of the third layer was broken by the attraction of partially acidic carbon surfaces. The carbon surface surrounding the metal lost its electrons (electron acceptor properties) because of the strong electron-acceptor characteristics of the metallic catalysts [158,159]. This environment acts as a driving force for hydrogen affinity properties on carbon surfaces via electron-acceptor-donor interaction behaviors.

Conclusions
Modern society is adopting the gradual change to a carbon-free world with the development of novel green energy storage and conversion technologies. Hydrogen has Figure 12. Comparison of two representatives: Yang's spillover model and Park's modified spillover model in H 2 storage (d n : distance between the metal and the number of the hydrogen layers; BE dn : binding energy between two d n s).

Conclusions
Modern society is adopting the gradual change to a carbon-free world with the development of novel green energy storage and conversion technologies. Hydrogen has been considered a significant alternative for fossils fuels for a veritably sustainable society. To date, the development of a highly efficient and stable hydrogen storage technique is urgent. To summarize, the US DOE has annually set up detailed targets for hydrogen fuel cell applications. The current state of hydrogen storage techniques is challenging; what is used is a high-pressure compressed tank that has underlying problems because of safety concerns and a lack of energy efficiency. Researchers have examined promising techniques using physical adsorption materials with light weights, high storage densities, and fast charging-discharging kinetics. Regarding highly efficient hydrogen storage capacities, it is important to understand the interfacial interactions between adsorbent surfaces and adsorbates. Here, we briefly addressed the recent progress on adsorbent materials, which can be carbonaceous or non-carbonaceous materials. Furthermore, the spillover effects of hydrogen molecules on solid-state adsorbents are suggested to achieve highly efficient hydrogen storage, which could be an important key point for designing hydrogen adsorbents.

Conflicts of Interest:
The authors declare no conflict of interest.