Hydrogen Storage Properties of Metal-Modified Graphene Materials

Abstract

The absence of adequate methods for hydrogen storage has prevented the implementation of hydrogen as a major source of energy. Graphene-based materials have been considered for use as solid hydrogen storage, because of graphene’s high specific surface area. However, these materials alone do not meet the hydrogen storage standard of 6.5 wt.% set by the United States Department of Energy (DOE). They can, however, be easily modified through either decoration or doping to alter their chemical properties and increase their hydrogen storage capacity. This review is a compilation of various published reports on this topic and summarizes results from theoretical and experimental studies that explore the hydrogen storage properties of metal-modified graphene materials. The efficacy of alkali, alkaline earth metal, and transition metal decoration is examined. In addition, metal doping to further increase storage capacity is considered. Methods for hydrogen storage capacity measurements are later explained and the properties of an effective hydrogen storage material are summarized.

1. Introduction

Fossil fuels, such as coal, oil, and natural gas, continue to service much of the world’s energy needs. These three sources of energy account for roughly 80% of the world’s energy [1]. However, they also contribute significantly to the growing threat of climate change and represent the largest reservoir of greenhouse gas (GHG) emissions [2].

More sustainable energy sources, such as solar, wind, and tidal power generators, are developing as renewable and cleaner substitutes for conventional fossil fuels. While there is much promise for these technologies, they have limitations at this time. The most relevant and persistent challenge is the intermittent nature of renewables during energy generation. Given the current state of the respective technologies, the power generation of most renewable energy sources is beholden to factors such as time of day, seasonality, weather conditions, and geographical location [3]. The ability to produce, store, and utilize energy on demand remains a major challenge for many renewable energy sources.

Hydrogen fuel has demonstrated promise as a clean and renewable source of energy that is not subject to these same issues. Hydrogen has a high energy density, which is approximately 120 MJ/kg, a value roughly three times that of gasoline [4]. There are also relatively few challenges associated with location-specific resource generation, since hydrogen can be extracted from a wide variety of materials, including water, biofuels, and sludges [5]. The electrolysis of water is a commonly cited method to reliably produce hydrogen [6]. Finally, the combustion of hydrogen is clean, producing only pure water and no GHGs [5]. In fact, the end result, namely the production of water, also gives hydrogen reactions the potential to be recyclable as well as free of emissions [6].

Perhaps the greatest barrier to using hydrogen as an efficient, ubiquitous, completely trustworthy, and sustainable energy source is its storage. Hydrogen storage remains a practical challenge, due to hydrogen’s small size, low volumetric density, and high diffusivity [6]. Storage becomes a greater challenge at room temperature and modest pressure, though such storage is likely necessary for applications in the transportation sector, given technical and safety limitations. To meet this end, the storage of hydrogen within porous solids has shown the best results. Specifically, nanoporous materials, such as zeolites and metal–organic frameworks (MOFs), have been utilized for the storage of gases, including hydrogen. Carbon-based materials, such as graphene and its derivatives, have been a major source of interest for hydrogen storage as well [7]. The United States Department of Energy (US DOE) has set a storage capacity of 0.065 kg H₂/kg (6.5 wt.%) as the ultimate goal for adequate hydrogen storage [8]. Hydrogen molecules (H₂) must also be able to adsorb onto the material with a binding energy within the range of 0.2 to 0.7 eV [4]. While some materials have shown this capacity, the storage process itself must not only be feasible at moderate temperatures and pressures but also be relatively easy to reverse, since the hydrogen must be safely extracted and retrieved from the storage matrix through easy and relatively mild means in order for it to be effectively processed and ultimately reused for additional applications and products.

Porous materials, such as those derived from graphene, can utilize two modes for hydrogen storage. Physical adsorption, or physisorption, is dominated by weak van der Waals forces. The hydrogen is stored in the molecular form on the surface of the material [9]. The adsorption energy is low, given the weak attractive forces governing it, and the hydrogen storage efficiency increases with the material’s specific surface area [10]. By contrast, chemisorption involves the formation of chemical bonds—namely metallic, ionic, or covalent—between the hydrogen and the storage material. As such, chemisorption necessitates the dissociation of hydrogen molecules into their atomic form for storage [10]. Chemisorption is generally, though not exclusively, described in terms of a “spill-over” effect. The spill-over effect involves the splitting of molecular hydrogen into constituent individual atoms on a catalyst, the subsequent migration of these atoms to the substrate’s surface, and ultimately, the diffusion of the hydrogen across and throughout the material’s surface [11]. Because a catalyst is involved, the chemisorption of hydrogen on graphene materials generally requires the decoration of these materials with metals, particularly transition metals. Pristine graphene does not demonstrate sufficient physisorption and/or chemisorption of hydrogen on its own to meet the standards set by the US DOE; as such, chemical doping and functionalization have been proposed as a possible solution to increase its hydrogen storage potential [6]. Taking advantage of the enhanced chemisorption effect enabled by metal decoration therefore represents a prime objective of hydrogen storage technology.

Another challenge underlying chemisorption revolves around the kinetics underpinning the storage mechanism. By contrast with the relatively low-energy physisorption process, chemisorption is a high-energy and irreversible process. This reality has led chemisorption to be initially disregarded as a possible means for storing hydrogen [12]. Specifically, the energy requirements for chemisorption are demanding. Breaking the H–H bond, a key aspect of the spill-over effect, requires a relatively high amount of 3 to 4 eV, thereby making it difficult to achieve under standard operating conditions. Moreover, the binding strength of the hydrogen on the adsorbent surface must exceed half the H–H bond energy within molecular hydrogen, which is 2.31 eV per hydrogen atom, in order to achieve stable adsorption [13]. By contrast, the binding energy needed for physisorption is much less, i.e., approximately 0.1 eV per atom [14]. The adsorption energy therefore must be kept as close to 2.31 eV as feasible in order to render it practical to release the hydrogen for further use. Lastly, any such adsorbent must have a large surface area in which the hydrogen may deposit [13].

These energy challenges, especially related to the magnitude of the adsorption energies involved, can be mitigated by the introduction of a catalyst [15]. A strong degree of interaction between hydrogen and catalyst particles can promote dissociation and reduce the energy barrier to a level below 1 eV [13]. Catalysis between metals and hydrogen has had a history of success in other hydrogen storage materials, including MOFs and metal hydrides [13,15]. However, the unique porosity and the immense surface area of graphene, as well as the ability of graphene to be doped and decorated with a wide variety of functional groups and components, render it as a strong candidate in terms of advancing the potential and promise of hydrogen storage technology at room temperature.

2. Systems and Strategies

2.1. Carbon-Based Materials

Graphene, graphyne, graphite, graphene oxide, reduced graphene oxide, fullerenes, carbon nanotubes (CNTs), and other carbon-based materials have all been identified as possible hydrogen storage systems (Figure 1). This is primarily due to their large surface area, which theoretically allows for a high capacity of hydrogen storage.

As shown in Figure 2, these materials contain many pores, which can serve as active sites for hydrogen storage. Additionally, these materials are lightweight, possess low density, exhibit high chemical stability, maintain good reversibility, and are characterized by fast kinetics [17]. In a study directly comparing pore size to hydrogen uptake, it was found that the total hydrogen uptake at −196 °C and 20 bar for pore sizes of 31 Å and 12 Å increased from 5.4 wt.% to 7.3 wt.%, respectively [18]. This indicates that the hydrogen storage capacity of these graphene-based materials depends substantially on the projected and expected pore size.

Graphene is an allotrope of carbon that exists as a two-dimensional honeycomb lattice. Within the graphene monolayer are three types of geometrically distinctive adsorption sites for hydrogen or other atoms to adsorb onto: hollow, bridge, and top [20]. Graphene can be modified to incorporate defects into the structure. Introducing these defects can affect the chemical properties of the material, including reactivity, magnetic properties, and modulus of elasticity, among others. Vacancy defects represent the most common defects typically found in crystalline materials. “Single-vacancy”, also known as mono-vacancy, graphene can be formed when a single carbon atom is removed from the graphene structure. A single dangling bond will always exist within this structure. Multiple vacancies can also be generated. Specifically, “double-vacancy” graphene is produced when two atoms are removed. The Stone–Wales (SW) defects, or point defects, denote the incorporation of non-hexagonal (i.e., pentagonal or heptagonal) rings within the graphene structure. Atoms are neither added nor removed in this process [6]. Instead, SW defects can be created by the in-plane 90° rotation of carbon–carbon bonds. This will create two heptagons and two pentagons from the inherent hexagonal lattice structure of the graphene. The Stone–Wales, single-vacancy, and double-vacancy defects described above are shown in Figure 3 [21].

Theoretically, the maximum gravimetric hydrogen density in graphene through chemisorption is 8.3 wt.% and can be ascribed to the formation of a completely saturated graphene sheet. However, experimental results have found that the hydrogen storage capacity of graphene sheets tends to be much lower. In particular, at 25 bar and room temperature, the storage capacity was noted to be 2.7 wt.% [22]. Moreover, graphene can be routinely, reliable, and successfully prepared, such that its intrinsic and unique structural, electronic, chemical, and optical properties can be duly exploited. Based upon a true ‘two-dimensional (2D)’ lattice-based structure, graphene is the poster child of a novel class of 2D-based materials, which has expanded to encompass systems such as but not limited to boron nitride nanosheets, transition metal dichalcogenides (TMDCs), mono-elemental 2D semiconductors (such as but not limited to silicene, germanene, stanene, and phosphorene), MXenes, and 2D oxide/hydroxide materials [23]. Continuous theoretical and experimental investigations are being conducted to understand the nature of 2D materials more fully in order to reap more benefits from their inherent structure-based advantages. Specifically, as per the scope of this review, defects in graphene can be considered as a viable and practical means with which to tune the favorable structure and associated characteristics of this key material.

Extrinsic defects can be introduced to mitigate the low hydrogen storage capacity of these materials. These defects may involve the surface modification of the graphene sheet and can include the incorporation of either carbon, foreign adatoms, or substitutional impurities. Substitutional impurity defects are created by replacing either one or more carbon atoms in the underlying lattice with a foreign adatom. These atoms can be either metal or nonmetals and are introduced into the graphene structure through doping or decoration. This paper will focus in particular on the introduction of metal atoms onto graphene and associated graphene derivatives.

In this review, therefore, we summarize recent, significant studies with respect to the role of defects in graphene in the context of hydrogen storage. We will discuss distinct types of defects, how these can be deliberatively generated, their subsequent influence on different properties of graphene, and their corresponding implications for hydrogen storage applications. Thus, this review presents a thoughtful and hopefully comprehensive study in probing the fundamental science of a diverse array of defective graphene structures [22]. These insights are particularly invaluable since pristine carbon-based nanomaterials are unable to meet the hydrogen storage goals set by the US DOE, especially under relatively modest room temperature and ambient/low-pressure conditions.

2.2. Chemical Modification and Dopant Introduction

Graphene-based materials can be modified either by doping or introducing foreign adatoms through decoration. The terms “doping” and “decoration” are often incorrectly used interchangeably, so we aim to clarify the distinction here. Doping typically involves the substitution of carbon atoms with foreign adatoms. The simplest process leaves the molecular geometry of the graphene effectively unchanged. This is typically achieved with B, N, Al, and S atoms. To integrate a transition metal into the surface of graphene, or to dope it, we necessarily need to affect the morphology and topology of the graphene sheet; indeed, the graphene surface will no longer be flat, and the doped metal will project itself from the surface in one direction or another as the planar bonds within graphene are distorted and broken through metal–carbon bond formation. Moreover, as a further nuance, the doping of transition metals in defective graphene can occur with distinctly different mechanisms as compared with pristine graphene itself. Defects, particularly vacancies, or holes, in the graphene layer, provide favorable sites for metal atoms to chemically bond. The presence of these vacancies attracts metal atoms from a substrate, thereby allowing them to “fill” the vacancy and stabilize their presence on the graphene surface [24,25]. Several other defect types that may be present, such as ring defects and vacancy complexes, can also influence how these transition metals integrate into the graphene lattice and, as such, should be considered as well [26]. Current methods used to effectively dope carbon-based materials include but are not limited to protocols based on hydrothermal, solvothermal, chemical vapor deposition (CVD), plasma-enhanced CVD, microwave plasma CVD, thermal annealing, microwave, calcination, pyrolysis, and ball-milling processes [27,28,29].

Decoration represents an analogous form of surface modification, but it involves various forms of adsorption of metal atoms onto a surface. Decorating graphene does not necessarily require or imply the removal of a carbon atom [30]. Decoration can usually be achieved using either solvothermal, hydrothermal, or electroless deposition methods. There are three adsorption modalities that govern the formation of metal-decorated graphene: (a) the aforementioned van der Waals physisorption, (b) metal adatom-facilitated polarization, and (c) Kubas adsorption. (a) Van der Waals physisorption can account for the non-specific interactions between hydrogen and metal atoms. It is caused by the intermolecular forces between atoms. Because they are very weak, they can essentially allow for relatively fast adsorption and desorption. (b) The second mechanism involves the creation of an electric field by alkali metals, which polarize the H₂. This induced polarization of H₂ leads to binding energies of around 0.2 eV, and in principle, this can facilitate hydrogen storage potential. (c) Finally, Kubas adsorption involves the hybridization of H₂ σ or σ* orbitals with the corresponding electronic states of transition metals. This process is associated with hydrogen binding energies in the range of 0.2 to 0.6 eV [25].

2.3. Alkali and Alkaline Earth Metal Decoration

Graphene-based nanomaterials can be modified with metals to improve their hydrogen storage capacity. Alkali metals, specifically Li, Na, and K, have been identified as possible candidates, because they are lightweight elements that can strengthen the adsorption of H₂. Alkaline earth metals, such as Ca and Sr, have also been found to improve the storage capacity of graphene materials. Alkali and alkaline earth metals are prime candidates, because they maintain a smaller cohesive energy as compared with their transition metal analogues. This prevents clustering of metal atoms onto the graphene-based material surface. Several first-principles studies have been conducted which have predicted the hydrogen storage capacities of alkali- and alkaline earth metal-decorated graphene, thereby suggesting that high storage can be achieved [31].

2.3.1. Lithium

Lithium-decorated graphene is touted as one of the most successful storage systems, because hydrogen adsorption is exceptionally strong in this material. Additionally, Li is a lightweight material, rendering high gravimetric storage capacity possible [32]. On pristine graphene, Li can adsorb onto either a hollow, bridge, or top site. Seenithurai et al. found that the most stable configuration for Li is for it to be localized at the hollow site, wherein the binding energy of Li on pristine graphene is 2.09 eV and the adsorption distance between C and Li is 1.668 Å.

H₂ adsorbs onto the Li–graphene with an average binding energy of 0.30 eV and a corresponding adsorption distance of 1.96 Å. Figure 4 illustrates that up to five H₂ can adsorb onto the Li-decorated graphene in a symmetric pattern, yielding a storage capacity of 4.32 wt.% [33]. A different study conducted by Huang et al. found that Li-decorated oxidized porous graphene gave rise to a storage capacity of 9.43 wt.% [34]. A further study by Liu et al. found that Li-decorated carbon nanotubes possessed a reversible storage capacity of 4.2 wt.% at room temperature and 10 MPa [32].

A major issue in decorating pristine carbon-based nanomaterials is the clustering of metal atoms onto the external surface of the carbon support. This will occur if the bulk cohesive energy of the metal is lower than the interaction binding energy of the adatoms. To mitigate this issue, transition metal doping can be performed. By doping the carbon support, the interaction binding energy of Li on the support increases. Typically, boron, nitrogen, or sulfur atoms are doped onto carbon supports. Lithium decoration can also be initiated on defective graphene and other carbon derivatives. On defective graphene, the hollow sites are the most stable for Li as well. The binding energy of Li doped onto mono-vacancy (MV) graphene is 2.64 eV, whereas that of the Li ions doped onto double-vacancy (DV) graphene is 2.61 eV [32].

2.3.2. Sodium

No sodium-doped or decorated graphene-based nanomaterials or derivatives have been experimentally evaluated to the best of our knowledge. However, many simulations and calculations have been completed. A calculation based on density functional theory (DFT) was performed by Wang et al. to measure the high-capacity hydrogen storage of Na-decorated graphene with boron substitution. It was found that each sodium atom decorated on the graphene was able to absorb up to 5 H₂, which resulted in a calculated 8.1 wt.% and 12.8 wt.% of H₂ storage capacity for single-sided and double-sided Na-decorated graphene, respectively. Considering the partial electron density of states of Na, B, C, and H₂, it was observed that the Na atoms become less positively charged by the presence of ever-increasing amounts of H₂. AIMD simulations were performed on a 12 H₂-2Na-BSG system to verify the stability of the system.

It was observed that if more than three hydrogen molecules adsorb onto the BSG layer, the molecules will rearrange into two layers (Figure 5). Three H₂ can escape from the boron-substituted graphene layer, incorporating a 11.7 wt.% of H₂, which resulted in a maximum of nine H₂ per boron atom [35].

An analogous study carried out by Liu et al. was completed on the hydrogen storage potential of Na-decorated graphyne and its boron nitride analogue. The calculations performed were based on DFT in a manner similar to the previous study. Specifically, single-sided Na-decorated graphyne bonded with H₂, two H₂, and three H₂ was reported to yield computed adsorption energies of −0.202 eV, −0.204 eV, and −0.204 eV, respectively. The enhancement of binding between the H₂ and graphyne was said to have been caused by the polarization of the H₂. Similarly, double-sided Na-decorated graphene with H₂, 2 H₂, and 3 H₂ bonded to it give rise to adsorption energies of −0.257 eV, −0.258 eV, and −0.254 eV, respectively. The average adsorption energies calculated for Na-decorated double-sided graphyne and BN-yne were noted to be −0.25 eV/H₂ and −0.17 eV/H₂, respectively.

Experimentally, the reported hydrogen storage capacities calculated for Na-decorated single-sided and double-sided graphyne were found to be 3.49 wt.% and 5.98 wt.%, respectively. Similarly, the measured hydrogen storage capacities calculated for Na-decorated double-sided BN-yne were determined to be in the range of 5.84 wt.%. No corresponding value was reported for Na-decorated single-sided BN-yne [36].

2.3.3. Potassium

It is worth stating that no potassium-doped or decorated graphene-based nanomaterials or derivatives have been experimentally assessed to the best of our knowledge either. By analogy with sodium, many calculations and simulations have also been performed to measure the hydrogen storage capacity of these theoretically attractive materials. A study by Tokarev et al. was initiated on the modeling of hydrogen storage in boron-substituted graphene, decorated with potassium metal atoms. As compared with potassium-modified graphene, it was found that the binding energy values of the first layer of adsorbed hydrogen molecules were much higher when bound onto the boron-substituted sheet. In the bottom layer, each hydrogen molecule interacted with two hydrogen molecules. Conversely, each potassium atom intermingled with 6 H₂. Both scenarios involved the sharing of their hydrogen molecules with other metal atoms. This situation yielded a hexagonal structure with potassium atoms localized in the center of the cells. These metal atoms possessed notably elevated hydrogen adsorption energy values of up to 40 kJ/mol. Four complete layers of hydrogen molecules were observed on both sides of the surface, giving rise to as much as 22 H₂ per K atom, which yielded an equivalent hydrogen storage capacity in principle of 22.5 wt.% [37].

However, certain factors need to be considered. First, one should not consider the first layer of hydrogen as part of a reversibly adsorbed layer of hydrogen, due to its exceedingly and unusually high adsorption energy (37 kJ/mol). This scenario lowered the hydrogen storage capacity to 17.4 wt.%. In addition, the calculations were performed without considering temperature which incorporates the entropy term and reduces the free energy of adsorption. The actual hydrogen storage capacity will hence be much lower due to this entropy term. However, the model did provide evidence that the material can be an encouraging candidate as a viable hydrogen storage system. Though the calculations are technically correct thus far, the next step ought to involve the validation of the reported results by applying more precise post-Hartree–Fock methods, such as a Moller–Plesset scheme [37].

An analogous study by Shams et al. was performed on the structural and electronic properties of potassium-decorated γ-graphyne as a viable hydrogen storage medium. The calculations were completed using the spin-polarized density functional method implemented in the DMol3 module of the Materials Studio software. To determine the effect that potassium has on H₂ adsorption, H₂ was modeled on the graphyne surface. It was found that the interactions between the graphyne and H₂ were very weak with adsorption energies in the range of 0.04 to −0.07 eV. The K-decorated graphyne possessed an adsorption energy of −0.212 eV. The amount of hydrogen capable of being adsorbed onto the surface of the K-decorated graphyne was determined by increasing the number of hydrogen molecules from one to nine in a stepwise fashion. Specifically, 8.95 wt.% hydrogen was able to be adsorbed onto one-sided K-decorated graphyne, whereas 13.95 wt.% was noted for two-sided K-decorated graphyne. The possibility of involving more than nine H₂ per atom was not investigated further, due to the adsorption energy reaching 0.181 eV. The best sites for adsorption were found to be at the center of the 12-membered ring, the core of the hexagonal ring, and the top of the acetylenic bond, with corresponding adsorption energies equal to −5.86, −5.48, and −5.47 eV, respectively. Due to the magnitude of these adsorption energies, the predominant adsorption mechanism was determined to be chemisorption [38].

2.3.4. Calcium

Alkaline earth metals (AEMs) can also be used to modify graphene. Calcium is one AEM that has become an attractive candidate for decoration onto carbon-based materials, because it is the only lightweight AEM with empty d orbitals and it gives rise to a strong interaction with carbon, due to back-bonding effects [6]. A first-principles study conducted by Yoon et al. [39] involved the decoration of carbon fullerenes, C₆₀, with various AEMs. The adsorption energies and binding mechanisms for each AEM onto C₆₀ varied significantly. Ca was found to bind most strongly on top of the hexagonal ring with a binding energy of ~1.3 eV, whereas Be and Mg yielded an average binding energy of ~60 meV. When Ca binds to C₆₀, Ca readily donates s electrons because of its relatively low ionization potential. These electrons partially fill the π* orbitals of C₆₀. The binding energy of hydrogen on M-Ca₆₀ was subsequently examined. It was determined that up to 5 H₂ can be adsorbed by a single Ca or Sr atom. Each hydrogen binds to Ca₃₂C₆₀ with a binding energy of ~0.2 eV. It was found that the hydrogen adsorption on Ca₃₂C₆₀ was >8.4 wt.%, associated with one of the highest hydrogen storage capacities of AEMs [39]. While Ca decoration on fullerenes seems promising, Ca decoration on graphene is more challenging and problematic. Most significantly, individual Ca atoms tend to aggregate onto pristine graphene.

Doping graphene with B atoms before decoration with Ca atoms can prevent aggregation. Double-sided Ca-decorated graphene doped with individual boron atoms of 12 atom% can theoretically achieve a gravimetric capacity of 8.38 wt.% hydrogen, corresponding to a binding energy of 0.46 eV. This binding energy is within the range needed to not only allow for hydrogen to stably bind onto the surface but also permit for it to desorb under ambient conditions [39].

2.3.5. Strontium

As another member of the Group 2 elements, strontium (Sr) exhibits similar hydrogen sorption properties to those of calcium. The same study by Yoon et al. found that Sr also bonds to C₆₀ on top of the hexagonal ring with a binding strength of ~1.3 eV. Sr atoms also highly prefer to adsorb as a monolayer coating. This strong adsorption is also due to the empty d levels of Sr. Specifically, there is a strong hybridization between the carbon π and π^* orbitals and the Sr d orbitals. There is also a complete depletion of Sr s orbitals, indicating that two electrons have migrated over from the metal to C₆₀. The charge redistribution that occurs creates an electric field around the fullerene, allowing it to attract molecular hydrogen and effectively store it. However, differences occurred when examining the interactions between these materials and hydrogen. Specifically, Sr-decorated C₆₀ gives rise to a slightly lower hydrogen binding energy, as compared with Ca₃₂C_60, of around 0.191 eV, which is on the boundary of the lower desired limit set by the DOE [39].

2.3.6. Beryllium and Magnesium

As previously stated, in and of themselves Be and Mg do not significantly improve the hydrogen storage capacity of graphene-based materials. They very poorly bind onto carbon, due to weak van der Waals interactions. On C₆₀, their average binding energy is ~60 meV with an average metal–carbon bond distance of ~3.5 Å [39].

Although alkali and alkaline earth metals are lightweight, they exhibit certain properties that hinder effective hydrogen sorption. For example, alkali metals can block microporous carbon adsorption sites, which can decrease the surface area and overall hydrogen storage capacity. Moreover, using alkaline earth metals to decorate carbon materials also has its disadvantages. Mg and Be adatoms were found to give rise to very weak adsorption, thereby rendering them as poor candidates overall for graphene decoration [31]. To overcome these issues with using alkali and alkaline metal decoration, transition metals and their corresponding alloys can be alternatively considered.

2.4. Transition Metal Decoration

Transition metals have been studied since the 1970s for use in hydrogen storage systems, with work focused mainly on metal hydride alloys, including both rare earth–transition metal alloys and non-rare-earth intermetallics. There was also a relatively big push towards developing high-weight-capacity materials utilizing highly abundant transition metals (mainly Fe, Ni, and Co) [40,41]. However, intensive research and relevant studies investigating the effects of hydrogen adsorption on graphene (and other carbon-based supports) decorated or doped with various transition metals are still relatively unexplored. Herein, we review each transition metal’s most recent simulations and experimental studies regarding their potential for hydrogen storage with graphitic materials. In particular, we analyze associated reported weight percents, hydrogen binding energy ranges, mechanisms of adsorption, and stable temperature ranges.

2.4.1. Scandium

Scandium represents the first and lightest transition metal element with a predominant oxidation state of +3; it possesses an adequate number of empty d orbitals for hydrogen binding through Kubas-like interactions [42,43]. Introducing impurity atoms can give rise to the formation of pore defects within graphene, which can prevent the transition metal atoms from aggregating [44]. This scenario can help to improve upon the hydrogen storage capacity of the material by a weaker interaction between the Sc and the underlying imperfect graphene surface, while increasing the adsorption energy of the Sc atom and that of the adjoining H₂, via weak Kubas-type interactions [45]. When Sc is decorated onto N-doped graphene, as shown in Figure 6, up to six hydrogen molecules can be physiosorbed onto the Sc atom. However, the system is most stable with four H₂ bonds [38].

Using density functional theory, one group determined that the average binding energy of H₂ to the decorated Sc was −0.32 eV, which is in the range of the suggested DOE guidelines. Computations also yielded an average bond length of 2.03 Å and 0.82 Å for Sc-H and H-H bonds, respectively [47]. The average bond length of Sc to C was 2.33 Å and when H₂ was bonded to Sc, there was an elongation of the H–H bond from 0.75 Å to 0.76 Å. This finding is an important consideration, since it indicates that the mechanism for adsorption to be “intermediate” in nature is between physisorption and chemisorption [43].

2.4.2. Titanium

A study conducted by Liu et al. found that Ti atoms can bond to graphene through Dewar coordination [48]. Dewar coordination describes the process in which the graphene back-donates part of its π* electrons to the 3d_xz and 3d_yz orbitals of Ti, which reinforces the bonding between Ti and graphene. When hydrogenated, a single Ti atom can strongly bind to up to four H₂ with a binding strength of 0.23 to 0.60 eV. This can be ascribed to the hybridization of the 3d orbitals of Ti with the σ and σ* orbitals of H₂, also known as Kubas adsorption [42]. Additionally, the surface dipole and the dipole of the polarized H₂ create another form of attraction. According to this DFT study, Ti-decorated graphene possesses a maximum storage capacity of 7.8 wt.%. Storing H₂ in Ti–graphene oxide (Ti-GO) uniquely utilizes all three types of interactions that may be the key to achieving the DOE’s energy storage goals [49]. Yuan et al. found that titanium-decorated porous graphene (Ti-PG) maintains a hydrogen adsorption energy of −0.457 eV and a hydrogen storage capacity of 6.11 wt.%. Molecular dynamics simulations showed that a total of six hydrogen molecules can be adsorbed on both sides of the Ti-PG [6].

Experimental results highlighted significantly lower hydrogen storage capacities. The maximum hydrogen storage capacity achieved by Ti-PG was found to be 2.5 wt.%. This finding can be attributed to the high cohesive energies of the Ti atoms, which can lead to the development of large islands of Ti atoms immobilized onto graphene [49]. The previously mentioned DFT studies calculated hydrogen storage capacities under the assumption that each Ti atom remained isolated and spatially localized at the center of each graphene hollow site, which is not the case, experimentally [50]. In fact, Ti atoms tend to form aggregates, or islands, which can decrease the number of sites for hydrogen adsorption.

To resolve this aggregation, Ti-decorated samples can be sputtered. Experimental results found that sputtering the sample can increase the amount of Ti islands present and decrease the diameter of each as-formed island by a factor of two, which can be observed in Figure 7 [49]. The method used to achieve a highly uniform dispersion of metal atoms on graphene may be the answer to meeting more ambitious target storage capacity goals.

2.4.3. Chromium

There has been little work performed in the context of investigating chromium in graphene-based materials. One group initiated DFT first-principles studies on graphene nanoflakes (GNFs) to investigate the hydrogen storage performance of Cr-doped GNFs [51]. The presence of the Cr molecule puts stress on the underlying GNF, and the optimized geometry revealed the existence of Cr atoms outwardly protruding from the nanoflake sheet at a height of 1.766 Å.

The maximum number of stable hydrogen atoms that can be adsorbed on the Cr graphene nanoflake is three. The binding energy of Cr on graphene oxide nanoflakes versus that of penta-graphene differed by ~2 eV with a value of −4.407 eV for the former and a value of ~−6.93 eV for the latter. Computationally, the H₂ adsorption energy for penta-graphene has been reported to be −0.25 eV when adsorbed onto the Cr site and −0.574 eV for Cr doped GNF. The hydrogen storage capacity adsorbed by graphene nanoflakes should increase by up to 9 H₂ per metal atom by doping with Cr, according to the DFT simulations. Weight percentages were not reported in either of these simulation studies [51].

2.4.4. Iron

Iron has been a popular transition metal used in the decoration of graphene-based materials. It is relatively inexpensive and widely available, as compared with other materials used for similar decorative purposes [52] Preliminary studies have shown that iron can outperform other metals with respect to improving the hydrogen storage of graphene-based materials. Goharibajestani et al. studied the adsorption of H₂ and CO₂ at 298 K and 328 K onto reduced graphene oxide (RGO) decorated with Fe₃O₄. It was found that decoration with iron increased the measured adsorption strength of RGO to a greater extent than a comparable decoration level with copper. The group obtained a maximum hydrogen uptake of ~0.4 wt.% at 298 K and 9 bar [53]. By contrast, Moradi achieved a hydrogen storage capacity of 2.1 wt.% for Fe₃O₄-reduced graphene oxide under conditions of 77 K and atmospheric pressure [54]. Additional studies need to be conducted on Fe-decorated graphene-based supports at room temperature and low pressures in order to make accurate conclusions about how the introduction of Fe compares with other transition metals.

2.4.5. Nickel

Nickel represents a promising candidate for graphene oxide decoration, because of its catalytic effect on the breakdown of H₂ to 2H and its enhancement of the hydrogen spill-over effect. Multiple hydrogen molecules are able to adhere to the surface of the Ni nanoparticles and dissociate into atomic hydrogen [55]. In addition, Ni nanoparticles have been observed to give rise to less clumping and appear to be evenly distributed, both in the presence of or in the absence of a boron dopant [55,56].

There have been multiple experimental studies examining the impact of nickel on hydrogen storage capacity in graphene-based materials, all of which have concluded that it is most effective at lower temperatures. For example, Liu et al. found that the maximum hydrogen storage capacity for three-dimensional hierarchical porous graphene with nickel nanoparticles (3DHPG-Ni) was 4.22 wt.% at 77 K as compared with 1.95 wt.% at 298 K [55]. Similarly, Flamina et al. noted that Ni nanoparticles dispersed onto boron-doped reduced graphene oxide (Ni-B-rGO) yielded a maximum hydrogen storage capacity of 6.9 wt.% at 77 K as compared with 0.16 wt.% at 298 K [56]. This latter study also suggested that because of its large atomic size, the addition of nickel decreased the overall hydrogen storage capacity of RGO, since pristine reduced graphene oxide (rGO) was reported to possess a capacity of 9.8 wt.% at 77 K in the absence of any metal decorations or dopants [56]. However, a different report by Ismail et al. reported that the presence of Ni likely increased the hydrogen storage capacity of reduced graphene oxide from 2 wt.% to 2.7 wt.% at 80 K [57]. Hence, conflicting reports necessitate further study to more closely evaluate the effects and impacts of nickel content upon the measured hydrogen storage capacity. These studies could perhaps be performed at different temperatures and wt.% of Ni. At higher temperatures, the storage capacity has been found to decrease with the addition of Ni, because it exists in the form of oxides and hydroxides that hinder hydrogen adsorption [57]. Varying the weight% of Ni nanoparticles is also a factor in terms of systematically altering the hydrogen storage capacity. Indeed, multiple studies have determined that raising the wt.% correspondingly increased hydrogen adsorption to a certain extent [55,57].

2.4.6. Yttrium

A study conducted by Chakraborty and his associates examined the hydrogen storage capacity within yttrium-decorated SWCNTs through molecular dynamics (MD) simulations. They found that a single yttrium (Y) atom on a SWCNT can physisorb levels of up to six H₂ with 100% desorption, which is equivalent to 6.1 wt.% of hydrogen [58]. The Y atom stably adsorbs at the hollow site of the SWCNT with a binding energy of 2.20 eV and forms bonds with all six C atoms of the hexagonal C ring. The Y–C bond lengths are 2.65 Å for four of the carbon atoms and 2.36 Å for the other two carbon atoms.

The researchers in this study were also interested in the stability and formation of Y atom clusters on SWCNT. They discovered that there is no cluster formation at room temperature. Additionally, atoms do not drift with time, implying that the Y-SWCNT structure is reasonably stable. Currently, there have been no experimental studies conducted on either Y-SWCNT or other Y-decorated graphene materials [58]. However, the theoretical storage capacity of Y-SWCNT is remarkably close to the target value set by the DOE, and the stability of this material is high. Thus, experimental studies regarding the hydrogen storage capacity of this material would be of great value and should be investigated.

2.4.7. Zirconium

Another benefit of doping is that it can control the magnetic character of a system. A first-principles study conducted by Yadav et al. determined that the magnetic moment of a system can impact the desorption temperature, T_D, of the H₂. Specifically, the authors found that decreasing the magnetic moment of a system can lower the T_D to a value suitable for ambient condition applications, which incidentally meets the technical requirements of the DOE. Incorporating a Zr atom onto the graphene surface increasingly reduces the magnetic moment of the system as more hydrogen molecules are adsorbed [59].

The most stable position for the Zr atom is located at the hollow site bonded to six C atoms. The strong hybridization between the Zr d orbitals and C p orbitals gives rise to a binding energy of 2.4 eV, suggestive of the formation of a strong bond between the Zr atom and the surface of the graphene [59]. This allows for the Zr atom to sufficiently bind to graphene at ambient conditions and at the accessible temperatures needed to desorb H₂. Theoretically, a single Zr atom attached onto a graphene surface can adsorb up to a maximum of 9 H₂ with an average binding energy of 0.34 eV and an average desorption temperature of 433 K, thereby leading to as much as 11 wt.% of H incorporation [59].

2.4.8. Ruthenium

Ruthenium has long been considered to be a viable option for transition metal decoration, because of its ability to hold up to four hydrogen molecules when acting as an adsorption focal point [60]. Figure 8 highlights the hexagonal structure of a SWCNT with a Ru atom in the center. Each atom surrounding it is labeled 1 through 8 and represents the maximum of four hydrogen atoms adsorbing onto the center. However, this metal species only promotes adsorption in the hexagons directly above it when doped onto the underlying carbon lattice [60]. Through a DFT study, it was found that when Ru is noted as an adsorption center on a SWCNT, it maintains an adsorption energy of −0.93 eV/H₂ and when uniformly added onto a SWCNT, up to five Ru atoms are able to be absorbed without any apparent clustering [61]. Like other metals, however, problems arise when Ru forms clusters on the carbon, which decreases the binding energy of the hydrogen molecules, thereby rendering the structure as inherently unstable.

To mitigate this issue, a boron-doped carbon nanotube (CNT) has been proposed. Theoretically, the hydrogen storage capacity increases from 1.057 eV/H₂ for a pure CNT system to a more favorable 1.151 eV/H₂ for a boron-doped CNT system [62]. This finding can be ascribed to the formation of a strong covalent bond between the boron and the hydrogen atoms closest to it, thereby leading to the possibility of a more facile transfer of electrons from the Ru to the adjoining hydrogen atoms.

An experimental study by Wang and Yang noted that another way to increase hydrogen storage capacity in practice is through high-temperature thermal reduction, which increases the contact between the Ru and the underlying carbon support [60]. It was shown that the hydrogen storage capacity of the Ru-doped templated carbon generated by thermal reduction increased to 1.56 wt.% at 298 K and 10.3 MPa as compared with an untreated composite analogue with a lower hydrogen reduction capacity of 1.43 wt.% [60].

2.4.9. Palladium

One of the most promising elements used for decorating onto graphene is palladium. Pd-functionalized graphene and its associated derivatives are known to give rise to a high hydrogen storage capacity, even at room temperature and moderate pressure. According to Ma et al., the most favorable adsorption site for a single Pd atom on graphene is at the bridge location, where the binding energy is 1.09 eV. Decorating graphene with palladium also creates no significant structural deformations within the graphene surface [20]. In an experimental study performed by Huang et. al., graphene was treated with nitric acid to introduce functional groups that would serve as nucleation sites for metal decoration [63]. The nitric acid-treated graphene (Gr-HNO₃) was then decorated with Pd to yield Gr-Pd, thereby increasing the measured hydrogen storage capacity from 0.067 wt.% to 0.156 wt.% at 5.7 MPa and 303 K [63].

One issue when decorating graphene with Pd atoms is the presence of strong metal adhesion, which can lead to a certain level of aggregation. In fact, doping a Pd–graphene composite with a heteroatom can impact the electronic density of the material and increase the binding strength between the metal and the graphene support, thereby decreasing the metal–metal cohesion and the associated possibility of aggregation. Pd-N-HEG can give rise to a hydrogen storage capacity of 4.40 wt.% at 25 °C and 4 MPa. Pd₃Co-D(100)-G yields a corresponding hydrogen storage capacity of 4.83 wt.% at 25 °C and ~20 bar H₂ equilibrium pressure [64].

2.4.10. Platinum

In theory, platinum-decorated graphene denotes a preferred transition metal for graphene decoration, due to its high catalytic activity and its promotion of hydrogen adsorption in all neighboring hexagons on the graphene structure. In the same experimental study carried out by Huang et al., Gr-HNO₃ was decorated with Pt to yield Gr-Pt, which increased the hydrogen storage capacity of the pristine graphene sheet from 0.067 wt.% to 0.15 wt.% at 5.7 mPa and 303 K, thereby representing an increase in the measured storage capacity by a factor of 2.23 [63]. Figure 9 highlights the observed hydrogen storage capacities of Gr-Pt as compared with those of Gr-Pd, Gr-HNO, and pristine graphene (Gr-200).

However, similar to Ru and Pd, the high cohesivity of the graphene causes the Pt to form clusters, rather than individual atoms, which are needed for effective graphene decoration. The tendency to form clusters results from the fact that metal–metal interactions tend to be stronger than metal–host interactions [65]. To overcome this issue, nitrogen and boron atoms have been proposed as means with which to dope the decorated graphene and to improve the interactions between the platinum clusters and the underlying substrate. As reported by a computational study carried out by Chen et al., the interactions between the platinum clusters and the substrate can be increased by 0.84 eV and 0.23 eV for B-doped and N-doped graphene, respectively [65]. A different study performed by Tan et al. examined the effectiveness of Pt-decorated graphene for photocatalytic H₂ generation in which the same principle of Pt decoration on N-doped graphene was utilized [66]. Figure 10 shows the results of this doping process. It is apparent that the Pt nanoparticles are uniformly dispersed onto the N-doped graphene, while those decorated onto the corresponding pristine graphene are larger and more aggregated.

2.5. Other Metal Dopants

Aluminum

Aluminum has been investigated as a potential dopant for graphene-based hydrogen storage materials. To the best of our knowledge, no experimental results for Al-doped graphene materials have been discussed in the literature, and most of these analyses are theoretical in nature. As an example, a study by Ao et al. probed the hydrogen storage capacity of Al-doped graphene at room temperature. This work utilized DFT and involved calculations based on the DMol3 code. The concentration utilized for the Al-doped graphene was 12.5 atom%. An additional constraint in the calculations was to limit only one Al atom per graphene hexagonal ring in order to avoid clustering. The calculations found that the most favorable site for H₂ adsorption was at the center of the carbon ring for the normal, underivatized graphene, which yielded a binding energy of −0.159 eV. This small value implies the presence of physisorption, albeit weak. Thus, not surprisingly, pristine graphene is not suitable for hydrogen storage due to its low binding energy. By contrast, Al-doped graphene gave rise to a binding energy of −0.427 eV. This finding is suggestive of a contribution by physisorption as well, but this value is more conducive to the promotion of hydrogen storage, since this metal-based composite can hold more H₂ [67].

The previous study went further in depth by probing the projected electronic density of states (DOSs). For the H₂/graphene system, the main peaks of the H₂ were situated at −4.37 eV and +6.92 eV, while the characteristic peaks of the graphene were located at 9 and 13 eV. The non-overlapping nature of these two substances suggests the presence of very weak interactions. For the H₂/Al-doped graphene system, the main peaks associated with the composite’s electronic DOS are positioned at −8.15 eV, +5.74 eV, +6.52 eV, and +7.51 eV. That is, the H₂ interacts with the doped Al and C atoms simultaneously, a scenario which evinces a strong interaction between the two substances. This highlights a more stable system for H₂ adsorption. Furthermore, illustrations of the electron density distribution show that no electron exists in the region between the H₂ and C layer within the H₂/graphene system, but some electrons appear amongst the H₂, Al, and C atoms within the H₂/Al-doped graphene system. This finding further supports the stronger adsorptive capability of Al-doped graphene towards H atoms. Moreover, it was deduced that the Al-doped graphene could maintain as much as six H₂ per Al atom and accommodate a 5.13 wt.% hydrogen storage capacity at a binding energy of −0.260 eV under a 0.1 GPa pressure and at room temperature (T = 300 K) [67].

An analogous study performed by Akbari et al. was conducted on the electronic and structural properties of single Al and N atoms and on Al-N co-doped graphyne towards hydrogen storage. The DFT study was performed using the DMol3 package, similar to what had been previously reported with respect to Al doping alone. The most stable structures for the Al-doped, N-doped, and N-Al co-doped graphyne were found to be localized on the acetylenic carbon bond, hexagonal bond, and acetylenic carbon bond, respectively. These calculations yielded corresponding cohesive energies for these optimal positions of −8.188 eV, −8.267 eV, and −8.204 eV, respectively. Based on these data, it was determined that N doping is likely to be more appropriate than Al doping [68].

To confirm the previous findings, a hydrogen storage study was conducted on these same structures along with pristine graphyne as a basis of comparison. Pristine graphyne and Al-doped, N-doped, and Al-N co-doped graphyne were shown to hold a maximum of 4, 9, 7, and 16 H₂ per atom, respectively. Furthermore, pristine graphyne, Al-doped, N-doped, and Al-N co-doped graphyne gave rise to apparent adsorption energies of −0.146 eV, −0.141 eV, −0.139 eV, and −0.163 eV, respectively. The rationale for why the variously functionalized types of graphyne cannot accommodate for more hydrogen atoms can be attributed to the fact that the adsorption energy of H₂ to the structure falls below 0.1 eV. Pristine graphyne and Al-doped, N-doped, and Al-N co-doped graphyne were shown to bring forth hydrogen storage capacities of 5.26 wt.%, 10.17 wt.%, 8.75 wt.%, and 16.67 wt.%, respectively. Analyzing the corresponding electronic DOS diagrams, it can be observed that the Al peaks coupled with H₂ are much larger than the N peaks attached to H₂. These findings further cement the importance of Al in determining the hydrogen storage ability of Al-N co-doped graphyne [68].

2.6. Heteroatom Dopants

As previously stated, the addition of heteroatom dopants into the lattice of metal-modified graphene and its derivatives allows for improved metal decoration and thus a higher hydrogen storage capacity. Heteroatom doping involves replacing a carbon atom within the graphene lattice with an atom that is neither carbon nor hydrogen. When impurity atoms are introduced, concomitant vacancies are often inserted as well. Various types of heteroatoms can be used for doping, including but not limited to light alkali metals, alkaline earth metals, transition metals, nonmetals, and semimetals [69,70]. The effects of doping graphene-based materials with all of these heteroatoms on hydrogen storage capacity are thoroughly discussed by Ghotia et al. [69]. This review will briefly discuss the most popular heteroatoms used for doping and their consequential effects.

Nitrogen and boron have both been widely used to dope graphene-based materials. In particular, nitrogen-doped graphene and other related materials have likely undergone the most extensive studies. By introducing N atoms into the graphene network, π-electron density and hence catalytic activity can be significantly improved. This process directly increases the amount of hydrogen that can be adsorbed [69]. Moreover, B doping can improve hydrogen storage by increasing the enthalpy of adsorption of hydrogen on carbon. A first-principles study by Kim et al. examined the total energy calculations on impurity-doped C₃₆. They found that an increase in the enthalpy of adsorption occurs due to interactions between the strongly localized empty p_z orbital of B and the occupied σ orbital of H₂ [69]. As discussed in this paper, doping also improves the hydrogen storage capabilities of transition metal-decorated graphene-based materials. Specifically, the high cohesive energy of transition metals encourages the development of nanoparticle clusters since the transition metals are more attracted to each other than to the carbon atoms of graphene. Doping a graphene-based material with a heteroatom will limit the clustering effect by creating a stronger interaction between the heteroatom and the adjacent transition metal nanoparticle [65,69].

A fascinating area of interest is co-doping, which does not involve the inclusion of cobalt atoms, but rather the dual doping of two different heteroatoms within the same material. Al-N co-doped graphyne, as mentioned in the previous section, has been shown to be more effective than either purely Al-doped or N-doped graphyne [68]. Other combinations of heteroatoms, such as N-Pd-doped graphene, have demonstrated significant increases in hydrogen storage capacity. The combination of multiple heteroatoms appears to create a favorable synergistic effect that is not well defined but is consistently observed in several materials. The clear improvements ascribed to the presence of both heteroatoms render it possible to achieve a storage capacity even greater than that if a single dopant had been used instead [69].

2.7. Hydrogen Storage Capacity Characterization

An essential part of experimental studies revolves around measuring the hydrogen storage capacity of as-synthesized materials. A comprehensive summary of prior work in this area is provided in

Table 1. Hydrogen storage properties of various carbon and metal-modified carbon derivatives.

Material	H2 Storage Capacity (wt.%)	Bond Length (Å)	Binding Energy (eV)	Conditions	Reference
Pristine graphene	2.7			Room temperature, 25 bar	[19]
Li@graphene	4.32		0.30		[26]
Li@CNT	4.2			Room temperature, 10 MPa	[25]
Li@oxidized porous graphene	9.43				[27]
Li@MV graphene		2.64			[25]
Li@DV graphene		2.61			[25]
Single-sided Na@graphene	8.1				[28]
Double-sided Na@graphene	12.8				[28]
Na@B-graphene	11.7				[28]
Single-sided Na@graphyne	3.49				[29]
Double-sided Na@graphyne	5.98				[29]
Double-sided Na@BN-yne	5.84				[29]
K@B-graphene	22.5				[30]
K@graphyne	8.95		−0.212		[31]
Two-sided K@graphyne	13.95				[31]
Ca32C60	>8.4		~0.2		[32]
Double-sided Ca@B-graphene	8.38		0.46		[32]
Sr@C60			0.191		[32]
Be@C60, Mg@C60		3.5			[32]
Ti@graphene	7.8				[41]
Ti-PG	6.11		−0.457		[6]
Ti-PG	2.5				[41]
Cr@GNF			−0.574		[43]
Cr@pentagraphene			−0.25		[43]
Fe3O4@RGO	0.4			298 K, 9 bar	[45]
Fe3O4@RGO	2.1			77 K, atmospheric pressure	[46]
3DHPG-Ni	4.22			77 K	[47]
3DHPG-Ni	1.95			298 K	[47]
Ni-B-rGO	6.9			77 K	[48]
Ni-B-rGO	0.16			298 K	[48]
rGO	9.8			77 K	[48]
Ni@rGO	2.7			80 K	[49]
Y@SWCNT	6.1				[50]
Zr@graphene	11		0.34		[51]
Ru@TC	1.56			298 K, 10.3 MPa	[52]
Gr-HNO3-Pd	0.156			303 K, 5.7 MPa	[55]
Pd-N-HEG	4.40			25 °C, 4 MPa	[56]
PdCo-D(100)-G	4.83			25 °C, 20 bar	[56]
Pt@graphene	0.15			303 K, 5.7 MPa	[55]
Pt@B-graphene			0.84		[57]
Pt@N-graphene			0.23		[57]
Al@graphene			−0.427		[59]
Pristine graphyne	5.26				[60]
Al-doped graphyne	10.17				[60]
N-doped graphyne	8.75				[60]
Al-N co-doped graphyne	16.67				[60]

. Hydrogen sorption studies are typically conducted using a gas sorption analyzer. The simplest of these analyzers is the Sieverts apparatus (Figure 11). The Sieverts technique represents a volumetric method for measuring hydrogen uptake within a solid material. In this procedure, the sample material is loaded into a cell at atmospheric pressure. The cell is connected to a reference volume cell via a valve. The reference volume is pressurized with hydrogen gas, and the valve is subsequently opened. The amount of hydrogen that can be adsorbed into the material can be calculated from the change in the gas pressure of the system [71]. A Sieverts apparatus can be relatively easily constructed in the lab, thereby rendering it as a popular choice for characterization. This process, when performed properly, is simple, robust, accurate, and cost-effective [71]. However, large errors can arise if (i) the apparatus is not properly constructed, (ii) the sample has not been adequately degassed, and/or (iii) atmospheric adsorbates are present [72]. Additionally, the maximum sample size in a volumetric apparatus is limited by the volume of the sample and reference volume cell. The minimum sample size can also be constrained not only by the balance used to measure the initial mass of the sample but also by the resolution of the pressure transducer.

The extent of hydrogen sorption can also be determined gravimetrically. The gravimetric method assesses the hydrogen uptake in a material by measuring the change in mass after hydrogenation. A vacuum microbalance can be used to evaluate the change for samples that are a few grams in mass. In this technique, a sample material is loaded into a cell. The cell is then pressurized with hydrogen until an equilibrium mass is reached. The hydrogen uptake can be subsequently determined from the change in the mass of the sample. Buoyancy effect corrections must be considered to avoid errors in this method [72]. The sensitivity of the gravimetric method is limited by the sensitivity of the balance used to gauge the sample mass. A balance with a relatively high precision must be used to measure the change in mass of the sample. Due to the complexity of the latter, the volumetric method of analysis is more frequently used.

Lastly, thermal desorption spectroscopy can also be utilized to assess the level of hydrogen uptake. In this protocol, the sample material is hydrogenated and held at a temperature where minimal hydrogen desorption occurs. The sample is then heated, and the amount of hydrogen desorbed is measured. A thermogravimetric analysis/mass spectroscopy (TGA/MS) instrument can be used to heat the sample in a controlled manner and to accurately evaluate the amount of hydrogen that was adsorbed [72]. This approach can be more costly as compared with using a simple Sieverts approach but it will still yield reasonably reliable results.

3. Discussion

The hydrogen storage capacity of various materials can be modified by decorating and doping metals onto its surface. In order to meet the US DOE’s storage goals, the metal species being introduced into the graphene-based material’s surface must allow for more hydrogen atoms to bind to the material. For most of the metals discussed in this review, once a metal atom has adsorbed onto a support, several hydrogen atoms can subsequently bind onto the metal. Thus, decorating a support with metal atoms increases the material’s hydrogen storage capacity. Choosing the appropriate metal for either decoration or doping depends on many factors, including magnetic character, cohesive energy, binding energy, and stability. These properties must be carefully considered to create the most optimal hydrogen storage material.

Optimizing the magnetic character of a material allows for the adsorption/desorption of hydrogen at the desired ambient conditions of temperature and pressure. The magnetic moment of a system can influence the degree of charge transfer from a metal atom to a hydrogen molecule. This will indirectly impact the binding energy of the adsorbed H₂. Certain elements, such as Zr, can decrease the magnetic moment of a storage system which will, in turn, lower the desorption temperature of hydrogen to a value necessary for ambient condition applications. The charge transfer also impacts the modality by which hydrogen can adsorb onto the system. If the charge transfer is large enough, it can cause a dissociation of the H-H bond in molecular hydrogen, thereby leading to chemisorption. Smaller levels of charge transfer weaken the H-H bond, causing bond elongation, but the hydrogen remains in its molecular form for physisorption. In many cases, the hydrogen within a transition metal-decorated system is adsorbed in both forms. For Ti, of the four hydrogens able to be adsorbed, one hydrogen is in its dissociative form, while the remaining three are molecular in nature [73,74].

Achieving a high level of dispersion will also improve the observed hydrogen storage capacity. For example, agglomeration of metal nanoparticles on the surface of the material will affect the electronic properties of the composite material and reduce the number of active sites for hydrogen storage. To avoid unwanted agglomeration and aggregation, the metal should possess a low cohesive energy. Metals that have been shown to evince a low cohesive energy include Na, K, Mg, Ca, Ti, Fe, Cu, and Zn [75]. Alternatively, doping of the graphene-based material can also prevent agglomeration. Elements typically used for doping graphene-based materials include B, N, and S. When these atoms are introduced into the graphene or graphene-derivative lattice, the lattice’s charge distribution and electronic properties are altered. This scenario can help repel the metal nanoparticles from one another and prevent clustering.

4. Conclusions

The biggest challenge to realizing a hydrogen-centric economy is to find optimal materials that balance a low binding energy and an efficient hydrogen desorption at a desired ambient temperature while preventing premature desorption, coupled with the potential for high hydrogen storage capacities which are attractive for commercial applications. This review summarizes recent advances and the current status of various decorated and doped graphene-based materials for hydrogen storage, with a focus on meeting the DOE’s stated performance target. The fundamental mechanisms and calculations underlying hydrogen physisorption and chemisorption have been discussed, and characteristics of materials that show special promise as feasible hydrogen storage systems have been identified. Out of the plethora of materials that have been discussed in this review article, from a theoretical perspective, the greatest hydrogen storage capacity was achieved by K-decorated B-doped graphene, followed by Al-N co-doped graphyne and two-sided K-decorated graphyne. It is apparent that the use of metal modification is essential for improving the hydrogen storage capacity of graphene and of its derivatives. In addition, the synergistic effects of decorating and doping with multiple elements, particularly B, Al, and N, can further enhance hydrogen storage capacity. We postulate that an effective candidate for hydrogen storage must utilize metal modification in accordance with heteroatom co-doping in order to not only achieve the hydrogen storage targets set by the DOE but also, with future work and directed effort, possibly surpass them.

Since many previous studies have only conducted simulations on decorated and doped metals on graphene, there is a need to validate the theoretical findings through experiments involving material synthesis, testing, and comprehensive performance evaluations. This process entails (i) determining storage capacities under real-world application temperatures, (ii) enabling the attainment of achievable gravimetric density as compared with reported values, and most importantly, (iii) ensuring complete reversibility to the greatest extent possible. Integrating material development with real-world tests and fundamental studies is crucial for designing practical hydrogen storage systems. These hydrogen storage systems will play a critical role in applications of not only stationary and portable power but also transportation. Perhaps most notably, this research will aid in the practical and facile implementation of fuel cell vehicles by providing for safe, lightweight, and energy-efficient fuel options [4,76]. The need for enabling a resourceful, effective, and more carbon-neutral means of transportation is evident as challenges associated with ongoing climate crises continue to pose serious existential challenges. It is hoped therefore that future work in this field will help us to understand and to verify the underlying mechanisms and phenomena associated with disparate types of adsorption reactions in order to optimize and achieve the formulation of notably improved, efficient, and competitive hydrogen storage systems for ubiquitous practical applications.