Graphene Supports for Metal Hydride and Energy Storage Applications

Abstract

Energy production, distribution, and storage remain paramount to a variety of applications that reflect on our daily lives, from renewable energy systems, to electric vehicles and consumer electronics. Hydrogen is the sole element promising high energy, emission-free, and sustainable energy, and metal hydrides in particular have been investigated as promising materials for this purpose. While offering the highest gravimetric and volumetric hydrogen storage capacity of all known materials, metal hydrides are plagued by some serious deficiencies, such as poor kinetics, high activation energies that lead to high operating temperatures, poor recyclability, and/or stability, while environmental considerations related to the treatment of end-of-life fuel disposal are also of concern. A strategy to overcome these limitations is offered by nanotechnology, namely embedding reactive hydride compounds in nanosized supports such as graphene. Graphene is a 2D carbon material featuring unique mechanical, thermal, and electronic properties, which all recommend its use as the support for metal hydrides. With its high surface area, excellent mechanical strength, and thermal conductivity parameters, graphene can serve as the support for simple and complex hydrides as well as RHC (reactive hydride composites), producing nanocomposites with very attractive hydrogen storage properties.

1. Introduction

Energy storage is a key driver and supporter of the everyday needs of society. Within this context, metal hydrides are promising systems with the ability to store and release hydrogen gas, the sole element promising a sustainable, emission-free future [1,2,3,4,5,6,7,8,9]. While there are many binary and complex hydrides known, only those belonging to lightweight metals have practical implications due to their high gravimetric and volumetric H₂ content, in line with DOE’s current targets. However, their slow reaction kinetics, possible side reactions, and poor recyclability have prompted researchers to find mitigation strategies to overcome these shortcomings. The recent advances in SSHS (solid-state hydrogen storage) materials have been summarized, and they entail both physisorption and chemisorption of hydrogen [10]. In contrast to conventional systems based on high-pressure gas phase or low-temperature liquid-state hydrogen, SSHS systems have the tangible potential to offer high hydrogen storage capacity, sustainable performance, recyclability with little loss in storage capacity, as well as manageable production costs [1,2,3,4,5,6,7,8,9,10].

Among recent advances, nanoconfinement in a properly chosen support has shown important improvements regarding stability, thermodynamic parameters, and overall kinetics, oftentimes affording reversible systems where pristine hydrides showed little or no reversibility. For instance, MgH₂, which arguably is the metal hydride most investigated to date, features too high energy barriers (ΔH = −75 kJ/mol, T_des = 573 K) to be considered in its neat form in hydrogen storage tanks as such. Additionally, the dehydrogenated material (Mg) is a very reactive metal (ε_ox = +2.37 V), susceptible to oxidation by even minute oxygen traces, which would lead to MgO and, upon moisture exposure, to Mg(OH)₂; this way, the reversibility of the cycle Mg/MgH₂ would be lost. Additionally, bare MgH₂ is also plagued by particle agglomeration during cycling, which can lead to a continuous decrease in H₂ storage capacity and even slower kinetics. These downsides observed for the model system Mg/MgH₂ have prompted scientists to further tune the thermodynamic parameters by altering the morphological features of MgH₂. Nanostructuring, alloying, and compounding with additives or catalysts are among the most utilized mitigating strategies.

Confinement in nano-sized supports would afford better particle size control upon absorption/desorption a/d cycling, along with increased stability due to the confinement matrix (typically silica or carbon). Furthermore, additional hydride may support interactions that further decrease total system energy. While many types of carbo materials have been investigated as supports (OMC-ordered mesoporous carbon, AC activated carbon, carbon nanotubes CNTs, etc.), graphene has emerged as a two-dimensional carbon material with unique electronic, mechanical, and thermal properties that can act as an efficient support material for metal hydrides. Several properties are essential to accomplishing this task: its large surface area (in excess of 2500 m²/g), the layered structure forbidding hydride NP from agglomeration, high electrical conductivity, and excellent mechanical strength. The inclusion of hydride materials into nanoporous scaffolds is also an important aspect; while ball milling (BM) seems to be the prevalent method, the incipient wetness/infiltration method and in situ generation of hydride materials inside the matrix are gaining momentum, especially due to the better dispersion and polydispersity of NPs produced by the latter methods. With a higher surface-to-volume ratio, hydride NPs are expected to behave differently in hydrogenation studies, with lower enthalpies compared to bulk and enhanced kinetics due in part to more efficient H₂ diffusion in the thin hydride layer of a few nm [1,2,3]. Solvent infiltration/evaporation is more commonly used in the case of complex hydride solutions, given their higher solubility in ethereal solutions such as MTBE and similar inert ethers [2,3,4,5,6].

The current review aims to discuss the recent advances in graphene-supported metal hydrides for energy storage applications, covering the main features of composite systems based on graphene (G, GO, and related) and metal hydrides (LiH, MgH₂, LiBH₄, LiAlH₄, NaBH₄, NaAlH₄, Mg(BH₄)₂, Mg₂NiH₄, Mn(BH₄)₂, and others) [11]. The applications of the generated nanocomposites have also been briefly described, with an emphasis on the energy storage properties of said composites. The conclusions and future outlook directions identify some areas where improvement strategies may yield enhanced energy storage properties of hydride/graphene composites, as well as raising awareness regarding the proper handling and processing of end-of-life composite energy materials, a prerequisite for a sustainable future built around a hydrogen economy.

2. Overview of Metal Hydrides and Graphene Supports

In an effort to reduce CO₂ emissions and the subsequent greenhouse effect, H₂ production can be achieved from renewable sources (biomass, water) by electrolysis, thermolysis, or photocatalytic splitting [11]. However, the lightweight and limited space requirements for a viable vehicular fuel have shifted attention towards solid-state hydrogen materials, and metal hydrides are an excellent example of such materials. However, metal hydrides do not cumulatively meet the requirements for high gravimetric content (~6.5 wt.%, ideally), fast and reliable recycling behavior (over 1000 cycles without significant degradation), and moderate thermodynamic parameters (enthalpies around 30 kJ/mol) that would allow operation under the expected range (−40…100 °C). Some of these shortcomings can be alleviated by the inclusion of the active hydrogen-storing compound in a nanoporous matrix such as graphene [12]. Graphene is a 2D material with very appealing properties, highlighting its potential use as support for various reactive species, including metals and metal hydrides [13]. By embedding hydride species into graphene supports, valuable nanocomposites can be obtained with direct use for energy storage applications (Figure 1) [11,12,13].

Engineering MH_x@G composites has been no easy task, but a vast majority of researchers have turned their attention towards the Mg/MgH₂ system, in part due to its favorable thermodynamic properties (ΔH = 65.8–75.2 kJ/mol), high H₂ storage capacity (7.7 wt.%, 109 g H₂/L), and most important, due to the reversible nature of the hydrogenation/dehydrogenation process [14,15,16]. Assessing the support contribution to H₂ storage is equally importantly, and functionalization strategies have yielded interesting results in ball-milled Mg-B-electrochemically synthesized reduced graphene oxide (erGO) with enhanced H-uptake kinetics [17]. B-doped graphene has also been investigated by density-functional theory DFT studies [18]. Tan et al. have shown, using principal calculations, that the adsorption energy of H₂…B-doped G can be tuned, enhancing the interaction by positively charging the support. BC5, for instance, can store up to 5.3 wt.% H₂. More significantly, the study sheds light on the heteroatom doping of graphene; B-doped G forms easier than N-doped G (5.6 eV/atom for the former vs. 8.0 eV/atom for the latter) [18]. Combining metal doping (Ni) with B-doped carbon-based materials, Wang et al. studied and compared various forms of carbon with those of 3D graphene-based materials, obtaining composites with surfaces functionalized with O-containing groups that further enhanced the spillover effect with a positive effect on H₂ storage properties [19]. Edge-functionalization of G for kinetic control of HSM (hydrogen storage materials) has also been reported [20].

Among various strategies to improve the behavior of hydrogen storage materials at the nanoscale, nanoconfinement and nanostructuring were shown to be very effective [2,21,22]. Nanostructured metal hydrides MH_x showed a tangible improvement in H₂ storage properties [2,21], while nanostructures of type hydride @ C in particular were proven to be a model system for the future solid-state hydrogen storage fuel, as evidenced by DFT computation by Shevlin and Guo in 2008 [22]. DFT simulations have played a pivotal role in the expansion of the hydrogen storage field, the above-mentioned resource comparing a few of the most important hydrides: MgH₂, LiNH₂, LiBH₄, and derived reactive hydride composites RHCs, as well as their interaction with various carbon systems [22].

2.1. Metal-Decorated Graphene

An important strategy for tuning graphene for energy storage applications has been to decorate the support with metals or clusters of metals. Among the investigated materials are metallized siligraphene nanosheets (SiG) with varying light metal decorations (Li, Na, K, Mg, Ca, Sc, Ti) [23], metal-decorated graphene (Li, Na, Mg)/G, DFT study [24], or K in K @ B-substituted G [25]. Given the potential of AlH₃ for hydrogen storage, Al-doping has also been explored in Al/G composites [26,27], Al_n clusters supported by coronene and graphene G (DFT study) [27], and Al/Si –SLG (SLG, single layer graphene + Si +Al) [28].

Palladium is well known for its high affinity for H₂, and many studies have been devoted to the theoretical modelling of this interaction; Pd-decorated N-doped G, DFT study [29]; Pd_n @ G (n = 1–4) in BC3 variant [30], and Cu- and Pd-decorated G, DFT study [31]. Titanium and its clusters have also been investigated: Ti₃ clusters [32]; Ti₄—decorated B/N-doped G [33]; Ti₄ & Ni₄–doped G nanoplatelets [34]; and Ti–Al subnanoclusters on G [35].

Most reports, however, focus on Mg-doped porous carbonaceous materials, such as Mg@ G flake nanocomposites (H₂ generation from H₂O) [36], Mg@graphite for comparison purposes [37], Mg@rGO layers [38,39], Mg@Heteroatom–doped G [40], Mg@B–doped G [41], and Mg/defected GO [42]. Additionally, various alloys have also been studied for graphene supports: Mg alloy @rGO–V₂O₃ [43], rGO–EC@AB5 hybrid material (EC = ethyl cellulose, AB5 = La(Ni_0.95Fe_0.05)₅) (LNF) [44], or MmNi_3.55Co_0.75Mn_0.4Al_0.3/G nanoplatelets (Mm denotes mischmetals) [45].

2.2. Mechanistic Insight and Kinetics of H₂…Support Interaction

Pristine graphene can chemically absorb H₂ and its theoretical storage capacity is 7.7 wt.%; the hydrogenated graphene (graphane, (CH)_n, a sp³ hybridized analog of graphene) releases H₂(g) at ~400 °C, with an Ea = 158 kJ/mol (1.64 eV) [46]. While an intriguing material in its pristine form, its thermodynamic parameters make it less feasible for scaling up processes aimed at vehicular applications; however, it is worth noting the similarity of the activation energy deduced for graphane and that of metal hydrides.

The fundamental understanding of the adsorption/desorption mechanism of H₂ in graphene is paramount to developing new materials aimed at this task; a pertinent comparison between physisorption and chemisorption on graphene was reported in 2011, where the physical limitations of G (5 wt.% H₂ storage) were correlated to the entropy contribution TΔS and the large van der Waals distance between two H₂ molecules (0.3 nm), further preventing the increase in gravimetric storage capacity of pristine graphene [7]. The interaction H₂…G was studied by DFT in single- and double-vacancy graphene by Wu et al., with direct implications for the behavior of defected graphene during hydrogenation studies [47]. The mechanism of H₂ interaction with Al-doped porous graphene has been reported by Ao et al., showing by DFT that Al/G can store up to 10.5 wt.% H₂, with a relatively low H₂ adsorption energy of −1.11 to −0.41 eV/H₂, which would potentially allow hydrogen absorption/desorption near room temperature conditions, in agreement with the findings from the analysis of atomic charges, electronic distribution, and density of states (DOS) of the system [26]. The enhanced interaction was potentially due to the polarization of the adsorbed H₂ molecules.

Akilan et al. have studied by DFT the adsorption of H₂ molecules on B/N-doped defected (5-8-5, 55-77, 555-777 and 5555-6-7777 defects) graphene sheets [48]. The N-atom addition (donor behavior, n-type semiconductor) increases the delocalized electrons, while the B atoms (acceptor, p-type semiconductor) increase the localized electrons in the considered system. The most efficient adsorption was modelled when the H₂ molecule approached the sheet in a perpendicular direction (−80 meV), while the least efficient interaction was observed in a parallel orientation (−9 meV), while the delocalized electron density was higher on the fusion points of the pentagonal and hexagonal rings and would therefore enhance H₂ adsorption [48]. Another supporting DFT study of H₂ storage on TM-doped defected graphene (TM = transition metal) revealed that in the case of TM = Sc, the 555-777/Sc structure doped with Sc showed the maximum H₂ capacity, with H₂ binding energies in the range 0.2–0.4 eV/H₂ [49].

The advances regarding TM-loaded Mg-based alloys/G have been reviewed recently [8]. A few important points are attributed to graphene: It can inhibit grain growth, thus aiding the overall cyclability of the composite, and it can (co-)catalyze the hydrogenation process, in which the electron transfer between Mg and C plays a key role [8].

The cyclic behavior of metal hydrides can be affected by issues related to grain growth. This can be partly overcome with the formation of G layers encapsulating MgH₂ to prevent grain growth [50]. In this report, Lototskyy et al. used various carbon sources (graphite, AC, MWCNTs, etc.) and showed that the formation of graphene sheets during high-energy reactive ball milling in hydrogen (HRBM) is responsible for the encapsulation of MgH₂, noting an increase in a/d cycling behavior along with a more reduced size of the MgH₂ crystallites (40–125 nm vs. 180 nm in pristine form) [50]. The catalytic role of graphene nanoplatelets (GNP) over H₂ storage kinetics in Mg has been studied by Ruse et al. [51]. The enhancement of more than an order of magnitude was attributed to GNP properties (size, thickness, defect density, and specific surface area), and these can be further tuned to alter H₂ storage kinetics in Mg–GNP nanocomposites [51]. A carbon-neutral, reversible, and sustainable process that produces H₂ is the formate-bicarbonate system, where graphene has also served as a support of Pd and Ru metals [52].

2.3. Manufacturing Techniques

Several techniques have been utilized to introduce metal catalysts into graphene, synthesizing (nano)composites containing graphene and carbonaceous materials [9]. While ball milling and its variants remain a key technique, other options have been explored: Electrostatic layer-by-layer self-assembled G/MWCNTs [53], Uranium U-decorated G (H₂ and D₂ adsorption) [54], and plasma-assisted milling in Mg@FLG composites (few-layer graphene nanosheets) [55]. Given the remarkable properties of 2D graphene on hydride storing materials, the synthesis of 2D MgH₂ has also been proposed in DFT studies [56].

3. Applications of Graphene-Based Hydride Nanocomposites

Graphene-supported metal hydrides have potential applications in hydrogen storage, battery electrodes, and fuel cells, among others. These are mainly the contributions of the embedded metal hydrides that have been utilized in energy storage devices such as batteries and fuel cells. By embedding metal hydrides in high-surface-area graphene, the obtained nanocomposites can further enhance the performance and efficiency of pristine hydrides, potentially yielding competitive novel energy storage devices with improved cycling stability and durability [2,21].

3.1. Batteries—Battery Electrodes

Graphene-supported metal hydrides have also been studied as electrode materials in batteries and fuel cells. The addition of graphene to metal hydride electrodes can improve their cycling stability and reduce their electrode polarization. This improves the efficiency and lifespan of the batteries and fuel cells. The high surface area of graphene also enhances the electrochemical performance of the electrodes by providing more active sites for electrochemical reactions.

Composites such as MH_x@G have been utilized for batteries [57,58,59]. Mixed-valence oxides (Co₃O₄) and hydroxides (Ni(OH)₂) have also been explored in Co₃O₄ nanocubes on N-doped G for Ni-metal hydride batteries [60] and Ni(OH)₂-coated N-doped G aerogel as replacement NiCd and NiMH batteries [61]. Various anode materials based on graphene have recently emerged, such as MgH₂ self-assembled on G as anode material for Li-ion batteries [62].

3.2. (Super)capacitors, Electrochemical Storage

Supercapacitors are another energy storage device that can benefit from the addition of graphene-supported metal hydrides in MH_x@G composites [57,58]. Supercapacitors are electrochemical devices that can store energy by electrostatically adsorbing ions at the surface of the electrode [53]. The improvement of using Li-ion (120–170 Wh/Kg) vs. Ni-metal hydride (40–100 Wh/Kg) has to be considered when designing new storage devices [58]. The high surface area and conductivity of graphene can enhance the electrochemical performance of supercapacitors. The addition of metal hydride nanoparticles to the graphene electrodes can further enhance the energy storage capacity and stability of supercapacitors. Other materials based on graphene have also been investigated, for instance Fe₃O₄ @ G composite for high-performance Li-ion hybrid capacitors [63]. Electrochemical storage properties have also been studied on heteroatom-substituted materials, such as N-doped C [64], but also on La_0.7Mg_0.3(Ni_0.85Co_0.15)_3.5@Ni/Graphene [65].

3.3. Solar Cells and Portable Electronic Devices

Graphene-supported metal hydrides have also been studied for their application in solar cells. The addition of metal hydrides to the graphene electrodes in solar cells can improve their photovoltaic performance by enhancing charge separation and transport properties. The high conductivity and large surface area of graphene also facilitate electron transfer and reduce recombination losses in the solar cells. In addition to the above applications, graphene-supported metal hydrides have also been studied for their potential use in energy storage for portable electronic devices, power grid stabilization, and energy harvesting. The unique properties of graphene and metal hydrides can be tailored to meet the specific requirements of these applications and enhance their performance [2].

3.4. Energy Storage

One of the most promising applications of graphene-supported metal hydrides is in hydrogen storage for fuel cell vehicles. Fuel cell vehicles are gaining popularity due to their high efficiency and low emissions. However, the limited availability of hydrogen storage materials has hindered their widespread adoption. This aspect will be discussed in more detail in the following section (Section 4) [2,21].

4. Hydrogen Storage Properties of Composites MH_x@G

Metal hydrides have shown promise as hydrogen storage materials, but their low hydrogen storage capacity and slow kinetics have limited their application. The addition of graphene as a support material can enhance the hydrogen storage capacity and kinetics of metal hydrides, making them a promising candidate for hydrogen storage in fuel cell vehicles.

4.1. Binary Hydrides—The Case of MgH₂

While light metal hydrides show great promise as H₂-storing materials, the literature and scientific research are still focused on MgH₂ due to its advantageous gravimetric content, activation energies for desorption/adsorption, and multiple synthesis methods. The following subsections explore the H₂ storage properties of non-catalyzed and catalyzed nanocomposites such as MgH₂@G.

4.1.1. Non-Catalyzed Support

A great variety of reports in the literature focus on the Mg/MgH₂ system embedded in various supports, including graphene [1] and GLM (graphene-like materials) (Figure 2) [16], although some other binary hydrides (layer-by-layer lithiated composites such as LiH/G for instance) have also been reported [66]. For instance, MgH₂ NPs nanoparticles have been embedded in GNS (graphene nanosheets) to yield MgH₂ NPs@GNS composites [67], MgH₂ NPs nanoparticles uniformly-grown@G [68].

The mechanism of MgH₂@G composite synthesis by reactive mechanical grinding (under 2 MPa H₂) has been studied by Jang et al., who concluded that G played a catalytic role in H₂ absorption, considering the absorption kinetics and PCT (pressure-composition-temperature) curves [69]. The composite MgH₂–5 wt.% graphene could store 3.69 wt.% H₂ at 423 K, while increasing the graphene amount in MgH₂–5 wt.% graphene allowed 5.08 wt.% at the same temperature (423 K) [69]. Comparing the PCI isotherms for the two samples, it seemed that MgH₂–10 wt.% graphene displays a lower activation energy Ea and better cyclic stability compared to MgH₂–5 wt.% graphene [69].

Another strategy to enhance the stability of MgH₂ during cycling was to create Mg/rGO multilaminates to produce Mg/MgH₂@rGO during cycling. These composites exhibited a very high 6.5 wt.% reversible H₂ storage and a capacity of 0.105 Kg H₂/L [70]. When MgH₂ interacts with 5 wt.% GO incorporated in MgH₂ during hydrogenation studies, it also causes a reduction in the porous support (GO→rGO), as highlighted by a study by Pukazhselvan et al. (Figure 3) [71].

The reduction occurs even during composite preparation, namely during mechanical milling, and it affects the dehydrogenation temperature of MgH₂ (a reduction of 60 °C vs. pristine) by lowering the dehydrogenation activation energy (by 20 kJ/mol). Considering the affinity of Mg for oxygen, this could also be used as a method to obtain MgO-decked rGO composites [71]. Various carbon scaffolds have been used for MgH₂, including but not limited to graphite, graphene derivatives, and carbon fiber [72]. Combined experimental and DFT studies on the helical form of graphene nanofibers (HGNF) by Singh et al. [73] revealed that the extra carbon sheet edges exposed would further improve MgH₂ dehydrogenation compared to G-catalyzed MgH₂, and the experimental results showed a 45 °C improvement due to HGNF use in MgH₂ + 7 wt.% HGNF nanocomposites. The DFT results supported this finding by revealing a weakening of Mg-H bonds by interaction with zigzag and armchair type HGNF, forming C–Mg–H and C–H bonds [73]. Combined DFT and experimental computations have also been performed on other MgH₂@G variants, which confirmed once again the catalytic role of G in reducing the dehydrogenation enthalpy and dehydrogenation activation energy of MgH₂ [74].

Other approaches tackle graphene support functionalization, as is the case of MgH₂–FG (FG = fluorographene nanosheets). Using fluorographene, the nanocomposite MgH₂–FG absorbed 6.0 wt.% within 5 min, and desorbed 5.9 wt.% within 50 min, at 300 °C [75].

4.1.2. Catalyzed Support

Various catalysts have been utilized to improve the behavior of hydride@graphene composites. For instance, MgH₂ catalyzed by TiH₂@Gr (desorption onset at 204 °C), with comparative improvements relative to the catalytic activity of Ti@Gr, TiO₂@Gr, and TiH₂@Gr [76]. Other Ti-based salts such as TiCl₃ have been used in MgH₂ catalyzed by TiCl₃ and GO-based porous carbon (GC) in composites such as MgH₂/GC-TiCl₃, obtained by isothermal hydrogenation of MgBu₂/GC, first to yield MgH₂/GC and then ball milling with TiCl₃ to form the final composite with about 78% MgH₂ [77]. The MgH₂/GC-TiCl₃ composite can release reversibly up to 7.6 wt.% H₂ within 9 min at 300 °C, a result superior to even the MgH₂-GC-TiCl₃ mixture (weight ratio of 1:1:0.08, produced by direct ball milling for 5 h) (Figure 4) [77]. This highlights the advantages of the stepwise synthesis procedure outlined above.

Among the metals showing the most striking improvements in hydrogenation studies, Ni has a distinct role. For instance, MgH₂ has shown improvements in dehydrogenation behavior when catalyzed by Ni@N-doped C spheres [78], or by NiCo–functionalized G [79]. MgH₂–Ni@NCS composites produced by Wang et al. were obtained by hydriding combustion and high energy ball milling Ni@NCS into Mg [78]. These composites showed good stability even after 10 a/d cycles, and moderate H₂ storage parameters (a: 5. wt.%, d: 4.3 wt.% H₂ within 8 min at 623 K; a: 4.2 wt.% in 60 min at 373 K (Figure 5)) [78].

When NiCo bimetallic NPs (4–6 nm) were used to functionalize graphene oxide supports, NiCo/rGO were obtained via a one-pot synthesis and produced energy storing nanocomposites MgH₂–NiCo/rGO with excellent cycling stability, featuring reduced Ea = 105 kJ/mol, absorbing 6.1 wt.% H₂ within 350 s at 300 °C under 0.9 MPa H₂ [79]. The metal sources were the corresponding metal chlorides (NiCl₂.6H₂O and CoCl₂.6H₂O) that underwent sonication with GO/EG, reflux, and reduction with N₂H₄.H₂O/NaOH [79].

Other support functions have also been pursued, such as MgH₂@PTh core-shell NPs on G (PTh = polythiophene) for use in Li-ion batteries [59], or MgH₂@NiPc (NiPc = nickel phthalocyanine)/G [80].

A summary of the main parameters in other doped-graphene supports for binary hydrides or alloys is given in

Table 1. Doped-graphene supports for Mg/MgH₂ and related alloys.

Hydrogen Storing Species	Graphene Dopant	Nanocomposite	Observations	Ref.
Mg	5 wt.% NiS	Mg—5 wt.% NiS/rGO (by HPMR, hydrogen plasma metal reaction)	ball milling Mg and NiS/rGO, the NiS-catalyzed support was prepared by co-reduction of Ni2+—impregnated GO support; des: 3.7 wt.% H2 in 10 min and 4.5 wt.% H2 in 60 min; Ea, abs = 44.47 kJ/mol; Ea, des = 63.02 kJ/mol	[81]
MgH2	Ni	MgH2@Ni/G	(MgH2 + 10 wt.%G + 10 wt.%Ni) composites were investigated from a combined DFT and experimental viewpoint; des. onset at 339.5 °C	[82]
MgH2	Ni	MgH2@Ni–Gn	nickel-decorated graphene support (Ni–Gn) afforded abs: 6.28 wt.%, 100 min, 373 K, and des: 5.73 wt.%, 1800 s, 523 K	[83]
MgH2	ZrCo	MgH2@10 wt.% ZrCo/G	The ZrCo dopant was present as 2D nanosheets; des: 6.3 wt.% H2, 5 min, 300 °C, Ea,des = 90.4 kJ/mol; abs: 4.4 wt.% H2, 10 min, 120 °C, 3 MPa H2, Ea,ads = 57.6 kJ/mol.	[84]
MgH2	KOH	MgH2 + KOH/graphene	KMgH3, MgO formed in-situ; Ea = 109.89 kJ/mol; 5.43 wt.% H2 reversibly. Graphene provides more H diffusion channels and better disperses the catalysts	[85]
Alloys HSAs	–	(LaCeY)(NiMnCoAl)5@rGO	HSAs = hydrogen storage alloys; retention rate 51.25% at a discharge current density 3000 mA/g	[86]
Mg–Al alloy	Ni	Mg90Al10-x (80 wt.%Ni@Gn)	Mg–Al alloy in graphene–supported Ni; x = 0,4, 8, 12 wt.%; for x = 8, a: 5.11 wt.% in 400 s, 523 K; d: 5.81 wt.%, 1800 s, 573 K	[87]
Mg–Al alloy	TiF3	Mg–Al alloy–TiF3@G	Mg–Al-M (M = G, TiF3, and TiF3@G) composites; Ea = 139.8 kJ/mol for TiF3@G support; Mg–Al–TiF3@G released 5.41 wt.% at 350 °C	[88]
Mg–Al alloy	Y2O3	Mg–Al alloy—Y2O3@G	Mg-Al-Y2O3@rGO reversibly store/release H2 at 250 °C; with 5 wt.% of Y2O3@rGO loading, Mg–Al composite had Ea,release = 145.9 (vs. 162.6 kJ/mol for pristine Mg–Al alloy). Ea,uptake = 54.3 kJ/mol for Y2O3 catalyzed G.	[89]
MgH2	FeNi	MgH2@FeNi/rGO	5 wt.% FeNi/rGO modified MgH2 released 6.5 wt.% H2 at 300 °C (onset 230 °C); uptake: 5.4 wt.%, 20 min, 125 °C, 32 bar H2	[90]
MgH2	Ni–CeOx	MgH2–Ni–CeOx/GNS	MgH2 catalyzed by Ni–CeOx/GNS; graphene nanosheets supported nanoscale Ni&CeOx (x = 1.69)	[91]
MgH2	TiB2	MgH2–TiB2/GNS	MgH2 catalyzed by TiB2/GNS	[92]
MgH2	NiCu	MgH2–NiCu/rGO	MgH2 catalyzed by NiCu/rGO from double layer hydroxide	[93]
MgH2	Fe–Ni	MgH2 + 10 wt.% Fe–LiCo 3D G	MgH2@Fe–Ni/G, namely MgH2 + 10 wt.% Fe–LiCo 3D G (3D-graphene)	[94]
MgH2	NiFe–LDH	MgH2–Ni3Fe/rGO	NiFe–LDH (layered double hydroxide precursor)/GO yield (Ni3Fe/rGO) active catalyst	[95]
MgH2	Al, Cu	Mg90Al10–Cu@G	MgH2→Mg–Al alloys with Cu introduced in situ: Mg90Al10–Cu@G nanoplates	[96]
MgH2	–	MgH2@CA microspheres	abs: 6.2 wt.% H2 within 5 min at 275 °C; des: 4.9 wt.% H2 within 100 min at 350 °C; Ea,des = 114.8 kJ/mol.	[97]
MgH2	TiC	MgH2–TiC@G	desorption at 180 °C by the plasma carbon–modified MgH2/TiC containing FLGS (few layer graphene sheets)	[98]
MgH2	SrF2 and SrF2	MgH2–SrF2(SrF2)@Gr	MgH2 cat. by SrF2 and SrF2@Gr additives	[99]
MgH2	FeCoNi	MgH2@FeCoNi NPs/G	MgH2@FeCoNi NPs/G as new catalyst	[4]
MgH2	FeOOH	MgH2@FeOOH nanodots/G	MgH2@FeOOH nanodots NDs @G, release H2 at 229.8 °C (ΔT = 106.8 °C lower than pristine MgH2), showing good cycling stability (over 20 cycles, 98.5% of initial capacity maintained, while also reducing the activation energy Ea)	[100]
Mg	Ni	Mg/MgH2 + Ni/GLM	Mg@Ni/G nanocomposites with 5–60 wt.% Ni loading: Mg/MgH2 + Ni/GLM (graphene-like material)	[101]
MgS	–	MH2@G, MgS@G	MgS@G prepared by reaction: MH2@G + S → MgS@G + H2; for advanced Li-storage	[102]

.

For instance, Zhang et al. used 2D ZrCo nanosheets to catalyze MgH₂ decomposition [84]. When 10 wt.% ZrCo was added, the composite MgH₂@10 wt.% ZrCo/G released 6.3 wt.% H₂ at 300 °C in 5 min. (Figure 6). When bimetallic catalysts such as ZrCo are homogeneously dispersed on G, they play a role similar to that of Mg₂Ni/Mg₂NiH₄ (as a “hydrogen pump”), easing the dissociation of H₂ as well as its recombination, along with weakening the Mg–H bonds, further improving the hydrogen storage properties of MgH₂ [84].

4.2. Complex Hydrides Embedded in Graphene Supports: M(BH₄)_n, M(AlH₄)_n

Nanoconfined complex hydrides have emerged as prime candidates in energy storage applications. They possess the highest H₂ gravimetric storage capacity, and careful tuning of their composition, additives, and morphology could yield highly reversible systems with real prospects in vehicular applications [2]. Graphene (non-catalyzed or catalyzed) has served as the support for manufacturing such stable, high-performance nanocomposites of complex hydride@G. A summary of more representative examples of such composites is given in

Table 2. Doped-graphene supports for complex hydrides M(BH₄)_n, M(AlH₄)_n and related hydrides.

Hydrogen Storing Species	Graphene Dopant	Nanocomposite	Observations	Ref.
LiBH4, LiAlH4, NaAlH4	(C3N4)	(Li, Na)XH4 (X = B/Al)/C3N4	LiBH4, LiAlH4, NaAlH4/C3N4 studied by DFT; support offers suitable adsorption site for AlHx/BHx (x = 3, 2, 1)	[103]
LiBH4, LiAlH4, NaAlH4	–	(Li, Na)XH4 (X = B/Al)/G	LiBH4, LiAlH4, NaAlH4/Graphene and Graphdiyne; DFT study shows strong support interaction due to well-defined pore structure	[104]
LiBH4	N	LiBH4–N-doped G/MC	LiBH4 in 10 at%N-doped Graphene/resorcinol formaldehyde; impregnation degree: 30, 50 and 70 vol%; XRD diffraction 2θ = 12.6° that can be attributed to Li–B–N(G)–H interaction	[105]
LiBH4	Ni/Co	LiBH4–Ni/Co NPs–N-doped G aerogels	LiBH4 catalyzed by NiCo NPs–N-doped G aerogels; Co-decorated, des.: 8 wt.% H2 at 325 °C (1st cycle; with 1 wt.% at 226 °C). Ni-decorated, des: 8 wt.% H2,	[106]
LiBH4	–	LiBH4–G	LiBH4 wrapped in G	[107]
LiBH4	Fe3O4	LiBH4–Fe3O4/porous G	LiBH4 catalyzed by (Fe3O4 dispersed on porous G)	[108]
LiBH4	–	LiBH4–(mesoporous resorcinol-formaldehyde/G)	LiBH4 nanoconfined in (mesoporous resorcinol-formaldehyde/G) entangled supports; des: 13 wt.% at 400 °C; recharging at 400 °C, 5 h, 60 bar H2; 6 wt.% reversible storage capacity;	[109]
LiBH4	Ni nanocrystals (2–4 nm)	LiBH4/G	LiBH4 (5–10 nm)–Ni(2–4 nm)/G, affording 9.2 wt.% H2 due to by-passing B2H6 and B12H122– anion formation	[110]
LiAlH4	NiCo2O4	LiAlH4–NiCo2O4 @ rGO	LiAlH4 dehydrogenation by NiCo2O4 nanorods@rGO nanocomposites by ball milling; LiAlH4 + 7 wt.% NiCo2O4@rGO des. onset at 62.7 °C (in total 6.28 wt.% H2); 4.0 wt.% hydrogen within 20 min at 150 °C (isothermal)	[111]
LiBH4	NiFe2O4	LiBH4-graphene-NiFe2O4 (Ar)	EA = 127 kJ/mol (vs. 170 kJ/mol for pristine LiBH4); after 5 a/d cycles. H2 storage was ~6.14 wt.%; des. onset at 349 °C	[112]
NaBH4	–	nano-NaBH4@GNs	Ultrasmall (6–10 nm) nano-NaBH4@GNs by MFSP (mechanical-force-driven self-printing, a technique similar to 3D printing), for scalable fabrication of 0D complex hydrides in 2D supports; ~5 wt.% stable H2 storage capacity	[113]
NaAlH4	–	NaAlH4@G	NaAlH4 (12 nm)@G–50; G weakens Al–H bonds of NaAlH4; des.: 5.6 wt.% at 300 °C; 3.8 wt.% (120 °C); Ea = 68.23 kJ/mol (vs. 128 kJ/mol for bulk)	[114]
NaAlH4	NP–TiH2	NaAlH4– 7 wt.% NP–TiH2@ G	NaAlH4 catalyzed by 7 wt.% NP–TiH2@G as active catalyst; 5 wt.% H2 is achieved reversibly, with onset below 80 °C; TiH2 (~50 nm lateral, ~15 nm thick) produced my metathesis TiCl4 + LiH in THF with G support	[115]
NaAlH4	N. CNTs	NaAlH4@N-doped G/CNTs	NaAlH4@N-doped G and CNTs (produced by NH3 treatment of G, 600 °C, 30 min); 1.8 wt.% reversible H2 storage	[116]
NaAlH4	–	NaAlH4/GNs	NaAlH4/GNs graphene nanosheets, C60 fullerenes and MC mesoporous carbon; NaAlH4–support interactions revealed by FE–SEM and 27Al solid state NMR.	[117]
NaAlH4	–	NaAlH4@rGO	NaAlH4 @rGO framework, GOF with NaAlH4@GOF (1 M), desorbing 1.01 wt.% (20.0 wt.% NaAlH4), 16.6% of bulk NaAlH4 due to oxygen functional groups in the GOF reduced by BH4−.,	[118]
NaAlH4	TiO2	TiO2–NaAlH4@G	Layer-by-layer TiO2–NaAlH4@G composites with 90% high loading (des. peak at 191.6 °C vs. 286.5 °C for bulk NaAlH4)	[119]
NaAlH4	–	NaAlH4@G	NaAlH4@G nanofibers	[120]
NaAlH4	Al, TiCl3	Al, Ti-doped NaAlH4@G	Ti-doped NaAlH4@G: NaAlH4 co-doped 2 mol% TiCl3, 10 mol% G, 5 mol% Al (ball milling)	[121]
NaAlH4	CeH2.51	NaAlH4@FLG/Ce (CeH2.51)	NaAlH4@FLG/Ce (CeH2.51); onset at 85 °C, 5.06 wt.% H2 at 200 °C; 4.91 wt.% reversible after 8 a/d cycles. Ce activated G surface.	[122]
NaAlH4	Heteroatoms	NaAlH4@dopant/G	NaAlH4@dopant/G (dopant: S-, N-, vacancies, B, N, O-, P-, F-, HO-), investigated by means of DFT	[123]
Mg(BH4)2	–	Mg(BH4)2@G	Mg(BH4)2@G synthesized by [MgH2@G + B2H6]; it was shown that G weakens the Mg–H and B–H bonds, affording a desorption of H2 starting at 154 °C (onset) and up to 225 °C (end)	[124]
Mg(BH4)2	–	Mg(BH4)2@rGO	Mg(BH4)2@rGO (1–4 wt.%), investigation of mechanism and phase evolution	[125]
Mg(BH4)2	–	Mg(BH4)2@G	Mg(BH4)2@G; the 3 polymorphs of Mg were produced from MgBu2 by tuning reaction conditions (11.2, 10.3, and 9.9 wt % H for the γ, β, and α phases)	[126]
NaMgH3	–	NaMgH3 @GO	NaMgH3 @GO and NaMgH3 @ MWCNTs	[127]
K2NaAlH6	–	K2NaAlH6–GS	GS graphene sheets as catalysts for sodium potassium alanate K2NaAlH6	[128]
Mn(BH4)2	Ni, LiNH2	Mn(BH4)2@(Ni, LiNH2)/G	Mn(BH4)2@G (Ni, LiNH2 additives) by solvent infiltration/extraction	[129]
Mg2Ni	–	Mg2Ni–rGO/MWCNTs	Mg2Ni–rGO/MWCNTs for battery—supercapacitor hybrid device (high discharge capacity 644 mAh/g)	[130]
Mg2Ni	–	Mg2Ni/rGO	rGO/Mg2Ni—enhanced cycling stability	[131]
Mg2NiH4	–	Mg2NiH4@GS	Mg2NiH4@GS (surface graphene nanosheets)	[132]

.

Huang et al. have described an interesting method to prepare NaAlH₄-based composites with an average hydride size of 12.4 nm, while highlighting the advantage of supporting the complex hydride on graphene (0.31 eV vs. 1.34 eV) (Figure 7) [114]. With high theoretical storage capacities (7.5 wt.% gravimetric, 94 g H₂/L volumetric), NaAlH₄ could desorb H₂ in three steps (via Na₃AlH₆—3.7 wt.%, further decomposition to NaH—1.9 wt.% and the often overlooked third step to Na metal); however, due to thermodynamic considerations, only the first two steps are achievable within reasonable temperature ranges.

While mechanochemistry or solvent infiltration are typically utilized to confine metal hydrides in nanoporous supports, it is widely accepted that the in situ generation of hydride species can provide more intimate contact between the active hydrogen storage species and the support, resulting in enhanced hydrogenation parameters. Zhang et al., for instance, synthesized the Mg(BH₄)₂@G (MBH–Gr) composite, which has the ability to start releasing H₂ at 154 °C vs. mechanochemically milled BM–MBH–Gr (257 °C) or pristine Mg(BH₄)₂ (296 °C), showing a tangible ~6 wt.% storage around 200 °C (Figure 8). The synthesis proceeded via the hydrogenation of MgBu₂, which shows favorable adsorption of graphene, and β–MgH₂ NPs of ~8 nm are formed highly dispersed on the substrate (“MH-Gr”) as a result of this interaction. The large spacing between graphene layers could be responsible for the lack of agglomeration during the solid–gas reaction with B₂H₆ in order to generate Mg(BH₄)₂ NPs (reaction (1)) [124].

$$MgB{u}_2\stackrel{50 bar{H}_2,170°\mathrm{C} }{\to }Mg{H}_2\stackrel{+B_2{H}_6,G}{\to }Mg{\left(B{H}_4\right)}_2$$

(1)

However, the authors also noted simultaneous formation of MgB₁₂H₁₂ (2478 cm⁻¹ in FTIR spectra), and DFT data showed that the synthesis of MgB₁₂H₁₂ (∆H = −320 kJ/mol) and Mg(BH₄)₂ ((∆H = −308 kJ/mol) is fairly similar. These values suggest that the presence of graphene favors the formation of Mg(BH₄)₂. The optimized geometries (Figure 8d) also reveal that graphene weakens Mg–H and B–B bonds (they are longer in the presence of graphene vs. pristine), congruent with the easier formation of Mg(BH₄)₂ [124].

4.3. Reactive Hydride Composites (RHCs) in G Hosts

There are only a few examples of MgH₂-based nancomposites; for instance, the MgH₂–LiBH₄ RHC [133] and 2 LiBH₄–MgH₂ @ G (80 wt.% loading capacity) [134]. In the case of the latter composite, the synthesis method started from MgH₂ @ G [134]. Not surprisingly, the MgH₂ was compounded with LiBH₄, another key component and widely–explored hydrogen storage complex hydride (18.5 wt.%, 121 kg H₂/m³), with the positive results making use of LiBH₄ destabilization by binary metal hydrides such as MgH₂, in addition to the advantageous nanoconfinement effect in the porous graphene support [133]. The mutual destabilization of the system 2LIBH₄–MgH₂ is depicted in reaction (2), where the final decomposition products are LiH and MgB₂. Even without graphene supports, the RHC system 2LIBH₄–MgH₂ showed reversibility under moderate conditions (50 bar H₂, 300 °C), considerably lower than the harsh rehydrogenation requirements from spent LiBH₄ (200 bar H₂, 700 °C).

$$2{ \mathrm{LiBH}}_4+{\mathrm{MgH}}_{2 }\leftrightarrow 2 \mathrm{LiH}+{\mathrm{MgB}}_2+4{ \mathrm{H}}_2$$

(2)

Xia et al. have demonstrated the solvothermal procedure to impregnate graphene with RHC precursors that finally yielded the binary composite 2LiBH₄–MgH₂, with homogeneous particle size (10.5 nm) uniformly dispersed on graphene support even at very high loadings, which afforded a reversible 9.1 wt.% hydrogen capacity that did not degrade below 8.9 wt.% after 25 a/d cycles (Figure 9) [134].

While other C-based materials supporting RHCs are rather common, the same cannot be said about G-supported RHCs. The improvements observed in lowered dehydrogenation temperature and parameters can be also traced back to the thermal conductivity of the graphene support, which enables the effective dissipation of heat and uniform heating throughout the composite [133]. Interestingly, nanoconfinement might not be solely responsible for the observed effects, as XRD data showed characteristic hydride peaks; hence, these were not fully confined, and the main role might actually be reserved to the highly specific surface area of graphene as well as to the nanosized metal hydrides.

4.4. Other Hydride Systems Confined in 2D-Graphene

Among hydrogen storing materials, ammonia borane (AB) plays a special role. While it has been investigated thoroughly, the mechanism of dehydrogenation is still not very clear, presumably due to polymerization processes occurring during thermal decomposition, and, perhaps more importantly, the system is not reversible. However, there have been attempts to incorporate AB into the porosity of catalyzed graphene supports, which yielded composites such as AB–Ni-Co/rGO catalyst [135], AB/Cu@CuCoOx supported on G [136], or AB/rGO (or GO) produced by ice-templating sheets of GO, serving as supports for the one-step integration of AB. The AB@rGO nanocomposite released H₂ as low as 70 °C [137]. A closely-related lithiated species of AB, namely lithium amidoborane LiNH₂BH₃, has been utilized to produce LiNH₂BH₃/G nanocomposites that released 10.9 wt.% H₂ without (BN)_x byproducts such as borazine [138].

Other 2D materials such as MXenes have also been incorporated in hydrogen storing materials such as hydrides, providing excellent improvements, presumably due to the presence of catalytic Ti sites and superior morphological features such as its highly-specific surface area [139].

Liquid organic hydrides can also release H₂ under G–CNT composite catalysts [140]. Substituted alkyl (R = Me) and aryl (R = Ph) silanes R_xSiH_4−x (R = Ph, Me) have also been employed in H₂ generation from MeOH. In this instance, however, the Ru-based catalyst was immobilized in the rGO support, producing the highly-active catalyst [Ru]–rGO (0.05 mol%), where [Ru] = [Ru(p-cym)Cl₂(NHCl)] complex [141]. To sum up, there are various approaches spanning metal salt catalysts [142], heterostructured supports [143], Ni-based catalysts supported by DFT computations [144], trimetallic catalysts FeCoNi NPs [145], or bimetallic PdRu [146] that have all played an essential role in the development of efficient catalysts and supports for metal hydrides and magnesium hydride in particular.

5. Conclusions

Featuring high hydrogen storage capacity and electrochemical properties, metal hydrides embedded in graphene supports have emerged as a new class of nanocomposite systems with promising features for energy storage systems. However, their development and integration in practical systems still face challenges to be overcome for their widespread adoption and long-term viability. The cost-effectiveness of these composite systems is an essential factor in their widespread adoption and increased lifespan viability. The cost of raw materials as well as production and energy costs are still too high, prompting the development of cost-effective and scalable fabrication processes.

Regarding the environmental impacts and ways to limit their availability, the use of CRM (critical raw materials) such as Mg, Ni, and Ti and their continuing depletion worldwide are of concern. The development of alternative materials that do not rely on CRMs is crucial for the sustainability of this technology. The long-term stability and durability of metal hydrides embedded in graphene supports are crucial for their sustainability, as frequent replacements can result in the generation of more waste and have a significant environmental impact. The optimization of the synthesis process and the development of durable and stable materials can improve the lifespan and performance of metal hydrides embedded in graphene supports, making them more sustainable. The disposal of metal hydrides embedded in graphene supports at the end of their lifecycle is also a concern. The development of recycling processes for metal hydrides and graphene supports can reduce the environmental impact of their disposal.

6. Current Challenges and Future Outlook

Energy storage systems are today in need of technological breakthroughs to revolutionize the way energy is produced, distributed, and stored. Various reviews have presented the critical shortcomings of current technology [3]. Nanocomposites based on metal hydrides and graphene currently feature hydrogen storage capacities lower than the targets set by the Department of Energy (DOE) for practical hydrogen storage (6.5 wt.% as the ultimate goal). Moreover, the cycling stability of metal hydrides needs to be improved to ensure the long-term performance of the composite material, while the potential side reactions occurring at the nanoscale must be further tuned and reduced. The scalability and cost-effectiveness of the synthesis methods used to prepare graphene-metal hydride composites need further refinement. Current methods such as ball milling (most commonly used) and in situ growth are time-consuming and require expensive equipment with high associated energy bills, which limits their practical application for large-scale production, which is compulsory for industrial adoption of this technology.

Future research efforts addressing these challenges ought to focus on developing new metal hydrides with higher hydrogen storage capacities and improved cycling stability. In addition, new synthesis methods should be explored to improve the scalability and cost-effectiveness of the process. Advances in computational modeling and simulation can also be used to predict the performance of new metal hydride candidates and optimize their synthesis routes. In this regard, thermodynamic considerations are an essential starting point for the successful selection of future hydride-based materials.

Additionally, while graphene does feature many desirable properties, other carbon-based materials can be explored for the development of new hybrid materials with enhanced properties for energy storage applications, such as carbon nanotubes (MWCNTs) or other forms of porous carbon (OMC, CMK-3, etc.). Carbon materials can be further activated to enhance hydrogen-support interactions, which in turn will lead to a more effective H₂ storage material. Hence, current strategies need to focus on enhancing H₂-support interactions [4] by any of the following approaches: heteroatom-doping of porous G/other nanoporous materials; coordinatively unsaturated metal sites in MOFs; nanoconfinement of H₂ into nanopores (<1 nm); and enhanced orbital interactions H₂-adsorbent [4].

As the global demand for clean energy solutions continues to grow, the development of efficient energy storage systems such as graphene-metal hydride composites can play a crucial role in accelerating the transition towards a sustainable energy future.