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Graphene Supports for Metal Hydride and Energy Storage Applications

National Institute of Materials Physics, Atomistilor Str. 405 A, 077125 Magurele, Romania
Crystals 2023, 13(6), 878;
Submission received: 15 May 2023 / Revised: 24 May 2023 / Accepted: 25 May 2023 / Published: 27 May 2023
(This article belongs to the Special Issue Advanced Technologies in Graphene-Based Materials)


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.

Graphical Abstract

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, Tdes = 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 (εox = +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 MHx@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 MHx 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], Aln 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]; Pdn @ 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(Ni0.95Fe0.05)5) (LNF) [44], or MmNi3.55Co0.75Mn0.4Al0.3/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 MHx@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 MHx@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 La0.7Mg0.3(Ni0.85Co0.15)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 MHx@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.
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.
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].
M g B u 2 50   b a r H 2 , 170 ° C   M g H 2 + B 2 H 6 , G M g B H 4 2
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   LiBH 4 + MgH 2   2   LiH + MgB 2 + 4   H 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 RxSiH4−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.


This work was supported by the Romanian Ministry of Research and Innovation through the Project No. PN-III-P1-1.1-TE-2021-1657 (TE 84/2022) and by the Core Program of the National Institute of Materials Physics, granted by the Romanian Ministry of Research, Innovation and Digitization through the Project PC3-PN23080303.

Data Availability Statement

Not applicable.


This work was supported by the Romanian Ministry of Research and Innovation through the Project No. PN-III-P1-1.1-TE-2021-1657 (TE 84/2022) and Project PC3-PN23080303.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.


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Figure 1. Graphene sits at the core of various improvement strategies for better and more effective embedding of active hydrogen storage materials.
Figure 1. Graphene sits at the core of various improvement strategies for better and more effective embedding of active hydrogen storage materials.
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Figure 2. Synthesis of metal-carbon-graphene composites. Reprinted from ref. [16] under Creative Commons Attribution (CC BY) license.
Figure 2. Synthesis of metal-carbon-graphene composites. Reprinted from ref. [16] under Creative Commons Attribution (CC BY) license.
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Figure 3. Dehydrogenation kinetics profiles obtained for 5 wt.% GO incorporated MgH2. Profile “a’’ corresponds to the as-prepared sample, and profile “b” corresponds to the hydrogen-restored sample. Dehydrogenation temperature/pressure: 315 °C/1 bar. Reprinted from reference [71] under Creative Commons Attribution (CC BY) license.
Figure 3. Dehydrogenation kinetics profiles obtained for 5 wt.% GO incorporated MgH2. Profile “a’’ corresponds to the as-prepared sample, and profile “b” corresponds to the hydrogen-restored sample. Dehydrogenation temperature/pressure: 315 °C/1 bar. Reprinted from reference [71] under Creative Commons Attribution (CC BY) license.
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Figure 4. Cyclic dehydriding (DH) curves of the MgH2-TiCl3, MgH2/GC-TiCl3 and MgH2-GC-TiCl3 composite samples at 300 C under 0.1 bar H2. Reprinted with permission from Ref. [77].
Figure 4. Cyclic dehydriding (DH) curves of the MgH2-TiCl3, MgH2/GC-TiCl3 and MgH2-GC-TiCl3 composite samples at 300 C under 0.1 bar H2. Reprinted with permission from Ref. [77].
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Figure 5. (a) Preparation process of the MgH2–Ni@NCS composite. (b) Absorption and desorption cycling of MgH2–Ni@NCS composite at 623 K; (c) Isothermal hydrogenation curves of the MgH2–Ni@NCS composite and milled MgH2 at 423 K; (d) Isothermal dehydrogenation curves of the MgH2–Ni@NCS composite and milled MgH2 at 598 K. Reprinted with permission from Ref. [78].
Figure 5. (a) Preparation process of the MgH2–Ni@NCS composite. (b) Absorption and desorption cycling of MgH2–Ni@NCS composite at 623 K; (c) Isothermal hydrogenation curves of the MgH2–Ni@NCS composite and milled MgH2 at 423 K; (d) Isothermal dehydrogenation curves of the MgH2–Ni@NCS composite and milled MgH2 at 598 K. Reprinted with permission from Ref. [78].
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Figure 6. (a) DSC curves of MgH2 + x (x = 5, 10, 15) wt.% ZrCo composites; (b) Kissinger’s plots (b) of the MgH2 with and without 10 wt.% ZrCo composites; (c) Isothermal dehydrogenation/hydrogenation curves of the MgH2 + 10 wt.% ZrCo and MgH2 + 10 wt.% ZrCo/5 wt.% Graphene composites as a function of cycling. Reproduced with permission from ref. [84].
Figure 6. (a) DSC curves of MgH2 + x (x = 5, 10, 15) wt.% ZrCo composites; (b) Kissinger’s plots (b) of the MgH2 with and without 10 wt.% ZrCo composites; (c) Isothermal dehydrogenation/hydrogenation curves of the MgH2 + 10 wt.% ZrCo and MgH2 + 10 wt.% ZrCo/5 wt.% Graphene composites as a function of cycling. Reproduced with permission from ref. [84].
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Figure 7. (a) Schematic illustration of the synthesis of SAH@G-50. Isosurface of electron density of the NaAlH4 on graphene and (NaAlH4)6 on graphene; (b) Energies required for the removal of hydrogen from NaAlH4 with (0.31 eV) and without the presence of graphene (1.34 eV); (c) Isothermal dehydrogenation of SAH@G-50 in comparison with NaAlH4 and the ball-milled NaAlH4/G composite at various temperatures. Reprinted with permission from Ref. [114].
Figure 7. (a) Schematic illustration of the synthesis of SAH@G-50. Isosurface of electron density of the NaAlH4 on graphene and (NaAlH4)6 on graphene; (b) Energies required for the removal of hydrogen from NaAlH4 with (0.31 eV) and without the presence of graphene (1.34 eV); (c) Isothermal dehydrogenation of SAH@G-50 in comparison with NaAlH4 and the ball-milled NaAlH4/G composite at various temperatures. Reprinted with permission from Ref. [114].
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Figure 8. (a) Schematic illustration of the fabrication of MBH-Gr via a space-confined solid–gas reaction: the graphene-tailored solid–gas reaction between MH-Gr and B2H6 to synthesize Mg(BH4)2 NPs that are uniformly distributed on graphene (MBH-Gr) under the protection of hydrogen; (bd): Relaxed atomic configurations of MgH2 and B2H6, and calculated reaction enthalpies for the formation of (b) Mg(BH4)2 and (c) MgB12H12 from the reaction between MgH2 and B2H6 with the support of graphene. (d) Relaxed atomic configurations of MgH2 and B2H6 without and with the support of graphene. Gray, white, green, and pink spheres are C, H, Mg, and B atoms, respectively; (e) Mass spectra of the as-synthesized MBH-Gr compared with BM-MBH-Gr and pure Mg(BH4)2; (f) Isothermal dehydrogenation of the as-synthesized MBH-Gr at various temperatures, with BM-MBH-Gr and pure Mg(BH4)2 at 175 °C included for comparison. Reprinted with permission from Ref. [124].
Figure 8. (a) Schematic illustration of the fabrication of MBH-Gr via a space-confined solid–gas reaction: the graphene-tailored solid–gas reaction between MH-Gr and B2H6 to synthesize Mg(BH4)2 NPs that are uniformly distributed on graphene (MBH-Gr) under the protection of hydrogen; (bd): Relaxed atomic configurations of MgH2 and B2H6, and calculated reaction enthalpies for the formation of (b) Mg(BH4)2 and (c) MgB12H12 from the reaction between MgH2 and B2H6 with the support of graphene. (d) Relaxed atomic configurations of MgH2 and B2H6 without and with the support of graphene. Gray, white, green, and pink spheres are C, H, Mg, and B atoms, respectively; (e) Mass spectra of the as-synthesized MBH-Gr compared with BM-MBH-Gr and pure Mg(BH4)2; (f) Isothermal dehydrogenation of the as-synthesized MBH-Gr at various temperatures, with BM-MBH-Gr and pure Mg(BH4)2 at 175 °C included for comparison. Reprinted with permission from Ref. [124].
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Figure 9. (a) Schematic illustration of the fabrication of graphene-supported 2LiBH4-MgH2 nanocomposite: (1) self-assembly of uniform MgH2 NPs on graphene via solvothermal treatment; (2) infiltration of LiBH4 solution; (3, 4) removal of solvent and heterogeneous nucleation of LiBH4 on MgH2 NPs.; (b) Thermogravimetric analysis curves of the as-prepared 2LiBH4-MgH2 nanocomposites anchored on graphene with various loadings compared with the ball-milled composite (bulk); (c) isothermal dehydrogenation of LBMH80@G at various temperatures under a back pressure of 0.3 MPa.; (d) Long-term cycling performance of the dehydrogenation (under a back pressure of 0.3 MPa) and hydrogenation for LBMH80@G and bulk 2LiBH4-MgH2 composite at 350 °C; (e) normalized H2 capacity as a function of cycle number, where the hydrogen capacities are normalized to the theoretical value of 2LiBH4-MgH2 composite; (f) SEM and (g) TEM images of LBMH80@G after 15 cycles of dehydrogenation. Reprinted with permission from Ref. [134].
Figure 9. (a) Schematic illustration of the fabrication of graphene-supported 2LiBH4-MgH2 nanocomposite: (1) self-assembly of uniform MgH2 NPs on graphene via solvothermal treatment; (2) infiltration of LiBH4 solution; (3, 4) removal of solvent and heterogeneous nucleation of LiBH4 on MgH2 NPs.; (b) Thermogravimetric analysis curves of the as-prepared 2LiBH4-MgH2 nanocomposites anchored on graphene with various loadings compared with the ball-milled composite (bulk); (c) isothermal dehydrogenation of LBMH80@G at various temperatures under a back pressure of 0.3 MPa.; (d) Long-term cycling performance of the dehydrogenation (under a back pressure of 0.3 MPa) and hydrogenation for LBMH80@G and bulk 2LiBH4-MgH2 composite at 350 °C; (e) normalized H2 capacity as a function of cycle number, where the hydrogen capacities are normalized to the theoretical value of 2LiBH4-MgH2 composite; (f) SEM and (g) TEM images of LBMH80@G after 15 cycles of dehydrogenation. Reprinted with permission from Ref. [134].
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Table 1. Doped-graphene supports for Mg/MgH2 and related alloys.
Table 1. Doped-graphene supports for Mg/MgH2 and related alloys.
Hydrogen Storing SpeciesGraphene DopantNanocompositeObservationsRef.
Mg5 wt.% NiSMg—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]
MgH2NiMgH2@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]
MgH2NiMgH2@Ni–Gnnickel-decorated graphene support (Ni–Gn) afforded abs: 6.28 wt.%, 100 min, 373 K, and des: 5.73 wt.%, 1800 s, 523 K[83]
MgH2ZrCoMgH2@10 wt.% ZrCo/GThe 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]
MgH2KOHMgH2 + KOH/grapheneKMgH3, 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@rGOHSAs = hydrogen storage alloys; retention rate 51.25% at a discharge current density 3000 mA/g[86]
Mg–Al alloyNiMg90Al10-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 alloyTiF3Mg–Al alloy–TiF3@GMg–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 alloyY2O3Mg–Al alloy—Y2O3@GMg-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]
MgH2FeNiMgH2@FeNi/rGO5 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]
MgH2Ni–CeOxMgH2–Ni–CeOx/GNSMgH2 catalyzed by Ni–CeOx/GNS; graphene nanosheets supported nanoscale Ni&CeOx (x = 1.69) [91]
MgH2TiB2MgH2–TiB2/GNSMgH2 catalyzed by TiB2/GNS [92]
MgH2NiCuMgH2–NiCu/rGOMgH2 catalyzed by NiCu/rGO from double layer hydroxide [93]
MgH2Fe–NiMgH2 + 10 wt.% Fe–LiCo 3D GMgH2@Fe–Ni/G, namely MgH2 + 10 wt.% Fe–LiCo 3D G (3D-graphene) [94]
MgH2NiFe–LDHMgH2–Ni3Fe/rGONiFe–LDH (layered double hydroxide precursor)/GO yield (Ni3Fe/rGO) active catalyst [95]
MgH2Al, CuMg90Al10–Cu@GMgH2→Mg–Al alloys with Cu introduced in situ: Mg90Al10–Cu@G nanoplates [96]
MgH2MgH2@CA microspheresabs: 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]
MgH2TiCMgH2–TiC@Gdesorption at 180 °C by the plasma carbon–modified MgH2/TiC containing FLGS (few layer graphene sheets)[98]
MgH2SrF2 and SrF2MgH2–SrF2(SrF2)@GrMgH2 cat. by SrF2 and SrF2@Gr additives [99]
MgH2FeCoNiMgH2@FeCoNi NPs/GMgH2@FeCoNi NPs/G as new catalyst [4]
MgH2FeOOHMgH2@FeOOH nanodots/GMgH2@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]
MgNiMg/MgH2 + Ni/GLMMg@Ni/G nanocomposites with 5–60 wt.% Ni loading: Mg/MgH2 + Ni/GLM (graphene-like material) [101]
MgSMH2@G, MgS@GMgS@G prepared by reaction: MH2@G + S → MgS@G + H2; for advanced Li-storage[102]
Table 2. Doped-graphene supports for complex hydrides M(BH4)n, M(AlH4)n and related hydrides.
Table 2. Doped-graphene supports for complex hydrides M(BH4)n, M(AlH4)n and related hydrides.
Hydrogen Storing SpeciesGraphene DopantNanocompositeObservationsRef.
LiBH4, LiAlH4, NaAlH4(C3N4)(Li, Na)XH4 (X = B/Al)/C3N4LiBH4, 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)/GLiBH4, LiAlH4, NaAlH4/Graphene and Graphdiyne; DFT study shows strong support interaction due to well-defined pore structure [104]
LiBH4NLiBH4–N-doped G/MCLiBH4 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]
LiBH4Ni/CoLiBH4–Ni/Co NPs–N-doped G aerogelsLiBH4 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]
LiBH4LiBH4–GLiBH4 wrapped in G [107]
LiBH4Fe3O4LiBH4–Fe3O4/porous GLiBH4 catalyzed by (Fe3O4 dispersed on porous G) [108]
LiBH4LiBH4–(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]
LiBH4Ni nanocrystals (2–4 nm)LiBH4/GLiBH4 (5–10 nm)–Ni(2–4 nm)/G, affording 9.2 wt.% H2 due to by-passing B2H6 and B12H122– anion formation [110]
LiAlH4NiCo2O4LiAlH4–NiCo2O4 @ rGOLiAlH4 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]
LiBH4NiFe2O4LiBH4-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]
NaBH4nano-NaBH4@GNsUltrasmall (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]
NaAlH4NaAlH4@GNaAlH4 (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]
NaAlH4NP–TiH2NaAlH4– 7 wt.% NP–TiH2@ GNaAlH4 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]
NaAlH4N. CNTsNaAlH4@N-doped G/CNTsNaAlH4@N-doped G and CNTs (produced by NH3 treatment of G, 600 °C, 30 min); 1.8 wt.% reversible H2 storage [116]
NaAlH4NaAlH4/GNsNaAlH4/GNs graphene nanosheets, C60 fullerenes and MC mesoporous carbon; NaAlH4–support interactions revealed by FE–SEM and 27Al solid state NMR. [117]
NaAlH4NaAlH4@rGONaAlH4 @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]
NaAlH4TiO2TiO2–NaAlH4@GLayer-by-layer TiO2–NaAlH4@G composites with 90% high loading (des. peak at 191.6 °C vs. 286.5 °C for bulk NaAlH4) [119]
NaAlH4NaAlH4@GNaAlH4@G nanofibers [120]
NaAlH4Al, TiCl3Al, Ti-doped NaAlH4@GTi-doped NaAlH4@G: NaAlH4 co-doped 2 mol% TiCl3, 10 mol% G, 5 mol% Al (ball milling)[121]
NaAlH4CeH2.51NaAlH4@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]
NaAlH4HeteroatomsNaAlH4@dopant/GNaAlH4@dopant/G (dopant: S-, N-, vacancies, B, N, O-, P-, F-, HO-), investigated by means of DFT [123]
Mg(BH4)2Mg(BH4)2@GMg(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)2Mg(BH4)2@rGOMg(BH4)2@rGO (1–4 wt.%), investigation of mechanism and phase evolution [125]
Mg(BH4)2Mg(BH4)2@GMg(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]
NaMgH3NaMgH3 @GONaMgH3 @GO and NaMgH3 @ MWCNTs [127]
K2NaAlH6K2NaAlH6–GSGS graphene sheets as catalysts for sodium potassium alanate K2NaAlH6 [128]
Mn(BH4)2Ni, LiNH2Mn(BH4)2@(Ni, LiNH2)/GMn(BH4)2@G (Ni, LiNH2 additives) by solvent infiltration/extraction [129]
Mg2NiMg2Ni–rGO/MWCNTsMg2Ni–rGO/MWCNTs for battery—supercapacitor hybrid device (high discharge capacity 644 mAh/g)[130]
Mg2NiMg2Ni/rGO rGO/Mg2Ni—enhanced cycling stability[131]
Mg2NiH4Mg2NiH4@GSMg2NiH4@GS (surface graphene nanosheets) [132]
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Comanescu, C. Graphene Supports for Metal Hydride and Energy Storage Applications. Crystals 2023, 13, 878.

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Comanescu C. Graphene Supports for Metal Hydride and Energy Storage Applications. Crystals. 2023; 13(6):878.

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Comanescu, Cezar. 2023. "Graphene Supports for Metal Hydride and Energy Storage Applications" Crystals 13, no. 6: 878.

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