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Review

Advancements in Ti3C2 MXene-Integrated Various Metal Hydrides for Hydrogen Energy Storage: A Review

by
Adem Sreedhar
and
Jin-Seo Noh
*
Department of Physics, Gachon University, 1342 Seongnam-daero, Sujeong-gu, Gyeonggi-do, Seongnam-si 461-701, Republic of Korea
*
Author to whom correspondence should be addressed.
Nanomaterials 2025, 15(9), 673; https://doi.org/10.3390/nano15090673 (registering DOI)
Submission received: 3 April 2025 / Revised: 26 April 2025 / Accepted: 27 April 2025 / Published: 28 April 2025
(This article belongs to the Special Issue Graphene-Like Nanomaterials: Simulation, Preparation, Characterization and Applications)
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Abstract

:
The current world is increasingly focusing on renewable energy sources with strong emphasis on the economically viable use of renewable energy to reduce carbon emissions and safeguard human health. Solid-state hydrogen (H2) storage materials offer a higher density compared to traditional gaseous and liquid storage methods. In this context, this review evaluates recent advancements in binary, ternary, and complex metal hydrides integrated with 2D Ti3C2 MXene for enhancing H2 storage performance. This perspective highlights the progress made in H2 storage through the development of active sites, created by interactions between multilayers, few-layers, and internal edge sites of Ti3C2 MXene with metal hydrides. Specifically, the selective incorporation of Ti3C2 MXene content has significantly contributed to improvements in the H2 storage performance of various metal hydrides. Key benefits include low operating temperatures and enhanced H2 storage capacity observed in Ti3C2 MXene/metal hydride composites. The versatility of titanium multiple valence states (Ti0, Ti2+, Ti3+, and Ti4+) and Ti-C bonding in Ti3C2 plays a crucial role in optimizing the H2 absorption and desorption processes. Based on these promising developments, we emphasize the potential of solid-state Ti3C2 MXene interfaces with various metal hydrides for fuel cell applications. Overall, 2D Ti3C2 MXenes represent a significant advancement in realizing efficient H2 storage. Finally, we discuss the challenges and future directions for advancing 2D Ti3C2 MXenes toward commercial-scale H2 storage solutions.

Graphical Abstract

1. Introduction

To meet the target of carbon neutrality and navigate the sustainable world, researchers have continuously focused on minimizing carbon emissions. Here, the world is heading towards the development of efficient batteries and fuel cells, which deliver convenient electrical energy to divert the usage of depleting fossil fuels. Therefore, H2 has been considered as a promising renewable and environmentally friendly chemical fuel alternative to current fossil fuels. The major advantage of H2 and oxygen combination in fuel cells is water vapor (by-product) generation at zero carbon emission, which suppresses the prospect of toxic nitrogen oxides ejection into the environment [1]. This process observes electrical energy generation for its formidable utilization in moving vehicles, industries, and household purposes [2]. Due to gaining attention in the technological revolutions, H2 fuel vehicles are progressively expanding worldwide to achieve eco-friendly nature [3]. Furthermore, this significance lies in their potential to suppress the direct emission of greenhouse gases into the atmosphere and noise pollution. In this manner, the global demand and target of H2 energy production are set to 80 EJ by 2050 [4]. Thus, the scaling up of H2 fuel vehicles aims to increase, which necessitates controlling global warming. Majorly, H2 renewable energy systems can be utilized in transportation and industries, which contribute to sustainable decarbonization processes. Thus, facile H2 fuel storage systems and materials aligned with increasing demand. To run the H2 fuel vehicles, one crucial aspect of H2 storage material is necessary. It should be noted that H2 is used as a potential energy carrier in fuel cells with high energy density [5]. This feature ensures effective electrical energy generation in fuel cells. To minimize the level of H2 gas volume (high density) at room temperature and pressure, the following convention methods have been implemented for many years. Firstly, H2 has been stored in physical-based cylinders and liquid forms under high pressure and low temperatures, respectively [6]. This process is crucial for compressing the H2 gas and maintaining high density. In addition to the above techniques, H2 storage through material-based sorption (physisorption and chemisorption) offers a higher density compared to gaseous and liquid storage forms. Two different types of materials explain the physical (porous materials) and chemical (metal hydrides) absorption processes. In detail, chemisorption refers to the dissociation of H2 molecules into individual hydrogen atoms, which form a chemical bond with the material. This results in a higher volumetric density for H2 storage in metal hydrides [7]. In contrast, physisorption involves the absorption of H2 molecules onto a material surface without any chemical bonding, typically occurring at lower temperatures, while chemisorption is generally observed at higher temperatures. Metal hydrides are prime examples, which offer superior volumetric density compared to other storage methods. Notably, both physisorption and chemisorption processes do not require catalysts as H2 molecules are directly absorbed and desorbed from the material surface. The absorption and desorption efficiency strongly depends on the bonding energy of H2 atoms and the material’s thermal stability, and both are critical factors in optimizing H2 storage performance. This feature highlights the potential use of H2 storage materials in fuel cell vehicles. Interestingly, more H2 can be stored in small vessels using solid H2 storage materials than in liquid compressed cylinders. One promising method for H2 production is through photocatalytic and photoelectrochemical water splitting techniques [8,9]. As mentioned earlier, the generation of water vapor as a by-product in the fuel cells can be harnessed to produce H2 fuel and O2 through photocatalytic processes. This H2 fuel can be stored in H2 storage materials. Interestingly, the integration of the photocatalytic effect (H2 fuel generation), H2 storage, and H2 combustion (energy generation with water vapor generation as by-product) offers a potential path toward a sustainable clean energy future [10]. Furthermore, the cost of manufacturing H2 fuel cells is lower than that of current battery technologies [11]. Additionally, H2 energy content (120 MJ/kg) is three times higher than that of gasoline (44 MJ/kg) [12].
Conventional H2 storage systems require low density and low temperature conditions, such as the extremely low temperature of −252.87 °C. Metal hydrides, carbonaceous materials, and metal–organic frameworks (MOFs) are promising candidates for solid H2 storage. Among these, metal hydroxides have demonstrated superior H2 storage capacity compared to liquid and pressurized conditions. This can be attributed to the chemisorption process, which allows H2 to be stored within solid materials under moderate temperature and pressure. During absorption and desorption cycles, the H2 atoms in the metal hydrides induce rearrangements in the crystal structure, leading to volume expansion [13]. Notably, the highest H2 density has been observed in the following order: chemisorption > physisorption > liquid form > pressure form [14]. Metal hydrides offer a safe approach for achieving high H2 storage densities, efficient desorption at low temperatures, and enhanced cyclic stability. Several metal hydrides and mixed metal hydrides, such as MgH2, LiBH4, NaMgH3, NaAlH4, NaBH4, and Mg(BH4)2 have been extensively investigated. Recent research on MgH2 has highlighted its potential for H2 storage, particularly when combined with a single-atom Ni supported on TiO2, which achieved H2 absorption of 6.53 wt% in just 10 s [15]. Moreover, when MgH2 was combined with LiBH4, Ding et al. [16] demonstrated a reversible H2 storage capacity of 5.0 wt% at temperatures below 265 °C, outperforming other MgH2 + LiBH4 systems. Additionally, MgH2 combined NaAlH4 exhibited a H2 storage capacity of 7.42 wt% after 60 min [17]. The combination of MgH2 with NaBH4 showed promising rehydrogenation results, reaching 5.89 wt% in 12 h at 600 °C [18]. These studies represent significant advancements in H2 storage capacity through the development of complex hybrid structures. Many researchers provided the reviews on the importance of magnesium (Mg)-based materials [19], rare earth-Mg based alloys [20], which explained the role of rare earth-Mg based alloys for H2 storage performance. Furthermore, Li et al. [21] provided the systematic information on thermodynamics and kinetics relating to the dehydriding and hydriding process of Mg based materials. Interestingly, the combination of NbC and Nb4C3 achieved the superior H2 storage performance of the Li–Mg–B–H composite [22]. In another study, Lu et al. [23] also studied the H2 storage performance of Li–Mg–B–H composite (2LiBH4 + MgH2) by the addition of Nb2C. The role of fluorine-functionalized intercalation in graphene was explained for H2 storage of LiBH4 by adjusting the interlayer space [24].
The H2 storage capabilities of metal hydrides mentioned above can be significantly enhanced by incorporating a novel Ti metal, particularly in the form of two-dimensional (2D) Ti3C2 MXene. A linear improvement in H2 uptakes has been observed as the surface area of the resultant material increases [25]. The 2D Ti3C2 MXene interacts, notably, with CO2 and N2 during photocatalytic and electrocatalytic reduction processes [26]. This highlights the critical role of Ti3C2 MXene, with its high surface area and layer structure, in facilitating the H2 storage process. The theoretical studies using density functional theory indicated that the accessibility of Ti2C MXene can achieve a H2 storage capacity of 8.5 wt% [27]. Experimental results confirmed that multilayer Ti2C MXene exhibited a H2 uptake of 8.8 wt% at room temperature [28]. Furthermore, Ti3C2 has demonstrated its potential in promoting a sustainable clean energy environment [29]. In addition to Ti3C2, V2C also proved potential H2 storage performance [30]. Previously, the role of MXenes towards H2 storage of various metal hydrides has been successfully reported [31,32]. Therefore, the 2D Ti3C2 MXene community offers distinct advantages for developing an efficient H2 storage system.

2. Importance of Ti3C2 MXene for H2 Storage

The hydrogen storage (absorption) process in MgH2 can be enhanced by considering the layered structure and titanium-rich characteristics of Ti3C2. Specifically, titanium (Ti) metal forms through the strategic interaction between MgH2 and Ti3C2, which act as promising materials for H2 absorption. It should be noted that the Ti functioned [2,2,2] paracyclophane explained the H2 storage performance under the strategic transition from chemisorption to physisorption [33]. Additionally, the distinct 2D layered structure and high surface area of Ti3C2 provide more active sites for further improving hydrogen storage capabilities. Recently, pure 2D Ti3C2Tx MXene achieved an experimental hydrogen storage capacity of 10.47 wt% under 25 bar and 77 K for the first time. Theoretical studies on H2 storage were explained by the various MXene bilayers [34]. Figure 1 shows the significance of the layered 2D Ti3C2 MXene structure in enhancing the H2 storage performance of different metal hydrides. Research on Ti3C2 MXene-based metal hydrides for H2 storage is still in its early stages. Meanwhile, there are few reviews addressing the potentiality of MXenes for H2 storage in fuel cell applications [4,35,36]. However, there is a lack of detailed focus on the evaluation of Ti3C2 MXene-based metal hydrides, specifically for H2 storage. This review aims to explore the potential role of 2D Ti3C2 MXene in enhancing the H2 storage performance of various metal hydrides.
Based on the above key insights, we have investigated the integration of various metal hydrides with 2D Ti3C2 MXene to enhance H2 storage performance at lower operating temperatures. Figure 2 illustrates a schematic representation of ball milled metal hydrides combined with 2D Ti3C2 MXene, aimed at future fuel cell applications. This combination is designed to improve the dehydrogenation and hydrogenation kinetics at lower operating temperatures.

3. Role of Ti3C2 MXene/Metal Hydride Interface for H2 Storage

Due to the key structural and surface termination features of Ti3C2 MXene, the combination of Ti3C2 MXene and metal hydrides strengthens the overall H2 storage capacity. Specifically, the Ti3C2 MXene has the ability to reduce the desorption temperature of metal hydrides as a potential catalyst by the creation of Ti metal and various Ti valance states (Ti0, Ti2+, Ti3+, and Ti4+). Moreover, Ti3C2 MXene stabilizes the H2 desorption of metal hydrides and increases the cyclic stability of the resultant composite. It should be noted that the diffusion of hydrogen molecules can be significantly improved in metal hydrides by the addition of Ti3C2 MXene to improve the absorption/desorption kinetics. Thus, the Ti3C2 MXene/metal hydride combination results in improved H2 storage performance at lower temperatures, compared with pure metal hydrides, especially for its applicability in automobile industry.

4. Ti3C2 MXene-Based H2 Storage Materials

As we explained before, metal hydrides and mixed metal hydrides play a crucial role in the H2 storage process. The integration of layer-structured Ti3C2 MXene with metal hydrides has further attracted the efficiency of H2 storage. Researchers have continuously focused on the unique physical and chemical properties of Ti3C2 MXene to better understand its role in hydrogen storage and its potential applications in fuel cell vehicles. Given the significance of various metal hydride interaction networks with 2D Ti3C2 MXene, we have conducted a thorough evaluation of the efficiency of the H2 absorption and desorption processes.

4.1. Binary Metal Hydrides

4.1.1. MgH2

The ball milling technique has been successfully used to create a synergy between MgH2 and few- or multi-layer Ti3C2 MXene, leading to significant improvements in the H2 absorption and desorption processes compared to pure MgH2. Additionally, the presence of Ti in various oxidation states within Ti3C2 MXene helps to lower the operating temperatures in contrast to pure MgH2. Sharp H2 release and absorption observed through the formation of MgH2 involved Ti3C2 MXene. The potential role of Ti3C2 in optimizing H2 storage performance of MgH2 is explained below.
The potential of Ti metal in Ti3C2 for enhancing the rapid dehydrogenation and hydrogenation process at various operating temperatures was investigated [37]. In this study, a selective combination of ball milled Ti3C2 (5 wt%) with MgH2 (MgH2-5 wt% Ti3C2) altered both the operating temperature and dehydrogenation process. During ball milling, metallic Ti was generated in situ, which significantly promoted the dissociation and recombination of molecular H2 on the MgH2-5 wt% Ti3C2 composite. Specifically, Ti3C2 contents (0, 1, 3, 5, and 7 wt%) were varied to explore their effects on the non-isothermal and isothermal dehydrogenation/hydrogenation processes. This optimized sample reduced the dehydrogenation operating temperature from 278 °C (pure MgH2) to 185 °C. Under isothermal conditions at 300 °C, the H2 desorption reached approximately 6.2 wt% within 1 min. Furthermore, with the dehydrogenated samples exposed to 50 bar hydrogen pressure, a H2 uptake of 6.1 wt% was achieved in just 30 s at 150 °C under isothermal conditions. Notably, H2 absorption started at room temperature and 5.5 wt% uptake was achieved at 100 °C under non-isothermal conditions. Overall, the combination of MgH2 and Ti-rich Ti3C2 (without Ti-C bonding) was highly effective in enhancing the cyclic stability of hydrogen storage, achieving 95% retention capacity after 10 cycles.
In another study, Gao et al. [38] explained the reduction in the operating temperature of the dehydrogenation process through the synergistic combination of carbon (C)-supported 5 wt% Ti3C2/TiO2-C and MgH2. In this case, Ti3C2 was partially oxidized to TiO2 and C under CO2 at 600 °C. The presence of multiple valance states of Ti (Ti0, Ti2+ Ti3+, and Ti4+) in the Ti3C2/TiO2-C composite facilitated rapid charge transfer, which enabled the conversion between Mg2+ and Mg or H− and H. This effect significantly enhanced the H2 absorption and desorption process. After 5 wt% Ti3C2/TiO2-C was ball milled with MgH2 in Ar atmosphere, the composite released 5 wt% H2 in 1700 s at 250 °C under 0.05 MPa pressure. The rate constant observed was 0.258 wt% min−1 for MgH2-5 wt% Ti3C2/TiO2 (A)-C, which was 1.48 and 9.6 times higher than that of MgH2-5 wt% Ti3C2 and MgH2-5 wt% Ti3C2(A)-C, respectively. Additionally, these dehydrogenated samples absorbed 4 wt% of H2 in 800 s at 125 °C under isothermal conditions, outperforming MgH2-5 wt% Ti3C2 (3 wt%) and MgH2-5 wt% TiO2 (A)-C (2.65 wt%). This study clearly demonstrated that the Ti metal preset in Ti3C2 plays a crucial role in triggering H2 absorption and desorption at lower operating temperatures.
In addition to the conventional multilayer Ti3C2Tx network, few-layer (FL) Ti3C2Tx MXene supported MgH2 has been explored for the first time in the H2 desorption/absorption process without altering its surface features [39]. In this study, an electrostatic self-assembly reduction process was used for in situ growth of 20–140 nm nano-Ni particles (at 20, 30, and 40 wt%) within FL Ti3C2Tx MXene (1.42 nm interlayer distance), which was then ball milled with MgH2. As in previous studies [38], the potential contributions of Ti0, Ti2+, Ti3+, and Ti4+ valence states were discussed in relation to H2 storage performance. The 5 wt% Ni30/FL-Ti3C2Tx doped MgH2 demonstrated dehydrogenation of 5.83 wt% in 1800 s 250 °C under 0.005 MPa (isothermal dehydrogenation) as shown in Figure 3a. The same sample also exhibited improved isothermal hydrogenation, reaching 5 wt% in 1700 s under 3.0 MPa H2 pressure at 100 °C (Figure 3b). Furthermore, stable cyclic hydrogenation and dehydrogenation were observed over 10 cycles at 100 °C with minimal attenuation of approximately 0.06 wt% per cycle. A new phase of MgO was formed during the testing process, which may explain the decrease in cyclic stability.
Similarly, Ni nanoparticles supported on monolayer Ti3C2 MXene (Ti-MX) were synthesized using a self-assembly technique and combined with MgH2 through ball milling (MgH2 + Ni@Ti-MX) to facilitate H2 absorption and release at lower operating temperatures [41]. During the dehydrogenation of MgH2 in the presence of Ni along with Ti, the formation of Mg2Ni was observed. The interfaces between Mg/Mg2Ni, Mg/TiO2, Mg/Ti, and Mg/C were identified, playing a key role in triggering the H2 absorption process of MgH2. The H2 absorption of dehydrogenated MgH2 + Ni@Ti-MX was observed at 125 °C under 3 MPa, releasing 5.4% H2 in 25 s (4.0 wt% at 75 °C in 60 min). The rapid absorption process (25 s) was significantly faster than the previously studied Ni30/FL-Ti3C2Tx (1700 s) [39]. Notably, room temperature H2 absorption (4.0 wt% in 5 h) was also observed. The activation energy of 56 ± 4 KJ/mol H2 was achieved, which was higher than other MXene composites, such as MgH2−5 wt % Nb4C3Tx (27.8 kJ/mol H2) [42] and MgH2−10 wt % Ni/CMK-3 (37 kJ/mol H2) [43]. The isothermal dehydrogenation process reached 5.2 wt% in 15 min at 250 °C and 5.0 wt% H2 at 300 °C in 200 s. The 2D Ti3C2 MXene facilitated H2 diffusion during the absorption/release cycle, demonstrating excellent cyclic stability over 10 cycles at 275 °C. Overall, the presence of Ni nanoparticles further enhanced H2 storage performance in the MgH2 + Ni@Ti-MX composite.
Additionally, Ti3C2 MXene-derived K2Ti6O13 (Hamamelis-like structure) in a KOH + H2O2 environment enhanced the H2 storage performance of MgH2 [44]. The H2 storage behavior of MgH2-K2Ti6O13 was studied at 0, 3, 5, and 10 wt% concentrations of K2Ti6O13. According to the temperature-programmed desorption (TPD) results, the ball milled MgH2-K2Ti6O13 was subjected to 1.5 MPa of H2, and showed a reduction in the onset and termination temperature to 175 °C and 220 °C, respectively, with 5 wt% of K2Ti6O13 compared to pure MgH2 (287 °C–onset). The isothermal dehydrogenation performance exhibited a 6.7 wt% H2 release at 280 °C in 3 min, which was 101.5 times higher than that of pure MgH2. Subsequently, the dehydrogenated sample absorbed approximately 6.5 wt% H2 in 30 s at 200 °C at 2.2 MPa (4.8 wt% at 100 °C in 5 min). The cyclic stability of dehydrogenation (280 °C) and hydrogenation (200 °C) under 2.2 MPa remained stable after 10 cycles. The charge (electron) transfer between Ti and Ti2+ accelerated the diffusion of H2. Moreover, KMgH3 formation during ball milling process further accelerated the H2 diffusion during both absorption and desorption processes.
In another study, the combination of dual V2C and Ti3C2 MXenes enhanced the H2 storage capacity of MgH2 at 10 wt % of 2V2C/Ti3C2 [45]. After successful ball milling, H2 desorption and H2 absorption processes, V-C and Ti-C bonding were observed, highlighting the critical role V2C and Ti3C2 as catalysts in the H2 storage process. The uniform distribution of Mg, Ti, and V elements in the MgH2-V2C/Ti3C2 composite promoted the H2 absorption activity of MgH2. Furthermore, the presence of various Ti valance states contributed to improving the H2 absorption capacity of MgH2. The addition of 2V2C/Ti3C2 reduced the starting desorption temperature of MgH2 from 320 °C to 180 °C. The non-isothermal studies showed a 6.3 wt% H2 release at 250 °C. The non-isothermal H2 desorption performance of MgH2-2V2C/Ti3C2 resulted in 5.1 wt% H2 release within 60 min at 225 °C and 5.8 wt% in just 2 min at 300 °C. Accordingly, MgH2-2V2C/Ti3C2 demonstrated 5.1 wt% of H2 absorption in 20 s at 40 °C under isothermal conditions. This configuration maintained 10 cycles of desorption stability at 300 °C with 6.3 wt% H2 release. The activation energy of H2 desorption in MgH2-2V2C/Ti3C2 was 79.4 kJ mol−1 H2. Overall, the dual 2D MXene configuration in MgH2 reduced H2 absorption time to 20 s and the operating temperature to 40 °C with 5.1 wt% absorption.
Wu et al. [46] highlighted the significance of multilayer Ti3C2 in enhancing the H2 storage of MgH2 potentiality through the involvement of Ti metal. The Ti metal, formed in situ during the ball milling process of Ti3C2 and MgH2, played a crucial role in improving H2 absorption and desorption. In this study, Ti3C2 at 6 wt% was optimized in MgH2 (MgH2-6 wt.% ML-Ti3C2). During the non-isothermal process, the initial dehydrogenation temperature was 142 °C with a 6.56 wt% H2 release. Following the dehydrogenation, the non-isothermal H2 absorption efficiency of 6.3 wt% was observed at room temperature (30 °C) compared to MgH2 (70 °C). Under isothermal conditions, the H2 storage and desorption capacities were about 6.47 and 6.45 wt% at 150 and 240 °C, respectively. The activation energy for the dehydrogenation process of MgH2-6 wt.% ML-Ti3C2 was about 99.11 kJ/mol, lower than that of MgH2 (153.09 kJ/mol). This suggests that the addition of Ti3C2 facilitates faster H2 release in MgH2. During the dehydrogenation processes, MgH2 was converted into Mg and MgO. Whereas Mg was converted back into MgH2 during the rehydrogenation process.
In addition to the nano-Ni particles [39] and Ni nanoparticles [41], Gao et al. [47] investigated the interaction of Ni particles (<50 nm) with Ti3C2 (Ni/Ti3C2-WE) to enhance the H2 storage capacity of MgH2. Specifically, ball milled MgH2 with 5 wt% Ni/Ti3C2 was optimized to study the hydrogenation/dehydrogenation processes. The absorption and desorption kinetics of MgH2 were significantly influenced by the excellent interface between Ni and Ti3C2. The presence of multiple Ti valence states (Ti0, Ti2+ Ti3+, and Ti4+) again proved electron transfer, which enhanced the catalytic activity of Ni/Ti3C2-WE. Non-isothermal studies showed that, at 240 °C, a dehydrogenation of 3.02 wt% was achieved. Under isothermal conditions, the dehydrogenated MgH2-5 wt% Ni/Ti3C2 absorbed 4.59 wt% H2 in 1200 s at 100 °C (MgH2-1.67 wt% at 200 °C). Remarkably, the system also demonstrated impressive low temperature (50 °C) H2 absorption, with 4.51 wt% in 5.5 h, meeting the U.S. Department of Energy (DOE) criteria for light-duty vehicle applications. The desorption of H2 reached 5.87 wt% and 6.73 wt% in 2400 s at 250 °C and 300 °C, respectively. Additionally, the MgH2-5 wt% Ni/Ti3C2 demonstrated stable absorption/desorption cycles over 10 cycles at 275 °C. Overall, the dissociation and recombination of H2 were facilitated by the Ti metal formation and mitigation of surface passivation of MgH2 in the MgH2-5 wt% Ni/Ti3C2.
In another study, the uniform distribution of praseodymium fluoride (PrF3) nanoparticles on Ti3C2 (PrF3/Ti3C2) enhanced the H2 storage capacity of MgH2 [40]. The valence states of Ti (Ti2+, Ti3+, and Ti4+) created the active sites for charge (electron) transfer between Mg2+ and H−, facilitating a significant dehydrogenation process. Specifically, ball milling of PrF3/Ti3C2 with MgH2 resulted in the formation of Ti metal and various Ti valence states (Ti0, Ti2+, Ti3+, and Ti4+), which improved the dissociation and recombination of H2 molecules. Additionally, enhanced charge transfer between Ti3+ and Ti2+ was observed during both the hydrogenation and dehydrogenation processes. Under non-isothermal conditions, the hydrogen desorption of 7.2 wt% was achieved at 230 °C, as shown in Figure 3c. Isothermal studies on MgH2-5 wt% PrF3/Ti3C2 demonstrated 7.0 wt% H2 desorption in 3 min at 260 °C and H2 absorption of 6.16 wt% at 150 °C within 10 min, as shown in Figure 3d. The non-isothermal dehydrogenation achieved a 7.2 wt% capacity starting at 180 °C. The dehydrogenation activation energy for MgH2-5 wt% PrF3/Ti3C2 was 78.11 kJ mol−1, which was lower than that of pure MgH2 (117.98 kJ mol−1). Furthermore, the composite retained 92.5% of its capacitance after 10 cycles of dehydrogenation.
The formation of internal edge planes on porous Ti3C2 MXene (MX-P) was shown to enhance the H2 storage in MgH2 by increasing the specific surface area and creating internal metallic Ti active sites [48]. The exposed internal Ti edge sites raised the Ti content in MgH2-5 wt% MX-P to 33.3% compared to pure Ti3C2 MXene (20.3%) in MgH2-5 wt% MX. In situ oxidation and etching processes created small holes on the surface of MX-P-TiO2, which partially converted the Ti active sites into TiO2. By adjusting the etching time, the number of holes and internal edge sites could be significantly increased. The isothermal hydrogenation process revealed a 2.55 wt% H2 uptake at 100 °C in 1200 s (Figure 4a). Through these beneficial surface features, both MgH2-5 wt% MX-P-TiO2 and MgH2-5 wt% MX-P exhibited H2 storage capacities of 6.6 and 6.5 wt%, respectively, after 10 cycles at 275 °C. The isothermal desorption behavior of MgH2-5 wt% MX-P-TiO2 at various temperatures is shown in Figure 4b. Additionally, the absorption activation energy for MgH2-5 wt% MX-P-TiO2 (33.95 kJ mol−1) was lower than that of MgH2-5 wt% MX-P (36.22 kJ mol−1) and MgH2-5 wt% MX (43.10 kJ mol−1). Overall, the creation of internal edge planes in Ti3C2 MXene significantly increased the Ti active sites for H2 absorption in MgH2-5 wt% MX-P-TiO2.
Furthermore, the synergistic interaction between MnO2 nanoparticles and 2D Ti3C2 significantly influenced the H2 storage performance of MgH2 in the MgH2 + Ti3C2@MnO2 composite at room temperature [50]. In this system, Mn, MnO2, MnO, TiO2, and Ti3C2 act as common catalysts during the hydrogenation and dehydrogenation process. During hydrogenation, Mg was converted into MgH2. The formation of multiple interfaces, including Ti3C2/MgH2, TiO2/MgH2, MnO2/MgH2, MnO/MgH2, and Mn/MgH2 created numerous channels for hydrogen atom diffusion, enhancing the H2 absorption and desorption processes of the Mg/MgH2 system. Isothermal hydrogenation achieved 4.4 wt% H2 absorption at 30 °C within 150 s and 5.13 wt% at 75 °C in 400 s (MgH2-0.28 wt% at 150 °C in 400 s). Dehydrogenation reached 6.4 wt% at 275 °C within 484 s. Remarkably, these interfaces exhibited 20-cycle stability at 275 °C with hydrogen absorption of 6.37 wt%, confirming the stability of the developed composite and its excellent hydrogen absorption/desorption performance.
In another study, a combination of Ni nanoparticles and Ti3C2 MXene enhanced the H2 desorption behavior of MgH2 [51]. Ball milling of Mg, Ni, and Ti3C2 produced Mg-xNi/Ti3C2 composites at different weight ratios (5 wt% of xNi/Ti3C2, x = 0.5, 1, 2, and 3) with 95 wt% Mg. The Ni content in Ti3C2 (Ti3C2/Ni = 1:2) and 5 wt% Ti3C2 content significantly influenced the H2 storage performance. At 300 °C, the Mg-2Ni/Ti3C2 composite exhibited H2 absorption and released values of 4.46 wt% and 3.96 wt%, respectively. The isothermal dehydrogenation of Mg-2Ni/Ti3C2 was 4.54 wt% in 5 min compared to the 3.76 wt% for pure Mg after in 70 min at 375 °C. The synergy between Ni and Ti3C2 lowered the activation energy of Mg-2Ni/Ti3C2 to about 75.0 kJ mol−1 (Mg-135.4 kJ mol−1). During hydrogenation, Mg was converted into MgH2 and during dehydrogenation, the system formed Mg, MgO, and Mg2Ni. The characteristic feature of Ti3+ disappeared after the dehydrogenation process, leaving Ti, Ti-C, and Ti2+ as the remaining phases. The metallic Ti developed during ball milling facilitated H2 desorption, with Ti3C2 acting as a H2 pump. Additionally, Ti3C2 played a crucial role in controlling the expansion of the Mg pellet, ensuing stability in the H2 absorption and desorption cycles.
In another study, Ti3C2 was coordinated with 3d transition metal (Fe, Co, Ni) particles to enhance the H2 storage performance of the MgH2-TiCrV composite [49]. Ball milling of Ti3C2 with Fe, Co, Ni, and Mg-TiCrV resulted in the formation of Mg-TiCrV/Ti3C2-Fe, Co, Ni composites, which reduced the layer structure and promoted the development of cluster morphology. These composites were evaluated for dehydrogenation at temperatures of 498, 523, 543, and 573 K under 0.1 MPa. At 523 K, the Mg-TiCrV/Ti3C2-Ni released the H2 about 4.982 wt% in 60 min, outperforming other temperatures and TiCrV/Ti3C2-Fe (3.598 wt%) and TiCrV/Ti3C2-Co (3.789 wt%), as shown in Figure 4c. The H2 release capability was further enhanced at 543 K, reaching 5.447 wt% for TiCrV/Ti3C2-Ni. Furthermore, TiCrV/Ti3C2-Ni exhibited a rapid isothermal H2 absorption of 5.72 wt% in just 1 min at 453 K, surpassing the absorption of the TiCrV/Ti3C2-Fe (5.53 wt%) and TiCrV/Ti3C2-Co (5.34 wt%) composites (Figure 4d). Based on these findings, it was concluded that Ni coordination was more effective than Fe and Co (Ni > Fe > Co) in enhancing the performance of Mg-TiCrV/Ti3C2-Fe, Co, Ni composites. Furthermore, the activation energy for H2 release was lower for TiCrV/Ti3C2-Ni (80.54 kJ mol−1) compared to TiCrV/Ti3C2-Fe (88.28 kJ mol−1) and TiCrV/Ti3C2-Co (88.90 kJ mol−1). Overall, the inclusion of 3D transition metals significantly improved the H2 storage performance of Mg-TiCrV composite.
The above results clearly demonstrated the role of Ti metal, its valance states (Ti0, Ti2+, Ti3+, and Ti4+), and Ti-C bonds in the 2D Ti3C2 MXene for enhancing the H2 storage performance of MgH2.

4.1.2. AlH3

In addition to MgH2, AlH3 has been observed with high H2 densities of 10.1 mass% and 149 gH2 L−1 [52]. The H2 storge performance of AlH3 was observed at low temperatures, ranging from 150 to 200 °C [53]. The H2 volume density of AlH3 is double that of liquid H2, providing significant potential for H2 storage. However, AlH3 is highly reactive with oxygen and water, which limits its broader applicability H2 storage. To improve its H2 storage performance, effective decomposition of AlH3 is necessary.
In this context, the potential of H2 storage performance was significantly enhanced by the strategic air-ball milling of AlH3 and Ti3C2, which resulted in the formation of an Al2O3 layer on AlH3 for the first time [54]. This strategy facilitated close contact between the AlH3 oxide layers and Ti3C2, which effectively minimized the loss of H2 storage capacity. The extended air-milling process (an additional 60 min) led to a reduction in the initial decomposition temperature (61 °C) and increased H2 release (8.1 wt%) at 4 wt% Ti3C2. Under isothermal conditions (100 °C), the AlH3 + 4 wt% Ti3C2 mixture released 6.9 wt% H2 in 20 min and 4.9 wt% in 10 min. In comparison, milled AlH3 alone released 0.1 wt% and 0.3 wt% at 100 °C in 10 and 20 min, respectively. The dehydrogenation activation energy of AlH3 + 4 wt% Ti3C2 was 40 kJ mol−1 (milled AlH3-99 kJ mol−1). The process of AlH3 and Ti3C2 combination triggered the dehydrogenation kinetics performance. The combination of AlH3 and Ti3C2 accelerated the dehydrogenation kinetics, while the Al2O3 layer on AlH3 was protected during the air-ball milling process, preserving the catalyst. Moreover, the layer structured Ti3C2 with a high surface area and active sites created a robust connection with AlH3, enabling efficient H2 storage at low temperatures.
Overall, the 2D Ti3C2 MXenes had a significant impact on the H2 storage performance of MgH2 and AlH3 by reducing the operating temperature. The binary metal hydrides (MgH2 and AlH3) integrated Ti3C2 MXenes at a specific weight percentage and key factors for H2 storage performance are summarized in Table 1.

4.2. Ternary Metal Hydrides

4.2.1. Mg(BH4)2

Zhang et al. [55] developed VF4 nanoparticles anchored on 2D Ti3C2 to evaluate the H2 storage performance of Mg(BH4)2. The presence of Ti3C2 MXene significantly reduced the aggregation of Mg(BH4)2 during the absorption and desorption processes. In the TPD process, VF4@Ti3C2 (20 wt%) demonstrated that the dehydrogenation began at 90 °C and reached a 13 wt% H2 release by 500 °C. The SEM image of Mg(BH4)2-20VF4@Ti3C2 is shown in Figure 5a. The isothermal desorption process maintained a H2 release of 8.2 wt% at 275 °C in 300 min for Mg(BH4)2-4.5 wt% (Figure 5b). The activation energy for the first step of dehydrogenation of Mg(BH4)2-20 VF4@Ti3C2 was 172.099 kJ mol−1, lower than that of pure Mg(BH4)2 (374.1 kJ mol−1). This reduction in activation energy was attributed to the destabilization of the B–H bonds in the composite. Ti3C2 remained stable during the de/hydrogenated processes, functioning as a scaffold and exhibiting synergistic behavior with VH2.01 and Mg(BH4)2 for enhancing the H2 release and uptake processes. Overall, the formation of metallic Ti and VH2.01 was observed during both the H2 release and absorption processes.

4.2.2. NiAlH4

NiAlH4 is also a promising complex hydride that can facilitate the dehydrogenation/hydrogenation process. It has been observed to exhibit low operating temperatures and high hydrogen storage capacity [56]. Compared to other complex borohydrides, alanates, and amides, NiAlH4 demonstrates impressive gravimetric and volumetric hydrogen densities of 7.5 wt% and 94 g L−1, respectively [57]. The H2 storage performance of NiAlH4 can be further enhanced by incorporating Ti-based catalysts, which help to reduce the operating temperature and lower the activation energies during H2 absorption/desorption process, while maintaining high cyclic stability [58,59]. In this study, we have investigated the role of Ti metal-doped 2D Ti3C2 MXene in improving the H2 storage performance of NiAlH4.
The role of pure 2D Ti3C2 MXene in enhancing H2 storage performance was effectively demonstrated by introducing it into NiAlH4 for the first time [60]. The addition of 7 wt% Ti3C2 significantly lowered the dehydrogenation temperature of NiAlH4. Furthermore, the incorporation of Ti3C2 MXene improved the stability of the dehydrogenation process. Following ball milling and the chemical interaction between NiAlH4 and Ti3C2, the Ti3C2 was reduced to metallic Ti and Ti3+ through the breaking of Ti–C bonds. The non-isothermal dehydrogenation of 7 wt% Ti3C2-doped NiAlH4 began at 100 °C, which was significantly lower than the 190 °C observed for pure NiAlH4. The hydrogenation onset temperature was 50 °C, with a H2 uptake of 4.9 wt% achieved at 150 °C under non-isothermal conditions. In isothermal dehydrogenation tests at 140 °C for 100 min, a H2 release of 4.7 wt% was observed. Additionally, the dehydrogenated sample at 100 bar absorbed 4.6 wt% of H2 at 120 °C (NiAlH4-0.4 wt% H2). Overall, the introduction of 2D Ti3C2 significantly enhanced the H2 storage capacity by achieving 4.8 wt% after 10 cycles, with reduced operating temperatures compared to pure NiAlH4.
Ti3C2(OH0.8F1.2)2 MXene derivatives have also gained attention for enhancing the H2 storage capacity of NiAlH4, specifically by forming a composite of 90% anatase and 10% rutile TiO2 in the flower-shaped (FS) FS-A0.9R0.1-TiO2/C (FS-A0.9R0.1-TC) for the first time [61]. This study demonstrated that Ti3C2 MXene-derived carbon-supported TiO2 promotes H2 desorption at lower temperatures. The TPD results showed that the combination of FS-A0.9R0.1-TC and NiAlH4 reduced the onset temperature to 95 °C compared to the 155 °C required for pure NiAlH4. The isothermal dehydrogenation was observed with a H2 release of 3.11 wt% in 90 min at 100 °C and 4.6 wt% in 20 min at 140 °C. Additionally, stable TPD curves (over 10 cycles) indicated a dehydrogenation of 4.5 wt% after the second cycle. The presence of abundant anatase TiO2 (001), graphene-like carbon, and carbon-doped TiO2 enhanced the dehydrogenation performance of NiAlH4 at lower temperatures when FS-A0.9R0.1-TC was added.
In another study, the role of partially derived anatase TiO2 (A-TiO2) at the interface of Ti3C2 MXene (MXene/A-TiO2) was found to enhance the low-temperature H2 storage performance of NiAlH4 [62]. Different weight percentages of MXene/A-TiO2 (0, 5, 10, and 15 wt%) were used as additives to NiAlH4 during the ball milling process. TPD graphs revealed that the onset dehydrogenation temperature of NaAlH4 + 15 wt% MXene/A-TiO2 decreased to 80 °C compared to the 155 °C for pure NiAlH4 as shown in Figure 5c. However, the dehydrogenation capacity of 4.21 wt% was achieved, which was lower than the 5.34 wt% observed for NaAlH4. The isothermal dehydrogenation process of NaAlH4 + 10 wt% MXene/A-TiO2 showed a rapid 3 wt% H2 release within 7 min, reaching 4.8 wt% after 200 min at 140 °C (Figure 5d). The activation energies for NaAlH4 + 15 wt% MXene/A-TiO2 were 78.32 and 65.31 kJ mol−1 for the first and second dehydrogenation steps, respectively. It should be noted that the porous structure NaAlH4 + 10 wt% MXene/A-TiO2 facilitated the diffusion and transport of H2. Overall, the homogeneous distribution of Ti and C in NiAlH4 promoted the Ti-H and TiC formation, which enhanced the H2 storage performance.
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Figure 5. (a) Surface morphology of Mg(BH4)2-20VF4@Ti3C2, (b) isothermal H2 desorption of Mg(BH4)2-20VF4@Ti3C2 at 275 °C (reprinted from Ref. [55] copyright 2023, with permission from Elsevier), (c) non-isothermal dehydrogenation process of NaAlH4 + MXene/A-TiO2 at various weight ratio of MXene/A-TiO2, and (d) isothermal H2 dehydrogenation of NaAlH4 + 10 wt% MXene/A-TiO2 at 140 °C (Reprinted from Ref. [62] copyright 2018, with permission from Elsevier).
Figure 5. (a) Surface morphology of Mg(BH4)2-20VF4@Ti3C2, (b) isothermal H2 desorption of Mg(BH4)2-20VF4@Ti3C2 at 275 °C (reprinted from Ref. [55] copyright 2023, with permission from Elsevier), (c) non-isothermal dehydrogenation process of NaAlH4 + MXene/A-TiO2 at various weight ratio of MXene/A-TiO2, and (d) isothermal H2 dehydrogenation of NaAlH4 + 10 wt% MXene/A-TiO2 at 140 °C (Reprinted from Ref. [62] copyright 2018, with permission from Elsevier).
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Jiang et al. [63] demonstrated the enhanced low operating temperature, reduced activation energy, and improved cyclic stability of 2D Ti3C2 MXene-doped NiAlH4. After the ball milling process with Ti3C2, the NiAlH4 phase remained intact, indicating a strong interaction between NaH, Al, and H2. In this study, Ti3C2-doped NaAlH4 (NaAlH4-8 wt% Ti3C2) and Ti3C2-doped NaH/Al (NaH/Al-8 wt% Ti3C2) were prepared by the ball milling technique to evaluate H2 storage performance. The non-isothermal dehydrogenation temperature of hydrided NaH/Al-8 wt% Ti3C2 was 76 °C, which was lower than that of pure NaAlH4 (146 °C). Under isothermal conditions, the hydrided NaH/Al-8 wt% Ti3C2 released 4.1 wt% H2 in 119 min at 110 °C (NaAlH4-8 wt% Ti3C2-174 min). Furthermore, the dehydrogenated NaH/Al-8 wt% Ti3C2 absorbed 4.2 wt% H2 in just 4.5 min at 110 °C. The authors emphasized the importance of the high surface area of 2D Ti3C2 MXene in enhancing H2 absorption performance. The NaH/Al-Ti3C2 composite exhibited a more distinct and stronger valence state of Ti (Ti3+) than Ti0. The distribution of Ti and TiFx particles on NaAlH4 facilitated rapid H2 absorption and release. The above features revealed the activation energies of NaH/Al-8 wt% Ti3C2 were 92.5 ± 4.6 kJ mol−1 (first step) and 58.1 ± 2.9 kJ mol−1 (second step).
In another study, the formation of Ti-F-Ce bonds at the interface of 10 wt% CeF3/Ti3C2 was found to influence the hydrogenation and dehydrogenation behavior of NaAlH4 [64]. Interestingly, the CeF3/Ti3C2 interface facilitated improved low-temperature H2 absorption and release. The observed Ti-F-Ce bonding remained stable after H2 absorption and desorption, enhancing the H2 storage performance of NaAlH4. Additionally, the chemical state of Ti (Ti0) in the NaAlH4 + 10CeF3/Ti3C2 composite was more stable compared to NaAlH4 + 10Ti3C2 due to the formation of the Ti-F-Ce structure. As a result, the NaAlH4 + 10CeF3/Ti3C2 composite promoted the stability of Ti0 species, which improved catalytic activity by preventing the formation of Ti-based alloys. With the stability of Ti0 species confirmed, non-isothermal dehydrogenation of 4.95 wt% H2 was achieved at an initial temperature of 87 °C, significantly lower than the 155 °C for pure NaAlH4. Under isothermal conditions, 3.0 wt% H2 was released in 80 min, reaching 3.9 wt% after 6 h at 100 °C. Overall, the dehydrogenation process at 100 °C demonstrated practical potential for mobile vehicle applications.
In another study, ball milling of carbon-coated anatase TiO2 nanoparticles interfaced with 2D Ti3C2 (10 wt% C@TiO2/Ti3C2) and NaAlH4 resulted in the formation of Ti valence states such as Ti0 and Ti3+, which were stabilized by suppressing Ti-O and Ti-C bonds for enhancing the H2 storage performance at low temperatures [65]. Here, the combination of Ti-containing TiO2, Ti3C2, and carbon proved to be highly effective additives for improving the H2 storage performance of NaAlH4. Specifically, Ti0 and Ti3+ played a crucial role in H2 desorption, while the carbon in C@TiO2/Ti3C2 significantly contributed to the dehydrogenation process. The presence of 2D Ti3C2 and TiO2 effectively lowered both the dehydrogenation temperature and improved hydrogen storage capacity, respectively. For example, the dehydrogenation temperature of NaAlH4 + 10 wt% Ti3C2 was reduced to 85 °C compared to the NaAlH4 (155 °C) under non-isothermal conditions. Isothermal studies showed that approximately 4 wt% of H2 was released in 13 min at 140 °C (4.91 wt% after 200 min). Furthermore, the activation energies of dehydrogenation were 72.41 kJ mol−1 (first step) and 64.27 kJ mol−1 (second step), which were lower than the values reported previously for NaH/Al-8 wt% Ti3C2 (92.5 ± 4.6 kJ mol-1 for first step) [63]. Thus, the catalytic effect of C@TiO2/Ti3C2 significantly influenced the H2 storage performance of NaAlH4.
The potential of Ti0 and Ti3+ in enhancing H2 storage in NaAlH4 was further emphasized by the development of N-doped carbon-coated Ti3C2 (Ti3C2/NC) [66]. The strategic interaction between pyridinic-N and Ti0 significantly improved the H2 storage performance. The Ti3C2/NC composite (10 wt%), mixed with NaAlH4, accelerated the non-isothermal dehydrogenation at lower temperatures with onset temperatures of 85 °C, 126 °C (first step), and 168 °C (second step), releasing 4.84 wt% H2, which was lower than the NaAlH4 dehydrogenation temperature (155 °C). The onset temperature of 85 °C was slightly lower than that observed in CeF3/Ti3C2 (87 °C). Under isothermal conditions (140 °C), the Ti3C2/NC + NaAlH4 mixture released H2 in amounts of 3.0, 4.0, and 4.61 wt% over 4, 16, and 60 min, respectively. Furthermore, the composite exhibited remarkable dehydrogenation cyclic stability, retaining 96.3% of its capacity (4.66 wt%) after 15 cycles. The stable H2 absorption and desorption characteristics were attributed to the formation of Ti0 and Ti3+ during ball milling. The electron transfer between Ti0 and pyridinic-N, as well as from pyridinic-N to Ti0, was responsible for the release of H2 and absorption, respectively.

4.2.3. LiBH4

Lithium borohydride (LiBH4) has been explored as a promising lightweight material for H2 storage, offering an ultrahigh volumetric density of 121 kg H2 m−3 and gravimetric density of 18.4 wt% [67]. However, the dehydrogenation/hydrogenation process occurred at high temperatures (>400 °C) due to the strong chemical bonding in LiBH4 [68]. As a result, significant efforts have been made to lower the dehydrogenation temperature and improve reversibility [69]. The inclusion of 2D Ti3C2 has shown to play a crucial role in enhancing the H2 storage capacity of LiBH4. Although research on Ti3C2-based LiBH4 is still in its early stages, the following studies highlight the potential of Ti3C2 to improve the H2 storage performance of LiBH4.
The merit of reversibility of the H2 storage process was demonstrated by developing the LiBH4@Ti3C2 composite [70]. Unlike the ball milling methods mentioned earlier, the LiBH4@2Ti3C2 composite was prepared using a simple impregnation method at room temperature, with varying mass ratios (2:1, 1:1, 1:2, and 1:3). Nanosized LiBH4 particles were dispersed on and between the Ti3C2 MXene layers. The TPD curves revealed that the LiBH4@2Ti3C2 (1:2) composite reduced the H2 desorption temperature to 172.6 °C, compared to 220 °C for pure LiBH4. The isothermal dehydrogenation behavior of LiBH4@2Ti3C2 was 8.2, 11.3, and 12.6 wt% after 8 h at 300, 350, and 380 °C, respectively, with 9.6 wt% H2 release at 380 °C within 1 h. However, the dehydrogenation cyclic stability of LiBH4@2Ti3C2 declined from 10.6 wt% in the first cycle to 5.5 wt% in the third cycle after 6 h at 350 °C, showing a 48% decrease after three cycles. This loss in cyclic stability was attributed to the unavoidable agglomeration of LiBH4 particles on Ti3C2, which reached about 100 nm in size, adversely affecting the performance of LiBH4@2Ti3C2.
In another study, ball milling of LiBH4 with various amounts of Ti3C2 (x = 0, 20, and 40 wt%) was investigated to enhance the H2 storage performance of LiBH4 in the LiBH4 + x Ti3C2 [71]. The TPD studies showed a significant reduction in the onset temperature from 300 °C for pure LiBH4 to 120 °C for LiBH4 with 40 wt% Ti3C2. The SEM image and elemental mapping of the LiBH4 + 40 wt% Ti3C2 are shown in Figure 6a. Furthermore, isothermal dehydrogenation of LiBH4 + 40 wt% Ti3C2 at 300 °C resulted in a H2 release of 3 wt% over 6 h compared to 0.5 wt% for pure LiBH4. At 350 °C, the H2 release reached 5.37 wt% in 1 h, as shown in Figure 6b. The activation energy for dehydrogenation was also significantly reduced to 70.3 kJ mol⁻¹ for LiBH4 + 40 wt% Ti3C2, compared to pure LiBH4 (187 ± 24 kJ mol−1). Among various additives (40 wt% TiC, TiO2, and Ti3C2), Ti3C2 showed the most dominant effect on the dehydrogenation process, exhibiting the lowest onset temperature. This was attributed to the unique properties of Ti3C2, including its layered structure and carbon sheets in its 2D form. The improved dehydrogenation at lower temperatures was explained by the substitution of H by F and the formation of TiB2.

4.2.4. NaMgH3

Doping of NaMgH3 with 7 wt% Ti3C2 significantly lowered the dehydrogenation temperature compared to pure NaMgH3 [72]. The Ti3C2 MXene at 7 wt% facilitated a rapid dehydrogenation process (TPD) at 350 °C than other concentrations (3, 5, and 9 wt%). However, higher Ti3C2 content in the NaMgH3-Ti3C2 composite led to the formation of MgO, which resulted in a decrease in H2 desorption efficiency due to the excess “dead weight” of Ti3C2. The isothermal dehydrogenation of the 7 wt% Ti3C2 composites released 3.4 wt% H2 at 350 °C within 5 min (compared to NaMgH3-0.2 wt%) as shown in Figure 6c. The activation energy for the first dehydrogenation step decreased to 114.08 kJ mol-1 for NaMgH3 with 7 wt% Ti3C2, compared to 158.45 kJ mol−1 for pure NaMgH3 (Figure 6d). In terms of isothermal absorption, NaMgH3-7 wt% Ti3C2 absorbed 3.5 wt% H2 at 300 °C in just 6 s, while pure NaMgH3 absorbed only 3.0 wt% H2 in 15 min. Furthermore, the cyclic stability of NaMgH3 improved significantly with the addition of 7 wt% Ti3C2, increasing from poor stability after five cycles for pure NaMgH3. Overall, Ti3C2 played a crucial role in enhancing the H2 storage performance of NaMgH3.
Overall, the 2D Ti3C2 material significantly boosted the H2 storage performance in Mg(BH4)2, NiAlH4, LiBH4, and NaMgH3. The systematic H2 storage performance of Mg(BH4)2, NiAlH4, LiBH4, and NaMgH3 with Ti3C2 additives is summarized in Table 2.

5. Complex Metal Hydrides

In addition to the previously discussed binary and ternary metal hydride combinations for H2 storage performance, the inclusion of 2D Ti3C2 played a significant role in enhancing the performance of complex metal hydrides by reducing the activation energy and operating temperature. The following quaternary metal hydride combinations also demonstrated promising H2 storage performance with the incorporation of 2D Ti3C2.

5.1. LiNa2AlH6 and Li1.3Na1.7AlH6

The addition of 5 wt% Ti3C2 to LiNa2AlH6 reduced the dehydrogenation temperature by approximately 68 K compared to pure LiNa2AlH6 [73]. The Ti0 species were actively involved during the dehydrogenation process, where they transformed into Ti3+. During the absorption process, Ti3+ further converted to Ti2+. Notably, the carbon in Ti3C2 did not interact with H in LiNa2AlH6. A gradual increase in Ti3C2 content (up to 5 wt%) effectively lowered the dehydrogenation temperature to 385 K, compared to the higher temperature of 453 K in pure LiNa2AlH6. The addition of Ti3C2 induced thermal destabilization of LiNa2AlH6, which contributed to the lowering of dehydrogenation temperature. This process resulted in improved dehydrogenation and reduction in the activation energy of LiNa2AlH6. The hydrogenation activation energy for LiNa2AlH6 with 5 wt% Ti3C2 was 58.28 kJ mol−1, which was slightly lower than that of pure LiNa2AlH6 (63.19 kJ mol−1). However, the dehydrogenation activation energy for LiNa2AlH6 with 5 wt% Ti3C2 (191 kJ mol−1) was higher than that of pure LiNa2AlH6 (163 kJ mol−1). Overall, the hydrogen absorption kinetics performance was superior to the dehydrogenation process.
Fan et al. [74] investigated the active role of 2D Ti3C2 (1 wt%, 3 wt%, and 5 wt%) in enhancing the H2 storage performance of Li1.3Na1.7AlH6. The dehydrogenation activation energy of Li1.3Na1.7AlH6 with 5 wt% Ti3C2 increased to 231.9 kJ mol−1 compared to 138.1 kJ mol−1 for pure Li1.3Na1.7AlH6 (Figure 7a). This increase suggests a negative impact on the dehydrogenation kinetics of Li1.3Na1.7AlH6 + 5 wt% Ti3C2. However, the desorption temperature was slightly reduced in the Li1.3Na1.7AlH6 + 5 wt% Ti3C2 (388 K) compared to pure Li1.3Na1.7AlH6 (423 K), as shown in Figure 7b. On the other hand, no significant difference was observed in the H2 absorption activation energies of Li1.3Na1.7AlH6 + 5 wt% Ti3C2 (56.3 kJ mol−1) compared to pure Li1.3Na1.7AlH6 (59.8 kJ mol−1). After the hydrogenation process, both Ti2+ and Ti0 demonstrated their importance for H2 absorption. Overall, Ti3C2 played a crucial role in improving the H2 storage performance of complex metal hydrides.

5.2. MgH2-LiAlH4

The incorporation of the MgH2-LiAlH4 complex hydride with Ti3C2 significantly enhanced the de/re-hydrogenation kinetics by promoting the development of metallic Ti and C [75]. The surface morphology of the 4MgH2-LiAlH4-Ti3C2 system is shown in Figure 7c. The dehydrogenation temperature of 4MgH2-LiAlH4-Ti3C2 reduced to 336 K compared to as-milled 4MgH2 (400 K) and 4MgH2-LiAlH4 (610 K). The system achieved a hydrogen release of 6.6 wt% from 4MgH2-LiAlH4-Ti3C2. After dehydrogenation, the Ti0 and Ti-O, through the disappearance of Ti-C, revealed the transformation of Ti3C2 into metallic Ti, which interacted with MgH2 for the formation of TiH1.942 (Ti2+). Here, the dehydrogenation activation energy of 4MgH2-LiAlH4-Ti3C2 was 128.4 kJ mol−1, lower than that of 4MgH2-LiAlH4-176.2 kJ mol−1 as shown in Figure 7d. The re-hydrogenation activation energy of 4MgH2-LiAlH4-Ti3C2 (65.7 kJ mol−1) was also lower than that of 4MgH2-LiAlH4 (99.2 kJ mol−1) with a H2 absorption of 3.5 wt% at 582 K in 1000 s under 4 MPa pressure. Overall, the addition of 2D Ti3C2 played a pivotal role in enhancing the H2 storage performance of the 4MgH2-LiAlH4-Ti3C2 system.
In conclusion, the 2D Ti3C2 significantly improved the H2 storage performance of LiNa2AlH6, Li1.3Na1.7AlH6, and 4MgH2-LiAlH4. The H2 storage performance of these systems with the addition of 2D Ti3C2 MXene is summarized in Table 3.

6. Challenges and Future Perspectives

Despite the promising potential of 2D Ti3C2 MXenes for H2 storage, several factors still need further investigation. To date, only a limited number of studies have focused on Ti3C2 MXene-based metal hydrides. Therefore, the following perspectives are essential to advance the H2 storage performance of 2D Ti3C2 MXene-based metal hydrides:
(i)
The role of physisorption or chemisorption of Ti3C2 MXenes is still challenging. An in-depth discussion on the chemisorption versus physisorption of Ti3C2 MXenes is needed to improve the H2 storage capacity of metal hydrides.
(ii)
Surface termination groups provide the room for active sites for hydrogen molecules. The role of surface termination groups in the H2 absorption/desorption process of Ti3C2 MXenes remains insufficiently explored. The role of -OH and -O terminated Ti3C2 MXene should be studied to strengthen the H2 storage performance of metal hydride.
(iii)
The precise involvement of Ti3C2 MXene content in the H2 storage process at the interface of metal hydrides is not well understood. Specifically, the roles of edge sites, interlayer spacing, and surface terminations should be addressed.
(iv)
A detailed comparison between monolayer, few-layer, and multilayer Ti3C2 MXenes is necessary to enhance the H2 storage process through the variation in the surface area, binding strength, storage and release ability. The direction of morphological tuning has the ability to strengthen the H2 absorption and release kinetics during the H2 storage process.
(v)
Expansion of interlayer spacing and surface area of Ti3C2 MXene by the doping of single metal atoms have a high chance of accommodating the hydrogen molecules between the layers. These features provide a chance to enhance the H2 absorption/desorption processes at the interface of metal hydrides.
(vi)
Metal oxides such as SnO2, WO3, MnO2, MoO3, and ZrO2 gained their importance in H2 storage [76]. Thus, the integration of metal oxides with Ti3C2 MXene has potential room for further improvement in H2 storage of various metal hydrides.
The integration of Ti3C2 MXenes into metal hydrides demonstrates considerable promise for achieving low-temperature adaptable H2 storage performance. Thus, the role of 2D Ti3C2 MXenes as catalysts is crucial for realizing stable and efficient H2 storage capabilities.

7. Conclusions

The potential of 2D Ti3C2 MXenes in enhancing the H2 storage performance of various metal hydrides has been systematically investigated. Among all the metal hydrides, doping Ti3C2 into MgH2 resulted in remarkable H2 storage performance at lower temperatures and in a significantly shorter time. The presence of Ti metal and the various valence states of Ti greatly enhanced the H2 storage capabilities. Specifically, the incorporation of Ti3C2 MXene reduced both the operating and onset temperatures of the metal hydrides. Additionally, the exposure of a few layers and internal edge planes contributed to an increase in active sites, further boosting the H2 storage performance. Sharp responses in isothermal dehydrogenation were observed with 6.2 wt% released in 60 s [37], 6.7 wt% in 3 min [43], and 7.0 wt% in 3 min for the addition of pure Ti3C2, Ti3C2 MXene-derived K2Ti6O13, and PrF3/Ti3C2 into MgH2, respectively. The inclusion of 2D Ti3C2 MXenes in various metal hydrides also demonstrated enhanced cyclic de/re-hydrogenation stability with 10 cycles for 5 wt% Ni30/FL-Ti3C2Tx doped MgH2 [39], 10 cycles for Ti3C2 MXene derived K2Ti6O13 + MgH2 [43], 20 cycles for MgH2 + Ti3C2@MnO2 [49], 15 cycles for Ti3C2/NC + NaAlH4 [66], and 5 cycles for NaMgH3-7 wt% Ti3C2. These findings strongly emphasize the crucial role of the 2D layered structure of Ti3C2 MXenes involved in improving the H2 storage performance of various metal hydrides, highlighting their potential applicability in future fuel cell technologies.

Author Contributions

Conceptualization, A.S.; Writing-original draft preparation, A.S.; validation, A.S.; Writing-review and editing, J.-S.N.; supervision, J.-S.N.; funding acquisition, J.-S.N. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2019R1A2C1008746).

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Figure 1. Schematic illustration of metal hydrides mixed with 2D Ti3C2 MXene for improvement in H2 storage performance.
Figure 1. Schematic illustration of metal hydrides mixed with 2D Ti3C2 MXene for improvement in H2 storage performance.
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Figure 2. Potentiality of various metal hydrides mixed 2D Ti3C2 MXenes for H2 storage performance.
Figure 2. Potentiality of various metal hydrides mixed 2D Ti3C2 MXenes for H2 storage performance.
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Figure 3. (a) Isothermal dehydrogenation behavior of 5 wt% Ni30/FL-Ti3C2Tx doped MgH2 at 250 °C, (b) isothermal hydrogenation of 5 wt% Ni30/FL-Ti3C2Tx doped MgH2 at 100 °C (reprinted from Ref. [39] copyright 2020, with permission from ACS Publication), (c) non-isothermal dehydrogenation behavior of PrF3/Ti3C2 interfaced MgH2 compared to pristine, PrF3, and Ti3C2, and (d) isothermal H2 desorption at 260 °C (reprinted from Ref. [40] copyright 2022, with permission from Elsevier).
Figure 3. (a) Isothermal dehydrogenation behavior of 5 wt% Ni30/FL-Ti3C2Tx doped MgH2 at 250 °C, (b) isothermal hydrogenation of 5 wt% Ni30/FL-Ti3C2Tx doped MgH2 at 100 °C (reprinted from Ref. [39] copyright 2020, with permission from ACS Publication), (c) non-isothermal dehydrogenation behavior of PrF3/Ti3C2 interfaced MgH2 compared to pristine, PrF3, and Ti3C2, and (d) isothermal H2 desorption at 260 °C (reprinted from Ref. [40] copyright 2022, with permission from Elsevier).
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Figure 4. (a) Isothermal hydrogenation behavior of porous Ti3C2 MXene (MX) mixed MgH2 at 100 °C, (b) isothermal desorption behavior of MgH2-5 wt% MX-P-TiO2 (reprinted from Ref. [48] copyright 2023, with permission from Elsevier), (c) isothermal desorption behavior of pure Ti3C2, and TiCrV/Ti3C2-(Fe, Co, and Ni) at 523 K, and (d) isothermal H2 absorption of pure Ti3C2, and TiCrV/Ti3C2-(Fe, Co, and Ni), at 453 K (reprinted from Ref. [49] copyright 2024, with permission from Elsevier).
Figure 4. (a) Isothermal hydrogenation behavior of porous Ti3C2 MXene (MX) mixed MgH2 at 100 °C, (b) isothermal desorption behavior of MgH2-5 wt% MX-P-TiO2 (reprinted from Ref. [48] copyright 2023, with permission from Elsevier), (c) isothermal desorption behavior of pure Ti3C2, and TiCrV/Ti3C2-(Fe, Co, and Ni) at 523 K, and (d) isothermal H2 absorption of pure Ti3C2, and TiCrV/Ti3C2-(Fe, Co, and Ni), at 453 K (reprinted from Ref. [49] copyright 2024, with permission from Elsevier).
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Figure 6. (a) Surface morphology and elemental mapping of LiBH4 + 40 wt% Ti3C2, (b) isothermal H2 desorption of pure LiBH4, LiBH4 + 20 wt% Ti3C2, and LiBH4 + 40 wt% Ti3C2, (reprinted from Ref. [71] copyright 2019, with permission from Elsevier), (c) isothermal dehydrogenation behavior of pure NaMgH3 and NaMgH3-Ti3C2 at 3, 5, 7, and 9 wt% of Ti3C2, (d) first step of dehydrogenation activation energy of NaMgH3 and NaMgH3-7 wt%Ti3C2 (reprinted from Ref. [72] copyright 2021, with permission from Elsevier).
Figure 6. (a) Surface morphology and elemental mapping of LiBH4 + 40 wt% Ti3C2, (b) isothermal H2 desorption of pure LiBH4, LiBH4 + 20 wt% Ti3C2, and LiBH4 + 40 wt% Ti3C2, (reprinted from Ref. [71] copyright 2019, with permission from Elsevier), (c) isothermal dehydrogenation behavior of pure NaMgH3 and NaMgH3-Ti3C2 at 3, 5, 7, and 9 wt% of Ti3C2, (d) first step of dehydrogenation activation energy of NaMgH3 and NaMgH3-7 wt%Ti3C2 (reprinted from Ref. [72] copyright 2021, with permission from Elsevier).
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Figure 7. (a) Dehydrogenation activation energy of Li1.3Na1.7AlH6 + 5 wt% Ti3C2, (b) variation in the desorption temperature of Li1.3Na1.7AlH6 and Li1.3Na1.7AlH6 + Ti3C2 at various wt% (1, 3, and 5) of Ti3C2 (reprinted from Ref. [74] copyright 2018, with permission from Elsevier), (c) surface morphology of 4MgH2-LiAlH4-Ti3C2, and (d) dehydrogenation activation energy of 4MgH2-LiAlH4-Ti3C2 (reprinted from Ref. [75] copyright 2019, with permission from Elsevier).
Figure 7. (a) Dehydrogenation activation energy of Li1.3Na1.7AlH6 + 5 wt% Ti3C2, (b) variation in the desorption temperature of Li1.3Na1.7AlH6 and Li1.3Na1.7AlH6 + Ti3C2 at various wt% (1, 3, and 5) of Ti3C2 (reprinted from Ref. [74] copyright 2018, with permission from Elsevier), (c) surface morphology of 4MgH2-LiAlH4-Ti3C2, and (d) dehydrogenation activation energy of 4MgH2-LiAlH4-Ti3C2 (reprinted from Ref. [75] copyright 2019, with permission from Elsevier).
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Table 1. Hydrogen storage performance of binary metal hydrides (MgH2 and AlH3) integrated Ti3C2 MXenes at a selective weight percentage of Ti3C2.
Table 1. Hydrogen storage performance of binary metal hydrides (MgH2 and AlH3) integrated Ti3C2 MXenes at a selective weight percentage of Ti3C2.
Additive
(Selective Content)		Isothermal DehydrogenationIsothermal
Hydrogenation		Activation Energy
(kJ mol−1)
Absorption/Desorption		Key Parameters
For H2
Storage		Ref.
Temperature—H2
Proportion—Time		Temperature—H2
Proportion—Time
MgH2
Ti3C2
(5 wt%)		300 °C—6.2 wt%—60 s150 °C—6.1 wt%—30 s---/98.9 (MgH2-155)Metallic Ti formation [37]
Ti3C2/TiO2-C
(5 wt%)		250 °C—5.0 wt%—1700 s125 °C—4.0 wt%—800 s42.32 (MgH2-71)/77.69Anatase TiO2, Ti0, Ti2+ Ti3+, and Ti4+[38]
Ni/Ti3C2Tx
(5 wt%)		250 °C—5.83 wt%—1800 s100 °C—5.0 wt%—1700 s41.36 (MgH2-71)/96.36 Ti0, Ti2+ Ti3+, and Ti4+[39]
Ni@Ti3C2250 °C—5.2 wt%—15 min125 °C—5.4 wt%—25 s73 ± 3.5 (MgH2-141 ± 5.3)/56 ± 4Metallic Ti along with Ni [41]
Ti3C2 MXene derived K2Ti6O13
(5 wt%)		280 °C—6.7 wt%—3 min200 °C—6.5 wt%—30 s---/105.67 (MgH2-175.34)Ti and Ti2+[44]
2V2C/Ti3C2
(10 wt%)		225 °C—5.1 wt%—60 min40 °C—5.1 wt%—20 s---/79.4 (MgH2-127.7)Uniform distribution of Mg, Ti, and V[45]
Ti3C2
(6 wt%)		240 °C—6.45 wt%—10 min150 °C—6.47 wt%—480 s---/99 (MgH2-153)Metallic Ti formation[46]
Ni/Ti3C2
(5 wt%)		100 °C—4.59 wt%—1200 s250 °C—5.87 wt%—2400 s42.38 (MgH2-71)/91.64Ti valence states (Ti0, Ti2+ Ti3+, and Ti4+)[47]
PrF3/Ti3C2
(5 wt%)		260 °C—7.0 wt%—3 min150 °C—6.16 wt%—10 min---/78.11(MgH2-117.98)Ti valence states (Ti0, Ti2+, Ti3+, and Ti4+)[40]
Ti3C2
(5 wt%)		---275 °C—6.6 wt%---36.22/79.46Internal metallic Ti active edge sites [48]
Ti3C2@MnO2275 °C—6.4 wt%—484 s30 °C—4.4 wt%—150 s---/61.8 ± 2.2 (MgH2-142.4 ± 0.9)Multiple interfaces, Ti3C2/MgH2, TiO2/MgH2, MnO2/MgH2, MnO/MgH2, and Mn/MgH2[50]
Ni/Ti3C2
(Ti3C2 @5 wt%)		300 °C—3.96 wt%---300 °C—4.46 wt%------/75.0(Mg-135.4)Synergy between Ni and Ti3C2 [51]
Ti3C2-Ni523 K—4.982 wt%—60 min453 K—5.72 wt%—1 min80.54/---Mg-TiCrV/Ti3C2-Ni
interface		[49]
AlH3
Ti3C2
(4 wt%)		100 °C—6.9 wt%—20 min------/40High surface area and active sites of Ti3C2 [54]
Table 2. Hydrogen storage performance of ternary metal hydrides- (Mg(BH4)2, NiAlH4, and LiBH4) integrated Ti3C2 MXenes at an optimized weight percentage of Ti3C2.
Table 2. Hydrogen storage performance of ternary metal hydrides- (Mg(BH4)2, NiAlH4, and LiBH4) integrated Ti3C2 MXenes at an optimized weight percentage of Ti3C2.
Additive
(Content)		Isothermal
Dehydrogenation		Isothermal
Hydrogenation		Activation Energy
(kJ mol−1)
Absorption/Desorption		Key Parameters for H2
Storage		Ref.
Temperature—H2
Proportion—Time		Temperature—H2
Proportion—Time
Mg(BH4)2
VF4@Ti3C2
(20 wt%)		275 °C—8.2 wt%—300 min------/172.9 (Mg(BH4)2-374.1)Formation of metallic Ti and VH2.01[55]
NiAlH4
Ti3C2
(7 wt%)		140 °C—4.7 wt%—100 min120 °C—4.6 wt%—60 min---/87.3 ± 6.7 (First step) Ti metal and Ti3+[60]
Ti3C2
(OH0.8F1.2)2		100 °C—3.11 wt%—90 min------Ti3C2 MXene-derived alanates and rutile TiO2 [61]
MXene/A-TiO2
(15 wt%)		140 °C—3.0 wt%—7 min------/78.32 (First step)Homogeneous distribution of Ti and C[62]
Ti3C2
(8 wt%)		110 °C—4.1 wt%—119 min110 °C—4.2 wt%—4.5 min---/92.5 (First step)Ti and TiFx particles[63]
CeF3/Ti3C2100 °C—3.0 wt%—80 min------/81.39 (First step)Ti-F-Ce bonding[64]
C@TiO2/Ti3C2
(10 wt%)		140 °C—4.0 wt%—13 min------/72.41 (First step)Ti0 and Ti3+ states[65]
Ti3C2/N doped carbon
(10 wt%)		140 °C—4.61 wt%—60 min------/76.66 (First step)Interaction between pyridinic-N and Ti0[66]
LiBH4
Ti3C2300 °C—8.2 wt%—8 h------/94.44 (50% of LiBH4)Ti-containing defect sites[70]
Ti3C2
(40 wt%)		300 °C—3.0 wt%—6 h------/70.3 (LiBH4-187 ± 24)Ti metal and high surface area[71]
NaMgH3
Ti3C2
(7 wt%)		350 °C—3.4 wt%—5 min300 °C—3.5 wt%—6 s---/114.08 (NaMgH3-158.45) (First step)Lamellar-structure Ti3C2[72]
Table 3. Complex metal hydrides (LiNa2AlH6, Li1.3Na1.7AlH6, and 4MgH2-LiAlH4) integrated Ti3C2 MXenes for H2 storage performance.
Table 3. Complex metal hydrides (LiNa2AlH6, Li1.3Na1.7AlH6, and 4MgH2-LiAlH4) integrated Ti3C2 MXenes for H2 storage performance.
Complex Metal HydridesKey Parameters for H2
Storage		Ref.
AdditiveDehydrogenation Temperature Hydrogenation Activation Energy
(kJ mol−1)		H2
Release/
Absorption
(wt%)
Ti3C2LiNa2AlH6 +
5 wt% Ti3C2
(385 K)		LiNa2AlH6
(453 K)		LiNa2AlH6 +
5 wt% Ti3C2
(58.28)		LiNa2AlH6
(63.19)		---Ti0 species[73]
Ti3C2Li1.3Na1.7AlH6 + 5 wt% Ti3C2
(388 K)		Li1.3Na1.7AlH6
(423 K)		Li1.3Na1.7AlH6 +
5 wt% Ti3C2
(56.3)		Li1.3Na1.7AlH6
(59.8) 		---Ti2+ and Ti0[74]
Ti3C24MgH2-LiAlH4-Ti3C2
(400 K)		4MgH2-LiAlH4 (610 K)4MgH2-LiAlH4-Ti3C2
(65.7)		4MgH2-LiAlH4
(99.2)		6.6/3.5metallic Ti, TiH1.942 (Ti2+)[75]
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Sreedhar, A.; Noh, J.-S. Advancements in Ti3C2 MXene-Integrated Various Metal Hydrides for Hydrogen Energy Storage: A Review. Nanomaterials 2025, 15, 673. https://doi.org/10.3390/nano15090673

AMA Style

Sreedhar A, Noh J-S. Advancements in Ti3C2 MXene-Integrated Various Metal Hydrides for Hydrogen Energy Storage: A Review. Nanomaterials. 2025; 15(9):673. https://doi.org/10.3390/nano15090673

Chicago/Turabian Style

Sreedhar, Adem, and Jin-Seo Noh. 2025. "Advancements in Ti3C2 MXene-Integrated Various Metal Hydrides for Hydrogen Energy Storage: A Review" Nanomaterials 15, no. 9: 673. https://doi.org/10.3390/nano15090673

APA Style

Sreedhar, A., & Noh, J.-S. (2025). Advancements in Ti3C2 MXene-Integrated Various Metal Hydrides for Hydrogen Energy Storage: A Review. Nanomaterials, 15(9), 673. https://doi.org/10.3390/nano15090673

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