Hydrogen Storage Behavior and Performance of Multiple Cold-Rolled MgH₂/Nb₂O₅ Nanocomposite Powders

Abstract

The global interest in MgH₂ is due to the natural availability of Mg and its capacity to retain hydrogen at a concentration of up to 7.60 wt.%. Despite its appealing characteristics and ease of production on an industrial scale at ambient temperature using the reactive ball milling (RBM) technique, MgH₂ is a highly stable chemical with sluggish hydrogenation and dehydrogenation rates below 300 °C. Among the different methods used to improve the hydrogenation/dehydrogenation kinetic behavior of MgH₂, mechanical treatment and/or catalysis are regarded to be the most effective methods. The purpose of this research was to explore the effectiveness of several cold rolling (CR) stages on the hydrogenation properties of recycled magnesium rods, as well as the effect of the resulting RBM on the final product. For this process, the as-received waste Mg-rods were firstly cold-rolled 200 times and then remilled under H₂ atmosphere for 100 h. The as-RBM powders were then cold-rolled for 100 passes and then ball-milled with 10 and 15 wt.% Nb₂O₅ powders for 50 h. The results showed that when the materials were subjected to different types of defects (dislocations, stacking faults, and twining) generated by CR and RBM, their gas absorption/desorption kinetics were improved. This was indexed by their ability to achieve a long cycle lifetime at lower temperatures when compared with the as-received materials. The powders were further improved in terms of kinetics and decomposition temperature upon RBM with Nb₂O₅ for 50 h. The nanocomposite MgH₂/10 wt.% and 15 wt.% Nb₂O₅ exhibit good hydrogen storage capabilities at a comparatively low temperature (225 °C) with a long cycle life that extended from 110 h to 170 h, without serious degradation in storage capacity and kinetics.

1. Introduction

Because of the significant worldwide environmental changes caused by carbon dioxide emissions, and the massive use of finite fossil fuel resources, finding alternative and renewable energy sources is critical for a secure and sustainable future. Concerns regarding combustion engine pollution, as well as other difficulties associated with large-scale mining, transportation, processing, and use of fossil fuels, are growing. [1]. The increased risks posed by global warming as a result of the use of fossil fuels necessitate the adoption of innovative methods to harness the limitless sources of energy [2]. Hydrogen is an energy carrier option that has great potential as a new renewable and clean energy source [3]. It is an efficient, secure, and flexible fuel source that can readily be transformed into the required type of energy while emitting no hazardous pollutants. As a consequence, hydrogen has been selected as the optimal fuel for mobile and stationary applications [4,5].

It is widely expected that hydrogen will become the primary fuel for the majority of cars and portable devices within a few decades for a variety of reasons, the majority of which are related to hydrogen’s fascinating and pioneering characteristics, which include the following: (i) hydrogen is a colorless, odorless, nontoxic, carbon-free gas whose only byproduct is water; (ii) hydrogen is the energy vector that could provide a bridge from the fossil energy economy to the renewable energy economy; and (iii) it can be stored [6]. Notably, when hydrogen gas is burned, the sole emission is water vapor. As a consequence, hydrogen combustion produces no carbon dioxide (CO₂) [7]. Obviously, hydrogen is a less polluting fuel than gasoline since it produces relatively little exhaust emissions. Hydrogen has the potential to outperform an internal combustion engine in terms of fuel cell engine efficiency. The same quantity of hydrogen will propel a fuel cell vehicle at least twice as far as a gasoline-powered vehicle. In comparison to fossil fuels, the potential alternative fuel with its previously stated favorable characteristics also displays attractive properties [5].

Hydrogen storage, which encompasses both hydrogen production and consumption and so plays a critical role in establishing a hydrogen economy [8], has been the subject of much research for many years. Metals and alloys have the ability to absorb hydrogen in its molecular state, therefore functioning as hydrogen sponges. Around 50 metallic elements in the periodic table have variable degrees of hydrogen absorption, resulting in a diverse variety of hydrogen storage materials [9]. Metal hydrides have a higher hydrogen storage density than hydrogen gas (0.99 hydrogen atoms/cm³) or liquid hydrogen (4.2 hydrogen atoms/cm³). [10] Thus, metal hydride storage enables the safe and volume-efficient storage of hydrogen for onboard vehicle applications. Magnesium hydride (MgH₂) has been identified as one of the most promising hydrogen storage compounds for use in real-world light automobile applications. This is because magnesium is a light, cheap metal capable of storing up to 7.60 wt.% hydrogen.

Despite these attractive properties, the slow hydrogenation and dehydrogenation rates of MgH₂ pose considerable barriers to its use in fuel cell applications [11]. The kinetics of MgH₂ powder absorption and desorption may be increased by adding catalysts to make the hydrogenation properties fast enough for practical applications [12] or by milling the powders for a prolonged length of time [13]. In fact, prolonged ball milling often boosts the impact of grain boundaries and hydrogen transport [14]. As a result of the introduction of different types of defects, such as lattice and point defects, into the lattice of ball-milled powders, the stable phase of MgH₂ destabilizes [15]. As Floriano et al. have shown, similar flaws may also occur during the cold rolling of magnesium metal [16]. It has been suggested [17,18] that increasing the length of ball milling time lowers powder grain sizes, hence increasing both grain boundaries and active surface area. As a consequence, a more direct path for hydrogen diffusion throughout the bulk area is expected.

Meanwhile, and in addition to the mechanically induced method, doping MgH₂ with catalytic agents has been identified as a realistic scenario used to enhance the slow kinetics of this metal hydride. Since the 1990s, a lengthy list of pure transition metals (TM), such as Ti, Zr, V, Nb, Fe, Co, Ni, and Mn [19,20,21], as well as their alloys, such as TiV [22], CrTi [23], TiMn₂ [24], and ZrNi₅ [25], have been developed. Almost all of these reported catalytic agents have resulted in considerable advantages, as measured by reduced hydrogen sorption/desorption temperatures and quicker hydrogenation/dehydrogenation kinetics [2]. In contrast to the extensive list of advantageous long-range order catalysts, El-Eskandarany presented the first research using short-range order material of an amorphous-Zr₇₀Ni₂₀Pd₁₀ powder for enhancing MgH₂ [26]. In addition, it has been reported that mechanical treatment of bulk Mg has shown significant improvement of the hydrogenation properties of MgH₂ [27]. It was reported that using such a metastable amorphous phase resulted in a substantial improvement in the kinetics of MgH₂.

Table 1. Selected MgH₂-nanocomposite hydrogen storage systems.

Material	Preparation	Properties	References
MgH2	As-received	Storage density of up to 7.6 wt.% with slow kinetics and poor thermodynamic characteristics necessitating dehydrogenation temperatures of 300 °C.	[28]
MgH2	Pure Mg powders were ball-milled under 50 bar (reactive ball milling, RBM) of hydrogen pressure for 200 h	Single, stable phase of β-MgH2 was obtained upon milling for 25 h. After 200 h of RBM time, the decomposition temperature and activation energy were recorded to be 399 °C and 131 kJ/mol, respectively. The times required for complete absorption and desorption of 7 wt.% of hydrogen at 250 °C were recorded to be 3140 s and 35,207 s under 10 and 0 bar, respectively. Complete 600 absorption/desorption cycles were achieved.	[13]
MgH2-10 wt.% TM (TM; Fe, Co, Ni, Cu nanoparticles)	Ar atmosphere/high-energy ball milling (HEBM)/4 h	Mg–Fe: Absorption/10 bar/350 °C: 5.1 wt.%/9.1 min, Ea = 86 kJ/mol Mg–Co: Absorption/10 bar/350 °C: 4.3 wt.%/7.7 min, Ea = 75 kJ/mol Mg–Ni: Absorption/10 bar/350 °C: 5 wt.%/6.1 min, Ea = 72 kJ/mol Mg–Cu: Absorption/10 bar/350 °C: 4.6 wt.%/15.3 min, Ea = 76 kJ/mol	[29]
MgH2-1 at % Zr	H2-RBM/300 rpm/20 h + annealing at 100 °C/40 bar H2/24 h	Absorption: 5.5 wt.%/4000 s/300 °C Absorption of nanocomposite: 3.5 wt.% H2/6000 s/100 °C Dehydrogenation: 5.5 wt.% H2/2000 s/350 °C	[30]
MgH2-10 wt.% Mn nanoparticles	H2-RBM/450 rpm/5 h	Onset desorption temperature: 175 °C and 6.7, 6.5, and 6.1 wt.% hydrogen released within 5, 10, and 25 min at 300, 275, and 250 °C, respectively.	[31]
MgH2/8 wt.% Nb2O5+2 wt.% Ni	Disks of nanocomposite coating with Ni powder (3 layers)	5.5 wt.% hydrogen storage density and fast absorption/desorption kinetics of 2.6/3 min at 250 °C with 400 h life cycle. MgH2-20 wt.% TiO2	[32]
MgH2-20 wt.% TiO2 nanoparticles	H2-RBM/1 h/8 bar	Absorption/20 bar/350 °C: 3.8 wt.%/2 min Desorption/1 bar/350 °C: 4.4 wt.%/8.5 min	[33]
MgH2-15 wt.% TiFe	H2-RBM/450 rpm/5 h	Onset desorption temperature: 175 °C Hydrogen released: 6.5 wt.% H2/10 min at 300 °C Ea: 72.5 kJ/mol	[34]
MgH2-40 wt.% TiFeMn	Ar-HEBM/40 h	Absorption/40 bar/80 °C: 4 wt.%/3600 s Desorption/ 3 bar/240 °C: 2.57 wt.%/7200 min	[35]

provides some further selected published findings from several categories for MgH₂, as well as MgH₂-based hydrogen storage systems.

Aim of the Present Study

As can be realized from the literature, authors employed either high-energy ball milling or cold rolling [36] to improve the hydrogen storage characteristics of the MgH₂ system. The use of reactive ball milling (RBM) in tandem with multistep cold rolling has not yet been reported, to the best of our knowledge. In contrast to the conventional practice of introducing lattice defects into Mg- bulk materials in order to improve their hydrogenation/dehydrogenation conditions through a cold rolling process [27], the present work aimed to investigate the synergetic effect of high-energy reactive ball milling of plastically deformed cold-rolled Mg flakes on hydrogenation/dehydrogenation as well as decomposition processes. When compared with the cold rolling process, which results in severe plastic deformation of magnesium lattices, high-energy ball milling results in different types of lattice imperfections, such as point and lattice defects, which are introduced through the action of the milling media, whereby the Mg flakes are trapped and nipped between the steel balls. The combination of the two methods may result in better lattice imperfection and significant improvements in the behavior of Mg metal in terms of absorption/desorption kinetics when compared with either approach alone. Secondly, we also investigated the behavior of as-prepared MgH₂ powders, which have been milled for an extended period of time, when subjected to significant plastic deformation caused by the cold rolling procedure. In addition, the impact of doping the cold-rolled MgH₂ milled powders with a common catalytic agent, Nb₂O₅, was investigated as well. Moreover, the current research sought to investigate the impact of cold pressing on the hydrogenation/dehydrogenation kinetics and cyclability of MgH₂.

2. Materials and Methods

Figure 1 displays the experimental procedure of preparing MgH₂ and the mechanical treatment procedure of cold rolling and high-energy ball milling.

2.1. Materials and Sample Preparations

A batch of 30 bars (200 mm in length, 5 mm in diameter) of recycled Mg (99.5 wt.%) chips were acquired from Shanghai Xinglu Chemical Technology Co. Ltd., Shanghai, China, and utilized as feedstock materials. Niobium pentoxide (Nb₂O₅) powders (99.9 wt.%, 10 μm) purchased from Sigma-Aldrich (Sigma-Aldrich, Inc., St. Louis, MO, USA) were used as catalytic agent at a concentration of 10–20 wt.% to enhance the hydrogen storage behavior of Mg and to improve its hydrogen storage kinetic behavior.

The Mg-rods were snipped into shorter rods (~100 mm in length) and sonicated in a cold acetone solution for 10 min. The rods were then washed with pure ethanol before being dried in an oven at 180 °C for 18 h. This treatment stage was required to eliminate any undesired hydrocarbon impurities from the rod’s surfaces. The materials were then cold-rolled (CR) on a typical two-tool steel drum cold roller machine for 50–200 passes. After CR, the snipped strips were cleaned with acetone and ethanol and then dried in an oven at 180 °C overnight. The as-snipped Mg flakes (Figure 1) were charged and sealed within a helium (He) glove box (UNILAB Pro Glove Box Workstation, mBRAUN, Glove Box Workstation UNILAB Pro, Dieselstr, 31, D-85748 Garching, Germany) in a stainless steel (SUS304) tube (8 cm diameter and 200 mm length). The tube containing Mg flakes was severely CR for a variety of passes (50 to 200 passes) using the manual cold roller.

The as-CR tube system was opened inside the glove box to discharge the CR Mg materials flakes. An amount of 5 g of Mg flakes was charged into a tool steel vial (200 mL capacity) supplied by evico GmbH, Großenhainer Str. 101, 01127 Dresden, Germany, together with 60 tool steel balls (10 mm in diameter) at a 45:1 ball-to-powder weight ratio. The sealed vial was then evacuated to a pressure of 10⁻³ bar and pressurized with 50 bar of pure (99.999 wt.%) H₂ gas. The vial was then placed on a high-energy ball mill (PM 400) provided by Retsch GmbH, Retsch-Allee 1–5, 42781 Haan, Germany, and operated at a speed of 250 rpm for 100 h.

A separate set of experiments was performed to determine the effect of the CR process on Mg powders produced after 100 h of reactive ball milling (RBM) in the presence of H₂. The powders that became MgH₂ were charged into a stainless steel (SUS304) tube (8 cm diameter and 200 mm length) and then cold-rolled for 100 passes. The as-CR MgH₂ powders were doped with 10 and 15 wt. % of Nb₂O₅ and then RBM under 50 bar of H₂ for 50 h.

2.2. Sample Characterizations

2.2.1. Crystal Structure

X-ray diffraction (XRD) analysis was conducted using a SmartLab-Rigaku XRD (Rigaku Corporation, 3-9-12, Matsubara-cho, Akishima-shi, Tokyo, Japan) equipped with CuKα radiation at a power output of 9 kW. Field emission high-resolution transmission electron microscopy (HRTEM, JOEL-2100F, Otemachi Nomura Bldg. 13F, 2-1-1, Otemachi, Chiyoda, Tokyo, Japan) was used in conjunction with energy dispersive spectroscopy (EDS) using a scanning transmission electron microscope (STEM) from Oxford Instruments (Asylum Research, NanoAnalysis, 25.2 mi, High Wycombe, UK, HP12 3SE) outfitted with a JEOL-2100F. The objective lens of this microscope has a spherical aberration coefficient (Cs) of 0.5 mm, a resolution of 0.19 nm at the point, and a lattice resolution of 0.12 nm at the lattice. The spot sizes produced by nanobeam diffraction (NBD) were 0.5 and 25 nm, respectively.

2.2.2. Thermal Analysis

Shimadzu Thermal Analysis System/TA-60WS (Shimadzu Manufacturing Co., Ltd., 1 Nishinokyo Kuwabaracho, Nakagyo-ku, Kyoto 604-8511, Japan) was used to study the decomposition temperatures of MgH₂ powders using a differential scanning calorimeter (DSC) at a heating rate of 10 °C/min and under 1 bar of He.

2.2.3. Hydrogenation/Dehydrogenation Behaviors

The kinetics of hydrogen absorption/desorption were studied using Sievert’s technique utilizing a PCTPro-2000 supplied by Setaram Instrumentation (11, rue de Sainte Assise, F-77240 Seine-Port, France) at hydrogen gas pressures ranging from 200 mbar (for dehydrogenation) to 10 bar (for hydrogenation). Temperatures of 225, 250, 275, and 350 °C were used to analyze the materials.

3. Results and Discussions

3.1. Morphology and Crystal Structure

Figure 2a illustrates the X-ray diffraction (XRD) patterns and field emission high-resolution transmission electron microscopy (FE-HRTEM) images of Mg-rods following various stages of CR (0 to 200 passes). Clear Bragg peaks matching to hcp-Mg were observed in the sample (PDF# 00-004-0770), with no indication of incompatibility with the given values. (Figure 2a). This may be owing to the absence of lattice imperfections in the MS–Mg ribbons. Meanwhile, low-magnification bright field imaging (BFI) of the as-received Mg-rods (0 passes) revealed twin-free grain boundaries with no indication of lattice defects (Figure 2b), which was in good accordance with the XRD pattern exhibited in Figure 2a. Additionally, the Mg-rods at this starting stage of CR contained large uniaxial grains with irregular size, ranging from 600 nm to 2200 nm, as shown in Figure 2b.

After 25 cycles of CR, these grains tended to disintegrate along their grain boundaries, forming finer grains (75 nm to 190 nm) of polycrystalline Mg, as presented in Figure 2c,d, respectively. Further CR (50–200 passes) resulted in tremendous lattice defects, as shown by the considerable mismatching of the Bragg peaks for crystallographic planes (002) and (101) in Figure 2e,g. This requires the formation of a high level for the fiber texture’s plane (002) [28].

At this stage of CR, Mg grains tended to follow the shear force produced by the process, resulting in plastic deformation (Figure 3b,c) along the CR direction, culminating in elongated Mg grains with apparent sizes between 50 nm to 240 nm, as shown in Figure 3a.

To prepare MgH₂ powders, the snipped Mg flakes obtained after 100 times of CR were high-energy ball-milled under H₂ pressure of 50 bar for 100 h. The FE-HRTEM dark-field image (DFI) and corresponding selected area diffraction pattern (SADP) of the powders obtained after 100 h of RBM are displayed in Figure 4a,b, respectively. The as-RBM powders composited of ultrafine nanocrystalline particles with sizes in the range of 8 nm to 18 nm, as shown in Figure 4a. The SADP of this end-product (100 h) revealed continuous Debye–Scherrer overlapped rings related to tetragonal (β) and orthorhombic (γ)—MgH₂ phases, as indexed in Figure 4b. Formation of MgH₂ powders after 100 h of RBM was also confirmed by XRD (Figure 5a). Figure 5a displays the XRD patterns of CR Mg-flakes for 100 passes after RBM for 100 h.

The formation of the -MgH₂ phase occurred in the presence of minute molecular fractions of the metastable -MgH₂ phase. The lack of Bragg peaks associated with pure hcp-Mg suggests that the RBM process was complete (Figure 5a). The considerable widening of the Bragg peaks observed after 100 h of RBM (Figure 5a) is ascribed to internal strain caused by a high dislocation density and grain refining. Figure 5b illustrates the XRD pattern of Nb₂O₅ powder used as a catalytic agent for MgH₂. The powder exhibited prominent Bragg peaks consistent with the orthorhombic-Nb₂O₅ phase as described (PDF File# 00-027-1003). To increase the hydrogenation/dehydrogenation kinetics and cycle life of MgH₂ produced after 100 h of RBM, 10% and 15% Nb₂O₅ powders were combined and then mechanically ball-milled for 50 h with the metal hydride powders. Figure 5c,d illustrates the XRD patterns of nanocomposite MgH₂/10 wt. percent Nb₂O₅ and MgH₂/15 wt. % Nb₂O₅ powders after 50 h of RBM. Both samples had large Bragg peaks, indicating the creation of nanocrystalline structures.

FE-HRTEM was utilized to visualize the local structure of the as-RBM nanocomposite powders produced after 50 h of milling. The FE-HRTEM image of MgH₂/10 wt.% Nb₂O₅ is displayed in Figure 6a. The sample after this stage of milling revealed numerous nanosized grains with different orientations (Figure 6a). Examining the interplanar spacing (d) presented by the Moiré-fringe images indicated that the presence of β- and metastable γ-MgH₂ phases coexisted with orthorhombic-Nb₂O₅, as displayed in Figure 6a. Moreover, the sharp ring SADP pattern shown in Figure 6b, which confirmed the existence of these phases, indicates the formation of nanoscaled grains. This is implied by the lack of sharp spots (Figure 6b).

3.2. Thermal Stability

The thermal stability of the samples was examined under flow (50 mL/min) of He gas and a heating rate of 10 °C/min. All Mg-rods obtained after different stages of CR process (0–200 passes) were first hydrogenated for 3 h at 350 °C under 50 bar of H₂. The DSC traces of as-received Mg-rods and as-CR samples for 50, 100, and 200 passes are presented collectively in Figure 7a. Each of the thermograms in the figure revealed a single endothermic event associated with the decomposition of the MgH₂ phase. As demonstrated in Figure 6a, the non-CR sample released hydrogen at a relatively high temperature (470 °C). The decomposition temperature (T^dec) tended to decrease from 390 °C to 378 °C upon increasing the CR from 50 to 200 passes, respectively, as displayed in Figure 7a. It can be argued that by adding extensive imperfections to the Mg lattice via the CR process, beneficial defects were created at the grain boundaries, resulting in the refinement of the magnesium grains. Due to the formation of such fine grains, the hydrogen diffusion distance was reduced, therefore improving the decomposition kinetics and, hence, reducing the T^dec temperature.

In contrast to the CR Mg samples, the thermograms of the sample, which was first CR for 200 passes and then RBM for 100 h, revealed two endothermic events, as displayed in Figure 7b(i). The low-temperature endothermic peak centered at 335 °C was related to the decomposition of metastable γ-MgH₂ phase, where the high-temperature (394 °C) peak corresponded to the decomposition of β-MgH₂ (Figure 7b(i)). The DSC trace of as-CR Mg rods for 200 passes, which were snapped into tiny flakes, and then RBM for 100 h is displayed in Figure 7b(ii). Obviously presented both T^dec temperatures related to γ- and β-MgH₂ phases were significantly decreased to 318 °C and 365 °C, respectively (Figure 7b(ii)). This significant deducing in T^dec temperatures was attributed to the introduction of further lattice imperfections to the powders that led to the defragmentation of large MgH₂ grains into fine grains. In order to conduct further reduction of the thermal stability for MgH₂ powders, the processed MgH₂ powders were mechanically ball-milled with 10 wt.% (Figure 7b(iii)) and 15 wt.% (Figure 7b(iv)) of Nb₂O₅ powders for 50 h. Doping the metal hydride powders with 10 wt.% Nb₂O₅ led to further decreasing in grain size and this led to further improving in T^dec temperatures, which decreased to the level of 310 °C (γ-phase) and 342 °C (β-phase) respectively, as displayed in Figure 7b(iii).

These values of T^dec temperatures revealed further decreasing to 289 °C and 335 °C upon mixing with 15 wt.% Nb₂O₅ powders. It has been realized that Nb₂O₅ abrasive powders played the role of micro-milling media, which led to significant reduction in the MgH₂ grain size and hence reduce the T^dec temperatures.

3.3. Cycle-Life-Time Performance

The cycle-life-time (CLT) test is the most critical evaluation of the hydrogen storage material since it confirms the material’s capabilities to function rapid hydrogenation/dehydrogenation cycles without failure. Figure 8a illustrates the CLT conducted at 350 °C under 10 bar/400 mbar of H₂ for the as-received Mg-rods. Even at a high temperature of 350 °C, the defect-free Mg with its large grain size (Figure 2b) demonstrated extremely sluggish hydrogenation/dehydrogenation rates. This is indicated by the limited number of hydrogenation/dehydrogenation cycles (10 cycles) carried out during a 70 h period. Moreover, this starting material had a very limited capability for hydrogen storage (less than 0.5 wt.% H₂), as shown in Figure 8a. When the Mg-rods were subjected to 200 passes of CR, their hydrogenation/dehydrogenation behavior was improved considerably, as indicated by the increased number of completed absorption/desorption cycles conducted (~90 cycles) at 350 °C under 10 bar/400 mbar of H₂ (Figure 8b). However, its hydrogen storage capacity (~3 wt.% H₂) was still far below the theoretical value of MgH₂ (7.6 wt.% H₂).

When the as-CR Mg rods were snipped into flakes and concurrently RBM for 100 h and then CR for another 100 passes, the CLT performance was significantly improved. Although the CLT of this sample was measured at a lower temperature of 275 °C, it demonstrated a high number of cycles of up to 500 in 600 h without apparent deterioration of the storage capacity, which remained constant at 7.3 weight percent H₂, as shown in Figure 9.

The impact of catalyzed MgH₂ powders with two different Nb₂O₅ concentrations (10 and 15 wt.%) was investigated in the present study. For this set of experiments, the CR-MgH₂ powders produced after 200 passes + 100 h RBM + 100 passes of CR were individually RBM for 50 h with the appropriate Nb₂O₅ concentrations. Figure 9 displays the CLT curve conducted under 10 bar/400 mbar at 225 °C of the catalyzed MgH₂ powders with 10 wt.% Nb₂O₅ obtained after RBM for 50 h. At this low temperature, the sample was able to absorb/desorb ~6.1 wt % of H₂, which remained almost constant (6 wt.% H₂) after processing for ~170 h. Neither degradation in the storage capacity nor retreating in the kinetic of hydrogenation/dehydrogenation could be detected even after the completion of 230 continuous cycles, as displayed in Figure 10a.

The current research demonstrated that the hard Nb₂O₅ nanoparticles functioned well as a nanomilling medium, significantly reducing the particle and grain size of MgH₂. Due to the presence of such fine grains, the hydrogenation/dehydrogenation kinetics in this binary system were improved. Similarly, Simchi et al. [11] observed this result when they ascribed the MgH₂/Nb₂O₅ system’s rapid desorption kinetics to grain refining caused by the so-called Nb₂O₅ cracking agent [30]. Due to the well-distributed nature of the hard Nb₂O₅ nanoparticles at the MgH₂ matrix, superior/absorption desorption catalytic effects were produced by these metal oxide powders. According to [31], Nb₂O₅ increases electron exchange reactions with hydrogen molecules, thus initiating the gas–solid reaction. Barkhordarian et al. [10] hypothesized that Nb₂O₅ might also serve as an efficient catalyst for hydrogen recombination, thus accelerating the development of a gas–solid reaction.

The CLT curve for MgH₂ powders doped with 15% Nb₂O₅ after 50 h of RBM is shown in Figure 10b. Excellent hydrogenation properties, as indicated by high cyclic stability even after approximately 110 h (~150 cycles) with no discernible degradation in hydrogen storage capacity, were indicated by nearly constant absorption and desorption values of +4.96 and −5.0 wt.% H₂, respectively, as shown in Figure 10b. It is realized that using a high concentration (15 wt.%) of Nb₂O₅, which has a molecular weight of 265.81 g/mol, led to a significant degradation in MgH₂ storage capacity (~5 wt.%), as indicated in Figure 10b.

4. Conclusions

The present work was conducted in order to investigate the synergetic effect of high-energy reactive ball milling of plastically deformed cold-rolled recycled Mg flakes on hydrogenation/dehydrogenation as well as decomposition processes. For the purpose of this work, the as-received solid waste Mg-rods were firstly cold-rolled 200 times and high-energy ball-milled under an H₂ atmosphere for 100 h. In order to increase the lattice imperfections, the as-ball-milled powders were then cold-rolled for 100 passes and then ball-milled with 10 and 15 wt.% Nb₂O₅ powders for 50 h. As a consequence of being treated to several forms of defects (dislocations, stacking faults, and twining) induced by cold rolling and ball milling, the findings demonstrated that the materials’ gas absorption and desorption kinetics were enhanced. When compared with the as-received materials, their capacity to produce a long cycle life at a reduced temperature was used to determine their effectiveness. When treated to RBM with Nb₂O₅ for 50 h, the powders exhibited significant improvement in terms of kinetics and decomposition temperature, indicating that the process was successful. At a relatively moderate temperature (225 °C), the nanocomposite MgH₂/10 and 15 wt.% Nb₂O₅ demonstrated strong hydrogen storage capabilities with a long cycle life that ranged from 110 h to 170 h without a significant decrease in storage capacity and kinetics.