1.   Introduction

Currently, fossil fuels provide 80% of the global energy demand. Unfortunately, they also cause environmental pollution and greenhouse effects [1]. The need to find renewable and clean energy to replace fossil energy has become a global consensus [2]. Hydrogen has attracted extensive interest from all over the world as an efficient and sustainable secondary energy. The so-called hydrogen economy consists of the production, storage, and transportation of hydrogen, as well as hydrogen energy applications. However, hydrogen storage has always been a bottleneck that hampers the application of hydrogen [3–7].

During the past decade, many hydrogen storage materials and technologies have been developed [8]. MgH₂ has received much attention among these solid-state hydrogen storage materials because of its light weight, abundant reserves, non-toxic nature, and large hydrogen storage capacity, which has considerable potential for use in hydrogen fuel cells [9–12]. Excellent solid-state hydrogen storage materials should be capable of absorbing or releasing a large amount of hydrogen rapidly under low pressure and ambient temperature [13]. However, due to the high thermodynamic stability and retarded reaction kinetics of MgH₂, the system delivers a high operating temperature and sluggish hydrogen absorption/desorption rate, making it difficult to meet the requirements of practical applications [14–18]. Many efforts, such as alloying, nanosizing, and catalyst doping, have been made to overcome these issues [19–25]. Zhang et al. [26] successfully synthesized ultrafine MgH₂ nanoparticles of 4–5 nm, which can achieve a reversible hydrogen storage of 6.7wt% at room temperature. Adding catalysts could effectively lower the working temperature of hydrogen storage, enhance the desorption, and increase the reversible absorption rate of hydrogen [27–30]. Theoretically, the doped catalyst can provide favorable charge transfer and promote heat transfer in the MgH₂ system by generating many defects on the surface of MgH₂ [31–32].

In general, nanoscale catalysts can be in close contact with MgH₂ to create more active sites. Therefore, nanoscale catalysts can improve the hydrogen storage performance of MgH₂ [33]. Many studies have been conducted on transition metal catalysts in recent years [34–35]. Specifically, the transition metals Fe, Co, Ni, and Cu, for example, have been proven to play an important role in enhancing the hydrogen storage properties of MgH₂ [36]. The Ni-doped MgH₂ composite, in particular, exhibits exceptional catalytic performance [37]. Yu et al. [38] and Xie et al. [39] found that introducing the transition metals Fe, Co, Ni, Cu, and Zn into MgH₂ during the ball milling or dehydrogenation/absorption cycle processes will form numerous defects at the interfaces of Fe/MgH₂, Co/MgH₂, Ni/MgH₂, etc., respectively. Such defects considerably aid the splitting of H₂ molecules and the recombination of hydrogen atoms. Moreover, Chen et al. [40] investigated the hydrogen storage performance of the MgH₂–Ni/TiO₂ system and found that metallic Ni particles could easily react with Mg to yield Mg₂Ni compound during the dehydrogenation process, and the in-situ produced Mg₂Ni is converted into Mg₂NiH₄ during the subsequent rehydrogenation, acting as a hydrogen pump. Shao et al. [41] prepared a stable Ni-metal organic framework (MOFs) catalyst with uniform and dispersed Ni atoms that can improve the hydrogen storage performance of the MgH₂ system. Huang et al. [42] created highly dispersed metal-supported catalysts, including a series of 3d transition elements, La and Ce, on N-doped carbon (M–N–C). The kinetics of MgH₂–M–N–Cs were correlated with the electronegativity of M in M–N–Cs (V, Cr, Fe, Co, Ni, Cu, Zn), indicating that M–N–Cs with high electronegativity core elements can enhance the kinetics. In addition, MgH₂–Ni–N–C500 with the highest electronegativity (Ni, 1.91) was demonstrated remarkable kinetic performance. Zhang et al. [43] synthesized a series of nickel-based compounds (Ni₃C, Ni₃N, NiO, and Ni₂P) and found that the catalyst obtained by combining Ni with low electronegativity elements (Ni₃C) better enhanced the hydrogen storage performance of the MgH₂ system. In addition, El-Eskandarany et al. [44] employed Ni spheres as a grinding medium, progressively doping Ni spheres into MgH₂ powder. Their samples after ball milling exhibited a low dehydrogenation temperature (491 K) and a dehydrogenation activation energy (75 kJ·mol⁻¹). Recent research has found that the size of the Ni particles has a major impact on the adsorption and dehydrogenation kinetics of MgH₂. Si et al. [45] discovered that the initial dehydrogenation temperature of the MgH₂–5wt% nano-Ni/C system was significantly reduced by 453 K, and the hydrogen absorption kinetics of the system was noticeably increased by 16-fold compared with that of the original MgH₂. Gao et al. [46] created Ni/Ti₃C₂ catalysts with interfacial differences and discovered that when Ni had the smallest particle size and best dispersibility at the Ti₃C₂ matrix interface, the most effective catalytic activity was produced. Chen et al. [47] synthesized Ni nanofibers with a uniform diameter of 50 nm and a porous structure composed of many Ni nano-crystallites, which were easily broken into superfine Ni nanoparticles with an average diameter of 17 nm by ball milling and uniformly dispersed on the surface of MgH₂. The hydrogen storage significantly improves the performance of MgH₂. Zhu et al. [48] designed a self-assembled two-dimensional MXene-based catalyst (2D-Ni@Ti₃C₂) whose Ni particles had an average size of less than 50 nm, and smaller particles were in the range of 5 nm. The MgH₂ + Ni@Ti–MX composite absorbed 5.4wt% H₂ in 25 s at 398 K and released 5.2wt% H₂ in 15 min at 523 K, demonstrating enhanced hydrogen storage performance. Therefore, the particle size of the catalyst and its close contact with MgH₂ determine its catalytic performance. Rahmalina et al. [49] also proved that reducing the size of Ni particles is the best approach to enhance the thermodynamic and kinetic performance of MgH₂. However, the best size of Ni and its size effect on the de/rehydrogenation performance still remain unclear.

In this paper, we focus on the effect and mechanism of nano-Ni particles on the thermal stability and catalytic performance of MgH₂. Ni particles with size of 1.5–2.5 nm have been successfully synthesized in this work. The effect of nano-Ni particle size on the MgH₂ system has been systematically studied, along with the catalytic mechanism of nano-Ni on the MgH₂ system.

2.   Experimental

2.1   Preparation of the MgH₂–nano-Ni

The overall synthesis process of nano-Ni particles is shown in Fig. 1. First, 1.8 g sodium hydroxide (97%, Aladdin), 0.2241 g nickel acetate tetrahydrate (99%, Aladdin), 2.05 g oleic acid (analytical reagent (AR), Macklin), 0.5 g polyvinylpyrrolidone (PVP, guaranteed reagent (GR), Sinopharm), and 150 mL of 1,2-propylene glycol (AR, Sinopharm) were placed in a round-bottom flask. The temperature was increased to 389 K, and the reagents completely dissolved with continuous magnetic stirring. The semi-products were obtained by dropping 30 mL of 1,2-propylene glycol solution containing 0.486 g of potassium borohydride (98%, Aladdin) into the solution in a glove box filled with high-purity argon. Finally, the Ni nanoparticles were collected by centrifugation and vacuum drying at 353 K for 10 h.

Figure  1.  Illustration of the synthesis of the nano-Ni composite.

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The purchased MgH₂ (95%, Alfa Aesar) was mixed (QM-1SP, Nanjing) with the prepared xwt% nano-Ni (x = 0, 3, 5, 10, 15) at 450 r/min with a 40:1 ball-to-powder ratio for 5 h. All samples were treated in a glovebox (Mikrouna Super 1220/750), and their H₂O and O₂ levels were below 0.0001‰.

2.2   Structural and morphology characterization

The phase structure of the composite system was determined by X-ray diffraction (XRD) and in-situ XRD (Rigaku) patterns. The morphology was characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. High-magnification images were obtained by using a high-resolution transmission electron microscope (HRTEM, FEI Tecnai G2 T20). The microstructure of the phases was analyzed by selected area electron diffraction (SAED).

2.3   De/rehydrogenation performances

The non-isothermal dehydrogenation performance was evaluated by using the volume release method (VR), from which the initial dehydrogenation temperature, the final dehydrogenation temperature, and the dehydrogenation capacity of the samples were identified. Dehydrogenation onset, peak, and cut-off temperatures were determined by temperature-programmed desorption (TPD) with online mass spectrometry (MS, Hiden, UK). The isothermal hydrogenation curves of the MgH₂–nano-Ni system were measured at 394, 421, 450, and 482 K, respectively, under 3 MPa. The pressure-composition isotherm curves (PCI) of MgH₂ and MgH₂–nano-Ni were tested at different temperatures. Differential scanning calorimetry (DSC) was performed on a TA Q2000 instrument, and approximately 5 mg of each sample was heated from room temperature to 773 K at 5, 10, 15, and 20 K·min⁻¹, respectively.

3.   Results and discussion

3.1   Characterization of nano-Ni

The TEM images in Fig. 2(a) and (b) show that the prepared catalyst powders are nanoclusters of extremely fine particles formed by agglomeration. The calculated average particle size is 2.14 nm, as indicated by the Nano Measure software, and the particle size is concentrated at 1.5–2.5 nm. This finding indicates that the particle size is relatively uniform, much smaller than that reported in many previous works [44–45,49–50], which should benefit the catalytic activity of the catalyst. The XRD pattern of the nanoparticles is displayed in Fig. 2(c), and all diffraction peaks correspond to Ni (111), Ni (200), and Ni (220), respectively. This finding demonstrates that the Ni precursor is completely reduced to metallic Ni after preparation. In addition, the diffraction peaks broaden while their intensities weaken, further indicating that the Ni nanoparticles are relatively small.

Figure  2.  (a) TEM image of the as-prepared nano-Ni. (b) Particle size distribution of the as-prepared nano-Ni. (c) XRD pattern of the as-prepared nano-Ni.

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3.2   Effects of nano-Ni on the MgH₂ system

Fig. 3(a) displays the VR curves at a heating rate of 5 K·min⁻¹ of MgH₂–xwt% nano-Ni (x = 0, 3, 5, 10, 15) samples. Its initial desorption temperature, final desorption temperature, and desorption capacity can be calculated as summarized in Table 1. Evidently, the system MgH₂–xwt% nano-Ni (x = 3, 5, 10, 15) starts to release hydrogen in the temperature range of 492–518 K, which is remarkably lower than the 580 K for the original MgH₂ sample. MgH₂–3wt% nano-Ni begins to release hydrogen at approximately 518 K, which is 62 K lower than that of undoped MgH₂. In addition, the dehydrogenation temperatures of MgH₂–5wt% nano-Ni, MgH₂–10wt% nano-Ni, and MgH₂–15wt% nano-Ni are further lowered to 510, 497, and 492 K, respectively. Furthermore, the three samples are dehydrogenated by the addition of nano-Ni before 672 K, with hydrogen releases of 6.5wt%, 6.2wt%, and 5.9wt%, respectively. The MgH₂–15wt% nano-Ni sample delivers an end temperature higher than the MgH₂–10wt% nano-Ni sample; thus, the MgH₂–10wt% nano-Ni sample seems to be the best choice for this work.

Figure  3.  (a) Volumetric release curves of as-milled MgH₂–xwt% nano-Ni (x = 0, 3, 5, 10, 15); (b) TPD-MS curves of the samples of MgH₂–10wt% nano-Ni and MgH₂. The temperature ramping rates are all 5 K·min⁻¹. DSC curves of the (c) as-milled MgH₂ and (d) MgH₂–10wt% nano-Ni, respectively; the insertion plots show the activation energies of the samples. (e) Isothermal hydrogenation curves of MgH₂–10wt% nano-Ni at different temperatures; inset is their Arrhenius plots. (f) The fitted lines reflecting the relationship of ln[−ln(1−α)]–lnt built on the basis of the JMA equation; the slope stands for the activation energy ${\boldsymbol{E}}_{\mathbf{a}}^{\mathbf{a}}$.

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Table  1.  Volumetric release curves of as-milled MgH₂–xwt% nano-Ni (x = 0, 3, 5, 10, 15)

Sample	Initial desorption temperature / K	End desorption temperature / K	Dehydrogenation capacity / wt%
MgH2	580	672	7.2
MgH2–3wt% nano-Ni	518	621	6.6
MgH2–5wt% nano-Ni	510	605	6.5
MgH2–10wt% nano-Ni	497	583	6.2
MgH2–15wt% nano-Ni	492	585	5.9

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Fig. 3(b) shows the TPD-MS curves of the MgH₂ and MgH₂–10wt% nano-Ni samples at a heating rate of 5 K·min⁻¹. For the sample doped by the nano-Ni catalyst, the dehydrogenation peak is apparently shifted to a lower temperature. The peak dehydrogenation temperature of the MgH₂–10wt% nano-Ni sample is significantly reduced by 87 K compared with that of pure MgH₂. In addition, a minor dehydrogenation peak occurs at 673 K after ball milling of pure MgH₂, which is probably caused by the heterogeneous distribution of magnesium hydride particle size according to Ref. [51].

To further investigate the desorption kinetics, DSC measurements of MgH₂ and MgH₂–10wt% nano-Ni samples were performed, as displayed in Fig. 3(c) and (d). The dehydrogenation temperature of MgH₂ is significantly lowered by nano-Ni. The endothermic peak of MgH₂–10wt% nano-Ni is 112 K lower than that of the original MgH₂. According to the different heating rates of DSC curves, the activation energy for desorption was calculated by Kissinger’s equation [52], as shown in Eq. (1),

where β represents the heating rate, ${E}_{\mathrm{a}}^{\mathrm{d}}$ denotes the activation energy for desorption, T_p stands for the endothermic peak temperature, and R is the gas constant. The activation energy of the MgH₂–10wt% nano-Ni system decreased to (88 ± 2) kJ·mol⁻¹, indicating that the nano-Ni additive could alleviate the kinetic barrier of MgH₂ dehydrogenation and greatly improve the MgH₂ dehydrogenation kinetics.

Aside from the remarkable improvement in the dehydrogenation properties, the hydrogen adsorption kinetics of the MgH₂–10wt% nano-Ni sample were also investigated. Isothermal hydrogen absorption measurements are shown in Fig. 3(e) and (f). An Arrhenius equation [53] could be established according to the following equation (Eq. (2)):

where k is the reaction rate constant, ${E}_{\mathrm{a}}^{\mathrm{a}}$ denotes the activation energy of hydrogen absorption, T is the temperature, R stands for the gas constant, and A is the pre-exponential factor. In Eq. (3), α is the hydrogen absorption rate, which represents the ratio of the sample hydrogen absorption capacity to the saturation hydrogen absorption capacity at a certain time, n is Avrami index, and t is the reaction time. The activation energy (${E}_{\mathrm{a}}^{\mathrm{a}}$) was calculated as (87 ± 1) kJ·mol⁻¹ by using the Arrhenius formula and the John–Mehl–Avrami (JMA) formula (Eq. (3)) [54]. As shown in Fig. 3(e), as the hydrogen absorption temperature of MgH₂–10wt% nano-Ni increases, the saturated hydrogenation capacity gradually increases. In particular, the sample could rapidly absorb 5.3wt% H₂ within 1000 s at 482 K and under 3 MPa hydrogen pressure. In addition, the sample could absorb a total of 3wt% H₂ at 394 K. However, Fig. 4 shows that under the same conditions (482 K, 3 MPa H₂), reabsorbing hydrogen is difficult for MgH₂ after dehydrogenation, which indicates that the hydrogen absorption performance of MgH₂ can be significantly improved by the addition of nano-Ni.

Figure  4.  Isothermal rehydrogenation curves of as-milled MgH₂ and MgH₂–10wt% nano-Ni composite at a temperature of 482 K under 3 MPa H₂.

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The PCI curves of MgH₂ were measured at 647, 676, 681, and 698 K, and the MgH₂–10wt% nano-Ni sample was measured at 565, 587, 591, and 604 K, as shown in Fig. 5(a) and (b). Their Van ’t-Hoff plots are shown in the insets of Fig. 5(a) and (b). The dehydrogenation enthalpy of MgH₂–10wt% nano-Ni and original MgH₂ were calculated by using the Van ’t Hoff equation (Eq. (4)) [55].

Figure  5.  PCI curves of (a) as-milled MgH₂ and (b) MgH₂–10wt% nano-Ni sample; the inserted Van ’t Hoff plots show the dehydrogenation enthalpy values of the samples.

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where P and P₀ represent the equilibrium atmosphere and the normal atmosphere (100 kPa), respectively; T is the temperature; ΔH means the enthalpy of desorption; R is the gas constant; C represents a constant whose value is equal to ΔS/R, where ΔS is the entropy change (the value of the metal hydride is usually 130 J·mol⁻¹·K⁻¹. The dehydrogenation enthalpy value shifts slightly from (77.7 ± 0.5) kJ per mol H₂ for the original MgH₂ to (72.2 ± 0.5) kJ per mol H₂ for the MgH₂–10wt% nano-Ni sample, as shown in Fig. 5(a). Hence, additional nano-Ni should be the main cause of the lower initial dehydrogenation temperature of the dehydrogenation. Therefore, we can conclude that nano-Ni might play a role in destabilizing MgH₂ during dehydrogenation.

3.3   Reaction mechanism

The diffraction peaks of the metallic Mg phase appear at 503 K, indicating that the MgH₂–10wt% nano-Ni sample begins to decompose and release hydrogen at this temperature, as shown in Fig. 6. The diffraction peaks of the Mg phase (PDF: 017-0902) gradually become noticeable with increasing temperature. Meanwhile, the MgH₂ phase (PDF: 002-6624) gradually weakens and finally disappears at 673 K, suggesting that the dehydrogenation is completed. Small traces of MgO and Mg(OH)₂ are also detected in the XRD patterns, which could be attributed to the sample’s brief exposure to air while transferred from the glove box to the holder.

Figure  6.  In-situ XRD patterns of MgH₂–10wt% nano-Ni from room temperature to 673 K.

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The SAED patterns from TEM image and inverse Fourier transform are shown in Fig. 7(a) and (b). The plane with a spacing of 0.225 nm can be considered as lattice fringes of the MgH₂ (110) planes (Fig. 7(c)). Crystal planes with spacing up to 0.205 and 0.178 nm correspond to the (111) and (200) planes of Ni, respectively. Such results confirm that no intermediate phase is formed during the ball milling and the ball milling products are Ni and MgH₂. On account of the high surface free energy of small particles, the sample after ball milling presents an aggregation distribution of small particles, as shown in Fig. 7(d). The TEM and Fourier transform images of the MgH₂–10wt% nano-Ni composite after dehydrogenation are shown in Fig. 7(e) and (f). They confirm the presence of Mg (101), Mg (103), Ni (220), and Mg₂Ni (114) planes in the dehydrogenated products, as shown by the in-situ XRD in Fig. 6. We can speculate that nano-Ni is involved in the MgH₂ dehydrogenation.

Figure  7.  (a) TEM, (b) SAED patterns, (c) HRTEM, and (d) SEM images of MgH₂–10wt% nano-Ni after preparation. (e) TEM image and (f) SAED patterns of the sample after dehydrogenation; those clusters are identified as Mg, Mg₂Ni, and Ni according to their SAED image. (g) TEM image and (h) SAED patterns of the sample after rehydrogenation remaining MgH₂, Mg₂NiH₄, and Ni phases.

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Furthermore, as shown in Fig. 7(g) and (h), the (002) plane of MgH₂, the (111) plane of Ni, and the (220) and (400) planes of Mg₂NiH₄ can be recognized in the SAED patterns of the rehydrogenated sample. The appearance of the Mg₂NiH₄ phase indicates that the product Mg₂Ni, as an intermediate phase in the absorption/desorption process, could promote the reversible absorption and desorption of the system. This result is consistent with the PCI measurement results, where the system is slightly destabilized during dehydrogenation. For MgH₂–Ni, the dehydrogenation is as follows:

The rehydrogenation could be expressed as follows:

For this system, the dehydrogenation for the second time is described as follows:

In addition, the Ni phase is identified during the dehydrogenation and rehydrogenation processes, indicating that some nano-Ni still plays a catalytic role in the system.

Previous work by Chen et al. [50] indicated that the onset dehydrogenation temperature of MgH₂ doped with 20–30 nm Ni particles is 490 K, and the final temperature is about 643 K. As shown in Table 2, compared with their study, the final temperature in this work is decreased by 60 K, although the onset dehydrogenation temperature of MgH₂ catalyzed by nano-Ni with an average particle size of 2.14 nm is 497 K. This finding suggests that the Ni nanoparticles tend to agglomerate in the reaction and weaken the catalytic effect because of the loss of great active surfaces.

Table  2.  Initial desorption temperature, end desorption temperature, and ${\boldsymbol{E}}_{\mathbf{a}}^{\mathbf{d}}$ of MgH₂–10wt% nano-Ni, MgH₂–10wt% Ni, and MgH₂

Sample	Initial desorption temperature / K	End desorption temperature / K	${E}_{\mathrm{a}}^{\mathrm{d}}$ / (kJ·mol−1)	ΔH / (kJ per mol H2)
MgH2 (This work)	580	672	170	77.7
MgH2–10wt% nano-Ni (This work)	497	583	88	72.2
MgH2–10wt% Ni [50]	490	643	121.8	—
MgH2–10wt% Ni/CMK-3 [50]	433	568	43.4	74.7

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From the results obtained by Chen et al. [50], the dehydrogenation enthalpy of the MgH₂–Ni/CMK-3 system is decreased by 3 kJ per mol H₂, which is still 2.5 kJ per mol H₂ higher than that of the MgH₂–nano-Ni system. This result means that after ball milling, the small Ni nanoparticles should have a larger interface and be in closer contact with MgH₂, thus more actively participating in the reaction with MgH₂ and more effectively destabilizing the MgH₂ system than the large Ni nanoparticles can.

4.   Conclusion

Nano-Ni with an average particle size of 2.14 nm has been synthesized via the reduction method. The optimal MgH₂–10wt% nano-Ni starts to release hydrogen at 497 K and offers a dehydrogenation capacity of 6.2wt% below 583 K. The dehydrogenated MgH₂–10wt% nano-Ni system can absorb 5.3wt% H₂ in 1000 s at 482 K under 3 MPa hydrogen pressure. More remarkably, even at 394 K and 3 MPa hydrogen pressure, MgH₂–10wt% nano-Ni can uptake 3wt% hydrogen. In summary, the synthesized nano-Ni mainly exhibits excellent performance in reducing the enthalpy of dehydrogenation of MgH₂ and its decomposition. However, the catalytic activity is degraded as a result of the easy agglomeration of particles. Supporting materials such as CMK-3, reduced graphene oxide (rGO), and other carbon materials can be added to the support to disperse the catalysts and produce steady catalytic activity. Further work to avoid particle agglomeration is underway.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 52071177), the Natural Science Foundation of Guangxi, China (No. 2020GXNSFAA297074), the Jiangsu Key Laboratory for Advanced Metallic Materials (No. BM2007204), and the Guangxi Key Laboratory of Information Materials (No. 211021-K).

Conflict of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.