MoSe2 hollow nanospheres decorated with FeNi3 nanoparticles for enhancing the hydrogen storage properties of MgH2

Journal of Alloys and Compounds 830 (2020) 154631

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MoSe2 hollow nanospheres decorated with FeNi3 nanoparticles for enhancing the hydrogen storage properties of MgH2 Shichao Gao a, 1, Hui Wang a, 1, Xinhua Wang a, *, Haizhen Liu b, **, Ting He a, Yuanyuan Wang a, Chen Wu a, Shouquan Li a, Mi Yan a, *** a

State Key Laboratory of Silicon Materials, School of Materials Science & Engineering, Zhejiang University, Hangzhou, 310027, PR China Guangxi Colleges and Universities Key Laboratory of Novel Energy Materials and Related Technology, Guangxi Novel Battery Materials Research Center of Engineering Technology, School of Physical Science and Technology, Guangxi University, Nanning, 530004, PR China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 December 2019 Received in revised form 15 February 2020 Accepted 3 March 2020 Available online 4 March 2020

The FeNi3, MoSe2 and MoSe2@FeNi3 hollow nano-spheres synthesized by wet chemical method are used for the first time to improve the hydrogen storage properties of the MgH2. The results show that they all possess excellent catalytic activity in the hydrogen desorption/sorption reactions of MgH2. Because of the synergistic catalytic effect of MoSe2 and FeNi3, the MoSe2@FeNi3 has the most outstanding catalytic effect. In detail, the onset dehydrogenation temperature of MgH2 is reduced from 310
C to 194
C, and its maximum dehydrogenation rate is increased by 19.8 times by the introduction of 10 wt% MoSe2@FeNi3. Furthermore, the 10 wt% MoSe2@FeNi3-doped MgH2 sample, 5.8 wt% H2 can be absorbed in 0.5 min at 150
C. However, the un-doped MgH2 sample hardly absorbs hydrogen under the same conditions. The onset hydrogen absorption temperature of the composite is only 73
C. More importantly, it has an excellent cycling stability. These improvements are because the in situ formed MgSe, Mo, Mg2Ni and Fe can serve as active species, and they as well as their homogeneous distribution on MgH2 matrix can remain stable during dehydrogenation/hydrogenation. In addition, the presence of Mg2Ni can convert MgH2 to an easier pathway of hydrogen absorption/desorption. These help to facilitate the hydrogen storage performance of MgH2. © 2020 Elsevier B.V. All rights reserved.

Keywords: Hydrogen storage materials Magnesium hydride Three nickel iron Molybdenum diselenide Catalysis

1. Introduction Hydrogen is considered as an ideal clean energy, and its applications mainly include production and storage. At present, there have been many studies on the preparation of hydrogen, and outstanding results have been achieved [1e3]. However, hydrogen storage is still a bottleneck technology for its large-scale utilization. Magnesium hydride (MgH2) is a promising hydrogen storage material because it has high abundance, better reversibility, high gravimetric (7.6 wt%) and volumetric (0.1 g/cm3) capacity [4,5]. However, high operating temperature, sluggish kinetics and poor cycling stability at high temperatures are the major obstacles for

* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (X. Wang), [email protected] (H. Liu), mse_ [email protected] (M. Yan). 1 Shichao Gao and Hui Wang contributed equally to this work. https://doi.org/10.1016/j.jallcom.2020.154631 0925-8388/© 2020 Elsevier B.V. All rights reserved.

practical applications [6,7]. Over the past decades, many methods, such as reducing the grain or particle size of the MgH2 to nanoscale [8e10], alloying [11e13] and adding catalysts/dopants [14e17] etc., have been utilized to enhance the hydrogen storage performance of MgH2. Transition metal compounds of molybdenum (Mo) have attracted great interests from researchers all over the world because of their excellent catalytic effect on the dehydrogenation/ hydrogenation of MgH2. Xia et al. [18] studied the catalytic effect of 10 wt% MoCl3 additive on LiBH4eMgH2 composites is attributed to the formation of the metallic Mo in the composites. Recently, Wang et al. [19] investigated the mechanically milled MgH210 wt% MoS2, and showed that MoS2 has the favorable effect on the reduction of hydrogen desorption temperature and acceleration of the desorption kinetics. Jia et al. [20] revealed that the addition of MoO2 and MoS2 could significantly enhance the hydrogen absorption/desorption kinetics and decrease the desorption temperature of MgH2. Although MoSe2 exhibits excellent catalytic effects on

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photocatalysis [21e23], hydrogen evolution [24e27], dyesensitized solar cells [28] etc., and can be also synthesized into the flower-like [29e31] with a larger specific surface area, its catalytic effect on hydrogen absorption/desorption properties of MgH2 is not clear. Therefore, in this work, in order to investigate the catalytic effect of MoSe2 on MgH2 hydrogen storage material, the as-prepared hollow spherical shape MoSe2 was introduced into MgH2 system for the first time. In addition, Liu et al. [32] reported that the NiSe and carbon decorated with MoSe2 (MoSe2eNiSe@carbon) fabricated by the moderate one-step hydrothermal approach possesses excellent catalytic effect on the electro-catalytic hydrogen evolution reaction. Sharma et al. [24] demonstrated that rhodium (Rh) and palladium (Pd) nanoparticles (NPs) were evenly distributed on the surface of MoSe2 by a simple wet-chemical method, and the Rh and Pd NPs decorated MoSe2 shows more efficient activity towards hydrogen evolution reaction than that of the pure MoSe2. The results indicated that the combined catalyst formed by MoSe2 and other substances has a better effect on facilitating the hydrogen storage properties of MgH2. Many reports indicated that not only iron and nickel can enhance the hydrogen storage properties of Mg-based hydrogen storage materials [33], but also the MgeFeeNi alloy formed by the reaction of iron and nickel with Mg can be used as an outstanding hydrogen storage material [34e38]. However, to the best of our knowledge, the effect of FeNi3 alloy on the hydrogen storage performance of MgH2 has not been reported. Therefore, in this work, FeNi3 nanoparticles (NPs) were synthesized by a modified wet chemical method and were used for the first time to enhance the hydrogen storage properties of MgH2. Moreover, according to previous reports [39e41], dual-tuning effects of the thermodynamics and kinetics for Mg-base materials or compounds is one of the key issues for hydrogen storage materials. Therefore, in order to further improve the catalytic activity of MoSe2 and achieve the dual-tuning effects, we used a chemical method to obtain a combined catalyst whose surface was modified by FeNi3 NPs, and systematically studied its effect on the hydrogen absorption/ desorption performance of MgH2. In summary, MoSe2 hollow nano-spheres were first synthesized following a slightly modified hydrothermal method [42], and then were decorated with as-prepared FeNi3 nanoparticles via a slightly modified solution reduction method to get MoSe2@FeNi3 [43]. Then, the as-prepared FeNi3 nanoparticles, MoSe2 and MoSe2@FeNi3 hollow nano-spheres were introduced respectively into the MgH2 hydrogen storage system by milling, and their effects on MgH2 were systematically studied for the first time. The mechanism in the catalyzed materials was also discussed. 2. Experimental section As-received Mg powder (Alfa Aesar, 98%) was first hydrogenated at 350
C and 5 MPa H2 for 12 h, and then was milled for 2 h in the planetary ball mill (Nanjing Nanda Instrument Plant, QM-3SP4) with rotation speed of 500 rpm. The above hydrogenation and milling process were repeated three times to obtain the pure MgH2. The reaction equation is shown in Eq. (1). 2 Mg þ H2 / MgH2

(1)

The silica nano-spheres as the template for the preparation of the MoSe2 hollow nano-spheres were first prepared according to a previous reported method with slight modification [44]. In a typical synthesis, 10 mL of tetraethyl orthosilicate (Shanghai Aladdin, 99.9%), 10 mL of deionized water, and 100 mL of ethanol (Sinopharm Group, 99.8%) were mixed in a 200 mL polyethylene container. Subsequently, the mixture was stirred vigorously for

12 h at room temperature, during which 4 mL of ammonium hydroxide (Alfa Aesar, 28e30 wt%) was added to the solution. Then, the solution was centrifuged and washed with ethanol repeatedly. After dried in vacuum oven at room temperature for 12 h, the silica nano-spheres could be obtained. The reaction equation is shown in Eq. (2).

NH3$H2O

CH3 CH2 OSiðOCH2CH3 Þ3 þ 2H2 O ƒƒƒ! SiO2 þ 4CH3 CH2 OH (2) Following a slightly modified hydrothermal method [42], MoSe2 hollow nano-spheres were synthesized using the as-prepared silica nano-spheres as templates. Firstly, 8 mmol of selenium powder (Shanghai Aladdin, 99.9%) and 0.4 g of silica nano-spheres were dissolved in 20 mL of hydrazine hydrate (Sinopharm Group, 85 wt %) solution and then treated ultrasonically for 2 h. The mixed solution was treated hydrothermally for 24 h at 180
C. The resultant black precipitates were collected by centrifugation washing with deionized water for several times, and then dried at 80
C in vacuum for 12 h. At this point, the MoSe2 solid nano-spheres was successfully synthesized and were then used as the precursor to synthesize the MoSe2@FeNi3 hollow nano-spheres in the following. The reaction equation is shown in Eq. (3). 2Na2MoO4$2H2O þ 4Se þ 3N2H4$H2O / 2MoSe2 þ 4NaOH þ11H2O þ 3N2 [

(3)

To prepare the MoSe2 hollow nano-spheres, 0.5 g of MoSe2 solid nano-spheres were first mixed with 50 mL 5 mol/L sodium hydroxide (Shanghai Aladdin, 97%) solution. Subsequently, the solution was kept at 60
C for 12 h. Then black solid precipitation from the solution were collected by centrifugation and washing with deionized water repeatedly, and then dried at 80
C in vacuum for 12 h. Finally, the MoSe2 hollow nano-spheres were gotten by annealing the black precipitation at 600
C in Ar for 5 h. The reaction equation is shown in Eq. (4). SiO2 þ 2NaOH / Na2SiO3 þ H2O

(4)

After that, the MoSe2@FeNi3 hollow nano-spheres were synthesized by a slightly modified wet chemical reduction method utilizing hydrazine hydrate as the reducing agent [43]. As a typical experiment, 2 mmol of ferrous sulfate heptahydrate (Alfa Aesar, 98%), 6 mmol of nickel dichloride hexahydrate (Meryer, 98%) and 2 mmol of the solid MoSe2 nano-spheres were dissolved in 80 ml of ethanol-water solution (volume ratio ¼ 2:3). Then, the solution was heated to the set temperature (80
C) by a thermostatic bath. After reaching 80
C, the sodium hydroxide (Sinopharm Group,  96%) solution of 5 M was added dropwise to the solution in order to adjust the pH value to 11. Then, 10 mL of hydrazine hydrate (Sinopharm Group,  85%) was added into the mixture during continuous stirring. After the addition, the solution was kept at the temperature (80
C) and stirred for 30 min in order to complete the reaction. The black precipitation from reaction was centrifuged and washed with deionized water for several times, and dried at 80
C in vacuum for 12 h. Then, the MoSe2@FeNi3 hollow nano-spheres were gotten via annealing the black precipitation in Ar atmosphere at 600
C for 5 h. The pure nanoparticles of FeNi3 without MoSe2 were prepared following the same process above except the addition of MoSe2. The reaction equations are shown in Eqs. (4)e(6). FeSO4$7H2O þ 3NiCl2$6H2O þ 8NaOH / Fe(OH)2 þ 3Ni(OH)2 þ Na2SO4 þ 6NaCl þ 25H2O [

(5)

S. Gao et al. / Journal of Alloys and Compounds 830 (2020) 154631

Fe(OH)2 þ 3Ni(OH)2 þ 2N2H4$H2O / FeNi3 þ 10H2O þ 2N2 [ (6) The MgH2 powders were mechanically milled for 2 h with 10 wt % FeNi3 nanoparticles, MoSe2 and MoSe2@FeNi3 hollow nanospheres in a planetary ball mill (Nanjing Nanda Instrument Plant, QM-3SP4), respectively. The milling process was paused every 6 min with 1 min rest. The ball to sample weight ratio was 120:1 and the rotation speed of the milling vial was 500 rpm. For comparison, the pure MgH2 powder was treated under the same conditions. The samples which contains MgH2 were treated in a glove box (MIKROUNA) filled with argon and the H2O and O2 contents were below 0.1 ppm. The dehydrogenation/hydrogenation properties of the samples were tested by using a homemade Sievert’s type apparatus. Generally, about 200 mg of sample was sealed into the reactor, and that was connected to a pressure sensor and a thermocouple to detect the pressure and temperature inside the reactor. The hydrogen capacities of the samples were calculated by the ideal gas equation. In the non-isothermal examination, the samples were gradually heated to 500
C at 2
C/min in vacuum for dehydrogenation and to 400
C at 2
C/min with an initial H2 pressure of 5 MPa for hydrogenation. About isothermal mode, the samples were quickly heated to the set temperature and kept during the whole test in vacuum for dehydrogenation (300
C) and 5 MPa H2 for hydrogenation (150
C). Cycling performance measurement was carried out under the isothermal mode. Thermal analyses were carried out by a differential scanning calorimeter (DSC, Netzsch STA449F3). The phase analyses of the samples were tested by powder X-ray diffraction (XRD, Bruker AXS D8 Advance, Co-Ka, 35 kV, 28 mA), and the samples were sealed with an amorphous membrane to prevent oxidation. The state of the element on the surface of the sample was characterized by X-ray photoelectron spectroscopy (XPS, VG Multilab 2000). Scanning electron microscopy (SEM, Carl Zeiss Jena Utral 55) was used to investigate the morphologies of the samples, and the elemental distribution was tested by an attached energy dispersive spectroscopy instrument (EDS, Oxford EDS Inca Energy Coate). High-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100, FEI) was employed to research the detailed microstructures of the samples. Brunauer̶ Emmett̶ Teller (BET) surface areas and porosities of the additives were characterized by N2 adsorption/desorption using a MicroActive for TriStar II Plus 2.02 analyzer. 3. Results and discussion 3.1. Characterization of additives and samples The as-prepared FeNi3, MoSe2 and MoSe2@FeNi3 were characterized by XRD, and the results are shown in Fig. 1(ac). The diffraction peaks in the XRD patterns are all consistent with the standard peaks of MoSe2 and FeNi3. It indicates the pure FeNi3, MoSe2 and MoSe2@FeNi3 additives were successfully synthesized by using the moderate chemical method. Moreover, N2 adsorption/ desorption isotherms and porous size distribution (insets) curves of the as-prepared FeNi3, MoSe2 and MoSe2@FeNi3 are shown in Fig. 1(df). The adsorption/desorption isotherms of the asprepared FeNi3, MoSe2 and MoSe2@FeNi3 all possess small hysteresis loop in the P/P0 range between 0.8 and 1.0, demonstrating that they have mesoporous structure [24]. These are further verified by the porous size distribution curves of the as-prepared FeNi3, MoSe2 and MoSe2@FeNi3 in the insets in Fig. 1(df), respectively. BrunnerEmmet-Teller (BET) surface area is further calculated from N2 adsorption and desorption curves and the results is displayed in Table 1. The MoSe2 shows the highest specific surface area (32.43 m2 g1) compared to FeNi3 (10.18 m2 g1) and MoSe2@FeNi3

3

(13.41 m2 g1), which may be ascribed to the hollow nanospheres structure composed of many thin-layer MoSe2, and this is observed by SEM in Fig. 2(d). The morphological features of the as-prepared FeNi3, MoSe2 and MoSe2@FeNi3 were clearly characterized using SEM and HRTEM, and shown in Fig. 2, respectively. As can be seen from Fig. 2(a and b), the as-prepared FeNi3 are nanoparticles with the diameter of around 100 nm. Moreover, a sets of well-resolved lattice fringes from Fig. 2(c) is clearly observed with interplanar distance of 0.204 nm indexed to the (111) planes of hexagonal FeNi3. In addition, it is apparent from Fig. 2(d) that the as-prepared MoSe2 are spherical and comprised of numerous loose and ultrathin flowerlike nanosheets with high specific surface area (32.43 m2 g1) (from Table 1). This is beneficial both to increase the catalytic performance and to load FeNi3. Moreover, some broken area demonstrates that the structure of the MoSe2 is hollow nanospheres. The TEM image in Fig. 2(e) further confirms the hollow structure of the as-prepared MoSe2 spheres. In addition, the transparent centers in the TEM picture as shown in Fig. 2(e) indicate that the MoSe2 is hollow nanospheres with small thickness. Furthermore, a sets of wellresolved lattice fringes from Fig. 2(f) is clearly observed with interplanar distance of 0.646 nm indexed to the (002) planes of hexagonal MoSe2. As shown in Fig. 2(g), SEM image shows MoSe2@FeNi3 also has a spherical structure similar to MoSe2, but the average diameter of MoSe2@FeNi3 is slightly larger than that of MoSe2. In addition, the uniformly dispersed FeNi3 nanoparticles (NPs) are anchored on the surface of MoSe2. Although subjected to an intense sonication during the SEM sample preparation, the FeNi3 NPs are still immobilized on the surface of the MoSe2 sphere, suggesting a strong interaction between FeNi3 NPs and the MoSe2 sphere substrate. From Fig. 2(h), the TEM image of MoSe2@FeNi3 shows that MoSe2@FeNi3 is also a hollow structure and has a larger thickness than MoSe2, which causes MoSe2@FeNi3 to have a larger diameter. The interplanar distances of MoSe2@FeNi3 from Fig. 2(i) are 0.204 nm and 0.646 nm, corresponding to the (111) plane of FeNi3 and (002) plane of hexagonal MoSe2, respectively. The XRD patterns of the as-milled MgH2, MgH210 wt% FeNi3, MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3 samples are presented in Fig. 3(a). It can be observed that all patterns contain the XRD peaks of MgH2 and Mg. The peaks for Mg are weak, which means that the content of Mg is small. It should be noted that the existence of unreacted Mg is hard to avoid due to the slow hydriding kinetics of large particles Mg. However, no apparent diffraction peaks of MoSe2 were identified from the patterns of MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3 samples due to its poor crystallinity [45]. The diffraction peaks at 2q z 51.9 and 60.6
from the patterns of MgH210 wt% FeNi3 and MgH210 wt% MoSe2@FeNi3 samples are corresponding to the peaks of FeNi3 nanoparticles. No new phases were found from all samples, indicating that no chemical reactions occurred during the ball milling. It is notable that the diffraction peaks of doped-MgH2 are significantly wider than those of pure MgH2, and these wider diffraction peaks indicate that smaller grain sizes were obtained during the mechanical milling [14]. Such small grains can offer more surface area for the hydrogenation and dehydrogenation processes, which is beneficial to improve hydrogen storage performance. Fig. 3(be) gives the SEM morphologies of the as-milled samples, respectively. It can be clearly seen that the average particle sizes in Fig. 3(ce) are obviously smaller than that in Fig. 3(b). These smaller particles can shorten the diffusion distance of hydrogen, and this will contribute to the improvement of the Habsorption/desorption properties of the MgH210 wt% FeNi3, MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3 mixtures. A similar positive effect has been previously reported during MgH2

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Fig. 1. XRD patterns of the as-prepared FeNi3 (a), MoSe2 (b) and MoSe2@FeNi3 (c); N2 adsorption/desorption isotherms and porous size distribution (insets) curves of the asprepared FeNi3 (d), MoSe2 (e) and MoSe2@FeNi3 (f).

Table 1 Brunauere-Emmette-Teller (BET) surface areas of the as-prepared FeNi3, MoSe2 and MoSe2@FeNi3. Sample

FeNi3

MoSe2

MoSe2@ FeNi3

BET surface area (m2 g1)

10.18

32.43

13.41

containing FeF3 as additive by R. Floriano et al. [46]. In addition, according to the report [47], as the particle size of the sample decreases, its specific surface area and defects will increase. The increase in specific surface area and defects also contributes to the improvement of the hydrogen storage performance of the samples. 3.2. The hydrogen storage performance of samples The non-isothermal dehydriding performance of the as-milled MgH2, MgH210 wt% FeNi3, MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3 were investigated by temperatureprogrammed-desorption (TPD) tests from room temperature to 500
C with a heating rate of 2
C/min, and the results were shown in Fig. 4(a). The un-doped MgH2 starts to dehydrogenate at around 310
C and can release around 7.3 wt% of H2 when heated to 440
C. However, the MgH210 wt% FeNi3, MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3 possess onset dehydrogenation temperatures of 224
C, 260
C and 194
C, and their the hydrogen decomposition are completed at 395
C, 402
C and 357
C, respectively. These show that the onset dehydrogenation temperatures of MgH210 wt% FeNi3, MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3 are about 86
C, 50
C and 116
C lower than that of the pure MgH2, respectively. The dehydrogenation capacity of MgH210 wt% FeNi3, MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3 are all 6.5 wt%. Obviously, FeNi3, MoSe2 and MoSe2@FeNi3 are effective on improving the dehydrogenation performance of MgH2. It is worth noting that the experimental hydrogen desorption capacity of the pure MgH2 is 7.3 wt%,

which is less than the theoretical value of 7.6 wt%. This is because that there is a small amount of un-hydrogenated Mg in the original sample according to the XRD results in Fig. 3(a). Thermal analyses were also conducted to investigate the hydrogen desorption performance of the samples. The DSC curves of the samples are shown in Fig. 4(b). It can be clearly seen that the peak dehydrogenation temperature are reduced from 348
C for the pure MgH2 to 290
C for MgH2 with FeNi3, to 337
C for MgH2 with MoSe2 and to 277
C for MgH2 with MoSe2@FeNi3. It is surprising that the peak desorption temperature of MgH210 wt% MoSe2@FeNi3 is about 71
C lower than that of the pure MgH2. The above results demonstrate that FeNi3, MoSe2 and MoSe2@FeNi3 all can significantly reduce the dehydrogenation temperature of MgH2, and the effect of MoSe2@FeNi3 is the best among all additives possible due to synergistic effect between MoSe2 and FeNi3. To study the hydrogen desorption kinetics of the samples, the isothermal dehydrogenation measurements of the samples were performed at 300
C in vacuum, and the results are presented in Fig. 5(a). As can be seen, the maximum hydrogen decomposition rates of the MgH210 wt% FeNi3, MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3 samples are 33.0 wt% H2/h, 7.1 wt% H2/ h and 35.7 wt% H2/h, and they are 18.3, 3.9 and 19.8 times that of un-doped MgH2, respectively. Moreover, only 0.5 wt% of H2 was released in the first 30 min for the pure MgH2. In contrast, the values increased to 6.2 wt%, 3.0 wt% and 6.2 wt% for MgH2 with FeNi3, MoSe2 and MoSe2@FeNi3, respectively. These results indicate that FeNi3, MoSe2 and MoSe2@FeNi3 can also obviously improve the hydrogen desorption kinetics of MgH2, and the effect of MoSe2@FeNi3 is still the best. This is similar to the effect on the nonisothermal dehydrogenation performance of MgH2 in the above section. To further confirm the effect of the additives on the dehydrogenation kinetics of MgH2, DSC tests of the MgH2, MgH210 wt% FeNi3, MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3 were performed at several different heating rates (c ¼ 2, 4, 8, 16
C/min).

S. Gao et al. / Journal of Alloys and Compounds 830 (2020) 154631

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Fig. 2. SEM, TEM and HRTEM images of the as-prepared FeNi3 (aec), MoSe2 (def), and MoSe2@FeNi3 (gei). Insets from (c) and (i) are the enlarged corresponding HRTEM micrographs.

Fig. 3. XRD patterns (a) of the as-milled MgH2, MgH210 wt% FeNi3, MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3; SEM images of the as-milled MgH2 (b), MgH210 wt% FeNi3 (c), MgH210 wt% MoSe2 (d) and MgH210 wt% MoSe2@FeNi3 (e) samples.

Moreover, the apparent activation energy (Ea) for the dehydrogenation reaction of MgH2 was calculated using Kissinger method

[48] with the parameters gotten from the DSC tests. The results are shown in Fig. 5(bf). The Kissinger’s equation can be described as

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Fig. 4. Non-isothermal dehydrogenation (a) and DSC (b) curves of the MgH2, MgH210 wt% FeNi3, MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3 with heating rate is 2
C/ min.

Fig. 5. Isothermal (300
C) (a) dehydrogenation curves of the MgH2, MgH210 wt% FeNi3, MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3; DSC curves of the MgH2 (b), MgH210 wt% FeNi3 (c), MgH210 wt% MoSe2 (d) and MgH210 wt% MoSe2@FeNi3 (e) composites at various heating rates (c ¼ 2, 4, 8, 16
C/min), and Kissinger plot for evaluating the apparent activation energy (Ea) (f).

Eq. (1),

ln

c Ea þA ¼  RTp Tp2

(7)

where c is the heating rate adopted in the DSC measurement, and Tp is the peak Kelvin temperature of sample dehydrogenation at different heating rates. A is a constant, and R is the universal gas constant. From the Kissinger plot, the apparent activation energy for the MgH2 with FeNi3, MoSe2 and MoSe2@FeNi3 are calculated to be 98.1 kJ/mol, 114.7 kJ/mol and 97.1 kJ/mol, respectively. They are 45.3 kJ/mol, 28.0 kJ/mol and 45.6 kJ/mol lower than that of the undoped MgH2. These reductions demonstrate that the hydrogen decomposition energy barrier of MgH2 can be decreased by the FeNi3, MoSe2 or MoSe2@FeNi3, which suggests the enhancement of

the hydrogen decomposition kinetics of MgH2 from the samples. More importantly, the apparent activation energy of the MgH2 with MoSe2@FeNi3 (97.1 kJ/mol) is significantly lower than that of MgH2 with SrTiO3 (109.0 kJ/mol) [49], MgH2 with Na3AlF6 (129.0 kJ/mol) [50], MgH2 with CeH2.73 (103.7 kJ/mol) [51] and MgH2 with CeCl3 (129.0 kJ/mol) [52]. Such a low apparent activation energy indicates that the composite has excellent dehydrogenation kinetics. In short, the above results further confirm that FeNi3, MoSe2 and MoSe2@FeNi3 benefit the improvement of hydrogen decomposition kinetics of MgH2. Moreover, in order to calculate the enthalpy change of the samples, according to the DSC curves of MgH2, MgH210 wt% MoSe2, MgH210 wt% FeNi3 and MgH210 wt% MoSe2@FeNi3 at 2
C/min, we can get the integrated values of the endothermic peaks of MgH2, MgH210 wt% MoSe2, MgH210 wt% FeNi3 and MgH210 wt% MoSe2@FeNi3 are 2207 J/g, 1921 J/g,

S. Gao et al. / Journal of Alloys and Compounds 830 (2020) 154631

1254 J/g and 1159 J/g, and they can be transformed into 58.1 kJ/mol, 56.2 kJ/mol, 36.7 kJ/mol and 33.9 kJ/mol, respectively. These results show that the addition of catalysts (MoSe2, FeNi3, MoSe2@FeNi3) can significantly reduce the enthalpy change of MgH2, which is beneficial to the improvement of the thermodynamics of MgH2. To further investigate the hydrogen decomposition mechanism, XRD tests were performed. The XRD patterns of non-isothermal and isothermal hydrogen decomposition products of MgH2, MgH210 wt% FeNi3, MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3 are respectively shown in Fig. 6(a and b). It can be clearly seen from Fig. 6(a) that there are no peaks of MgH2 in the four samples, which demonstrates that all samples can completely dehydrogenate at 300
C. This is well consistent with the result in Fig. 4(a). However, only the XRD patterns of the isothermal hydrogen decomposition products of MgH210 wt% FeNi3 and MgH210 wt% MoSe2@FeNi3 does not include peaks of MgH2 in Fig. 6(b). And there are traces of MgH2 in the isothermal dehydrogenation products of MgH2 and MgH210 wt% MoSe2, and the peaks of MgH2 is more obvious in that of un-doped MgH2. These results indicate that only MgH210 wt% FeNi3 and MgH210 wt% MoSe2@FeNi3 achieves completely hydrogen decomposition, which is in good agreement with the results Fig. 5(a). As shown in Fig. 6(a and b), the non-isothermal/isothermal hydrogen decomposition products of MgH210 wt% FeNi3 include Mg2Ni and Fe phases, which indicates that MgH2 may have reacted with FeNi3 to form Mg2Ni and Fe during dehydrogenation. For the MgH210 wt% MoSe2, the dehydrogenation products contain MgSe phase, which means that MgH2 may also react with MoSe2 to generate MgSe. As for the dehydrogenation products of MgH210 wt% MoSe2@FeNi3, it contains Mg2Ni, Fe and MgSe phases. However, no phases containing Mo element is found in the XRD

7

patterns of the dehydrogenation products of MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3. Therefore, we conduct further research using XPS tests to reveal the chemical state of Mo element. Fig. 6(c) shows the XPS higheresolution scans of Mo 3d of the nonisothermal hydrogen decomposition products of the MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3. The results were calibrated with the binding energy of C 1s at 284.8 eV. The deconvoluted Mo 3d binding energy spectrums show that there are four peaks in XPS pattern of MgH210 wt% FeNi3 sample. One doublet peaks at lower binding energy position are due to the Mo phase of Mo 226.1 eV (3d5/2), 231.1 eV (3d3/2) and the peaks at 229.1 (3d5/2) eV and 232.4 eV (3d3/2) are due to the MoSe2 phase, and they match well with the literatures [24,53e56]. These information indicates that the Mo element in the non-isothermal dehydrogenation product of the MgH210 wt% MoSe2 is mainly in the form of Mo and MoSe2 phases. As described above, Mo may be the reaction product of MgH2 and MoSe2, and MoSe2 should be the unreacted additive. Significantly, it can be observed that the binding energy of Mo 3d from the non-isothermal dehydrogenation product of MgH210 wt% MoSe2@FeNi3 shows a slight increase of 0.4 eV compared with that of the non-isothermal dehydrogenation product of MgH210 wt% MoSe2. The shift indicates that there are electrons in MoSe2 matrix transferred to the FeNi3 nano-particles, which further confirm the interaction between MoSe2 and FeNi3 in the MoSe2@FeNi3 composite [32,57]. Furthermore, the interaction is consistent well with the synergistic catalytic effect of MoSe2 and FeNi3 in MoSe2@FeNi3 mentioned above, and these are the reasons why the catalytic effect of MoSe2@FeNi3 on the decomposition of MgH2 is significantly better than that of single FeNi3 or MoSe2. Based on XRD and XPS analyses, the possible reaction pathways

Fig. 6. XRD patterns of non-isothermal (2
C/min) (a) and isothermal (300
C, 3 h) (b) dehydrogenation products of the MgH2, MgH210 wt% FeNi3, MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3; X-ray photoemission spectroscopy (XPS) (c) higheresolution scans of Mo 3d of non-isothermal dehydrogenation products of MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3; SEM images of the non-isothermal dehydrogenation products of MgH2 (d), MgH210 wt% FeNi3 (e), MgH210 wt% MoSe2 (f) and MgH210 wt% MoSe2@FeNi3 (g).

8

S. Gao et al. / Journal of Alloys and Compounds 830 (2020) 154631

can be concluded. The formations of MgSe, Mo, Mg2Ni and Fe are shown in reactions (1) and (2), respectively. 2MgH2 þ MoSe2 / 2MgSe þ Mo þ2H2 [

(8)

6MgH2 þ FeNi3 / 3Mg2Ni þ Fe þ 6H2 [

(9)

As mentioned above, the improvement of the dehydrogenation properties of MgH2 may be due to the fact that MgH2 can react with FeNi3, MoSe2 or MoSe2@FeNi3 and these will change the hydrogen desorption pathway of the MgH2. The reactions can effectively lead to the dehydrogenation of the whole system. Meanwhile, the in situ formed Fe and Mg2Ni, MgSe and Mo or MgSe, Mo, Fe and Mg2Ni can act as catalysts to promote the dehydrogenation performance of MgH2 [19,20,33e35,37,43]. Moreover, because of the synergistic effect of MoSe2 and FeNi3 in MoSe2@FeNi3, the catalytic effect of MoSe2@FeNi3 is significantly better than that of single FeNi3 or MoSe2. The SEM images of the non-isothermal decomposition products of the samples are shown in Fig. 6(dg). The average particle sizes in Fig. 6(eg) are significantly smaller than that in Fig. 6(d), and the particle surfaces in Fig. 6(eg) are coarse. These rough and relatively small particles can improve the specific surface area of the samples, thus enhancing the release of hydrogen. To evaluate the effect of FeNi3, MoSe2 and MoSe2@FeNi3 on hydrogen absorption, the fully dehydrogenated MgH2, MgH210 wt % FeNi3, MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3 samples were hydrogenated in non-isothermal modes from room temperature to 400
C with a heating rate of 2
C/min under 5 MPa H2, and the results are shown in Fig. 7. It was observed that the dehydrogenated MgH210 wt% FeNi3, MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3 samples obviously start taking up H2 at 80
C, 127
C and 73
C, which are 67
C, 20
C and 74
C lower than that of the un-doped sample, respectively. When the temperature rises to 150
C, the hydrogen absorption capacities of the samples containing FeNi3, MoSe2 and MoSe2@FeNi3 are accumulated to 4.9 wt%, 1.4 wt% and 5.6 wt%. However, only 0.3 wt% of H2 was recharged into the un-doped sample under the identical conditions. In addition, the maximum hydrogenation capacities of the dehydrogenated MgH210 wt% FeNi3, MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3 samples can reach respectively 6.5 wt %, 6.3 wt% and 6.4 wt%. Further isothermal measurements (150
C, 1 h) in Fig. 7(b) demonstrate that the hydrogen absorption rate of the MgH210 wt % FeNi3 and MgH210 wt% MoSe2@FeNi3 composites are much faster than MgH210 wt% MoSe2 and MgH2. Moreover, only MgH210 wt% FeNi3 and MgH210 wt% MoSe2@FeNi3 composites could be fully hydrogenated under the above conditions with 6.5 wt

% and 6.1 wt% of hydrogen absorbed within 7 min. In addition, from Fig. 7(c), MgH210 wt% FeNi3 and MgH210 wt% MoSe2@FeNi3 composites can absorb 5.4 wt% and 5.8 wt% of hydrogen in the first 0.5 min, while MgH2 þ 10 wt% MoSe2 and MgH2 can hardly absorb hydrogen. These results are in well agreement with the nonisothermal hydrogenation results that the MgH210 wt% FeNi3 and MgH210 wt% MoSe2@FeNi3 composites have the lower onset hydrogen absorption temperature among the four samples. Therefore, hydrogenation properties were also remarkably improved due to the presence of FeNi3, MoSe2 or MoSe2@FeNi3, and the comprehensive effect of MoSe2@FeNi3 is still greater than that of single FeNi3 or MoSe2. In order to evaluate the phases of the non-isothermal and isothermal hydrogenation products of the samples, the XRD tests were performed, and the results are shown in Fig. 8(a and b). It is worth noting that there are no detectable peaks of Mg in Fig. 8(a), and the result indicates that the samples can be fully hydrogenated. In Fig. 8(b), the isothermal hydrogenation products of MgH2 and MgH210 wt% MoSe2 include peaks of Mg, and the intensity of Mg peaks in isothermal hydrogenation products of MgH2 is significantly stronger than that of MgH210 wt% MoSe2. However, there are no observable traces of Mg in isothermal hydrogenation products of MgH210 wt% FeNi3 and MgH210 wt% MoSe2@FeNi3. The above results indicate that only. MgH210 wt% FeNi3 and MgH210 wt% MoSe2@FeNi3 samples can achieve complete hydrogenation in all samples. Moreover, MgSe and Fe phases generated during the dehydrogenation of the samples still remain stable after non-isothermal or isothermal hydrogenation. Whereas, the Mg2Ni obtained during the dehydrogenation of the samples convert to Mg2NiH4 after non-isothermal hydrogenation, and to Mg2NiH0.3 and Mg2NiH4 after isothermal hydrogenation, which are consistent well with previous reported results [58e60]. Thus, the improvement of the hydrogen absorption performance of MgH210 wt% FeNi3, MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3 are ascribed to the existence of Fe or MgSe or MgSe and Fe which can act as catalysts during the hydriding reaction. Jia et al. [20] and Saita et al. [61] found the similar catalytic effect of MgS and Fe, respectively. As for the MgH210 wt% FeNi3 and MgH210 wt% MoSe2@FeNi3 samples, in addition to the catalysis of Fe or MgSe and Fe, the presence of Mg2Ni can change the hydrogenation pathway of the samples, and this makes the hydrogenation of the samples easier to occur than that of MgH210 wt% MoSe2 and un-doped MgH2. In addition, some reports have shown that Fe or Ni can significantly improve the hydrogen storage performance of MgH2 [33,37,62,63]. Therefore, FeNi3 and MoSe2@FeNi3 possess greater catalytic effect than MoSe2. XPS was conducted to measure the binding energies of Mo in the non-isothermal hydrogenation products of MgH210 wt%

Fig. 7. Non-isothermal (2
C/min) (a) and isothermal hydrogenation (150
C, 1 h) (b) curves of the non-isothermal dehydrogenation products of MgH2, MgH210 wt% FeNi3, MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3 at 5 MPa H2; isothermal hydrogenation curves in the time range from 0 to 5 min for (b) (c).

S. Gao et al. / Journal of Alloys and Compounds 830 (2020) 154631

9

Fig. 8. XRD patterns of non-isothermal (2
C/min, 5 MPa H2) (a) and isothermal (150
C, 5 MPa H2) (b) hydrogenation products of MgH2, MgH210 wt% FeNi3, MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3; X-ray photoemission spectroscopy (XPS) (c) high-resolution scans of Mo 3d of non-isothermal hydrogenation products of MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3; SEM images of the non-isothermal hydrogenation products of the non-isothermal dehydrogenation products of MgH2 (d), MgH210 wt% FeNi3 (e), MgH210 wt% MoSe2 (f) and MgH210 wt% MoSe2@FeNi3 (g).

MoSe2 and MgH210 wt% MoSe2@FeNi3, and the results are shown in Fig. 8(c). Both the high resolution deconvoluted XPS spectrums of non-isothermal hydrogenation products of MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3 only show two dominant peaks, and they are attributed to the doublet Mo 3d5/2 and Mo 3d3/2, respectively. Compared to Fig. 6(c), there is no existence of MoSe2 in Fig. 8(c), and this shows that MoSe2 is consumed to an undetectable content in the hydrogenation process. It is noteworthy that the binding energy of Mo 3d in the non-isothermal hydrogenation products of MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3 have respectively the slight decreases of 0.3 and 0.7 eV compared with that of Fig. 6(c). The shifts illuminate that Mo can get electrons from the samples with the density of electron clouds around it increasing [29,32], and this implies that there is an interaction between Mo and the sample matrix. This is beneficial to improve the catalytic effect of Mo on the hydrogenation of MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3. The SEM images of the non-isothermal hydrogenation products of the samples are presented in Fig. 8(dg). The average particle sizes of Fig. 8(eg) are significantly smaller than that of Fig. 8(d), suggesting the FeNi3, MoSe2 and MoSe2@FeNi3 can effectively prevent particles agglomeration during hydrogen absorption and desorption. These smaller particles can increase the BET surface area of the samples, improving the hydrogenation performance of the samples. As mentioned above, MgH210 wt% MoSe2@FeNi3 composite possessed the best hydrogen storage performance in all samples. Thus, we further investigated the cycling property of MgH210 wt% MoSe2@FeNi3 at isothermal dehydrogenation (300
C, 1 h) and isothermal hydrogenation (150
C, 0.5 h) modes, as shown in Fig. 9(a). It can be clearly seen that not only the capacity, but also the hydrogen storage kinetics stay almost stable in 10 cycles,

suggesting the outstanding cyclic stability of the composite. In order to reveal the phase of the composite in hydrogenated/dehydrogenated states after ten cycles, the XRD was carried out, and the results are shown in Fig. 9(b), respectively. It is clear that the phases of the hydrogen absorption/desorption products obvious agglomeration. Therefore, the improvement of the sample cycling property can be attributed to the stability of the phase as well as the uniform distribution of elements during the cycling. 4. Conclusions The catalytic effect of the as-synthesized FeNi3 NPs, MoSe2 and MoSe2@FeNi3 hollow nano-spheres additives on the hydrogen storage properties of MgH2 is systematically studied. The researched MgH210 wt% FeNi3, MgH210 wt% MoSe2 and MgH210 wt% MoSe2@FeNi3 composites all possess better hydrogen storage properties as compared to the pure MgH2. The best composite, MgH210 wt% MoSe2@FeNi3 is able to decompose at 194
C with a total capacity of 6.5 wt%. The composite is able to release up to 6.2 wt% of hydrogen at 300
C in 30 min and its decomposition apparent activation energy is only 97.1 kJ/mol, which is 45.6 kJ/mol lower than the pure MgH2. On the other hand, the dehydrogenated MgH210 wt% MoSe2@FeNi3 sample obviously started absorbing hydrogen at 73
C, and this is 71
C lower than that of the un-doped sample. In addition, at 150
C and 5 MPa H2, it can absorb up to 5.8 wt% H2 in 0.5 min. The cycling performance study shows that the capacities and kinetics of hydrogen absorption/desorption can stay almost stable in the 10 cycles. The phase composition study based on the XRD and XPS analyses found the formation of MgSe, Mo, Mg2Ni and Fe after dehydrogenation of the composite. The formed substances are believed to be the active species that help to improve the hydrogen storage properties of the

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S. Gao et al. / Journal of Alloys and Compounds 830 (2020) 154631

Fig. 9. Cycling profiles (a) of MgH210 wt% MoSe2@FeNi3 with dehydrogenation conditions of 300
C and 1 h, and hydrogenation conditions of 150
C and 0.5 h under 5 MPa of H2; XRD patterns (b) of the composite in dehydrogenated and hydrogenated states after 10 cycles; SEM image in the EDS mode and corresponding EDS mapping (Mg, Fe, Ni, Se and Mo elements) (c) of the composite in dehydrogenated state after 10 cycles.

composite. In addition, the presence of Mg2Ni can convert MgH2 to an easier the hydrogen absorption/desorption pathway. Moreover, both the formed products and homogeneous distribution of elements can remain stable during the cycling, improving cycling stability of the composite. In short, the excellent catalytic effect of MoSe2@FeNi3 is due to the synergy between MoSe2 and FeNi3. Declaration of competing 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.

CRediT authorship contribution statement Shichao Gao: Writing - review & editing, Writing - original draft, Software, Project administration, Formal analysis, Data curation, Conceptualization. Hui Wang: Methodology, Software. Xinhua Wang: Funding acquisition, Investigation, Visualization. Haizhen Liu: Funding acquisition. Ting He: Validation. Yuanyuan Wang: Software, Validation. Chen Wu: Methodology. Shouquan Li: Supervision. Mi Yan: Resources.

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51771171, 51971199); Program for Innovative Research Team in University of Ministry of Education of China (No. IRT13037); and Education Department of Guangxi Zhuang Autonomous Region (No. 2019KY0021).

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