Remarkably Improved Hydrogen Storage Performance of MgH2

Subscriber access provided by SUNY DOWNSTATE

Article

Remarkably Improved Hydrogen Storage Performance of MgH Catalyzed by Multi-valence NbH Nanoparticles 2

x

Liuting Zhang, Xuezhang Xiao, Chenchen Xu, Jiaguang Zheng, Xiulin Fan, Jie Shao, Shouquan Li, Hongwei Ge, Qidong Wang, and Lixin Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b01532 • Publication Date (Web): 01 Apr 2015 Downloaded from http://pubs.acs.org on April 3, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Remarkably Improved Hydrogen Storage Performance of MgH2 Catalyzed by Multi-valence NbHx Nanoparticles Liuting Zhang, Xuezhang Xiao*, Chenchen Xu, Jiaguang Zheng, Xiulin Fan, Jie Shao, Shouquan Li, Hongwei Ge, Qidong Wang and Lixin Chen* State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China

Abstract Magnesium hydride is widely investigated due to its high hydrogen storage capacity. However, the unfavorable thermodynamic and kinetic barriers hinder its practical application. To ease these problems, three kinds of NbHx nanoparticles were prepared by wet-chemical methods, and then introduced into MgH2 for catalytically enhancing its hydrogen storage properties in this work. The results show that all the NbHx nanoparticles are effective in promoting the de/rehydrogenation kinetics of MgH2, and the three NbHx doped MgH2 composites can desorb 7.0 wt% H2 within 9 min at 300 °C while ball milled MgH2 only releases 0.2 wt% H2 in 9 min and 4.1 wt% H2 even in 200 min. Interestingly, the significant hydrogen absorption by NbHx doped MgH2 under lower temperature ranging from 50 to 100 ºC was observed, thus MgH2/c-NbHx sample can uptake about 4.0 wt% H2 at 100 ºC. It is found that the more disordered the structure and smaller the size of the NbHx particles, the better catalytic effect on hydrogen storage performances of MgH2. Analyses of XRD, XPS and TEM results indicate that the NbHx remain stable in the ball milling and following de/rehydrogenation process, and act as active catalytic specie in improving hydrogen storage performance of MgH2. Moreover, a mechanism is proposed to understand how the nanosized NbHx acted as charge transfer between Mg2+ and H-, which contributes to the significantly improved hydrogen storage performances of MgH2. It is believed that the use of Nb-based nanoparticles as catalysts would greatly promote the

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

development of the practical applications of MgH2 for hydrogen storage.

1. Introduction Hydrogen is widely regarded as a potential alternative to fossil fuel as it produces almost zero emission of pollutants when used in power generators. To achieve a future ‘‘hydrogen economy’’, a safe hydrogen storage technology with a high energy density is urgently required worldwide.1 Recently, intensive interest has been focused on hydrogen storage materials with high hydrogen content,2 especially for MgH2, owing to its high gravimetric and volumetric hydrogen storage capacity (7.6 wt% H2 and 110 g H2/L).3 However, due to the undesirable thermodynamic and kinetic properties, proper strategies are still needed to improve the hydrogen storage properties of MgH2. Mechanical milling is a very effective technique to improve the de/rehydrogenation properties of MgH2 by creating fresh surfaces and reducing particle size to nanoscale.4 But it is very difficult to prepare nano-sized MgH2 powder with current milling equipment, and it remains a tough task to prevent such fine particles from growing up in the de/rehydrogenation cycling. Another widely used strategy to improve the kinetics of MgH2 is to use catalysts or additives,6-28 which could enable Mg to absorb and MgH2 to release hydrogen at fairly low temperatures, respectively. Liang et al.6 found that TMs (Ti, V, Mn, Fe, and Ni) could improve the hydrogen storage properties of MgH2, and V and Ti showed superior catalytic effects on de/rehydrogenation properties. Nb2O5 is another widely investigated catalyst23-26 and it was shown that the addition of 1 mol % Nb2O5 to MgH2 can achieve a rapid room temperature absorption of hydrogen as well as a very low desorption temperature (200 ºC).23 Friedrichs et al.24 discovered that numerous pathways composed of ternary magnesium-niobium oxides were formed in the dehydrogenation process, which facilitated the hydrogen diffusion. They deduced that with the ability of Nb to change its valence state, metastable niobium hydride species


Page 2 of 29

Page 3 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


may possibly formed when the hydrogen enter through these pathway to the sample. Yet, it is pity that the mechanism of hydrogen sorption improvement by Nb2O5 is still unclear. Inspired by the above investigations, it seems that Nb hydrides maybe directly responsible for the excellent hydrogen storage properties of MgH2 when doped with Nb based compounds or metals. Besides, to our knowledge, the direct use of Nb hydrides on the hydrogen storage performances of MgH2 has never been studied before. Herein, NbHx nanoparticles with different degree of crystallization and particle sizes were prepared via a wet-chemical method, and the effect of these NbHx nanoparticles on improving the hydrogen storage properties of MgH2 has been investigated. Based on the microstructure analyses and superior hydrogen storage performances, the catalytic mechanism is also proposed.

2. Experimental NbCl5 (99%, Aladdin), LiCl (99%, Aladdin), LiH (98%, Alfa Aesar) and MgH2 (98%, Alfa Aesar) were all used as received. 2.1 Synthesis of NbHx nanoparticles NbHx was synthesized by ball milling NbCl5 and LiH with mole ratio of 1: 5 in a 100 ml stainless steel jar (see details in Supporting Information). The NbCl5 and excess LiCl buffer agent (0 wt%, 30 wt% and 50 wt%) were pre-milled together at 400 rpm for 20 h. Then LiH was added to the pre-milled composite and another 20 h milling was carried out to ensure reaction completeness. After milling, the mixture was washed with free-water tetrahydrofuran (THF) by centrifugation to remove LiCl byproduct. Then the NbHx nanoparticles were collected after vacuation at room temperature for 3 h. During the synthesis, washing and centrifugation processes, Ar was used to avoid oxidization. Three as-prepared NbHx samples, which prepared with different weight percent of LiCl buffer agent (0, 30 and 50 wt%), were referred as



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a-NbHx, b-NbHx and c-NbHx, respectively. 2.2 Synthesis of MgH2/NbHx composite The as-prepared a-NbHx, b-NbHx and c-NbHx nanoparticles were mixed with the commercially available MgH2 according to the mass ratio of 5 : 95, respectively. The corresponding mixtures were milled at 400 rpm with 1 MPa H2 pressure for 2 h, referred as MgH2/a-NbHx, MgH2/b-NbHx and MgH2/c-NbHx, respectively. For comparison, pure MgH2 was milled under identical conditions, referred as BM MgH2. 2.3 Characterization X-ray diffraction (XRD) measurements were carried out on an X'Pert Pro X-ray diffractometer (PANalytical, the Netherlands) with Cu Karadiation at 40 kV and 40 mA. X-ray Photoelectron Spectroscopy (XPS) was performed on a VGESCALAB MARK II system using Mg Kα radiation (1253.6 eV) under a base pressure of 1×10-8 Torr. All binding energy (BE) values were standardized with an uncertainty of ±0.2 eV (refer to the C1s peak at 284.6 eV). Scanning electron microscopy (SEM, Hitachi SU-70), transmission electron microscopy (TEM, JEOL JEM-1200EX) and high resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F20 S-Twin) equipped with an EDX detector (Oxford Microanalysis 6767) were used to observe the morphology of the samples. During the measurement, Ar protective gas was applied to prevent H2O and O2 contamination. The hydrogen storage properties of the samples were carried out in a homemade Sievert's type apparatus. In detail, hydrogen desorption was performed under 3.3 kPa H2 at 275, 280 and 300 ºC, respectively. The dehydrogenated samples were rehydrogenated under an initial hydrogen pressure of 2.2 MPa at 50, 100, 200 and 300 ºC, respectively. To better evaluate the hydrogen storage performance, the hydrogen capacity is presented as weight percent of the whole composite (i.e. MgH2 plus NbHx). All the sample handling was carried out in an Ar-filled glovebox (MBraun), where the H2O/O2 levels were kept below 1 ppm.


Page 4 of 29

Page 5 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3. Results and discussion 3.1 Characterization of NbHx nanoparticles Three different NbHx nanoparticles were prepared by the ion exchange reaction between NbCl5 and LiH, the XRD patterns of the as-synthesized NbHx samples are displayed in Figure 1. It can be found that, except a small amount of Fe come from the ball milling jar, the NbH and NbH2.7 phases are clearly existed in the a-NbHx sample in Figure 1(a). With the increasing amount of LiCl buffer agent, the peaks become broader, as shown in Figure 1(b) and (c). Broad peaks indicate that the grains may become extremely fine and/or the crystal particles turn to be amorphous. According the XRD results, different amount of LiCl buffer agent led to different degree of crystallization: the more amount of LiCl buffer agent, the worse degree of the crystallization. Moreover, the results also indicate that the synthesized particles may contain at least two kinds of Nb-hydride phases (NbH (JCPDS: 39-1327) and NbH2.7 (JCPDS: 39-1165)), thus the particles were referred as NbHx. In addition, SEM images (Figure S3) demonstrate that the particle size becomes smaller with increasing amount of LiCl. To further present the fine microstructure of the synthesized NbHx nanoparticles, TEM with SAED measurement was carried out, as shown in Figure 2. The nanoparticles of a-NbHx presented in Figure 2(a) are made up of 30-100 nm particles and the particles of as-synthesized b-NbHx (Figure 2(b)) have size of 20-50 nm, indicating that the added LiCl buffer agent can not only reduce the particle size of NbHx but also narrow down the size distribution. Moreover, the appearance of crystalline diffraction spots in the SAED (inset picture in Figure 2(a) and (b)) indicates that a-NbHx and b-NbHx were polycrystalline, which in line with the XRD results presented in Figure 1(a) and (b). The particle size of as-synthesized c-NbHx ranges from 10 to 50 nm (Figure 2(c) and SEM, Figure S3) and the diffuse diffraction rings in the related selective area electron diffraction (SAED) patterns (Figure 1(c), inset) demonstrates as-synthesized c-NbHx an amorphous phase, which agrees well with XRD results. By introducing these Nb-based nanoparticles into the MgH2 system via ball milling, homogenous dispersion would be obtained, which will



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 29

improve the de/rehydrogenation properties of MgH2.

3.2 Catalytic effects of NbHx nanoparticles on the hydrogen storage properties of MgH2 In order to investigate the structure of BM MgH2, MgH2/a-NbHx, MgH2/b-NbHx and MgH2/c-NbHx in the as-milled state, XRD measurement was carried out, as shown in Figure 3. Due to the impurity of the as-received MgH2, all XRD files show the signal of metal Mg. It can be clearly seen from Figure 3 (b) that only NbH2.7 could be readily distinguished in MgH2/a-NbHx while no phases of NbHx could be detected in MgH2/b-NbHx and MgH2/c-NbHx. This result was caused by the low amounts and poor crystallization of the NbHx additive. The relation between the size of NbHx nanoparticles and dehydrogenation properties of MgH2-NbHx composites was studied by DSC measurement under a heating rate of 5 ºC min-1. Figure 4 shows the dehydrogenation performances of BM MgH2, MgH2/a-NbHx, MgH2/b-NbHx and MgH2/c-NbHx, respectively. Obviously, both the onset and peak dehydrogenation temperature shift to lower ones after adding NbHx nanoparticles, see details in Table 1. For the pure MgH2 milled for 2 h, the peak temperature is 365.5 ºC, while those of MgH2/a-NbHx, MgH2/b-NbHx and MgH2/c-NbHx shift to 281.0 ºC, 277.4 ºC and 273.0 ºC, respectively. The existence of side peaks in the DSC curves may be due to the ball milling caused bimodal particle size distribution of particels.29 Experimental data of previous studies are also included in table 1. The dehydrogenation properties of MgH2/c-NbHx can compare that of MgH2/10wt% Nb2O5 (particle size: 15 nm),25 but what should be noted that the amount of Nb2O5 in MgH2/10wt% Nb2O5 is twice that of NbHx in MgH2/c-NbHx. The DSC results confirm that NbHx exerts an excellent catalysis for enhancing the dehydrogenation property of MgH2. To

present

clear

comparison

in

dehydrogenation

kinetics,

isothermal

dehydrogenation measurements were carried out to investigate the dehydrogenation kinetics of these as-milled MgH2-NbHx composites. Figure 5 shows the desorption kinetics of BM MgH2, MgH2/a-NbHx, MgH2/b-NbHx and MgH2/c-NbHx at 300 ºC,


Page 7 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


respectively. As shown in Figure 5, NbHx would greatly improve the dehydrogenation kinetics of MgH2. It is found that all the MgH2/NbHx composites can desorb 7.0 wt% H2 within 9 min at 300 ºC while BM MgH2 only releases 0.2 wt% H2 in 9 min and 4.1 wt% H2 even in 200 min. The inset figure shows only slight difference on the desorption kinetics of MgH2 at 300 ºC among the three NbHx doped MgH2 samples. When the desorption temperature goes down to 270 ºC (see Figure S4), the MgH2/c-NbHx composite shows better desorption performance than the other two, while the BM MgH2 could hardly release any hydrogen at the same condition. According to isothermal dehydrogenation performance, MgH2/c-NbHx displays the best kinetics among the three doped samples, which is in agreement with DSC results. Thus, the following study would mainly focus on the MgH2/c-NbHx composite. As well as the improved dehydrogenation kinetics, the MgH2/c-NbHx also exhibits the improved hydrogenation kinetics compared to BM MgH2 sample. The hydrogenation kinetics measurement of the as-milled MgH2/c-NbHx and BM MgH2 were further investigated at 300 ºC, 200 ºC, 150 ºC and 50 ºC, respectively. As shown in Figure 6, the increased reaction temperature would not only promote the hydrogenation kinetics, but also increase the hydrogenation capacity. It is found that MgH2/c-NbHx can absorb 5.3 wt%, 4.9 wt%, 3.7 wt% and 0.7 wt% H2 within 60 s at 300 ºC, 200 ºC, 100 ºC and 50 ºC, respectively, whereas for the pure MgH2 milled for 2 h, only 3.3 wt% H2 was absorbed even at 300 ºC. The excellent hydrogenation property of MgH2/c-NbHx sample could be attributed to the catalytic effect of NbHx. The hydrogen-saturated NbHx contacted closely with Mg particles and acted as nucleation and growth sites of the Mg hydride, which is similar to that of MgH2-0.1TiH2 system in the hydrogen absorption catalytic process.30 The above investigation demonstrates that the NbHx additive has great beneficial effects on the hydrogen storage performances of MgH2, it is also suggested that such NbHx with nanosize/amorphous state may be an ideal material for catalysts to promote the hydrogen storage performances of MgH2.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3.3 Dehydrogenation Kinetic Mechanism of the MgH2/c-NbHx Composite. For identifying the dehydrogenation kinetics mechanism of MgH2/c-NbHx, the isothermal dehydrogenation kinetic curves of MgH2/c-NbHx in the temperatures range of 275-300 ºC (Figure 7) were measured. It is obvious that with increasing temperature, MgH2/c-NbHx exhibits better desorption kinetic. About 4.3 wt%, 5.4 wt% and 6.1 wt% H2 can be released from MgH2/c-NbHx composite dehydrogenated at 270 ºC, 285 ºC and 300 ºC for 5 min, respectively. To have a in-depth investigation on the kinetics mechanism of MgH2/c-NbHx composite, it is necessary to choose a appropriate model which contains all essential chemical steps. Earlier，Sharp and Jone’s group31,32 had proposed a practical approach to rapidly select an rational rate equation for the kinetics of an isothermal solid-state reaction. In brief, the experimental values of (t/t0.5)exp are plotted against the theoretical ones (t/t0.5)theo to get a fitted line and the fitted slope value of an acceptable model must be very close to 1 (t0.5 means the reaction time when the reaction fraction α reaches 0.5). To determine the dehydrogenation mechanism of the MgH2/c-NbHx system, nine different relationships of (t/t0.5)exp versus (t/t0.5)theo for kinetic mechanisms (Table S133) are presented in Figure S5, and the detailed slope values of each model are also listed in the figure. For the MgH2/c-NbHx composite, it is clearly shown that the R3 model exhibits the desirable linear relationship with slope value of 0.99846. In this context, we can conclude that the hydrogen desorption reaction of MgH2/c-NbHx composite can be reasonably interpreted by the R3 model (three-dimensional phase boundary controlled kinetic mechanism). To further demonstrate the accuracy of the kinetic model, results of the plots of proposed kinetics mechanism versus time for the MgH2/c-NbHx composite are presented. It can be seen clearly from Figure S6 that all the three curves show a good linearity with linear coefficient R2> 0.997, indicating the reaction behaviors at different temperatures can be well explained by the three dimensional phase boundary controlled model. To realize more deeply the enhancement of the dehydrogenation kinetics, the hydrogen desorption apparent activation energies (Ea) is calculated using the Arrhenius equation K=Aexp(−Ea/RT) combined with the rate constants derived from


Page 8 of 29

Page 9 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure S6. It is found that the hydrogen desorption Ea is 50.4±0.6 kJ mol-1 (Figure S7) for MgH2/c-NbHx. In addition, the apparent activation energy was further investigated by Kissinger's method. DSC curves of MgH2/c-NbHx composite at different heating rates (2, 5, 8 and 10 ºC/min) were presented in Figure S8(a) and the corresponding Estimation of the apparent activation energy derived from Kissinger's method is 53.9 ±0.1 kJ mol-1(seen in Figure S8(b)), which is very close to 50.4±0.6 kJ mol-1. In this context, the hydrogen desorption Ea of MgH2/c-NbHx composite is about 90 kJ mol-1 lower compared with that of bulk MgH2 (141±5 kJ mol-1),34 indicating that the amorphous nano-catalyst can greatly reduce the activation energy and improve the kinetics under identical dehydrogenation conditions.

3.4 Evolvement of NbHx nanoparticles and the catalytic mechanism As demonstrated above, amorphous nano-sized c-NbHx has excellent effects on both desorption and absorption kinetics of MgH2, however, the catalytic mechanism of MgH2/c-NbHx composite remains unknown. Possibly due to their low amounts and poor crystallization, no signal of NbHx could be detected in MgH2/c-NbHx. In order to illuminate the catalytic mechanism of NbHx nanoparticles, the amount of c-NbHx was increased to 30 wt% to see the evolution of c-NbHx in the dehydrogenation process of MgH2. Figure 8 presents the XRD profiles of as-synthesized c-NbHx, ball-milled MgH2/30 wt% c-NbHx, dehydrogenated and rehydrogenated MgH2/30 wt% c-NbHx samples. Comparing the ball-milled MgH2/30 wt% c-NbHx with as-synthesized c-NbHx, both samples show an broad peak centered at around 2θ=36º, which belongs to amorphous NbHx, conforming that the ball-milled MgH2/30 wt% c-NbHx was just a physical mixture of MgH2 and c-NbHx. The broad peak centered at around 2θ=36° still exists in both dehydrogenated and rehydrogenated samples (Figure 8(c) and (d)), although this broad peak becomes slightly narrower after cycling. This result suggests that the stability of amorphous c-NbHx in the subsequent de/rehydrogenation process. As can been seen from the XRD results, no diffraction signal pertinent to other Nb phases appears after the de/rehydrogenation cycle, indicating the stability of NbHx during the reversible hydrogen storage process of MgH2.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

To further elucidate the reaction mechanism of the MgH2/30 wt% c-NbHx system, X-ray photoelectron spectroscopy (XPS) measurement was applied to characterize and identify the Nb-containing species in cycling products. Figure 9 shows the XPS spectra of the ball-milled MgH2/30 wt% c-NbHx, desorbed MgH2/30 wt% c-NbHx at 300 °C, rehydrogenated MgH2/30 wt% c-NbHx at 300 °C as well as the spectra of as-synthesized c-NbHx and MgH2/30 wt% Nb. The Nb 3d spectra of the as-synthesized c-NbHx and MgH2/30 wt% c-NbHx samples (Figure 9(a) and (b)) can be matched quite well with the spin-orbit split doublet from NbHx. The peaks at 203.2 eV and 205.9 eV should be assigned to Nb 3d5/2 and Nb 3d3/2 of the NbHx, respectively,24,35 which shift to higher positions compared to that of metal Nb36 (202.2 eV and 205.0 eV, Figure 9(e)).The peaks at 207.2 eV and 210 eV in MgH2/30 wt% c-NbHx should be assigned to the Nb2O5,37 which may be formed during the sample transfer process. During the first dehydrogenation and rehydrogenation cycle, the shapes of the Nb 3d spectra have no obvious change, indicating that NbHx remains stable in the cycling process. Jin et al.38 concluded that the Nb metal or other Nb compounds like halide and oxide were just precursors or intermediate phases that produced and/or evenly dispersed “active” Nb hydrides in MgH2 during ball milling and the NbH (or other Nb hydrides) was the actual catalyst in the MgH2 system. Moreover, Pelletier et al.39 demonstrated that a metastable phase of NbH0.6, which appeared during the dehydrogenation process of MgH2-Nb nanocomposite, may act as a gate way for hydrogen flow. Besides, Schimmel et al.40 also found that the catalytic properties of NbH rather than those of Nb metal were important for the hydrogen storage performances under pressures around 0.5-5 bar. In Figure 11, the XPS profile directly demonstrates the good stability of the doped NbHx, which makes it believable that NbHx nanoparticles are the active species in improving the hydrogen storage properties of MgH2. Now, we try to deeply illuminate the catalytic mechanism in the MgH2/c-NbHx composite. TEM and HRTEM measurements were carried out to present the morphology and microstructure of the MgH2/c-NbHx composite, shown in Figure 10. It can be found from Figure 10(a) that, the composites are composed of 0.2~2 µm


Page 10 of 29

Page 11 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


sized particles after ball milling, much smaller compared to that of the as-received MgH2 (~20 µm, shown in Figure S9). It is widely known that the hydrogen storage properties of magnesium hydride are size dependent, wherein a significant decrease in MgH2 particle size would greatly facilitate the dehydriding/hydriding process.41 In addition, dark spots can be readily distinguished from the bulk and are homogeneously dispersed in the composite. In the corresponding SAED pattern of Figure 10(a), only MgH2 phase is observed but no sign of NbHx, this is due to the amorphous state of NbHx. The HRTEM image presented in Figure 10(b) further shows that the dark spots have no crystal structure while the lattice spacing of 0.250 nm can be indexed as the (101) plane of MgH2. In order to explore what the dark spots were in the TEM and HRTEM images, EDX measurement was conducted. In the STEM-HAADF image in Fig 10(c), the dark spots shown in Figure 10(a) and (b) turn to be bright ribbons and these bright ribbons are confirmed by EDS results (Figure 10(d)) as Nb element. Combined with the above XRD and XPS analyses, it can be concluded that the well distributed nanoscale amorphous NbHx active species can provide favorable contact of composites together with more diffusion channels for hydrogen, which plays a favorable role for hydrogen boundaries/interface diffusion along the Mg/MgH2 interfaces. Furthermore, as mentioned above, the synthesized c-NbHx has two key features: first, c-NbHx contains the multiple valence Nb (0 ~ +5) elements; second, the amorphous c-NbHx is controlled within nanoscale. Barkhordarian et al. found that the multivalency of the transition metal played a critical role for the efficient catalytic behavior of the componds.42 As Nb has the medium electronegativity between Mg and H (Nb (1.6), Mg (1.31) and H2 (2.2)), it is easier for Nb ions to gain electrons (e-) than Mg ions and to lose e- than H- ions. Hence, multi-valence Nb can act as an intermediate carrier and promote the electron transfers between Mg2+ and H- during the de/rehydrogenation reaction. In order to in-depth understand the catalytic effect of NbHx active specie in the Mg-based composite, the schematic illustration of the catalytic mechanism is displayed in Figure 11. In detail, the ribbonlike NbHx first separates MgH2 during the ball milling process, thus provided favorable contact



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

between NbHx and MgH2 and created more diffusion channels for hydrogen, as shown in Figure 13(a). Then, the high valence Nb at the interface between MgH2 and NbHx gain e- from H-, the electron transfers between multi-valence Nb and Mg2+ gain efrom low valence Nb at the same time. In this way, Mg-H bond is easier to break down in dehydrogenation reaction. After that, Mg nucleates and grows together with the formation of H2. Also, the amorphous state of c-NbHx enable e- easily flows among multi-valence Nb. In this way, the activation energy of MgH2/c-NbHx composite is greatly reduced to 50.4±0.6 kJ mol-1 H2. Conversely, in the rehydrogenation process, H dissociated easily at the surface of ribbonlike NbHx, low valance Nb gives e- to H and high valance Nb obtains e- from Mg at the same time. Thus, Mg-H bonding is formed and the rehydrogenation reaction is occurred. As discussed above, the dissociation/recombination of hydrogen molecules is easy on the surface of Mg/MgH2 after catalyzed by c-NbHx. In addition, the nanosized c-NbHx can also provide more diffusion channels, which promote the hydrogen boundaries/interface diffusion along the Mg/MgH2 interfaces. According to above discussions, the rate limiting step of the isothermal decomposition process in the MgH2/c-NbHx composite is the Mg-MgH2 phase boundary movement under current experimental conditions.

4. Conclusion In summary, three kinds of NbHx nanoparticles with different degree of crystallization and particle size ranges were synthesized and doped into MgH2 through ball milling. The microstructure and hydrogen storage properties of the catalyzed MgH2 were investigated systematically. It was found that the hydriding-dehydriding kinetics of MgH2 can be significantly enhanced by these NbHx nanoparticles and the catalytic efficiency was ordered as crystalline a-NbHx (30~100 nm) < crystalline b-NbHx (20~50 nm) < amorphous c-NbHx (10~50 nm) in general. It can be concluded that the more disordered the structure and smaller the particle size of the NbHx, the better catalytic effect on hydrogen storage performances of MgH2. Thermal analysis


Page 12 of 29

Page 13 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


measurements showed that a reduction of 92.5 ºC in peak temperature was obtained in the MgH2 doped with 5 wt% c-NbHx sample, compared with the undoped MgH2. The apparent activation energy for the MgH2/c-NbHx was estimated to be 50.4±0.6 kJ mol-1 H2 and kinetic mechanism analysis revealed that reaction behaviors at different temperatures can be reasonably explained by the three dimensional phase boundary controlled model and the rate limiting step was the Mg-MgH2 phase boundary movement under current experimental conditions. Moreover, the MgH2/c-NbHx sample can absorb ~4.0 wt% H2 at 100 ºC at a reasonable rate, while only 3.3 wt% H2 was absorbed even at 300 ºC for the undoped MgH2 sample. Analyses of XRD, XPS and TEM demonstrated that the amorphous c-NbHx remains stable after ball milling and the following de/rehydrogenation cycle, which acts as active catalytic specie in promoting hydrogen storage performance of MgH2. Based on the experiment results, a mechanism was proposed to understand how the amorphous nanosized NbHx acted as charge transfer between Mg2+ and H-, which contributed to the good hydrogen storage performance of MgH2. It is believed that the directly use of amorphous nanosized catalytic species as catalysts would pave way for the practical application of the MgH2 system for hydrogen storage.

Acknowledgements The authors gratefully acknowledge the financial supports for this research from the National High

Technology Research & Development Program

of China

(2012AA051503), the National Natural Science Foundation of China (51471151, 51171173), the Program for Innovative Research Team in University of Ministry of Education of China (IRT13037), and the Zhejiang Provincial Science & Technology Program of China (2014C31134)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

AUTHOR INFORMATION Corresponding Author *Tel/Fax: +86 571 8795 1152. E-mail: [email protected]; [email protected].

Supporting Information Paragraph Supporting Information Available: Table of kinetic models of nanoconfined samples; Synthesis of NbHx nanoparticles; Figure of MS profile of gas inside the jar after ball milling LiH and NbCl5 for 5h; Figure of XRD of products inside the jar after ball milling LiH and NbCl5 for 20h; Figure of SEM images of (a) a-NbHx, (b) b-NbHx and (c) c-NbHx powders; Figure of Isothermal desorption curves of (a) BM MgH2, (b) MgH2/a-NbHx, (c) MgH2/b-NbHx and (d) MgH2/c-NbHx samples under 3 kPa hydrogen back pressure at 270 ºC; Figure of (t/t0.5)theo vs (t/t0.5)exp of MgH2/c-NbHx composite for various kinetic models; Figure of Time dependence of ƒ(α) of MgH2/c-NbHx composite for various temperatures; Figure of Arrhenius plots for the dehydrogenation of MgH2/c-NbHx composite; Figure of DSC curves (a) of MgH2/c-NbHx composite at various heating rates and the corresponding Estimation of the apparent activation energy (b) using the Kissinger's method with the parameters obtained from DSC measurements; Figure of SEM image of as-received MgH2. This material is available free of charge via the Internet at http://pubs.acs.org.

Reference (1) Felderhoff, M.; Weidenthaler, C.; Helmolt, R. von; Eberle, U. Hydrogen Storage: the Remaining Scientific and Technological Challenges. Phys. Chem. Chem. Phys. 2007, 9, 2643-2653. (2) Liu, C.; Li, F.; Ma, L. P.; Cheng, H. M. Advanced Materials for Energy Storage. Adv. Mater. 2010, 22, E28-E62.


Page 14 of 29

Page 15 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(3) Aguey-Zinsou, K.; Ares-Fernández, J. Hydrogen in Magnesium: New Perspectives toward Functional Stores. Energy Environ. Sci. 2010, 3, 526-543. (4) Sakintuna, B.; Lamari-Darkrim, F.; Hirscher, M. Metal Hydride Materials for Solid Hydrogen Storage: A Review. Int. J. Hydrogen Energy 2007, 32, 1121-1140. (5) Song, M. Y. Hydriding Kinetics of A Mechanically Alloyed Mixture Mg-10wt% Ni. Int. J. Hydrogen Energy, 2003, 28, 403-408. (6) Liang, G.; Huot, J.; Boily, S.; Van Neste, A.; Schulz, R. Catalytic Effect of Transition Metals on Hydrogen Sorption in Nanocrystalline Ball Milled MgH2-Tm (Tm=Ti, V, Mn, Fe and Ni) Systems. J. Alloy Compd. 1999, 292, 247-252. (7) Hanada, N.; Ichikawa, T.; Fujii, H. Catalytic Effect of Nanoparticle 3d-transition Metals on Hydrogen Storage Properties in Magnesium Hydride MgH2 Prepared by Mechanical Milling. J. Phys. Chem. B 2005, 109, 7188-7194. (8) Shang, C. X.; Bououdina, M.; Song, Y.; Guo, Z. X. Mechanical Alloying and Electronic Simulations of (MgH+M) Systems (M=Al, Ti, Fe, Ni, Cu and Nb) for Hydrogen Storage. Int. J. Hydrogen Energy 2004, 29, 73-80. (9) Cui, J.; Wang, H.; Liu, J.; Ouyang, L.; Zhang, Q.; Sun, D.; Yao, X.; Zhu, M. Remarkable Enhancement in Dehydrogenation of MgH2 by a nano-coating of Multi-valence Ti-based Catalysts. J. Mater. Chem. A 2013, 1, 5603-5611. (10) Song, M.; Bobet, J.; Darriet, B. Improvement in Hydrogen Sorption Properties of Mg by Reactive Mechanical Grinding with Cr2O3, Al2O3 and CeO2. J. Alloy Compd. 2002, 340, 256-262. (11) Friedrichs, O.; Aguey-Zinsou, F.; Fernandez, J. R.; Sanchez-Lopez, J. C.; Justo, A.; Klassen, T.; Bormann, R.; Fernandez, A. MgH2 with Nb2O5 as Additive for Hydrogen Storage: Chemical, Structural and Kinetic Behavior with Heating. Acta Mater. 2006, 54, 105-110. (12) Wang, P.; Wang, A. M.; Zhang, H. F.; Ding, B. Z.; Hu, Z. Q. Hydrogenation Characteristics of Mg-TiO2 (rutile) Composite. J. Alloy Compd. 2000, 313, 218-223.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(13) Milošević, S.; Rašković-Lovre, Ž.; Kurko, S.; Vujasin, R.; Cvjetićanin, N.; Matović, L.; Grbović Novaković, J. Influence of VO2 Nanostructured Ceramics on Hydrogen Desorption Properties from Magnesium Hydride. Ceram Int. 2013, 39, 51-56. (14) Guoxian, L.; Erde, W.; Shoushi, F. Hydrogen Absorption and Desorption Characteristics of Mechanically Milled Mg-35wt% FeTi2 Powders. J. Alloy Compd. 1995, 223, 111-114. (15) Orimo, S.; Fujii, H.; Horie, S. Investigations of Interfacial Materials Design: the Effect of Interface Microstructures in ZrCr1.8Cu0.3/Mg on MgH2 Formation as A Result of Hydrogen Interdiffusion. J. Alloy Compd. 1995, 231, 766-772. (16) Liang, G.; Boily, S.; Huot, J.; Neste, A. V.; Schulz, R. Hydrogen Absorption Properties of A Mechanically Milled Mg-50 wt.% LaNi5 Composite. J. Alloy Compd. 1998, 268, 302-307. (17) Terzieva, M.; Khrussanova, M.; Peshev, P. Hydriding and Dehydriding Characteristics of Mg-LaNi5 Composite Materials Prepared by Mechanical Alloying. J. Alloy Compd. 1998, 267, 235-239. (18) Wang, P.; Wang, A.; Zhang, H.; Ding, B.; Hu, Z. Hydriding Properties of A Mechanically Milled Mg-50 wt.% ZrFe1.4Cr0.6 Composite. J. Alloy Compd. 2000, 297, 240-245. (19) Fernández, J. F.; Bodega, J.; Sánchez, C. R. Hydriding/dehydriding Properties of Magnesium-ZrCr2 Composites. J. Alloy Compd. 2003, 356, 343-347. (20) Mao, J.; Guo, Z.; Yu, X.; Liu, H.; Wu, Z.; Ni, J. Enhanced Hydrogen Sorption Properties of Ni and Co-catalyzed MgH2. Int. J. Hydrogen Energy 2010, 35, 4569-4575. (21) Mao, J.; Guo, Z.; Yu, X.; Ismail, M.; Liu, H. Enhanced Hydrogen Storage Performance of LiAlH4-MgH2-TiF3 Composite. Int. J. Hydrogen Energy 2011, 36, 5369-5374. (22) Yu, X. B.; Yang, Z. X.; Liu, H. K.; Grant, D. M.; Walker, G. S. The Effect of A Ti-V-based Bcc Alloy as A Catalyst on the Hydrogen Storage Properties of MgH2. Int. J. Hydrogen Energy 2010, 35, 6338−6344.


Page 16 of 29

Page 17 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(23) Hanada, N.; Ichikawa, T.; Hino, S.; Fujii, H. Hydrogen Absorption Kinetics of the Catalyzed MgH2 by Niobium Oxide. J. Alloy Compd. 2006, 420, 46-49. (24) Friedrichs, O.; Sánchez-López, J. C.; López-Cartes, C.; Klassen, T.; Bormann, R.; Fernandez, A. Nb2O5 “Pathway Effect” on Hydrogen Sorption in Mg. J. Phys. Chem. B 2006, 110, 7845-7850. (25) Barkhordarian, G.; Klassen, T.; Bormann, R. Fast Hydrogen Sorption Kinetics of Nanocrystalline Mg Using Nb2O5 as Catalyst. Scripta Mater. 2003, 49, 213-217. (26) Aguey-Zinsou, K. F.; Ares-Fernandez, J. R.; Klassen, T.; Bormann, R. Effect of Nb2O5 on MgH2 Properties During Mechanical Milling. Int. J. Hydrogen Energy 2007, 32, 2400-2407. (27) Zhang, L.; Chen, L.; Xiao, X.; Fan, X.; Shao, J.; Li, S.; Ge, H.; Wang, Q. Superior Dehydrogenation Performance of Nanoscale Lithium Borohydride Modified with Fluorographite. Int. J. Hydrogen Energy 2014, 39, 12716-12726. (28) Song, M.; Bobet, J.; Darriet, B. Improvement in Hydrogen Sorption Properties of Mg by Reactive Mechanical Grinding with Cr2O3, Al2O3 and CeO2. J. Alloy Compd. 2002, 340, 256-262. (29) R de Castro, J. F.; F Santos, S.; M Costa, A. L.; R Yavari, A.; J Botta F, W.; T Ishikawa, T. Structural Characterization and Dehydrogenation Behavior of Mg-5 at.%Nb Nano-composite Processed by Reactive Milling. J. Alloy Compd. 2004, 376, 251-256. (30) Lu, J.; Choi, Y. J.; Fang, Z. Z.; Sohn, H. Y.; Rönnebro, E. Hydrogenation of Nanocrystalline Mg at Room Temperature in the Presence of TiH2. J. Am. Chem. Soc. 2010, 132, 6616–6617. (31) Sharp, J. H.; Brindley, G. W.; Achar, B. N. Comparison of Experimental Kinetic Decomposition Data with Master Data Using A Linear Plot Method. J. Am. Ceram. Soc. 1966, 49, 379-382. (32) Jones, L. F.; Dollimore, D.; Nicklin, T. Numerical Data for Some Commonly Used Solid State Reaction Equations. Thermochim. Acta 1975, 13, 240-245. (33) Zhang, Y.; Tian, Q.; Zhang, J.; Liu, S.; Sun, L. The Dehydrogenation Reactions



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and Kinetics of 2LiBH4-Al Composite. J. Phys. Chem. C 2009, 113, 18424-18430. (34) Bazzanella, N.; Checchetto, R.; Miotello, A. Catalytic Effect on Hydrogen Desorption in Nb-doped Microcristalline MgH2. Appl. Phys. Lett. 2004, 85, 5212-5214. (35) Sasaki, T. A.; Baba, Y. Chemical-state Studies of Zr and Nb Surfaces Exposed to Hydrogen ions. Phys. Rev. B 1985, 31, 791. (36) Moulder, J. F.; Stickle, W. F.; Sool, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer, Eden Prairie, 1992. (37) McGuire, G. E.; Schweitzer, G. K.; Carlson, T. A. Core Electron Binding Energies in Some Group IIIA, VB, and VIB Compounds. Inorg. Chem. 1973, 12, 2450-2453. (38) Jin, S.; Shim, J.; Ahn, J.; Cho, Y. W.; Yi, K. Improvement in Hydrogen Sorption Kinetics of MgH2 with Nb Hydride Catalyst. Acta Mater. 2007, 55, 5073-5079. (39) Pelletier, J. F.; Huot, J.; Sutton, M.; Schulz, R.; Sandy, A. R.; Lurio, L. B.; Mochrie, S. Hydrogen Desorption Mechanism in MgH2-Nb Nanocomposites. Phys. Rev. B 2001, 63, 52103. (40) Schimmel, H. G.; Huot, J.; Chapon, L. C.; Tichelaar, F. D.; Mulder, F. M. Hydrogen Cycling of Niobium and Vanadium Catalyzed Nanostructured Magnesium. J. Am. Chem. Soc. 2005, 127, 14348-14354. (41) Aguey-Zinsou, K. F.; Ares-Fernandez, J. R. Synthesis of Colloidal Magnesium: A Near Room Temperature Store for Hydrogen. Chem. Mater. 2008, 20, 376–378. (42) Barkhordarian, G.; Klassen, T.; Bormann, R. Catalytic Mechanism of Transition Metal Compounds on Mg Hydrogen Sorption Reaction. J. Phys. Chem. B 2006, 110, 11020-11024.


Page 18 of 29

Page 19 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FIGURE CAPTIONS Table 1. The onset and peak temperatures of DSC for different samples. Figure 1. XRD patterns of (a) a-NbHx, (b) b-NbHx and (c) c-NbHx powders. Figure 2. TEM images of (a) a-NbHx, (b) b-NbHx and (c) c-NbHx powders. Figure 3. XRD patterns of (a) BM MgH2, (b) MgH2/a-NbHx, (c) MgH2/b-NbHx and (d) MgH2/c-NbHx samples. Figure 4. DSC curves of (a) BM MgH2, (b) MgH2/a-NbHx, (c) MgH2/b-NbHx and (d) MgH2/c-NbHx samples. Figure 5. Isothermal desorption curves of (a) BM MgH2, (b) MgH2/a-NbHx, (c) MgH2/b-NbHx and (d) MgH2/c-NbHx samples under 3 kPa hydrogen back pressure at 300 ºC. Figure 6. Isothermal absorption curves of MgH2/c-NbHx at different temperatures (a) 50 ºC, (b) 100 ºC, (c) 200 ºC and (d) 300 ºC under 2.2 MPa hydrogen pressure. For comparison, isothermal absorption curve of BM MgH2 at 300 ºC (e) is also compiled. Figure 7. Isothermal desorption curves of MgH2/c-NbHx at different temperatures (a) 270 ºC, (b) 285 ºC and (c) 300 ºC under 3 kPa hydrogen pressure. Figure 8. XRD patterns for (a) as-synthesized c-NbHx, (b) ball-milled MgH2/30 wt% c-NbHx and (c) MgH2/30 wt% c-NbHx dehydrogenated at 300 ºC and (d) MgH2/30 wt% c-NbHx rehydrogenated at 300 ºC. Figure 9. XPS spectra in the energy levels of Nb 3d for (a) as-synthesized c-NbHx, (b) ball-milled MgH2/30 wt% c-NbHx and (c) MgH2/30 wt% c-NbHx dehydrogenated at 300 °C and (d) MgH2/30 wt% c-NbHx rehydrogenated at 300 °C. For comparison, XPS spectra of MgH2/30 wt% Nb (e) is also compiled. Figure 10. TEM image (a) of the MgH2/c-NbHx composite with the corresponding SAED pattern inset, (b) HRTEM image MgH2/c-NbHx composite, (c) HAADF-STEM image MgH2/c-NbHx composite and (d) EDX spectrum of selected red rectangle areal in (c). Figure 11. Schematic illustration of the catalytic mechanism for the MgH2/NbHx composite.



Table 1. The onset and peak temperatures of DSC for different samples Sample

crystal structure

particle size

Tonset (ºC)

Tpeak (ºC)

BM MgH2

-

-

331.9

365.5

MgH2/a-NbHx

crystal

30~100 nm

246.3

281.0

MgH2/b-NbHx

crystal

20~50 nm

241.1

277.4

MgH2/c-NbHx

amorphous

10~50 nm

237.2

273.0

15 nm

233.3

282.0

100 nm

290.0

341.0

-

270.0

320.0

MgH2/10wt% crystal

Nb2O525 MgH2-5at.%

crystal

Nb29

♣

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 29

♣ NbH ♦ NbH2.7

JCPDS: 39-1327 JCPDS: 39-1165

♥ Fe

JCPDS: 85-1410

♦

(a)

♦

♣♣

♣ ♥

♦

♦ ♦

♣

(b) (c) 20

30

40

50

60

70

80

2 θ (° )

Figure 1. XRD patterns of (a) a-NbHx, (b) b-NbHx and (c) c-NbHx powders.


Page 21 of 29

Figure 2. TEM images of (a) a-NbHx, (b) b-NbHx and (c) c-NbHx powders.

♥ NbH2.7

♣

♦ Mg ♣ MgH2

♣ ♣ ♣

(d)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


♦♦ ♦

♣

♣ ♣ ♣ ♣♣

♣

♣ ♣ ♣ ♣♣

♣

♣ ♣ ♣ ♣♣

♣

♣

♣

(c)

♣ ♣ ♣ ♣♣

♦ ♦♦ ♣ ♦ ♣ ♦♥ ♦ ♣ ♦ ♦♦

(b) (a)

10

20

30

♣

40

50

60

70

80

2θ (°)

Figure 3. XRD patterns of (a) BM MgH2, (b) MgH2/a-NbHx, (c) MgH2/b-NbHx and (d) MgH2/c-NbHx samples.



(d) 273.0 °C Heat flow (a.u.)

(c) 277.4 °C (b) 281.0 °C (a) 365.5 °C 50

100

150

200

250

300

350

400

450

Temperature (°C)

Figure 4. DSC curves of (a) BM MgH2, (b) MgH2/a-NbHx, (c) MgH2/b-NbHx and (d)

8

@300°C 7 6 5

Hydrogen Desorption (wt%)

MgH2/c-NbHx samples.

Hydrogen Desorption (wt%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 29

7 6 5 4 3 2 1 0 0

4

1

2

3

4

5

6

Time (min)

3

(a) (b) (c) (d)

2 1 0 0

25

50

75

100

125

150

175

200

Time (min)

Figure 5. Isothermal desorption curves of (a) BM MgH2, (b) MgH2/a-NbHx, (c) MgH2/b-NbHx and (d) MgH2/c-NbHx samples under 3 kPa hydrogen back pressure at 300 ºC.


Page 23 of 29

7 6

Hydrogen Absorbed (wt%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5.3

5

4.9 4

3.7 3

3.3

(a) (b) (c) (d) (e)

2 1

0.7 0 0

2

4

6

8

10

12

14

16

18

20

Time (min)

Figure 6. Isothermal absorption curves of MgH2/c-NbHx at different temperatures (a) 50 ºC, (b) 100 ºC, (c) 200 ºC and (d) 300 ºC under 2.2 MPa hydrogen pressure. For comparison, isothermal absorption curve of BM MgH2 at 300 ºC (e) is also compiled.



7

Hydrogen Desorped (wt%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 29

6.1

6

5.4 (a) (b) (c)

5

4.3

4 3 2 1 0 0

5

10

15

20

25

Time (min) Figure 7. Isothermal desorption curves of MgH2/c-NbHx at different temperatures (a) 270 ºC, (b) 285 ºC and (c) 300 ºC under 3 kPa hydrogen pressure.


Page 25 of 29

♣

♠ NbHx ♣ ΜgΗ2

♣

♦ Μg

♦ ♣ ♣ Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(d)

♣ ♦ ♦ ♣

(c)

♦ ♣

♣

(b)

♣

(a)

20

♦

♣

♠

25

30

35

40

45

50

55

60

2θ (°)

Figure 8. XRD patterns for (a) as-synthesized c-NbHx, (b) ball-milled MgH2/30 wt% c-NbHx and (c) MgH2/30 wt% c-NbHx dehydrogenated at 300 ºC and (d) MgH2/30 wt% c-NbHx rehydrogenated at 300 ºC.



Nb 3d3/2

Nb 3d5/2

(e)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 29

(d)

(c) Nb2O5 3d3/2

Nb2O5 3d5/2

(b) NbHx 3d5/2

NbHx 3d3/2

(a) 198

200

202

204

206

208

210

212

214

Binding energy / ev

Figure 9. XPS spectra in the energy levels of Nb 3d for (a) as-synthesized c-NbHx, (b) ball-milled MgH2/30 wt% c-NbHx and (c) MgH2/30 wt% c-NbHx dehydrogenated at 300 °C and (d) MgH2/30 wt% c-NbHx rehydrogenated at 300 °C. For comparison, XPS spectra of MgH2/30 wt% Nb (e) is also compiled.


Page 27 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10. TEM image (a) of the MgH2/c-NbHx composite with the corresponding SAED pattern inset, (b) HRTEM image MgH2/c-NbHx composite, (c) HAADF-STEM image MgH2/c-NbHx composite and (d) EDX spectrum of selected red rectangle areal in (c).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 11. Schematic illustration of the catalytic mechanism for the MgH2/NbHx composite.


Page 28 of 29

Page 29 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents (TOC) Image

The ribbonlike NbHx separates MgH2 to provide favorable contact between NbHx and MgH2. In addition, NbHx acts as charge transfer between Mg2+ and H-, thus contributes to the good hydrogen storage performance of MgH2.


Remarkably Improved Hydrogen Storage Performance of MgH2

Recommend Documents