Superior Catalytic Effect of Nickel Ferrite Nanoparticles in Improving

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Superior Catalytic Effect of Nickel Ferrite Nanoparticles in Improving Hydrogen Storage Properties of MgH 2

Qi Wan, Ping Li, Jiawei Shan, Fuqiang Zhai, Ziliang Li, and Xuanhui Qu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp508528k • Publication Date (Web): 20 Jan 2015 Downloaded from http://pubs.acs.org on January 29, 2015

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The Journal of Physical Chemistry

Superior Catalytic Effect of Nickel Ferrite Nanoparticles in Improving Hydrogen Storage Properties of MgH2 Qi Wan a, Ping Li a∗, Jiawei Shan a, Fuqiang Zhai b, Ziliang Li a, Xuanhui Qu a a

State Key Laboratory for Advanced Metals and Materials ,Institute for Advanced

Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China b

Departament Física Aplicada, EETAC, Universitat Politècnica de Catalunya BarcelonaTech, 08860 Castelldefels, Spain

Corresponding author: [email protected] (Ping Li); Tel: +86-10-82377286; Fax: +86-10-62334311. Abstract: The catalysis of NiFe2O4 nanoparticles on the hydrogen storage performances of magnesium hydride synthesized by high-energy ball milling were studied for the first time. The H2 storage performances and catalytic mechanism were studied by pressure-composition-temperature (PCT), differential scanning calorimetry (DSC), X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The non-isothermal dehydrogenation results display that the initial dehydrogenation temperature of 7 mol% NiFe2O4 doped MgH2 is 191 °C, which is 250 °C lower than that of pristine MgH2. The desorption kinetics displays that MgH2+7 mol% NiFe2O4 sample could desorb 3.79 wt% H2 within 1 h at 300 °C under H2 pressure of 0.1 MPa. The absorption kinetics displays that MgH2+7 mol% NiFe2O4 sample could absorb 2.06 wt% H2 within 3 h near room temperature under

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H2 pressure of 4 MPa. The desorption activation energy of MgH2+7 mol% NiFe2O4 sample is 59.6 kJ/mol, decreasing 195.3 kJ/mol as compared with pristine magnesium hydride. The reaction enthalpy and entropy of MgH2+7 mol% NiFe2O4 sample during the dehydrogenation process are improved. The enhancement in the H2 storage performances of MgH2 by adding NiFe2O4 nanoparticles are primarily ascribed to intermetallic Fe7Ni3 and (Fe,Ni) phases during the desorption procedure, which act as the real catalyzer in 7 mol% NiFe2O4-doped sample. Keywords: Nickel ferrite, H2 storage, MgH2, Desorption activation energy, Dehydriding temperature Introduction: The solid hydrogen materials with great volumetric (≥40 g/L) and gravimetric (5.5 wt%) density are highly desirous by human being for on-board H2 storage in fuel cell vehicles on the basis of the U.S. DOE’s 2017 goal.1 For improving the harmful effects on the environment and lessening the dependence on fossil fuels, MgH2 is seem as a hopeful candidate for fully satisfying the demand of various applications because of its comparative advantages including 7.6 wt% H2 storage amount, abundant resources and cheap cost.2 Nevertheless, the relatively high dehydrogenation temperature (>400 °C) and inferior hydrogenation-dehydrogenation properties have hindered MgH2 from being utilized in the practical applications.2-5 During the past decades, researchers were trying diverse approaches to conquer these obstacles. Extensive endeavors have been devoted to ameliorate H2 storage property of MgH2, covering the preparation nanocrystal MgH2 by using high-energy ball milling,

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chemical synthesis method6 and doping additives. To date, the documented additives for MgH2 contain metals such as Al,7 Ni,8 Fe,9 Pd,10 Nb,2 In,11 V12 and Ti;13 metal oxides such as Nb2O5,14 TiO2,15 CeO2,16 La2O317 and Cr2O3;14 metal halides such as NbF5,18 TiCl3,19 ZrF420 and TiF5;19 and other compounds such as Zr8Ni21,21 BaRuO3,4 SiC,22 TiH2,23 TiAl24 and TiC.25 Because transition metals have multiple valence in their relevant transition metal oxides, transition metal oxides have better reaction efficiency for enhancing the H2 storage performance of magnesium hydride.26-27 So, investigating H2 interactions between MgH2 and transition metal oxides can better understand the specific role of transition metal oxides in MgH2 hydrogen absorption-desorption reactions.28-29 Furthermore, it was also reported that some transition metal oxides, NiO30 and Fe2O331-32 could improve hydrogen storage performance of MgH2, and recently Li et al.33 reported that NiFe2O4 nanoparticles could remarkably improve the dehydrogenation behaviors of lithium aluminum hydride. So, it is rational to believe that NiFe2O4 nanoparticles could display a super potential as the effective catalyzer to improve MgH2 hydrogen storage properties. Inspired by these results, in this study, NiFe2O4 nanoparticle was employed as catalyzer to study its effect on the H2 storage performances of MgH2 prepared by ball milling. Experimental: MgH2 (≥98% pure), NiO (≥99.99% pure, 20 nm) and Fe2O3 (≥99.99% pure, 20 nm) were bought from Sigma Aldrich Company, NiFe2O4 (≥99.99% pure, 20 nm) was synthesized by the auto-combustion process. The details of the synthesis process can

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obtain in my previous studies.34-36 All raw materials were used directly. All operations including weight and loading were conducted in a ZKX 2 glove box (Nanjing University, China) filled with high purity Ar for the purpose of avoiding oxidation and moisture for the undoped/doped MgH2 samples. About 5 g of MgH2 was mixed with various ratios (3, 5, 7 and 9 mol%) NiFe2O4 nanoparticles and milled for 30 minutes in a high-energy ball mill, respectively. The undoped/doped samples were put into the stainless steel pot in the glove box filled with Ar. ZrO2 balls were mixed with a ball-to-powder weight ratio of 15:1. The samples were ball milled for 10 minutes, and then the steel pot has to stop for 5 minutes to prevent the temperature of steel pot from increasing. For comparison, MgH2+7 mol% (NiO+Fe2O3) with the molar ration 1:1 between NiO and Fe2O3 sample was prepared in the same conditions. The dehydrogenation/hydrogenation performances of pristine magnesium hydride,

ball milled

MgH2 and

doped

MgH2

samples

were

tested

by

pressure-composition-temperature (PCT) apparatus (General Research Institute for Nonferrous Metal, China). For the measurement of nonisothermal desorption, 0.5 g sample was put into the stainless steel vessel, and warmed up to 500 °C at a raping rate of 6 °C/min under the H2 pressure of 0.01 MPa. Prior to heating, the measuring system was pumped to a hard vacuum. After the first desorption, the samples were rehydrogenated at 350 °C under H2 pressure of 4 MPa. The rehydrogenation samples were desorbed at 350 °C, 300 °C and 200 °C under H2 pressure of 0.1 MPa, respectively. The isothermal rehydrogenation was done at 300 °C, 100 °C, 50 °C and room

temperature

under

H2

pressure

of

4

MPa,

respectively.

The

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dehydrogenation/rehydrogenation cycles were done at 350 °C/300 °C under 0.1 MPa/4 MPa. For comparing the H2 storage properties of MgH2 doped with NiFe2O4 nanoparticles, the dehydrogenation/rehydrogenation properties of MgH2+7 mol% (NiO+Fe2O3) sample were carried out. In order to count the dehydrogenation activation energy of undoped/doped MgH2 samples by the Kissinger method, the differential scanning calorimetry (DSC) measurement was measured by using NETZSCH STA 449C in a high-purity argon with flow rate of 50 mL/min. Typically, 5 mg sample was loaded into a alumina crucible in the glove box, and heated at various raping rates of 4 °C/min, 7 °C/min and 10 °C/min from 50 °C to 500 °C. The Pressure-Composition-Temperature (PCT) curves were plotted at 300 °C, 350 °C and 380 °C, respectively. The morphology and phase constitution of all samples after ball milling and desorption were observed by Field-Emission Scanning Electron Microscopy (FESEM, ZEISS ULTRA55, Germany) and Transmission Electron microscopy (TEM, JEM-2010, Japan). The phase structure of the samples after ball milling and desorption was examined by MXP21VAHF X-ray diffractometer (XRD with Cu Kα radiation, 40 kV, 200 mA) and the 2θ angle was tested from 10° to 90° with a scanning step of 0.02°. X-ray photoelectron spectroscopy (XPS) measurement was done by using the PHI-5300 spectrometer. Results and Discussion: Figure 1 displays the thermal dehydrogenation process of pristine magnesium hydride, ball milled magnesium hydride and MgH2 doped with 3, 5, 7 and 9 mol%

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NiFe2O4 nanoparticles. We can observe that NiFe2O4 nanopowder can significantly enhance the desorption performances of magnesium hydride, resulting from that onset dehydrogenation temperature of doped sample dramatically reduces as compared with that of the pristine MgH2. For the pristine MgH2 sample, it commences to release hydrogen at 441 °C, and a H2 release amount of 7.31 wt% is acquired after heating to 550 °C. While the ball milled MgH2 starts to desorb at 350 °C, which is 91 °C lower than that of pristine MgH2 introduced by the ball milling. 2,22 After doping NiFe2O4 nanoparticles to MgH2, the initial dehydrogenation temperature of MgH2 further decreases. For the MgH2+3 mol% NiFe2O4 sample, the desorption starts at 211 °C. With increasing the additive content to 5 mol%, its onset desorption temperature decreases to 200 °C. Compared with pristine MgH2, doping 3 mol% and 5 mol% NiFe2O4 brings about 230 °C and 241 °C decline in the initial desorption temperature, respectively. For the dehydrogenation procedure, the 3 mol% NiFe2O4 doped sample releases 5.82 wt% H2, while 5.07 wt% H2 is released for the MgH2+5 mol% NiFe2O4 sample. Further increasing the addition amount to 7 mol%, the initial dehydriding temperature declines to 191 °C, suggesting a further reduction as compared with the MgH2+3 mol% NiFe2O4 and MgH2+5 mol% NiFe2O4 and a decrease by 250 °C as compared with pristine MgH2. At the same time, the MgH2+7 mol% NiFe2O4 sample releases 4.48 wt% hydrogen. When the NiFe2O4 addition amount increases to 9 mol%, the initial desorption temperature reduces to 179 °C, indicating that NiFe2O4 contributes to improving the MgH2 dehydrogenation onset temperature as compared with other reported catalysts.8,25,37 Nevertheless, the

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dehydriding hydrogen amount for 9 mol% doped sample drops to 3.49 wt%, indicating a significant drop in desorption amount due to an superfluous addition of NiFe2O4 nanoparticles. Through comprehensively considering the dehydriding hydrogen properties, the 7 mol% NiFe2O4 doped MgH2 sample presents the optimal dehydriding properties. Therefore, the MgH2+7 mol% NiFe2O4 sample is employed to study the NiFe2O4 catalysis for the H2 storage properties of MgH2 in this study. For further comparing the desorption process of undoped/doped magnesium hydride samples. Figure 2 depicts DSC curves of pristine magnesium and MgH2+7 mol% NiFe2O4 sample with the temperature ranging from 50 °C to 500 °C at the raping rate of 4 °C/min. As displayed in Figure 2 (a), the DSC curve of pristine MgH2 has one endothermic peak at 411.2 °C assigned to the decomposition of MgH2. For MgH2+7 mol% NiFe2O4 sample, the DSC curve also presents one endothermic peak ascribing to the decomposition of magnesium hydride. However, the endothermic peak of 7 mol% doped sample appears at 289.5 °C, which is much lower than that of pristine MgH2 and various catalysts in other reported literature.2,18,23,38 According to the above analysis, the remarkable reduction in the dehydrogenation temperature reflects the excellent catalysis of NiFe2O4 nanopowders on significantly accelerating the desorption performances of MgH2. However, it is worth to note that the initial dehydrogenation temperature tested by DSC is lower than that from PCT measurement. The analogous phenomenon was also observed in other papers.25,37 In order to exhibit the effectively catalysis of NiFe2O4 nanoparticles on improving the desorption kinetics of MgH2, Figure 3 shows the isothermal

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dehydrogenation of MgH2 samples with and without NiFe2O4 additive at various temperatures under H2 pressure of 0.1 MPa. As presented in Figure 3, the 5 mol% and 7 mol% NiFe2O4 doped samples release 3.27 wt% and 3.79 wt% hydrogen within 3600 s at 300 °C under H2 pressure of 0.1 MPa, respectively. When the temperature increases to 350 °C, 4.33 wt% and 4.28 wt% H2 is desorbed for MgH2+5 mol% NiFe2O4 and MgH2+7 mol% NiFe2O4 samples within 3600 s under 0.1 MPa, respectively. However, the ball milled MgH2 sample can only desorb 0.83 wt% hydrogen within 4000 s at the same conditions. It is notable that the MgH2+5 mol% and 7 mol% NiFe2O4 samples can release 0.86 wt% and 1.48 wt% hydrogen at 200 °C, respectively. However, nothing else can be released for as-milled MgH2 heated at 300 °C and 200 °C. As a consequence, NiFe2O4 nanoparticles possess a more excellent catalytic effect on boosting MgH2 dehydrogenation kinetics properties as compared with other additives reported in literatures.4,7,23,32,34,39-40 On the purpose of investigating the reversibility of ball milled MgH2 and MgH2 +7 mol% NiFe2O4 samples, the rehydrogenation performance of the dehydrogenated samples was measured at various temperatures under H2 pressure of 4 MPa. Figure 4 shows the rehydrogenated curves of ball milled MgH2 and NiFe2O4 doped MgH2 samples at various temperatures under 4 MPa H2 pressure. It can be distinctly observed that NiFe2O4 doped sample can absorb hydrogen at or close to room temperature, as shown in Figure 4 (b) and (c). 7 mol% NiFe2O4 doped sample can absorb 2.06 wt% and 2.89 wt% hydrogen at room temperature and 50 °C within 3 h under hydrogen pressure of 4 MPa, respectively, while as-milled sample cannot

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absorb any amount of hydrogen at or close to room temperature, indicating that NiFe2O4 nanopowders have a better catalytic effect than other reported catalysts.3 When the heating temperature is set to 100 °C, as-milled MgH2, 5 mol% doped and 7 mol% doped samples can absorb 0.84 wt%, 3.48 wt% and 3.32 wt% hydrogen within 11700 s, 8400 s and 8400 s, respectively. With temperature up to 300 °C, as-milled MgH2, 5 mol% doped and 7 mol% doped samples absorb 7.44 wt%, 5.09 wt% and 4.50 wt% hydrogen under 4 MPa hydrogen pressure, respectively. However, the doped samples have a faster absorption rate than that of undoped sample. Thus significant enhancement of MgH2 rehydrogenation kinetics performances could be obtained by doping NiFe2O4. For investigating the influence of addition of NiFe2O4 nanoparticles on the H2 storage properties MgH2 with increasing the cycle numbers, the cycle stability of ball-milled MgH2 doped with 7 mol% NiFe2O4 sample is investigated as shown in Figure S1. We could find that H2 storage amount of the MgH2+7 mol% NiFe2O4 sample is 4.53 wt% after ten cycles, which is a little bit lower than 4.54 wt% for the first absorption at 300 °C under H2 pressure of 4 MPa. The hydrogen storage capacity only decreases by 0.01 wt% after ten cycles as compared with that of the first time, indicating that the MgH2+7 mol% NiFe2O4 sample exhibits a good cycles properties.25,40 Figure 5 shows the pressure-composition-temperature (PCT) curves of MgH2 +7 mol% NiFe2O4 sample measured at 300 °C, 350 °C and 380 °C, respectively. As presented in Figure 5, the maximum hydrogen hydrogenation capacity at 300 °C is

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4.50 wt%, which is lower than the theoretical H2 storage amount of pure MgH2. This is due to the adding of NiFe2O4 nanoparticles in sample and the incompletely hydrogenated Mg particles. The maximum hydrogen absorption capacity at 350 °C and 380 °C are 4.54 wt% and 4.56 wt%, respectively. It can be also observed that the desorption plateau pressure at 300 °C, 350 °C and 380 °C is 0.232 MPa, 0.656 MPa and 1.201 MPa, respectively, which can be utilized to calculate the reaction formation enthalpy and entropy for MgH2+7 mol% NiFe2O4 sample according to the following equation:41

ln p =

∆H ∆S − RT R

(1)

Where p is the desorption plateau pressure, ∆H is the reaction formation enthalpy, ∆S is the reaction formation entropy, R is the gas constant and T is the testing temperature. Thus the reaction formation enthalpy and entropy during the dehydrogenation process can be achieved from the slope and intercept of van’t Hoff plot, as shown in Figure S2. The reaction formation enthalpy and entropy for MgH2+7 mol% NiFe2O4 sample are calculated to be 69.4 k/mol and 127.9 J/(mol⋅K), respectively, which is lower than the theoretical values of pure MgH2 (∆H=74.5 kJ/mol, ∆S=135.4 J/(mol⋅K))41 and also lower than those obtained through addition of various catalysts in MgH2 matirx.17,21,23 Through the above quantitative evidence on reducing the dehydrogenation thermodynamics barriers of MgH2 by addition of NiFe2O4 nanoparticles, it is reasonable to believe that NiFe2O4 nanoparticles can improve the dehydriding thermodynamics properties of MgH2, Mg2Ni plays the real role to improve the thermodynamics properties of MgH2, which also reported in

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literature 24. For analyzing MgH2 dehydrogenation mechanism after adding NiFe2O4 nanoparticles, the desorption activation energy (Ea) of pristine MgH2 and MgH2+7 mol% NiFe2O4 is gained from DSC measured at various raping rates (Figure S3) by using the Kissinger method.42 Figure S4 shows the Kissinger plots of pristine MgH2 and MgH2+7 mol% NiFe2O4 sample. The Ea value of the pristine MgH2 is 254.9 kJ/mol, while the Ea value of the MgH2+7 mol% NiFe2O4 sample is 59.6 kJ/mol. So, there is a significantly decline by 195.3 kJ/mol in Ea for magnesium hydride by doping NiFe2O4 nanoparticles, demonstrating that the desorption kinetics of MgH2 is significantly enhanced by adding NiFe2O4 nanoparticles. To further clearly demonstrate the NiFe2O4 catalytic effect on MgH2 desorption, the comparison of Ea for MgH2 doped with different catalysts is listed in Table 1.4,8,14,16,18,20,25,34,37-38,43-44 The NiFe2O4 doped sample has the lowest desorption activation energy, demonstrating the superiority of NiFe2O4 in enhancing the dehydrogenation properties of magnesium hydride as compared with other catalysts. In order to analyze the phase constitution of undoped/doped samples after ball milling, Figure 6 presents the XRD patterns of pristine MgH2, ball milled MgH2 and MgH2+5 mol% NiFe2O4 and MgH2+7 mol% NiFe2O4 samples after high energy ball milling. For the diffraction pattern of pristine MgH2 sample, all diffraction patterns belong to β-MgH2 except a few diffractions peaks of Mg, and no additional diffraction peaks are found. However, for the ball milled MgH2 sample, the γ-MgH2 phase is observed due to ball milling,45 and minor Mg phase still exists. Meanwhile, minor

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MgO phase is also detected, which results from the reaction between Mg and oxygen introduced during preparing sample. Compared with pristine MgH2, the peaks of ball milled MgH2 and NiFe2O4-doped samples are comparatively broadened due to the reduction in particle size. For ball milled samples doped with the NiFe2O4, not only NiFe2O4 phase is observed but also γ-MgH2 phase is detected for the MgH2+5 mol% NiFe2O4 sample. The β-MgH2 and Mg phases both still exist. With increasing the amount of NiFe2O4 nanoparticles to 7 mol%, the peak intensities of NiFe2O4 increase and peak intensities of MgH2 decline compared with the diffraction pattern of the MgH2+5 mol% NiFe2O4 sample, indicating that no reaction between MgH2 and NiFe2O4 occurs during ball milling process. For determining the phase structures of all samples after the dehydrogenation, the XRD patterns are carried out on ball milled MgH2, the MgH2+5 mol% NiFe2O4 and MgH2+7 mol% NiFe2O4 samples after complete desorption, as presented in Figure 7. For the diffraction pattern of ball milled MgH2, there are only the diffraction peaks corresponding to Mg phase except minor MgO phase after complete dehydrogenation. For the MgH2+5 mol% NiFe2O4 samples, the XRD patterns reflect that not only magensium and magnesium oxides phases, but also Mg2Ni, Fe7Ni3 and (Fe,Ni) phases are observed. Further raising the NiFe2O4 addition amount to 7 mol%, the diffraction peaks of MgO, Fe7Ni3, Mg2Ni and (Fe,Ni) phases gradually enhance, while the diffraction peaks of Mg decline, which demonstrates that MgH2 may react with NiFe2O4, and MgO, Fe7Ni3, Mg2Ni and (Fe,Ni) phases form during the heating procedure, and this reaction becomes more severely with the increase of NiFe2O4

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addition amount. Considering the significant improvement on the dehydrogenation performance of MgH2 by adding NiFe2O4 nanoparticles, it can be concluded that in-situ formed MgO, Fe7Ni3 and (Fe,Ni) acted as catalysts playing the important role in improving MgH2 desorption. For

the

investigation

on

the

phase

structures

during

dehydrogenation/rehydrogenation cycles after adding NiFe2O4 nanoparticles, the XRD patterns of MgH2+7 mol% NiFe2O4 after ten cycles is shown in Figure S5. As can be seen, MgO, Fe7Ni3 and (Fe,Ni) phases stably exist in the desorption/absorption cycles, which could further verify that MgO, Fe7Ni3 and (Fe,Ni) phases play the vital role in enhancing the hydrogen storage performances of NiFe2O4-doped samples. To prove the superior catalytic effect of NiFe2O4 on MgH2 resulting from forming MgO, Fe7Ni3 and (Fe,Ni) phases, the MgH2+7mol% (NiO+Fe2O3) sample is prepared to compare the catalytic mechanism with MgH2+7mol% NiFe2O4. Figure S6 presents the isothermal desorption curves of MgH2+7mol% (NiO+Fe2O3) sample at 300 °C and 350 °C under H2 pressure of 0.1 MPa, respectively. It releases 1.68 wt% H2 in 4000 s at 300 °C and 3.98 wt% hydrogen in 2700 s at 350 °C, which is 2.11 wt% and 0.30 wt% lower than those of MgH2+7 mol% NiFe2O4 in the same conditions, respectively. The average desorption rate for MgH2+7mol% (NiO+Fe2O3) is 0.0251 wt%/min and 0.0884 wt%/min at 300 °C and 350 °C, respectively, which is lower than those of MgH2+7 mol% NiFe2O4 (0.0632 wt%/min and 0.0713 wt%/min) in the same conditions. Figure S7 shows the isothermal absorption curves of MgH2+7 mol% (NiO+Fe2O3) at various temperatures. We can observe that 0.57 wt%, 1.99 wt%,

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3.11 wt% and 4.49 wt% are absorbed at room temperature, 50 °C, 100 °C and 300 °C, which decreases 1.49 wt%, 0.90 wt%, 0.21 wt% and 0.10 wt% hydrogen as compared with these of MgH2+7 mol% NiFe2O4 sample, respectively. The average absorption rate for MgH2+7mol% (NiO+Fe2O3) is 0.0032 wt%/min, 0.0111 wt%/min, 0.0222 wt%/min, and 0.1321 wt%/min at room temperature, 50 °C, 100 °C and 300 °C, respectively, which is lower than those of MgH2+7 mol% NiFe2O4 (0.0114 wt%/min, 0.0161 wt%/min, 0.0237 wt%/min, and 0.1324 wt%/min) in the same conditions. As a result, it is concluded that the superiority of NiFe2O4 on improving desorption performance of MgH2 is larger than that of NiO and Fe2O3 mixtures. Figure S8 displays the XRD patterns of MgH2+7mol% (NiO+Fe2O3) sample after ball milling and complete dehydrogenation. For the diffraction pattern of ball-milled sample as presented in Figure S8 (a), only β-MgH2, γ-MgH2, Mg, NiO and Fe2O3 phases are detected and no other diffraction peaks are observed, indicating that no reaction among MgH2 and NiO or Fe2O3 nanoparticles occurs during ball milling process. For the dehydrogenated samples, not only Mg and Mg2Ni phases but also MgO and Fe appear, which is different from that of dehydrogenated MgH2+7 mol% NiFe2O4 sample. The 7 mol% NiFe2O4-doped sample has the better hydrogen storage properties than that of 7 mol% (NiO+Fe2O3)-doped sample, implying that intermetallic Fe7Ni3 and (Fe,Ni) play a synergetic effect in enhancing MgH2 desorption performances. Only adding NiFe2O4 nanoparticles can obtain the excellent hydrogen storage properties for MgH2 rather than addition of NiO and Fe2O3 mixture. To further reveal the catalytic mechanism of NiFe2O4 nanopowders on improving

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the hydrogen storage performances of MgH2, it is necessary to observe the microstructure of updoped/doped sample. The FESEM images of pristine MgH2, ball milled MgH2 and MgH2+7 mol% NiFe2O4 are exhibited in Figure 8. The particle size of pristine MgH2 distributes between 60 µm and 120 µm. Nevertheless, the particle size of ball milled MgH2 and doped-MgH2 sample changes from 1 µm to 4 µm. Microscopically, the large particle shown in Figure 8(b,d) consists of lots of small particles with particle size between 400 nm and 900 nm, indicating that the agglomeration occurs among these small nanosized particles, and the distribution of average particle size become smaller and more uniform after ball milling by adding NiFe2O4 nanoparticles. The NiFe2O4 nanoparticles cannot be observed at the surface of MgH2 matrix due to their extremely small particle size or embeded to the surface of MgH2 matrix. For the purpose of obtaining the distribution of the constituent elements in the NiFe2O4 doped-MgH2 sample, the EDS mappings of MgH2+7 mol% NiFe2O4 after ball milling and complete dehydrogenation are shown in Figure S9 and S10, respectively. Through observation the distribution of Mg, O, Fe and Ni elements, all elements distribute uniformly, suggesting that the catalyzers are well blended with MgH2 after ball milling and complete dehydrogenation, and exists a good contact with MgH2 matrix resulting in the significantly enhanced hydriding/dehydriding properties of MgH2. According to the above analysis, the dehydrogenation performances of MgH2 doped with NiFe2O4 are notably improved due to the interaction between MgH2 matrix and NiFe2O4 nanoparticles during ball milling or dehydrogenation process. The distribution of dispersive and uniform catalyst in the matrix and the decline in

15



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Page 16 of 36

particle and crystal size result in a high surface defect density and more crystal boundaries, which introduce plenty of nucleation sites and much shorter H2 diffusion paths for absorption/desorption processes.2,20,46 Moreover, The intermetallic compounds are usually strong hydride formers. Adding intermetallic nanoparticles facilitate the surface adsorption and hydrogen dissociation. The diffusion of H2 though intermetallic is much faster than that in Mg lattice, which could also contribute to increasing the rate of hydrogenation/dehydrogenation. 24 TEM micrograph observation was performed to analyze the condition of NiFe2O4 were distributed on the matrix of MgH2 in a nanometer scale. Figure 9 (a) presents the bright field picture of TEM for ball-milled MgH2+7 mol% NiFe2O4 sample. Through observation Figure 9 (a), it is found that amounts of black particles with smaller sizes than 20 nm are inseted in the big bright particles. Figure 9 (c) and (d) show the EDS measurements of black region (A) and bright region (B), respectively. For the black regions, magnesium, oxygen, iron and nickel elements are identified by the EDS analysis, where the detested magnesium is derived from the basis material. However, for the bright region, only magnesium element is detected by the EDS measurement, which indicates that the big bright particles are magnesium hydride and the black particles correspond to NiFe2O4 doped in MgH2 matrix. Therefore, through the above analysis, it can be concluded that the NiFe2O4 nanoparticles are rather uniformly distributed in the MgH2 matrix, which leads to the enhancement of H2 storage performances. The result is in good accord with the EDS mapping analysis in Figure S9. The microstructure of 7 mol% NiF2O4 doped sample

16


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is further investigated by High Resolution TEM (HRTEM), as presented in Figure 9 (b). The NiFe2O4 component is proved to be existed through the measurement of the lattice spacings of the HRTEM figure, as presented in Figure 9 (d), which is in line with the XRD analysis. The NiFe2O4 nanocrystallines are evenly distributed in the magnesium hydride matrix during the ball milling process. Figure 10 presents the TEM micrograph and the corresponding SAED pattern of MgH2+7 mol% NiFe2O4 after complete dehydrogenation. The catalyst is still uniformly distributed in the MgH2 matrix after the complete dehydrogenation process, as shown in Figure 10 (a). The SAED is performed to confirm the structure of the nanoparticle. The diffraction rings could be well indexed with crystal planes of (002), (101), (103) for Mg, (102) for Mg2Ni, (200) for MgO, (110) for Fe7Ni3 and (102) for (Fe,Ni), which is in good agreement with XRD analysis. XPS of MgH2+7 mol% NiFe2O4 is performed to study the valence states of Ni and Fe species after ball milling and after complete dehydrogenation. Figure 11 presents the XPS curves of Ni 2p for milled and dehydrogenated samples. The photoemission spectrum of Ni 2p for milled sample lies in 855.40 eV and 873.80 eV 47

corresponding to NiFe2O4. The photoemission spectrum of Ni 2p for

dehydrogenated sample appears at 852.65 eV and 870.10 eV, which are attributed to the Mg2Ni/(Fe,Ni) and Fe7Ni3, respectively. Figure 12 presents the XPS curves of Fe 2p for milled and dehydrogenated samples. The photoemission spectrum of Fe 2p for milled sample is located in 710.50 eV and 724.00 eV corresponding to NiFe2O4. The photoemission spectrum of Fe 2p for dehydrogenated sample is observed at 707.20

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eV and 710.70 eV,

47

Page 18 of 36

which are due to the Fe7Ni3 and (Fe,Ni), respectively. XPS

results further demonstrate that NiFe2O4 nanoparticles can remarkably enhance the H2 storage performances of magnesium hydride by the formed intermetallic Fe7Ni3 and (Fe,Ni) catalytic species. Conclusions: The H2 storage performances of MgH2 are remarkably enhanced by adding NiFe2O4 nanoparticles. The non-isothermal dehydrogenation curves present that the initial dehydrogenation temperature of 7 mol% NiFe2O4 doped sample is 191 °C and release 4.48 wt% hydrogen, resulting in 250 °C decrease as compared with pristine magnesium hydride. The isothermal desorption results present that MgH2+7 mol% NiFe2O4 sample could desorb 3.79 wt% H2 within 3600 s at 300 °C under H2 pressure of 0.1 MPa, while ball milled MgH2 only releases 0.83 wt% H2 within 4000 s at 350 °C under H2 pressure of 0.1 MPa. It is worth notably that MgH2+7 mol% NiFe2O4 sample can emit 1.48 wt% hydrogen within 2400 s at 200 °C under H2 pressure of 0.1 MPa, whereas ball milled MgH2 sample can not desorb hydrogen at 300 °C. Rehydrogenation kinetics measurement results show that MgH2+7 mol% NiFe2O4 sample can absorb 2.06 wt% within 3 h near room temperature under H2 pressure of 4 MPa. However, as-milled MgH2 only absorbs 0.84 wt% hydrogen within 11700 s at 100 °C under H2 pressure of 4 MPa. The hydrogen storage capacity of MgH2+7 mol% NiFe2O4 sample only decrease 0.01 wt% after ten cycles, compared with that of the first time, indicating that the MgH2+7 mol% NiFe2O4 sample has a good cycles properties. According to the differential scanning calorimetry and Kissinger results,

18


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the desorption activation energy of the MgH2+7 mol% NiFe2O4 sample is 59.6 kJ/mol, leading

to

195.3

kJ/mol

decrease,

compared

with

pristine

MgH2.

The

pressure-composition-temperature curves show that the enthalpy and entropy of formation for MgH2+7 mol% NiFe2O4 sample during the desorption process are 69.4 kJ/mol and 127.9 J/(mol⋅K), respectively, which is lower than the theoretical value of pure MgH2 (∆H=74.5 kJ/mol, ∆S=135.4 J/(mol⋅K)). The high catalytic activity of NiFe2O4 is related with formed Fe7Ni3 and (Fe,Ni) phases from the reaction between NiFe2O4 and MgH2 during the desorption process. Therefore, in situ formed intermetallic Fe7Ni3 and (Fe,Ni) phases during the dehydrogenation process act as real catalyzers, which improves the H2 storage properties of 7 mol% NiFe2O4-doped MgH2 sample. Acknowledgment: The authors thank the financial support from the National High-Tech R&D Program (863 Program) of China. Fuqiang Zhai acknowledges China Scholarship Council (CSC) for providing the scholarship. Supporting Information Figure S1 shows hydrogenation curves of MgH2+7 mol% NiFe2O4 in different cycles at 300 °C under H2 pressure of 4 MPa. Figure S2 shows the van’t Hoff plot displaying equilibrium hydrogen pressures with temperatures for 7 mol% NiFe2O4 doped sample. Figure S3 presents the DSC curves of pristine MgH2 and MgH2 +7 mol% NiFe2O4 at raping rates of 4 °C/min, 7 °C/min, and 10 °C/min. Figure S4 shows the Kissinger curves for (a) the pristine MgH2 and (b) MgH2+7 mol% NiFe2O4. Figure S5 shows XRD pattern of MgH2+7 mol% NiFe2O4 after ten cycles. Figure S6 displays

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Page 20 of 36

isothermal dehydrogenation curves of MgH2+7mol% (NiO+Fe2O3) at (a) 300 °C and (b) 350 °C under H2 pressure of 0.1 MPa. Figure S7 presents isothermal rehydrogenation curves of MgH2+7mol% (NiO+Fe2O3) near (a) room temperature, (b) 50 °C, (c) 100 °C, and (d) 300 °C under 4 MPa pressure. Figure S8 presents XRD patterns of MgH2+7 mol% NiO+Fe2O3 (a) after ball milling and (b) after complete dehsorption. Figure S9 FESEM image of MgH2+7 mol% NiFe2O4 after ball milling and EDS maps of (a) Mg, (b) O, (c) Fe and (d) Ni. Figure S10 displays FESEM image of MgH2+7 mol% NiFe2O4 after complete dehydrogenation and EDS maps of (a) Mg, (b) O, (c) Fe and (d) Ni. This information is available free of charge via the Internet at http://pubs.acs.org. References: [1] Dalebrook, A. F.; Gan, W. J.; Grasemann, M.; Moret, S.; Laurenczy, G. Hydrogen Storage: Beyond Conventional Methods. Chem. Commun. 2013, 49, 8735-8751. [2] Gasan, H.; Celik, O. N.; Aydinbeyli, N.; Yaman, Y. M. Effect of V, Nb, Ti and Graphite Additions on the Hydrogen Desorption Temperature of Magnesium Hydride. Int. J. Hydrogen Energy 2012, 37, 1912-1918. [3] Pitt, M. P.; Paskevicius, M.; Webb, C. J.; Sheppard, D. A.; Buckley, C. E.; Gray, E. M. The Synthesis of Nanoscopic Ti Based Alloys and Their Effects on the MgH2 System Compared with the MgH2+0.01Nb2O5 Benchmark. Int. J. Hydrogen Energy 2012, 37, 4227-4237. [4] Baricco, M.; Rahman, M. W.; Livraghi, S.; Castellero, A.; Enzo, S.; Giamello, E. Effects of BaRuO3 Addition on Hydrogen Desorption in MgH2. J. Alloys Compd.

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2012, 536, S216-S221. [5] Jia, Y.; Sun, C. H.; Cheng, L. N.; Wahab, M. A.; Cui, J.; Zou, J.; Zhu, M.; Yao, X. D. Destabilization of Mg-H Bonding through Nano-Interfacial Confinement by Unsaturated Carbon for Hydrogen Desorption from MgH2. Phys. Chem. Chem. Phys. 2013, 15, 5814-5820. [6] Liu, T.; Qin, C. G.; Zhang, T. W.; Cao, Y. R.; Zhu, M.; Li, X. G. Synthesis of Mg@Mg17Al12 Ultrafine Particles with Superior Hydrogen Storage Properties by Hydrogen Plasma-metal Reaction. J. Mater. Chem. 2012, 22, 19831-19838. [7] Xin, G. B.; Yang, J. Z.; Zhang, G. Q.; Zheng, J.; Li, X. G. Promising Hydrogen Storage Properties and Potential Applications of Mg-Al-Pd Trilayer Films under Mild Conditions. Dalton Trans. 2012, 41, 11555-11558. [8] Mao, J. F.; Guo, Z. P.; Yu, X. B.; Liu, H. K.; Wu, Z.; Ni, J. Enhanced Hydrogen Sorption Properties of Ni and Co-catalyzed MgH2. Int. J. Hydrogen Energy 2010, 35, 4569-4575. [9] Puszkiel, J. A.; Larochette, P. A.; Gennari, F. C. Hydrogen Storage Properties of MgxFe (x: 2, 3, and 5) Compounds Produced by Reactive Ball Milling. J. Power Sources 2009, 186, 185-193. [10] Xin, G. B.; Yang, J. Z.; Wang, C. Y.; Zheng, J.; Li, X. G. Superior (De)hydrogenation Properties of Mg-Ti-Pd Trilayer Films at Room Temperature. Dalton Trans. 2012, 41, 6783-6790. [11] Zhou, C. S.; Fang, Z. Z.; Lu, J.; Zhang, X. Y. Thermodynamic and Kinetics Destabilization of Magnesium Hydrogen Using Mg-In Solid Solution Alloys. J. Am.

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Chem. Soc. 2013, 135, 10982-10985. [12] He, Y. P.; Zhao, Y. P. Improved Hydrogen Storage Properties of a V Decorated Mg Nanoblade Array. Phys. Chem. Chem. Phys. 2009, 11, 255-258. [13] Lu, H. B.; Poh, C. K.; Zhang, L. C.; Guo, Z. P.; Yu, X. B.; Liu, H. K. Dehydrogenation Characteristics of Ti- and Ni/Ti-catalyzed Mg Hydrides. J. Alloys Compd. 2009, 481, 152-155. [14] Patah, A.; Takasaki, A.; Szmyd, J. S. Influence of Multiple Oxide (Cr2O3/Nb2O5) Addition on the Sorption Kinetics of MgH2. Int. J. Hydrogen Energy 2009, 34, 3032-3037. [15] Li, F. B.; Jiang, L. J.; Du, J.; Wang, S. M.; Liu, X. P.; Zhan, F. Investigations Synthesis and Hydrogenation Properties of Mg-20 wt% Ni-1 wt% TiO2 Composite Prepared by Reactive Mechanical Alloying. J. Alloys Compd., 2008, 452, 421-424. [16] Gulicovski, J.; Lovre, Ž. R.; Kurko, S.; Vujasin, R.; Jovanović, Z.; Matović, L.; Novaković, J. G. Influence of Vacant CeO2 Nanostructured Ceramics on MgH2 Hydrogen Desorption Properties. Ceram. Int. 2012, 38, 1181-1186. [17] Gupta, R.; Agresti, F.; Russo, S. L.; Maddalena, A.; Palade, P.; Principi, G. Structure and Hydrogen Storage Properties of MgH2 Catalysed with La2O3. J. Alloys Compd. 2008, 450, 310-313. [18] Luo, Y.; Wang, P.; Ma, L. P.; Cheng, H. M. Hydrogen Sorption Kinetics of MgH2 Catalyzed with NbF5. J. Alloys Compd. 2008, 453, 138-142. [19] Ma, L. P.; Wang, P.; Cheng, H. M. Hydrogen Sorption Kinetics of MgH2 Catalyzed with Titanium Compounds. Int. J. Hydrogen Energy 2010, 35, 3046-3050.

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[20] Malka, I. E.; Pisarek, M.; Czujko, T.; Bystrzycki, J. A Study of the ZrF4, NbF5, TaF5, and TiCl3 Influences on the MgH2 Sorption Properties. Int. J. Hydrogen Energy 2011, 36, 12909-12917. [21] Pighin, S. A.; Capurso, G.; Russo, S. L.; Peretti, H. A. Hydrogen Sorption Kinetics of Magnesium Hydride Enhanced by the Addition of Zr8Ni21 Alloy. J. Alloys Compd. 2012, 530, 11-115. [22] Imamura, H.; Nakatomi, S.; Hashimoto, Y.; Kitazawa, I.; Sakata, Y.; Mae, H.; Fujimoto, M. Synthesis and Hydrogen Storage Properties of Mechanically Ball-milled SiC/MgH2 Nanocomposites. J. Alloys Compd. 2009, 488, 265-269. [23] Shao, H.; Federhoff, M.; Schüth, F. Hydrogen Storage Properties of Nanostructured MgH2/TiH2 Composite Prepared by Ball Milling under High Hydrogen Pressure. Int. J. Hydrogen Energy 2011, 36, 10828-10833. [24] Zhou, C. S.; Fang, Z. Z.; Ren, C.; Li, J. Z.; Lu, J. Effect of Ti Intermetallic Catalysts on Hydrogen Storage Properties of Magnesium Hydride. J. Phys. Chem. C 2013, 117, 12973-12980. [25] Fan, M. Q.; Liu, S. S.; Zhang, Y.; Zhang, J.; Sun, L. X.; Xu, F. Superior Hydrogen Storage Properties of MgH2-10 wt% TiC Composite. Energy 2010, 35, 3417-3421. [26] Barkhordarian, G.; Klassen, T.; Bormann, R. Fast Hydrogen Sorption Kinetics of Nanocrystalline Mg Using Nb2O5 as Catalyst. Scripta Mater. 2003, 49, 213-217. [27] Fukai, Y. The Metal-hydrogen System: Basic Bulk Properties; Springer: Berlin, 1993.

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[28] Dolci, F.; Chio, M. D.; Baricco, M.; Giamello, E. The Interaction of Hydrogen with Oxidic Promoters of Hydrogen Storage in Magnesium Hydride. Mater. Res. Bull. 2009, 44, 194-197. [29] Nieto, S.; Polanco, R.; Malherbe, R. R. Absorption Kinetics of Hydrogen in Nanocrystals of BaCe0.95Yb0.05O3-δ Proton-conducing Perovskite. J. Phys. Chem. C 2007, 111, 2809-2818. [30] Lei, Z. L.; Liu, Z. Y.; Chen, Y. B. Cyclic Hydrogen Storage Properties of Mg Milled with Nickel Nano-powders and NiO. J. Alloys Compd. 2009, 470, 470-472. [31] Song, M. Y.; Kwon, S.; Mumm, D. R.; Hong, S. H. Enhancement of Hydriding and Dehydriding Kinetics of Mg by the Addition of Ni and Nano-structured Fe2O3. Curr. Appl. Phys. 2009, 9, S118-S120. [32] Song, M. Y.; Kwak, Y. J.; Lee, B. S.; Park, H. R.; Kim, B. G. Effects of Ni, Fe2O3, and CNT Addition by Reactive Mechanical Grinding on the Reaction Rates with H2 of Mg-based Alloys. Int. J. Hydrogen Energy 2012, 37, 1531-1537. [33] Li, P.; Li, Z. L.; Wan, Q.; Li, X. Q.; Qu, X. H.; Volinsky, A. A. NiFe2O4 Nanoparticles Catalytic Effects of Improving LiAlH4 Dehydrogenation Properties. J. Phys. Chem. C 2013, 117, 25917-25925. [34] Li, P.; Wan, Q.; Li, Z. L.; Zhai, F. Q.; Li, Y. L.; Cui, L. Q.; Qu, X. H.; Vonlinsky, A. A. MgH2 Dehyddrogenation Properties Improved by MnFe2O4 Nanoparticles. J. Power Sources 2013, 239, 201-206. [35] Wan, Q.; Li, P.; Li, Z. L.; Zhai, F. Q.; Qu, X. H.; Volinsky, A. A. Improved Hydrogen Storage Performance of MgH2-LiAlH4 Composite by Addition of MnFe2O4.

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J. Phys. Chem. C 2013, 117, 26940-26947. [36] Wan, Q.; Li, P.; Li, Z. L.; Zhao, K. F.; Liu, Z. W.; Wang, L.; Zhai, F. Q.; Qu, X. Q.; Vonlinsky, A. A. NaAlH4 Dehydrogenation Properties Enhanced by MnFe2O4 Nanoparticles. J. Power Sources 2014, 248, 388-395. [37] 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. [38] Verón, M. G.; Troiani, H.; Gennari, F. C. Synergetic Effect of Co and Carbon Nanotubes on MgH2 Sorption Properties. Carbon 2011, 49, 2413-2423. [39] Song, M. Y.; Kwon, S. N.; Park, H. R.; Mumm, D. R. Effects of Fine Cr2O3 Addition on Mg’s Hydrogen-storage Performance. J. Ind. Eng. Chem. 2011, 17, 167-169. [40] Liu, D. M.; Fang, C. H.; Zhang, Q. A. Hyderogen Storage Properties of MgH2-(Sr,Ca)2AlH7 Composite. J. Alloys Compd. 2009, 485, 391-395. [41] Cuevas, F.; Korablov, D.; Latroche, M. Synthesis Structural and Hydrogenation Properties of Mg-rich MgH2-TiH2 Nanocomposites Prepared by Reactive Ball Milling under Hydrogen Gas. Phys. Chem. Chem. Phys. 2012, 14, 1200-1211. [42] Kissinger, H. E. Reaction Kinetics in Differential Thermal Analysis. Anal. Chem. 1957, 29, 1702-1706. [43] Milosevic, S.; Raskovic-Lovre, Z.; Kurko, S.; Vujasin, R.; Cvjeticanin, N.; Matovic, L.; Novakovic, J. G. Inﬂuence of VO2 Nanostructured Ceramics on Hydrogen Desorption Properties from Magnesium Hydride. Ceram. Int. 2013, 39,

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51-56. [44] Polanski, M.; Bystrzycki, J. Comparative Studies of the Influence of Different Nano-sized Metal Oxides on the Hydrogen Sorption Properties of Magnesium Hydride. J. Alloys Compd. 2009, 486, 697-701. [45] Mahmoudi, N.; Kaﬂou, A.; Simchi, A. Hydrogen Desorption Properties of MgH2-TiCr1.2Fe0.6 Nanocomposite Prepared by High-energy Mechanical Alloying. J. Power Sources 2011, 196, 4604-4608. [46] din, R. U.; Qu, X. H.; Li, P.; Zhang, L.; Wan, Q.; Iqal, M. Z.; Rafique, M. Y.; Farooq, M. H.; din, I. U. Superior Catalytic Effects of Nb2O5, TiO2, and Cr2O3 Nanoparticles in Improving the Hydrogen Sorption Properties of NaAlH4. J. Phys. Chem. C 2012, 116, 11924-11938. [47] http://srdata.nist.gov/xps/

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Figure captions Figure 1 Thermal desorption profiles of (a) pristine, (b) ball milled MgH2 and MgH2 doped with (c) 3, (d) 5, (e) 7 and (f) 9 mol% NiFe2O4 nanoparticles. Figure 2 DSC curves of (a) pristine MgH2 and (b) MgH2 doped with 7 mol% NiFe2O4 nanoparticles within temperature range of 50-500 °C at raping rate of 4 °C/min. Figure 3 The isothermal dehydrogenation curves of as-milled MgH2 and doped-MgH2 at various temperatures under H2 pressure of 0.1 MPa, (a) ball milled MgH2 desorption at 350 °C, MgH2+5 mol%NiFe2O4 desorption at (b) 200 °C, (d) 300 °C, and (f) 350 °C, MgH2+7 mol%NiFe2O4 desorption at (c) 200 °C, (e) 300 °C and (g) 350 °C. Figure 4 Isothermal rehydrogenation curves of as-milled MgH2 and doped-MgH2 at various temperatures under H2 pressure of 4 MPa, ball milled MgH2 hydrogenation at (a) 100 °C, (f) 300 °C, MgH2+7 mol% NiFe2O4 absorption near (b) room temperature, (c) 50 °C, (e) 100 °C, and (h) 300 °C, (d) MgH2+5 mol% NiFe2O4 absorption at 100 °C (g) 300 °C. Figure 5 Pressure-Composition-Temperature (PCT) curves of MgH2+7 mol% NiFe2O4 at 300, 350 and 380 °C, respectively. Figure 6 XRD of (a) pristine MgH2, (b) ball milled MgH2, ball-milled MgH2 doped with (c) 5 and (d) 7 mol% NiFe2O4 samples. Figure 7 XRD of (a) ball milled MgH2, (b) MgH2+5 mol% NiFe2O4 and (c) MgH2+7 mol% NiFe2O4 after complete dehydrogenation. Figure 8 FESEM images of (a) pristine MgH2, (b) ball milled MgH2, and (d)

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Page 28 of 36

ball-milled MgH2 doped with 7 mol% NiFe2O4, (c) a magnified image of (b), (e) a magnified image of (d). Figure 9 (a) TEM and (b) HRTEM micrographs, EDS ((c). black region, (d). bright region) of MgH2+7 mol% NiFe2O4 after ball milling. Figure 10 (a) TEM micrograph and (b) the corresponding SAED pattern of MgH2+7 mol% NiFe2O4 after complete dehydrogenation. Figure 11 Narrow scan Ni 2p XPS of MgH2+7 mol% NiFe2O4 (a) after ball milling, (b) after complete desorption. Figure 12 Narrow scan Fe 2p XPS of MgH2+7 mol% NiFe2O4 (a) after ball milling, (b) after complete desorption. Table captions Table 1 Desorption activation energy of MgH2 doped with various catalysts, calculated by the Kissinger method.

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Hydrogen desorption (wt%)

0 (c)

-1

(a)

(b)

-2 -3

as-received MgH2

-4

MgH2+3mol% NiFe2O4

(e) (f)

as-milled MgH2 MgH2+5mol% NiFe2O4

-5

MgH2+7mol% NiFe2O4

(d)

MgH2+9mol% NiFe2O4

-6 -7 -8 100

200

300 400 Temperature (°C)

500

Figure 1 411.2 °C

(a)

Heat flow (mW/mg)

Exothermic

100


400

500

(b) Exothermic Heat flow (mW/mg)

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


289.5 °C

100


400

500

Figure 2

29



MgH2+7mol% NiFe2O4 350 °C

Hydrogen desorption (wt%)

4

(g)


(f)


3 (e)

MgH2+5mol% NiFe2O4 300 °C 2

(d) MgH2+7mol% NiFe2O4 200 °C

(c) 1


(b) (a)

as-milled MgH2 350 °C

0 0

1000

2000 Time (s)

3000

4000

Figure 3 as-milled MgH2 300 °C

Hydrogen absorption (wt%)

7 (h)

6 5

(g)

4

(f)

MgH2+5mol% NiFe2O4 300 °C MgH2+7mol% NiFe2O4 300 °C MgH2+5mol% NiFe2O4 100 °C

(e)

3


(d) 2 1


(c) (b)

MgH2+7mol% NiFe2O4 RT

(a)

as-milled MgH2 100 °C

0 0

2000

4000

6000 Time (s)

8000

10000

12000

Figure 4 10

380 °C Pressure (MPa)

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 30 of 36

1

350 °C 300 °C

0.1

0.01 0

1

2 3 Hydrogen capacity (wt%)

4

5

Figure 5 30


Page 31 of 36

Intensity (a.u.)

--β-MgH2

--γ-MgH2 --Mg --NiFe2O4 --MgO

(d)

(c)

(b)

(a) 20

40

60

80

2θ (degrees)

Figure 6 --Mg --MgO --Mg2Ni --Fe7Ni3 --(Fe,Ni)

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) 20

40

60

80

2θ (degrees)

Figure 7

31



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 32 of 36

(a)

(b)

(c)

(d)

(e)

Figure 8 (a)

32


Page 33 of 36

(a)

(b) (220)

A

B

Mg

(d)

O

Fe Ni Fe

Ni Fe

0

Mg

Intensity (a.u.)

(c) 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


2

4 6 Energy (keV)

Ni

8

10

0

2

4 6 Energy (keV)

8

10

Figure 9 (a)

(b)

Figure 10

33



Ni 2p

Intensity (a.u.)

(a)

(b)

890

880

870

860

850

Binding Energy (keV)

Figure 11 Fe 2p

(a)

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 34 of 36

(b)

750

740

730

720

710

700

Binding Energy (keV)

Figure 12

34


Page 35 of 36

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 1 Catalyst BaRuO3 VO2(B) Cr2O3 TiO2 Fe2O3 Fe3O4 Nb2O5 ZrF4 NbF5 TaF5 TiCl3 CoCl2 NiCl2 NbF5 CeO2 TiC C Ti0.4Cr0.15Mn0.15V0.3 MnFe2O4 NiFe2O4

Ea (kJ/mol) Before doping 140 160 206 240 240 240 240 160 160 120 191.27 179.7 254.9 254.9

After doping 90 139 84 94 124 115 197 77 83 97 97 121.3 102.6 90 60 144.62 136 71.2 64.6 59.6

References [4] [43] [44] [44] [44] [44] [14] [20] [20] [20] [20] [8] [8] [18] [16] [25] [38] [37] [34] This work

35



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

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TOC

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Superior Catalytic Effect of Nickel Ferrite Nanoparticles in Improving

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