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Materials Science Forum Vols. 654-656 (2010) pp 2847-2850 Online available since 2010/Jun/30 at www.scientific.net © (2010) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.654-656.2847
The Effects of Cryogenic Milling and Catalytic Additives on the Hydrogen Desorption Behaviour of Nanostructured MgH2 Chan Yeol Seoa, Xiaodong Wub and Kiyonori Suzukic Department of Materials Engineering, Monash University, Clayton, Victoria 3800, Australia a
, [email protected]
, c [email protected]
Keywords: MgH2, hydrogen storage, kinetics, DTA, TGA, mechanical milling
Abstract. It is well known that catalytic additives and mechanical milling are effective in improving hydrogen desorption kinetics of MgH2. In this study, the effect of catalytic additives including BaCa1-xNdxO3-δ (BCN) on the desorption behavior of MgH2 was investigated. It was found that BCN can improve the desorption kinetics, but not as effective as other known additives such as Nb2O5. The effect of milling temperature was also studied. It was found that the cryogenic milling is not as effective as room temperature milling primarily due to the inhomogeneous particle size distribution. Introduction Magnesium hydride MgH2 is one of the most attractive hydrogen storage materials because it is directly formed from the reaction of bulk Mg with gaseous hydrogen and reaches a high hydrogen capacity (7.6 wt.%). The hydrogen sorption kinetics of Mg or Mg-based alloys is sluggish and this remains a challenging problem yet to be resolved . Hence, the focus of alloy development in Mg-based hydrogen storage alloys is to improve the absorption and desorption kinetics. Promising approaches to this problem reported to date include additions of metal oxides  and nanoscale microstructural refinement by ball milling . It was reported that the desorption temperature of hydrogen in ball milled MgH2 could be reduced by up to 150 K through additions of Nb2O5 . Lattice defect-type ACa1-xBxO3-δ (A = Sr and Ba; B = Nd, Sm, Gd, Dy, Y and Yb) ceramics are well known as proton conducting materials which exhibit high hydrogen solubility and mobility under a hydrogen atmosphere. The substitution of A site or B site in the perovskite with lower valent cations can produce oxygen defects. The dopant and oxygen defect concentrations have a significant effect on the proton conductivity of these ceramics . The proton conductive ceramic catalysts in Mg nanocomposites may improve the hydrogen desorption kinetics of MgH2 if triple phase boundaries of the hydrogen gas, the proton conductive ceramic catalyst and the MgH2 are created by mechanical milling due to enhanced hydrogen diffusion at the surfaces of MgH2 nanograins. Milling temperature often plays an important role in the microstructural properties of mechanically alloyed materials. It is know that grain refinement occurs when milled at cryogenic temperatures. Furthermore, a recent study on a LiNH2 - LiH2 system showed that low temperature milling can greatly enhance the desorption kinetics . Therefore, cryogenic milling of MgH2 based alloys could be an effective approach to reduce H desorption temperatures. In this report, we have studied the effect of BCN addition on the desorption behavior of H2 in MgH2. The effect of Nb2O5 was also studied for comparison. In addition, we have prepared nanostructured MgH2 and MgH2/Nb2O5 powders by ball milling at room and cryogenic temperatures in order to clarify the effect of cryogenic milling on the desorption kinetics. Experimental Procedures The starting material was commercial magnesium hydride powders with a purity of 99 wt.% from Sigma–Aldrich. The powders were milled with a Nisshin Giken Super MISUNI vibration mill with a ball to weight ratio of 10:1 at 10.5 Hz for different times. The samples before and after milling were always handled in a glove box filled with purified Ar gas so as to minimize the oxidation of the samples. Low temperature milling was conducted by immersing the milling vial in liquid nitrogen. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 184.108.40.206-13/01/11,06:54:01)
Commercial Nb2O5 from Sigma-Aldrich and BCN were added to the MgH2 powders and also ball milled; the ratio between the additives and MgH2 was 17%. The particle size was examined by Phenom scanning electron microscopy (SEM). The powder samples were characterized on a Rigaku RINT2000 X-ray diffractometer with the Cu Kα radiation. The hydrogen desorption behaviors were examined by Perkin-Elmer TGA7 thermogravimetric and DTA7 differential thermal analyzers. Results and Discussion The X-ray diffraction (XRD) profiles of MgH2/BCN and MgH2/Nb2O5 powders are shown in Fig. 1. All the reflection peaks on the patterns are attributable to the phases of the initial powder mixtures before milling and no new phases appear after milling for 100 h. This suggests that the reactions between MgH2 and the catalytic additives are limited. Substantial peak broadening is observed for both MgH2/BCN and MgH2/Nb2O5 powders after ball milling for 100h, indicating that the average grain size is greatly reduced by ball milling.
Fig. 1 XRD patterns of MgH2/BCN and MgH2/Nb2O5 [(○) Nb2O5, (*) β-MgH2 and (Δ) BCN].
Fig. 2 DTA (a) and TGA (b) curves of MgH2, MgH2/BCN and MgH2/Nb2O5. Figure 2 shows the DTA and TGA curves for MgH2, MgH2/Nb2O5 and MgH2/BCN milled for 100 h. An endothermic peak is seen at 664 K on the DTA curve of MgH2, reflecting the hydrogen desorption reaction which is also evident in the weight loss on the corresponding TGA curve. The endothermic peak on the DTA curves for MgH2/Nb2O5 and MgH2/BCN are both clearly lower than that 664 K, showing the effectiveness of these additives in accelerating the desorption kinetics, although the effect of BCN is weaker than Nb2O5. The TGA results show that the weight loss after desorption of H2 is 4.9% for MgH2, 4.5% for MgH2/BCN and 4.7% for MgH2/Nb2O5. Backscattered SEM images of MgH2/Nb2O5 and MgH2/BCN powders milled for 100 h are shown in Fig. 3. It can be found that BCN and Nb2O5 particles, which appear brighter in the image due to the larger atomic numbers of Nb and Nd, are randomly distributed in the MgH2 matrix. In comparison to Nb2O5 particles, BCN particles are larger and the population density is limited. This could be the reason for the lower catalytic effect of BCN as compared with that of Nb2O5.
Materials Science Forum Vols. 654-656
Fig. 3 SEM micrographs of (a) MgH2/BCN and (b)MgH2/Nb2O5 milled for 100 h.
Fig. 4 XRD patterns of MgH2 and MgH2/Nb2O5 milled for 6 h at different temperatures [(o) Nb2O5, (*) β-MgH2 and (Δ) α-MgH2]. Figure 4 shows the XRD patterns of MgH2 and MgH2/Nb2O5 milled for 6 h at room temperature and the cryogenic temperature. In Fig. 5 we also show the DTA and TGA curves acquired from these four samples. Although no obvious effect of the milling temperature on the phase constitution is evident on the XRD patterns, the samples milled at the cryogenic temperature show higher desorption temperature for both MgH2 and MgH2/Nb2O5 samples judging from the DTA curves in Fig.4 (a). The TGA results showed that the weight loss of MgH2 milled at room temperature is 5.8 %, while the value for the sample milled at low temperature is 5.2 %. The weight losses of the MgH2/Nb2O5 powders milled at room temperature and the cryogenic temperature are 5.5 % and 4.8 %, respectively. Thus, the low temperature milling is not as effective as room temperature milling.
Fig.5 Effect of milling temperatures on the (a) DTA and (b) TGA results of MgH2 and MgH2/Nb2O5. Figure 6 shows the particle size distributions of the MgH2/Nb2O5 powders prepared at room temperature and low temperature. The distributions are given in two different manners; Figs. 6(b) and (e) show the simple population density while Figs. 6(c) and (f) indicate the volume fractions in the vertical axis. Given the fact that the hydrogen storage capacity of each particle is directly related to the volume, the contributions from the large particles to the entire desorption rate could be significant relative to those from small particles even if the number density of the large particles is small. Hence,
attention must be paid to the volume fractions of the particles when the overall desorption kinetics is discussed. Although, the distributions based on the simple population density for the two samples look similar, it is clear in Figs. 6(c) and (f) that the volume fractions of large particles are far greater in the samples milled at the cryogenic temperature. Since the hydrogen desorption kinetics in MgH2 is accelerated by reducing the particle size which results in a larger surface to volume ratio, the slower kinetics observed for the MgH2/Nb2O5 powders prepared at the cryogenic temperature can be attributed to the higher volume fractions of the large particles.
Fig.6 (a) The SEM image, (b) particle size distribution, and (c) particles volume fraction distribution of MgH2/Nb2O5 milled for 6h at room temperature. (d) The SEM image, (e) particle size distribution, and (f) particles volume fraction distribution of MgH2/Nb2O5 milled for 6h at cryogenic temperature. Summary The effect of low temperature milling and BCN addition on the hydrogen desorption behavior of MgH2 was studied. It was found that the BCN additives are effective in accelerating desorption kinetics. However, it is not as effective as Nb2O5. The low temperature milling is not as effective as room temperature milling in improving the desorption kinetics because of the inhomogeneous particle size. References    
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PRICM7 doi:10.4028/www.scientific.net/MSF.654-656 The Effects of Cryogenic Milling and Catalytic Additives on the Hydrogen Desorption Behaviour of Nanostructured MgH2 doi:10.4028/www.scientific.net/MSF.654-656.2847