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J. Phys. Chem. C 2007, 111, 10700-10706
In Situ Energy-Dispersive XAS and XRD Study of the Superior Hydrogen Storage System MgH2/Nb2O5 Oliver Friedrichs,*,†,§ Diego Martı´nez-Martı´nez,† Gemma Guilera,‡ Juan Carlos Sa´ nchez Lo´ pez,† and Asuncio´ n Ferna´ ndez† Instituto de Ciencia de Materiales de SeVilla (CSIC-UniVersidad SeVilla), AVenida Ame´ rico Vespucio 49, 41092 SeVille, Spain, and European Synchrotron Radiation Facility, F-38043 Grenoble 09, France ReceiVed: NoVember 15, 2006; In Final Form: May 3, 2007
By in situ energy-dispersive X-ray absorption spectroscopy (EDXAS) and X-ray diffraction (XRD), we analyzed the evolution of niobium in a MgH2/Nb2O5 system based on high-energy ball milling during hydrogen cycling. The high time resolution of the EDXAS method allowed us to monitor fast sample changes during this process. Thereby, we demonstrated that the Nb2O5 is already partially reduced during the milling process with the MgH2. Further reduction occurs during the heating and cycling processes, in which a lower limit of oxidation state is reached. Hereby, a reaction between the niobium oxide and the Mg/MgH2 leads to a decrease of crystalline Nb2O5 and the formation of a ternary oxide phase MgxNbyO. During the cycling processes a repetitive Nb oxidation-reduction process was observed, which may indicate hydrogen diffusion along the ternary oxide by the formation of metastable niobium hydrides. This points to a mechanism of kinetic sorption improvement by diffusion of hydrogen through pathways of ternary Mg-Nb oxides, which may also reduce the activation energy of the Mg-MgH2 transition.
1. Introduction Magnesium is a very attractive material for hydrogen storage. It is an abundant and low-cost metal and shows a very high volumetric storage capacity in its hydride form, like many other metal hydrides. Its great advantage however lies in its low weight. As one of the lightest metals, it is able to store a high amount of hydrogen without increasing much the overall weight of the storage material. Hereby it shows a theoretical storage capacity in weight of 7.6%. Its limitation for practical application lies however in the slow hydrogen sorption kinetics and in the high thermodynamic stability of its hydride.1 A strong tendency of magnesium to oxidation, which hinders the hydrogen sorption,2 and the slow diffusion of hydrogen through MgH23 represent two factors impeding faster hydrogen sorption kinetics. Major progress toward technical application has been achieved by using magnesium hydride in nanocrystalline form obtained by mechanical milling4 and by using transition metal oxides during the milling process as additives.5,6 This has led to big improvements in the hydrogen sorption kinetics. In particular, Nb2O5 has been shown to behave as one of the best additives7,8 at the present time. However, the mechanism of the hydrogen sorption improvement by Nb2O5 is still not understood. Hereby, the long milling time necessary to get good hydrogen sorption kinetics is disadvantageous for understanding the process involved. Many different effects, for example, changes in the microstructure9,10 or a particle size reduction and formation of * Corresponding author. Telephone: (+41) 44 823 4153. Fax: (+41) 823 4022. E-mail:
[email protected]. † CSIC-Universidad Sevilla. ‡ European Synchrotron Radiation Facility. § Present address: Laboratory for Hydrogen & Energy, Empa, Swiss Federal Laboratories for Materials Testing and Research, U ¨ berlandstrasse 129, CH-8600 Du¨bendorf, Switzerland.
nanoparticles of MgH2 by mechanical milling with the additive,11 may be responsible for the improvement of the kinetics. In recent work we showed that the use of Nb2O5 nanoparticles (ca. 15 nm) drastically reduces the milling time (by more than a factor of 200) leading to excellent hydrogen sorption kinetics.12 The short additional milling time (15 min) allowed us to study the Nb2O5/MgH2 system with excellent kinetic properties under exclusion of long-term milling effects, i.e., changes of microstructure, such as particle size or crystal size. Extensive microstructural, chemical, and crystalline characterization led us to propose a pathway model, in which a reaction between niobium oxide and Mg/MgH2 forms pathways of Mg-Nb oxides14 in the sample which facilitate the hydrogen release by improving the hydrogen diffusion. Through these pathways the hydrogen can enter the sample, possibly by formation of a metastable niobium hydride species6 using the ability of Nb to change its oxidation state in a range from 0 to +5. The formation of the metastable niobium hydrides along these pathways may also lower the activation energy of the Mg hydride dissociation and so could also improve the desorption kinetics. To further investigate the evolution and role of the niobium in the cycling process, we performed energy-dispersive X-ray absorption spectroscopy (EDXAS)15 and in situ X-ray diffraction (XRD)16 studies of a MgH2/Nb2O5 nanopowder system under hydrogen cycling, which are presented in the present work. These studies allowed us to monitor the changes of the oxidation state of the niobium and sample composition in situ during all processes thanks to the high time resolution of the EDXAS method. In this way, the influences of the different cycle steps could be investigated and the role of the niobium in the improvement of the sorption kinetics could be elucidated. 2. Experimental Section 2.1. Sample Preparation. The samples under investigation were prepared from MgH2, which had been mechanically milled
10.1021/jp0675835 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/22/2007
EDXAS and XRD Study of MgH2/Nb2O5
J. Phys. Chem. C, Vol. 111, No. 28, 2007 10701
Figure 1. Schematic illustration of the experimental setup for hydrogen cycling. The valves are indicated by “On/Off” (two-position valve) and “R” (remote-controlled two-position valve).
for 20 h. This MgH2 was physically mixed with 10 wt % nanocrystalline Nb2O5 powder (PM sample) and additionally ball milled for 50 h (BM sample) to ensure a good homogeneity of the sample with a well-distributed and dispersed Nb2O5 necessary for the experiment. These two samples show very different kinetic behaviors, whereby the BM sample is more than 1 order of magnitude faster in hydrogen desorption than the PM sample as we presented in a prior work.12 MgH2 powder with a purity of 95% (the rest being Mg) was purchased from Goldschmidt AG, and the Nb2O5 powder used as milling additive was supplied by Novacentrix Corp., where it was produced by a pulsed plasma technique17 with a mean particle size of 15 nm. The milling was performed under argon atmosphere (
