J. Phys. Chem. B 2006, 110, 7845-7850
7845
Nb2O5 “Pathway Effect” on Hydrogen Sorption in Mg Oliver Friedrichs,*,† Juan C. Sa´ nchez-Lo´ pez,† Carlos Lo´ pez-Cartes,† Thomas Klassen,‡ Ruediger Bormann,‡ and Asuncio´ n Ferna´ ndez† Instituto de Ciencia de Materiales de SeVilla, AVda. Ame´ rico Vespucio 49, 41092 SeVille, Spain, and Institute for Materials Research, GKSS Research Centre, Max-Planck-Strasse 1, D-21502 Geesthacht, Germany ReceiVed: December 22, 2005; In Final Form: March 7, 2006
In the present work we investigate the hydrogen sorption mechanism in a MgH2/Nb2O5 composite and analyze why Nb2O5 could strongly improve hydrogen sorption kinetics in magnesium. Hereby we make use of the fact that Nb2O5 nanoparticles are able to reduce the milling time significantly with the achievement of excellent sorption kinetics, and can so exclude effects occurring at long-term milling that make difficult the study of the mechanism. On the basis of extensive chemical, crystalline, and microstructural characterization of the MgH2/Nb2O5 nanopowder system, a “pathway model” is proposed, which explains the kinetic hydrogen sorption improvement by a formation of pathways of niobium oxide species with lower oxidation state that facilitate the hydrogen transport into the sample. This mechanism is shown to be supported by additional oxidation experiments, which indicate increased oxygen diffusion through these pathways.
1. Introduction
2. Experimental Procedures
Magnesium as a light, abundant, and low-cost metal with a high storage capacity of hydrogen represents a very attractive material for hydrogen storage. Its limitation for practical application however lies 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. By using magnesium hydride in nanocrystalline form obtained by mechanical milling4 and transition-metal oxides as milling additives,5,6 big improvements of the sorption kinetics have been achieved. Nb2O5 has been shown to behave as one of the best additives7,8 at the present time. However, the mechanism of hydrogen sorption improvement by Nb2O5 is still unknown. The long milling time necessary to get good hydrogen sorption kinetics results is disadvantageous for understanding the process involved due to changes in the microstructure9,10 or a particle size reduction and formation of nanoparticles of MgH2 by mechanical milling11 with the additive, for example. In a recent work we reported that the use of Nb2O5 nanoparticles reduces drastically the milling time, leading to excellent hydrogen sorption kinetics.12 Therefore, Nb2O5 nanoparticles have enabled us to study systems with excellent kinetic properties under exclusion of long-term milling effects. In the present work we show a study of the hydrogen sorption mechanism by an extensive microstructural, chemical, and crystalline characterization of samples based on the MgH2/ Nb2O5 system. On the basis of this study a model is proposed to explain the improvement in hydrogen sorption kinetics by the Nb2O5 additive. Additional oxidation experiments on a system based on microcrystalline Nb2O5 are presented, which support this model.
2.1. Sample Preparation. MgH2 powder with a purity of 95% (the rest being Mg) was purchased from Goldschmidt AG. As milling additives, Nb2O5 micropowder (Nb2O5-micro) purchased from Sigma Aldrich with >99.99% purity and a particle size >100 nm and Nb2O5 nanopowder (Nb2O5-nano) supplied by Nanotechnologies Inc. with a mean particle size of 15 nm were used. The samples were milled MgH2 (20 h), additionally milled with and without 10 wt % additives (additional milling times: 15 min, 5 h, 50 h), a physical mixture of milled MgH2 and 10 wt % Nb2O5 nanopowder (MgH2/Nb2O5-PM), and MgH2 samples milled for 200 h with about 17 wt % (2 mol %) Nb2O5 before and after one cycle of hydrogen sorption. A hydrogen sorption cycle consists of hydrogen desorption at 300 °C and 0.1 kPa and hydrogen absorption at 300 °C and 1 MPa. The milling was performed under an argon atmosphere (
