Hydrogen diffusion and effect of grain size on hydrogenation kinetics in magnesium hydrides

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Z.H. Zhu Australian Research Council (ARC) Center for Functional Nanomaterials, University of Queensland, QLD 4072, Australia

H.M. Cheng National Laboratory of Materials Science, Institute of Metals Research, Shenyang 110015, China

G.Q. Lu Australian Research Council (ARC) Center for Functional Nanomaterials, University of Queensland, QLD 4072, Australia (Received 22 May 2007; accepted 23 July 2007)

Hydrogenation and dehydrogenation of metal hydrides are of great interest because of their potential in on-board applications for hydrogen vehicles. This paper aims to study hydrogen diffusion in metal hydrides, which is generally considered to be a controlling factor of hydrogenation/dehydrogenation. The present work first calculated temperature-dependent hydrogen diffusion coefficients by a theoretical model incorporated with experimental data in a Mg-based system and accordingly the activation energy. The grain size effect on diffusion in nanoscale was also investigated. I. INTRODUCTION

Economical and safe hydrogen storage is critical to the viability of a future hydrogen economy. Magnesium and magnesium-based alloys have been considered among the most promising materials for hydrogen storage because of their low cost and potentially high capacity, and they have been extensively studied in recent years.1–18 Several novel approaches have successfully been proposed to improve the hydrogen absorption and desorption kinetics, which is one of the key limitations of application of Mg-based materials, such as nanostructuring and defect inducing by the ball-milling technique,1–7 doping with catalytic elements,4–12 alloying,13,14 and hybriding with other hydrogen-storage materials.15–17 Yao et al.18 reported that enhancement of diffusion by carbon nanotubes (CNTs) additives increased the absorption kinetics significantly at low temperatures, which indicated that the hydrogenation of MgH2 is diffusion limiting at low temperatures. Recently, the dissociative behavior of molecular hydrogen at the surface of magnesium, which is believed to be an important step for hydrogenation, was studied by the first-principle calculations.19–22 Results also show that diffusion of the dissociated hydrogen atoms into the bulk magnesium is the controlling step in a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0063 336

J. Mater. Res., Vol. 23, No. 2, Feb 2008

the formation of hydrides. However, the diffusion behavior of the hydrogen atoms and the effect of grain size on hydrogenation have not been reported so far. This work aims to develop a mathematical model that describes the diffusion of hydrogen atoms in magnesium and firstly predicts the temperature-dependent hydrogen diffusion coefficient in magnesium hydrides.

II. MODEL DESCRIPTION

With assumptions of: (i) the geometry of grains inside Mg particles is spherical; (ii) the hydrogen diffusion along grain boundaries is much faster than diffusion inside grains so that the initial hydrogen concentration around each grain is same and c

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