Elsevier

Journal of Power Sources

Volume 183, Issue 2, 1 September 2008, Pages 693-700
Journal of Power Sources

Enhanced hydrogen absorption kinetics for hydrogen storage using Mg flakes as compared to conventional spherical powders

https://doi.org/10.1016/j.jpowsour.2008.05.033Get rights and content

Abstract

Practical challenges of metal hydrides for hydrogen storage such as magnesium hydride, lie predominantly in improving hydrogen absorption kinetics and capacity. Approaches to improving kinetics of Mg commonly include changing particle geometry, reducing crystallite size and coating with catalytic transition metals such as Ni. This study is aimed at improving the hydrogen storage kinetics and capacity of Mg by a novel approach utilizing high aspect ratio powders (thin metal flakes with large diameters) coated with a Ni nano-catalyst. A high speed orbiting ball media (HSOBM) processor was utilized to fabricate the flake-shaped materials. Mg flakes effectively coated with Ni nano-catalyst using dry mechanical coating were found to possess more favorable hydrogen absorption/desorption characteristics and improved hydrogen storage capacity than traditional spherical particles. Geometric shape was shown to be a vital factor in hydrogen absorption kinetics and its effects proved to be dominant over those of crystallite size.

Introduction

Among the various light metals and alloys that are capable of absorbing/desorbing large amounts of hydrogen, magnesium has been considered a promising candidate for solid hydrogen storage due to its high volumetric/gravimetric capacity, ease of availability and low cost [1], [2]. The hydrogenation process starts from physisorption of hydrogen molecules onto the surface of magnesium particles, followed by dissociation into hydrogen atoms. The hydrogen atoms thus obtained diffuse into magnesium lattices to initiate nucleation and growth of magnesium hydride [3], [4], [5]. However, the major disadvantage of utilizing pure magnesium is its slow absorption of hydrogen, which is due to its low affinity for hydrogen physisorption, and the formation of a dense magnesium hydride layer [1], [4], [6]. The hydride layer becomes a resistive barrier for subsequent hydrogen diffusion into the bulk and therefore results in limited material utilization. This ultimately becomes a crucial rate-limiting mechanism in the hydrogenation process [5], [7], [8].

Various attempts have been undertaken to chemically improve the kinetics of Mg by incorporating transition metals such as Ni, V, Fe and Ti [8], [9], [10], [11], [12], [13], [14], [15]. The addition of transition metals with high affinities for chemisorption results in interfacial catalysis of H2 dissociation thus reducing the activation energy required for dissociation of hydrogen molecules [16], [17]. The resultant hydrogen atoms can more easily diffuse along grain boundaries of magnesium than H2 [18], [19]. Consequentially, it has been demonstrated that the onset temperature for hydrogen absorption can be lowered by approximately 170 °C by adding a small amount of Ni onto Mg [9]. Studies report that a homogeneous distribution of nano-catalysts on the metal drastically reduces the activation energy of hydrogen diffusion and results in faster formation of metal hydride even in the presence of oxides and hydroxides [20], [21], [22], [23]. However, it should be noted that when hydrogen diffusion was the rate-limiting mechanism, excessive Ni wt.% loading showed negligible impact on H2 absorption rate and capacity [8]. In addition, the particle diameter of the catalyst has been also recognized as an influential parameter. Varin et al. [15] investigated the effect of Ni particle size (micro, submicron and nano-sizes) on the hydrogen absorption/desorption kinetics of 44 μm Mg powder. The addition of nano-Ni was observed to greatly improve the hydrogen absorption/desorption rate as compared to micron/submicron sized catalysts.

The kinetics of hydrogen absorption and desorption of metal hydrides have also been improved mechanically by increasing the specific surface area and number of grain boundaries, reducing the crystallite size, and altogether enhancing diffusion of hydrogen atoms within the metal bulk leading to increased hydride formation [9], [10], [11], [12], [13], [15], [24], [25], [26], [27], [28]. Most past research studies such as these have been conducted based on magnesium particles processed by conventional ball milling due to their ability to change the microstructure of the magnesium.

A better understanding of the overall process of hydrogenation is needed to further develop the practicality of magnesium as a storage option. In the beginning of the process, nucleation of MgH2 is the rate-liming mechanism; however, hydrogen diffusion through the impermeable hydride layer eventually becomes the rate-liming mechanism once the hydride layer reaches a critical thickness. The maximum penetration depth for nucleation-limited condition for a 44 μm sample was calculated to be 6 μm [8]. Since a short fuelling time is critical to practical applications, it is advantageous to accomplish the majority of hydrogen uptake while it is in the most rapid nucleation-limited stage. In order to reduce the limiting effect of diffusion through the compact MgH2 layer and thereby extend the nucleation-limited stage, particles with high specific surface area are favored. This is because they increase the number of surface reaction sites and decrease the necessary diffusion depth required to utilize the entire bulk volume. Nano-sized Mg particles and thin Mg films have been used to minimize this problem; however, practical production costs of these materials may be a concern.

The effect of geometry on the hydrogen absorption kinetics of magnesium particles has not yet been explored in depth. Developments of new material production technologies that improve material utilization and increase production efficiency are very much sought after. Recently, a high speed orbiting ball media (HSOBM) processor was successfully introduced as a fast and efficient method to fabricate flake-shaped magnesium particles with high specific surface areas [29], although its impact on hydrogen uptake has not been evaluated yet.

The objective of this study was to maximize the nucleation-limited regime, to improve material utilization and to enhance hydrogen uptake by changing either geometry or crystallite size of Mg. Spherical and flake-shaped particles were characterized by crystallite size and hydrogenation characteristics. The HSOBM processor was also tested for its ability to coat the nano-catalyst on magnesium flakes in order to evaluate its potential for combined flake/coating processor. The coating efficiency of both the HSOBM processor and a dry mechanical coating processor (Theta Composer) were assessed. Finally, the effects of geometry on hydrogen absorption kinetics were analyzed from hydrogenation data.

Section snippets

Fabrication of Mg flake–nano-Ni composites

Mg flakes were prepared using a HSOBM processor. The coating of nano-Ni catalysts was carried out using either a HSOBM processor or a Theta Composer (Tokuju, Corp.) under an argon environment. In-depth details of the mechanistic aspects of the processes have been described elsewhere [8], [29], [30], and only a brief description is provided here.

The Theta Composer consists of an elliptic rotor encased in a vessel. The rotor operates at high revolution while the vessel counter-rotates at a lower

Determination of minimum Ni loading

Prior to experimentation, the minimum nano-Ni loading for spherical and flake-shaped particles was theoretically investigated. A minimum nano-Ni wt.% is defined as the Ni wt.% required to form monolayer coverage on the surface of Mg powder. For spherical/flake particles, the minimum Ni loading can be expressed by Eqs. (1), (2) respectively:Xsph=1(1/π)(dm/dn)(ρm/ρn)+1Xfla=1(3/π)(dmh/(dn(dm+2h)))(ρm/ρn)+1

The average thickness and diameter of flakes, measured to be 11.73 μm and 442 μm respectively

Conclusion

This study aimed at enhancing hydrogen uptake kinetics and storage efficiency by utilizing thin Mg flakes with large diameters coated with nano-Ni catalysts. XRD analyses of the product processed by the HSOBM process showed a change in the orientation of crystal structure of Mg, and a reduction in crystallite size; however, this change was not large enough to hydrogenate uncoated Mg flakes. The individual crystalline phase of Mg and Ni was preserved and no XRD peaks of MgxNiy alloy were

Acknowledgements

Ki-Joon Jeon is grateful to the University of Florida Alumni Fellowship and Korean Science & Engineering Foundation's Graduate Study Abroad Scholarship Program (M06-2003-000-10264-0). Alexandros Theodore is grateful to Dr. Dale Lundgren for the EPA Air Pollution Training Scholarship and to the Particle Engineering Research Center (PERC) at University of Florida for the Undergraduate Research Scholarship. We would like to acknowledge Major Analytical Instrument Center (MAIC) and PERC at

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    Present address: Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.

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