## 1 Introduction
Figure 2 shows the shows volumetric and gravimetric capacities of various storage materials and compares them to the best-known system capacities using these materials. Figure 1: Comparison of kWhr/kg of hydrogen and total system volume for different hydrogen storage technologies. In practical applications, the rate of the hydrogen absorption/desorption reaction is often limited by how quickly heat can be added to, or removed from, the hydride as opposed to the intrinsic kinetics of hydriding and dehydriding [1; 19]. While metal hydrides are basically alloys with good thermal conductivity, they break down into micron size particles after a few hydrogenation/dehydrogen cycles in a hydrogen system. As a result, thermal conductivity is greatly affected and decreases significantly; typically, the thermal conductivity is around 1 W/(m\(\cdot\)K) for metal hydrides [20; 21]. In addition, when hydrogen is introduced into a hydride bed, its atoms fill the pores in the bed and result in even lower conduction rates. The decreased rate of heat transfer in a bed will eventually result in extended hydriding times [14]. Figure 3 shows the effect of the thermal conductivity of the bed on its fill time. It can be observed that for the typical thermal conductivity of metal hydrides, i.e., 1 W/(m\(\cdot\)K), the fill time is significantly higher and can be reduced if the thermal conductivity can be at least doubled. It is very important that the heat transfer is efficient in order for the hydriding reaction to proceed at a given pressure, and it can be achieved by using proper techniques for enhancing the thermal conductivity of the storage system. Enhancing heat conductivity is more a technical issue than a theoretical one, and measures have been presented experimentally to address this problem. Most of  
Figure 3: Effect of metal hydride thermal conductivity on hydrogen fill time [14]; fill time can be greatly reduced for thermal conductivity higher than 2 W/(m\(\cdot\)K). Figure 2: Capacity comparison of different storage techniques. Adapted with permission from [14], ©Elsevier, 2009.  
the methods are developed by adding solid matrices like fins or by compacting metal hydrides [22]. Some of these techniques are: using porous tubes/sheets for instantaneous hydrogen supply along the bed, porous bodies encapsulating the hydride powder, e.g., graphite structures, Porous Metal Hydride (PMH-compacts), metal-coated hydride particles, and the insertion of heat transfer enhancement matrices, e.g., metal screens and bands, metal foams, radial and axial metallic fins, and metallic wire nets [13; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36]. The problem which arises by implementing any of these techniques is the increased thermal mass of the system, which affects the overall hydrogen storage capacity of the system. Authors of this work also performed a study on the different reactor and heat exchanger configurations in metal hydride hydrogen storage systems [37] and recently proved experimentally that applying a magnetic field can enhance the effective thermal conductivity in porous beds [38; 39]. Owing to the fact that metal hydride storage systems are similar to porous beds, we have tested the effect of the magnetic field on improving heat transfer and reaction kinetics in a metal hydride storage system. Considering the desired hydrogenation properties of AB\({}_{5}\) hydrides, and that LaNi\({}_{5}\) is a paradigm of this type of metal hydride, in addition to its magnetic properties, this material is selected for this study. The goal of this work is to test the effect of the magnetic field on improving the heat transfer, and hence the rate of hydrogenation, in a metal hydride hydrogen storage system. Though LaNi\({}_{5}\) does not meet the DOE hydrogen storage targets, this material was selected because of its commercial availability and magnetic properties. Magnetic fields can exert a force on materials with magnetic properties and manipulate them without being in contact with those materials [40].