Preparation and Characterization of Mg-based Hydrogen Storage Materials

Abstract

Nowadays, the use of portable electronic devices is increasing tremendously. The continuous rise in the amount of built-in functionality, e.g in mobile phones, makes the power consumption of these devices ever higher. This puts very high demands on the portable energy source that is used for a particular application with regard to the operating time of the equipment, cycle life etc. On a larger scale, the availability of energy is also becoming an increasingly important issue. Traditional energy sources such as fossil fuels are not infinitely available and the resulting emissions are environmentally unfriendly. The use of hydrogen produced in a sustainable manner, such as from solar or wind power, has been proposed as an alternative, environmentally friendly, energy carrier. Stored hydrogen can be used in two forms; as a gas in e.g. PEM fuel cells or electrochemically in rechargeable Nickel-MetalHydride (NiMH) batteries, which can be used in portable electronic devices or in Hybrid Electric Vehicles (HEVs). At present, a number of different technologies to store gaseous hydrogen are under intense investigation. It can be stored under very high pressures in containers, as a liquid at cryogenic temperatures, physisorbed on large surface area materials such as activated carbons and Metal-Organic-Frameworks or in the form of reversible metal hydrides (MHs). For a hydrogen storage technology to be viable, it must store at least 6 wt.% of hydrogen as stated in the U.S. Department of Energy (DoE) target for 2010. The present-day AB5 type storage materials that are used as the negative electrode in NiMH batteries can store only 1.2 wt.%, which makes them unsuitable for use in fuel cell applications. Therefore, materials that can store a substantially higher amount of hydrogen, both as a gas and electrochemically, are intensively being investigated. Magnesium-based alloys have been identified as a promising class of materials, as the capacity of pure MgH2 is 7.6 wt.% Electrochemistry is used as the main tool to investigate the hydrogen storage properties of the alloys. The basic principles of electrochemistry are discussed in Chapter 2. The electrochemical hydrogen storage reactions as well as a numerous different measurement techniques such as constant-current (CC) measurements, galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS) and the experimental electrochemical setup are described. The preparative methods used to synthesize the Mgbased alloys and hydrides by casting and ball-milling are also discussed in Chapter 2. The remainder of this chapter is devoted to theoretical introductions on X-ray Diffraction (XRD), Nuclear Magnetic Resonance (NMR) and Density Functional Theory (DFT) calculations. XRD, as well as neutron diffraction, is extensively used to study the crystallographic properties of the alloys and hydrides. NMR gives insight into the direct chemical environment of the hydrogen atoms on nearest-neighbor level and in this way complements the information obtained from diffraction measurements. Using DFT, it is possible to predict hydrogenation enthalpies of metals and alloys and to compare the relative stabilities of different crystallographic modifications of the hydride. XRD, NMR and DFT are introduced to a level that is sufficient to understand how the results presented in subsequent chapters are obtained and how the conclusions drawn from those results are reached. Chapter 3 is an overview of the state-of-the-art in hydrogen storage materials. Some general properties of metal hydrides as well as a number of interesting application areas are highlighted in this chapter. Four classes of hydrogen storage materials are discussed, complex hydrides, nitride-based materials, interstitial metal hydrides and Mg-based alloys. The complex hydrides, in particular the borohydrides, have very high hydrogen storage capacities.