Hydrogen storage in Mg-Ti thin film alloys : an in situ characterization study

Abstract

Depleting fossil fuel reserves and growing climate threats urge us towards a sustainable society. Moreover, we should preferably not solely rely on fossil fuels for our primary energy needs as part of the fossil fuels is imported from politically unstable regions. We should therefore think of new ways to ensure our energy needs are met in the near future. Most likely, a mixture of different sources will be used. These resources are preferable renewable in nature, e.g. solar, biomass, wind, water and geothermal, which can typically be used for stationary applications. For mobile applications, however, the use of an on-board energy storage system is indispensible. Especially for the latter, hydrogen is expected to play a dominant role. One of the important aspects of hydrogen is that only environmentally friendly products are emitted in the exothermic reaction of hydrogen with oxygen in a fuel cell. However, the feasibility of hydrogen production, storage and finally the use in fuel cells are still under debate. In prototype applications, such as fuel cell-driven automobiles, hydrogen is generally stored in high-pressure cylinders. New lightweight composite cylinders have been developed that are capable of withstanding pressures of up to 800 bars. Even though hydrogen cylinders are expected to withstand even higher pressures in the near future, their large volumes and the energy required to compress hydrogen will limit their practical applicability. As opposed to storing molecular hydrogen it can also be stored atomically in a metal hydride (MH), which can reduce the volume significantly. In addition, MHs provide relatively safe storage as they can be handled without extensive safety precautions unlike, for example, compressed hydrogen gas. Currently, the foremost problem of solid state hydrogen storage is to find a metal-hydrogen system with a gravimetric capacity that exceeds 6 wt.% H and absorbs/desorbs hydrogen at atmospheric pressures at ambient temperatures. One of the most promising elements that can reversible absorb and desorb a significant amount of hydrogen is magnesium, which has an intrinsic gravimetric storage capacity of 7.7 wt.% H. In spite of its excellent gravimetric storage capacity, the high desorption temperature (279 °C) and extremely slow hydrogen (de)sorption kinetics prevent Mg from being employed commercially. Mg is, however, often a large constituent of new hydrogen storage materials as it lowers the weight of the material and therefore increases the gravimetric capacity, which is necessary to fulfill the weight restrictions. In this thesis the hydrogen storage characteristics of Mg alloyed with other metals are addressed. The primary aim is to enforce a high absorption and desorption rate, and limit the weight of the alloys. Chapter 2 describes the experimental settings of the thin films preparation methods and characterization techniques. The thin films were prepared by means of electron beam deposition and magnetron co-sputtering and hereafter investigated by means of Rutherford Backscattering Spectroscopy to accurately determine the film thickness and composition. Electrochemistry was used as the main tool to investigate the hydrogen storage properties of the films in detail. One of the advantages of using electrochemistry is that the electrochemical equilibrium potential can be used to calculate the equivalent hydrogen partial pressure, which gives information about the thermodynamics of the metal-hydrogen system. The electrochemical setup is not straightforward as it requires a special three-electrode setup to obtain reliable experimental data. The experimental pitfalls and solutions, like for instance the need of an oxygen scrubber, to avoid incorrect electrochemical analyses are described in detail. By applying a fixed current, which is equivalent to a fixed (de)hydrogenate rate, the possibility to rapidly insert or extract hydrogen from the hydrogen absorbing medium can be addressed. Electrochemical control also offers the possibility to calculate and tune the hydrogen content in the films with high precision. The former was used to determine if the materials are interesting from a gravimetric point-of-view, while the latter was adopted in combination with other characterization techniques, like for example X-ray diffraction, which provides new insights into the effects of the hydrogen content on the host material. The theoretical background and experimental settings of several electrochemical techniques, e.g. amperometry, cyclic voltammetry, Galvanostatic Intermittent Titration Technique and impedance spectroscopy, were discussed. X-ray diffraction was used throughout the thesis to resolve the crystallography of the phases in the as-prepared samples. To acquire crystallographic data as a function of the hydrogen content custom made in situ X-ray diffraction setups were used. The theoretical background of X-ray diffraction and a detailed description of the experimental setups and settings are described. A Pd topcoat is often applied to hydride-forming thin film materials to protect them from oxidation and catalyze the dissociation of H2 or electrocatalyze the reduction of H2O. As a 10 nm Pd caplayer was applied to all Mg-based alloys described in this thesis, it is useful to determine its thermodynamic and electrocatalytic properties separately, which is presented in Chapter 3. A lattice gas model was presented recently and successfully applied to simulate the absorption/desorption isotherms of various hydride-forming materials. The simulation results are expressed by parameters corresponding to several energy contributions, e.g. interaction energies. The use of a model-system is indispensable in order to show the strength of these simulations. The palladium-hydrogen system is one of the most thoroughly described metal hydrides found in the literature and is therefore ideal for this purpose. The effects of decreasing the Pd thickness on the pressure-composition isotherms were monitored experimentally and subsequently simulated. An excellent fit of the lattice gas model to the experimental data was obtained and the corresponding parameters were used to describe several thermodynamic properties. It was found that the contribution of H-H interaction energies to the total energy and the influence of the host lattice energy are significantly and systematically changing as a function of Pd thickness. Conclusively, it was verified that the lattice gas model is a useful tool to analyze the thermodynamic properties of hydrogen storage materials. Also, the electrocatalytic properties of a 10 nm thick Pd film were determined by means of electrochemical impedance spectroscopy, which revealed that the best electrocatalytic properties are found for ??-phased Pd hydride. Determining the properties of a single-layer 10 nm thick Pd film was valuable as it was used to determine its influence on the Pd-coated Mg-based thin film alloys that were the topic of investigation for the remainder of the thesis. Recently, a thin film approach revealed that new lightweight alloys of Mg with Ti, V or Cr can be prepared that cannot be synthesized via standard alloying techniques, because the alloys are thermodynamically unstable. Electrochemical measurements showed that especially the Mg-Ti system possesses the ability to reversibly store a considerable amount of hydrogen, which can be absorbed and desorbed at relatively high rates compared to pure Mg. The systematic investigation of hydrogen storage properties of the binary MgyTi1-y alloy composition is described in Chapter 4. It is shown from X-ray diffraction (XRD) measurements that as-prepared electron-beam deposited and sputtered MgyTi1-y thin films with y ranging from 0.50 to 1.00 are crystalline and single-phase. Galvanostatic (de)hydrogenation measurements were performed to unveil the effects of the Mg-to-Ti ratio on the hydrogen absorption and desorption rates. Increasing the Ti-content up to 15 at.% does not change these rates much and hydrogen can only be desorbed at a relatively low rate. Beyond 15 at.% Ti, however, the hydrogen desorption rate increases substantially. A superior reversible hydrogen storage capacity that exceeds 6 wt.% H, along with excellent hydrogen absorption and desorption rates, was found for the Mg0.80Ti0.20 alloy. The close analogy of the electrochemical behavior of MgyTi1-y and MgySc1-y alloys points to a face-centered cubic-structured hydride for the alloys showing fast hydrogen uptake and release rates, whereas for the hydrides of alloys rich in Mg (>80 at.%), that show a slow desorption rate, probably crystallize into the common MgH2 body-centered tetragonal structure. The cycling stability of electron-beam deposited and sputtered thin film Mg0.80Ti0.20 alloys was found to be constant over the first 10 cycles, hereafter it decreased sharply caused by delamination of the film from the substrate. The intrinsic cycling stability is therefore expected to be higher. Isotherms of MgyTi1-y thin films showed that the desorption plateau pressure is not strongly affected by the Mg-to-Ti ratio and is almost equal to the equilibrium pressure of the magnesium-hydrogen system. Impedance analyses showed that the surface kinetics can be fully attributed to the Pd-topcoat. The impedance, when the MgyTi1-y thin film electrodes are in their hydrogen-depleted state, was found to be dominated by the transfer of hydrogen across the Pd/MgyTi1-y interface. In Chapter 4 it was argued that the symmetry of the crystal lattice of the host material probably strongly affects the hydrogen uptake and release rates. The largest difference for the (de)hydrogenation rates was found for MgyTi1-y alloys containing 70 to 90 at.% Mg. Therefore, the crystallography of these alloy compositions was resolved by in situ XRD and the results are presented in Chapter 5. Firstly, in situ gas phase XRD measurements were performed to identify the crystal structures of as-deposited and hydrogenated MgyTi1-y thin film alloys. The preferred crystallographic orientation of the films in both the as-prepared and hydrogenated state made it difficult to unambiguously identify the crystal structure and therefore the identification of the symmetry of the unit cells was achieved by in situ recording XRD patterns at various tilt angles. The results reveal a hexagonal closed packed structure for all alloys in the as-deposited state. Hydrogenating the layers under 1 bar H2 transforms the unit cell into face-centered cubic for the Mg0.70Ti0.30 and Mg0.80Ti0.20 compounds, whereas the unit cell of hydrogenated Mg0.90Ti0.10 has a body-centered tetragonal symmetry. The (de)hydrogenation kinetics changes along with the crystal structure of the hydrides from rapid for face-centered cubic-structured hydrides to sluggish for hydrides with a body-centered tetragonal symmetry and emphasized the influence of the symmetry of the crystal lattice on the hydrogen transport properties.