Thin films on hollow micro glass spheres

The applications of materials with catalytic properties are wide spread and manifold. The basic functionality of a catalyzing material is to accelerate a chemical reaction without being consumed. This leads e.g. to the possibility to perform chemical processes at significantly lower temperatures than it would be predicted from classical reaction theory. The basic principle of catalysis is the involvement of active surface sites of the catalytic material which intricately influence the reaction path by modifying activation barriers. Many experimental data concerning these phenomena have been acquired and led to the development of a semiquantitative understanding of the catalytic process. Nonetheless the pathway towards predictive methods which allow for the optimization of catalytic materials in regard to a special application is hampered by several obstacles. The reason for this can be tracked back to the fact that catalysis is (i) mainly a surface effect, (ii) is critically influenced by the type and concentration of surface defects and (iii) by the supporting material the catalyst is situated on. Often powders or granulates are used in application to guarantee a maximum in exposed surface area. This is contrary to common samples for surface analytics where well defined plane thin films are preferred. Thin films of supporting material and/or catalyst also allow for a full exploitation of the wide material spectrum which is accessible by various deposition methods. It is the aim of this project to merge the possibility to access a wide range of material classes (catalytic metals or oxides, metallic or oxidic support materials and heat conductive materials) by thin film deposition with the capability to deposit these films on granular or powder shaped substrates. The deposition method chosen will be magnetron sputtering in non-reactive and reactive mode. In previous projects a process which allows the coating of granulates by sputtering was developed by the proposer and his co-workers. The aim of the present project is the functionalization of hollow glass micro spheres for hydrogen and helium storage by optimized catalytic coatings which may either be deposited as single layers, multilayers or gradient materials. A further aim of the project is to improve the thermal conductivity of glass micro spheres, since it is a critical parameter for the gas releasing process.

A safe supply with electrical energy from grids which are subjected to strongly fluctuating input and consumption has become a critical issue because of the increasing amount of sources with high temporal variation in production levels like e. g. solar or wind power. Within the project "CatSphere - Thin films on hollow micro glass spheres" an inherently safe system for the energy storage by hydrogen gas within hollow glass microspheres was investigated on a scientific basis and tested in regard to its basic feasibility. As opposed to large high-pressure vessels, hydrogen gas is stored in tiny, thin walled glass microspheres with a diameter of less than 1/100 of a millimetre. These microspheres can therefore be regarded as microscopic hydrogen tanks which, in contrary to a high pressure container, pose no harm in the case of an eventual explosion. To release the hydrogen gas the microspheres have to be heated to a temperature of around 100C. If this is done, the walls of the microcontainers become permeable for the gas and it can be extracted from the storage system. To provide a heat source to generate this temperature, the present project kicks in. With the help of a chemical reaction heat can be released, which enables the hydrogen diffusion through the microsphere walls. By means of a catalyst this chemical reaction can be accelerated and the peak temperature can be increased. As an additional benefit the reactants can be chosen in a way that hydrogen is generated by the reaction, thus increasing the total efficiency of this hybrid system. The challenge in this approach lies in directly applying the catalyst substance to the surface of the microspheres to generate the heat directly at the position where it is needed most. For this purpose a special coating technique was developed which allows for the deposition of uniform coatings on small, fragile particles. The technique is based on a physical vapour deposition approach which has the benefit of an almost arbitrary choice of the material of the catalyst. It could be a metal, a metal oxide or another chemical compound. Unfortunately an uniform coating of the particles is hard to achieve, since the coating material does not surround the glass spheres but is aimed towards them as a directional beam of atoms. This necessitates a clever system of intermixing which was developed within the project. With this mechanism it was possible to apply uniform catalyst coatings to the microspheres. In consequence, it was possible to take the efficiency of the chemical reaction to its theoretical limit. It could be shown that the coating is well-adherent and that it can even sustain several cycles of re-use, which is important for recycling issues associated to the described hydrogen storage system. In conclusion, it was proven that the described hybrid hydrogen storage system is theoretically and practically feasible and might even be improved to a level which might satisfy the high demands imposed by the US Department of Energy (DOE) for hydrogen storage systems in automotive systems.