Au vu des mesures de confinement actuelles, la défense aura lieu par vidéo-conférence via le lien suivant :
Promoteur: Monsieur Rony Snyders
Résumé de la dissertation:
Since many years, hydrogen is considered as a promising energy vector that could facilitate the mutation of our fossil fuel-based societies towards sustainable energy systems. Indeed, hydrogen can be produced by various electrochemical and biological methods, has a higher chemical energy when compare to fossil fuels and can generates electricity during fuel cell operations leaving water vapor as the only exhaust gas. Nevertheless, several issues related to the production, distribution and storage of hydrogen have to be fixed before its economically viable utilization as a fuel. Particularly, the hydrogen storage is an important issue related to the low volumetric density of hydrogen.
Today, among the solutions developed to store hydrogen, the utilization of solid state materials is foreseen because of its higher volumetric density (as compared with gaseous and liquid solutions) and for safety reasons. Within the considered solid state materials, the hydride materials appears to be good candidates. Among these materials, Mg-based hydrides and specifically MgH2 are often considered as the most promising because Mg is abundant, low cost, has a light density, a low toxicity and, overall, present a higher hydrogen capacity and reversibility in comparison with other hydrides. Nevertheless, this material suffers of two main drawbacks which are a high desorption temperature and a slow hydrogen sorption kinetic. In addition, Mg can easily be oxidized. These problems can be solved by reducing the dimensions of the metal particles to the “nano” size and/or by using catalysts and, in addition, by protecting the metal from oxidation often designing a composite materials.
In this work, we aimed to contribute to the development of air-stable Mg-based nanocomposites by industrially scalable technologies. In this context, two approaches have been developed: (i) the wet-chemistry synthesis of Mg/PMMA nanocomposites by in-situ reduction reactions and (ii) the development of a Mg/plasma polymer composites by a full plasma synthesis road combining magnetron sputtering and plasma polymerization.
Concerning the wet chemistry strategy, two composites have been prepared: PMMA-Mg NPs and MWCNTs-PMMA-Mg NPs for which, multiwalled carbon nanotubes (MWCNTs) have been added. In both these compounds, the PMMA matrix protects the Mg particles from oxidation while allowing the diffusion of hydrogen. For PMMA-Mg NPs, it was found that Mg particles did not oxidize within 30 days while for a temperature of 200℃ and a hydrogen pressure of 30 bar, without adding catalyst, the hydrogen absorption of the material can reach 4.8 wt%, while a mass fraction of hydrogen of 4.15 wt% was released at 270 ℃. When adding the MWCNTs to the system, Mg NPs are even better dispersed in the PMMA matrix which leads to a reduction of the average diameter of the NPs. For this composite, at 200℃ and for a hydrogen pressure of 20 bar the amount of hydrogen absorption reaches 6.7 wt%, and the mass fraction of hydrogen is 3.7 wt% at 150°C. Therefore, it appears that a reduction of the Mg particles sizes and a better dispersion allow for better performances of the system. In addition the study of continuous hydrogen adsorption and desorption cycle reveals a very good stability of both systems.
Considering the “plasma road”, the composite material consists on a nano-columnar Mg film prepared by magnetron sputtering in glancing angle geometry covered by a anti-corrosion conformal plasma polymer layer. We first experimentally and theoretically studied the growth of the nano-columnar Mg. This study reveals that it is possible to finely control the structural dimensions (at the nanoscale) of the material which are controlled by surface diffusion phenomena during the growth by adapting the experimental parameters. In addition, a complete study of the impact of the oxidation of the growing Mg films demonstrated the detrimental effect of this phenomenon on the final surface area of the films potentially strongly impacting the hydrogen storage properties. Finally, we have developed an ethylene plasma polymer coating in order to protect the Mg structure from oxidation. This layer is grown on the Mg structure before venting. Our data reveal that by adapting the experimental conditions, it is possible to conformally cover the nano-sculpted Mg films by a protective polymer-like layer for which the anti-corrosion properties are demonstrated. We believe that this study paves the way to the development a “full plasma” based synthesis road of air-stable hydrogen storage materials.
To conclude, in this work, air-stable nano-Mg /polymers composites were prepared by both wet chemistry and plasma-based technologies. Both experimental strategies are promising in terms of hydrogen storage capability and air stability. In addition some important phenomenon and mechanisms have been highlighted which would allow for the development of highly efficient hydrogen storage material in the near future by using clean and industry transferable technologies.
Keywords: Hydrogen storage materials, Mg/polymer nanocomposites; In situ reduction; Magnetron sputtering; Plasma polymerization; air-stability