Lucas-Alexandre Stern
At the dawn of the 21st century, mankind is facing an important energy challenge. Future energy demand can only be met by large scale exploitation of renewable energy resources. Solar energy is abundant, with a sufficient capacity for the growing energy demand. However, it is necessary to implement renewable energy storage means. Hydrogen is considered as the energy carrier of the future. An energy economy based on hydrogen is viable only if hydrogen is produced from sustainable means. Water electrolysis is the most promising technique to produce hydrogen directly from water. Yet, the efficiency is limited and electrocatalysts are required to drive both the hydrogen evolution reaction and oxygen evolution reaction. Scarce and expensive materials are currently used to drive these reactions. It is, thus, imperative that efficient Earth-abundant catalysts are developed. Nanostructuring enhances the performance of inexpensive materials for water splitting and several catalysts were studied for this goal. Chapter 2 describes the application of nanostructuring technique to the archetypical catalysts that are nickel oxides for oxygen evolution. The prepared nanoparticles show superior activity towards oxygen evolution than that of their bulk counterpart. The ultrafine size of nanoparticles synthesized allowed the exposure of a higher number of active sites. The activity for oxygen evolution was, thus, significantly enhanced. We also detailed that short conditioning of the electrode improved the performance of the evaluated materials. In chapter 3 we carefully studied nanoparticles and nanowires of nickel phosphide. The catalysts is known to be really active for hydrogen evolution. Our group suspected that this material was, also, a potential oxygen evolution catalyst. We proved, for the first time, that nickel phosphide is a remarkable oxygen evolution catalyst. Under alkaline conditions, used for oxygen evolution, we observed an in-situ formation of a core-shell heterostructure, a typical nanostructure architecture. The surface oxidation of this material allowed high oxygen evolution capabilities. We concluded that careful oxidation of nanostructured materials, as observed for nickel phosphide, is essential for the preparation of future outstanding oxygen evolution catalysts. Chapter 4 details the fabrication of direct solar-to-fuel electrode using cobalt phosphide as hydrogen evolving catalyst. Instead of using the electrical energy provided by solar energy, solar irradiation on the developed assembly allows direct hydrogen production. The careful design of the light-sensitive electrode (photocathode) is detailed. Simple incorporation of cobalt phosphide by means of photodeposition alleviates the cost of the electrode fabrication. The assembled electrode is active for hydrogen evolution under visible light irradiation. In the chapter 5 we sought to fabricate a 3-dimensional hydrogen evolving catalyst. This nanostructure is based on polymer brushes on a flat conducting substrate. Once the brushes are grown on the substrate, a molybdenum sulfide catalyst is then incorporated by simple soaking technique. We were able to tune the loading of the catalyst and the corresponding activity by changing the polymer properties. The height of the polymer was modified as well as its packing density. The resulting activity proved superior to similar approaches and to previous reports on carefully engineered molybdenum sulfide catalysts.
EPFL2017
