Molybdenum Based Electrocatalysts for Hydrogen Evolution in Water

The limited fossil fuel resources and the increasing levels of greenhouse gases in the atmosphere are the driving forces in the research of alternative ways of energy production and storage. The use of solar energy is expected to grow as our society claims for a renewable and sustainable source of energy. Renewable energy is intermittent. Efficient ways of storing energy must be developed to allow a smooth supply of electricity. Hydrogen is considered as one alternative, since it can be made from water electrolysis during periods of overproduction of electricity and it can be converted back when needed, by the use of fuel cells. Efficient and low-cost catalysts for the hydrogen evolution reaction (HER) are needed to increase the efficiency of the process and allow the widespread use of hydrogen. Chapter two is dedicated to the electrochemical characterization of an amorphous molybdenum sulfide film, prepared by electrodeposition using cyclic voltammetry from ammonium tetrathiomolybdate solution. The mechanism of film formation is rather complex since several redox processes are present. Gravimetric studies on the film formation during electrodeposition were performed by using an electrochemical quartz crystal microbalance that was built by us. The nature of the film was studied by X-ray photoelectron spectroscopy (XPS) prior and after electrocatalysis. A fair comparison in the activity of several materials towards HER was made possible by depositing similar amounts of catalyst with the help of the quartz microbalance. In Chapter three, we investigated the possibility to prepare by chemical methods a similar material to the electrodeposited films studied in Chapter two. MoS3 has proved to be a good “pre-catalyst” for HER; the material requires an activation process prior to hydrogen evolution. The nature of this activation was probed by XPS. Furthermore, a limited electronic conductivity was observed for MoS3. To overcome this problem, the pre-catalyst was synthesized on the surface of highly porous carbon black support. An important improvement in the catalytic activity was observed. A novel form of molybdenum sulfide was prepared from ammonium tetrathiomolybdate by reduction with sodium borohydride. This catalyst has improved catalytic activity and electronic conductivity when compared to pure MoS3. XPS analysis revealed that the chemical composition of this new catalyst is very similar to the one formed upon electroreduction of MoS3. Chapter four is dedicated to the electrochemical investigation of the catalytic properties of Molybdenum Carbide and Molybdenum Boride towards HER. Both materials exhibit good performances for HER in both acidic and basic solutions. XPS analyses have shown interesting transformations for the samples subjected to catalytic tests. Appendix I relates in detail the technical improvements of instrumental methods developed during this thesis. A full description of the equipment built as well as the design of glassware, s