Local Study on Hydrogen and Hydrogen Gas Bubble Formation on a Platinum Electrode

This work investigates the effect of current density on the hydrogen gas bubble formation during electrolysis on a platinum electrode. Local detection of bubble evolution was performed electrochemically by using a hydrogen-collecting microelectrode placed in close proximity of a larger hydrogen-generating platinum electrode. The micro-electrode probe locally measured the concentration of dissolved molecular hydrogen produced during electrolysis. In acidic conditions, it was found that at low electrolysis currents the concentration of dissolved hydrogen could temporarily rise up to 50 times the saturation level, while once a large current was reached, the concentration dropped, increasing the concentration of hydrogen inside gas bubbles, and became independent of the current. Through laser reflectance, the onset of bubble formation was found (similar to-0.7 mA/cm(2)), which was lower than the first bubble observable by naked eye or detected by the microelectrode probe.

Molybdenum Based Electrocatalysts for Hydrogen Evolution in Water

Heron Vrubel

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, software and signal acquisition techniques is given.

EPFL2013

Nanostructured Electrocatalysts for Water Splitting Reactions

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

Non-Precious Electrodes for Practical Alkaline Water Electrolysis

Alberto Battistel, Alejandro Nicolás Colli, Hubert Girault

Water electrolysis is a promising approach to hydrogen production from renewable energy sources. Alkaline water electrolyzers allow using non-noble and low-cost materials. An analysis of common assumptions and experimental conditions (low concentrations, low temperature, low current densities, and short-term experiments) found in the literature is reported. The steps to estimate the reaction overpotentials for hydrogen and oxygen reactions are reported and discussed. The results of some of the most investigated electrocatalysts, namely from the iron group elements (iron, nickel, and cobalt) and chromium are reported. Past findings and recent progress in the development of efficient anode and cathode materials appropriate for large-scale water electrolysis are presented. The experimental work is done involving the direct-current electrolysis of highly concentrated potassium hydroxide solutions at temperatures between 30 and 100 °C, which are closer to industrial applications than what is usually found in literature. Stable cell components and a good performance was achieved using Raney nickel as a cathode and stainless steel 316L as an anode by means of a monopolar cell at 75 °C, which ran for one month at 300 mA cm−2. Finally, the proposed catalysts showed a total kinetic overpotential of about 550 mV at 75 °C and 1 A cm−2.