Renewable energy carriers from sun light, water and CO2

Climate change as a consequence of the increasing CO2 concentration in the atmosphere has been identified as one of the most critical challenges facing mankind and requires immediate action: “The Paris Agreement aims to strengthen the global response to the threat of climate change, ( ) by low greenhouse gas emissions development, in a manner that does not threaten food production” (United Nations, 2015). Switzerland has committed to transform its energy technology from a fossil to a renewable basis (Energy Strategy 2050). Numerous studies and publications have indicated that the sun’s energy and its derivatives (wind, water) are by far sufficient to supply world’s energy demand; but the large daily and seasonal power variation of renewable energy is an additional complication for a wide spread replacement of fossil energy by renewable energy. Our research is devoted to this challenge by identifying the bottlenecks and forward technology to overcome them.
Water splitting

Harvesting of solar light and the conversion of the energy into an electrochemical potential is the crucial step of the production of a renewable energy carrier. Hydrogen production may be seen as the starting point in a closed circle using hydrogen as a renewable energy carrier. The fundamental processes of renewable hydrogen production by electrolysis are relatively well understood and pilot plants have demonstrated the feasibility on the industrial scale. Nevertheless, today’s hydrogen stems to 95% from fossil sources due to economic reasons. Research and development of hydrogen production is needed to make renewable hydrogen cost competitive to fossil hydrogen. This generates demands for fundamental research on new concepts and materials in electrolysis.

Our focus is in developing accessible spectroscopic techniques for a comprehensive understanding of the electronic structure and its dynamics in materials for solar water splitting. This includes the electronic structure of single molecules as well as extended solid frameworks. Our approach centres around using operando techniques, especially, photoelectron spectroscopy and magneto-optical spectroscopy. The former is the membrane-based approach for X-ray photoelectron spectroscopy (XPS) analysis of catalytic systems at ambient pressure. This includes, but is not limited to, the reactions in aqueous environment. The membrane device, which is composed of a polymer impregnated with a photocatalytic system for water splicing, is required to bridge the desired analytical pressure of 1 bar to the ultra-high vacuum (UHV), which is required for the XPS analysis (for details on the system see next section). We develop water permeable membranes for the XPS membrane approach based on Co-nanoparticles embedded in polydimethylsiloxane. A different technique, time resolved magneto-optical spectroscopy, is developed in our lab for studying homogeneous as well as heterogeneous solar water splitting systems. The work is embedded and partially financed by the LightCheC project at UZH.

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Hydrogen may be used directly or bound to other elements for storage. The interaction of hydrogen with matter is thus of pivotal importance to produce this energy carrier and convert it to work or heat. In this project, we develop a novel measurement method being able to unravel the mechanisms driving the hydrogen – matter interaction taking place in hydrogen storage materials, hydrogen selective membranes, and the catalysis of hydrogenation reactions. Although apparently simple, a direct photoemission measurement of the hydrogen induced changes of the electronic structure, which are the origin of the binding of hydrogen with metals, is possible in a few cases only. The reason for this is purely technical: the electronic structure of hydrogen chemisorbed to surfaces can be measured using standard surface science techniques, because the required hydrogen pressure is compatible with the UHV-technology. However, processes relevant for energy conversion and storage take place at several atmospheres hydrogen pressure; and thus valuable information on these systems is not accessible by commonly used surface science methods due to their incompatibility with high pressures. The UHHX project relies on a membrane approach for high pressure XPS under development in our laboratory. The method is based on a new type of specimen holder, which is a metallic, hydrogen permeable membrane fed on one side with a high hydrogen pressure and exposed on the other side to the X-ray beam at UHV-pressures. In first papers, we introduced the fundamental idea and demonstrated the feasibility of the method in some well-studied cases, paving the way for its use on relevant questions in energy storage. In this project, we want to utilize the membrane approach to prepare and measure in-situ various ionic and intermetallic hydrides as a function of the chemical potential of hydrogen, which will deliver insightful knowledge on the electronic structure of hydrides. The strong impact of the surface properties on permeation, as measured on Pd membranes, inspired us to develop Pd membranes with enhanced hydrogen permeability by coating them with PTFE. As a result of the investigations, we demonstrated the spectacular effect, which raises numerous questions and motivates more fundamental research in this scientific area as planned in this project.
CO2 reduction

The transformation from the excessive consumption of fossil energy towards a sustainable energy circle is most easily marketable by not changing the underlying energy carrier but generating it from renewable energy. Hydrocarbons can be principally produced from renewable hydrogen and carbon dioxide, as the corresponding technical processes are already established. The infrastructure, e.g., the natural gas pipelines, fuel stations, heating in households, gas power plants exists and could be further used without extra effort. However, primarily goal of most industrialized chemical processes are aiming at the production of a chemical compound with highest economic efficiency. The energy efficiency is important, too, but usually ranks as a secondary parameter. This is different in chemical processes aiming for the production of a chemical energy carrier. A chemical energy carrier, e.g. hydrogen or methane, is a vector, i.e., it is most important how much energy can be delivered with it considering also the conversion losses; an energy carrier costing more energy for production than it delivers does not make any sense. The energy efficiency of the production and conversion reactions is thus the most relevant parameter. In our we are addressing this issue from various perspectives, i.e.,

  • investigation of reaction mechanisms using advanced spectroscopy such as diffusive reflectance infrared spectroscopy, inelastic neutron spectroscopy, etc.: where and why do energy losses occur? NFP 70 project “Catalytic methanation of industrially derived CO2
  • development of new catalysts following novel approaches such as transition metal sulfides for CO2 reduction. NFP 70 project “Catalytic methanation of industrially derived CO2
  • development of new processes: a success story is the sorption enhanced methanation, BFE/FOGA project “SmartCat”
  • demonstration and optimization of pilot plants, BFE/FOGA project “SmartCat”
  • new conversion devices: state-of-the-art is the combustion of hydrocarbons at high temperatures (internal combustion engines, gas turbines, high temperature fuel cells). We are exploring new ways such as thermo-photovoltaic energy conversion.
  • Identification of CO2 sources and development of technology for energy efficient CO2 capture

 

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References

 

  • Borgschulte, O. Sambalova, R. Delmelle, S. Jenatsch, R. Hany, F. Nüesch, Hydrogen reduction of molybdenum oxide at room temperature, Sci. Rep. 7, 40761 (2017).
  • R. Delmelle, P. Ngene, B. Dam, D. Bleiner, and A. Borgschulte, Promotion of hydrogen desorption from Pd surfaces by fluoropolymer coating, ChemCatChem. 2016, 8, 1646 – 1650.