The production of fuels from renewable energy sources is of interest both for sustainable mobility applications and for medium and long-term storage of energy. The simplest synthetic fuel is hydrogen, an environmentally benign energy carrier which can be applied in fuel cells or combustion devices directly or after conversion e.g. to hydrocarbons (power-to-gas).
Photo-electrochemical water
splitting
Over the last decades photoelectrochemical water splitting became of increasing importance for fundamental and applied research, since the direct conversion of sunlight into chemical energy via the production of H2 has the potential to contribute to the world’s energy needs without CO2 generation. One of the unsolved challenges consists of finding a highly efficient photocatalysts, which is cheap, environmentally friendly, contains exclusively abundant elements, is (photo)chemically stable and absorbs visible light. Photocatalytic efficiency is closely connected to both structural properties like crystallinity, particle size and surface area and to electronic properties like the band gap and the quantum efficiency. Hence extensive control over a large parameter field is necessary to design a good photocatalyst. After having achieved a good photocatalyst design, the next step in the process consists in developping scalable fabrication processes for efficient photoelectrodes, since it was established in techno-economic studies that the fabrication costs match or even outweigh materials costs in PEC systems. With our research we aim to contribute to the resolution of both challenges.
The production of elemental hydrogen (H2) from water by alkaline electrolysis is an industrially well-established process, which has recently regained attention due to increasing interests in the storage of electrical energy from intermittent renewable sources. Alkaline water electrolysis is a proven method for large scale hydrogen production in the MW range. The most efficient electrolyzers are equipped with zero-gap cells, where a OH- conducting separator is sandwiched between two porous electrodes (e.g. nickel-coated stainless steel meshes). The alkaline electrolysis technology features high technical standards and over 100 years of experience at constant-current operation. Nevertheless, in order to become cost-efficient, the system and material costs need to be decreased, and the process efficiency needs to be increased. Furthermore, application with variable current loads from intermittent renewable energy sources poses new challenges to cell and system design.
Fuels can also be produced using concentrated solar energy and thermochemical redox cycles based on non-stoichiometric ceria (CeO2). In this technology, syngas (CO and H2) is produced from CO2 and H2O in two-step thermochemical cycles. First, ceria is reduced to oxygen-deficient ceria CeO2-δ in an endothermic proces driven by concentrated solar radiation. Second, the oxygen-deficient ceria CeO2-δ reacts to stoichiometric ceria CeO2 with water vapor and CO2 to produce syngas (CO and H2).
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