School of Mathematics and Natural Sciences

Photoredoxsystems for Converting and Storing Solar Power Chemically

In this thesis, photoredox reactions for the conversion of light energy and its storage in chemical systems as well as their possible applications in photogalvanic cells will be investigated based on the current state of scientific research. The starting point is a system consisting of the photocatalyst proflavine, the sacrificial donor EDTA and the redox mediator methylviologen. In the course of the experimental investigations further organic dyes, which can act as photocatalysts, as well as inorganic photocatalytic semiconductors, have been investigated for their possible applications in the system consisting of photocatalyst, sacrificial donor and redox mediator. Further substitutes for the redox mediator methylviologen and the sacrificial donor EDTA have also been found and investigated. All chemicals used were selected according to the principles of "green chemistry", in particular the solvents as well as chemicals used and, if applicable, their degradation products had to be harmless during experimentation and for the environment.

Methylviologen, which is used together with a photocatalyst and a sacrificial donor in photogalvanic cells, can be replaced by various other viologens. It turned out that the reaction process couldn’t be followed with the aid of UV/Vis spectroscopy when using benzylviologens and phenylviologens because no absorption maxima could be determined in contrast to the useage of methyl or ethylviologens. Furthermore, the redox potentials of benzyl and phenyl viologens are higher than those of methyl- and ethylviologens. The use of ethylviologens is therefore preferable, as ethylviologens are, according to the current state of knowledge, non-toxic. The values for voltage and current generated in a photogalvanic cell are comparable between methyl- and ethylviologens. Another argument for the use of the ethylviologens is its good solubility in water. The non-polar, aromatic moiety of phenyl and benzyl viologens reduces their solubility in water, so that a non-polar solvent would have to be used. The most effective photocatalysts under the selected conditions are the acridine dyes proflavine and acridine orange. The irradiation of an aqueous solution containing ethyl and methyl viologens with EDTA in the presence of the photocatalytically active xanthene dyes rose bengal and eosin Y did not result in a conversion of the viologen from a dication to a monocation. A photocatalytic conversion of the viologen could only be observed with the addition of ethanol or methanol. The acridine dyes are therefore the more suitable photocatalysts for the aims of this work. Acridine orange, however, has the GHS hazard statement 341 (Suspected of causing genetic defects), which is why proflavine is preferable as a photocatalyst. Using proflavine it could be shown that the photocatalyst is degraded in side reactions.

Semiconductors such as titanium dioxide in the anatase modification and zinc oxide can also be used as photocatalysts, resulting in a heterogeneous system. The advantage of using inorganic semiconductors is that the solid can be recovered by filtration after the reaction, but a UV LED with an emission maximum at 365 nm has to be used because both semiconductors cannot absorb visible light due to their large band gap.

A microscale structure of a photogalvanic cell was successfully developed and investigated. Different salt bridges, electrodes and light sources were used. As a salt bridge, a piece of filter paper in a PVC tube proved to be the best solution for the intended use of these microscale apparatuses, as the salt bridge wouldn’t dry out even during prolonged irradiation. The use of a platinum electrode provided the best voltage values in this test setup. A stainless steel electrode, on the other hand, provides lower values, but represents a good alternative for the intended field of application of the microscale apparatus as they are more affordable. As a light source, a UV LED torch can be used, as it emits light close to the absorption maximum of the photocatalyst and it is safer to handle for the experimenters than a halogen lamp.

Get the thesis (in German): urn:nbn:de:hbz:468-20170308-113043-5

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