Artificial photosynthesis with Prussian blue type redox catalysts

2017 edition

Franziska Hegner

Population growth and technological progress leads to a rapid increase of the world’s energy consumption. In the near future the limited amount of fossil fuels will no longer be able to compensate its dramatically rising demand. Hence the development of a viable alternative to fossil fuels is of uttermost importance. Artificial photosynthesis, in which high energy compounds, such as hydrogen, are produced from water by using sunlight, seems to be a very promising solution regarding its huge advantages of sustainability and abundance.

Although large progress has been made in this field of research during the last decades, a large-scale implementation of solar water splitting is still hampered by the expenses of the necessary photo-catalytic devices. The bottleneck step to realizing artificial photosynthesis the development of an efficient, cheap and robust water-oxidation catalyst.

Materials based on Prussian blue (iron(III)hexacyanoferrate(II)), which fulfill all those requirements, have shown high catalytic activities with exceeding long-term stabilities.

The cobalt analogues of Prussian blue promote water oxidation with excellent quantum yield (88 %) and are therefore competitive with state-of-the-art cobalt oxide water-splitting catalysts, while remaining highly active even in acidic medi. In addition, cobalt hexacyanometallates match all important criteria of an industrially applicable catalysts, such as facile processability, flexibility, low density and large surface area. While these compounds exhibit remarkable catalytic properties, their underlying photo-physical mechanisms are not (yet) well-understood and, hence, ripe for investigation. In my project, I try to gain insight into the important catalytic processes happening during water oxidation, which is crucial to improve the efficiency of the catalyst. By combining experimental methods with theoretical calculations, I shed light on important photo-catalytic mechanisms, such as electron excitation and transfer, electron-hole recombination or surface processes.