Research Interests of Peter BRÜGGELLER

 

Energy is one of today’s burning topics in society, economics and politics. There are a lot of considerations everybody has to be aware of.

Firstly, there is continuous increase of the global energy demand. Today’s resources provide about 160 TWh per year. A lot of studies describe the history of the global energy demand of the last century. Many prognoses support the increasing trend as well.
As a consequence ideas to guarantee adequate energy supply are absolutely necessary.

Secondly, environmental damage is increasing and poses a serious threat to our well-being. Greenhouse effect and global warming are already serious problems and reality on the entire planet earth, resulting in floods, landslides, melting glaciers and other climate catastrophes. For several years there has been increasing effort to stop or at least to minimise this climate change. Even political agreements like the Kyoto protocol in 1997 and the Paris protocol in 2015 could not limit emissions in a satisfying dimension. A lot of new strategies for generating power by renewable sources are in work or progress. Commonly known sources are wind, hydropower, solar and geothermal energy, biofuels and biomass. Another special green fuel is hydrogen, in terms of its high energy density and interdisciplinary generation and use. Applications are for example car fuels or energy carriers for storing.

Combustion of hydrogen with oxygen could be a real alternative to carboniferous fossil fuels when two elements would be produced from an appropriate source, for example from water through photoinduced water splitting. With this process indeed it is possible to produce molecular hydrogen in a fully environmental friendly way using a green source, the water, and a completely clean energy source, the sun. The earth’s practically infinite and widely available supply of water makes this pathway toward solar fuel particularly attractive, however several problems are still to be solved for developing an efficient water splitting device.

More in detail, the water splitting mechanism is the redox process that splits water into molecular hydrogen and oxygen. The reaction can be divided in the oxidative path, which produces oxygen in a 4 electron step, and the reductive path, which yields hydrogen in a 2 electron step.

In order to achieve an economic efficiency a catalyst is needed. Since the water is transparent to most of the solar spectrum it is necessary to develop photosensitizers able to harvest the sunlight and transfer it to a catalyst. Exploitation of solar light to rearrange water bonds in oxygen and hydrogen is a truly common process normally done by plants named as photosynthesis.

The advantages of using organometallic or coordination compounds in this photocatalytic system are their classical photophysical and photochemical properties: Excitation in the visible range, high efficiency of intersystem crossing, long lifetime triplet excited states that can undergo reductive electron transfer and therefore hydrogen production.

The ideal system of the artificial photosynthesis is illustrated in Figure 1; however it is very difficult to combine the 4 e- step of the oxidation to produce oxygen and the 2 e- step of the reduction to produce hydrogen. If the interaction of the two different active systems does not fit, some very reactive species like peroxide or radicals will be produced and can destroy the whole system.

 Brüggeller Research

 

Figure 1: prototype of the artificial photosynthesis; the photosensitizer harvests the energy of the sunlight (middle) and transfers an electron from the excited state to the water reduction catalyst, which produces elementary hydrogen (right), the photosensitizer is regenerated from the electrons of the production of elementary oxygen (left); both parts are separated with an electron permeable membrane (copyright by V. Balzani).

To analyse the two different systems, the system is separated into the reductive part (hydrogen production) and the oxidative part (oxygen production). In our work the main focus is on the reductive part.

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