How does water create light?

H2 from light and water

In contrast to catalysts that have already been found, it can use the entire light spectrum of the sun, including the least energetic red and infrared components. In the field of basic research, the increased efficiency is a major step forward - even if it is still a long way from being used. Firstly, they want to get away from expensive rhodium and, secondly, the model has so far only worked with an acid as a proton supplier, but not with pure water.

Mind games with H2

In a broader sense, the concept could fit into the energy transition. If electricity is to come increasingly from renewable sources in the future, there are appropriate ways to store and distribute energy in the interests of security of supply. In the mind games, hydrogen has a fixed role as a possible energy carrier. It could store excess electricity on windy and / or sunny days and, if necessary, generate electricity and heat in fuel cells, for example.

It could also be fed directly into the natural gas network or methanized, i.e. converted into synthetic natural gas. Energy suppliers could also supply industry with sustainable hydrogen - today it is still obtained from fossil methane with the emission of CO2. With increasing sector coupling, efficiency and energy supply security increased. None of this exists today on a sufficient scale for a hydrogen economy.

Hydrogen from light can complement the H2 economy

Within certain niches of a possible hydrogen economy, it could be interesting to generate hydrogen directly from light without the detour through electricity production, which would reduce efficiency. The development of energy generation then determines whether, from when, or for how long this is worthwhile. Numerous institutes and scientists are already working on solutions. Last year, for example, scientists from the TU Ilmenau achieved an efficiency of over 19 percent in a solar cell that electrolyzes water with the electricity generated, up from a maximum of around 23 possible in their concept.

Most of these approaches to solar H2 production rely on a combination of molecules to break down into oxygen and hydrogen. The hydrogen side of this process generates excited electrons with a light-absorbing photosensitizer (in plants this is chlorophyll). These electrons are assembled into hydrogen gas with two protons in a hydrogen generating catalyst. However, energy is lost when the charge is transferred from the light absorber to the catalytic converter.

Two steps at the same time

Turro therefore tried to achieve both steps in the same molecule, which did not work well with the catalysts for hydrogen production investigated so far. They proved to be unstable, inefficient and completely inactive in the spectral range of red or infrared light. The newly developed catalyst contains two rhodium atoms bonded together, flanked by two pairs of ligands, benzo [c] cinnoline and N, N-diphenylformamidate.

These contribute to the shortening of the rhodium atom bond, which changes the energy level of the complex. At the same time, it extends the duration of the excited state, a key factor in its improved performance, as C&EN summarizes. According to the scientists' publication, a single catalyst molecule should be able to produce up to 28 H2 molecules per hour under red light and an average of 170 hydrogen molecules (+/- 5) over a whole day.

Looking for a cheaper transition element

Basic research has so far only shown that such catalysts could offer a significantly higher degree of efficiency than those in which an additional sensitizer is necessary. If they should work in the future, they will first have to catch up with other methods of producing renewable hydrogen, for example using electricity from photovoltaics. The extent to which they may be needed depends on the development of the entire system of energy production. Obviously, niche applications are clearly expected.

A special feature of basic research is its relative independence. Photocatalysis projects are financially supported, but the team is now taking the next, tiny step without any secure prospects for a possible hydrogen economy. It tries to replace rhodium with a less rare, cheaper transition element. (fpi)

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