Das Forschungsprogramm „Solar Technologies go Hybrid“ wird mit 1,7 Millionen Euro weiterhin vom Freistaat Bayern gefördert. Insgesamt wurde es bisher mit 70 Millionen Euro gefördert. Nun geht es darum, aus Kohlendioxid synthetische Brennstoffe zu erzeugen. Laut Wissenschaftsminister Blume sind Investitionen in Fortschritt die beste Klimaschutz-Strategie. Um zu dem kompletten Artikel des Bayerischen Staatsministeriums für Wissenschaft und Kunst zu gelangen, klicken Sie bitte hier.
Wasserstoff gewinnen, Kohlendioxid für Brennstoffe nutzen: Das sind Ziele des Forschungsverbundes Solar Technologies go Hybrid. (Bild: Forschungsverbund SolTech)
Auf dem Weg zur sonnenlichtgetriebenen Produktion von Wasserstoff ist ein Fortschritt gelungen. Ein Team aus der Chemie präsentiert einen enzymähnlichen molekularen Katalysator für die Wasseroxidation.
Die Menschheit steht vor einer zentralen Herausforderung: Sie muss den Übergang zu einer nachhaltigen und kohlendioxidneutralen Energiewirtschaft bewältigen.
Wasserstoff gilt als vielversprechende Alternative zu fossilen Brennstoffen. Er lässt sich unter Einsatz von elektrischem Strom aus Wasser herstellen. Stammt der Strom aus regenerativen Quellen, spricht man von grünem Wasserstoff. Noch nachhaltiger wäre es aber, könnte man Wasserstoff direkt mit der Energie des Sonnenlichts produzieren.
In der Natur läuft die lichtgetriebene Wasserspaltung bei der Photosynthese der Pflanzen ab. Diese verwenden dafür einen komplexen molekularen Apparat, das sogenannte Photosystem II. Dessen aktives Zentrum nachzuahmen ist eine vielversprechende Strategie, um eine nachhaltige Produktion von Wasserstoff zu realisieren. Daran arbeitet ein Team von Professor Frank Würthner am Institut für Organische Chemie und dem Zentrum für Nanosystemchemie der Julius-Maximilians-Universität Würzburg (JMU).
Wasserspaltung ist keine banale Reaktion
Wasser (H2O) besteht aus einem Sauerstoff- und zwei Wasserstoffatomen. Der erste Schritt der Wasserspaltung ist eine Herausforderung: Um den Wasserstoff freizusetzen, muss aus zwei Wassermolekülen der Sauerstoff entfernt werden. Dafür ist es zunächst nötig, den beiden Wassermolekülen vier Elektronen und vier Protonen zu entziehen.
Diese oxidative Reaktion ist nicht banal. Pflanzen nutzen dafür ein komplexes Gebilde als Katalysator, bestehend aus einem Cluster mit vier Mangan-Atomen, über die sich die Elektronen verteilen können.
Würthners Team hatte in einem ersten Durchbruch eine ähnliche Lösung entwickelt, eine Art „künstliches Enzym“, das den ersten Schritt der Wasserspaltung erledigen kann. Dieser Wasseroxidations-Katalysator, bestehend aus drei miteinander agierenden Ruthenium-Zentren innerhalb eines makrozyklischen Konstrukts, katalysiert erfolgreich den thermodynamisch anspruchsvollen Prozess der Wasserspaltung. Publiziert wurde das 2016 und 2017 in den Journalen Nature Chemistry und Energy & Environmental Science.
Zum Erfolg mit einer künstlichen Tasche
Nun ist es den Chemikerinnen und Chemikern der JMU gelungen, die anspruchsvolle Reaktion mit einem einzigen Ruthenium-Zentrum effizient ablaufen zu lassen. Dabei wurden sogar ähnlich hohe katalytische Aktivitäten wie im natürlichen Vorbild erreicht, dem Photosyntheseapparat der Pflanzen.
„Möglich wurde dieser Erfolg, weil unser Doktorand Niklas Noll eine künstliche Tasche um den Ruthenium-Katalysator geschaffen hat. Darin werden die Wassermoleküle für den gewünschten protonengekoppelten Elektronentransfer vor dem Ruthenium-Zentrum in einer genau definierten Anordnung arrangiert, ähnlich wie es in Enzymen geschieht“, sagt Frank Würthner.
Publikation in Nature Catalysis
Die JMU-Gruppe präsentiert die Details ihres neuartigen Konzepts nun im Fachjournal Nature Catalysis. Das Team aus Niklas Noll, Ana-Maria Krause, Florian Beuerle und Frank Würthner ist davon überzeugt, dass sich dieses Prinzip auch zur Verbesserung anderer katalytischer Prozesse eignet.
Das langfristige Ziel der Würzburger Gruppe ist es, den Wasseroxidations-Katalysator in ein künstliches Bauteil einzubauen, das mit Hilfe von Sonnenlicht Wasser in seine beiden Bestandteile Wasserstoff und Sauerstoff zerlegt. Das wird noch seine Zeit dauern, denn dafür muss der Katalysator mit weiteren Komponenten zu einem funktionierenden Gesamtsystem gekoppelt werden – mit lichtsammelnden Farbstoffen und mit sogenannten Reduktionskatalysatoren.
Der Europäische Forschungsrat (European Research Council, ERC) hat die beschriebenen Arbeiten im Rahmen eines ERC Advanced Grant für Frank Würthner gefördert (grant agreement No. 787937). Weitere Fördermittel stammen vom Bayerischen Wissenschaftsministerium im Rahmen des Forschungsnetzwerks „Solar Technologies go Hybrid“.
Enzyme-like water preorganization in a synthetic molecular cleft for homogeneous water oxidation catalysis. Nature Catalysis, 3. Oktober 2022, DOI: 10.1038/s41929-022-00843-x
It is crucial for photovoltaics and other technical applications how efficiently energy spreads in a small volume. With new methods, the path of energy in the nanometer range can now be followed precisely.
Plants and bacteria lead the way: They can capture the energy of sunlight with light-harvesting antennas and transfer it to a reaction centre. Transporting energy efficiently and in a targeted fashion in a minimum of space – this is also of interest to mankind. If scientists were to master it perfectly, they could significantly improve photovoltaics and optoelectronics.
Two new spectroscopic methods
But how can the flow of energy be observed? This is
what Tobias Brixner’s group at the Institute of Physical and Theoretical
Chemistry at Julius-Maximilians-Universität (JMU) Würzburg in Bavaria,
Germany, is working on.
In the journal Nature Communications, the
team now presents two new spectroscopic methods with which energy
transport on the nanoscale can be observed. According to the JMU
professor, the new findings provide valuable information for the design
of artificial light-harvesting antennas.
These research successes were achieved in
cooperation with the working groups of Christoph Lambert and Todd Marder
(JMU Würzburg), Uwe Bunz and Andreas Dreuw (University of Heidelberg)
as well as Jasper Knoester and Maxim Pshenichnikov (University of
Nanotubes imitate nature
Using the new methods, the research teams have
succeeded in deciphering the energy transport in double-walled nanotubes
made up of thousands of dye molecules. These tiny tubes serve as models
for the light-harvesting antennas of photosynthetically active
At low light intensities, the energetic excitations
are transported from the outer to the inner wall of the tubes. At high
intensities, on the other hand, the excitations only move along the
outer wall – if two excitations meet there, one of them disappears.
“This effect, which has been known for some time, can be made directly
visible with our method for the first time,” says Brixner.
The measurements could be carried out by combining
the exciton-exciton-interaction-two-dimensional spectroscopy (EEI2D
spectroscopy) method developed in the Brixner group with a microfluidic
arrangement of the Groningen group.
Data acquisition is much faster
In the second paper, the research teams also demonstrate a new approach to measuring energy flows. The highlight: The speed of the data recording could be extremely increased compared to the state of the art. Within just eight minutes, it was possible to measure up to 15 different 3D spectra simultaneously in a single experiment. Traditional methods, on the other hand, typically require several hours for only a single spectrum.
As a basis for measuring coherent spectra over three frequency dimensions, the researchers employed a fast method of varying the temporal sequence of ultrashort laser pulses. “The expansion from 2D to 3D frequency analysis and the increase in the number of light-matter interactions from the four usual in the literature to six now provides detailed insights into the dynamics of highly excited states,” says Brixner.
This work was funded by the Solar Technologies Go Hybrid research network of the Free State of Bavaria, the German Research Foundation (DFG), and the European Research Council (ERC) as part of the ERC consolidator grant “MULTISCOPE”.
B. Kriete, J. Lüttig, T. Kunsel, P. Malý, T. L.
C. Jansen, J. Knoester, T. Brixner, and M. S. Pshenichnikov, “Interplay
between structural hierarchy and exciton diffusion in artificial light
harvesting”, Nature Communications, https://doi.org/10.1038/s41467-019-12345-9 (2019)
S. Mueller, J. Lüttig, P. Malý, L. Ji, J. Han,
M. Moos, T. B. Marder, U. H. F. Bunz, A. Dreuw, C. Lambert, and T.
Brixner, “Rapid multiple‐quantum three‐dimensional fluorescence spectroscopy disentangles quantum pathways”, Nature Communications, https://doi.org/10.1038/s41467-019-12602-x (2019)
Chemical Data Mining Boosts Search for New Organic Semiconductors
Organic semiconductors are lightweight, flexible and easy to manufacture. But they often fail to meet expectations regarding efficiency and stability. Researchers at the Technical University of Munich (TUM) are now deploying data mining approaches to identify promising organic compounds for the electronics of the future.
Producing traditional solar cells made of silicon is very energy intensive. On top of that, they are rigid and brittle. Organic semiconductor materials, on the other hand, are flexible and lightweight. They would be a promising alternative, if only their efficiency and stability were on par with traditional cells.
Together with his team, Karsten Reuter, professor of Theoretical Chemistry at the Technical University of Munich, is looking for novel substances for photovoltaics applications, as well as for displays and light-emitting diodes – OLEDs. The researchers have set their sights on organic compounds that build on frameworks of carbon atoms.
Contenders for the Electronics of Tomorrow
Depending on their structure and composition, these molecules, and the materials formed from them, display a wide variety of physical properties, providing a host of promising candidates for the electronics of the future.
“To date, a major problem has been tracking them down: It takes weeks to months to synthesize, test and optimize new materials in the laboratory,” says Reuter. “Using computational screening, we can accelerate this process immensely.”
Computers instead of Test Tubes
The researcher needs neither test tubes nor Bunsen burners to search for promising organic semiconductors. Using a powerful computer, he and his team analyze existing databases. This virtual search for relationships and patterns is known as data mining.
“Knowing what you are looking for is crucial in data mining,” says PD Dr. Harald Oberhofer, who heads the project. “In our case, it is electrical conductivity. High conductivity ensures, for example, that a lot of current flows in photovoltaic cells when sunlight excites the molecules.”
Algorithms Identify Key Parameters
Using his algorithms, he can search for very specific physical parameters: An important one is, for example, the “coupling parameter.” The larger it is, the faster electrons move from one molecule to the next.
A further parameter is the “reorganization energy”: It defines how costly it is for a molecule to adapt its structure to the new charge following a charge transfer – the less energy required, the better the conductivity.
The research team analyzed the structural data of 64,000 organic compounds using the algorithms and grouped them into clusters. The result: Both the carbon-based molecular frameworks and the “functional groups”, i.e. the compounds attached laterally to the central framework, decisively influence the conductivity.
Identifying Molecules Using Artificial Intelligence
The clusters highlight structural frameworks and functional groups that facilitate favorable charge transport, making them particularly suitable for the development of electronic components.
“We can now use this to not only predict the properties of a molecule, but using artificial intelligence we can also design new compounds in which both the structural framework and the functional groups promise very good conductivity,” explains Reuter.
C. Kunkel, C. Schober, J. T. Margraf, K. Reuter, H. Oberhofer, Finding the Right Bricks for Molecular Legos: A Data Mining Approach to Organic Semiconductor Design, Chem. Mater. 2019, 31, 969-978. (DOI: 10.1021/acs.chemmater.8b04436)
The project was funded by the “Solar Technologies go Hybrid” research initiative of the Bavarian state government and is part of the new Cluster of Excellence e-conversion of the Munich universities funded by the German Research Foundation.
The structural data for the analysis were taken from the Cambridge Structural Database. The conductivity data was generated in sophisticated electronic structure calculations on Super-MUC, the supercomputer of the Leibniz Supercomputing Center in Garching. The new computer-designed molecules will be produced in a laboratory within the Cluster of Excellence e-conversion.
The smallest building blocks within the power stations of organisms which get their energy directly from the sun are basically miniature reactors surrounded by collectors which capture photons and forward them to the centre. The close correlation between structure and interaction of the components boosts productivity, a strategy which an international team of researchers is using for increasing the efficiency of solar technology. At FAU, research is being carried out in this area by the Chair of Physical Chemistry I, and the latest results have been published in the prestigious journal Nature Chemistry (https://doi.org/10.1038/s41557-018-0172-y).
Green plants, algae and some bacteria use sunlight to convert energy. The pigments in chlorophyll absorb electromagnetic radiation which induces chemical reactions in electrons. These reactions take place in the nucleus of complex protein structures, referred to by experts as photosystems I and II. The processes which take place in these photosystems are induced by catalysts in a certain order. In the first step, oxygen is released from water. The following reaction prepares the production of carbohydrates for which no further source of energy is needed.
The reaction centres of the photosystems are encircled by light-absorbing pigments grouped into consolidated complexes. These antennae increase the area available for light rays to hit and extend the spectrum of usable wavelengths, both prerequisites for a favourable energy balance. Each reactor core is surrounded by approximately 30 antennae. Experiments conducted by scientists are still far from replicating the complexity of nature. In general, a ratio of 1:1 is the best that can be achieved: one light-absorbing molecule in combination with one catalyst for oxidising water.
The group of researchers led by Prof. Dr. Dirk Guldi and his former employee Dr. Konstantin Dirian hope to revolutionise solar technology by synthesising modules based on the correlation between structure and function in photosystem II. In the newly developed systems, light-absorbing crystals such as those which are already used in LEDs, transistors and solar cells are layered into a network of hexagonal honeycombs around a water-oxidising catalyst with four ruthenium metal atoms in the centre. When shown in a rather simplified manner, the compact, stable units made up of two components with a common long axis are reminiscent of cylindrical batteries. In the self-assembling chemical process, such ‘miniature power stations’ create two dimensional slats. Like layers in a gateau, they form a common block which collects the energy won from the sun’s rays.
This is not an entirely accurate reproduction of the ideal arrangement found in the natural photosystem, but the principle is the same. Five macromolecules in the shape of a honeycomb with the ability to capture light create a sheath around each reactor core, and it has been shown that these small power stations are efficient and successful at harvesting sun energy. They have an efficiency of over 40 percent, losses are minimal. Wavelengths from the green portion of the colour spectrum, which plants reflect, can also be used. These research results nourish the hope that solar technology can one day make use of the sun’s energy as efficiently as nature.
Solar-powered water splitting is a promising means of generating clean and storable energy. A novel catalyst based on semiconductor nanoparticles has now been shown to facilitate all the reactions needed for “artificial photosynthesis”.
In the light of global climate change, there is an urgent need to develop efficient ways of obtaining and storing power from renewable energy sources. The photocatalytic splitting of water into hydrogen fuel and oxygen provides a particularly attractive approach in this context. However, efficient implementation of this process, which mimics biological photosynthesis, is technically very challenging, since it involves a combination of processes that can interfere with each other.
Now, LMU physicists led by Dr. Jacek Stolarczyk and Professor Jochen Feldmann, in collaboration with chemists at Julius-Maximilians-Universität Würzburg (JMU) in Bavaria, led by Professor Frank Würthner, have succeeded in demonstrating the complete splitting of water with the help of an all-in-one catalytic system for the first time. Their new study appears in the journal Nature Energy.
Mimicking Natural Photosynthesis
Technical methods for the photocatalytic splitting of water molecules use synthetic components to mimic the complex processes that take place during natural photosynthesis. In such systems, semiconductor nanoparticles that absorb light quanta (photons) can, in principle, serve as the photocatalysts.
Absorption of a photon generates a negatively charged particle (an electron) and a positively charged species known as a ‘hole’, and the two must be spatially separated so that a water molecule can be reduced to hydrogen by the electron and oxidized by the hole to form oxygen. “If one only wants to generate hydrogen gas from water, the holes are usually removed rapidly by adding sacrificial chemical reagents,” says Stolarczyk. “But to achieve complete water splitting, the holes must be retained in the system to drive the slow process of water oxidation.”
The problem lies in enabling the two half-reactions to take place simultaneously on a single particle – while ensuring that the oppositely charged species do not recombine. In addition, many semiconductors can be oxidized themselves, and thereby destroyed, by the positively charged holes.
Nanorods with Spatially Separated Reaction Sites
“We solved the problem by using nanorods made of the semiconducting material cadmium sulfate, and spatially separated the areas on which the oxidation and reduction reactions occurred on these nanocrystals,” Stolarczyk explains. The researchers decorated the tips of the nanorods with tiny particles of platinum, which act as acceptors for the electrons excited by the light absorption.
As the LMU group had previously shown, this configuration provides an efficient photocatalyst for the reduction of water to hydrogen. The oxidation reaction, on the other hand, takes place on the sides of the nanorod. To this end, the LMU researchers attached to the lateral surfaces a ruthenium-based oxidation catalyst developed by Würthner‘s JMU team.
The compound was equipped with functional groups that anchored it to the nanorod. “These groups provide for extremely fast transport of holes to the catalyst, which facilitates the efficient generation of oxygen and minimizes damage to the nanorods,” says Dr. Peter Frischmann, one of the initiators of the project in Würzburg.
SolTech: Concepts for the Conversion of Solar Energy
The study was carried out as part of the interdisciplinary project “Solar Technologies Go Hybrid” (SolTech), which is funded by the state of Bavaria.
“SolTech’s mission is to explore innovative concepts for the conversion of solar energy into non-fossil fuels,” says Professor Jochen Feldmann, holder of the Chair of Photonics and Optoelectronics at LMU.
“The development of the new photocatalytic system is a good example of how SolTech brings together the expertise available in diverse disciplines and at different locations. The project could not have succeeded without the interdisciplinary cooperation between chemists and physicists at two institutions,” adds Professor Frank Würthner who heads the JMU Chair of Organic Chemistry II. Together with Feldmann, Würthner initiated SolTech in 2012
C. M. Wolff, P. D. Frischmann, M. Schulze, B. J. Bohn, R. Wein, P. Livadas, M. T. Carlson, F. Jäckel, J. Feldmann, F. Würthner, J. K. Stolarczyk, All-in-one visible-light-driven water splitting by combining nanoparticulate and molecular co-catalysts on CdS nanorods, Nature Energy2018, 3, 862-869. (DOI: 10.1038/s41560-018-0229-6)
NIM scientists from LMU Munich have found a new effect regarding the optical excitation of charge carriers in a solar semiconductor. It could facilitate the utilization of infrared light, which is normally lost in solar devices.
Semiconductors are nowadays the most prominent materials to convert solar light into usable electric energy. The International Energy Agency (IEA) reported that half a million solar panels were installed every day around the world last year. However, semiconductor-based solar cells still suffer from relatively low energy conversion efficiencies. The reason for that mainly lies in the fact that semiconductors efficiently convert the light from a quite small portion of the solar spectrum into electrical power. The spectral position of this window of light that can be efficiently converted is strongly related to a property of the semiconductor involved (that is, its band-gap). This means that, if the semiconductor is designed to absorb yellow light, longer-wavelength light (such as red and infrared light), will pass through the material without producing currents. Additionally, shorter-wavelength light (green, blue and UV light), that is more energetic than yellow light, will lose its additional amount of energy into heat. Obtaining higher energy conversion efficiencies from semiconductors is therefore still a big challenge.
Perovskite Nanocrystals for Energy Conversion
To study these limitations, Aurora Manzi, a PhD student from the Chair for Photonics led by Prof. Jochen Feldmann, has measured the charge carrier density created by the absorption of multiple photons in perovskite nanocrystals, a novel and promising material for photovoltaic applications.
“Multiple photon absorption of long-wavelength light with an energy lower than the semiconductor absorption window is usually very inefficient”, highlights Manzi, first author of the publication in Nature Communications and a student of the NIM graduate program. “I was therefore totally surprised to observe that for specific excitation wavelengths the efficiency of this process becomes drastically enhanced. At the beginning this did not make any sense to us!”
Light and Exciton “Overtones” in Resonance
After intense discussions, the team of LMU scientists realized that these resonances occur when multiples of two distinct fundamental frequencies become equal, namely that of the frequency of the primary light oscillation and that of the frequency of the band gap or more precisely of the exciton at the band-gap.
One could draw an analogy to resonance or overtone phenomena in acoustics, commonly used in music instruments. When intense red light impinges on nano-structured perovskite nanocrystals, a process similar to the generation of overtones in a guitar string takes place. The fundamental light wavelength generates higher order optical harmonics, that are overtones whose frequencies are integer multiples of the primary light oscillation. When such a “light overtone” becomes resonant with an overtone of the excitonic band-gap, the energy exchange is enhanced leading to an increased generation of charge carriers or more precisely of multiple excitons at the band gap.
Starting Point for Further Research
“The resonances observed are analogous to the physical phenomena taking place in two different strings of a guitar”, continues Manzi. “If we associate the first string to the light excitation and the second string to the semiconductor excitonic band-gap, we know from acoustics that they will get into resonance if a certain harmonic of the first string will match another harmonic of the second string.”
“The observation of this novel resonance phenomenon for optical excitations in excitonic semiconductors could pave the way for solar cells to more efficiently convert long-wavelength light into usable electric power”, adds Prof. Feldmann, the leader of the research team. “This is an exciting new finding with a possible impact for future solar devices. Together with our colleagues from the Research Network “Solar Technologies Go Hybrid” (SolTech), we will now try to develop innovative applications by playing with such overtones.”
A. Manzi, Y. Tong, J. Feucht, E.-P. Yao, L. Polavarapu, A. S. Urban, J. Feldmann, Resonantly enhanced multiple exciton generation through below-band-gap multi-photon absorption in perovskite nanocrystals, Nature Communications2018, 9, 1518.
For further information on Jochen Feldmann’s work, see: