Solar energy revolution

Excited by light

Mimicking 'plant power' through artificial photosynthesis: an approach that could drive the solar energy revolution. Researchers at the University of Vienna are in a bid to chase this dream by trying to capture the sun's energy and storing it in chemical compounds. A report on how this works and why computer simulations are indispensable in the process.
The teams led by Leticia González and Davide Bonifazi from the University of Vienna are looking for compounds that are efficient light absorbers – a puzzle piece in the quest for artificial photosynthesis. The picture showst est tubes containing different molecules that can fluoresce – photons hitting these molecules excite electrons, leading to the emission of light. The properties of the molecules determine the colour. The tube in the middle contains a control compound that does not fluoresce. © Alexander Bachmayer

Yellow, blue and pink: The liquids in the test tubes shine in bright, vibrant colours. PhD candidate El Czar Galleposo carefully fills one test tube after the other with a pipette. The chemist is about to test how the molecules produced in the laboratory react to light. The samples are ready, Galleposo darkens the room and switches on a lamp. Bright blue light floods the laboratory. The colour of the liquids changes instantly; the pink sample now gleams in intense orange. An impressive display for us visitors and confirmation for Galleposo: The synthesis has gone according to plan, the molecules produced can actually be excited by light and fluoresce.

The research groups of Leticia González from the Institute for Theoretical Chemistry and Davide Bonifazi from the Institute of Organic Chemistry open their doors for us and give us an insight into their research. Their common goal is to artificially replicate what nature effortlessly accomplishes: harvesting energy from sunlight and storing it in chemical compounds. To achieve this, materials made from 'designer molecules' with very specific properties are required. For example, those that efficiently absorb light, thus making its energy usable.

A "perfect match"

Bonifazi and González are something of a perfect match. While González's lab simulates molecules and their properties on the computer, the researchers in the Bonifazi group synthesize these molecules in such a way that the previously calculated properties ultimately come into effect.

Our tour begins in the Bonifazi lab. The organic chemist Bonifazi quickly equips his theoretical colleague, Leticia González, with a lab coat. She is a rare visitor here, as her work typically takes place in front of a computer screen. This is exactly how we imagined a classical chemistry laboratory: We see shelves and work surfaces full of glass containers, hoses, metal rods, clamps and scales. The glass walls of some of the fume hoods are covered in scribbles of chemical structural formulae. The room is a hive of activity: Master's student Jan Heckhausen takes a container with a brownish liquid out of the fume hood and places it in a preheated water bath. Meanwhile, PhD candidate Davide Zanetti separates a bright yellow substance from a blue one under the fume hood. "I purified my target compound via flash-column chromatography," explains the early stage researcher.

"I think that in future we will have many materials that are capable of producing the energy we need for our mobile phones or our cars, and we will be able to do so easily and along the way," says González, outlining the shared vision of making solar technology something ordinary and integrating it into our lives.

The quest for an artificial leaf

The energy transition is one of the greatest challenges of our time. And chemistry plays a key role in this. How do we replace fossil fuels, such as coal, gas and oil? In the future, we may resort to hydrogen, methanol or synthetic liquid fuels to power our vehicles and industries. However, a lot of energy will be needed to provide these chemicals, and it will have to come from renewable sources. Solar power is one of our greatest hopes. In just one hour, the sun radiates as much energy to the Earth as humanity uses in a year. What if we could tap into at least part of this inexhaustible resource and convert it directly into chemical energy carriers?

Nature has been mastering this clever trick for around 3.5 billion years – we know this process as photosynthesis. This is the complex process that captures the energy of sunlight in the leaves of plants and uses it to convert inhaled CO2 into sugar molecules. The plant then uses the sugar as fuel for its growth and development. What plants do is therefore nothing more than converting solar energy into chemical energy and using it for their own purposes when required.

Leaves of a tree sunlight shining through
In photosynthesis, plants convert light energy into chemical energy. This way, they utilise the sun's energy to build carbohydrates. Breakthroughs in artificial photosynthesis would have revolutionary consequences: Like plants, we could convert solar energy into a wide variety of chemical products, as energy sources or as raw materials for the chemical industry. © Kumiko Shimizu via Unsplash

If this ingenious principle from nature could be applied to mankind's energy-hungry industrial processes, the possible applications would be versatile and promising. "It is a scenario that the whole world should be dreaming of: taking cheap raw materials, such as water, air and sunlight and extracting storable energy and chemical products from them," says Leticia González. For example, water could be split directly into hydrogen fuel without the need for electricity as an intermediate product. Or CO2 molecules from the air could be converted into usable hydrocarbons, such as methanol or other carbon-based chemical products, and converted into fuel.

The challenge of mimicking photosynthesis

Recreating photosynthesis in the laboratory is a hard nut to crack. This is because many sequential processes take place in the cell of a plant, all perfectly coordinated with each another.

"Let us think through what happens in the plant," says Bonifazi: Every cell in a leaf is packed with chloroplasts. These are the cell organelles in which the photosynthetic action takes place. Here, molecules of the green pigment chlorophyll act like antennae by absorbing photons. They excite electrons that are passed on like a relay stick until their energy is stored in chemical bonds that the cell can utilise. A manganese catalyst plays a role in the process, splitting water into hydrogen and oxygen, thus making the electrons available. A catalyst is a substance that accelerates chemical reactions. If this only happens under the influence of light, it is called a photocatalyst.

The holy grail is to find efficient photocatalysts for an ’artificial leaf’. But that is not all there is to it: "We cannot do anything with the best photocatalyst if we do not harvest the light efficiently," clarifies González. Therefore, we also need efficient antennae to harvest the light. The challenge facing the top researchers González and Bonifazi is to coordinate the antennae and photocatalyst so that the overall process works.

'Playing Lego' with molecules: Creativity and collaboration are key

"What we want to create is basically an artificial leaf," says González, "a system that harvests light energy and makes it directly available for the production of all kinds of chemicals." What the researchers need for this are the chemical building blocks that can do just that.

The researchers are tackling this challenge as a team. To explain how this works, the two produce a piece of paper with drawings of chemical structural formulae. It shows a series of molecules that are potentially useful for the research project. Each molecule differs from the others in one detail and each of these variations can change its properties. In case of complex compounds, an almost infinite number of configurations is possible.

The Bonifazi lab is trying to identify the best candidates and test them in the laboratory. It would be far too time-consuming and costly to go about this at random. This is where the input of González, the theorist, comes into play: Using computer simulations, her team can predict the properties of candidate molecules and thus filter out the most promising of the many theoretically possible combinations. "We try to identify the candidates with the best properties. Davide's team then tests them in the lab to see whether they actually have the predicted properties," González explains.

These computer predictions are particularly important when things get even more complicated: when many molecules come together to form a new material that is supposed to fulfil a function. "It is like building a Lego house," says Bonifazi, "the individual components have no function on their own, but you need to know how they behave in the overall structure so that you can understand the properties of the end product." The two research groups are therefore investigating the components that are theoretically possible. However: "There is no such thing as the perfect molecule," emphasises González. Rather, the aim is to optimise molecules for specific functions.

Computer simulations enable the researcher to try out also unorthodox ideas. "Davide sometimes says: ‘You can get creative when it comes to possible candidates", says González. "This creativity, this search for new approaches and ways of thinking, is fundamental in science if you want to solve complex problems.”

Cluster of Excellence MECS: Research for the energy storage of the future

In the age of renewable energy, the issue of storage is crucial. This is because sustainable energy sources such as solar and wind power have a flaw: They are not available around the clock. Therefore, we need ways to store energy and make it available again in calm weather or at night. The researchers of the new collaborative, cross-university Cluster of Excellence Materials for Energy Conversion and Storage (MECS), including Leticia González and Davide Bonifazi (Faculty of Chemistry), as well as Jani Kotakoski and Georg Kresse (Faculty of Physics) and their teams, are taking on this challenge. (More information on Jani Kotakoski's research in the podcast and on Georg Kresse's research in the interview).

The MECS scientists are pooling their multidisciplinary research power to develop innovative approaches to develop materials that can produce and store energy in chemical compounds. "The research cluster will provide a fantastic opportunity to collaborate on ambitious projects at a larger scale. It is very important to have such clusters of excellence in Austria," González, who represents the University of Vienna on the Board of Directors, is pleased to say. And Bonifazi emphasises how essential regular exchange with other scientists is: "It is very important to be exposed to the ideas of others. I always come back from conferences with dozens of new ideas."

More info and video about the Cluster of Excellence Materials for Energy Conversion & Storage

Gateway to the supercomputer

The offices of Leticia González and her team are located at Währingerstrasse 17, just a few minutes' walk from Bonifazi's lab. The theoretical chemists do not need a laboratory for their research – they work on the computer. Instead of test tubes and Bunsen burners, it is books, folders and papers that pile up in their rooms.

Using their laptops and PCs, the scientists connect to the servers located at the Arsenal in Vienna. In addition to their own customised hardware, which was especially configured and acquired for the calculations of the working group, they also use the Vienna Scientific Cluster – a supercomputer that the University of Vienna operates together with other Austrian universities for research projects requiring a lot of computing resources.

Both when using their own hardware and when using the supercomputer for their calculations, the scientists have to share the resources with other researchers. Therefore, it may sometimes take hours or days until they can start their simulations. Calculations via the Vienna Scientific Cluster can run no longer than 72 hours, so the scientists use their own hardware for simulations that take several months. "We can only hope that there are no major problems with the hardware, because this would mean that we have to start all over again," says González with a wink.

Portrait of Leticia Gonzalez at the Vienna Scientific Cluster
Leticia González at the Vienna Scientific Cluster 5, a supercomputer integral to the work of theoretical physicists and chemists. Being the most powerful supercomputer in Austria, the VSC5 has about 99,000 cores and can run quadrillions of computing operations per second. © Alexander Bachmayer

These lengthy calculations provide the researchers with information about how the molecules react when they are exposed to light. "As a result, we get something like a film that shows us how the molecules virtually dance when hit by light," González explains enthusiastically. Special computer programs, some of which the group writes themselves, convert the data that describe the molecules into three-dimensional, colourful illustrations that evolve in time like a movie. And then the feedback loop begins and the proposed compounds go to Bonifazi and his colleagues in the laboratory for synthesizing and testing.

Staying optimistic and seeking solutions: The golden era of chemistry

The clock is ticking, so "chemistry must find solutions," says González, putting in a nutshell her personal motivation. The two researchers consider pushing the era of solar technology forward "a fantastic opportunity to give something back to society", because: "As scientists, we have to remain optimistic – that is the only way we can move forward and answer new questions," González is convinced.

Whether it is about recycling polymers and breaking them back down into their basic components or about preventing the release of CO2 into the atmosphere by means of intelligent chemical synthesis processes, chemistry definitely plays a key role. "This will be the golden age of chemistry," says Bonifazi.

In the attempt to finding these solutions, the two research groups will keep putting their heads together. So it comes as no surprise that Bonifazi bids his colleague farewell at the end of our visit with the somewhat unusual words: "Until the next problem, Leticia!"

© Alexander Bachmayer
© Alexander Bachmayer
Leticia González joined the University of Vienna in 2011 and is Professor of Theoretical Chemistry. Born in Spain, she is a member of the Board of Directors of the new Cluster of Excellence Materials for Energy Conversion and Storage. She and her team are currently conducting research into artificial photosynthesis as the energy source of the future.
© Barbara Mair
© Barbara Mair
Davide Bonifazi joined the University of Vienna in 2020 as Professor of Organic Chemistry after career stages in Italy, Belgium and the UK. His research focuses on target-oriented synthetic organic chemistry to develop supramolecular architectures for various applications. His research group is a member of the new Cluster of Excellence Materials for Energy Conversion and Storage.