Photosynthesis is a powerful process. Drawing on sunlight, it enables a plant to power through air and water, taking in carbon and hydrogen and giving out oxygen as it grows. In times of carbon concern, it offers an ideal solution.
Nature, even when easy to understand, can be incredibly difficult to copy, but recent developments in mimicking the process in order to produce a fuel for human consumption have bought us a step closer.
In a study titled Visualizing the non-equilibrium dynamics of photoinduced intreamolecular electron transfer with femtosecond X-ray pulses, an international team of scientists led by Villy Sundstrom, professor of chemical physics at Lund University, proved that sunlight can be converted very quickly. The results, which were gathered with the help of one of only two X-ray free-electron lasers in existence, showed that a molecule can travel between two metal atoms at more than ten times the speed of sound.
Here, Sundstrom explains how the research has progressed to this point and what the future holds for artificial photosynthesis and solar fuels.
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Adam Leach: What is the overall aim of the project?
Villy Sundstrom: The ultimate goal is to create a molecule or material that can produce solar fuel by, as the name says, mimicking photosynthesis but probably doing it in a much more simple way than what nature does in photosynthesis. In that, there are proteins, there are membranes and all that stuff. If you look at the end result it is to use solar light to split water to take the electrons and produce a fuel with their energy. The simplest fuel would be hydrogen – just reduce protons to make hydrogen – but these days people think, of course, of carbon dioxide to help reduce the carbon dioxide and produce a carbon-based fuel like methanol or something.
AL: How did the project to investigate the potential of photosynthesis to create a solar fuel develop and who is involved in your project?
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VS: If you really want the roots of it, it goes back to the Consortium for Artificial Photosynthesis, which is a multi-university network; so us here in Lund, people in Uppsala and people in Stockholm at the KTH covering chemistry, biochemistry and physical chemistry. That’s where this artificial photosynthesis and solar fuel collaboration started and from the node here in Lund it has branched out for this particular type of experiment.
Currently, it is a multinational collaboration involving scientists from Sweden, Denmark, Germany, Hungary, with the experiments done in Japan at the SACLA X-ray Free Electron Laser. That’s more or less needed because you need the expertise from the chemists building the molecules to the people expert in carrying out the X-ray experiments, which are pretty complicated.
AL: How do the recently published findings play into achieving that aim?
VS: This paper is not doing that, it’s not really reaching the holy grail to produce a fuel, but we are studying a relatively simple model system which contains a light absorbing unit of ruthenium and a cobalt unit that mimics the catalytic part of photosynthesis.
This particular one doesn’t do full catalysis, but when you shine light on it, it has all the fundamental processes; it moves an electron from the ruthenium to the cobalt. There are structural changes at the cobalt and there are changes of spin state at the cobalt; we wanted to develop techniques to be able to monitor these initial fundamental light-driven processes that we know will be present in a fully functional system.
That we did, and we could see all the steps that you can imagine happen, so we characterize all those different processes now in a functional system. So that will be the next step – to go to really functional catalysts and really functional materials.
AL: What is the next stage for the project?
VS: The next stage is to use these techniques on a functioning catalytic system where, when you shine light on the material, you see that it does the chemistry that you wanted – that it splits water and it produces a fuel. Since this is developing, nobody really knows what such a material should look like, but the knowledge we have gained from the recent experiments will be funnelled back into the development of the molecules. Then, in steps of increasing complexity, we will look at more and more functioning systems and characterise the processes for feedback to the chemists who design the molecules.
AL: The paper explains that the experiments showed that the electron travelled through the molecule in half a trillionth of a second. What does such a high speed of transfer mean for the project?
VS: This electron transfer basically happens faster than anything else, faster than the molecule gives up energy to the environment. That means that all the energy in the light, or photon, can be transferred to the catalytic side.
AL: You expressed surprise that the transfer happened so quickly. Why was that?
VS: We had started with a slightly different molecule, where the two metal centres were the same but the bridge connecting them was different – basically two carbon atoms. Then, the electron transfer is more than a hundred times slower, so this shows that when you change the bridge to a more rigid structure, or ‘a conjugated system with double bonds’ in the words of the chemists, the bridge works almost like a wire. It will transfer much faster without having to jump through a vacuum. Again, that shows us how the molecule should be designed if you want to use the light energy in the most efficient way.
AL: How did you measure the energy transfer and how important was the use of an X-ray free-electron laser?
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VS: With the first laser pulse you start the reaction, like shooting the gun when you start a 100m race, and then with a slight delay of a few picoseconds you shoot the second pulse which takes a snapshot of the reaction that started with the first pulse. By taking many such snapshots over increasing time you get the sequence. We have done that for a long time by using visible or infrared or ultraviolet laser pulses, but this time the pulses that we used to take the snapshot were X-ray pulses and that means we can get the full structure of the molecule rather than getting a visible spectrum. We get more structural information by using the X-ray pulses.
AL: When do you think the potential of a solar fuel generated through artificial photosynthesis might be proven?
VS: There are catalysts which can generate hydrogen but then you need to feed them with electrons from somewhere else, not from splitting water. Also, there are catalysts that split water, but they are not connected. So in principle, we have the two ‘half reactions’, with some working at a very poor yield and some at a better yield, but we do not yet have a complete system that does precisely what photosynthesis does by taking electrons from water, feeding them into a catalyst and producing a fuel. To be optimistic, I think maybe five years until somebody has really demonstrated the feasibility, where someone shows that they can produce a coupled system which does what you really want.
AL: And when do you think it might be possible to produce it on a commercial basis?
VS: In terms of a practical system, through which you can produce a solar fuel at the end that is economically interesting to produce at a high yield and compete with present fuels, well that is much more difficult to say. There are lots of people working on this right now and it is a rapidly growing field, but I think this will take at least another 10-15 years before we’ll have it.
AL: How big an impact could it have if developed?
VS: We need to solve our energy problem and there is lots of work on solar cells, including work we are doing ourselves on new solar cell materials, but they produce electricity when the sun shines. We need electricity, but much of our society needs fuel, so the question is are we going to do it with very efficient solar cells and electrolysis or can this way of producing fuels from sunlight be more efficient, cheaper? We don’t know yet, but that is why people work on both things. We don’t know what will be the final solution or the final material, but we have to use the sun, that is for sure.
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