Turning sunlight into fuel

Ordinary photovoltaic solar panels turn sunlight into an electrical current. Quite useful, but the energy is difficult to store. Plants use photosynthesis to turn sunlight into solid fuels, such as sugar, which are easy to store. That’s why scientists want to build artificial photosynthesis units. A recent paper showing how part of such a system works should help design a sunlight-to-fuel solar panel. And maybe even quantum computers.

Jasper Knoester, Professor of Theoretical Physics at the University of Groningen, collaborated on this paper, which was published online by the Nature Chemistry journal on 1 July. ‘It’s about light-harvesting nanotubes that consist of tightly packed dye molecules,’ he says. These tubes, a few nanometres (one millionth of a millimetre) in diameter and up to a few micrometres (one thousandth of a millimetre) in length, can be used to capture energy from sunlight and transfer it along their length.

‘It’s the process of this energy transfer that we’ve been studying. Unlike in photovoltaic cells, these nanotubes don’t produce an electrical current but rather a moving energy packet produced by oscillating charges within the dye molecules.’ Knoester likens the dye molecules that form the nanotubes to tuning forks. ‘If you have a row of tuning forks and make one vibrate, the tuning forks nearby will take up the same vibration energy, thus transferring it.’ In the natural photosynthesis complex, energy thus travels from light-harvesting molecules to an active centre, where it is used to produce fuel.

‘In recent years the suggestion has been made that in a natural photosynthesis complex quantum mechanical effects play a part in energy transport,’ says Knoester. ‘This is remarkable, because quantum effects were thought to be restricted to smaller structures at very low temperatures.’ The quantum effect in question is known as ‘coherence’. To put it simply, solar energy striking the light-harvesting molecules wants to travel to the active centre by the shortest route. The energy packet finds this route because it simultaneously takes all possible routes before ‘deciding’ which one is best. This may contribute to the staggering 95 per cent efficiency with which plants transfer such energy packets to the active centre.

The nanotubes, which Knoester and his colleagues from Germany and the US have studied for some ten years now, consist of an inner and an outer layer of dye molecules. One question they had was whether these two layers work separately or as a whole.

‘We particularly wanted to know if there is coherence between the two layers,’ says Knoester. ‘The evidence so far has been conflicting.’ In a crucial experiment, the outer layer was slowly oxidized while the absorption of light by the nanotubes was measured, because the oxidation changes the properties of the light-absorbing dye.

The conclusion was that the layers seem to function independently. ‘At least at room temperature.’ A second experiment confirmed this result.

‘It gives us a better understanding of the relationship between the molecular structure of the nanotubes and how they function,’ explains Knoester. This will help in the design of novel light-harvesting sunlight-to-fuel systems. ‘But it also gives us a fundamental understanding of the role of quantum mechanical effects in these nanotubes and provides us with a model system to study them further.’

This may not only benefit research into light-harvesting systems, but it may also help the development of quantum computers. Thanks to coherence, a quantum bit can have different values at the same time, which is why quantum computing can, in theory, be much faster than working with normal binary bits. But so far, constructing quantum bits that work at room temperature has been a major challenge. ‘That’s why it is important to study quantum effects in both biological and synthetic photosynthetic systems.’

See also the press releases by the University of Groningen and by MIT.