Capturing and storing solar energy for later use

A molecule that absorbs energy from sunlight and stores it in chemical bonds has been developed by researchers at Linköping University (LiU), Sweden.

It is thought that, in the long term, this molecule could be used to capture solar energy efficiently and store it for later consumption. The researchers’ results have been published in the Journal of the American Chemical Society.

One of the challenges of solar energy is to store it efficiently, and enable its use when the sun is not shining.

“Our molecule can take on two different forms: a parent form that can absorb energy from sunlight, and an alternative form in which the structure of the parent form has been changed and become much more energy-rich, while remaining stable. This makes it possible to store the energy in sunlight in the molecule efficiently,” said study leader Bo Durbeej, Professor of Computational Physics in the Department of Physics, Chemistry and Biology at Linköping University.

The molecule belongs to a group known as ‘molecular photoswitches’. These are always available in two different forms — isomers — which differ in their chemical structures. The two forms have different properties — in the case of the molecule developed by the LiU researchers, this difference is in the energy content. The chemical structures of all photoswitches are influenced by light energy. This means that the structure, and thus the properties, of a photoswitch can be changed by illuminating it. One possible area of application for photoswitches is molecular electronics, in which the two forms of the molecule have different electrical conductivities. Another area is photopharmacology, in which one form of the molecule is pharmacologically active and can bind to a specific target protein in the body, while the other form is inactive.

Advanced computer simulations showed that the molecule the researchers had developed would undergo the chemical reaction they required, and that it would take place extremely fast — within 200 femtoseconds. Their colleagues at the Research Centre for Natural Sciences in Hungary were then able to build the molecule and perform experiments that confirmed the theoretical prediction.

In order to store large amounts of solar energy in the molecule, the researchers have attempted to make the energy difference between the two isomers as large as possible. The parent form of their molecule is extremely stable — a property that within organic chemistry is denoted by saying that the molecule is ‘aromatic’. The basic molecule consists of three rings, each of which is aromatic. When it absorbs light, however, the aromaticity is lost, such that the molecule becomes much more energy-rich. The LiU researchers show in their study that the concept of switching between aromatic and non-aromatic states of a molecule shows major potential in the field of molecular photoswitches.

“Most chemical reactions start in a condition where a molecule has high energy and subsequently passes to one with a low energy. Here, we do the opposite — a molecule that has low energy becomes one with high energy. We would expect this to be difficult, but we have shown that it is possible for such a reaction to take place both rapidly and efficiently,” said Prof Durbeej.

The researchers will now examine how the stored energy can best be released from the energy-rich form of the molecule.

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