An artist’s impression of the novel, lead-free double perovskite splitting water. Image: George Volonakis.
An artist’s impression of the novel, lead-free double perovskite splitting water. Image: George Volonakis.

Solar energy is clean and abundant, but only when the sun is shining. To take advantage of solar energy when the sun isn’t shining, it must be stored in batteries or through a process called photocatalysis – in which solar energy is used to make fuels. In photocatalytic water splitting, sunlight is used to power the separation of water into hydrogen and oxygen, which can then be recombined in a fuel cell to release energy.

Now, Feliciano Giustino and George Volonakis at the University of Oxford in the UK have found that a new class of materials – halide double perovskites – may have just the right properties to split water. They report their findings in a paper in Applied Physics Letters.

"If we can come up with a material that can be useful as a water-splitting photocatalyst, then it would be an enormous breakthrough," said Giustino.

Researchers have already experimented with many photocatalytic materials, such as titanium dioxide (TiO2). While TiO2 can harness sunlight to split water, it's inefficient because it doesn't absorb visible light very well. Up to now, no photocatalytic material for general water splitting has become commercially available.

Using supercomputers to calculate the quantum energy states of four halide double perovskites, Giustino and Volonakis found that two of them – Cs2BiAgCl6 and Cs2BiAgBr6 – should be able to absorb visible light much better than TiO2. They should also generate electrons and holes (the positive charges produced by the absence of electrons) with sufficient energy (or nearly ideal energies) for splitting water into hydrogen and oxygen.

Very few other materials possess this mix of features. "We can't say this will work for sure, but these compounds seem to have all the right properties," said Giustino

Giustino and his research team originally discovered this type of perovskite while looking for materials to make solar cells. Over the past few years, perovskites have garnered interest as materials that can boost the efficiency of silicon-based solar cells through tandem designs that integrate a perovskite cell directly onto a high-efficiency silicon cell. But these perovskites contain a small amount of lead, which could pose a potential environmental hazard.

In 2016, using computer simulations to identify alternative materials, the researchers found a new type of lead-free perovskite with potential for use in high-efficiency solar cells. The present paper shows that these new materials also show potential for splitting water. "These new double perovskites are not only promising as a complementary material for tandem solar cells, but they can also be promising in areas like photocatalysis," Volonakis said.

Still, the new analysis is purely theoretical, as it assumes the compounds form perfect crystals. The next step, the authors said, is for experimentalists to see if the material works in the real world as well as predicted. In the meantime, the researchers are using their computational techniques to explore whether these double perovskites have properties useful for other applications like light detectors.

This story is adapted from material from the American Institute of Physics, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.