Solar power is one of the most plentiful and readily-available energy on Earth. Photocatalytic water splitting technology can separate water into hydrogen and oxygen, and then uses the hydrogen in the solar storage (aka solar fuel cell). Therefore, solar water splitting is very promising but it is not easy to apply in the real world. Because the bottleneck is to find the proper photocatalyst materials, this technology is still in the testing phase.

However, recently Oxford University published a paper on Applied Physics Letters. Their researchers found out that halide double perovskites can be used in solar cells and might be able good candidates for splitting water into hydrogen and oxygen.

Professor Feliciano Giustino, from Department of Materials, Oxford University, expressed that if these photocatalysts are effective, the research team has reached a major milestone.

Research teams from many countries have tested various types of photocatalyst materials, such as GaP, GaAs, and TiO₂. However, the results of them are not matching expectation.

Currently, scientists can use titanium dioxide (TiO₂) to do solar water splitting. Unfortunately, TiO₂ couldn't absorb visible light effectively to a commercial level, and its light conversion efficiency couldn't increase. To be brief, none of photocatalyst materials above are commercially ready.

In order to find out materials with great potentials, Oxford University research team uses supercomputers to calculate quantum energy states for four types of halide double perovskites. The team discovered that double perovskites (Cs₂BiAgCl₆ and Cs₂BiAgBr₆) are great candidates of photocatalysts. Both of their ability to absorb visible light are better than that of TiO₂, and they both are able to generate electrons and holes. They have sufficient energy to carry out reduction-oxidation reaction, so they can split water into hydrogen and oxygen.

Giustino said, very few materials can equip these features above in the same time. The team cannot guarantee how effective these materials above can be, but these compounds seem to have all the necessary properties of a photocatalyst.

Surprisingly found perovskites' different applications

It was a surprise to find these materials can be photocatalyst. Giustino's team was looking for solar cell materials, but ended up realizing that these types of perovskites can work as photocatalysts.

In recent years, scientists have discovered perovskites' great potential in the optoelectronic segment. Perovskites' energy conversion efficiency has been raised to 6 folds within 9 years. Currently, there are quite some research teams combining silicon and perovskites via tandem designs. This is how they significantly increase optoelectronic conversion efficiency.

However, typically, in perovskites cells, there are lead elements. If perovskites cells are used in PV power plants in a massive scale, the lead solar cells can be harmful to the environment. Hence, scientists began to use computer simulations in 2016 to look for alternative materials. They were able to develop new type of lead-free perovskites.

The Oxford University research team pointed out that there are good potential of these materials in terms of optoelectronic conversion, and they can be used as photocatalysts. George Volonakis, a post-doc researcher from Department of Materials, University of Oxford, expressed that the new type of double perovskites can be used in solar tandem cells, and its developmental potential in the solar storage segment (aka photocatalyst) is also huge.

However, the current analysis is only theoretical. The team's next step is to put these materials into real world usage, and to see if the reality matches their predictions. These researchers will also use computational calculations to test whether the double perovskites can work as light detectors.

(Article by Daisy Chuang; Image: pixabay)