Splitting the water atom

Materials World magazine
,
1 Aug 2018

A new theoretical material could split water into hydrogen and oxygen, lending new strengths for solar energy. Khai Trung Le reports.

‘And when the sun doesn’t shine’ is an oft-repeated mantra of solar sceptics, but a real current concern. The opportunities for solar are somewhat hampered by battery technologies, and photocatalysis – using sunlight to produce fuels by separating water into hydrogen and oxygen, which is subsequently recombined into a fuel cell – is viewed by some as the true test of solar energy. A research team from the University of Oxford, UK, believes it has found the perfect photocatalyst in another extremely promising material – perovskites.

One major consideration is that their analysis is entirely theoretical. The Oxford team used supercomputers to calculate the quantum energy states of four halide double perovskites, but its analysis is centred on the assumption that the perovskite compounds will form perfect crystals. 

Experiments conducted with other photocatalytic materials, including titanium dioxide (TiO2), have been mired by low efficiency due to low light absorption. As such, the Oxford research team states that, so far, no photocatalytic material for general water splitting has become commercially available.

While exploring materials for solar cells, Feliciano Giustino and George Volonakis, co authors of the study, Surface properties of lead-free halide double perovskites: Possible visible-light photo-catalysts for water splitting, published in Applied Physics Letters, found that halide double perovskites are suitable due to their superior light absorption, as well as their ability to generate electrons and holes, the positively charged absence of electrons, that have sufficient energy to split water.

The paper states Cs2BiAgCl6 and Cs2BiAgBr6 are the most promising synthesised compounds for photocatalytic water splitting, while Cs2InAgCl6 and Cs2SbAgCl6 would ‘require controlling their surface termination to obtain energy levels appropriate for water splitting’.

In a statement to Materials World, Giustino and Volonakis said, ‘If we can come up with a material that can be useful as a water-splitting photcatalyst, then it would be an enormous breakthrough. These new double perovskites are not only promising as a complementary material for tandem solar cells, but they also have potential in areas like photocatalysis.’

Promises, promises

The potential of perovskites as the next great technology has been viewed with anticipatory glares for years. Materials World itself has reported on its seemingly vast potential as next generation data storage (see Materials World, January 2017), an alternative to radiation detection (see November 2016), and, most frequently, its promise as the next standard for solar cells (see July 2018).

Perovskites have also found enthusiasm for their use in tandem designs – boosting the performance of high-efficiency silicon-based solar cells. However, this has prompted concerns regarding the small amounts of lead within perovskites that could become an environmental hazard. This could be circumvented with the Oxford team’s lead-free perovskites.

The authors state the next step is for experimentalists to see if the material works in real world conditions. They will also continue to use computational techniques to explore other uses for double perovskites, including as light detectors.