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2D WSe2 shows potential for cheaper solar energy storage

Swiss scientists develop new method for direct solar-to-hydrogen conversion

Scientists at à‰cole Polytechnqiue Federale de Lausanne (EPFL) in Switzerland have developed a simple method to make high-quality, efficient solar panels for direct solar hydrogen production using the 2D compound semiconductor WSe2 (tungsten diselenide). The work is published in Nature Communications.

Many different materials have been considered for use in direct solar-to-hydrogen conversion technologies but 2D materials have recently been identified as particularly promising candidates due to their extraordinary electronic properties

However, harvesting usable amounts of solar energy requires large areas of solar panels, and it is difficult and expensive to fabricate thin films of 2D materials at such a scale and maintain good performance.

Kevin Sivula and colleagues at EPFL addressed this problem with an innovative and cheap method that uses the boundary between two non-mixing liquids. Before making a thin film of WSe2, the scientists first had to achieve an even dispersion of the material. To do this, they mixed the WSe2 powder with a liquid solvent using sonic vibrations to exfoliate it into thin, 2D flakes, and then chemically stabilised the mix.

Developed by Sivula's lab (2014), this technique produces an even dispersion of the flakes that is similar to an ink or a paint. The researchers  injected the WSe2 ink at the boundary between two liquids that do not mix. Exploiting this oil-and-water effect, they used the interface of the two liquids as a "˜rolling pin' that forced the 2D flakes to form an even and high-quality thin film with minimal clumping and restacking.

The liquids were then carefully removed and the thin film was transferred to a flexible plastic support, which is much less expensive than a traditional solar panel.

At this proof-of-concept stage, the solar-to-hydrogen conversion efficiency was around 1 percent - already a vast improvement over thin films prepared by other methods, according to the researchers, and with considerable potential for higher efficiencies in the future.

More importantly, the team think this liquid-liquid method can be scaled up on a commercial level. "It is suitable for rapid and large-area roll-to-roll processing," says Sivula. "Considering the stability of these materials and the comparative ease of our deposition method, this represents an important advance towards economical solar-to-fuel energy conversion."

The work was funded by the Swiss Competence Centers for Energy Research (SCCER Heat and Electricity Storage) and the European Commission's Framework Project 7 (FP7) through a Marie-Curie Intra-European Fellowship (COCHALPEC).


'Self-assembled 2D WSe2 thin films for photoelectrochemical hydrogen production' by Xiaoyun Yu et al;  Nature Communications, 6 ( 2015)   DOI: 10.1038/ncomms8596

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