New world record for direct solar water-splitting efficiency

July 6, 2018, Helmholtz Association of German Research Centres
The transparent anti-corrosion layer contains Rhodium nanoparticles as a catalyst. Credit: ACS Energy Letters.

Hydrogen will play a central role as a storage medium in sustainable energy systems. An international team of researchers has now succeeded in raising the efficiency of producing hydrogen from direct solar water-splitting to a record 19 percent. They did so by combining a tandem solar cell of III-V semiconductors with a catalyst of rhodium nanoparticles and a crystalline titanium dioxide coating. Teams from the California Institute of Technology, the University of Cambridge, Technische Universität Ilmenau, and the Fraunhofer Institute for Solar Energy Systems ISE participated in the development work. One part of the experiments took place at the Institute for Solar Fuels in the Helmholtz-Zentrum Berlin.

Photovoltaics are a mainstay of renewable-energy supply systems, and sunlight is abundantly available worldwide – but not around the clock. One solution for dealing with this fluctuating power generation is to store sunlight in the form of chemical energy, specifically by using sunlight to produce hydrogen. This is because hydrogen can be stored easily and safely, and used in many ways – whether in a fuel cell to directly generate electricity and heat, or as feedstock for manufacturing combustible fuels. If you combine solar cells with catalysts and additional functional layers to form a "monolithic photoelectrode" as a single block, then splitting water becomes especially simple: the photocathode is immersed in an aqueous medium and when light falls on it, hydrogen is formed on the front side and oxygen on the back.

Transparent anti-corrosion layer

For the monolithic photocathode investigated here, the research teams combined additional functional layers with a highly efficient tandem cell made of III-V semiconductors developed at Fraunhofer ISE. This enabled them to reduce the surface reflectivity of the cell, thereby avoiding considerable losses caused by parasitic light absorption and reflection. "This is also where the innovation lies", explains Prof. Hans-Joachim Lewerenz, Caltech, USA: "Because we had already achieved an efficiency of over 14 percent for an earlier cell in 2015, which was a world record at the time. Here we have replaced the anti-corrosion top layer with a crystalline titanium dioxide layer that not only has excellent anti-reflection properties, but to which the catalyst particles also adhere." And Prof. Harry Atwater, Caltech, adds: "In addition, we have also used a new electrochemical process to produce the rhodium nanoparticles that serve to catalyse the water-splitting reaction. These particles are only ten nanometres in diameter and are therefore optically nearly transparent, making them ideally suited for the job."

Under simulated solar radiation, the scientists achieved an efficiency of 19.3 percent in dilute aqueous perchloric acid, while still reaching 18.5 percent in an electrolyte with neutral pH. These figures approach the 23 percent theoretical maximum efficiency that can be achieved with the inherent electronic properties for this combination of layers.

"The crystalline titanium-dioxide layer not only protects the actual solar cell from corrosion, but also improves charge transport thanks to its advantageous electronic properties", says Dr. Matthias May, who carried out part of the efficiency determination experiments at the HZB Institute for Solar Fuels in the forerunner laboratory to the Solar-Fuel Testing Facility of the Helmholtz Energy Materials Foundry (HEMF). The record figure now published is based on work that May had already begun as a doctoral student at the HZB and for which he was awarded the Helmholtz Association Doctoral Prize for the field of energy research in 2016. "We were able to increase the operating life to almost 100 hours. This is a major advance compared to previous systems that had already corroded after 40 hours. Nevertheless, there is still a lot to be done", May explains.

That is because it is still fundamental research on small, high-priced systems in the laboratory. However, the researchers are optimistic: "This work shows that tailor-made tandem cells for direct solar water-splitting have the potential to achieve efficiencies beyond 20 percent. One approach for this is to choose even better band-gap energies for the two absorber materials in the tandem cell. And one of the two could even be silicon", explains Prof. Thomas Hannappel, TU Ilmenau. Teams at Fraunhofer ISE and TU Ilmenau are working to design that combine III-V semiconductors with lower-priced silicon, which could considerably reduce costs."

Explore further: New efficiency record for solar hydrogen production is 14 percent

More information: Wen-Hui Cheng et al, Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency, ACS Energy Letters (2018). DOI: 10.1021/acsenergylett.8b00920

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1 / 5 (4) Jul 06, 2018
Make H2 Filled Containers To Move Automatically To the Nearest Sea shores just like The stranded Whales on Beaches !
1 / 5 (4) Jul 06, 2018
Make H2 Filled Containers To Move Automatically To the Nearest Sea shores just like The stranded Whales on Beaches !

As Humans, We can do better than those Birds and Fish that Migrate thousands of miles !
Turtles lay their Eggs in far Away Seashores & Leave to get back to the Sea & Baby Turtles immediately head Directly Towards the Sea on Hatching !
1.5 / 5 (4) Jul 06, 2018
Make H2 Filled Containers To Move Automatically To the Nearest Sea shores just like The stranded Whales on Beaches !

Authors' concern is about the expensive materials that are being used. So, Meanwhile try to create a container prototype for the suggested mechanism. Once it is done in bulk, it gets cheaper....particularly if new materials that will replace them will come into force meanwhile. Any nation that comes across the container and wants to use the Stored H2 should pay/get permission first before being able to break the seal of the container ! JUST LITTER THE OCEANS with these containers. Make them Reusable, though !
2.3 / 5 (3) Jul 06, 2018
Make H2 Filled Containers To Move Automatically To the Nearest Sea shores just like The stranded Whales on Beaches !

H2 has potential to replace Fossil fuels /Crude Oil etc., as most important energy carrier in near future
4.2 / 5 (5) Jul 06, 2018
betterexists, a couple of things to keep in mind:
1. Hydrogen is extremely corrosive.
2. Salt water is also very corrosive.
3. This study shows pure H20, which is not quite the same as salt water.
4. Hydrogen and oxygen are extremely explosive/flammable, especially if mixed together.
5. Lightning might be an issue if not properly insulated.
6. If these things collide with ships/boats that would be very bad.
Jul 06, 2018
This comment has been removed by a moderator.
4.5 / 5 (2) Jul 06, 2018
It is Prudent to convert it into H2 and O2
We are messing up Seas & Lands drilling for oil and natural gas !
3 / 5 (2) Jul 06, 2018
Sooo, instead of taking a methodical, conservative approach of increasing efficiency, using recyclable materials and learning to be a responsible adult by moderating one's own expectations?

You want to extravagantly litter the oceans with floating mines constructed of precious metals in an irresponsible orgy of selfish demands?

I always wondered who could be more ethically challenged, morally reprehensible. grossly wasteful immature loons than denier shills?

See preceding comments ....
Jul 07, 2018
This comment has been removed by a moderator.
3.4 / 5 (5) Jul 07, 2018
I am sure that better does exist. I am not anywhere near as sure that betterexists is it.
Da Schneib
3 / 5 (3) Jul 08, 2018
It's a pretty good idea but needs work on the corrosion problem.
3 / 5 (4) Jul 08, 2018
better to build 100 1000 megawatt fission plants and flash sea water to steam.

problem, if there actually is a problem, solved
not rated yet Jul 09, 2018
Hydrogen for Health and Renewable Energy Source -

1 liter of H+ every 9 sec. @ 360 watts from seawater, brine water, or fresh, 3 yrs so far.

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