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September 25, 2014
Ecole Polytechnique Fédérale de Lausanne
Scientists have a new efficient way of producing hydrogen fuel fromsunlight and water. By combining a pair of solar cells made with amineral called perovskite and low cost electrodes, scientists haveobtained a 12.3 percent conversion efficiency from solar energy tohydrogen, a record using Earth-abundant materials as opposed to raremetals.
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Efficiently harvesting hydrogen fuel from Sun using Earth-abundantmaterials
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Credit: EPFL / LPI / Alain Herzog [Click to enlarge image]
T
When an electrical current is applied, water splits into hydrogen and oxygen.
he race is on to optimize solar energy's performance. More efficientsilicon photovoltaic panels, dye-sensitized solar cells, concentratedcells and thermodynamic solar plants all pursue the same goal: to
produce a maximum amount of electrons from sunlight. Those electrons canthen be converted into electricity to turn on lights and power your refrigerator.
At the Laboratory of Photonics and Interfaces at EPFL, led by Michael Grätzel, wherescientists invented dye solar cells that mimic photosynthesis in plants, they have alsodeveloped methods for generating fuels such as hydrogen through solar water splitting.
To do this, they either use photoelectrochemical cells that directly split water intohydrogen and oxygen when exposed to sunlight, or they combine electricity-generatingcells with an electrolyzer that separates the water molecules.
By using the latter technique, Grätzel's post-doctoral student Jingshan Luo and hiscolleagues were able to obtain a performance so spectacular that their achievement isbeing published today in the journal Science. Their device converts into hydrogen 12.3percent of the energy diffused by the sun on perovskite absorbers -- a compound thatcan be obtained in the laboratory from common materials, such as those used inconventional car batteries, eliminating the need for rare-earth metals in the productionof usable hydrogen fuel.
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Bottled sun
This high efficiency provides stiff competition for other techniques used to convert solarenergy. But this method has several advantages over others:
"Both the perovskite used in the cells and the nickel and iron catalysts making up theelectrodes require resources that are abundant on Earth and that are also cheap,"explained Jingshan Luo. "However, our electrodes work just as well as the expensiveplatinum-based models customarily used."
On the other hand, the conversion of solar energy into hydrogen makes its storagepossible, which addresses one of the biggest disadvantages faced by renewableelectricity -- the requirement to use it at the time it is produced.
"Once you have hydrogen, you store it in a bottle and you can do with it whatever youwant to, whenever you want it," said Michael Grätzel. Such a gas can indeed beburned -- in a boiler or engine -- releasing only water vapor. It can also pass into a fuelcell to generate electricity on demand. And the 12.3% conversion efficiency achievedat EPFL "will soon get even higher," promised Grätzel.
More powerful cells
These high efficiency values are based on a characteristic of perovskite cells: theirability to generate an open circuit voltage greater than 1 V (silicon cells stop at 0.7 V,for comparison).
"A voltage of 1.7 V or more is required for water electrolysis to occur and to obtainexploitable gases," explained Jingshan Luo. To get these numbers, three or moresilicon cells are needed, whereas just two perovskite cells are enough. As a result,there is more efficiency with respect to the surface of the light absorbers required."This is the first time we have been able to get hydrogen through electrolysis with onlytwo cells!" Luo adds.
The profusion of tiny bubbles escaping from the electrodes as soon as the solar cellsare exposed to light say it better than words ever could: the combination of sun andwater paves a promising and effervescent way for developing the energy of the future.
Video: http://www.youtube.com/watch?v=hkGAqk-TXw8&feature=youtu.be
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The above story is based on materials provided by Ecole Polytechnique Fédéralede Lausanne. Note: Materials may be edited for content and length.
Journal Reference:
1. Jingshan Luo, Jeong-Hyeok Im, Matthew T. Mayer, Marcel Schreier, MohammadKhaja Nazeeruddin, Nam-Gyu Park, S. David Tilley, Hong Jin Fan, and MichaelGrätzel. Water photolysis at 12.3% efficiency via perovskite photovoltaicsand Earth-abundant catalysts. Science, 26 September 2014: 1593-1596 DOI:
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Ecole Polytechnique Fédérale de Lausanne. "Efficiently harvesting hydrogen fuelfrom Sun using Earth-abundant materials." ScienceDaily. ScienceDaily, 25September 2014. <www.sciencedaily.com/releases/2014/09/140925141232.htm>.
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