Efficiently
harvesting hydrogen fuel from Sun using Earth-abundant materials
The race is on to optimize solar energy's performance. More
efficient silicon photovoltaic panels, dye-sensitized solar cells, concentrated
cells and thermodynamic solar plants all pursue the same goal: to produce a
maximum amount of electrons from sunlight.
Those electrons can then 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, where scientists invented dye solar cells that mimic
photosynthesis in plants, they have also developed methods for generating fuels
such as hydrogen through solar water splitting.
To do this, they either use photoelectrochemical cells that
directly split water into hydrogen and oxygen when exposed to sunlight, or they
combine electricity-generating cells with an electrolyzer that separates the
water molecules.
Their device
converts into hydrogen 12.3 percent of the energy diffused by the sun on
perovskite absorbers -- a compound that can be obtained in the laboratory from
common materials, such as those used in conventional car batteries, eliminating
the need for rare-earth metals in the production of usable hydrogen fuel.
Bottled
sun
This high efficiency provides stiff competition for other
techniques used to convert solar energy. But this method has several advantages
over others:
"Both the perovskite used in the cells and the nickel and
iron catalysts making up the electrodes require resources that are abundant on
Earth and that are also cheap," explained Jingshan Luo.
"However, our
electrodes work just as well as the expensive platinum-based models customarily
used."
On the other hand, the conversion of solar energy into hydrogen
makes its storage possible, which addresses one of the biggest disadvantages
faced by renewable electricity -- 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 you want to, whenever you want it," said Michael
Grätzel. Such a gas can indeed be burned -- in a boiler or engine -- releasing
only water vapor. It can also pass into a fuel cell to generate electricity on
demand. And the 12.3% conversion efficiency achieved at EPFL "will soon
get even higher," promised Grätzel.
More
powerful cells
These high efficiency values are based on a characteristic of
perovskite cells: their ability 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 obtain exploitable gases," explained Jingshan
Luo. To get these numbers, three or more silicon 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 only two
cells!" Luo adds.
The profusion of tiny bubbles escaping from the electrodes as
soon as the solar cells are exposed to light say it better than words ever
could: the combination of sun and water paves a promising and effervescent way
for developing the energy of the future.
Story
Source:
The above story is based on materials provided by Ecole
Polytechnique Fédérale de Lausanne. Note: Materials may be edited for
content and length.
Journal
Reference:
Jingshan Luo, Jeong-Hyeok Im, Matthew T. Mayer, Marcel Schreier,
Mohammad Khaja Nazeeruddin, Nam-Gyu Park, S. David Tilley, Hong Jin Fan, and
Michael Grätzel. Water
photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant
catalysts. Science,
26 September 2014: 1593-1596 DOI:10.1126/science.1258307
Cite
This Page:
Ecole Polytechnique Fédérale de Lausanne. "Efficiently
harvesting hydrogen fuel from Sun using Earth-abundant materials." Science Daily, 25 September 2014.
<www.sciencedaily.com/releases/2014/09/140925141232.htm>.
