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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 Grtzel.
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 Grtzel.
More powerful cells
These high efficiency values are based on a char-
acteristic 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 electroly-
sis 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.
(Source: Ecole Polytechnique F¨¦d¨¦rale de Lausanne. ¡°Efficiently harvest-
ing hydrogen fuel from Sun using Earth-abundant materials.¡± Science-
Daily. ScienceDaily, 25 September 2014. www.sciencedaily.com/releas-
es/2014/09/140925141232.htm)
New Device Efficiently Harvests Hydrogen Fuel From Sun
Spectacular performance converts 12.3% through solar water splitting
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Scientists have a new efficient way of produc-
ing hydrogen fuel from sunlight and water.
By combining a pair of solar cells made with a
mineral called perovskite and low cost elec-
trodes, scientists have obtained a 12.3 percent
conversion efficiency from solar energy to hy-
drogen, a record using Earth-abundant materi-
als as opposed to rare metals.
September 25, 2014: 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 Grtzel, where scientists in-
vented dye solar cells that mimic photosynthesis in
plants, they have also developed methods for gen-
erating 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
2014 OCTOBER #5-9
separates the water molecules.
By using the latter technique, Grtzel¡¯s post-doctoral student Jingshan
Luo and his colleagues were able to obtain a performance so spectacu-
lar that their achievement was published September 25 in the journal
Science. 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 elec-
trodes work just as well as the expensive platinum-based models
When an electrical current is applied,
water splits into hydrogen and oxy-
gen. Credit: EPFL / LPI / Alain Herzog
FREE
¡°Once you have hydrogen, you
store it in a bottle and you can
do with it whatever you want
to, whenever you want it.¡±
- Michael Grtzel, Ecole Polytechnique
F¨¦d¨¦rale de Lausanne