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Researchers have found a cheap and easy way to store the energy made by solar power.

     Splitting water: Daniel Nocera poses with a device for breaking down water into hydrogen and oxygen. The device uses an inexpensive catalyst that he has developed.
     Researchers have made a major advance in inorganic chemistry that could lead to a cheap way to store energy from the sun. In so doing, they have solved one of the key problems in making solar energy a dominant source of electricity.
     Daniel Nocera, a professor of chemistry at MIT, has developed a catalyst that can generate oxygen from a glass of water by splitting water molecules. The reaction frees hydrogen ions to make hydrogen gas. The catalyst, which is easy and cheap to make, could be used to generate vast amounts of hydrogen using sunlight to power the reactions. The hydrogen can then be burned or run through a fuel cell to generate electricity whenever it's needed, including when the sun isn't shining.
     Solar power is ultimately limited by the fact that the solar cells only produce their peak output for a few hours each day. The proposed solution of using sunlight to split water, storing solar energy in the form of hydrogen, hasn't been practical because the reaction required too much energy, and suitable catalysts were too expensive or used extremely rare materials. Nocera's catalyst clears the way for cheap and abundant water-splitting technologies.

HOW IT WORKS
Illustration of Dr. Nocera's "artificial photosynthesis" system.

     Nocera's advance represents a key discovery in an effort by many chemical research groups to create artificial photosynthesis--mimicking how plants use sunlight to split water to make usable energy. "This discovery is simply groundbreaking," says Karsten Meyer, a professor of chemistry at Friedrich Alexander University, in Germany. "Nocera has probably put a lot of researchers out of business." For solar power, Meyer says, "this is probably the most important single discovery of the century."
     The new catalyst marks a radical departure from earlier attempts. Researchers, including Nocera, have tried to design molecular catalysts in which the location of each atom is precisely known and the catalyst is made to last as long as possible. The new catalyst, however, is amorphous--it doesn't have a regular structure--and it's relatively unstable, breaking down as it does its work. But the catalyst is able to constantly repair itself, so it can continue working.
     In his experimental system, Nocera immerses an indium tin oxide electrode in water mixed with cobalt and potassium phosphate. He applies a voltage to the electrode, and cobalt, potassium, and phosphate accumulate on the electrode, forming the catalyst. The catalyst oxidizes the water to form oxygen gas and free hydrogen ions. At another electrode, this one coated with a platinum catalyst, hydrogen ions form hydrogen gas. As it works, the cobalt-based catalyst breaks down, but cobalt and potassium phosphate in the solution soon re-form on the electrode, repairing the catalyst.
     Nocera created the catalyst as part of a research program whose goal was to develop artificial photosynthesis that works more efficiently than photosynthesis and produces useful fuels, such as hydrogen. Nocera has solved one of the most challenging parts of artificial photosynthesis: generating oxygen from water. Two more steps remain. One is replacing the expensive platinum catalyst for making hydrogen from hydrogen ions with a catalyst based on a cheap and abundant metal, as Nocera has done with the oxygen catalyst.
     Finding a cheaper catalyst for making hydrogen shouldn't be too difficult, says John Turner, a principal investigator at the National Renewable Energy Laboratory, in Golden, CO. Indeed, Nocera says that he has promising new materials that might work, and other researchers also have likely candidates. The second remaining step in artificial photosynthesis ihttp://www.sciam.com/article.cfm?id=hydrogen-power-on-the-cheap&sc=WR_20080805
http://www.sciam.com/article.cfm?id=hydrogen-power-on-the-cheap&sc=WR_20080805s developing a material that absorbs sunlight, generating the electrons needed to power the water-splitting catalysts. That will allow Nocera's catalyst to run directly on sunlight; right now, it runs on electricity taken from an outlet.
     There's also still much engineering work to be done before Nocera's catalyst is incorporated into commercial devices. It will, for example, be necessary to improve the rate at which his catalyst produces oxygen. Nocera and others are confident that the engineering can be done quickly because the catalyst is easy to make, allowing a lot of researchers to start working with it without delay. "The beauty of this system is, it's so simple that many people can immediately jump on it and make it better," says Thomas Moore, a professor of chemistry and biochemistry at Arizona State University.

     C'est un rêve d'écologiste: produire de l'énergie avec du soleil et de l'eau, à la manière de la photosynthèse réalisée par les plantes. Ce rêve pourrait devenir réalité, si l'on en croit les travaux, publiés dans la revue Science du 1er août, de deux chimistes américains du Massachusetts Institute of Technology (MIT), Daniel Nocera et Matthew Kanan. Dans la nature, les végétaux utilisent la lumière comme source d'énergie pour fabriquer du sucre à partir de gaz carbonique et d'eau, dont les molécules sont décomposées entre d'un côté l'oxygène et de l'autre l'hydrogène. C'est cette réaction qu'imite l'électrolyse de l'eau, consistant à dissocier les molécules liquides en oxygène et hydrogène gazeux, à l'aide d'un courant électrique circulant entre deux électrodes. Un procédé connu de tous les écoliers et maîtrisé de longue date, puisque la première électrolyse de l'eau a été effectuée en 1800 par deux chimistes britanniques.
     L'usage à grande échelle de cette technique est toutefois limité par des obstacles économiques. Les installations industrielles d'électrolyse de l'eau sont en effet complexes et coûteuses, notamment parce qu'elles nécessitent des catalyseurs (activant les réactions) qui sont habituellement faits de platine, métal cher. L'idée des chercheurs du MIT est d'exploiter, pour provoquer l'électrolyse, le rayonnement solaire converti en électricité par les cellules photovoltaïques de panneaux équipant les maisons et les bâtiments publics. Et de recourir à des matériaux moins onéreux.

PILE À COMBUSTIBLE
     Daniel Nocera et Matthew Kanan ont montré que la dissociation de l'eau en oxygène et hydrogène pouvait être réalisée avec une électrode en oxyde d'indium (métal proche de l'aluminium que l'on trouve en petites quantités dans les minerais de zinc) dopé à l'étain, placée dans un bain d'eau additionnée de cobalt et de phosphate de potassium. Lesquels s'avèrent, en présence d'un courant électrique, des catalyseurs efficaces.
     Tout l'intérêt de l'opération est d'obtenir, in situ et à moindre coût, de l'hydrogène. Celui-ci pourra ensuite être recombiné à de l'oxygène pour produire de l'électricité, selon le procédé inverse de l'électrolyse mis en oeuvre dans les piles à combustible. Dans la pratique, les auteurs imaginent des habitations dotées de capteurs photovoltaïques qui, pendant les heures d'ensoleillement, les alimenteraient en électricité. L'excès d'électricité servirait à produire de l'hydrogène qui, la nuit, serait recombiné à de l'oxygène dans une pile à combustible. L'énergie nécessaire au foyer - voire à une voiture électrique - serait ainsi fournie en permanence et à demeure.
     Ces résultats sont jugés "très intéressants" par Paul Lucchese, directeur du programme "Nouvelles technologies de l'énergie" au Commissariat à l'énergie atomique (CEA), qui travaille également sur la filière hydrogène. Ils constituent "une brique supplémentaire dans un ensemble de recherches menées depuis quelques années sur les systèmes biomimétiques, s'inspirant de la photosynthèse naturelle pour produire de l'hydrogène". Toutefois, souligne-t-il, "il ne s'agit encore que d'une expérience de laboratoire et il reste un énorme travail technologique à accomplir avant de disposer de systèmes fonctionnels".

In Situ Formation of an Oxygen-Evolving Catalyst in Neutral Water Containing Phosphate and CO₂⁺
Matthew W. Kanan 1 and Daniel G. Nocera 1* 1 Department of Chemistry, 6-335, Massachusetts Institute of Technology, Cambridge, MA 02139–4307, USA.

* To whom correspondence should be addressed.
Daniel G. Nocera , E-mail: nocera@mit.edu

     The utilization of solar energy on a large scale requires its storage. In natural photosynthesis, energy from sunlight is used to rearrange the bonds of water to O₂ and H₂-equivalents. The realization of artificial systems that perform similar "water splitting" requires catalysts that produce O₂ from water without the need for excessive driving potentials. Here, we report such a catalyst that forms upon the oxidative polarization of an inert indium tin oxide electrode in phosphate-buffered water containing CO₂⁺. A variety of analytical techniques indicates the presence of phosphate in an approximate 1:2 ratio with cobalt in this material. The pH dependence of the catalytic activity also implicates HPO₄^2– as the proton acceptor in the O₂-producing reaction. This catalyst not only forms in situ from earth-abundant materials but also operates in neutral water under ambient conditions.

     The fuel of the future could be hydrogen—if it can be made cheaply enough. Currently, electrolyzers (machines that split water into its constituent hydrogen and oxygen) need a catalyst, namely platinum, to run; ditto fuel cells to recombine that hydrogen with oxygen, which produces electricity. The problem is that the precious metal costs about $1,700 to $2,000 per ounce, which means that hydrogen would be an uneconomical fuel source unless a less costly catalyst can be found. But researchers from the Massachusetts Institute of Technology (M.I.T.) and Monash University in Australia report in Science today that they may have a cost-effective solution.
     Chemist Daniel Nocera, head of the M.I.T.'s Solar Revolution Project, focused on one side of the equation: splitting water into its constituent hydrogen and oxygen molecules. This can be done well, but it remains difficult to actually separate the molecules. But Nocera and postdoctoral fellow Matthew Kanan discovered it could be accomplished by simply adding the metals cobalt and phosphate to water and running a current through it. In contrast to platinum, cobalt and phosphate cost roughly $2.25 an ounce and $.05 an ounce, respectively.
     "We [have] figured out a way just using a glass of water at room temperature, under atmospheric pressure," Nocera says. "This thing [a thin film of cobalt and phosphate on an electrode] just churns away making [oxygen] from water."