author: Eric Hamonou
HYDROGEN
A new miracle alloy
Producing hydrogen at will, through a simple process of oxidation: at long last this is possible due to a new unheard-of alloy of aluminium. Of note: the first uses of this clean form of energy are expected as soon as 2008.
Take a piece of metal, put it in a recipient, add a bit of water... and hydrogen appears! As simple as that. Better yet, this hydrogen is ready for immediate use: to power electrical appliances, a desalination system, even an automobile! A miracle? No, but a recipe for a new alloy, called Alumifuel, being developed by Global Hydrofuel, a Canadian enterprise from Vancouver. An alloy that might well transform hydrogen the mythological energy vector into an everyday reality, this starting next year. Keeping in mind that the Canadians are not the only ones following this promising lead: last September, Jerry Woodall and his team from Purdue University in Indiana, the United States, also published a version of the recipe, based on the same ingredients. What makes these new alloys interesting is that they resolve two difficulties at once: first of all they eliminate the need to store hydrogen, a perilous task for this highly inflammable and not very dense gas; secondly, they present an economical and rational solution to the question of how to produce this gas.
Finding this miracle alloy...
While hydrogen represents a formidable energy vector for the future, because it is little or not polluting (its use produces no CO2) and one finds it in abundance in nature, it presents the formidable difficulty of always being associated with other atoms, as in the case of water ( H20) or methane (CH4). To use it, then, one first has to break up these molecules, which man is actually incapable of doing without recourse to hydrocarbons (see "the Stakes are High")... Nonetheless, nature has its own ways of breaking up water. The nec plus ultra approach? Doing exactly what plants do, which use sunlight and an ad hoc catalyst, veritable magician's powder, to accomplish this operation.
Yet research in this domain is still far from the goal (see insert). Global Hydrofuel and Jerry Goodall's team have, each on their own, chosen a simpler alternative method and in fine, very efficient: using the mechanism which makes a metal rust in the presence of humidity to produce hydrogen from water. The idea is not new and the chemical principles which underlie it have been known since the discovery of oxygen in the XVIII th century: oxidation. In theory, one merely needs to plunge a bloc of metal in a glass of water so that oxygen separates from hydrogen and links up with metallic atoms to from oxides.
Problem: this reaction is either too slow for us to hope gathering appreciable quantities of hydrogen, or it is too violent. Hydrogen immediately catches fire because of the heat generated, so that one captures little, if any at all. At this oxidation game there is one exception metal, aluminium. It does oxidize rather quickly yet not much heat is generated. But there is a drawback, as Jerry Woodall explains. 'With this material, the oxidation reaction doesn't last long. The surface quickly becomes coated with a layer of alumina, the oxidized version of aluminium, which protects it against corrosion, and thus stops oxidation'. Insurmountable? Not necessarily. 'If the alumina layer forms a particularly efficient shield, it is because aluminium and its oxidized form have pretty much the same density' adds Jerry Woodall. 'In a flash, the surface coat that forms has no faults, it is perfectly non-porous. If these two species had different densities, like iron and its oxidized form, the surface would be full of small cracks through which water could infiltrate and continue the oxidation process, until all the aluminium had been used up'.
Thus, the idea among researchers to use not pure aluminium, but an alloy that contains a small percentage of a catalyst which does not react with water, but which changes the density of aluminium with respect to its oxidized form. Which catalyst? In what quantity in the alloy? Here, solutions differ...so many industrial secrets. Only Jerry Woodall agreed to confide his miracle recipe: 80% aluminium, and 20% gallium, (by percentage weight). The result? 'With 27 grams of aluminium, we have managed to produce 3 grams of hydrogen in a few minutes' time' proudly reports the researcher. Admittedly, this solution is less productive than the "reforming" of methane, the classical method of producing hydrogen. But it is still adequate to look forward to feeding electrical appliances and even a car: 'To feed a sedan equipped with a fuel cell traveling at 100 kilometers per hour for a distance of 560 kilometers, one would need to bring aboard some 80 kilograms of alloy and water, which is roughly the same weight as that of the necessary gasoline', argues Jerry Woodall. ' But the trip's cost would be reduced to a third'.
An abundant metal on earth
This technique thus has the immense merit of solving the principles obstacles to using hydrogen in automobiles. First advantage: no need to store the gas in cumbersome reservoirs under high pressure, veritable ambulatory bombs. The other asset is abundance: aluminium is one of the most widely found metals in the earth's crust. Finally, the residual products are totally recyclable. Gallium is intact at the end of the reaction and ready to serve again. Alumina can be recycled in the traditional operations producing aluminium.
One last problem: these processes are very expensive in electricity (1 kilogram of aluminium requires between 13 and 17 kWh of electricity...) But Jerry Woodall remains optimistic. In his estimation, recycling alumina in the United States would cost 30 Euro centimes per kilogram: still competitive with gas at 50 Euro centimes per liter. Jerry Woodall is looking, for the moment,to restrained applications, such as security systems for dependent people. Global Hydrofuel has just signed on, in France, for a partnership for the production of safety vests equipped with geolocation functioning with Alumifuel, this in 2008. Thus giving reality to Jules Verne's dream of water being "the coal of the future". Clean coal, this time.
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The Stakes are high!
More abundant, more energy-packed, less polluting: hydrogen could be a "clean" alternative to petrol-fuels and gasoline. Unless one is forced extract it from water or methane before using it! A true Herculean feat: one must go beyond 2 000 C to start breaking up water molecules in good quantities...Electrolysis or the "cracking" of methane (thermochemical dissociation of the CH4 molecule) take up too much energy. Anew alloy which would produce ready-to-use hydrogen would thus change the playing field...
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What if we imitated plants?
Rarely a month passes without someone publishing a new method for obtaining hydrogen. From every part of the globe, researchers are struggling to define the miracle solution which will allow us to produce hydrogen in an ecological and economical manner. From this perspective, it is surely the photolysis of water which appears to be the most promising approach, since it proposes the use of the sun to break down the water molecule and produce hydrogen. This is what plants and bacteria which perform photosynthesis accomplish every day. Yet for this reaction to be efficient, one must choose the right catalyst, the substance which allows the reaction to begin. Here is the nub of the problem. Researchers are advancing in a trial and error fashion but to date, no candidate for the perfect catalyst is really appropriate. The most efficient only use UV rays, which is a mere 5% of incident solar energy. As a consequence the production of hydrogen is too little to permit concrete applications. An alternative solution would be to copy what plants themselves already do,using a molecule consisting of four atoms of manganese and one calcium, whose structure we have just found. Which permits us the hope of being able to synthesize it. We still don't know when...
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