Why are metal hydrides unstable
The space saver in the tank
Hydrogen could make driving a car cleaner - but so far there has been a lack of suitable storage materials for the gas. The candidates for a hydrogen tank are being studied by researchers at the Max Planck Institute for Metals Research, the Max Planck Institute for Coal Research and the Max Planck Institute for Colloids and Interfaces.
Sometimes there is a huge gap between desire and reality. It was dead quiet in the lecture hall when Michael Hirscher presented his laboratory results in November 2000. He had repeated his experiments for months, and yet he had always come to the same result: carbon nanotubes, carbon nanotubes, previously praised as hydrogen storage, were extremely reluctant to absorb hydrogen. Hirscher was able to charge them with just under two percent by weight. More was not possible with the best will. The publications of US researchers in respected magazines had looked very different. They attested the seemingly miracle tubes storage capacities of ten, sometimes even 67 percent by weight - fascinating, hardly imaginable measured values.
No question about it: the turn of the millennium was the era of carbon nanotubes. The carbon tubes were already enthusiastically celebrated as signposts to the future of hydrogen. The fall that followed was deep. And it began in November 2000 with Hirscher's lecture at a conference of the US Materials Research Society in Boston. Hirscher had carried out the experiments of the US colleagues in his own laboratory and found out that the exorbitant hydrogen content in no way came from the nanotubes, but rather from microscopically fine titanium splinters from an ultrasound rod with which the US researchers prepared their tubes at the beginning of their experiments .
Hirscher's presentation hit like a hammer, it made it clear that the incredible memory values were not due to scientific ingenuity, but to unbelievable sloppiness. The consequences were severe. The US Department of Energy stopped the funding and said goodbye to hydrogen storage in carbon nanotubes. Michael Hirscher, meanwhile, earned his detective work the reputation of running one of the best analytical laboratories in the world.
The fuel cell car sets the bar
Michael Hirscher works as a metal physicist at the Max Planck Institute for Metals Research in Stuttgart. Despite the sobering results, he remained loyal to carbon for a long time. He meticulously checked what it was really doing. “Today we are convinced that carbon nanotubes actually do not store much more than two percent by weight. And that is clearly not enough for the future of hydrogen, ”says Hirscher. The benchmark for hydrogen storage is the fuel cell car. Because that should open the largest mass market for energy-rich gas in the future. The advantages are well known: In a fuel cell, hydrogen and oxygen molecules combine on a membrane to form water and generate electricity in the process. Only hot steam smokes from the exhaust. It doesn't get any cleaner than that.
Car manufacturers have been sending hydrogen vehicles on test drives for more than ten years. The automobiles have already covered several million kilometers. But they still have a long way to go before they can go into large-scale production. One of the biggest stumbling blocks is hydrogen storage. Although the energy density of hydrogen is around four times greater than that of gasoline or diesel, the hydrogen gas has so far only been able to be stowed away in the car with heavy tank systems, i.e. not really compact.
The current state of affairs is high-pressure tanks that compress hydrogen gas to an impressive 700 bar, 700 times atmospheric pressure. They absorb around five kilograms of hydrogen in this way. In this way, a car actually achieves the range of 500 kilometers required by automobile manufacturers. But the five kilograms need space. Such a high-pressure tank system has a volume of around 260 liters. That corresponds to two voluminous suitcases that engineers have to stow as inconspicuously as possible in the car. A diesel vehicle needs around 33 kilograms of fuel, around 37 liters, for the same distance. With all the trimmings, a suitable tank system measures just 46 liters, as much as a small trolly. To make matters worse, the 700 bar tanks are currently still extremely expensive. The demands on the developers of hydrogen storage systems are: more compact, lighter and cheaper!
It is precisely these requirements that Max Planck scientist Michael Hirscher wants to meet. For several years he has been using MOFs, a strange, fascinating class of crystals. These Metal-Organic Frameworks, metal-organic framework structures, are undoubtedly different from other crystals. MOFs form a hybrid being between organic and inorganic chemistry, the world of plastics and the world of metals. Or, as some MOF researchers put it, “between a PET bottle and sunscreen”.
MOFs consist of a regular porous crystal lattice. At the corners of the grid are metal compounds such as zinc oxide, which is added to sunscreen as a protective pigment.
The corners are linked by molecular bridges similar to plastic, the linkers, as we know them from PET beverage bottles. MOFs are extremely porous and surprisingly light. MOFs weigh as little in the hand as styrofoam balls. A 20 liter drum can easily be lifted with one hand. Their porosity makes MOFs a promising alternative for hydrogen storage, because where there are many pores there is potentially a lot of space for hydrogen molecules. The more pores or breaklines run through a substance, the more surface it offers in the smallest of spaces. And that's ultimately not just about MOFs, but all modern storage materials. Because the larger the surface, the more molecules can settle. MOFs achieve impressive values of up to 4,000 square meters per gram. Hirscher measures exactly how many molecules a MOF can absorb in his laboratory in Stuttgart. Shiny barrels as thick as wine barrels with cryogenic liquid helium are enthroned there. They cool the MOFs down to operating temperature: minus 196 degrees Celsius. Because, as Hirscher and his colleagues have found in a seemingly endless series of measurements, hydrogen is stored in the MOFs above all at very low temperatures.
A measurement like the one from TÜV
Above all, Hirscher's achievement is to be able to measure tiny amounts of MOFs that various research teams send him. A measurement in Stuttgart is equivalent to a TÜV seal. MOFs are a relatively new class of substances that researchers did not become aware of until the mid-1990s. If they discover new compounds, they can usually only synthesize a few milligrams. Hirscher therefore often has to be content with less than a knife point of MOF for his measurements; it is an art to generate reliable analysis values from it. In the meantime, he has worked out measuring methods that register very precisely how many hydrogen molecules settle on the MOF surface at different temperatures. The bonding of the hydrogen to the MOFs is relatively weak. The molecules are not bound chemically, but held by physical forces, van der Waals forces - just like blotting paper with its capillary force simply soaks up ink. This bond to surfaces is called physisorption. Only at low temperatures and a pressure of around 20 bar is the kinetic energy of the hydrogen so low that it even adheres to the MOF surface.
MOF researchers around the world are currently investigating the influence of pore size and different metals on the binding of hydrogen. Hirscher works with experts from BASF in Ludwigshafen, who can now produce certain MOFs in kilograms and who have been researching MOFs since the mid-1990s. "MOFs have the advantage that you can create different molecules and use numerous metals for them," says Ulrich Müller, research director in the field of catalysis at BASF. “We can draw on a lot to design new, more powerful MOFs.” The best MOFs currently store between five and seven percent by weight of hydrogen. That is still not enough for the car. For a practical application you would need at least nine. Nonetheless, Japan in particular is currently making courageous progress when it comes to testing MOFs in prototypes - in the first tank systems. Because the storage material is only part of the whole.
Anyone who wants to use new types of hydrogen storage in the car also needs the appropriate trimmings. And there all new storage concepts still have their shortcomings. MOFs, for example, need a temperature of minus 196 degrees. For the MOF car of the future, the hydrogen would first have to be cooled down with liquid nitrogen when refueling. In addition, the vehicle would need a hermetically sealed cold tank, a cryotank. "The cooling costs energy and the cryogenics would require additional volume," says Hirscher. "If you look at the whole system, it becomes clear that today's MOFs are not yet powerful enough." The 700 bar tank marks the state of the art. The goal is to beat it, although that would be possible in principle, because in a solid storage medium such as MOFs, hydrogen can actually be packed more densely than in the gaseous state. However, neither the MOFs nor other compounds found to date - the complex metal hydrides, for example - achieve the goal.
Looking for light metal hydrides
Michael Felderhoff and Ferdi Schüth at the Max Planck Institute for Coal Research in Mülheim an der Ruhr are working on these storage substances. Metal hydrides consist of light metals and hydrogen, which react with one another when a catalyst is added. Simple metal hydrides have been around for 30 years. And they are actually already in use as storage materials - on modern submarines, for example, which switch to whisper-quiet fuel cell operation when diving. The storage capacity of these compounds is limited to two percent by weight of hydrogen. To store five kilograms of hydrogen, you need around 250 kilograms of metal hydride. In the submarine, which already needs a lot of mass for the dive trip, the additional weight is definitely welcome. For car manufacturers who are fighting for every gram of weight savings with aluminum bodies and magnesium sheets, however, the classic metal hydrides are completely out of the question.
Felderhoff and Schüth are therefore working on the new complex metal hydrides in which they combine different metals with one another. "We try to create compounds of the classic light metals such as sodium or magnesium with so-called transition metals such as titanium, which can bind more hydrogen," says Ferdi Schüth, director of the Heterogeneous Catalysis department. This means that the weight of the molecules hardly increases, while the hydrogen storage capacity increases significantly. The manufacturing process appears amazingly simple. The researchers use ball mills. In it, balls hurling back and forth pulverize small light metal hydride lumps together with transition metal crumbs. If the powder is fine enough, the transition metals slowly migrate into the light metal hydrides.
With the help of small, robust radio sensors in the mill, the researchers can tell whether the newly obtained substance actually stores hydrogen adequately during the grinding process. If an effective metal hydride is formed and hydrogen is bound, the hydrogen pressure in the ball mill decreases. Michael Felderhoff has already succeeded in producing complex metal hydrides from magnesium, calcium and aluminum that store more than nine percent by weight of hydrogen. But the compounds hardly give off the chemically tightly bound hydrogen.
In contrast to physisorption, the chemical bond in the metal hydride splits the hydrogen molecule into two ions, which are, as it were, incorporated into the hydride. "Our goal is therefore to create more unstable hydrides that bind the hydrogen less tightly," says Felderhoff. But that's tricky, because some hydrides are so unstable that they disintegrate again immediately at room temperature and pressure. The people of Mülheim therefore work under higher pressures. “We hope to find complex hydrides that bind enough hydrogen at around 300 bar.” 300 bar is something like a magical limit, because car tanks for 300 bar have been around for a long time. They are significantly cheaper than the newer 700-bar models and would therefore currently be more conceivable for series use in cars.
And the metal hydride experts have to clear another hurdle: If metal hydrides absorb hydrogen, the chaotically floating hydrogen atoms change into an orderly and thus low-energy state. This creates heat. This can be used to free the hydrogen from the hydride again: the hydride is simply heated so that the reaction proceeds in reverse and the hydride breaks down into metal and hydrogen. However, the heat released when refueling would heat the material to several hundred degrees. A vehicle tank would need huge heat exchanger plates to dissipate the heat. Too difficult. But Felderhoff also hopes to get the heat problem under control with the help of new unstable metal hydrides - if hydrogen atoms are more loosely bound, less binding energy is released. The Institute for Energy and Environmental Technology in Duisburg has constructed prototypes of hydrogen storage systems with metal hydrides.
In the search for the ideal hydrogen trap, Felderhoff and Schüth also work closely with General Motors' Research Center for Alternative Drives in Mainz-Kastel. "We are happy about this cooperation, after all, the Mülheim working group is the most outstanding in the world when it comes to complex metal hydrides," says GM project manager Ulrich Eberle. Eberle and his employees are currently advancing all three storage technologies in parallel - the 700 bar tank, the MOFs and the complex metal hydrides. In addition, the car manufacturers are developing their own storage substances.
Refueling along the lines of the lungs
“With a 700 bar tank, a hydrogen vehicle can already travel 500 kilometers - about two to three times as far as with battery operation,” says Eberle. “Our goal, however, is to further increase the energy density of the tank with new technologies; At the moment, however, we cannot say with any certainty which technology is the best. GM has already constructed the first test tanks, demonstrators, in which the new materials are tested. Eberle: “We want to know how well and, above all, how quickly the various materials absorb hydrogen and release it again - and how often they survive such tank cycles.” The fact that the storage tank absorbs hydrogen quickly and then releases it again is essential for use in cars crucial. Because nobody wants to wait 15 minutes until the MOF or metal hydride is finally filled up.
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