A tank - with a cat

Hydrogen is one of the energy sources of the future. But just when this environmentally friendly substance will be adopted on a large scale to fuel our cars remains to be seen. Not least because it is as yet unclear how sufficient hydrogen (H2) can be produced to keep the world’s growing fleet of vehicles on the move. Nor has an ideal solution yet been found for the onboard storage of this volatile chemical. Usually, vehicles are tanked up with liquid hydrogen. But liquefying the gas costs money and energy. In addition, the hydrogen gradually runs low as a result of vaporization. On the other hand, gaseous hydrogen must be stored in exceptionally stable containers – a challenge for car makers, as only cylindrical tanks can withstand the high gas pressures of up to 350 bar (350 times atmospheric pressure). This limits manufacturers’ freedom of design.

For some years, researchers have therefore been tinkering with an alternative hydrogen reservoir, the basic principles of which were developed about 10 years ago at the Max Planck Institute for Coal Research – a light metal hydride reservoir, which is a complex of metals and a variable number of hydrogen atoms. The idea is that, when the tank is filled, with the aid of a catalyst, the H2 is bound with the light metal hydride, to be released during the journey and fed into a fuel cell that turns the hydrogen into energy. Thus the hydrogen is stored in neither a liquid nor a gaseous state, but is chemically packaged – in a solid state.

As long as 30 years ago, motor manufacturers were testing the suitability of metal hydrides as H2 reservoirs. But the heavy metals tried out in those days were too weighty for efficient use. The hydrogen accounted for only around 1.5 percent of the total weight of the reservoir. The use of light metal hydrides has changed all that: the scientists in Mülheim turned to lighter sodium alanate (NaAlH4) and hiked the hydrogen content up to 5 percent by weight. This made these substances an attractive prospect for vehicular use. Meanwhile, the Max Planck researchers are cooperating with an international automotive group that intends to use light metal hydrides in a prototype tank. This could be ready for its maiden voyage in around two years – even though the process still requires some optimization. After all, whether a reservoir system of this kind prevails as a patented future transportation solution depends, above all, on the time it takes to fill up.

No one wants to wait more than 5 minutes to fill their tank. So the catalyst must bind the hydrogen with light metal hydride with enormous speed. Fill-up times of 10 minutes are already a possibility – albeit only in the laboratory, at high pressures. With the necessary moderate pressure of below 50 bar, waiting times are presently around an hour. Catalyst experts are therefore working on more effective mixtures of substances, changing the external structure and the chemical – stoichiometric – composition of their catalytic particles. Besides sodium alanate, tests are also being carried out with calcium, lithium and magnesium alanates, which can store more hydrogen. Just when the team in Mülheim will find the perfect H2 reservoir with a better than 5.5 percent hydrogen content and the right catalyst remains to be seen.

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