Hydrogen Storage Materials for Clean Cars

Researchers from the University of Alberta use neutron beams to discover how thin metal layers enhance store hydrogen effectively, aiming to one day enable hydrogen-powered cars without the safety issues of pressurized tanks.

Source: Canadian Neutron Beam Centre (CNBC)
Contact: cnbc@cnl.ca
Image: The 2008 Honda FCX Clarity was the first hydrogen fuel cell vehicle available to retail customers.

If we want to stop using carbon dioxide-emitting fossil fuels in our cars, an environmentally-friendly alternative may be to use hydrogen as a fuel, because it only produces water and energy when it is “burned,” that is, when it is combined with oxygen. To advance this technology, we need materials that store and release hydrogen efficiently, as an alternative to the pressurized tanks in the first hydrogen cars that have arrived on the market recently.

Neutron beams are effective probes for studying materials that are candidates for storing hydrogen, because they can detect hydrogen within metals. Prof. David Mitlin and his research group from the University of Alberta use neutron reflectometry at the CNBC to examine thin films of these materials under realistic conditions.

One candidate material for hydrogen storage is magnesium because it has a very high storage capacity for hydrogen (7.6 wt.%), but it is limited by its slow response in accepting and releasing the hydrogen. The slow response is due in part to a build-up of magnesium hydride at the surface, which blocks further movement of the hydrogen in and out of the magnesium film.

Illustration of hydride formation (in the form of deuterium, D) in thin films of magnesium in which a blocking layer forms (left), and of magnesium-chromium-vanadium in which the hydrogen moves more freely (right).

Illustration of hydride formation (in the form of deuterium, D) in thin films of magnesium in which a blocking layer forms (left), and of magnesium-chromium-vanadium in which the hydrogen moves more freely (right).

Recently, Mitlin observed much faster responses when a combination of other metals was added (chromium and vanadium), and then accessed the CNBC to understand the improvement. The results showed that the addition of chromium and vanadium had the effect of preventing the blocking layer of magnesium hydride from forming and allowing the hydrogen to move much faster throughout the magnesium.

The elucidation of the underlying mechanism led to an examination of the effect of adding other metals like chromium and iron together to the magnesium. This alloy turned out to be another promising hydrogen storage system, in which the blocking layer is prevented in a manner similar to the magnesium-chromium-vanadium system.