Pocket-size PEMs

Hydrides Pack
Away the H₂

By Paul Sharke, Associate Editor

A big issue in automotive fuel cells is the difficulty of storing sufficient hydrogen safely on board a car while giving it adequate range. For powering portable electronic devices, however, hydrogen storage is much less of a question.

A metal hydride alloy before activation. As a preparatory step, activation improves hydrogen absorption and release.

Metal hydride canisters using lanthanum-nickel or titanium-iron alloys are available as standard commercial products for fuel cell developers seeking a means of storing hydrogen in a small space. The composition of these alloys can be tailored to maximize the plateau zone of the hydride. That is the region where 80 to 90 percent of the hydrogen that goes into the metal lattice also comes out, said David Tragna, project manager at Ergenics Inc., a builder of metal hydride storage devices in Ringwood, N.J.

An important consideration in designing hydride storage for portable devices, said Tragna, is the range of temperatures that the device might see. A cell phone user, for instance, could easily inflict temperatures upon the device from -10°C to 70°C, by calling in a tow for a snow-entombed car, or leaving a phone on the dashboard under the summertime sun. As a rule of thumb, Tragna said, the pressure of the hydrogen doubles for every 20°C temperature increase—though the pressure actually increases exponentially with heating. Manufacturers of metal hydride vessels reinforce the containers or modify the hydride structure to accommodate these pressure swings.

Another consideration is the limitation that impure hydrogen can impose on the number of times a hydride can absorb and discharge a gas. Metal hydrides actually make good "getters," a term Tragna used to describe a medium that purifies gases. When the hydride metal is exposed to contaminants such as carbon monoxide or carbon dioxide, the impurities stick to the alloying particles. Used in the manner of a filter, a hydride eventually will absorb its fill of contaminants. So, a hydrogen source only 99 percent pure might limit a hydride to a hundred charging cycles, whereas an ultrapure source could stretch the hydride cycling count to 100,000 or more. That brings up the question of charging. Is it practical for consumers to refill their own cartridges using an activation appliance, or would they be better off picking up fresh cartridges at the store? Any appliance must have its hydrogen lines cleaned and purged before charging a metal hydride vessel; if not, contamination of the cartridge is almost a certainty. Ergenics is exploring both scenarios.