Article published Fuel cell
A material which stores and generates hydrogen without risk
A hurdle to the development of fuel cells is the issue of hydrogen storage. The CEA is seeking an industrial partner to design a prototype. A key advantage for nomadic technologies and backup systems.
This is a neat white powder which has the advantage of containing hydrogen. It can release the hydrogen progressively so that it is consumed slowly. It is ideal for storing precious gases and supplying fuel cells (PAC) for mobile phones or back-up systems on board aircraft, ships, spacecraft, etc.
“With one gram of this material, that can be shaped into pellets of various sizes, a portable telephone can be supplied with power for 24 hours”, states Jérôme Saillard, a research engineer in the CEA Military Applications Division. The material in question is ammonia-borane (chemical formula: NH3BH3). It belongs to the family of borohydrides, synthetic chemical compounds which have high hydrogen densities: Ammonia-borane contains 19.6 % hydrogen by mass and can release two-thirds of this under the action of heat, by thermolysis, which is an excellent return in comparison to other materials. It is then just a question of know-how, of understanding and controlling the auto-combustion phenomena of the material, of measuring the quantity and degree of purity of the hydrogen released - a key point for fuel cells - and finally of designing the prototypes.
“We have constructed a small demonstration unit where the released hydrogen is stored in a stainless steel canister and used to supply a fuel cell which runs a small fan”, indicated the researcher. The hydrogen is generated by initiating auto-combustion (more correctly termed “autopyrolysis”) by heating the material to above 100 °C using, for example, a small heating wire with a current flowing through it. The reaction propagates by itself and, very slowly, the material is entirely consumed. “We have filed a patent on this subject", adds Jérôme Saillard. One of the main manufacturing secrets relates to the thermal auxiliaries which ensure combustion and the purity of the hydrogen generated.”
A first generation of prototypes designed for nomadic applications (GSM, PDA, etc.) will be ready this year. Then, in partnership with a fuel cell manufacturer, the CEA intends to develop a second generation for these applications and also for aeronautic and naval back-up systems, with stores of several kilograms, to supply a one kilowatt-hour fuel cell for around ten hours. In fact the applications are as varied as the packaging possibilities: from several grams in the shape of slabs for a mobile telephone, or a little larger for military field equipment, to several kilograms for a back-up system or a power supply which is independent from the grid. “This solution has the advantage of storing hydrogen at ambient pressure over a large range of temperatures (-40°C to + 60°C) and is able to generate it on demand and at low pressure (1 to 2 bars), which considerably limits the maintenance and safety issues for this equipment”, concludes Jérôme Saillard. This technology provides a complementary solution to current processes developed by the CEA for hydrogen storage.
