Magnesium for reversible hydrogen storage

Hydrogen can be used in fuel cells to produce electricity via electrochemical reaction with air, without any emission of greenhouse gases. Fuel cell technology is still under development, but hydrogen fuelled thermal engines could provide a reasonable intermediate solution. Although hydrogen is sourced from fossil hydrocarbons at present, it could represent a clean alternative for storage of energy involving zero CO2 emission if it were itself produced by renewable energy sources. Nevertheless, development of a true “hydrogen economy” requires an efficient and safe method to store hydrogen. Current proposals such as very high (700 Bars) pressure or cryogenic liquid storage are not yet satisfactory in terms of safety, volume density and the energy consumption for densification. Conversely, storage in reversible metal hydrides, from which hydrogen easily desorbs by heating or by lowering the overpressure, makes a much safer solution. The metal is loaded under a H2 pressure of only 10 Bars, and any brutal release of hydrogen is self-interrupting because the desorption reaction is endothermic.

Magnesium Hydride MgH2 is an excellent hydrogen storage system since Mg is an abundant, low cost, non-toxic element. Its mass storage capacity is remarkable : up to 7.6% by weight. If the kinetics of hydrogen absorption and desorption are slow for standard Mg powders, the powders can be strongly activated by aggressive ball-milling them together with transition metal elements. We have investigated the processes occurring in energetic co-milling of poorly reacting MgH2 with small amounts of transition metals (TMs) such as Ti, V, Nb, etc. We find that the initially micron size crystal grains are restructured at the nanometre scale and that TM nanoparticles are deposited as hydrides TMHx. This produces a dramatic enhancement of the hydrogen sorption kinetics. The initating role of the TM hydride was seen in neutron diffraction studies at the Institut Laue-Langevin. During hydrogenation, MgH2 appears only in a second step : the transition metal hydride TMHx forms first and acts as catalyst for dissociation of hydrogen molecules and the diffusion of the hydrogen into the magnesium. Two patent applications have been made and the techniques developed here have been transferred to Metals Composites Powders Mg-Serbien, a company specialized in magnesium granulation. At present, 1 kg batches of highly reactive powders are produced at a semi-industrial scale. These powders retain very stable properties through successive absorption/desorption cycles.

A prototype tank to contain MgH2 has also been developed. The main challenge was to measure and control the heat gradient in the bed of MgH2, which results from the exothermic nature of the Mg-H reaction and the poor thermal conductivity of MgH2 powder. That is, charging with hydrogen sharply increases the temperature so that Mg-MgH2 equilibrium is quickly reached, stopping further hydrogenation. Optimizing heat exchangers has given a much reduced loading time. This first reservoir stores 170 litres gas volume of hydrogen at a volume density comparable to that of liquid hydrogen. A Proton Exchange Membrane Fuel Cell (PEMFC) was fuelled from the reservoir, lighting a halogen lamp for one week.
Fig. 1 : Mg nano-crystallites, as observed by HREM after co-milling with transition metals, after hydrogen desorption.
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