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R. Bachofner, G. Schmid, L. Stöttinger, C. Eisenmenger-Sittner:
"Design, optimization and experimental evaluation of a device to measure hydrogen release from coated hollow glass microspheres";
Vortrag: 15th Joint Vacuum Conference (JVC15), Hotel Kaiserwasser, Wien; 15.06.2014 - 20.06.2014.

Hydrogen and other gases can be stored in hollow glass microspheres (diameter approx. 50µm, wall strength approx. 1 µm) under high pressures and elevated temperatures. To release the gas the spheres have to be subjected to temperatures of around 100°C to trigger the diffusion process of the trapped gas through the wall of the spheres. This temperature can be generated by an exothermic reaction. In the case of hydrogen storage an aqueous solution of NaBH4 (VenpureTM) can be used. In the presence of a catalyst water chemically reacts with NaBH4 according to the reaction equation NaBH4 + 2 H20  NaBO2 + 4 H2 + Q. As it can be seen readily, this produces hydrogen and an amount of heat, Q. The catalyst can be applied to the microspheres by uniformly coating them with a special magnetron sputtering process [1].
To evaluate the efficiency of the chemical reaction given above, a device was designed and constructed which is capable of measuring the hydrogen release and the temperature increase in the presence of catalyst coated microspheres. It consists of a reaction chamber in which the aqueous solution of NaBH4 and the coated microspheres can be mixed. The temperature rise in the mixture is monitored by a thermocouple and the produced hydrogen is funneled into an initially water filled vessel. The water is forced out from the vessel into a surrounding reservoir and the mass change due to the addition of hydrogen can be measured by a balance.
To evaluate the thermal characteristics of the device heat transport and heat losses from the reaction chamber to the ambient were modeled by a finite element model to get information on the influence of the chamber size on the temperature maximum measured by the thermocouple. After this theoretical evaluation the device was put into operation and several batches of microspheres coated with Pt were evaluated. It was found that the thickness of the Pt coating is a crucial parameter for the efficiency of the reaction.
The financial support of the Austrian Science Fund (FWF) under Grant Nr. P-22718 is gratefully acknowledged.
References
[1] G. H. S. Schmid, C. Eisenmenger-Sittner, Surface & Coatings Technology 236 (2013) 353.