Improved hydrogen release from LiB0.33N0.67H2.67 with metal additives:: Ni, Fe, and Zn

We have examined the effect of adding small quantities of Fe, Ni, and Zn, or their dichlorides, on the dehydrogenation temperature of the new quaternary hydride material LiB0.33N0.67H2.67. NiCh proved to be an especially effective dehydrogenation promoter. The hydrogen release temperature, represented by the temperature T-1/2, at which the hydrogen release reaction is half completed, decreased by AT(1/2) = -104 degrees C for 5 wt% NiCl2 addition and by Delta T-1/2 = -112 degrees C for 11 wt% NiCl2 addition compared to that of additive-free LiB0.33N0.67H2.67. This represents a significant improvement over the maximum temperature reduction of Delta T-1/2 = similar to 90 degrees C achieved previously with Pt/Vulcan carbon additive (Pt nanoparticles supported on a Vulcan carbon substrate). Transmission electron microscopy on LiB0.33N0.67H2.67 + 11 wt% NiCl2 revealed uniformly dispersed nanoparticles with diameters less than 8 nm within the LiB0.33N0.67H2.67 matrix, consistent with reduction of the NiCl2 to metallic Ni during synthesis by ball milling. Mass spectrometry of the evolved gas showed that the total amount of NH3 released concurrently during dehydrogenation of LiB0.33N0.67H2.67 + 5 wt% NiCl, was reduced by an order of magnitude compared to the additive-free material, and by a factor of four compared to LiB0.33N0.67H2.67 + 5 wt% Pt[Vulcan carbon. In contrast to additive-free LiB0.33N0.67H2.67, which melts completely above 190 degrees C and releases hydrogen from the liquid state only above about 250 degrees C, hydrogen release from LiB0.33N0.67H2.67 + 5 wt% NiCl, is accompanied by partial melting above 190 degrees C plus transformation to a new, hydrogen-poor solid intermediate phase. Addition of 5 wt% FeCl2 gave Delta T-1/2 = -36 degrees C, but Fe, Zn, and ZnCl2 additives did not produce significant improvement. (c) 2006 Elsevier B.V. All rights reserved.