Improved hydrogen release from LiB 0.33N 0.67H 2.67 with metal additives: Ni, Fe, and Zn

Abstract

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 LiB 0.33N 0.67H 2.67. NiCl 2 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 Δ T 1/2 = −104 °C for 5 wt% NiCl 2 addition and by Δ T 1/2 = −112 °C for 11 wt% NiCl 2 addition compared to that of additive-free LiB 0.33N 0.67H 2.67. This represents a significant improvement over the maximum temperature reduction of Δ T 1/2 = −90 °C achieved previously with Pt/Vulcan carbon additive (Pt nanoparticles supported on a Vulcan carbon substrate). Transmission electron microscopy on LiB 0.33N 0.67H 2.67 + 11 wt% NiCl 2 revealed uniformly dispersed nanoparticles with diameters less than 8 nm within the LiB 0.33N 0.67H 2.67 matrix, consistent with reduction of the NiCl 2 to metallic Ni during synthesis by ball milling. Mass spectrometry of the evolved gas showed that the total amount of NH 3 released concurrently during dehydrogenation of LiB 0.33N 0.67H 2.67 + 5 wt% NiCl 2 was reduced by an order of magnitude compared to the additive-free material, and by a factor of four compared to LiB 0.33N 0.67H 2.67 + 5 wt% Pt/Vulcan carbon. In contrast to additive-free LiB 0.33N 0.67H 2.67, which melts completely above 190 °C and releases hydrogen from the liquid state only above about 250 °C, hydrogen release from LiB 0.33N 0.67H 2.67 + 5 wt% NiCl 2 is accompanied by partial melting above 190 °C plus transformation to a new, hydrogen-poor solid intermediate phase. Addition of 5 wt% FeCl 2 gave Δ T 1/2 = −36 °C, but Fe, Zn, and ZnCl 2 additives did not produce significant improvement.