Hydrogen Sorption in Erbium Borohydride Composite Mixtures with LiBH4 and/or LiH

Michael Heere,Torben Jensen,Matteo Brighi,Christoph Frommen,Radovan Černý,Bjørn Hauback,Seyed Gharibdoust,Magnus Sørby

doi:10.3390/inorganics5020031

Abstract

Rare earth (RE) metal borohydrides have recently been receiving attention as possible hydrogen storage materials and solid-state Li-ion conductors. In this paper, the decomposition and reabsorption of Er(BH4)3 in composite mixtures with LiBH4 and/or LiH were investigated. The composite of 3LiBH4 + Er(BH4)3 + 3LiH has a theoretical hydrogen storage capacity of 9 wt %, nevertheless, only 6 wt % hydrogen are accessible due to the formation of thermally stable LiH. Hydrogen sorption measurements in a Sieverts-type apparatus revealed that during three desorption-absorption cycles of 3LiBH4 + Er(BH4)3 + 3LiH, the composite desorbed 4.2, 3.7 and 3.5 wt % H for the first, second and third cycle, respectively, and thus showed good rehydrogenation behavior. In situ synchrotron radiation powder X-ray diffraction (SR-PXD) after ball milling of Er(BH4)3 + 6LiH resulted in the formation of LiBH4, revealing that metathesis reactions occurred during milling in these systems. Impedance spectroscopy of absorbed Er(BH4)3 + 6LiH showed an exceptional high hysteresis of 40–60 K for the transition between the high and low temperature phases of LiBH4, indicating that the high temperature phase of LiBH4 is stabilized in the composite.

Highlights

One of the most promising candidates for solid state hydrogen storage applications is LiBH4, which has a theoretical capacity of 18.5 wt % H2
The composites react during ball milling, storage as well as decomposition in a two-step reaction where Er-hydrides and LiBH4 are formed in the first step
ErB4 is formed in a second step during thermal decomposition

Summary

Introduction

One of the most promising candidates for solid state hydrogen storage applications is LiBH4 , which has a theoretical capacity of 18.5 wt % H2. It requires very tough conditions for rehydrogenation [3] and suffers from capacity loss on cycling due to the formation of higher boranes [4]. Another class of materials to be considered is rare earth (RE) borohydrides, with hydrogen capacities varying between 9.0 wt % for Y(BH4 ) and 5.5 wt %. RE borohydrides have shown novel properties such as luminescence, and a magnetocaloric effect has been recently published [24,25,26]

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