Abstract
A systematic investigation of phase transitions in unmilled and milled LiBH4 has been performed by Pressurized Differential Scanning Calorimetry (PDSC). It was found that a large exotherm is present below the low temperature (LT) → high temperature (HT) phase transition. This exotherm is not caused by air contamination but seems to originate from hydrogen release from a solid solution in the matrix of LiBH4 low temperature phase. The exotherm activation energy has been measured to be 100 kJ mol–1. Calorimetric measurements under argon and hydrogen have shown that for the milled sample, the endothermic peak of the LT → HT transition is split in two when the PDSC scan is performed under hydrogen atmosphere. Synchrotron X-ray powder diffraction on the milled LiBH4 sample revealed only a single-step transition from the LT to HT phase, both under vacuum and under 2 and 40 bar of hydrogen pressure. The axial ratios for the LT LiBH4 below 300 K are significantly altered by milling; they are also considerably different under 40 bar of hydrogen, indicating an interaction between the hydrogen gas and the LT LiBH4 solid phase.
Highlights
In the perspective of a future hydrogen economy, there is an obvious need for hydrogen storage material that has high volumetric and gravimetric capacity as well as low cost and low temperature of operation
It should be pointed out that, because LiBH4 reacts with water vapour contained in air, a sealed sample holder had to be used for X-ray powder diffraction measurements
From Pressurized Differential Scanning Calorimetry (PDSC) measurements under argon and hydrogen atmospheres, it was revealed that this exotherm is closely related to the presence of hydrogen and has an activation energy of
Summary
In the perspective of a future hydrogen economy, there is an obvious need for hydrogen storage material that has high volumetric and gravimetric capacity as well as low cost and low temperature of operation. In this respect, the potential of lithium boro-hydride LiBH4 as hydrogen storage material has been recently exposed by Züttel et al [1]. –74 kJ mol–1 and a hydrogen capacity of 13.8 wt% [2] As this hydrogen reaction enthalpy is comparable to magnesium hydride, similar temperatures of the order of 573 K are required for desorption at around one bar of hydrogen pressure. Elevated temperatures of around 873 K and pressures of the order of 150 bar are needed for LiBH4 formation from the decomposition products [3,4,5]
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