Promotion of Hydrogen Release from Ammonia Borane with Mechanically Activated Hexagonal Boron Nitride
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Abstract:
Nanoscale hexagonal BN additive for ammonia borane, AB, is shown to decrease the onset temperature for hydrogen release. Both the nano-BN and the AB:nano-BN samples are prepared by ball milling. The materials are characterized by X-ray powder diffraction, 11B muclear magnetic resonance, thermogravimetric analysis, differential scanning calorimetry, and mass spectrometry, and the hydrogen release is measured by a volumetric gas burette system. Several effects of the mixtures of AB:nano-BN are shown to be beneficial in comparison with neat AB. These are the decrease of the dehydrogenation temperature, the decrease in NH3 formation, as well as the decrease of the exothermicity of hydrogen release with increasing the nano-BN concentration.Keywords:
Ammonia Borane
Thermogravimetric analysis
Boranes
Ammonia Borane
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Cryo-adsorption
Boranes
Gravimetric analysis
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Potassium hydride (KH) was directly added to a Mg(NH2)2–2LiH system to improve the hydrogen storage properties; the corresponding mechanisms were elucidated. The Mg(NH2)2–2LiH–0.08KH composite displays optimized hydrogen-storage properties, reversibly storing approximately 5.2 wt% hydrogen through a two-stage reaction and a dehydrogenation onset at 70 °C. The 0.08KH-added sample fully dehydrogenated at 130 °C begins to absorb hydrogen at 50 °C, and takes up approximately 5.1 wt% of hydrogen at 140 °C. Adding KH significantly enhances the de-/hydrogenation kinetic properties; however, an overly rapid hydrogenation rate enlarges the particle size and raises the dehydrogenation temperature. A cycling evaluation reveals that the KH-added Mg(NH2)2–2LiH system possesses good reversible hydrogen storage abilities, although the operational temperatures for de-/hydrogenation increase during cycling. Detailed mechanistic investigations indicate that adding KH catalytically decreases the activation energy of the first dehydrogenation step and reduces the enthalpy of desorption during the second dehydrogenation step as a reactant, significantly improving the hydrogen storage properties of Mg(NH2)2–2LiH.
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