Nanoscale Mg-B via surfactant ball milling of MgB 2: morphology, composition and improved hydrogen storage properties

Abstract

Metal borides have attracted the attention of researchers due to their useful physical properties and unique ability to form high hydrogen-capacity metal borohydrides. We demonstrate improved hydrogen storage properties of a nanoscale Mg-B material made by surfactant ball milling MgB2 in a mixture of heptane, oleic acid and oleylamine. TEM data shows that Mg-B nanoplatelets are produced with sizes ranging from 5 � 50 nm, which agglomerate upon ethanol washing to produce an agglomerated nanoscale Mg-B material of micron-sized particles with some surfactant still remaining. XRD measurements reveal a two-component material where 32% of the solid is a strained crystalline solid maintaining the hexagonal structure with the remainder being amorphous. FTIR shows the oleate binds in a "bridge bonding" fashion preferentially to magnesium rather than boron, which is confirmed by density functional theory calculations. The Mg-B nanoscale material is deficient in boron relative to bulk MgB2, with a Mg:B ratio of ~1:0.75. The nanoscale MgB0.75 material has a disrupted B-B ring network as indicated by x-ray absorption measurements. Hydrogenation experiments at 700 bar and 280 �C show it partially hydrogenates at temperatures 100 ?C below the threshold for bulk MgB2 hydrogenation. In addition, upon heating to 200 ? C, the H-H bond breaking ability increases ~10-fold according to hydrogen-deuterium exchange experiments due to desorption of oleate at the surface. This behavior would make the nanoscale Mg-B material useful as an additive where rapid H-H bond breaking is needed.