Daphiny Pottmaier

NaBX 4-MgX 2 Composites (X= D, H) Investigated by In Situ Neutron Diffraction

Light element complex hydrides (e.g. NaBH 4) together with metal hydrides (e.g. MgH 2) are considered two primary classes of solid state hydrogen storage materials. In spite of drawbacks such as unfavorable thermodynamics and poor kinetics, enhancements may occur in reactive hydride composites by nanostructuring of reactant phases and formation of more stable product phases (e.g. MgB 2) which lower overall reaction enthalpy and allow reversibility. One potential system is based on mixing NaBH 4 and MgH 2 and subsequent ball milling, which in a 2:1 molar ratio can store considerable amounts of hydrogen by weight (up to 7.8 wt%). A study of the 2NaBX 4 + MgX 2 → MgB 2 + 2NaX + 4X 2 (X=D,H) reaction is assessed by means of insitu neutron diffraction with different combinations of hydrogen and deuterium on the X position. The desorption is established to begin at temperatures as low as 250 °C, starting with decomposition of nanostructured MgX 2 due to joint effects of nanostructured MgX 2 and its reducing effect at NaBX 4. Analyses of background profile, due to the high incoherent neutron scattering of hydrogen, as a function of temperature demonstrate direct correlation of H/D desorption reactions with relative phases amount.

Hydrogen Desorption Reactions of the Na-Mg-B-H System

Hydrogen storage in the solid state has shown increasing research and development, and recently an approach in mixing two hydride systems together by ball milling (reactive hydride composites) has been investigated in more detail, e.g. NaBH4 plus MgH2. Thermodynamic destabilization may occur by new compounds formation during dehydrogenation, e.g. MgB2. A study of the the role of O2/H2O contamination for the reaction 2NaBH4 + MgH2 ↔ 2NaH + MgB2 + 4H2 was conducted using in-situ X-ray powder diffraction. Desorption reaction is observed to begin by a competition of MgH2 and NaBH4 decomposition due to higher reactivity promoted by ball milling processing summed to O2/H2O contamination. Oxidation of NaBH4 into NaBO2 is observed to happen in higher degree than MgH2/Mg into MgO for the Na-Mg-B-H system.