Understanding the catalysis of chromium trioxide added magnesium hydride for hydrogen storage and Li ion battery applications

Abstract

This study explores how the chemical interaction between magnesium hydride (MgH2) and the additive CrO3 influences the hydrogen/lithium storage characteristics of MgH2. We have observed that a 5 wt.% CrO3 additive reduces the dehydrogenation activation energy of MgH2 by 68 kJ/mol and lowers the required dehydrogenation temperature by 80 °C. CrO3 added MgH2 was also tested as an anode in an Li ion battery, and it is possible to deliver over 90% of the total theoretical capacity (2038 mAh/g). Evidence for improved reversibility in the battery reaction is found only after the incorporation of additives with MgH2. In depth characterization study by X-ray diffraction (XRD) technique provides convincing evidence that the CrO3 additive interacts with MgH2 and produces Cr / MgO byproducts. Gibbs free energy analyses confirm the thermodynamic feasibility of conversion from MgH2/CrO3 to MgO/Cr, which is well supported by the identification of Cr(0) in the powder by X ray photoelectron spectroscopy (XPS) technique. Through high resolution transmission electron microscopy (HRTEM) and energy dispersive spectroscopy (EDS) we found evidence for the presence of 5 nm size Cr nanocrystals on the surface of MgO rock salt nanoparticles. There is also convincing ground to consider that MgO rock salt accommodates Cr in the lattice. These observations support the argument that creation of active metal – metal dissolved rock salt oxide interface may be vital for improving the reactivity of MgH2, both for the improved storage of hydrogen and lithium.