An insight into the catalytic mechanism of perovskite ternary oxide for enhancing the hydrogen sorption kinetics of MgH2

Abstract

Solid-state hydrogen storage has emerged as an efficient and reliable technique to commercialize hydrogen energy on a large scale. Magnesium hydride (MgH2), amongst other materials, shows excellent hydrogen storage capability. However, it suffers from setbacks like sluggish kinetics and high thermodynamic stability. Several studies have shown that doping with suitable materials is an effective method to improve the sorption kinetics. Current work is concerned with doping of MgH2 with perovskite-type ternary metal oxide NaNbO3 at 5,10 & 15 wt% doping concentration via ball milling and study of its sorption properties. Thermal desorption mass spectra (TDMS) with thermogravimetry (TG) and Differential scanning calorimetry (DSC) validate the optimum doping concentration to be 10 wt% NaNbO3 in MgH2. According to the isothermal hydrogenation plots, 10 wt% catalyzed sample was capable of absorbing 5.29 wt% hydrogen in just 4.2 min at 150ºC which by far outperforms the 2 h milled MgH2 sample which under the same set of conditions absorbs only 0.67 wt% H2. The catalyst starts affecting the absorption rate right from the room temperature whereas the milled sample has minimal absorption throughout the experiment. At room temperature, the average rate of absorption grows by factor 7 which is staggeringly high. The Kissinger analysis reveals activation energy for hydrogen release as 73.12 kJ/mol for 10 wt% doped NaNbO3-MgH2 system while 137.13 kJ/mol for as milled MgH2. The pressure-temperature isotherm (PCI) at four different temperatures gives a quantitative measure of the thermodynamic stability of the system. To fully comprehend the catalytic process, XRD, SEM, and XPS analysis were conducted after each stage of experiment. XPS suggested possible reduction of Nb valance state from + 5 to + 2 due to surface reduction reaction which further accelerated the sorption kinetics due to the electron transfer process.