Understanding the kinetic enhancement of hydrogen sorption in MgH2 via KNbO3 through the perspective of the catalytic mechanism

Abstract

MgH₂ possesses the attributes of a superior hydrogen storage material because of its excellent efficiency, safety, large hydrogen storage capacity, and cheap cost. However, it still has certain flaws, including a high working temperature and sluggish desorption kinetics, which restrict its applicability and open possibilities to device new methodologies to circumvent these obstacles. The current work is concerned with the study of the hydrogen storage capabilities of MgH₂ - x wt% KNbO₃ (x = 5,10,15) and the underlying mechanism. Among these composites, the MgH₂+ 10 wt% KNbO₃ sample has the finest performance, which was concluded after evaluating the desorption temperature and isothermal absorption kinetics. Compared to the as-milled and as-received MgH₂, the 10 wt% doped sample began to emanate hydrogen at about 232 °C (onset temperature) with a peak temperature of 238.5 °C; a reduction of almost 96 °C and 180 °C, respectively. Furthermore with the addition of 10 wt% KNbO₃, the apparent activation energy for dehydrogenation was decreased from 170 kJ/mol for as received MgH₂ to 70 kJ/mol for catalyzed one. The catalysis yields a 60 % decrement in the kinetic barrier, which is a rather remarkable result compared to the pristine one. Hydrogen absorption at room temperature sees a remarkable upgradation in terms of both increased rate and higher wt% stored by the catalyzed sample (4.28 wt% in 4 h) compared to milled one (0.22 wt% in 4 h). X-ray diffraction (XRD), Scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS) were performed after each step to gain an in-depth understanding of the catalytic mechanism. XPS confirmed that the higher valency Nb (+5) in KNbO₃ was converted to the lower valency speceies, which fastened the transformation of electrons. Additionally, the formation of oxygen vacancies was confirmed which act as active centers and promote electron mobility and hydrogen diffusivity. This event created a multielement multivalent catalytic environment, which improves the sorption kinetics of MgH₂. Our research might serve as a blueprint for future high-performance, complex catalyst creation for magnesium hydride as reversible hydrogen storage.