Low-Temperature Dehydrogenation of Vapor-Deposited Magnesium Borohydrides Imaged Using Identical Location Microscopy

Abstract

Complex metal hydrides are promising hydrogen storage materials with their high volumetric and gravimetric capacities, but they are limited by high temperatures for hydrogen release and slow kinetics for hydrogen uptake. These limitations necessitate modifications to allow hydride materials to become more technologically viable. It has been shown that chemical additives can reduce the hydrogen desorption temperature and improve reaction rates of a hydride matrix. Currently, the most common method to integrate chemical additives is through ball milling. In contrast, this work investigates the vapor-phase delivery of chemical additives to the gamma phase of Mg(BH4)2, adapted from atomic layer deposition which uses sequential precursor exposures on a material surface to grow thin films. The modified Mg(BH4)2 materials demonstrated up to 7.6 wt % hydrogen release at temperatures as low as 100 °C. The materials in this work were characterized extensively using temperature-programmed desorption, the Sieverts method, and X-ray diffraction. Additionally, identical location scanning transmission electron microscopy was conducted to identify the chemical complexities of the modifications introduced from the vapor-phase delivery process and through the hydrogen desorption process. Findings from the microscopy study were combined with the aforementioned characterization techniques to illuminate the mechanism of decomposition and allow for a better understanding of the vapor-phase-modified materials. Overall, this work demonstrates how combining a diverse suite of characterization techniques, especially electron microscopy, enables the discovery and understanding of the sorption and chemical processes that take place in hydrogen storage materials and contribute to accelerate research in this field.