Mg2FeH6 formation and hydrogenation mechanisms in Mg-32 wt. %Fe nanocomposite produced by cold rolling

• Grant number: 18/21231-4
• Support Opportunities: Scholarships abroad - Research Internship - Scientific Initiation
• Effective date (Start): January 03, 2019
• Effective date (End): March 29, 2019
• Field of knowledge: Engineering - Materials and Metallurgical Engineering - Physical Metallurgy
• Principal Investigator: Daniel Rodrigo Leiva
• Grantee: Lucas Varoto
• Supervisor: Jacques Huot
• Host Institution: Centro de Ciências Exatas e de Tecnologia (CCET). Universidade Federal de São Carlos (UFSCAR). São Carlos , SP, Brazil
• Research place: Université du Québec à Trois-Rivières (UQTR), Canada
• Associated to the scholarship: 17/17952-5 - Mg-based nanocomposites for hydrogen storage with additives containing Zr or Fe, BP.IC

Abstract

Hydrogen storage is an important topic of applied research, in a way that H2 becomes viable as a cleaner and renewable energy source. Some of the major recent advances in this area refer to the development of solid hydrogen tanks using metallic hydrides, in particular based on MgH2. This material presents as main advantages its high volumetric density of energy and the low cost of starting metal. The elaboration of MgH2 nanocomposites by high energy milling (HEBM) promotes important improvements in the kinetics of hydrogen absorption/desorption at temperatures around 300 °C. However, HEBM processing consists and requires parameters that are not practical and fully viable for the commercial production of alloys, composites and others. On the other hand, severe plastic deformation (SPD) processing methods, more specifically cold rolling (CR), have a significant cost and process feasibility advantage over HEBM. This project is based on the expansion of the research conducted in the first year of the scientific initiation grant. Significant results referring to the hydrogen storage properties, especially the desorption at lower temperatures, of mixtures of MgH2 with high content of additives were obtained. The mixture containing 32 wt% Fe shows very positive and expressive results not yet related in the literature. Therefore, the investigation of the H-absorption/desorption behavior of this same mixture, processed now by cold rolling, is a relevant research question. Also, the high content of iron in the mixture and the influence of the microstructure and morphology imposed by cold rolling are interesting aspects to be considered for the mechanisms of complex hydride formation and hydrogenation. During the milling process under hydrogen atmosphere, in Mg-Fe-H systems, the formation of Mg2FeH6 involves two steps: MgH2 formation at shorter times, and reaction between MgH2 and Fe to produce Mg2FeH6 as milling time increases. The presence of Fe in the mixture also produces a catalytic effect on hydrogen desorption kinetics from MgH2. Among all complex hydrides, the Mg2FeH6 shows the highest known volumetric hydrogen density and has a high gravimetric hydrogen density of 5.47 wt.%, resulting in a promising hydrogen storage material. However, this ternary hydride has the particularity that Mg and Fe do not form stable binary compounds between themselves. As a consequence of this, the synthesis of Mg2FeH6 is difficult to carry out. In this project it's expected to study and identify by fitting kinetics curves the mechanisms of the kinetics reactions and the formation of Mg2FeH6 by processing Mg with 32 wt.% of Fe in cold rolling, differently than the common processing route, ball milling. As cold rolling has less energy involved in its process and the interface contact between bulk Mg and additive Fe is different than in powders conditions, the formation mechanism and the effects of Mg2FeH6 is an aim to understanding its' behavior under others conditions. Thus, the solid state hydrogen kinetics of products made by these types of processing can be described and better understood by fitting in theoretical kinetics mechanism models combined with structural characterization. (AU)