Understanding Charge Transfer at Mg/MgH2 Interfaces for Hydrogen Storage

Abstract

Three-phase boundaries (solid-solid-gas) significantly influence the thermodynamics and kinetics of hydrogenation and dehydrogenation processes in metal hydrides for hydrogen storage. Interfacial processes govern hydrogen dissociation, mass transport, phase nucleation and growth during cycling. These properties are critically influenced by chemical composition, structure, interface thickness, energetics, and etc. Unfortunately, strategies for controlling these parameters in order to optimize hydrogen storage performance are challenging to explore both in experiments and computations due to the complicated heterogenous nature of the interfaces. In experiments, for instance, direct observation and quantitative characterization of interfaces often require destroying a sample; furthermore, isolating interface-derived data from bulk properties is extremely difficult [1,2]. For this reason, a computational approach to model the interface structure, energetics, and composition becomes a valuable tool. Moreover, the formation and decomposition of metal hydrides is accompanied by electron transfer as well as chemical adsorption and desorption of H2molecules on surfaces/interfaces. However, the role of electron transfer and heterogeneous interfaces has received little attention in the hydrogen storage community, unlike more conventional electrochemical systems such as batteries. In fact, metal hydrides for hydrogen storage share much in common with electrochemical devices. For example, in hydrogen storage hydrogen gas reacts with solid metal phases and forms metal hydrides during hydrogenation, similar to the way oxygen gas reacts with metal ions in metal-air batteries during discharge. In subsequent dehydrogenation or charging processes, these materials likewise release hydrogen or oxygen gases, respectively. Processes in hydrogen storage are driven by hydrogen pressure- and temperature-dependent chemical potentials, while chemical potential and voltage are analogous driving forces in electrochemical systems. As a part of the DOE Hydrogen Storage Materials — Advanced Research Consortium (HyMARC), we have been leveraging the similarities between these systems to introduce new modeling concepts to study heterogeneous interfaces in metal hydrides for hydrogen storage. We have categorized interfaces into three groups: intercalation, sharp, and diffuse interfaces, based on whether lattice deformation occurs between phases and the extent of variations of composition and strain profiles (see Table 1). In this talk, we will provide examples of how different computational techniques can be deployed for each of these three different types of interfaces. In addition, we will show how a description of chemical reaction kinetics can be included to model reactive and diffuse interfaces for systems comprised of cations and reactive molecular anions. The chemical reaction kinetics of molecular phases at the interfaces and the interface heterogeneity issue are addressed using large-scale ab initio molecular dynamics, enabling us to study spatial and temporal properties of the reactive diffuse interface. In this manner, we expect to better understand the energetics, composition, and structure of metal hydride interfaces, and further studies on electron transfer, hydrogen reaction and transport at and along the interfaces will enable us to optimize system performance.