Theoretical investigation of the surface orientation impact on the hydrogen vacancy formation of MgH₂

Highlights

• Formation of hydrogen vacancies with all the possible combinations on MgH₂(110), (100), (101), (001) surfaces within certain sizes are investigated.

• The relative high barriers for vacancy H formation, especially on the most stable surface, MgH₂(110), reflect the slow kinetics of H release.

• The H diffusion path of H from its original site to a nearby metastable site is studied to investigate the initial process of H vacancy formation.

• H vacancy formation gets easier as the vacancy concentration inceases in general.

Abstract

MgH₂ is one of the most promising materials for solid-state hydrogen storage, but its slow kinetics and relatively high dehydrogenation temperature have yet to be overcome. We have theoretically investigated surface energy and surface H vacancy formation on various MgH₂ surfaces, including all possible H vacancy configurations in the studied size of MgH₂(110), (100), (101), and (001), up to (3 × 2), (2 × 3), (2 × 2) and (2 × 2) slabs respectively, based on density functional theory. Consequently, the hydrogen diffusion paths on the surfaces are also studied. The most unstable surface, MgH₂(001), has the most drastic change in vacancy formation energy. The relative high barriers for vacancy H formation, especially on the most stable surface, MgH₂(110), reflect the slow kinetics of H release. The introduction of vacancy into MgH₂ surfaces often induces defect levels in the energy-gap region. In general, the release of surface H atoms exhibits a pronounced dependence on the surface stability and the concentration of surface H vacancy, in line with the remarkable effect of ball milling in experiments.

Introduction

Nowadays, non-renewable fossil fuels remain to be the dominant source of energy. It was reported that current reserves of fossil fuels would not last for longer than a few decades, although unconventional oil exploitation in the Arctic may extend existing reserves [1]. It is a global demand to develop economically and environmentally sustainable renewable energy sources to replace fossil fuels. Hydrogen cleanly and efficiently transforms the chemical energy of hydrogen fuel into electrical energy with water as the only exhaust product. Moreover, hydrogen energy can be gained from various sources, such as hydro, wind, wave, and solar energies [2]. These advantages make hydrogen a considerable media for energy carrier material used for fuel cell-driven portable, mobile, and stationary applications. However, the conventional gaseous state storage system as pressurized hydrogen gas and liquid state storage system pose safety and cost problems to on board applications. Fortunately, solid state storage systems based on metal hydrides (e.g. MgH₂ and LiBH₄) have demonstrated great potentials to store hydrogen in large quantities in a quite secure, compact, and repeatedly reversible manner. Thus, metal hydrides have become an increasingly attractive option for hydrogen applications [3, 4]. Among the rest, using magnesium-based hydrogen storage materials have remained at the center of investigations, and MgH₂ is one of the most researched and promising materials. It has the potential for safe, non-toxic, low cost, high gravimetric, and volumetric storage density (7.7% and 55 kg/m³, respectively) materials for reversibly utilizing hydrogen. Although hydrogen is often viewed as competitive technology to the popular battery, several drawbacks, such as slow kinetics of hydrogen uptake and release [5], and high temperatures for hydrogen release (1 bar, T_d = 300°C) prevent its practical application. To improve hydrogen kinetics and thermodynamics, many approaches have been proposed, for examples, nanosizing of MgH₂ by mechanical milling [6], [7], [8], adding various effective catalysts [9], [10], [11], alloying Mg with elements that form less stable hydrides [5, [12], [13], [14], [15]], pulsed laser deposition and magnetron sputtering for the preparation of Mg thin films with Pd capping [16], [17], [18].

Mechanical milling is the most common method to modify the properties of Mg, as a result of the formation of special microballstructures, metastable phases and modified surfaces. Mg surfaces surely are involved in the well-reported promotion of hydrogenation/dehydrogenation properties of Mg-H systems via the ball-milling treatment [19]. For instance, the energy barrier for hydrogen dissociation has a significant improvements when Fe is used as dopant on Mg(0001) surface [20]. Further, energies, relaxations, and H adsorption mechanisms of various Mg surfaces have been systematically studied [19, 21].

Kelekar et al. [22] reported various orientation relationships between MgH₂ and Mg according to an XRD analysis, in which epitaxial Mg thin films were deposited on different substrates and were then hydrogenated. When an Al₂O₃ substrate was used, the relationship was MgH₂(110)[001]//Mg(001)[100]. However, when a LiGaO₂ substrate was used, the relationship was MgH₂(200)[001]//Mg(110)[−111]. Schober [23] prepared a thin Mg disc and then hydrogenated it for TEM observation. He summarized the relationship as MgH₂(100)//Mg(0001) and MgH₂[001]//Mg[−1–120]. Various MgH₂ surfaces form from Mg after hydrogen absorption.

However, the theoretical study of MgH₂ dehydrogenation is relatively less than the hydrogenation of Mg. It has been reported that the (110) MgH₂ surface has the lowest energy barrier for the recombinative hydrogen desorption, and the activation barriers for hydrogen desorption from the magnesium hydride surfaces vary depending on the crystalline surfaces that are exposed [24]. Dehydrogenation of MgH₂ is also affected by hydrogen vacancy formation energy and diffusion energy barriers higher in the crystal bulk than the surface. Surface activity could be also enhanced by doping transition metals and/or creating defects on it [25, 26]. Dai et al. investigated dehydrogenation properties of Al, Ti, Mn, and Ni doped MgH₂ (110) and (001) surface by first principles calculations [27, 28]. Ti and Fe co-substitution in MgH₂ (110) surface could be more effective method to improve hydrogen desorption properties of MgH₂ [29]. As the formation of vacancies of H or Mg on MgH₂ surface breaks the Mg-H bond, it is a critical step in dehydrogenation of MgH₂. German et al. studied the effect of Zr-, Nb-doped and vacancy-like defects on hydrogen desorption properties on MgH₂ (001) and (110) surfaces. They showed the doping and vacancy-like defects play positive roles on the hydrogen desorption from surface [30, 31]. In addition to doping, strain, and a combination with doping can also destabilize surface and reduce dehydrogenation energies [32, 33]. The energies for creating adjacent surface divancacies at two in-plane sites and at an in-plane and a bridge site are even smaller than that for forming a single H-vacancy, a fact that is attributed to the strong vacancy-vacancy interactions [34, 35] Different vacancy configurations of surface and sub-surface influence on the H-vacancy formation energy, which suggests surface hydrogen concentration and distribution is decisive for the H-desorption kinetics. It should be noted that defects often promote the initial process of dehydrogenation. It has been established that near-surface defects have a predominant influence on decreasing the H-desorption temperature [36]. Nevertheless, there is little work on the relation between the surface stability and H surface vacancies of various configurations/concentrations, and a comprehensive comparison between various surfaces.

In this work, we have considered hydrogen vacancies with all the possible combinations on the relative stable MgH₂(110), (100), (101), (001) surfaces within certain sizes, aided with ab initio calculations. The kinetics of H vacancy formation is also compared between these surfaces with detailed kinetic barriers, calculated from the CI-NEB method. It is clearly demonstrated that dehydrogenation is easier on unstable surfaces, and interestingly that it is even easier to form as hydrogen vacancy concentration increases. The relative high barriers for vacancy H formation, especially on the most stable surface, MgH₂ (110), reflect the slow kinetics of H release. Detailed bond breaking on the four surfaces was also discussed, as the bond breaking clearly dominates the dehydrogenation process and the stability of the surface.