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Modeling magnesium surfaces and their dissolution in an aqueous environment using an implicit solvent model

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Abstract

Magnesium has attracted a growing interest for its use in various applications, primarily due to its, abundance, lightweight properties and relatively low-cost. However, one major drawback to its widespread use remains its reactivity in aqueous environments, which is poorly understood at the atomistic level. Ab initio density functional theory methods are particularly well suited to bridge this knowledge gap, but the explicit simulation of electrified water/metal interfaces is often too costly from a computational viewpoint. Here we investigate water/Mg interfaces using the computationally efficient implicit solvent model VASPsol. We show that the Mg (0001), (10-10), and (10-11) surfaces each form different electrochemical double layers due to the anisotropic smoothing of the electron density at their surfaces, following Smoluchowski rules. We highlight the dependence that the position of the diffuse cavity surrounding the interface has on the potential of zero charge and the electron double layer capacitance, and how these parameters are also affected by the addition of explicated water and adsorbed OH. Lastly, we calculate the equilibrium potential of Mg ²⁺ / Mg ⁰ in an aqueous environment as 2.46 V vs. standard hydrogen electrode in excellent agreement with experiment.

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his paper focuses on of the use of implicit solvent in electrochemical density functional theory (DFT) calculations. We investigate both the necessity and limits of an implicit solvent polarizable continuum model (PCM). In order to recover the proper electrochemical behavior of the surface and in particular a proper potential scale, the solvent model is found mandatory: in the limit of a high dielectric constant, the surface capacitance becomes independent on the inter-slab space used in the model and therefore the electrochemical properties are only dominated by the interface structure. We show that the computed surface capacitance is not only dependent on the implicit solvent dielectric constant, but also on the solvent cavity parameter that should be precisely tuned. This model is then applied to the Li/electrolyte interface in order to check its ability to compute thermodynamic equilibrium properties. The use of a purely implicit solvent approach allows recovering a more reasonable equilibrium potential for the Li+/Li redox pair compared to vacuum approaches, but that it is still off by 1.5 V. Then, the inclusion of explicit solvent molecules improves the description of the solvent-Li+ chemical bond in the first solvation shell and allows recovering the experimental value within 100 mV. Finally, we show that the redox active center involves the first solvation shell of Li+, suggesting a particular pathway for the observed solvent dissociation in Li/Ion batteries.
Article
The increase in the rate of hydrogen evolution (HE) on dissolving Mg surfaces with increasing anodic current density or potential, which is sometimes called the negative difference effect, has been the topic of much discussion in recent years. A review of the very recent contributions to this subject is given in this paper. Increased catalytic activity of the corrosion product layer, either from the accumulated impurities or from the Mg oxy-hydroxide itself, is shown to have a minor influence on the anodic HE observed on dissolving Mg at high anodic current densities and potentials. Al exhibits similar characteristics during anodic polarization in concentrated HCl, although the anodic HE rate on Al is less than on Mg. Possible mechanisms for the anodic hydrogen are provided and implications in the area of intergranular corrosion and environmental cracking are discussed.
Article
The origin of high rates of hydrogen evolution (HE) on dissolving Mg, the so-called negative difference effect, remains of practical interest. Recent studies have suggested that the ability of Mg to support the cathodic reaction is enhanced during dissolution and that enrichment of noble impurity elements at the dissolving Mg surface may also play a role in enhanced rates of HE. To begin to uniquely address the role played by other elements, Mg was intentionally alloyed with Li, Ca, or Fe. Hydrogen collection was performed during anodic galvanostatic polarization tests. An Mg-Li alloy containing 33 at% Li exhibited similar rates of anodic HE as pure Mg. Because Li dissolves with a valence of 1, any mechanism of anodic HE involving a lower valence state such as Mg+, is therefore ruled out. Mg-Ca and Mg-Fe alloys exhibited higher rates of anodic HE than pure Mg. They also exhibited a minimum of the HE rate under cathodic polarization instead of at the open-circuit potential, the latter being the case for pure Mg and Mg-Li. The role of alloying elements in the HE on Mg and the implications for the mechanism of anodic HE are discussed.
Article
Density functional theory (DFT) was used to study water dissociation on the Mg(0001) surface. The metal/water interface was modeled with a supercell approach, consisting of an extended metal surface coupled to an implicit solvent medium. Several electrochemical reactions were studied on the Mg surface, and it was found that dissociation of adsorbed water is thermodynamically favorable, and that the Mg(0001) surface has multiple ‘active sites’ that can accommodate adsorbed hydroxyl groups (*OH). This is similar to previous first principles findings of oxygen adsorption on Mg(0001). It was also found that the local structure of an adsorbed hydroxyl monolayer mimics that of the crystal structure of brucite, Mg(OH)2. Lastly, DFT-calculated reaction enthalpies were used to reproduce the bulk Mg Pourbaix diagram, and Pourbaix's formalism was extended to develop a theoretical Mg surface Pourbaix diagram. From this, it was shown that the enthalpy of hydroxylation of Mg(0001) becomes more negative with increasing surface coverage of *OH groups. This indicates that the presence of adsorbed *OH species provides an energetic driving force for water dissociation on Mg(0001). Furthermore, the corrosive region of the Mg Pourbaix diagram can be suppressed if *OH adsorption is limited to certain low-energy active sites, where they form a stable hydroxide surface.
Article
This paper provides a review of recent developments in the field of Mg corrosion and puts those into context. This includes considerations of corrosion manifestations, material influences, surface treatment, anodization, coatings, inhibition, biodegradable medical applications, stress corrosion cracking, flammability, corrosion mechanisms for HP Mg, critical evaluation of corrosion mechanisms, and concluding remarks. There has been much research recently, and much research continues in this area. This is expected to produce significantly better, more-corrosion-resistant Mg alloys.
Article
Magnesium (Mg) dissolution is distinct from other engineering metals, as Mg can support cathodic hydrogen evolution on its surface during anodic polarisation. The phenomenon of cathodic hydrogen evolution upon anodically polarised Mg is characterised by the rate of the hydrogen evolution reaction (HER) increasing with anodic polarisation, a phenomenon called the negative different effect (NDE). Mg has a tendency to aggressively corrode in aqueous solutions, impairing its application as a durable engineering material or a predictable electrode material, which is also influenced by the NDE. Over the last century a number of different theories have sought to explain the NDE. However, recent progress in research upon Mg utilising contemporary methods including advanced electrochemical techniques, on-line elemental analysis and cross-sectional electron microscopy, have not only refined the understanding of Mg dissolution, but discredited almost a century of alternate theories. During anodic polarisation, a bilayered MgO/Mg(OH)2 film forms on Mg, appearing as a dark region on visual inspection. This film gradually occupies the bulk of the previously pristine Mg surface, and importantly sustains (and enhances) the HER. This phenomenon of cathodic activation may also be catalysed by an enrichment of noble elements or impurities on the Mg surface, which could play an important role in promoting the HER. A phenomenological model for the dissolution of Mg encompassing the current opinion of many researchers is presented herein.
Article
The effect of 0.5, 1.0, and 1.5 wt% Ca additions on the microstructure and corrosion resistance of the heat-treated Mg–Al–Zn alloy was investigated. Addition of 0.5 wt% Ca did not form any new phase but suppressed the discontinuous precipitation of the β-Mg17Al12 phase by being dissolved in both the second phase and magnesium matrix. In the materials containing higher amounts of Ca, however, metallographic investigation shows that Ca added to Mg–Al–Zn can obviously decrease the size of β-Mg17Al12 and forms Al4Ca intermetallic compounds in the shape of bone-like morphology. The corrosion tests used include constant immersion technique, and potentiodynamic polarization experiments and salt spray test. Surface examination and analytical studies were carried out using optical and scanning electron microscopy, EDX, and XRD. The results of corrosion tests show that magnesium alloy Mg–Al–Zn with 1.0 wt% Ca addition has the best corrosion resistance behavior.
Article
VESTA is a three-dimensional visualization system for crystallographic studies and electronic state calculations. It has been upgraded to the latest version, VESTA 3 , implementing new features including drawing the external morphology of crystals; superimposing multiple structural models, volumetric data and crystal faces; calculation of electron and nuclear densities from structure parameters; calculation of Patterson functions from structure parameters or volumetric data; integration of electron and nuclear densities by Voronoi tessellation; visualization of isosurfaces with multiple levels; determination of the best plane for selected atoms; an extended bond-search algorithm to enable more sophisticated searches in complex molecules and cage-like structures; undo and redo in graphical user interface operations; and significant performance improvements in rendering isosurfaces and calculating slices.
Article
We present a theoretical study of the structural response of model water monolayers on a Pd(1 1 1) surface upon various electrochemical conditions. Whereas the water molecules pointing toward the vacuum are mostly electrochemically inactive, those lying parallel to the surface undergo an oxidative adsorption under oxidizing conditions, and those pointing toward the surface show a reductive adsorption under reducing conditions. The oxidative adsorption is shown to result from the interaction of the H2O 1b1 orbital with the metal surface dz2 band which becomes bonding only under oxidative conditions. The reductive adsorption arises from the interaction of the H2O 4a1–2b2 orbitals with the dz2 band of the Pd surface. This electronic analysis of electrochemical effects is further validated using the Fukui function associated with the water monolayers. The Fukui function is shown to be a powerful tool for studying electrochemical effects as it is directly linked with the electrochemical bond reorganization and other parameters like the surface capacitance. The electrochemical stability diagram for these water monolayers is computed using a new correction for the DFT electrochemical calculations and compared to previous results. The phase transformation from a positively charge H-up to a negatively charged H-down phase is here confirmed with, however, a potential shifted up to 4.5 V compared to our previous report (4.3 V). The neutral phase is found to consist in a positively charged H-up phase and a negatively charged H-down phase. This phenomenon is confirmed by large super-cell calculations with different ratio of H-up and H-down phases. The stability of the mixed charged phases is then rationalized in terms of an electrochemical disproportionation whose origin and consequences are further discussed in regards to the results previously obtained on the basis of the ice rules.
Article
This complete review of double-layer data obtained with single-crystal faces of sp metals includes seven tables summarizing all data obtained to date and 158 references. Comparison of these data is made, and comparison with data obtained with polycrystalline electrodes is discussed.
Article
Work function is experimentally known to be different for different faces of a crystal by amounts ranging from one-tenth to half a volt. For tungsten the faces can be arranged according to decreasing work function as follows: 110, 211, 100 and finally 111. The explanations so far suggested for the differences of the work function are discussed and shown to give either an incorrect sequence or a wrong order of magnitude of the observed differences. The author uses the picture of Wigner and Bardeen according to which the work function is a sum of a volume contribution and a contribution due to a double layer on the surface of the metal. The origin of the latter can be described in the following manner. With every atom one can associate a polyhedron ("s-polyhedron") with the atom at its center, such that it contains all points nearer to the atom under consideration than to any other atom. If the distribution of the electron density within these polyhedra of the surface atoms was the same as for the inside atoms then there would be no double layer on the surface. However, this is not the case since the total energy is lowered by a redistribution of the electron cloud on the surface. There are two effects: the first is a partial spread of the charge out of the s-polyhedra and the second is a tendency to smooth out the surface of the polyhedra. In consequence of the second effect the surfaces of equal charge density are more nearly plane than in the original picture. The two effects have opposite influences and since they are comparable in magnitude, it is not possible to predict the sign of the total double layer without numerical computations. Some general formulae for the double layers are derived and discussed more fully in the case of a simple cubic and a body-centered cubic lattice. The minimum problem of the surface energy is solved for four faces of a body-centered crystal and the results are applied to the case of tungsten. One obtains the differences between the work functions for different directions. The results agree satisfactorily with the experimental data: assuming a reasonable density of the free electrons, one obtains the correct sequence of faces and the correct differences of the work function. The surface energies are calculated an d found in agreement with the observed stability of certain crystal faces.
Article
New LEED (low-energy electron diffraction) results are reported for Fe{310} and Fe{210} surfaces, including multilayer relaxation of atomic planes both perpendicular and parallel to the surface. The structures of six different surfaces of iron are now known. A comparison of the results yields relaxation trends: top-layer relaxation is found to increase monotonically as the surfaces become more open; for the higher-index surfaces {211}, {310} and {210} “decay curves” of relaxation as functions of depth into the surface show a surface-independent decay length (the depth at which the crystal returns to a bulk-like arrangement) of approximately 2 Å.
Article
The corrosion behavior of four Mg–5Al–xCa alloys (x = 0.0 to 2.0 wt.%) was evaluated in an alkaline NaCl solution. Surface analyses indicated that the benefits of Ca addition are the refinement of the precipitates and a decrease in grain size. Furthermore, the refinement of the precipitates (Mg2Ca, Al2Ca) became more complete with increasing of Ca content. The electrochemical tests revealed that the pitting resistance was improved in Ca-containing specimens. In addition, the polarization resistance of the Mg–5Al specimens increased with increasing Ca content. This is due to the fact that precipitation which is expected to act as a barrier is more continuous over the Mg matrix with a smaller grain size and higher precipitation density.