Electrochemical Stability of Magnesium Surfaces in an Aqueous

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Electrochemical Stability of Mg Surfaces in an Aqueous Environment Jodie A Yuwono, Nick Birbilis, Kristen Smith Williams, and Nikhil V. Medhekar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09232 • Publication Date (Web): 03 Nov 2016 Downloaded from http://pubs.acs.org on November 12, 2016

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The Journal of Physical Chemistry

Electrochemical Stability of Mg Surfaces in an Aqueous Environment Jodie A. Yuwono1*, Nick Birbilis1, Kristen S. Williams2 and Nikhil V. Medhekar1* 1

Department of Materials Science and Engineering, Monash University, Clayton, VIC 3800, Australia 2

Boeing Research & Technology, 499 Boeing Blvd. SW, Huntsville, AL 35824, USA

ABSTRACT

An insight into the electrochemical stability of Mg surfaces is of practical importance for improving the corrosion resistance of Mg as well as its performance as a battery electrode. The present paper employs first principles density functional theory (DFT) simulations to study the electrochemical stability of magnesium (Mg) surfaces in aqueous environments. A number of electrochemical reactions that describe the interactions between Mg(0001) surface and water were analyzed. It was verified that water dissociation is favored upon the Mg surface in agreement with recent works; however, it is also shown that the previously unstudied Heyrovsky reaction may play an important role in controlling the surface stability. Furthermore, it was found that the surface stability also crucially depends on the concentration of adsorbed hydroxyl groups. Specifically, the surface work function was determined to vary as the function of hydroxyl coverage, which has ramifications for the catalytic behavior of the Mg surface. The influences of

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surface doping with Ca (a reactive element) and Fe (a comparatively noble element) were also studied to provide an atomic level understanding on how the dopants influence surface properties and subsequent electrochemical reactions. With a keen recent empirical interest in Mg surface stability given the industrial relevance of Mg, the present study provides key new insights into the physical processes related to the enhanced catalytic activity of Mg and its alloys.

INTRODUCTION The use of magnesium (Mg) alloys continues to increase across a wide-range of applications, including transportation (automotive and aerospace), consumer goods, electronics and biomedical applications1–3. The benefits of using Mg alloys are driven by their high strength to weight ratio, along with high volumetric capacities for energy storage applications4–11, and the advantages associated with their low cost8,12,13. In spite of the increasing use of Mg and its alloys, its electrochemistry and surface stability remains poorly understood even in the simple case of the Mg-water (H2O) electrochemical system14. The stability of Mg and its alloys is determined by the complex chemical/electrochemical reactions occurring at the interface of the Mg surface and an aqueous electrolytic environment. A complete understanding of these interfacial reactions is essential for developing high performance corrosion-resistant Mg alloys and predictable Mg electrodes. Furthermore, investigating these reactions at the atomic level is also important for clarification and quantification of the so called “cathodic activation” of Mg alloys, where the dissolution of Mg from the surface is reported to enhance the rate of the cathodic water reduction and/or hydrogen evolution reactions15,16. This phenomenon results in the persistence of a cathodic reaction upon the Mg surface, the rate of which increases with time (i.e. Mg dissolution) or anodic polarization (which accelerates Mg dissolution). In particular, it also gives rise to the


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negative difference effect (NDE), a persistent hydrogen evolution that occurs during anodic polarization of Mg surface. While there is an increasing interest in fully understanding these phenomena at a fundamental level, their atomic level origins are not yet definitively determined. Earlier works in this regard have suggested three distinct hypotheses to explain experimental observations of cathodic activation, namely: (i) noble metals accumulated upon the dissolving Mg surface may serve as persistent cathodes17–21; (ii) the Mg(OH)2 dissolution products that form on the Mg surface catalyze hydrogen evolution22,23, or (iii) the dissolution of Mg changes the exchange current density associated with water reduction24. While these hypotheses are all reasonable, determining which mechanisms are dominant, or designing experiments to determine the attendant atomic scale phenomena is challenging. In light of the challenges in atomic level experimental investigations of the electrochemical stability of Mg alloy surfaces, atomistic simulations based on first principles methods are increasingly being used to compliment experiments. Moreover, the recent experimental studies have suggested a unique role played by other elements at the Mg surface towards their electrochemical stability, i.e. Ca and Fe25,26. Realizing the complexity in modeling Mg alloys systems this work introduces alloying elements as surface dopants. This is based on an assumption that alloying elements significantly affect Mg surface stability while they reside near to or at the outermost layer. Such an assumption can be founded on the basis that recent detailed experiments using nuclear microprobe analysis have identified surface alloying enrichment in dissolving Mg19,27. The empirically researched Mg-Fe and Mg-Ca alloys were used as the baseline to validate the precision of theoretical studies herein. As such the physical processes in the Mg alloys/water system can also contribute in revealing the critical factors in Mg surface stability that are difficult to understand through experimental studies.



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For the case of Mg alloys in aqueous environments, very few studies have investigated the stability of surfaces using first principles methods. Recently, Williams and co-workers studied the adsorption of hydroxyls on Mg surfaces and showed that the reduction of water is energetically more favourable on a partially hydroxylated Mg surface than on the bare Mg surface15. While this early work provided crucial insight into surface electrochemical reactions into Mg/H2O systems, the atomic level origins of the enhanced catalytic behavior in Mg followed by OH- adsorption remains unanswered. This present work, which is a key effort in this regard, builds on prior knowledge of Mg surface stability obtained from both experimental and theoretical studies. Herein, systematic investigations have been performed for the lowest energy (0001) surface of pure Mg, as well as Mg alloyed with Ca and Fe exposed to an aqueous environment. Specifically, the surface hydroxylation and hydrogen evolution reaction (HER) mechanisms were also considered. Prior DFT work has focused on exploring the Tafel step16 and hence the results related to a number of previously un-explored reactions are reported herein for the first time. By determining the surface work functions as a measure of the surface stability, the enhanced catalytic activity behavior of Mg can be rationalized. Finally, the analysis presented here shows how the alloying elements Ca and Fe alter the electrochemical reactions on Mg(0001) surface, which can also be explored by DFT-calculated Pourbaix diagrams.


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COMPUTATIONAL METHODS AND ELECTROCHEMISTRY OF MG SURFACES First principles calculations were executed using spin-polarized, plane wave density functional theory methods as implemented in the Vienna Ab Initio Simulation Package (VASP)28,29 The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional form was used to approximate electron exchange and correlation30, while core and valence electronic interactions were modeled using projector augmented wave (PAW) potentials31,32. A plane wave kinetic energy cutoff of 500 eV and a Gamma-centered k-points grid for sampling the Brillouin zone were employed. Geometrical optimization of all geometries considered was achieved by relaxing all ionic positions as well as supercell vectors until the Hellman-Feynman force was less than 0.01 eV/ Å. The calculated optimized lattice constants for hcp Mg are a=3.20 Å; c=5.15 Å, whilst for hcp Sc are a=3.29 Å; c=5.21 Å; and for fcc Al are a=c=4.04 Å, all in good agreement with previously reported experimental and theoretical values15,33–35. Surface-environment interactions were calculated using the "slab model" in which a semiinfinite surface is modeled using six layers of metal atoms periodically extended in the x and y directions and separated by 20 Å vacuum layer in the z direction. The vacuum then was implicitly solvated with water to support the study of Mg in aqueous environment36,37. A dipole correction in the z-direction38,39, with a dielectric constant equal to that of bulk water at standard conditions, i.e. 80, was applied. For the slab model, the spatial coordinates of the bottom two layers were fixed, whilst the four uppermost layers were allowed to relax. This method allows the model to have a surface-bulk like configuration. The adsorption geometries and energies of environmental species, such as OH-, H+, and H2O, on the surface were then determined through a series of DFT calculations. The model solvated ionic substance, Mg cation (Mg2+), was created with a single Mg surrounded by six explicit H2O molecules in implicit water medium and positively charged mode. A (6x6x3) supercell of Mg(0001) was used in this study, with dimensions: a = b = 18.91



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Å, c = 35.49 Å (with 20 Å vacuum); α = β = 90°, γ = 120°. In the supercell, there are a maximum of 36 possible adsorption sites (FCC or HCP hollow sites) that, when covered, constituted a single monolayer (ML). In studies of Sc(0001) and Al(100), slab models of (2x2x2) and (2x1x2) supercells were used, respectively. The Ca- and Fe-doped Mg(0001) surfaces were modeled by substituting a single Mg atom in the first or second layer with the dopant atom, as shown in Figure 1. With this configuration, the concentration of dopant at the Mg(0001) surface is ~0.46 at.%. In principle, the anodic (R. 1) and cathodic (R. 2) reaction below represents the primary Mg interactions with H2O. Mg ↔ Mg2+ + 2e-

(R. 1)

2H2O + 2e- ↔ 2OH-+ H2(g)

(R. 2)

The quantity of hydrogen gas (H2) evolved through the recombination of H+ is shown to correlate with the extent of Mg dissolution under free corrosion conditions40. Experimental studies have suggested that suppressing the cathodic reaction can decrease the Mg anodic activity, and thus retard dissolution26,41. Better understanding on HER enables the suppression of this reaction that will beneficially enhance the chemical stability (i.e. corrosion resistance) of Mg. H2 evolution upon the Mg surface may occur via Volmer - Tafel, and/or Heyrovsky reactions42,43. The process of hydrogen recombination to form H2 starts with water dissociation into H+ and OH-. H+ adsorbs on the surface through the Volmer reaction. This Had can then combine with either another Had (Tafel) or with a proton from solution and a surface electron (Heyrovsky) to form H2 gas. The corresponding reactions can be defined as: Volmer: H+ + e- ↔ Had

(R. 3)

Tafel: 2Had ↔ H2

(R. 4)


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Heyrovsky: Had + H+ + e- ↔ H2

(R. 5)

Besides that, at the interface, there are three competing Mg electrochemical reactions that may occur on the (0001) surface: the reaction of Mg and H2O to form hydroxylated Mg (R. 6); the dissolution of hydroxylated Mg to give solvated Mg2+ complex (R. 7); and the dissolution of solid Mg to produce solvated Mg2+ complex (R. 8). The double arrows indicate that these three reactions are reversible. Mg(0001)1v represents the Mg(0001) surface with single atom vacancy in the surface (denotes the dissolution of single Mg atom). The reactions below are listed together with the corresponding surface and active species models involve in the process. Mg(0001) + n H2O ↔ Mg(0001)/(OHad)n + n H+ + n e-

(R. 6)

Mg(0001) + n H2O ↔ Mg(0001)/(OHad)n + (n/2) H2

Mg(0001)/(OHad)n + 2 H+ ↔ Mg(0001)1v + [Mg(H2O)6]2+ + (n-6) H2O

(R. 7)

Mg(0001)/(OHad)n + 2 (H3O.H2O)+ + (1-n/2) H2 + (1-n/2) O2 ↔ Mg(0001)1v + [Mg(H2O)6]2+

Mg(0001) + 6 H2O ↔ Mg(0001)1v + [Mg(H2O)6]2+ + 2e-

(R. 8)

Mg(0001) + 2 (H3O.H2O)+ + 2 H2O ↔ Mg(0001)1v + [Mg(H2O)6]2+ + H2 The reaction enthalpy (ΔHr in kJ/mol) was used to characterize thermodynamic stability and to construct DFT-calculated Pourbaix diagrams of the Mg(0001) surface. ΔHr of a given structure is defined as the difference between the total energies of its products and reactants (Eq. 1), as obtained from DFT calculations. ΔHr = HProduct(s) – HReactant(s)

(Eq. 1)

For each reaction, the entropic contributions were assumed to be small and the approximation ΔGr = ΔHr was used (this term will be used interchangeably throughout the text). Negative ΔHr



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indicates exothermic reactions, which are more thermodynamically favorable than those with positive ΔHr. The rate of H2 gas evolution on the magnesium surface was investigated under zero charge conditions. Different mechanisms of HER were assessed thermodynamically. The structure of different adsorbates (OHad and Had) combinations was first calculated to find the lowest energy configurations. For each adsorption configuration, we also confirmed that the contributions from the van der Waals (vdW) interactions were insignificant — we found that these interactions contributed to less than 5% to the ΔHr values. The thermodynamic stability of the passivated (or hydroxylated) Mg(0001) surface was determined by calculating the enthalpy of the hydroxylation reaction, as a function of surface coverage, defined by Eq. 2. Here the model of clustered OHad had been chosen over the evenly distributed OHad for the coverage studies; since the preliminary calculations indicated that OHad are energetically more stable when they adsorb close to each other. Therefore, the adsorption and growth of OH- as a cluster of OHad is believed to be the preferential mechanism on Mg(0001) surface. ΔHr = Hr [Mg(0001)/(OH)x] + Hr (H2) – [Hr [Mg(0001)] + x Hr (H2O)]

(Eq. 2)

For the doped Mg(0001) surfaces, where Ca or Fe were added to the system, the substitution energy, as well as surface energy, was used to measure the segregation preference of the alloying element within the Mg surface. Substitution energy (Esub in eV/atom) was calculated as the difference between the energy of the doped surface (EMg-X(0001)) and that of the clean surface (EMg(0001)), with contributing factors of energy from Mg atom (EMg) and dopant atom (EX), according to Eq. 3. Esub = EMg-X(0001) + EMg – EX – EMg(0001)


(Eq. 3)

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The surface energy (Esurface in eV/atom) was also used to measure the stability of new doped Mg(0001) surfaces. The surface energy defines the energy required to create a new surface or cleave an infinite crystal into two. If the doped surface has smaller surface energy than the clean surface, then the doped surface, Mg-X(0001), was deemed more stable. Surface energy can be calculated as described by Eq. 4. Esurface =

(Eq. 4)

Negative substitution energy and smaller surface energy than that of the clean Mg(0001) surface imply that the dopant is more stable when surrounded by Mg atoms (relative to in its pure bulk state); that is, there should be no segregation. Changes in surface properties with Ca and Fe dopant are discussed and related to the hydroxylation process and Mg dissolution in water. Work function (Φ) has recently been used as an indicator of chemical/electrochemical reactions occurring in an aqueous electrolytic environment. It is defined as the energy needed to take one electron away from the surface44,45. Higher Φ value means that it is unfavorable for the electron to be removed from the surface, while lower Φ implies favorable electron donation, and thus facilitates subsequent reactions that require electron(s) to take place. The surface characteristics of Mg are then compared for discussion of OHad catalytic role with those of scandium (Sc) and aluminum (Al), the results can be found in Figure S2 and Table S4 in the Supporting Information [SI]. Sc and Al were chosen essentially because they exhibit different thermodynamic propensity towards hydroxide layer formation on the surface46. Calculations were performed with OHad adsorbed at different sites (such as top, bridge, and hollow) on Mg(0001), Sc(0001), and Al(100) surfaces. The most energetically favorable site configuration was then used to study the effect of increasing OHad concentration on the work function. Herein, Φ according to Eq. 5 is defined as



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the difference between the energy of an electron in the solvent (far away from the surface) and the Fermi level. Φ = Esol – εF

(Eq. 5)

To construct chemical phase stability diagrams (or Pourbaix diagrams) of the clean and doped Mg(0001) surfaces in water, three competing electrochemical reactions (R. 6 to R. 8) were considered. A similar method used in previous works to construct Pourbaix diagrams was applied in the present study15,47,48. The Nernst equation can be calculated from the formation enthalpies determined through the first principles DFT calculations. All competing reactions were plotted as functions of potential and pH. In all cases, the potential is relative to that of the Standard Hydrogen Electrode (SHE), 2H+ + 2e- ↔ H2.

RESULTS AND DISCUSSION Water dissociation and HER on the clean Mg(0001) surface Thermodynamic stability of different forms of water (i.e. molecular or dissociated) on the clean Mg(0001) surface was studied by the implicit water method in first principle calculations. An explicit single water molecule was introduced on the surface and was found to adsorb at the top of an Mg atom. The Mg-O interatomic distance of 2.45 Å was determined, and is in relatively good agreement with previous report of 2.26 Å16. Water molecules adsorbed with an energy of 0.39 eV/mol and are relatively parallel to the surface with H atoms tilted upward by 19.8°. Nonetheless, the water molecule is found to lift the associated Mg atom up by 0.27 Å. This suggests that water may assist the Mg dissolution, during the oxidation process. Several possible reactions may occur at the Mg(0001) surface in the following water dissociation, such as: H+ adsorption; OH- adsorption, and OH- and H+ coadsorption. The surface reactions involving oxygen only is not considered in this study because MgO will be


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spontaneously reduced to form Mg(OH)2 in aqueous solution49. The hydrogen adsorption energies were calculated at different adsorption sites on the Mg surface, and hydrogen atom was found to show a slight preference (~0.01 eV) to adsorb at HCP hollow sites than at FCC hollow sites. On HCP hollow sites, H+ adsorbs with an adsorption length of 0.72 Å relative to the topmost Mg layer, with the energy of -0.25 eV. These findings are consistent with previous studies of hydrogen adsorption on Mg surface in vacuum50–53. Other reactions include the formation of Mg(0001)/OHad and Mg(0001)/OHad/Had. The enthalpies of reaction of these products from Mg and H2O suggest that the transformation from H2O to OHad or OHad/Had occurs spontaneously. As schematically shown in Figure 2, in the zero charge potential (PZC) or at the open circuit potential (OCP), water molecules prefer to dissociate on the Mg(0001) surface. The most stable configuration of the OHad and Had pair is obtained when both species adsorb at FCC hollow sites15 with adsorption lengths of 1.08 (for OHad) and 0.90 (for Had) Å. The calculations imply their configurations (Had and OHad) from the most stable to the least stable state as: FCC hollow - FCC hollow; HCP hollow - HCP hollow; HCP hollow FCC hollow; and FCC hollow - HCP hollow, respectively. Furthermore, adsorbed OH- on the surface is also found to be far more stable than adsorbed H2O. The OH- adsorption energy in the most favorable site, FCC hollow, was -4.43 eV/OHad. Compared to the adsorption energy of H+, this FCC hollow site is more thermodynamically stable when filled with OHad than with Had. However, coadsorbed OHad and Had is found as the most thermodynamically stable state of Mg(0001) surface in the water at the simulation conditions, pH=0 at PZC, as shown in Figure 2. Table 1 lists the Mg surface reactions with water, showing that the incorporation of more water molecules into those particular Mg surface reactions resulted in more exothermic or lower water dissociation enthalpies. The hydroxylation of the Mg(0001) surface is also considered to be a dominant mechanism, even without the coadsorption with H+. The reaction enthalpies also show



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the unfavorable O2 formation, since oxygen atoms and hydroxyl groups have relatively high adsorption energies33. On this basis, it can be established that oxygen atoms will always take part in surface reactions, either in the formation of MgO and/or Mg(OH)2. Increasing coverage of OHad and Had is known to cause potential shifts and local changes in pH40. Williams et. al. reported that increasing OHad concentration will cause the potential of the Mg(0001) surface to become more negative15. This means the on-going continuous water dissociation and Mg oxidation process at the surface will create instability in the system (as more electrons build up at the surface); and one way of compensating for this excess charge is achieved via the HER. Consistent with the lower adsorption energy of Had relative to OHad, reactions that require electrons in their process will be favored. The cathodic reactions of water reduction and/or Had atom recombination should maintain the overall system balance between anodic and cathodic reactions at the Mg(0001) surface at the PZC. This assumption is well supported through the experimental observation of substantial H2 evolution in Mg alloys at PZC/ OCP24,25. The presence of Had on the surface means that the H2 evolution reaction is half way to completion. There are two subsequent processes that have been considered in this study: the Tafel and Heyrovsky reactions. All possible reactions that may follow the intermediate state of coadsorbed Had and OHad on the Mg(0001) surface are listed in Table 2. When ignoring the effects of polarization and pH, at the simulation conditions (pH=0 at PZC), H2 evolution via the Heyrovsky reaction is calculated to occur spontaneously (exothermic reaction), in contrast to the unspontaneous Tafel reaction (endothermic reaction). The HER pathway via Tafel reaction has been modeled recently by Williams and co-workers, who reported an endothermic Tafel reaction involving two Had species at the surface of Mg(0001), with ΔG° of 0.36 eV16. The participation of OH- adsorption was also found to decrease the Heyrovsky reaction enthalpy by up to 0.32 eV. Such a trend was not found in the case of the Tafel reaction,


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of which the enthalpy rises by up to 0.1 eV. Thus, the Heyrovsky reaction merits consideration as a potential candidate towards so-called persistent hydrogen evolution upon Mg, whilst noting that the effect is also constrained/moderated by the presence of a hydroxide layer. It is hypothesized in the case of anodic polarization followed by the water dissociation; there will be an ongoing hydroxylation process on the surface. This will obviously allow the pH at the interface to evolve to a more basic condition40. Nonetheless, based on the calculations presented herein and the associated elaboration of adsorbed species, conditions favoring the Heyrovsky reaction in preference to the Tafel reaction are shown in Table 2. These presented scenarios supporting the concept of enhanced cathodic activation due to an increase in anodic activity of Mg, i.e. hydroxylation and/or dissolution20,22,54. The proposed pathways described above only defines the possible routes for HER on the Mg(0001) surface at the simulation conditions. The present findings are not sufficient to clearly describe the origin of the negative difference effect (NDE) alone, as different pathways of HER and favorability between the Tafel and/or Heyrovsky reaction will alter based on their preference to occur under the variation of potential and pH

55–58

. However, Had is expected to participate in

the HER. This is based on the notion that both OHad and Had preferentially adsorb at FCC hollow sites, and the adsorption energy of Had is much weaker than that of OHad. With the desorption of Had from the surface, a passive hydroxide layer will develop and grow on the surface.

Thermodynamic stability of hydroxyl adsorption on the clean Mg(0001) surface Hydroxylated Mg(0001) surfaces with various OHad concentrations were studied and the results are compiled in Figure 3 and Table 3. Mg(0001)/OHad, as discussed in the previous section, is known the second most stable state in the Mg and water interactions. OHad at HCP hollow sites



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has a shorter average Mg-O interatomic distance than at FCC hollow sites, 0.9571 vs. 1.0783 Å respectively. This difference in the interatomic spacing arises from attractive interaction between subsurface Mg atom(s) and O adatom(s) of OHad group(s). The HCP hollow sites are positioned directly above Mg atoms in the second atomic layer, while the FCC hollow sites are not. The calculated O-H distance is ~0.98 Å, and the H atoms of OHad were found to repel each other, as can be seen from the topmost Mg layer–O–H angle. The presence of a neighboring OHad group indicates a stronger attractive interaction between particular OHad group and the substrate. The reaction enthalpies (in eV/adatom) are slightly more negative in the presence of a “nearest neighbor” than in the presence of a “next nearest neighbor”, - 1.53 vs. -1.50 and -1.52 vs. -1.48 at the HCP hollow sites and the FCC hollow sites, respectively. Nearest neighbor coadsorbed OHad configuration, with an oxygen atom separation of 3.17 Å, was found to be thermodynamically favorable. Observations of OHad clusters (of 2, 3, 7, and 36 OHad) on the Mg(0001) surface denote FCC hollow sites as the favorable adsorption sites with the increasing OHad concentration, as shown in Figure 4. Cluster calculations indicated that the hydroxide layer formation on the surface is a dynamic process. Initial adsorption of OH- may occur at HCP hollow sites then followed by a surface diffusion (reconfiguration) process to occupy FCC hollow sites. Therefore, OHad behavior, especially in terms of surface diffusion on the Mg surface will need to be validated in future works. Herein, a unique behavior was found in the hydroxylation reaction enthalpy in vacuum and implicit water, which is in contrast to a prior study15. The hydroxylation enthalpy of a single OHad in the implicit water indicates thermodynamic favorability. Although, continuous hydroxylation improves the stability of the Mg surface, the adsorption of many OH- groups is still considered to be unfavorable for such conditions. Similar to H2O adsorption on the surface, OHad was found to assist the dissolution of Mg atoms. There are competing Mg dissolution pathways,


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which highly depends on the pH, between H2O and/or OHad assisted mechanisms. From the plot in Figure 3, it can be realized that in the simulation conditions, pH=0 at PZC, the Mg dissolution process is mainly facilitated by water molecules. The effects of different numbers and configurations of OHad on work function are also tabulated in Table 3. When two OHad are in the “nearest neighbor” configuration, more energy is required to remove an electron away from the system to when they are in the “next nearest neighbor” configuration. This implies that the adsorption of OH- can significantly affect the surface chemical stability, as shown from the changes in the work function. In the next nearest neighbor configuration, adsorption of two separated hydroxyl groups will create a larger area affected by electronic structure redistribution59,60. However, thermodynamically, OHad prefers to form a cluster on the Mg(0001) surface. This indicates the complexity and significance of OHad in altering the Mg(0001) surface properties. The complete profile on how OHad decreases Mg work function will be discussed and elaborated with other metals. Hydroxylation of three different metal surfaces (Al, Sc and Mg) was investigated, focusing on the role of OHad and how it changes the work function. According to the Pourbaix diagrams, Mg and Sc form a stable hydroxide layer on the surface, and Al does not46. Work function was used to measure the amount of energy required to remove an electron from the metal surfaces; thus indicating if such electrons can be used for other electrochemical reactions, i.e. cathodic reactions at the metal/water interface. Detailed results of the computed thermodynamic characteristics of OHad on the metal surfaces (Sc and Al) can be found in Table S4 (in the SI); showing similar observations as those seen in the respective experimental Pourbaix diagrams for the corresponding metals46. With the growth of a surface layer (OHad), the reaction enthalpies become more exothermic for Mg(0001) and less exothermic for Sc(0001) and Al(100) surfaces.



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This suggests the hydroxylation of Mg(0001) is favored with further OH- adsorption on the surface, of which this trend is not found for the Sc(0001) and Al(100) surface. Work functions were calculated in both vacuum and implicit water. The vacuum work function was always greater than that in implicit water. The work function of the clean Al(100) surface was the highest, followed by the clean Sc(0001) and then the clean Mg(0001) surface. Based on the clean surface work function itself, the associated cathodic reaction, i.e. HER can be determined. The higher the metal work function, the slower HER kinetics will be observed, vice versa61. With the following OH- adsorption, as shown in Figure 5, the work functions of Mg(0001) are significantly reduced. These imply that OHad facilitates the electrons donation from the Mg surface. As electron transfer becomes easier, the subsequent surface electrochemical reactions, such as cathodic reaction (HER), can be facilitated. Thus, hydroxide layer formation on Sc and Mg is found not only passivizing and/or dissolving the surface, but also promoting the surface cathodic reaction22,54,62,63. Such mechanism was not found in the Al(100) surface, where OHad had no significant effect towards work function. In particular, this difference is consistent with the physical structure of the adsorbed species. Since the O atom(s) of OHad adsorbed very closely, < 1.25 Å to the Mg(0001) and Sc(0001) surface, the charge accumulation layer was created beneath the metal overspill region, which then formed a small layer of charge depletion right outside the metal surface. The changes in the charge distribution then contribute to the decrease in work function45,59,64 as shown in Figure 6. The change in work function and corresponding HER promotion as the result of surface hydroxylation provide a new understanding of the negative difference effect (NDE) that is observed in highly active metals, i.e. Mg, Ca, Gd, La and Sc. The enhanced cathodic activity and its relation to the work function can provide a theoretical explanation of the catalytic effect of hydroxide layers on the Mg surface as proposed by Salleh et. al.22. The present finding also


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postulates a favorability of HER to occur in the basic pH for those metals with NDE behavior 22,54

.

The Pourbaix diagram of the Mg(0001) surface as a function of potential and pH, is plotted in Figure 7; giving a general overview of water interaction on the undoped Mg(0001) surface. This plot is extrapolated from the calculations obtained at the controlled conditions of pH=0 at PZC. Three electrochemical reactions, R. 6 to R. 8, were considered, and their calculated Nernst equations are also given in Figure 7. The Pourbaix diagram was constructed at the OHad surface concentrations of 0.06, projecting the condition in the early stages of hydroxylation process on the Mg(0001) surface with the pH of 6.8 - 7.1 and dissolution potential of -1.934 VSHE. It suggests the hydroxylation pH to be more neutral and/or acid than in conditions previously studied15. In the higher OHad concentration, the equilibrium phase line of Mg2+|Mg/(OHad)n and Mg|Mg/(OHad)n is still predicted to follow the same principle: a hydroxylation pH shift to the more acid conditions15. The present finding gives a representation of the Pourbaix diagram for the Mg surface with what may considered an empirically more realistic value of hydroxylation pH and Mg dissolution potential65–67.



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Thermodynamic stability of Ca- and Fe- doped Mg(0001) surfaces Ca and Fe dopant atoms were substituted into the first or second-topmost Mg layer of Mg(0001) surfaces, to understand the role of specific alloying elements in altering Mg surface properties in water. These dopants were chosen because experimental data was available that could be rationalized by the present study. Experimentally, Ca increases the rate of the anodic reaction, while Fe supports the cathodic reaction25,27,68–70. The Ca and Fe dopants were assessed thermodynamically in the Mg(0001) surface, and these results are compiled in Table 4. The positive substitution energy of Fe-doped Mg(0001) surface suggests that Fe prefers to segregate in Mg. This is in agreement with the surface energies of the corresponding systems (0.31 eV), which are slightly higher than that of the undoped surface (0.29 eV). In contrast, the Ca-doped Mg(0001) surface was found to be free from any segregation, as shown by the negative substitution energies and relatively similar surface energies than that of the undoped Mg surface. In principle, an increase in work function was observed when there are electrons transfer from the substrate (Mg atoms) to the dopant/substitutional (Fe or Ca), while a decrease is observed when electrons are partially transferred from the dopant to the substrate59. Ca dopant in the firsttopmost Mg layer of Mg(0001) increases the surface work function, while Ca dopant resides in the second-topmost Mg layer decreases it. The increase in work function when the Ca is in the first-topmost Mg layer is caused by the difference in electronegativity of Ca and Mg, which reduces the overall tendency of Mg atoms at the surface to donate an electron. Due to the presence of the first-topmost Mg layer, which acts a physical barrier between Ca and water, the effect of Ca in the second-topmost Mg layer is less robust. Figure 8 shows the electron build-up and depletion regions in the Mg-Ca(0001) and Mg-Fe(0001) surface. The electron flow initiates from the Ca atom towards the surrounding Mg atoms leads to a decrease in work function. Whilst, in the Fe-doped Mg(0001) surface, regardless of whether the Fe is in the first or second-


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topmost Mg layer, the surface work function decreases. This trend is observed because Fe is less electronegative than Mg. The free electron population changes in the surface will eventually affect the electrochemical properties of the Mg(0001) surface. The OH- on the doped Mg(0001) surfaces was adsorbed at FCC hollow sites and their corresponding hydroxylation reaction enthalpies at 0.06 OHad surface coverage were evaluated in the presence of alloying elements. From Table 4, it is shown that Ca dopant (in the first and second-topmost Mg layer) makes the Mg surface hydroxylation reaction less exothermic. A similar effect was also noticed in the first-topmost Mg layer Fe dopant. However, the secondtopmost Mg layer Fe dopant exhibits a different effect, as it favors hydroxylation on the Mg(0001) surface. From the physical structure, it was noticed the hydrogen of OHad was attracted towards Fe and repelled by Ca. The effect on H displacement is much stronger when the dopant presents in the first-topmost Mg layer. It was also observed that the presence of a Ca dopant increases the average interatomic distance of the topmost Mg layer-O atom(s), while Fe makes the average interatomic distance for topmost Mg layer-O atoms(s) shorter. Based on the electronegativity and the work function behavior, the increase of adsorption distance will result in weaker interactions between the charge build-up and overspill region in Mg; thus the decrease in work function will be smaller. When the adsorbate, OHad binds closer to the Mg surface, the interaction between these two contributing factors will be larger and the decrease in work function will be larger too. The Pourbaix diagrams of the doped Mg(0001) surfaces are presented in Figures 9 and 10. In the first-topmost Mg layer, Ca and Fe dopants have opposite effects on the hydroxylation at the surface. Ca promotes OH- adsorption on the surface, while Fe does not. Fe causes Mg to dissolve at a lower potential. This effect of Fe is often referred to in the empirical literature as “noble element enrichment” — since Fe is less active than Mg at the characteristic OCP, with Mg



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tending to dissolve first and Fe accumulating19,27. The reverse of this mechanism was observed when Ca was in the first-topmost Mg layer of Mg(0001) surface, because Ca is more active than Mg. Overall, the Fe dopant in the first-topmost Mg layer diminishes the immunity and passivation regions of Mg; while the Ca dopant improves both immunity and passivation of Mg. When the dopant atoms are in the second-topmost Mg layer, the changes in surface properties are different. Ca shifts the dissolution of Mg to a lower potential, and no longer enhances hydroxylation of the Mg(0001) surface. This suggests the presence of Ca in the second-topmost Mg layer promotes the dissolution of Mg atoms at the surface. Such a mechanism indicates the physical barrier (Mg) removal, so that Ca atom(s) can directly interact with water. Whilst, the effect of Fe in the second-topmost Mg layer on dissolution potential is insignificant; although Fe may affect the extent of the passivation region. In relation to the HER, both Mg-Ca and Mg-Fe alloys were found to have a high HER rate at the OCP25. Through experiment, Samaniego et al. suggested the increase of HER rate in Mg-Ca alloy was based on the enhanced dissolution mechanisms that are accompanied by enhanced cathodic reactions25. In contrast, Fe in Mg-Fe alloys supported the cathodic reaction at the interface through “noble element enrichment”25. From the previous discussion, it was shown that, energetically, Volmer-Heyrovsky reactions are the predominant mechanisms for HER on clean Mg(0001) surfaces at the PZC/OCP. This heterolytic reaction type suggests that the electron transfer from the surface plays a key role in determining the rate of H2 gas evolution. The Fe dopant in the first-topmost Mg layer provides both additional adsorption sites for Had right at the top of Fe atom and reduces the work function of Mg(0001) surface by 0.18 eV; while Fe dopant in the second-topmost Mg layer reduces the surface work function by up to 0.21 eV. These findings are in support of the notion that when Fe is present in Mg, this is accompanied by an


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enhanced rate of the cathodic reaction — Fe dopants reduce the work function of the Mg(0001) surface that facilitates the electron transfer required for the cathodic reaction. The findings herein from DFT have been shown to be very useful in providing a molecular level description for a number of evolving empirical works that have emerged in the past 5 years and have been cited throughout the present study. For example, the work herein can provide some illumination on the mechanistic interpretation suggested by Samaniego et al. for enhanced dissolution of Mg in the presence of Ca, whereby it was determined herein that Ca dopant in the second-topmost Mg layer promotes Mg dissolution. Such a scenario is likely to occur in the case of a dynamic surface evolving under polarization. While, a Ca dopant in the first-topmost Mg layer reveals that Ca dissolves into the water, promoting water dissociation, as realized from the increase of hydroxylated region stability over a wide range of pH in Figure 9. This suggests that the Ca dopant increases the HER kinetics through the surface hydroxylation process. Furthermore, Ca appeared to have essentially no effect in the Mg work function.

General discussion The work herein is a focused examination of Mg surface stability in water, addressing a number of key aspects that contribute to the surface electrochemistry of Mg. In particular, the physical origins of so-called cathodic activation of Mg have not to date been fully illuminated. From experimental work, it is clear that the rate of cathodic reactions upon Mg increase with Mg dissolution62,71–73. While the scenario of anodic polarization is not discussed herein, the present work makes an important contribution to understanding Mg and H2O interactions. DFT calculated surface Pourbaix diagrams reveal that the surface may form a hydroxide layer (purported to contribute to be a contributor to “catalytic” properties22) at significantly lower pH



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values (i.e. neutral pH ≈ 7) than those in the classical Pourbaix diagrams (i.e. where hydroxides are deemed to form at highly alkaline condition). Although there are a number of possible surface reactions that can occur upon Mg, including the Tafel and Heyrovsky reaction, it was found that neither of these reactions can fully explain the “catalytic” properties of the surface that are seen in cathodic activation. Instead this study finds that in the simulation conditions, the reduction reaction followed by the Heyrovsky reaction is energetically more favorable after OH- adsorption on the surface. This is an important finding in its own right, given the debate in previous studies16,20,22,54. Further, what was shown in the present work is that the “catalytic” properties of the Mg surface are also enhanced by increasing OHad concentration on the surface, supported by the observation that increasing OHad surface coverage leads to a monotonic decrease in the work function. A decrease in the work function corresponds to greater ease of electron removal from the surface, and it is this electron that is then consumed in the cathodic reaction – again important in the context of recent studies22. Surface doping by alloyed Ca or Fe can be summarized as follows. Adding a Ca atom to the surface layer resulted in an apparent electron shielding effect, where Ca depletes electrons from surrounding Mg – this may be a beneficial scenario for the fate of Mg atoms, but does not necessarily suppress alloy oxidation. Ca present in the second-topmost Mg layer makes the dissolution of Mg surface atom(s) easier – as the Ca seeks to preferentially emerge at the surface. Concomitantly, from experimental work, very high rates of dissolution of Mg-Ca alloys have been shown experimentally68. Fe dopants show a different mechanism of dissolution enhancement. In the surface layer, the Fe atom serves as a local site of reduction (local cathode). In subsequent atomic layers, Fe promotes the hydrogen evolution reaction on the Mg surface. These notions are consistent with trends in recent experimental work as reported and summarized by Lysne and co-workers21. Moreover, the alloying elements may also form intermetallic


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compounds with Mg, which may exhibit a different electrochemical behavior than their solid solution counterpart. However the Mg alloys model with alloying elements in low concentration as dopants has already showed an accurate and reliable description of the overall electrochemical performance of Mg alloys. This study shows that the influence of the alloying element is significantly decreased when the dopants (and/or the intermetallic compounds) go deeper into the bulk or move further away from the surface. Further studies on the intermetallic compounds of Mg alloys can provide a quantitative description of their electrochemical performance.



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CONCLUSIONS Here, a systematic theoretical investigation of the chemical stability of the Mg(0001) surface in water, including an exploration of the effects of dopant Ca and Fe atoms has been successfully presented. The viability of various forms of water on the clean Mg(0001) surface was examined. This work finds that the dissociation of H2O, followed by the adsorption of OH- on the surface is dependent on pH and potential. The reaction enthalpy of OH- adsorption and the extrapolated DFT Pourbaix diagram showed that adsorbed hydroxyl groups are thermodynamically stable at neutral pH of ~7. Hydroxyl group(s) adsorption was also shown to reduce the surface work function, and therefore is expected to promote hydrogen evolution. The physical adsorption of OH- on the Mg surface affects electronic configuration, and consequently, the alteration of work function was found to be a prominent factor towards the enhanced catalytic behavior of Mg. This behavior was also exhibited in Sc and potentially in all the hydroxide-forming metals due to identical physical OH- adsorption characteristics. The DFT results indicate that under open circuit potential condition, the predominant pathways for HER favor the Volmer-Heyrovsky reaction. Finally, this work also performed the first principle studies of Mg(0001) Ca- and Fe-doped surfaces representing the Mg-Ca and Mg-Fe alloys systems in aqueous environments. The dopants, Ca and Fe, were found to affect the surface electrochemical properties by modifying the surface work functions. The DFT results were found to be in qualitative agreement with previously reported experimental studies of corrosion rates of such Mg alloys, thus providing a more in depth understanding in the atomistic details at the interface. Fe dopants, either in the first or second layer, supports the cathodic reaction at Mg(0001) surface by lowering the work function (and therefore the energy required to transfer the electron used in the HER) and providing the additional adsorption sites for hydrogen. In contrast, Ca dopants in the firsttopmost Mg layer promote the formation of passive hydroxide layer on the surface. Reduction of


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the work function, along with enhancement of Mg dissolution, was observed when the Ca dopants were in the second-topmost Mg layer of the Mg(0001) surface.

ASSOCIATED CONTENT Supporting Information. Figure S1. Atomistic model of the clean Mg(0001) surface with 20 Å vacuum separation, which then filled with implicit water. Figure S2. Work function vs. OHad surface coverage (θ) upon the different metal surfaces: Sc(0001) - left and Al(100) - right. Table S1. Computed physical characteristics of hydroxylated Mg(0001) surface. Table S2. Computed physical characteristics of hydroxylated Ca- and Fe-doped Mg(0001) surfaces. Table S3. Computed physical characteristics of hydroxylated metal surfaces: Mg(0001), Sc(0001), and Al(100). Table S4. Computed thermodynamic characteristics of hydroxylated metal surfaces: Mg(0001), Sc(0001), and Al(100).



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AUTHOR INFORMATION Corresponding Author *Jodie A. Yuwono, Department of Materials Science and Engineering, Monash University, Melbourne, Australia. [email protected] *Dr. Nikhil V. Medhekar, Department of Materials Science and Engineering, Monash University, Melbourne, Australia. [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Jodie A. Yuwono is supported by the Monash International Postgraduate Scholarship and Monash Graduate Scholarship. Nick Birbilis and Nikhil V. Medhekar received funding from Australian Research Council DP 160103661 Scheme. ACKNOWLEDGEMENT The authors thank Dr. Kate Nairn for helpful discussions. J.A.Y. gratefully acknowledge computational support from the Monash University Sun Grid, the Australian National Computing Infrastructure (NCI), and the Pawsey Supercomputing Centre; also the financial support from


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Mg-(Mn,

Zr)-Fe

System

and

Experimental

Comparison

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with

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Alloy

Microstructures as Relevant to Impurity Driven Corrosion of Mg-Alloys. Mater. Chem. Phys. 2014, 143 (3), 1082–1091. (71) Taheri, M.; Kish, J. R.; Birbilis, N.; Danaie, M.; McNally, E. A.; McDermid, J. R. Towards a Physical Description for the Origin of Enhanced Catalytic Activity of Corroding Magnesium Surfaces. Electrochim. Acta 2014, 116, 396–403. (72) Curioni, M. The Behaviour of Magnesium during Free Corrosion and Potentiodynamic Polarization Investigated by Real-Time Hydrogen Measurement and Optical Imaging. Electrochim. Acta 2014, 120, 284–292. (73) Thomas, S.; Izquierdo, J.; Birbilis, N.; Souto, R. M. Possibilities and Limitations of Scanning Electrochemical Microscopy of Mg and Mg Alloys. Corrosion 2015, 71 (2), 171–183.


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LIST OF TABLES AND FIGURES

Table 1. Water dissociation and hydrogen evolution reaction (HER) at pH=0 and PZC/OCP

Surface Electrochemical Reactions

ΔHr (eV/mol H2O)

Mg(0001) + H2O  Mg(0001)/H2Oad

-0.39

Mg(0001) + H2O  Mg(0001)/OHad + 1/2 H2

-1.59

Mg(0001) + 2 H2O  Mg(0001)/2OHad + H2

-1.52 (-1.54 16)

Mg(0001) + H2O  Mg(0001)/Had + 1/2 H2 + 1/2 O2 Mg(0001) + H2O  Mg(0001)/Had/OHad

2.60 -1.65 (-1.78 16)

Mg(0001) + 2 H2O  Mg(0001)/Had /2OHad + 1/2 H2

-1.68

Mg(0001) + H2O  Mg(0001)/2Had + 1/2 O2

2.32

Mg(0001) + 2 H2O  Mg(0001)/2Had/OHad +1/2 H2 + 1/2 O2

0.47

Mg(0001) + 2 H2O  Mg(0001)/2Had/2OHad


-1.78 (-1.85 16)


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Table 2. Reaction enthalpies of various HER subsequent pathways at pH=0 and PZC/OCP Heyrovsky Pathway: Had + H+ + e-  H2 Mg(0001)/Had + H3O.H2O+ + e-  Mg(0001) + H2 + 2 H2O +

-

Mg(0001)/Had/OHad + H3O.H2O + e  Mg(0001)/OHad + H2 + 2 H2O +

-3.66 -3.85

-

Mg(0001)/H/2OHad + H3O.H2O + e  Mg(0001)/2OHad + H2 + 2 H2O +

ΔHr (eV)

-3.89

-

Mg(0001)/2Had + H3O.H2O + e  Mg(0001)/Had + H2 + 2 H2O +

-3.74

-

Mg(0001)/2Had/OHad + H3O.H2O + e  Mg(0001)/Had/OHad + H2 + 2 H2O +

-

-3.66

Mg(0001)/2Had/2OHad + H3O.H2O + e  Mg(0001)/Had/2OHad + H2 + 2 H2O

-3.42

Tafel Pathway: 2Had  H2

ΔHr (eV) 0.42 (0.54 16)

Mg(0001)/2Had  Mg(0001) + H2 Mg(0001)/2Had/OHad  Mg(0001)/OHad + H2 Mg(0001)/2Had/2OHad  Mg(0001)/2OHad + H2

0.21 0.52 (0.61 16)

Note: Bold is the most stable Mg(0001) surface in respect to dissociated H2O, as listed in Table 1.


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Table 3. Computed physical and thermodynamic characteristics of hydroxylated Mg(0001) surface

Number of OHad Properties 2N

2NN

3

7

36

Adsorption sites HCP Hollow Sites Reaction enthalpy (eV/adatom)

-1.53

-1.50

-1.50

-1.55

-1.53

Work function (eV)

3.06

3.01

2.99

2.82

2.33

Adsorption sites FCC Hollow Sites Reaction enthalpy (eV/adatom)

-1.52

-1.48

-1.55

-1.54

-1.56

Work function (eV)

3.11

2.98

2.92

2.82

2.00

Note: N: nearest neighbor; NN: next nearest neighbor



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Table 4. Computed physical and thermodynamic properties of the clean and hydroxylated Caand Fe-doped Mg(0001) surfaces Ca-doped Mg

Fe-doped Mg

Properties 1st layer

2nd layer

1st layer

2nd layer

Surface energy (eV)

0.29

0.29

0.31

0.31

Substitution energy (eV)

-1.13

-0.77

1.81

1.03

Work function (eV)

3.21

3.13

3.02

2.99

Dopant displacement in z axis (Å)

0.60

0.19

-0.48

-0.05

Hydroxylation reaction enthalpy (eV/adatom) FCC Hollow Sites

-1.41

-0.85

-1.75

-1.04

HCP Hollow Sites

-1.40

-0.92

-1.37

-1.56


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Figure 1. Top view of the clean and the first topmost layer doped Mg(0001) surface



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Figure 2. Reaction enthalpies of various H2O forms on clean Mg(0001) surface, as listed in Table 1.


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Figure 3. The hydroxylation reaction enthalpy vs. surface coverage on the clean Mg(0001) surface. The filled marks denote the reaction enthalpy in implicit water and hollow marks denote the reaction enthalpy in vacuum.



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Figure 4. The Mg slabs with hydroxide clusters consist of three, seven and thirty-six (1 ML) OHad.


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Figure 5. Work function vs. OHad surface coverage (θ) upon the Mg(0001) surface. The solid lines indicate the vacuum work function, and dashed lines indicate the aqueous work function



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Figure 6. Cross-sectional view of electron density difference ρdiff (r) in a clean Mg(0001) surface (left) and with a single OH adsorbed at an FCC hollow site (right). Red contours denote regions of electron build-up and blue contours denote regions of electron depletion.


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Figure 7. DFT-calculated Pourbaix diagram for the clean Mg(0001) surface with 0.06 surface coverage of OHad adsorbed at FCC (red) and HCP (blue) hollow sites.


47


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Figure 8. Top and cross-sectional views of electron density difference ρdiff (r) in Mg(0001) surfaces with Ca (left) and Fe (right) dopants. Red contours denote regions of electron build-up and blue contours denote regions of electron depletion.


48

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Figure 9. DFT-calculated Pourbaix diagram for 1st topmost layer doped Mg(0001) surfaces (solid) and clean Mg(0001) surfaces (dashed) with 0.06 surface coverage of OH groups adsorbed at FCC hollow sites.


49


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Figure 10. DFT-calculated Pourbaix diagram for 2nd topmost layer doped Mg(0001) surfaces (solid) and clean Mg(0001) surfaces (dashed) with 0.06 surface coverage of OH groups adsorbed at FCC hollow sites.


50

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TOC FIGURE


51

Electrochemical Stability of Magnesium Surfaces in an Aqueous

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