Quasi-"In Situ" Analysis of the Reactive Liquid-Solid Interface during Magnesium Corrosion Using Cryo-Atom Probe Tomography.

The early stages of corrosion occurring at liquid-solid interfaces control the evolution of the material's degradation process, yet due to their transient state, their analysis remains a formidable challenge. Here corrosion tests are performed on a MgCa alloy, a candidate material for biodegradable implants using pure water as a model system. The corrosion reaction is suspended by plunge freezing into liquid nitrogen. The evolution of the early-stage corrosion process on the nanoscale by correlating cryo-atom probe tomography (APT) with transmission-electron microscopy (TEM) and spectroscopy, is studied. The outward growth of Mg hydroxide Mg(OH)2 and the inward growth of an intermediate corrosion layer consisting of hydrloxides of different compositions, mostly monohydroxide Mg(OH) instead of the expected MgO layer, are observed. In addition, Ca partitions to these newly formed hydroxides and oxides. Density-functional theory calculations suggest a domain of stability for this previously experimental unreported Mg(OH) phase. This new approach and these new findings advance the understanding of the early stages of magnesium corrosion, and in general reactions and processes at liquid-solid interfaces, which can further facilitate the development of corrosion-resistant materials or better control of the biodegradation rate of future implants.

Comparative analysis of surface phase diagrams in aqueous environment: Implicit vs explicit solvation models.

Identifying the stable surface phases under a given electrochemical conditions serves as the basis for studying the atomistic mechanism of reactions at solid/water interfaces. In this work, we systematically compare the performance of the two main approaches that are used to capture the impact of an aqueous environment, implicit and explicit solvent, on surface energies and phase diagrams. As a model system, we consider the magnesium/water interface with (i) Ca substitution and (ii) proton and hydroxyl adsorption. We show that while the implicit solvent model is computationally very efficient, it suffers from two shortcomings. First, the choice of the implicit solvent parameters significantly influences the energy landscape in the vicinity of the surface. The default parameters benchmarked on solvation in water underestimate the energy of the dissolved Mg ion and lead to spontaneous dissolution of the surface atom, resulting in large differences in the surface energetics. Second, in systems containing a charged surface and a solvated ion, the implicit solvent model may not converge to the energetically stable ionic charge state but remain in a high-energy metastable configuration, representing the neutral charge state of the ion. When these two issues are addressed, surface phase diagrams that closely match the explicit water results can be obtained. This makes the implicit solvent model highly attractive as a computationally-efficient surrogate model to compute surface energies and phase diagrams.

Laterally Resolved Free Energy Profiles and Vibrational Spectra of Chemisorbed H Atoms on Pt(111).

A scheme to compute laterally resolved free energy surfaces and spectral signatures of specifically adsorbed ions on electrode surfaces from their ab initio molecular dynamics (AIMD) trajectories is proposed. Considering H-covered Pt(111) electrodes, both in contact with water and vacuum and for various H coverages, we systematically explore the impact of explicit water and H-coverage on site occupancy, providing direct insight into the proportion of underpotential and overpotential deposited hydrogen adsorbates. Extending this approach further, we can obtain laterally resolved vibrational spectra of the Pt-H stretch modes. We discuss how the difference between the free energy basins of the on-top and fcc-hollow adsorption sites explains the features of the experimentally observed spectral fingerprints in this system. These fingerprints do not contain only information about the stable and metastable adsorption sites but also about intermediate short-lived adsorbate configurations. Our results also show that for these properties chemisorbed H2O acts as a spectator and does not qualitatively influence the relative stabilities of the adsorption sites and their spectral fingerprints.

Controlled Doping of Electrocatalysts through Engineering Impurities.

Fuel cells recombine water from H2 and O2 thereby can power, for example, cars or houses with no direct carbon emission. In anion-exchange membrane fuel cells (AEMFCs), to reach high power densities, operating at high pH is an alternative to using large volumes of noble metals catalysts at the cathode, where the oxygen-reduction reaction occurs. However, the sluggish kinetics of the hydrogen-oxidation reaction (HOR) hinders upscaling despite promising catalysts. Here, the authors observe an unexpected ingress of B into Pd nanocatalysts synthesized by wet-chemistry, gaining control over this B-doping, and report on its influence on the HOR activity in alkaline conditions. They rationalize their findings using ab initio calculations of both H- and OH-adsorption on B-doped Pd. Using this "impurity engineering" approach, they thus design Pt-free catalysts as required in electrochemical energy conversion devices, for example, next generations of AEMFCs, that satisfy the economic and environmental constraints, that is, reasonable operating costs and long-term stability, to enable the "hydrogen economy."

Understanding Alkali Contamination in Colloidal Nanomaterials to Unlock Grain Boundary Impurity Engineering.

Metal nanogels combine a large surface area, a high structural stability, and a high catalytic activity toward a variety of chemical reactions. Their performance is underpinned by the atomic-level distribution of their constituents, yet analyzing their subnanoscale structure and composition to guide property optimization remains extremely challenging. Here, we synthesized Pd nanogels using a conventional wet chemistry route, and a near-atomic-scale analysis reveals that impurities from the reactants (Na and K) are integrated into the grain boundaries of the poly crystalline gel, typically loci of high catalytic activity. We demonstrate that the level of impurities is controlled by the reaction condition. Based on ab initio calculations, we provide a detailed mechanism to explain how surface-bound impurities become trapped at grain boundaries that form as the particles coalesce during synthesis, possibly facilitating their decohesion. If controlled, impurity integration into grain boundaries may offer opportunities for designing new nanogels.

Impact of Water Coadsorption on the Electrode Potential of H-Pt(1 1 1)-Liquid Water Interfaces.

Density functional theory molecular dynamics simulations of H-covered Pt(111)-H_{2}O interfaces reveal that, in contrast to common understanding, H_{2}O coadsorption has a significant impact on the electrode potential of and plays a major role in determining the stability of H adsorbates under electrochemical conditions. Based on these insights, we explain the origin behind the experimentally observed upper limit of H coverage well below one monolayer and derive a chemically intuitive model for metal-water bonding that explains an unexpectedly large interaction between coadsorbed water and adsorbates.

First-Principles Approach to Model Electrochemical Reactions: Understanding the Fundamental Mechanisms behind Mg Corrosion.

Combining concepts of semiconductor physics and corrosion science, we develop a novel approach that allows us to perform ab initio calculations under controlled potentiostat conditions for electrochemical systems. The proposed approach can be straightforwardly applied in standard density functional theory codes. To demonstrate the performance and the opportunities opened by this approach, we study the chemical reactions that take place during initial corrosion at the water-Mg interface under anodic polarization. Based on this insight, we derive an atomistic model that explains the origin of the anodic hydrogen evolution.

Selective Solvent-Induced Stabilization of Polar Oxide Surfaces in an Electrochemical Environment.

The impact of an electrochemical environment on the thermodynamic stability of polar oxide surfaces is investigated for the example of ZnO(0001) surfaces immersed in water using density functional theory calculations. We show that solvation effects are highly selective: They have little effect on surfaces showing a metallic character, but largely stabilize semiconducting structures, particularly those that have a high electrostatic penalty in vacuum. The high selectivity is shown to have direct consequences for the surface phase diagram and explains, e.g., why certain surface structures could be observed only in an electrochemical environment.

Ab Initio Determined Phase Diagram of Clean and Solvated Muscovite Mica Surfaces.

Focusing on muscovite mica, the most significant phyllosilicate in the mica series, we determine its surface phase diagram employing density functional theory. Surfaces in vacuum and in more realistic environmental conditions, that is, the surface in contact with water or an ionic liquid, are considered. These results naturally explain experimental observations such as the swelling of mica when it comes into contact with water.

Identification of bulk oxide defects in an electrochemical environment.

We discuss how electronic-structure calculations can be used to identify the dominant point defects that control the growth and dissolution of the oxide barrier layer formed if a metal comes into contact with a corrosive environment. Using the example of the Zn/ZnO/H(2)O interface we develop and apply a theoretical approach that is firmly based on ab initio computed defect formation energies and that unifies concepts of semiconductor defect chemistry with electrochemical concepts. Employing this approach we find that the commonly invoked and chemically intuitive defects such as the doubly negatively charged oxygen vacancy in electrochemically formed ZnO films may not be present. Rather, hitherto not discussed defects such as the oxygen interstitial or unexpected charge states, such as the neutral oxygen vacancy, are found. These new defect types will be shown to critically impact our understanding of fundamental corrosion mechanisms and to provide new insight into strategies to develop alloys with higher corrosion resistance.

Negatively charged ions on Mg(0001) surfaces: appearance and origin of attractive adsorbate-adsorbate interactions.

Adsorption of electronegative elements on a metal surface usually leads to an increase in the work function and decrease in the binding energy as the adsorbate coverage rises. Using density-functional theory calculations, we show that Cl adsorbed on a Mg(0001) surface complies with these expectations, but adsorption of {N,O,F} causes a decrease in the work function and an increase in the binding energy. Analyzing the electronic structure, we show that the presence of a highly polarizable electron spill-out in front of Mg(0001) causes this unusual adsorption behavior and is responsible for the appearance of a hitherto unknown net-attractive lateral electrostatic interaction between same charged adsorbates.

Temperature stabilized surface reconstructions at polar ZnO(0001).

The atomic structure of the polar ZnO(0001) surfaces in a dry and humid oxygen environment is studied combining diffraction experiments and density-functional theory. Our results indicate that for similar stoichiometries a large number of very different, but energetically almost degenerate reconstructions exist. Thus vibrational entropy, which could be safely neglected for most semiconductor surfaces becomes dominant, giving rise to a hitherto not reported strong dependence of surface phase diagrams on temperature. Based on this insight we are able to consistently describe and explain the experimentally observed surface structures on polar ZnO(0001) surfaces.