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International materials reviews (2021). doi:10.1080/09506608.2021.1930734, Published by Taylor & Francis, London

Fuel cells recombine water from H-2 and O-2 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."

Journal of the American Chemical Society 144(2), 987-994 (2022). doi:10.1021/jacs.1c11680, Published by American Chemical Society, Washington, DC

To support and accelerate the development of simulation protocols in atomistic modelling, we introduce an integrated development environment (IDE) for computational materials science called pyiron ( http://pyiron.org ). The pyiron IDE combines a web based source code editor, a job management system for build automation, and a hierarchical data management solution. The core components of the pyiron IDE are pyiron objects based on an abstract class, which links application structures such as atomistic structures, projects, jobs, simulation protocols and computing resources with persistent storage and an interactive user environment. The simulation protocols within the pyiron IDE are constructed using the Python programming language. To highlight key concepts of this tool as well as to demonstrate its ability to simplify the implementation and testing of simulation protocols we discuss two applications. In these examples we show how pyiron supports the whole life cycle of a typical simulation, seamlessly combines ab initio with empirical potential calculations, and how complex feedback loops can be implemented. While originally developed with focus on ab initio thermodynamics simulations, the concepts and implementation of pyiron are general thus allowing to employ it for a wide range of simulation topics.

Gold nanoparticles supported on reduced TiO 2 (110) surfaces are widely used as catalysts for oxidation reactions. Despite extensive studies, the role of oxygen vacancies in such systems remains elusive and is controversially discussed. Combining ab initio molecular dynamics simulations with methods originally developed to describe defects in semiconductor physics we study how the electronic charge originally located at the vacancy modifies the charge on the cluster. Despite differences resulting from the employed level of density functional theory (namely semilocal/GGA, GGA + U, and hybrid functionals), we consistently find that the Au clusters remain either neutral or acquire a positive charge. The intuitively expected electron transfer from the oxygen vacancy to the gold cluster can be safely ruled out. Analyzing these findings, we discuss the role of the oxygen vacancy in the bonding between Au clusters and support and the catalytic activity of the system. © 2019 American Chemical Society.

Density functional theory molecular dynamics simulations of H-covered $\mathrm{Pt}(111)\text{\ensuremath{-}}{\mathrm{H}}_{2}\mathrm{O}$ interfaces reveal that, in contrast to common understanding, ${\mathrm{H}}_{2}\mathrm{O}$ 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.

Author(s): Yoo, Su-Hyun; Todorova, Mira; Wickramaratne, Darshana; Weston, Leigh; Van de Walle, Chris G; Neugebauer, Joerg | Abstract: AbstractThe repeated slab approach has become a de facto standard to accurately describe surface properties of materials by density functional theory calculations with periodic boundary conditions. For materials exhibiting spontaneous polarization, we show that the conventional scheme of passivation with pseudo hydrogen is unable to realize a charge-neutral surface. The presence of a net surface charge induces via Gauss’s law a macroscopic electric field through the slab and results in poor size convergence with respect to the thickness of the slab. We propose a modified passivation method that accounts for the effect of spontaneous polarization, describes the correct bulk limits and boosts convergence with respect to slab thickness. The robustness, reliability, and superior convergence of energetics and electronic structure achieved by the proposed method are demonstrated using the example of polar ZnO surfaces.

International audience; It has recently been shown that the ab initio anharmonic free energy of fcc crystals can be approximated to meV/atom accuracy by a lattice of anharmonic nearest-neighbor bonds, where the bonding potential can be efficiently parametrized from the target system. We develop a mean-field approach for the free energy of a general bond lattice, analytically accounting for strong bond-bond correlations while enforcing material compatibility and thermodynamic self-consistency. Applying our fundamentally anharmonic model to fcc crystals yields free energies within meV/atom of brute force thermodynamic integration for core seconds of computational effort. Potential applications of this approach in computational materials science are discussed.

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.

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.

Using density-functional theory calculations we study the embrittling behaviour of liquid Zn in the Σ 3 [ 1 1 1 ] 60 ° and the Σ 5 [ 1 0 0 ] 36.8 ° symmetric tilt grain boundaries in bcc iron (ferrite). Investigating Zn induced changes in energetics of the grain boundaries and their associated free surfaces we utilise both a canonical and a grand-canonical thermodynamic model to discuss the embrittling behaviour of liquid zinc. Our results show that Zn wetting significantly destabilises the grain boundaries, but the conclusion which GB is more susceptible to embrittlement depends on the mechanism, as well as on the concentration.

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/H2O interface we develop and apply a theoretical approach that is firmly based onab initiocomputed 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.

To gain an initial understanding of the copper-based catalysts in commercially important chemical reactions such as the oxygenassisted water–gas shift reaction, we performed density-functional theory calculations, investigating the interaction of oxygen and copper, focusing on the relative stability of surface oxides and oxide surfaces of the O/Cu system. By employing the technique of ‘‘ab initio atomistic thermodynamics’’, we show that surface oxides are only metastable at relevant pressures and temperatures of technical catalysis, with no stable chemisorption phase observed even at very low coverage. Although exhibiting only metastability, these surface oxides resemble the bulk oxide material both geometrically and electronically, and may serve as a precursor phase before onset of the bulk oxide phase. Having identified the bulk oxide as the most stable phase under realistic catalytic conditions, we show that a Cu2O(1 1 1) surface with Cu vacancies has a lower free energy than the stoichiometric surface for the considered range of oxygen chemical potential and could be catalytically relevant. � 2007 Elsevier B.V. All rights reserved.

Experimental studies of nitrogen adsorbed on a Cu(1 1 1) surface show that the surface layer undergoes a reconstruction to form a pseudo-(1 0 0) structure. We use ab initio techniques to demonstrate the theoretical stability of this reconstructed surface phase over a range of conditions. We systematically investigate the chemisorption of N on the Cu(1 1 1) surface, from 0.06 to 1 ML coverage. A peculiar atomic relaxation of N atoms for 0.75 ML is identified, which results in the formation of a (metastable) “N-trimer cluster” on the surface. We have also investigated surface nitride formation, as suggested from experiments. A surface nitride-like structure similar to the reported pseudo-(1 0 0) reconstruction is found to be highly energetically favored. Using concepts from “ab initio atomistic thermodynamics”, we predict that this surface nitride exists for a narrow range of nitrogen chemical potential before the formation of bulk Cu3N.

Gerald Frankel, Janine Mauzeroll, Geoffrey Thornton, Hendrik Bluhm, Jonathan Morrison, Vincent Maurice, Trevor Rayment, David Williams, Angus Cook, Gaurav Joshi, Alison Davenport, Simon Gibbon, Denis Kramer, Matthew Acres, Markus Tautschnig, Hiroki Habazaki, Philippe Marcus, David Shoesmith, Clara Wren, Tom Majchrowski, Rob Lindsay, Mary Wood, Mira Todorova, John Scully, Frank Renner, Anton Kokalj, Christopher Taylor, Sannakaisa Virtanen and Julian Wharton

[Cook, Angus; Davenport, Alison] Univ Birmingham, Birmingham B15 2TT, W Midlands, England. [Frankel, Gerald] Ohio State Univ, Columbus, OH 43210 USA. [Hughes, Trevor] Schlumberger Gould Res, Cambridge, England. [Gibbon, Simon] AkzoNobel RDI Marcus, Philippe] CNRS Chim ParisTech, Paris, France. [Lyth, Stephen] Kyushu Univ, Fukuoka 812, Japan. [Shoesmith, David; Wren, Clara] Western Univ, London, ON, Canada. [Wharton, Julian] Univ Southampton, Southampton SO9 5NH, Hants, England. [Hunt, Gregory] Lubrizol, Wickliffe, OH USA. [Lyon, Stuart; Lindsay, Rob; Morsch, Suzanne] Univ Manchester, Manchester M13 9PL, Lancs, England. [Majchrowski, Tom] Natl Nucl Lab, Workington, Cumbria, England. [Williams, Geraint] Swansea Univ, Swansea, W Glam, Wales. [Oller, Beatriz Rico; Nixon, Sonja] Airbus Grp Innovat, Ottobrunn, Germany. [Todorova, Mira] Max Planck Inst Eisenforsch GmbH, Dusseldorf, Germany. [Cheng, Su-Ting] Max Planck Inst Iron Res, Dusseldorf, Germany. [Scully, John] Univ Virginia, Charlottesville, VA 22903 USA. [Wilson, Ann] Univ W Indies, St Augustine, Trinid & Tobago. [Renner, Frank] Hasselt Univ, Inst Mat Res IMO, B-3590 Diepenbeek, Belgium. [Taylor, Christopher] DNV GL, Katy, TX USA. [Taylor, Christopher] Ohio State Univ, Columbus, OH 43210 USA. [Habazaki, Hiroki] Hokkaido Univ, Sapporo, Hokkaido 060, Japan. [Michaelides, Angelos] UCL, London, England. [Visser, Peter] Delft Univ Technol, NL-2600 AA Delft, Netherlands. [Kyhl, Line] Aarhus Univ, DK-8000 Aarhus C, Denmark. [Kokalj, Anton] Jozef Stefan Inst, Ljubljana, Slovenia.

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.

By use of density-functional theory we analyze the on-surface adsorption of oxygen on Pd(111) for coverages up to 1 monolayer and compare the results with corresponding data for the other late 4d transition metals, namely, Ru, Rh, and Ag. Besides the known effect of the continued d-band filling on the oxygen-metal bond strength, we also discern trends in the adsorption geometries, work functions, and electron density of states. The repulsive lateral interactions in the overlayer give rise to a pronounced reduction of the adsorption

Combining high-resolution core-level spectroscopy (HRCLS), scanning tunneling microscopy (STM) and density-functional theory (DFT) calculations we reanalyze the Pd(100)-(SQRT(5) x SQRT(5) R27^o)-O surface oxide phase. We find that the prevalent structural model, a rumpled PdO(001) film suggested by previous low energy electron diffraction (LEED) work (M. Saidy et al., Surf. Sci. 494, L799 (2001)), is incompatible with all three employed methods. Instead, we suggest the two-dimensional film to consist of a strained PdO(101) layer on top of Pd(100). LEED intensity calculations show that this model is compatible with the experimental data of Saidy et al.

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 ${\mathrm{N},\mathrm{O},\mathrm{F}}$ 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.

We study the effect an aqueous electrolyte can have on the concentration and electronic character of native point defects in a semiconducting electrode by utilising a recently derived grand canonical approach. Constructing defect phase diagrams which show the majority defect species as function of applied bias and chemical potential we identify which native point defects of ZnO become important when this semiconductor comes into contact with an aqueous electrolyte at varying pH and bias. Our results show that in thermodynamic equilibrium Zn rather than O vacancies are stable under electrochemical conditions.

Despite similarities between their fundamental building blocks---charged defects/ions---theoretical and modeling concepts in semiconductor defect chemistry and electrochemistry have little overlap. Theorists present a unified approach based on a fully grand-canonical description of both ions and electrons, connecting the respective concepts and providing surprising new insights into apparently ``old'' problems.

Presented is an approach based on the seamless transmission of information obtained from modern electronic structure methods like density-functional theory to thermodynamic functions. It allows to circumvent short term dynamics in a system, instead focusing on the long-term impact an environment can have on it. It allows the identification of stable and metastable phases at relevant (T,p)-conditions. After outlining the basic principles of the methodology, examples of surfaces in contact with realistic environments are used to emphasise or illustrate different aspects of the method. Particular attention is paid to possible approximations and their justification is discussed. The use of a direct screening approach, in which relevant structures are directly compared, as a possibility to deal with the configurational diversity of a system is discussed. Its advantages and disadvantages are discussed, and alternative routes are suggested. The importance of surface vibrational entropy is discussed in some depth, since it can have a large impact on formation Gibbs free energies and thus on the competition between relevant structures to gain thermodynamic stability at a given temperature. The assumption underlying the thermodynamic approach are reviewed and routes to extend the thermodynamic formalism to account for kinetically stabilised surface reconstructions are presented. Experimental situations, which may give rise to chemical potentials beyond the thermodynamic limit are mentioned. Examples of equilibrium, extended and meta-stable surface phase diagrams are shown and discussed. Keywords: ab-initio thermodynamics; Gibbs free energy; chemical potential; surface vibrational entropy; (extended and meta-stable) surface phase diagrams

Combining density-functional-theory calculations and atomistic thermodynamics we study hydrogen adsorption on the Zn-terminated polar ZnO(0001) surface. We extend the concept of equilibrium surface phase diagrams to include kinetically stabilized surface reconstructions by constructing metastable phase diagrams and going beyond the thermodynamic limit. Under these conditions we find a monolayer H coverage to be extremely favorable on triangular-shaped reconstructions due to a simultaneous protonation of step-edge oxygen atoms and surface terminating Zn atoms. Experimental situations, which realize hydrogen chemical potentials outside the stability range of the ${\text{H}}_{2}$ molecule are discussed.

To obtain insight into the structure and surface stoichiometry of copper-based catalysts in commercially important chemical reactions such as the oxygen-assisted water-gas shift reaction, we perform density-functional theory calculations to investigate the relative stability of low-index copper oxide surfaces. By employing the technique of ``ab initio atomistic thermodynamics,'' we identify low-energy surface structures that are most stable under realistic catalytic conditions are found to exhibit a metallic character. Three surfaces are shown to have notably lower surface free energies compared to the others considered and could be catalytically relevant; in particular, under oxygen-rich conditions, they are the ${\mathrm{Cu}}_{2}\mathrm{O}(110):\mathrm{Cu}\mathrm{O}$ surface, which is terminated with both Cu and O surface atoms, and the ${\mathrm{Cu}}_{2}\mathrm{O}(111)\text{\ensuremath{-}}{\mathrm{Cu}}_{\mathrm{CUS}}$ surface, which contains a surface (coordinatively unsaturated) Cu vacancy, while for the oxygen-lean conditions, the ${\mathrm{Cu}}_{2}\mathrm{O}(111)$ surface with a surface interstitial Cu atom is found to be energetically most favorable, highlighting the importance of defects at the surface.

As a first step towards gaining microscopic understanding of copper-based catalysts, e.g., for the low-temperature water-gas shift reaction and methanol oxidation reactions, we present density-functional theory calculations investigating the chemisorption of oxygen, and the stability of surface oxides on $\mathrm{Cu}(111)$. We report atomic geometries, binding energies, and electronic properties for a wide range of oxygen coverages, in addition to the properties of bulk copper oxide. Through calculation of the Gibbs free energy, taking into account the temperature and pressure via the oxygen chemical potential, we obtain the $(p,T)$ phase diagram of $\mathrm{O}∕\mathrm{Cu}(111)$. Our results show that for the conditions typical of technical catalysis the bulk oxide is thermodynamically most stable. If, however, formation of this fully oxidized surface is prevented due to a kinetic hindering, a thin surface-oxide structure is found to be energetically preferred compared to chemisorbed oxygen on the surface, even at very low coverage. Similarly to the late $4d$ transition metals (Ru, Rh, Pd, Ag), sub-surface oxygen is found to be energetically unfavorable.

Pd(111) has recently been shown to exhibit a propensity to form a sub-nanometer thin surface oxide film already well before a full monolayer coverage of adsorbed O atoms is reached on the surface. Aiming at an atomic-scale understanding of this finding, we study the initial oxygen incorporation into the Pd(111) surface using density-functional theory. We find that oxygen incorporation into the sub-surface region starts at essentially the same coverage as formation of the surface oxide. This implies that the role of sub-surface oxygen should be considered as that of a metastable precursor in the oxidation process of the surface. The mechanisms found to play a role towards the ensuing stabilization of an ordered oxidic structure with a mixed on-surface/sub-surface site occupation follow a clear trend over the late 4d transition metal series, as seen by comparing our data to previously published studies concerned with oxide formation at the basal surface of Ru, Rh and Ag. The formation of a linearly aligned O-TM-O trilayered structure (TM = Ru, Rh, Pd, Ag), together with an efficient coupling to the underlying substrate seem to be key ingredients in this respect., Comment: 8 pages including 5 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.html

The oxidation of the Pd(100) surface at oxygen pressures in the 10^-6 to 10^3 mbar range and temperatures up to 1000 K has been studied in-situ by surface x-ray diffraction (SXRD). The results provide direct structural information on the phases present in the surface region and on the kinetics of the oxide formation. Depending on the (T,p) environmental conditions we either observe a thin sqrt(5) x sqrt(5) R27 surface oxide or the growth of a rough, poorly ordered bulk oxide film of PdO predominantly with (001) orientation. By either comparison to the surface phase diagram from first-principles atomistic thermodynamics or by explicit time-resolved measurements we identify a strong kinetic hindrance to the bulk oxide formation even at temperatures as high as 675 K., 4 pages including 4 figures, Related publications can be found at http://www.fhi-berlin.mpg.de/th/paper.html