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Density functional theory calculations together with ab initio molecular dynamics (AIMD) simulations have been used to study the solvation, diffusion and transformation of Li+ and LiO2 upon O2 reduction in three organic electrolytes. These processes are critical for the performance of Li-air batteries. Apart from studying the structure of the solvation shells in detail, AIMD simulations have been used to derive the diffusivity and together with the Blue Moon ensemble approach to explore LiO2 formation from Li+ and O2- and the subsequent disproportionation of 2LiO2 into Li2O2 + O2. By comparing the results of the simulations to gas phase calculations the impact of electrolytes on these reactions is assessed which turns out to be more pronounced for the ionic species involved in these reactions.

Electrochemical stability is a critical performance parameter for the materials used as electrolytes and electrodes in batteries. Using first-principles electronic structure calculations, we have determined the electrochemical stability windows of magnesium binary and ternary spinel compounds. These materials are candidates for protective coating, solid electrolytes and cathodes in Mg-ion batteries, which represent a promising sustainable alternative to Li-ion batteries that still dominate the battery market. Furthermore, we have applied and assessed two different criteria for the chemical stability of compounds. For the spinel materials, we identify the critical role of the ionic radii of the transition metal for the stability of the compounds. In addition, we determine the ion mobility in these materials using a recently developed descriptor. We thus provide guidelines for the choice of promising solid materials for Mg-ion batteries with improved properties.

The modelling of electrochemical interfaces between a liquid electrolyte and an electrode from a quantum chemical perspective is typically done by performing ab initio molecular dynamics simulations. Thus the statistically nature of the electrolyte structure can be taken into account by performing the proper averages. However, in order to obtain reliable results for such electrochemical interfaces, the simulations should be performed for sufficiently large systems and sufficiently long times under potential control. These requirements lead to significant challenges for running such simulations which will be addressed in this contribution, together with possible approaches to address these challenges.

The transition from lithium-based energy storage to post lithium systems plays a crucial part in achieving an environmentally sustainable energy infrastructure. Prime candidates for the replacement of lithium are sodium and potassium batteries. Despite being critical to battery performance, the solid electrolyte interphase (SEI) formation process for Na and K batteries remains insufficiently understood, especially compared to the well-established lithium systems. Using ab initio molecular dynamics (AIMD) simulations based on density functional theory (DFT) calculations, we study the first steps of SEI formation upon the decomposition of typical solvent molecules on lithium, sodium and potassium metal anodes. We find that two dominant products form during the early SEI formation of cyclical carbonates on alkali metal anodes, carbon monoxide and alkali-carbonate. The carbonate-producing reaction is thermodynamically favorable for all tested metals, however, Na and K exhibit a much stronger selectivity than Li towards carbonate formation. Furthermore, we propose a previously unknown reaction mechanism for the CO polymerization on metallic lithium.

Structures and processes at water/metal interfaces play an important technological role in electrochemical energy conversion and storage, photoconversion, sensors, and corrosion, just to name a few. However, they are also of fundamental significance as a model system for the study of solid-liquid interfaces, which requires combining concepts from the chemistry and physics of crystalline materials and liquids. Particularly interesting is the fact that the water-water and water-metal interactions are of similar strength so that the structures at water/metal interfaces result from a competition between these comparable interactions. Because water is a polar molecule and water and metal surfaces are both polarizable, explicit consideration of the electronic degrees of freedom at water/metal interfaces is mandatory. In principle, ab initio molecular dynamics simulations are thus the method of choice to model water/metal interfaces, but they are computationally still rather demanding. Here, ab initio simulations of water/metal interfaces will be reviewed, starting from static systems such as the adsorption of single water molecules, water clusters, and icelike layers, followed by the properties of liquid water layers at metal surfaces. Technical issues such as the appropriate first-principles description of the water-water and water-metal interactions will be discussed, and electrochemical aspects will be addressed. Finally, more approximate but numerically less demanding approaches to treat water at metal surfaces from first-principles will be briefly discussed.

The theoretical modeling of metal/water interfaces centers on an appropriate configuration of the electric double layer (EDL) under grand canonical conditions. In principle, ab initio molecular dynamics (AIMD) simulations would be the appropriate choice for treating the competing water-water and water-metal interactions and explicitly considering the atomic and electronic degrees of freedom. However, this approach only allows simulations of relatively small canonical ensembles over a limited period (shorter than 100 ps). On the other hand, computationally efficient semiclassical approaches can treat the EDL model based on a grand canonical scheme by averaging the microscopic details. Thus, an improved description of the EDL can be obtained by combining AIMD simulations and semiclassical methods based on a grand canonical scheme. By taking the Pt(111)/water interface as an example, we compare these approaches in terms of the electric field, water configuration, and double-layer capacitance. Furthermore, we discuss how the combined merits of the approaches can contribute to advances in EDL theory.

To model a NaOTF Water-in-Salt (WiS) electrolyte using classical Molecular Dynamics (MD) simulations, we explore various force fields where atomic polarization is accounted for at three different levels: a non-polarizable all-atom force field where polarization is only implicitly included in its Van der Waals interaction parameters, the same force field with uniformly scaled ionic charges mimicing electron polarization within a mean-field approximation, and an explicit polarizable force field where polarization is modeled via Drude oscillators. We also probe combining different polarization levels for salt ions and water: when ion polarization is described by the Drude method, water is modeled by either the non-polarizable SPC/E model or the polarizable SWM4-NDP model. The main goal is to achieve simulation stability for different force fields and investigate the influence of the force field parameters on the electrolyte properties. Force field parameters that adjust the interactions of cations or Drude pairs are found to significantly affect the electrolyte structure and its dynamic properties. This effect is primarily due to the strong dependence of the degree of salt dissociation on these parameters. Among the force fields studied in this work, we identify an efficient combination of the Drude polarizable force field with non-polarizable water models, which is sufficiently flexible to reproduce various properties of WiS solutions, while the computational cost is affordable and simulation stability is ensured over a relatively wide range of force field parameters.

Based on quantum chemical calculations, we predict strong solvatochromism in a light-driven molecular photocatalyst for hydrogen generation, i.e., we show that the electronic and optical properties of the photocatalyst strongly depend on the solvent it is dissolved in. Our calculations in particular indicate a solvent dependent relocation of the highest occupied molecular orbital (HOMO). Ground-state density functional theory and linear response time dependent density functional theory calculations are applied in order to investigate the influence of implicit solvents on the structural, electronic and optical properties of a molecular photocatalyst. Only at high dielectric constants of the solvent, the HOMO is located at the metal center of the photosensitizer, whereas at low dielectric constants the HOMO is centered at the metal atom of the catalytically active complex. We elucidate the electronic origins of this strong solvatochromic effect and sketch the consequences of these insights for the use of photocatalysts in different environments.

Mixing dopants into oxide catalysts can improve the catalytic activity, as shown in the dramatic boost of the NH3 selective catalytic reduction (SCR) activity on vanadia catalysts upon doping by tungsten. Thus, the design and optimization of oxide catalysts require a precise understanding of the role of dopants and their influence on catalytic reactions. Here, we employ first-principles calculations to study the influence of selected dopants (Ce, Zr, Nb, Mo, and W) in tungsta-vanadia on the SCR activity in terms of dopant concentration, distribution, and species. We demonstrate how the dopants affect the stoichiometry of the catalyst and thus fine-tune the local electron distribution and polarization in the catalytic layer. In addition, we address the relation between dopant concentration and the population of the active vanadyl configuration on the surface. Finally, we propose the generalized surface stoichiometry of the doped vanadia catalysts as a descriptor for the SCR catalytic activity, which promises to be instrumental in identifying oxide catalysts with improved properties also for other important catalytic reactions.

Electrocatalytic activity is influenced by the surface charge on the solid catalyst. Conventionally, our attention has been focused on how the surface charge shapes the electric potential and concentration of ionic reactant(s) in the local reaction zone. Taking H2O2 redox reactions at Pt(111) as a model system, we reveal a peculiar surface charge effect using ab initio molecular dynamics simulations of electrified Pt(111)-water interfaces. In this scenario, the negative surface charge on Pt(111) repels the O–O bond of the reactant (H2O2) farther away from the electrode surface. This leads to a higher activation barrier for breaking the O–O bond. Incorporating this microscopic mechanism into a microkinetic-double-layer model, we are able to semi-quantitatively interpret the pH-dependent activity of H2O2 redox reactions at Pt(111), especially the anomalously suppressed activity of H2O2 reduction with decreasing electrode potential. The relevance of the present surface charge effect is also examined in wider scenarios with different electrolyte cations, solution pHs, crystal facets of the catalyst, and model parameters. In contrast with previous mechanisms focusing on how surface charge influences the local reaction condition at a fixed reaction plane, the present work gives an example in which the location of the reaction plane is adjusted by the surface charge. This work was supported by the National Natural Science Foundation of China (21802170). J.H. acknowledges the financial support for a Europe Research Stay at Alicante University from the Alexander von Humboldt Foundation, and the National Natural Science Foundation of China under the grant number of 21802170. Computer time provided by the state of Baden-Württemberg through bwHPC and the German Research Foundation (DFG) through grant no INST 40/575-1 FUGG (JUSTUS 2 cluster) are gratefully acknowledged. This work contributes to the research performed at CELEST (Center for Electrochemical Energy Storage Ulm-Karlsruhe). J.M.F and V.C. acknowledge financial support from Ministerio de Ciencia e Innovación (Project PID2019-105653GB-100) and Generalitat Valenciana (Project PROMETEO/2020/063).

The full understanding of electrocatalytic reactions requires a complete knowledge of the elementary steps occurring in these reactions together with the corresponding rate constants as a function of the parameters controlling the electrochemical environment. Given the complexity of electrochemical electrode/electrolyte interfaces, the modelling of elementary steps in electrocatalysis represents a substantial challenge, and there is still a need for reliable benchmark studies. Nevertheless, tremendous progress has been made in this field in recent years. Here the current status of this field will be briefly sketched, possible routes to meet the challenges will be suggested and open questions will be discussed.

Tungsten-doped vanadia-based catalysts supported on anatase TiO2 are used to reduce hazardous NO emissions through the selective catalytic reduction of ammonia, but their exact atomistic structure is still largely unknown. In this computational study, the atomistic structure of mixed tungsta-vanadia monolayers on TiO2 support under typical operating conditions has been addressed by periodic density functional theory calculations. The chemical environment has been taken into account in a grand-canonical approach. We evaluate the stable catalyst structures as a function of the oxygen chemical potential and vanadium and tungsten concentrations. Thus we determine structural motifs of tungsta-vanadia/TiO2 catalysts that are stable under operating conditions. Furthermore, we identify active sites that promise high catalytic activity for the selective catalytic reduction by ammonia. Our calculations reveal the critical role of the stoichiometry of the tungsta-vanadia layers with respect to their catalytic activity in the selective catalytic reduction.

Li-air batteries are a promising alternative to Li-ion batteries as they theoretically provide the highest possible specific energy density. Mainly, Li2O2 (lithium peroxide) and to a lesser extent, Li2O (lithium oxide) are assumed to be the discharge products of these batteries formed with the soluble LiO2 (lithium superoxide) considered to be an intermediate product. Bulk Li2O2 is an electronic insulator, and the precipitation of this compound on the cathode is thought to be the main limiting factor in achieving high capacities in lithium-oxygen cells. For the most promising electrolytes including solvents with high donor numbers, microscopy observations frequently reveal crystallite morphologies of Li2O2 compounds, rather than uniform layers covering the electrode surface. The precise morphologies of Li2O and Li2O2 particles, and their effect and their extent of contact with the electrode, which may all affect the capacity and rechargeability, however, remain largely undetermined. Here, we address the stability of various Li2O and Li2O2 surfaces and consequently, their crystallite morphologies using density functional theory calculations and ab initio thermodynamics. In contrast to previous studies, we also consider high-index surface terminations, which exhibit surprisingly low surface energies. We carefully analyze the reasons for the stability of these high-index surfaces, which also prominently influence the equilibrium shape of the particles, at least for Li2O2, and discuss the consequences for the observed morphology of the reaction products.

Employing density functional theory (DFT) calculations and x-ray photoelectron spectroscopy (XPS), we identify products of the reaction of the ionic liquid N,N - butylmethylpyrrolidinum bis(trifluoromethylsulfonyl)imide (BMP-TFSI) with lithium in order to model the initial chemical processes contributing to the formation of the solid electrolyte interphase in batteries. Besides lithium oxide, sulfide, carbide and fluoride, we find lithium cyanide or cyanamide as possible, thermodynamically stable products in the Li-poor regime, whilst Li3N is the stable product in the Li-rich regime. The thermodynamically controlled reaction products as well as larger fragments of TFSI persisting due to kinetic barriers could be identified by a comparison of experimentally and computationally determined core level binding energies.

In this review, a discussion on renewable sources of energy with clear focus on solar cell applications is presented. Especially, possible future directions for development of dye-sensitized solar cells (DSSCs) are discussed. Dye-sensitized solar cells have become an important topic of research due to its high importance in energy conversion. Current DSSCs are based on either metal dye sensitizers, metal-free organic dyes or natural dyes. They have been extensively studied due to their low cost, simple preparation methodology, low toxicity, and ease of production. Still there is a need to find more abundant DSSC materials that at same time exhibit long-term stability. Computational studies have been an important ally for developing/designing new dye sensitizers. They are reviewed here with a special emphasis on the benefit of such studies. The conceptual understanding of development and working principle of photoactive DSSC materials are the primary feature of the review followed by examples of studies on different dye sensitizers using scarce to abundant metal based dyes and metal free organic dyes with donor-π-acceptor geometries for both n- and p-type DSSCs. The proper choice of organic dyes including donor, spacer, or acceptor is discussed and a prospective on dual donor based dyes is presented.

We have performed density functional theory calculations to explore the possibility to overcome the linear scaling relations in the oxygen reduction reaction (ORR) using local inhomogeneities on Pt...

Recently, a surprisingly fast diffusion of O atoms on a fully CO-covered Ru(0001) surface has been observed using video-rate scanning tunneling microscopy. This finding has been explained by a so-c...

To strain or not to strain: We calculate the strain dependence of metal anode surface properties from first principles to explore whether surface stress can be related to dendrite growth in metal-ion batteries on the atomistic level. We only find a weak dependence of these properties on the applied strain, which suggests that strain does not influence dendrite growth on the atomistic scale, but rather dendrite-related morphologies on longer-length scales. Dendrite growth poses a significant problem in the design of modern batteries as it can lead to capacity loss and short-circuiting. Recently, it has been proposed that self-diffusion barriers might be used as a descriptor for the occurrence of dendrite growth in batteries. As surface strain effects can modify dendritic growth, we present first-principles DFT calculations of the dependence of metal self-diffusion barriers on applied surface strain for a number of metals that are used as charge carriers in batteries. Overall, we find a rather small strain dependence of the barriers. We mainly attribute this to cancellation effects in the strain dependence of the initial and the transition states in diffusion., publishedVersion

While the Mo6S8 chevrel phase is frequently used as cathode material in Mg--ion batteries, theoretical studies on this material are comparatively scarce. The particular structure of the Mo6S8 phase, with rather loosely connected cluster entities, points to the important role of dispersion forces in this material. However, so far this aspect has been completely neglected in the discussion of Mo6S8 as cathode material for mono- and multivalent-ion batteries. In this work we therefore have studied the impact of dispersion forces on stability and kinetics of Mo6S8 intercalation compounds. For this purpose, a series of charge carriers (Li, Na, K, Mg, Ca, Zn, Al) has been investigated. Interestingly, dispersion forces are observed to only slightly affect the lattice spacing of the chevrel phase, nevertheless having a significant impact on insertion voltage and in particular on the charge carrier mobility in the material. Moreover, upon varying the charge carriers in the chevrel phase, their diffusion barriers are observed to scale linearly with the ion size, almost independent of the charge of the considered ions. This indicates a rather unique and geometry dominated diffusion mechanism in the chevrel phase. The consequences of these findings for the ion mobility in the chevrel phase will be carefully discussed.