Modeling interfacial electrochemistry: concepts and tools.

This paper presents a grand canonical formalism to treat electrochemical effects at interfaces. This general formalism is linked with the classical chemical hydrogen electrode (CHE) approximation and an improved approximation is proposed. This new approximation including a higher order correction that (i) keeps the low computational cost of classical CHE approach, (ii) does not require to know the type of reaction (electrochemical/not electrochemical) and (iii) should give better estimates in many problematic cases. Beyond the applicability domain of this new approximation, the homogeneous background method (HBM) which is a potential dependent density functional theory method is then presented. HBM allows computing and extracting, at ab initio level, electrochemical properties of molecules either adsorbed or in the double layer. In particular, quantitative redox potential and number of exchanged electrons can be computed giving access to non-integer electron exchange or decoupled electron/proton transfer reactions. Tools for rationalizing electrochemical reactivity consisting of potential dependant projected density of states, Fukui function and metallicity index are defined. The methodology and tools are applied to examples relevant to the energy domain in order the compare reactivity in the outer Helmholtz plane and at the surface. Then, the combination of HBM and reactivity create a toolbox usable to predict and investigate the different redox, degradation and ageing processes occurring at an electrochemical interface such as the one found in energy materials but also in all electrochemical applications.

Unraveling the Origin of the Repulsive Interaction between Hydrogen Adsorbates on Platinum Single-Crystal Electrodes.

Hydrogen adsorption on platinum (Pt) single-crystal electrodes has been studied intensively in both experiments and computations. Yet, the precise origin and nature of the repulsive interactions observed between hydrogen adsorbates (Hads) have remained elusive. Here, we use first-principles density functional theory calculations to investigate in detail the interactions between Hads on Pt(111), Pt(100), and Pt(110) surfaces. The repulsive interaction between Hads on Pt(111) is deconvoluted into three different physical contributions, namely, (i) electrostatic interactions, (ii) surface distortion effect, and (iii) surface coordination effect. The long-range electrostatic interaction, which is generally considered the most important source of repulsive interactions in surface adsorption, was found to contribute less than 30% of the overall repulsive interaction. The remaining >70% arises from the other two contributions, underscoring the critical influence of surface-mediated interactions on the adsorption process. Surface distortion and coordination effects are found to strongly depend on the coverage and adsorption geometry: the effect of surface distortion dominates when adsorbates reside two or more Pt atoms apart; the effect of surface coordination dominates if hydrogen is adsorbed on neighboring adsorption sites. The above effects are considerably less pronounced on Pt(100) and Pt(110), therefore resulting in weaker interactions between Hads on these two surfaces. Overall, the study highlights the relevance of surface-mediated effects on adsorbate-adsorbate interactions, such as the often-overlooked surface distortion. The effect of these interactions on the hotly debated adsorption site for the adsorbed hydrogen intermediate in the hydrogen evolution reaction is also discussed.

Advancement of the Homogeneous Background Method for the Computational Simulation of Electrochemical Interfaces.

Computational studies of electrochemical interfaces based on density-functional theory (DFT) play an increasingly important role in the present research on electrochemical processes for energy conversion and storage. The homogeneous background method (HBM) offers a straightforward approach to charge the electrochemical system within DFT simulations, but it typically requires the specification of the active fraction of excess electrons based on a certain choice of the electrode-electrolyte boundary location, which can be difficult in the presence of electrode-surface adsorbates or explicit solvent molecules. In this work, we present a methodological advancement of the HBM, both facilitating and extending its applicability. The advanced version requires neither energy corrections nor the specification of the active fraction of excess electrons, providing a versatile and readily available method for the simulation of charged interfaces when adsorbates or explicit solvent molecules are present. Our computational DFT results for Pt(111), Au(111), and Li(100) metal electrodes in high-dielectric-constant solvents demonstrate an excellent agreement in the interfacial charging characteristics obtained from simulations with the advanced HBM in comparison with the (linearized) Poisson-Boltzmann model (PBM).

Importance of Halide Ions in the Stabilization of Hybrid Sn-Based Coatings for Lithium Electrodes.

The properties of hybrid Sn-based artificial solid electrolyte interphase (SEI) layers in protecting Li-metal electrodes toward surface instabilities were investigated via a combined experimental and theoretical approach. The performance of coating layers can be coherently explained based on the nature of the coating species. Notably, when starting from a chloride precursor, the hybrid coating layer is formed by an intimate mixture of Li7Sn2 and LiCl: the first ensures a high bulk ionic conductivity, while the second forms an external layer allowing a fast surface diffusion of Li+ to avoid dendrite growth, a low surface tension to guarantee the thermodynamic stability of the protective layer, and a negative underneath plating energy (UPE) to promote lithium plating at the interface between the Li metal and the coating layer. The synergy between the two components and, in particular, the crucial role of LiCl in the promotion of such an underneath plating mechanism are shown to be the key properties to improve the performance of artificial SEI layers.

Ab initiomodelling of interfacial electrochemical properties: beyond implicit solvation limitations.

First-principles calculations are an important tool to investigate the complex processes occurring at solid/liquid interfaces which are at the heart of modern technologies. Currently, capturing the whole electrochemical environment at an interface, including the applied potential and solvation, still remains challenging as it necessitates to couple different approaches whose interactions are not fully understood. In this work, a grand canonical density functional theory approach is coupled with solvation models to investigate the electrochemical interfaces under applied potential. We show that a parametrized polarizable continuum model (PCM) which represent solvation in a mean field approach by a continuous polarizable media, possesses catastrophic limitations for the modelling of ionic and charged interfaces. We reveal the origin of PCM instabilities under chemical or electrochemical strong oxidation to be the consequence of a phase transition in the surface Li electronic structure. Thus, PCM undergoes an unphysical response to this phase transition by penetrating within the atomic radius of surface Li atoms. To recover a physical response, an explicit first solvation shell has to be included in addition to the PCM in order to properly describe the electrochemistry of the interface. The Fukui functions show that the first solvation shell becomes involved in the redox process as solvent electron doublet is transferred to the acidic Li+. If another explicit solvent layer is added, the interface electrochemical properties become independent of the PCM parameters: in particular capacitance can then be computed from a parameter-free electrochemical approach. This is an important conclusion as the experimental electrochemical capacitance are not easily found and thus the parametrization of the PCM for electrochemical interface can be difficult. This approach can easily be applied to investigate electrochemical properties at the atomic scale and generalized to any electrochemical device for which interfaces play a crucial role.