Quantum study of electrochemical interfaces

Les phénomènes électrochimiques sont de plus en plus présents dans notre vie quotidienne : ils sont au cœur des batteries de nos ordinateurs, de nos téléphones, dans les piles (à combustibles), etc ... L'objectif de cette thèse est de mieux comprendre les interfaces métal-solvant qui interviennent aux niveaux de ces systèmes (pile à combustible, batteries Li-ion …). Cependant si ceux-ci sont fortement étudiés expérimentalement à la fois dans les laboratoires universitaires et industriels, leur compréhension à l'échelle atomique reste encore imparfaite. Le but de cette thèse est donc d'améliorer la compréhension de l'interface entre l'électrolyte et l'électrode ainsi que sa modification avec le potentiel appliqué à l‘aide de calculs ab initio basés sur la théorie de la fonctionnelle de la densité (DFT). Pour cela au moyen de méthodes de simulation électrochimiques utilisant des approches théoriques spécifiques des surfaces et interfaces développées au sein du laboratoire, nous nous sommes d'abord intéressés à une interface Ruthénium-eau. Le ruthénium est un métal qui crée des interactions fortes avec l'eau : l'eau se trouve alors non dissociée ou partiellement dissociée sur cette surface selon les conditions d'étude. Ainsi, en partant des contradictions apparentes entre résultats expérimentaux présents dans la littérature, nous avons étudiés plus d'une dizaine de phase d'eau sur le ruthénium, dont nous avons extrait le diagramme de phase en fonction du potentiel, de la température, et en tenant compte du fort effet isotopique présent dans ces systèmes. La comparaison de nos résultats calculés avec les résultats expérimentaux nous a permis de rationaliser les observations et d'apporter des réponses sur la cause de l'effet isotopique géant associé à la dissociation de l'eau sur le ruthénium.Dans un second temps nous nous sommes intéressés à une interface lithium-solvant (éthylène carbonate) présente dans les batteries Li-métal. Contrairement à l'étude de l'interface métal-eau précédente nous nous sommes intéressés à la modélisation de la surface et de l'électrolyte liquide. La modélisation du solvant liquide étant plus complexe que celle d'une monocouche d'eau solide adsorbée sur la surface, nous avons donc utilisé une méthode de solvatation implicite que nous avons dû adapter à nos calculs électrochimiques. Nous avons aussi modélisé des molécules de solvant de façon explicite pour pallier aux limites du modèle implicite et inclure les effets de première sphère de solvatation. Notre modèle de solvatation mixte implicite/explicite nous a permis d'étudier les processus électrochimique de réorganisation de surface, de réduction de Li+ ainsi que l'hystérésis/nucléation de surface de Li.The electrochemical phenomena are increasingly present in our daily lives: they are at the heart of the batteries of our computers, our phones, in batteries (fuels), etc ... The objective of this thesis is to better understand metal-solvent interfaces involved levels of these systems (fuel cell, Li-ion batteries ...). However if they are heavily studied experimentally both in academic and industrial laboratories, understanding at the atomic scale is still imperfect. The aim of this thesis is to improve understanding of the interface between the electrolyte and the electrode and its modification with the potential applied using ab initio calculations based on the Density Functional Theory (DFT).For this simulation using electrochemical methods using specific theoretical approaches of developed surfaces and interfaces in the laboratory, we primarily interested in a ruthenium-water interface. Ruthenium is a metal that creates strong interactions with water: water is then undissociated or partially dissociated on this surface under the conditions of study. Thus, starting from the apparent contradictions between experimental results present in the literature, we have studied more than a dozen water phase on ruthenium, we extract the phase diagram based on potential, temperature, and taking into account the strong isotope effect present in these systems. Comparison of our calculated results with the experimental results allowed us to rationalize the observations and provide answers on the cause of the giant isotope effect associated with the dissociation of water on ruthenium.In a second step we are interested in a lithium-solvent interface (ethylene carbonate) present in the Li-metal batteries. Unlike the previous study of metal-water interface we are interested in modeling the surface and the liquid electrolyte. The modeling of the liquid solvent is more complex than a single layer of solid water adsorbed on the surface, so we used a method of implicit solvation that we had to adapt our electrochemical calculations. We also modeled explicitly solvent molecules to overcome the limitations of the implicit model and include the effects of the first solvation sphere. Our model of mixed implicit solvation / explicitly allowed us to study the electrochemical surface reorganization process, Li + reduction as well as hysteresis / Li surface nucleation

Using the electrochemical dimension to build water/Ru(0001) phase diagram

International audienceThe water monolayer/Ru(0001) electrochemical phase diagram as a function of surface potential and temperature is built using a DFT approach. The monolayer structure with temperature is extracted following the zero-charge line in good agreement with experiments. Below 140 K, a mix of oppositely charged hydroxyl/water and hydride/water domains is found stable; above 140 K, water molecules desorb from the hydride phase leading to a mixture of oppositely charged surface hydride and hydroxyl/water phases; above 280 K, all the residual adsorbed water desorbs. For undissociated water, a Chain structure is found stable and desorbs above 150 K. The observed nano-sized domains are suggested to be the balance between hydroxyl/hydride repulsion that tends to create two well separated domains and opposite charging that tends to favor a domain mix. An isotopic effect is computed to reduce by a factor of 160 the kinetic rate of D2O dissociation (compared to H2O) and is linked to the reduction of the ZPE in the transition state caused by a proton transport chain. Water monolayer/Ru(0001) has a specific reactivity and its organization is highly sensitive to the surface potential suggesting that under electrochemical conditions, the potential is not only tuning directly the chemical reactivity but also indirectly through the solvent structure

Using Implicit Solvent in Ab Initio Electrochemical Modeling: Investigating Li+/Li Electrochemistry at a Li/Solvent Interface

International audienceThis paper focuses on the use of implicit solvent in electrochemical density functional theory (DFT) calculations. We investigate both the necessity and limits of an implicit solvent polarizable continuum model (PCM). In order to recover the proper electrochemical behavior of the surface and, in particular, a proper potential scale, the solvent model is determined to be mandatory: in the limit of a high dielectric constant, the surface capacitance becomes independent of the interslab space used in the model and, therefore, the electrochemical properties become intrinsic of the interface structure. We show that the computed surface capacitance is not only dependent on the implicit solvent dielectric constant, but also on the solvent cavity parameter that should be precisely tuned. This model is then applied to the Li/electrolyte interface in order to check its ability to compute thermodynamic equilibrium properties. The use of a purely implicit solvent approach allows the recovery of a more reasonable equilibrium potential for the Li+/Li redox pair, compared to vacuum approaches, but a potential that it is still off by 1.5 V. Then, the inclusion of explicit solvent molecules improves the description of the solvent–Li+ chemical bond in the first solvation shell and allows recovery of the experimental value within 100 mV. Finally, we show that the redox active center involves the first solvation shell of Li+, suggesting a particular pathway for the observed solvent dissociation in Li-ion batteries

Holistic Strategy to Study Nanoparticles and Metallic Trace Elements in Surface Waters

Emergent Contaminants (ECs) including nanoparticles (NPs) are preoccupation cause for concern for the scientific community, industrials, territorial communities and the general public. High production volume, potential release from consumer product, environmental exposure, we need to measure the environmental concentrations of NPs and their behavior in natural waters. This is too complicated given the complexity of natural particles and the similarity of Engineering NanoParticles (ENPs) with Natural NanoParticles (NNPs) called here NPs. Thus, we need new methods/protocols dedicated to NPs as ECs which cover the sampling strategy, the analysis and the data valorization using new approaches, theory, paradigms! Our objective is to propose a global approach addressing the above mentioned concerns conveying: sampling, analysis and concept(s) dedicated to Metallic Trace Elements (MTEs) and Nanoparticles (NPs). • Sampling: A crucial point due to the representativeness of the samples regarding a water mass (notion of flux) whose matrix integrates organic matter, chemical elements (from the major to the trace) ligands, biological compounds and contaminants such as MTEs and NPs? • Analysis: Once a representative sample is obtained, how to analyze with minimum artefacts (fractionation, metastable complexes over time, endogenous reaction between the different components, etc.)? What are the basic parameters necessary for understanding the fate of TMEs and NPs in the water column? • Concept(s): it should be holistic which means able to describe the main scenario allowing for consideration of the parameters controlling the fate and behavior of MTE and NPs. Holistic approach consists of sampling devices adapted to the target elements, user-friendly, easy to analyze using a dedicated analytical platform that fits the concept of physical chemical speciation

L'expérience dans l'art

L’œuvre d’art est de plus en plus incertaine, fluctuante, éphémère, immatérielle. Marges revient dans ce numéro sur une thématique qui court tout au long du 20e siècle : la volonté de remettre l’art au sein de la vie quotidienne. La période récente permet de reformuler cette ambition, à un moment où l’expérience esthétique en vient à se fixer sur de nouveaux types d’objets