arXiv subjects

Mathieu Salanne

Publications and source records attributed to Mathieu Salanne.

At least 19 recordsLinked to original sources

arXiv2026

Competing Ring-Opening and Hofmann Elimination Pathways in Aqueous TEMPO Catholytes: A First-Principles Study

Aqueous redox-flow batteries based on TEMPO derivatives are promising for large-scale energy storage, but their practical use is limited by the chemical instability of the oxidized N -oxoammonium state. In this work, we investigate the degradation of five TEMPO derivatives using ab initio molecular dynamics combined with enhanced sampling. Two proposed degradation mechanisms, ring opening and Hofmann elimination, are examined and their corresponding activation free energies are compared. For all derivatives considered, ring opening exhibits a lower activation free energy than Hofmann elimination, identifying it as the kinetically preferred degradation pathway. The magnitude of the ring-opening barrier, however, varies significantly between molecules, showing that different functionalizations strongly influence its stability toward degradation. The predicted preference for ring opening is consistent with available experimental studies, which have identified or inferred ring-opening degradation for several TEMPO-based catholytes. These results provide an atomistic picture of degradation pathways that are difficult to resolve experimentally and highlight the importance of molecular structure in controlling the kinetic stability of TEMPO derivatives in aqueous electrolytes.

physics.chem-ph↗

arXiv2026

Overcoming sampling limitations using machine-learned interatomic potentials: the case of water-in-salt electrolytes

Machine-learned interatomic potentials hold the promise to enable the modeling of highly concentrated liquids over meaningful timescales, far from reach for current ab initio electronic structure methods. Here we evaluate the performances of various MACE potentials in modeling a $21 m$ water-in-salt electrolyte based on lithium bis(trifluoromethanesulfonyl)imide. We test out-of-the-box foundation models, as well as both fine tuning and from scratch training strategies. Our simulations demonstrate that surrogate models allow to overcome sampling limitations of ab initio molecular dynamics, reaching an excellent agreement with experimental observables such as the structure factor. We also demonstrate the benefit of fine tuning a foundation model over training from scratch: in terms of data efficiency, but most importantly as a means to provide information regarding configurations hard to sample, such as short Li$^+$--Li$^+$ distances. Finally, we show that depending on the reference exchange-correlation functional, empirical dispersion correction schemes can be detrimental. All in all, our work shows that machine-learned interatomic potentials are a good fit for the modeling of highly concentrated electrolytes over long timescales.

physics.chem-ph↗

arXiv2026

LPC3D: An Enhanced Parallel Software for Large-Scale Simulation of Adsorption in Porous Carbons and Supercapacitors

Simulations of electrochemical double layer capacitors based on porous carbon electrodes, energy storage systems which accumulate and release energy through reversible ion adsorption at electrode/electrolyte interfaces, are often performed at the microscopic scale, using molecular dynamics. Such simulations provide crucial information to understand the adsorption of ions and the effect of confinement on some electrochemical properties. However, their computational cost limits the size of the systems studied to a few nanometers and a few pores while experimental materials are highly heterogeneous with a distribution of particle and pore sizes. LPC3D is a software designed for mesoscopic simulations of porous carbon particles and carbon-based supercapacitors which allow for the inclusion of such heterogeneity. The code calculates quantities of adsorbed ions, diffusion coefficients and NMR spectra of ions / molecules adsorbed in porous carbon matrices. In this work, we report on a new implementation of LPC3D, written in Python using the PyStencils module which can generate optimized C++ and CUDA code. This implementation is parallel, can be run on CPU and GPU, and allows one to simulate systems going from a single carbon particle to a supercapacitor with hundreds of micrometers in length. Here, we apply the new implementation of LPC3D to the simulation of supercapacitors with porous carbon electrodes represented as monoliths or carbon films to investigate the influence of the microstructure on the resulting adsorption and spectroscopic properties.

cond-mat.mtrl-sci↗

arXiv2025

Electrically driven first-order phase transition of a 2D ionic crystal at the electrode/electrolyte interface

Liquid electrolytes adsorbed at the surface of metallic electrodes display a multitude of structures that can largely differ from the parent bulk system, both in terms of composition and local organization. In particular, the existence of disorder-order or order-order transitions has been increasingly reported in experimental and simulation studies, and the electrode potential identified as the corresponding driving force. The microscopic mechanisms and the stages of the process are, however, poorly understood, and the free energy variation during the transition remain insufficiently characterized. To fill this gap, we investigate the crystallization of the adsorbed layer in a prototypical molten salt-metal interface. We demonstrate that the transition from the disordered to ordered structures proceeds in two stages. Pre-ordering effects are observed across a wide range of potentials, resulting in the formation of a poly-crystalline structure on the electrode surface, before an abrupt ordering transition finally occurs. The pre-ordering displays signature of a continuous transition. On the other hand, finite-size effects analysis proves the first-order character of the transition towards the mono-crystalline state. Upon increasing the system size, a shift in the onset applied voltage is observed, accompanied by a dramatic increase in the free energy barrier. The latter reflects in the interfacial capacitance, which displays a peak that sharpens with increasing system size.

physics.chem-ph↗

arXiv2025

Learning the Electrostatic Response of the Electron Density through a Symmetry-Adapted Vector Field Model

A current challenge in atomistic machine learning is that of efficiently predicting the response of the electron density under electric fields. We address this challenge with symmetry-adapted kernel functions that are specifically derived to account for the rotational symmetry of a three-dimensional vector field. We demonstrate the equivariance of the method on a set of rotated water molecules and show its high efficiency with respect to number of training configurations and features for liquid water and naphthalene crystals. We conclude showcasing applications for relaxed configurations of gold nanoparticles, reproducing the scaling law of the electronic polarizability with size, up to systems with more than 2000 atoms. By deriving a natural extension to equivariant learning models of the electron density, our method provides an accurate and inexpensive strategy to predict the electrostatic response of molecules and materials.

cond-mat.mtrl-sci↗

arXiv2024

Accelerating QM/MM simulations of electrochemical interfaces through machine learning of electronic charge densities

A crucial aspect in the simulation of electrochemical interfaces consists in treating the distribution of electronic charge of electrode materials that are put in contact with an electrolyte solution. Recently, it has been shown how a machine-learning method that specifically targets the electronic charge density, also known as SALTED, can be used to predict the long-range response of metal electrodes in model electrochemical cells. In this work, we provide a full integration of SALTED with MetalWalls, a program for performing classical simulations of electrochemical systems. We do so by deriving a spherical harmonics extension of the Ewald summation method, which allows us to efficiently compute the electric field originated by the predicted electrode charge distribution. We show how to use this method to drive the molecular dynamics of an aqueous electrolyte solution under the quantum electric field of a gold electrode, which is matched to the accuracy of density-functional theory. Notably, we find that the resulting atomic forces present a small error of the order of 1 meV/{\AA}, demonstrating the great effectiveness of adopting an electron-density path in predicting the electrostatics of the system. Upon running the data-driven dynamics over about 3 ns, we observe qualitative differences in the interfacial distribution of the electrolyte with respect to the results of a classical simulation. By greatly accelerating quantum-mechanics/molecular-mechanics approaches applied to electrochemical systems, our method opens the door to nanoseconds timescales in the accurate atomistic description of the electrical double layer.

cond-mat.mtrl-sci↗

arXiv2024

Accounting for the Quantum Capacitance of Graphite in Constant Potential Molecular Dynamics Simulations

Molecular dynamics simulations at a constant electric potential are an essential tool to study electrochemical processes, providing microscopic information on the structural, thermodynamic, and dynamical properties. Despite the numerous advances in the simulation of electrodes, they fail to accurately represent the electronic structure of materials such as graphite. In this work, we introduce a simple parameterization method that allows to tune the metallicity of the electrode based on a quantum chemistry calculation of the density of states. As a first illustration, we study the interface between graphite electrodes and two different liquid electrolytes, an aqueous solution of NaCl and a pure ionic liquid, at different applied potentials. We show that the simulations reproduce qualitatively the experimentally-measured capacitance; in particular, they yield a minimum of capacitance at the point of zero charge, which is due to the quantum capacitance contribution. An analysis of the structure of the adsorbed liquids allows to understand why the ionic liquid displays a lower capacitance despite its large ionic concentration. In addition to its relevance for the important class of carbonaceous electrodes, this method can be applied to any electrode materials (e.g. 2D materials, conducting polymers, etc), thus enabling molecular simulation studies of complex electrochemical devices in the future.

cond-mat.mtrl-sci↗

arXiv2024

Dynamics and energetics of ion adsorption at the interface between a pure ionic liquid and carbon electrodes

Molecular dynamics simulations have been used extensively to determine equilibrium properties of the electrode-electrolyte interface in supercapacitors held at various potentials. While such studies are essential to understand and optimize the performance of such energy storage systems, investigations of the dynamics of adsorption during the charge of the supercapacitors is also necessary. Dynamical properties are especially important to get an insight into the power density of supercapacitors, one of their main assets. In this work, we propose a new method to analyze the trajectories of adsorbing ions. We focus on pure 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide in contact with planar carbon electrodes. We characterize the evolution of the ion orientation and ion-electrode distance during adsorption and show that ions reorientate as they adsorb. We then determine the forces experienced by the adsorbing ions and demonstrate that Coulomb forces dominate at long range while van der Waals forces dominate at short range. We also show that there is an almost equal contribution from the two forces at an intermediate distance, explaining the peak of ion density close to the electrode surface.

cond-mat.soft↗

arXiv2023

Data-driven path collective variables

Identifying optimal collective variables to model transformations, using atomic-scale simulations, is a long-standing challenge. We propose a new method for the generation, optimization, and comparison of collective variables, which can be thought of as a data-driven generalization of the path collective variable concept. It consists in a kernel ridge regression of the committor probability, which encodes a transformation's progress. The resulting collective variable is one-dimensional, interpretable, and differentiable, making it appropriate for enhanced sampling simulations requiring biasing. We demonstrate the validity of the method on two different applications: a precipitation model, and the association of Li$^+$ and F$^-$ in water. For the former, we show that global descriptors such as the permutation invariant vector allow to reach an accuracy far from the one achieved \textit{via} simpler, more intuitive variables. For the latter, we show that information correlated with the transformation mechanism is contained in the first solvation shell only, and that inertial effects prevent the derivation of optimal collective variables from the atomic positions only.

physics.chem-ph↗

arXiv2023

Predicting the Charge Density Response in Metal Electrodes

The computational study of energy storage and conversion processes calls for simulation techniques that can reproduce the electronic response of metal electrodes under electric fields. Despite recent advancements in machine-learning methods applied to electronic-structure properties, predicting the non-local behavior of the charge density in electronic conductors remains a major open challenge. We combine long-range and equivariant kernel methods to predict the Kohn-Sham electron density of metal electrodes in response to various kinds of electric field perturbations. By taking slabs of gold as an example, we first show how the non-local electronic polarization generated by the interaction with an ionic species can be accurately reproduced in electrodes of arbitrary thickness. A finite-field extension of the method is then introduced, which allows us to predict the charge transfer and the electrostatic potential drop induced by the application of a homogeneous and constant electric field. Finally, we demonstrate the capability of the method to reproduce the charge-density response in a gold/electrolyte capacitor under an applied voltage, predicting the system polarization with a greater accuracy than state-of-the-art classical atomic-charge models.

cond-mat.mtrl-sci↗

arXiv2021

Multi-scale simulation of the adsorption of lithium ion on graphite surface: from Quantum Monte Carlo to Molecular Density Functional Theory

The structure of the double-layer formed at the surface of carbon electrodes is governed by the interactions between the electrode and the electrolyte species. However, carbon is notoriously difficult to simulate accurately, even with well-established methods such as electronic Density Functional Theory and Molecular Dynamics. Here we focus on the important case of a lithium ion in contact with the surface of graphite, and we perform a series of reference Quantum Monte Carlo calculations that allow us to benchmark various electronic Density Functional Theory functionals. We then fit an accurate carbon--lithium pair potential, which is used in molecular Density Functional Theory calculations to determine the free energy of the adsorption of the ion on the surface in the presence of water. The adsorption profile in solution differs markedly from the gas phase results, which emphasize the role of the solvent on the properties of the double-layer.

cond-mat.mtrl-sci↗

arXiv2021

Effect of the metallicity on the capacitance of gold - aqueous sodium chloride interfaces

Electrochemistry experiments have established that the capacitance of electrode-electrolyte interfaces is much larger for good metals such as gold and platinum than for carbon-based materials. Despite the development of elaborate electrode interaction potentials, to date molecular dynamics simulations were not able to capture this effect. Here we show that changing the width of the Gaussian charge distribution used to represent the atomic charges in gold is an effective way to tune its metallicity. Larger Gaussian widths lead to a capacitance of aqueous solutions (pure water and 1 molar NaCl) in good agreement with recent ab initio molecular dynamics results. For pure water, the increase in the capacitance is not accompanied with structural changes, while in the presence of salt the Na$^+$ cations tend to adsorb significantly on the surface. For a strongly metallic gold electrode, these ions can even form inner sphere complexes on hollow sites of the surface.

cond-mat.mtrl-sci↗

arXiv2021

Computational Amperometry of Nanoscale Capacitors in Molecular Simulations

In recent years, constant applied potential molecular dynamics has allowed to study the structure and dynamics of the electrochemical double-layer of a large variety of nanoscale capacitors. Nevertheless it remained impossible to simulate polarized electrodes at fixed total charge. Here we show that combining a constant potential electrode with a finite electric displacement fills this gap by allowing to simulate open circuit conditions. The method can be extended by applying an electric displacement ramp to perform computational amperometry experiments at different current intensities. As in experiments, the full capacitance of the system is obtained at low intensity, but this quantity decreases when the applied ramp becomes too fast with respect to the microscopic dynamics of the liquid.

cond-mat.mtrl-sci↗

arXiv2020

Size-dependence of hydrophobic hydration at electrified gold/water interfaces

Hydrophobic hydration at metal/water interfaces actively contributes to the energetics of electrochemical reactions, e.g. CO$_2$ and N$_2$ reduction, where small hydrophobic molecules are involved. In this work, constant applied potential molecular dynamics is employed to study hydrophobic hydration at a gold/water interface. We propose an extension of the Lum-Chandler-Weeks (LCW) theory to describe the free energy of hydrophobic hydration at the interface as a function of solute size and applied voltage. Based on this model we are able to predict the free energy cost of cavity formation at the interface directly from the free energy cost in the bulk plus an interface-dependent correction term. The interfacial water network contributes significantly to the free energy yielding a preference for outer-sphere adsorption at the gold surface for ideal hydrophobes. We predict an accumulation of small hydrophobic solutes of sizes comparable to CO or N$_2$, while the free energy cost to hydrate larger hydrophobes, above 2.5 Angstrom radius, is shown to be greater at the interface than in the bulk. Interestingly, the transition from the volume dominated to the surface dominated regimes predicted by the LCW theory in the bulk is also found to take place for hydrophobes at the Au/water interface, but occurs at smaller cavity radii. By applying the extended LCW theory to a simple model addition reaction, we illustrate some implications of our findings for electrochemical reactions.

physics.chem-ph↗

arXiv2020

Structural and dynamic properties of soda-lime-silica in the liquid phase

Soda-lime-silica is a glassy system of strong industrial interest. In order to characterize its liquid state properties, we performed molecular dynamics simulations employing an aspherical ion model that includes atomic polarization and deformation effects. They allowed to study the structure and diffusion properties of the system at temperatures ranging from 1400 to 3000 K. We show that Na$^+$ and Ca$^{2+}$ ions adopt a different structural organization within the silica network, with Ca$^{2+}$ ions having a greater affinity for non-bridging oxygens than Na$^+$. We further link this structural behavior to their different diffusivities, suggesting that escaping from the first oxygen coordination shell is the limiting step for the diffusion. Na$^+$ diffuses faster than Ca$^{2+}$ because it is bonded to a smaller number of non-bridging oxygens. The formed ionic bonds are also less strong in the case of Na$^+$.

cond-mat.mtrl-sci↗

arXiv2020

Molecular Simulation of Electrode-Solution Interfaces

Many key industrial processes, from electricity production, conversion and storage to electrocatalysis or electrochemistry in general, rely on physical mechanisms occurring at the interface between a metallic electrode and an electrolyte solution, summarized by the concept of electric double layer, with the accumulation/depletion of electrons on the metal side and of ions on the liquid side. While electrostatic interactions play an essential role on the structure, thermodynamics, dynamics and reactivity of electrode-electrolyte interfaces, these properties also crucially depend on the nature of the ions and solvent, as well as that of the metal itself. Such interfaces pose many challenges for modeling, because they are a place where Quantum Chemistry meets Statistical Physics. In the present review, we explore the recent advances on the description and understanding of electrode-electrolyte interfaces with classical molecular simulations, with a focus on planar interfaces and solvent-based liquids, from pure solvent to water-in-salt-electrolytes.

physics.comp-ph↗

arXiv2020

Mass-zero constrained molecular dynamics for electrode charges in simulations of electrochemical systems

Classical molecular dynamics simulations have recently become a standard tool for the study of electrochemical systems. State-of-the-art approaches represent the electrodes as perfect conductors, modelling their responses to the charge distribution of electrolytes via the so-called fluctuating charge model. These fluctuating charges are additional degrees of freedom that, in a Born-Oppenheimer spirit, adapt instantaneously to changes in the environment to keep each electrode at a constant potential. Here we show that this model can be treated in the framework of constrained molecular dynamics, leading to a symplectic and time-reversible algorithm for the evolution of all the degrees of freedom of the system. The computational cost and the accuracy of the new method are similar to current alternative implementations of the model. The advantage lies in the accuracy and long term stability guaranteed by the formal properties of the algorithm and in the possibility to systematically introduce additional kinematic conditions of arbitrary number and form. We illustrate the performance of the constrained dynamics approach by enforcing the electroneutrality of the electrodes in a simple capacitor consisting of two graphite electrodes separated by a slab of liquid water.

cond-mat.stat-mech↗

arXiv2019

A first-principles investigation of the structural and electrochemical properties of biredox ionic species in acetonitrile

Biredox ionic liquids are a new class of functionalized electrolytes that may play an important role in future capacitive energy storage devices. By allowing additional storage of electrons inside the liquids, they can improve device performance significantly. However current devices employ nanoporous carbons in which the diffusion of the liquid and the adsorption of the ions could be affected by the occurrence of electron-transfer reactions. It is therefore necessary to understand better the thermodynamics and the kinetics of such reactions in biredox ionic liquids. Here we perform ab initio molecular dynamics simulations of both the oxidized and reduced species of several redox-active ionic molecules (used in biredox ionic liquids) dissolved in acetonitrile solvent and compare them with the bare redox molecules. We show that in all the cases, it is necessary to introduce a two Gaussian state model to calculate the reaction free energies accurately. These reaction free energies are only slightly affected by the presence of the IL group on the molecule. We characterize the structure of the solvation shell around the redox active part of the molecules and show that in the case of TEMPO-based molecules strong reorientation effects occur during the oxidation reaction.

cond-mat.mtrl-sci↗