arXiv subjects

Céline Merlet

Publications and source records attributed to Céline Merlet.

At least 19 recordsLinked to original sources

arXiv2026

Long-time Brownian dynamics of hard and sticky colloids by geometric projection

Particle-resolved colloid simulations are often limited by the fastest microscopic time scale retained in the model, which can be many orders of magnitude shorter than the collective process of interest. This is especially restrictive for aggregating colloids, where explicit near-contact potentials force the time integration to resolve the rapid positional relaxation of bonded pairs within narrow attractive wells. Here we introduce an overdamped constraint-based Brownian dynamics method that replaces the time-resolved integration of stiff near-contact forces by a geometric projection step. After each free Brownian update, hard-sphere exclusion, sticky bonds, and hard-wall interactions are enforced by projecting the particle positions onto the constrained configuration set. The method preserves hard-sphere structure and diffusion, recovers the aggregation kinetics of sticky hard spheres, and extends the physical time reached at fixed computational cost by one to two orders of magnitude relative to the explicit-potential simulations tested here. This acceleration makes Brownian-dominated, low-Péclet electrophoretic deposition accessible to particle-scale simulations, revealing a pathway in which the suspension first densifies and only subsequently develops a bonded network. Implemented as a LAMMPS fix, the framework provides a modular route to long-time Brownian simulations of dense constrained colloidal systems.

cond-mat.soft↗

arXiv2026

The effect of Van der Waals interaction on the microstructure of EPD deposits: a simulation study

Electrophoretic deposition is a method of choice for generating coatings thanks to its ease of implementation and its ability to produce coatings of relatively large thicknesses in a single step process. While this process also benefits from a large number of tunable parameters to adapt the coating to each application (applied electric field, particle concentration, viscosity of the suspension, etc...), such a freedom can lead to the selection of parameters being an overwhelming task. A better fundamental understanding of the microscopic phenomena and mechanisms at play during deposition can provide clues for the more efficient design of optimized coatings. Particle-based models, allowing for the simulation of deposit microstructures for various process parameters, are particularly interesting to get insights in such systems. Nevertheless, such studies are rare and usually do not involve the possibility of self-cohesion between particles, while it seems crucial for the final structure of the deposit. Here, we use particle-based simulations to study the influence of aggregation on the deposit formed for different applied electric fields. We show that the self-cohesion indeed leads to different microstructures, both in the close vicinity of the substrate and in the bulk of the deposit, and relate this to the mechanical signature of the deposits. Our results reveal that at high electric field, the influence of self-cohesion on resulting microstructures essentially vanishes beyond a critical field strength. This marks the transition between a deposition regime affected by aggregation to a regime largely dominated by volume exclusion effects.

cond-mat.mtrl-sci↗

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↗

arXiv2024

Investigating the effect of particle size distribution and complex exchange dynamics on NMR spectra of ions diffusing in disordered porous carbons through a mesoscopic model

Ion adsorption and dynamics in porous carbons is crucial for many technologies such as energy storage and desalination. Nuclear Magnetic Resonance (NMR) spectroscopy is a key method to investigate such systems thanks to the possibility to distinguish adsorbed (in-pore) and bulk (ex-pore) species in the spectra. However, the large variety of magnetic environments experienced by the ions adsorbed in the particles and the existence of dynamic exchange between the inside of the particles and the bulk renders the intepretation of the NMR experiments very complex. In this work, we optimise and apply a mesoscopic model to simulate NMR spectra of ions in systems where carbon particles of different sizes can be considered. We demonstrate that even for monodisperse systems, complex NMR spectra, with broad and narrow peaks, can be observed. We then show that the inclusion of polydispersity is essential to recover some experimentally observed features, such as the co-existence of peaks assigned to in-pore, exchange and bulk. Indeed, the variety of exchange rates between in-pore and ex-pore environments, present in experiments but not taken into account in analytical models, is necessary to reproduce the complexity of experimental NMR spectra.

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

Investigating particle size effects on NMR spectra of ions diffusing in porous carbons through a mesoscopic model

Characterizing ion adsorption and diffusion in porous carbons is essential to understand the performance of such materials in a range of key technologies such as energy storage and capacitive deionisation. Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful technique to get insights in these systems thanks to its ability to distinguish between bulk and adsorbed species and to its sensitivity to dynamic phenomena. Nevertheless, a clear interpretation of the experimental results is sometimes rendered difficult by the various factors affecting NMR spectra. A mesoscopic model to predict NMR spectra of ions diffusing in carbon particles is adapted to include dynamic exchange between the intra-particle space and the bulk electrolyte surrounding the particle. A systematic study of the particle size effect on the NMR spectra for different distributions of magnetic environments in the porous carbons is conducted. The model demonstrates the importance of considering a range of magnetic environments, instead of a single chemical shift value corresponding to adsorbed species, and of including a range of exchange rates (between in and out of the particle), instead of a single timescale, to predict realistic NMR spectra. Depending on the pore size distribution of the carbon particle and the ratio between bulk and adsorbed species, both the NMR linewidth and peak positions can be largely influenced by the particle size.

cond-mat.mtrl-sci↗

arXiv2022

Understanding the chemical shifts of aqueous electrolyte species adsorbed in carbon nanopores

Interfaces between aqueous electrolytes and nanoporous carbons are involved in a number of technological applications such as energy storage and capacitive deionization. The disordered nature of the carbon materials makes it challenging to characterize ion adsorption and relationships between materials properties and performance. Nuclear magnetic spectroscopy can be very helpful in that respect thanks to its nuclei specificity and ability to distinguish between ions in the bulk and in pores. Nevertheless, several factors can affect the measured chemical shifts making it difficult to interpret experimental results. We use complementary methods, namely density functional theory calculations, molecular dynamics simulations and a mesoscopic model, to investigate various factors affecting the chemical shifts of aqueous electrolyte species. We show that the relative importance of these factors depend on the ion nature. In particular, the hydration shell has a much more pronounced effect for large polarizable ions such as Rb$^+$ and Cs$^+$ compared to Li$^+$.

cond-mat.mtrl-sci↗

arXiv2021

Carbon-carbon supercapacitors: Beyond the average pore size or how electrolyte confinement and inaccessible pores affect the capacitance

Carbon-carbon supercapacitors are high power electrochemical energy storage systems which store energy through reversible ion adsorption at the electrode-electrolyte interface. Due to the complex structure of the porous carbons used as electrodes, extracting structure-property relationships in these systems remains a challenge. In this work, we conduct molecular simulations of two model supercapacitors based on nanoporous electrodes with the same average pore size, a property often used when comparing porous materials, but different morphologies. We show that the carbon with the more ordered structure, and a well defined pore size, has a much higher capacitance than the carbon with the more disordered structure, and a broader pore size distribution. We analyze the structure of the confined electrolyte and show that the ions adsorbed in the ordered carbon are present in larger quantities and are also more confined than for the disordered carbon. Both aspects favor a better charge separation and thus a larger capacitance. In addition, the disordered electrodes contain a significant amount of carbon atoms which are never in contact with the electrolyte, carry a close to zero charge and are thus not involved in the charge storage. The total quantities of adsorbed ions and degrees of confinement do not change much with the applied potential and as such, this work opens the door to computationally tractable screening strategies.

cond-mat.mtrl-sci↗

arXiv2021

Mesoscopic simulations of the in situ NMR spectra of porous carbon based supercapacitors: Electronic structure and adsorbent reorganisation effects

In situ NMR spectroscopy is a powerful technique to investigate charge storage mechanisms in carbon-based supercapacitors thanks to its ability to distinguish ionic and molecular species adsorbed in the porous electrodes from those in the bulk electrolyte. The NMR peak corresponding to the adsorbed species shows a clear change of chemical shift as the applied potential difference is varied. This variation in chemical shift is thought to originate from a combination of ion reorganisation in the pores and changes in ring current shifts due to the changes of electronic density in the carbon. While previous Density Functional Theory calculations suggested that the electronic density has a large effect, the relative contributions of these two effects is challenging to untangle. Here, we use mesoscopic simulations to simulate NMR spectra and investigate the relative importance of ion reorganisation and ring currents on the resulting chemical shift. The model is able to predict chemical shifts in good agreement with NMR experiments and indicates that the ring currents are the dominant contribution. A thorough analysis of a specific electrode/electrolyte combination for which detailed NMR experiments have been reported allows us to confirm that local ion reorganisation has a very limited effect but the relative quantities of ions in pores of different sizes, which can change upon charging/discharging, can lead to a significant effect. Our findings suggest that in situ NMR spectra of supercapacitors may provide insights into the electronic structure of carbon materials in the future.

cond-mat.mtrl-sci↗

arXiv2021

On the effect of surface functional groups on the structural and dynamical properties of an ionic liquid confined in zeolite templated carbons studied through molecular simulations

Porous carbons are used in a wide range of applications, including electrochemical double layer capacitors for energy storage, in which electrolyte ion properties under confinement are crucial for the performance of the systems. While many synthesis techniques lead to the presence of surface functional groups, their effect on the adsorption and diffusion of electrolyte ions is still poorly understood. In this study we investigate the effect of surface chemistry on dynamical and structural properties of adsorbed ions through molecular dynamics simulations of a neat ionic liquid in contact with several zeolite templated carbons. The steric and specific influence of functional groups is explored using three structures: a structure without any functional groups; a structure with ester, hydroxyl, anhydride acid and carboxyl functional groups; and a structure where the oxygen and hydrogen atoms are replaced by carbon atoms. We show that the functionalization has a limited impact on the structure of the confined electrolyte but affects the diffusion coefficients, and that the relative importance of the structure and chemical nature of the functional groups is not the same depending on the ion type and property considered.

cond-mat.mtrl-sci↗

arXiv2020

NMR studies of adsorption and diffusion in porous carbonaceous materials

Porous carbonaceous materials have many important industrial applications including energy storage, water purification, and adsorption of volatile organic compounds. Most of their applications rely upon the adsorption of molecules or ions within the interior pore volume of the carbon particles. Understanding the behaviour and properties of adsorbate species on the molecular level is therefore key for optimising porous carbon materials, but this is very challenging owing to the complexity of the disordered carbon structure and the presence of multiple phases in the system. In recent years, NMR spectroscopy has emerged as one of the few experimental techniques that can resolve adsorbed species from those outside the pore network. Adsorbed, or "in-pore" species give rise to resonances that appear at lower chemical shifts compared to their free (or "ex-pore") counterparts. This shielding effect arises primarily due to ring currents in the carbon structure in the presence of a magnetic field, such that the observed chemical shift differences upon adsorption are nucleus-independent to a first approximation. Theoretical modelling has played an important role in rationalising and explaining these experimental observations. Together, experiments and simulations have enabled a large amount of information to be gained on the adsorption and diffusion of adsorbed species, as well as on the structural and magnetic properties of the porous carbon adsorbent. Here, we review the methodological developments and applications of NMR spectroscopy and related modelling in this field, and provide perspectives on possible future applications and research directions.

cond-mat.mtrl-sci↗

arXiv2020

Local distortions and dynamics in hydrated Y-doped BaZrO$_3$

Y-doped BaZrO$_3$ is a promising proton conductor for intermediate temperature solid oxide fuel cells. In this work, a combination of static DFT calculations and DFT based molecular dynamics (DFT-MD) was used to study proton conduction in such a material. Geometry optimisations of 100 structures with a 12.5% dopant concentration allowed us to identify a clear correlation between the bending of the metal-oxygen-metal angle and the energies of the simulated cells. Depending on the type of bending, two configurations, designated as inwards bending and outwards bending, were defined. The results demonstrate that a larger bending decreases the energy and that the lowest energies are observed for structures combining inwards bending with protons being close to the dopant atoms. These lowest energy structures are the ones with the strongest hydrogen bonds. DFT-MD simulations in cells with different yttrium distributions provide complementary microscopic information on proton diffusion as they capture the dynamic distortions of the lattice caused by thermal motion. A careful analysis of the proton jumps between different environments confirmed that the inwards and outwards bending states are relevant for the understanding of proton diffusion. Indeed, intra-octahedral jumps were shown to only occur starting from an outwards configuration while the inwards configuration seems to favor rotations around the oxygen. On average, in the DFT-MD simulations, the hydrogen bond lengths are shorter for the outwards configuration which facilitates the intraoctahedral jumps. Diffusion coefficients and activation energies were also determined and compared to previous theoretical and experimental data showing a good agreement with previous data corresponding to local proton motion.

cond-mat.mtrl-sci↗

arXiv2020

Ionic liquids under confinement: From systematic variations of the ion and pore sizes towards an understanding of structure and dynamics in complex porous carbons

We use molecular simulations of an ionic liquid in contact with a range of nanoporous carbons to investigate correlations between ion size, pore size, pore topology and properties of the adsorbed ions. We show that diffusion coefficients increase with the anion size and, surprisingly, with the quantity of adsorbed ions. Both findings are interpreted in terms of confinement: when the in-pore population increases additional ions are located in less confined sites and diffuse faster. Simulations in which the pores are enlarged while keeping the topology constant comfort these observations. The interpretation of properties across structures is more challenging. An interesting point is that smaller pores do not necessarily lead to larger confinement. In this work, the highest degrees of confinements are observed for intermediate pore sizes. We also show a correlation between the quantity of adsorbed ions and the ratio between the maximum pore diameter and the pore limiting diameter.

cond-mat.mtrl-sci↗

arXiv2020

Efficient prediction of Nucleus Independent Chemical Shifts for polycyclic aromatic hydrocarbons

Nuclear Magnetic Resonance (NMR) is one of the most powerful experimental techniques to characterize the structure of molecules and confined liquids. Nevertheless, the complexity of the systems under investigation usually requires complementary computational studies to interpret the NMR results. In this work we focus on polycyclic aromatic hydrocarbons (PAHs), an important class of organic molecules which have been commonly used as simple analogues for the spectroscopic properties of more complex systems, such as porous disordered carbons. We use Density Functional Theory (DFT) to calculate 13C chemical shifts and Nucleus Independent Chemical Shifts (NICS) for 34 PAHs. The results show a clear molecular size dependence of the two quantities, as well as the convergence of the 13C NMR shifts towards the values observed for graphene. We then present two computationally cheap models for the prediction of NICS in simple PAHs. We show that while a simple dipolar model fails to produce accurate values, a perturbative tight-binding approach can be successfully applied for the prediction of NICS in this series of molecules, including some non-planar ones containing 5- and 7-membered rings. This model, one to two orders of magnitudes faster than DFT calculations, is very promising and can be further refined in order to study more complex systems.

physics.chem-ph↗

arXiv2018

Charge storage in nanoporous carbons: The molecular origin of supercapacitance

Supercapacitors are electric devices able to deliver a large power, enabling their use e.g. for the recovery of breaking energy in cars. This is achieved by using two carbon electrodes and an electrolyte solution or a pure ionic liquid (Room Temperature Ionic Liquid, RTIL). Energy is stored by the adsorption of ions at the surface of the electrodes, but the microscopic mechanism underlying the exceptional performance of Carbide Derived Carbon (CDC) electrodes remained unknown. Using molecular simulation with realistic electrode structures and under constant voltage conditions, we investigate the effect of confinement and solvation on the microscopic charging mechanism. We further analyse the dynamics of the charging process and make the link with equivalent circuit models used by electrochemists.

physics.chem-ph↗

arXiv2014

Lattice simulation method to model diffusion and NMR spectra in porous materials

A coarse-grained simulation method to predict NMR spectra of ions diffusing in porous carbons is proposed. The coarse-grained model uses input from molecular dynamics simulations such as the free-energy profile for ionic adsorption, and density-functional theory calculations are used to predict the NMR chemical shift of the diffusing ions. The approach is used to compute NMR spectra of ions in slit pores with pore widths ranging from 2 to 10 nm. As diffusion inside pores is fast, the NMR spectrum of an ion trapped in a single mesopore will be a sharp peak with a pore size dependent chemical shift. To account for the experimentally observed NMR line shapes, our simulations must model the relatively slow exchange between different pores. We show that the computed NMR line shapes depend on both the pore size distribution and the spatial arrangement of the pores. The technique presented in this work provides a tool to extract information about the spatial distribution of pore sizes from NMR spectra. Such information is diffcult to obtain from other characterisation techniques.

cond-mat.mtrl-sci↗

arXiv2014

The electric double layer has a life of its own

Using molecular dynamics simulations with recently developed importance sampling methods, we show that the differential capacitance of a model ionic liquid based double-layer capacitor exhibits an anomalous dependence on the applied electrical potential. Such behavior is qualitatively incompatible with standard mean-field theories of the electrical double layer, but is consistent with observations made in experiment. The anomalous response results from structural changes induced in the interfacial region of the ionic liquid as it develops a charge density to screen the charge induced on the electrode surface. These structural changes are strongly influenced by the out-of-plane layering of the electrolyte and are multifaceted, including an abrupt local ordering of the ions adsorbed in the plane of the electrode surface, reorientation of molecular ions, and the spontaneous exchange of ions between different layers of the electrolyte close to the electrode surface. The local ordering exhibits signatures of a first-order phase transition, which would indicate a singular charge-density transition in a macroscopic limit.

physics.chem-ph↗

arXiv2013

Computer simulations of ionic liquids at electrochemical interfaces

Ionic liquids are widely used as electrolytes in electrochemical devices. In this context, many experimental and theoretical approaches have been recently developed for characterizing their interface with electrodes. In this perspective article, we review the most recent advances in the field of computer simulations (mainly molecular dynamics). A methodology for simulating electrodes at constant electrical potential is presented. Several types of electrode geometries have been investigated by many groups in order to model planar, corrugated and porous materials and we summarize the results obtained in terms of the structure of the liquids. This structure governs the quantity of charge which can be stored at the surface of the electrode for a given applied potential, which is the relevant quantity for the highly topical use of ionic liquids in supercapacitors (also known as electrochemical double-layer capacitors). A key feature, which was also shown by atomic force microscopy and surface force apparatus experiments, is the formation of a layered structure for all ionic liquids at the surface of planar electrodes. This organization cannot take place inside nanoporous electrodes, which results in a much better performance for the latter in supercapacitors. The agreement between simulations and electrochemical experiments remains qualitative only though, and we outline future directions which should enhance the predictive power of computer simulations. In the longer term, atomistic simulations will also be applied to the case of electron transfer reactions at the interface, enabling the application to a broader area of problems in electrochemistry, and the few recent works in this field are also commented upon.

cond-mat.mtrl-sci↗