A better understanding of interfacial mechanisms is needed to improve the performances of electrochemical devices. Yet, simulating an electrode surface at fixed electrolyte composition remains a challenge. Here we apply a finite electric field to a single electrode held at constant potential and in contact with an aqueous ionic solution, using classical molecular dynamics. The polarization yields two electrochemical interfaces on opposite sides of the same metal slab. While the net charge on one electrode surface is the opposite of the net charge on the other, maintaining overall charge neutrality of the metal. The electrode surface charges fluctuations are compensated by the adsorption of ions from the electrolyte, forming a pair of electric double layers with aligned dipoles. This opens the way towards the efficient simulation of electrochemical interfaces using any flavor of molecular dynamics, from classical to first principles-based methods. PACS numbers:Despite many advances over the past decades [1,2], the efficient simulation of full electrochemical cells at the molecular scale, using electronic structure based calculations, remains a daunting task. This is due to their slab structure, since the minimal experimental setup consists of an electrolyte between two electrodes. The system is generally simplified by simulating one interface only, but the main conceptual difficulty is to find a way to charge the electrode surface at fixed composition of the electrolyte. Several methods have recently emerged [3-5], where the system is allowed to exchange electrons with a reservoir at fixed voltage (grand-canonical approach), but they all rely on the use of continuum descriptions for the electrolyte. These models, which are generally based on a Poisson-Boltzmann theory [6], remain mostly qualitative and an atomistic description would be preferable (this is also true because the solvent may actively participate to electrochemical reactions [7,8]). This is almost impossible to do since it would be necessary to remove/insert ions to counterbalance the electrode charge fluctuations.Here we propose an alternative route to simulate electrochemical cells. Our approach is based on the coupling of a finite field with a system consisting in an electrolyte and a single electrode. Finite fields methods, developed in the framework of the modern theory of polarization, consist in imposing a macroscopic field (electric field [9], polarization [10] or electric displacement [11]) via an extended Hamiltonian accounting for the interaction between the system and the fixed field. They have recently been adapted and applied to the study of electrical double layers at solid/liquid interfaces, and more precisely charged [12] or polar [13] insulators/electrolyte interfaces. Electrochemical systems are by nature more complex since they involve metallic electrodes whose charge distribution is not fixed but depends on the surrounding medium and the applied potential. Due to the long sim-ulation times related to the relaxation of the electric...

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Applied electrochemistry plays a key role in many technologies, such as Li-ion batteries, fuel cells, supercapacitors, solar cells, etc. It is therefore at the core of many research programs all over the world. However, fundamental electrochemical investigations remain scarce. In particular, electrochemistry is among the fields for which the gap between theory and experiment is the largest. From the computational point of view, there is no classical molecular dynamics (MD) software devoted to the simulation of electrochemical systems while other fields such as biochemistry or material science have dedicated tools. "MetalWalls" (MW), a MD code dedicated to electrochemistry, fills this gap. Its main originality is the inclusion of a series of methods which allow a constant electrical potential to be applied to the electrode materials. It also allows the simulation of bulk liquids or solids using the polarizable ion model and the aspherical ion model. MW is designed to be used on high-performance computers and it has already been employed in a number of scientific publications. It was for example used to study the charging mechanism of supercapacitors, nanoelectrowetting and water desalination devices.

In recent years, constant applied potential molecular dynamics has allowed researchers to study the structure and dynamics of the electrochemical doublelayer of a large variety of nanoscale capacitors. Nevertheless, it has 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 us 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.

Despite its low abundance, water has a great influence on the geodynamics of the Earth's upper mantle. Indeed, water has the ability to modify the phase relations and to affect in a significant way the rheological properties of minerals and melts. However the mechanisms of water incorporation in silicate melts and the impact on the melt properties is still not fully understood. To improve our understanding of hydrous silicate melts, we have performed a series of molecular dynamics simulations to evaluate the H 2 O solubility, the liquid-vapour coexistence, the surface tension, the water speciation, the equation of state, the viscosity, the electrical conductivity, the diffusion of silicate elements and protonated species, as well as the melt structure of various magmatic liquids representative of the Earth's upper mantle (rhyolite, andesite, MORB, peridotite, and kimberlite). For that, we introduce a new force field for water, which is compatible with an accurate force field for silicates recently developed (Dufils et al., 2018). A comparison between MD calculations and experimental data (when they exist) shows that the MD simulations are reliable. Among all the results obtained in this study, the following points may be emphasized. (1) The solubility of water changes very little when the melt composition evolves from rhyolitic to andesitic and basaltic, but it is strongly enhanced in ultramafic melts. (2) When hydrous melt and aqueous fluid are coexisting with each other, the oxide content of the aqueous fluid increases rapidly with the pressure. (3) A consequence of point ( 2) is that water has a large influence on the surface tension, as the latter one drops by a factor of 2 4 when the water pressure increases from 1 bar to a few kbar. (4) Concerning the water speciation, an important point is that the MD simulation probes the liquid phase, when most of the experimental studies are dealing with glasses. Thus at magmatic temperatures the concentration in hydroxyl groups and the one in molecular water are crossing for a water content of about 15 wt%, a value much higher than 2 the one observed in glasses ( -4 wt%). ( 5) MD calculations show that the molar volume of the melt is a linear function of the water content, and so for all the chemical compositions investigated. Therefore the water partial molar volume () is virtually independent of total water content and of water speciation. A by-product of this result is that an ideal mixing rule between water and the silicate component leads to an accurate estimate of the melt molar volume. (6) At fixed T and P, the melt viscosity decreases with water content, more depolymerized the melt the smaller the influence of water on the viscosity. However, at the high temperatures investigated in this study (T 1673 K), the decrease in viscosity induced by water does not exceed one or two orders of magnitude, as compared with many orders of magnitude near the glass transition temperature. ( 7) The diffusivity of ions increases exponentially with water content. As for the prot...

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International audienceA new atom-atom interaction potential is introduced for describing by classical molecular dynamics (MD) simulation the physical properties of natural silicate melts. The equation of state, the microscopic structure, the viscosity, the electrical conductivity, and the self-diffusion coefficients of ions in a mid-oceanic ridge basalt (MORB) melt are evaluated by MD over a large range of temperature and pressure (1673-3273 K and 0-60 GPa). A detailed comparison with experimental data shows that the model reproduces the thermodynamic, structural and transport properties of a MORB with an unprecedented accuracy. In particular, it is shown that the MORB melt crystallizes at lower mantle conditions into a perovskite phase whose the equation of state (EOS) is compatible with those proposed in the experimental literature. Moreover, in accordance with experimental findings, the simulation predicts not only that the MORB viscosity exhibits a (slight) minimum with the pressure, but also that the viscosity at high temperature remains very low (<100 mPa.s for T > 2273 K) even at high pressure (up to 40 GPa). However the evolution of the electrical conductivity with temperature and pressure is not always the symmetrical of that of the viscosity. In fact, the relationship between viscosity and electrical conductivity shows a crossover at around 2073 K