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1.
J Chem Phys ; 157(18): 184801, 2022 Nov 14.
Artigo em Inglês | MEDLINE | ID: mdl-36379806

RESUMO

Electrochemistry is central to many applications, ranging from biology to energy science. Studies now involve a wide range of techniques, both experimental and theoretical. Modeling and simulations methods, such as density functional theory or molecular dynamics, provide key information on the structural and dynamic properties of the systems. Of particular importance are polarization effects of the electrode/electrolyte interface, which are difficult to simulate accurately. Here, we show how these electrostatic interactions are taken into account in the framework of the Ewald summation method. We discuss, in particular, the formal setup for calculations that enforce periodic boundary conditions in two directions, a geometry that more closely reflects the characteristics of typical electrolyte/electrode systems and presents some differences with respect to the more common case of periodic boundary conditions in three dimensions. These formal developments are implemented and tested in MetalWalls, a molecular dynamics software that captures the polarization of the electrolyte and allows the simulation of electrodes maintained at a constant potential. We also discuss the technical aspects involved in the calculation of two sets of coupled degrees of freedom, namely the induced dipoles and the electrode charges. We validate the implementation, first on simple systems, then on the well-known interface between graphite electrodes and a room-temperature ionic liquid. We finally illustrate the capabilities of MetalWalls by studying the adsorption of a complex functionalized electrolyte on a graphite electrode.

2.
ACS Nano ; 16(11): 18658-18666, 2022 Nov 22.
Artigo em Inglês | MEDLINE | ID: mdl-36269844

RESUMO

Metallic 1T-MoS2 is a promising electrode material for supercapacitor applications. Its layered structure allows the efficient intercalation of ions, leading to experimental volumetric capacitance as high as 140 F/cm3. Molecular dynamics could in principle be used to characterize its charging mechanism; however, unlike conventional nanoporous carbon, 1T-MoS2 is a multicomponent electrode. The Mo and S atoms have very different electronegativities so that 1T-MoS2 cannot be simulated accurately using the conventional constant potential method. In this work, we show that controlling the electrochemical potential of the atoms allows one to recover average partial charges for the elements in agreement with electronic structure calculations for the material at rest, without compromising the ability to simulate systems under an applied voltage. The simulations yield volumetric capacitances in agreement with experiments. We show that due to the large electronegativity of S, the co-ion desorption is the main charging mechanism at play during the charging process. This contrasts drastically with carbon materials for which ion exchange and counterion adsorption usually dominate. In the future, our method can be extended to the study of a wide range of families of 2D layered materials such as MXenes.

3.
J Chem Phys ; 157(9): 094103, 2022 Sep 07.
Artigo em Inglês | MEDLINE | ID: mdl-36075704

RESUMO

Quinones are redox active organic molecules that have been proposed as an alternative choice to metal-based materials in electrochemical energy storage devices. Functionalization allows one to fine tune not only their chemical stability but also the redox potential and kinetics of the electron transfer reaction. However, the reaction rate constant is not only determined by the redox species but also impacted by solvent effects. In this work, we show how the functionalization of benzoquinone with different functional groups impacts the solvent reorganization free energies of electron transfer half-reactions in acetonitrile. The use of molecular density functional theory, whose computational cost for studying the electron transfer reaction is considerably reduced compared to the state-of-the-art molecular dynamics simulations, enables us to perform a systematic study. We validate the method by comparing the predictions of the solvation shell structure and the free energy profiles for electron transfer reaction to the reference classical molecular dynamics simulations in the case of anthraquinone solvated in acetonitrile. We show that all the studied electron transfer half-reactions follow the Marcus theory, regardless of functional groups. Consequently, the solvent reorganization free energy decreases as the molecular size increases.

4.
ACS Appl Mater Interfaces ; 14(18): 20835-20847, 2022 May 11.
Artigo em Inglês | MEDLINE | ID: mdl-35481776

RESUMO

Li-ion batteries are the electrochemical energy storage technology of choice of today's electrical vehicles and grid applications with a growing interest for Na-ion and K-ion systems based on either aqueous or non-aqueous electrolyte for power, cost, and sustainable reasons. The rate capability of alkali-metal-ion batteries is influenced by ion transport properties in the bulk of the electrolyte, as well as by diverse effects occurring at the vicinity of the electrode and electrolyte interface. Therefore, identification of the predominant factor affecting the rate capability of electrodes still remains a challenge and requires suitable experimental and computational methods. Herein, we investigate the mechanistic of the K+ insertion process in the Prussian blue phase, Fe4III[FeII(CN)6]3 in both aqueous and non-aqueous electrolytes, which reveals drastic differences. Through combined electrochemical characterizations, electrochemical-quartz-crystal-microbalance and ac-electrogravimetric analyses, we provide evidences that what matters the most for fast ion transport is the positioning of the partially solvated cations adsorbed at the material surface in aqueous as opposed to non-aqueous electrolytes. We rationalized such findings by molecular dynamics simulations that establish the K+ repartition profile within the electrochemical double layer. A similar trend was earlier reported by our group for the aqueous versus non-aqueous insertion of Li+ into LiFePO4. Such a study unveils the critical but overlooked role of the electrode-electrolyte interface in ruling ion transport and insertion processes. Tailoring this interface structuring via the proper salt-solvent interaction is the key to enabling the best power performances in alkali-metal-ion batteries.

5.
Chem Rev ; 122(12): 10860-10898, 2022 Jun 22.
Artigo em Inglês | MEDLINE | ID: mdl-35389636

RESUMO

Electrochemical double-layer capacitors (EDLCs) are devices allowing the storage or production of electricity. They function through the adsorption of ions from an electrolyte on high-surface-area electrodes and are characterized by short charging/discharging times and long cycle-life compared to batteries. Microscopic simulations are now widely used to characterize the structural, dynamical, and adsorption properties of these devices, complementing electrochemical experiments and in situ spectroscopic analyses. In this review, we discuss the main families of simulation methods that have been developed and their application to the main family of EDLCs, which include nanoporous carbon electrodes. We focus on the adsorption of organic ions for electricity storage applications as well as aqueous systems in the context of blue energy harvesting and desalination. We finally provide perspectives for further improvement of the predictive power of simulations, in particular for future devices with complex electrode compositions.

6.
J Chem Phys ; 156(9): 094709, 2022 Mar 07.
Artigo em Inglês | MEDLINE | ID: mdl-35259900

RESUMO

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 aqueous solution differs markedly from the gas phase results, which emphasize the role of the solvent on the properties of the double-layer.

8.
Angew Chem Int Ed Engl ; 61(5): e202112679, 2022 Jan 26.
Artigo em Inglês | MEDLINE | ID: mdl-34796598

RESUMO

The electrical double-layer plays a key role in important interfacial electrochemical processes from catalysis to energy storage and corrosion. Therefore, understanding its structure is crucial for the progress of sustainable technologies. We extract new physico-chemical information on the capacitance and structure of the electrical double-layer of platinum and gold nanoparticles at the molecular level, employing single nanoparticle electrochemistry. The charge storage ability of the solid/liquid interface is larger by one order-of-magnitude than predicted by the traditional mean-field models of the double-layer such as the Gouy-Chapman-Stern model. Performing molecular dynamics simulations, we investigate the possible relationship between the measured high capacitance and adsorption strength of the water adlayer formed at the metal surface. These insights may launch the active tuning of solid-solvent and solvent-solvent interactions as an innovative design strategy to transform energy technologies towards superior performance and sustainability.

9.
Adv Mater ; 34(4): e2107439, 2022 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-34699650

RESUMO

Ion adsorption inside electrified carbon micropores is pivotal for the operation of supercapacitors. Depending on the electrolyte, two main mechanisms have been identified so far, the desolvation of ions in solvents and the formation of superionic states in ionic liquids. Here, it is shown that upon confinement inside negatively charged micropores, transition-metal cations dissolved in water associate to form oligomer species. They are identified using in situ X-ray absorption spectroscopy. The cations associate one with each other via hydroxo bridging, forming ionic oligomers under the synergic effect of spatial confinement and Coulombic screening. The oligomers display sluggish dissociation kinetics and accumulate upon cycling, which leads to supercapacitor capacitance fading. They may be dissolved by applying a positive potential, so an intermittent reverse cycling strategy is proposed to periodically evacuate micropores and revivify the capacitance. These results reveal new insights into ion adsorption and structural evolution with their effects on the electrochemical performance, providing guidelines for designing advanced supercapacitors.

10.
J Chem Phys ; 155(19): 194506, 2021 Nov 21.
Artigo em Inglês | MEDLINE | ID: mdl-34800945

RESUMO

The structure of a concentrated solution of NaCl in D2O was investigated by in situ high-pressure neutron diffraction with chlorine isotope substitution to give site-specific information on the coordination environment of the chloride ion. A broad range of densities was explored by first increasing the temperature from 323 to 423 K at 0.1 kbar and then increasing the pressure from 0.1 to 33.8 kbar at 423 K, thus mapping a cyclic variation in the static dielectric constant of the pure solvent. The experimental work was complemented by molecular dynamics simulations using the TIP4P/2005 model for water, which were validated against the measured equation of state and diffraction results. Pressure-induced anion ordering is observed, which is accompanied by a dramatic increase in the Cl-O and O-O coordination numbers. With the aid of bond-distance resolved bond-angle maps, it is found that the increased coordination numbers do not originate from a sizable alteration to the number of either Cl⋯D-O or O⋯D-O hydrogen bonds but from the appearance of non-hydrogen-bonded configurations. Increased pressure leads to a marked decrease in the self-diffusion coefficients but has only a moderate effect on the ion-water residence times. Contact ion pairs are observed under all conditions, mostly in the form of charge-neutral NaCl0 units, and coexist with solvent-separated Na+-Na+ and Cl--Cl- ion pairs. The exchange of water molecules with Na+ adopts a concerted mechanism under ambient conditions but becomes non-concerted as the state conditions are changed. Our findings are important for understanding the role of extreme conditions in geochemical processes.

11.
ACS Nano ; 15(10): 15422-15428, 2021 Oct 26.
Artigo em Inglês | MEDLINE | ID: mdl-34546032

RESUMO

In this Perspective, we sketch out a vision of fast charging and self-healable energy systems that are primarily organic, feature only abundant elements, and operate with ions other than lithium. Using conductive oligomers as highly configurable building blocks, it is possible to create intrinsically adaptable conductive polymeric networks that can be rejuvenated and recycled using simple and safe chemical treatments. Using the versatile organic chemistry toolbox, these oligomers can be further functionalized, for example, with redox-active side chains for high charge storage capacity and ligands capable of complexing metal centers. Cross-linking with metal ions converts the soluble oligomers into insoluble supramolecular networks to yield high-performing electrode materials. The oligomer-based approach can thus provide an exceptional level of control to the design of organic-based battery materials.

12.
J Chem Phys ; 155(4): 044703, 2021 Jul 28.
Artigo em Inglês | MEDLINE | ID: mdl-34340400

RESUMO

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 are 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 M NaCl) in good agreement with recent ab initio molecular dynamics results. For pure water, the increase in the capacitance is not accompanied by 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.

13.
J Chem Phys ; 155(7): 074504, 2021 Aug 21.
Artigo em Inglês | MEDLINE | ID: mdl-34418918

RESUMO

Redox-active molecules are of interest in many fields, such as medicine, catalysis, or energy storage. In particular, in supercapacitor applications, they can be grafted to ionic liquids to form so-called biredox ionic liquids. To completely understand the structural and transport properties of such systems, an insight at the molecular scale is often required, but few force fields are developed ad hoc for these molecules. Moreover, they do not include polarization effects, which can lead to inaccurate solvation and dynamical properties. In this work, we developed polarizable force fields for redox-active species anthraquinone (AQ) and 2,2,6,6-tetra-methylpiperidinyl-1-oxyl (TEMPO) in their oxidized and reduced states as well as for acetonitrile. We validate the structural properties of AQ, AQ•-, AQ2-, TEMPO•, and TEMPO+ in acetonitrile against density functional theory-based molecular dynamics simulations and we study the solvation of these redox molecules in acetonitrile. This work is a first step toward the characterization of the role played by AQ and TEMPO in electrochemical and catalytic devices.

14.
J Phys Chem B ; 125(20): 5365-5372, 2021 05 27.
Artigo em Inglês | MEDLINE | ID: mdl-33988996

RESUMO

Aqueous biphasic systems (ABSs) can form when mixing water with two compounds such as polymers, ionic liquids, or simple salts. While this phenomenon has been known for decades and found applications in various fields such as biology, recycling, or even more recently electrochemistry, the physics behind the formation of ABSs remains ill-understood. It was recently demonstrated that ABSs can be composed of two salts sharing the same cation (Li+) but different anions (sulfonamide and halide). Interestingly, their formation could not be explained by the position of the anions within the chaotropic/kosmotropic series and was rather proposed to originate from an anion size mismatch, albeit the size for these anions was never measured yet owing to the lack of a proper experimental methodology. Here, we combine experimental techniques and molecular simulations to assess the specific effects (size, shape, hydrophobic/hydrophilic character) of a series of anions and correlate them with the formation of ABSs. We demonstrate that while the anion size mismatch is a prerequisite for the formation of Li-salts based ABSs, their shape can also play an important role, providing general guidelines for forming new ABSs with potential future applications.

15.
Proc Natl Acad Sci U S A ; 118(15)2021 Apr 13.
Artigo em Inglês | MEDLINE | ID: mdl-33876767

RESUMO

Hydrophobic hydration at metal/water interfaces actively contributes to the energetics of electrochemical reactions, e.g. [Formula: see text] and [Formula: see text] 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 adaptation 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 [Formula: see text], while the free energy cost to hydrate larger hydrophobes, above 2.5-Å 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 adapted LCW theory to a simple model addition reaction, we illustrate some implications of our findings for electrochemical reactions.

16.
J Phys Chem Lett ; 12(18): 4357-4361, 2021 May 13.
Artigo em Inglês | MEDLINE | ID: mdl-33929860

RESUMO

In recent years, constant applied potential molecular dynamics has allowed researchers to study the structure and dynamics of the electrochemical double-layer 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.

17.
Acc Chem Res ; 54(4): 1034-1042, 2021 Feb 16.
Artigo em Inglês | MEDLINE | ID: mdl-33530686

RESUMO

ConspectusThe recent discovery of "water-in-salt" electrolytes has spurred a rebirth of research on aqueous batteries. Most of the attention has been focused on the formulation of salts enabling the electrochemical window to be expanded as much as possible, well beyond the 1.23 V allowed by thermodynamics in water. This approach has led to critical successes, with devices operating at voltages of up to 4 V. These efforts were accompanied by fundamental studies aiming at understanding water speciation and its link with the bulk and interfacial properties of water-in-salt electrolytes. This speciation was found to differ markedly from that in conventional aqueous solutions since most water molecules are involved in the solvation of the cationic species (in general Li+) and thus cannot form their usual hydrogen-bonding network. Instead, it is the anions that tend to self-aggregate in nanodomains and dictate the interfacial and transport properties of the electrolyte. This particular speciation drastically alters the presence and reactivity of the water molecules at electrified interfaces, which enlarges the electrochemical windows of these aqueous electrolytes.Thanks to this fundamental understanding, a second very active lead was recently followed, which consists of using a scarce amount of water in nonaqueous electrolytes in order to control the interfacial properties. Following this path, it was proposed to use an organic solvent such as acetonitrile as a confinement matrix for water. Tuning the salt/water ratio in such systems leads to a whole family of systems that can be used to determine the reactivity of water and control the potential at which the hydrogen evolution reaction occurs. Put together, all of these efforts allow a shift of our view of the water molecule from a passive solvent to a reactant involved in many distinct fields ranging from electrochemical energy storage to (electro)catalysis.Combining spectroscopic and electrochemical techniques with molecular dynamics simulations, we have observed very interesting chemical phenomena such as immiscibility between two aqueous phases, specific adsorption properties of water molecules that strongly affect their reactivity, and complex diffusive mechanisms due to the formation of anionic and aqueous nanodomains.

18.
Annu Rev Phys Chem ; 72: 189-212, 2021 Apr 20.
Artigo em Inglês | MEDLINE | ID: mdl-33395545

RESUMO

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 an 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 in 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 in 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.

19.
J Chem Phys ; 153(21): 214505, 2020 Dec 07.
Artigo em Inglês | MEDLINE | ID: mdl-33291914

RESUMO

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 us to study the structure and diffusion properties of the system at temperatures ranging from 1400 K to 3000 K. We show that Na+ and Ca2+ ions adopt a different structural organization within the silica network, with Ca2+ 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 Ca2+ 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+.

20.
J Chem Phys ; 153(17): 174704, 2020 Nov 07.
Artigo em Inglês | MEDLINE | ID: mdl-33167657

RESUMO

Spurred by the increasing needs in electrochemical energy storage devices, the electrode/electrolyte interface has received a lot of interest in recent years. Molecular dynamics simulations play a prominent role in this field since they provide a microscopic picture of the mechanisms involved. The current state-of-the-art consists of treating the electrode as a perfect conductor, precluding the possibility to analyze the effect of its metallicity on the interfacial properties. Here, we show that the Thomas-Fermi model provides a very convenient framework to account for the screening of the electric field at the interface and differentiating good metals such as gold from imperfect conductors such as graphite. All the interfacial properties are modified by screening within the metal: the capacitance decreases significantly and both the structure and dynamics of the adsorbed electrolyte are affected. The proposed model opens the door for quantitative predictions of the capacitive properties of materials for energy storage.

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