Selective adsorption of divalent and trivalent cations in porous electrodes.

The capacitive deionization technology uses the electrochemical adsorption of ions in porous electrodes to desalinate seawater or brackish water. Recently, capacitive deionization has gained significant attention as a technology for selective adsorption of ionic species from multicomponent aqueous electrolytes. To investigate the mechanism of selective adsorption at the molecular level, we performed molecular dynamics simulations of aqueous electrolytes and porous electrodes with different divalent or trivalent ions, electrode pore sizes, and applied voltages. We calculated the free energy barriers preventing ions from entering the pores of the electrode and the structure of the water molecules near the ions and the electrode surface under various conditions. Our results suggest that, when the pore and ion sizes are comparable, the steric and electrostatic interactions between the hydrated ions and electrode pores are comparable in magnitude. Moreover, the relative importance of the two interactions can be reversed by slight changes in the external conditions, such as the ion size, valence of the ions, electrode pore size, and applied voltage. Thus, by finely tuning the electrode pore size and the applied voltage, it may be possible to selectively adsorb a particular ionic species from a multicomponent electrolyte through capacitive deionization using a porous electrode.

Molecular dynamics simulation to reveal the transport mechanism of LiPF6 in ethylene carbonate + dimethylcarbonate binary solvent.

This paper presents the molecular dynamics simulation of 1 mol kg-1 LiPF6 in a binary solvent of ethylene carbonate (EC) and dimethylcarbonate, which is a representative electrolyte solution for lithium-ion batteries. The simulation successfully reproduced the diffusion coefficient, ionic conductivity, and shear viscosity as functions of EC content at 300 K, which had been experimentally determined in our previous study. The Yukawa potential was adopted to model intercharge interactions to reduce computational costs, which consequently allowed us to precisely calculate the conductivity and viscosity by directly integrating time-correlation functions without explicitly modeling the molecular polarization. Breaking down microscopic current correlation functions into components revealed that, whereas the cation-anion attractive interaction dominantly impedes the conduction when the EC content is low, it is the cation-cation and anion-anion repulsive interactions that reduce the conductivity at a high EC content. An analysis of the pressure correlations revealed that all components positively contribute to the viscosity in the binary solvent without the electrolyte. On the other hand, negative terms are observed in five out of six cross correlations in the presence of the electrolyte, implying that these correlations negatively contribute to the shear stress and entropy production, both of which are net positive.

Transport coefficients of gel electrolytes: A molecular dynamics simulation study.

The responses of gel electrolytes to stimuli make them useful in applications such as sensors and actuators. However, few studies have explored their transport properties from a molecular viewpoint. We studied the transport coefficients of gel electrolytes based on perfluorinated sulfonic acid using molecular dynamics simulations. The transport coefficients for electric and pressure fields, namely, the ionic conductivity, Darcy permeability, and cross coupling constant, were calculated based on Kubo's linear response theory from the corresponding velocity correlation functions and mean square displacements. The effects of the water content of the gel electrolyte and those of the monovalent cationic species were also analyzed. The calculated transport coefficients qualitatively agree with the reported experimental results. The role of the cross coupling constants in determining the functional efficiency of gel electrolytes as pressure sensors or electroactive actuators is discussed.

Selective adsorption of monovalent cations in porous electrodes.

To clarify the mechanisms involved in the electrochemical adsorption of ions of aqueous electrolytes in porous electrodes, we performed molecular dynamics simulations of systems composed of porous carbon electrodes with various pore sizes and aqueous solutions containing a Li+, Na+, K+, or Cs+ cation and a Cl- anion. The free energy barrier preventing the cation from entering the pore in the electrode and the hydration structure around the cation were calculated for each cation species and each pore size of the electrode. As the cation moved toward the porous electrode from the bulk electrolyte, rearrangement of the hydration network occurred. The energetic cost of this rearrangement of the hydration network was identified as the cause of the free energy barrier. We estimated the likelihood of cations becoming adsorbed by the porous electrode for different pore sizes and applied voltages and found that the specificity of the magnitude of the free energy barrier for different ions is determined by two factors: ion size (Li+ < Na+ < K+ < Cs+) and the strength of hydration (Li+ > Na+ > K+ > Cs+). With no or a low applied voltage, the ion size dominates the selectivity, and with a high applied voltage, the strength of hydration dominates, although there were some exceptions. The ion specificity of the free energy barrier could be utilized in the selective adsorption of ions from multi-component electrolytes by controlling the pore size of the electrode and the applied voltage.

Hydration of monovalent and divalent cations near a cathode surface.

Hydration of monovalent (Li+, Na+, K+, and Cs+) and divalent (Mg2+, Ca2+, Sr2+, and Ba2+) cations on a cathode surface was studied by a classical molecular dynamics simulation. The potential of mean force (PMF) for each cation species was calculated as a function of the distance from the cathode surface, and the potential barriers for dehydrating the first and second hydration shells near the cathode surface were estimated. The positions of the minimum of the PMF closest to the cathode surface were found to be in the order Li+ < Na+ < Mg2+ < Ca2+ < Sr2+ < Ba2+ < K+ < Cs+. It was found that Li+, Mg2+, Ca2+, Sr2+, and Ba2+ ions are most likely doubly hydrated when they are adsorbed on the cathode surface without an applied voltage, whereas Na+, K+, and Cs+ ions are most likely singly hydrated at room temperature. On the other hand, when a voltage of 1 V was applied to the electrodes, all the cation species that we studied appeared most likely to be singly hydrated on the cathode surface. The depths of the potential well closest to the cathode surface under an applied voltage of 1 V were found to be in the order Ba2+ < Sr2+ < Ca2+ < Mg2+ for the divalent cations and Li+ < Na+ < K+ < Cs+ for the monovalent cations in the set of models that we used. These orders coincide with the Hofmeister series from the kosmotropic to the chaotropic.

Ionic conductivity of molten alkali-metal carbonates A2CO3 (A = Li, Na, K, Rb, and Cs) and binary mixtures (Li1-xCsx)2CO3 and (Li1-xKx)2CO3: A molecular dynamics simulation.

Based on experimental data, we optimized the potential parameters for the classical molecular dynamics simulation to reproduce the volume and ionic conductivity of the molten alkali-metal carbonates A2CO3 where A = Li, Na, K, Rb, and Cs at T/K = 1223 and ambient pressure. The force field was then applied to the binary mixtures (Li1-xCsx)2CO3 and (Li1-xKx)2CO3. In (Li1-xCsx)2CO3, the diffusion coefficient DCs exceeds DLi at x > 0.6, testifying to the Chemla effect. The net ionic conductivity was broken down into the contributions from the velocity auto- and cross-correlations of each ionic species. The significant negative deviation of the real conductivity of (Li1-xCsx)2CO3 from the one estimated by the Nernst-Einstein (NE) relation is clearly explained by the contribution from the cross correlations; specifically, the cross term between Li+and CO3 2-, which is negative at x = 0, significantly shifts to the positive side when x increases, which is dominantly responsible for dampening the conductivity from the NE conductivity. A similar behavior was observed in (Li1-xKx)2CO3 with a less pronounced manner than in (Li1-xCsx)2CO3. These observations corroborate the precedent studies pointing to the trapping of Li+ by the anion when a lithium salt is mixed with another salt of which the cation size is greater than that of Li+.

Hydration and dehydration of monovalent cations near an electrode surface.

The mechanism of hydration and dehydration of monovalent ions, Li+, Na+, K+, and Cs+, in a dilute solution near an electrode surface was studied by molecular dynamics simulations. The potentials of mean force for these ions were calculated as a function of the distance from the electrode surface and the potential barriers for dehydrating the first and the second hydration shell near the electrode surface and were estimated for each ion species. It was found that the mechanism of hydration for Li+ is distinct from those for Na+, K+, and Cs+. Penetration of ions into the first layer of water molecules on the electrode surface is unlikely to occur for the case of Li+, while that would occur with certain probabilities for the case of Na+, K+, or Cs+, whether or not voltage is applied to the electrode. Li+ ions would be adsorbed on the electrode surface in a doubly hydrated form with a significant probability, while Na+, K+, and Cs+ ions would be adsorbed most likely in a singly hydrated form. Furthermore, the theory of ionic radii, which has been successfully used in the analysis of bulk solutions, was applied to the electrode/electrolyte interface. It was found that the theory of ionic radii is also useful in explaining the structural behaviors of ions near an electrode surface. The distance between an ion and the layers of water molecules on the electrode surface showed almost linear dependence on the radius of the ion, as predicted by the theory of ionic radii. Analysis of the deviation from the linearity showed that Li+ ions are most likely adsorbed in the first layer of water molecules on the electrode surface, while Na+, K+, and Cs+ ions are adsorbed on the second layer of water molecules. These analyses indicate that Li+ is a structure maker, while Na+, K+, and Cs+ are structure breakers, which is consistent with the widely accepted idea in explaining the behaviors of the bulk solutions.

Ferroelectric Phase Behaviors in Porous Electrodes.

The phase behavior of ions in porous electrodes is qualitatively different from that in the bulk because of the confinement effect and the interaction between the electrode surface and the electrolyte ions. We found that porous electrodes of which the pore size is close to the size of the electrolyte ions can show ferroelectric phase behaviors in some conditions by Monte Carlo simulations of simple models. The phase behavior of the porous electrodes dramatically changes as a function of the pore size of the porous electrode and that is compared to the phase behavior of typical ferroelectric materials, for which the phase behavior changes as a function of the temperature or the composition. The origin of the phase behavior is discussed in terms of the molecular interaction and the ionic structure inside the porous electrodes. We also found that the density of counterions and that of co-ions inside porous electrodes changes in a nonlinear fashion as a function of the applied voltage, which is in agreement with the experimental results.

Electrochemical and structural properties of the electrical double layer of two-component electrolytes in response to varied electrode potential.

The electrochemical and structural properties of the electrical double layers for two-component electrolytes were studied by Monte Carlo simulations using simple models. When the electrolyte contains two species of cations that have different diameters, the capacitance on the cathode dramatically increases as a large negative potential is applied. This behavior is qualitatively similar to the one reported in an experimental work that has used Li-containing ionic liquid as the electrolyte [M. Yamagata et al., Electrochim. Acta 110, 181-190 (2013)], in which it has also been reported that addition of Li ions to the electrolyte enhances the potential window to the negative side. The analysis of the ionic structure showed that the electrical double layer on the cathode is dominantly formed by the larger cations under small negative potentials, while they are replaced by the smaller cations under large negative potentials. This transition of the ionic structure with electrode potential is also consistent with the enhancement of the potential window that was found in the experimental work, which suggests that the organic cations are expelled from the electrical double layer under large negative potentials and the chance of decomposition is reduced.

Phase transition in porous electrodes. III. For the case of a two component electrolyte.

The electrochemical thermodynamics of electrolytes in porous electrodes is qualitatively different from that in the bulk with planar electrodes when the pore size is comparable to the size of the electrolyte ions. In this paper, we discuss the thermodynamics of a two component electrolyte in a porous electrode by using Monte Carlo simulation. We show that electrolyte ions are selectively adsorbed in porous electrodes and the relative concentration of the two components significantly changes as a function of the applied voltage and the pore size. This selectivity is observed not only for the counterions but also for the coions.

Phase transition in porous electrodes. II. Effect of asymmetry in the ion size.

The electrochemical thermodynamics of electrolytes in porous electrodes is qualitatively different from that in the bulk with planar electrodes when the pore size is comparable to the size of the electrolyte ions. In this study, the effect of the ion size asymmetry on the thermodynamics in porous electrodes was studied by using Monte Carlo simulation. We used the electrolyte ions for which the size of the cations and that of anions is different. Due to the asymmetry in the ion size, the ionic structure and the way the surface charge is distributed on the electrode surfaces were found to be qualitatively different in the cathode and in the anode. In particular, for some ranges of applied voltage, the distribution of the surface charge induced on the electrode planes shows inhomogeneity, which is not intrinsic to the structure of the porous electrodes. The transition from the homogeneous to the inhomogeneous distribution of surface charge on changing the voltage is a second order phase transition.

Phase transition in porous electrodes.

It is shown by Monte Carlo simulation that electrochemical thermodynamics of electrolytes in a porous electrode is qualitatively different from that in the bulk with a planar electrode. In particular, first order phase transitions occur in porous electrodes when the pore size is comparable to the ion size of the electrolytes: as the voltage is increased from zero, the surface charge density and the ion density in the porous electrodes discontinuously change at a specific voltage. The critical points for those phase transitions are identified.

Electrolytes in porous electrodes: Effects of the pore size and the dielectric constant of the medium.

Monte Carlo simulations in the constant voltage ensemble were performed for electrolytes in porous electrodes. It was found that the electrical and mechanical properties in porous electrodes dramatically change depending on the pore size and the dielectric constant of the medium. For a low dielectric constant of the medium, the capacitance of porous electrodes tends to increase as the pore size decreases and the pressure in the porous electrodes is positive or negative depending on the pore size. For a high dielectric constant of the medium, on the contrary, the capacitance tends to decrease as the pore size decreases and the pressure is positive for all the conditions studied here. Such pore size dependencies are explained in terms of the balance between the electrostatic interaction and the volume exclusion interaction in the porous electrode.

Monte Carlo simulation of electrolytes in the constant voltage ensemble.

The authors studied the structural, electrostatic, and electromechanical properties of the terlamellar structure composed of the anode, the cathode, and the electrolyte layer separating them. They used the Monte Carlo simulation technique in the constant voltage ensemble, where the electrical potential difference between the anode and the cathode is introduced as an external field. For ions, they used the primitive models of different sizes and valences in order to investigate how they affect the physical properties when an electrical field is applied between the electrodes. For electrodes, they used impermeable and permeable models, which mimic planar and porous electrodes, respectively. The asymmetry between the anions and the cations in size or valence was found to be responsible for the asymmetry in the concentration profile, the potential drop, and the stress distribution, in comparing the anode and the cathode sides. The charging/discharging process in the planar and porous electrodes is discussed at molecular level.

Surface anchoring of rodlike molecules on corrugated substrates.

We studied the mechanism of surface anchoring of rodlike molecules on substrates with the surfaces corrugated at molecular scale by molecular-dynamics simulation. We constructed a model for substrates that can have anisotoropic topographical patterns such as corrugation. The structural and thermodynamic properties of rodlike molecules on the corrugated surfaces, including the elastic and anchoring properties, were calculated and the influence of the surface structure on the anchoring was discussed. We found that the rodlike molecules are aligned along the grooves of the corrugated surfaces guided by the anisotropic molecular interaction between the molecules and the corrugated surface. The strength of anchoring was found to be increased when the period of corrugation is decreased at molecular level.