Two-Component Electrolyte Solutions with Dipolar Cations on a Charged Electrode: Theory and Computer Simulations

Abstract

The development of advanced electrochemical devices for energy conversion and storage requires fine tuning of electrode reactions, which can be accomplished by altering the electrode/solution interface structure. Particularly, in case of an alkali-salt electrolyte the electric double layer (EDL) composition can be managed by introducing organic cations (e.g. room temperature ionic liquid cations) that may possess polar fragments. To explore this approach, we develop a theoretical model predicting the 1 arXiv:2007.00378v1 [cond-mat.soft] 1 Jul 2020 efficient replacement of simple (alkali) cations with dipolar (organic) ones within the EDL. For the typical values of the molecular dipole moment (2 − 4 D) the effect manifests itself at the surface charge densities higher than 30 µC/cm 2. We show that the predicted behavior of the system is in qualitative agreement with the molecular dynamics simulation results.

Theory of ionic liquids with polarizable ions on a charged electrode

“…The presence of static polarizability or a permanent dipole moment on the cations makes their attraction to the negatively charged electrode stronger than that of nonpolarizable anions to the positively charged electrode surface. The stronger attraction of the cations to the electrode is the result of an additional dielectrophoretic attractive force acting on the induced or permanent dipole placed in an inhomogeneous electric field 4,17,23 . Fig.…”

“…The authors introduced a new length scale related to the excess ion polarizability of ions and showed that when it is comparable to the effective Debye length, ion static polarizability can significantly influence double layer properties. It is also important to discuss several papers [20][21][22][23] , where the static polarizability or permanent dipole moment of ions were taken into account in the context of the theory of liquid electrolytes on charged interfaces. In paper 20 , the author incorporated ion static polarizabilities into the Poisson-Boltzmann equation by modifying the effective dielectric constant and the Boltzmann distribution of ions and thus formulated the so-called polarizable Poisson-Boltzmann equation.…”

“…A comparison with the case of nonpolarizable counterions showed that static polarizability of ions brings important differences to the counterion density distribution as well as the counterion mediated electrostatic disjoining pressure. In paper 23 , the authors developed a self-consistent field theory of a two-component electrolyte solution on a negatively charged electrode (cathode), predicting the efficient replacement of simple (alkali) cations with dipolar (organic) ones within the EDL. For the typical values of the molecular dipole moment, the replacement manifests itself at reasonable surface charge densities of the electrode.…”

Modified Poisson-Boltzmann equations and macroscopic forces in inhomogeneous ionic fluids

“…The modified PB equations obtained up to now take into account the individual properties of ionic species and the chemical specifics of the surface interacting with them. In this case it is necessary to especially emphasize the consideration of the non-zero size of ions (steric interactions) [1][2][3][4] , their specific and structural interactions 5,6 , polarizability/quadrupolarizability and permanent dipole moment of ionic species and solvent (co-solvent) molecules [7][8][9][10][11][12][13][14][15][16][17] , surface charge regulaton 18 , specific ion-electrode interactions 12,19 and electrostatic correlations [20][21][22] .…”

Electric double layer theory for room temperature ionic liquids on charged electrodes: milestones and prospects

“…It is significant that for the 5 case of the ideal ionic gas reference model, self-consistent field equation ( 8) transforms into the known polarizable PB equation [41]. We also mention that similar formalism [42] was also utilized to describe a two-component electrolyte solution on a charged electrode, where one of the cations possesses a sufficiently large dipole moment. Using the first integral of the self-consistent field equation, which is reduced to the condition of the RTIL mechanical equilibrium, and the expression for the surface charge density σ = (0)E(0), the authors obtained the following analytical expression for the DC…”