Mechanism of oscillation of aqueous electrical double layer capacitance: Role of solvent

Capacitive Behavior of Aqueous Electrical Double Layer Based on Dipole Dimer Water Model

“…The neutral cohesive interactions between fluid phase particles and those between the fluid phase particles and constitute particles of the solid surface are shown to cause significant changes in the density profiles, which necessarily produce a variation of the capacitive behaviors [ 61 , 63 ]; they are not considered in the present study because our main goal is to study the changes caused by the explicit consideration of the hard core and electric dipole of the water solvent. So, the inorganic salt ions are modeled by appropriately charged hard spheres and the water molecule is approximated as a dimer consisting of two hard sphere sites (representing the hydrogen atom and oxygen atom, respectively) tangentially bonded together and rotating freely on each other’s surfaces.…”

“…Usually, the aqueous electrolyte solution is modeled by so-called primitive model (PM) [ 20 , 22 , 50 , 51 , 52 ], in which a water solvent is considered as a structureless continuum and shows its existence only through a high dielectric constant completely neglecting its hard core; some researches [ 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 ] pick up the hard core missed in the PM and use an appropriately high dielectric constant to reflect the electric dipole moment of the water molecule (such a model is called the solvent primitive model abbreviated as SPM). There are studies [ 61 , 62 ] that consider a semi-primitive model where the solvent is represented by hard spheres with a Yukawa attraction and a dielectric permittivity is also introduced to reflect the electrostatic shielding effect of water molecules; in a very recent study [ 63 ], one Lennard–Jones (LJ) sphere with a higher energy parameter is used to model the water molecule to reflect its strong polarity, both the LJ energy and size parameters are determined by reproducing the two experimentally measured a and b parameters in the van der Waals equation of the state of water. More advanced models (the so-called “civilized” model) have been proposed, where both the polar nature and hard core of the solvent molecule are explicitly considered at the same time, by a dipole hard sphere (hard sphere carrying a point dipole at its center) [ 64 , 65 , 66 ] or dipole dimer hard sphere with positive and negative polarized charges residing in the centers of the two constituent spheres [ 67 ].…”

On Capacitance Enhancement at Decreasing Pore Width and its Relation with Solvent Concentration and Polarity

“…Relationship between the C d and other parameters such as electrolyte composition, [1][2][3] ion structure, [4][5][6][7] and temperature, [8][9][10] etc. has become a research focus since a large number of studies [11][12][13][14][15][16][17] have shown that the C d value of the EDLC using ionic liquids or organic ions as working electrolytes obviously increases as the electrode pore tends to order of the ion dimension, and exhibits an oscillatory character with the electrode pore width. However, it has also been reported that the C d changes only slightly with the electrode pore width.…”

“…It has been shown that increasing the size of counter-ions significantly reduces their differential capacitance. [4][5][6][7]17,47 Obviously, the oscillation of voltage -C d curve and increase of the C d in small electrode pores are affected by solvent and counter-ion characteristics. Understanding these effects has a theoretical guiding role in optimization of the EDLC.…”

“…However, influencing factors have not been studied systematically in current researches. [11][12][13][14][15][16][17][18][19][20] Purpose of this paper is to study how solvent polarity and concentration, counter-ion size, and width and voltage of electrode pore influence the differential capacitance and its enhancement in counter-ion sized electrode pore (CISEP). In Model and method section, we describe one coarse-grained model for ions solvent, electrode pore, and one classical density functional theory (cDFT) used to deal with the classical thermodynamic system, and present and analyze our results for the C d behaviors and microscopic configurations in Results and discussion section; Finally, we summarize the paper in Summary section.…”