Influence of ion structure and solvent electric dipole on ultrananoporous supercapacitor: a lattice model study

Abstract

A five-state modified BEG model is proposed for describing aqueous electrolyte in cylindrical pore so narrow that only one row of particles can be stationed. Two properties the closest to the actual situation of the aqueous electrolyte are integrated into the model: cation is dimer comprised of two tangentially tethered hard spheres of different diameters: one is charged, while the other is neutral, and solvent is modeled as neutral hard sphere with an electric dipole. The system partition function is obtained by numerically solving maximum real root of characteristic equation of Kramers-Wannier transfer matrix. The present theory reduces to that by the Rochester et al by making both the cation neutral sphere size and electric dipole moment of the solvent equal to zero. The model is used to investigate effects of cation shape and electric valence, solvent electric dipole moment, and cylindrical pore radius on specific differential capacitance and specific energy storage in electric double layer supercapacitor formed inside the cylindrical pore. Main findings are briefly described below. (i) With dimer counter-ion electric valence changing from 1 to 2, the peak becomes higher, and the peak position becomes nearer to the zero charge potential; as a comprehensive effect of these changes, the dimer counter-ion electric valence does not exert obvious influence on the saturation value , but reduces the threshold electrode potential strength , beyond which the value is just and approximately reached. (ii) Whether the dimer counter-ion is monovalent or bivalent, the peak height and the value tend to rise with the dimer neutral site size decreasing, but influence of the value on the value is very small. Only when the value is small enough, the value rises a little with decrease of the value. (iii) Increase of solvent electric dipole moment (wheth

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

“…The electrode voltage also affects the C d growth rate, the maximum of which always corresponds to the electrode voltage of 1 to 1.5 V. The inverse correlation between C d strength and counter-ion size is consistent with previous studies. [4][5][6][7]17,47 However, the inverse correlation between the C d enhancing rate and the counter-ion size and the nonmonotonicity of the ψ − C s d curve for larger counter-ion size are new findings in this paper.…”

“…Differential capacitance C d is a measurable quantity in experiments; it can describe responses of electric double layer (EDL) structure to the electrode potential, and plays a key role in determining the energy storage density and energy output mode. 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.…”

“…Although the theory is approximate, this does not prevent it from giving qualitatively or even quantitatively correct conclusions. 1,[5][6][7][32][33][34][35][36][37][38][39][40] In aqueous inorganic electrolyte solution, solvent water is generally the main component (unless it is water in salt [41][42][43][44] ); organic solvents may also exist in ionic liquids or organic electrolytes. Literature 45 shows that low ion diffusion rate in pure ionic liquids can be improved by reducing the number of ion pairs by diluting the electrolyte in organic solvent(s), thereby curing the weakened EDLC power density.…”

“…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.…”

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

“…In fact, the limitation of the narrow electrochemical window can be overcome in highly concentrated aqueous electrolytes, also known as water-in-salt (WiS) electrolytes [ 8 , 9 , 10 , 11 , 12 , 13 ], which have been shown to significantly widen the electrochemical stability window to over 3.0 V, via interface modification [ 14 , 15 ], or the development of chemically stable, high surface area carbon structures [ 16 , 17 , 18 ]. As a result, aqueous electrolytes are still popular in the study of EDLCs [ 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 ].…”