Ion Sieving Effects in the Electrical Double Layer of Porous Carbon Electrodes:  Estimating Effective Ion Size in Electrolytic Solutions

Abstract

Certain microporous carbons have pores of molecular dimensions, the size of which can be adjusted over a wide range to fit the dimensions of a large variety of molecules. With knowing the dimensions of these molecules, size-calibrated porous carbons can be produced. In this work, we took advantage of the electronic conductivity of carbon in using it as an electrode in electrolytic solutions. By changing the electrode potential, we could induce electroadsorption and electrodesorption of ions of different dimensions into pore-calibrated carbons, thus enabling us to estimate effective ion sizes. A few fundamental questions such as whether ions accommodated in the electrodes' pores are in a solvated state, which is important for novel electrochemical energy storage devices and for microbiological systems, could be addressed. Analysis of the results indicates that all the cations that are employed are electroadsorbed in the electrode pores in hydrated states. Compared with the monovalent cations, the bivalent cations exhibit dimensions that make them almost twice as big. Monovalent anions are basically adsorbed in a nonhydrated state. The doubly charged sulfate anion is adsorbed in the hydrated state. The nitrate, a multiatom planar anion, has a smaller effective dimension than the monoatomic halogen anions, in accordance with the observation that the pores in many activated carbons are slit-shaped. The analysis of the results enabled us to establish a scale of ionic effective dimensions in aqueous media, as presented below (MTBE stands for methyl tert-butyl ether): cations, [3.62 Å, N2] < Cs+ < K+ < Na+ < Li+ < [4.21 Å, CF4] < [5.05 Å, SF6] < [5.8 Å, MTBE] < Ba2+, Ca2+, and Mg2+; anions, [3.62 Å, N2] < NO3 - < Cl- < F- < Br- < [4.21 Å, CF4] < ClO4 - < [5.05 Å, SF6] < [5.8 Å, MTBE] < SO2-. We discovered that as long as the pore size is considerably greater than the ion size, the electric double-layer capacity at moderate concentrations is independent of both the size and charge of the ions in solutions.

Combined Effect of Nitrogen‐ and Oxygen‐Containing Functional Groups of Microporous Activated Carbon on its Electrochemical Performance in Supercapacitors

“…[51] The corresponding sizes of SO 2À 4 (H 2 O) 12 and H 3 O þ were reported to be 5.33 Å and 4.2 Å , respectively. [51,52] Thus, the pore size of 5.2 Å perfectly matches the size of negatively charged sulphate ions and is not too big to ''waste'' the space during accommodation of hydronium ions. Lota et al recently reported on the unchanged mobility of SO 2À 4 ions in constrained pores with a diameter of 6 Å , [53] which is in agreement with this result.…”

General Properties of Electrochemical Capacitors

“…For a given porous material, the smaller the ion size, the higher the accessible surface area available to the ions. Thus, as aqueous electrolytes often have a smaller ion size than nonaqueous electrolytes, they provide, in general, higher specific capacitance values (Tables 2.2 confirm the accessibility of fine micropores, as small as 0.4-0.5 nm in diameter, to aqueous electrolytes [51,52]. As ions in electrolyte solutions are stabilized by a solvation sheath, many early studies argued that micropores were difficult to access and that mesopores, (2-50 nm) were more useful in EDLCs.…”

High capacitance B/C/N composites for capacitor electrodes synthesized by a simple method

“…The shape of CV resembles that for nitrogen-enriched carbons in 6 mol dm -3 KOH solution [29] and it has been explained as pseudo-capacitive interactions between cations and the nitrogen atoms of carbon, though detailed background was not given [6,29]. It was reported that alkali ions in aqueous solutions adsorb into pores of carbon materials together with sizable hydration sheath [30]. A scale of ionic effective dimensions was estimated to be 0.362 nm < K + < Na (Figs.…”