Insight into Manipulation Mechanism of Ion Transport through Angström-Scale Channels in Ti₃C₂T_x Membrane by Controlling Surface Potential Using Electrochemical Quartz Crystal Microbalance with Dissipation

Abstract

A lamellar membrane assembled by parallel restacking of two-dimensional nanosheets provides a novel platform for studying the electrostatic manipulation of ion mobility under angstrom-scale confinement. The membrane nanostructure in aqueous solution is unstable due to intercalation. However, because accurate measurements of structural changes under different charged conditions are difficult, their effect on the ion modulation was seldom fully addressed, and the electrostatic modulation mechanism was not good. Here, we report the efficient manipulation of ion diffusion in the confined channels through a Ti 3 C 2 T x membrane and gain insight into the mechanism by an in situ measurement of the lamellar nanostructure using an electrochemical quartz crystal microbalance with dissipation (EQCM-D). By precisely tuning the external potential directly applied to the membrane, the ion permeation rate can be effectively and reversibly increased by 6 times or decreased by 15 times. The EQCM-D measurement provides empirical evidence to demonstrate that the anomalous ionflow modulation is determined by both the channel diameter and an ion rearrangement in the electrical double layers (EDLs), particularly, in the Stern layers. Although the channel expanded at low negative voltage, the ion permeation rate decelerated due to the counterion distribution in the Stern layers.

Selective Ion Transport in Two‐Dimensional Lamellar Nanochannel Membranes

“…Based on the test results of the QCM-D, which could monitor the real-time changes in membrane structures when different voltages were applied, the structural variations in the Ti 3 C 2 T x membranes generally showed a corresponding relationship with the ion permeation rate under different applied voltages. [49] It should be noted that the interlayer spacing when À 0.2 V was applied was higher than that at 0 V; however, the ion permeability decreased slightly compared with the original state, which was different from the graphene-based membrane. We concluded that the increased charge density attracted more cations to move into the interlayer spacing.…”

“…For instance, the Stern layer occupied considerable space in the angstrom-scale channels and showed a significant impact on the effective diameter of the channel for ion transport. [49,211] Indeed, further investigation is necessary to confirm if the ion distribution still follows the layered structure as described in EDL theory in the highly confined spacing. Recently, novel concepts, i.e., "quantumconfined superfluid", have been proposed to describe the relationship between ultrafast mass transport and wettability of the nanochannel surface.…”

Conductive Polymer Intercalation Tunes Charge Transfer and Sorption–Desorption Properties of LDH Enabling Efficient Alkaline Water Oxidation

“…However, Δ D for LDH was slightly increased, while it exhibited a decreasing trend for LDH-ppy (Figure E, bottom), suggesting that the surface of LDH became softer while LDH-ppy became more rigid. This difference is likely due to the modified surface physicochemical and sorption properties with the intercalation of ppy, which would make the surface/interface bound OH – /H 2 O species in a more ordered arrangement. , In addition, when the external potential was set at 0.2 V (vs Ag/AgCl), the increase of Δ f / n on LDH-ppy (∼0.67) was 3 times that on LDH (∼0.18, Figure F), suggesting the much faster desorption/transportation of OH – /H 2 O on LDH-ppy, leading to a larger weight loss. Moreover, by comparing the value of Δ f / n at step 3 with that at step 1, we found that the change of Δ f / n on LDH-ppy (∼2.29) was doubled over that on LDH (∼1.09).…”

Selective Ion Transport in Two‐Dimensional Lamellar Nanochannel Membranes

“…Moreover, unlike the diffusion layer in the EDL, the influence of the Stern layer and first water layer has also been ignored for a long time in nanofluidic studies. For instance, the Stern layer occupied considerable space in the angstrom‐scale channels and showed a significant impact on the effective diameter of the channel for ion transport [49, 211] . Indeed, further investigation is necessary to confirm if the ion distribution still follows the layered structure as described in EDL theory in the highly confined spacing.…”