Diffusion kinetics of ionic charge carriers across Ti3C2Tx MXene-aqueous electrochemical interfaces

Abstract

Titanium carbide MXene -Ti3C2Tx - is a representative and well-employed electrode material for a variety of electrochemical energy storage applications. Despite exhibiting metal-like electronic conductivity, restacked Ti3C2Tx films show mediocre rate performance. Here, we report on the estimation of diffusion coefficients of aqueous cations (H+, Li+, Na+and Ca+2) across Ti3C2Tx MXene interlayer spaces by employing electrochemical impedance spectroscopy. Proton diffusivity across 2D slits is found to be 2.2 * 10−8 ± 0.4 * 10−8 cm2/s, which is three orders of magnitude slower than in bulk water. Such a sluggish diffusive behavior is attributed to interfacial confinement effects in the 2D space. It is found that protons diffuse three times faster than hydrated metal cations (Li+, Na+and Ca+2) through Ti3C2Tx slits and exhibit strong potential dependent behavior. Therefore, the rate performance of Ti3C2Tx MXene electrodes is limited by the ionic transportation in the 2D slits rather than electronic transport. This study provides insights into the engineering of open 2D electrode architectures for better ionic diffusive pathways towards the design of high rate MXene based energy storage devices.