Asymmetric Multi-Site Ion Exchange in Porous Carbon Electrodes

Ion transport in porous carbon electrodes underpins the performance of electrochemical energy-storage and separation technologies, yet exchange dynamics within heterogeneous pore networks remain difficult to quantify. Here, we present a rigorous analytical framework for extracting quantitative ion-exchange kinetics in confined electrolytes from two-dimensional exchange spectroscopy (2D EXSY) NMR, explicitly accounting for asymmetric ion populations, relaxation effects, and microscopic reversibility. By describing exchange with a lognormal distribution of rates, the framework captures the intrinsic heterogeneity of ion motion and overcomes the limitations of conventional discrete-site models. Applied to aqueous LiTFSI electrolytes confined in mesoporous CMK-3 and hierarchically structured ST-CMK-3 carbons, the method resolves a hierarchy of ion-exchange processes spanning fast near-surface exchange to slow in-pore diffusion in two and three-site exchange systems. We report the first observation of three-site ion exchange in porous electrodes, revealing enhanced directional ion mobility and higher effective diffusion coefficients at interconnected micro–mesopore boundaries in hierarchical carbons. These findings establish direct, quantitative links between pore architecture and ion-transport efficiency. This distribution-based framework provides a generalizable route to identifying rate-limiting transport pathways in complex porous energy materials and offers design principles for optimizing porous carbons for high-rate supercapacitors, capacitive deionization, and gas-storage applications.