Ion pairing dynamics leading to a non-linear electrochemical structure of an electric double layer on a carbon electrode

A non-linear trend in piezoionic effects of an electric double layer (EDL) on ion concentration has been widely observed experimentally, yet its atomic-scale origin remains unclear. In this study, ion pairing that destroy polarized Helmholtz planes were unlocked for multi-walled carbon nanotube (MWCNT) energy harvesters with different ion concentrations of LiCl electrolyte. LiCl electrolyte with low concentrations (≤2 M) allowed Li⁺ and Cl⁻ to form independently hydrated structures, facilitating a well-defined EDL with clearly polarized Helmholtz planes. At high concentrations (>2 M), the hydrogen-bonded water network was weakened, leading to stronger electrostatic interactions between ion clusters. This resulted in the formation of Li(H₂O)₄-Cl(H₂O)₈ ion pairs, where Li⁺ partially shared its water ligand with Cl⁻. Such ion pairing disrupted inner-sphere adsorption of Li⁺ onto the MWCNT surface, thereby degrading the EDL structure. These results indicate that excessive ion pairing homogenize the spatial charge distributions in the EDL, thereby weakening the piezoionic effects. These findings provide atomic insights into the structural evolution of EDLs as a function of ion concentration, and offer critical guidelines for optimizing MWCNT-based energy storage and harvesting devices.