Molecular Mechanism of Ionic Conductivity Enhancement by Heterogeneous Interface in Composite Hydrogel Electrolytes

Enhancing ion transport in polymer hydrogels is essential for the development of hydrogel-based electrochemical devices. Herein, this study investigates the molecular mechanisms by which embedded SiO₂ nanoparticles enhance the ionic conductivity of poly(acrylic acid) (PAA) hydrogels. Upon hydration, the deprotonated PAA chains expand the intermolecular space through electrostatic repulsion. Concurrently, the strong surface energy of SiO₂ drives the formation of solvent-enriched interfacial water channels. These interfacial structures facilitate ion transport via two synergistic effects: 1) Zn²⁺ ions near the nanoparticle interface experience reduced structural constraints from the polymer network, and 2) the hydration shells of interfacial Zn²⁺ ions are partially disturbed and asymmetric, weakening the ion-water binding. These nanoscale alterations reduce both steric hindrance and solvation energy barriers, resulting in enhanced Zn²⁺ mobility within the hydrogel domains. This work provides a mechanistic framework for understanding nanoparticle–hydrogel interactions and offers insights into the design of composite hydrogel electrolytes with enhanced ion-transport performance.