Graphene oxide-induced free volume effects in proton conductive chitosan/polyvinyl alcohol (CS/PVA)-based composite electrolytes: a positron annihilation lifetime study

In this study, chitosan/polyvinyl alcohol (CS/PVA)-based composite electrolytes containing ionic liquid (IL) and graphene oxide (GO) as conductivity-enhancing additives were prepared and characterized in detail in terms of free volume (FV) parameters, mechanical strength, and proton conductivity. FV parameters were determined by positron annihilation lifetime spectroscopy (PALS) and correlated with dynamic mechanical analysis (DMA), ionic conductivity, and dielectric measurements. According to PALS analyses, the CS-5 sample (4 wt% GO) exhibited the highest FV fraction (2.45% at 40 °C) with a corresponding ortho-positronium lifetime (τ₃) of 2.12 ns, and FV hole radius (R) of 2.96 nm. This microstructural expansion was directly associated with the maximum proton conductivity of 1.64 × 10⁻3 S/m at 1 MHz, demonstrating the role of GO in facilitating continuous proton pathways. Moreover, CS-5 achieved superior structural reinforcement, with a high-temperature storage modulus of 12.38 MPa, confirming that GO simultaneously enhances both conduction and mechanical stability. The strong correlation between FV fraction and proton conductivity experimentally validates the FV-based conduction theory, offering mechanistic evidence for the design of next-generation polymer electrolytes. These results highlight that tuning FV distribution through GO optimization provides a reliable strategy for advancing solid-state biopolymer electrolytes in energy storage and environmental applications.