Terahertz Time-Domain Spectroscopy Insights into Ionic Transport and Hydration Stability of Dual- Functionalised Nafion Membranes
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Dual-functionalised Nafion membranes, obtained via glutaraldehyde crosslinking followed by post-sulfonation (N-GS), were investigated to enhance ionic transport, hydration stability, and chemical durability, which are relevant to polymer electrolyte membrane systems. A combination of FTIR and FT-Raman spectroscopy, mechanical analysis, accelerated oxidative ageing, and terahertz time-domain spectroscopy (THz-TDS) was employed to establish quantitative structure–property relationships governing proton conduction and water-mediated transport. FTIR revealed enhanced hydrophilic character and increased density of proton-exchange sites in N-GS membranes, as evidenced by strengthened O-H (0.1140) and SO 3 - (0.1265) band intensities relative to pristine Nafion. The modified membranes exhibited increased water uptake (28.7%), stable long-term retention (3.6%),moderate swelling (14.6%), and consistent porosity (3.9-5.8% after 16 h), indicating improved hydration control. Mechanical testing demonstrated a balanced combination of stiffness (Young’s modulus ≈81.5 MPa) and toughness (≈21.8 MJ/m 3 ), while peroxide-assisted ageing in 30% H 2 O 2 confirmed enhanced oxidative resistance, with a limited thickness loss of 1.94%. THz-TDS measurements provided direct insight into hydration-state dynamics, yielding an average absorption coefficient (α avg ≈100.1 cm -1 ) and indicating a 1.19% improvement in water retention behaviour compared to unmodified Nafion. These terahertz-derived hydration descriptors were incorporated into an electrochemical transport model, demonstrating that conductivity losses associated with dehydration could be mitigated and, under optimised conditions, converted into performance gains across different membrane thicknesses. The results highlight THz-TDS as a sensitive, operando-capable technique for probing dynamic water-ion interactions in proton-conducting membranes. The dual-functionalisation strategy, combined with terahertz-guided analysis, provides a robust framework for designing ionically efficient and chemically durable Nafion-based membranes for electrochemical energy applications.
