Water- and Alkali-Stable Cellulose Nanocrystal Films via Synergistic Citric Acid Cross-Linking and Poly(vinyl alcohol) Incorporation for Hydroxide-Conducting Membranes

Cellulose nanocrystals (CNC) are promising reinforcing materials for biopolymer films, but their poor stability in water and alkaline environments limits their application in alkaline energy-conversion systems. In this study, CNC-based composite films (CNC/CP) were fabricated through a simple aqueous suspension-casting method using citric acid (CA) as a green cross-linker and poly(vinyl alcohol) (PVA, 10 wt%) as a secondary polymer. The cross-linking degree was controlled by varying the CA content from 10 to 30 wt% relative to CNC, followed by heat treatment to promote esterification. The combined effect of CA cross-linking and PVA incorporation greatly improved the film performance. The CNC/CP films showed excellent water stability, with low swelling values of ∼6.66% and 20% after 960 h at room temperature and 80 °C. Mechanical performance was also improved, with the optimized CNC/CP film showing a 109.9% increase in tensile strength and a 590.3% increase in toughness compared with the pristine CNC film. The hydroxide-ion conductivities of CNC/CP films reached up to 122.64 mS cm–1 at 25 °C and 157.99 mS cm–1 at 80 °C in 1 M KOH. Moreover, the CNC/CP films showed good alkaline stability, retaining hydroxide-ion conductivities of 29.32–48.25 mS cm–1 after soaking in 1 M KOH for 720 h. These results show that citric acid cross-linking combined with PVA incorporation is an effective strategy to produce stable, mechanically robust, and conductive CNC-based films for further alkaline energy-conversion applications.