Nafion membranes reinforced by cellulose nanocrystals for fuel cell applications: aspect ratio and heat treatment effects on physical properties

This study explored the improvement of the physicochemical properties (mechanical resistance, water uptake, swelling, etc.) of Nafion membranes by cellulose nanocrystals (CNCs). These composite membranes were prepared from Ramie and Tunicate nanocrystals with respective aspect ratios of about 28 and 106. It was demonstrated that, regardless of the type of nanocrystals, increasing the cellulose weight content from 0 to 10 wt% increased the water uptake and the thickness swelling and decreased the in-plane swelling of the composite membranes during water immersion. The mechanical performances of the composite membranes (tensile strength, elongation at break and Young’s modulus) were also enhanced, with the best compromise for Tunicate nanocrystals found to be 5 wt%. The effect of thermal annealing up to 150 °C on the Nafion-Tunicate 5 wt% composite was tested and compared to that of pure Nafion. With thermal annealing, a small decrease in water uptake capacity, protonic conductivity, ion exchange capacity and hydration number was observed for both membranes. At the sub-molecular level, FTIR data suggest that the heat treatment of Nafion-Tunicate membranes induces cross-linking reactions between sulfonic groups of Nafion chains and surface functional groups of CNCs, leading for example to sulfonic ester links. The exothermic peak observed by DSC can assign an increase of the crystalline phase of Nafion chains and especially in the vicinity of CNCs. All cross-linkages led to an improvement in the mechanical resistance of the membranes when thermal annealing was below 130 °C.