Thiol-ene crosslinked solution-processable polynorbornene ionomers for scalable production of anion exchange membranes

State-of-the-art anion exchange membranes (AEMs) used in alkaline water electrolysis, fuel cells and other devices are often cast from the pre-polymer and later converted to the ion conducting form due to the low solubility of the ion conducting polymer in the casting solvent. In this study, the tradeoffs between solution functionalization of the prepolymer, crosslinking, and high membrane conductivity were addressed through a novel method of membrane fabrication of a vinyl-addition polynorbornene (PNB). Random copolymers of vinyl norbornene (VNB) and bromobutyl norbornene (BBNB) were synthesized and subsequently quaternized in solution by reacting the pendant bromo-alkyl functionality with N,N-dimethyldecylamine through the Menshutkin reaction. The resulting pre-quaternized ionomers were solvent cast and crosslinked by UV-initiated thiol-ene click reaction, which was found to be more efficient than diamine crosslinking through the Menshutkin reaction. The ion exchange capacity, ionic conductivity and mobility, and mechanical properties of the solution processable membranes were investigated at different crosslink density by controlling the mole fraction of the VNB in the prepolymer. The optimal copolymer composition had a hydroxide conductivity of 50 mS cm -1 at 20 °C. The solution-quaternized, ion-conducting membranes had 35% higher hydroxide mobility (1.95 × 10 -4 cm 2 V -1 s -1 ) compared to membranes quaternized after membrane casting due to improved phase segregation as indicated by x-ray scattering. This results in higher ion conductivity at all ion exchange capacity values for the solution quaternized membranes. In a water electrolysis cell, the pre-quaternized membrane with moderate crosslinking achieved 2.14 V at 1.5 A cm -2 and maintained stable operation over 500 h at constant current of 1.5 A cm -2 with minimal degradation of 23 μ V h -1 .