Polycyclic norbornene based anion-exchange membranes with high ionic conductivity and chemical stability

Abstract

Polycyclic norbornene derivatives, dicyclopentadiene (DCPD) and tricyclopentadiene (TCPD), have rigid structures with ease of accessibility and high chemical reactivity. After vinylic-addition polymerization, the resultant polynorbornenes (PNBs) possess high thermal stability, excellent mechanical integrity, and chemical inertness due to the retention of the rigid polycyclic norbornene motifs along the polymer backbones. Herein, we first report polycyclic PNBs composing DCPD or TCPD and an alkyl-bromide-substituted norbornene as comonomers to prepare high performance anion-exchange membranes (AEMs). The two double bonds in DCPD/TCPD can be used for vinylic-addition polymerization and thiol-ene click cross-linking reactions, respectively. By comparing with previously reported cross-linked PNB AEMs without polycyclic structures, it is unambiguously confirmed that the involvement of DCPD/TCPD significantly improved the mechanical strength (the stress-at-break of 34 MPa vs 28 MPa), the hydroxide conductivity (212 mS cm⁻¹ vs 199 mS cm⁻¹ at 90 °C), and the alkaline stability (93 % vs 81 % conductivity remaining after treating in 1 M KOH at 80 °C for 1200 h). The successful application of the AEMs with excellent durability for over 500 h prove that the involvement of polycyclic structures is a convenient strategy to enhance the performance of aromatic-free AEMs.