(Invited) Poly(norbornene) Hydroxide Conductive Polymers: Stable, High Conductivity Hydroxide Conducting Block Copolymer Membranes and Their Applications

Abstract

Block copolymer, anion-exchange membranes (AEM) were synthesized using the vinyl-addition polymerization and ring-opening metathesis polymerization of norbornene monomers. The intended use is for use electrochemical devices operating at high pH including fuel cells, electrolyzers, and flow batteries. The focus of this talk is on the impact very high conductivity AEM with low water uptake on device performance. The low molecular weight of the norbornene monomers enabled the design of very high ion exchange capacity (IEC) membranes, up to 4.73 meq/g. A wide variety of norbornene monomers are available allowing for the synthesis of membranes with optimized properties. The norbornene backbone is an all-hydrocarbon structure giving the polymers excellent chemical stability in 1 M NaOH at 80°C. The monomers were synthesized into a multi-block copolymer with hydrophilic and hydrophobic blocks. AEM membranes were solvent cast with and without a supporting polymer reinforcement. The hydrophobic, ion-conducting block of the copolymers formed a lamellar phase during casting. Tetrablock copolymers synthesized by vinyl-addition reaction had an IEC of 1.88 meq/g and conductivity of 123 mS/cm at 80°C. Light cross-linking was found to provide a critical control of water uptake allowing the incorporation of very high IEC without unwanted water uptake. Without light cross-linking, unwanted water uptake would cause swelling and softening of the membrane at high IEC values, as with other AEMs. Thus, the path to achieving exceptionally high ionic conductivity includes high mobility ion channels (achieved here through the use of block copolymers), and light crosslinking to permit the use of high IEC values (e.g. approaching 4 meq/g). The best performing membrane had an ionic conductivity of 212 mS/cm at 80°C. There was little or no drop in ionic conductivity over 1,000 h of aging in 1 M NaOH at 80°C. This is the highest reported hydroxide conductivity at 80°C to date for a chemically stable AEM. The number of bound and free water molecules per ion pair were measured in order to analyze the role of water. The membranes are suitable for electrochemical devices and were used in AEM fuel cells and electrolyzers.