Synthesis of Poly(norbornene) Anion Conducting Membranes for Alkaline Electrochemical Devices: Structure-Property Relationships

Abstract

Anion-exchange membranes (AEMs) are used in electrochemical energy storage and conversion devices such as batteries, fuel cells, and electrolyzers. The development of AEMs with long-term alkaline stability and high anion conductivity (i.e. hydroxide, carbonate and bicarbonate) is of current interest. The benefits over proton-exchange membrane (PEM) devices include the use of non-precious catalysts, facile reaction kinetics, and reduced fuel crossover. However, the issues with the low ionic conductivity, poor stability at high pH, and high water uptake are need to be addressed. A series of homopolymers and block copolymers containing non-hydrolysable backbone and long-chain head-group tethers based on vinyl addition and ring-opening metathesis polymerization (ROMP) of norbornenes were synthesized for use in AEM fuel cells. The focus of this talk is on the structure-property relationships of poly(norbornene) anion exchange membranes. Polymers were cross-linked with N,N,N′,N′-Tetramethyl-1,6-hexanediamine (TMHDA) to control the water uptake, and improve the mechanical properties. The structure property relationship of AEMs towards high ionic conductivity was correlated with ion-exchange capacity (IECs), number and size of the blocks, molecular weight, and polydispersity. The membranes had record high conductivity of 212 mS/cm at 80 °C and low water uptake. The long-term alkaline stability showed <1% conductivity loss in over 1200 h at 80 °C. The membranes have been used in anion-exchange membranes fuel cells and electrolyzers.