Effect of hydration on the mechanical properties of anion exchange membranes

Abstract

Anion exchange membranes (AEM) are promising solid polymer electrolytes for use in alkali fuel cells and electrochemical conversion devices. The dynamic nature of the fuel cell environment requires that AEMs operate at a range of hydration levels. Water sorption is critical for ion conduction, but excess water uptake causes dimensional swelling and mechanical instability. Ion conduction is slower in AEMs, compared to proton exchange membranes (PEM), making it important to minimize overall transport resistance by reducing membrane thickness; however, maintaining mechanical durability is difficult as thickness is reduced. Achieving an AEM with high conductivity and good mechanical durability is a difficult balance, which was the focus of this thesis. Various polymer chemistries were investigated with respect to ion conduction, morphology, swelling, and mechanical properties as potential AEMs. The success of perfluorosulfonic acid PEMs inspired synthesis of perfluorinated AEMs, but cation functionalization was low, and proved chemically unstable, resulting in poor performance. Random polyiosoprene copolymers with high ion concentration were solution processed into films and subsequently crosslinked to generate solid AEMs. Diblock copolymers were studied due to their ability to phase separate into organized morphologies for efficient ion transport, but polymer chemistry greatly influenced mechanical performance. A polystyrene based diblock resulted in stiff, brittle AEMs with insufficient strength, but a polyethylene based diblock AEM produced large, flexible films. Mechanical performance was investigated by extensional and dynamic mechanical testing. The addition of cation functionalities increased membrane stiffness, leading to brittle films. Water in the membrane acts as a plasticizer increasing elasticity and elongation, but also weakening membranes. Changing polymer chemistry to a polyethylene based diblock and optimizing casting conditions produced large (~300 cm2) area membranes of consistent (10 um) thickness. These membranes were flexible and showed good mechanical performance. Mechanical softening, due to hydration level, was identified by dynamic mechanical analysis. Conductivity measured as a function of humidity suggested increased ion conduction correlated with the hygromechanical softening point. Understanding the relationship between ion conduction and mechanical properties is critical to the development of robust, well-performing AEMs for use in fuel cells and electrochemical devices.