Robust Polymer Electrolytes via Polymerization-Induced Microphase Separation

Abstract

Polymer electrolyte membranes (PEMs) are polymeric matrices that facilitate efficient transport of charged species, and are critical components of various electrochemical devices, such as lithium (Li) ion batteries, flexible organic solar cells, and fuel cells. Irrespective of the application, the outstanding challenge in advancing PEM performance is to maximize the ionic conductivity while simultaneously addressing orthogonal mechanical properties, such as high modulus, toughness, or high-temperature stability. Nanostructured PEMs, such as block polymers selectively incorporating electrolytes into one of the domains, are capable of exhibiting the desired decoupled mechanical and conducting characteristics. This thesis details development of block polymer-based, robust, and high conducting PEMs targeted for specific applications, prepared via a versatile synthetic strategy–termed polymerization-induced microphase separation (PIMS). The PIMS route involves simultaneous growth and in situ crosslinking of the polystyrene (PS) block which kinetically arrests the emerging system in a co-continuous morphology that is locally correlated but does not exhibit long-range order. Chapter 2 discusses development and characterization of nanostructured PIMS PEMs with poly(ethylene oxide) (PEO) domains incorporating a protic ionic liquid. In addition to high proton conductivity, the PEMs exhibit high-temperature mechanical stability furnished by the cross-linked polystyrene (PS) scaffold, which is desirable for high-temperature, anhydrous fuel cell applications. Chapter 3 presents a design of robust PS-b-PEO PEMs based on the PIMS platform, incorporating a Li salt and succinonitrile plasticizer into the PEO domains. A systematic study of the PEM ionic conductivity demonstrated that network defects such as dead ends and isolated domains are rare in PIMS PEMs, and the PEMs manifest conducting nanochannels with long-range continuity. Finally, Chapter 4 details preparation of ready-to-use IL-based reference electrodes with a hydrophobic polymeric matrix. The solvent-free, one-step design capitalized on another virtue of the PIMS strategy – the ease of processing a liquid reaction mixture, followed by in situ solidification to obtain solid PEMs. Throughout all this work, the goal is to better understand structure-property relationships in nanostructured PEMs in order to optimize the macroscopic performance in terms of conductivity and mechanical properties.