Pentablock copolymer morphology dependent transport and its impact upon film swelling, proton conductivity, hydrogen fuel cell operation, vanadium flow battery function, and electroactive actuator performance

Abstract

Ionomer composition and morphology impacts functional group distribution, water and ion-transport, and physical properties related to toughness, and degradation resistance. NEXAR MD9100 a pentablock copolymer (PBC) film morphology was dramatically altered when solution-cast into a film using tetrahydrofuran (THF) versus a cyclohexane: heptane (C:H) mixture. Film property and morphology changes were evaluated using Transmission Electron Microscopy (TEM), Small-Angle X-ray Scattering (SAXS), and electrochemical impedance spectroscopy. These changes were compared to Nafion 117 and Nafion 212. Average sulfonated inter-domain spacing through the film's thickness increased from 22.3nm (C:H cast) to 30.5nm (THF cast) that was estimated using SAXS. TEM revealed that PBC solution-cast films from C:H contained a random distribution of discrete sulfonated domains. An ordered PBC morphology consisting of lamella and hexagonally packed ion groups were created from a THF solution-cast film. These changes were attributed to favorable solvent-ionomer interactions during solvent evaporation and film densification. This ordered morphology led to increased conductivity (4.5mS/cm versus 47.8mS/cm), improved fuel cell power (19mW/cm2 versus 160mW/cm2), enhanced ionomer actuation (3.0cm versus 6.9cm), and modest self-discharge improvements for a vanadium redox-flow battery. This study demonstrates that morphology impacts ionomer physical properties, transport, and device function.