Investigation of Morphology and Conductance of Anionic Block Copolymer Membranes for Anion Exchange Membrane Fuel Cells Using Atomic Force Microscopy

Hydroxide-conducting anion exchange membranes (AEMs) have attracted significant research interest for fuel cell application. Although AEM fuel cells offer improved oxygen reduction reaction kinetics in alkaline media, the mobility of OH^- in particular in the presence of CO₂ is low. Similar to proton exchange membranes (PEMs), AEMs are copolymers with hydrophobic and hydrophilic components that phase separate to form ~10 nm diameter conducting channels. The conductivity of the membrane is critically tied to channel connectivity. We investigate anionic block copolymer electrolyte membranes previously synthesized by Coughlin et al. The polymers self-assemble into well-ordered morphologies, such as hexagonal and lamellar phases. The structural order of these polymers can better elucidate channel connectivity over random copolymer counterparts. We use conductive probe atomic force microscopy to investigate the nanoscale conductance in a closed fluid cell and find the hexagonal phase has improved channel connectivity over lamellar. Despite perpendicularly aligned hexagonal phase in the bulk, we find cylindrical aggregates prefer to align parallel to the surface. We hypothesize that the reorientation of these domains at the surface improves the mechanical properties of the membrane.