Fast potassium ion transport in sub-1 nm pores of self-assembled polymer membranes derived from linoleic acid

The transport of ionic species in nanoporous polymers is central to the development of membranes for electrochemical devices. Here, we devise a strategy for fabricating highly ordered nanoporous polymers using sustainable materials and study the transport of potassium ions in the uniform, sub-1 nm pores of the system. Self-assembly of conjugated linoleic acid in a glycerol-water mixture yields a direct hexagonal lyotropic mesophase. Photoinduced mesophase crosslinking produces a nanoporous polymer membrane in which the continuous yet structured aqueous domain provides pores within which ion transport can occur. The membrane features a transport limiting dimension, or effective pore size of 0.65 nm, with 0.70 nm separation of negatively charged carboxylate headgroups on the pore walls. Despite the small pores size relative to hydrated K+, we observe K+ conductivities of approximately 1 mS/cm and 10 mS/cm at 50% and 80% relative humidity (RH) at 30°C. The activation energy for K+ transport decreases with hydration from about 45 kJ/mol to 25 kJ/mol as the hydration number increases from ≈2 to 17 on changing relative humidity from 30% to 80%, while the membranes show less than 10% swelling. The activation energy at 80% RH is ≈1.5× that for K+ conduction in bulk electrolyte, suggesting that confinement and ion pairing interactions do not significantly hinder transport. The well-defined pore size may offer opportunities for sustainably-derived membranes for electrochemical devices where size-based restriction of species crossover is required alongside appreciable conductivities.