Proton Conductivity of Metallo-Supramolecular Polymer Boosted by Lithium Ions

This is a report to reveal the role of alkali metal ions on proton transfer in metallo-supramolecular polymers (MSPs). We successfully synthesized europium(III)-based MSPs with different counter cations (Li+, Na+, and K+) (polyEu-M+, M = Li, Na, and K, respectively) via 1:1 complexation of Eu(III) salt and tetracarboxylic acid-substituted bisterpyridine in the presence of metal hydroxide (MOH, M = Li, Na, and K). We found that the proton conductivity was dramatically changed by the countercation species of the polymer. Among the three polymers, polyEu-Li+ showed the highest proton conductivity (1.54 × 10–2 S cm–1) at 95% relative humidity (RH) and 75 °C and also exhibited the lowest activation energy 0.14 eV at 95% RH. PolyEu-Li+ presented high water uptake (14.58), which was measured by the mass change (%) with respective reference mass, compared with polyEu-Na+ (13.08) and polyEu-K+ (11.50) at 95% RH. The results indicate that effective proton channels enabled by water molecules were formed in the presence of Li+.