Engineering ionic conductivity of ionomeric membranes: influence of Van der Waals volume of counterions and temperature

Ionomeric membranes are essential constituents of many ionic devices including fuel cells, lithium-ion polymer batteries, and ionomeric polymer transducers. Understanding the dynamics of ionic conductivity of ionomeric membranes is significant in better understanding and optimizing ionic devices. In this work, effects of temperature and Van der Waals volume of Nafion counterions on ionic conductivity of Nafion ionomeric membrane are studied. Ionic conductivity of lithium (Li⁺), potassium (K⁺), and 1-Ethyl-3-methylimidazolium (EMI⁺) ion-exchanged Nafion membranes at temperatures varying from 30 °C–70 °C was investigated and a direct correlation between ionic conductivity and both temperature and Van der Waals volume of counter ions was observed. Ionic conductivity as a function of temperature (for all three counterions) exhibited two distinct slopes below and above ∼50 °C which is an indication of changes in activation energy and confirms previously claims of ionic decomposition of EMI-Tf ionic liquid around ∼50 °C. (paper)