Thermal stability and gas absorption characteristics of ionic liquid-based solid polymer electrolytes

Ionic liquid (IL)-based solid polymer electrolytes (SPE) with stable thermal properties and low electrical resistivity have been evaluated. Two candidates for the polymer component of the SPE, poly(ethylene glycol) diacrylate (PEGDA) and Nafion, were considered. Differential scanning calorimetry analysis and electrical resistivity tests revealed that PEGDA, in comparison to Nafion, enables the formation of uniform SPEs with lower electrical resistivity and better thermal stability within a range of 25 °C-170 °C. Therefore, PEDGA was selected for further evaluation of the IL component effect on the resulting SPE. Six IL candidates, including 1-butyl-3-methylimidazolium methanesulfonate ± methanesulfonic acid (BMIM.MS ± MSA), diethylmethylammonium triflate ±bis(trifluoromethanesulfonyl)imine (Dema.OTF±HTFSI), and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ± bis(trifluoromethanesulfonyl)imine (BMIM.TFSI ± HTFSI), were selected to test the effect of hydrophobicity/hydrophilicity of the IL on the resulting SPE. Fourier transformation infrared spectrometer analysis revealed that the BMIM.MSA-based electrolytes have the highest tendency to absorb from the environment and keep the moisture, while Dema.OTF has the fastest curing time. The SPE candidates were further evaluated for absorption characteristics of different gasses and vapors, such as N₂, O₂, ethanol vapor, and diluted CO/N₂, that were tested with the in situ quartz crystal microbalance (QCM) technique. Among all six candidates, BMIM.MS showed the largest N₂ and O₂ absorption capacity from the environment. Dema.OTF + HTFSI, meanwhile, demonstrated a higher level of interactions with the ethanol vapor. In the case of CO/N₂, QCM analysis revealed that BMIM.MS+MSA has the largest, ∼13 µg/cm², absorption capacity that is reached within 400 s of being exposed to the gas mixture.