Designing conductive polymers using anion-polymerized ionic liquids

Polymeric ionic liquids (poly-ILs) are of substantial interest as polyelectrolyte proton conduction membranes in fuel cells. Polymeric electrolytes limit leakage and reduce risks owing to flammability and toxicity, but conductivity can be lower. Battery and fuel cell polymeric electrolytes are almost exclusively fossil fuel derived and non-biodegradable. However, there is a growing interest in using renewable, sustainable, bio-derived and/or bio-degradable polymers. Wearable electronic devices are also increasingly desirable, they require less current but electrolyte materials need to be flexible, transparent and bio-compatible. We are interested in poly-ILs based on a poly-acrylate anion backbone paired with 'free' imidazolium cations. Herein we undertake a combined experimental and computational study, tuning the chemical structure of the acrylate/imidazolium functionality and varying the synthesis procedure to optimize conductivity. Little is known about the molecular level characteristics or proton conduction mechanism within these poly-ILs. We undertake molecular density functional theory (DFT) calculations to probe the fundamental inter-ion interactions and explore the proton conduction mechanism. The computational insights obtained allow us to rationalize the experimental results and provide insight into key features of the proton conduction mechanism. Overall, the combined experimental and computational study has delivered an increase in conductivity spanning eight orders of magnitude for the poly-ILs studied, from 10-12 to 10-4 S cm-1 at 25°C. This article is part of the discussion meeting issue 'Ionic liquids and the future of soft materials'.