Highly Conductive Nonstoichiometric Protic Poly(ionic liquid) Electrolytes

Abstract

Nonstoichiometric protic ionic liquids have recently gained interest because proton acceptors are crucial in the electrolyte of many electrochemical applications such as fuel cells, proton batteries and supercapacitors. In the present work we have prepared and studied several protic poly(ionic liquid)s doped with acid in nonstoichiometric amounts. The poly(ionic liquid)s were based on polydimethylsiloxane backbones functionalized with imidazole, 1,2,4-triazole and benzimidazole groups, respectively, via flexible alkyl spacer units. These polymers did not show any observable glass transition above -50 °C, resulting in high charge mobility and conductivities of up to 19 µS/cm at 25 °C. Additional doping with free imidazole significantly increased the conductivity, to 0.15 mS/cm at 25 °C, more than four orders of magnitude higher than in the nondoped state. The origins of the fast charge transport in these polymer electrolytes were investigated by electrochemical impedance spectroscopy and electrode polarization analysis. The results indicate that these kinds of nonstoichiometric protic poly(ionic liquid)s have potential as high-performance electrolytes for room temperature electrochemical cells, such as organic proton batteries, where the availability of both proton donors and acceptors is important.