Journal of Materials Chemistry A vol:1 issue:6 pages:2238-2247
Imidazolium methanesulfonate (1) has been studied as a model proton conductor for high temperature polymer electrolyte membrane fuel cells (PEMFCs). It is found that 1 undergoes transformation from crystalline to plastic crystalline and then molten states successively from ambient temperature to 200 °C. The solid–solid phase transition of 1 at 174 °C has been preliminarily verified by differential scanning calorimetry (DSC) and temperature-dependent X-ray diffraction (XRD). At the melting point of 188 °C, 1 displays a low entropy of fusion of around 24 J mol−1 K−1. In particular, a high ionic conductivity of 1.0 × 10−2 S cm−1 is reached at 185 °C in the plastic phase. The activation energy for ionic conduction decreases as 1 is heated from the crystal phase to the melt phase. In the molten state, the contribution of protons to the ionic conductivity of 1 was corroborated electrochemically. In addition, 1 is electrochemically active for H2 oxidation and O2 reduction at a Pt electrode while it shows a high electrochemical window of 2.0 V. Furthermore, a Nafion® membrane has been successfully doped with 1, as identified by infrared spectroscopy, powder XRD, grazing incidence XRD and thermogravimetric analysis. To the best of our knowledge, this may be the first report on a protic organic ionic plastic crystal (OIPC) consisting of protonated imidazole (C3H5N2+) and an organic anion. The good thermal stability, high ionic conductivity, wide electrochemical window, favorable plastic crystal behavior and simple synthesis make 1 a highly interesting model proton conductor for high temperature PEMFCs.