Proton-Conducting Properties of a Brønsted Acid−Base Ionic Liquid and Ionic Melts Consisting of Bis(trifluoromethanesulfonyl)imide and Benzimidazole for Fuel Cell Electrolytes

Abstract

A novel protic ionic liquid and ionic melts consisting of a Brønsted acid and base were prepared with the combination of bis(trifluoromethanesulfonyl)imide (HTFSI) and benzimidazole (BIm) at various molar ratios. The thermal properties, ionic conductivities, 1H-NMR chemical shifts, Raman spectra, 1H and 19F self-diffusion coefficients, and electrochemical polarization curves were explored. A mixture at the equivalent molar ratio formed a protic neutral salt, and its thermal stability was higher than 350 °C. The phase diagram of the BIm−HTFSI binary mixtures revealed that stoichiometric complexes other than the neutral salt were found at [BIm]/[HTFSI] = 2/1 and 6/1. In these BIm excess compositions, fast proton exchange reactions between protonated BIm (HBIm+) and free BIm were observed at 140 °C, where BIm and HBIm+ were indistinguishable by 1H-NMR but were distinguishable by Raman spectroscopy. The proton transfer became faster and also the proton transference number increased with increasing BIm mole fraction. The neutral and base-rich BIm−HTFSI melts exhibited electroactivities for H2 oxidation and O2 reduction at a Pt electrode. The neutral salt was hydrophobic and stable for the electrode reactions of H2 oxidation and O2 reduction even in the presence of water at 150 °C. The neutral and base-rich BIm−HTFSI melts can serve as H2/O2 fuel cell electrolytes under entirely nonhumid conditions and at temperatures higher than 100 °C. The polarization curves were compared with those of conventional electrolytes, such as anhydrous H3PO4 and an aqueous H2SO4 solution; these indicate highly favorable characteristics of the BIm−HTFSI melts as fuel cell electrolytes.