Proton-conducting properties of a bronsted acid-base ionic liquid and ionic melts consisting of bis(trifluoromethanesulfonyl)imide and benzimidazole for fuel cell electrolytes

A novel protic ionic liquid and ionic melts consisting of a Bronsted 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, H-1-NMR chemical shifts, Raman spectra, H-1 and F-19 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 degrees 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 degrees C, where BIm and HBIm(+) were indistinguishable by IH-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 H-2 oxidation and O-2 reduction at a Pt electrode. The neutral salt was hydrophobic and stable for the electrode reactions of H-2 oxidation and O-2 reduction even in the presence of water at 150 degrees C. The neutral and base-rich BIm-HTFSI melts can serve as H-2/O-2 fuel cell electrolytes under entirely nonhumid conditions and at temperatures higher than 100 degrees 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 Blm-HTFSI melts as fuel cell electrolytes.