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.

Applying neutron diffraction with isotopic substitution to the structure and proton-transport pathways in protic imidazolium bis{(trifluoromethyl)sulfonyl}imide ionic liquids

“…Such regions of Such regions of the PIL phase space can correspond to the important regions of 'acidity' and 'basicity' in aprotic chlorometallate ionic liquids. 2 Watanabe and coworkers 3 and Angell and co-workers 4 have both made major contributions to the study of PILs formed with combinations of strong acids and bases (empirically with differences in pKa values in excess of [8][9][10], demonstrating that interesting, and useful, ionic liquids can be generated at compositions that do not have notional 1:1 acid:base stoichiometry. either additional H[NTf 2 ] or imidazole, two extended liquid regions with depressed melting points ( Fig.…”

Protic ionic liquids: an alternative proton-conducting electrolyte for high temperature proton exchange membrane fuel cells

“…3), the same as the results of many literature studies. 20,[24][25][26] In order to explain this phenomenon, CV curves of blank Pt/C and Pt/C electrodes dyed with ionic liquids were The other important factor responsible for the cell performance is the loading content of the ionic liquid to the neat matrix. As the matrix, PP-NW has a high loading content of ionic liquid, as mentioned above.…”

Design and Materialization of Ionic Liquids Based on an Understanding of Their Fundamental Properties

“…45 Our initial interest in H + -conducting electrolytes concerned the transport properties of H + in protic ILs. [43][44][45][46][47] At that time, K.-D. Kreuer at the Max Planck Institute for Solid State Research was carrying out pioneering research with non-aqueous proton carriers; [48][49][50][51] his research focused on the autoprotolysis reactions of neutral species such as imidazole and pyrazole, which form proton defects in the systems and accelerate Grotthuss transport. The remarkable changes in conductivity that occur on changing the composition of systems of imidazole or pyrazole and H 2 SO 4 opened a new route to the design of non-aqueous proton conductors.…”

“…50 This 43,44 However, in the base-rich conditions, the thermal stability is disappointingly low due to evaporation of the neutral species. 43,47 The thermal stability and electrochemical activity of protic ILs for fuel cell reactions are greatly affected by the chemical structure of protic ILs. The most important factor affecting the thermal stability of protic ILs is the difference in pK a values ("pK a ) between the constituent acids and the conjugate acids of the constituent bases.…”