In this paper, the characterization of a protic ionic liquid, diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]), as a proton conductor for a fuel cell and the fabrication of a membrane-type fuel cell system using [dema][TfO] under nonhumidified conditions at intermediate temperatures are described in detail. In terms of physicochemical and electrochemical properties, [dema][TfO] exhibits high activity for fuel cell electrode reactions (i.e., the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR)) at a Pt electrode, and the open circuit voltage (OCV) of a liquid fuel cell is 1.03 V at 150 degrees C, as has reported in ref 27. However, diethylmethylammonium bis(trifluoromethane sulfonyl)amide ([dema][NTf(2)]) has relatively low HOR and ORR activity, and thus, the OCV is ca. 0.7 V, although [dema][NTf(2)] and [dema][TfO] have an identical cation ([dema]) and similar thermal and bulk-transport properties. Proton conduction occurs mainly via the vehicle mechanism in [dema][TfO] and the proton transference number (t(+)) is 0.5-0.6. This relatively low t(+) appears to be more disadvantageous for a proton conductor than for other electrolytes such as hydrated sulfonated polymer electrolyte membranes (t(+) = 1.0). However, fast proton-exchange reactions occur between ammonium cations and amines in a model compound. This indicates that the proton-exchange mechanism contributes to the fuel cell system under operation, where deprotonated amines are continuously generated by the cathodic reaction, and that polarization of the cell is avoided. Six-membered sulfonated polyimides in the diethylmethylammonium form exhibit excellent compatibility with [dema][TfO]. The composite membranes can be obtained up to a [dema][TfO] content of 80 wt % and exhibit good thermal stability, high ionic conductivity, and mechanical strength and gas permeation comparable to those of hydrated Nafion. H(2)/O(2) fuel cells prepared using the composite membranes can successfully operate at temperatures from 30 to 140 degrees C under nonhumidified conditions, and a current density of 250 mA cm(-2) is achieved at 120 degrees C. The protic ionic liquid and its composite membrane are a possible candidate for an electrolyte of a H(2)/O(2) fuel cell that operates under nonhumidified conditions.

A simple protic ionic liquid obtained from the combination of diethylmethylamine and trifluoromethanesulfonic acid exhibits the remarkable results as a medium temperature fuel cell electrolyte under non-humidifying conditions, affording a higher and stable open-circuit potential, wide liquid temperature range, and high thermal stability.

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.

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A super-strong acid, bis(trifluoromethanesulfonyl)amide, was combined with 4,4′-trimethylenedipyridine at various molar ratios to prepare a novel series of Brønsted acid–base ionic liquids. The protic neutral salt was electroactive for H2 oxidation and O2 reduction at a platinum electrode under non-humidifying condition, which shows the potential of pyridinal systems as new fuel cell electrolytes for elevated temperature operation and foretells the polymeric model of solid-state anhydrous proton conductors.