Nonhumidified Intermediate Temperature Fuel Cells Using Protic Ionic Liquids

In this paper, the characterization of a protic ionic liquid, diethylmethylammonium trifluoromethanesulfonate (demaTfO), as a proton conductor for a fuel cell and the fabrication of a membrane-type fuel cell system using demaTfO under nonhumidified conditions at intermediate temperatures are described in detail. In terms of physicochemical and electrochemical properties, demaTfO 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 (demaNTf(2)) has relatively low HOR and ORR activity, and thus, the OCV is ca. 0.7 V, although demaNTf(2) and demaTfO have an identical cation (dema) and similar thermal and bulk-transport properties. Proton conduction occurs mainly via the vehicle mechanism in demaTfO 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 demaTfO. The composite membranes can be obtained up to a demaTfO 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.