Nonhumidified Intermediate Temperature Fuel Cells Using Protic Ionic Liquids

Abstract

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.

Checkerboard Self-Patterning of an Ionic Liquid Film on Mercury

“…Their favourable chemical properties, such as [1,2] nonvolatility, wide electrochemical window, high ion mobility, etc. renders them outstanding "green" replacements for volatile, corrosive and polluting solvents, reaction media, and working fluids in the chemical industries [1], and in energy-and electronics-related applications in batteries, solar and fuel cells [3], and MEMSs [4].…”

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

“…64 The maximum current density and maximum power density of these fuel cells reach 400 mA cm ¹2 and 150 mW cm ¹2 , respectively, at 120°C under non-humidified conditions. 45,65 The following reactions may occur in the fuel cell: 64 because there are no proton vacant sites due to the complete proton transfer; however, the situation is different when the fuel cell reactions proceed at the electrode/electrolyte interfaces. At the cathode, the free base is continuously generated and is shuttled to the anode to be protonated, regenerating the cation.…”

“…6). 45,60,[63][64][65] The obtained polymer electrolyte membranes are mechanically reliable even at a high doping of [dema][TfO], making it possible to construct polymer electrolyte fuel cells that can operate under non-humidified conditions and at temperatures greater than 100°C. 64 The maximum current density and maximum power density of these fuel cells reach 400 mA cm ¹2 and 150 mW cm ¹2 , respectively, at 120°C under non-humidified conditions.…”

“…45,60,[63][64][65] The obtained polymer electrolyte membranes are mechanically reliable even at a high doping of [dema][TfO], making it possible to construct polymer electrolyte fuel cells that can operate under non-humidified conditions and at temperatures greater than 100°C. 64 The maximum current density and maximum power density of these fuel cells reach 400 mA cm ¹2 and 150 mW cm ¹2 , respectively, at 120°C under non-humidified conditions. 45,65 The following reactions may occur in the fuel cell: 64 because there are no proton vacant sites due to the complete proton transfer; however, the situation is different when the fuel cell reactions proceed at the electrode/electrolyte interfaces.…”

“…However, proton exchange reactions between BH + and B by Grotthuss transport appear to proceed, and we have succeeded in obtaining evidence for this. 64 …”