High-temperature proton conducting membranes based on perfluorinated ionomer membrane-ionic liquid composites

Composite membranes that exhibit fast proton transport at elevated temperatures are needed for proton-exchange-membrane fuel cells and other electrochemical devices operating in the 100 to 200 degrees C range. Traditional water-swollen proton conducting membranes such as the Nafion membrane suffer from the volatility of water in this temperature range leading to a subsequent drop in conductivity. Here we demonstrate that perfluorinated ionomer membranes such as the Nafion membrane can be swollen with ionic liquids giving composite free-standing membranes with excellent stability and proton conductivity in this temperature range while retaining the low volatility of the ionic liquid. Ionic conductivities in excess of 0.1 S/cm at 180 degrees C have been demonstrated using the ionic liquid 1-butyl, 3-methyl imidazolium trifluoromethane sulfonate. Comparison between the ionic-liquid-swollen membrane and the neat liquid itself indicate substantial proton mobility in these composites. (C) 2000 The Electrochemical Society. S0013-4651(99)06-126-1. All rights reserved.