Abstract
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°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°C have been demonstrated using the ionic liquid 1-butyl, 3-methyl imidazolium trifluoromethane sulfonate. Comparisons between the ionic‐liquid‐swollen membrane and the neat liquid itself indicate substantial proton mobility in these composites. © 2000 The Electrochemical Society. All rights reserved.