The Effect of Na+ Impurities on the Conductivity and Water Uptake of Nafion 115 Polymer Electrolyte Fuel Cell Membranes

Water uptake and ionic conductivities are reported for Nafion 115 membranes as functions of water activity and percentage of sulfonic groups occupied by sodium impurities. Water content was determined gravimetrically under liquid hydration and at 100, 75.3, and 11.3% relative humidity (RH). Water content exponentially decreased from the H+-form membrane water uptake isotherm to the Na+-form isotherm when hydrated by water vapor. Ninety percent of this decrease is reached at a substitution level of 0.2Na(+)/SO3-. Water uptake under liquid water hydration decreased more gradually, only 50% to completion at 0.2Na(+)/SO3-. Four-probe conductivity testing of Nafion 115 membranes, normalized against dry dimensions, revealed that although hydration decreases immediately with the introduction of sodium impurities, ionic conductivity at 100% RH remains constant up to 0.15Na(+)/SO3-. Above 0.15Na(+)/SO3- an exponential decrease in ionic conductivity is observed with higher sodium content. The dependence of ionic conductivity on water content is also reported for sodium contents of 0, 0.27, 0.62 and 1Na(+)/SO3-. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3479188] All rights reserved.