Investigating the Electro-Osmotic Coefficients of PFSA and Anion-Exchange Ionomers Using Microelectrodes

Abstract

The electro-osmotic coefficient of ionomer membranes is a significant water management property that quantifies the number of water molecules dragged with the mobile ion (typically a proton) when that ion moves due to the operation of an electrochemical cell or an electric field. This coefficient becomes critical when attempting to model water distribution and movement in fuel cells and electrolyzers, where water is an important factor influencing device performance. However, there is disagreement on the value of the electro-osmotic coefficient for PFSA membranes and little significant study of the coefficient in anion exchange membranes (AEMs). Here we present an electrochemical, two-electrode method of determining the electro-osmotic coefficient of ionomers using differential relative humidity (RH) measurements. This approach allows for more accurate determination of the electro-osmotic coefficient. We present the electro-osmotic coefficient as a function of temperature and mobile ion for a variety of ionomers, including Nafion, sulfonated polystyrene, Versogen,, and Sustainion. In addition, a model based on the Maxwell-Stefan-Onsager framework is developed for the AEMs, enabling calculation of the membrane water permeability via fitting of the electro-osmotic coefficient. These coefficients will allow for more accurate models of water transport in electrochemical systems, leading to a greater understanding of the inefficiencies in these systems and, hopefully, insight into how to improve them. This work was supported by the HydroGEN Advanced Water Splitting Materials consortium, which is supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office, under contract number DE-AC02-05CH11231.