Through-Plane Conductivity of Anion Exchange Membranes at Sub-Freezing Temperatures—Hydroxide vs (Bi-)Carbonate Ions

Abstract

Anion exchange membrane fuel cells (AEMFCs) are considered as potential future alternative for proton exchange membrane fuel cells (PEMFCs) due to their potential to not require platinum. However, many properties of alkaline ionomers/membranes are not yet well-characterized. The goal of this study is to evaluate the suitability of current AEMs for application in a wide range of operating conditions, especially at temperatures below the freezing point of water. For this, a method was developed to reversibly convert the counter ion of the cationic group in the membrane electrode assembly (MEA) from (bi-)carbonate to hydroxide and vice versa. Subsequently, the through-plane membrane conductivity in an AEMFC was evaluated by electrochemical impedance spectroscopy at different temperatures (−20°C to 50°C) and water contents, whereby the electrical resistance contribution (contact and through-plane) to the high frequency resistance of the cell was determined in an ex-situ experiment. The results obtained in this study were compared to a standard PEM (Nafion 212) and to a sulfonic acid based membrane with a hydrocarbon backbone. The here acquired conductivity data suggest that the conductivity of the evaluated anion exchange membrane, particularly in its (bi-)carbonate form, would be too low at sub-zero temperature to meet automotive freeze start requirements.