Transport properties of anion exchange membranes for fuel cell applications

Abstract

Low temperature fuel cells are emerging technology for efficient energy conversions. Polymer membrane electrolytes used in the fuel cell devices should be highly conductive, chemically and mechanically stable under extreme operating conditions of temperature and humidity, and cost effective. Although proton exchange membranes (PEM), such as Nafion® are highly conductive and chemically and mechanically stable, PEMs are not widely used in fuel cells due to the higher cost of fluoropolymers and the platinum metal catalysts. Anion exchange membranes (AEM) offer lower cost of both polymers and metal catalysts used in fuel cells. Unfortunately, AEMs are still in the earlier phase of development and demand in-depth understanding of transport, chemical, and mechanical properties. The goal of this thesis is to understand ions and water transport, morphology, chemical, and mechanical properties of AEMs under fuel cell-operating conditions. A unique protocol was developed to measure pure OH- conductivity with a complete CO2 exclusion as functions of both humidity and temperature. Grafted and diblock AEMs with hydrocarbon backbones and benzyl trimethyl ammonium functional groups showed sufficient alkaline stability at 80 °C. Both of these membranes were highly conductive (>100 mS.cm-1 at 95%RH and 60 °C) in alkaline forms. After processing of the diblock AEM by solvent cast and melt pressing technique, the alkaline conductivity performance was improved significantly (75 ± 13 mS.cm-1 at room temperature in water) and was comparable to the proton conductivity in Nafion®. Environmental control systems for FTIR, small angle x-ray scattering, and Sentmanat Extensional Rheometer (SER) were developed to understand the effect of water in morphology, chemical and mechanical properties. Using the control system in FTIR, heterogeneous water distributions in AEMs were discovered, polymer-water interactions were detected, and water and CO2 uptake kinetics was explored.