Systematic design of chemically stable, high-performance anion exchange membranes for fuel cell applications

Abstract

Over the years, increased attention in the use of alternative energy conversion devices has resulted in order to reduce the ever-increasing consumption of fossil fuels. Fuel cells are an excellent option among alternative energy technologies because they have the ability to convert chemical energy stored in fuels directly into electrical energy without the emission of polluting chemicals. In particular, anion exchange membrane (AEM) fuel cells offer significant advantages over other types of fuel cells because they can mitigate fuel leakages, device failures, and most importantly reduce production costs as they can utilize non-platinum-based metal catalysts.

The results from these extensive structure-property relationship studies have revealed that both polymer backbone and functional group architectures are crucial in yielding AEMs with high alkaline stability. It was concluded that the following key structural parameters are needed for chemically stable and high performance polymer backbones: (i) polymers without aryl-ether bonds, (ii) block copolymer structures, and (iii) flexible, elastomeric polymer systems. For improving the chemical stability and ion aggregation of the quaternary ammonium functional groups the following structural parameters are needed: (i) small cation size, such as the –N(CH3)3 trimethylammonium and (ii) a flexible, alkyl-tethered cation. Combination of these parameters has facilitated the design of chemically stable, high-performance anion exchange membranes for fuel cell applications.

However, the most significant challenges currently preventing the advancement of AEM fuel cells are poor chemical and mechanical stabilities of anion-conducting polymer membranes under strong alkaline environment and low hydroxide ion conductivities. It was therefore the intention of this dissertation to identify key structural parameters needed for improving long-term chemical stability of AEMs. Identification of these parameters was accomplished by a three-step approach: (1) systematic stability studies of small molecule cation functional groups, (2) optimization of synthetic procedures to functionalize aromatic polymers, and (3) systematic polymer backbone stability studies.