Alkaline-stable anion exchange membranes: A review of synthetic approaches

In honor of the 100th anniversary of Staudinger's "Polymer Hypothesis", we write this trend review to describe some key developments in the synthesis of cationic polymers. Cationic polymers provided early evidence in support of the Polymer Hypothesis and have been recognized for their tremendous potential in various sub-disciplines of chemistry and biology. This article is focused on a specific class of cationic polymers that conduct hydroxide anions, namely alkaline anion exchange membranes (AAEMs). These polymers are essential components in anion exchange membrane fuel cell (AEMFC) energy conversion devices that can operate with zero-emission fuels (H2 and O2) and do not require precious metals (e.g. platinum) as electrocatalysts for oxygen reduction. One of the critical considerations for AAEMs is their long-term stability to alkaline media, and as such, a wide range of polymer backbones and appended cations have been evaluated as potential constructs for this purpose. Herein, an overview of the different classes of cations is presented, along with a description of the typical synthetic methods for the preparation of AAEMs. By organizing the review according to the synthetic strategies, we hope to inspire the design and preparation of new cationic materials that are suitable for AEMFCs.