Anion exchange membranes: Current status and moving forward

Abstract

This short review is meant to provide the reader with highlights in anion exchange membrane research, describe current needs in the field, and point out promising directions for future work. Anion exchange membranes (AEMs) provide one possible route to low platinum or platinum-free fuel cells with the potential for facile oxidation of complex fuels beyond hydrogen and methanol. AEMs and related stable cationic polymers also have applications in energy storage and other electrochemical technologies such as water electrolyzers and redox flow batteries. While anion exchange membranes have been known for a long time in water treatment applications, materials for electrochemical technology with robust mechanical properties in thin film format have only recently become more widely available. High hydroxide and bicarbonate anion conductivity have been demonstrated in a range of AEM formats, but intrinsic stability of the polymers and demonstration of long device lifetime remain major roadblocks.Novel approaches to stable materials have focused on new types of cations that employ delocalization and steric shielding of the positive center to mitigate nucleophilic attack by hydroxide. A number of promising polymer backbones and membrane architectures have been identified, but limited device testing and a lack of understanding of the degradation mechanisms in operating devices is slowing progress on engineered systems with alkaline fuel cell technology. Our objective is to spur more research in this area to develop fuel cell systems that approach the costs of inexpensive batteries for large-scale applications. V C 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013Phys. , 51, 1727Phys. -1735 KEYWORDS: anion exchange membrane; cationic polymer; conductivity; fuel cell; hydrophilic polymers; membranes; phase separation; stability Anion exchange membrane fuel cell (AMFC) technology has seen a resurgence with new interest in high pH solid polymer membranes to bring alkaline fuel cells into a membrane format similar to now-common proton exchange membrane fuel cells (PEMFC), Figure 1. Major interest in AMFCs was generated by the early reports of Varcoe and Slade 1-4 and commercial advances in materials and devices have been pursued by Tokuyama Corporation, 5 Daihatsu Motor Company, [6][7][8] and CellEra, among others. There are now a number of research groups in China, Europe, Japan, Canada, and the United States that are investigating AMFC technology with the objective of providing a new platform for innovation in fuel cells that can break the cost, fuel flexibility, and materials constraints posed by acidic perfluorinated polymers and precious metal catalysts, Figure 2.The tremendous advances in PEMFC technology over the last couple of decades in the United States, Japan, and Europe have brought fuel cells to the cusp of commercialization in automotive applications. Despite the lack of major consumer market adoption at this time, acid-based fuel cells have seen commercial use in some stationary a...

Elucidating Performance Limitations in Alkaline-Exchange- Membrane Fuel Cells

“…CO 2 reacts with OH − to produce HCO 3 − ions, converting the membrane from OH − form to HCO 3 − form, thereby increasing the membrane ionic resistance. 13,43,44 In addition, this has an exacerbating impact on the HOR kinetic losses due to insufficient reactant OH − ions (see Eqn. 8).…”

Mechanical Characterization of Anion Exchange Membranes by Extensional Rheology under Controlled Hydration

“…Hydroxide present in the AEM degrades many of the proposed cationic groups and some polymer backbones, making development of chemically stable AEMs difficult. 9,11,12 Additionally, transport of hydroxide in AEMs is inherently slower than protons in PEMs, 13 to compensate, the concentration of ionic groups is often increased in AEMs.…”

“…6 Increasing durability of the polymer membrane is necessary for application in fuel cells, 5,10,11 particularly in AEMs where thinner films are needed. A number of design factors can influence membrane strength and durability.…”

“…Hydroxide present in the AEM degrades many of the proposed cationic groups and some polymer backbones, making development of chemically stable AEMs difficult. 9,11,12 Additionally, transport of hydroxide in AEMs is inherently slower than protons in PEMs, 13 to compensate, the concentration of ionic groups is often increased in AEMs. 8 Increasing ion concentration in AEMs increases water sorption in the polymer and can result in significant dimensional swelling, which can alter the mechanical integrity of the membrane.…”

“…AEM fuel cells benefit from increased kinetics in an alkali media allowing more complex fuels then hydrogen and have the potential to utilize non-platinum catalysts to reduce costs. [7][8][9][10][11] However, a number of challenges must be overcome before AEMs reach the performance and durability necessary for fuel cells and other electrochemical energy conversion devices. Hydroxide present in the AEM degrades many of the proposed cationic groups and some polymer backbones, making development of chemically stable AEMs difficult.…”