Anion exchange membranes (AEMS) have been an active area of research for over a decade. AEMS can be used for fuel cells, redox flow batteries, electrolyzers, and even water desalination membranes. The electrolyte layer is the “heart” of electrochemical cells such as fuel cells, batteries, and because it transports ions from one side of the membrane to the other side while allowing the ion to stay in its the ionic form. AEMs are an attractive electrolyte layer due to their low-cost and ease of manufacturing in comparison with PEM membranes. The fuel cell type that uses AEMs is the AEMFC. The previous version of the AEMFC is the AFC, which used a liquid alkaline electrolyte matrix, and was used in NASA’s Gemini, Apollo, and Space Shuttle space missions. The AEMFC has solid electrolyte layer, similar to PEMFCs. AEMs are usually composed of polymers that are covalently bonded to different types of cations. The choices for cations include quaternary ammonium cations, sulfonium types, imidazoliums, phosphoniums as well as others.
In alkaline fuel cells and electrolyzers, the gaseous fuel get broken into protons and electrons at the catalyst layers. The electrons travel to the external circuit to power the load, and the ions travel through the electrolyte until it reaches the anode to combine with oxygen to form water. The chemical reactions that occur in an AFC/AEMFC are as follows (Figure 1):
Anode: 2H2 (g) + 4(OH)- (aq) → 4H2O (l) + 4e-
Cathode: O2 (g) + 2H2O (l)+ 4e- → 4(OH)-(aq)
Overall: 2H2 (g) + O2 (g) → 2H2O (l)
Figure 1. Chemical Reactions in an AFC or AEMFC.
The transport of ions in electrolyte solutions works the same way as in polymer electrolyte membranes (PEMs). The most popular PEM is Nafion®, which consists of a hydrophobic PTFE backbone with fluoroether sidechains, the ends of which are capped by a sulfonic acid group. The general consensus in the literature is that the hydrophilic sulfonic acid groups phase separate from the hydrophobic backbone, forming water-rich regions.
PEM membranes uptake water from dissociation and solvation of salt moieties. This hydration and swelling results in water-rich regions where ion transport can occur in a similar manner to a dilute solution. The aggregation of these regions results in a hydrophilic network that creates a path for proton transport. Ions are transferred through direct transfer of protons or hydroxide ions between ion headgroups. Therefore, the overall conductivity of a PEM is highly dependent on its water content and relative humidity. The polymer membrane system creates some constraints that are not present in a dilute solution, such as tortuosity of the water-rich regions, Coloumbic interaction between solvated ions and pendant counterions, or even electrostatic condensation of the free ions, all of which can affect ion transport. Thus, the ionic conductivity of a polymer electrolyte material is contingent on several factors, including degree of ionization, relative humidity and morphology.
Ion transport in an electrolyte occurs via (1) diffusion by a concentration gradient of ions, (2) convection from flow of the electrolyte, or (3) migration driven by an electrical potential across the electrolyte. However, these known methods of ion transport do not adequately capture the high mobilities of protons and hydroxide, but they transfer by a structural diffusion process. Protons are known to undergo Grotthus transport (Figure 2) in aqueous solution, wherein proton migration occurs as a result of interconversion between tri-coordinated H3O+(H2O)3 and H5O2+ via hydrogen bond breaking and formation at the second hydration shell. Hydroxide is hyper coordinated by 4.5 water molecules, while protons are hypocoordinated by only 3 water molecules. This can be visualized as protons rapidly “hopping” across a hydrogen-bonded network of water molecules.
Figure 2. Illustration of Grotthus Transport of a Proton.
Even though this is the currently accepted explanation for the hydroxide transport, it does not adequately explain why hydroxide mobility has been empirically found to be only 57% of proton mobility. There have been numerous experiments that have been conducted in order to understand the ion transport mechanisms. The hyper-coordinated OH- complex has been observed in neutron and X-ray scattering experiments, while FTIR spectroscopy data supports the existence of the weak hydrogen bond between the hydrogen on OH- and the oxygen on H2O. However, still other studies report that, although hyper-coordinated OH- is prominent, its population is not as dominant as believed and there exists a substantial fraction of tri-coordinated OH-. There have been other proposed mechanisms and the exact details of hydroxide transport are in debate. While the detailed transport mechanisms are not fully understood, it is generally agreed that water solvation is critical to efficient migration of the hydroxide anion.
There are three major limitations of current AEM materials: (1) hydroxide conductivities, (2) poor chemical stability, and (3) degradation of the electrolyte at elevated temperatures. Since the hydroxide transport is approximately 50% slower than proton transport in water, the cell conductivity is expected to be lower for membranes with the same thickness as PEMs. Many AEMs also exhibit poor chemical stability under alkaline conditions, which results in insufficient operating lifespans for electrochemical devices using AEMs. Fuel cell and electrolyzer efficiencies increase at elevated temperatures, but the degradation of the electrolyte is pronounced at higher temperatures.
Low Ionic Conductivity: As discussed previously, the lower conductivity of AEMs compared to PEMs is because the hydroxide anion has a dilute solution mobility that is only 57% of that of the proton, leading to a fundamental kinetic limitation. Therefore, the charge density of an AEM must be approximately two times greater than that of a comparable PEM material. To increase hydroxide transport, we can increase the number of charged groups in the membrane (ion exchange capacity (IEC)) and increase and maintain the hydration level to create water channels to facilitate hydroxide transport. There is a delicate balance though because larger charge densities result in excessive swelling, charge dilution and diminished mechanical integrity. Hydroxide conductivity of > 100mS/cm is required to achieve competitive power densities and some of the membranes have actually succeeded in reaching this target.
Stability: The stability of AEMs is a primary concern for electrochemical cells because they need to work for at least 5000 hours in order to compete with legacy power technologies. Many AEMs exhibit poor chemical stability due to the alkaline nature of the material – the strong nucleophilic nature of hydroxide anion leads to multiple degradation pathways for both the pendant cation as well as the polymer backbone. Alkaline conditions cause the polymer backbone and cations to degrade with multiple byproducts such as alcohols and tertiary amines. Also, depending upon the chemistry, some of the membranes undergo dehydration reactions that liberate a water molecule and tertiary amine. Also, in many AEM systems, the hydroxide attack of cationic moieties leads to charge neutralization and rapid conductivity loss. Researchers have been working on making AEMs more stable through several methods such as altering chain lengths attached to the quaternary ammonium to form a more stable cation or by creating a less “bulky” molecule so that the hydroxide can move more readily. There are numerous other methods that are currently under development to improve the stability.
Degradation. at High Temperatures: The degradation at high temperatures is due to degradation of the quaternary ammonium groups by the nucleophilic OH- ions through (1) a direct nucleophilic displacement and/or (2) a Hofmann elimination reaction, forming a tertiary amine and methanol. The amount of degradation is dependent upon temperature. The hydroxide conductive form of the membrane can also convert to the carbonate form if it is exposed to 400 ppm of CO2 in air. The conversion of the membrane to the carbonate or bicarbonate form results in less mobility of the OH- groups.
The AEM should ideally have high hydroxide conductivity, high chemical stability, no electronic conductivity, and good mechanical integrity with the appropriate water uptake. To create an improved ion exchange membrane for a particular application, both the transport and stability properties need to be considered simultaneously. AEMs have a lot of potential for future applications due to their low-cost and multiple chemical moieties under development.