Providing Molecular Insight for Understanding Anion Exchange Membrane Conductivity

Anion exchange membranes (AEMs) are notorious for having both low alkaline stability and poor ion conductivity in fuel cell operation conditions, with solutions to these two challenges often being developed independent of each other. The chemical instability of an AEM is viewed through degradation of the polymer backbone and the cationic species and improving a material’s stability is approached by altering the polymer backbone, the cation, or both. On the other hand, poor ion conductivity is typically addressed by modifying bulk membrane properties such as increasing the ion exchange capacity (IEC), changing the morphology, or increasing the water uptake. These modifications are most often accomplished by altering the polymers incorporated, the architecture of the polymers, and increasing the number of cations in the network. However, as a deeper understanding of these challenges is gained, the connection between the solutions to alkaline stability and ion conductivity has become clearer. Both the cation’s identity and the polymer backbone incorporated into the membrane influence properties of the AEM, such as chemical stability, morphology, and water uptake, which results in differences in ion conduction. Therefore, developing parameters that can decouple the cation’s and polymer’s impact on AEM properties from their impact on AEM ion conduction is critical to understanding and developing highly conductive AEMs. Here, the impact of cation identity on AEM conductivity was explored using metal cation-based AEMs. Ring-opening metathesis polymerization (ROMP) was performed on di-norbornene functionalized, bis(terpyridine) metal complexes in the presence of norbornene, dicyclopentadiene (DCPD) and a di-norbornene functionalized poly(ethylene oxide) (PEO) crosslinker to control the crosslink density and IEC of these materials. Six different metal cations were studied (ruthenium, nickel, cobalt, iron, manganese, and zinc) along with four different counterions (chloride, bicarbonate, hydroxide, and acetate). Bicarbonate and hydroxide counterions were only utilized to explore the impact of the counterion on the water uptake of these metal cation-based AEMs, while chloride and acetate ions were used to explore the impact of counterion identity on ion conduction. Interestingly, changing the identity of the metal center resulted in minimal changes to AEM properties such as mechanical stability and water uptake, while significant differences in chemical stability and ion conduction were observed. Commonly used bulk-level parameters, such as the hydration number, ion concentration, activation energy, and ion diffusion, offered important insights into AEM ion conduction, but they ultimately failed to fully capture the ion conduction phenomenon. Therefore, isothermal titration calorimetry (ITC) was used to quantify the dissociation thermodynamics for analogous small molecule metal cations through a counterion exchange reaction from either chloride or acetate to bicarbonate ions. This characterization technique showed that the enthalpic response to counterion exchange (ΔHtot) corresponded to a molecular-level parameter termed the cation-counterion association strength (CCAS), where a smaller endothermic ΔHtot corresponds to a weaker CCAS. That ΔHtot value can then be used to understand and predict AEM ion conduction since a weaker CCAS results in better ion conduction through a larger dissociation of counterions. The CCAS for different cation-counterion pairs is controlled by the degree of ion hydration, where the electrostatic interaction between the cation and water molecule stabilizes the charge, allowing for more facile dissociation of the counterion as the ion hydration increases. This ITC approach was then expanded to characterize organic, nitrogen-based cations. A series of quaternary ammonium-based cations demonstrated that having a long alkyl chain spacer (10 atoms or more) between an aromatic group and the cation can weaken the CCAS. These parameters shed light on the importance of understanding the various facets of ion conduction and represent a significant step towards elucidating ion conduction trends for different cations through molecular-level interactions