Understanding the degradation mechanisms of organic cations under basic conditions is extremely important for the development of durable alkaline energy conversion devices. Cations are key functional groups in alkaline anion exchange membranes (AAEMs), and AAEMs are critical components to conduct hydroxide anions in alkaline fuel cells. Previously, we have established a standard protocol to evaluate cation alkaline stability within KOH/CD 3 OH solution at 80 °C. Herein, we are using the protocol to compare 26 model compounds, including benzylammonium, tetraalkylammonium, spirocyclicammonium, imidazolium, benzimidazolium, triazolium, pyridinium, guanidinium, and phosphonium cations. The goal is not only to evaluate their degradation rate, but also to identify their degradation pathways and lead to the advancement of cations with improved alkaline stabilities.

Herein, a simple synthetic method to functionalize norbornene at the 5-position with a base-stable tetraaminophosphonium cation was developed. Upon confirmation that the cationic monomer could be polymerized in a living fashion, statistical and diblock copolymers were synthesized with norbornene as a comonomer. After hydrogenation, the statistical copolymers were effective anion-exchange materials, while the diblock did not produce a free-standing film. Differential scanning calorimetry and atomic force microscopy indicated that crystallinity was mostly suppressed by the bulky phosphonium cations in the statistical copolymers, with the impact somewhat dependent on the amino groups bound to the phosphorus atom. Small-angle X-ray scattering profiles revealed a two-phase morphology with 3 nm domains arising from ion clustering in the film. Altogether, the study revealed the large impact these novel phosphonium cations have on polymer organization and packing, which is a critical consideration when targeting larger delocalized cations in anion-transport materials.

Block copolymers have shown promise in ion-exchange membranes as they can phase separate into well-defined nanostructures which promote transport. Herein, a systematic study of multiblock copolymers containing cationic blocks is presented (diblock up to pentablock), and these were contrasted against a statistical copolymer. A series of vinyl addition polynorbornene anion-exchange membranes were prepared by copolymerization of 5-n-hexyl-2-norbornene and 5-(4-bromobutyl)-2-norbornene, followed by conversion of the halide to a trimethylammonium group. The hydroxide conductivities of all synthesized block copolymers were higher than the statistical copolymer, with the tetra and pentablock copolymers being the most conductive. The higher conductivity of the multiblocks is likely a combination of the increased surface-to-volume ratio (smaller domain sizes) improving the connectedness of ionic domains. Water uptake of the block copolymers was also dependent on the number and order of blocks. Copolymers with ionic blocks at one chain end took up more water than those where the ionic segments were confined to the chain interior. Finally, a method was developed to attach alkaline-stable tetraaminophosphonium cations to the bromo-functionalized statistical and pentablock polynorbornene. Interestingly, the synthesized phosphonium polymers had double the water uptake of their ammonium counterparts, which was attributed to the larger occupied volume of the phosphonium as compared to the ammonium group.

In this study, we aimed to decrease segmental mobility in ring-opened polynorbornene anion-exchange membranes (AEMs) as a strategy to suppress swelling. This was accomplished by copolymerization of a norbornene-anthracene cycloadduct with cationic tetraaminophosphonium norbornene monomers to afford AEMs with glass transition temperatures above 100 °C. The anthracenyl side groups appended to the poly(vinylene-1,3-cyclopentylene) chain resulted in lower water uptake for the phosphonium-functionalized AEM when compared to an analogue without the arene moiety. Though chemical aging of these AEMs was noted over time, the benefits of rigidifying the polymer backbone were clear. Additionally, we have noted that the choice of dialkylamino groups around the phosphorus center plays a key role in alkaline stability of these phosphonium-based AEMs.

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