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One of the important challenges in designing robust alkaline anion exchange membranes is the difficulty associated with the chemical stability of covalently bound cationic units. Here, a systematic study exploring alkaline stabilities of polymerizable hexyltrimethylammonium cations is presented, where the hexyl chain is linked to a phenyl ring through a direct carbon‐carbon, phenyl ether, or benzyl ether functionality. For this work, small molecule model compounds, styrenic monomer analogs, and their homopolymers are synthesized. Alkaline stabilities of the small molecule cations and their homopolymers are compared to alkaline stability of benzyltrimethylammonium (BTMA) cation and its homopolymer poly(BTMA), respectively. All the hexyl‐tethered cations and their homopolymers are significantly more stable under strongly alkaline conditions (2 m KOD at 80 °C). Moreover, ether‐linked cations show comparable stability to the direct carbon‐carbon linked cation. Via 1H NMR analyses, possible degradation mechanisms are investigated for each small molecule cation. Findings of this study strongly suggest that the alkaline stability is dictated by the steric hindrance around the β‐hydrogen. This study expands beyond the limits of general knowledge on alkaline stability of alkyl‐tethered ammonium cations via the Hofmann elimination route, highlights important design parameters for stable ammonium cations, and demonstrates accessible directly polymerizable alkaline stable ammonium cations. [ABSTRACT FROM AUTHOR]

Ionic conductivity in a polymeric fuel cell requires water uptake. Previous theoretical studies of water uptake used idealized parameters. We report a parameter-free prediction of the water-swelling behavior of a model fuel cell membrane. The model polymers, poly(methyl-butylene)-block-poly(vinylbenzyl-trimethylamine), form lamellar domains that absorb water in humid air. We use the Scheutjens-Fleer methodology to predict the resulting change in lamellar structure and compare with x-ray scattering. The results suggest locally uniform water distributions. However, under conditions where a PVBTMA and water mixture phase-separate, the two phases arrange into stripes with a dilute stripe sandwiched between two concentrated stripes. A small amount of water enhances conductivity most when it is partitioned into such channels, improving fuel-cell performance. [ABSTRACT FROM AUTHOR]

The effect of anion exchange ionomer (AEI) chemistry on the kinetics of the oxygen evolution reaction (OER) was systematically studied in both half-cell and single-cell electrolysis experiments. OER was studied in 1 M K 2 CO 3 at 50 °C using an array of ionomer-silver catalyst inks deposited on Ni foam. Different ionomer modifications were investigated to optimize the OER performance. The AEI used in this work features a block copolymer backbone of polychloromethylstyrene- b -polycyclooctene- b -polychloromethylstyrene which was functionalized with either benzyltrimethylammonium (TMA) or benzylmethylpiperidinium (MPRD) quaternary ammonium cations. Using an MPRD quaternary ammonium cation shifted the rate determining step at high overpotentials, ultimately providing enhanced performance. Increasing the catalyst ink dilution was also found to substantially improve mass activity and catalyst utilization (increase from 35 to 88 A g−1 Ag), likely by reducing the ionomer thickness and decreasing transport resistances. In the final ionomer modification, the Ni foam substrate was leveraged to partially hydrogenate the polycyclooctene midblock to be polyethylene-like at the Ni interface. A significant increase in the electrochemical surface area (C dl increased from 8.8 to 20.8 mF cm−2) and performance (by 47 mA cm−2 or 29 A g−1 Ag) was observed with the incorporation of polyethylene. Kinetic results from half-cell experiments were validated via single-cell anion exchange membrane electrolysis experiments, where the optimized electrode displayed enhanced performance in both 1 M K 2 CO 3 and DI H 2 O. [ABSTRACT FROM AUTHOR]

Development of improved durable, conductive, and selective anion exchange membranes (AEMs) are critical for electrodialysis. A series of three ABA triblock copolymers derived from polychloromethylstyrene- b -polyethylene- b -polychloromethylstyrene (PCMS- b -PE- b -PCMS), designated as large, 0.8:1, medium, 0.2:1, and small 0.1:1, block AEMs by their PCMS:PE block size were quaternized with methylpiperidine. The water uptake decreases from 100% for the large block AEM to 5% for the small block AEM. At this low water content in liquid water, the small block AEM shows complete ion dissociation, with similar diffusion (2.6 × 10−6 mol m−2 s−1) and E a (24 kJ/mol) as the larger block AEMs. However, a 34% increase in apparent permselectivity was achieved with the small block AEM. In electrodialysis, the small block AEM improved salt removal by 300% compared with a commercial AEM. Under further investigation, migration was shown to be enhanced by 600% in the small block AEM. The small block AEM has a λ = 5 in liquid water, which is too low to explain the chloride conductivity behavior. Furthermore, the permselectivity in the small block AEM is much higher than expected from its small-angle X-ray scattering morphology, indicating there must be a water-rich region of smaller dimensions where cation/anion pairs are fully solvated. [Display omitted] • A triblock copolymer AEM was developed with decreasing hydrophilic block lengths. • Small block has low water uptake and charge density, but full ion dissociation. • Small block achieves enhanced permselectivity and migrative flux. • Small block improves electrodialysis by nearly 300x compared to large block. [ABSTRACT FROM AUTHOR]