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The chemical stability and ion transport properties of quaternized chitosan (QCS)-based anion exchange membranes (AEMs) were explored using Density Functional Theory (DFT) calculations and all-atom molecular dynamics (MD) simulations. DFT calculations of LUMO energies, reaction energies, and activation energies revealed an increasing stability trend among the head groups: propyl trimethyl ammonium chitosan (C) < oxy propyl trimethyl ammonium chitosan (B) < 2-hydroxy propyl trimethyl ammonium chitosan (A) at hydration levels (HLs) of 0 and 3. Subsequently, all-atom MD simulations evaluated the diffusion of hydroxide ions (OH-) through mean square displacement (MSD) versus time curves. The diffusion coefficients of OH- ions for the three types of QCS (A, B, and C) were observed to increase monotonically with HLs ranging from 3 to 15 and temperatures from 298 K to 350 K. Across different HLs and temperatures, the three QCS variants exhibited comparable diffusion coefficients, underlining their effectiveness in vehicular transport of OH- ions. [ABSTRACT FROM AUTHOR]

The development of membranes with rapid and selective ionic transport is imperative for diverse electrochemical energy conversion and storage systems, including fuel cells and flow batteries. However, the practical application of membranes is significantly hindered by their limited conductivity and stability under strong alkaline conditions. Herein, a unique composite membrane decorated with functional Cu2+ cross‐linked chitosan (Cts‐Cu‐M) is reported and their high hydroxide ion conductivity and stability in alkaline flow batteries are demonstrated. The underlying hydroxide ions transport of the membrane through Cu2+ coordinated nano‐confined channels with abundant hydrogen bonding network via Grotthuss (proton hopping) mechanism is proposed. Consequently, the Cts‐Cu‐M membrane achieves high hydroxide ion conductivity with an area resistance of 0.17 Ω cm2 and enables an alkaline zinc‐based flow battery to operate at 320 mA cm−2, along with an energy efficiency of ≈80%. Furthermore, the membrane enables the battery for 200 cycles of long‐cycle stability at a current density of 200 mA cm−2. This study offers an in‐depth understanding of ion transport for the design and preparation of high‐performance membranes for energy storage devices and beyond. [ABSTRACT FROM AUTHOR]

This work studied the polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS) triblock copolymers functionalized by butyl quaternary ammonium (C4Q) groups and alkyl side chains of different chain lengths (Cn, n = 0 to 24). The hydrated membrane morphology was modeled by dissipative particle dynamics simulation at hydration levels from 10 to 30. A hydroxide model was devised to characterize the diffusivity of anions under the coarse-grained framework. In general, the ionomers with alkyl side chains provided ion conductivity of a similar level at a lower ion exchange capacity. All hydrated SEBS–C4Q–Cn ionomers showed clear phase separation of the hydrophobic and hydrophilic domains, featuring 18.6 mS/cm to 36.8 mS/cm ion conductivity. The hydrophilic channels expanded as the water content increased, forming more effective ion conductive pathways. Introducing excess alkyl side chains enhanced the nano-segregation, leading to more ordered structures and longer correlation lengths of the aqueous phase. The membrane morphology was controlled by the length of alkyl side-chains as well as their tethering positions. Ionomers with functionalized side chains tethered on the same block resulted in well-connective water networks and higher conductivities. The detailed structural analysis provides synthesis guidelines to fabricate anion exchange membranes with improved performances. [ABSTRACT FROM AUTHOR]

Anion exchange membrane fuel cells (AEMFCs) offer several advantages over proton exchange membrane fuel cells, such as the use of a non‐precious metal catalyst, but these cells suffer from various issues related to OH–‐conducting electrolytes, including low conductivity and the formation of K2CO3 salt. These issues need to be resolved for the widespread use of AEMFCs. Recently, many studies have focused on developing excellent ion‐conductive electrolytes using porous materials based on metal–organic and covalent organic frameworks. However, most of this research is biased toward proton‐conducting electrolytes; to the best of the authors' knowledge, reviews addressing OH–‐conducting electrolytes using porous materials have not been reported thus far. This review discusses OH–‐conducting porous crystalline materials and their membranes in terms of different synthetic strategies, conduction mechanisms, and experimental modalities for the design and development of future anion conductive electrolytes in fuel cells. [ABSTRACT FROM AUTHOR]

Alkaline water electrolysis (AWE) refers to a representative water electrolysis technology that applies electricity to synthesize hydrogen gas without the production of carbon dioxide. The ideal polymer electrolyte membranes for AWE should be capable of transporting hydroxide ions (OH−) quickly in harsh alkaline environments at increased temperatures. However, there has not yet been any desirable impedance measurement method for estimating hydroxide ions’ conduction behavior across the membranes, since their impedance spectra are significantly affected by connection modes between electrodes and membranes in the test cells and the impedance evaluation environments. Accordingly, the measurement method suitable for obtaining precise hydroxide ion conductivity values through the membranes should be determined. For this purpose, Zirfon®, a state-of-the-art AWE membrane, was adopted as the standard membrane sample to perform the impedance measurement. The impedance spectra were acquired using homemade test cells with different electrode configurations in alkaline environments, and the corresponding hydroxide ion conductivity values were determined based on the electrochemical spectra. Furthermore, a modified four-probe method was found as an optimal measurement method by comparing the conductivity obtained under alkaline conditions.

[Display omitted] • A zero-gap MFC with AEM and no catholyte allowed selective transport of OH– • Facilitating OH– transport using an AEM enabled 8.8 ± 0.5 W m−2. • Limiting anode acidification increased the bioanode current by 584% • CEM and UFM produced one-third of the AEM-MFC power due to transport of Na and Mg. • An AEM avoids cation transport and salt precipitation and increases MFC stability. High current densities have been obtained by bioanodes in electrochemical cells, but this performance has not previously been translated into higher power densities in microbial fuel cells (MFCs). The most critical factor for boosting power generation in MFCs was proven here to be OH– ion transfer from the cathode to the bioanode by comparing performance of an MFC with an anion exchange membrane (AEM), in a near zero-gap configuration and no bulk catholyte, to that obtained using a cation exchange membrane (CEM) or non-ion selective ultrafiltration membrane (UFM). In the AEM-MFC hydroxide ion transport primarily balanced charge transfer between the electrodes, enabling 8.8 ± 0.5 W m−2 (at 42 ± 1 A m−2) which is highest power density ever achieved using this anolyte (acetate in a 50 mM phosphate buffer medium). The power density with the AEM was 3 × higher than that obtained using a CEM (3.1 ± 0.1 W m−2) or UFM (2.4 ± 0.1 W m−2) as these other two membranes allowed cations (sodium and magnesium) to be transported to the cathode for balancing charge. The lack of cation transport into the cathode using the AEM without a bulk catholyte also avoided salt precipitation in the cathode, and thus enabling more stable power production over time. The large differences in power obtained using the AEM, compared to the CEM or UFM conclusively demonstrate the critical role of OH– ion transport in MFCs. [ABSTRACT FROM AUTHOR]