Designing Anion Exchange Membranes with Enhanced Hydroxide Ion Conductivity by Mesoscale Simulations

Abstract

This work studied polyphenylene oxide tetramethylammonium (PPO-TMA) anion exchange membrane by dissipative particle dynamics (DPD) simulations. The simulation method is validated by semiquantitatively reproducing the ion conductivity of a standard PPO-TMA and then applied to systematically explore the microstructure and ion diffusivity of modified PPO-TMA influenced by alkyl chain length, side-chain structure, and side-chain distribution. The nanosegregation of hydrophobic and hydrophilic domains is driven by alkyl sidechain modifiers, and the ionic pathways formed in lamellar structure are observed if the side chains are distributed normally as comblike structure. In these systems, ion conductivity has been elevated to 17 mS/cm compared to 11 mS/cm for nonmodified PPO-TMA. The solvation of cationic groups is also crucial for forming effective ion transport pathways. The percolated water domain breaks into smaller clusters if the charged TMA groups are moved toward PPO backbones, indicating that the alkyl spacer modification outperforms the alkyl extender design. By further altering the tethering style of side chains from comblike to block-copolymer-like, the lamellar water percolation transforms into interconnective water framework, which promotes the ion conductivity to 22 mS/cm.

Molecular Modeling in Anion Exchange Membrane Research: A Brief Review of Recent Applications

“…The various head-group-based PPO AEM including trimethylamine, dimethyl butylamine, dimethyl octylamine, dimethyl hexylamine, tripropylamine, and dimethyl methoxy butyl amine hydrated with ion and water molecules were, also, modeled and studied via the implementation of ReaxFF (reactive) and APPLE&P (non-reactive) classical polarizable force fields [ 168 , 169 , 177 ]. The diffusion of ion is improved by forming a water channel under high water content ( Figure 9 ).…”

“…The loss of coordinated water molecules from ion, observed during the vehicular transport mechanism (non-reactive MD) through bottlenecks in the water channel, creates a more significant kinetic barrier for such an event. As was, also, found, the Grotthuss mechanism is essential for understanding -ion diffusion, by water channels in non-blocky polymer-structure-based AEMs, and ions can be transported without damage or loss of the -ion-hydration structure or loss of its coordinated water molecules, with a lower transition barrier (the bottleneck is an easy transportation pathway) [ 36 , 168 , 169 , 177 ].…”

“…A dissipative particle dynamics (DPD) simulation was applied, to study the nanostructure and ion diffusivity of PPO matrix-based AEM, via various process parameters, including alkyl-chain length, side-chain structure, and side-chain distribution, as shown in Figure 10 [ 177 ].…”

Dissipative Particle Dynamics Simulation: A Review on Investigating Mesoscale Properties of Polymer Systems

“…[ 87 ] An anion exchange membrane (AEM) is the core of an AEMFC; however, the requirements for designing membranes with high OH − conductivities are not fully understood. [ 88–91 ] To overcome the low mobility of hydroxide ions (relative to that of protons), extensive efforts have been made to improve the OH − conductivity of AEMs. Zhu et al.…”

Molecular dynamics study on water and hydroxide transfer mechanisms in PSU-g-alkyl-TMA membranes at low hydration: Effect of side chain length

“…In the case of the QA group, the graft (also named side chain) can be tuned by increasing the length of the intermediate alkyl chain (spacer unit) or by substituting one of the methyl group for a longer chain (extender chain) [17,18,37,39,43,45]. Coarse grained MD with a PPO (polyphenylene oxide) backbone show that hydrophilic morphology can be designed by the pattern of the tethering from the lamellar structure to a more interconnected one [46,47]. Adding several cationic groups in the same side chain or tuning the hydrophobicity of the side chain are another way to increase the variety of the aqueous morphology [48e51].…”