Tunable OH^– Transport and Alkaline Stability by Imidazolium-Based Groups of Poly(2,6-dimethyl-1,4-phenylene oxide) Anion Exchange Membranes: A Molecular Dynamics Simulation

Abstract

Imidazolium-based groups are promising organic cations in anion exchange membrane (AEM) materials. To investigate the effect of the imidazolium structure on OH − transport and alkaline stability of AEMs, we performed molecular dynamics simulation studies on hydrated poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) AEMs with imidazoliums modified by various substituents and/or alkyl pendent/spacer chains. Imidazoliums with the methyl or phenyl substituent at the C2, C4, and C5 sites or alkyl pendant chains at the N3 site show a steric effect on the distribution of water and OH − around imidazoliums, which inhibits the OH − transport but increases the alkaline stability of AEMs. By introducing alkyl spacer chains, the enhanced hydration structure of imidazolium promotes OH − transport, but the weakened steric effect of PPO on imidazolium decreases alkaline stability. We elucidate that the PPO AEMs modified by 1,2,4,5-tetramethylimidazolium and alkyl spacer chains with six or eight aliphatic carbons show good balance between OH − transport and alkaline stability of AEMs. Moreover, the complete hydration shells of both imidazolium and OH − enhance the OH − transport efficiency and decrease the possibility of imidazolium degradation with the hydration level more than six. Our work provides a design principle of imidazolium-based AEMs in fuel cell applications.

Improving the Properties of Anion Exchange Blend Membranes via Optimizing the NPBI Content and Deprotonation Degree: A Theoretical Study

“…To analyze the effect of the coherence of ion transport channels on OH – transport, the bottleneck volume fraction (FBV) and surface-to-volume ratio (SA/WV) of ion transport channels were calculated (Figure ). In general, a lower FBV indicated more continuous ion transport channels . In AEBMs with 100% NPBI deprotonation degree, the FBV of SI/NP-100-20 was smaller than those of SI/NP-100-10 and SI/NP-100-30.…”

“…In general, a lower FBV indicated more continuous ion transport channels. 37 In AEBMs with 100% NPBI deprotonation degree, the FBV of SI/NP-100-20 was smaller than those of SI/NP-100-10 and SI/NP-100-30. This was due to the increased NPBI content facilitating enhanced interactions between NPBI and SI in AEBMs with a lower NPBI content, which facilitated the formation of efficient ion transport channels.…”

Insight into the Alkaline Stability of Arylene-Linked Bis-Benzimidazoles and Polybenzimidazoles

“…It is important to note that the effective energy barrier was approximated with specific static configurations and a limited amount of explicit water molecules around the model compound structure. Recently, Wu et al 50 have investigated the distribution of water molecules and hydroxide ions around imidazolium-based molecules by molecular dynamic (MD) simulations with about 100−400 explicit water molecules. They have found significantly different distribution around the different imidazolium structures due to the modification with various substitutions at the same hydration level.…”

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

“…In MD simulations, the total forces on all the atoms are calculated, and then the dynamics of designed systems are simulated by discrete integration of Newton’s equations of motions with a shorter timescale, to determine the movement atom’s response to those forces [ 125 , 126 , 127 , 128 ]. There is a wide range of MD simulations: rational design and development of drugs, catalysts, conformational analysis of proteins, polymers, the molecular formation mechanism of green solvents, aerosols, transportation of lithium-ion in electrode, transportation of ion in the cationic-head-group-based AEM, and others [ 120 , 129 , 130 , 131 , 132 , 133 , 134 , 135 , 136 , 137 , 138 , 139 , 140 ]. The standard software for MD simulations includes, to name a few, in alphabetical order: AMBER SUITE, CP2K, GROMACS, NAMD, LAMMPS, and others [ 139 , 140 , 141 , 142 , 143 , 144 , 145 , 146 , 147 , 148 , 149 , 150 , 151 ].…”

“…In addition, the imidazolium-based head group structure’s effect on ion diffusion and chemical stability of AEM were, also, investigated by classical all-atom MD simulation using Materials Studio, COMPASS II Force field [ 140 ]. The results of classical all-atom MD simulations with COMPASS II Force field yielded that the PPO AEM with 1,2,4,5-tetramethylimidazolium and alkyl spacer chain with six or eight aliphatic carbons at the HL 6 illustrated an excellent balance between chemical stability and ion diffusivity for AEM.…”