Understanding of imidazolium group hydration and polymer structure for hydroxide anion conduction in hydrated imidazolium-g-PPO membrane by molecular dynamics simulations. (31st December 2018)

• Record Type: Journal Article
• Authors: Zhang, Ning; Huo, Jun; Yang, Boyun; Ruan, Xuehua; Zhang, Xiaopeng; Bao, Junjiang; Qi, Wenxu; He, Gaohong
• Is Part Of: Chemical engineering science. Volume 192(2018)
• Page Start: 1167
• Page End: 1176
• Publication Date: 2018-12-31
• Subjects: Imidazolium-grafted PPO -- Molecular dynamics simulation -- Hydroxide ion -- Hydration structure -- Affinity; Chemical engineering -- Periodicals; Génie chimique -- Périodiques; Chemical engineering; Periodicals; Electronic journals; 660
• DOI: 10.1016/j.ces.2018.08.051
• Languages: English
• ISSNs: 0009-2509

Abstract

Graphical abstract: Highlights: We investigated imidazolium-grafted PPO membrane with various water uptake. We described the effect of water uptake on the hydration structure of imidazolium group. Enhancing the hydration structure of the imidazolium group could weaken the affinity to the surrounding OH − . Critical water saturation of imidazolium group could produce proper affinity to the surrounding OH − . Abstract: In an anion exchange membrane, OH − conduction is closely related to the affinity of the functional group and membrane morphology. This relationship can change when the water uptake of the membrane varies. To explore the effect of the water uptake on the affinity of the functional group to OH − and the membrane morphology, a series of molecular dynamics simulations based on an all-atom force field were performed for the imidazolium-grafted PPO (Im-g-PPO) membranes with different water uptakes. The simulation results of the membrane density, water and OH − self-diffusivity, and OH − conductivity verify the accuracy of the simulation systems. The local distributions of OH − and water around the imidazolium group indicate that increasing water uptake enhances the hydration structure of the imidazolium group and weakens the affinity of the imidazolium group to OH − . The critical water saturation of imidazolium group could produces suitable affinity to the surrounding OH − . When the imidazolium group is water saturated, further increasing water uptake is not conducive to retaining the affinity to OH − . As a result, the critical water saturation of the imidazolium group balances between the affinity to OH − and the transfer of OH − in the hydrated Im-g-PPO membrane. Furthermore, it also produces a percolated hydrophilic channel and maintains a relatively high mechanical strength of the Im-g-PPO membrane. Therefore, the imidazolium groups should be maintained under the critical water saturation, where two and eight water molecules are retained in the first and second hydration shells, respectively. This work provides a molecular-level understanding of the effect of the hydration structure of the imidazolium group on OH − conduction and the morphology of the Im-g-PPO membrane. It also provides potential guidance for maintaining high performance in anion exchange membrane fuel cells.