Effect of side chain on the electrochemical performance of poly (ether ether ketone) based anion-exchange membrane: A molecular dynamics study

doi:10.1016/j.memsci.2020.118105

Journal of Membrane Science

Volume 605, 15 June 2020, 118105

https://doi.org/10.1016/j.memsci.2020.118105 Get rights and content

Highlights

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Coarse-grained molecular dynamics simulations of the SQ and GQ AEMs were conducted.
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The ions diffusion has little contribution to the improved ionic conductivity of GQ.
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The improved ionic conductivity should attribute to the obvious promotion of IECs.
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More H₂O molecules wrap around OH⁻ in GQ, leading to the better alkaline stability.

Abstract

Ionic conductivity and alkaline stability are the key properties that limited the widespread application of anion-exchange membranes (AEM) in electrochemical energy conversion/storage systems. In recent years, quaternary ammonium functionalized poly (ether ether ketone) (PEEK) membranes serve as a promising solution due to the good mechanical and chemical properties. Varied ionic conductivity and alkaline stability could be obtained when the membrane contains different functionalized side chains. However, it's still a challenge to understand the mechanism from the experimental study because various parameters could affect the electro-chemical performance of the membranes. In this work, we conduct coarse-grained molecular dynamics simulations to investigate two PEEK-based membranes, in which the side chains contain one (SQ) or two (GQ) quaternary ammonium groups. The simulation results indicate the self-diffusion coefficients in SQ and GQ are quite similar which should not be the main reason for the improved ionic conductivity of GQ, while the obviously increased ion-exchange capacity of GQ should result in the improved ionic conductivity. Furthermore, the simulation reveals that more water molecules wrap around the OH⁻ in GQ, which could lead to the improved alkaline stability in comparison to that of SQ. This work provides a deeper understanding for the design of grafted copolymer based AEM with QA functional side chains.

Graphical abstract

Introduction

The anion-exchange membranes (AEMs) play an important role in the electrochemical energy conversion/storage systems, such as the anion-exchange membrane fuel cells, alkaline water electrolyzers, redox flow batteries, electrodialysis and bio-electrochemical systems. It is generally recognized that the anion (usually OH⁻) conductivity and the stability in alkaline operation condition are the key properties of AEMs [[1], [2], [3]]. For the transport of OH⁻ in AEMs, both the vehicular (diffusion) mechanism and the Grotthuss hopping mechanism are contributed to the conductivity [[4], [5], [6]]. For the alkaline stability of AEMs, the chemical degradation of the cationic functional groups of the anion-conducting polymers dominants the loss of OH⁻ transport property [[7], [8], [9], [10], [11], [12]]. The improvement of OH- conductivity by simply increasing the positive-charged groups in the AEMs might lead to the increase of OH- concentration, which could be unfavorable to the alkaline stability since there would be more opportunity for OH⁻ to be involved in the chemical degradation. Thus, a trade-off effect emerges.

To achieve the rational design of AEMs, it is crucial to have an overall consideration on polymeric regimes, functional groups and the balance of properties. Poly (ether ether ketone) (PEEK) is a robust engineering plastic with good thermal and chemical stability. The quaternary ammonium (QA) functionalized PEEK provides anion transport ability with the introduction of hydrophilic QA groups or QA-based side chains. QA-PEEK membrane has been a promising solution for alkaline electrochemical systems in recent years thanks to the improvement of synthesis method [2,13,14] and the abundance of grafting groups [[15], [16], [17], [18], [19], [20]]. Besides, the divergence of hydrophilicity between backbones and side chains of QA-PEEK could result in the spontaneous microphase separation, which benefits the improvement of the OH⁻ conduction [1]. In our previous work [16], the OH⁻ conductivity of the functionalized PEEK grafted by QA-based side chains with two QA groups (Gemini Quaternary, abbreviated as GQ in Fig. 1) is higher than that with only one QA group (Single Quaternary, abbreviated as SQ in Fig. 1) under the same grafting degree (GD). Besides, SQ and GQ was successfully used to assemble anion-exchange membrane fuel cells (AEMFC). However, the mechanism of the improved ionic conductivity and stability still needs more investigation because various factors could affect the ionic conductivity and stability, for example, the ion exchange capacity (IEC), micro-phase separation morphology, as well as effective ions diffusion coefficient. Furthermore, the investigation of these various factors of PEEK-based AEM could provide insightful information for the design of other grafted copolymer based AEM through the regulation of the side chains, including the chemical structure [[20], [21], [22], [23], [24]], the functional degree [15,16,25], and the number of ionic groups on the side chains [16,26].

Generally, the micro-structure and ions transport process of the membrane could be effectively investigated by the molecular dynamics (MD) simulations, which could provide in-depth information beyond the experiments [[27], [28], [29], [30], [31], [32], [33], [34]]. In the present work, we carried out the MD simulations on SQ and GQ as shown in Fig. 1, where SQ and GQ represent functionalized PEEK grafted by the common quaternary ammonium side group (-CH₂N⁺(CH₃)₃) and the side chain with two quaternary ammonium coexist (-CH₂N⁺(CH₃)₂CH₂CH₂CH(OH)CH₂N⁺(CH₃)₃) respectively. The all-atomic (AA) molecular dynamics simulation was conducted firstly, whose sufficiently equilibrious geometry provides the parameters for the further coarse-grained (CG) models, ensuring the reappearance of the structure characteristics of the polymers. The mapping scheme of the transfer from the AA model to the CG model is shown in Fig. 1. Different grafting degrees (GD) of 60%, 80% and 100% of the polymers were considered, as GD = x/(x + y) × 100%, where x and y are the number of repeat units with and without the QA side chains respectively. The radial distribution function (RDF) and structure factor (SF) S(q) were calculated from MD trajectory files. The simulation results indicate the substantially identical self-diffusion coefficients should have little contribution to the improved ionic conductivity of GQ, while the obviously increased ion exchange capacity of GQ should result in the improved ionic conductivity. Furthermore, the simulation reveals that more water molecules wrap around the OH⁻ in GQ, which could lead to the improved alkaline stability in comparison to that of SQ.

Section snippets

All-atomic MD simulations

The initial all-atomic (AA) MD simulations were carried out using GROMACS [35] package. Parameters of Generated Amber force field (GAFF) for all related molecules were attained using Antechamber [36] in Ambertools18 package and ACPYPE software [37]. RESP charges are generated as follows: All molecules are optimized in B3LYP/6-311g(d,p) DFT level using Gaussian 09 software [38], where solvent effect is corrected using PCM method. Then, RESP charges are fitted from the optimized geometry and wave

Structure characteristics

After sufficient simulating time for equilibrium, the hydrophilic beads aggregated spontaneously to generate the microphase separation and connected water channels, as the snapshots of CGMD models of membranes shown in Fig. 2a and b. From the snapshots, it is illustrated that most of the hydroxyl ion beads distributed close to the QA groups of polymers due to the strong electrostatic interaction between the charged beads. Some isolated hydroxyl ion beads are also observed due to the

Conclusions

In summary, the coarse-grained molecular dynamics simulations of the SQ and GQ AEMs were conducted to investigate the mechanism of the improved ionic conductivity and alkaline stability of GQ. The simulated results indicate the substantially identical self-diffusion coefficients should have little contribution to the improved ionic conductivity, while the obvious difference of IECs should result in the improved ionic conductivity. Furthermore, the RDF analysis reveals that more water molecules

Author statement

All the authors are listed below as the same sequence of the uploaded paper. The individual author contributions are recognized using the CRediT (Contributor Roles Taxonomy) method.

CRediT authorship contribution statement

Sian Chen: Conceptualization, Software, Validation, Formal analysis, Writing - original draft, Writing - review & editing, Visualization. Haining Wang: Conceptualization, Resources, Writing - original draft, Writing - review & editing, Project administration, Funding acquisition. Jin Zhang: Resources, Project administration. Shanfu Lu: Conceptualization, Resources, Project administration, Funding acquisition. Yan Xiang: Resources, Supervision, Project administration, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The research was support by grants from the National of Key R&D Program of China (No. 2018YFB1502303), National Natural Science Foundation of China (No. 21722601) and the Fundamental Research Funds for the Central Universities.

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