Side Chain Piperidinium Functionalized AEMs with an Ethylene Oxide Spacer for Improving Ion Conductivity and Alkaline Stability

Abstract

In this work, grafting alkaline stable piperidinium cations via ethylene oxide (EO) spacers onto an aryl ether-free poly(oxindole terphenylene) backbone was adopted as a strategy for designing self-aggregating side chain AEMs with optimized alkaline stability. Aryl ether-free poly(oxindole terphenylene) backbones were synthesized via superacid-catalyzed step-growth polycondensation and were subsequently functionalized with either piperidinium containing hydrophilic, dipolar EO or hydrophobic alkyl spacer, aiming to explore the effect of side chain-engineering on conductivity and alkaline stability of the resulting AEMs. The AEM membrane containing dipolar ethylene oxide spacer, despite its lower ion exchange capacity (IEC), exhibited a more pronounced microphase separated morphology as evidenced by TEM, and higher ionic conductivity (reaching up to 30.5 mS cm⁻¹ at 80 °C) compared to the hydrophobic alkyl spacer-containing AEM membrane. This was attributed to its higher water uptake stemming from the EO hydrophilic nature and the formation of interconnected ion-conducting channels due to piperidinium-EO interactions. Additionally, the hydrophilic nature of the ethylene oxide groups endowed the membrane with enhanced alkaline stability, preserving its mechanical integrity and retaining 71.5% of its initial conductivity after 3 weeks of immersion in 2 M KOH at 80 °C. In contrast, the AEM with an alkyl spacer experienced severe degradation under the same conditions. These results suggest that incorporating flexible alkoxy-containing spacers onto an aryl ether-free backbone is a promising and simple route for fabricating mechanically and chemically robust AEMs with sufficient conductivity.

Keywords

Anion exchange membranes (AEMs) / Alkaline water electrolysis / Ethylene oxide / Piperidinium / Ionic conductivity / Alkaline stability

Supplementary Materials

The following supporting information can be found at: https://www.sciepublish.com/article/pii/688, Figure S1: ¹H NMR spectra of (a) 1,2-bis (2-iodoethoxy)ethane and (b) 1-(2-(2-(2-iodoethoxy)ethoxy)ethyl)-methylpiperidinium iodide (I-OPip); Figure S2: ¹H NMR spectra of Br-Pip; Figure S3: ATR-FTIR spectrum of the P(OpT80-dBrac) copolymer; Figure S4: Photographs of synthesized P(OpTx-dBrac) copolymers; Figure S5: Water uptake and swelling behavior of (a) P(OpT70-dBrac)-Pip and (b) P(OpT70-dBrac)-OPip in the OH⁻ form as a function of temperature; Figure S6: ATR spectra of the untreated P(OpT70-dBrac)-OPip membrane and the corresponding membrane after alkaline aging for 3 weeks in 2 M KOH solution at 80 °C in the whole spectral region (3700-650 cm⁻¹) and in the magnified region from 1800 to 625 cm⁻¹; Figure S7: ATR spectra of the untreated P(OpT70-dBrac)-Pip membrane and the corresponding membrane after alkaline aging for 3 weeks in 2 M KOH solution at 80 °C in the whole spectral region (3700-650 cm⁻¹) and in the magnified region from 1800 to 625 cm⁻¹.

Acknowledgments

The authors are grateful to M. Kolia (Laboratory of Electron Microscopy and Microanalysis, University of Patras) for TEM measurements.

Author Contributions

Conceptualization, V.D.; Methodology, V.D.; Validation, S.G. and V.D.; Investigation, S.G.; Resources, V.D.; Data Curation, V.D.; Writing—Original Draft Preparation, S.G.; Writing—Review & Editing, V.D.; Visualization, S.G.; Supervision, V.D.

Ethics Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Research data are contained within the article and supplementary materials.

Funding

This research was funded by the European Union’s Horizon 2020 Research and Innovation Action program “Materials for next generation of alkaline electrolyzer” (NEXTAEC), grant number 862509.

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.

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Funding

This research was funded by the European Union’s Horizon 2020 Research and Innovation Action program “Materials for next generation of alkaline electrolyzer”(NEXTAEC)

grant number(862509)