High Ion Conductivity and Diffusivity in the Anion Exchange Membrane Enabled by Tethering with Multication Strings on the Poly(biphenyl alkylene) Backbone

Abstract

Aryl ether-free polyaromatics tethered with quaternary ammoniums (QAs) offer a promising candidate as an anion exchange membrane (AEM) for use in alkaline fuel cells. However, the traditional trade-off dilemma between the ion conductivity and water swelling of the AEM still remains. Herein, we report a series of poly­(biphenyl alkylene) (PBPA) polymers tethered with multiple QAs per side chain that combine the advantages of a high ion conductivity, high ion diffusion coefficient, and suppressed water uptake. Upon increasing the number of QAs on each side chain, the degree of phase separation of the resulting AEM increased, thereby improving the ion diffusion coefficient from 2.03 × 10–6 cm2 s–1 for a single-QA-based PBPA100-1QA to 2.26 × 10–6 cm2 s–1 for a triple-QA-based PBPA35-3QA. The side chain triply-charged membranes show much improved performances. Specifically, at a given ion exchange capacity (IEC) of 2.5 mmol g–1, despite a notably low water uptake that was observed, the PBPA35-3QA exhibited a conductivity ∼40% higher than that of PBPA100-1QA. The highest conductivity was observed for PBPA50-3QA (IEC = 3.1 mmol g–1), achieving a 63 mS cm–1 chloride conductivity at 80 °C, while the corresponding water uptake was well-suppressed within 116 wt %. Owing to the high conductivity and ion diffusivity as well as the suppressed water uptake, a fuel cell fabricated with the 25 μm thick PBPA35-3QA membrane showed a low high-frequency resistance of 0.03 Ω cm2. However, the triple-QA-based membrane showed an 18% cation degradation after aging for 300 h in 2 M NaOH at 80 °C, which is a significantly inferior value than those of the counterparts with a low number of QAs on the side chains.

A covalent organic framework membrane with enhanced directional ion nanochannels for efficient hydroxide conduction

“…Therefore, enormous efforts have been directed towards the construction of high-performance hydroxide-conducting materials, including nanophase-separated polymers and organic-inorganic hybrids. 6–10 Notably, membranes with well-defined hydrophilic/hydrophobic microphase separation structures usually generate multi-connected ion nanochannels, resulting in enhanced conductivity. 11–15…”

“…Therefore, enormous efforts have been directed towards the construction of high-performance hydroxide-conducting materials, including nanophase-separated polymers and organic-inorganic hybrids. [6][7][8][9][10] Notably, membranes with well-dened hydrophilic/hydrophobic microphase separation structures usually generate multi-connected ion nanochannels, resulting in enhanced conductivity. [11][12][13][14][15] In this regard, two-dimensional (2D) materials are ideal building blocks for hydroxide-conducting membranes, giving rise to their continuous one-dimensional (1D) channels, low electrical resistance and excellent stability.…”

Highly Stable and Conductive Multicationic Poly(biphenyl indole) with Extender Side Chains for Anion Exchange Membrane Fuel Cells

“…In addition, the maximum power density of PPHIN-N8C is slightly higher than some AEMs reported in recent studies. 47,58 The result suggests that the prepared membrane holds promise for AEMFC applications. Since the performance of fuel cells is affected by many factors, we will optimize the structure of ionomers, the manufacturing of MEA, and the activation process (such as humidity, gas flow, and current density) in future work to improve the single-cell performance.…”

Alkaline membrane fuel cells: anion exchange membranes and fuels

“…However, such membranes have shown an inferior alkaline durability in comparison to AEMs with a low number of quaternary ammonium on the side chains. 84 Several studies on cation backbone type AEMs have also been performed, including a study by Tan et al 80 which have investigated main-chain polymers with pyrrolidinium cations tethered with different methylene groups. They conducted that increasing cation charge density of the pyrrolidinium compounds provided better alkaline stability, but a long spacer between the pyrrolidinium cations accelerated cations degradation in alkaline solutions.…”