Alkaline Stability Of Anion Exchange Membranes Research Articles

High-temperature inorganic anion exchange membranes based on layered double hydroxides for anion exchange membrane fuel cell applications

Anion-exchange membrane fuel cells (AEMFCs) have garnered substantial attention over the past two decades due to their numerous advantages, positioning them as promising alternatives to proton exchange membrane fuel cells (PEMFCs). However, their rapid development faces notable challenges, including the limited high-temperature and alkaline stability of anion-exchange membranes due to the incorporation of cationic functional groups, as well as intricate preparation processes. In this study, we propose a high-temperature inorganic anion exchange membrane based on layered double hydroxide doped with K2CO3 prepared by a simple process. This innovative approach optimizes the ionic transport pathway within the LDHs inorganic membrane and enhances water retention performance at elevated temperatures. The maximum cross-plane ionic conductivity of the membrane can reach up to 10−2 S cm−1. Remarkably, the peak power density attains an impressive level of 215.2 mW cm−2 at 110 °C and 200 kPa during the fuel cell testing.

Alkaline Stability of Anion-Exchange Membranes.

Recently, the development of durable anion-exchange membrane fuel cells (AEMFCs) has increased in intensity due to their potential to use low-cost, sustainable components. However, the decomposition of the quaternary ammonium (QA) cationic groups in the anion-exchange membranes (AEMs) during cell operation is still a major challenge. Many different QA types and functionalized polymers have been proposed that achieve high AEM stabilities in strongly alkaline aqueous solutions. We previously developed an ex situ technique to measure AEM alkaline stabilities in an environment that simulates the low-hydration conditions in an operating AEMFC. However, this method required the AEMs to be soluble in DMSO solvent, so decomposition could be monitored using 1H nuclear magnetic resonance (NMR). We now report the extension of this ex situ protocol to spectroscopically measure the alkaline stability of insoluble AEMs. The stability ofradiation-grafted (RG) poly(ethylene-co-tetrafluoroethylene)-(ETFE)-based poly(vinylbenzyltrimethylammonium) (ETFE-TMA) and poly(vinylbenzyltriethylammonium) (ETFE-TEA) AEMs were studied using Raman spectroscopy alongside changes in their true OH- conductivities and ion-exchange capacities (IEC). A crosslinked polymer made from poly(styrene-co-vinylbenzyl chloride) random copolymer and N,N,N',N'-tetraethyl-1,3-propanediamine (TEPDA) was also studied. The results are consistent with our previous studies based on QA-type model small molecules and soluble poly(2,6-dimethylphenylene oxide) (PPO) polymers. Our work presents a reliable ex situ technique to measure the true alkaline stability of AEMs for fuel cells and water electrolyzers.

Measuring the alkaline stability of anion-exchange membranes

One significant barrier in developing durable, robust anion-exchange membranes (AEMs) for liquid-electrolyte-free fuel cells (AEMFCs) and water electrolyzers (AEMWEs) is their limited chemical stability to alkali. To measure the alkaline stability of AEMs, ex-situ tests are commonly used where the AEMs are immersed for long durations in aqueous alkali solutions. However, such tests do not adequately simulate the liquid-electrolyte-free environment of AEMFCs and AEMWEs, as the hydration and alkaline conditions do not always mimic actual operando conditions, yielding misleading and inaccurate indications of degradation rates for relatively low hydration conditions. We recently reported a unique ex-situ method which determines the alkaline stability of AEMs under conditions that mimic in-situ operating environments. In this study, we apply this technique to determine the alkaline stability of several AEMs containing different functional group and backbone chemistries. The alkaline stability of HDPE-based radiation-grafted (RG)-AEMs containing different functional group chemistries follows the trend: TMA ≥ MPY ∼ MPIP ≫ DEMA > TEA. Radiation-grafted AEMs (and a non-radiation-grafted PPO benchmark) containing different backbones and the same stable TMA group follows the stability order: ETFE ≥ LDPE > HDPE ≫ PPO. This technique is recommended for ex-situ testing of the alkaline stability of AEMs for both AEMFC and AEMWE applications.

Side-chain manipulation of poly (phenylene oxide) based anion exchange membrane: Alkoxyl extender integrated with flexible spacer

Anion exchange membranes (AEMs) are an important component in anion exchange membrane fuel cells, and their side chain structures have a significant impact on membrane performance. Here, we construct a series of poly(phenylene oxide) (PPO)-based AEMs with varied side chain structures and investigate the effects of alkyl or alkoxyl extenders (or “tail” of the cation) and spacers (or the link between backbone and cation head group) on AEM performance. Our investigations demonstrate that the simultaneous introduction of alkoxyl extender and spacer can significantly improve water absorption and ion conductivity of membrane; this is attributed to the good hydrophilicity of alkoxyl group and flexibility of spacer that benefit the formation hydrophilic domains. At an IEC (ion exchange capacity) of 1.13 mmol/g, the AEM containing both spacer and alkoxyl extender exhibits a hydroxide conductivity of 87.3 mS/cm (80 °C), which is much higher than that of other spacer-containing membranes at similar IECs. The H2/O2 single cell constructed with this membrane exhibits a good peak power density of 292 mW/cm2 at 60 °C. In addition, the steric hindrance of the extender gives rise to higher alkali stability of the AEM than that of the membrane which does not have the extender. Our work provides a meaningful direction of side chain design which can effectively improve conductivity and alkaline stability of AEMs.

Flexible cationic side chains for enhancing the hydroxide ion conductivity of olefinic-type copolymer-based anion exchange membranes: An experimental and theoretical study

The performance of anion exchange membranes (AEMs) is closely related to their cationic groups, hydration degree, and nano-phase morphology. Here, mono- (and di-) quaternary ammonium, pyrrolidinium, and imidazolium cation-based AEMs were synthesized by one-step photo-polymerization. Through combination of experimental and molecular dynamics (MD) approaches, the effects of mono- (and di-) cationic functional groups on the properties of the AEMs, in terms of physicochemical, electrochemical properties, and alkaline stability were comprehensively investigated. It was concluded by the experimental data that AEMs with di-cations exhibit better water uptake, swelling ratio, alkaline stability, and higher ionic conductivity than those with mono-cations. Furthermore, mathematical models of the synthesized AEMs were established for MD simulations, and the calculated radial distribution function and mean square displacements between cationic functional groups, hydroxide ions, and water molecules obtained using the simulated molecular trajectories were examined for the solvation, transport properties, and alkaline stability of AEMs, which are in good qualitative agreement with the experimental results.

Hydrophilic side chain assisting continuous ion-conducting channels for anion exchange membranes

A high performance of anion exchange membranes (AEMs) is highly demanded in anion exchange membrane fuel cells (AEMFCs) and has been extensively studied. However, the poor ionic conductivities and the alkaline stability might limit their applications. Herein, we developed a novel strategy to improve the ionic conductivity and alkaline stability of AEMs by introducing hydrophilic oligo(ethylene glycol)(OEG) groups. Continuous ion-conducting channels were efficiently formed thanks to the dynamic aggregation of water molecules assisted by the hydrophilic side chains. The resultant AEMs with low IEC values showed significant enhancement in hydroxide conductivity compared to the AEMs functionalized by the conventional 1,2-dimethylimidazolium (PSf-Im), as the hydroxide conductivity of PSf-2 membrane with an IEC of 1.05 mmol g−1 was 59.5 mS·cm−1 at 60 °C while the hydroxide conductivity of PSf-Im membrane with an IEC of 1.27 mmol g−1 was 36.7 mS·cm−1. In addition, the alkaline stability and the mechanical properties of the resultant AEMs were improved remarkably. The density functional theory (DFT) study showed that the introduction of OEG part can significantly increase the LUMO energy of functional group and then enhance the alkaline stability.

Tethering Cations to Aromatic Polymers via Flexible Spacers to Enhance the Performance of Alkaline Fuel Cell Membranes

Conventional anion-exchange membranes based on aromatic polymers with cationic groups placed in benzylic positions on the polymer backbone normally suffer from poor alkaline stability and limited OH− conductivity. However, recently it has been shown that both the conductivity and alkaline stability of anion-exchange membranes can be significantly enhanced by instead placing the cationic groups on flexible and hydrolytically stable alkyl spacers. The present contribution briefly reviews the latest results and developments concerning this synthetic approach to improved alkaline membranes for fuel cells.