Comb-shaped Anion Exchange Membranes Research Articles

Impact of side chain length on the properties and alkaline fuel cell performance of OEG-grafted poly(terphenyl piperidinium) anion exchange membranes

In designing comb-shaped anion exchange membranes (AEMs), replacing alkyl side chains with ethylene glycol (EG)-based ones significantly enhances membrane properties and boosts AEM fuel cell performance. Although much research has focused on optimizing the placement of EG segments within the polymer matrix, the effect of EG chain length remains less explored. In this work, a series of poly(terphenyl piperidinium) AEMs with N-oligo(ethylene glycol) (OEG) terminal pendants having two to five EG repeating units were prepared by simple post-polymerization quaternization. The membrane properties showed a strong correlation with the length of the OEG side chains, influenced by both chemical composition and phase behavior. Among the OEG-grafted AEMs, PTP-OEG3 with moderate three EG repeating units, exhibited the best properties due to a careful balance between EG and cationic groups, leading to a favorable microphased separation. It achieved high hydroxide conductivity (114 mS cm−1 at 80 °C in water), robust mechanical strength (tensile strength >52 MPa), and good ex situ alkaline stability. As a result, the AEM fuel cells with the property-balanced PTP-OEG3 membrane achieved the highest peak power density of 976 mW cm−2. The in situ stability of these AEMs was also investigated. Evidently, understanding the optimal EG side chain length is an important criterion for designing AEM materials for alkaline fuel cells.

Comb-shaped anion exchange membrane with segmented hydrophilic/hydrophobic side chain

A flexible hydrophilic chain segment with ether bonds and a long hydrophobic tail chain with multiple carbon atoms are designed and grafted on the same side of the polymer backbone, which can not only improve the alkaline stability by preventing the benzyl carbon from being attacked by OH-, but also reduce the hydration degree of the membrane, giving it excellent conductivity, alkaline stability and low swelling ratio. These results show that the prepared anion exchange membranes (AEMs) with a hydrophilic chain segment containing 2 ether bonds exhibits the best overall performance. Its ionic conductivity can reach 90.4 mS cm−1 at 80 °C, the swelling ratio is less than 8%, the tensile strength can reach 14.5 MPa, and the elongation at break is 79.0%. Even after being soaked under alkaline conditions for 672 h, the conductivity of the original membrane still remains over 85%. The peak power density of the fuel cell using this AEM can reach as high as 582 mW cm−2. The segmented hydrophilic and hydrophobic side chain proposed in this work provides a novel idea to design the AEM materials and improve the performance of anion exchange membrane fuel cell.

A novel anion exchange membrane based on poly (2,6-dimethyl-1,4-phenylene oxide) with excellent alkaline stability for AEMFC

We report a novel comb-shaped anion exchange membrane based on poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) and 4-(dimethylamino)butyraldehyde diethyl acetal (DABDA). The Menshutkin reaction successfully introduced DABDA into the brominated PPO backbone, which was proved through FT-IR and 1H NMR. The distinct hydrophilic/hydrophobic microphase separation structure observed by transmission electron microscope (TEM). As the increase of grafting degree, so does the water uptake, swelling ratio, hydroxide conductivity and alkaline stability. The membranes also possess good mechanical property with tensile strength from 14.2 to 38.11 MPa. This is due to the increasing number of hydroxide and unique steric hindrance effect caused the higher water uptake and higher dimensional stability. Simultaneously, especially PPO/DABDA-60 in comb-shaped membranes demonstrate an excellent long-term alkali resistance stability. In a 576-h alkali resistance stability test, the retained ionic conductivity of the PPO/DABDA-60 membrane is 96% of the initial value. The PPO/DABDA-60 is a potential candidate material for AEM.

Comb-shaped anion exchange membranes: Hydrophobic side chains grafted onto backbones or linked to cations?

In order to investigate the relationship between the side chain topology of comb-shaped anion exchange membranes (AEMs) and membranes properties, two types of hydrophobic side chains attached AEMs that based on QAPPO are synthesized, namely side chains grafted onto backbones and side chains tethered to cations, resulting in AxSB-QAPPO and AxSC-QAPPO AEMs, respectively. Compared with AxSC-QAPPO, the side chains in AxSB-QAPPO AEMs are more inclined to facilitate the membranes to form broad and interconnected ion channels, promoting ion conduction and providing more free volume for the external water to enter the membranes, which are benefit to improving the ionic conductivity and chemical stability of the AxSB-QAPPO samples. At 80 °C, the OH− conductivity of A8SB-QAPPO is 101.2 mS cm−1, while the values for A8SC-QAPPO and QAPPO are only 56.1 and 48.5 mS cm−1 respectively. After the stability test, the weight and IC retentions of A8SB-QAPPO are 83.0% and 77.0%, respectively, while the values for A8SC-QAPPO are 76.5% and 68.0%, respectively, and the values for QAPPO are 56.0% and 42.0%, respectively. Besides, the influence of side chain length on the membranes properties is studied. The chemical stability for both AxSB-QAPPO and AxSC-QAPPO AEMs is enhanced by increasing the side chain length from 5 to 14. After the stability test, the IEC retentions of A5SB-QAPPO, A8SB-QAPPO, A14SB-QAPPO, A5SC-QAPPO, A8SC-QAPPO and A14SC-QAPPO are 60.0%, 78.0%, 90.0%, 49.0%, 70.0% and 75.0%, respectively. While for ionic conductivity, the AEMs with the side chain length of 8 exhibit the highest OH− conductivity. Hence, we speculate that the comb-shaped AEMs with hydrophobic side chains attached to backbones (AxSB-QAPPO) are superior to the membranes with the side chains linked to cations (AxSC-QAPPO), and the membranes performance can be further optimized by adjusting the side chain length.

Preparation and characterization of high conductivity comb polymer anion exchange membranes

A comb-shaped anion exchange membrane was successfully constructed by simultaneously introducing a hydrophobic aliphatic long-chain bromododecane (BA) and a hydrophilic short-chain bromoethanol (BE) as a quaternizing agent into the side chain of poly-4-vinyl-pyridine (P4VP). The aforementioned membrane chemical structure was investigated by FT-IR, and confirmed that the BA and BE were successfully grafted onto the P4VP backbone. A series of performance test results reveal that the water uptake (WU) and swelling ratio (SR) are effectively controlled with the BA content increased from 0% to 75.0%, which the value was corresponding to the water uptake decreases from 92.3% to 31.8%, and the swelling ratio decreases from 78.4% to 7.8%. Interestingly, the conductivity of synthesized membrane in this paper is much increased with substantially the same ion exchange capacity (IEC) (from 50.0 mS·cm−1 for BE100.0%-BA0%/P4VP to 75.0 mS·cm−1 for BE25.0%- BA75.0%/P4VP at 70 °C), which can be attributed to the microphase-separated structure. On the other hand, the membrane BE25.0%-BA75.0%/P4VP has the best overall performance, which with the conductivity of 75.0 mS·cm−1 at 70 °C, which the temperature closest to the alkaline fuel cell operating temperature. And it also had excellent tensile strength (16.0 MPa) and elongation at break (5.9%), indicating sufficient flexibility and processability, and it has excellent chemical stability immersed it into 3 mol·L−1 KOH for up to 240 h, which was the conductivity can still reach more than 70% of the initial value. And the aforementioned membrane also with the better thermal stability (the thermal decomposition temperatures above 200 °C). The single cell test result reveal that the membrane displayed good cell performance with a power density of 47.52 mW·cm−2 at 70 °C. In summary, the comprehensive performance of BE25.0%-BA75.0%/P4VP is expected to be applied to alkaline polymer fuel cells.

Comb-shaped anion exchange membrane with densely grafted short chains or loosely grafted long chains?

Anion exchange membranes (AEMs) are crucial components for advanced energy and environment processes including alkaline fuel cells, redox flow batteries, and industrial effluent treatment; while low anionic conductivity and poor stability remain the major challenges for the widespread implementation of AEMs. Through molecular engineering, comb-shaped AEMs have been proved possessing the capability of delivering both high conductivity and good alkaline stability. However, how to precisely control the side chain topology and how the chain topology would influence the membrane properties need to be further elucidated. We hereby propose a radically novel and readily scalable route towards the controllable synthesis of comb-shaped AMEs and we were able to determine the length of side chains and the number of ionic groups along the side chains. To probe the effect of side chain topology on membrane properties, two types of AEMs of densely grafted short side chains or loosely grafted long side chains with similar ion exchange capacity (IEC, ∼1.7 mmol/g) were synthesized and compared. We found that the comb-shaped AEMs with loosely grafted long chains (LG-LS-DIm), with higher hydroxide conductivity (55 mS cm−1 at 30 °C) and better alkaline stability (∼80 % of IEC retention after soaking in 2 mol L−1 NaOH solution at 60 °C for 25 days), outperform those with densely grafted short chains (HG-SS-DIm) and the benchmark main-chain type AEMs (DIm-PPO), which is also superior to those of conventional linear AEMs with densely functionalized structure.

Comb-shaped anion exchange membrane to enhance phosphoric acid purification by electro-electrodialysis

Comb-shaped polysulfone anion exchange membranes (AEMs) containing various pendant alkyl side chains were synthesized and characterized. Ionic clusters aggregation is observed in transmission electron microscopy (TEM) image and small-angle X-ray scattering (SAXS). The comb-shaped quaternized ammonium polysulfone (Cx-QAPSU) membranes show enhanced dimensional stability and ultra-low membrane area resistance (Rm). Notably, the C16-QAPSU with 16 carbon atoms has the best hydrophilic-hydrophobic phase separation structure and lowest Rm of 2.5 Ω cm2 despite low ion exchange capacity (IEC) of 1.55 mmol/g and swelling ratio of 5.3%. Moreover, comb-shaped AEMs show much higher tensile strength (TS) compared to QAPSU membrane under the same IEC level. The C16-QAPSU with lowest Rm of 2.5 Ω cm2 exhibits the highest TS of 16.7 MPa. These results indicate that the long alkyl side chains especially the length of the pendant side chain of Cx-QAPSU up to C16 achieve an electrochemical-mechanical balance of AEMs. In electro-electrodialysis systems, the comb-shaped AEMs exhibit higher purification efficiency of wet phosphoric acid than non-comb-shaped AEMs.

N3-adamantyl imidazolium cations: Alkaline stability assessment and the corresponding comb-shaped anion exchange membranes

In the pursuit of alkaline stable cationic ions as promising candidates for high performance anion exchange membranes, two novel imidazoliums with N3-adamantyl substituent along with four imidazoliums with N3-methyl and butyl-substituents were synthesized and systematically assessed the impact of N3- and C2-substitutions on their chemical stabilities. Substituent at the N3-position prevented the degradation of imidazolium, and bulky and rigid adamantyl substituent was the most effective. The 1-adamantyl-2-methyl-3-ethylimidazolium cation (AdMEIm) exhibited the highest alkaline stability among the six cationic ions, which is stable under 5M NaOH aqueous solution at 80°C for 168h. Combined with the highly chemical stability of N3-adamantyl substituted imidazolium and excellent chemical and thermal stability of poly(2,6-dimethyl phenylene oxide) (PPO), the PPO-based AEMs functionalized with N3-adamantyl substituted imidazolium were further synthesized and characterized. However, unlike the small-molecule imidazolium salt with exceptional alkaline stability, a considerable ring-opening degradation of imidazolium was taken place for AEM functionalized with the stable N3-adamantyl substituent when exposed to 1M NaOH aqueous solution at 60°C for over 48h, demonstrating that the alkaline stabilities of modular cation and the corresponding AEM seems less correlated. A single H2-O2 fuel cell using PPO-AdIm-34 membrane showed a peak power density of 46.0mWcm−2 and an open circuit voltage (OCV) of 0.90V at 45°C without any optimization of the electrodes, demonstrating that the as-prepared membranes have potential application in AEMFCs.

Dual-cation comb-shaped anion exchange membranes: Structure, morphology and properties

Anion exchange membranes (AEMs) are employed as the gas separator and hydroxide ion conductor between the anode and the cathode of alkaline polyelectrolyte fuel cell (APEFC). Highly conductive and stable AEMs are urgently needed in order to achieve satisfactory fuel cell performance. Nevertheless, the low hydroxide conductivity remains major challenge that limits the development and application of APEFC. In order to improve the hydroxide conductivity of AEMs, highly ordered ion conducting channels must be constructed within the membrane matrix. In this study, an AEM with particular polymer structure was designed to achieve fine ion conducting channels by facilitating the hydrophilic-hydrophobic phase separation. Concretely, the side chain of this polyelectrolyte contains two quaternary ammonium groups and terminates with one long hydrophobic tail. Ascribing to the enhanced nano-phase separation ability, inter-connected ion conducting channels were observed by AFM as well as SAXS. A high hydroxide conductivity of 47mS/cm at 25°C and 85mS/cm at 80°C was achieved. In addition, when applied in an APEFC system, the synthesized AEM showed an outstanding peak power density of 369.3mW/cm2. Good alkaline tolerance, lowered water uptake and swelling ratio were also observed attributing to its carefully designed polymer structure.

Cross-linked comb-shaped anion exchange membranes with high base stability

A unique one-step cross-linking strategy that connects quaternary ammonium centers using Grubbs II-catalyzed olefin metathesis was developed. The cross-linked anion exchange membranes showed swelling ratios of less than 10% and hydroxide conductivities of 18 to 40 mS cm(-1). Cross-linking improved the membranes' stability to hydroxide degradation compared to their non-cross-linked analogues.