Orderly branched anion exchange membranes bearing long flexible multi-cation side chain for alkaline fuel cells

Ether-Free Alkaline Polyelectrolytes for Water Electrolyzers: Recent Advances and Perspectives

Anion exchange membrane (AEM) water electrolyzers (AEMWEs) have attracted great interest for their potential as sustainable, environmentally friendly, low-cost sources of renewable energy. Alkaline polyelectrolytes play a crucial role in AEMWEs, determining their performance and longevity. Because heteroatom-containing polymers have been shown to have poor durability in alkaline conditions, this review focuses on ether-free alkaline polyelectrolytes, which are more chemically stable. The merits, weaknesses, and challenges in preparing ether-free AEMs are summarized and highlighted. The evaluation of synthesis methods for polymers, modification strategies, and cationic stability will provide insights valuable for the structural design of future alkaline polyelectrolytes. Moreover, the in situ degradation mechanisms of AEMs and ionomers during AEMWE operation are revealed. This review provides insights into the design of alkaline polyelectrolytes for AEMWEs to accelerate their widespread commercialization.

High Free Volume Polyelectrolytes for Anion Exchange Membrane Water Electrolyzers with a Current Density of 13.39 A cm-2 and a Durability of 1000 h

The rational design of the current anion exchange polyelectrolytes (AEPs) is challenging to meet the requirements of both high performance and durability in anion exchange membrane water electrolyzers (AEMWEs). Herein, highly-rigid-twisted spirobisindane monomer is incorporated in poly(aryl-co-aryl piperidinium) backbone to construct continuous ionic channels and to maintain dimensional stability as promising materials for AEPs. The morphologies, physical, and electrochemical properties of the AEPs are investigated based on experimental data and molecular dynamics simulations. The present AEPs possess high free volumes, excellent dimensional stability, hydroxide conductivity (208.1 mS cm-1 at 80 °C), and mechanical properties. The AEMWE of the present AEPs achieves a new current density record of 13.39 and 10.7 A cm-2 at 80 °C by applying IrO2 and nonprecious anode catalyst, respectively, along with outstanding in situ durability under 1 A cm-2 for 1000 h with a low voltage decay rate of 53 µV h-1 . Moreover, the AEPs can be applied in fuel cells and reach a power density of 2.02 W cm-2 at 80 °C under fully humidified conditions, and 1.65 W cm-2 at 100 °C, 30% relative humidity. This study provides insights into the design of high-performance AEPs for energy conversion devices.

Triptycene Branched Poly(aryl-co-aryl piperidinium) Electrolytes for Alkaline Anion Exchange Membrane Fuel Cells and Water Electrolyzers

Alkaline polymer electrolytes (APEs) are essential materials for alkaline energy conversion devices such as anion exchange membrane fuel cells (AEMFCs) and water electrolyzers (AEMWEs). Here, we report a series of branched poly(aryl-co-aryl piperidinium) with different branching agents (triptycene: highly-rigid, three-dimensional structure; triphenylbenzene: planar, two-dimensional structure) for high-performance APEs. Among them, triptycene branched APEs showed excellent hydroxide conductivity (193.5 mS cm-1 @80 °C), alkaline stability, mechanical properties, and dimensional stability due to the formation of branched network structures, and increased free volume. AEMFCs based on triptycene-branched APEs reached promising peak power densities of 2.503 and 1.705 W cm-2 at 75/100 % and 30/30 % (anode/cathode) relative humidity, respectively. In addition, the fuel cells can run stably at a current density of 0.6 A cm-2 for 500 h with a low voltage decay rate of 46 μV h-1 . Importantly, the related AEMWE achieved unprecedented current densities of 16 A cm-2 and 14.17 A cm-2 (@2 V, 80 °C, 1 M NaOH) using precious and non-precious metal catalysts, respectively. Moreover, the AEMWE can be stably operated under 1.5 A cm-2 at 60 °C for 2000 h. The excellent results suggest that the triptycene-branched APEs are promising candidates for future AEMFC and AEMWE applications.

Energy Applications of Ionic Liquids: Recent Developments and Future Prospects

Ionic liquids (ILs) consisting entirely of ions exhibit many fascinating and tunable properties, making them promising functional materials for a large number of energy-related applications. For example, ILs have been employed as electrolytes for electrochemical energy storage and conversion, as heat transfer fluids and phase-change materials for thermal energy transfer and storage, as solvents and/or catalysts for CO2 capture, CO2 conversion, biomass treatment and biofuel extraction, and as high-energy propellants for aerospace applications. This paper provides an extensive overview on the various energy applications of ILs and offers some thinking and viewpoints on the current challenges and emerging opportunities in each area. The basic fundamentals (structures and properties) of ILs are first introduced. Then, motivations and successful applications of ILs in the energy field are concisely outlined. Later, a detailed review of recent representative works in each area is provided. For each application, the role of ILs and their associated benefits are elaborated. Research trends and insights into the selection of ILs to achieve improved performance are analyzed as well. Challenges and future opportunities are pointed out before the paper is concluded.

Effects of the crown ether cavity on the performance of anion exchange membranes

Anion exchange membrane fuel cells (AEMFCs) have emerged as a promising alternative to proton exchange membrane fuel cells (PEMFCs) due to their adaptability to low-cost stack components and non-noble-metals catalysts. However, the poor alkaline resistance and low OH^- conductivity of anion exchange membranes (AEMs) have impeded the large-scale implementation of AEMFCs. Herein, the preparation of a new type of AEMs with crown ether macrocycles in their main chains via a one-pot superacid catalyzed reaction was reported. The study aimed to examine the influence of crown ether cavity size on the phase separation structure, ionic conductivity and alkali resistance of anion exchange membranes. Attributed to the self-assembly of crown ethers, the poly (crown ether) (PCE) AEMs with dibenzo-18-crown-6-ether (QAPCE-18-6) exhibit an obvious phase separated structure and a maximum OH^- conductivity of 122.5 mS cm^-1 at 80 °C (ionic exchange capacity is 1.51 meq g^-1). QAPCE-18-6 shows a good alkali resistance with the OH^- conductivity retention of 94.5% albeit being treated in a harsh alkali condition. Moreover, the hydrogen/oxygen single cell equipped with QAPCE-18-6 can achieve a peak power density (PPD) of 574 mW cm^-2 at a current density of 1.39 A cm^-2.

Pyrrolidinium-Based Hyperbranched Anion Exchange Membranes with Controllable Microphase Separated Morphology for Alkaline Fuel Cells

It is well acknowledged that the microphase-separated morphology of anion exchange membranes (AEMs) is of vital importance for membrane properties utilized in alkaline fuel cells. Herein, a rigid macromolecule poly(methyldiallylamine) (PMDA) is incorporated to regulate the microphase morphology of hyperbranched AEMs. As expected, the hyperbranched poly(vinylbenzyl chloride) (HB-PVBC) is guided to distribute along PMDA chains, and longer PMDA cha leads to a more distinct microphase morphology with interconnected ionic channels. Consequently, high chloride conductivity of 10.49 mS cm^-1 at 30 °C and suppressed water swelling ratio lower than 30% at 80 °C are obtained. Furthermore, the β-H of pyrrolidinium cations in the non-antiperiplanar position increases the energy barrier of β-H elimination, leading to conformationally disfavored Hofmann elimination and increased alkaline stability. This strategy is anticipated to provide a feasible way for preparing hyperbranched AEMs with clear microphase morphology and good overall properties for alkaline fuel cells.

Strong and Flexible High-Performance Anion Exchange Membranes with Long-Distance Interconnected Ion Transport Channels for Alkaline Fuel Cells

Anion exchange membrane fuel cells (AEMFCs), which operate on a variety of green fuels, can achieve high power without emitting greenhouse gases. However, the lack of high ionic conductivity and long-term durability of anion-exchange membranes (AEMs) as their key components is a major obstacle hindering the commercial application of AEMFCs. Here, a series of homogeneous semi-interpenetrating network (semi-IPN) AEMs formed by cross-linking a copolymer of styrene (St) and 4-vinylbenzyl chloride (VBC) with branched polyethylenimine (BPEI) were designed. The pure carbon copolymer skeleton without sulfone/ether bonds accompanied by the semi-IPN endows the AEMs with excellent chemical stability. Moreover, the cross-linking effect of flexible BPEI chains is supposed to promote the "strong-flexible" mechanical properties, while the presence of multiquaternary ammonium groups can boost the formation of microphase separation, thereby enhancing the ionic conductivity of these AEMs. Consequently, the optimized (S₁V₁)₃Q AEM exhibits an excellent hydroxide conductivity of 106 mS cm^-1 at 80 °C, as well as more than 81% residual conductivity after soaking in 1 M NaOH at 60 °C for 720 h. Furthermore, the H₂/O₂ fuel cell assembled with (S₁V₁)₃Q AEM delivers a peak power density of 150.2 mW cm^-2 at 60 °C and 40% relative humidity. All results indicate that the approach of combining a pure carbon backbone polymer with a semi-IPN structure may be a viable strategy for fabricating AEMs that can be used in AEMFCs for long-term applications.

Nanofiltration Membranes Modified with a Clustered Multiquaternary Ammonium-Based Ionic Liquid for Improved Magnesium/Lithium Separation

Lithium separation is of great significance to overcome the lithium supply shortage resulting from a heightened demand in the energy sector. The low selectivity of polymer nanofiltration membranes for lithium extraction from concentrated Mg/Li mixtures caused by miniaturized pore structures and weak and unstable positive surface charges limits their practical implementation. To address the surface charge strength and stability, a novel ionic liquid monomer, N¹-(6-aminohexyl)-N¹,N¹,N⁶,N⁶,N⁶-pentamethylhexane-1,6-diaminium bromide (denoted as DABIL), is first synthesized and covalently anchored on a pristine polyamide thin-film composite (TFC) membrane via a secondary amidation reaction for improved selective lithium separation from Mg/Li mixtures. DABIL modification of the polyamide network contributes to increased surface hydrophilicity, an enlarged membrane pore structure, and reinforced Donnan exclusion effects. Molecular dynamics simulation confirmed that the difference of the interaction energies between water and the multication groups dominates the surface properties. The DABIL membrane exhibits sixfold enhancement of water permeability compared to the unmodified membrane and outperforms the recently reported state-of-the-art positively charged membranes. It presents an improved Li⁺/Mg²⁺ selectivity of 26.49, suggesting the membranes' potential for lithium recovery. Moreover, the membrane shows efficient antibacterial activity for mitigating biofilm formation. We establish that functionalization of TFC membranes with ionic liquids containing multication side chains could be a promising approach to achieve improved and sustainable permselectivity for the recovery of critical metal resources.

Side-Chain-Type Anion Exchange Membranes Based on Poly(arylene ether sulfone)s Containing High-Density Quaternary Ammonium Groups

To obtain anion exchange membranes with both high ionic conductivity and good dimensional stability, a series of side-chain-type poly(arylene ether sulfone)s (PAES-QDTPM-x) were designed and synthesized. Quaternary ammonium (QA) groups were densely aggregated and grafted onto the main chain via flexible hydrophobic spacers. Well-defined microphase separation was confirmed by small-angle X-ray scattering. PAES-QDTPM-0.30 exhibited reasonably high conductivity (39.4 mS cm^-1 at 20 °C and 76.1 mS cm^-1 at 80 °C) and excellent dimensional stability at 80 °C (11.9% in length, 11.2% in thickness) due to the concentration of ion clusters and the side-chain-type structure. All membranes maintained over 82% of the conductivity after alkali treatment for 14 days. In the H₂/O₂ fuel cell performance test, the maximum power density of PAES-QDTPM-0.30 at 60 °C was 225.8 mW cm^-2.

Enhanced Hydroxide Conductivity and Dimensional Stability with Blended Membranes Containing Hyperbranched PAES/Linear PPO as Anion Exchange Membranes

A series of novel blended anion exchange membranes (AEMs) were prepared with hyperbranched brominated poly(arylene ether sulfone) (Br-HB-PAES) and linear chloromethylated poly(phenylene oxide) (CM-PPO). The as-prepared blended membranes were fabricated with different weight ratios of Br-HB-PAES to CM-PPO, and the quaternization reaction for introducing the ionic functional group was performed by triethylamine. The Q-PAES/PPO-XY (quaternized-PAES/PPO-XY) blended membranes promoted the ion channel formation as the strong hydrogen bonds interconnecting the two polymers were maintained, and showed an improved hydroxide conductivity with excellent thermal behavior. In particular, the Q-PAES/PPO-55 membrane showed a very high hydroxide ion conductivity (90.9 mS cm⁻¹) compared to the pristine Q-HB-PAES membrane (32.8 mS cm⁻¹), a result supported by the morphology of the membrane as determined by the AFM analysis. In addition, the rigid hyperbranched structure showed a suppressed swelling ratio of 17.9–24.9% despite an excessive water uptake of 33.2–50.3% at 90 °C, and demonstrated a remarkable alkaline stability under 2.0 M KOH conditions over 1000 h.

Comb-shaped cardo poly(arylene ether nitrile sulfone) anion exchange membranes: significant impact of nitrile group content on morphology and properties

A series of comb-shaped cardo poly(arylene ether nitrile sulfone) (CCPENS-x) materials were synthesized by varying the content of nitrile groups as anion exchange membranes (AEMs). The well-designed architecture of cardo-based main chains and comb-shaped C10 long alkyl side chains bearing imidazolium groups was responsible for the clear microphase-separated morphologies, as confirmed by atomic force microscopy. The ion exchange capacity (IEC) of the AEMs ranged from 1.56 to 1.65 meq. g⁻¹. With strong dipole interchain interactions, the effects of nitrile groups on the membrane morphology and properties were investigated. With the nitrile group content increasing from CCPENS-0.2 to CCPENS-0.8, CCPENS-x revealed larger and more interconnected ionic domains to form more efficient ion-transport channels, thus increasing the corresponding ionic conductivity from 25.8 to 39.5 mS cm⁻¹ at 30 °C and 58.6 to 83 mS cm⁻¹ at 80 °C. Furthermore, CCPENS-x with a higher content of nitrile groups also exhibited lower water uptake (WU) and swelling ratio (SR), and better mechanical properties and thermal stability. This work presents a promising strategy for enhancing the performance of AEMs.

A series of comb-shaped cardo poly(arylene ether nitrile sulfone) (CCPENS-x) materials were synthesized by varying the content of nitrile groups as anion exchange membranes (AEMs).