High-performance tetracyclic aromatic anion exchange membranes containing twisted binaphthyl for fuel cells

Engineering Robust Triazine Crosslinked and Pyridine Capped Anion Exchange Membrane for Advanced Water Electrolysis

Exploring high-performance anion exchange membranes (AEM) for water electrolyzers (AEMWEs) is significant for green hydrogen production. However, the current AEMWEs are restricted by the poor mechanical strength and low OH- conductivity of AEMs, leading to the low working stability and low current density. Here, we develop a robust AEM with polybiphenylpiperidium network by combining the crosslinking with triazine and the capping with pyridine for advanced AEMWEs. The AEM exhibits an excellent mechanical strength (79.4 MPa), low swelling ratio (19.2 %), persistent alkali stability (≈5,000 hours) and high OH- conductivity (247.2 mS cm-1) which achieves the state-of-the-art AEMs. Importantly, when applied in AEMWEs, the corresponding electrolyzer equipped with commercial nickel iron and nickel molybdenum catalysts obtained a current density of up to 3.0 A cm-2 at 2 V and could be stably operated ~430 h at a high current density of 1.6 A cm-2, which exceeds the most of AEMWEs. Our results suggest that triazine crosslinking and pyridine capping can effectively improve the overall performance of the AEMWEs.

Tuning polar discrimination between side chains to improve the performance of anion exchange membranes

Anion exchange membranes (AEMs) are the heart of alkaline fuel cells and water electrolysis, and have made a great progress in recent years. However, AEMs are still unable to satisfy the needs of high conductivity and stability, hindering their widespread commercialization. Side chain regulations have been widely used to prepare highly conductive and durable AEMs. Here, we construct a series of polyaromatic AEMs grafted with fluorinated cation side chains and cation-free alkyl chains with different end groups to explore the polar discrimination of side chains on membrane performance. This work demonstrates that AEMs grafting the cation side chains with superhydrophobic fluorine pendent and alkyl side chains with hydrophilic pendent enhance water content and ion conductivity. This is due to the strong immiscibility between the hydrophilic and hydrophobic head groups which promotes the establishments of microphase separation and ion highways. Specifically, poly(binaphthyl-co-terphenyl piperidinium) containing fluorinated piperidinium side chains and alkyl chains with methoxy pendent (QBNTP-QFM) possesses a satisficed OH- conductivity (170.6 mS cm-1 at 80 °C) and can tolerate 5 M hot NaOH for 2100 h with only 3.4 % conductivity loss. Expectedly, the single cell with QBNTP-QFM yields a prominent maximum power density of 1.62 W cm-2 and the water electrolysis cell with QBNTP-QFM achieves a pronounced current density of 3.0 A cm-2 at 1.8 V, both cells also display a prominent durability for 120 h operation. The results prove that this side chain optimization can improve ion conductivity and is a promising method for AEM development.

Aryl ether-free polymer electrolytes for electrochemical and energy devices

Anion exchange polymers (AEPs) play a crucial role in green hydrogen production through anion exchange membrane water electrolysis. The chemical stability of AEPs is paramount for stable system operation in electrolysers and other electrochemical devices. Given the instability of aryl ether-containing AEPs under high pH conditions, recent research has focused on quaternized aryl ether-free variants. The primary goal of this review is to provide a greater depth of knowledge on the synthesis of aryl ether-free AEPs targeted for electrochemical devices. Synthetic pathways that yield polyaromatic AEPs include acid-catalysed polyhydroxyalkylation, metal-promoted coupling reactions, ionene synthesis via nucleophilic substitution, alkylation of polybenzimidazole, and Diels-Alder polymerization. Polyolefinic AEPs are prepared through addition polymerization, ring-opening metathesis, radiation grafting reactions, and anionic polymerization. Discussions cover structure-property-performance relationships of AEPs in fuel cells, redox flow batteries, and water and CO2 electrolysers, along with the current status of scale-up synthesis and commercialization.

Separators and Membranes for Advanced Alkaline Water Electrolysis

Traditionally, alkaline water electrolysis (AWE) uses diaphragms to separate anode and cathode and is operated with 5-7 M KOH feed solutions. The ban of asbestos diaphragms led to the development of polymeric diaphragms, which are now the state of the art material. A promising alternative is the ion solvating membrane. Recent developments show that high conductivities can also be obtained in 1 M KOH. A third technology is based on anion exchange membranes (AEM); because these systems use 0-1 M KOH feed solutions to balance the trade-off between conductivity and the AEM's lifetime in alkaline environment, it makes sense to treat them separately as AEM WE. However, the lifetime of AEM increased strongly over the last 10 years, and some electrode-related issues like oxidation of the ionomer binder at the anode can be mitigated by using KOH feed solutions. Therefore, AWE and AEM WE may get more similar in the future, and this review focuses on the developments in polymeric diaphragms, ion solvating membranes, and AEM.

Beyond Small Molecular Cations: Elucidating the Alkaline Stability of Cationic Moieties at the Membrane Scale

A major hindrance in the commercialization of alkaline polyelectrolyte-based electrochemical energy conversion devices is the development of durable anion exchange membranes (AEMs). Despite many alkali-stable cations that have been explored, the stability of these cationic moieties at the membrane scale is in the blind. Herein, we present a molecularly designed polyaromatic AEM with cationic moieties in an alternating manner to unbiasedly compare the alkaline stability of piperidinium and ammonium groups at the membrane state. Using nuclear magnetic resonance spectroscopy, we demonstrate that the pentyltrimethyl group is about 2-fold more stable than piperidinium within a polyaromatic scaffold, either in ex-situ alkaline soaking or in-situ cell operation. This finding challenges the judgment extrapolated from the stability trend of cations, that is, the piperidinium-functionalized AEM is more alkali-stable than the counterparts based on quaternary ammoniums. Moreover, the deterioration mechanism of piperidinium moiety after being embedded in polyaromatic backbone is rationalized by density functional theory.

"Fishbone" Design of Amino/N-Spirocyclic Cations toward High-Performance Poly(triphenylene piperidine) Anion-Exchange Membranes for Fuel Cells

N-Spirocyclic cations have excellent alkali resistance stability, and precise design of the structure of N-spirocyclic anion-exchange membranes (AEMs) improves their comprehensive performance. Here, we design and synthesize high-performance poly(triphenylene piperidine) membranes based on the "fishbone" design of amino/N-spirocyclic cations. The "fishbone" design does not disrupt the overall stabilized conformation but promotes a microphase separation structure, while exerting the synergistic effect of piperidine cations and spirocyclic cations, resulting in a membrane with good conductivity and alkali resistance stability. The hydroxide conductivity of the QPTPip-ASU-X membrane reached up to 133.5 mS cm-1 at 80 °C. The QPTPip-ASU-15 membrane was immersed in a 2 M NaOH solution at 80 °C for 1200 h, and the conductivity was maintained at 91.02%. In addition, the QPTPip-ASU-5 membrane had the highest peak power density of 255 mW cm-2.

Recent Development of Anion Exchange Membrane Fuel Cells and Performance Optimization Strategies: A Review

Anion exchange membrane fuel cells (AEMFCs) are the most promising low-temperature fuel cells and have received extensive attention. Compared to PEMFCs, the cost per unit of power can be significantly reduced for AEMFCs because, in theory, they allow the usage of non-precious metal catalysts and low-cost cell components. Owing to the development of advanced materials and performance improvement strategies, AEMFCs have achieved new records in both initial performance and durability. However, the high performance currently achieved is contingent on certain conditions, e. g., high Pt loading, large gas flowrates, and operation in pure O2 , which are far from practical applications. Therefore, the transition to commercially relevant performance and durability is the next goal of AEMFCs. This paper reviews the performance data of H2 -fueled AEMFCs since 2010 and summarizes possible performance optimization schemes, which can provide useful insights for developing next-generation AEMFCs.

Fluorinated poly(p-triphenyl piperidine) anion exchange membranes with robust dimensional stability for fuel cells

Anion exchange membrane fuel cells (AEMFCs), which are more economical than proton exchange membrane fuel cells (PEMFCs), stand out in the context of the rapid development of renewable energy. Superacid-catalyzed ether-free aromatic polymers have recently received a lot of attention due to their exceptional performance, but their development has been hampered by the trade-off between the dimensional stability and ionic conductivity of anion exchange membranes (AEMs). Here, we introduced fluoroketones containing different numbers of fluorinated groups (x = 0, 3 and 6) in the main chain of p-terphenyl piperidine because of the favorable hydrophobic properties of fluorinated groups. The results show that fluorinated AEMs can enhance OH- conductivity by building more aggregated hydrophilic channels while ensuring dimensional stability. The PTF6-QAPTP AEM with more fluorinated groups has the most excellent performance at 80 °C with an OH- conductivity of 142.7 mS cm-1 and a swelling ratio (SR) of only 4.55 %. Additionally, it exhibits good alkali durability, with the OH- conductivity and quaternary ammonium (QA) cation retaining at 93.45% and 92.6%, respectively, after immersion in a 2 M NaOH solution at 80 °C for 1200 h. In addition, the power density of the PTF6-QAPTP based single cell reaches 849 mW cm-2 when the current density is 1600 mA cm-2. The PTF6-QAPTP based cell has a voltage retention of 88% after 80 h of stability testing at a constant current density of 300 mA cm-2 at 80 °C.

Poly(Ethylene Piperidinium)s for Anion Exchange Membranes

The lack of anion exchange membranes (AEMs) that possess both high hydroxide conductivity and stable mechanical and chemical properties poses a major challenge to the development of high-performance fuel cells. Improving one side of the balance between conductivity and stability usually means sacrificing the other. Herein, we used facile, high-yield chemical reactions to design and synthesize a piperidinium polymer with a polyethylene backbone for AEM fuel cell applications. To improve the performance, we introduced ionic crosslinking into high-cationic-ratio AEMs to suppress high water uptake and swelling while further improving the hydroxide conductivity. Remarkably, PEP80-20PS achieved a hydroxide conductivity of 354.3 mS cm-1 at 80 °C while remaining mechanically stable. Compared with the base polymer PEP80, the water uptake of PEP80-20PS decreased by 69 % from 813 % to 350 %, and the swelling decreased substantially by 85 % from 350.0 % to 50.2 % at 80 °C. PEP80-20PS also showed excellent alkaline stability, 84.7 % remained after 35 days of treatment with an aqueous KOH solution. The chemical design in this study represents a significant advancement toward the development of simultaneously highly stable and conductive AEMs for fuel cell applications.

Alkaline Membranes toward Electrochemical Energy Devices: Recent Development and Future Perspectives

Anion-exchange membranes (AEMs) that can selectively transport OH-, namely, alkaline membranes, are becoming increasingly crucial in a variety of electrochemical energy devices. Understanding the membrane design approaches can help to break through the constraints of undesired performance and lab-scale production. In this Outlook, the research progress of alkaline membranes in terms of backbone structures, synthesis methods, and related applications is organized and discussed. The evaluation of synthesis methods and description of membrane stability enhancement strategies provide valuable insights for structural design. Finally, to accelerate the deployment of relevant technologies in alkaline media, the future priority of alkaline membranes that needs to be addressed is presented from the perspective of science and engineering.