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

A twisted imidazole-tethered aromatic polymer for high-performance membranes in vanadium-based redox flow batteries

We report on a twisted aromatic polymer, the first xanthene-based example bearing protonated imidazole groups. Its special architecture enables superior ion selectivity, conductivity, and chemical stability, providing excellent performance in both all-vanadium and iron-vanadium redox flow batteries.

Hydrogen Bond Network Assisted Ultrafast Ion Transport of Anion Exchange Membrane Grafting with Covalent Organic Frameworks for Hydrogen Conversion

The development of anion exchange membranes (AEMs) capable of facilitating rapid hydroxide ion transport, while maintaining robust mechanical stability, is considered a key direction for advancing hydrogen energy conversion systems. Herein, we synthesized a series of AEMs by grafting covalent organic frameworks (COFs) onto triphenylpiperidine copolymer and systematically evaluated the performance of AEMs. The tailored COFs, characterized by an extensive hydrogen bond network and high micro-porosity, created interconnected high-speed ion transport channels, significantly reducing the resistance to hydroxide ion conduction. Remarkably, the COF-grafted membranes exhibited superior ionic conductivity compared to pristine triphenylpiperidine, even at lower ion exchange capacities. Additionally, the crystalline and highly rigid structure of the grafted COFs effectively preserved the mechanical stability of the membranes. The optimized COF-grafted AEMs demonstrated outstanding performance, achieving a peak power density of 1.54 W cm-2 in H2-O2 fuel cells and exceptional current densities of 4.5 A cm-2 at 2.0 V in 1 m KOH and 1.1 A cm-2 at 2.0 V in pure water at 80 °C. The present work provides an effective strategy for enhancing AEM performance through the grafting of COFs.

Recent Advances in Direct Synthesis of Functional Polymers of Intrinsic Microporosity Based on (Super)Acid Catalysis

Polymers of intrinsic microporosity (PIMs) are an emerging class of amorphous organic porous materials with solution processability, which are widely used in a multitude of fields such as gas separation, ion conduction, nanofiltration, etc. PIMs have adjustable pore structure and functional pore wall, so it can achieve selective sieving for specific substances. In order to meet the functional requirements of PIMs, two principal methods are used to synthesize functional PIMs, namely, post-modification of PIMs precursors and functionalization of monomers. A number of post-modification routes have been reported, however, the direct synthesis of functional PIMs with diverse groups still remains a challenge. The synthesis of PIMs through the acid-catalyzed polyhydroxyalkylation has been demonstrated to be an effective solution, exhibiting the advantages of wider substrates range, milder reaction conditions, and higher molecular weight. Recently, a series of functional substrates for direct synthesis of PIMs have been proposed. This article presents a review and summary of recent advances in synthesizing PIMs via acid-catalyzed polyhydroxyalkylation, and the synthesis route and structure-activity relationship are emphasized, which provides a versatile platform for the direct synthesis of functional PIMs.

Alkali-Stable Cations and Anion Exchange Membranes

Anion exchange membranes (AEMs) based energy conversion and storage devices have attracted attention as an innovative technology due to their advantageous alkaline catalytic kinetics and cost-effectiveness. AEMs play a crucial role in these devices and have shown significant progress in terms of ionic conductivity, mechanical properties, alkaline stability, and other essential characteristics. Nevertheless, their durability remains a limiting factor preventing the large-scale deployment of AEMs based devices. The attack of hydroxide ions on the cations is an inherent issue that needs to be addressed to enhance the lifetime of the AEMs. Therefore, the design of more stable cationic groups is essential to maintain the initial properties of AEMs and extend the device lifetime. This concept systematically summarizes the development and stability enhancement strategies of the cationic groups for AEMs in recent years, with particular emphasis on the emerging cyclic cationic groups. Furthermore, the stability differences of cations in small molecules versus AEMs are systematically discussed, as well as prospective research toward stable AEMs.

Promoting in-situ stability of hydroxide exchange membranes by thermally conductive network for durable water electrolysis

Hydroxide exchange membrane (HEM) water electrolysis is promising for green hydrogen production due to its low cost and excellent performance. However, HEM often has insufficient stability in strong alkaline solutions, particularly under in-situ electrolysis operation conditions, hindering its commercialization. In this study, we discover that the in-situ stability of HEM is primarily impaired by the locally accumulated heat in HEM due to its low thermal conductivity. Accordingly, we propose highly thermally conductive HEMs with an efficient three-dimensional (3D) thermal diffusion network to promote the in-situ stability of HEM for water electrolysis. Based on the 3D heat conductive network, the thermal conductivity of polymeric HEM is boosted by 32 times and thereby reduce the HEM temperature by up to 4.9 °C in a water electrolyzer at the current density of 1 A cm-2. Thus, the thermally conductive HEM exhibits negligible degradation after 20,000 start/stop cycles and reduces the degradation rate by 6 times compared to the pure polymeric HEM in a water electrolyzer. This study manifests the significance of thermal conductivity of HEM on the durability of water electrolysis, which provides guidelines on the rational design of highly durable HEMs in practical operation conditions for water electrolysis, fuel cells, and beyond.

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.

Anion Exchange Membrane Water Electrolysis at 10 A ⋅ cm-2 Over 800 Hours

Anion exchange membrane water electrolyzer (AEMWE) is a potentially cost-effective technology for green hydrogen production. Although the normal current densities of AEMWEs are below 3 A ⋅ cm-2, operating them at higher current densities represents an efficient, but little-explored approach to decrease the total cost of hydrogen production. We show here that a benchmark AEMWE has an operational lifetime of only seconds at an ultrahigh current density of 10 A ⋅ cm-2. By using a more conductive and robust AEM, and judicious choices of ionomers, catalyst, and porous transport layer, we have developed AEMWEs that stably operate at 10 A ⋅ cm-2 with extended lifetimes. The optimized AEMWE has an operational lifetime of more than 800 hours, a 5-order magnetite improvement over the current benchmark. The cell voltage is only 2.3 V at 10 A ⋅ cm-2, comparable to those of the state-of-the-art devices operating at current densities lower than 3 A ⋅ cm-2. This work demonstrates the potential of ultrahigh current density AEMWEs.

Green Hydrogen Production by Low-Temperature Membrane-Engineered Water Electrolyzers, and Regenerative Fuel Cells

Green hydrogen (H2) is an essential component of global plans to reduce carbon emissions from hard-to-abate industries and heavy transport. However, challenges remain in the highly efficient H2 production from water electrolysis powered by renewable energies. The sluggish oxygen evolution restrains the H2 production from water splitting. Rational electrocatalyst designs for highly efficient H2 production and oxygen evolution are pivotal for water electrolysis. With the development of high-performance electrolyzers, the scale-up of H2 production to an industrial-level related activity can be achieved. This review summarizes recent advances in water electrolysis such as the proton exchange membrane water electrolyzer (PEMWE) and anion exchange membrane water electrolyzer (AEMWE). The critical challenges for PEMWE and AEMWE are the high cost of noble-metal catalysts and their durability, respectively. This review highlights the anode and cathode designs for improving the catalytic performance of electrocatalysts, the electrolyte and membrane engineering for membrane electrode assembly (MEA) optimizations, and stack systems for the most promising electrolyzers in water electrolysis. Besides, the advantages of integrating water electrolyzers, fuel cells (FC), and regenerative fuel cells (RFC) into the hydrogen ecosystem are introduced. Finally, the perspective of electrolyzer designs with superior performance is presented.

Ultramicroporous crosslinked polyxanthene-poly(biphenyl piperidinium)-based anion exchange membranes for water electrolyzers operating under highly alkaline conditions

Anion exchange membrane water electrolyzers (AEMWEs) suffer from low efficiencies and durability, due to the unavailability of appropriate anion exchange membranes (AEM). Herein, a rigid ladder-like polyxanthene crosslinker was developed for the preparation of ultramicroporous crosslinked polyxanthene-poly(biphenyl piperidinium)-based AEMs. Due to the synergetic effects of their ultramicroporous structure and microphase-separation morphology, the crosslinked membranes showed high OH- conductivity (up to 163 mS cm-1 at 80 °C). Furthermore, these AEMs also exhibited moderate water uptake, excellent dimensional stability, and remarkable alkaline stability. The single-cell AEMWE based on QPBP-PX-15% and equipped with non-noble catalysts achieved a current density of 3000 mA cm-2 at 2.03 V (compared to PiperION's 2.26 V) in 6 M KOH solution at 80 °C, which outperformed many AEMWEs that used platinum-group-metal catalysts. Thus, the crosslinked AEMs developed in this study showed significant potential for application in AEMWEs fed with concentrated alkaline solutions.

Microporous Electrode Binders for Anion Exchange Membrane Water Electrolyzers

In anion exchange membrane (AEM) water electrolyzers, AEMs separate hydrogen and oxygen, but should efficiently transport hydroxide ions. In the electrodes, catalyst nanoparticles are mechanically bonded to the porous transport layer or membrane by a polymeric binder. Since these binders form a thin layer on the catalyst particles, they should not only transport hydroxide ions and water to the catalyst particles, but should also transport the nascating gases away. In the worst case, if formation of gases is >> than gas transport, a gas pocket between catalyst surface and the binder may form and hinder access to reactants (hydroxide ions, water). In this work, the ion conductive binder SEBS-DABCO is blended with PIM-1, a highly permeable polymer of intrinsic microporosity. With increasing amount of PIM-1 in the blends, the permeability for water (selected to represent small molecules) increases. Simultaneously, swelling and conductivity decrease, due to the increased hydrophobicity. Ex situ data and electrochemical data indicate that blends with 50% PIM-1 have better properties than blends with 25% or 75% PIM-1, and tests in the electrolyzer confirm an improved performance when the SEBS-DABCO binder contains 50% PIM-1.

High-Performance Anion Exchange Membrane Water Electrolyzers Enabled by Highly Gas Permeable and Dimensionally Stable Anion Exchange Ionomers

Designing suitable anion exchange ionomers is critical to improving the performance and in situ durability of anion exchange membrane water electrolyzers (AEMWEs) as one of the promising devices for producing green hydrogen. Herein, highly gas-permeable and dimensionally stable anion exchange ionomers (QC6xBA and QC6xPA) are developed, in which bulky cyclohexyl (C6) groups are introduced into the polymer backbones. QC650BA-2.1 containing 50 mol% C6 composition shows 16.6 times higher H2 permeability and 22.3 times higher O2 permeability than that of QC60BA-2.1 without C6 groups. Through-plane swelling of QC650BA-2.1 decreases to 12.5% from 31.1% (QC60BA-2.1) while OH- conductivity slightly decreases (64.9 and 56.2 mS cm-1 for QC60BA-2.1 and QC650BA-2.1, respectively, at 30 °C). The water electrolysis cell using the highly gas permeable QC650BA-2.1 ionomer and Ni0.8Co0.2O in the anode catalyst layer achieves two times higher performance (2.0 A cm-2 at 1.69 V, IR-included) than those of the previous cell using in-house ionomer (QPAF-4-2.0) (1.0 A cm-2 at 1.69 V, IR-included). During 1000 h operation at 1.0 A cm-2, the QC650BA-2.1 cell exhibits nearly constant cell voltage with a decay rate of 1.1 µV h-1 after the initial increase of the cell voltage, proving the effectiveness of the highly gas permeable and dimensionally stable ionomer in AEMWEs.

Anion Exchange Membrane with Pendulous Piperidinium on Twisted All-Carbon Backbone for Fuel Cell

As a central component for anion exchange membrane fuel cells (AEMFCs), the anion exchange membrane is now facing the challenge of further improving its conductivity and alkali stability. Herein, a twisted all-carbon backbone is designed by introducing stereo-contorted units with piperidinium groups dangled at the twisted sites. The rigid and twisted backbone improves the conduction of hydroxide and brings down the squeezing effect of the backbone on piperidine rings. Accordingly, an anion exchange membrane prepared through this method exhibits adapted OH- conductivity, low swelling ratio and excellent alkali stability, even in high alkali concentrations. Further, a fuel cell assembled with a such-prepared membrane can reach a power density of 904.2 mW/cm2 and be capable of continuous operation for over 50 h. These results demonstrate that the designed membrane has good potential for applications in AEMFCs.

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.

Stabilizing the Catalyst Layer for Durable and High Performance Alkaline Membrane Fuel Cells and Water Electrolyzers

Anion exchange membrane (AEM) fuel cells (AEMFCs) and water electrolyzers (AEMWEs) suffer from insufficient performance and durability compared with commercialized energy conversion systems. Great efforts have been devoted to designing high-quality AEMs and catalysts. However, the significance of the stability of the catalyst layer has been largely disregarded. Here, an in situ cross-linking strategy was developed to promote the interactions within the catalyst layer and the interactions between catalyst layer and AEM. The adhesion strength of the catalyst layer after cross-linking was improved 7 times compared with the uncross-linked catalyst layer due to the formation of covalent bonds between the catalyst layer and AEM. The AEMFC can be operated under 0.6 A cm-2 for 1000 h with a voltage decay rate of 20 μV h-1. The related AEMWE achieved an unprecedented current density of 15.17 A cm-2 at 2.0 V and was operated at 0.5, 1.0, and 1.5 A cm-2 for 1000 h.