Designing the next generation of proton-exchange membrane fuel cells.

With the rapid growth and development of proton-exchange membrane fuel cell (PEMFC) technology, there has been increasing demand for clean and sustainable global energy applications. Of the many device-level and infrastructure challenges that need to be overcome before wide commercialization can be realized, one of the most critical ones is increasing the PEMFC power density, and ambitious goals have been proposed globally. For example, the short- and long-term power density goals of Japan's New Energy and Industrial Technology Development Organization are 6 kilowatts per litre by 2030 and 9 kilowatts per litre by 2040, respectively. To this end, here we propose technical development directions for next-generation high-power-density PEMFCs. We present the latest ideas for improvements in the membrane electrode assembly and its components with regard to water and thermal management and materials. These concepts are expected to be implemented in next-generation PEMFCs to achieve high power density.

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.

Aminal-Linked Covalent Organic Framework Membranes Achieve Superior Ion Selectivity.

High-salinity wastewater treatment is perceived as a global water resource recycling challenge that must be addressed to achieve zero discharge. Monovalent/divalent salt separation using membrane technology provides a promising strategy for sulfate removal from chlor-alkali brine. However, existing desalination membranes often show low water permeance and insufficient ion selectivity. Herein, an aminal-linked covalent organic framework (COF) membrane featuring a regular long-range pore size of 7 Å and achieving superior ion selectivity is reported, in which a uniform COF layer with subnanosized channels is assembled by the chemical splicing of 1,4-phthalaldehyde (TPA)-piperazine (PZ) COF through an amidation reaction with trimesoyl chloride (TMC). The chemically spliced TPA-PZ (sTPA-PZ) membrane maintains an inherent pore structure and exhibits a water permeance of 13.1 L m-2 h-1 bar-1, a Na2SO4 rejection of 99.1%, and a Cl-/SO4 2- separation factor of 66 for mixed-salt separation, which outperforms all state-of-the-art COF-based membranes reported. Furthermore, the single-stage treatment of NaCl/Na2SO4 mixed-salt separation achieves a high NaCl purity of above 95% and a recovery rate of ≈60%, offering great potential for industrial application in monovalent/divalent salt separation and wastewater resource utilization. Therefore, the aminal-linked COF membrane developed in this work provides a new research avenue for designing smart/advanced membrane materials for angstrom-scale separations.

Interface-Engineered NiFe/Ni-S Nanoparticles for Reliable Alkaline Oxygen Production at Industrial Current: A Sulfur Source Confinement Strategy.

Using powder-based ink appears to be the most suitable candidate for commercializing the membrane electrode assembly (MEA), while research on the powder-based NPM catalyst for anion exchange membrane water electrolyzer (AEMWE) is currently insufficient, especially at high current density. Herein, a sulfur source (NiFe Layered double hydroxide adsorbed SO 4 2 - ${\mathrm{SO}}_4^{2 - }$ ) confinement strategy is developed to integrate Ni3S2 onto the surface of amorphous/crystalline NiFe alloy nanoparticles (denoted NiFe/Ni-S), achieving advanced control over the sulfidation process for the formation of metal sulfides. The constructed interface under the sulfur source confinement strategy generates abundant active sites that increase electron transport at the electrode-electrolyte interface and improve ability over an extended period at a high current density. Consequently, the constructed NiFe/Ni-S delivers an ultra-low overpotential of 239 mV at 10 mA cm-2 and 0.66 mA cm ECSA - 2 ${\mathrm{cm}}_{{\mathrm{ECSA}}}^{ - 2}$ under an overpotential of 300 mV. The AEMWE with NiFe/Ni-S anode exhibits a cell voltage of 1.664 V @ 0.5 A cm-2 and a 400 h stability at 1.0 A cm-2.

ZIF-62 glass foam self-supported membranes to address CH4/N2 separations.

Membranes with ultrahigh permeance and practical selectivity could greatly decrease the cost of difficult industrial gas separations, such as CH4/N2 separation. Advanced membranes made from porous materials, such as metal-organic frameworks, can achieve a good gas separation performance, although they are typically formed on support layers or mixed with polymeric matrices, placing limitations on gas permeance. Here an amorphous glass foam, agfZIF-62, wherein a, g and f denote amorphous, glass and foam, respectively, was synthesized by a polymer-thermal-decomposition-assisted melting strategy, starting from a crystalline zeolitic imidazolate framework, ZIF-62. The thermal decomposition of incorporated low-molecular-weight polyethyleneimine evolves CO2, NH3 and H2O gases, creating a large number and variety of pores. This greatly increases pore interconnectivity but maintains the crystalline ZIF-62 ultramicropores, allowing ultrahigh gas permeance and good selectivity. A self-supported circular agfZIF-62 with a thickness of 200-330 µm and area of 8.55 cm2 was used for membrane separation. The membranes perform well, showing a CH4 permeance of 30,000-50,000 gas permeance units, approximately two orders of magnitude higher than that of other reported membranes, with good CH4/N2 selectivity (4-6).

Large-Area Ultrathin Metal-Organic Framework Membranes Fabricated on Flexible Polymer Supports for Gas Separations.

Ultrathin continuous metal-organic framework (MOF) membranes have the potential to achieve high gas permeance and selectivity simultaneously for otherwise difficult gas separations, but with few exceptions for zeolitic-imidazolate frameworks (ZIF) membranes, current methods cannot conveniently realize practical large-area fabrication. Here, we propose a ligand back diffusion-assisted bipolymer-directed metal ion distribution strategy for preparing large-area ultrathin MOF membranes on flexible polymeric support layers. The bipolymer directs metal ions to form a cross-linked two-dimensional (2D) network with a uniform distribution of metal ions on support layers. Ligand back diffusion controls the feed of ligand molecules available for nuclei formation, resulting in the continuous growth of large-area ultrathin MOF membranes. We report the practical fabrication of three representative defect-free MOF membranes with areas larger than 2,400 cm2 and ultrathin selective layers (50-130 nm), including ZIFs and carboxylate-linker MOFs. Among these, the ZIF-8 membrane displays high gas permeance of 3,979 GPU for C3H6, with good mixed gas selectivity (43.88 for C3H6/C3H8). To illustrate its scale-up practicality, MOF membranes were prepared and incorporated into spiral-wound membrane modules with an active area of 4,800 cm2. The ZIF-8 membrane module presents high gas permeance (3,930 GPU for C3H6) with acceptable ideal gas selectivity (37.45 for C3H6/C3H8).

Defect-Engineered ZIF-Derived Non-Pt Cathode Catalyst at 1.5 mg cm-2 Loading for Proton Exchange Membrane Fuel Cells.

Due to the sluggish kinetics of the oxygen reduction reaction (ORR) by non-Pt based catalyst, high loading of catalyst is required to achieve satisfactory fuel cell performance, which inevitably leads to the increase of the catalyst layer thickness with serious mass transport resistance. Herein, a defective zeolitic imidazolate framework (ZIF) derived Co/Fe-N-C catalyst with small mesopores (2-4 nm) and high density of CoFe atomic active sites are prepared by regulating the Fe dosage and pyrolysis temperature. Molecular dynamics simulation and electrochemical tests indicate that > 2 nm mesopores show insignificant influence on the diffusion process of O2 and H2 O molecules, leading to the high utilization of active sites and low mass transport resistance. The proton exchange membrane fuel cell (PEMFC) shows a high-power density of 755 mW cm-2 with only 1.5 mg cm-2 of non-Pt catalyst in the cathode. No apparent performance loss caused by concentration difference can be observed, in particular in the high current density region (1 A cm-2 ). This work emphasizes the importance of small mesopore design in the Co/Fe-N-C catalyst, which is anticipated to provide essential guidance for the application of non-Pt catalysts.

Host-Guest Recognition Boosts Biomimetic Mono/Multivalent Cation Separation.

Biomimetic ion permselective membranes with ultrahigh ion permeability and selectivity represent a research frontier in ion separation, yet the successful fabrication of such membranes remains a formidable challenge. Here, we demonstrate a 4-sulfocalix[4]arene (4-SCA)-modified graphene oxide (GO) membrane that shows extraordinary performance in separating mono-from multivalent cations, as well as having reversible pH-responsiveness. The resulting 4-SCA-modified GO (SCA-GO) membrane preferentially transports potassium ions (K+) over radionuclide cations (Co2+, UO22+, La3+, Eu3+, and Th4+). The ion selectivities are an order of magnitude higher than that of the unmodified GO membrane. Theoretical calculations and experimental investigations demonstrate that the much-improved ion selectivity arises from the specific recognition between 4-SCA and radionuclide cations. The transport of multivalent radionuclides is impeded by a binding-obstructing mechanism from the host-guest interactions. Interestingly, the host-guest interactions are responsive to the protonation/deprotonation transformation of the 4-SCA. Therefore, the SCA-GO membrane mimics pH-regulated ion selective behavior found in biological ion channels. Our strategy of designing a biomimetic permselective GO membrane may allow efficient nuclear wastewater treatment and, more importantly, deepen our understanding of biomimetic ion transport mechanisms.

Metal-Organic Framework Crystal-Glass Composite Membranes with Preferential Permeation of Ethane.

Metal-organic framework (MOF) glass is an easy to process and self-supported amorphous material that is suitable for fabricating gas separation membranes. However, MOF glasses, such as ZIF-62 and ZIF-4 have low porosity, which makes it difficult to obtain membranes with high permeance. Here, a self-supported MOF crystal-glass composite (CGC) membrane was prepared by melt quenching a mixture of ZIF-62 as the membrane matrix and ZIF-8 as the filler. The conversion of ZIF-62 from crystal to glass and the simultaneous partial melting of ZIF-8 facilitated by the melt state of ZIF-62 make the CGC membrane monolithic, eliminating non-selective grain boundaries and improving selectivity. The thickness of CGC membrane can be adjusted to fabricate a membrane without the need of a support substrate. CGC membranes exhibit a C2 H6 permeance of 41 569 gas permeation units (GPU) and a C2 H6 /C2 H4 selectivity of 7.16. The CGC membrane has abundant pores from the glassy state of ZIF-62 and the crystalline ZIF-8, which enables high gas permeance. ZIF-8 has preferential adsorption for C2 H6 and promotes C2 H6 transport in the membrane, and thus the GCG membrane exhibits ultrahigh C2 H6 permeance and good C2 H6 /C2 H4 selectivity.

Single Solution-Phase Synthesis of Charged Covalent Organic Framework Nanosheets with High Volume Yield.

Covalent organic framework nanosheets (COF-NSs) are emerging building blocks for functional materials, and their scalable fabrication is highly desirable. Current synthetic methods suffer from low volume yields resulting from confined on-surface/at-interface growth space and complex multiple-phase synthesis systems. Herein, we report the synthesis of charged COF-NSs in open space using a single-phase organic solution system, achieving magnitudes higher volume yields of up to 18.7 mg mL-1 . Charge-induced electrostatic repulsion forces enable in-plane anisotropic secondary growth from initial discrete and disordered polymers into large and crystalline COF-NSs. The charged COF-NS colloidal suspensions are cast into thin and compact proton exchange membranes (PEMs) with lamellar morphology and oriented crystallinity, displaying outstanding proton conductivity, negligible dimensional swelling, and good H2 /O2 fuel cell performance.

Fast Current-Driven Synthesis of ZIF-Derived Catalyst Layers for High-Performance Zn-Air Battery.

As a core component, the catalyst layer (CL) is widely used in fuel cell, metal-air battery, and other energy conversion devices. Herein, a highly efficient method for CL preparation via fast current-driven synthesis followed by pyrolysis is proposed. Compared with previously reported fabrication procedures of zeolite imidazolate frameworks (ZIF)-based CLs, this method directly deposits the ZIF precursor onto the conductive substrate in a very short time (≤15 min). The self-supporting CL, converted from ZIF membrane by simple single-step pyrolysis, is assembled with the gas diffusion layer to obtain cathode. Electrochemical tests exhibit a small potential gap (0.83 V) between the oxygen reduction and evolution reactions, as well as high performance and excellent stability for Zn-air battery (241 mW cm-2 at 390 mA cm-2 ), due to the unique design of a bi-continuous framework (interconnected pores and long carbon nanotubes) and Co-based active sites. This work may provide new directions for the fast fabrication of non-platinum group metal CLs for metal-air batteries or fuel cell applications.

Nanocrack-regulated self-humidifying membranes.

The regulation of water content in polymeric membranes is important in a number of applications, such as reverse electrodialysis and proton-exchange fuel-cell membranes. External thermal and water management systems add both mass and size to systems, and so intrinsic mechanisms of retaining water and maintaining ionic transport in such membranes are particularly important for applications where small system size is important. For example, in proton-exchange membrane fuel cells, where water retention in the membrane is crucial for efficient transport of hydrated ions, by operating the cells at higher temperatures without external humidification, the membrane is self-humidified with water generated by electrochemical reactions. Here we report an alternative solution that does not rely on external regulation of water supply or high temperatures. Water content in hydrocarbon polymer membranes is regulated through nanometre-scale cracks ('nanocracks') in a hydrophobic surface coating. These cracks work as nanoscale valves to retard water desorption and to maintain ion conductivity in the membrane on dehumidification. Hydrocarbon fuel-cell membranes with surface nanocrack coatings operated at intermediate temperatures show improved electrochemical performance, and coated reverse-electrodialysis membranes show enhanced ionic selectivity with low bulk resistance.

Oil-Water-Oil Triphase Synthesis of Ionic Covalent Organic Framework Nanosheets.

Ionic covalent organic framework nanosheets (iCOFNs) with long-range ordered and mono-dispersed ionic groups hold great potential in many advanced applications. Considering the inherent drawbacks of oil-water biphase method, herein, we explore an oil-water-oil triphase method based on phase engineering strategy for the bottom-up synthesis of iCOFNs. The middle water phase serves as a confined reaction region, and the two oil phases are reservoirs for storing and supplying monomers to the water phase. A large aqueous space and low monomer concentration lead to the anisotropic gradual growth of iCOFNs into few-layer thickness, large lateral size, and high crystallinity. Notably, the resulting three cationic and anionic iCOFNs exhibit ultra-high aspect ratios of up to 20,000. We further demonstrate their application potential by processing into ultrathin defect-free COF membranes for efficient biogas separation. Our triphase method may offer an alternative platform technology for the synthesis and innovative applications of iCOFNs.

Metal-induced ordered microporous polymers for fabricating large-area gas separation membranes.

Metal-induced ordered microporous polymers (MMPs), a class of porous polymer, are synthesized from amine-bearing polymers, small organic linkers and divalent metal ions using a polymer-directed chemical synthesis process. Specifically, small organic linkers first coordinate to metal ions, with the resulting unit cells then self-assembling along the extension of polymer chains to construct three-dimensional frameworks. The MMPs demonstrate good controllability of crystal and framework size, as well as hydrolytic stability. MMP dispersions were coated on a modified polysulfone substrate to fabricate MMP/mPSf membranes with an ultrathin selective layer (below 50 nm) and surface areas of >100 cm2. The MMPs are readily fabricated into defect-free thin selective-layered membranes with high CO2 permeance (3,000 GPU) and stable CO2/N2 selectivity (78) under both humid and dry gas feed conditions, demonstrating promising CO2 membrane separation performance. This synthetic methodology could be extended to other polymers, potentially enabling facile synthesis of membrane materials.

Harnessing Filler Materials for Enhancing Biogas Separation Membranes.

Biogas is an increasingly attractive renewable resource, envisioned to secure future energy demands and help curb global climate change. To capitalize on this resource, membrane processes and state-of-the-art membranes must efficiently recover methane (CH4) from biogas by separating carbon dioxide (CO2). Composite (a.k.a. mixed-matrix) membranes, prepared from common polymers and rationally selected/engineered fillers, are highly promising for this application. This review comprehensively examines filler materials that are capable of enhancing the CO2/CH4 separation performance of polymeric membranes. Specifically, we highlight novel synthetic strategies for engineering filler materials to develop high-performance composite membranes. Besides, as the matrix components (polymers) of composite membranes largely dictate the overall gas separation performances, we introduce a new empirical metric, the "Filler Enhancement Index" ( Findex), to aid researchers in assessing the effectiveness of the fillers from a big data perspective. The Findex systematically decouples the effect of polymer matrices and critically evaluates both conventional and emerging fillers to map out a future direction for next-generation (bio)gas separation membranes. Beyond biogas separation, this review is of relevance to a broader community with interests in composite membranes for other gas separation processes, as well as water treatment applications.

Flexible Superhydrophobic Metal-Based Carbon Nanotube Membrane for Electrochemically Enhanced Water Treatment.

Treatment of highly saline wastewaters via conventional technology is a key challenging issue, which calls for efficient desalination membranes featuring high flux and rejection, low fouling, and excellent stability. Herein, we report a high-strength and flexible electro-conductive stainless steel-carbon nanotube (SS-CNT) membrane, exhibiting significantly enhanced anticorrosion and antifouling ability via a microelectrical field-coupling strategy during membrane distillation. The membrane substrates exhibited excellent mechanical strength (244.2 ± 9.8 MPa) and ductility, thereby overcoming the critical bottleneck of brittleness of traditional inorganic membranes. By employing a simple surface activation followed by self-catalyzed chemical vapor deposition, CNT was grown in situ on SS substrates via a tip-growth mechanism to finally form robust superhydrophobic SS-CNT membrane. To address the challenging issues of significant corrosion and fouling, using a negative polarization microelectrical field-coupling strategy, simultaneously enhanced antifouling and anticorrosion performance was realized for treatment of organic high salinity waters while exhibiting stable high flux and rejection via an electrostatic repulsion and electron supply mechanism. This application-oriented rational design protocol can be potentially used to extend toward high performance composite membranes derived from other electro-conductive metal substrates functionally decorated with CNT network and to other applications in water treatment.

Hydrocarbon-Based Polymer Electrolyte Membranes: Importance of Morphology on Ion Transport and Membrane Stability.

A fundamental understanding of polymer microstructure is important in order to design novel polymer electrolyte membranes (PEMs) with excellent electrochemical performance and stabilities. Hydrocarbon-based polymers have distinct microstructure according to their chemical structure. The ionic clusters and/or channels play a critical role in PEMs, affecting ion conductivity and water transport, especially at medium temperature and low relative humidity (RH). In addition, physical properties such as water uptake and dimensional swelling behavior depend strongly on polymer morphology. Over the past few decades, much research has focused on the synthetic development and microstructural characterization of hydrocarbon-based PEM materials. Furthermore, blends, composites, pressing, shear field, electrical field, surface modification, and cross-linking have also been shown to be effective approaches to obtain/maintain well-defined PEM microstructure. This review summarizes recent work on developments in advanced PEMs with various chemical structures and architecture and the resulting polymer microstructures and morphologies that arise for potential application in fuel cell, lithium ion battery, redox flow battery, actuators, and electrodialysis.

Stable Superhydrophobic Ceramic-Based Carbon Nanotube Composite Desalination Membranes.

Membrane distillation (MD) is a promising process for the treatment of highly saline wastewaters. The central component of MD is a stable porous hydrophobic membrane with a large liquid-vapor interface for efficient water vapor transport. A key challenge for current polymeric or hydrophobically modified inorganic membranes is insufficient operating stability, resulting in some issues such as wetting, fouling, flux, and rejection decline. This study presents an overall conceptual design and application strategy for a superhydrophobic ceramic-based carbon nanotube (CNT) desalination membrane having specially designed membrane structures with unprecedented operating stability and MD performance. Superporous and superhydrophobic surface structures with CNT networks are created after quantitative regulation of in situ grown CNT. The fully covered CNT layers (FC-CNT) exhibit significantly improved thermally and superhydrophobically stable properties under an accelerated stability test. Due to the distinctive structure of the superporous surface network, providing a large liquid-vapor superhydrophobic interface and interior finger-like macrovoids, the FC-CNT membrane exhibits a stable high flux with a 99.9% rejection of Na+, outperforming existing inorganic membranes. Under simple and nondestructive electrochemically assisted direct contact MD (e-DCMD), enhanced antifouling performance is observed. The design strategy is broadly applicable to be extended toward fabrication of high performance membranes derived from other ceramic or inorganic substrates and additional applications in wastewater and gas treatment.

Biomimetic Nanocones that Enable High Ion Permselectivity.

Artificial counterparts of conical-shaped transmembrane protein channels are of interest in biomedical sciences for biomolecule detection and selective ion permeation based on ionic size and/or charge differences. However, industrial-scale applications such as seawater desalination, separation of mono- from divalent cations, and treatment of highly-saline industrial waste effluents are still big challenges for such biomimetic channels. A simple monomer seeding experimental approach is used to grow ionically conductive biomimetic charged nanocone pores at the surface of an acid-functionalized membrane. These readily scalable nanocone membranes enable ultra-fast cation permeation (Na+ =8.4× vs. Mg2+ =1.4×) and high ion charge selectivity (Na+ /Mg2+ =6×) compared to the commercial state-of-the-art permselective membrane (CSO, Selemion, Japan) owing to negligible surface resistance and positively charged conical pore walls.

Graphene Oxide Membranes with Heterogeneous Nanodomains for Efficient CO2 Separations.

Achieving high membrane performance in terms of gas permeance and carbon dioxide selectivity is an important target in carbon capture. Aiming to manipulate the channel affinity towards CO2 to implement efficient separations, gas separation membranes containing CO2 -philic and non-CO2 -philic nanodomains in the interlayer channels of graphene oxide (GO) were formed by intercalating poly(ethylene glycol) diamines (PEGDA). PEGDA reacts with epoxy groups on the GO surface, constructing CO2 -philic nanodomains and rendering a high sorption capacity, whereas unreacted GO surfaces give non-CO2 -philic nanodomains, rendering low-friction diffusion. Owing to the orderly stacking of nanochannels through cross-linking and the heterogeneous nanodomains with moderate CO2 affinity, a GO-PEGDA500 membrane exhibits a high CO2 permeance of 175.5 GPU and a CO2 /CH4 selectivity of 69.5, which is the highest performance reported for dry-state GO-stacking membranes.