Self-assembly of anthraquinone covalent organic frameworks as 1D superstructures for highly efficient CO2 electroreduction to CH4

Interlayer synergistic reaction of radical precursors for ultraefficient 1O2 generation via quinone-based covalent organic framework

Singlet oxygen (1O2) is important in the environmental remediation field, however, its efficient production has been severely hindered by the ultrafast self-quenching of the as-generated radical precursors in the Fenton-like reactions. Herein, we elaborately designed lamellar anthraquinone-based covalent organic frameworks (DAQ-COF) with sequential localization of the active sites (C═O) at molecular levels for visible-light-assisted peroxymonosulfate (PMS) activation. Theoretical and experimental results revealed that the radical precursors (SO5·-) were formed in the nearby layers with the migration distance less than 0.34 nm, via PMS donating electrons to the photogenerated holes. This interlayer synergistic effect eventually led to ultraefficient 1O2 production (14.8 μM s-1), which is 12 times that of the highest reported catalyst. As an outcome, DAQ-COF enabled the complete degradation of bisphenol A in 5 min with PMS under natural sunlight irradiation. This interlayer synergistic concept represents an innovative and effective strategy to increase the utilization efficiency of ultrashort-lived radical precursors, providing inspirations for subtle structural construction of Fenton-like catalysts.

Photocatalytic Extraction of Uranium from Seawater Using Covalent Organic Framework Nanowires

In the push to achieve net-zero emissions by 2050, nuclear power will play an essential role alongside renewable wind and solar power, and correspondingly global interest and investment in this well-established technology is accelerating. The uranium present in seawater could support nuclear power generation for centuries, but traditional adsorptive separation strategies have proven ineffective for the selective extraction of uranium from this vast resource. Here, we report the synthesis of nanowires of a triazine-linked two-dimensional covalent organic framework via a solvent modulation approach, which can be used to access nanowire external diameters ranging from 50 to 200 nm. The 100 nm nanowires are exceptionally promising for the capture of uranium(VI) via photocatalytic reduction. Under simulated sunlight and without the use of sacrificial agents, the nanowires achieve a uranium uptake of 10.9 g/g from a 100 ppm uranyl(VI) solution, which is the highest reported to date among materials studied for photo and electrocatalytic uranium capture. Significantly, these nanowires exhibit a uranium adsorption capacity of 34.5 mg/g after exposure to seawater under irradiation for 42 days, a record among all materials reported to date for uranium capture.

Self-Charging Aqueous Zn//COF Battery with UltraHigh Self-Charging Efficiency and Rate

Self-charging zinc batteries that combine energy harvesting technology with batteries are candidates for reliable self-charging power systems. However, the lack of rational materials design results in unsatisfactory self-charging performance. Here, a covalent organic framework containing pyrene-4,5,9,10-tetraone groups (COF-PTO) is reported as a cathode material for aqueous self-charging zinc batteries. The ordered channel structure of the COF-PTO provides excellent capacity retention of 98% after 18 000 cycles at 10 A g-1 and ultra-fast ion transfer. To visually assess the self-charging performance, two parameters, namely self-charging efficiency (self-charging discharge capacity/galvanostatic discharge capacity, η) and average self-charging rate (total discharge capacity after cyclic self-charging/total cyclic self-charging time, ν), are proposed for performance evaluation. COF-PTO achieves an impressive η of 96.9% and an ν of 30 mAh g-1 self-charge capacity per hour in 100 self-charging cycles, surpassing the previous reports. Mechanism studies reveal the co-insertion of Zn2+ and H+ double ions in COF-PTO of self-charging zinc batteries. In addition, the C═N and C═O (on the benzene) in COF-PTO are ortho structures to each other, which can easily form metal heterocycles with Zn ions, thereby driving the forward progress of the self-charging reaction and enhancing the self-charging performance.

Linkage Engineering in Covalent Organic Frameworks for Metal-Free Electrocatalytic C2H4 Production from CO2

Electrocatalytic carbon dioxide reduction reaction (CO2RR) to produce ethylene (C2H4) is conducive to sustainable development of energy and environment. At present, most electrocatalysts for C2H4 production are limited to the heavy metal copper, meanwhile, achieving metal-free catalysis remains a challenge. Noted piperazine with sp3 N hybridization is beneficial to CO2 capture, but CO2RR performance and mechanism have been lacking. Herein, based on linkage engineering, we construct a novel high-density sp3 N catalytic array via introducing piperazine into the crystalline and microporous aminal-linked covalent organic frameworks (COFs). Thanks to its high sp3 N density, strong CO2 capture capacity and great hydrophilicity, aminal-linked COF successfully achieves the conversion of CO2 to C2H4 with a Faraday efficiency up to 19.1 %, which is stand out in all reported metal-free COF electrocatalysts. In addition, a series of imine-linked COFs are synthesized and combined with DFT calculations to demonstrate the critical role of sp3 N in enhancing the kinetics of CO2RR. Therefore, this work reveals the extraordinary potential of linkage engineering in COFs to break through some catalytic bottlenecks.

Nanofibrous Porous Organic Polymers and Their Derivatives: From Synthesis to Applications

Engineering porous organic polymers (POPs) into 1D morphology holds significant promise for diverse applications due to their exceptional processability and increased surface contact for enhanced interactions with guest molecules. This article reviews the latest developments in nanofibrous POPs and their derivatives, encompassing porous organic polymer nanofibers, their composites, and POPs-derived carbon nanofibers. The review delves into the design and fabrication strategies, elucidates the formation mechanisms, explores their functional attributes, and highlights promising applications. The first section systematically outlines two primary fabrication approaches of nanofibrous POPs, i.e., direct bulk synthesis and electrospinning technology. Both routes are discussed and compared in terms of template utilization and post-treatments. Next, performance of nanofibrous POPs and their derivatives are reviewed for applications including water treatment, water/oil separation, gas adsorption, energy storage, heterogeneous catalysis, microwave absorption, and biomedical systems. Finally, highlighting existent challenges and offering future prospects of nanofibrous POPs and their derivatives are concluded.

Remodeling the Electronic Structure of Metallic Nickel and Ruthenium via Alloying in a Molecular Template for Sustainable Hydrogen Evolution

The reasonably constructed high-performance electrocatalyst is crucial to achieve sustainable electrocatalytic water splitting. Alloying is a prospective approach to effectively boost the activity of metal electrocatalysts. However, it is a difficult subject for the controllable synthesis of small alloying nanostructures with high dispersion and robustness, preventing further application of alloy catalysts. Herein, we propose a well-defined molecular template to fabricate a highly dispersed NiRu alloy with ultrasmall size. The catalyst presents superior alkaline hydrogen evolution reaction (HER) performance featuring an overpotential as low as 20.6 ± 0.9 mV at 10 mA·cm-2. Particularly, it can work steadily for long periods of time at industrial-grade current densities of 0.5 and 1.0 A·cm-2 merely demanding low overpotentials of 65.7 ± 2.1 and 127.3 ± 4.3 mV, respectively. Spectral experiments and theoretical calculations revealed that alloying can change the d-band center of both Ni and Ru by remodeling the electron distribution and then optimizing the adsorption of intermediates to decrease the water dissociation energy barrier. Our research not only demonstrates the tremendous potential of molecular templates in architecting highly active ultrafine nanoalloy but also deepens the understanding of water electrolysis mechanism on alloy catalysts.

Pore Space Partition Synthetic Strategy in Imine-linked Multivariate Covalent Organic Frameworks

Partitioning the pores of covalent organic frameworks (COFs) is an attractive strategy for introducing microporosity and achieving new functionality, but it is technically challenging to achieve. Herein, we report a simple strategy for partitioning the micropores/mesopores of multivariate COFs. Our approach relies on the predesign and synthesis of multicomponent COFs through imine condensation reactions with aldehyde groups anchored in the COF pores, followed by inserting additional symmetric building blocks (with C2 or C3 symmetries) as pore partition agents. This approach allowed tetragonal or hexagonal pores to be partitioned into two or three smaller micropores, respectively. The synthesized library of pore-partitioned COFs was then applied for the capture of iodine pollutants (i.e., I2 and CH3I). This rich inventory allowed deep exploration of the relationships between the COF adsorbent composition, pore architecture, and adsorption capacity for I2 and CH3I capture under wide-ranging conditions. Notably, one of our developed pore-partitioned COFs (COF 3-2P) exhibited greatly enhanced dynamic I2 and CH3I adsorption performances compared to its parent COF (COF 3) in breakthrough tests, setting a new benchmark for COF-based adsorbents. Results present an effective design strategy toward functional COFs with tunable pore environments, functions, and properties.

Efficient solar-driven CO2-to-fuel conversion via Ni/MgAlO x @SiO2 nanocomposites at low temperature

Solar-driven CO2-to-fuel conversion assisted by another major greenhouse gas CH4 is promising to concurrently tackle energy shortage and global warming problems. However, current techniques still suffer from drawbacks of low efficiency, poor stability, and low selectivity. Here, a novel nanocomposite composed of interconnected Ni/MgAlO x nanoflakes grown on SiO2 particles with excellent spatial confinement of active sites is proposed for direct solar-driven CO2-to-fuel conversion. An ultrahigh light-to-fuel efficiency up to 35.7%, high production rates of H2 (136.6 mmol min-1g- 1) and CO (148.2 mmol min-1g-1), excellent selectivity (H2/CO ratio of 0.92), and good stability are reported simultaneously. These outstanding performances are attributed to strong metal-support interactions, improved CO2 absorption and activation, and decreased apparent activation energy under direct light illumination. MgAlO x @SiO2 support helps to lower the activation energy of CH* oxidation to CHO* and improve the dissociation of CH4 to CH3* as confirmed by DFT calculations. Moreover, the lattice oxygen of MgAlO x participates in the reaction and contributes to the removal of carbon deposition. This work provides promising routes for the conversion of greenhouse gasses into industrially valuable syngas with high efficiency, high selectivity, and benign sustainability.

Modulated Connection Modes of Redox Units in Molecular Junction Covalent Organic Frameworks for Artificial Photosynthetic Overall Reaction

The precise tuning of components, spatial orientations, or connection modes for redox units is vital for gaining deep insight into efficient artificial photosynthetic overall reaction, yet it is still hard achieve for heterojunction photocatalysts. Here, we have developed a series of redox molecular junction covalent organic frameworks (COFs) (M-TTCOF-Zn, M = Bi, Tri, and Tetra) for artificial photosynthetic overall reaction. The covalent connection between TAPP-Zn and multidentate TTF endows various connection modes between water photo-oxidation (multidentate TTF) and CO2 photoreduction (TAPP-Zn) centers that can serve as desired platforms to study the possible interactions between redox centers. Notably, Bi-TTCOF-Zn exhibits a high CO production rate of 11.56 μmol g-1 h-1 (selectivity, ∼100%), which is more than 2 and 6 times higher than those of Tri-TTCOF-Zn and Tetra-TTCOF-Zn, respectively. As revealed by theoretical calculations, Bi-TTCOF-Zn facilitates a more uniform distribution of energy-level orbitals, faster charge transfer, and stronger *OH adsorption/stabilization ability than those of Tri-TTCOF-Zn and Tetra-TTCOF-Zn.

Selective oxidation of amines powered with green light and oxygen over an anthraquinone covalent organic framework

The exploration of emerging photocatalysts like covalent organic frameworks (COFs) is an essential but challenging endeavor to find sustainable solutions for selective organic transformations. Anthraquinones are envisaged to construct COFs for visible light photocatalysis because their derivatives are employed industrially as oxidation catalysts or organic dyes. Herein, an anthraquinone COF, TpAQ-COF, is successfully constructed with 1,3,5-triformylphloroglucinol (Tp) and 2,6-diaminoanthraquinone (AQ). Then, the selective oxidation of amines over TpAQ-COF is implemented. Amines can be effectively converted into corresponding imines over TpAQ-COF powered with green light and oxygen, during which superoxide radical anion is discerned as the pivotal reactive oxygen species. This work suggests that COFs could inherit the advantages of molecular building blocks for selective reactions powered with broad visible light.

A rational design of functional porous frameworks for electrocatalytic CO2 reduction reaction

The electrocatalytic CO₂ reduction reaction (ECO₂RR) is considered one of the approaches with the most potential to achieve lower carbon emissions in the future, but a huge gap still exists between the current ECO₂RR technology and industrial applications. Therefore, the design and preparation of catalysts with satisfactory activity, selectivity and stability for the ECO₂RR have attracted extensive attention. As a classic type of functional porous framework, crystalline porous materials (e.g., metal organic frameworks (MOFs) and covalent organic frameworks (COFs)) and derived porous materials (e.g., MOF/COF composites and pyrolysates) have been regarded as superior catalysts for the ECO₂RR due to their advantages such as designable porosity, modifiable skeleton, flexible active site structure, regulable charge transfer pathway and controllable morphology. Meanwhile, with the rapid development of nano-characterization and theoretical calculation technologies, the structure-activity relationships of functional porous frameworks have been comprehensively considered, i.e., metallic element type, local coordination environment, and microstructure, corresponding to selectivity, activity and mass transfer efficiency for the ECO₂RR, respectively. In this review, the rational design strategy for functional porous frameworks is briefly but precisely generalized based on three key factors including metallic element type, local coordination environment, and microstructure. Then, details about the structure-activity relationships for functional porous frameworks are illustrated in the order of MOFs, COFs, composites and pyrolysates to analyze the effect of the above-mentioned three factors on their ECO₂RR performance. Finally, the challenges and perspectives of functional porous frameworks for the further development of the ECO₂RR are reasonably proposed, aiming to offer insights for future studies in this intriguing and significant research field.

Porous Polymer Materials for CO2 Capture and Electrocatalytic Reduction

Efficient capture of CO₂ and its conversion into other high value-added compounds by electrochemical methods is an effective way to reduce excess CO₂ in the atmosphere. Porous polymeric materials hold great promise for selective adsorption and electrocatalytic reduction of CO₂ due to their high specific surface area, tunable porosity, structural diversity, and chemical stability. Here, we review recent research advances in this field, including design of porous organic polymers (POPs), porous coordination polymers (PCPs), covalent organic frameworks (COFs), and functional nitrogen-containing polymers for capture and electrocatalytic reduction of CO₂. In addition, key issues and prospects for the optimal design of porous polymers for future development are elucidated. This review is expected to shed new light on the development of advanced porous polymer electrocatalysts for efficient CO₂ reduction.

Boosting CO2 electroreduction on a Zn electrode via concurrent surface reconstruction and interfacial surfactant modification

Herein, we report an effective strategy for improving the electrocatalytic CO₂ reduction reaction (CO₂RR) performance of a Zn foil electrode via concurrent surface reconstruction and interfacial surfactant modification. The oxide-derived and CTAB-modified Zn electrode (OD-Zn-CTAB) prepared by electrochemically reducing the air-annealed Zn foil electrode in the presence of CTAB exhibits high electrocatalytic activity and selectivity for CO production with a CO partial current density (j_CO) of 8.2 mA cm^-2 and a CO faradaic efficiency (FE_CO) of 90% at -1.0 V vs. the reversible hydrogen electrode (RHE), greatly outperforming the pristine Zn foil (FE_CO = 32.0%; j_CO = 0.5 mA cm^-2) and OD-Zn (FE_CO = 77.6%; j_CO = 5.0 mA cm^-2) obtained by electroreduction of annealed Zn. The greatly enhanced CO₂RR performance of OD-Zn-CTAB can be attributed to the increased number of active sites originating from the surface reconstruction and the formation of a favorable CTAB-modified electrode/electrolyte (E/E) interface that can efficiently adsorb and activate CO₂ while inhibiting the competitive H₂ evolution reaction.

A Honeycomb-Like Porous Crystalline Hetero-Electrocatalyst for Efficient Electrocatalytic CO2 Reduction

Porous heterostructured electrocatalysts with multifunctionality and synergistic effect have much benefit for efficient electrocatalytic CO₂ reduction reaction (CO₂ RR), yet it still remains a daunting challenge to explore heterostructures based on covalent organic frameworks (COFs) and metal-organic frameworks (MOFs) in this field. Here, a series of honeycomb-like porous crystalline hetero-electrocatalysts (MCH-X, X = 1-4, X stands for the numbered sample obtained from different MOF doses in the synthesis of the MCH) are synthesized, and these are successfully applied in electrocatalytic CO₂ RR. The specially designed heterostructures with integrated porous MOF-template and ultrathin COF-coating enable efficient CO₂ adsorption/activation and conversion into CH₄ . The best of them, MCH-3, shows greatly inhibited H₂ evolution, excellent current density (-398.1 mA cm^-2 ), and superior FE CH 4 ${\rm{F}}{{\rm{E}}_{{\rm{C}}{{\rm{H}}_4}}}$ (76.7%) to the physical mixture (38.0%), the MOF@COF without the honeycomb-like morphology (47.7%), and the bare COF (37.5%) and MOF (15.9%) at -1.0 V. Based on the density functional theory calculations and various characterizations, the vital roles of the MOF in facilitating CO₂ adsorption/activation, stabilizing intermediates, and conquering the energy barrier of rate-determining step are intensively studied.

Metalated covalent organic frameworks: from synthetic strategies to diverse applications

Covalent organic frameworks (COFs) are a class of organic crystalline porous materials discovered in the early 21st century that have become an attractive class of emerging materials due to their high crystallinity, intrinsic porosity, structural regularity, diverse functionality, design flexibility, and outstanding stability. However, many chemical and physical properties strongly depend on the presence of metal ions in materials for advanced applications, but metal-free COFs do not have these properties and are therefore excluded from such applications. Metalated COFs formed by combining COFs with metal ions, while retaining the advantages of COFs, have additional intriguing properties and applications, and have attracted considerable attention over the past decade. This review presents all aspects of metalated COFs, from synthetic strategies to various applications, in the hope of promoting the continued development of this young field.

Recent progress on covalent organic framework materials as CO2 reduction electrocatalysts

CO2 emission caused by fuel combustion and human activity has caused severe climate change and other subsequent pollutions around the world. Carbon neutralization via various novel technologies to alleviate the CO2 level in the atmosphere has thus become one of the major topics in modern research field. These advanced technologies cover CO2 capture, storage and conversion, etc., and electrocatalytic CO2 reduction reaction (CO2RR) by heterogeneous catalysts is among the most promising methods since it could utilize renewable energy and generate valuable fuels and chemicals. Covalent organic frameworks (COFs) represent crystalline organic polymers with highly rigid, conjugated structures and tunable porosity, which exhibit significant potential as heterogeneous electrocatalysts for CO2RR. This review briefly introduces related pioneering works in COF-based materials for electrocatalytic CO2RR in recent years and provides a basis for future design and synthesis of highly active and selective COF-based electrocatalysts in this direction.

Toward High-Performance CO2 -to-C2 Electroreduction via Linker Tuning on MOF-Derived Catalysts

Copper (Cu)-based metal-organic frameworks (MOFs) and MOF-derived catalysts are well studied for electroreduction of carbon dioxide (CO₂ ); however, the effects of organic linkers for the selectivity of CO₂ reduction are still unrevealed. Here, a series of Cu-based MOF-derived catalysts is investigated with different organic linkers appended, named X-Cu-BDC (BDC = 1,4-benzenedicarboxylic acid, X = NH₂ , OH, H, F, and 2F). It is found that the linkers affect the faradaic efficiency (FE) for C₂ products with an order of NH₂  < OH < bare Cu-BDC < F < 2F, thus tuning the FE_C2 :FE_C1 ratios from 0.6 to 3.8. As a result, the highest C₂ FE of ≈63% at a current density of 150 mA cm^-2 on 2F-Cu-BDC derived catalyst is achieved. Using operando Raman measurements, it is revealed that the MOF derives to Cu₂ O during eCO₂ RR but organic linkers are stable. The fluorine group in organic linker can promote the H₂ O dissociation to *H species, further facilitating the hydrogenation of *CO to *CHO that helps CC coupling.

Covalent Organic Framework Based Functional Materials: Important Catalysts for Efficient CO2 Utilization

As hot topics in the chemical conversion of CO₂ , the photo-/electrocatalytic reduction of CO₂ and use of CO₂ as a supporter for energy storage have shown great potential for the utilization of CO₂ . However, many obstacles still exist on the road to realizing highly efficient chemical CO₂ conversion, such as inefficient uptake/activation of CO₂ and mass transport in catalysts. Covalent organic frameworks (COFs), as a kind of porous material, have been widely explored as catalysts for the chemical conversion of CO₂ owing to their unique features. In particular, COF-based functional materials containing diverse active sites (such as single metal sites, metal nanoparticles, and metal oxides) offer great potential for realizing CO₂ conversion and energy storage. This Minireview discusses recent breakthroughs in the basic knowledge, mechanisms, and pathways of chemical CO₂ conversion strategies that use COF-based functional catalysts. In addition, the challenges and prospects of COF-based functional catalysts for the efficient utilization of CO₂ are also introduced.

Anthraquinone Covalent Organic Framework Hollow Tubes as Binder Microadditives in Li-S Batteries

The exploration of new application forms of covalent organic frameworks (COFs) in Li-S batteries that can overcome drawbacks like low conductivity or high loading when typically applied as sulfur host materials (mostly ≈20 to ≈40 wt % loading in cathode) is desirable to maximize their low-density advantage to obtain lightweight, portable, or high-energy-density devices. Here, we establish that COFs could have implications as microadditives of binders (≈1 wt % in cathode), and a series of anthraquinone-COF based hollow tubes have been prepared as model microadditives. The microadditives can strengthen the basic properties of the binder and spontaneously immobilize and catalytically convert lithium polysulfides, as proved by density functional calculations, thus showing almost doubly enhanced reversible capacity compared with that of the bare electrode.