Covalent Organic Framework (COF)-Based Hybrids for Electrocatalysis: Recent Advances and Perspectives

Covalent Organic Frameworks Based Electrocatalysts for Two-Electron Oxygen Reduction Reaction: Design Principles, Recent Advances, and Perspective

Hydrogen peroxide (H2O2) is an environmentally friendly oxidant with a wide range of applications, and the two-electron pathway (2e-) of the oxygen reduction reaction (ORR) for H2O2 production has attracted much attention due to its eco-friendly nature and operational simplicity in contrast to the conventional anthraquinone process. The challenge is to design electrocatalysts with high activity and selectivity and to understand their structure-activity relationship and catalytic mechanism in the ORR process. Covalent organic frameworks (COFs) provide an efficient template for the construction of highly efficient electrocatalysts due to their designable structure, excellent stability, and controllable porosity. This review firstly outlines the design principles of COFs, including the selection of metallic and nonmetallic active sites, the modulation of the electronic structure of the active sites, and the dimensionality modulation of the COFs, to provide guidance for improving the production performance of H2O2. Subsequently, representative results are summarized in terms of both metallic and metal-free sites to follow the latest progress. Moreover, the challenges and perspectives of 2e- ORR electrocatalysts based on COFs are discussed.

Porphyrin-based covalent organic frameworks from design, synthesis to biological applications

Covalent organic frameworks (COFs) constitute a class of highly functional porous materials composed of lightweight elements interconnected by covalent bonds, characterized by structural order, high crystallinity, and large specific surface area. The integration of naturally occurring porphyrin molecules, renowned for their inherent rigidity and conjugate planarity, as building blocks in COFs has garnered significant attention. This strategic incorporation addresses the limitations associated with free-standing porphyrins, resulting in the creation of well-organized porous crystal structures with molecular-level directional arrangements. The unique optical, electrical, and biochemical properties inherent to porphyrin molecules endow these COFs with diversified applications, particularly in the realm of biology. This review comprehensively explores the synthesis and modulation strategies employed in the development of porphyrin-based COFs and delves into their multifaceted applications in biological contexts. A chronological depiction of the evolution from design to application is presented, accompanied by an analysis of the existing challenges. Furthermore, this review offers directional guidance for the structural design of porphyrin-based COFs and underscores their promising prospects in the field of biology.

Engineering organic polymers as emerging sustainable materials for powerful electrocatalysts

Cost-effective and high-efficiency catalysts play a central role in various sustainable electrochemical energy conversion technologies that are being developed to generate clean energy while reducing carbon emissions, such as fuel cells, metal-air batteries, water electrolyzers, and carbon dioxide conversion. In this context, a recent climax in the exploitation of advanced earth-abundant catalysts has been witnessed for diverse electrochemical reactions involved in the above mentioned sustainable pathways. In particular, polymer catalysts have garnered considerable interest and achieved substantial progress very recently, mainly owing to their pyrolysis-free synthesis, highly tunable molecular composition and microarchitecture, readily adjustable electrical conductivity, and high stability. In this review, we present a timely and comprehensive overview of the latest advances in organic polymers as emerging materials for powerful electrocatalysts. First, we present the general principles for the design of polymer catalysts in terms of catalytic activity, electrical conductivity, mass transfer, and stability. Then, the state-of-the-art engineering strategies to tailor the polymer catalysts at both molecular (i.e., heteroatom and metal atom engineering) and macromolecular (i.e., chain, topology, and composition engineering) levels are introduced. Particular attention is paid to the insightful understanding of structure-performance correlations and electrocatalytic mechanisms. The fundamentals behind these critical electrochemical reactions, including the oxygen reduction reaction, hydrogen evolution reaction, CO2 reduction reaction, oxygen evolution reaction, and hydrogen oxidation reaction, as well as breakthroughs in polymer catalysts, are outlined as well. Finally, we further discuss the current challenges and suggest new opportunities for the rational design of advanced polymer catalysts. By presenting the progress, engineering strategies, insightful understandings, challenges, and perspectives, we hope this review can provide valuable guidelines for the future development of polymer catalysts.

Bimetallic atom synergistic covalent organic framework for efficient electrochemical nitrate reduction

Electrochemical reduction has emerged as an effective method to remove nitrate from industrial wastewater. Nevertheless, this method has been largely restricted by the lack of low-cost and efficient electrocatalysts. Here, we demonstrate a porous two-dimensional covalent organic framework (2D COF) material as a promising electrocatalyst, which is obtained via a Schiff base reaction by combining copper phthalocyanine with bipyridine sites for precise copper coordination. The bidentate coordinated COF material has a robust framework and stable chemical property, allowing the isolated Cu sites to be embedded into the regular pores with controlled and uniformly dispersed active centers. The well-defined design of the reaction monomers makes the COF material to trap nitrate ions more easily from aqueous solution. By rationally combining the synergistic effect of 2D COF and Cu active sites, the CuTAPc-CuBPy-COF electrocatalyst shows much higher nitrate reduction efficiency than CuTAPc-BPy-COF under low superpotential and different nitrate concentrations. The high NO3- conversion (90.3 %) and NH3 selectivity (69.6 %) are achieved. To our best acknowledge, this is the first demonstration of bi-copper-based COF material for NO3-RR electrocatalysis, which provides a new direction for the rational design of COFs as significant electrocatalysts for nitrate reduction.

Structural Regulation of Photocatalyst to Optimize Hydroxyl Radical Production Pathways for Highly Efficient Photocatalytic Oxidation

Ring-opening of phenol in wastewater is the pivotal step in photocatalytic degradation. The highly selective generation of catalytical active species (•OH) to facilitate this process presents a significant scientific challenge. Therefore, a novel approach for designing photocatalysts with single-atom containment in metal-covalent organic frameworks (M-COFs) is proposed. The selection of imine-linked COFs containing abundant N and O-chelate sites provides a solid foundation for anchoring metal atom. These dispersed metal atom possess rapid accumulation and transfer capabilities for photogenerated electrons, while the periodic π-conjugated structure in 2D-COFs establishes an effective platform. Additionally, the Lewis acid properties of imine bonds in COFs can enhance the adsorption capacity toward gases with Lewis base properties, such as O2 and N2 . It is demonstrated that the Pd2+ @Tp-TAPT, designed based on this concept, exhibits efficient oxygen adsorption and follows the reaction pathway of O2 →•O2 - →H2 O2 →•OH with high selectivity, thereby achieving completely degradation of refractory phenol through photocatalysis within 10 min. It is anticipated that the selective generation of catalytic active species via advanced material design concepts will serve as a significant reference for achieving precise material catalysis in the future.

COF-supported zirconium oxyhydroxide as a versatile heterogeneous catalyst for Knoevenagel condensation and nerve agent hydrolysis

A composite of catalytic Lewis acidic zirconium oxyhydroxides (8 wt %) and a covalent organic framework (COF) was synthesized. X-ray diffraction and infrared (IR) spectroscopy reveal that COF's structure is preserved after loading with zirconium oxyhydroxides. Electron microscopy confirms a homogeneous distribution of nano- to sub-micron-sized zirconium clusters in the COF. 3D X-ray tomography captures the micron-sized channels connecting the well-dispersed zirconium clusters on the COF. The crystalline ZrOx(OH)y@COF's nanostructure was model-optimized via simulated annealing methods. Using 0.8 mol % of the catalyst yielded a turnover number of 100-120 and a turnover frequency of 160-360 h-1 for Knoevenagel condensation in aqueous medium. Additionally, 2.2 mol % of catalyst catalyzes the hydrolysis of dimethyl nitrophenyl phosphate, a simulant of nerve agent Soman, with a conversion rate of 37% in 180 min. The hydrolytic detoxification of the live agent Soman is also achieved. Our study unveils COF-stabilized ZrOx(OH)y as a new class of zirconium-based Lewis + Bronsted-acid catalysts.

Solvothermally designed Pr-MOF/Fe2O3 based nanocomposites for efficient electrocatalytic water splitting

To meet the energy demand of modern civilization, efforts to find renewable, safe, and highly effective fuel generation are still a big challenge. The oxygen evolution reaction (OER) is one of many modern technologies for hydrogen generation, and a number of new electrode materials have been created to increase the effectiveness of O2 evolution. This project utilizes a range of high performance nanomaterials, such as Pr-MOF, Fe2O3, and Pr-MOF/Fe2O3, to carry out the oxygen evolution reaction. This study shows that Pr-MOF/Fe2O3 exhibits exceptional electrocatalytic activity in alkaline solution with 238 mV overpotential at the current density of 10 mA cm-2 and a Tafel slope of 37 mV dec-1 which is much lower when compared to pure Pr-MOF and Fe2O3. The enhanced electrochemical results are due to the higher electrochemical surface area of 237 cm2. This work will lay the foundation for an approach to enhance the crystalline nature of surface-active nanoparticles made from rare earth MOFs for a range of electrochemical energy applications.

Large-Scale Synthesis of Covalent Organic Frameworks: Challenges and Opportunities

Connecting organic building blocks by covalent bonds to design porous crystalline networks has led to covalent organic frameworks (COFs), consequently transferring the flexibility of dynamic linkages from discrete architectures to extended structures. By virtue of the library of organic building blocks and the diversity of dynamic linkages and topologies, COFs have emerged as a novel field of organic materials that propose a platform for tailor-made complex structural design. Progress over the past two decades in the design, synthesis, and functional exploration of COFs in diverse applications successively established these frameworks in materials chemistry. The large-scale synthesis of COFs with uniform structures and properties is of profound importance for commercialization and industrial applications; however, this is in its infancy at present. An innovative designing and synthetic approaches have paved novel ways to address future hurdles. This review article highlights the fundamental of COFs, including designing principles, coupling reactions, topologies, structural diversity, synthetic strategies, characterization, growth mechanism, and activation aspects of COFs. Finally, the major challenges and future trends for large-scale COF fabrication are outlined.

Molecular Design of Porous Organic Polymer-Derived Carbonaceous Electrocatalysts for Pinpointing Active Sites in Oxygen Reduction Reaction

The widespread application of fuel cells is hampered by the sluggish kinetics of the oxygen reduction reaction (ORR), which traditionally necessitates the use of high-cost platinum group metal catalysts. The indispensability of these metal catalysts stems from their ability to overcome kinetic barriers, but their high cost and scarcity necessitate alternative strategies. In this context, porous organic polymers (POPs), which are built up from the molecular level, are emerging as promising precursors to produce carbonaceous catalysts owning to their cost-effectiveness, high electrical conductivity, abundant active sites and extensive surface area accessibility. To enhance the intrinsic ORR activity and optimize the performance of these electrocatalysts, recognizing, designing, and increasing the density of active sites are identified as three crucial steps. These steps, which form the core of our review, serve to elucidate the link between the material structure design and ORR performance evaluation, thereby providing valuable insights for ongoing research in the field. Leveraging the precision of polymer skeletons based on molecular units, POP-derived carbonaceous catalysts provide an excellent platform for in-depth exploration of the role and working mechanism for the specific active site during the ORR process. In this review, the recent advances pertaining to the synthesis techniques and electrochemical functions of various types of active sites, pinpointed from POPs, are systematically summarized, including heteroatoms, surficial substituents and edge/defects. Notably, the structure-property relationship, between these active sites and ORR performance, are discussed and emphasized, which creates guidelines to shed light on the design of high-performance ORR electrocatalysts.

Sulfonic acid functionalized covalent organic frameworks for lithium-sulfur battery separator and oxygen evolution electrocatalyst

Covalent organic frameworks (COFs) are considered as a class of potential candidates for energy storage and catalysis. In this work, a COF containing sulfonic groups was prepared to be a modified separator in lithium-sulfur batteries (LSBs). Benefiting from the charged sulfonic groups, the COF-SO₃ cell exhibited higher ionic conductivity (1.83 mS⋅cm^-1). Moreover, the modified COF-SO₃ separator not only inhibited the shuttle of polysulfide but also promoted Li⁺ diffusion, thanks to the electrostatic interaction. The COF-SO₃ cell also showed excellent electrochemical performance that the initial specific capacity of the battery was 890 mA h g^-1 at 0.5 C and demonstrated 631 mA h g^-1 after 200 cycles. In addition, COF-SO₃ with satisfactory electrical conductivity was also used as an electrocatalyst toward oxygen evolution reaction (OER) via cation-exchange strategy. The electrocatalyst COF-SO₃@FeNi possessed a low overpotential (350 mV at 10 mA cm^-2) in an alkaline aqueous electrolyte. Furthermore, COF-SO₃@FeNi exhibited exceptional stability, and the overpotential increased about 11 mV at a current density of 10 mA cm^-2 after 1000 cycles. This work facilitates the application of versatile COFs in the electrochemistry field.

Novel Microwave-Assisted Synthesis of COFs: 2020–2022

Covalent organic frameworks (COFs) have emerged as a new type of crystalline porous polymers of great interest. However, their preparation requires long reaction times. Microwave-assisted synthesis (MAS) offers an interesting approach to increasing the reaction rate of chemical processes. Thus, microwaves can be a key tool for the fast and scalable synthesis of COFs. Since our previous review on the topic, the preparation of COFs with microwaves has been evolving. Herein, we present a compilation of COFs studies and experiments published in the last three years on the synthesis of COFs using microwave-assisted synthesis as a source of energy. The articles include imine, triazine, and other 2D COFs synthesized using MAS. The 3D COFs have also been compiled. The chemical structure of the monomers and the COFs and their main parameters of synthesis and application are summarized for each article.

Covalent organic frameworks (COFs): a promising CO2 capture candidate material

Covalent organic frameworks (COFs) are an emerging kind of porous crystal material.

Microporous Polymelamine Framework Functionalized with Re(I) Tricarbonyl Complexes for CO2 Absorption and Reduction

A mixture of polymeric complexes based on the reaction between Re(CO)₅Cl and the porous polymeric network coming from the coupling of melamine and benzene-1,3,5-tricarboxaldehyde was obtained and characterized by FTIR, NMR, SEM, XPS, ICP, XRD, and cyclic voltammetry (CV). The formed rhenium-based porous hybrid material reveals a noticeable capability of CO₂ absorption. The gas absorption amount measured at 295 K was close to 44 cm³/g at 1 atm. An interesting catalytic activity for CO₂ reduction reaction (CO₂RR) is observed, resulting in a turn over-number (TON) close to 6.3 under 80 min of test at -1.8 V vs. Ag/AgCl in a TBAPF₆ 0.1 M ACN solution. A possible use as filler in membranes or columns can be envisaged.

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.

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.