2D Redox-Active Covalent Organic Frameworks for Supercapacitors: Design, Synthesis, and Challenges

Ion transport mechanisms in covalent organic frameworks: implications for technology

Covalent organic frameworks (COFs) have emerged as promising materials for ion conduction due to their highly tunable structures and excellent electrochemical stability. This review paper explores the mechanisms of ion conduction in COFs, focusing on how these materials facilitate ion transport across their ordered structures, which is crucial for applications such as solid electrolytes in batteries and fuel cells. We discuss the design strategies employed to enhance ion conductivity, including pore size optimization, functionalization with ionic groups, and the incorporation of solvent molecules and salts. Additionally, we examine the various applications of ion-conductive COFs, particularly in energy storage and conversion technologies, highlighting recent advancements and future directions in this field. This review paper aims to provide a comprehensive overview of the current state of research on ion-conductive COFs, offering insights into their potential to design highly ion-conductive COFs considering not only fundamental studies but also practical perspectives for advanced electrochemical devices.

Porous Crystalline Materials Based on Tetrathiafulvalene and Its Analogues: Assembly, Charge Transfer, and Applications

ConspectusThe directed synthesis and functionalization of porous crystalline materials pose significant challenges for chemists. The synergistic integration of different functionalities within an ordered molecular material holds great significance for expanding its applications as functional materials. The presence of coordination bonds connected by inorganic and organic components in molecular materials can not only increase the structural diversity of materials but also modulate the electronic structure and band gap, which further regulates the physical and chemical properties of molecular materials. In fact, porous crystalline materials with coordination bonds, which inherit the merits of both organic and inorganic materials, already showcase their superior advantages in optical, electrical, and magnetic applications. In addition to the inorganic components that provide structural rigidity, organic ligands of various types serve as crucial connectors in the construction of functional porous crystalline materials. In addition, redox activity can endow organic linkers with electrochemical activity, thereby making them a perfect platform for the study of charge transfer with atom-resolved single-crystal structures, and they can additionally serve as stimuli-responsive sites in sensor devices and smart materials.In this Account, we introduce the synthesis, structural characteristics, and applications of porous crystalline materials based on the famous redox-active units, tetrathiafulvalene (TTF) and its analogues, by primarily focusing on metal-organic frameworks (MOFs) and covalent organic frameworks (COFs). TTF, a sulfur-rich conjugated molecule with two reversible and easily accessible oxidation states (i.e., radical TTF•+ cation and TTF2+ dication), and its analogues boast special electrical characteristics that enable them to display switchable redox activity and stimuli-responsive properties. These inherent properties contribute to the enhancement of the optical, electrical, and magnetic characteristics of the resultant porous crystalline materials. Moreover, delving into the charge transfer phenomena, which is key for the electrochemical process within these materials, uncovers a myriad of potential functional applications. The Account is organized into five main sections that correspond to the different properties and applications of these materials: optical, electrical, and magnetic functionalities; energy storage and conversion; and catalysis. Each section provides detailed discussions of synthetic methods, structural characteristics, the physical and chemical properties, and the functional performances of highlighted examples. The Account also discusses future directions by emphasizing the exploration of novel organic units, the transformation between radical cation TTF•+ and dication TTF2+, and the integration of multifunctionalities within these frameworks to foster the development of smart materials for enhanced performance across diverse applications. Through this Account, we aim to highlight the massive potential of TTF and its analogues-based porous crystals in chemistry and material science.

Unlocking the Charge-Storage Potential of a Phenanthraquinone-based Two-Dimensional Covalent Organic Framework (2D COF)

The high surface area, open pore-structure and atomic-level organization inherent in many covalent organic frameworks (COFs) make them an attractive polymer platform for developing functional materials. Herein, a chemically robust 2D COF (TpOMe-DAPQ COF) containing phenanthraquinone moieties was prepared by condensing 2,4,6-trimethoxy-1,3,5-benzenetricarbaldehyde (TpOMe) and 2,7-diamino-9,10-phenanthraquinone (DAPQ) using the convenient mechanochemical method. The poor charge-storage capacity of the pristine TpOMe-DAPQ COF was substantially improved by first investigating its redox-site accessibility (RSA) using different conductivity-enhancement methods, and then optimizing the amount of EDOT needed to perform an in-situ polymerization. The resulting composite (0.4EDOT@TpOMe-DAPQ) was characterized and its enhanced charge-storage capabilities enabled it to be used as an anode material in an aqueous Mn beaker-cell battery capable of delivering 0.76 V. This work outlines the rational design approach used to develop a functional charge-storage material utilizing a COF-based polymerization platform.

Redox-Active Phenanthrenequinone Molecules and Nitrogen-Doped Reduced Graphene Oxide as Active Material Composites for Supercapacitor Applications

Carbon-based supercapacitor electrodes are generally restricted in energy density, as they rely exclusively on electric double-layer capacitance (EDLC). The introduction of redox-active organic molecules to obtain pseudocapacitance is a promising route to develop electrode materials with improved energy densities. In this work, we develop a porous nitrogen-doped reduced graphene oxide and 9,10-phenanthrenequinone composite (N-HtrGO/PQ) via a facile one-step physical adsorption method. The electrochemical evaluation of N-HtrGO/PQ using cyclic voltammetry showed a high capacitance of 605 F g-1 in 1 M H2SO4 when the composite consisted of 30% 9,10-phenanthrenequinone and 70% N-HtrGO. The measured capacitance significantly exceeded pure N-HtrGO without the addition of redox-active molecules (257 F g-1). In addition to promising capacitance, the N-HtrGO/30PQ composite showed a capacitance retention of 94.9% following 20,000 charge/discharge cycles. Based on Fourier transform infrared spectroscopy, we postulate that the strong π-π interaction between PQ molecules and the N-HtrGO substrate enhances the specific capacitance of the composite by shortening pathways for electron transfer while improving structural stability.

Recent advances in two-dimensional polymers: synthesis, assembly and energy-related applications

Two-dimensional polymers (2DPs) are a class of 2D crystalline polymer materials with definite structures, which have outstanding physical-chemical and electronic properties. They cleverly link organic building units through strong covalent bonds and can construct functional 2DPs through reasonable design and selection of different monomer units to meet various application requirements. As promising energy materials, 2DPs have developed rapidly in recent years. This review first introduces the basic overview of 2DPs, such as their historical development, inherent 2D characteristics and diversified topological advantages, followed by the summary of the typical 2DP synthesis methods recently (including "top-down" and "bottom-up" methods). The latest research progress in assembly and processing of 2DPs and the energy-related applications in energy storage and conversion are also discussed. Finally, we summarize and prospect the current research status, existing challenges, and future research directions of 2DPs.

Rapid Charge Transfer Enabled by Noncovalent Interaction through Guest Insertion in Supercapacitors based on Covalent Organic Frameworks

Covalent organic frameworks (COFs) have been proposed for electrochemical energy storage, although the poor conductivity resulted from covalent bonds limits their practical performance. Here, we propose to introduce noncovalent bonds in COFs through a molecular insertion strategy for improving the conductivity of the COFs as supercapacitor. The synthesized COFs (MI-COFs) establish equilibriums between covalent bonds and noncovalent bonds, which construct a continuous charge transfer channel to enhance the conductivity. The rapid charge transfer rate enables the COFs to activate the redox sites, bringing about excellent electrochemical energy storage behavior. The results show that the MI-COFs exhibit much better performance in specific capacitance and capacity retention rate than those of most COFs-based supercapacitors. Moreover, through simply altering inserted guests, the mode and strength of noncovalent bond can be adjusted to obtain different energy storage characteristics. The introduction of noncovalent bonds is an effective and flexible way to enhance and regulate the properties of COFs, providing a valuable direction for the development of novel COFs-based energy storage materials.

Exploring the Charge Storage Dynamics in Donor-Acceptor Covalent Organic Frameworks Based Supercapacitors by Employing Ionic Liquid Electrolyte

Two donor-acceptor type tetrathiafulvalene (TTF)-based covalent organic frameworks (COFs) are investigated as electrodes for symmetric supercapacitors in different electrolytes, to understand the charge storage and dynamics in 2D COFs. Till-date, most COFs are investigated as Faradic redox pseudocapacitors in aqueous electrolytes. For the first time, it is tried to enhance the electrochemical performance and stability of pristine COF-based supercapacitors by operating them in the non-Faradaic electrochemically double layer capacitance region. It is found that the charge storage mechanism of ionic liquid (IL) electrolyte based supercapacitors is dependent on the micropore size and surface charge density of the donor-acceptor COFs. The surface charge density alters due to the different electron acceptor building blocks, which in turn influences the dense packing of the IL near its pore. The micropores induce pore confinement of IL in the COFs by partial breaking of coulomb ordering and rearranging it. The combination of these two factors enhance the charge storage in the highly microporous COFs. The density functional theory calculations support the same. At 1 A g-1 , TTF-porphyrin COF provides capacitance of 42, 70, and 130 F g-1 in aqueous, organic, and IL electrolyte respectively. TTF-diamine COF shows a similar trend with 100 F g-1 capacitance in IL.

A Redox-Active Covalent Organic Framework with Highly Accessible Aniline-Fused Quinonoid Units Affords Efficient Proton Charge Storage

Owing to their intrinsic safety and sustainability, aqueous proton batteries have emerged as promising energy devices. Nevertheless, the corrosion or dissolution of electrode materials in acidic electrolytes must be addressed before practical applications. In this study, a cathode material based on a redox-active 2D covalent organic framework (TPAD-COF) with aniline-fused quinonoid units featuring inherently regular open porous channels and excellent stability is developed. The TPAD-COF cathode delivers a high capacity of 126 mAh g-1 at 0.2 A g-1 , paired with long-term cycling stability with capacity retention of 84% after 5000 cycles at 2 A g-1 . Comprehensive ex situ spectroscopy studies correlated with density functional theory (DFT) calculations reveal that both the -NH- and C=O groups of the aniline-fused quinonoid units exhibit prominent redox activity of six electrons during the charge/discharge processes. Furthermore, the assembled punch battery consisting of a TPAD-COF//anthraquinone (AQ) all-organic system delivers a discharge capacity of 115 mAh g-1 at 0.5 A g-1 after 130 cycles, implying the potential application of the TPAD-COF cathode in aqueous proton batteries. This study provides a new perspective on the design of electrode materials for aqueous proton batteries with long-term cycling performance and high capacity.

Design strategies of covalent organic framework-based electrodes for supercapacitor application

Supercapacitors (SCs) have been recognized as a promising electrochemical energy storage (EES) device, thanks to their high-power density, long lifespan, fast charge-discharge capability, and eco-friendliness. The breakthrough of electrode materials that determine the electrochemical performance of SCs is urgently desired. Covalent organic frameworks (COFs), an emerging and burgeoning class of crystalline porous polymeric materials, have been found to have huge potential for application in EES devices by virtue of their unique properties including atomically adjustable structures, robust and tunable skeletons, well-defined and open channels, high surface areas, etc. In this feature article, we aim at summarizing the design strategies of COF-based electrode materials for SCs based on the representative advances. The current challenges and future perspectives of COFs for SC application are highlighted as well.

Multiple Impact-Resistant 2D Covalent Organic Framework

Exploring and designing two-dimensional (2D) nanomaterials for armor-piercing protection has become a research focus. Here, by molecular dynamics simulation, we revealed that the ultralight monolayer covalent organic framework (COF), one kind of novel 2D crystalline polymer, possesses superior impact-resistant capability under high-velocity impact. The calculated specific penetration energy is much higher than that of other traditional impact-resistant materials, such as steel, poly(methyl methacrylate), Kevlar, etc. It was found that the hexagonal nanopores integrated by polymer chains have large deformation compatibility resulting from flexible torsion and stretching, which can remarkably contribute to the energy dissipation. In addition, the deformable nanopores can effectively restrain the crack propagation, enable COF to resist multiple impacts. This work uncovers the extreme dynamic responses of COF under high-velocity impact and provides theoretical guidance for designing superstrong 2D polymer-based crystalline nanomaterials.

Amide Linkages in Pyrene-Based Covalent Organic Frameworks toward Efficient Photocatalytic Reduction of Uranyl

The bond linkages in covalent organic frameworks (COFs) partly determine its physical and chemical properties, thus affecting the photoreactive activity by influencing the generation of photoelectrons and the separation of excitons. Herein, pyrene-based amide COF 4,4',4″,4‴-(pyrene-1,3,6,8-tetrayl)tetrabenzaldehyde-3,8-diamino-6-phenylphenanthridine (TFPPy-DP) was synthesized by postsynthetic modification of imine COFs. Due to the introduction of oxygen atoms into the framework and the change in polarity, an increased number of photogenerated electrons and a wide band gap for amide COFs were found, hydrophilicity and dispersibility were prompted as well. Both imine and amide COF TFPPy-DP were applied in the photocatalytic reduction and removal of toxic U(VI) under visible light, the catalytic reduction equilibrium (91% removal percentage of 238 ppm U at pH 3) was achieved by imine COFs with 10 h of irradiation, while amide COFs only took 2 h of irradiation (82% removal percentage). The much faster photocatalytic reduction rate of U(VI) can be attributed to the fact that amide COF TFPPy-DP retained crystallinity and permanent porosity and exhibited lower electrochemical impedance and enhanced charge separation and accumulation. Further electronic excitation analysis based on time-dependent density functional theory calculations revealed that the intramolecular charge-transfer effect in amide TFPPy-DP enhanced its photocatalytic rate.

An Ultrastable Porous Polyhedral Oligomeric Silsesquioxane/Tetraphenylthiophene Hybrid as a High-Performance Electrode for Supercapacitors

In this study, we synthesized three hybrid microporous polymers through Heck couplings of octavinylsilsesquioxane (OVS) with 2,5-bis(4-bromophenyl)-1,3,4-oxadiazole (OXD-Br₂), tetrabromothiophene (Th-Br₄), and 2,5-bis(4-bromophenyl)-3,4-diphenylthiophene (TPTh-Br₂), obtaining the porous organic–inorganic polymers (POIPs) POSS-OXD, POSS-Th, and POSS-TPTh, respectively. Fourier transform infrared spectroscopy and solid state ¹³C and ²⁹Si NMR spectroscopy confirmed their chemical structures. Thermogravimetric analysis revealed that, among these three systems, the POSS-Th POIP possessed the highest thermal stability (T₅: 586 °C; T₁₀: 785 °C; char yield: 90 wt%), presumably because of a strongly crosslinked network formed between its OVS and Th moieties. Furthermore, the specific capacity of the POSS-TPTh POIP (354 F g⁻¹) at 0.5 A g⁻¹ was higher than those of the POSS-Th (213 F g⁻¹) and POSS-OXD (119 F g⁻¹) POIPs. We attribute the superior electrochemical properties of the POSS-TPTh POIP to its high surface area and the presence of electron-rich phenyl groups within its structure.

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.

Triangular Topological 2D Covalent Organic Frameworks Constructed via Symmetric or Asymmetric "Two-in-One" Type Monomers

Most of the reported covalent organic frameworks (COFs) so far are prepared from highly symmetric building blocks, which to some extent limits the expansion of COF diversity and complexity. Low-symmetric building blocks can be designed through a desymmetrized vertex strategy, which might be used to construct new topological COFs. But reports of COFs constructed by asymmetric building blocks are thus far very rare. Here, a feasible strategy to design asymmetric building blocks for COF synthesis is introduced, by simply varying the positions of functional groups in the monomer. As a proof of concept, two isomeric hexaphenylbenzene-based "two-in-one" type monomers (1,2,4-HPB-NH₂ and 1,3,5-HPB-NH₂ ) are designed and synthesized. To the authors' surprise, self-polycondensation of the asymmetric 1,2,4-HPB-NH₂ (i.e., the isomer of common C₃ -symmetric 1,3,5-HPB-NH₂ ) also affords highly crystalline COF (1,2,4-HPB-COF) similar to the symmetric 1,3,5-HPB-NH₂ counterpart with identical topological structure. The triangular porous structures of both HPB-based COFs are well resolved by powder X-ray diffraction (PXRD), theoretical simulations, nitrogen sorption, and morphologies analysis. This work demonstrates the "two-in-one" type asymmetric building blocks can also produce highly crystalline frameworks and thus provides a new structural design strategy for reticular chemistry.

Porous organic polymers for high-performance supercapacitors

With the aim of addressing the global warming issue and fossil energy shortage, eco-friendly and sustainable renewable energy technologies are urgently needed. In comparison to energy conversion, studies on energy storage fall behind and remain largely to be explored. By storing energy from electrochemical processes at the electrode surface, supercapacitors (SCs) bridge the performance gap between electrostatic double-layer capacitors and batteries. Organic electrode materials have drawn extensive attention because of their special power density, good round trip efficiency and excellent cycle stability. Porous organic polymers (POPs) have drawn extensive attention as attractive electrode materials in SCs. In this review, we present and discuss recent advancements and design principles of POPs as efficient electrode materials for SCs from the perspectives of synthetic strategies and the structure-performance relationships of POPs. Finally, we put forward the outlook and prospects of POPs for SCs.

Aqueous Organic Zinc-Ion Hybrid Supercapacitors Prepared by 3D Vertically Aligned Graphene-Polydopamine Composite Electrode

A three-dimensional vertical-aligned graphene-polydopamine electrode (PDA@3DVAG) composite with vertical channels and conductive network is prepared by a method of unidirectional freezing and subsequent self-polymerization. When the prepared PDA@3DVAG is constructed as the positive electrode of zinc-ion hybrid supercapacitors (ZHSCs), excellent electrochemical performances are obtained. Compared with the conventional electrolyte, PDA@3DVAG composite electrode in highly concentrated salt electrolyte exhibits better multiplicity performance (48.92% at a current density of 3 A g⁻¹), wider voltage window (−0.8~0.8 V), better cycle performance with specific capacitance from 96.7 to 59.8 F g⁻¹, and higher energy density (46.14 Wh kg⁻¹).

Chemically Stable Carbazole-Based Imine Covalent Organic Frameworks with Acidochromic Response for Humidity Control Applications

Isoreticular chemically stable two-dimensional imine covalent organic frameworks (COFs), further denoted as DUT-175 and DUT-176, are obtained in a reaction of 4,4′-bis(9H-carbazol-9-yl)biphenyl tetraaldehyde with phenyldiamine and benzidine. The crystal structures, solved and refined from the powder X-ray diffraction data and confirmed by high-resolution transmission electron microscopy, indicate AA-stacked layer structures. Both structures feature distorted hexagonal channel pores, assuring remarkable porosity (S_BET = 1071 m² g^–1 for DUT-175 and S_BET = 1062 m² g^–1 for DUT-176), as confirmed by adsorption of gases and vapors. The complex conjugated π system of the COFs involves electron-rich carbazole building units, which in combination with the imine groups allow reversible pH-dependent protonation of the frameworks, accompanied by charge transfer and shift of the absorption bands in the UV–vis spectrum. The sigmoidal shape of the water vapor adsorption and desorption isotherms with a steep adsorption step at p/p₀ = 0.4–0.6 in combination with excellent stability over dozens of adsorption and desorption cycles ranks these COFs among the best materials for indoor humidity control applications.

Photo-induced synthesis of ternary Pt/rGO/COF photocatalyst with Pt nanoparticles precisely anchored on rGO for efficient visible-light-driven H2 evolution

Covalent organic frameworks (COFs) have been recognized as a new type of promising visible-light-driven photocatalysts for H₂ evolution, while it still is a key point to facilitate the separation and transfer of photoinduced charges for further enhancing their activities. In this work, we fabricated a new type of ternary Pt/rGO/COF photocatalysts with Pt cocatalyst precisely anchored on rGO serving as electron collector for largely enhanced H₂ evolution. A series of ternary hybrid materials were obtained via one-pot photoreduction of Pt⁴⁺ and GO under visible-light irradiation in a solution the same as photocatalytic H₂ evolution reaction and simultaneous self-assembling of rGO/COF heterostructure. No need isolation, the synthetic system could be further used for photocatalytic H₂ evolution reaction and the results show the H₂ evolution rate of Pt/rGO(20%)/TpPa-1-COF hybrid material is 19.59 mmol·g^-1·h^-1, 6.51 times higher than that of Pt/TpPa-1-COF. The essential role of the exclusively distributed Pt nanoparticles on rGO to the high H₂ evolution activity was confirmed by various comparisons of activity for the samples with diverse Pt distribution.

Three-dimensional covalent organic frameworks based on a π-conjugated tetrahedral node

The construction of three-dimensional (3D) covalent organic frameworks (COFs), especially fully conjugated 3D COFs, is a long-standing challenge. Herein, we report a saddle-like, π-conjugated cyclooctatetrathiophene (COTh) as a tetrahedral node to construct fully conjugated 3D COFs. The present work enriches the structural diversities and potential applications of 3D COFs.

Construction of Interlayer Conjugated Links in 2D Covalent Organic Frameworks via Topological Polymerization

Two-dimensional covalent organic frameworks (2D COFs) are well-defined polymeric sheets that usually stack in an eclipsed mode via van der Waals forces. Extensive efforts have been made to manipulate interlayer interactions, yet there still lack a way to construct conjugated connections between adjacent layers, which is important for (opto)electronic-related applications. Herein, we report an interlayer topological polymerization strategy to transform the well-organized diacetylene columnar arrays in three different 2D COFs (TAPFY-COF, TAPB-COF, and TAPP-COF) into conjugated enyne chains upon heating in the solid state. The resultant COFs (COF-P) with retained high crystallinity possess broadened absorption bands and narrowed band gaps. The newly formed conjugated chains provide extra charge carrier pathways through direct π-electron delocalization. As a proof-of-concept, after topological polymerization, the conductivity of the TAPFY-COF film achieves 2.8 × 10^-4 S/cm without doping, and the photothermal, photoacoustic, and oxygen reduction catalytic performance of TAPP-COF is significantly improved.

A Tröger's Base-Derived Covalent Organic Polymer Containing Carbazole Units as a High-Performance Supercapacitor

Porous organic polymers have been received considerable attention due to their heteroatom-containing structures and high surface areas, which can offer high electrochemical performance in energy applications. The majority of reported Tröger's base-functionalized porous organic polymers have been applied as effective candidates for sensing and gas separation/adsorption, while their use as electrode materials in supercapacitors is rare. Here, a novel covalent microporous organic polymer containing carbazole and Tröger's base CzT-CMOP has been successfully synthesized through the one-pot polycondensation of 9-(4-aminophenyl)-carbazole-3,6-diamine (Cz-3NH₂) with dimethoxymethane. The polycondensation reaction's regioselectivity was studied using spectroscopic analyses and electronic structure calculations that confirmed the polycondensation occurred through the second and seventh positions of the carbazole unit rather than the fourth and fifth positions confirmed by first-principles calculations. Our CzT-CMOP exhibited high thermal stability of approximately 463.5 °C and a relatively high Brunauer-Emmett-Teller surface area of 615 m² g^-1 with a nonlocal density functional theory's pore size and volume of 0.48 cm³ g^-1 and 1.66 nm, respectively. In addition, the synthesized CzT-CMOP displayed redox activity due to the existence of a redox-active carbazole in the polymer skeleton. CzT-CMOP revealed high electrochemical performance when used as active-electrode material in a three-electrode supercapacitor with an aqueous electrolyte of 6 M KOH, and it showed specific capacitance of 240 F g^-1 at a current density of 0.5 A g^-1 with excellent stability after 2000 cycles of 97% capacitance retention. Accordingly, such porous organic polymer appears to have a variety of uses in energy-related applications.

Synthesis methods of microporous organic polymeric adsorbents: a review

MOPs can be synthesized in a large variety of ways, which affect their pores and surface area. Variation in synthesis and porosity has a significant effect on their adsorption properties.