The advancement in materials chemistry promoted the growth of energy storage systems such as capacitors, supercapacitors and batteries. Covalent organic frameworks and nanomaterials have significantly improved the performance of various energy storage systems. Because of the unique properties of these materials, like high surface area, tunable architectures, and enhanced conductivity, researchers have developed effective and durable energy storage solutions for multiple applications. These findings are significant for meeting the demand for reliable and sustainable energy storage materials in order to save energy for a better future of mankind. As the demand for reliable and sustainable energy storage materials is increasing, the scientific community is more focussed towards the development of covalent organic frameworks (COFs). The high surface area, thermal and chemical stability, structural tunability, porosity, and low density of COFs make them appropriate for energy storage applications. Their potential to produce advanced energy storage devices with better performance and durability is further reinforced by their ability to be customized for specific applications and amplified for conductive materials. This review covers the designs and synthetic techniques of COFs and their composites specifically suitable for energy storage uses. It further highlights their use as cathode and anode materials in supercapacitors, COF based electrolytes and batteries. The review further includes the flexibility and efficiency of COFs in energy storage applications. Furthermore, it addresses the challenges and their potential solutions regarding the use of COFs in energy storage devices. By providing a comprehensive understanding of the advantages and limitations of COFs, this review aims to inform and inspire future advancements in energy storage technologies.

Covalent organic frameworks (COFs) are highly porous, thermally and chemically stable organic polymers. Their high porosity, crystallinity, and adjustable properties make them suitable for numerous applications. However, COFs encounter critical challenges, such as their difficult processability, self-stacking propensity, low electrical conductivity, pore blockage which limits their ionic conductivity, and high recombination rates of photoinduced electrons and holes. To overcome these issues, the hybridization of COFs with 2D materials (2DMs) has proven to be an effective strategy. 2DMs including graphene-like materials, transition metal dichalcogenides, and MXenes are particularly advantageous because of their unique physicochemical properties, such as exceptional electrical and optical characteristics, and mechanical resilience. Over the past decade, significant research efforts have been focused on hybrid 2DMs-COFs materials. These hybrids leverage the strengths of both materials, making them suitable for advanced applications. This Review highlights the latest advancements in 2DM-COF hybrids, examining the physicochemical strengths and weaknesses of the pristine materials, together with the synergistic benefits of their hybridization. Moreover, it emphasizes their most remarkable applications in chemical sensing, catalysis, energy storage, adsorption and filtration, and as anticorrosion agents. Finally, it discusses future challenges and opportunities in the development of 2DM-COFs for new disruptive technologies.

Covalent organic frameworks (COFs) with high porosity and redox-active properties have been advanced as electrode materials for energy storage systems, although poor electron conductivity and inaccessibility of active pore sites hinder their performance as electrode material for supercapacitor application. Thus, incorporation of a COF with various conductive materials can be an effective strategy to improve their electrochemical performances for supercapacitors. Herein, we report the synthesis of POR-COF@g-C3N4 composites by fabricating a porphyrin-based COF on g-C3N4 via in situ solvothermal conditions. A series of COF composites, known as POR-COF@g-C3N4-X, were prepared by incorporating different ratios of COF (X = 10%, 20%, 30%, and 40%) into g-C3N4 and investigated as electrode materials for supercapacitor performance. The optimized COF composite (POR-COF@g-C3N4-30) achieved a specific capacitance of 788 F g-1 at 4 A g-1, much higher compared to pristine COF. Further, the asymmetric device assembled using POR-COF@g-C3N4-30 demonstrated a high energy density of 24.4 Wh kg-1 at a power density of 1152 W kg-1 and a long-term cycling life of 74% capacity retention after 6000 cycles. Our work highlights the scope of COF-based composites as active electrode materials for highly efficient supercapacitor performance.

Redox-active porous organic polymers (POPs) demonstrate significant potential in supercapacitors. However, their intrinsic low electrical conductivity and stacking tendencies often lead to low utilization rates of redox-active sites within their structural units. Herein, polyimide POPs (donated as PMTA) are synthesized in situ on multi-walled carbon nanotubes (MWCNTs) from tetramino-benzoquinone (TABQ) and 1,4,5,8-naphthalene tetracarboxylic dianhydride (PMDA) monomers. The strong π-π stacking interactions drive the PMTA POPs and the MWCNTs together to form a PMTA/MWCNT composite. With the assistance of MWCNTs, the stacking issue and low conductivity of PMTA POPs are well addressed, leading to the obvious activation and enhanced utilization of the redox-active groups in the PMTA POPs. PMTA/MWCNT then achieves a high capacitance of 375.2 F g-1 at 1 A g-1 as compared to the pristine PMTA POPs (5.7 F g-1) and excellent cycling stability of 89.7% after 8000 cycles at 5 A g-1. Cyclic voltammetry (CV) and in situ Fourier-Transform Infrared (FT-IR) results reveal that the electrode reactions involve the reversible structural evolution of carbonyl groups, which are activated to provide rich pseudocapacitance. Asymmetric supercapacitors (ASCs) assembled with PMTA/MWCNTs and activated carbon (AC) offer a high energy density of 15.4 Wh kg-1 at 980.4 W kg-1 and maintain a capacitance retention of 125% after 10,000 cycles at 5 A g-1, indicating their good potential for practical applications.

Eco-friendly and efficient energy production and storage technologies are highly demanded to address the environmental and energy crises. Porous organic polymers (POPs) are a class of lightweight porous network materials covalently linked by organic building blocks, possessing high surface areas, tunable pores, and designable components and structures. Due to their unique structural and compositional advantages, POPs have recently emerged as promising electrode materials for energy storage devices, particularly in the realm of supercapacitors and ion batteries. In this work, a comprehensive overview of recent progress and applications of POPs as electrode materials in energy storage devices, including the structural features and synthesis strategies of various POPs, as well as their applications in supercapacitors, lithium batteries, sodium batteries, and potassium batteries are provided. Finally, insights are provided into the future research directions of POPs in electrochemical energy storage technologies. It is anticipated that this work can provide readers with a comprehensive background on the design of POPs-based electrode materials and ignite more research in the development of next-generation energy storage devices.

In order to expand the applicability of materials and improve their performance, the combined use of different materials has increasingly been explored. Among these materials, inorganic-organic hybrid materials often exhibit properties superior to those of single materials. Covalent organic frameworks (COFs) are famous crystalline porous materials constructed by organic building blocks linked by covalent bonds. In recent years, the combination of COFs with other materials has shown interesting properties in diverse fields, and the composite materials of COFs and TiO2 have been investigated more and more. These two outstanding materials are combined through covalent bonding, physical mixing, and other methods and exhibit excellent performance in various fields, including photocatalysis, electrocatalysis, sensors, separation, and energy storage and conversion. In this Review, the current preparation methods and applications of COF-TiO2 hybrid materials are introduced in detail, and their future development and possible problems are discussed and prospected, which is of great significance for related research. It is believed that these interesting hybrid materials will show greater application value as research progresses.

Covalent organic frameworks (COFs) with customizable geometry and redox centers are an ideal candidate for supercapacitors and hybrid capacitive deionization (HCDI). However, their poor intrinsic conductivity and micropore-dominated pore structures severely impair their electrochemical performance, and the synthesis process using organic solvents brings serious environmental and cost issues. Herein, a 2D redox-active pyrazine-based COF (BAHC-COF) was anchored on the surface of graphene in a solvent-free strategy for heterointerface regulation. The as-prepared BAHC-COF/graphene (BAHCGO) nanohybrid materials possess high-speed charge transport offered by the graphene carrier and accelerated electrolyte ion migration within the BAHC-COF, allowing ions to effectively occupy ion storage sites inside BAHC. As a result, the BAHCGO//activated carbon asymmetric supercapacitor achieves a high energy output of 61.2 W h kg-1 and a satisfactory long-term cycling life. More importantly, BAHCGO-based HCDI possesses a high salt adsorption capacity (SAC) of 67.5 mg g-1 and excellent long-term desalination/regeneration stability. This work accelerates the application of COF-based materials in the fields of energy storage and water treatment.

To address current energy crises and environmental concerns, it is imperative to develop and design versatile porous materials ideal for water purification and energy storage. The advent of covalent organic frameworks (COFs), a revolutionary terrain of porous materials, is underscored by their superlative features such as divinable structure, adjustable aperture, and high specific surface area. However, issues like inferior electric conductivity, inaccessible active sites impede mass transfer and poor processability of bulky COFs restrict their wider application. As a herculean stride forward, COF/graphene hybrids amalgamate the strengths of their constituent components and have in consequence, enticed significant scientific intrigue. Herein, the current progress on the structure and properties of graphene-based materials and COFs are systematically outlined. Then, synthetic strategies for preparing COF/graphene hybrids, including one-pot synthesis, ex situ synthesis, and in situ growth, are comprehensively reviewed. Afterward, the pivotal attributes of COF/graphene hybrids are dissected in conjunction with their multifaceted applications spanning adsorption, separation, catalysis, sensing, and energy storage. Finally, this review is concluded by elucidating prevailing challenges and gesturing toward prospective strides within the realm of COF/graphene hybrids research.

Covalent organic frameworks (COFs) with flexible periodic skeletons and ordered nanoporous structures have attracted much attention as potential candidate electrode materials for green energy storage and efficient seawater desalination. Further improving the intrinsic electronic conductivity and releasing porosity of COF-based materials is a necessary strategy to improve their electrochemical performance. Herein, the employed graphene as the conductive substrate to in situ grow 2D redox-active COF (TFPDQ-COF) with redox activity under solvent-free conditions to prepare TFPDQ-COF/graphene (TFPDQGO) nanohybrids and explores their application in both supercapacitor and hybrid capacitive deionization (HCDI). By optimizing the hybridization ratio, TFPDQGO exhibits a large specific capacitance of 429.0 F g-1 due to the synergistic effect of the charge transport highway provided by the graphene layers and the abundant redox-active centers contained in the COF skeleton, and the assembled TFPDQGO//activated carbon (AC) asymmetric supercapacitor possesses a high energy output of 59.4 Wh kg-1 at a power density of 950 W kg-1 and good cycling life. Furthermore, the maximum salt adsorption capacity (SAC) of 58.4 mg g-1 and stable regeneration performance is attained for TFPDQGO-based HCDI. This study highlights the new opportunities of COF-based hybrid materials acting as high-performance supercapacitor and HCDI electrode materials.

Due to their excellent stability, ease of modification, high specific surface area, and tunable redox potentials, covalent organic frameworks (COFs) as potential electrodes in supercapacitors (SCs) have raised much research interest because these materials can enable the achievement of high electric double-layer supercapacitance and high pseudocapacitance. Here, the design strategies and SC applications of COF-based electrode materials are summarized. The detailed principles are introduced first, followed by discussions on strategies with diverse examples. The updated advances in design and applications are also discussed. Finally, in the outlook section, we provide some guidelines on the rational design of COF-based electrode materials for high-performance SCs, which we hope will inspire novel concepts for COF-based supercapacitors.

Benefiting from the merits of low cost, nonflammability, and high operational safety, aqueous rechargeable batteries have emerged as promising candidates for large-scale energy-storage applications. Among various metal-ion/non-metallic charge carriers, the proton (H+ ) as a charge carrier possesses numerous unique properties such as fast proton diffusion dynamics, a low molar mass, and a small hydrated ion radius, which endow aqueous proton batteries (APBs) with a salient rate capability, a long-term life span, and an excellent low-temperature electrochemical performance. In addition, redox-active organic molecules, with the advantages of structural diversity, rich proton-storage sites, and abundant resources, are considered attractive electrode materials for APBs. However, the charge-storage and transport mechanisms of organic electrodes in APBs are still in their infancy. Therefore, finding suitable electrode materials and uncovering the H+ -storage mechanisms are significant for the application of organic materials in APBs. Herein, the latest research progress on organic materials, such as small molecules and polymers for APBs, is reviewed. Furthermore, a comprehensive summary and evaluation of APBs employing organic electrodes as anode and/or cathode is provided, especially regarding their low-temperature and high-power performances, along with systematic discussions for guiding the rational design and the construction of APBs based on organic electrodes.

Asymmetric supercapacitors (ASCs), employing two dissimilar electrode materials with a large redox peak position difference as cathode and anode, have been designed to further broaden the voltage window and improve the energy density of supercapacitors. Organic molecule based electrodes can be constructed by combining redox-active organic molecules with conductive carbon-based materials such as graphene. Herein, pyrene-4,5,9,10-tetraone (PYT), a redox-active molecule with four carbonyl groups, exhibits a four-electron transfer process and can potentially deliver a high capacity. PYT is noncovalently combined with two different kinds of graphene (Graphenea [GN] and LayerOne [LO]) at different mass ratios. The PYT-functionalized GN electrode (PYT/GN 4-5) possesses a high capacity of 711 F g^-1 at 1 A g^-1 in 1 M H₂ SO₄ . To match with the PYT/GN 4-5 cathode, an annealed-Ti₃ C₂ T_x (A-Ti₃ C₂ T_x ) MXene anode with a pseudocapacitive character is prepared by pyrolysis of pure Ti₃ C₂ T_x . The assembled PYT/GN 4-5//A-Ti₃ C₂ T_x ASC delivers an outstanding energy density of 18.4 Wh kg^-1 at a power density of 700 W kg^-1 . The PYT-functionalized graphene holds great potential for high-performance energy storage devices.

As a rising class of crystallographic organic polymers, covalent-organic frameworks (COFs) have high specific surface areas, ordered pore structures, and designability, which exhibit broad application prospects in the energy storage sector. However, their low electrical conductivity hinders their potential use in supercapacitors. To improve the electrical conductivity, we introduced carboxylated multi-walled carbon nanotubes to obtain a series of carbon nanotube@COF composites by a facile one-pot method, in which 2D TFA-COFs are in situ grown on the surface of carboxylated multi-walled carbon nanotubes. Among them, the CNT@TFA-COF-3 composite exhibits good crystallinity, regular pores, excellent stability and a specific surface area of 1034 m² g^-1. As expected, as a capacitive electrode material, the CNT@TFA-COF composite shows improved electrochemical performance. Notably, the value of specific capacitance of the CNT@TFA-COF-3 composite (338 F g^-1) is about 8.5, 4.9, and 7.5 times higher than those of TFA-COFs, CNTs, and the CNT/TFA-COF physically mixed complex at a current density of 1.0 A g^-1, respectively. Furthermore, the CNT@TFA-COF-3 supercapacitor exhibits long-term cycle chemical stability and splendid rate capability even after 7000 charge-discharge cycles. The successful preparation of the CNT@TFA-COF-3 composite can provide new ideas for the construction of new COF-based composites and the development of new materials for energy storage.

The poor compatibility between the polymer matrix and complex modification processes greatly affects the excellent tribological properties of graphene in the polymer matrix. In this study, a covalent organic framework (COF)-coated graphene hybrid lubricating filler (G/COFs) was synthesized in situ using a sample one-step mechanochemical synthesis process. This was used to improve the tribological properties of bismaleimide (BMI) resin. The morphology and microstructure of the G/COFs hybrid were characterized, and the effect of the added amount on the tribological properties of the G/COFs/BMI composites was studied. The results showed that the G/COFs hybrid could improve the stability of the friction coefficient and decrease the volume wear rate of BMI composites. Compared to the neat BMI, the 0.6 wt% G/COFs/BMI composites showed optimal tribological performance, with the friction coefficient and volume wear rate decreasing from 0.35 to 0.14 and from 48 × 10^-6 to 10.6 × 10^-6 mm³/(N‧m), respectively. In addition, the G/COFs/BMI composites showed lower friction coefficient fluctuations and volume wear rates than G/BMI composites. This is mainly attributed to the fact that the deposition of COFs can not only effectively prevent the aggregation of graphene nanosheets, but can also significantly improve the compatibility and interfacial bond between the graphene and BMI matrix. Moreover, the good synergistic effect between the lamellar COFs and graphene nanosheets can generate high-quality self-lubricating transfer films during the friction process. The excellent dispersibility, efficient chemical functionalization, better friction reduction and wear-resistance properties, and facile preparation method make graphene/COFs hybrid nanoparticles promising as an excellent lubricating filler.