Cobalt-Doped Porous Carbon Nanosheets Derived from 2D Hypercrosslinked Polymer with CoN₄ for High Performance Electrochemical Capacitors

Macrocycle-Based Covalent Organic Frameworks

Macrocycles with well-defined cavities and the ability to undergo supramolecular interactions are classical materials that have played an essential role in materials science. However, one of the most substantial barriers limiting the utilization of macrocycles is their aggregation, which blocks the active regions. Among many attempted strategies to prevent such aggregation, installing macrocycles into covalent organic frameworks (COFs), which are porous and stable reticular networks, has emerged as an ideal solution. The resulting macrocycle-based COFs (M-COFs) preserve the macrocycles' unique activities, enabling applications in various fields such as single-atom catalysis, adsorption/separation, optoelectronics, phototherapy, and structural design of forming single-layered or mechanically interlocked COFs. The resulting properties are unmatchable by any combination of macrocycles with other substrates, opening a new chapter in advanced materials. This review focuses on the latest progress in the concepts, synthesis, properties, and applications of M-COFs, and presents an in-depth outlook on the challenges and opportunities in this emerging field.

2D Organic Materials: Status and Challenges

In the past few decades, 2D layer materials have gradually become a central focus in materials science owing to their uniquely layered structural qualities and good optoelectronic properties. However, in the development of 2D materials, several disadvantages, such as limited types of materials and the inability to synthesize large-scale materials, severely confine their application. Therefore, further exploration of new materials and preparation methods is necessary to meet technological developmental needs. Organic molecular materials have the advantage of being customizable. Therefore, if organic molecular and 2D materials are combined, the resulting 2D organic materials would have excellent optical and electrical properties. In addition, through this combination, the free design and large-scale synthesis of 2D materials can be realized in principle. Furthermore, 2D organic materials exhibit excellent properties and unique functionalities along with great potential for developing sensors, biomedicine, and electronics. In this review, 2D organic materials are divided into five categories. The preparation methods and material properties of each class of materials are also described in detail. Notably, to comprehensively understand each material's advantages, the latest research applications for each material are presented in detail and summarized. Finally, the future development and application prospects of 2D organic materials are briefly discussed.

Organic Crosslinked Polymer-Derived N/O-Doped Porous Carbons for High-Performance Supercapacitor

Supercapacitors, as a new type of green electrical energy storage device, are a potential solution to environmental problems created by economic development and the excessive use of fossil energy resources. In this work, nitrogen/oxygen (N/O)-doped porous carbon materials for high-performance supercapacitors are fabricated by calcining and activating an organic crosslinked polymer prepared using polyethylene glycol, hydroxypropyl methylcellulose, and 4,4-diphenylmethane diisocyanate. The porous carbon exhibits a large specific surface area (1589 m2·g−1) and high electrochemical performance, thanks to the network structure and rich N/O content in the organic crosslinked polymer. The optimized porous carbon material (COCLP-4.5), obtained by adjusting the raw material ratio of the organic crosslinked polymer, exhibits a high specific capacitance (522 F·g−1 at 0.5 A·g−1), good rate capability (319 F·g−1 at 20 A·g−1), and outstanding stability (83% retention after 5000 cycles) in a three-electrode system. Furthermore, an energy density of 18.04 Wh·kg−1 is obtained at a power density of 200.0 W·kg−1 in a two-electrode system. This study demonstrates that organic crosslinked polymer-derived porous carbon electrode materials have good energy storage potential.

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.

Enhancing Functionality of Epoxy-TiO2-Embedded High-Strength Lightweight Aggregates

With the increasing trend of high-rise, large-scale, and functional modern architectural structures, lightweight aggregate (LWA) concrete that exhibits excellent strength and high functionality has garnered active research attention. In particular, as the properties of concrete vary considerably with the raw materials and the proportions of aggregates in the mix, in-depth research on weight reduction, strength improvement, and functional enhancements of aggregates is crucial. This study used the negative pressure coating of a mixed solution comprising epoxy (mixture of epoxy resin and crosslinker), hyper-crosslinked polymer, and titanium oxide (TiO₂) nanoparticles on the LWA, and achieved an improvement in the strength of the LWA as well as a reduction in air pollutants such as NO_x and SO_x. Compared to a normal LWA with an aggregate impact value (AIV) of 38.7%, the AIV of the proposed epoxy–TiO₂-embedded high-strength functional LWA was reduced by approximately half to 21.1%. In addition, the reduction rates of NO_x and SO_x gases resulting from the photocatalytic properties of TiO₂ nanoparticles coated with epoxy were approximately 90.9% and 92.8%, respectively. Epoxy–TiO₂, embedded in LWAs through a mixture, exhibited stability, high strength, and a reduction in air pollutant characteristics, despite repeated water washing. The LWA proposed herein offers excellent structural and functional properties and is expected to be used in functional lightweight concrete that can be practically applied in high-rise and large-scale architectural structures.

Bimetallic MOF Nanosheets Decorated on Electrospun Nanofibers for High-Performance Asymmetric Supercapacitors

The rational design of metal-organic framework (MOF)-based materials with a huge specific surface area, high redox activity, and favorable conductivity is currently a hot subject for their potential usage in supercapacitor electrodes. Herein, novel bimetallic MOFs with a flowerlike nanosheet structure grown on the electrospun nanofibers (PPNF@M-Ni MOF, M = Co, Zn, Cu, Fe) have been prepared by controlling the incorporation of various types of metal ions, which display superior electrochemical performance. For example, PPNF@Co-Ni MOF possesses a large specific capacitance of 1096.2 F g^-1 (specific capacity of 548.1 C g^-1) at 1 A g^-1 and excellent rate performance. In addition, an asymmetric solid-state device composed of PPNF@Co-Ni MOF (positive materials) and KOH-activated carbon nanofibers embedded with reduced graphene oxide (negative materials) reaches a maximum energy density of 93.6 Wh kg^-1 at the power density of 1600.0 W kg^-1 and long cycling life. This work may greatly advance the research toward the design of supported MOF-based electrode materials for a promising prospect in energy conversion and storage.

Construction of Polymer Electrolyte Based on Soybean Protein Isolate and Hydroxyethyl Cellulose for a Flexible Solid-State Supercapacitor

Supercapacitors are a very active research topic. However, liquid electrolytes present several drawbacks on security and packaging. Herein, a gel polymer electrolyte was prepared based on crosslinked renewable and environmentally friendly soybean protein isolate (SPI) and hydroxyethyl cellulose (HEC) with 1.0 mol L⁻¹ Li₂SO₄. Highly hydrophilic SPI and HEC guaranteed a high ionic conductivity of 8.40 × 10⁻³ S cm⁻¹. The fabricated solid-state supercapacitor with prepared gel polymer electrolyte exhibited a good electrochemical performance, that is, a high single electrode gravimetric capacitance of 91.79 F g⁻¹ and an energy density of 7.17 W h kg⁻¹ at a current density of 5.0 A g⁻¹. The fabricated supercapacitor exhibited a flexible performance under bending condition superior to liquid supercapacitor and similar electrochemical performance at various bending angles. In addition, it was proved by an almost 100% cycling retention and a coulombic efficiency over 5000 charge–discharge cycles. For comparison, supercapacitors assembled with commercial aqueous PP/PE separator, pure SPI membrane, and crosslinked SPI membrane were also characterized. The obtained gel polymer electrolyte based on crosslinked SPI and HEC may be useful for the design of advanced polymer electrolytes for energy devices.

Recent Advances in Organic Thermoelectric Materials: Principle Mechanisms and Emerging Carbon-Based Green Energy Materials

Thermoelectric devices have recently attracted considerable interest owing to their unique ability of converting heat to electrical energy in an environmentally efficient manner. These devices are promising as alternative power generators for harvesting electrical energy compared to conventional batteries. Inorganic crystalline semiconductors have dominated the thermoelectric material fields; however, their application has been restricted by their intrinsic high toxicity, fragility, and high cost. In contrast, organic thermoelectric materials with low cost, low thermal conductivity, easy processing, and good flexibility are more suitable for fabricating thermoelectric devices. In this review, we briefly introduce the parameters affecting the thermoelectric performance and summarize the most recently developed carbon-material-based organic thermoelectric composites along with their preparation technologies, thermoelectric performance, and future applications. In addition, the p- and n-type carbon nanotube conversion and existing challenges are discussed. This review can help researchers in elucidating the recent studies on carbon-based organic thermoelectric materials, thus inspiring them to develop more efficient thermoelectric devices.