Isometric Covalent Triazine Framework-Derived Porous Carbons as Metal-Free Electrocatalysts for the Oxygen Reduction Reaction

Covalent triazine frameworks (CTFs) and their derivative N-doped carbons have attracted much attention for application in energy conversion and storage. However, previous studies have mainly focused on developing new building blocks and optimizing synthetic conditions. The use of isometric building blocks to control the porous structure and to fundamentally understand structure-property relationships have rarely been reported. In this work, two isometric building blocks are used to produce isometric CTFs with controllable pore geometries. The as-prepared CTF with nonplanar hexagonal rings demonstrates higher surface area, larger pore volume, and richer N content than the planar CTF. After pyrolysis, nonplanar porous CTF-derived N-doped carbons exhibit admirable catalytic activity for oxygen reduction in alkaline media (half-wave potential: 0.86 V; Tafel slope: 65 mV dec^-1 ), owing to their larger pore volume and the abundance of pyridinic and graphitic N species. When assembled into a zinc-air battery, the as-made electrocatalysts show high capacities of up to 651 mAh g^-1 and excellent durability.

Graphene Oxide-Assisted Covalent Triazine Framework for Boosting Photocatalytic H2 Evolution

Covalent triazine frameworks (CTFs) with two-dimensional structures have exhibited promising visible-light-induced H₂ evolution performance. However, it is still a challenge to improve their activity. Herein, we report π-conjugation-linked CTF-1/GO for boosting photocatalytic H₂ evolution. The CTF-1/GO hybrid material was obtained by a facile low-temperature condensation of 1,4-dicyanobenzene in the presence of GO. The results of photocatalytic H₂ evolution indicate that the optimum hybrid, CTF-1/GO-3.0, exhibited an H₂ evolution rate of 2262.4 μmol ⋅ g^-1  ⋅ h^-1 under visible light irradiation, which was 9 times that of pure CTF-1. The enhanced photocatalytic performance could be attributed to the fact that GO in CTF-1/GO hybrids not only acts as an electron collector and transporter like a "bridge" to facilitate the separation and transfer of photogenerated charges but also shortens the electron migration path due to its thin sheet layer uniformly distribution over CTF-1. This work could help future development of novel conjugated CTF-based composite materials as high-efficiency photocatalyst for photocatalysis.

The synthesis of alternating donor-acceptor polymers based on pyrene-4,5,9,10-tetraone and thiophene derivatives, their composites with carbon, and their lithium storage performances as anode materials

Two conjugated polymer@activated carbon composites were synthesized by the in situ polymerization of two donor-acceptor type polymers including poly[(thiophene-2,5-yl)-((pyrene-4,5,9,10-tetraone)-2,7-yl)] (PTPT) and poly[((2,3-dihydrothieno[3,4-b][1,4]dioxine)-5,7-yl)-((pyrene-4,5,9,10-tetraone)-2,7-yl)] (POTPT) on activated carbon (AC) by one-step cross-coupling reaction catalyzed by an organometallic catalyst. Cyclic voltammetry showed that both polymers exhibited ambipolar properties, low bandgaps, and low electrode potentials, which could be useful for their application as anodes in lithium-ion battery cells (LIBs). For PTPT@AC and POTPT@AC anodes, they showed a high capacity of 253.9 and 370.5 mA h g-1 at 100 mA g-1. Besides, the capacities of pure polymers were calculated to be 693.5 and 1276.5 mA h g-1 for PTPT and POTPT, respectively, at 100 mA g-1. Compared with PTPT, the introduction of the 3,4-ethylenedioxy unit into the side chain of the thiophene unit leads to substantially improved performance of POTPT due to the lowered LUMO energy levels of POTPT and the electron-rich feature of the EDOT unit. It is suggested that the structure-tuning strategy might be an effective method to prepare the new polymer-based anode for next generation LIBs with high performance and high safety.

B/N-Enriched Semi-Conductive Polymer Film for Micro-Supercapacitors with AC Line-Filtering Performance

Microsupercapacitors (MSCs) have drawn great attention for use as miniaturized electrochemical energy storage devices in portable, wearable, as well as implantable electronics. Many materials have been developed as electrodes for MSCs. However, the thin-film fabrication for most of these materials involves multistep operations, including filtration, spray coating, and sputtering. Most importantly, these methods present challenges for the preparation of thin films at the atomic or molecular scale. Therefore, the understanding of performance of ultrathin-film-based MSCs remains challenge. Herein, a B/N-enriched polymer film is successfully prepared using the photoassisted interfacial approach. The as-synthesized polymer film exhibits typical semiconductive characteristics and can be easily scaled up to a large area of up to tens of square centimeters. This ultrathin polymer film can be directly transferred to silicon wafers to fabricate MSC through laser scribing. The prepared MSC exhibits specific volumetric capacitance as high as 20.9 F cm^-3, corresponding to volumetric energy density of 2.9 mWh cm^-3 (at 0.1 V s^-1). Moreover, the volumetric power density can reach 1461 W cm^-3, surpassing most existing semiconductive polymer film-based MSC devices. In addition, the prepared MSC exhibits typical AC line-filtering ability (-67° at 120 Hz). This study offers a facile interfacial approach to preparing semiconductive polymer films with aromatic moieties for microsized energy storage devices.

Ionic Polyimide Derived Porous Carbon Nanosheets as High-Efficiency Oxygen Reduction Catalysts for Zn-Air Batteries

Two-dimensional (2D) porous carbon nanosheets (2DPCs) have attracted great attention for their good porosity and long-distance conductivity. Factors such as templates, precursors, and carbonization-activation methods, directly determine their performance. However, rational design and preparation of porous carbon materials with controlled 2D morphology and heteroatom dopants remains a challenge. Therefore, an ionic polyimide with both sp² - and sp³ -hybridized nitrogen atoms was prepared as a precursor for fabricating N-doped hexagonal porous carbon nanosheets through a hard-template approach. Because of the large surface area and efficient charge-mass transport, the resulting activated 2D porous carbon nanosheets (2DPCs-a) displayed promising electrocatalytic properties for oxygen reduction reaction (ORR) in alkaline and acidic media, such as ultralow half-wave potential (0.83 vs. 0.84 V of Pt/C) and superior limiting current density (5.42 vs. 5.14 mA cm^-2 of Pt/C). As air cathodes in Zn-air batteries, the as-developed 2DPCs-a exhibited long stability and high capacity (up to 614 mA h g^-1 ), which are both higher than those of commercial Pt/C. This work provides a convenient method for controllable and scalable 2DPCs fabrication as well as new opportunities to develop high-efficiency electrocatalysts for ORR and Zn-air batteries.

Exfoliation of 2D Materials for Energy and Environmental Applications

The fascinating properties of single-layer graphene isolated by mechanical exfoliation have inspired extensive research efforts toward two-dimensional (2D) materials. Layered compounds serve as precursors for atomically thin 2D materials (briefly, 2D nanomaterials) owing to their strong intraplane chemical bonding but weak interplane van der Waals interactions. There are newly emerging 2D materials beyond graphene, and it is becoming increasingly important to develop cost-effective, scalable methods for producing 2D nanomaterials with controlled microstructures and properties. The variety of developed synthetic techniques can be categorized into two classes: bottom-up and top-down approaches. Of top-down approaches, the exfoliation of bulk 2D materials into single or few layers is the most common. This review highlights chemical and physical exfoliation methods that allow for the production of 2D nanomaterials in large quantities. In addition, remarkable examples of utilizing exfoliated 2D nanomaterials in energy and environmental applications are introduced.

Nitrogen-Linked Hexaazatrinaphthylene Polymer as Cathode Material in Lithium-Ion Battery

Nitrogen-linked hexaazatrinaphthylene polymer (N² -HATN) as organic cathode material with low HOMO-LOMO gap was synthesized and was observed to possess reversible high capacity and unexpected long-term cycling stability. The pre-treated N² -HATN and pRGO combination demonstrated good structure compatibility and the resultant cathode exhibited a constant increment of capacity during the redox cycles. The initial capacity at 0.05 A g^-1 was 406 mA h^-1  g^-1 , and increased to 630 mA h^-1  g^-1 after 70 cycles. At 0.5 A g^-1 discharging rate, the capacity increased from an initial value of 186 mA h^-1  g^-1 to 588 mA h^-1  g^-1 after 1600 cycles. The pseudocapacitance-type behavior is postulated to be attributed to the structure compatibility between the active material and pRGO.

Two-Dimensional Porous Polymers: From Sandwich-like Structure to Layered Skeleton

Inorganic porous materials have long dominated the field of porous materials due to their stable structure and wide applications. In the past decade, porous polymers have generated considerable interest among researchers because of their easily tunable porosity, carbon-rich backbones, and prominent physical properties. These attributes enable porous polymers to be used in various applications such as sensing, gas separation and storage, catalysis, and energy storage. However, poor dispersibility has long hindered the development of porous polymers. A majority of the reported porous polymers can only be synthesized with amorphous structure through direct precipitation from solutions during reactions. The rational design and synthesis of porous polymers with controllable morphology, such as two-dimensional (2D) morphology, remains great challenge. Two-dimensional nanomaterials have attracted considerable interest because of their unique properties, which originate from the intrinsic chemical structures and 2D dimensionality. Among 2D nanomaterials, 2D porous polymers, which possess the advanced features of polymers, porous materials, and 2D nanomaterials, have been a rising star. Conventionally, polymerization strategies for generating 2D porous polymers mainly include the cross-linking of multiarmed monomers in 2D-space-confined environments, such as crystalline solid surfaces, liquid-liquid interfaces, and liquid-air interfaces. However, these methods always involve complicate operations, e.g., under vacuum, sophisticated equipment, film transfer technology, exfoliation, and so on and, most importantly, are difficult to scale up. To overcome this synthesis obstacle, 2D nanomaterials, such as graphene, can be used as 2D templates for synthesis of sandwich-like 2D porous polymers and porous carbon nanosheets. p-Bromobenzene-, p-cyanobenzene-, polyacrylonitrile-, and amino-functionalized graphene are used as templates for direct surface polymerization through reactions such as Sonogashira-Hagihara coupling reaction, condensation reaction, ionothermal reaction, reversible addition-fragmentation chain transfer polymerization, Friedel-Crafts reaction, and oxidation reaction. Therefore, sandwich-like 2D conjugated microporous polymers, Schiff-base type porous polymers, covalent triazine frameworks, hyper-cross-linked porous polymers, and mesoporous conducting polymers can be easily prepared. Beyond graphene, other excellent 2D nanomaterials, e.g., MoS₂, can also act 2D templates to construct 2D porous polymers and corresponding hybrid materials. In addition, 2D morphology for porous polymer can be achieved without 2D templates in a few cases. For instance, olefin-linkage-linked covalent organic frameworks can be synthesized through Knoevenagel condensation reaction. As is known, porous polymers can serve as carbon-rich precursors to generate heteroatom doped porous carbons for energy storage and catalysis. Thus, one benefit of 2D porous polymers is new access toward porous carbon nanosheets through direct pyrolysis without using inorganic porous templates. In this Account, we summarize recent research on 2D porous polymers and corresponding porous carbon nanosheets.

High Capacitive Storage Performance of Sulfur and Nitrogen Codoped Mesoporous Graphene

Mesoporous graphene is synthesized based on the chemical vapor deposition methodology by using heavy MgO flakes as substrates in a fluidized-bed reactor. Subsequently, sulfur and nitrogen coincorporation into graphene frameworks is realized by the reaction between carbon atoms and thiourea molecules. The as-obtained sulfur and nitrogen codoped mesoporous graphene (SNMG) exhibits remarkable capacitive energy-storage behavior, as a result of well-developed pore channels, in terms of that in a symmetric supercapacitor and lithium-ion hybrid capacitor (LIHC). The ultrahigh durability of the SNMG/SNMG symmetric supercapacitor is demonstrated by long-term cycling, for which no capacitance decay is found after 20 000 cycles. A LIHC constructed from commercial Li₄ Ti₅ O₁₂ (LTO) as the anode and SNMG as the cathode is capable of delivering much enhanced lithium-storage ability and better rate capability than that of activated carbon (AC)/LTO LIHC. Moreover, SNMG/LTO LIHC exhibits maximum energy and power densities of 86.2 Wh kg^-1 and 7443 W kg^-1 and maintains 87 % capacitance retention after 2000 cycles.

Conductive Microporous Covalent Triazine-Based Framework for High-Performance Electrochemical Capacitive Energy Storage

Nitrogen-enriched porous nanocarbon, graphene, and conductive polymers attract increasing attention for application in supercapacitors. However, electrode materials with a large specific surface area (SSA) and a high nitrogen doping concentration, which is needed for excellent supercapacitors, has not been achieved thus far. Herein, we developed a class of tetracyanoquinodimethane-derived conductive microporous covalent triazine-based frameworks (TCNQ-CTFs) with both high nitrogen content (>8 %) and large SSA (>3600 m²  g^-1 ). These CTFs exhibited excellent specific capacitances with the highest value exceeding 380 F g^-1 , considerable energy density of 42.8 Wh kg^-1 , and remarkable cycling stability without any capacitance degradation after 10 000 cycles. This class of CTFs should hold a great potential as high-performance electrode material for electrochemical energy-storage systems.

Thin-walled graphitic nanocages with nitrogen-doping as superior performance anodes for lithium-ion batteries

N-Doped nanocages were prepared with thin-walled graphitic shells (∼1.2 nm), which enhanced electrochemical performance of the nanocages.

Polydopamine-graphene oxide derived mesoporous carbon nanosheets for enhanced oxygen reduction

Composite materials combining nitrogen-doped carbon (NC) with active species represent a paramount breakthrough as alternative catalysts to Pt for the oxygen reduction reaction (ORR) due to their competitive activity, low cost and excellent stability. In this paper, a simple strategy is presented to construct graphene oxide-polydopamine (GD) based carbon nanosheets. This approach does not need to modify graphene and use any catalyst for polymerization under ambient conditions, and the obtained carbon nanosheets possess adjustable thicknesses and uniform mesoporous structures without using any template. The thickness of GD hybrids and the carbonization temperature are found to play crucial roles in adjusting the microstructure of the resulting carbon nanosheets and, accordingly their ORR catalytic activity. The optimized carbon nanosheet generated by a GD hybrid of 5 nm thickness after 900 °C carbonization exhibits superior ORR activity with an onset potential of -0.07 V and a kinetic current density of 13.7 mA cm(-2) at -0.6 V. The unique mesoporous structure, high surface areas, abundant defects and favorable nitrogen species are believed to significantly benefit the ORR catalytic process. Furthermore, it also shows remarkable durability and excellent methanol tolerance outperforming those of commercial Pt/C. In view of the physicochemical versatility and structural tunability of polydopamine (PDA) materials, our work would shed new light on the understanding and further development of PDA-based carbon materials for highly efficient electrocatalysts.

Conjugated microporous polymers with dimensionality-controlled heterostructures for green energy devices

Dimensionality for conjugated micro-porous polymers (CMP-nD, n = 0, 1, 2) is proven to be of great importance for tailoring their photophysical properties. Moreover, CMP-nD can further be converted into boron and nitrogen co-doped porous carbons (nDBN, n = 0, 1, 2) with maintained 0D, 1D, and 2D nano-structures and highly efficient catalytic performance.