Hollow melamine resin-based carbon spheres/graphene composite with excellent performance for supercapacitors

Purification of Waste-Generated Biogas Mixtures Using Covalent Organic Framework's High CO2 Selectivity

Development of crystalline porous materials for selective CO2 adsorption and storage is in high demand to boost the carbon capture and storage (CCS) technology. In this regard, we have developed a β-keto enamine-based covalent organic framework (VM-COF) via the Schiff base polycondensation technique. The as-synthesized VM-COF exhibited excellent thermal and chemical stability along with a very high surface area (1258 m2 g-1) and a high CO2 adsorption capacity (3.58 mmol g-1) at room temperature (298 K). The CO2/CH4 and CO2/H2 selectivities by the IAST method were calculated to be 10.9 and 881.7, respectively, which were further experimentally supported by breakthrough analysis. Moreover, theoretical investigations revealed that the carbonyl-rich sites in a polymeric backbone have higher CO2 binding affinity along with very high binding energy (-39.44 KJ mol-1) compared to other aromatic carbon-rich sites. Intrigued by the best CO2 adsorption capacity and high CO2 selectivity, we have utilized the VM-COF for biogas purification produced by the biofermentation of municipal waste. Compared with the commercially available activated carbon, VM-COF exhibited much better purification ability. This opens up a new opportunity for the creation of functionalized nanoporous materials for the large-scale purification of waste-generated biogases to address the challenges associated with energy and the environment.

Competition among Refined Hollow Structures in Schiff Base Polymer Derived Carbon Microspheres

Synthetic polymer-derived hollow carbon spheres have great utilitarian value in many fields for which the synthesis of proper polymer precursors is a key process. The exploration of new suitable polymer precursors and the construction of refined hollow structures in emerging polymers are both of great significance for synthetic methodology and novel carbon materials. Here, for the first time Schiff base polymer (SBP) colloid spheres with refined hollow structures were synthesized by tandem gradient growth and confined polymerization processes. The Hill equation was employed as a mathematical model to explain the gradient growth of SBP spheres. The size-dependent inner structure of SBP spheres can be adjusted from hollow to multichamber-surrounded hollow, and then to a multichamber structure. SBP-derived carbon spheres having similar surface area and chemical composition but different inner structures provide an effective way to investigate the relationship between inner structure and performance.

Preparation of an N-doped mesoporous carbon sphere and sheet composite as a high-performance supercapacitor

Carbon-based materials with multidimensional structures generally exhibit improved properties compared with single-morphology carbon materials for various applications including catalysis, adsorption, and energy storage. Here, an N-doped mesoporous carbon sphere and sheet composite is prepared by a co-assembly strategy using an ionic liquid ([C18Mim]Br) as the structure-directing agent, ethylenediamine as the catalyst, tetraethyl orthosilicate as the pore-forming agent, and resorcinol formaldehyde resin as the carbon precursor. [C18Mim]Br and ethylenediamine not only induce formation of the unique structure but also lead to in situ nitrogen doping on the N-doped mesoporous carbon skeleton. The obtained N-doped mesoporous carbon shows a unique composite structure of thin sheets embedded with carbon spheres, having high a specific surface area and uniform mesopore distribution. When used as an electrode material, the N-doped mesoporous carbon shows a good specific capacity of 273 F g−1 at a current density of 0.5 A g−1 and a good rate capability (82.1% of the capacitance is retained at a high current density of 10 A g−1). Moreover, the N-doped mesoporous carbon exhibited ideal stability behavior (91.6% capacitive retention after 10,000 cycles), indicating a promising role as an electrode material for excellent performance supercapacitors.

Oriented Carbon Nanostructures from Plasma Reformed Resorcinol-Formaldehyde Polymer Gels for Gas Sensor Applications

Oriented carbon nanostructures (OCNs) with dominant graphitic characteristics have attracted research interest for various applications due to the excellent electrical and optical properties owing to their vertical orientation, interconnected structures, electronic properties, and large surface area. Plasma enhanced chemical vapor deposition (PECVD) is considered as a promising method for the large-scale synthesis of OCNs. Alternatively, structural reformation of natural carbon precursor or phenol-based polymers using plasma-assisted surface treatment is also considered for the fabrication of OCNs. In this work, we have demonstrated a fast technique for the synthesis of OCNs by plasma-assisted structure reformation of resorcinol-formaldehyde (RF) polymer gels using radio-frequency inductively coupled plasma (rf-ICP). A thin layer of RF polymer gel cast on a glass substrate was used as the carbon source and treated with rf plasma under different plasma discharge conditions. Argon and hydrogen gases were used in surface treatment, and the growth of carbon nanostructures at different discharge parameters was systematically examined. This study explored the influence of the gas flow rate, the plasma power, and the treatment time on the structural reformation of polymer gel to produce OCNs. Moreover, the gas-sensing properties of as-prepared OCNs towards ethanol at atmospheric conditions were also investigated.

Facile synthesis of graphene-based hyper-cross-linked porous carbon composite with superior adsorption capability for chlorophenols

In this work, we proposed a green and cost-effective method to prepare a graphene-based hyper-cross-linked porous carbon composite (GN/HCPC) by one-pot carbonization of hyper-cross-linked polymer (HCP) and glucose. The composite combined the advantages of graphene (GN) and hyper-cross-linked porous carbon (HCPC), leading to high specific surface area (396.93 m²/g) and large total pore volume (0.413 cm³/g). The resulting GN/HCPC composite was applied as an adsorbent to remove 2,4-dichlorophenol (2,4-DCP) from aqueous solutions. The influence of different solution conditions including pH, ionic strength, contact time, system temperature and concentration of humic acid was determined. The maximum adsorption capacity of GN/HCPC composite (calculated by the Langmuir model) could reach 348.43 mg/g, which represented increases of 43.6% and 13.6% over those of the as-prepared pure GN and HCPC, respectively. The Langmuir model and pseudo-second-order kinetic model were found to fit well with the adsorption process. Thermodynamic experiments suggested that the adsorption proceeded spontaneously and endothermically. In addition, the GN/HCPC composite showed high adsorption performance toward other organic contaminants including tetracycline, bisphenol A and phenol. Measurement of the adsorption capability of GN/HCPC in secondary effluent revealed a slight decrease over that in pure water solution. This study demonstrated that the GN/HCPC composite can be utilized as a practical and efficient adsorbent for the removal of organic contaminants in wastewater.

Hierarchical porous carbons from carboxylated coal-tar pitch functional poly(acrylic acid) hydrogel networks for supercapacitor electrodes

A gel carbonization strategy for the synthesis of hierarchical porous carbons (HPCs) from carboxylated coal-tar pitches (CCP) functional poly(acrylic acid) (PAA) hydrogel networks for advanced supercapacitor electrodes was reported. The amphiphilic CCP and PAA polymer could be easily self-assembled to gel by the major driving force of hydrogen bonding and π–π stacking. The HPCs containing interconnected macro-/meso-/micropores were fabricated by direct carbonization of the dried hydrogels. The resultant HPCs with a high specific surface area and total pore volume of 1294.6 m² g⁻¹ and 1.34 cm³ g⁻¹ respectively, as a supercapacitor electrode exhibit a high specific capacitance of 292 F g⁻¹ at 1.0 A g⁻¹ in two-electrode system. The electrode also exhibits ultra-long cycle life with a capacitance retention as high as 94.2% after 10 000 cycles, indicating the good electrochemical stability. Furthermore, the concept of such hierarchical architecture and synthesis strategy would expand to other materials for advanced energy storage systems, such as Na-ion batteries and metal oxides for supercapacitors.

As a supercapacitor electrode exhibit a high specific capacitance of 292 F g⁻¹ at 1.0 A g⁻¹.

Nitrogen-doped hollow carbon spheres with tunable shell thickness for high-performance supercapacitors

Nitrogen-doped hollow carbon spheres (NHCSs) are well prepared by using Cu₂O microspheres as a hard template and 3-aminophenol formaldehyde resin polymer as carbon and nitrogen precursors. The thickness of the carbon shell can be easily controlled in the range of 15–84 nm by simply adjusting the weight ratios of the precursors to Cu₂O microspheres, and the Cu₂O templates can also be further reused. Physicochemical characterization demonstrates that the obtained NHCSs possess a well-developed hollow spherical structure, thin carbon shell and high nitrogen doping content. Due to these characteristics, when further utilized as electrodes for supercapacitors, the NHCSs with the carbon shell thickness of 15 nm show a high capacitance of 263.6 F g⁻¹ at 0.5 A g⁻¹, an outstanding rate performance of 122 F g⁻¹ at 20 A g⁻¹ and an excellent long-term cycling stability with only 9.8% loss after 1000 cycles at 5 A g⁻¹ in 6 M KOH aqueous electrolyte. This finding may push forward the development of carbon materials, exhibiting huge potential for electrochemical energy storage applications.

Nitrogen-doped hollow carbon spheres (NHCSs) are well prepared by using Cu₂O microspheres as a hard template and 3-aminophenol formaldehyde resin polymer as carbon and nitrogen precursors.

Nanoengineering Carbon Spheres as Nanoreactors for Sustainable Energy Applications

Colloidal carbon sphere nanoreactors have been explored extensively as a class of versatile materials for various applications in energy storage, electrochemical conversion, and catalysis, due to their unique properties such as excellent electrical conductivity, high specific surface area, controlled porosity and permeability, and surface functionality. Here, the latest updated research on colloidal carbon sphere nanoreactor, in terms of both their synthesis and applications, is summarized. Various synthetic strategies are first discussed, including the hard template method, the soft template method, hydrothermal carbonization, the microemulsion polymerization method, and extension of the Stöber method. Then, the functionalization of colloidal carbon sphere nanoreactors, including the nanoengineering of compositions and the surface features, is discussed. Afterward, recent progress in the major applications of colloidal carbon sphere nanoreactors, in the areas of energy storage, electrochemical conversion, and catalysis, is presented. Finally, the perspectives and challenges for future developments are discussed in terms of controlled synthesis and functionalization of the colloidal carbon sphere nanoreactors with tunable structure, and the composition and properties that are desirable for practical applications.

Facile synthesis of hierarchical mesopore-rich activated carbon with excellent capacitive performance

Mesoporous carbons attract increasing attention owing to their potential applications in supercapacitors. So far, controlled synthesis of mesoporous carbons with a narrow pore size distribution relies largely on the complicated template methods. To avoid the use of templates, a surfactant-free emulsion polymerization method is presented for the fabrication of a melamine-modified phenolic resin microrod (MPRR) assembled by micron-sized spherical cells and thin walls. In addition, one-step KOH activation strategy is adopted to synthesize hierarchical mesoporous activated carbon with 2-10 nm narrow mesopores by using MPRR as carbon precursors. The as-prepared mesoporous activated carbon has a high specific surface area of about 2758 m² g^-1 and a mesopore volume of 0.54 cm³ g^-1 (calculated by density functional theory), comprising ∼43.5% of total pore volume (∼1.43 cm³ g^-1). Hierarchical mesopores can significantly accelerate ion transfer and increase micropore accessibility, which endow the carbon with high specific capacitance equal to 409 F g^-1 at 0.1 A g^-1 and 268 F g^-1 at 100 A g^-1 in 6 M KOH electrolyte, with a high capacitance retention of 66%. Moreover, the assembled symmetric supercapacitor also exhibits good cycling stability in KOH electrolyte and delivers high power density equal to 12080 W kg^-1 when energy density is 5.02 Wh kg^-1. This finding provides an insight into directional tailoring of mesoporous structures of phenolic resin-based carbon materials at the molecular level for high-performance supercapacitors.

Fabrication of nitrogen-rich three-dimensional porous carbon composites with nanosheets and hollow spheres for efficient supercapacitors

N-Rich 3D porous carbon composites with nanosheets and hollow spheres have been fabricated for efficient supercapacitors.

Hierarchical porous carbon derived from carboxylated coal-tar pitch for electrical double-layer capacitors

Hierarchical porous carbons have been synthesized using amphiphilic carboxylated coal-tar pitch as a precursor via a simple KOH activation process. Amphiphilic carboxylated coal-tar pitch has a high content of hydrophilic carboxyl groups that enable it to be easily wetted in KOH solution and that facilitate the activation process. In the present study, the effect of the activation agent to precursor ratio on the porosity and the specific surface area was studied by nitrogen adsorption–desorption. A maximum specific surface area of 2669.1 m² g⁻¹ was achieved with a KOH to carboxylated pitch ratio of three and this produced a structure with micropores/mesopores. Among the various hierarchical porous carbons, the sample prepared with an activation agent to precursor ratio of two exhibited the best electrochemical performance as an electrode for an electrical double-layer capacitor in a 6 M KOH electrolyte. The specific capacitance of the sample was 286 F g⁻¹ at a current density of 2 A g⁻¹ and it had a capacitance-retention ratio of 93.9%, even after 10 000 cycles. Thus, hierarchical porous carbons derived from amphiphilic-carboxylated coal-tar pitch represent a promising electrode material for electrical double-layer capacitors.

The specific capacitance of the HPC-2 electrode retention of 93.9% was obtained after 10 000 cycles, indicating good electrochemical stability.

N-Doped Mesoporous Carbon Sheets/Hollow Carbon Spheres Composite for Supercapacitors

Composite carbon materials with multiple morphologies (such as spheres/sheets and spheres/tubes) that combine the merits of both structures have a wide range of applications in electrochemistry, catalysis, energy storage, and so on. Therefore, the development of an efficient and simple method for preparing carbonaceous composite materials is a research hot spot. On the basis of the inhomogeneity of 3-aminophenol/formaldehyde (3-AF) polymerization spheres, the hollow 3-AF spheres were obtained after the dissolution of the internal 3-AF oligomer. The dispersed 3-AF oligomer was reassembled with silicate oligomers on the hard template of Mg(OH)₂ sheets and hollow 3-AF spheres linked by CTAB through electrostatic force. The obtained N-MCS/HCS possessed both sheet and sphere structure, with a high specific surface area and uniform mesoporous distribution. As an electrode material, N-MCS/HCS exhibited a good specific capacity (270 F g^-1 at the current density of 1 A g^-1) and outstanding cycling life stability (96.3% after 5000 cycles) at a current density of 5 A g^-1 and could be used as a new electrode material.

N-Doped Hollow Carbon Spheres/Sheets Composite for Electrochemical Capacitor

Functional carbon materials with a combination of 0 dimension and 2 dimension are particularly interesting in the electrochemical field owing to the low density, high surface area, and strong ion-bearing capacity. Especially, hollow mesoporous carbon spheres (0 dimension) and nanosheet (2 dimension) composite hollow porous carbon spheres and nanosheet (HMCS/S) have received much attention as electrochemical capacitor electrode materials. However, it is challenging for effective preparation of this complex composite structure. In this work, a novel and simple procedure to prepare N-doping HMCS/S (N-HMCS/S) is presented. This approach adopted silica spheres as the core and employed [C₁₈Mim]Br and tetraethyl orthosilicate as the structural directing agent for the formation of the flaky/spherical hybrid structure. The unique structure of nanosheets embedded by hollow carbon spheres and N-doping characteristics endow N-HMCS/S with good performance in electrochemical capacitor with high specific capacity (196.5 F g^-1) in the three-electrode system and excellent high-rate capability with retention of 61.5% in the two-electrode system.

Three dimensional carbon-bubble foams with hierarchical pores for ultra-long cycling life supercapacitors

Design and synthesis of integrated, interconnected porous structures are critical to the development of high-performance supercapacitors. We develop a novel and facile synthesis technic to construct three-dimensional carbon-bubble foams with hierarchical pores geometry. The carbon-bubble foams are fabricated by conformally coating, via catalytic decomposition of ethanol, a layer of carbon coating onto the surfaces of pre-formed ZnO foams and then the removal of the ZnO template by a reduction-evaporation process. Both the wall thickness and the pore size can be well tuned by adjusting the catalytic decomposition time and temperature. The as-synthesized carbon-bubble foams electrode retains 90.3% of the initial capacitance even after 70 000 continuous cycles under a high current density of 20 A g^-1, demonstrating excellent long-time electrochemical and cycling stability. The symmetric device displays rate capability retention of 81.8% with the current density increasing from 0.4 to 20 A g^-1. These achieved electrochemical performances originate from the unique structural design of the carbon-bubble foams, which provide not only abundant transport channels for electron and ion but also high active surface area accessible by the electrolyte ions.

Assembly of Hollow Carbon Nanospheres on Graphene Nanosheets and Creation of Iron-Nitrogen-Doped Porous Carbon for Oxygen Reduction

Triblock copolymer micelles coated with melamine-formaldehyde resin were self-assembled into closely packed two-dimensional (2D) arrangements on the surface of graphene oxide sheets. Carbonizing these structures created a 2D architecture composed of reduced graphene oxide (rGO) sandwiched between two monolayers of sub-40 nm diameter hollow nitrogen-doped carbon nanospheres (N-HCNS). Electrochemical tests showed that these hybrid structures had better performance for oxygen reduction compared to physically mixed rGO and N-HCNS that were not chemically bonded together. Further impregnation of the sandwich structures with iron (Fe) species followed by carbonization yielded Fe_1.6-N-HCNS/rGO-900 with a high specific surface area (968.3 m² g^-1), a high nitrogen doping (6.5 at%), and uniformly distributed Fe dopant (1.6 wt %). X-ray absorption fine structure analyses showed that most of the Fe in the nitrogen-doped carbon framework is composed of single Fe atoms each coordinated to four N atoms. The best Fe_1.6-N-HCNS/rGO-900 catalyst performed better in electrocatalytic oxygen reduction than 20 wt % Pt/C catalyst in alkaline medium, with a more positive half-wave potential of 0.872 V and the same limiting current density. Bottom-up soft-patterning of regular carbon arrays on free-standing 2D surfaces should enable conductive carbon supports that boost the performance of electrocatalytic active sites.

One-step nanocasting synthesis of nitrogen and phosphorus dual heteroatom doped ordered mesoporous carbons for supercapacitor application

N and P dual heteroatom doped ordered mesoporous carbon was synthesized and exhibited enhanced specific capacitance (220 F g⁻¹ at 1 A g⁻¹), good rate capability (178 F g⁻¹ at 16 A g⁻¹ with 81% capacitance retention) and excellent cycling stability (91% after 3000 cycles).

Synthesis of hybrid carbon spheres@nitrogen-doped graphene/carbon nanotubes and their oxygen reduction activity performance

The hybrid architecture of carbon spheres@nitrogen-doped graphene/carbon nanotubes (CS@N-G/CNT) was synthesized by a hydrothermal and ultrasonic-assisted method.