Dense carbon monoliths for supercapacitors with outstanding volumetric capacitances

A commercially available dense carbon monolith (CM) and four carbon monoliths obtained from it have been studied as electrochemical capacitor electrodes in a two-electrode cell. CM has: (i) very high density (1.17 g cm⁻³), (ii) high electrical conductivity (9.3 S cm⁻¹), (iii) well-compacted and interconnected carbon spheres, (iv) homogeneous microporous structure and (v) apparent BET surface area of 957 m²g⁻¹. It presents interesting electrochemical behaviors (e.g., excellent gravimetric capacitance and outstanding volumetric capacitance). The textural characteristics of CM (porosity and surface chemistry) have been modified by means of different treatments. The electrochemical performances of the starting and treated monoliths have been analyzed as a function of their porous textures and surface chemistry, both on gravimetric and volumetric basis. The monoliths present high specific and volumetric capacitances (292 F g⁻¹ and 342 F cm⁻³), high energy densities (38 Wh kg⁻¹ and 44 Wh L⁻¹), and high power densities (176 W kg⁻¹ and 183 W L⁻¹). The specific and volumetric capacitances, especially the volumetric capacitance, are the highest ever reported for carbon monoliths. The high values are achieved due to a suitable combination of density, electrical conductivity, porosity and oxygen surface content.     

Simple fabrication of a Fe2O3/carbon composite for use in a high-performance lithium ion battery

A simple approach was developed for the fabrication of a Fe₂O₃/carbon composite by impregnating activated carbon with a ferric nitrate solution and calcinating it. The composite contains graphitic layers and 10 wt.% Fe₂O₃ particles of 20–50 nm in diameter. The composite has a high specific surface area of ∼828 m² g⁻¹ and when used as the anode in a lithium ion battery (LIB), it showed a reversible capacity of 623 mAh g⁻¹ for the first 100 cycles at 50 mA g⁻¹. A discharge capacity higher than 450 mAh g⁻¹ at 1000 mA g⁻¹ was recorded in rate performance testing. This highly improved reversible capacity and rate performance is attributed to the combination of (i) the formation of graphitic layers in the composite, which possibly improves the matrix electrical conductivity, (ii) the interconnected porous channels whose diameters ranges from the macro- to meso- pore, which increases lithium-ion mobility, and (iii) the Fe₂O₃ nanoparticles that facilitate the transport of electrons and shorten the distance for Li⁺ diffusion. This study provides a cost-effective, highly efficient means to fabricate materials which combine conducting carbon with nanoparticles of metal or metal oxide for the development of a high-performance LIB.     

NiO/C nanocapsules with onion-like carbon shell as anode material for lithium ion batteries

The synthesis of NiO/C nanocapsules with NiO nanoparticles as the core and onion-like carbon layers as the shell is reported. The NiO/C nanocapsules deliver an initial discharge capacity of 1689.4 mAh g⁻¹ at 0.5 C and maintain a high reversible capacity of 1157.7 mAh g⁻¹ after 50 cycles compared to the NiO nanoparticles of 383.5 mAh g⁻¹. As an anode material for lithium ion batteries, the NiO/C nanocapsules exhibit a remarkable discharge capacity, a high rate charge–discharge capability and an excellent cycling stability. The improvements are ascribed to the fact that the onion-like carbon shells not only can provide enough voids to accommodate the volume change of NiO nanoparticles but also can prevent the formation of solid electrolyte interface (SEI) films on the surface of the NiO nanoparticles and hence the direct contact of Ni and SEI films upon lithium extraction.     

Mesoporous carbons synthesized by direct carbonization of citrate salts for use as high-performance capacitors

A simple one-step synthesis methodology for the fabrication of mesoporous carbons with an excellent performance as supercapacitor electrodes is presented. The procedure is based on the carbonization of non-alkali organic salts such as citrate salts of iron, zinc or calcium. The carbonized products contain numerous inorganic nanoparticles (i.e. Fe, ZnO or CaO) embedded within a carbonaceous matrix. These nanoparticles act as endotemplate, which when removed, leaves a mesoporous network. The resulting carbon samples have a large specific surface area up to ∼1600 m² g⁻¹ and a porosity made up almost exclusively of mesopores. An appropriate heat-treatment of these materials with melamine allows the synthesis of N-doped carbons which have a high nitrogen content (∼8–9 wt.%), a large specific surface area and retain the mesoporous structure. The mesoporous carbon samples were employed as electrode materials in supercapacitors. They exhibit specific capacitances of 200–240 F g⁻¹ in 1 M H₂SO₄ and 100–130 F g⁻¹ in EMImTFSI/AN. More importantly, the carbon samples possess a good capacitance retention in both electrolytes (>50% in H₂SO₄ and >80% in EMImTFSI/AN at 100 A g⁻¹) owing to their mesoporous structure which facilitates the penetration and transportation of ions.     

Synthesis of nitrogen-doped porous graphitic carbons using nano-CaCO3 as template,graphitization catalyst,and activating agent

Nitrogen-doped porous graphitic carbons (NPGCs) with controlled structures were synthesized using cheap nano-CaCO₃ as template, melamine-formaldehyde resin as carbon precursor, and dilute HCl as template removing agent. In addition to its use as a template, the nano-CaCO₃ acted as an internal activating agent to produce micro- and mesopores, as an adsorbent to remove the released hazardous gases (i.e. HCN, NH₃), and as a mild graphitization catalyst. The obtained NPGCs with hierarchical nanopores contained as high as 20.9 wt% of nitrogen, had surface areas of up to 834 m² g^–1, and also exhibited high thermal stability with respect to oxidation. Using carbohydrate or phenolic resin as the carbon precursor, this simple approach was also capable of producing hierarchical porous graphitic carbons with high surface area (up to 1683 m² g^–1) and extremely large pore volumes (>6 cm³ g^–1). X-ray diffraction and infrared spectroscopy suggested that the intermediate CaCN₂ or CaC₂ generated during the carbonization plays a critical role in the formation of the graphitic structure.     

Porous nitrogen doped carbon sphere as high performance anode of sodium-ion battery

The carbon material is regarded as the most promising anode candidate for sodium ion battery. In this paper, we found that the porous structure is a critical factor for the improving of carbon anode material. Porous structure is successfully fabricated in nitrogen doped carbon sphere (N-CS) via the mature template-assisted method and the sodium storage property of the porous nitrogen doped carbon sphere (P-N-CS) and N-CS is investigated. The results show that the P-N-CS possesses super rate capability of 155 mAh g⁻¹ at 1 A g⁻¹, which is much higher than that of N-CS (18 mAh g⁻¹). In addition, the P-N-CS exhibits outstanding cycle stability with 206 mAh g⁻¹ after 600 cycles at 0.2 A g⁻¹ and the capacity of N-CS is only 96 mAh g⁻¹ at the same condition. The super electrochemical performance of P-N-CS could be attributed to the high content of pores. Moreover, the high content of pyridinic and graphitic N could facilitate the transfer of sodium ion and electron.     

Activated carbon xerogels with a cellular morphology derived from hydrothermally carbonized glucose-graphene oxide hybrids and their performance towards CO2 and dye adsorption

We report the preparation of micro-/mesoporous carbon monolithic xerogels by means of a two-step approach that comprises (1) hydrothermal carbonization of glucose in the presence of graphene oxide (GO) sheets as morphology-directing agents and (2) chemical activation of the resulting hydrothermal carbon (HTC) xerogels with KOH. The as-prepared HTC xerogels were made up of a random assembly of thin (<30 nm) carbon platelets, which were interpreted to arise via dehydration and condensation reactions of glucose at catalytically active (acidic) sites present on the surface of GO. The chemical activation afforded xerogels with large surface areas and pore volumes (up to ∼2000 m² g⁻¹ and 1.15 cm³ g⁻¹, respectively) and a cellular morphology, which could be attributed to the combined effect of the activating agent and the unusual, compliant nature of the HTC xerogel. Additionally, the use of different activation conditions allowed fine-tuning the porous texture of the activated xerogels. Finally, the activated carbon xerogels displayed CO₂ uptake capacities up to 4.9 mmol g⁻¹ at 0 °C and 1 bar, as well as an efficient performance (between 600 and 700 mg g⁻¹) in the adsorption of bulky dyes, thus demonstrating their application potential.     

Graphitic carbon nitride nanosheet-assisted preparation of N-enriched mesoporous carbon nanofibers with improved capacitive performance

N-enriched mesoporous carbon nanofibers (NMCNFs) were prepared by an electrospinning technique using graphitic carbon nitride (g-C₃N₄) nanosheets both as sacrificial template and N-doping source. The resultant NMCNF film has a high N-doping level of 8.6 wt% and a high specific surface area of 554 m² g⁻¹. When directly used as the electrode material for supercapacitor, the free-standing NMPCNF film shows a significantly improved capacitive performance including a higher specific capacitance (220 F g⁻¹ at 0.2 A g⁻¹) and a better rate capability (∼70% retention at 20 A g⁻¹) than those of microporous carbon nanofiber film prepared using the same process without using g-C₃N₄ nanosheets (145 F g⁻¹ at 0.2 A g⁻¹ and ∼45% retention at 20 A g⁻¹). Moreover, the NMCNFs show superior stability with only a ∼3% decrease of its initial capacitance after 1000 cycles at a high current density of 10 A g⁻¹. More significantly, the energy density of a symmetrical supercapacitor (SC) based on the NMPCNF film can reach 12.5 Wh kg⁻¹ at a power density of 72 W kg⁻¹.     

Structure and electrochemical performance of highly nanoporous carbons from benzoate–metal complexes by a template carbonization method for supercapacitor application

A series of highly nanoporous carbons have been prepared by converting benzoate–metal complexes, including zinc benzoate, magnesium benzoate and aluminium benzoate through a template carbonization process. The carbonization temperature plays a pivotal role in determining the carbon structures as well as the resultant electrochemical behaviors in supercapacitors. The carbon–Zn-900 sample derived from zinc benzoate complex has a high specific surface area (1466.4 m² g^–1), large pore volume (2.54 cm³ g^–1) and hierarchical pore size distribution. It can also deliver a large specific capacitance of 314.1 F g⁻¹ at a current density of 0.5 A g⁻¹, together with a large energy density of 67.2 Wh kg⁻¹ when measured in a three-electrode system using 6 mol L⁻¹ KOH as electrolyte. Besides, the carbon–Zn-900 sample has been tested in a two-electrode system using [EMIm]BF₄/AN as electrolyte at different operation temperatures of 25/50/80 °C.     

Preparation and carbon dioxide uptake capacity of N-doped porous carbon materials derived from direct carbonization of zeolitic imidazolate framework

The preparation, characterization and CO₂ uptake performance of N-doped porous carbon materials and composites derived from direct carbonization of ZIF-8 under various conditions are presented for the first time. It is found that the carbonization temperature has remarkable effect on the compositions, the textural properties and consequently the CO₂ adsorption capacities of the ZIF-derived porous materials. Changing the carbonization temperature from 600 to 1000 °C, the composites and the resulting porous carbon materials possess a tuneable nitrogen content in the range of 7.1–24.8 wt%, a surface area of 362–1466 m² g⁻¹ and a pore volume of 0.27–0.87 cm³ g⁻¹, where a significant proportion of the porosity is contributed by micropores. These N-doped porous composites and carbons exhibit excellent CO₂ uptake capacities up to 3.8 mmol g⁻¹ at 25 °C and 1 bar with a CO₂ adsorption energy up to 26 kJ mol⁻¹ at higher CO₂ coverages. The average adsorption energy for CO₂ is one of the highest ever reported for any porous carbon materials. Moreover, the influence of textural properties on CO₂ capture performance of the resulting porous adsorbents has been discussed, which may pave the way to further develop higher efficient CO₂ adsorbent materials.     

Onion-like carbon-encapsulated Co,Ni, and Fe magnetic nanoparticles with low cytotoxicity synthesized by a pulsed plasma in a liquid

We synthesized onion-like carbon-encapsulated Co, Ni, and Fe (Co–C, Ni–C, and Fe–C) magnetic nanoparticles with low cytotoxicity using pulsed plasma in a liquid. The pulsed plasma is induced by a low-voltage spark discharge submerged in a dielectric liquid. The face-centered cubic Co and Ni, and body-centered cubic Fe core nanoparticles showed good crystalline structures with an average size between 20 and 30 nm were encapsulated in onion-like carbon coatings with a thickness of 2–10 nm. Vibrating-sample magnetometer measurements revealed the ferromagnetic properties of as-synthesized samples at room temperature (Co–C = 360 Oe, Fe–C = 380 Oe, and Ni–C = 211 Oe). Raman-spectroscopy analysis found onion-like carbon shells composed of well-organized graphitic structures. Thermal gravimetric analysis showed a high stability of the as-synthesized samples under thermal treatment and oxidation. Cytotoxicity measurements showed higher cancer cell viability than samples synthesized by different methods.     

Mesoporous N-containing carbon nanosheets towards high-performance electrochemical capacitors

We developed a direct carbonization strategy to efficiently fabricate mesoporous N-containing carbon nanosheets (N-CNSs) by using polyaniline nanosheets as a carbon precursor. Physicochemical characterizations revealed that the as-synthesized N-CNSs with 5.9 wt.% N species possessed a well-developed mesoporous architecture with large specific surface area of 352 m² g⁻¹, high mesoporous volume of 0.32 cm³ g⁻¹, and average pore size of ∼5.2 nm. When further utilized as an electrode for electrochemical capacitors, the mesoporous N-CNSs delivered a large specific capacitance of 239 F g⁻¹ at 0.5 A g⁻¹, and even 197 F g⁻¹ at a high current load of 8 A g⁻¹, indicating its good rate behavior. Furthermore, the capacitance degradation of ∼4% over continuous 5000 charge–discharge cycles at 6 A g⁻¹ further verified its good electrochemical stability at high rates for long-term electrochemical capacitors application.     

Robust growth of herringbone carbon nanofibers on layered double hydroxide derived catalysts and their applications as anodes for Li-ion batteries

Herringbone carbon nanofibers (CNFs) were efficiently produced by chemical vapor deposition on Ni nanoparticles derived from layered double hydroxide (LDH) precursors. The as-obtained CNFs with a diameter ranging from 40 to 60 nm demonstrated herringbone morphologies when they grew on Ni/Al LDH derived catalysts both in the fixed-bed and fluidized-bed reactor. The Ni/Mg/Al, Ni/Cu/Al, as well as Ni/Mo/Mg/Al catalysts were also effective to grow herringbone CNFs. The diameter and specific surface area of the as-obtained CNFs highly depended on the catalyst composition and the growth temperature. When CNFs were grown at 550 °C on Ni/Al catalyst, the as-obtained products had an outer diameter of ca. 50 nm and a specific surface area of 242 m² g⁻¹, possessed a discharge capacity of 330 mAh g⁻¹ as the electrode in a two-electrode coin-type cell. With the increase of the surface area, the discharge capacity increased at a rate of 0.90 mAh cm⁻², while the initial coulombic efficiency decreased gradually on nanocarbon anodes. This is attributed to the fact that CNFs with higher surface area afford smaller sp² carbon layer that facilitated more Li ions to extract from the anodes.     

An advanced method to manufacture hierarchically structured carbide-derived carbon monoliths

The preparation of carbide-derived carbon (CDC) monoliths with a hierarchically structure in the nm and μm range is presented. Basis is the manufacturing of porous cellular SiC ceramics based on a biomorphous approach with μm porosity and subsequent conformal conversion to CDC by reactive extraction with chlorine. The SiC ceramics can be sintered at low temperatures and short times (1500 °C, 2 h) compared to classical preparation methods. The SiC ceramics show a macro pore volume (1–10 μm channel size) of 0.56 ml g⁻¹, which corresponds to 1.5 ml g⁻¹ in the resulting CDC. The final carbon material exhibits an additional nano pore volume of 0.525 ml g⁻¹ with a mean slit pore size of 0.86 nm. Mechanical stabilities of the highly porous CDC are excellent (bending strength 2.1 ± 0.2 MPa, corrected Weibull modulus 8.7, characteristic strength 2.2 MPa and Youngs modulus 10.0 ± 0.5 GPa). The reactive extraction of the carbide monoliths shows very high reaction rates, approx. two dimensions faster (95×) compared to non-porous samples. Thus the manufacturing of the structured carbide and CDC can be performed at lower costs.     

Carbon composite membrane derived from a two-dimensional zeolitic imidazolate framework and its gas separation properties

The carbonization of a newly reported two-dimensional zeolitic imidazolate framework (ZIF-L) with leaf-like morphology was investigated by TG, SEM, XRD and XPS. ZIF-L flakes were thermally stable at up to 200 °C, and completely transformed into an amorphous carbonaceous material after heat treatment in nitrogen at 550 °C. A carbon composite membrane was then prepared by deposition of ZIF-L flakes on a porous alumina support and then direct carbonization of ZIF-L film. During the carbonization, the ZIF-L membrane reorganized into a nanoporous carbon composite membrane composed of ZnO nanoparticles and leaf-like carbon flakes. The resulting nanoporous carbon composite membrane exhibited a narrow micropore size distribution, and it had higher BET surface area than the ZIF-L flakes. Gas separation permeation experiments showed that the carbon composite membrane had a high H₂ permeance of 3.5 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹, and moderate H₂/N₂ and H₂/CO₂ ideal selectivities of 6.2 and 4.9, respectively. This work presents a simple and effective method for preparing functional nanoporous carbon composite membranes from ZIFs (or MOFs) for many potential applications.     

A hierarchical carbon fiber/sulfur composite as cathode material for Li–S batteries

A novel hierarchical structure carbon/sulfur composite is presented based on carbon fiber matrices, which are synthesized by electrospinning. The fibers are constituted with hollow graphitized carbon spheres formed using catalytic Ni nano-particles as hard templates. Sulfur is loaded to the carbon substrates via thermal vaporization. The structure and composition of the hierarchical carbon fiber/S composite are characterized with X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and nitrogen adsorption isotherms. The electrochemical performance is evaluated by cyclic voltammetry and galvanostatic charge–discharge. The results exhibit an initial discharge capacity of 845 mA h g⁻¹ at 0.25 C (420 mA g⁻¹), with a retention of 77% after 100 cycles. A discharge capacity of 533 mA h g⁻¹ is still attainable when the rate is up to 1.0 C. The good cycling performance and rate capability are contributed to the uniform dispersion of sulfur, the conductive network of carbon fibers and hollow graphitized carbon spheres.     

High rate performance activated carbons prepared from ginkgo shells for electrochemical supercapacitors

Partially graphitized ginkgo-based activated carbon (GGAC) is fabricated from ginkgo shells by pyrolysis, KOH activation and heat treatment using cobalt nitrate as graphitization catalyst. The graphitization temperature is 900 °C. The GGAC has a microporous structure and its specific surface area is 1775 m² g⁻¹. XRD patterns show that the carbon becomes more graphitic after heat treatment. The specific capacitance of the GGAC reaches to 178 F g⁻¹ at a potential scan rate of 500 mV s⁻¹, which is superior to that of commercial activated carbons and ordered mesoporous carbons. The high electrochemical performance of the GGAC is attributed to its good electronic conductivity and high surface area. Partially graphitized activated carbon is a promising electrode material for electrochemical supercapacitors with high rate performance.     

Application of as-synthesized Co–Al layered double hydroxides for the preparation of the electroactive composites containing N-doped carbon nanotubes

A set of electrically conductive, porous and electrocatalytically active composites was prepared by catalytic chemical vapor deposition using Co–Al layered double hydroxides and acetonitrile. The effect of synthesis temperature, i.e. 600, 700 and 800 °C on their composition, structure and morphology was examined by means of X-ray diffraction, thermogravimetric analysis, X-ray photoelectron spectroscopy, nitrogen sorption and scanning electron microscopy. Electrochemical properties of the composites were evaluated by cyclic voltammetry (CV) in alkaline solution in the presence and absence of oxygen. The composites were composed of metallic cobalt, metal oxides and turbostratic/graphitic carbon. Graphite-like carbon was doped with nitrogen (according to XPS analysis N concentration is 2 at.%) and occurred as multi-walled carbon nanotubes with diameters ranging from 10 up to 55 nm. The composites were a mixture of compounds showing strongly temperature-dependent crystallinity therefore they showed various specific surface areas (125, 114 and 53 m² g^− 1) and different specific capacitances (9, 7 and 3 F g^− 1). The oxygen reduction peak in the CVs recorded in 0.1 M KOH was observed at − 0.26, − 0.28 and − 0.31 V versus Ag/AgCl/KCl_sat electrode for the samples prepared at 600, 700 and 800 °C, respectively.     

Composites of MnO2 nanocrystals and partially graphitized hierarchically porous carbon spheres with improved rate capability for high-performance supercapacitors

Manganese dioxide/carbon nanocomposites with partially graphitized hierarchical porous structure have been designed and synthesized. A high specific capacitance of 412 F g⁻¹ and excellent rate capability of these composites can be achieved owing to the interconnected meso- and micro-porous structure and the graphitic pore walls facilitating the ion diffusion and electron transportation, respectively, which is highly demanded for high-performance supercapacitor electrodes materials. Even at a high scan rate of 100 mV s⁻¹, a specific capacitance of 251 F g⁻¹ can be obtained, corresponding to 61% capacitance retention. Moreover, a long cycling stability with initial capacitance retention of ∼88% is obtained after over 4000 cycles at a current density of 1.0 A g⁻¹. This work presents an efficient electrode materials design and a novel composite which holds great promise in high-performance supercapacitor applications.