Hierarchical Carbon Nanostructures Research Articles

Spine-like nanostructured carbon interconnected by graphene for high-performance supercapacitors.

Recent studies on supercapacitors have focused on the development of hierarchical nanostructured carbons by combining two-dimensional graphene and other conductive sp2 carbons, which differ in dimensionality, to improve their electrochemical performance. Herein, we report a strategy for synthesizing a hierarchical graphene-based carbon material, which we shall refer to as spine-like nanostructured carbon, from a one-dimensional graphitic carbon nanofiber by controlling the local graphene/graphitic structure via an expanding process and a co-solvent exfoliation method. Spine-like nanostructured carbon has a unique hierarchical structure of partially exfoliated graphitic blocks interconnected by thin graphene sheets in the same manner as in the case of ligaments. Owing to the exposed graphene layers and interconnected sp2 carbon structure, this hierarchical nanostructured carbon possesses a large, electrochemically accessible surface area with high electrical conductivity and exhibits high electrochemical performance.

A hierarchical nanostructured carbon nanofiber-In2S3 photocatalyst with high photodegradation and disinfection abilities under visible light.

Photocatalytic degradation of pollutants under visible light provides a new door to solve the water contamination problem by utilizing free and renewable sunlight. The search for highly efficient photocatalysts with hierarchical nanostructures remains crucial for accessing this new door. In this work, a new hierarchical nanostructured photocatalyst is designed and synthesized, for the first time, by anchoring In2S3 flower-like nanostructures on non-woven carbon nanofiber (CNF). The nanostructures of these CNF-In2S3 composites were fine-tuned, with the aim of achieving the highest photocatalytic activity under visible light. The formation mechanism of the hierarchical nanostructure is also investigated. The results indicate that the optimized hierarchical CNF-In2S3 photocatalyst is superior in photodegradation and disinfection efficiency to that of pure In2S3 under visible-light irradiation. The prominent photocatalytic activities of these hierarchical CNF-In2S3 photocatalysts can be attributed to the excellent properties of enhanced light absorption, large surface area, and efficient charge separation, which are all derived from the special three-dimensional hierarchical nanostructures. Therefore, this work presents the great potential of this hierarchical nanostructured CNF-In2S3 photocatalyst in practical environmental remediation fields.

Sustainable one-pot aqueous route to hierarchical carbon–MoO2 electrodes for Li-ion batteries

A route towards carbon–MoO2 core–shell spheres has been developed, through hydrothermal decomposition of ascorbic acid combined with precipitation of MoO2 nanoparticles. In this one-pot and green process, carbon spheres originating from ascorbic acid act as seeds for the in situ deposition of a corona made of 30 nm molybdenum dioxide particles. The as-obtained hierarchical nanostructured carbon–MoO2 core–shell spheres exhibit an ideal combination of electrical conductivity and lithium reactivity for Li-ion battery electrodes. This nanocomposite offers the opportunity to master the collector-active material and active material–electrolyte interfaces. Direct transfer “from the beaker to the battery” without any additives nor thermal treatment yields storage capacity values of ca. 600 mA h·g−1 at C/5 rate with excellent stability that challenges state-of-the-art molybdenum oxide-based batteries.

Synthesis of carbon nanotube—nanodiamond hierarchical nanostructures and their polyurea nanocomposites

Hierarchical carbon nanostructures (HCNs) comprising functionalized nanodiamond particles (ND) covalently bonded to carbon nanotubes (CNTs) through urea or ethylene diamine linkers were synthesized using wet chemistry technique. Atomic force microscopy, transmission electron spectroscopy, and scanning electron spectroscopy reveal the pearl-necklace-like morphology of new HCNs with up to 50% of the CNT surface decorated by ND particles. Nanocomposites fabricated using polyuria/polyurethane hybrid polymer matrix and 0.2 wt.% of HCNs as a reinforcing filler show a 64% increase in tensile and elongation strength at break relatively to neat polymer.

High-performance quaternary PtRuIrNi electrocatalysts with hierarchical nanostructured carbon support

The PtRuIrNi quaternary system is explored by a robotic dispenser and combinatorial optical screening method, and some active ternary and quaternary electrocatalysts for methanol oxidation in direct methanol fuel cells are discovered. When combinatorial screening is employed, pH change allows one to differentiate active catalysts using fluorescent acid–base indicators. A quaternary catalyst with Pt34Ru30Ir13Ni23 composition is found to be the most active, demonstrating superior electrochemical catalytic activity and stability for methanol oxidation compared to commercial Pt50–Ru50 binary catalyst. The effect of carbon support on methanol oxidation is also investigated using hollow mesoporous carbon (HMC), mesoporous CMK-3, and commercial Vulcan XC-72 as supporting materials for the quaternary composition. The newly developed PtRuIrNi/VC, PtRuIrNi/CMK-3, and PtRuIrNi/HMC catalysts exhibit an enhancement in activity of ca. 26–50% and demonstrate better electrochemical stability toward methanol oxidation than the Vulcan carbon-supported Pt50–Ru50 binary alloy catalyst. About half of the total improvement in catalytic activity is attributed to a new active quaternary catalyst composition, whose activity can be explained by a bifunctional mechanism, electronic effect, and stability effect occurring from addition of Ni and Ir. An additional boost amounting to the other half of the total enhancement is obtained from a unique hierarchical nanostructured carbon support with high surface area and pore volume, which favors not only homogeneous dispersion of small catalyst nanoparticles, but also fast mass transport in the catalyst layer.

Chrysanthemum like carbon nanofiber foam architectures for supercapacitors

Abstract

Hierarchical Nanostructured Carbons with Meso–Macroporosity: Design, Characterization, and Applications

Nanostructured porous carbon materials have diverse applications including sorbents, catalyst supports for fuel cells, electrode materials for capacitors, and hydrogen storage systems. When these materials have hierarchical porosity, interconnected pores of different dimensions, their potential application is increased. Hierarchical nanostructured carbons (HNCs) that contain 3D-interconnected macroporous/mesoporous and mesoporous/microporous structures have enhanced properties compared with single-sized porous carbon materials, because they have improved mass transport through the macropores/mesopores and enhanced selectivity and increased specific surface area on the level of fine pore systems through mesopores/micropores. The HNCs with macro/mesoporosity are of particular interest because chemists can tailor specific applications through controllable synthesis of HNCs with designed nanostructures. An efficient and commonly used technique for creating HNCs is "nanocasting", a technique that first involves the creation of a sacrificial silica template with hierarchical porous nanostructure and then the impregnation of the silica template with an appropriate carbon source. This is followed by carbonization of the filled carbon precursor, and subsequent removal of the silica template. The resulting HNC is an inverse replica of its parent hierarchical nanostructured silica (HNS). Through such nanocasting, scientists can create different HNC frameworks with tailored pore structures and narrow pore size distribution. Generally, HNSs with specific structure and 3D-interconnected porosity are needed to fabricate HNCs using the nanocasting strategy. However, how can we fabricate a HNS framework with tailored structure and hierarchical porosity of meso-macropores? This Account reports on our recent work in the development of novel HNCs and their interesting applications. We have explored a series of strategies to address the challenges in synthesis of HNSs and HNCs. Through careful control of experimental parameters, we found we could readily create new HNSs and HNCs with tailored structure and hierarchical porosity. In this Account, we describe the applications of the HNCs in low-temperature fuel cells, in Li ion batteries, in quantum-dot-sensitized solar cells (QDSSCs) and as hydrogen storage materials. Fuel cell and QDSSC polarization performance data reveal that both the ordered HNC and spherical HNC with uniform macro- and mesoporosity demonstrate superior catalyst support effect and considerably enhanced photovoltaic performance due to their incredible structural characteristics. For hydrogen and lithium storage applications, primary experimental results show that spherical HNCs with uniform macroporous core/mesoporous shell and ordered HNC are highly beneficial in terms of a high hydrogen (or Li) uptake, good rate capability and excellent cycling retainability. These data suggest that the innovative HNCs with tailored nanostructure may find promising applications in the rapid and efficient storage of hydrogen (or Li).

Nanocasting of hierarchical nanostructured porous carbon in molecular dynamics simulation

We developed a model template method mimicking the nanocasting process by introducing cylindrical templates during the quenched molecular dynamics process [Y. Shi, J. Chem. Phys., 2008, 128, 234707] to generate hierarchical nanostructured porous carbons (HNPCs). Bimodal pore size distributions with both mesopores (from templates) and micropores (from the direct quenching of the carbon source) were well developed during the mimetic nanocasting process. The structure and properties of the nanocast HNPCs can be tailored by various control parameters, such as the template–carbon interaction strength (et), the template radius (rt), the number density of the carbon source (ρ0) and the quench rate (qr). The mesopore wall is made of graphitic carbons, whose morphology can be well-controlled by et and high temperature heat treatments, while the microporous structure can be engineered by ρ0 and qr. The crystallinity of the mesopore wall and the structural integrity of HNPCs (characterized by the number of bridging bonds between the mesopore wall and the microporous region) can be balanced by fine-tuning the control parameters. The mimetic nanocasting method allows us to generate atomic models to quantify the relationship between the structure and properties of HNPCs, and to provide guidelines for identifying the optimal synthesis parameters for HNPCs with desired properties.

A high-energy-density supercapacitor with graphene–CMK-5 as the electrode and ionic liquid as the electrolyte

A graphene-based supercapacitor electrode suitable for ionic liquid electrolytes was designed and prepared based on the electrostatic interactions between negatively charged graphene oxides (GO) and positively charged mesoporous carbon CMK-5 platelets. Thermal annealing of the GO–CMK-5 composite under an inert atmosphere yielded a hierarchical carbon nanostructure with CMK-5 platelets uniformly intercalated between the GO sheets. The electrochemical results demonstrated that the CMK-5 platelets with straight and short mesochannels served as a highway for the fast transport of electrolyte ions, while the separated graphene sheets with more exposed electrochemical surface area favored the formation of electrical double layer capacitance. The RGO–CMK-5 electrode exhibited a specific capacitance of 144.4 F g−1 in 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid electrolyte, which can be charged/discharged at an operating voltage of 3.5 V. As a result, an energy density of 60.7 W h kg−1 and a power density as high as 10 kW kg−1 were achieved, which outperforms most of the present graphene-based supercapacitors. Moreover, the superior rate performance together with the excellent cycle performance makes the RGO–CMK-5 composite a promising candidate for next generation supercapacitor electrodes.

Hierarchically aminated graphene honeycombs for electrochemical capacitive energy storage

Graphene with mediated surface properties and three-dimensional hierarchical architectures show unexpected performance in energy conversion and storage. To achieve advanced graphene electrode supercapacitors, manipulating the graphene building-blocks into hierarchical nanostructured carbon materials with large electrical double layer capacitance and pseudo-capacitance is a key issue. Here, it is shown that the hierarchically aminated graphitic honeycombs (AGHs) with large surface area for electrical double layer capacitance, tunable surface chemistry for pseudo-capacitance, mediated 3D macropores for ion buffering, and low-resistant pathways for ion diffusion are fabricated for electrochemical capacitive energy storage application through a facile high vacuum promoted thermal expansion and subsequent amination process. In the initial stage of amination (∼200 °C), NH3 reacts with carboxylic acid species to form mainly intermediate amides and amines through nucleophilic substitution. As the temperature increases, the intramolecular dehydration and decarbonylation will take place to generate thermally more stable heterocyclic aromatic moieties such as pyridine, pyrrole, and quaternary type N sites. The AGH exhibits a promising prospect in supercapacitor electrodes with high capacitance (e.g. maximum gravimetric capacitance 207 F g−1 and specific capacitance 0.84 F m−2 at a scan rate of 3 mV s−1) and extraordinary stability (e.g. 97.8% of capacitance retention after 3000 cycles, and 47.8% of capacitance maintaining at a high scan rate of 500 mV s−1 comparing with that at 3 mV s−1). This provides a novel structure platform for catalysis, separation, and drug delivery, which require fast mass transfer through mesopores, reactant reservoirs, and tunable surface chemistry.

Different Hierarchical Nanostructured Carbons as Counter Electrodes for CdS Quantum Dot Solar Cells

CdS quantum dot sensitized solar cells based on TiO(2) photoanode and nanostructured carbon as well as Pt as counter electrodes using iodide/triiodide and polysulfide electrolytes were fabricated to improve the efficiency and reduce the cost of solar cells. Compared with conventional Pt (η = 1.05%) and CMK-3 (η = 0.67%) counter electrodes, hollow core-mesoporous shell carbon (HCMSC) counter electrode using polysulfide electrolyte exhibits much larger incident photon to current conversion efficiency (IPCE = 27%), photocurrent density (J(sc) = 4.31 mA.cm(-2)) and power conversion efficiency (η = 1.08%), which is basically due to superb structural characters of HCMSC such as large specific surface area, high mesoporous volume, and 3D interconnected well-developed hierarchical porosity network, which facilitate fast mass transfer with less resistance and enable HCMSC to have highly enhanced catalytic activity toward the reduction of electrolyte shuttle.

Hierarchical carbon nanostructure design: ultra-long carbon nanofibers decorated withcarbon nanotubes

Hierarchical carbon nanostructures based on ultra-long carbon nanofibers (CNF)decorated with carbon nanotubes (CNT) have been prepared using plasma processes.The nickel/carbon composite nanofibers, used as a support for the growth ofCNT, were deposited on nanopatterned silicon substrate by a hybrid plasmaprocess, combining magnetron sputtering and plasma-enhanced chemical vapordeposition (PECVD). Transmission electron microscopy revealed the presence ofspherical nanoparticles randomly dispersed within the carbon nanofibers. The nickelnanoparticles have been used as a catalyst to initiate the growth of CNT by PECVD at600 °C. After the growth of CNT onto the ultra-long CNF, SEM imaging revealed the formationof hierarchical carbon nanostructures which consist of CNF sheathed with CNTs.Furthermore, we demonstrate that reducing the growth temperature of CNT to less than500 °C leads to the formation of carbon nanowalls on the CNF instead of CNT. This simplefabrication method allows an easy preparation of hierarchical carbon nanostructures over alarge surface area, as well as a simple manipulation of such material in order to integrate itinto nanodevices.

Ordered Hierarchical Nanostructured Carbon as an Efficient Anode Material in Li-Ion Battery

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Incorporating Hierarchical Nanostructured Carbon Counter Electrode into Metal-Free Organic Dye-Sensitized Solar Cell

Hierarchical nanostructured carbon with a hollow macroporous core of ca. 60 nm in diameter in combination with mesoporous shell of ca. 30 nm in thickness has been explored as counter electrode in metal-free organic dye-sensitized solar cell. Compared with other porous carbon counterparts such as activated carbon and ordered mesoporous carbon CMK-3 and Pt counter electrode, the superior structural characteristics including large specific surface area and mesoporous volume and particularly the unique hierarchical core/shell nanostructure along with 3D large interconnected interstitial volume guarantee fast mass transport in hollow macroporous core/mesoporous shell carbon (HCMSC), and enable HCMSC to have highly enhanced catalytic activity toward the reduction of I(3)(-), and accordingly considerably improved photovoltaic performance. HCMSC exhibits a V(oc) of 0.74 V, which is 20 mV higher than that (i.e., 0.72 V) of Pt. In addition, it also demonstrates a fill factor of 0.67 and an energy conversion efficiency of 7.56%, which are markedly higher than those of its carbon counterparts and comparable to that of Pt (i.e., fill factor of 0.70 and conversion efficiency of 7.79%). Furthermore, HCMSC possesses excellent chemical stability in the liquid electrolyte containing I(-)/I(3)(-) redox couples, namely, after 60 days of aging, ca. 87% of its initial efficiency is still achieved by the solar cell based on HCMSC counter electrode.

Hierarchical nanostructured spherical carbon with hollow core/mesoporous shell as a highly efficient counter electrode in CdSe quantum-dot-sensitized solar cells

Hierarchical nanostructured spherical carbon with hollow core/mesoporous shell (HCMS) was explored as a counter electrode in CdSe quantum-dot-sensitized solar cells. Compared with conventional Pt electrodes and commercially available activated carbon, the HCMS carbon counter electrode exhibits a much larger fill factor due to the considerably decreased charge transfer resistance at the interface of the counter electrode/polysulfide electrolyte. Furthermore, a solar cell with the HCMS carbon counter electrode presents a high power conversion efficiency of up to 3.90% as well as an incident photon-to-current conversion efficiency peak of 80%.

Ordered Hierarchical Nanostructured Carbon as a Highly Efficient Cathode Catalyst Support in Proton Exchange Membrane Fuel Cell

Ordered hierarchical nanostructured carbon (OHNC) has been fabricated through inverse replication of silica template and explored for the first time to support high loading of Pt nanoparticles as cathode catalyst in proton exchange membrane fuel cells (PEMFC). The superb characteristics such as ordered hierarchical nanostructure, large surface area, and mesopore volume enable the OHNC to support high loading (60 wt %) of Pt nanoparticles with uniform dispersion and small particle size. Compared to carbon black Vulcan XC-72 (VC)-supported Pt (60 wt %), the OHNC-supported one has exhibited greatly enhanced catalytic activity toward oxygen reduction reaction by ca. 53−88% and considerably improved PEMFC performance. More importantly, unlike the VC-supported Pt catalysts, the OHNC-supported Pt (40 or 60 wt %) outperforms the OHNC-supported Pt (20 wt %) even at a low catalyst loading of 0.2 mg Pt/cm2 in the cathode, which represents a significant breakthrough in fuel cell technology because of the expectable marked reduction in fabricating cost of the supported Pt catalyst and the corresponding system weight and considerably improved fuel cell performance.

Hierarchical nanostructured hollow spherical carbon with mesoporous shell as a unique cathode catalyst support in proton exchange membrane fuel cell

Hierarchical nanostructured spherical carbon with hollow macroporous core in combination with mesoporous shell has been explored to support Pt cathode catalyst with high metal loading in proton exchange membrane fuel cell (PEMFC). The hollow core-mesoporous shell carbon (HCMSC) has unique structural characteristics such as large specific surface area and mesoporous volume, ensuring uniform dispersion of the supported high loading (60 wt%) Pt nanoparticles with small particle size, and well-developed three-dimensionally interconnected hierarchical porosity network, facilitating fast mass transport. The HCMSC-supported Pt(60 wt%) cathode catalyst has demonstrated markedly enhanced catalytic activity toward oxygen reduction and greatly improved PEMFC polarization performance compared with carbon black Vulcan XC-72 (VC)-supported ones. Furthermore, the HCMSC-supported Pt(40 wt%) or Pt(60 wt%) outperforms the HCMSC-supported Pt(20 wt%) even at a low catalyst loading of 0.2 mg Pt cm(-2) in the cathode, which is completely different from the VC-supported Pt catalysts. The capability of supporting high loading Pt is supposed to accelerate the commercialization of PEMFC due to the anticipated significant reduction in the amount of catalyst support required, diffusion layer thickness and fabricating cost of the supported Pt catalyst electrode.

Growth of carbon nanostructures on carbonized electrospun nanofibers with palladiumnanoparticles

This paper studies the mechanism of the formation of carbon nanostructureson carbon nanofibers with Pd nanoparticles by using different carbonsources. The carbon nanofibers with Pd nanoparticles were produced bycarbonizing electrospun polyacrylonitrile (PAN) nanofibers includingPd(Ac)2. Such PAN-based carbon nanofibers were then used as substrates to grow hierarchicalcarbon nanostructures. Toluene, pyridine and chlorobenzine were employed as carbonsources for the carbon nanostructures. With the Pd nanoparticles embedded in thecarbonized PAN nanofibers acting as catalysts, molecules of toluene, pyridine orchlorobenzine were decomposed into carbon species which were dissolved into the Pdnanoparticles and consequently grew into straight carbon nanotubes, Y-shaped carbonnanotubes or carbon nano-ribbons on the carbon nanofiber substrates. X-ray diffractionanalysis and transmission electron microscopy (TEM) were utilized to capture themechanism of formation of Pd nanoparticles, regular carbon nanotubes, Y-shapedcarbon nanotubes and carbon nano-ribbons. It was observed that the Y-shapedcarbon nanotubes and carbon nano-ribbons were formed on carbonized PANnanofibers containing Pd-nanoparticle catalyst, and the carbon sources playeda crucial role in the formation of different hierarchical carbon nanostructures.

New Hierarchical Nanostructured Carbons as Efficient Catalyst Supports in PEM Fuel Cells

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Novel Hierarchical Nanostructured Carbons as High Efficient Catalyst Supports in Low Temperature Fuel Cells

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