Carbon Microtubes Research Articles

Kapok fibers-derived carbon microtubes as efficient electromagnetic wave absorption materials

The superior electromagnetic wave absorption materials have the characteristics of thin matching thickness, low density, wide absorption bandwidth and strong absorption intensity, which can effectively deal with electromagnetic pollution. With the further research, biomass-derived carbon has received great focus due to abundant biological reserves and unique natural structure. Herein, we put forward an excellent electromagnetic wave absorption material, carbon microtubes (CMTs) with a one-dimensional hollow tubular structure, which derived from kapok fibers (KFs) via a facile annealing method. The result shows that the effective absorption bandwidth (EAB) of CMTs is 6.78 GHz under 900 °C annealing conditions. Especially, the filling ratio is just 3 wt% and the matching thickness is 2.06 mm. The superior electromagnetic wave absorption properties of CMTs can be related to its microstructural characteristics. This one-dimensional tubular structure has a large specific surface area increasing the interaction area between the material and electromagnetic waves, while the hollow tube cavity can make the incident electromagnetic waves for multi-ply scattering and reflection strengthening the dissipation capability of the material. This work offers a convenient way to produce effective electromagnetic wave absorption materials with low cost and eco-friendly environment.

Robust and Highly Efficient Electrochemical Hydrogen Production from Hydrazine-Assisted Water Electrolysis Enabled by the Metal-Support Interaction of Ru/C Composites.

Hydrazine oxidation-assisted water electrolysis provides a promising way for the energy-efficient electrochemical hydrogen (H2) and synchronous decomposition of hydrazine-rich wastewater, but the development of highly active catalysts still remains a great challenge. Here, we demonstrate the robust and highly active Ru nanoparticles supported on the hollow N-doped carbon microtube (denoted as Ru NPs/H-NCMT) composite structure as HER and HzOR bifunctional electrocatalysts. Thanks to such unique hierarchical architectures, the as-synthesized Ru NPs/H-NCMTs exhibit prominent electrocatalytic activity in the alkaline condition, which needs a low overpotential of 29 mV at 10 mA cm-2 for HER and an ultrasmall working potential of -0.06 V (vs RHE) to attain the same current density for HzOR. In addition, assembling a two-electrode hybrid electrolyzer using as-prepared Ru NPs/H-NCMT catalysts shows a small cell voltage of mere 0.108 V at 100 mA cm-2, as well as the remarkable long-term stability. Density functional theory calculations further reveal that the Ru NPs serve as the active sites for both the HER and HzOR in the nanocomposite, which facilitates the adsorption of H atoms and hydrazine dehydrogenation kinetics, thus enhancing the performances of HER and HzOR. This work paves a novel avenue to develop efficient and stable electrocatalysts toward HER and HzOR that promises energy-saving hybrid water electrolysis electrochemical H2 production.

Reactive Template-Engaged Synthesis of NiSx/MoS2 Nanosheets Decorated on Hollow and Porous Carbon Microtubes with Optimal Electronic Modulation toward High-Performance Enzyme-like Performance

As a promising cost-effective nanozyme, MoS2 nanosheets (NSs) have been considered as a good candidate for the enzyme-like catalysis. However, their catalytic activity is still restricted by the insufficient active sites and poor conductivity, and thus, the comprehensive performances are still unsatisfactory. To address these issues, herein, we design and fabricate an intelligent tubular nanostructure of hierarchical hollow nanotubes, which are assembled by NiSx/MoS2 NSs encapsulated into N-doped carbon microtubes (NiSx/MoS2@NCMTs). The N-doped carbon microtubes (NCMTs) serve as a conductive skeleton, integrating with NiSx/MoS2 NSs and ensuring their well-distribution, thereby maximally exposing more active sites. Additionally, the tube-like structure is favorable for increasing the mass transfusion to ensure their excellent catalytic performance. Profiting from their component and structural advantages, the obtained NiSx/MoS2@NCMTs exhibit a surprisingly enhanced enzyme-like activity. Based on these, a facile colorimetric sensing platform to detect H2O2 and GSH has been developed. This proposed approach can be expected to synthesize a series of tubular heterostructured MoS2-based composites, which will be widely applied in catalysis, energy storage, disease diagnosis, etc.

Study on formation characteristics of carbon dioxide hydrate in modified carbon microtube system

The slow formation rate and low nucleation rate of carbon dioxide hydrate were common problems in current research. Many scholars focused on the promotion of gas hydrates by carbon nanotubes, whereas they ignored carbon microtubes, which were similar in structure and properties to carbon nanotubes and had more structural advantages. In this article, the effects of system temperature (1, 2, and 3 °C), pressure (3, 3.5, and 4 MPa), gas–liquid ratio (3:2, 4:1, and 5:1) and different types of carbon microtube concentration (0.01, 0.05, and 0.1 mg/mL) on the growth kinetics of carbon dioxide hydrate were investigated using orthogonal experiment method in a high-pressure reactor. The results revealed that compared to the pure carbon microtube system, amination, carboxylation, and hydroxylation carbon microtube systems improved the probability of carbon dioxide hydrate generation by 11–22%, with the shortest induction time of 0.23 hour. The highest gas storage capacity was 31.71 V/V under the carboxylated carbon microtube system. In the pure carbon, aminocarbon, carboxylated carbon and hydroxylated carbon microtube systems, the primary factors affecting the induction time were temperature, gas–liquid ratio, pressure and concentration. The gas–liquid ratio was the primary factor influencing the gas storage capacity of the carboxylated carbon microtubes; however, temperature was the primary factor affecting the gas storage capacity of the remaining three systems. Finally, we compared the effects of modified hydrophilic groups on the formation of carbon dioxide hydrate and made a conjecture.

Facile Preparation of Magnetic Nitrogen‐Doped Carbon Microtubes with Co Nanoparticles for Reduction of 4‐Nitrophenol

AbstractHeterogeneous catalysts with controllable interface structures are of great importance to demonstrate the relationship between structure and property. Herein, magnetic Nitrogen‐doped carbon microtubes with Co nanoparticles (Fe3O4@C−Co) were prepared, which could be used for the reduction of 4‐nitrophenol(4‐NP). In the process, MoO3@FeOOH core‐shell structure was fabricated, then ZIF‐67 was decorated on the MoO3@FeOOH. After adding 2‐methylimidazole, MoO3microrods were removed. Then the prepared FeOOH@ZIF‐67 microtubes are converted to Fe3O4@C−Co hybrid microtubes through pyrolysis. Due to the unique structures, the obtained Fe3O4@C−Co composites improved the dispersion of Co nanoparticles (NPs), diminished Co leaching, and promoted catalytic activity. The plentiful nitrogen improved electronic conductivity and brought additional active sites. The resulting Fe3O4@C−Co implies outstanding catalytic ability towards the reduction of 4‐NP. The synergistic effect between the hollow microtube and the fine dispersed Co NPs within Fe3O4@C−Co offered an excellent catalytic efficiency. More particularly, this achievement opens a new avenue to the targeted design of well‐defined nanocatalysts for practical applications in environmental remediation.

Green fabrication of hierarchically porous carbon microtubes from biomass waste via self-activation for high-energy-density supercapacitor

Developing high-efficiency electrode materials is always desired for supercapacitor, in order to improve the energy density. Herein, we found that hierarchically porous carbon microtubes (HPCMTs) derived from plane tree fruit fluff via self-activation method demonstrated prominent supercapacitor performance as electrode materials, since they possessed hierarchical pore structure consisting of abundant micro- and mesopores, high specific surface area (SSA) and good wettability with electrolyte. The HPCMT-1100-6 with the largest SSA (2805 m2/g) and maximal total pore volume (1.98 cm3/g) was obtained at 1100 °C with dwelling time of 6 h. The HPCMT-1100-6-based cell in organic electrolyte showed an ultrahigh energy density of 46.3 Wh/kg at high power density of 1106 W/kg in 1.0 M TEABF4/AN, as well as splendid rate capability and impressive long-term cyclic stability. The energy density still can retain 35 Wh/kg even at an ultrahigh power density of 83.8 kW/kg, the capacitance retention maintains 96.4% after 10000 cycles at a current density of 10 A/g. This study provided a novel practical sustainable strategy for converting the abundant and low-cost biomass waste into high-valued porous carbons by the green self-activation method for high-energy-density supercapacitor.

Multiscale hierarchical design of bismuth-carbon anodes for ultrafast-charging sodium-ion full battery

Bismuth (Bi) is an emerging anode material for sodium-ion batteries (SIBs) owing to favorable attributes of high volumetric capacity, fast internal diffusion of sodium ions, and low redox potential. However, the significant volume expansion exhibited by Bi during charge storage reaction trades off its remarkable energy storage properties. In this study, carbon microtubes coated with bismuth-carbon (CMT@Bi-C) cellular structure were synthesized using a facile and scalable process. The CMT@Bi-C delivers ultrafast charging performance with 78% capacity retention at 200 A g−1 (645 C) and a long cycle life of 82% capacity retention after 5,000 cycles at 10 A g−1. Moreover, full-cell SIBs consisting of CMT@Bi-C anode and Na3V2(PO4)3 cathode demonstrated a high reversible capacity of 105.0 mAh g−1 and an outstanding capacity retention of 90.3 % after 700 cycles at 1 A g−1 (10 C). A multiscale hierarchical design that circumvents the huge volume change in high-capacity anodes has been demonstrated in SIBs with a practical applicability.

Self-Assembly of Soot Nanoparticles on the Surface of Resistively Heated Carbon Microtubes in Near-Hexagonal Arrays of Micropyramids.

Almost regular hexagonal arrays of microscopic pyramids consisting of soot nanoparticles are formed on the surface of graphitized hollow filaments, which are resistively heated to ∼1800-2400 °C under an Ar atmosphere containing trace amounts of oxygen (∼300 ppm). At higher temperatures (T > 2300 °C, approximately) the soot particles are represented mainly by multishell carbon nano-onions. The height and width of the pyramids are strongly dependent on the temperature of the resistive heating, diminishing from 5 to 10 μm at T ≈ 1800 °C to ∼1 μm at 2300-2400 °C. Quasi-hexagonal arrays of the micropyramids are organized in the convex "craters" on the surface of the microtubes, which grow with the time of the thermal treatment. The pyramids always point normally to the surface of the craters, except at the boundaries between the craters, where the normal direction is not well-defined. The pyramids are soft and can be easily destroyed by touching them but can be hardened by heating them under an oxygen-free atmosphere. The pyramids are observed only on the exterior surface of the microtubes, not on their inner surface. This suggests that the thermophoretic force generated by a strong temperature gradient near the external surface of the tubes may be the cause of the micropyramid formation. Electrostatic charging of the soot nanoparticles due to thermionic emission may also be relevant to this phenomenon. The micropyramids can function as field emission point sources, as demonstrated with the use of a micronanoprobing station, mounted in a scanning electron microscope.

Radially aligned hierarchical N-doped porous carbon beads derived from oil-sand asphaltene for long-life water filtration and wastewater treatment

The application of waste-derived highly efficient adsorbent for organic pollutants removal from water and wastewater is presented. Highly porous carbon beads with radially aligned macrochannels were prepared from asphaltene. Well-ordered inwardly aligned macrovoids favored solute diffusion and maximized the liquid accommodation capacity. A further N-doping could modulate the sorbent hydrophilicity leading to an outstanding absorption performance for a range of organic solvents and oily chemicals. N-doped carbon beads were effective sorbents of lopinavir (LNV) and ritonavir (RNV) from water and wastewater. The process of sorption was fast, and the highest removal was noted for RNV than LPV. N-doping favored LNV and RNV adsorption due to the increased porous structure of N-doped asphaltene beads. The chemisorption of both LPV and RTV was a rate-limiting step. The presence of co-pollutants in treated wastewater enhanced LPV and RNV removal and an up to 470 % increase was noted. The presence of LPV or RTV in distilled water was not toxic to Aliivibrio fischeri or even can stimulate their growth. However, after the adsorption process, the solution of RTV reduced its toxicity significantly and the final solution was not toxic. The opposite effect was noted for LPV. Given the repeatability, high removal performance, and cost-effectiveness of the asphaltene-based carbon microtubes when compared to other well-known sorbents such as carbon nanotubes, they demonstrated great potential as a low-cost and effective agent for long-life water filtration and wastewater treatment.

Bulk preparation of free‐standing single‐iron‐atom catalysts directly as the air electrodes for high‐performance zinc‐air batteries

AbstractThe keen interest in fuel cells and metal‐air batteries stimulates a great deal of research on the development of a cost‐efficient and high‐performance catalyst as an alternative to traditional Pt to boost the sluggish oxygen reduction reaction (ORR) at the cathode. Herein, we report a facile and scalable strategy for the large‐scale preparation of a free‐standing and flexible porous atomically dispersed Fe–N‐doped carbon microtube (FeSAC/PCMT) sponge. Benefiting from its unique structure that greatly facilitates the catalytic kinetics, mass transport, and electron transfer, our FeSAC/PCMT electrode exhibits excellent performance with an ORR potential of 0.942 V at −3 mA cm−2. When the FeSAC/PCMT sponge was directly used as an oxygen electrode for liquid‐state and flexible solid‐state zinc‐air batteries, high peak power densities of 183.1 and 58.0 mW cm−2 were respectively achieved, better than its powdery counterpart and commercial Pt/C catalyst. Experimental and theoretical investigation results demonstrate that such ultrahigh ORR performance can be attributed to atomically dispersed Fe–N5 species in FeSAC/PCMT. This study presents a cost‐effective and scalable strategy for the fabrication of highly efficient and flexible oxygen electrodes, provides a significant new insight into the catalytic mechanisms, and helps to realize significant advances in energy devices.

BiPO4-coated carbon microtube electrodes: preparation and characterization of their properties and electrocatalytic degradation of methylene blue.

Bismuth phosphate (BiPO4), a very attractive candidate for organics electrodegradation, harbors tremendous potential on removing contaminants from water. Here, four carbon microtube electrodes were prepared from corncob, each coated with BiPO4 by a different method to study the electrodegradation of methylene blue (MB). A thorough insight into the composite features of four electrodes was characterized. Better reversibility and electrocatalytic activity of the fourth electrode (BCC4) prepared by digital signal generator was presented with a current density of 5.71mAcm-2 at a potential of 1.6V vs Ag/AgCl. The electrochemical impedances and actual lifetime of BCC4 were 125Ω and 833h, respectively. The effectiveness of each kind of BiPO4/carbon electrode was preliminarily evaluated by analyzing the actual conversion rate of the MB concentration, which confirms MB electrodegradation by the BiPO4/carbon electrode was mainly dominated by the hydroxyl radical oxidation. The mass transfer rate was increased by carbon microtube; thereby, electrocatalysis of BiPO4/carbon electrode increased as revealed by an increase in the MB degradation rate. The rate constants k obtained for the degradation of MB by BiPO4/carbon electrode at 20℃ was 0.0046mM-1s-1, which was 11 times than that of BiPO4. The diffusion layer was decreased by carbon microtube, resulting in MB electrodegradation rate increased. The BiPO4 coated on the surface of the carbon microtube electrodes strengthened their electrocatalytic performance, which shed new light on effective selection of suitable carbon electrode for degradation of organics. Therefore, BiPO4/carbon electrode could be potentially applied in the electrodegradation of organic pollutants.

Biomass-derived hard carbon microtubes with tunable apertures for high-performance sodium-ion batteries

Biomass-derived hard carbon microtubes with tunable apertures for high-performance sodium-ion batteries

Construction of tubular carbon matrix-supported NiCoP-NiS2 nanowires with heterointerfaces for overall water splitting

The design of inexpensive bifunctional electrocatalysts with excellent catalytic activity is crucial for the cost reduction from water splitting. As a kind of biomass with hollow tubular structure, Metaplexis japonica fluff is expected to be a natural substrate for the growth of nanostructured electrocatalysts. Herein, nanowire-like NiCo precursors are grown on Metaplexis japonica fluff biomass substrates by hydrothermal method, which are then converted into carbon microtube supported NiCoP-NiS2 composites (NiCoP-NiS2/CMT) by low-temperature phosphorization and sulfidation treatment. As expected, the biomass substrate provides a guiding role for the growth of NiCoP-NiS2 nanowires, which is conducive to expose more active areas and promote charge transfer during the reaction. As a result, the obtained NiCoP-NiS2/CMT exhibits good electrocatalytic performance for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in 1 M KOH, which only requires overpotentials of 134 and 308 mV at 10 mA cm−2, respectively. In addition, the constructed symmetric water splitting device exhibits a cell voltage of 1.62 V at 10 mA cm−2, and it also displays good stability during the 24 h test.

Ultrathin Fe2O3 shell-encapsulated NiFe alloy nanoparticles embedded in tubular carbon matrix for enhanced oxygen evolution reaction

The exploitation of efficient and low-cost electrocatalysts for the oxygen evolution reaction (OER) is the key to the industrialization of the hydrogen production technology from water splitting. Herein, Ni1Fe2@Fe2O3 composites with core-shell structure embedded in Metaplexis japonica fluff-derived carbon microtube matrix (Ni1Fe2@Fe2O3@C) are prepared by nitrate soaking and subsequent high-temperature calcination, which are used to catalyze the OER. The existence of carbon matrix greatly prevents the agglomeration of metal particles and creates an efficient electron transfer path for OER. The synergy among the carbon matrix, Ni1Fe2 core and Fe2O3 shell together enhances the OER performance, thus endowing Ni1Fe2@Fe2O3@C with the considerable catalytic activity with the overpotential of 271 mV at 10 mA cm−2 and a Tafel slope of 78 mV dec−1. Ni1Fe2@Fe2O3@C also shows good stability due to the double protection of metal particles by biomass carbon matrix and ultrathin Fe2O3 shell. The water splitting device composed of Pt/C as the cathode and Ni1Fe2@Fe2O3@C as the anode exhibits water splitting activity close to that of Pt/C || RuO2 system in alkaline medium, and the stability retention rate of the system reaches to 93 % after 30 h electrolysis.

Evaluation to possibility of cellulose fibers isolated from kapok randu (Ceiba Pentandra) as a precursor growth of carbon micro structures with the assist of metal catalysts

ObjectiveAs a hydrocarbon material, cellulose could be used as precursor in synthesizing carbon micro-structure (CMTs), and this study aims to investigate the potential use of cellulosic material from kapok randu as a precursor in synthesizing carbon-based structures. MethodsThe isolation of cellulose was carried out via alkaline treatment, followed by mechanical disintegration. Meanwhile, the growth of CMTs was performed via heating treatment for 12 h with various catalysts (i.e., Fe, Ni, and Cu). Chemical characteristics were confirmed by FTIR, XRD spectra, while TEM, SEM and Raman spectra were performed to determine the growth of CMTs. ResultsThe thermal characteristic suggested that the decomposition was initiated at 300 °C. The FTIR results confirmed the presence of functional groups in accordance to cellulose fiber, such as –OH (3418 cm−1), CH aliphatic group (2900 cm−1), OH (1635 cm−1), and CH (1334 cm−1). Whilst, the FTIR pattern also confirmed the presence of CC stretching at 1600 cm−1, –CH in between 2800 and 3000 cm−1, indicating the activated carbon. Raman shift indicated the growth on G-band in the interval of 1500–1700 cm−1, suggesting the presence of CC structures. Based on the morphological characteristics, the growth of CMTs were successfully obtained with different diameters and lengths due to the different catalysts used. The iron (Fe) catalysts produced CMTs with a diameter about 200 nm and average length of 2–3 µm, whereas the Ni catalysts formed tubes with around 150 nm of length and 50 nm of diameter. Meanwhile, the Cu catalysts formed amorphous particles with diameter below 10 nm. ConclusionFrom these results, the evaluation of cellulose isolated from kapok randu as a precursor in the growth of carbon micro-tube with distinguished characteristics was demonstrated.

Natural Hollow Fiber-Derived Carbon Microtube with Broadband Microwave Attenuation Capacity.

Constructing hierarchical structures is indispensable to tuning the electromagnetic properties of carbon-based materials. Here, carbon microtubes with nanometer wall thickness and micrometer diameter were fabricated by a feasible approach with economical and sustainable kapok fiber. The carbonized kapok fiber (CKF) exhibits microscale pores from the inherent porous templates as well as pyrolysis-induced nanopores inside the wall, affording the hierarchical carbon microtube with excellent microwave absorbing performance over broad frequency. Particularly, CKF-650 exhibits an optimized reflection loss (RL) of -62.46 dB (10.32 GHz, 2.2 mm), while CKF-600 demonstrates an effective absorption bandwidth (RL < -10 dB) of 6.80 GHz (11.20-18.00 GHz, 2.8 mm). Moreover, more than 90% of the incident electromagnetic wave ranging from 2.88 GHz to 18.00 GHz can be dissipated by simply controlling the carbonization temperature of KF and/or the thickness of the carbon-microtube-based absorber. These encouraging findings provide a facile alternative route to fabricate microwave absorbers with broadband attenuation capacity by utilizing sustainable biomass.

Lithium storage behaviors and properties of the pristine and activated catkin-derived carbon microtubes

Lithium storage behaviors and properties of the pristine and activated catkin-derived carbon microtubes

Iron and nitrogen anchored hierarchical hollow porous carbon microtubes for an electrocatalytic oxygen evolution reaction

The development of active nonprecious-metal and heteroatom-doped materials is critical for the oxygen evolution reaction (OER). Herein, asphaltene-derived carbon-coated Fe- and N-doped hierarchical hollow porous carbon microtubes were prepared through feasible freeze-drying and carbonization/activation strategies. Cotton with a fibrous morphology acted as a soft template and carbon precursor, while asphaltene with easy graphitization properties was utilized to stabilize the structure of microtubes and accelerate the charge conductivity. Meanwhile, Fe- and N-doping changed the electronic structure of carbon and increased the number of active sites. In particular, the optimum sample displayed excellent OER activity with a lower overpotential of 261.4 mV at 10 mA/cm2 and Tafel slope of 94.60 mV/dec under alkaline conditions. Moreover, the catalyst demonstrated excellent long-term durability with a 10% attenuation rate after continuous operation for 50 h by the chronoamperometry method at a constant potential of 1.5 V. This work provides a strategy for the effective resource utilization of biomass and asphaltene to prepare highly efficient OER electrocatalysts.

Sustainable Kapok Fiber-Derived Carbon Microtube as Broadband Microwave Absorbing Material.

The design of hierarchical structures from biomass has become one of the hottest subjects in the field of microwave absorption due to its low cost, vast availability and sustainability. A kapok-fiber-derived carbon microtube was prepared by facile carbonization, and the relation between the structure and properties of the carbonized kapok fiber (CKF) was systematically investigated. The hollow tubular structures afford the resulting CKF composites with excellent microwave-absorbing performance. The sample with a 30 wt.% loading of CKF in paraffin demonstrates the strongest microwave attenuation capacity, with a minimum reflection loss of −49.46 dB at 16.48 GHz and 2.3 mm, and an optimized effective absorption bandwidth of 7.12 GHz (10.64–17.76 GHz, 2.3 mm) that covers 34% of the X-band and 96% of the Ku-band. Further, more than 90% of the incident electromagnetic wave in the frequency from 4.48 GHz to 18.00 GHz can be attenuated via tuning the thickness of the CKF-based absorber. This study outlines a foundation for the development of lightweight and sustainable microwave absorbers with a high absorption capacity and broad effective absorption bandwidth.

(Ni,Co)Se2 nanoparticles on vertical graphene nanosheets@carbon microtubes for high-performance solid-state asymmetric supercapacitors

Much attention has been draw to transitional metal selenides (TMSe) for energy storage, especially the high-performance supercapacitor devices, so it is research hotspots to construct free-standing electrode with TMSe loading. In this work, 3D carbon microtubes (CMTs) wrapped by vertical graphene nanosheets (VGNs), were fabricated by plasma-enhanced chemical vapor deposition (PECVD) method using SiO2 nanowires as templates, which were served as free-standing, conductive and flexible substrates for supporting (Ni,Co)Se2 nanoparticles. Benefiting from the structural merits, such as the interwoven CMTs network, high conductive VGNs substrate, and the synergistical effect of nanostructured (Ni,Co)Se2, the fabricated CMTs@VGNs@(Ni,Co)Se2 membrane electrode exhibits excellent electrochemical performance in terms of high specific capacitance (1740 F g−1 at 1 A g−1), good rate capability (71.8 % capacitance retention at 20 A g−1), and superior cycling stability (89.6 % capacitance retention after 10,000 cycles). Notably, the solid-state asymmetric supercapacitor (ASC) assembled with CMTs@VGNs@(Ni,Co)Se2 membrane and activated carbon (AC), not only delivers a high energy density of 41.6 Wh kg−1 at a power density of 0.75 kW kg−1, but also shows long cycle life with excellent performance stability. More impressively, our work provides a new idea for structural construction of 3D carbon-based nano-substrates and self-supporting electrodes for supercapacitors.