Selective growth of carbon nanotube at low temperate using triode type plasma enhanced CVD method

The low temperature growth and the selective growth of the carbon nanotube (CNT) were studied using the triode-type radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) equipment. The aligned CNTs −2.6 μm in length and −10¹¹/cm² in density were vertically grown at 550 °C on the Si substrate. Moreover, the selective growth of the CNT was performed using the photolithography method.     

Pressure evolution in propylene carbonate based electrochemical double layer capacitors

Increasing the available cell voltage for electrochemical double layer capacitors (EDLC) is one route to simultaneously increase energy density and power density of the EDLC. Increased cell voltage may, however, introduce faradaic reactions such as ion insertion and electrolyte decomposition, which potentially limit the lifetime of the device. Using a purpose designed pressure cell, we have, for the first time, measured the pressure increase in capacitor cells based on real EDLC electrode coils in 1 M (C₂H₅)₄NBF₄/propylene carbonate electrolyte during cycling between 0 and 2.5 V and for constant cell voltages up to 3 V. During cycling a reversible pressure decrease was observed upon charging. An irreversible pressure increase was monitored during load tests at constant cell voltages above 2.5 V. The absolute amount of gases evolved could be determined by means of simultaneous compressibility monitoring.     

Electrochemical characterization of single-walled carbon nanotubes for electrochemical double layer capacitors using non-aqueous electrolyte

Single-walled carbon nanotubes (SWCNTs) were investigated by cyclic voltammetry and electrochemical impedance spectroscopy in a non-aqueous electrolyte, 1 M Et₄NBF₄ in acetonitrile, suitable for supercapacitors. Further, in situ dilatometry and in situ conductance measurements were performed on single electrodes and the results compared to an activated carbon, YP17. Both materials show capacitive behavior characteristic of high surface area electrodes for supercapacitors, with the maximum full cell gravimetric capacitance being 34 F/g for YP17 and 20 F/g for SWCNTs at 2.5 V with respect to the total active electrode mass. The electronic resistance of SWCNTs and activated carbon decreases significantly during charging, showing similarities of the two materials during electrochemical doping. The SWCNT electrode expands irreversibly during the first electrochemical potential sweep as verified by in situ dilatometry, indicative of at least partial debundling of the SWCNTs. A reversible periodic swelling and shrinking during cycling is observed for both materials, with the magnitude of expansion depending on the type of ions forming the double layer.     

Aging of electrochemical double layer capacitors with acetonitrile-based electrolyte at elevated voltages

Laboratory-scale electrochemical capacitor cells with bound activated carbon electrodes and acetonitrile-based electrolyte were aged at various elevated constant cell voltages between 2.75 V and 4.0 V. During the constant voltage tests, the cell capacitance as well as the capacitance and resistance of each electrode was determined. Following each aging experiment, the cells were analyzed by means of electrochemical impedance spectroscopy, and the individual electrodes were characterized by gas adsorption and X-ray photoelectron spectroscopy. At cell voltages above 3.0 V, the positive electrode ages much faster than the negative. Both the capacitance loss and resistance increase of the cell could be totally attributed to the positive electrode. At cell voltages above 3.5 V also the negative electrode aged significantly. X-ray photoelectron spectroscopy indicated the presence of degradation products on the electrode surface with a much thicker layer on the positive electrode. Simultaneously, a significant decrease in electrode porosity could be detected by gas adsorption.     

In situ grown carbon nanotubes on carbon paper as integrated gas diffusion and catalyst layer for proton exchange membrane fuel cells

In situ grown carbon nanotubes (CNTs) on carbon paper as an integrated gas diffusion layer (GDL) and catalyst layer (CL) were developed for proton exchange membrane fuel cell (PEMFC) applications. The effect of their structure and morphology on cell performance was investigated under real PEMFC conditions. The in situ grown CNT layers on carbon paper showed a tunable structure under different growth processes. Scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET) demonstrated that the CNT layers are able to provide extremely high surface area and porosity to serve as both the GDL and the CL simultaneously. This in situ grown CNT support layer can provide enhanced Pt utilization compared with the carbon black and free-standing CNT support layers. An optimum maximum power density of 670 mW cm⁻² was obtained from the CNT layer grown under 20 cm³ min⁻¹ C₂H₄ flow with 0.04 mg cm⁻² Pt sputter-deposited at the cathode. Furthermore, electrochemical impedance spectroscopy (EIS) results confirmed that the in situ grown CNT layer can provide both enhanced charge transfer and mass transport properties for the Pt/CNT-based electrode as an integrated GDL and CL, in comparison with previously reported Pt/CNT-based electrodes with a VXC72R-based GDL and a Pt/CNT-based CL. Therefore, this in situ grown CNT layer shows a great potential for the improvement of electrode structure and configuration for PEMFC applications.     

Catalytic CVD synthesis of double and triple-walled carbon nanotubes by the control of the catalyst preparation

We report the influence of catalyst preparation conditions for the synthesis of carbon nanotubes (CNTs) by catalytic chemical vapour deposition (CCVD). Catalysts were prepared by the combustion route using either urea or citric acid as the fuel. We found that the milder combustion conditions obtained in the case of citric acid can either limit the formation of carbon nanofibres (defined as carbon structures not composed of perfectly co-axial walls or only partially tubular) or increase the selectivity of the CCVD synthesis towards CNTs with fewer walls, depending on the catalyst composition. It is thus for example possible in the same CCVD conditions to prepare (with a catalyst of identical chemical composition) either a sample containing more than 90% double- and triple-walled CNTs, or a sample containing almost 80% double-walled CNTs.     

Aligned carbon nanotubes fabricated by thermal CVD at atmospheric pressure using Co as catalyst with NH3 as reactive gas

Co is used as a catalyst for chemical vapor deposition (CVD) of vertically aligned multi-walled carbon nanotubes (CNTs) in a tube furnace at atmospheric pressure. C₂H₂ and NH₃ were used for the carbon feedstock and reaction control, respectively. The CVD process parameters determine the chemical properties of the Co particles and subsequently the morphologies and field emission behavior of CNTs as they strongly depend upon the catalyst condition. The flow rate ratio of NH₃ to C₂H₂ is shown to be central to the synthesis of vertically aligned CNTs. Repeatable synthesis of vertically aligned CNTs at atmospheric pressure in a tube furnace is cost effective for large area deposition of such structures which may be used, for example, in vacuum field emission devices.     

Catalyst deactivation in CVD synthesis of carbon nanotubes

A computational fluid dynamics (CFD) model with multistep chemical reactions was applied to predict the yield of multiwalled carbon nanotubes produced from our xylene-based chemical vapor deposition (CVD) reactor. Two-step xylene decomposition in the gas phase and catalytic decomposition of hydrocarbons to nanotubes on the growth surfaces were adopted based on exhaust-gas composition measurements. Using the experimentally obtained exhaust-gas concentrations, we conducted inverse calculations to determine apparent rate constants of the catalytic surface reactions. During the CVD process, catalyst deactivation was observed probably due to carbon formation on the catalyst surface. Its effect on the apparent rate constant was empirically correlated with a simple exponential equation. Applying the CFD model, we predicted the local yielding rate of nanotubes along the axis of the reactor. The total yield was then computed by integrating the local yielding rate over the growth surfaces and compared favorably (∼95%) with the experimental results. The proposed model is expected to help researchers optimize the process parameters to achieve the maximum nanotube yield.     

Discharge capacitance of electric double layer capacitor with electrodes made of carbon nanotubes directly deposited on SUS304 plates

Carbon nanotubes were deposited directly on SUS304 plates by PECVD with acetylene and hydrogen as precursors under various deposition conditions. Raman spectroscopy showed that carbon nanotubes were not fully graphitized at the deposition temperatures, 600 to 750 ‡C, although defects decreased with increase of deposition temperature. SEM microscopy showed that carbon nanotubes were not straight, but their growth followed the tip growth model. Pretreatment of the substrate such as polishing and dipping in HF solution was required for the successful deposition. Using non-aqueous electrolyte we fabricated electrical double layer capacitance (EDLC) with SUS304 plates, on which carbon nanotubes were deposited, without any treatment, and measured charge/discharge characteristics. Discharge capacitance decreased with cycles from initial value of 128 F/g, but stabilized at 58 F/g after 50 cycles.     

Controllable growth of double wall carbon nanotubes in a floating catalytic system

Double-wall carbon nanotubes (DWCNTs) have been better prepared by thermally decomposing ferrocene mixed with sulphur in the mixture flows of argon and acetylene at 950-1150 °C. The product of DWCNTs has been systematically studied as a function of the reaction temperature, the partial pressure of hydrocarbon, the sublimed temperature of ferrocene, and the relative molar value of ferrocene to sulphur. Our efforts to optimize the catalyst composition by replacing the partial ferrocene with cobaltocene have been proved to be inappropriate. In this process, the role of sulphur on the preparation condition has also been discussed.     

A comparison of the aging of electrochemical double layer capacitors with acetonitrile and propylene carbonate-based electrolytes at elevated voltages

The aging behavior of electrochemical double layer capacitors (EDLCs) based on activated carbon electrodes bound with poly(tetrafluoroethylene) (PTFE) was tested in electrolyte solutions based on acetonitrile (AN) and propylene carbonate (PC) at a constant elevated cell voltage of 3.5 V. The aging was quantified in terms of capacitance loss and resistance increase for the full cell and the individual electrodes. It is shown that the enhanced aging rate of symmetric EDLCs in either solvent at elevated voltages is dominated by the aging of a single electrode, and that the polarity of this limiting electrode depends directly on the solvent. In AN, the positive electrode ages much more rapidly than the negative, while in PC the negative electrode exhibits faster aging than the positive. After aging, the electrodes were investigated by nitrogen adsorption and X-ray photoelectron spectroscopy, revealing significant modifications of the electrode surface and providing clear evidence for the deposition of electrolyte degradation products on the electrodes.     

Study on synthesis of low surface area activated carbons using multi-step activation for use in electric double layer capacitor

Low surface area activated carbon derived from compact mesocarbon microbeads (MCMB2010) was synthesized using a lower amount of KOH (1:1 weight ratio of KOH to MCMB) than normally used followed by electrochemical activation. The specific capacitance of the activated carbon heat treated at between 650 and 900 °C was increased up to ca. 118 F/cc (half cell base, 750 °C-heat treated sample) after electrochemical activation, even with a low surface area carbon (<50 m²/g). The morphology of low surface area activated MCMB determined by FE-SEM showed a smooth carbon surface without pores. The charge/discharge profiles were similar to those of conventional activated carbon. The specific capacitance of the activated samples increased with increasing heat treatment up to 850 °C after electrochemical activation. However, it was lower for the sample heat treated at 900 °C.     

Electrochemical characterization of carbon nanotubes as electrode in electrochemical double-layer capacitors

Carbon nanotubes uniformly 50 nm in diameter were directly grown on graphite foil. Cyclic voltammetry (CV) shows that the carbon nanotube/graphite foil electrode has a high specific capacitance (115.7 F/g at a scan rate of 100 mV/s) and exhibits typical double-layer behavior. A rectangular-shaped CV curve persists even at a scan rate of 100 mV/s in 1.0 M H₂SO₄ aqueous solution, which suggests that the carbon nanotube electrode could be an excellent candidate as the electrode in electrochemical double-layer capacitors. In addition, the influence of the potential scan rate, aging, and the electrolyte solution on the specific capacitance of nanotube electrodes was also studied.     

Decrease in equivalent series resistance of electric double-layer capacitor by addition of carbon nanotube into the activated carbon electrode

An electric double layer capacitor (EDLC) was fabricated with the addition of carbon nanotubes (CNTs) to the polarizable electrodes to act as a conducting material. This EDLC showed a low equivalent series resistance of 2.5 Ω. This value was lower than that of an EDLC fabricated with the addition of acetylene black, which is widely used in commercial EDLCs.     

Effect of electrochemical anodization and growth time on continuous growth of carbon nanotubes on carbon fiber surface

Carbon nanotubes (CNTs)/carbon fiber (CF) reinforcements were prepared by chemical vapor deposition after electrochemical anodization and catalyst impregnation. The results showed that after the electrochemical anodization, the CFs were oxidatively etched and the surface roughness increased, which is helpful to form a uniform catalyst coating on the surface of CF. Under the current of 0.4 A and 0.6 A, CNTs can grow evenly on the surface of CF. Within a certain range, with the increase of growth time, the density and length of CNTs are improved. The CNTs/CF reinforcement prepared at the current intensity of 0.4 A and the growth time of 8 min has the best comprehensive performances compared with other as-fabricated samples. The tensile strength of the sample can reach a high value of 4.56 GPa, and the wettability of resin has an effective improvement.     

Improvement of the structural and chemical properties of a commercial activated carbon for its application in electrochemical capacitors

The present paper shows that the performance of an inexpensive activated carbon used in electrochemical capacitors can be significantly enhanced by a simple treatment with KOH at 850 °C. The changes in the specific surface area, as well as in the surface chemistry, lead to high capacitance values, which provide a noticeable energy density.The KOH-treatment of a commercial activated carbon leads to highly pure carbons with effective surface areas in the range of 1300-1500 m² g⁻¹ and gravimetric capacitances as high as three times that of the raw carbon.For re-activated carbons, one obtains at low current density (50 mA g⁻¹) values of 200 F g⁻¹ in aqueous electrolytes (1M H₂SO₄ and 6M KOH) and around 150 F g⁻¹ in 1M (C₂H₅)₄NBF₄ in acetonitrile. Furthermore, the resulting carbons present an enhanced and stable performance for high charge/discharge load in organic and aqueous media.This work confirms the possibilities offered by immersion calorimetry on its own for the prediction of the specific capacitance of carbons in (C₂H₅)₄NBF₄/acetonitrile. On the other hand, it also shows the limitations of this technique to assess, with a good accuracy, the suitability of a carbon to be used as capacitor electrodes operating in aqueous electrolytes (H₂SO₄ and KOH).     

CVD synthesis of coal-gas-derived carbon nanotubes and nanocapsules containing magnetic iron carbide and oxide

Iron carbide-oxide filled carbon nanotubes and nanocapsules (CNCs) are separately synthesized by catalytic chemical vapor deposition of coal gas at 950 °C with ferrocene as catalyst. The products are examined using transmission electron microscopy and XRD techniques, showing that nanosized iron carbide-oxide particles are encapsulated by well ordered carbon layers. Magnetic moment measurement reveals that these carbon encapsulated iron carbide-oxides exhibit large magnetic coercivity at room temperature. It has been found that the filled CNCs are corrosion-proof and stable in hydrochloric acid. The effect and interaction between different gaseous components in the coal-gas during the formation of magnetic iron carbide-oxide filled carbon nanostructures are discussed.