High rate lithium intercalation properties of V2O5/carbon/ceramic-filler composites

Composite electrodes of amorphous vanadium pentoxide/carbon/ceramic filler were prepared by mixing vanadium oxide hydrosol, acetone, carbon and ceramic fillers, and by extension on aluminum foil. High rate charge/discharge property of the composite electrode was examined, and the effect of fillers was discussed. The composite electrode had a porous structure, in which pores were 0.5–3 μm in diameter and penetrated through the composite. The composite electrode showed a large capacity of 98 mA h/g-electrode at a high current density of 17.2 mA/cm² (270 A/g-electrode). The relation between discharge capacity and current density was calculated by solving the simplified diffusion equation. The apparent diffusion coefficient of lithium ion in the composite electrode was found to be 10 times larger than that of electrode without fillers.     

Carbon nanotubes modified with catalyst—Promising material for fuel cells

Carbon nanotubes (CNTs), modified with manganese dioxide, were synthesized. Manganese dioxide was obtained by reduction of solution of potassium permanganate with formic acid solution or with carbon of nanotubes. Electrochemical characteristics of the oxygen electrodes containing manganese dioxide modified nanotubes in active layer were investigated. It was shown, that deposition of manganese dioxide on surface of carbon nanotubes results in improvement of electrochemical characteristics in comparison with manganese dioxide as a separate phase. The optimum mass content of manganese dioxide is 50% from the sum (carbon nanotubes + MnO₂).     

Influence of carbon nanotubes on nucleate pool boiling heat transfer characteristics of refrigerant–oil mixture

Influence of carbon nanotubes (CNTs) on nucleate pool boiling heat transfer characteristics of refrigerant–oil mixture was investigated experimentally. Four types of CNTs with the outside diameters from 15 nm to 80 nm and the lengths from 1.5 μm to 10 μm were used in the experiments. Test conditions include CNTs mass fractions in the CNTs nanolubricant from 0 to 30 wt% and CNTs nanolubricant mass fractions from 0 to 5 wt%. The experimental results indicate that the presence of CNTs enhances the nucleate pool boiling heat transfer coefficient of R113-oil mixture by a maximum of 61% under the present test conditions, and the enhancement increases with the decrease of CNTs outside diameter or the increase of CNTs length. For fixed CNTs physical dimension, the enhancement increases with the increase of CNTs mass fraction in the CNTs nanolubricant or the decrease of CNTs nanolubricant mass fraction. A correlation for predicting the nucleate pool boiling heat transfer coefficient of refrigerant–oil mixture with CNTs is proposed, and it agrees with 96% of the experimental data within a deviation of ±10%.     

Resistance distribution in electrochemical capacitors with a bipolar structure

The distribution of internal resistance in a 5-cell electrochemical capacitor comprising an electrode material with 80 wt.% RuO₂·xH₂O and 20 wt.% activated carbon and an electrolyte of 38 wt.% H₂SO₄ acid solution was analyzed. It was found that over 94% of the internal resistance in the capacitor was contributed to various contacts with the current collector of the conducting plastic sheet. Careful analysis of three different contact sources including the current collector to the end metal plate, the current collector to the neighboring current collector, and the current collector to the electrode, it was found that the resistance was dominated by the contact resistance between the current collector and the electrode. A composite current collector, made with conducting plastic sheet coated with metal films on both surfaces, was proposed for reducing all kinds of contact resistances. Using the composite current collector, the internal resistance can be reduced to about only 26% of that using conventional conducting plastic sheet.     

Novel architecture of composite electrode for optimization of lithium battery performance

We show that the polymeric binder of the composite electrode may have an important role on the lithium trivanadate Li_1.2V₃O₈ electrode performance. We describe a new tailored polymeric binder combination with controlled polymer–filler (carbon black) interactions that allows the preparation of new and more efficient electrode architecture. Using this polymeric binder, composite electrodes based on Li_1.2V₃O₈ display a room temperature cycling capacity of 280 mAh g⁻¹ (C/5 rate, 3.3–2 V) instead of 150 mAh g⁻¹ using a standard-type (poly(vinylidene fluoride)–hexafluoropropylene (PVdF–HFP) binder) composite electrode. We have coupled scanning electron microscopy (SEM) observations, galvanostatic cycling and electrochemical impedance spectroscopy in order to define and understand the impact of the microstructure of the composite electrode on its electrochemical performance. Derived from these studies, the main key factors that provide efficient charge carrier collection within the composite electrode complex medium are discussed.     

Studies on preparation and performances of carbon aerogel electrodes for the application of supercapacitor

Carbon aerogel was prepared by the polycondensation of resorcinol (R) with formaldehyde (F), and sodium carbonate was added as a catalyst (C). Physical properties of carbon aerogel were characterized by infrared spectrometer (IR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is found that carbon aerogel is an amorphous material with a pearly network structure, and it consists of one or two diffuse X-ray peaks. The results of cyclic voltammetry indicated that the specific capacitance of a carbon aerogel electrode in 6 M KOH electrolyte was approximately 110.06 F g⁻¹. Through the galvanostatic charge/discharge measurement, it was found that the electrode is stable in KOH electrolyte, the maximum capacitance of the supercapacitor with carbon aerogel as the electrode active material was 28 F g⁻¹. Besides, the supercapacitor has long cycle life. Thus, it was thought that the carbon aerogel is an excellent electrode material for a supercapcitor.     

The capacitive characteristics of supercapacitors consisting of activated carbon fabric–polyaniline composites in NaNO3

A sandwich-type supercapacitor consisting of two similar activated carbon fabric–polyaniline (ACF–PANI) composite electrodes was demonstrated to exhibit excellent performance (i.e., highly reversibility and good stability) in NaNO₃. Polyaniline with the charge density of polymerization less than or equal to 9 C cm⁻² synthesized by means of a potentiostatic method showed a high specific capacitance of 300 F g⁻¹. Influences of the polymerization charge density (i.e., the polymer loading) on the capacitive characteristics of ACF–PANI composites were compared systematically. The capacity of an ACF–PANI electrode reach ca. 3.4 F cm⁻² (a 100% increase in total capacity) when the charge density of polymerization is equal to 9 C cm⁻². The surface morphology of these ACF–PANI composites was examined by a scanning electron microscope (SEM).     

Carbon anode for dry-polymer electrolyte lithium batteries

A spherical carbon material of meso-carbon microbead (MCMB) was examined as an anode in a polyethylene oxide (PEO) based polymer electrolyte lithium battery. The electrochemical performance of the carbon electrode with the polymer electrolyte depended on the electrode thickness and the particle size of MCMB. The 30 μm-thick electrode of MCMB with the particle size of 20–30 μm showed a reversible capacity comparable with that in a liquid electrolyte, but the 100 μm-thick electrode showed a half of the 30 μm-thick electrode. The smaller particle size of 5–8 μm exhibited a high irreversible capacity at the first charge–discharge cycle. The reaction heat between MCMB and the polymer electrolyte was 0.5 J mAh^?1, which was much lower compared to those between lithium metal and the polymer electrolyte, 1.2 J mAh^?1, and MCMB and conventional liquid electrolyte, 4.3 J mAh^?1.     

Electrochemical capacitance of nanocomposite films formed by loading carbon nanotubes with ruthenium oxide

This work reports the supercapacitive properties of composite films of multiwalled carbon nanotubes (MWNT) and ruthenium oxide (RuO₂). Transmission and scanning electron microscopy, cyclic voltammetry, and electrochemical studies revealed that the nanoporous three-dimensional arrangement of RuO₂-coated MWNT in these films facilitated the improvement of electron and ion transfer relative to MWNT films. The capacitance was measured for films of different RuO₂ loading, revealing specific capacitances per mass as high as 628 F g⁻¹. The energy storage density of the electrode has increased about three times as compared to MWNT treated with piranha solution.     

Optimization of synthesis condition and electrode fabrication for spinel LiMn2O4 cathode

We used cyclic voltammetry (CV) and galvanostatic cycling test to optimize the synthesis condition and electrode composition for spinel LiMn₂O₄ cathode. Based on a synthetic approach of solid reaction, the most appropriate Li–Mn source for the synthesis of LiMn₂O₄ was found to be LiOH/MnO₂ and the optimum synthesis condition was to react at 750 °C in air for 18 h. The LiMn₂O₄ such obtained has an initial specific capacity of 120–130 mAh/g between 3.5 and 4.2 V. In the electrode films, the carbon that was used as a conducting agent significantly affects performance of the LiMn₂O₄ electrode. Among the carbons examined in this work, we found that carbon black from Alfa Aesar was a better conducting agent and its appropriate content was around 10% in the LiMn₂O₄ electrode.     

Pressure-driven water flow through carbon nanotubes: Insights from molecular dynamics simulation

Pressure-driven water flow through carbon nanotubes (CNTs) is examined using molecular dynamics simulation. The results are compared to reported experimental flow rate measurements through similarly sized CNTs and larger carbon nanopipes. By using molecular dynamics simulation to predict the variation of water viscosity and slip length with CNT diameter, we find that flow through CNTs with diameters as small as 1.66 nm can be fully understood using continuum fluid mechanics. Potential mechanisms to explain the differences between the flow rates predicted from simulation and those measured in experiments are identified and discussed.     

Hybrid electrochemical capacitors based on polyaniline and activated carbon electrodes

The performance of a newly designed, polyaniline–activated carbon, hybrid electrochemical capacitor is evaluated. The capacitor is prepared by using polyaniline as a positive electrode and activated carbon as a negative electrode. From a constant charge–discharge test, a specific capacitance of 380 F g⁻¹ is obtained. The cycling behaviour of the hybrid electrochemical capacitor is examined in a two-electrode cell by means of cyclic voltammetry. The cycle-life is 4000 cycles. Values for the specific energy and specific power of 18 Wh kg⁻¹ and 1.25 kW kg⁻¹, respectively, are demonstrated for a cell voltage between 1 and 1.6 V.     

A high energy density lithium/sulfur–oxygen hybrid battery

In this paper we introduce a lithium/sulfur–oxygen (Li/S–O₂) hybrid cell that is able to operate either in an air or in an environment without air. In the cell, the cathode is a sulfur–carbon composite electrode containing appropriate amount of sulfur. In the air, the cathode first functions as an air electrode that catalyzes the reduction of oxygen into lithium peroxide (Li₂O₂). Upon the end of oxygen reduction, sulfur starts to discharge like a normal Li/S cell. In the absence of oxygen or air, sulfur alone serves as the active cathode material. That is, sulfur is first reduced to form a soluble polysulfide (Li₂S_x, x ≥ 4) that subsequently discharges into Li₂S through a series of disproportionations and reductions. In general, the Li/S–O₂ hybrid cell presents two distinct discharge voltage plateaus, i.e., one at ～2.7 V attributing to the reduction of oxygen and the other one at ～2.3 V attributing to the reduction of sulfur. Since the final discharge products of oxygen and sulfur are insoluble in the organic electrolyte, it is shown that the overall specific capacity of Li/S–O₂ hybrid cell is determined by the carbon composite electrode, and that the specific capacity varies with the discharge current rate and electrode composition. In this work, we show that a composite electrode composed by weight of 70% M-30 activated carbon, 22% sulfur and 8% polytetrafluoroethylene (PTFE) has a specific capacity of 857 mAh g^?1 vs. M-30 activated carbon at 0.2 mA cm^?2 in comparison with 650 mAh g^?1 of the control electrode consisting of 92% M-30 and 8% PTFE. In addition, the self-discharge of the Li/S–O₂ hybrid cell is expected to be substantially lower when compared with the Li/S cell since oxygen can easily oxidize the soluble polysulfide into insoluble sulfur.     

Electrochemical profile of nano-particle CoAl double hydroxide/active carbon supercapacitor using KOH electrolyte solution

A nano-structured CoAl double hydroxide with an average particle size of 60–70 nm was prepared by a chemical co-precipitation. It was used as a positive electrode for the asymmetric hybrid supercapacitor in combination with an active carbon negative electrode in KOH electrolyte solution. The electrochemical capacitance performance of this kind of hybrid supercapacitor was investigated by means of cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge–discharge tests. A specific capacitance of 77 F g⁻¹ with a specific energy density of 15.5 wh kg⁻¹ was obtained for the hybrid supercapacitor within the voltage range of 0.9–1.5 V. The supercapacitor also exhibits a good cycling performance and keep 90% of initial capacity over 1000 cycles.     

Electrically conductive LCP–carbon composite with low carbon content for bipolar plate application in polymer electrolyte membrane fuel cell

Lightweight polymer–carbon composites with high specific electrical conductivity at a carbon content below 40 vol.% were developed. The electrical and mechanical properties and the hydrogen permeability of carbon fiber and particle reinforced liquid crystalline polymers were examined. Vectra^® A 950, SIGRAFIL^® carbon fibers and Vulcan^® XC 72 R carbon black were employed. The composites are found to have sufficient mechanical properties and a hydrogen permeability low enough to be utilised as bipolar plate material in fuel cell applications. The density of the new composite is 20% lower than the density of commercial bipolar plates made from carbon reinforced polymeric composite materials, due to the lower carbon content. The current density at 0.5 V in an operating fuel cell is only 20% lower compared to commercial materials with more than 80 vol.% carbon content and meets the requirements for bipolar plate application.     

Preparation and characterization of composite electrodes of coconut-shell-based activated carbon and hydrous ruthenium oxide for supercapacitors

The relationship between the structure-specific capacitance (F g⁻¹) of a composite electrode consisting of activated coconut-shell carbon and hydrous ruthenium oxide (RuO_x(OH)_y) has been evaluated by impregnating various amounts of RuO_x(OH)_y into activated carbon that is specially prepared with optimum pore-size distribution. The composite electrode shows an enhanced specific capacitance of 250 F g⁻¹ in 1 M H₂SO₄ with 9 wt.% ruthenium incorporated. Chemical and structural characterization of the composites reveals a homogeneous distribution of amorphous RuO_x(OH)_y throughout the porous network of the activated carbon. Electrochemical characterization indicates an almost linear dependence of capacitance on the amount of ruthenium owing to its pseudocapacitive nature.     

Electrochemical characterization on MnFe2O4/carbon black composite aqueous supercapacitors

MnFe₂O₄–carbon black (CB) composite powders synthesized by a co-precipitation method have been characterized and optimized for their electrochemical properties for supercapacitor applications. The composite shows pseudocapacitance in electrolyte solutions of alkali and alkaline chlorides, sulfates and sulfites. For the chlorides and sulfates electrolytes, the pseudocapacitance has been identified, by in situ X-ray absorption near-edge spectroscopy study, to involve charge-transfer at both the Mn and Fe sites of the ferrite. In 1 M NaCl_(aq), the composite electrode exhibits an operating potential window of 1.0 V with a maximum leakage current of 0.3 mA F⁻¹, and it exhibits far superior cycling stability to amorphous MnO₂ electrode. Both the specific capacitance and self-discharge behavior of the composite electrode depend strongly on the composite composition. The optimum capacitance occurs at ferrite:CB weight ratio of 7:3, which gives reduced self-discharge rate as compared with CB. The composite electrode also demonstrates capability of high-power delivery.     

Evaluation of different approaches for improving the cycle life of MgNi-based electrodes for Ni-MH batteries

Several methods have been investigated to enhance the cycle life of amorphous MgNi used as the negative electrode for Ni-MH batteries. The first approach involves modifying its surface composition in different ways, including the electroless deposition of a chromate conversion coating, the addition of chromate salt or NaF into the electrolyte and the mechanical coating of the particles with various compounds (e.g. TiO₂). Another approach consists of developing (MgNi + AB₅) composite materials. However, the cycle life of these modified MgNi electrodes remains unsatisfactory. On the other hand, the modification of the bulk composition of the MgNi alloy with elements such as Ti and Al appears to be more effective. For instance, a Mg_0.9Ti_0.1NiAl_0.05 electrode retains 67% of its initial discharge capacity (404 mAh g⁻¹) after 15 cycles compared to 29% for MgNi. The charging conditions also have a great influence on the electrode cycle life as demonstrated by the existence of a charge input threshold below which minor capacity decay occurs. In addition, the particle size has a major influence on the electrode performance. We have developed an optimized electrode constituted of Mg_0.9Ti_0.1NiAl_0.05 particles with the appropriate size (>150 μm) showing a capacity decay rate as low as ∼0.2% per cycle when charged at 300 mAh g⁻¹.     

Morphology and electrochemical behaviour of ruthenium oxide thin film deposited on carbon paper

In order to improve the efficiency of ruthenium dioxide, RuO₂, as an electrochemical capacitor electrode, a RuO₂ thin film is deposited on carbon paper and its structure and properties are evaluated. This new composite material is prepared via solution dip-coating of a Ru-ethoxide precursor and heat conversion. The coating thickness is easily controlled by varying the number of repetitions of the preparation process. The resulting structure consists of a by homogeneously coated RuO₂ film on carbon paper which has a porous graphite matrix. Extensive electrochemical studies have been performed in 1 M H₂SO₄ electrolyte in order to evaluate the properties of the composite as an electrode in an electrochemical capacitor. The composite material shows not only high specific capacitance (620 F g⁻¹) but also good power characteristics.     

Influence of the microstructure on the supercapacitive behavior of polyaniline/single-wall carbon nanotube composites

Polyaniline/single-wall carbon nanotube (PANI/SWCNT) composites were prepared by in situ potentiostatic deposition of PANI onto SWCNTs at the potential of 0.75 V versus SCE, with the aim to investigate the influence of microstructure on the specific capacitance of PANI/SWCNT composites. It was found that the specific capacitance of the PANI/SWCNT composites is strongly influenced by their microstructure, which is correlated to the wt.% of the PANI deposited onto the SWCNTs. The optimum condition, corresponding to the highest specific capacitance, 463 F g⁻¹ (at 10 mA cm⁻²), was obtained for 73 wt.% PANI deposited onto SWCNTs. The specific capacitance of the PANI/SWCNT composite electrode was highly stable, with a capacitive decrease of 5% during the first 500 cycles and just 1% during the next 1000 cycles, indicative of the excellent cyclic stability of the composite for supercapacitor applications.