Preparation of Pt/CeO2/HCSs anode electrocatalysts for direct methanol fuel cells

Hollow carbon spheres (HCSs) were prepared through a simple hydrothermal method using silica particles and glucose as the template and carbon precursor, respectively. HCSs used as supports for platinum catalysts deposited with cerium oxide (CeO₂) were prepared for application as anode catalysts in direct methanol fuel cells. The composition and structure of the samples were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The electrocatalytic properties of the as-prepared catalysts for methanol oxidation were investigated by cyclic voltammetry (CV). The Pt/CeO₂/HCSs catalyst heated at 550 °C for 1 h exhibited the best catalytic activity for methanol oxidation.     

Pyrolytic polyurea encapsulated natural graphite as anode material for lithium ion batteries

Carbon encapsulated graphite was prepared by coating polyurea on the surface of natural graphite particles via interfacial polymerization followed by a pre-oxidation at 250 °C in air and a heat treatment at 850 °C in nitrogen. FT-IR spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to investigate the structure of the graphite before and after the surface modification. Galvanostatic cycling, dc impedance spectroscopy, and cyclic voltammetry were used to investigate the electrochemical properties of the modified graphite as the anode material of lithium cells. The modified graphite shows a large improvement in electrochemical performance such as higher reversible capacity and better cycleability compared with the natural graphite. It can work stably in a PC-based electrolyte with the PC content up to 25 vol.% because the encapsulated carbon can depress the co-intercalation of solvated lithium ion. The initial coulombic efficiency of C-NG and NG in non-PC electrolyte is 74.9 and 88.5%, respectively.     

The morphology and electrical resistance of long oriented vapor-grown carbon fibers synthesized from coal pitch

Long, oriented vapor-grown carbon fibers with high purity were synthesized from coal pitch by chemical vapor deposition using ferrocene as catalyst precursor and hydrogen as carrier gas. Field emission scanning electron microscopy, energy dispersive spectroscopy, high resolution transmission electron microscopy, X-ray diffraction, Raman spectroscopy and thermalgravimetric analysis were employed to characterize the morphology and structure of the products. The resistance of a single carbon fiber was also measured. Results revealed that the obtained oriented fibers had an average diameter of 1.0 μm and a bundle length of up to 6.0 cm. Electrical measurements showed a non-linear resistance, decreasing with increasing applied voltage between 0 V and 25 V.     

Electrochemical behavior of carbon-coated SnS2 for use as the anode in lithium-ion batteries

Carbon-coated SnS₂ nanoparticles were prepared by a simple solvothermal route at low temperature. A carbon coating with a thickness of about 5 nm was deposited on nano-sized SnS₂ particles to serve as the anode in lithium-ion batteries. Both the nanostructure and the morphology of the SnS₂ powders were characterized by X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM). The coated samples were used as active anode materials for lithium-ion batteries, and their electrochemical properties were examined by constant current charge-discharge cycling, cyclic voltammetry and electrochemical impedance spectroscopy. The reversible capacity of the carbon-coated SnS₂ after 50 cycles was 668 mAh/g, which was much higher than that of the uncoated SnS₂ (293 mAh/g). The carbon-coated SnS₂ also had a better rate capability than the uncoated SnS₂ in the range of 0.008-1 C. The capacity retention of the carbon-coated SnS₂ was improved due to its good conductivity and the effective buffer matrix that alleviated volume expansion during the charge-discharge process.     

Tin dioxide/carbon nanotube composites with high uniform SnO2 loading as anode materials for lithium ion batteries

SnO₂/multi-walled carbon nanotube (MWCNT) composites were prepared by the solvothermal method and subsequent heat treatment at 360 °C. The samples were characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). Results on the higher SnO₂ content composite sample indicate that a uniform layer of SnO₂ nanocrystals with crystal size around 5 nm was deposited on the surface of the carbon nanotubes. The composite demonstrates a reversible lithium storage capacity of 709.9 mAh g⁻¹ at the first cycle and excellent cyclic retention up to 100 cycles as anode for lithium ion batteries.     

Three-dimensional reticular tin-manganese oxide composite anode materials for lithium ion batteries

Tin-manganese oxide film with three-dimensional (3D) reticular structure has been prepared by electrostatic spray deposition (ESD). X-ray diffraction (XRD) and transmission electron microscopy (TEM) indicate that the film is amorphous. X-ray-photoemission spectroscopy (XPS) demonstrates that the 3D grid is composed of tin-manganese oxide. As an anode electrode for the lithium ion battery, the tin-manganese oxide film has 1188.3 mAh g⁻¹ of initial discharge capacity and very good capacity retention of 656.2 mAh g⁻¹ up to the 30th cycle. Such a composite film can be used as an anode for lithium ion batteries with higher energy densities.     

Synthesis of novel Y-junction hollow carbon nanotrees

A simple catalyst-dwindling route without additional templates has been developed, for the first time, to synthesize novel Y-junction hollow carbon nanotrees in a high-yield of 90% by the reaction of ferrocene with 1,2-dichlorobenzene at 180 °C. The nanotrees have an average length about 5 μm, an average external trunk diameter of about 500 nm, branch diameters ranging from 70 to 100 nm, and wall thickness ranging from 24 to 30 nm. Unlike traditional carbon nanotubes, these nanotrees have close-ended branches but open-ended roots. X-ray powder diffraction, field emission scanning electron microscopy, transmission electron microscope, high-resolution transmission electron microscopy, electron diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy were used to characterize the products. It is also found that proper reaction temperatures and appropriate proportion of ferrocene and 1,2-dichlorobenzene play important roles in this route. The formation mechanism of treelike carbon nanotubes was simply proposed.     

Synthesis at 250 °C of submicron hydrogenated amorphous carbon “test tubes”

Uniform submicron hydrogenated amorphous carbon “test tubes”, both empty and containing nickel particles, were synthesized via the self-catalysis-decomposition of Ni(Hdmg)₂ (Hdmg = dimethylglyoxime anion) at 250 °C. Since one end of the tubular carbon is always open but the other end is closed, they look like “test tubes”. Transmission electron microscopy images show they have an average inner (outer) diameter of 560 nm (740 nm) and an average length of around 5.4 μm. They were characterized by X-ray diffraction, scanning and transmission electron microscopy, energy-dispersive X-ray analysis, Fourier transform infrared spectroscopy, Raman spectroscopy and thermogravimetric analysis. The results indicated that the “test tubes” consist of amorphous hydrogenated carbon. The magnetic properties of the contained nickel reached a coercivity as high as H_c ≈ 153 Oe and showed a remarkable decrease of saturation magnetization. The reaction temperature and solvent are of great importance in determining the final structures. A possible formation mechanism was discussed.     

A study on the dissymmetrical microporous layer structure of a direct methanol fuel cell

The effect of carbon type, carbon loading and microporous layer structure in the microporous layer on the performance of a direct methanol fuel cell (DMFC) at low temperature was investigated using electrochemical polarization techniques, electrochemical impedance spectroscopy, scanning electron microscope and other methods. Vulcan XC-72 carbon was found to be most suitable as a microporous layer for low temperature DMFC. Maximum fuel cell performance was obtained utilizing a microporous layer with carbon loading of 1.0 mg cm⁻² when air was used as an oxidant. A membrane electrode assembly with 1.0 mg cm⁻² Vulcan XC-72 carbon with 20 wt.% Teflon in the cathode and no microporous layer in the anode showed a maximum power density of 36.7 mW cm⁻² at 35 °C under atmospheric pressure. The AC impedance study proved that a cell with a dissymmetrical microporous layer structure had lower internal resistance and mass transfer resistance, thus obtaining better performance.     

Synthesis of multi-branched porous carbon nanofibers and their application in electrochemical double-layer capacitors

Multi-branched carbon nanofibers with a porous structure have been synthesized on a Cu catalyst doped with Li, Na, or K. The products were characterized by field emission scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy and Raman spectroscopy. Using this new type of nanofiber as polarized electrodes, an electrochemical double-layer capacitor with a specific capacitance of ca. 297 F/g was obtained using 6 M KOH as the electrolyte.     

Effects of reactors on the deposition of DLC films using liquid electrochemical technique

Carbon films were deposited on silicon substrates by liquid electrochemical technique at low temperature (60 °C) in ambient atmosphere. Glass reactor, glass reactor with PTFE-coating inside, glass reactor with quartz-coating inside and quartz reactor were used with the same experimental setup to compare the effects of reactors on the deposition of carbon films. The applied potential, the distance between anode and substrate and the deposition time were fixed at 900 V (4.2 kHz, 50%), 6 mm and 5 h, respectively. The morphology and microstructure of the films were analyzed by scanning electron microscopy (SEM) and Raman spectroscopy. Energy Dispersive X-ray Spectrometry (EDX) was used to measure the composition of the films. The SEM observations showed that the films deposited using glass reactor were composed of crystals of several micrometers which contained nearly 10 at.% of Ca. Raman spectroscopy analysis confirmed that DLC films have been deposited, but with an obvious sharp peak at 1085 cm^− 1 which is assigned to calcium carbonate (CaCO₃) crystals. The glass reactor is the possible source of Ca because the electrolyte was composed of analytically pure acetone and deionized water with the proportion of Ca below the determination of AAS (atomic absorption emission spectrophotometer AA-6200). Using glass reactor with PTFE-coating inside could successfully avoid the impurity of Ca from the glass reactor, but new non-metallic impurities coming from the PTFE-coating made the films rough. Continuous and smooth films were deposited by using a glass reactor with quartz-coating inside and quartz reactor, which could avoid both Ca (< 1 at.%) and other impurities. Raman spectroscopy analysis confirmed typical DLC films without CaCO₃. It can be concluded that the materials of the reactors could play an important role not only in the composition, but also the morphology and microstructure of films deposited by liquid electrochemical technique.     

The effect of current density and temperature on the degradation of nickel cermet electrodes by carbon monoxide in solid oxide fuel cells

The oxidation of dry carbon monoxide (CO) in intermediate temperature solid oxide fuel cells (IT-SOFCs) has been studied using a three electrode assembly. Ni/CGO:CGO:LSCF/CGO three electrode pellet cells at 500, 550 and 600 °C were exposed to dry carbon monoxide for fixed periods of time, at open circuit and under load at 50 and 100 mA cm⁻², in an aggressive test designed to accelerate electrode degradation. It is shown that if the anode is kept under load during exposure to dry CO, degradation in anode performance can be minimised, and that under most conditions the anode showed significant irreversible degradation in performance after subsequent load cycling on dry H₂. Only at 500 °C and at 100 mA cm⁻² was the degradation in performance after operation on dry CO and subsequent load cycling on dry H₂ within the background degradation rates measured. Where anode performance was compromised, this appeared to be caused by a reduction in the exchange current density for hydrogen oxidation, and the relatively large degradation after load cycling on dry H₂ was primarily caused by an increase in the series resistance of the anode. It is suggested that this increase in series resistance is associated with the removal of carbon deposited in the non-electrochemically active region of the electrode during operation on dry CO, and that operation under load inhibits carbon deposition in the active region.     

A solvothermal-reduction method for the production of horn shaped multi-wall carbon nanotubes

Horn shaped multi-wall carbon nanotubes (HMWCNTs) with uniform diameter of ∼100 nm, an average wall thickness of 10 nm and length of about 1 μm were prepared using a solvothermal-reduction method employing carbon tetrachloride as a carbon source and metallic copper as reductant. X-ray diffraction pattern indicated the presence of an amorphous carbon phase. Functional groups attached to the surface of the HMWCNTs were identified by diffuse reflectance infrared Fourier transform spectroscopy. The nitrogen sorption isotherm was of Type IIb with the hysteresis typically found for aggregated powders which possess non-rigid slit-shaped pores. The formation of HMWCNTs, as observed by scanning electron microscopy, proceeded through an intermediate fish-bone like morphology.     

Pulsed laser deposition of glass-like cluster assembled carbon films

Carbon films have been synthesized at room temperature in helium atmosphere, at high pressure, on (1 0 0) Si substrates by pulsed KrF excimer laser ablation of highly oriented pyrolitic graphite. By changing laser power density (from 8.5 to 19 MW mm⁻²) and gas pressure (from 0.6 Pa to 2 kPa), nanometer sized cluster assembled films were obtained. Film morphology, as studied by scanning electron microscopy, changes with increasing helium pressure, from dense columns, to node-like morphology, then to an open dendritic structure. Carbon coordination was studied by visible Raman spectroscopy in all films. They are structurally disordered, sp² coordinated and belong to the family of glass-like carbons. The deduced film coherence length agrees with the average size of carbon aggregates that build up the films, as measured by transmission electron microscopy in representative samples. The average number of carbon atoms per cluster, that depends on helium (high) pressure, was obtained by a simple model.     

Electrochemically oxidized carbon anode in direct l-ascorbic acid fuel cells

The activity of electrochemically oxidized carbon electrode was investigated in the operation of a direct l-ascorbic acid fuel cell anode. The surface oxygen species placed on electrochemically oxidized carbon electrode were analyzed by X-ray photoelectron spectroscopy and cyclic voltammetry. The electrochemical oxidation process of carbon electrode can facilitate the pore-filling process (i.e., wetting) of the electrolyte into the microstructure of the carbon electrode by increasing the number of more polar functional groups on the electrode surface. The electrochemically oxidized carbon electrode exhibited significantly enhanced electro-catalytic oxidation activity of l-ascorbic acid compared to an unmodified carbon electrode. Moreover, the simplified electrode structure using carbon paper without an additional powder-based precious catalyst layer is very favorable in creating percolation network and generates power density of 18 mW/cm² at 60 °C.     

Effects of surrounding confinements of Si nanoparticles on Si-based anode performance for lithium ion batteries

We report herein the effects of various surrounding confinements of Si nanoparticles on the electrochemical performance of Si nanoparticles based anode for lithium ion batteries (LIBs). Three different types of surrounding confinements are incorporated onto or around the surface of Si nanoparticles: Si nanoparticles-embedded carbon nanofibers (Si@CNF) via electrospinning, carbon nitride encapsulated Si nanoparticles with core/shell structure (Si@CND), and binder enriched Si nanoparticles-based anode (Si@RBD). Morphology and microstructure of the samples are characterized using X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, and the results were discussed in relation to the electrochemical performances. It is found that Si@CNF which has conducting hard surrounding confinements and Si@RBD exhibited high reversible specific capacity of 620 mA hg⁻¹ and 1200 mA hg⁻¹, up to 30th cycle, respectively. Meanwhile, Si@CND that has mechanically hard but poor electrical conductivity exhibits low specific capacity compared with the other two samples.     

Structural evolutions and electrochemical behaviors of Co-B alloys as anode materials for alkaline secondary batteries

Co-B alloys were synthesized via a chemical reduction method, and adopted as anode materials for alkaline secondary batteries. The structural evolutions and the electrochemical behaviors of the as-prepared Co-B alloys with increasing heat-treating temperatures were analyzed with X-ray diffraction (XRD), scanning electronic microscopy (SEM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV) and charge-discharge test. It is found that the Co-B alloys treated at various temperatures show high reversibility. And the electrochemical activities were found to be dependent on the structural evolutions of the Co-B alloys. The amorphous Co-B alloy treated at 50 °C achieves an excellent discharge capacity of 304 mAh/g in the first cycle, while the Co-B alloy treated at 500 °C shows superior cyclicity.     

Performance degradation study of a direct methanol fuel cell by electrochemical impedance spectroscopy

A stability test of a direct methanol fuel cell (DMFC) was carried out by keeping at a constant current density of 150 mA cm⁻² for 435 h. After the stability test, maximum power density decreased from 68 mW cm⁻² of the fresh membrane-electrode-assembly (MEA) to 34 mW cm⁻² (50%). Quantitative analysis on the performance decay was carried out by electrochemical impedance spectroscopy (EIS). EIS measurement of the anode electrode showed that the increase in the anode reaction resistance was 0.003 Ω cm². From the EIS measurement results of the single cell, it was found that the increase in the total reaction resistance and IR resistance were 0.02 and 0.05 Ω cm², respectively. Summarizing the EIS measurement results, contribution of each component on the performance degradation was determined as follows: IR resistance (71%) > cathode reaction resistance (24%) > anode reaction resistance (5%). Transmission electron microscopy (TEM) results showed that the average particle size of the Pt catalysts increased by 30% after the stability test, while that of the PtRu catalysts increased by 10%.     

Decreasing resistances of membrane-electrode assemblies containing hydrocarbon-based ionomers for improving their anodic performances

To improve methanol-oxidation performances of membrane-electrode assemblies composed of a hydrocarbon-based ionomers, the resistances involved in the reaction were decreased. Electrochemical impedance spectroscopy revealed that the proton-conductive resistance (R_i) in the anode was decreased from 0.54 to 0.40 Ω cm² by increasing a loading ratio of platinum-ruthenium to carbon support of anode catalyst from 54 to 73 wt.%. In addition, R_i was decreased to be 0.25 Ω cm² by increasing ion-exchange capacity (IEC) of the ionomer from 1.4 to 2.9 mequiv. g⁻¹. Consequently, the polarization resistance of the anode was significantly decreased, in turn, increasing current density of methanol oxidation at the potential of 0.45 V from 0.110 to 0.244 A cm⁻².     

Electrochemical reaction of the SiMn/C composite for anode in lithium ion batteries

The SiMn-graphite composite powder was prepared by mechanical ball milling and its electrochemical performances were evaluated as the candidate anode materials for lithium ion batteries. It is found that the cyclic performance of the composite materials is improved significantly compared to SiMn alloy and pure silicon. The heat treatment of the electrodes is beneficial for enhancing the cyclic stabilities. The SiMn-20 wt.% graphite composite electrode after annealing at 200 °C has an initial reversible capacity of 463 mAh g⁻¹ and a charge-discharge efficiency of 70%. Moreover, the reversible capacity maintains 426 mAh g⁻¹ after 30 cycles with a coulomb efficiency of over 97%. The phase structure and morphology of the composite were analyzed by X-ray diffraction (XRD) and scanning electron microscopy. The lithiation/delithiation behavior was investigated by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry. The composite materials appear to be promising candidates as negative electrodes for lithium rechargeable batteries.