Determination of active surface areas of coal chars using a temperature-programmed desorption technique

A new dynamic method is described for measuring active surface areas (ASA) of carbons, involving oxygen chemisorption followed by temperature-programmed desorption of CO₂ and CO. The method gives similar results to the standard volumetric method. Under the chosen experimental conditions, it was shown that during chemisorption the carbon surface was saturated with oxygen and that during desorption a number of possible secondary reactions did not occur. ASA and total surface area (TSA) (measured by adsorption of argon at 77 K), are reported for a steam activated series prepared from a model char and a coal char. In the initial stages of gasification, the ASA/TSA ratio decreased and then remained constant for a wide range of burn-off. The initial decrease in the ASA/TSA ratio is attributed to a low value of TSA caus3d by activated diffusion of argon at 77 K into micropores.     

Correlation of the irreversible lithium capacity with the active surface area of modified carbons

Negative electrodes of lithium-ion batteries are generally based on graphite. Higher storage capacities can be obtained with disordered carbons, however they demonstrate a noticeable hysteresis and irreversibility, which can preclude a practical application. In this paper, the main parameters which may affect the irreversible capacity are analyzed and we show that the irreversible lithium consumption which occurs at the negative carbon electrode during the first charge (C_irr) is proportional to the active surface area (ASA). Composites with a reduced ASA have been obtained after coating a hard carbon or milled graphite samples with a thin film of pyrolytic carbon. Deactivating the surface by pyrolytic carbon deposition allows the irreversible capacity to be noticeably reduced, being lower than in graphite, while the reversible capacity is 50% higher than in graphite. The electrochemical properties of this new C/C composite are investigated by galvanostatic cycling at various current densities and by impedance spectroscopy. The main effect of the dense carbon coating is to hinder the diffusion of solvated lithium ions to the active sites of the carbon host during the first discharge, giving rise to a moderate development of the Solid Electrolyte Interphase (SEI).     

Kinetics of the reaction of oxygen with thin films of carbon

A new technique is described for the study of reactions of carbon with gaseous reactants. The optical absorbance of a thin film of carbon deposited on the surface of a reaction vessel by the pyrolysis of methane was measured in situ using a He-Ne laser. The reaction of the film with oxygen (30–120 Torr, 623°C) was followed by continuous monitoring of the absorbance of the film during the complete course of the reaction. The initial rate of reaction of the carbon was first order in oxygen pressure and the activation energy of the rate was 38 kcal/mole. The concentration of strongly-bound complexes on the surface of carbon films was determined from measurements of the yield of carbon monoxide released at 700°C after equilibration with oxygen at low temperatures and has been identified with the active surface area (ASA) of the film. The ASA has been measured as a function of the heat treatment and of the percentage conversion of the film. It is shown that both the ASA and the differential rate of gasification increase with increasing conversion of the film. The results are interpreted in terms of a mechanism involving the attack of oxygen molecules on carbon atoms in the neighbourhood of the complex. Changes in the mechanism at high temperatures and low pressures are discussed.     

The role of the surface complex in the kinetics of the reaction of oxygen with carbon

Using thin films of pyrolytic carbon the kinetics of the reaction with oxygen were studied in a static system over the temperature range 748–1173 K at pressures in the neighborhood of l Torr (0.13 kPa). The formation of the stable surface complex, measured by pressure change and by temperature-programmed desorption, and products carbon monoxide and carbon dioxide were monitored as a function of reaction time during the complete course of the reaction. At lower temperatures an induction period was observed for formation of carbon monoxide and carbon dioxide but not for formation of the complex. The presence of the complex on the surface before the start of a reaction shortened the induction period and increased slightly the over-all rate. It was concluded that the surface complex is an intermediate in the production of carbon monoxide and carbon dioxide. A mechanism for the reaction is proposed which involves secondary reaction of oxygen with the complex and which accounts qualitatively for the kinetics of the reaction. It is suggested that a turn-over number may be estimated based on a number of active carbon atoms on the surface.     

Carbonization and CO<Subscript>2</Subscript> activation of scrap tires: Optimization of specific surface area by the Taguchi method

This research demonstrates the production of activated carbon from scrap tires via physical activation with carbon dioxide. A newly constructed apparatus was utilized for uninterrupted carbonization and activation processes. Taguchi experimental design (L16) was applied to conduct the experiments at different levels by altering six operating parameters. Carbonization temperature (550–700 °C), activation temperature (800–950 °C), process duration (30–120 min), CO₂ flow rate (400 and 600 cc/min) and heating rate (5 and 10 °C/min) were the variables examined in this study. The effect of parameters on the specific surface area (SSA) of activated carbon was studied, and the influential parameters were identified employing analysis of variance (ANOVA). The optimum conditions for maximum SSA were: carbonization temperature=650 °C, carbonization time=60 min, heating rate=5 °C/min, activation temperature= 900 °C, activation time=60 min and CO₂ flow rate=400 cc/min. The most effective parameter was activation temperature with an estimated impact of 49%. The activated carbon produced under optimum conditions was characterized by pore and surface structure analysis, iodine adsorption test, ash content, scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The process yield for optimized activated carbon was 13.2% with the following properties: specific surface area=437 m²/g, total pore volume=0.353 cc/g, iodine number=404.7 mg/g and ash content=13.9% along with an amorphous structure and a lot of oxygen functional groups. These properties are comparable to those of commercial activated carbons.     

IR spectroscopy studies of oxygen surface compounds on carbon

Spectral studies of oxidation process were carried out on carbon films prepared by carbonization of polyfurfuryl alcohol and of cellulose. Investigation of the oxygen role in the initial stages of carbonization shows that oxygen surface compounds formed during such process have different chemical structure from those produced during oxidation of carbonic film previously desorbed at 600°C. Chemisorption of oxygen at room temperature on the carbonic film (desorbed previously at 600°C) occurs with the participation of π electrons of condensed aromatic systems. Iono-radical structures formed during that process show absorption bands in the range of 1590cm⁻¹. Oxidation of the carbon films carbonized at 800°C causes a decrease in the absorption coefficient, and oxygen surface compounds formed show absorption bands at about 1760, 1600 and 1260cm⁻¹. The carbon film as a model substance gives possibilities for broader application of IR spectroscopy in studies of carbon, carbonization and activation processes, sorption effects and catalytic mechanisms.     

Characteristics and surface energy of silicon-doped diamond-like carbon films fabricated by plasma immersion ion implantation and deposition

Diamond-like carbon (DLC) films doped with different silicon contents up to 11.48 at.% were fabricated by plasma immersion ion implantation and deposition (PIII-D) using a silicon cathodic arc plasma source. The surface chemical compositions and bonding configurations were determined by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The results reveal that the sp³ configuration including Si–C bonds increases with higher silicon content, and oxygen incorporates more readily into the silicon and carbon interlinks on the surface of the more heavily silicon-doped DLC films. Contact angle measurements and calculations show that the Si-DLC films with higher silicon contents tend to be more hydrophilic and possess higher surface energy. The surface states obtained by silicon alloying and oxygen incorporation indicate increased silicon oxycarbide bonding states and sp³ bonding states on the surface, and it can be accounted for by the increased surface energy particularly the polar contribution.     

Correlations between surface properties of graphite and the first cycle specific charge loss in lithium-ion batteries

We report the influence of two surface parameters, the active surface area (ASA) and the surface chemistry (oxygen functional groups) on the first electrochemical reduction of graphite. The experimental results highlight two important points. One, the ASA is the determining parameter which controls the exfoliation during the first electrochemical cycle. An ASA limit (ca. 0.2 m² g⁻¹) above which the exfoliation is suppressed was experimentally found. Below or above this limit, the specific charge loss remains almost constant even if the ASA changes. Two, for a sample having an ASA value higher than 0.2 m² g⁻¹, it is shown that the presence of oxygen groups at the surface is critical for the formation of an efficient solid electrolyte interphase (SEI) layer. The lack of oxygen groups during the first electrochemical reduction cycle hinders the electrolyte reduction process, and consequently increases the specific charge loss via exfoliation of the graphite electrode. This was confirmed by TPD measurement where significant release of CO gas occured above 400 °C, suggesting the presence of high-thermal-stable surface oxygen-containing groups of different natures in the as-received - SLX50 sample. Finally, it was found that H₂ treatment avoids the formation of oxygen-containing groups during air contact leading to exfoliation of the graphite sample.     

A study of the properties of pyrolytic carbons deposited from propane in a tumbling and stationary bed between 900 and 1230°C

In order to obtain low-temperature-isotropic (LTI) pyrolytic carbons, a new “Tumbling Bed” reactor has been developed. The characteristics of the pyrolytic carbons deposited in this tumbling bed have been studied by varying the gas composition, the total gas flow rate, the weight of bed particles and the rotational speed (RPM) of the reaction tube in the temperature range of 900–1230°C. It was found that all pyrolytic carbons deposited were isotropic in the temperature range of 1050–1230°C. The density of the Isotropie carbon increased slightly with temperature, but it was independent of the other variables with values of 1.9–2.0 g/cm³. The apparent crystallite size, L_e, of the isotropic carbon was about 30 Å regardless of coating conditions. The deposition rate increased with temperature, propane concentration, and total flow rate, showed a minimum with increasing RPM of reaction tube, and decreased with the weight of bed particles. The deposition mechanism of the isotropic pyrolytic carbon was suggested from the results. Additionally, a few experiments were carried out in a stationary bed in order to study the role of the rotating action in the tumbling bed reactor. Columnar, sooty and filamentous carbons were obtained in the stationary bed. From scanning electron micrographs of fracture surfaces of the filamentous carbons, it appeared that they are constructed by spiral growth of carbon flakes.     

Electrical properties and structural characterizations of polyphenylquinoxaline pyrolyzed at high temperature

Insulating polyphenylquinoxaline (PPQ) was converted into an electrical conductor by pyrolysis at high temperature in nitrogen. Room temperature conductivity was measured as a function of pyrolytic conditions, and it was found that it strongly depends on the pyrolytic temperature and time. A maximum of room temperature conductivity about 177 S cm⁻¹ for PPQ film pyrolyzed at 1200°C for 1 h was obtained, which is 18 orders of magnitude greater than that of the original PPQ film. The current voltage (I–V) curve of pyrolyzed PPQ films follows Ohm's law characteristics of metals. Anisotropy in conductivity along and perpendicular to the surface of the film indicates the formation of a graphite-like structure in pyrolyzed PPQ films. The structure of the pyrolyzed PPQ films was investigated by elemental analysis, X-ray photoelectron spectroscopy spectra, X-ray diffraction, and scanning electron microscopic image. The electrical property and the structural characterizations suggest that the pyrolysis of PPQ films consists of two processes (i.e., carbonization and graphitization), and the critical temperature is at about 800°C. During carbonization (T_p < 800°C), some H, N, and O atoms are removed and the temperature dependence of conductivity of pyrolyzed PPQ film can be expressed by the three-dimensional Variable-Range Hopping(3-D VRH) model. During graphitization (T_p > 800°C), most H, N, and O atoms are removed from the residue, and a polyconjugated structure forms in it. The temperature dependence of conductivity deviates somewhat from the 3-D VRH model and can be fitted with a modified 3-D VRH model. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 123–128, 1998     

Insight into high areal capacitances of low apparent surface area carbons derived from nitrogen-rich polymers

The energy storage mechanism of N-doped carbons with low apparent specific surface areas (Brunauer–Emmett–Teller specific surface area determined by N₂ adsorption) has puzzled the researchers in the supercapacitor field in recent years. In order to explore this scientific problem, such carbon materials were prepared through pyrolysis of N-rich polymers such as melamine formaldehyde resin and polyaniline. Although these carbons possess low apparent specific surface areas of no more than 60 m² g⁻¹, their areal capacitance could reach up to an abnormally high value of 252 μF cm⁻². The results of systematical materials characterizations and electrochemical measurements show that these carbons contain numerous ultramicropores which could not be detected by the adsorbate of N₂ but are accessible to CO₂ and electrolyte ions. These ultramicropores play dominant roles in the charge storage process for these low apparent surface area carbons, leading to an energy storage mechanism of electric double layer capacitance. The contribution of pseudocapacitance to the total capacitance is calculated to be less than 15%. This finding challenges the widely accepted viewpoint that the high capacitance of N-doped carbon is mainly attributed to the pseudocapacitance generated from the faradic reactions between nitrogen functionalities and electrolyte.     

Double layer capacity of graphite in cryolite-alumina melts and surface area changes by electrolyte consumption of graphite and baked carbon

The double layer (d.l.) capacity of pyrolytic graphite in cryolite-alumina melts at 1010°C was found to exhibit a minimum of 20F cm^–2 at 0·9 V positive to the aluminium electrode. The d.l. capacity attained a plateau of 60F cm^–2 at 1·1–1·4 V, while it rose steeply at potentials below 0·7 V. During electrolytic consumption involving CO₂ evolution the d.l. capacity of pyrolytic graphite remained unaffected, while that of baked carbon was changed, reflecting changes in surface area. At low current densities (cds) the surface area increased substantially and the surface was noticeably roughened, while the opposite behaviour was observed at above 2.5 A cm^–2.     

Carbons of high surface area. A study by adsorption and high resolution electron microscopy

Five carbons of high surface area, ~ 2000–3000 m²g^?1 are studied by adsorption of carbon dioxide at 195 and 273 K. Effective surface areas are calculated using Langmuir and Dubinin- Radushkevich equations. Structure in these carbons is assessed by phase contrast high resolution transmission electron microscopy. The lamellae or constituent layers of these carbons are resolved as fringe images. Careful examination of thin sections of these carbons shows significant differences in separation distances of lamellae which indicate differences in the size and shape of the supermicroporosity which exists as the space between the lamellae. These differences correlate closely with the effective surface areas. The supermicroporosity consists of cage-like voids 1–5 nm dia., the cages being separated by walls of 1–3 carbonaceous layers in thickness. The filling of such supermicroporosity by a mechanism of increasing adsorption potential or cooperative adsorption adequately accounts for high internal volumes of up to 1.7cm³g^?1 and of effective surface areas of about 3000 m²g^?1. The size and shape of supermicroporosity can be deduced from micrographs.     

Development and Characterisation of Electrically Conductive Polymeric‐Based Blends for Proton Exchange Membrane Fuel Cell Bipolar Plates

The main objective of this work was to develop films with controlled dimensions for proton exchange membrane fuel cell (PEMFC) bipolar plates (BPPs) using the twin‐screw extrusion process. These films consisted of a low‐viscosity polyethylene terephthalate (PET) in which a mixture of high specific surface area carbon black (CB) and synthetic flake graphite (GR) were dispersed. A third conductive additive, consisting of silver‐coated glass particles (SCG) or multi‐walled carbon nanotubes (MWCNT), was also added at a low concentration (5 wt.‐%) in order to study its synergistic effect on the PET‐based blend electrical conductivity. As the developed blends had to meet properties suitable for PEMFC bipolar plate applications, they were characterised for their electrical through‐plane resistivity, mechanical properties and oxygen permeability. Through‐plane electrical resistivity of about 0.3 Ω·cm and oxygen permeation rate of 3.5 × 10^–8 cc cm^–2 s^–1 were obtained for only 30 wt.‐% of a 60:40 mixture of CB/GR conductive additives. Although the substitution of 5 wt.‐% of CB/GR by the same amount of MWCNT had no significant effect on BPPs' electrical resistivity, it helped to improve their mechanical properties and especially their oxygen permeation, which was decreased from 3.5 × 10^–8 cc cm^–2 s^–1 to around 0.6 × 10^–8 cc cm^–2 s^–1.     

The influence of nitrogen on the dielectric constant and surface hardness in diamond-like carbon (DLC) films

In this work a carbon target was sputtered by a methane/argon/nitrogen plasma in order to produce nitrogenated diamond-like carbon films (a-C:H:N). As the N₂ content in the sputtering gas was increased, the deposition rate increased markedly. Rutherford backscattering spectrometry (RBS) was used to investigate the chemical composition of the films. This nitrogen incorporation modifies the chemical bonding structure of the films, as shown by the analysis of the Raman spectra, including the occurrence of two extra peaks at approximately 2200 and 690 cm⁻¹. Electrical properties were measured through capacitance–voltage (C–V) curves. The hardness of the films decreased with the N content as shown by measurements performed by indentation method. A correlation among the Raman studies, the N content in the films, the dielectric constant and the surface hardness is presented.     

Controlling the surface topology and hence the hydrophobicity of amorphous carbon thin films

Amorphous carbon films with different surface topologies were synthesized on a Si substrate by plasma enhanced chemical vapor deposition at various pressures using acetylene as a carbon precursor. The samples were characterized by scanning electron microscopy, atomic force microscopy and Fourier transformed infrared spectroscopy. The contact angles for water with the as-prepared carbon films were measured and found to vary in the range 40-145^o. The surface energies of the as-prepared carbon films have been calculated using the Owens method in the hydrophilic region for the two liquids namely water and glycerol. It was found that with the decrease of polar component of the surface energy the surface gradually became hydrophobic. The contact angle was also found to be independent of the pH of water from extremely acidic to extremely basic.     

Effects of carbon surface area on performance of lithium sulfur battery cathodes

The effect of carbon surface area on capacity is investigated in cathodes for lithium sulfur batteries. Carbon additives help overcome the low electrical conductivity of sulfur. Cathodes were prepared at 30 wt% sulfur on different activated carbons having unloaded BET surface areas of 1200–3200 m²/g. Sulfur utilization ranged from 33% to 83% of the theoretical capacity (1672 mAh/g) with a strong correlation to the accessible pore volumes having pore widths between 1 and 5 nm. Additionally, cathodes prepared at 12.5–68 wt% on an activated carbon having unloaded BET surface area of 3200 m²/g showed excessive sulfur loading provided little additional capacity.     

Deposition rates during the early stages of pyrolytic carbon deposition in a hot-wall reactor and the development of texture

Pyrolytic carbon layers were deposited from methane/oxygen/argon mixtures on planar substrates (silicon wafers) at a total pressure of 100 kPa, a maximum gas residence time of 2 s and a temperature of 1100 °C. The depositions were performed in a hot-wall reactor with the substrate oriented parallel to the gas flow. Particular attention was paid to factors that influence the reproducibility of the deposited layers. Scanning and transmission electron microscopy were applied to study the thickness profiles and the texture of the carbon layers. The surface topography was investigated by atomic force microscopy. For pyrolytic carbon deposited without oxygen, an alteration from medium- to high-textured carbon is observed with increasing residence time. Islands are observed on the surface of the layer whose size increases with the texture. For pyrolytic carbon deposited with 3% oxygen, lower deposition rates were obtained and a strong modification of the texture is found compared to gas mixtures without oxygen.     

Thermal stability and surface modifications of detonation diamond nanoparticles studied with X-ray photoelectron spectroscopy

Detonation nanodiamond (ND) particles were dispersed on silicon nitride (SiN_x) coated sc-Si substrates by spin-coating technique. Their surface density was in the 10¹⁰–10¹¹ cm^?2 range. Thermal stability and surface modifications of ND particles were studied by combined use of X-ray Photoelectron Spectroscopy (XPS) and Field Emission Gun Scanning Electron Microscopy (FEG SEM). Different oxygen-containing functional groups could be identified by XPS and their evolution versus UHV annealing temperature (400–1085 °C) could be monitored in situ. The increase of annealing temperature led to a decrease of oxygen bound to carbon. In particular, functional groups where carbon was bound to oxygen via one σ bond (C–OH, C–O–C) started decomposing first. At 970 °C carbon–oxygen components decreased further. However, the sp²/sp³ carbon ratio did not increase, thus confirming that the graphitization of ND requires higher temperatures. XPS analyses also revealed that no interaction of ND particles with the silicon nitride substrate occurred at temperatures up to about 1000 °C. However, at 1050 °C silicon nitride coated substrates started showing patch-like damaged areas attributable to interaction of silicon nitride with the underlying substrate. Nevertheless ND particles were preserved in undamaged areas, with surface densities exceeding 10¹⁰ cm^?2. These nanoparticles acted as sp³-carbon seeds in a subsequent 15 min Chemical Vapour Deposition run that allowed growing a 60–80 nm diamond film. Our previous study on Si(100) showed that detonation ND particles reacted with silicon between 800 and 900 °C and, as a consequence, no diamond film could be grown after Chemical Vapour Deposition (CVD). These findings demonstrated that the use of a thin silicon nitride buffer layer is preferable insofar as the growth of thin diamond films on silicon devices via nanoseeding is concerned.     

The faradaic impedance of the carbon anode in cryolite-alumina melt

'The anode reaction on pyrolytic graphite, regular graphite and baked carbon in cryolite-alumina melts was studied by impedance measurements within the cd range 0.05–0.7 A cm⁻². The anode process was found to be reaction controlled on pyrolytic graphite at above i_a = 0.25 A cm⁻² and on regular graphite at above i_a = 0.7 A cm⁻². At lower cds a combination of reaction and diffusion control was observed, and for baked carbon this was the case over the entire cd range. The diminishing influence of diffusion control with increasing cd may be caused by suppression of the anode reaction within pres due to blocking by anode gas. The double layer capacity (C_dl) increased considerably with increasing cd on pyrolytic and regular graphites, while on baked carbon it remained uneffected above i_a = 0.1 A cm⁻². The increase in C_dl was attributed to enlargement of the anode surface area during electrolysis. The anodic overvoltage, porosity and reactivity were also investigated. For all tested anodes Tafel slopes of about 0.3 V dec⁻¹ were found. The overpotential increased with decreasing porosity and reactivity.