High Area Carbon Research Articles

Structural Effects in Electrocatalysis: Formic Acid Oxidation at Model and Real Pt Catalysts

Formic acid oxidation was studied at low-index Pt single crystals (model systems) as well as at the platinum catalyst supported on high area carbon (real catalyst) in HClO4. The Pt single crystals were characterized by LEED. The LEED patterns obtained after a mild heating of flame-annealed crystals have shown clean, well ordered unreconstructured surfaces. Pt-C supported catalyst was analyzed by AFM and STM in air and by XRD. AFM and STM images revealed the presence of Pt-C agglomerates of several tenth of nm consisting of Pt particles ranged from 2 nm to 6 nm. The electrocatalytic activity of these catalysts in formic acid oxidation increased in a sequence: Pt(100) < Pt(110) < Pt-C/GC < Pt(111).

Electrogeneration of Hydrogen Peroxide in Acid Medium using Pyrolyzed Cobalt-based Catalysts: Influence of the Cobalt Content on the Electrode Performance

Cobalt tetramethoxyphenyl porphyrin (CoTMPP) adsorbed on a high area carbon support (Vulcan XC72-R) and heat-treated at 900 °C under inert atmosphere was studied as electrocatalyst for the reduction of O2 to H2O2 in acid medium. Experiments performed on rotating ring-disc electrode (RRDE) and gas diffusion electrode (GDE) show that the catalyst performance depends on the cobalt loading, going through a maximum at 0.2 wt. % Co. For higher cobalt loadings, a growing part of oxygen is reduced into water, decreasing therefore the selectivity of the catalyst. These results are interpreted in terms of a further reduction of H2O2 on Co-based catalytic sites before leaving the catalytic layer. For a GDE polarized at −150 mV vs. saturated calomel electrode (SCE) and loaded with 0.9 μg cm−2 of 0.2 wt. % Co-based catalyst, a H2O2 production rate of 300 μmol h−1 cm−2 was obtained which is five times higher than the H2O2 production rate measured with Vulcan. In these conditions, the selectivity of the Co-based catalyst for H2O2 production is 92%. The good agreement observed between RRDE and GDE results confirms the relevance of using RRDE experiment for screening these non-precious metal catalysts for further GDE applications.

Kinetics of sodium borohydride direct oxidation and oxygen reduction in sodium hydroxide electrolyte: Part I. BH 4− electro-oxidation on Au and Ag catalysts

The direct oxidation of sodium borohydride in concentrated sodium hydroxide medium has been studied by cyclic and linear voltammetry, chronoamperometry and chronopotentiometry for silver and gold electrocatalysts, either bulk and polycrystalline or nanodispersed over high area carbon blacks. Gold and silver yield rather complete utilisation of the reducer: around 7.5 electrons are delivered on these materials, versus 4 at the most for platinum as a result of the BH 4 − non-negligible hydrolysis taking place on this latter material. The kinetic parameters for the direct borohydride oxidation are better for gold than for silver. A strong influence of the ratio of sodium hydroxide versus sodium borohydride is found: whereas the theoretical stoichiometry does forecast that eight hydroxide ions are needed for each borohydride ion, our experimental results prove that a larger excess hydroxide ion is necessary in quasi-steady state conditions. When the above-mentioned ratio is unity (1 M NaOH and 1 M NaBH 4), the tetrahydroborate ions direct oxidation is limited by the hydroxide concentration, and their hydrolysis is no longer negligible. The hydrolysis products are probably BH 3OH − ions, for which gold displays a rather good oxidation activity. Additionally, silver, which is a weak BH 4 − oxidation electrocatalyst, exhibits the best activity of all the studied materials towards the BH 3OH − direct oxidation. Finally, carbon-supported gold nanoparticles seem promising as anode material to be used in direct borohydride fuel cells.

Enhancement of Nitrite Oxidation by Heat-Treated Cobalt Phthalocyanine Supported on High Area Carbon

Our ongoing effort is to develop efficient electrocatalysts and sensors for nitrite detection. Nitrite is one of the major components of wastewater from nuclear power production and involved in the corrosion and bacterial process known as the nitrogen cycle. It also plays important physiological roles in the form of NO, for example, as an intraand intercellular messenger, a neurotransmitter, and an immune system mediator. The detection of nitrite, therefore, is important from an environmental and biological point of view. We have been utilizing transition-metal (particularly Fe and Co) phthalocyanines and porphyrins for this purpose as they often display catalytic activities toward many important electrochemical reactions such as oxygen reduction and CO oxidation. We found that iron phthalocyanine (FePc) is a very effective catalyst for nitrite reduction, undergoing structural changes on the surface as a function of the redox state. Since the discovery by Jahnke et al. that the heat treatment of metal-N4 systems under an inert atmosphere can ensure both catalytic activity and stability, much effort has been poured to elucidate the nature of catalysis. Now there seems to be a consensus that different catalyst structures result depending on the heat-treatment temperature, thus leading to different catalytic activities. For low and medium pyrolysis temperatures, the metal-N4 moiety or its fragment of the macrocycles is responsible for the activity. With treatment at higher temperature, metal-N bonds are no longer found but rather metallic clusters surrounded by a graphite envelope are observed. In this paper, we have extended our previous research on nitrite reduction with FePc and its μ-oxo dimer, (FePc)2O to cover nitrite oxidation with CoPc this time. The changes in catalytic activity induced by heat-treatment at high temperatures (500-1000 C) have been correlated with structural aspects monitored by XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure).

Polycrystalline boron-doped diamond films as supports for methanol oxidation electrocatalysts

The high-area carbon supports that are often used for direct methanol fuel cell electrocatalysts (nanoparticles consisting of platinum, ruthenium and other metals) are being targeted for replacement with diamond in order to take advantage of the superior stability and chemical modifiability of diamond. As a preliminary study, we have examined the electrochemical behavior of boron-doped diamond (BDD) films in 0.5 M H 2SO 4 using cyclic voltammetry and have also sequentially electrodeposited platinum, and ruthenium by potential cycling in the aqueous solutions of the respective metals between 0.00 and 1.50 V vs. Ag/AgCl. The catalyst composites, i.e., Pt and Pt–Ru, deposited on BDD film substrates, were tested for methanol oxidation. The modified diamond surfaces were also characterized by SEM/EDS, XPS, and AFM.

Electrosorption of thiocyanate anions on active carbon felt electrode in dilute solution

Adsorption and electrosorption of thiocyanate (SCN −) anions on active carbon felt electrodes were measured by UV spectroscopy and cyclic voltammetry to provide more knowledge of the application of electrosorption on high-area carbon material in industrial waste water purification. Positive polarization caused increased adsorption of SCN −. SCN − showed the highest electrosorption capacity at about pH 3. Reversible electrosorption of SCN − to different extents was observed by changing the direction of polarization currents, which is possibly accompanied by the accessible surface area of C-felt being increased by positive polarization and electrochemical oxidation of SCN −. The enhanced capacity for adsorption by positive polarization will improve reuse of active carbon felt.

Kinetic study of formic acid oxidation on carbon-supported platinum electrocatalyst

Oxidation of formic acid on the platinum catalyst supported on high area carbon was investigated by potentiodynamic and quasi-steady-state polarization measurements. It was found that the poisoning of the reaction occurred both in the hydrogen region and in the double-layer region, but poisons were formed faster at lower potentials. Kinetics of the reaction was consistent with the dual path mechanism. At lower potentials HCOOH was oxidized to CO 2 at the Pt sites uncovered by CO ads. If high coverage by the poisoning species was attained, the reaction reached the limiting current plateau and further increase of the current densities started at the potential of CO ads oxidation. Kinetic parameters of the HCOOH oxidation suggested that the rate determining step was the transfer of the first electron from HCOOH ads, which was adsorbed under the Temkin conditions. Oxidation of formic acid became pH-dependent reaction in the electrolytes of pH < 1 with the reaction order with respect to H + ions of about −0.8.

Adsorption of benzoic acid onto high specific area activated carbon cloth

The adsorption of benzoic acid from aqueous solution onto high area carbon cloth at different pH values has been studied. Over a period of 125 min the adsorption process was found to follow a first-order kinetics and the rate constants were determined for the adsorption of benzoic acid at pH 2.0, 3.7, 5.3, 9.1, and 11.0. The extents of adsorption and the percentage coverage of carbon cloth surfaces were calculated at 125 min of adsorption. Adsorption isotherms at pH values of 2.0, 3.7, and 11.0 were derived at 25 °C. Isotherm data were treated according to Langmuir and Freundlich equations and the parameters of these equations were evaluated by regression analysis. The fit of experimental isotherm data to both equations was good. It was found that both the adsorption rate and the extent of adsorption at 125 min were the highest at pH 3.7 and decreased at higher or lower pH values. The types of interactions governing in the adsorption processes are discussed considering the surface charge and the dissociation of benzoic acid at different pH values.

Highly Active Nonplatinum Catalyst for Air Cathodes

Nonplatinum electrocatalysts for air cathodes showing higher activity than the conventional Pt/C catalysts were prepared by heat-treating cobalt hexacyanoferrate precursors dispersed on carbon support under an inert atmosphere. The activity for oxygen reduction was examined by polarization measurements with a gas-diffusion electrode floating on the surface of an electrolyte under an air atmosphere at room temperature. The electrolyte used was a neutral phosphate buffer solution. To investigate the effect of carbon support on the catalytic activity, different types of carbon supports were used with and without pretreatment with nitric acid prior to dispersion of the inorganic precursor. Among several carbon supports examined, the high-area carbon pretreated with nitric acid at higher concentrations than 8 M led to an enhanced performance for oxygen reduction in the gas-diffusion electrode. The effect of Co/Fe ratio in the precursor on the catalytic activity was also investigated. The catalyst prepared by heat-treating the inorganic precursor with a Co/Fe ratio of 1.2 dispersed on the high-area carbon showed the highest activity. Concerning the polarization voltage for oxygen reduction with the floating electrode, the best catalyst prepared in this study exhibited ca. a 200 mV higher potential at 100 mA cm−2 than the carbon-supported Pt-black catalysts. The remarkable enhancement in the performance of the air cathodes seems to be due to high dispersion on the high-area carbon, leading to effective utilization of the catalyst in addition to its intrinsic activity. © 2004 The Electrochemical Society. All rights reserved.

Enhanced Catalytic Activity of Non-platinum Catalyst for Oxygen Reduction in Alkaline Solution

Abstract Remarkable enhancement in catalytic activity of a non-platinum catalyst for oxygen reduction in alkaline solutions has been achieved by an oxidative pretreatment of a high-area carbon support before dispersion of catalyst precursor (cobalt hexacyanoferrate), followed by pyrolyzing the precursor under an inert atmosphere. The polarization potential for oxygen reduction at substantial current densities at room temperature under air was higher than a potential corresponding to a limit of relative predominance for O2/HO2− couple.

Mass transfer effect in electrochemical oxidation of methanol at platinum electrocatalysts

The influence of the mass transfer on the electrochemical oxidation of methanol was investigated at a rotating disk electrode with smooth polycrystalline Pt and high area carbon supported Pt (Pt/C) as the electrocatalysts. It was found that the reaction rate at the smooth Pt is suppressed significantly by the convection of the electrolyte. The Tafel slope and the reaction order with respect to methanol were also influenced by the convection of the electrolyte. On the contrary, mass transfer influenced neither the reaction rate nor the kinetic parameters of methanol oxidation at the Pt/C electrocatalyst. It was proposed that a reaction path via formaldehyde as a soluble intermediate is operative at both electrocatalysts, but with hindered diffusion of formaldehyde from the corrugated surface of the Pt/C.

Studies on removal of metribuzin, bromacil, 2,4- d and atrazine from water by adsorption on high area carbon cloth

The removal of pesticides such as metribuzin, bromacil, 2,4- d and atrazine from aqueous solutions was studied by adsorption on high area carbon cloth. The adsorption process was followed by in situ UV-spectrophotometric technique in a specially designed adsorption cell. Spectroscopic data of the pesticides were determined in separate experiments. The extent of adsorption was quantified by calculating the amount of adsorbate adsorbed per unit area of the carbon cloth and the percentage coverage at the carbon-cloth surface. The order of extent of adsorption of the pesticides studied was found as metribuzin < 2,4- d < bromacil < atrazine. The adsorption process was found to follow a first-order kinetics and the rate constants were determined. The competitive adsorption of pesticides was also examined by carrying out the adsorption process from a solution of equimolar mixture of bromacil and metribuzin.

Double-layer and pseudocapacitance types of electrochemical capacitors and their applications to the development of hybrid devices

The basis of the complementary use of electrochemical capacitors (so-called supercapacitors) in hybrid electric power generation by rechargeable batteries and fuel cells is explored. Electrochemical capacitors are of two types: one where the interfacial double-layer capacitance of high specific area carbon materials is the basis of electric charge storage (as ions and electrons); and the other where pseudocapacitance, associated with electrosorption and surface redox processes at high-area electrode materials, e.g. RuO2, or at conducting polymers, provides the basis of charge storage. The former, double-layer, type of capacitance stores charge non-faradaically while the latter type, pseudocapacitance, stores charge indirectly through faradaic chemical processes but its electrical behaviour is like that of a capacitor. Two types of hybrid battery/capacitor system are recognized: one based on combination of an electrochemical capacitor cell with a rechargeable battery or a fuel cell in a load-leveling function, e.g. in an electric vehicle power train; and the other based on combination of a faradaic battery-type electrode coupled internally with a capacitative electrode in a two-electrode hybrid module (termed an asymmetric capacitor). Optimization of operation of such systems in terms of balancing of active masses, of power and charge densities, and choice of maximum but limited states-of-discharge, is treated.

In situ Fe K-edge X-ray absorption fine structure of a nitrosyl adduct of iron phthalocyanine irreversibly adsorbed on a high area carbon electrode in an acidic electrolyte.

Key aspects of the microenvironment surrounding the Fe center in the nitrosyl adduct of iron phthalocyanine, [Fe(Pc)(NO)], have been elucidated from the analysis of the Fe K-edge extended X-ray absorption fine structure (EXAFS) of the material adsorbed on the surface of a high area carbon electrode recorded in situ, in 0.5 M H(2)SO(4). Statistical best fits to the EXAFS data place the Fe center in a five-coordinated square pyramidal configuration shifted away from the Pc plane toward the axially bound NO bent at an angle of ca. 40 degrees with respect to the normal to the Pc plane. This environment is analogous to that of Fe in the nitrosyl adduct of crystalline [Fe(TPP)], where TPP = meso-tetraphenylporphyrinato(2-), determined from X-ray diffraction.

Adsorptive and electrosorptive removal of aniline and bipyridyls from waste-waters

Removal of aniline, 2,2′-bipyridyl, 4,4′-bipyridyl and their protonated cations from dilute aqueous solutions, simulating polluted waste-waters, by adsorption and electrosorption at high-area carbon cloth (C-cloth) electrodes is described. The adsorption behaviour and kinetic parameters for rates of adsorption of these solutes at high-area porous-C electrodes were followed and evaluated in situ by means of UV spectrophotometry. The results obtained show that the above neutral adsorbates have stronger affinity over their protonated cations for the surface of C-cloth electrodes. Polarization of the C-substrate electrodes leads to a great increase in both rates and extents of adsorption for each of the adsorbates investigated but complete adsorptive removal of aniline and 2,2′-bipyridyl was attained only under certain conditions. The extents of adsorptive removal of the adsorbates and the rate-constants for such processes are summarized quantitatively. Polarization electrosorption under slow alternating positive/negative conditions or combination of open-circuit adsorption with polarizing electrosorption procedures dramatically enhanced the extents of adsorption in most cases, leading to complete removal of the adsorbate from solution in each of the cases of aniline, 2,2′-bipyridyl, 4,4′-bipyridyl and their singly protonated cations under various conditions. The latter observation is valuable for the development of methodologies for purification of industrial waste-waters.

Investigation of Removal of Cr(VI), Mo(VI), W(VI), V(IV), and V(V) Oxy-ions from Industrial Waste-Waters by Adsorption and Electrosorption at High-Area Carbon Cloth

Adsorption and electrosorption of Cr(VI), Mo(VI), W(VI), V(IV), and V(V) ions from water samples at low concentration were studied at high-area C-cloth electrodes. The concentrations of ions in the solution were monitored using in situ UV spectroscopy. All the investigated ions, except V(IV), showed better adsorption in acidic media. Positive polarization of the C-cloth caused increased adsorption of Cr(VI), Mo(VI), and V(V) ions. When previously adsorbed, Mo(VI) and V(V) ions were shown to be largely desorbable by negative polarization of the C-cloth. Since V(IV) does not become adsorbed significantly at the C-cloth in acidic media, the method provides an interesting means for separation of V(V) and V(IV) species in solution.

Development of techniques for purification of waste waters: removal of pyridine from aqueous solution by adsorption at high-area C-cloth electrodes using in situ optical spectrometry

The adsorption and electrosorption/desorption behavior of pyridine (Py) was studied for potential applications to the purification of industrial waste-waters. The use of high-area carbon cloth (C-cloth) electrodes as quasi-3-dimensional interfaces, coupled with in situ UV–Vis spectrophotometric techniques and scanning kinetics for quantitatively monitoring the adsorption/desorption processes and simultaneously obtaining kinetic parameters are described. The conversion of Py to PyH +, which decreases the removal efficiency, takes place in non-buffered electrolyte solution due to pH changes but is eliminated in a HOAc+NaOAc buffer (pH 6.8) solution. Rapid and complete removal of Py is then achieved by anodic polarization. Under controlled potential, Py removal rates are lower than those for galvanostatic polarization where changes of potential arise due to double-layer charging.

Removal of phenol, phenoxide and chlorophenols from waste-waters by adsorption and electrosorption at high-area carbon felt electrodes

The adsorption and electrosorption behavior of phenol, sodium phenoxide, 1-, 2- and 3-chlorophenol and 2,6-dichlorophenol were studied at high-area carbon-felt (C-felt) electrodes by in situ UV spectroscopy in relation to development of procedures for purification of industrial waste-waters. Spectral characteristics and calibration parameters were evaluated for the phenolic species studied. It was demonstrated that the initial concentrations (about 1 mM) of these compounds can be reduced by a factor of 3 or more from waste-waters by adsorption at the C-felt already at a level of small percentage coverage on the surface (<10%). It was also found that the rate of adsorption of these compounds can be enhanced by polarizing the C-felt electrode, especially in the presence of an inert supporting electrolyte such as Na 2SO 4. Suppression of hydrolysis of phenoxide in the presence of NaOH, associated with ionization of phenol, diminishes the adsorption of phenoxide probably due to its hydration, causing it to be more hydrophilic than phenol. However, in situ analysis of aqueous phenoxide solution, without NaOH, as a mixture of phenoxide anion and phenol, the hydrolysis product, is made possible by the experimental setup and the software used, and shows that both species were adsorbed and electrosorbed at the C-felt. Among the three monochlorophenols, 4-chlorophenol exhibited the lowest rate of adsorption with 2-chlorophenol having the highest according to a kinetically first-order treatment of the adsorption data, while 2,6-dichlorophenol was adsorbed at an intermediate rate.

Methanol oxidation on an ink type electrode using Pt supported on high area carbons

Methanol oxidation was investigated on the following Pt catalysts: bare Pt electrode, Pt covered by Nafion® film, and Pt catalysts supported on high area carbons in the form of an ink. The reaction was carried out using slow linear potential sweep and potentiostatic steps. Active surface area of Pt particles was estimated from hydrogen desorption charge on cyclic voltammograms. The influence of Nafion film, Pt loading, type of carbon support, and methanol concentration were studied. No influence of Nafion® film on kinetic parameters of methanol oxidation was found. Tafel slope on smooth Pt was found to be about 90 mV dec−1, while on supported Pt catalysts time dependent values between 140 and 90 mV dec−1 were estimated from the potentiostatic decays by using data between 0.5 and 5 min. Specific activity was slightly dependent on Pt loading with flat maximum at about 20% Pt. Activity of Pt/SAB catalyst was two to three times higher than that of Pt/BP and Pt/XC-72 catalysts.

Iron(III) tetramethoxyphenylporphyrin (FeTMPP-C1) as electrocatalyst for oxygen reduction in direct methanol fuel cells

Iron(III) tetramethoxyphenylporphyrins (FeTMPP-C1) adsorbed on high-area carbons were heat treated in an inert atmosphere at various temperatures ranging from 200–1000 °C to produce catalyst for the electroreduction of oxygen in acid electrolytes. It was found that the specific surface area of the FeTMPP-C1/C catalysts linearly decreases with increase of the FeTMPP-C1 loading on RB carbon. The electrical resistivity of the catalyst decreases with the increase of heat treatment temperature in the range of 200 to 800 °C, then it increases beyond 800 °C. The results obtained with elemental analysis, FTIR spectroscopy and XPS techniques indicate that the onset temperature for partial decomposition for the FeTMPP chelate occurs at temperatures of about 400–500 °C. The surface concentrations of both iron and nitrogen on the carbon support increase as the heat treatment temperature increases, and the maximum occurs at 700 °C. Some possibilities about the nature of active sites in the catalyst are discussed.