Ernest Yeager

Electrocatalysts for O2 reduction

The proposed mechanisms for O2 reduction on various catalysts are discussed, taking into account the possible role of superoxide radicals, hydogen peroxide and adsorbed dioxygen. Particular attention is focused on O2 reduction on carbon and graphite (both with and without surface modifications), the anomalous temperature dependence of the Tafel slope for O2 reduction on Pt, transition metal macrocycles and heat treated macrocycles. The latter offer considerable promise as O2 reduction catalysts combining high activity with good stability.

Heat-treated polyacrylonitrile-based catalysts for oxygen electroreduction

Polyacrylonitrile (PAN), mixed with Co(II) or Fe(II) salts and high-area carbon and then heat treated, has been found to yield very promising catalysts for O2 reduction in concentrated alkaline and acid solutions. The catalytic activities are comparable to those for the heat-treated corresponding transition metal macrocycles and polypyrrole black-based catalysts. The addition of the transition metal to the nitrogen-containing polymer, either before or after the heat treatment with carbon, is an important factor for good activity. The nitrile nitrogen of the PAN is probably retained and converted to pyridyl nitrogen during the heat treatment, and this nitrogen is believed to provide binding sites for the transition metal ions, which then act as catalytic sites for oxygen reduction to peroxide and its decomposition.

Identification of Surface Films Formed on Lithium in Propylene Carbonate Solutions

FTIR, IR, and XPS have been used to study the films formed on lithium in propylene carbonate solutions of ,, and . Over a range of conditions, the main components detected in the initial surface films were lithium alkyl carbonates . Another alkyl carbonate solvent, diethyl carbonate, was found to react with lithium to form lithium ethyl carbonate, . In addition to solvent reduction, XPS measurements gave indication of salt reduction reactions. , , and were reduced by lithium to form halide ions, which were detected on the lithium surface. Two possible mechanisms for the formation of alkyl carbonates are discussed. One is the nucleophilic reaction of propylene carbonate with basic species such as OH−, while the other involves one‐electron reduction of propylene carbonate by lithium metal, followed by free radical termination reactions. When high concentrations of water were present, lithium carbonate was formed by further reaction of the alkyl carbonates with water. On lithium surfaces without a mechanically stable surface film, such as those of lithium/mercury amalgams, the reduction reaction is believed to proceed by an overall two‐electron process, and the primary product is lithium carbonate.

Structural effects in electrocatalysis: oxygen reduction on platinum low index single-crystal surfaces in perchloric acid solutions

Oxygen reduction has been studied in 0.1 M HClO4 on the platinum low index surfaces, employing a hanging meniscus rotating-disk technique. A special cleaning procedure has been developed to remove impurities from the surface prior to the oxygen reduction measurements. Oxygen reduction was found to be sensitive to the crystallographic orientation of the platinum electrode surface. The activity for oxygen reduction discerned from the half-wave potential decreases in the sequence (110) & >; (111) & >; (100). The reaction proceeds on all well-ordered low index planes with exchange of four electrons per O2 molecule. Tafel slopes of −120 mV per decade at high current densities and −60 mV / per decade at low current densities were found for all three planes at room temperature. Oxygen-containing species chemisorbed on platinum appear to be the cause of the change in the Tafel slope.

The electrochemistry of noble metal electrodes in aprotic organic solvents containing lithium salts

Abstract The electrochemical behavior of various non-aqueous organic electrolyte systems has been investigated using inert metal electrodes. The systems studied included propylene carbonate, dimethoxyethane and tetrahydrofuran solutions of LiClO 4 , LiAsF 6 , LiSO 3 CF 3 and Bu 4 NClO 4 . The electrode metals included polycrystalline gold and silver. Various techniques including cyclic voltammetry, FTIR and XPS were used to characterize the main electrochemical reactions that occur in these systems. Several separate film forming processes have been identified, including reduction of solvent, the salt, and traces of oxygen and water. The surface films formed in these processes lead to the apparent stability of these systems at low potentials. Li UPD was also examined and was found to be controlled by the nature of the surface films through which lithium is deposited.

A Mechanistic Study of O2 Reduction on Water Soluble Phthalocyanines Adsorbed on Graphite Electrodes.

Abstract : Co and Fe tetrasulfonate phthalocyanines (M-TSP), adsorbed at monolayer levels on graphite surfaces, have been found to have a pronounced catalytic effect on the O2 reduction process in both acid and alkaline solutions. The kinetics have been examined with the rotating ring-disk electrode technique. Co-TSP promotes the O2 reduction process via 2-electrons to give peroxide whereas Fe-TSP promotes a 4-electron reduction to give water. (Author)

Electrochemical oxidation of glucose on single crystal gold surfaces

The electrooxidation of glucose has been studied extensively because of the interest in the development of the glucose sensor [l] and the glucose-oxygen fuel cell [2] for cardiac pacemakers and artifical hearts. The understanding of the reaction kinetics is far from complete. It has been established, however, that the oxidation of glucose is a typical electrocatalytic reaction whose kinetics depend on the nature of the electrode material [3,4]. Platinum, as the electrode material, has been the subject of most of the studies, although gold displays a higher activity in neutral and alkaline electrolytes [4]. The pronounced poisoning of the platinum electrode by some intermediates and/or products apparently suppresses its intrinsic activity for the initial stages of the reactions. The crystal plane dependence of the electrooxidation of small organic molecules with similar poisoning effects is now well documented [5,6]. Therefore, the oxidation of glucose is expected also to be structure sensitive. Kokkinidis et al. [7] indeed have found different catalytic activities for glucose oxidation on the low-index platinum surfaces in acid solution. The Pt (111) surface has a higher activity than the other two low-index planes as a consequence of a lower self generated poison effect on this surface. The present research has indicated that gold appears to be a more active electrocatalyst for glucose oxidation in neutral and alkaline electrolytes, which makes it an attractive choice for sensor application. This calls for a study of the glucose oxidation on single crystal gold electrodes in order to determine which

Kinetic studies of the oxygen—peroxide couple on pyrolytic graphite☆

The cathodic and anodic properties of the oxygen—peroxide couple have been studied on ordinary pyrolytic graphite, high-pressure annealed pyrolytic graphite and single crystal graphite in alkaline solutions using the rotating disk technique. Both the reduction of O2 and oxidation of peroxide are inhibited on the cleavage surface of the oriented graphite relative to the edge surface. This difference in behaviour is attributed to the lack of suitable adsorption sites on the cleavage surface for the reacting species and/or intermediates. The current/potential data, reaction orders, and stoichiometric number support the following mechanism: O2 → O2(ads); O2(ads) + HOH + e → HO2(ads) + OH−; 2HO2(ads) + OH− → HO2− + O2 + HOH, with the first and second steps rate-controlling for the cathodic reduction.

Differential capacitance study on the basal plane of stress-annealed pyrolytic graphite

Summary The non-faradaic differential electrode capacity of the basal plane of high-pressure stress-annealed pyrolytic graphite has been examined in aqueous solution using an a.c. impedance bridge. The capacity has a near parabolic dependence on electrode potential with a minimum of ∼3 μF cm−2 in concentrated electrolytes and is pH independent. Studies in NaF solutions at concentrations in the range 0.9–10−5 M indicate that in the range of potential studied (+0.5 to −0.5 V vs. NHE) the minimum capacity is best attributed to the space charge region within the graphite and not to the diffuse ionic layer. Apparently the potential of zero charge of the electrolyte is outside the potential range of the measurements. The capacity value on the basal plane of the stress-annealed pyrolytic graphite compares favorably with that estimated from the carrier concentration using the presently available theory of the space charge layer in semiconductor electrodes.

Differential Capacitance Study of Stress‐Annealed Pyrolytic Graphite Electrodes

Abstract : The non-faradaic capacity of electrodes prepared from high pressure stress-annealed pyrolytic graphite has been examined in aqueous solutions using an a.c. impedance bridge. SUCH MATERIALS HAVE A ROCKING ANGLE (x-ray diffraction) as small as 0.4 degrees and the properties of surfaces oriented parallel to the basal plane approach rather closely those of single crystal graphite. The differential capacity measured on this surface has a near parabolic dependence on electrode potential with no evidence of a hump and a minimum of about 3 microfarads/sq. cm in 0.9 M NaF. This low value is explained on the basis that a substantial fraction of the potential change between the electrode bulk and the solution bulk occurs across a space charge layer within the graphite.