Alfred B. Anderson

Charge Transfer Equilibria Between Diamond and an Aqueous Oxygen Electrochemical Redox Couple

Undoped, high-quality diamond is, under almost all circumstances, one of the best insulators known. However, diamond covered with chemically bound hydrogen shows a pronounced conductivity when exposed to air. This conductivity arises from positive-charge carriers (holes) and is confined to a narrow near-surface region. Although several explanations have been proposed, none has received wide acceptance, and the mechanism remains controversial. Here, we report the interactions of hydrogen-terminated, macroscopic diamonds and diamond powders with aqueous solutions of controlled pH and oxygen concentration. We show that electrons transfer between the diamond and an electrochemical reduction/oxidation couple involving oxygen. This charge transfer is responsible for the surface conductivity and also influences contact angles and zeta potentials. The effect is not confined to diamond and may play a previously unrecognized role in other disparate systems.

Hydrogen and Oxygen Evolution on Boron‐Doped Diamond Electrodes

The evolution of hydrogen and oxygen was studied on diamond electrodes containing approximately 1021 boron atom/cm3. Voltammetry showed a wide potential window [−1.25 to +2.3 V vs. standard hydrogen electrode (SHE)] without significant water decomposition. This window was much narrower for poor quality diamond films with appreciable sp2 content. A redox couple observed at +1.7 V indicates oxidation of the diamond surface prior to oxygen evolution. The extent of surface oxidation increased with sp2 content. Anodic polarization made the diamond surface hydrophilic; x‐ray photoelectron spectroscopy showed an increase in oxygen coverage and the presence of carbon‐oxygen bonds. The estimated capacitance of the interface ranged from 0.05 μF/cm2 for high quality diamond to 5 μF/cm2 for low quality diamond. Preliminary measurements of the exchange current densities for oxygen and hydrogen evolution indicated slow kinetics compared to metals or highly oriented pyrolytic graphite.

Applications of Diamond Thin Films in Electrochemistry

Electrochemical reactions typically involve electron transfer between an electrode and a dissolved chemical species at a solid-electrode/liquid-electrolyte interface. Three broad classes of electrochemical applications may be identified: (1) synthesis (or destruction), in which an applied potential is used to bring about a desired chemical oxidation or reduction reaction; (2) analysis, in which the current/potential characteristics of an electrode are used to determine the type and concentration of a species; and (3) power generation. These broad types of applications require stable, conductive, chemically robust, and economical electrodes. Diamond electrodes, fabricated by chemical vapor deposition, provide electrochemists with an entirely new type of carbon electrode that meets these requirements for a wide range of applications. The first reports of electrochemical studies using diamond were in the mid-1980s. During the past several years, the field has attracted increasing attention. This review summarizes the electrochemical properties of diamond that make it a unique electrode material and that distinguish it from conventional carbon electrodes.

Density functional theory study of O2 electroreduction when bonded to a Pt dual site

Abstract The B3LYP hybrid density functional theory (DFT) was used to study the four-electron reduction mechanism of oxygen on platinum in aqueous acid electrolytes. The calculations indicate that, (a) O2 bonded to a dual site on Pt2 does not dissociate before the first electron transfer and the product for this step, OOH, easily dissociates with a small 0.06 eV activation barrier to form adsorbed O and OH; (b) the first electron transfer step has the highest activation barrier and hence it is rate determining, in agreement with the proposed kinetic model in the literature; (c) the electric field of the reacting solvated hydronium ion significantly increases the electron affinities of the species being reduced, indicating that a proton is involved in the rate determining first, as well as in the subsequent steps of the reduction; (d) the symmetry factor β for the first step is 0.5 (or less) in the high over-potential region and about 1.0 in the low overpotential region, in approximate agreement with Tafel plots in the literature; (e) the calculated activation energy at 1.23 V (SHE) for the first step is 0.6 eV, close to the experimental effective activation energy value of 0.44 eV on Pt(111) in H2SO4.

Catalytic Effect of Platinum on Oxygen Reduction An Ab Initio Model Including Electrode Potential Dependence

The effects of bonding to a platinum atom are calculated for the reduction of oxygen to water. The electron-correlation corrected MP2 method is used, and the electrode potential is modeled by variations in values for the electron affinities of the reaction centers. Potential-dependent transition state structures and activation energies are reported for the one-electron reactions Pt-O 2 + H + (aq) + e - (U) → Pt-OOH [i] Pt-OOH + H + (aq) + e - (U) → Pt-(OHOH) [ii] Pt-(OHOH) + H + (aq) + e - (U) → Pt-OH + H 2 O [iii] Pt-OH + H + (aq) + e - (U) → Pt-OH 2 [iv] This is the predicted lowest energy pathway. An alternative, where step (ii) is replaced by Pt-OOH + H + (aq) + e - (U) → Pt-O + H 2 O [v] is excluded by the high activation energy calculated for it, though reduction of Pt-O to Pt-OH Pt-O + H + (aq) + e - (U) → Pt-OH [vi] has a very low activation energy. Compared to uncatalyzed outer-Helmholtz-plane values, bonding to the Pt has the effect of decreasing the calculated high reduction activation energies for O 2 and H 2 O 2 . Bonding to Pt also decreases the HOO. and increases the HO- activation energy values. The reverse reaction, oxidation of H 2 O to O 2 , is also discussed in light of these results. The issues of potential-dependent double-layer potential drops and adsorbate bond polarizations are discussed, and it is pointed out that the results of this study can be used to estimate the effects of such potential drops.

Molecular orbital studies of dissociative chemisorption of first period diatomic molecules and ethylene on (100) W and Ni surfaces

The extended Huckel molecular orbital method is used to examine interactions of Li2, B2, C2, N2, CO, NO, O2, and F2 with nine atom clusters representing W(100) and Ni(100) crystal surfaces. The following predictions are made and are corroborated by experimental facts when available: (1) A strong tendency for charge transfer between the substrate and adsorbate exists as would be expected from electronegativity differences. (2) The adsorbed molecules display a tendency to dissociate because of Coulombic repulsions and frequently because of the filling of antibonding levels as well. At the same time Li2 and F2 form ionic bonds with the surface and covalent character is also evident for the others. (3) Strong bonding interactions form between the adsorbates and surfaces for any adsorbate orientation or position. Thus a connection is made with physical theories which exclude atomic detail. Such surface homogeneity can be resolved into detail with molecular orbital methods, but the emphasis in this paper is on ...

Systematic Theoretical Study of Alloys of Platinum for Enhanced Methanol Fuel Cell Performance

The ability of substitutional atoms in the (111)Pt surface to attract a water molecule and activate the formation of OH ads on them is calculated using the ASED-MO theory. OH ads is believed to be the oxidant that removes the CO poison from Pt anode surfaces in organic fuel cells. A total of 42 alloying atoms is treated, Sc through Se from period 4, Y through Te from period 5, and La through Po from period 6. As surface substitutional atoms, no elements to the right of the Pt group are found to attract H 2 O strongly enough to activate OH dissociation. Some of these elements, including Sn, are known to be active in the electrocatalytic oxidation of CO ads but are believed to be atoms or complexes on or near the Pt surface. Of the elements to the left of the Pt group, a number from the first and second transition series attract and activate H 2 O with comparable or greater effectiveness than Ru, a known activator when present on Pt electrode surfaces. Whether these can be made stable alloy surfaces for organic fuel cell operation is an experimental issue. Past experimental work in the literature suggests promise for some of them.

Structures, binding energies, and charge distributions for two to six atom Ti, Cr, Fe, and Ni clusters and their relationship to nucleation and cluster catalysis

In a low spin molecular orbital approximation, binding energies and optimized structures for various two to six atom clusters of Ti, Cr, Fe, and Ni are calculated. Ti, with few d electrons, shows a preference for tightly packed clusters with positively charged corner atoms while Ni favors open and ringlike clusters with negatively charged corner atoms; Cr and Fe, with nearer to half‐filled d shells, prefer tightly packed configurations, but charge distributions are less predictable. The parameterization and spin models used in the theoretical procedure are considered. The binding energies, structures, and charge distributions are discussed in relation to transition metal cluster catalysis and experimental Fe nucleation studies. The photoemission spectra for O and CO on a nine atom Fe (100) model cluster are calculated and are found to be similar, within calculational and experimental resolution, to those for the Fe(100) surface.

O2 reduction and CO oxidation at the Pt-electrolyte interface. The role of H2O and OH adsorption bond strengths

Abstract The nature of surface sites on which OH(ads) forms in acid and basic electrolytes is discussed based on the analysis of experimental results in the literature and the results of quantum calculations. Theoretical evidence is given for OH(ads) forming in acid solution on Pt surfaces by the reaction Pt  OH 2 ⋯ OH 2 ( OH ) 2 ↔ Pt  OH ⋯ H +  OH 2 ( OH 2 ) 2 + e − ( U ) ( i ) and an analysis of experimental results suggests that in base it forms by the reaction Pt  OH 2 ⋯ OH − ( HOH ) 2 ↔ Pt  OH ⋯ H  OH ( HOH ) 2 + e − ( U ) ( ii ) The reversible potential for reaction (i) is calculated to be 0.62 V, which is essentially the onset potential for OH(ads) formation in weak acid electrolytes. It is suggested that OH(ads) forms at potentials as low as ∼0.17 V in weak basic electrolyte by reaction (ii) where H 2 O molecules bond by lone-pair donation to unblocked Pt δ + sites of the hydrided electrode surface, and that as the potential is increased to the double layer region, beginning at 0.4 V, H 2 O no longer bonds with the surface to participate in this reaction. This behavior would explain the ∼0.3 V prewave in CO(ads) oxidation by OH(ads). At the potential of zero charge, ∼0.6 V, H 2 O again adsorbs and the reaction resumes. It is concluded based on experimental and theoretical evidence that four-electron O 2 reduction on Pt electrodes at low overpotentials requires a special site characterized by a relatively small ratio of OH to H 2 O adsorption bond strengths and or a higher activation energy for OH(ads) formation by reaction (i), as well as a weak ability to adsorb anions.

The influence of electrochemical potential on chemistry at electrode surfaces modeled by MO theory

Abstract When, on an electrode surface, the applied potential is changed, electrons flow into or out of the surface and some ions from the electrolyte may adsorb specifically while others remain in the diffuse double layer. An equilibrium is reached with the surface work function changed by an amount equal to the applied potential change, according to work function measurements on emersed electrodes with intact double layers. This suggests a band shift model is appropriate for quantum calculations of electrode phenomena. The use of the atom superposition and electron delocalization molecular orbital (ASED-MO) theory to calculate properties of isolated adsorbate molecules as functions of band shifts is demonstrated and the use of perturbation theory to interpret the calculational results is explained in this paper. For adsorbed CO and CN, calculated CO and PtC and CM− and AgC bond vibrational frequency dependencies on potential changes are found to agree with experiment in a qualitative if not quantitative way. The phenomenological Stark-effect approach and semiempirical as well as self-consistent field single determinant cluster-in-a-field approaches from other laboratories for the potential dependencies of adsorbed CO and CN− vibrational frequencies are discussed and evaluated. Additional results of the ASED-MO band shift theory are presented. These include the potential dependence of CO adsorption sites on Pt and Pd surfaces, activation of CC and C-H bonds in C2H2 on Pt and Fe electrodes, the dissolution of FeOH+ from Fe anodes, leading to passive film formation, and the anodic generation of O2 on SrFeO3.