D.B. Sepa

Reaction intermediates as a controlling factor in the kinetics and mechanism of oxygen reduction at platinum electrodes

A rotating platinum disk electrode was used to study oxygen reduction over the entire pH range and to compare the kinetics in acid and alkaline solutions. In both solutions two Tafel regions are confirmed. At low cds ∂V/∂ln i = −RT/F and at high cds it is −2RT/F. In the low cd region, the reaction order with respect to H3O+ is 32 in acid, and 12 in alkaline solutions. For the high cd region the reaction orders are 1 and 0 in acid and alkaline solutions, respectively. Transition of the kinetics from the low to the high cd region, as well as the fractional reaction orders at low cds, are interpreted in terms of the first charge transfer step as rate determining in both cd regions but under Temkin adsorption conditions in the low, and Lagmuirian conditions in the high cd region. Coverages, θ, with oxygen intermediates are also determined as a function of potential and pH. At all pHs, substantial coverages are observed only at potentials in the low cd region. The usual Tafel slope of −120 mV for the first charge transfer step as rate determining is not observed at low cds since it is modified by the dependence of activation energy on θ, and hence on V. At high cd, θ is low and ineffective and the usual slope for the first charge transfer step is observed. At all pHs the transition from low to high cds occurs at potentials, VT, that decrease 60 mV as pH increases one unit. Dependence of VT and iT on pH is discussed. Fractional reaction orders at low cds are also related to θ. Real reaction orders with respect to H3O+ are modified by the dependence of activation energy on θ, and hence on pH. The change of the kinetics from the acid to the alkaline region is discussed. The range of cds accessible to measurements is limited at both high and low cds. At high cds it is limited by the supply of oxygen and/or H3O+ from the solutions. At low cds, V—log i relationship extends only to a rest potential which decreases 60 mV as pH increases one unit. The limits of measurements and the nature of the rest potential are discussed.

Different views regarding the kinetics and mechanisms of oxygen reduction at Pt and Pd electrodes

Abstract Two diametrically different pH dependences have been reported for oxygen reduction at prereduced Pt electrodes. In the current density (cd) region with the Tafel slope of −2.3 RT/F , the pH dependence expressed as d E /dpH is, according to one group of workers, −90 mV, ie −3 × 2.3 RT /(2 F ) and − 30 mV, ie −2.3 RT /(2 F ) in acid and alkaline solutions, respectively. The same dependences, according to another group of workers are −60 mV, ie −2.3 RT/F in both acid and alkaline solutions. In the cd region with the Tafel slope of −2 × 2.3 RT/F , d E /dpH according to the first group of workers is − 120 mV and 0 mV in acid and alkaline solutions, respectively. However, according to the second group, this dependence is −60 mV in both acid and alkaline solutions. The reasons for the variation in pH dependences is discussed. A previous analysis for the pH dependence of − 2.3 RT/F at all pHs in both cd regions is in error. An account of the pH dependences of −3 × 2.3 RT /(2 F ) and −2.3 RT /(2 F ) in terms of Temkin conditions of adsorption is satisfactory and allows for the change of Tafel slopes and pH dependences with increasing cd.

Kinetics and mechanism of O2 reduction at Pt IN alkaline solutions

Abstract A rotating disk electrode was used to examine the steady state kinetics of O 2 reduction at oxide free Pt in alkaline solutions of various pHs. Two linear V -log i regions are observed. At low current densities, the ∂V/∂ log i slope is close to −60 mV; at high cd s it is close to −120 mV. In the low cd region, the reaction order with respect to OH − is −1/2. In the high cd region, the reaction order is zero. In both regions the order with respect to O 2 is 1. The “unusual”, fractional reaction order of −1/2 cannot be accounted for in terms of simple electrode kinetics with Langmuirian conditions. It is suggested that in both regions the first electrochemical step is rate determining. In the low cd region, due to intermediate coverages with adsorbed oxygen species, Temkin conditions prevail and lead to the ∂ V /∂ log i slope of −60 mV and the “unusual” reaction order. At high cd s, the coverage is low and the “usual” kinetics with ∂ V /∂ log i = −120 mV and zero reaction order with respect to OH − follow.

Apparent enthalpies of activation of electrodic oxygen reduction at platinum in different current density regions—I. Acid solution

Oxygen reducton (OR) at Pt electrodes in aqueous HClO4 solution (pH = 1.9) is examined at temperature from 5 to 45°C. At all temperatures, two distinct Elogi regions are observed. At low current densities (cd), the Tafel slopes are close to −2.3 RT/F, while at high cds they are close to −2 × 2.3 RT/F. Though the apparent enthalpy of activation at the reversible potential in the low cd region (=60 kJ mole−1) is higher than the apparent enthalpy of activation in the high cd region (=43 kJ mol−1), it is shown that the actual enthalpies of activation, ie the enthalpies at †φ = 0, are the same in both cd regions. The rate determining step of the reaction is, therefore, the same in both cd regions and the observed changes in the Tafel slopes arise from different conditions of adsorption of reaction intermediates in these regions, as previously suggested.

Palladium electrode in oxygen-saturated aqueous solutions: potential dependent adsorption of oxygen containing species and their effect on oxygen reduction

Adsorption of oxygen species at palladium electrodes at various potentials, E, was determined by a programmed potentiodynamic method in solutions of different pH. In all solutions examined, a linear relationship between E and cover-age θ, with oxygen species was established in the range 0.1

Invariance with pH of enthalpies of activation for O2 reduction at Pt electrodes in acid solutions

The apparent enthalpies of activation, ΔH*a, for O2 reduction at Pt electrodes are determined in acid solutions of different pH. In the high current density region, characterized by a Tafel slope close to − 120 mV, ΔH*a,h is lower than ΔH*a,l in the low current density region characterized by a Tafel slope close to − 60 mV. Although ΔH*a,l > ΔH*a,h it is concluded that at every pH the enthalpies of activation at the zero Galvani potential difference are the same in both current density regions. Therefore, irrespective of the different Tafel slopes, the same mechanism is operative in both regions. In all solutions, ΔH*a for either current density region are independent of pH when they are determined at constant potentials vs rhe. This invariance with pH is unexpected in view of the suggested variance of the Galvani potential difference with pH. Alternative causes for the invariance are discussed.

Symmetry factor and transfer coefficient in analysis of enthalpies of activation and mechanisms of oxygen reduction at platinum electrodes

Abstract Oxygen reduction at Pt electrodes is characterized by two linear E -log i regions. Tafel slopes of - RT /β F in the high current density ( cd ) region and of - RT /α F in the low cd region increase with temperature as predicted for the temperature independent symmetry factor β(=1/2) and transfer coefficient α(=1). Enthalpies of activation based on these data support the conclusion that in both cd regions the first electron transfer step is rate determining irrespective of the difference in the Tafel slopes.

Kinetics of cementation of noble metals on nickel

Abstract A special technique was developed to observe the phenomena occurring at very short time intervals after immersion of an electrode into the solution of more electropositive ions. A gun-like arrangement driven by gunpowder gave the electrode a considerable speed and made the immersion complete in less than 50 μs. The change of potential of a nickel electrode with time of immersion into a solution of silver ions was recorded oscilloscopically. Two distinct regions were observed: at very short time intervals (up to 1 ms) a smooth shift of potential towards more positive values, and afterwards, sporadic fluctuations of potential. A theoretical explanation is given for the entire course of the curve. The rates of deposition of silver and dissolution of nickel as well as the concentration of silver adatoms is calculated from the initial part of the curves. The irregular behaviour in the second part is ascribed to the nucleation of silver grains.