Kinetics of Activation Controlled Consecutive Electrochemical Reactions: Anodic Evolution of Oxygen

The kinetics of activation controlled electrode reactions differ from those of chemical kinetics by the potential dependence of the rate constant of a step involving charge transfer and the constancy of concentration of reactants in the initial state. A method is proposed whereby an expression for the kinetics of an electrode reaction containing any number of intermediate steps may be obtained. The method is applied to five paths for the electrolytic evolution of oxygen at anodes and yields limiting expressions for the dependence of the current density (or rate) of the reaction upon electrode potential which are diagnostic of the various rate controlling steps assumed. If discharge of OH′ or H2O molecules is rate determining, the Tafel slope is 2RT/F at overpotentials > 50 mv. If some other reaction involving charge transfer is rate determining, two possible Tafel slopes, each independent of potential in its range of applicability, occur, depending upon degree of coverage of the electrode with intermediate products. If a reaction involving no charge transfer is rate determining, a number of Tafel slopes is indicated, each characteristic of the rate controlling reaction, and an activation controlled limiting current density is possible. Quantitative conditions for change of mechanism with current density are evaluated. For the initial reaction of a consecutive series to be rate determining it suffices that the rate constant for the backward partial reaction of this step be limitingly small compared with other rate constants in the series; for the last reaction to be rate controlling, the rate constant of its forward partial reaction must be limitingly small; for any other reaction in the series, both backward and forward rate constants of the rate determining reaction must be limitingly small. The rate of the anodic oxygen evolution reaction as a function of pH is evaluated, taking into account concentration and foreign electrolyte effects on the electrokinetic potential at the electrode-solution interface. The results enable the discharging entity in solution to be distinguished. Former data are reinterpreted from the criteria of mechanism deduced.