Kinetic theory of the open-circuit potential decay method for evaluation of behaviour of adsorbed intermediates: Analysis for the case of the H 2 evolution reaction

Abstract

In recent work we have shown how digital acquisition of open-circuit potential-decay transients, coupled with computer processing of the data, can lead to interesting information on the coverage behaviour of the “overpotential-deposited” intermediates that are involved in the kinetics of multistep Faradaic reactions proceeding at appreciable net current.s A fuller theoretical treatment was, however, required in order to interpret the results obtained and this forms the basis of the present paper. A new kinetic method for calculating the form of open-circuit potential decay transients has therefore been developed and is applied to the hydrogen evolution reaction, with electrosorption, electrodesorption and recombination steps considered. The equations are solved numerically and the forms of the solutions are discussed. The analysis shows that a representative transient has four regions: (A) where overpotentian η changes slowly with log (time, t) until a characteristic time τ; (B) where η decays linearly with log t with slope 2.3 RT/0.5 F until a time τ p; (C) where η levels off to a plateau with height η p and (D) where η decays finally to zero. The coverage by the adsorbed intermediate does not change significantly in regions A to C. An approximate equation is developed for regions A to C which predicts the values of τ, τ p and η p in terms of the rate constants. The plateau region C becomes less pronounced or is absent when recombination is significant or when the equilibrium constant for electrosorption is larger than that for electrodesorption. Three adsorption pseudocapacitance quantities are defined and their relationships to the coverage charges occurring during the transients are distinguished and discussed.