Effect of Metal Work Function on Electron Transfer Kinetics in Electrochemical Reactions

Abstract

The longstanding problem of explaining the observed dependence of the kinetic rates of electrochemical reactions on the work function of the electrode material (metal) is revisited, based on a recent paper.1 It was proposed that this dependence could be explained in terms of the effect of the work function of the metal on the free energies of activation of the forward and backward electron transfer reactions. In this presentation, we will explore details of the proposed mechanism.In essence, the mechanism is based on the effect of work function on the Galvani potential of the metal at any given applied potential. Even though the total electron energy (Fermi level) of an electron in the electrode must be the same, regardless of the metal, when it is at equilibrium with a given electrolyte, the manner in which that energy is partitioned between the effect of work function and the effect of potential is significantly different for different electrode materials. A higher work function leads to more stabilization of an electron in the metal (lower free energy) and, since the total free energy must remain the same, this lower free energy due to work function must be compensated by a more negative Galvani potential.The classical analysis of the effect of a more negative potential is based on the theory that a change in potential has a greater effect on the initial (reactant) free energy than it does on the free energy of the transition state. Consequently, the activation energy of the forward (cathodic) reaction is decreased by some fraction α of the corresponding increase in electron energy in the metal. This type of analysis is generally used to quantify the effect of changes in applied voltage and derive current-voltage relationships. In this paper we extend the analysis to take account of changes in the (Galvani) potential ϕm of the metal due to changes in work function Φm.It is reasonable to assume that a change in ϕm will have a similar effect whether it occurs due to a change in applied voltage or due to a change in the work function of the metal. This implies that an increase in work function with a corresponding negative shift in Galvani potential can lead to a lowering of activation energy and so to faster kinetics. However, we must also consider a possible change in activation energy arising from the decreased electron energy (increased stabilization) due to higher work function. If this caused a change in activation energy it would be expected to be in the opposite direction to the change due to Galvani potential, since it would be expected to lower the initial free energy by a greater amount than that of the transition state.The presentation will consider in detail the interplay of these effects and the consequent implications for modelling electrode kinetics. K. S. R. Dadallagei, D. L. Parr IV, J. R. Coduto, A. Lazicki, S. DeBie, C. D. Haas, and J. Leddy, J. Electrochem. Soc. 170, 086508 (2023)