Perspective on theories for reversible potentials for reactions taking place on electrode surfaces

Abstract

• Surface potential and activation energy from electron affinities: LRC. • Surface potential and activation energy from DFT with surface charging. • Surface potential from standard solution values and adsorption energies: LGER . • Surface potential standard hydrogen value and surface reaction energy: CHE. • A relationship between LGER and CHE explained: U LGER − U CHE = U CHE (aq) − U 0 . This article provides a brief overview of three models for calculating activation energies and reversible potentials for oxidation and reduction reactions involving protons at the electrochemical interface and advances the understanding of one of them. Two incorporate the electrode potential in the calculations, the local reaction center (LRC) model where an external electron source or sink is at the chosen potential, and slab-band density functional (DFT) codes which incorporate surface charging to set the potential. The simplest theory comes in two forms: the linear Gibbs energy relationship (LGER) and the computational hydrogen electrode (CHE) model. They have empirical components, inputting experimental data and predicting reversible potentials for surface reactions from calculations of energies for adsorbates at the vacuum interface. LGER uses experimentally measured standard reversible potentials as parameters and CHE employs the work function of the standard hydrogen electrode and the standard value of 0 V for this electrode as parameters. A relationship between LGER and CHE is derived. While LGER predictions depend on two calculated adsorption energies, CHE is demonstrated by means of a Haber cycle to effectively depend on the energy calculated for the reaction in solution as well as the two adsorption energies.