Many-body effects in electrochemisorption and electron transfer processes

Abstract

Two important phenomena that govern the behaviour of an electrochemical interface are (a) chemisorption and (b) electron transfer. The combination of these — electron transfer involving chemisorbed entities — is also of great interest, especially in electrocatalysis. The purpose here is to provide a quantum mechanical description of these phenomena through suitable model Hamiltonians and appropriate many?body techniques.

First, an analysis of electrochemisorption as a lone?adsorbate problem interacting with both condensed phases — electrode and electrolyte — is presented. The theory is then extended to the two?centre case for studying induced interactions between adsorbates. The ultimate objective is to calculate the partial charge associated with the adsorbate and the binding energy, as well as their variation with the potential difference across the interface. Experimental and theoretical studies show that adsorbates at an electrode surface can carry partial electronic charge due to electron donation or acceptance, which affects the surface dipole and binding energy.

Another aspect of the reported work pertains to electron transfer (to or from an electrode) when an adsorbed species acts as a bridge between the reactant and the metal. Models describing these processes are analysed to give expressions for the current as a function of fundamental parameters such as the electronic energy levels of the reactant, adsorbate and electrode surface, and the coupling constants between phases. A key part of electrochemical electron transfer theory, such as developed by Marcus and Hush, involves the interplay between solvent reorganization energy and electronic coupling. Finally, electron transfer through a random adsorbate layer is modelled. Specifically, the type of coverage dependence of the resulting current in this context is discussed. In the case of chemisorption, the model Hamiltonian is a generalisation of the Anderson–Newns Hamiltonian, including coupling with bosonic modes (such as solvent polarization and substrate plasmons) to capture quantum effects. Analysis is conducted using Green’s function techniques and the many?body superoperator approach. This analysis also leads to generalisations of earlier model theories (e.g., those of Newns and Anderson) in the non?electrochemical context.

Some calculations for the average occupation probability of alkali ions on electrodes are also indicated. The methodology adopted for analysing electron transfer rates uses the linear response approximation together with Green’s function methods and coherent potential approximations, enabling treatment of both weak and strong coupling regimes. Known results by Marcus, Schmickler and others emerge as limiting cases of the general expressions derived here.