Theory of the electrostatic surface potential and intrinsic dipole moments at the mixed ionic electronic conductor (MIEC)–gas interface

Abstract

The local activation overpotential describes the electrostatic potential shift away from equilibrium at an electrode/electrolyte interface. This electrostatic potential is not entirely satisfactory for describing the reaction kinetics of a mixed ionic–electronic conducting (MIEC) solid-oxide cell (SOC) electrode where charge transfer occurs at the electrode–gas interface. Using the theory of the electrostatic potential at the MIEC–gas interface as an electrochemical driving force, charge transfer at the ceria–gas interface has been modelled based on the intrinsic dipole potential of the adsorbate. This model gives a physically meaningful reason for the enhancement in electrochemical activity of a MIEC electrode as the steam and hydrogen pressure is increased in both fuel cell and electrolysis modes. This model was validated against operando XPS data from previous literature to accurately predict the outer work function shift of thin film Sm_0.2Ce_0.8O_1.9 in a H₂/H₂O atmosphere as a function of overpotential.