Understanding the charge transfer processes at solid oxide-fuel cell (SOFC) electrodes is critical to designing more efficient and robust materials. Activation losses at
SOFC electrodes have been widely attributed to the ambipolar migration of charges at
the mixed ionic–electronic conductor-gas interface. Empirical Butler-Volmer kinetics
based on transition state theory is often used to model the current-voltage relationship, where charged particles transfer classically over an energy barrier. However, the
hydrogen oxidation/water electrolysis reaction H2(g) + O2− ⇀↽ H2O(g) + 2e− must be
modelled through concerted electron and proton tunnelling events, where we unify the
theory of the electrostatic surface potential with proton-coupled electron transfer kinetics. We derive a framework for the reaction rate that depends on the electrostatic
surface potential, adsorbate dipole moment, the electronic structure of the electron
donor/acceptor, and vibronic states of the hydrogen species. This theory was used to
study the current-voltage characteristics of the Ni/gadolinium doped ceria electrode in
H2/H2O(g) where we find excellent validation of this novel model. These results yield
the first reported quantification of the solvent reorganisation energy for an SOFC material, and suggests that the three-phase boundary mechanism is the dominant pathway
for charge transfer at cermet electrodes.