where O 2-and e -represent oxide ions in the electrolyte and electrons in the electrode, respectively. The dipole at the electrode-liquid electrolyte interface, known as the activation overpotential (η act ), accounts for the energy barrier of charge transfer processes such as Li-ion intercalation. For SOFC systems these phenomena do not apply and little is known about the electrode-gas interactions that determine the activation overpotential. The ambipolar transfer of ions and electrons with the gas phase means this interface is chemical in nature. 9