From a mechanistic point of view, electrode reactions in solid state electrochemistry and in electrochemical systems with liquid electrolyte are significantly different. In case of liquid systems the electrolyte is usually both ionic conductor and solvent for the reactive species. Thus, the electrode|electrolyte two phase boundary (2PB) is the location where the electrochemically relevant reaction steps occur. In solid state electrochemistry, which is our main research field, such a situation is frequently not found, simply because reactants such as O2, H2, CO2, H2O (among others) do not dissolve in most oxides. Therefore, not only the electrode|electrolyte interface is relevant for the electrochemical reaction on solid oxide ion conductors, but also the electrode|gas two phase boundary (2PB) and the three phase boundary (3PB) where gas phase, electrode, and electrolyte meet. As a consequence, several different reaction pathways become possible for the electrode reactions in solid state electrochemistry, depending on the properties of the electrode and the electrolyte material.
In case of composite electrodes of a metal and an ion conductor, very often a so-called surface path is very relevant, typically with an electrochemical reaction at the 3PB. On mixed ionic and electronic conductors (MIECs), both ions and electrons can be provided via a single phase, thus favouring the reaction at the surface of the material (i.e. at the electrode|gas 2PB). We continuously develop novel or improved concepts to understand such electrochemical reactions. In contrast to liquid-phase electrochemistry, the redox reaction at a MIEC surface, for example, is not driven by an electrostatic potential difference. Rather, the overpotential causes a defect chemical polarization, which induces a change of the oxygen chemical potential and thus of the concentrations of the ionic and electronic charge carriers in the material. Consequently, the defect chemistry of a polarised MIEC electrode is not in equilibrium with the gas phase, which causes a net reaction rate. As a consequence, knowledge about the defect chemistry of MIEC electrodes is extremely relevant for an understanding of the reaction mechanisms on these materials, which is one of our ongoing research fields.