Solid oxide cells (SOCs) are electrochemical energy converters, whose components - as the name already suggests - consist of solid oxides. Most prominent examples of these devices are solid oxide fuel and solid oxide electrolysis cells (SOFCs and SOECs). To fulfil their function in a SOC, the materials used must have an appropriate conductivity. Depending on the role of the respective material, this conductivity is either purely ionic or electronic, or a mixture of both. Electrolyte materials, for example, should exhibit exclusively ionic conductivity. Most common electrolytes are oxide ion conductors such as Y-doped ZrO2, but also proton-conducting oxides are employed as SOC electrolytes. The existence of ionic charge carriers in these materials is provided by doping. Since the ionic conductivity of oxides depends on the mobility of the ionic charge carriers, elevated temperatures are typically needed to achieve sufficient conductivities.
As electrode materials, on the hand, metals such as Ni or Pt can be used - the latter for example in lambda sensors for automotive exhaust gas analysis. Alternatively, mixed ionic electronic conductors (MIECs) are employed as electrode materials. On such MIECs, the electrochemical reaction with the gas phase can proceed on the entire electrode surface, which typically makes them excellent electro-catalysts. Depending on composition, doping, oxygen partial pressure, and electrochemical polarisation the ratio of ionic and electronic conductivity in such mixed conductors can be largely different, which usually largely affects their electrochemical properties and especially their surface activity for the desired electrochemical reaction. Here it is important to mention that already small changes of the bulk can trigger large changes of the material’s surface (or an interface), which can have a severe impact on its electro-catalytic activity. This makes the field of mixed conducting SOC electrodes a huge and fascinating playground for basic researchers.
Our work on SOC materials is guided by the aim to explain the observed cell resistances on the level of physical elementary parameters or even further on an atomistic level. To reach this goal, we usually accompany the investigation of real SOC materials with measurements on model systems.