Solid oxide fuel cells (SOFC) have gained particular interest due to the demands for sustainable energy sources and reached a mature state of development. In contrast, there are still challenges in materials engineering for solid state electrolysis cells (SOEC). If an external electric current is driven through the cell in electrolysis mode, degradation effects due to electrochemical redox processes can occur, which may limit the lifetime. Electric current induced redox phenomena also gain importance for processing of ceramic materials using Field-Assisted Flash Sintering (FAST) and Spark Plasma Sintering (SPS). Intermediate reduction during the sintering process was re­cently reported. [1] Generally, when considering a contact between two mixed electronic and ionic conductors, electrochemical redox phenomena will occur, if the transference number of the electronic charge carriers is changing at the interface. This includes reduction phenomena as well as oxidative pore formation at electrode interfaces or in the interface between solid electrolytes and mixed conductors. [2, 3]

In this contribution we want to focus on the formation of blackened zirconia, known since the first reports by Casselton in 1968 and subject of various studies in recent decades. The cathodic electroreduction of yttria-stabilized zirconia (YSZ) will lead to a dendrite-shaped reaction front of a reduced phase moving to the anode side. In contrast to the solid electrolyte YSZ, reduced YSZ is a good electronic conductor, leading to an interface between reduced and unreduced phase with a large change of the electronic transference number. However, the composition and structure of the oxygen deficient phases is still not cleared in detail. In many studies it is reported that the mechanical properties of reduced single crystal or ceramic material degrades significantly (i.e. getting brittle).

Our new investigations on YSZ single crystals reveal strong strain fields leading to the for­mation of checkerboard-like structures on the surface above a highly oxygen deficient region below the surface. [4] Transmission electron microscopy (TEM), energy dispersive x-ray spectro­scopy (EDX) and x-ray photoelectron spectroscopy (XPS) have been employed to understand its physical nature. The lattice constant of one cubic axes has shrunk significantly, Zr⁴⁺ is reduced to lower valences, resulting in an approximate stoichiometry of ZrO ∙ Zr₂O. Moreover, there are many stacking faults in the reduced phase and misfit dislocations in the interface to the unreduced phase. The strong change of the molar volume may be the origin of the strain field resulting in the checkerboard-like surface structure and of the deterioration of the mechanical properties.