The review article covers all state-of-the art materials related to high-temperature electrolyzers based on oxygen-ion conductors. The focus lies on the cell materials, materials of additional components like interconnects and sealants are briefly described.

Perspective on recent improvements in experiment and theory towards realizing lithium metal electrodes with liquid electrolytes.

In recent years, the stability field of a monoclinic phase at the morphotropic phase boundary in lead zirconate titanate, $\mathrm{Pb}\left[{\mathrm{Zr}}_{1-x}{\mathrm{Ti}}_x\right]{\mathrm{O}}_3$, has been under discussion. In the present study, we investigated samples in the compositional range between $0.40⩽x⩽0.475$ and $x=0.55$ using high-resolution synchrotron x-ray diffraction in combination with transmission electron microscopy and electron paramagnetic resonance to correlate average structure and microstructural information. It is shown that the microstructure plays a crucial role in the analysis of diffraction data. The appearance of intensity in diffraction patterns formerly linked to a monoclinic phase [B. Noheda et al., Phys. Rev. B 61, 8687 (2000)] can directly be correlated to a miniaturization of the average domain structure of the material visible in the presence of nanodomains. The internal symmetry of the nanodomains is not necessarily monoclinic due to coherence effects in diffraction and is discussed with respect to martensitic theory.

Multi-frequency and pulsed electron paramagnetic resonance (EPR) provides a sensitive spectroscopic tool to elucidate the defect structure of transition-metal doped perovskite oxides, as well as to monitor dynamic processes of oxygen vacancies in these materials. In this regard, high-frequency EPR spectrometers and pulsed EPR techniques such as the hyperfine sublevel correlation experiment (HYSCORE) may now routinely be used for dedicated investigations, providing considerably more insight than the application of standard continuous-wave EPR. Recent results include the formation of defect complexes between acceptor-type transition-metal centers with either one or two oxygen vacancies for the reason of charge compensation. Furthermore, such defect complexes follow the domain switching upon poling ferroelectric compounds with correspondingly high electric fields. On the other hand, multi-valent manganese functional centers provide trapping centers for electronic and ionic charge carriers (e′, ) such that valency altered acceptor states or defect complexes are formed. Additionally, the trapping of charge carriers at the intrinsic ‘reduced’ B-site ions, and , can be observed by means of EPR spectroscopy.