  Oxygen activity and surface stability are two key parameters in the search
for advanced materials for intermediate temperature solid oxide electrochemical
cells, as overall device performance depends critically on them. In particular
$in$ $situ$ and $operando$ characterisation techniques have accelerated the
understanding of degradation processes and the identification of active sites,
motivating the design and synthesis of improved, nanoengineered materials. In
this short topical review we report on the latest developments of various
sophisticated $in$ $situ$ and $operando$ characterization techniques, including
Transmission and Scanning Electron Microscopy (TEM and SEM), surface-enhanced
Raman spectroscopy (SERS), Electrochemical Impedance Spectroscopy (EIS), X-ray
Diffraction (XRD) and synchrotron based X-ray photoelectron spectroscopy (XPS)
and X-ray absorption spectroscopy (XAS), among others. We focus on their use in
three emerging topics, namely: (i) the analysis of general electrochemical
reactions and the surface defect chemistry of electrode materials; (ii) the
evolution of electrode surfaces achieved by nanoparticle exsolution for
enhanced oxygen activity and (iii) the study of surface degradation caused by
Sr segregation, leading to reduced durability. For each of these topics we
highlight the most remarkable examples recently published. We anticipate that
ongoing improvements in the characterisation techniques and especially a
complementary use of them by multimodal approaches will lead to improved
knowledge of $operando$ processes, hence allowing a significant advancement in
cell performance in the near future.
