Influence of processing conditions on phase evolution, defect chemistry and proton-related electrical conductivity of medium-entropy BaZr0.25Ce0.25Nd0.25Zn0.25O3−δ

The development of clean energy technologies is essential for achieving a low-carbon energy system. Protonic ceramic fuel cells (PCFCs) are promising candidates due to their high efficiency in hydrogen conversion and intermediate-temperature operation; however, their performance is still limited by the lack of electrolytes combining high electrical conductivity under humidified conditions, chemical stability, and good sinterability. Here, a nominally medium-entropy BaZr0.25Ce0.25Nd0.25Zn0.25O3−δ perovskite-related ceramic was prepared by solid-state and co-precipitation routes to assess how processing affects phase evolution, densification, defect chemistry and proton transport. XRD analysis indicates the formation of a dominant cubic perovskite phase after high-temperature treatment, although minor secondary phases should be considered in the interpretation of the results. The solid-state route promoted markedly higher densification, reaching a relative density close to 99%, whereas the co-precipitated sample showed lower densification and spectroscopic features consistent with greater local structural disorder. Raman and XPS analyses indicate processing-dependent differences in local structural order and surface chemical environments, respectively. Electrical measurements under dry and humidified atmospheres showed a humidity-dependent increase in the apparent high-frequency conductivity calculated from the series resistance at the high-frequency intercept, consistent with an enhanced proton-related contribution under selected temperature and atmosphere conditions. The reported values are therefore expressed as apparent high-frequency conductivity, while the humidity dependence is discussed in terms of a proton-related contribution. The results demonstrate that the measured electrical response depends on the combined effects of processing history, phase assemblage, densification, local structural disorder, and defect chemistry. Because the SS and CO specimens were sintered under different conditions, the observed differences cannot be attributed exclusively to the synthesis route.