Knowledge of thermal behavior of electrolyte is important for fuel cell fabrication. In this study, using high‐temperature X‐ray diffraction analysis (HT‐XRD) and thermo‐mechanical analysis (TMA), a systematic investigation of lattice constants was performed on Y‐doped BaZrO3, which is a promising candidate for electrolyte in protonic ceramic fuel cells. The results revealed that a chemical expansion was observed between 300°C and 450°C during the heating process in HT‐XRD, and was attributed to the dehydration of BZY. Furthermore, it was found that the lattice constants of the samples doped with Y, Sm, Eu, and Dy were larger for the ones finally heat‐treated at 1600°C for sintering than those heat‐treated at 1300°C for synthesizing. The similar behavior was not observed in Sc‐doped samples.

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The precipitation condition of LaPO 4 in phosphoric acid solution was investigated under both atmospheric and humidified conditions. The crystal morphology, Sr doping level, H 2 O incorporation, and high temperature stability of Sr-doped LaPO 4 precipitates were also analyzed. Increasing pH 2 O from 0.01 to 1 atm extends the precipitation region of LaPO 4 in phosphoric acid solutions toward higher temperatures by nearly 100 °C. Under such conditions (e.g. T = 250 °C and pH 2 O = 1 atm), it is possible to obtain aggregated crystals up to 200 μm. The Sr doping level in LaPO 4 shows a strong dependence on the precipitation ratio of La, X La. The Sr doping level is higher in the low X La region, reaching 20 mol% at maximum. This value is one order of magnitude higher than the previously reported solubilities. It was confirmed H 2 O is certainly incorporated in the bulk by Sr doping. The solubility of Sr in LaPO 4 would be sufficiently high (at least 20.1 %) even at 600 °C; however, it becomes considerably lower at 1200 °C.

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Doped lanthanum polyphosphate (LaP 3 O 9 ) exhibits relatively high proton conductivity. For the practical applications such as the electrolyte of fuel cells, however, its conductivity must be improved by 2 orders of magnitude. Protons are introduced into matrix by lower-valent cation doping, and proton conductivity depends on dopant species. To date, LaP 3 O 9 has been doped with only Ca, Sr and Ba. In this work, we tried to dope LaP 3 O 9 with Na + , K + , Mg 2+ and Pb 2+ , as the new dopant species, due to their close ionic radii to La 3+ . Among them, only Pb could substitute for La at a comparable concentration to those of alkaline earth metals and its highest doping level was 6.4 mol% (Doping level is defined as the concentration ratio of dopant (M) to host cation (La) site in matrix (≡M/(La + M) × 100 (mol%))). Though Pb can exist as either divalent or tetravalent state, Pb in LaP 3 O 9 was identified to be divalent state by XPS analysis. Proton conduction was demonstrated by H/D isotope effect. The electrical conductivity of Pb-doped LaP 3 O 9 increased with Pb-doping level, owing to the increase in proton concentration. The conductivity of 4.5 mol% Pb-doped LaP 3 O 9 was about one order of magnitude lower than that of 7.9 mol% Sr-doped LaP 3 O 9 .