Systematic Water Uptake Energetics of Yttrium-Doped Barium Zirconate—A High Resolution Thermochemical Study

Abstract

A combination of surface area analyzer and microcalorimetry was employed to investigate the in situ water uptake energetics and the mechanism of proton incorporation in yttrium-doped barium zirconate in the temperature range 200−400 °C. The BaZr 1−x Y x O 3 solid solutions are made with variable yttrium content (x = 10, 20, and 30 mol %) by a controlled oxidant-peroxo synthesis method. The water uptake increases as the partial pressure of water increases; however, no saturation in the hydration isotherm is observed, implying further reaction at higher p H 2 O . The results suggest three distinct regions of hydration energies as a function of water content. The first water uptake enthalpy values showed high exothermicity, −140, −158, and −157 kJ mol −1 for BaZr 1−x Y x O 3 (x = 10, 20, and 30 mol %), respectively, at 400 °C, and the strong exothermic contribution supports the dissociative incorporation of water. The stepwise in situ hydration energetics is essential to understand the mechanisms of water incorporation and the role of H 2 O uptake in transport properties.

Novel Class of Proton Conducting Materials—High Entropy Oxides

“…Summing up, among the high entropy oxides studied in this work, only BaZr 0.2 Sn 0.2 ­Ti 0.2 Hf 0.2 ­Y 0.2 O 3−δ contains a significant number of oxygen vacancies, which explains why this sample exhibits the highest water uptake. The relative mass change for this composition corresponds to 1.6 × 10 –2 mol/mol proton defect concentration and is similar to other mixed proton–electron conductors like barium lanthanide cobaltites but is lower than in well-recognized proton conducting materials like barium zirconate . Proton defect concentration in BaZr 0.2 Sn 0.2 ­Ti 0.2 Hf 0.2 ­Ce 0.2 O 3−δ , BaZr 1/7 Sn 1/7 ­Ti 1/7 Hf 1/7 ­Ce 1/7 Nb 1/7 ­Y 1/7 O 3−δ , and BaZr 0.15 Sn 0.15 ­Ti 0.15 Hf 0.15 ­Ce 0.15 Nb 0.15 ­Y 0.10 O 3−δ is 1.5 × 10 –3 , 2.0 × 10 –3 , and 2.4 × 10 –3 mol/mol, respectively.…”

The effect of ionic defect interactions on the hydration of yttrium-doped barium zirconate

“…Fitting yields Dh1 = À75 AE 4 kJ mol À1 and Ds1 = À118 AE 4 J mol À1 K À1 for the short-range model, and Dh1 = À94 AE 5 kJ mol À1 and Ds1 = À132 AE 5 J mol À1 K À1 for the longrange model. The obtained values of Dh1 and Ds1 lie within the data range as reported in the literature 2,5,6 for experiments conducted above 500 1C. This is expected, as there are few interacting protons present and K int has a minor effect at high temperatures.…”

“…Various experimental results from literature which were obtained at comparable yttrium fraction and water partial pressure are shown for comparison. The following features can be identified: (i) the data including K int strongly deviates from the ideal curves improving or hindering the hydration at the high and low temperature regions, respectively; (ii) the favouring effect of the long-range model is smaller than for the short range model; and (iii) there is an excellent agreement of the BZY10 curves with the experimental data by Imai et al 4 and Kreuer et al 3 -measured at 23 mbar and 10 mol% Y content -and a good qualitative agreement of the BZY20 and BZY30 curves with the data by Gonçalves et al 5 -measured at 19 mbar and 20 mol% and 30 mol% yttrium, respectively. However, the deviation of their results for 10 mol% yttrium from the data by the other groups is unexpected considering the minor change in p H 2 O .…”

“…The differences between the yttrium fractions are much smaller than the divergences due to proton defects or temperature changes, which agrees with the experimental observation that x Y affects the equilibrium constants to a minor degree only. 3,5,32 The distinct zones of interest are K int 4 1, K int E 1, and K int o 1, which imply that proton incorporation is above, equal, or below the ideal case without defect interactions, respectively. Thus, the shown trends in K int are expected when compared to the ideal model, that is: (i) regardless of temperature, it is generally more Fig.…”

“…In recent years, multiple studies obtaining Dh1 and Ds1 by fitting eqn (4) to experimental data accumulated strong evidence that the assumption of an ideal solution is mostly inaccurate. [2][3][4][5][6] It was repeatedly shown that data obtained from experiments investigating the proton content, such as thermogravimetry, are not well described by eqn (3) and (4) when the bulk material is subjected to larger variations of temperature or water partial pressure. Kreuer obtained satisfactory fitting results for the hydration isobars only by considering an effective dopant concentration indicating that the ideal solution model is too simple.…”