Protonated BaZr_0.8Y_0.2O_3−δ: Impact of Hydration on Electrochemical Conductivity and Local Crystal Structure

Acceptor-doped BaZrO3 is a promising electrolyte candidate in fuel cells and electrolysis cells due to its high proton conductivity and excellent chemical stability. However, it turns to be partially hole-conductive in an oxidizing atmosphere, resulting in the degraded performance of these electrochemical devices. Based on the knowledge of defect chemistry on BaZrO3, its electrochemical properties can be tuned by simply controlling the partial pressure of oxygen (pO2) and water vapor (pH2O) in the atmosphere. In this work, we investigated the hydration behavior of BaZrO3 doped with Y or Yb under pH2O varying from 0.03 to 0.51 atm and found that both the proton concentration and lattice constants increased for BaZr0.8Y0.2O3−δ (BZY20) and BaZr0.8Yb0.2O3−δ (BZYb20) with increasing pH2O. Also, the chemical expansion coefficients of (0.882 ± 0.036) × 10–2 and (1.44 ± 0.067) × 10–2 per one proton per unit cell were estimated for BZY20 and BZYb20, respectively. The change in the local crystal structure due to such chemical expansion induced by water incorporation was probed by EXAFS analysis, and the result coincides with the reported insight that the oxide ions and protons are more favorable to reside in the vicinity to Y cations. Furthermore, it is effective to suppress the hole conduction by elevating pH2O and lowering pO2; for example, the ionic transport number (tion) at 600 °C can be increased to be higher than 0.9 by elevating pH2O over 0.20 atm, suggesting an effective and feasible strategy to improve the performance of the electrochemical devices with the BZY20 electrolyte.