Transport properties of the Ba(Zr,Ce,Y,Yb)O_3- proton conductor: the real role of co-substitution of Y and Yb

Proton conducting oxides are attracting increasing attention due to their promising application as electrolytes in solid-state electrochemical devices for energy conversion and chemical production, including protonic ceramic fuel cells, electrolyzers and membrane reactors. BaZrO3 and Ba(Zr,Ce)O-3 substituted by acceptor dopants are currently the most promising proton conductors. In principle, replacing Zr or Ce with one kind of trivalent element (e.g., Y or Yb) can generate oxygen vacancies, which are necessary for the hydration reaction to introduce charge carriers of protons. However, it is commonly recognized that substituting Zr or Ce with Y and Yb simultaneously leads to high performance of the devices, but the reason is not clear. In this work, we studied the transport properties of four different compositional classes: BaZr0.8-xCexY0.2O3-delta (BZCY20), BaZr0.8Y0.2-xYbxO3-delta (BZYYb), BaZr0.1Ce0.7Y0.2-xYbxO3-delta (BZCYYb) and BaCe0.8Y0.2-xYbxO3-delta (BCYYb), at 600, 650 and 700 degrees C. For BZCY20, the ionic conductivity decreases on replacing Zr in BZY20 with x = 0.1 of Ce, increases with increasing Ce content within x = 0.1-0.5, and decreases on further increasing Ce content from x = 0.5 to 0.8. The ionic transport number increases with increasing Ce content. The ionic conductivity, partial conductivity of oxide ions and hole conductivity of BZCYYb and BCYYb increase with increasing Yb content. Since the increment of hole conductivity is larger, the ionic transport number decreases. For BZYYb, the ionic conductivity decreases and the hole conductivity increases with increasing Yb content, leading to decreasing ionic transport number. Both the high ionic conductivity and transport number are vital factors governing the performance of protonic ceramic cells (PCCs). Therefore, the improved performance of PCCs by adding both Y and Yb into the electrolyte cannot be explained from the viewpoint of electrical properties of the electrolytes. Thereby, we summarized our recent series of studies on the chemical compatibility between the electrolyte and NiO negatrode substrate, and suggested that the loss of dopant was suppressed by partly replacing Y with Yb. The high ionic conduction of the electrolyte can be maintained after co-sintering at high temperature to fabricate PCCs. This might be the real reason for the high performance of the devices using Y and Yb co-substituted electrolytes, prepared by the co-sintering method.