Proton Conductive BaZr0.8-x Cex Y0.2 O3-δ : Influence of NiO Sintering Additive on Crystal Structure, Hydration Behavior, and Conduction Properties.

Y-doped BaZrO3 , BaCeO3 and BaZr1-x Cex O3 show high proton conductivity at intermediate temperature and are promising electrolyte candidates in electrochemical devices. However, in most cases, the present cell fabrication process seems to be unavailable to avoid the addition of NiO, which is either added to improve the sinterability of these electrolyte or diffuses from the electrode substrate during co-sintering. In this work, a systematic investigation was performed to study the effect of NiO on BaZr0.8-x Cex Y0.2 O3-δ (BZCY20) covering the full Ce range from 0 to 0.8. The results revealed that regardless of the composition of BZCY20, both the dehydration temperature and proton concentration decreased by adding NiO, which further greatly decreased the ionic conductivity and the transport number. And it is found that the redox cycles in Ce-rich samples containing Ni makes the grain boundary conductivity worse and the electrolyte brittle. The conclusion is that NiO is detrimental to the performance of the electrochemical cells using these materials as the electrolyte, although compromise might be achieved in certain degree by tuning the Ce content. However, it should be noted that to further improve the cell performance, a new sintering additive or new processing for cell fabrication is essential.

Detrimental Effect of Sintering Additives on Conducting Ceramics: Yttrium-Doped Barium Zirconate.

Y-doped BaZrO3 (BZY) is currently the most promising proton-conductive ceramic-type electrolyte for application in electrochemical devices, including fuel cells and electrolyzer cells. However, owing to its refractory nature, sintering additives, such as NiO, CuO, or ZnO are commonly added to reduce its high sintering temperature from 1600 °C to approximately 1400 °C. Even without deliberately adding a sintering additive, the NiO anode substrate provides another source of the sintering additive; during the co-sintering process, NiO diffuses from the anode into the BZY electrolyte layer. In this work, a systematic study of the effect of NiO, CuO, and ZnO on the electroconductive properties of BaZr0.8 Y0.2 O3-δ (BZY20) is conducted. The results revealed that the addition of NiO, CuO, or ZnO into BZY20 not only degraded the electrical conductivity but also resulted in enhancement of the hole conduction. Removal of these sintering additives can be realized by post-annealing in hydrogen at a mild temperature of 700 °C, but it is kinetically very slow. Therefore, the addition of NiO, CuO, and ZnO is detrimental to the electroconductive properties of BZY20, and significantly restrict its application as an electrolyte. The development of new sintering additives, new anode catalysts, or new methods for preparing BZY electrolyte-based cells is urgently needed.