The effect of a CuO sintering additive on the sinterability of Ba-based perovskite electrolytes for protonic ceramic electrochemical cell applications

The addition of sintering additives is frequently regarded as an effective approach for increasing the density of proton-conducting oxide materials for their subsequent application as thin-film electrolytes for high-efficiency and high-performance protonic ceramic fuel cells (PCFCs) and electrolysis cells (PCECs). Although the positive effects of sintering additives on the sinterability of materials have been repeatedly confirmed, the nature of their localization in produced ceramics remains unclear. In particular, an analysis of approximately 20 studies related to the use of CuO for the densification of Ba-based ceramics does not allow solid conclusions about the real role of CuO to be drawn. Some researchers believe that CuO acts as a dopant, whereas others detect no significant solubility but instead observe the formation of Cu-containing impurities. To overcome the ambiguity of the different experimental results, we selected a reference system of BaSn_0.8Y_0.2O_3−δ + x wt% CuO (BSYx, 0 ≤ x ≤ 2) and performed complex structural, microstructural, and electrochemical characterization using as-prepared (oxidized) and H₂-treated (reduced) ceramics. Our results indicate that CuO facilitates liquid-phase sintering and therefore cannot act as a dopant, as CuO segregates at the grain boundaries or surface ceramic sites upon cooling. Furthermore, the high-temperature sintering of the studied materials at 1500 °C allows for the evaporation of the majority of the CuO (∼90%), thereby reducing the potential negative effects of CuO on proton transport. Electrochemical impedance spectroscopy analysis demonstrated that the BSYx ceramics exhibit high grain boundary proton transport, which is beneficial for the prospective application of such or similar electrolyte systems in PCFCs and PCECs.