Design of a highly stable and conductive electrolyte by suppressing barium copper oxide formation at the grain interfaces in Cu_x-doped BaCe_0.7Zr_0.1Dy_0.2-xO_3-δ sintered at a low temperature (1200 °C) for SOFCs

Proton-conducting electrolytes with high conductivity and long-term stability, achievable at low sintering temperatures, are of paramount importance. In this study, we investigate the impact of Cu doping on the sintering mechanism, electrical performance, and stability of BaCe_0.7Zr_0.1Dy_0.2-xO_3-δ (BCZD) electrolyte. The morphology, composition, structure, and chemical state of BCZD electrolytes were investigated using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Remarkably, the sintering temperature of BCZD is reduced to 1200 °C through Cu doping Furthermore, the introduction of Cu as a dopant induces Ba exsolution from the BCZD crystal lattice alongside the generation of oxygen vacancies. XPS measurements confirmed that vacancy concentrations increase with increasing Cu doping concentrations. However, as the Cu content surpasses 2 mol%, the exsoluted Cu reacts with Ba, leading to the formation of BaCuO₂. Accumulation of BaCuO₂ at the grain boundaries adversely affects the conductivity and stability of Cu-doped BCZD, in a humidified atmosphere where it exhibits significant instability. Notably, BCZD with 2 mol% Cu content demonstrates a conductivity of 2.7 × 10^-2 S cm⁻¹ and maintains stability for up to 420 h in the H₂/3%H₂O atmosphere at 600 °C. In contrast, BCZD with 5 mol% Cu content exhibits a conductivity of 1.9 × 10^-2 S cm⁻¹ at 600 °C but experiences continuous degradation in a humidified atmosphere, ultimately leading to failure within 30 h. The 2 % Cu-doped BCZD exhibits high conductivity and stability at intermediate temperatures, rendering it highly suitable for solid oxide fuel cell (SOFC) applications.