Towards highly dense electrolytes at lower sintering temperature (∼1200 °C): Optimization strategies for BaCe_0.7Zr_0.1Cu_xY_0.2-xO_3-δ in SOFCs

High-conducting and long-term stable proton-conducting electrolytes are of utmost importance. This study investigates the synthesis and characterization of BaCe_0.7Zr_0.1Cu_xY_0.2-xO_3-δ (BCZCu_xY; x = 0, 0.005, 0.01, 0.02, 0.05, and 0.1) electrolytes tailored for proton-conducting solid oxide fuel cells (SOFCs). Through Cu²⁺ doping at the B-site of BaCe_0.7Zr_0.1Y_0.2O_3-δ (BCZY), a successful reduction of the sintering temperature to 1200 °C was achieved. BCZCu_0.02Y exhibited a high relative density of approx. 98.1% at this temperature, but beyond 2 mol% Cu doping, the appearance of a BaCuO₂ secondary phase adversely impacted conductivity. The electronic properties of BCZY and BCZCu_xY are elucidated via partial density of states (PDOS) analysis, revealing optimized crystal structures and band gap reductions (from approx. 1.9 eV–1.1 eV) upon Cu doping. Notably, BCZCu_0.02Y demonstrated a commendable conductivity, with values of 3.5 × 10⁻² S. cm⁻¹ in air and 4.8 × 10⁻² S. cm⁻¹ in a moist atmosphere at 750 °C. Remarkably, excellent electrochemical stability was observed in a moist hydrogen atmosphere for up to 450 h at 600 °C. Single cells incorporating BCZCu_0.02Y electrolytes exhibited peak power densities of 380 mW/cm² at 750 °C. The incorporation of 2 mol% Cu²⁺ in the BCZY lattice holds promise for achieving low-temperature sintering and high-performance proton-conducting SOFCs.