Thin-film X-ray diffractometry for evaluating effect of BaCO3 coating on the electrolyte of protonic ceramic fuel cells

Abstract

The manufacturing of protonic ceramic fuel cells (PCFCs) involves high-temperature sintering at ∼1500 °C to form a dense electrolyte film. This process results in Ba evaporation, which complicates the control of the electrolyte surface composition. To address this problem, we herein examined the effect of modifying the electrolyte (Ba_0.97Zr_0.8Yb_0.2O_3−δ, BZYb20d) surface in anode-supported PCFCs by BaCO₃ slurry coating followed by firing at 1300 °C. X-ray diffractometry (θ–2θ measurements) indicated a decrease in the amount of Yb₂O₃ precipitated on the surface of the thus treated BZYb20d, and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy revealed a decrease in the segregation of Yb₂O₃ at the treated electrolyte surface. Thin-film X-ray diffractometry (ω–2θ measurement) revealed a change in the lattice constant of the BZYb20d electrolyte (thickness = 10 μm) in a BZYb20d/NiO-BZYb20d half-cell as a function of the X-ray penetration depth from the surface to the bulk (i.e., near the BZYb20d/NiO-BZYb20d interface) at 25–900 °C in air, dry N₂, and wet N₂. At 900 °C, the lattice constant of BZYb20d after the BaCO₃ treatment hardly changed upon going from the surface to the bulk, which suggested that the Ba content at the BZYb20d electrolyte surface was almost the same as that in the bulk. The thermal expansion coefficients and chemical expansion rates of the BZYb20d film electrolyte bulk were lower (∼0.66 and ∼ 0.33 times, respectively) than those of BaZr_0.8Y_0.2O_2.9 and BaZr_0.8Yb_0.2O_3−δ bulk. The BaCO₃ treatment increased the maximum power density of the corresponding PCFC from ∼0.5 to ∼0.6 W cm⁻². The cathode fabricated using the modified BZYb20d electrolyte showed a lower polarization resistance (0.07 Ω cm²) than that based on the unmodified electrolyte (0.22 Ω cm²). The Ba deficiency of the BZYb20d electrolyte surface that developed during high-temperature sintering was alleviated by the BaCO₃ coating, and the interfacial resistance between the air electrode and electrolyte therefore decreased.