Optical absorption and XPS studies of (Ba_1-xSr_x)(Ce_0.75Zr_0.10Y_0.15)O_3-δ electrolytes for protonic ceramic fuel cells

Perovskite-type materials containing metal and cerium cations have been used as catalysts, membranes, electrodes in fuel cells, and hydrogen pumps. The effect of strontium doping on the perovskite-type structure has been evaluated and effectively maintains the cubic structure of (Ba_1-xSr_x)(Ce_0.75Zr_0.10Y_0.15)O_3-δ. X-ray photoelectron spectroscopy (XPS) was performed on the oxidation states of barium, strontium, cerium, zirconium, yttrium and oxygen. With strontium doping, the oxidation state of cerium ions in 4+ increases and results in the decrease in oxygen vacancy concentration. These are attributed to the strong binding energy of (A/B)-O bond which in turn, enhances the structural stability of electrolyte system. The free volume (V_f) is maximum for x=0.2 (V_f=33.1 ų) and then decreases with further strontium insertion. The optical absorption peaks are attributed to charge transfer transitions and energy band gap (increases from ∼4.13 to ∼4.35 eV). However, the increase of band gap is only ∼5% with partial replacement of barium by strontium. These characteristics are useful to make (Ba_1-xSr_x)(Ce_0.75Zr_0.10Y_0.15)O_3-δ a promising candidate for futuristic and reliable fuel cell technology.