Exploring the impact of sintering additives on the densification and conductivity of BaCe0.3Zr0.55Y0.15O3-δ electrolyte for protonic ceramic fuel cells

Highlights: • BaCe_0.3Zr_0.55Y_0.15O_3-δ perovskite is successfully synthesized by citrate-nitrate combustion. • Highly dense (> 95%) BCZY35 electrolyte membranes are obtained at 1400 °C. • The total conductivity of BCZY35 is found to be ~3.69 × 10⁻³ S cm⁻¹ at 500 °C. • An improved specific grain boundary conductivity is observed for ZnO-BCZY35. • A lowered space charge boundary height is noted for ZnO-BCZY35. -- Abstract: Ceramic proton conducting BaCeO₃-BaZrO₃ materials have received great interest as potential electrolyte candidates for intermediate temperature protonic ceramic fuel cells. In the current work, the impact of sintering additives ZnO, NiO and CuO on densification, microstructure and electrochemical properties of the state-of-the-art electrolyte composition BaCe_0.3Zr_0.55Y_0.15O_3-δ (BCZY35) is appraised. Highly dense (> 95%) BCZY35 electrolyte pellets are obtained at the temperature of 1400 °C by use of the sintering additives, whereas only ~86% relative density is achieved for the pristine material at this temperature. The BCZY35 composition is shown to offer high chemical stability against a pure CO₂ environment, as confirmed by thermogravimetric analysis. The electrical conductivity of BCZY35 with and without sintering additives is studied as a function of temperature in humid N₂, O₂ and H₂ by A.C. impedance spectroscopy. In general, slightly lowered total conductivities are observed for all sintering additive specimens than the pristine BCZY35 material. The bulk conductivity is shown to be significantly impaired by the addition of sintering additives, in all cases, while specific grain boundary conductivity is either relatively unaffected, NiO, CuO, or improved, ZnO. This interesting feature is analysed further by application of a space-charge model, highlighting the additive containing compositions to offer lower Schottky barrier heights than the base BCZY35 phase and with this factor being most significant for the ZnO-additive. The activation energy for total and bulk conductivities is found to be in the range of 0.4–0.6 eV under humid conditions in the temperature range 400–600 °C, while slightly higher Ea values ~0.72–0.77 eV are observed for the grain boundary conductivity. The total conductivity of BCZY35 reaches the value of ~3.69 × 10^-3 S cm⁻¹ at 500 °C in humid nitrogen and hydrogen, where protons are expected to be the majority charge carriers. The results show the BCZY35 electrolyte composition to be promising to use in intermediate temperature protonic ceramic electrochemical cell applications, with the peak sintering additive being that of ZnO.