Effect of Ni on electrical properties of Ba(Zr,Ce,Y)O3-δ as electrolyte for protonic ceramic fuel cells

NiO is known to promote sinterability and grain growth of the refractory proton conducting ceramic material Ba(Zr,Ce,Y)O3-δ. However, the NiO addition also decreases bulk proton uptake and conductivity. In this work, the influence of NiO addition on the electrochemical properties of Ba1.015Zr0.664Ce0.20Y0.136O3-δ is investigated. For 0.125–1.0 wt% NiO addition, the apparent grain-boundary conductivity increases significantly with NiO content. The largely increased grain size is the main reason for the improved apparent grain-boundary conductivity (the specific grain boundary conductivity does not correlate strongly with NiO content). The bulk conductivity is lower for higher NiO addition, both from decreased proton uptake and proton mobility. STEM-EDX line scans on a large number of grain boundaries show a systematically increased Y content in the boundary region. The local cation composition exhibits a significant scatter, which is expected to contribute to the observed variability of the grain boundary conductivity. STEM-EELS indicates the presence of Ce3+ in the grain boundary regions. Overall, as a good compromise 0.4–0.5 wt% NiO addition is recommended for future electrolyte membrane fabrication for protonic ceramic fuel cells.