Impact of calcium and copper co-doping on the oxygen transport of layered nickelates: a case study of Pr_1.6Ca_0.4Ni_1–yCu_yO_4+δ and a comparative analysis

This study aims at investigating oxygen content and diffusion in the Pr1.6Ca0.4Ni1‑yCuyO4+δ (y = 0.0–0.4) series. Single-phased materials were obtained via nitrate combustion using glycerol as a fuel. The materials exhibit an orthorhombic structure, which correlates well with the absolute oxygen content in the samples in the range of 4.15–3.97. The oxygen mobility in the samples was investigated by a temperature-programmed isotope exchange of oxygen (TPIE) with C18O2 in a flow reactor. The obtained curves, comprising few extrema, were fitted using a mathematical model that includes two or three distinct oxygen forms, each characterized by a specific diffusion coefficient value. The oxygen tracer diffusion coefficient values and the contribution of fast oxygen forms tend to decrease with increasing Cu content (except for Pr1.6Ca0.4Ni0.8Cu0.2O4+δ), which can be explained by the variation of highly mobile interstitial oxygen content since the effective activation energy values remains the same within the calculation error. Comparative analysis shows that this behavior is similar to the trends observed and discussed for Ca- and Cu-doped La and Nd nickelates. Nevertheless, the highest D* value for Ofast was demonstrated for the Pr1.6Ca0.4Ni0.8Cu0.2O4+δ sample (9.4·10–8 cm2/s at 700 °C), which correlates well with superior electrochemical properties of the electrodes on its basis.