Oxygen permeability, stability and electrochemical behavior of \( \Pr _{2} {\text{NiO}}_{{4 + \delta }} \)-based materials

The high-temperature electronic and ionic transport properties, thermal expansion and stability of dense \( \Pr _{2} {\text{NiO}}_{{4 + \delta }} ,\Pr _{2} {\text{Ni}}_{{0.9}} {\text{Fe}}_{{0.1}} {\text{O}}_{{4 + \delta }} \) and \( \Pr _{2} {\text{Ni}}_{{0.8}} {\text{Cu}}_{{0.2}} {\text{O}}_{{4 + \delta }} \) ceramics have been appraised in comparison with K₂NiF₄-type lanthanum nickelate. Under oxidizing conditions, the extensive oxygen uptake at temperatures below 1073–1223 K leads to reversible decomposition of Pr₂NiO₄-based solid solutions into Ruddlesden–Popper type Pr₄Ni₃O₁₀ and praseodymium oxide phases. The substitution of nickel with copper decreases the oxygen content and phase transition temperature, whilst the incorporation of iron cations has opposite effects. Both types of doping tend to decrease stability in reducing atmospheres as estimated from the oxygen partial pressure dependencies of total conductivity and Seebeck coefficient. The steady-state oxygen permeability of \( \Pr _{2} {\text{NiO}}_{{4 + \delta }} \) ceramics at 1173–1223 K, limited by both surface-exchange kinetics and bulk ionic conduction, is similar to that of \( {\text{La}}_{2} {\text{NiO}}_{{4 + \delta }} \). The phase transformation on cooling results in considerably higher electronic conductivity and oxygen permeation, but is associated also with significant volume changes revealed by dilatometry. At 973–1073 K, porous \( \Pr _{2} {\text{Ni}}_{{0.8}} {\text{Cu}}_{{0.2}} {\text{O}}_{{4 + \delta }} \) electrodes deposited onto lanthanum gallate-based solid electrolyte exhibit lower anodic overpotentials compared to \( {\text{La}}_{2} {\text{Ni}}_{{0.8}} {\text{Cu}}_{{0.2}} {\text{O}}_{{4 + \delta }} \), whilst cathodic reduction decreases their performance.