Evaluation of stability of Pr2−xNdxNiO4+δ by thermogravimetry under various oxygen partial pressures

The thermodynamic and kinetic stability of Pr2−xNdxNiO4+δ was evaluated by thermogravimetry (TG) under a varying oxygen partial pressure (P(O2)). This study was performed because Pr2−xNdxNiO4+δ has been anticipated as the new cathode material for solid oxide fuel cells. It was found that Pr2NiO4+δ decomposed to a mixture of PrO1.83 and Pr4Ni3O10+δ at a P(O2) of 1 atm above 750 °C. For specimens with x ≤ 0.5, similar decomposition was observed above 750 °C; however, this decomposition was not observed for specimens with x ≥ 0.75. Regeneration of Pr2−xNdxNiO4+δ from the decomposed species was observed as an abrupt mass loss in TG curves for x = 0.0–0.5 at a higher temperature; this could be regarded as the approximate equilibrium temperature for the decomposition reaction. Using the approximate equilibrium temperature under the examined P(O2), thermodynamically stable P(O2) and temperature region of Pr2NiO4+δ was clarified and represented using a van’t Hoff plot. It was revealed that the thermodynamically stable region expanded only slightly with the partial substitution of Nd at the Pr site. Reaction kinetics of the decomposition was evaluated by isothermal TG at various P(O2). With decreasing P(O2) or increasing Nd content, kinetic stability of Pr2−xNdxNiO4+δ significantly improved; this was in good agreement with the observation of no decomposition for specimens with x ≥ 0.75. A higher activation energy was observed for decomposition kinetics in the Nd substituted specimen than that obtained by solely decreasing P(O2); this indicated that Nd substitution was more effective in preventing oxidation of Pr, which induced decomposition.