Stable and efficient n = 2 Ruddlesden–Popper La/Pr nickelates as oxygen electrodes for SOC applications

Abstract

The La for Pr substitution in the Ruddlesden–Popper (RP) n = 2 La₃Ni₂O_7±δ nickelates was investigated for the first time in the whole possible substitution range for their use as oxygen electrodes in Solid Oxide Fuel Cells (SOFCs) and Electrolyzers (SOECs). In this work we evidenced that the solid solution is restricted up to the composition La_1.5Pr_1.5Ni₂O_7±δ. The synthesis conditions (temperature, atmosphere), the oxygen stoichiometry including its variation with temperature in air, as well as the corresponding cell parameters of the prepared materials were thoroughly determined. Long-term ageing experiments were performed both on pure materials and on mixtures prepared with gadolinium doped ceria (GDC20) powders, to determine the stability of the phases under the operating conditions of temperature and the chemical reactivity at the interface with GDC20, which is used as a barrier layer between the 8YSZ electrolyte and the nickelate. The latter acts as accelerated degradation conditions. Using XRD and TEM-EDX analysis, we were able to evidence the diffusion of Pr and La into GDC20 after the ageing of the nickelate/GDC mixture. The electrochemical performances measured on symmetrical cells are among the best reported in the literature, with a polarisation resistance, R_p, of 36 mΩ cm² at 800 °C for La_1.5Pr_1.5Ni₂O_7±δ, close to that of the reference La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ perovskite material (R_p = 10 mΩ cm²) and state of the art n = 1 RP Pr₂NiO_4+δ (R_p = 10 mΩ cm²). Although the extent of the impact of La/Pr diffusion into the barrier layer on the performances still needs to be determined, the slow degradation rate compared to Pr₂NiO_4+δ along with the low polarisation resistance of La_1.5Pr_1.5Ni₂O_7±δ makes this RP n = 2 nickelate compound an attractive alternative for oxygen electrode materials in SOFC/SOEC application.