Oxygen-Deficient Nd_0.8Sr_1.2Ni_0.8M_0.2O_4-δ (M = Ni, Co, Fe) Nickelates as Oxygen Electrode Materials for SOFC/SOEC

Ruddlesden-Popper Nd0.8Sr1.2Ni0.8M0.2O4-δ (M = Ni, Co, Fe) nickelates have been characterized as prospective oxygen electrode materials for solid electrolyte cells. XRD studies showed that these oxides retain tetragonal K2NiF4-type structure in air until at least 900°C. Average thermal expansion coefficients of Nd0.8Sr1.2Ni0.8M0.2O4-δ calculated from the structural data are in the range 14.5-15.8 ppm/K. TGA studies revealed that these nickelates are oxygen-deficient in air at temperature above 700°C but tends to oxygen stoichiometry or minor excess on cooling. Incorporation of cobalt or iron into nickel sublattice of Nd0.8Sr1.2NiO4-δ reduces oxygen deficiency and electrical conductivity. Electrochemical impedance spectroscopy studies of symmetrical cells showed that porous Nd0.8Sr1.2Ni0.8M0.2O4-δ electrodes applied onto Ce0.9Gd0.1O2-δ electrolyte exhibit quite similar performance, with lowest values of polarization resistance (0.8 Ω⸱cm2 at 800°C) observed for M = Ni. The polarization resistance can be further decreased (down to 0.04 Ω⋅cm2 at 800°C for M = Ni) by surface modification with PrOx.