Redox Behaviour of Potential SOFC Cathode Materials La_1.6Sr_0.4Ni_1-xCo_xO_4+δ (x = 0.6 and 0.8) Determined from In Situ Neutron Powder Diffraction under Flowing O₂ and 5%H₂

Abstract:

The n = 1 Ruddlesden-Popper (RP) oxides La_1.6Sr_0.4Ni_1-xCo_xO_4+δ (x = 0.6 and 0.8) have been prepared by the citrate-gel method and studied using thermal analysis and in situ neutron powder diffraction (NPD) under oxygen (x = 0.6) and subsequent 5%H₂ flow. On heating under O₂, the x = 0.6 sample loses oxygen from the interstitial site until it is emptied at 475 °C. Subsequent heating in 5%H₂ results in reduction which proceeds within two-steps; the first one occurs between ~170 and 300 °C and involves removal of the interstitial oxygen (O_int), the second step starting at ~ 400 °C and going on up to 600 °C (the highest temperature reached) involves oxygen deintercalation from the equatorial position (O_eq). After 20 min of isothermal heating at 600 °C, unless a thermodynamic equilibrium state was not reached there was no sign of impurity related to the decomposition of the sample and the composition refined to La_1.6Sr_0.4Ni_0.4Co_0.6O_3.85(3). The x = 0.8 sample exhibits similar behavior under hydrogen with slight shifts in temperature and the composition reached after the reducing cycle refined to La_1.6Sr_0.4Ni_0.2Co_0.8O_3.85(1). This study demonstrates that the Co-rich La_1.6Sr_0.4Ni_1-xCo_xO_4+δ oxides can withstand hydrogen reducing conditions with topotactic deintercalation of O_int and O_eq but are not stable enough at 600 °C in comparison to La_1.5Sr_0.5Ni_0.5Co_0.5O_4+δ owing to increased Sr/La ratio in the latter.