Factors governing surface exchange kinetics in undoped and strontium/iron co-substituted La2NiO4+δ

Surface oxygen exchange in the La2NiO4+δ and La1.5Sr0.5Ni1-yFeyO4+δ (y = 0.3, 0.4) oxides is analyzed using the data on oxygen permeability through the membranes with different thicknesses measured under various oxygen partial pressure P(O2) gradients in the 800–1000 °C range. The increase in P(O2) gradient induced surface limitations in La2NiO4+δ leading to a predominant role of surface exchange in the overall oxygen flux. The origin of surface exchange limitations in La2NiO4+δ is ascribed to a relatively fast decrease in oxygen excess and Ni3+ concentration with P(O2) reduction compared to La1.5Sr0.5Ni0.7Fe0.3O4+δ and La1.5Sr0.5Ni0.6Fe0.4O4+δ, which retained an oxygen excess. Faster surface exchange kinetics for La1.5Sr0.5Ni0.6Fe0.4O4+δ in comparison with that for La1.5Sr0.5Ni0.7Fe0.3O4+δ is interpreted on the basis of surface microstructure obtained by electron backscatter diffraction (EBSD). It is suggested that the observed changes in size, shape and crystallographic orientation of grains in La1.5Sr0.5Ni0.6Fe0.4O4+δ (compared to La1.5Sr0.5Ni0.7Fe0.3O4+δ) could result in a higher amount of 3d-metal cations in surface layers of the oxide.