Oxygen Reduction Reaction Kinetics in Sr-Doped La₂NiO_4+δRuddlesden-Popper Phase as Cathode for Solid Oxide Fuel Cells

In this paper, electro-catalytic reduction of oxygen in Sr-doped lanthanum nickelates, La 2-x Sr x NiO 4+δ (0 ≤ x ≤ 0.4) Ruddlesden-Popper (R-P) phase, has been investigated. The oxygen reduction reaction (ORR) kinetics is evaluated via electrochemical impedance spectroscopy (EIS) with the symmetric cell configuration. The maximum performance is achieved with the un-doped La 2 NiO 4+δ , ∼0.13 Ωcm 2 at 800°C. Sr doping decreases the electrode performance progressively. Further detailed analysis indicates that bulk ionic diffusion and surface oxygen exchange co-limit the ORR of those cathodes. Sr substitution leads to both lowered bulk diffusion and surface exchange rates. With high Sr content ( x = 0.3, 0.4), oxygen ion transfer resistance between nickelate/electrolyte is observed. As for the surface exchange process, oxygen adsorption is suggested to be the main rate-limiting step (RLS) according to the reaction orders, which is retarded further by the oxidation of Ni 2+ to Ni 3+ as Sr content increases. The possible role of incorporation process in determining the overall reaction rate is also discussed.