Surface Oxygen Exchange Properties of Sr Doped La2NiO4+δ as SOFC Cathode: Thin-Film Electrical Conductivity Relaxation Investigation

Roddlesden-Popper Phases, such as La2NiO4, have been proposed as promising intermediate temperature SOFC cathodes, due to their high surface oxygen exchange and bulk diffusivities. However, relatively low electrical conductivity of LNO phase is a concern for its application. It has been shown that Sr doping is an effective way to improve LNO’s electrical conductivity, but it is important to investigate the effect of such doping on other properties such as oxygen surface exchange coefficient. In this study, Sr doped lanthanum nickelates i.e., La_2-xSr_xNiO_4+δ (0≤x≤0.4) thin and dense films on dense thick GDC (~1mm) substrate, were prepared by using spray-modified pressing method. The surface oxygen transport kinetics was investigated by electrical conductivity relaxation (ECR) technique. Since the thickness of thin films is ~15μm, much less than the characteristic length of LNO, the oxygen transport kinetics are controlled by the surface exchange steps and the oxygen ion concentration is uniform in the film at any time during the ECR process. Such situation enables a more accurate fitting results compared to the traditional fitting procedure based on thick pellets, because only k value is the unknown parameter. The fitted k for LNO is 4.02×10^-5cm/s at 0.2atm 700^oC, which decreases with lowered oxygen partial pressure. Sr doping harmed the surface exchange kinetics, k of Sr40 is about one order of magnitude smaller than undoped one. Interstitial oxygen and Ni oxidation state are suggested to be predominant roles in determining the surface kinetics. Both Sr doping and lowered oxygen partial pressure lead to the loss of interstitial oxygen and the oxidation of Ni²⁺, resulting the lowered surface exchange rate.