Investigation of rate-determining step of LaNi_0.6Co_0.4O_3-δ film electrode

The rate-determining step (RDS) in the oxygen reduction process on a LaNi_0.6Co_0.4O_3-δ film electrode was analyzed using electrochemical impedance spectroscopy in varied temperatures (873–1073 K), oxygen partial pressures (1–10⁻³ bar), and DC bias. From the evaluation of the area-specific interfacial conductivity, σ_E, and chemical capacitance, C_E, it is suggested that the RDS is surface reaction under low p(O₂) (10⁻²–10⁻³ bar) but there appears a contribution of bulk diffusion under high p(O₂) in the film electrode at any given temperature. The change of the RDS was also confirmed using isotope exchange depth profile method where a significant change in the oxygen profile in the film electrode was observed. This is attributed to the rate of the bulk diffusion of O²⁻ which is almost constant when the oxygen nonstoichiometry of LaNi_0.6Co_0.4O_3-δ is small, whereas the surface reaction rate strongly depends on p(O₂). At low p(O₂), the surface reaction is slower than the bulk diffusion, so that the potential drop on the surface of the electrode film is large. In contrast, at high p(O₂), the surface reaction is faster than the bulk diffusion where the potential decay is sluggish inside the film electrode.