Investigation of rate-determining step of LaNiCoO film electrode

The rate-determining step (RDS) in the oxygen reduction process on a LaNiCoO film electrode was analyzed using electrochemical impedance spectroscopy in varied temperatures (873–1073 K), oxygen partial pressures (1–10 bar), and DC bias. From the evaluation of the area-specific interfacial conductivity, σ, and chemical capacitance, C, it is suggested that the RDS is surface reaction under low p(O) (10–10 bar) but there appears a contribution of bulk diffusion under high p(O) 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 O which is almost constant when the oxygen nonstoichiometry of LaNiCoO is small, whereas the surface reaction rate strongly depends on p(O). At low p(O), 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(O), the surface reaction is faster than the bulk diffusion where the potential decay is sluggish inside the film electrode.