Reliable Determination of Surface Exchange and Bulk Diffusion Coefficients of La2NiO4+Δ Cathode Materials for SOFC

Accurate assessment of the electrode candidates for solid oxide fuel cell (SOFC) is crucial in understanding the underlying mechanisms of the performance, and in focusing our effort to the most promising ones. The evaluation of those materials can be divided into individual criterion so that unambiguous conclusions can be made on different aspects of the materials, such as the bulk diffusion (D) and surface exchange (k) coefficients. In this study, k of lanthanum nickelates La₂NiO_4+δ is characterized by electrochemical impedance spectroscopy (EIS) on dense electrode symmetric cells, and by electrical conductivity relaxation (ECR) on GDC-supported thin layer samples. Surface-dominant situation is confirmed in both approaches. k derived from polarization resistance is ~8.6×10^-7 cm/s at 700^oC in air, in comparison to ~1.6×10^-5 cm/s fitted from ECR testing. The correlation between those two values is discussed based on the definitions and exchange mechanisms. To determine D accurately, pellet samples with D or D&k co-control are measured with ECR. The pre-known k values from the thin layer samples facilitate reliable fitting on D by reducing k variable in the equation. Such ECR strategy of avoiding D&k interplay successfully yields electrochemical parameters in good accuracy.