Combined experimental and modeling study of interaction between LSCF and CGO in SOFC cathodes

Abstract

The development of a high-performance oxygen electrode for SOFCs in order to achieve high power density at a stack level is still challenging. It is important to emphasize the factors controlling the efficiency of the cathode. Over the intrinsic electro-catalytic activity of the cathode material itself toward the oxygen reduction, the microstructural parameters such as the porosity, the tortuosity or the particle size are of major importance in the definition of the electrochemical active surface area. In this paper we focus on the behavior of La0.6Sr0.4Co0.2Fe0.8O3-α (LSCF)-Ce1-xGdxO2-α (CGO) composite cathodes with different development and characterization of composite cathodes produced by suspension spraying and sintering, with or without further surface modification by infiltration either LSCF or CGO nano-particles. Different symmetrical cells were produced by varying the LSCF/CGO ratio with an active surface area of about 12.57 cm2. Cells were contacted with a fine platinum mesh without any contacting paste and electrochemical impedance spectra (EIS) were recorded in static ambient air in the frequency range 10 mHz – 100 kHz between 500 °C and 800 °C. The serial resistance (Rs) and the total polarization resistance (Rp) were both quantified. The developed kinetic model of oxygen reduction at LSCF/CGO cathode, which incorporates elementary heterogeneous chemistry and electrochemical charge-transfer processes at two different electrochemical double layers, transport in the porous composite electrode, as well as gas supply, is used to explain the non-intuitive interaction between LSCF and CGO phases. This allows a mechanistic interpretation of the origin of different chemical, electrochemical and transport phenomena occurring in composite cathodes.