Composite cathodes for proton conducting electrolytes

Different composite materials made of mixed protonic/electronic conductors, SrCe_0.9Yb_0.1O3-d (10YbSC) or BaCe_0.9Yb _0.1O3-d (10YbBC), and a mixed oxygen-ion/electronic conductor, La_0.6Sr_0.4Co_0.2Fe_0.8O3 (LSCF), were investigated for cathode application in intermediate temperature solid oxide fuel cells (IT-SOFCs) using a high temperature proton conducting BaCe _0.8Y_0.2O3-d electrolyte. Only the LSCF/10YbBC composite was found to be chemically stable. Area specific resistance (ASR) measurements were performed in wet air for LSCF/10YbBC cathodes, changing the weight ratio between the phases and the sintering procedure. The best performance was obtained for the composite cathode containing 50 wt.-% of LSCF and 50 wt.-% of 10YbBC, sintered at 1,100 °C. Electrochemical impedance spectroscopy (EIS) measurements of the tested cathodes showed two depressed semicircles in the middle and low frequency range, respectively. Performing ASR measurements at different pO2 allowed us to attribute the two semicircles to charge transfer and oxygen diffusion processes, respectively. The microstructure of the LSCF/10YbBC(1:1) composite cathode was optimised changing the ratio of the particle sizes between the two phases. The lowest ASR values (0.14 X cm ² at 700 °C) were observed for the LSCF/ 10YbBC(1:1) composite cathode with different particle size (sub-micrometer particles for LSCF and nanometer particles for 10YbBC). Fuel cell polarisation curves demonstrated superior performance of the LSCF/10YbBC (1:1) cathode with respect to Pt.