Enhancement in Three-Phase Boundary of SOFC Electrodes by an Ion Impregnation Method: A Modeling Comparison

Abstract

Solid oxide fuel cells SOFCs are promising to be the nextgeneration energy-conversion devices due to their high efficiency and ultralow pollution emission. 1 Many efforts have been made recently to lower their operating temperatures from conventional 1000°C to 600–800°C, in order to significantly reduce the manufacturing cost and improve the stability of the SOFC system. At a reduced operating temperature, the electrode performance becomes the most important determinant of the overall cell output, especially when a thin-film electrolyte i.e., 5–20 m is applied. The electrode performance is believed to be determined by the sum of various polarizations typically associated with the length of the socalled three-phase boundary TPB where the electronic conductor, ionic conductor, and gases are in contact with each other so that the electrochemical reaction can take place. Therefore, a large TPB length is generally essential for high electrode performance. Several experimental studies demonstrated the inverse proportionality of activation overpotential with respect to the TPB length in standard composite electrodes such as Ni–yttria-stabilized zirconia YSZ anodes 2,3 and La,SrMnO3 LSM–YSZ cathodes. 4,5 A composite electrode is usually composed of an electronic conducting phase, an oxygen-ion conducting phase, and pores for gas transportation. Typical examples of the electronic phases are Ni and LSM, which also serve as the electrocatalyst in the anode and cathode, respectively. The ionic phase is generally an electrolyte material such as YSZ and doped ceria DCO. The TPB length of such a composite electrode is dominantly affected by its microstructure characteristics including particle size, porosity, and distribution state of the electronic and ionic conducting phases. Various electrode models have been established to predict and improve the performance of the composite electrode with regards to its microstructure parameters.