ABSTRACT
In solid oxide fuel cells (SOFCs), the cathode is the material where pure oxygen or oxygen from air is reduced through the following electrochemical reaction [1]
O V O
2 2 4 2+ + =-e (3.1)
where, in Kröger-Vink notation, V O •• is a vacant oxygen site, and O
on a regular oxygen site in the Y2O3-ZrO2 (YSZ) lattice. As illustrated by Equation (3.1), the oxygen reduction process requires the presence of oxygen and electrons as well as the possibility for generated oxide ions to be transported away from the reaction site into the bulk of the electrolyte. When the electrode material and the
3.1 Introduction ................................................................................................... 131 3.2 Lanthanum Manganite-Based Perovskites .................................................... 132
3.2.1 Structure, Oxygen Nonstoichiometry, and Defect Model ................. 132 3.2.2 Electronic Conductivity and Thermal Expansion Coefficient .......... 137 3.2.3 Oxygen Diffusion and Surface Exchange Coefficient ...................... 139 3.2.4 Polarization, Activation, and Microstructure Optimization ............. 141
3.3 Lanthanum Cobaltite and Ferrite Perovskites ............................................... 146 3.3.1 Structure, Oxygen Nonstoichiometry, and Defect Model ................. 146 3.3.2 Electronic Conductivity and Thermal Expansion Coefficient .......... 147 3.3.3 Oxygen Diffusion and Surface Exchange Coefficient ...................... 150 3.3.4 Electrochemical Polarization Performance ...................................... 150
3.4 Other Perovskite Oxides ............................................................................... 154 3.5 Interaction and Reactivity with Other SOFC Components ........................... 156
3.5.1 Interaction with the Electrolyte ......................................................... 157 3.5.1.1 Interaction with YSZ Electrolyte ....................................... 157 3.5.1.2 Interaction with LSGM ...................................................... 161
3.5.2 Interaction with Fe-Cr Alloy Metallic Interconnect ......................... 162 3.5.3 Interaction with Other SOFC Components ....................................... 165
3.6 Performance Stability and Degradation ........................................................ 167 3.7 Summary and Conclusions ........................................................................... 170 References .............................................................................................................. 171
electrolyte material possess only electronic and ionic conductivity, respectively, such as Sr-doped LaMnO3 (LSM) electrode and YSZ electrolyte, these criteria are fulfilled in the vicinity of the triple-phase boundary (TPB) between the electrode, electrolyte, and oxidant gas. If the electrode material possesses mixed electronic and ionic conductivity, for example, Sr-doped LaCoO3 (LSC), oxygen species may be transported through the bulk of the electrode and the reaction zone could be extended to the electrode surface away from the electrode-electrolyte interface. Under certain conditions, electronic conductivity may be induced in the electrolyte surface close to the TPB, which will also expand the reaction zone around the TPB. Despite the extensive studies there still exist considerable disagreements on the exact reaction sites for the O2 reduction even on the most common cathode such as LSM [1].
