Potential Distributions and the Corresponding Driving Forces for Transport in Cathodes of Solid Oxide Fuel Cells

The mixed ion-electron conductive (MIEC) cathode materials with extended reaction zone have been attracting increasing interest due to their superior performance than the conventional cathode with active three-phase boundary. In the MIEC cathode, a complete electrode process consists of gas diffusion, surface reaction, surface or bulk diffusion and interface exchange. The relevant potentials controlling those processes include electrostatic potential, electrochemical potential of electron and ion. The distributions of those potentials and resultant influence on electrode processes are treated with Nernst and drift diffusion equations. The correlation between those potentials and the measured electrochemical impedance spectra are also discussed. It is found that the change of oxygen electrochemical potential determines the impedance of the processes whichever involves oxygen species, even though the electron electrochemical potential determines the impedance response directly. The assumption of electroneutrality in electrode is disproven and the resultant electrostatic gradient is established in the dense electrode model due to the uneven distributions of ions and electrons. Besides the concentration gradient, electrical field force produced by the electrostatic potential is proposed for the first time as one of the driving forces in the ion diffusion in electrode.