The Course of Oxygen Partial Pressure and Electric Potentials across an Oxide Electrolyte Cell

Solid oxide fuel cells and electrolyzers are usually based on yttrium stabilized zirconia (YSZ) as electrolyte. Although the transport number of oxide ions in YSZ at normal conditions is not below 0.999 the electron and hole conductivity may be of practical importance. In long term operation it may cause oxygen pressure to build up in closed cavities inside the electrolyte. Fundamental concepts as electric and electrochemical potential are discussed, and revive the concept electromotive potential originally introduced by Bro̸nsted. This potential is equivalent to the Fermi level and is useful in the discussion of electronic conduction in mixed conductors. On the bases of a solution to the transport equations for oxide ions, electrons and holes, potential profiles across the cell are calculated for fuel cell and electrolyzer applications. One main result of the analysis is the verification that in steady state operation, the oxygen pressure inside a cavity can never exceed the value corresponding to the electrode potential.