Torben Jacobsen

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 Bronsted. 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.

A Critical Review of Models of the H2/H2O/Ni/SZ Electrode Kinetics

Various models of the H2/H2O/Ni/SZ (SZ = stabilized zirconia) electrode kinetics have been presented in the literature in order to explain the reported experimental data. However, there has been a strong tendency of using a limited set of data to “verify” a given model, disregarding other data sets, which do not fit the model. We have inspected some models in the literature, and problems (e.g. no quantitative model has explained the large variation in reported values of apparent activation energy of the electrode kinetics) as well as strengths of the models are discussed. We point out experimental findings that a useful model must be able to explain such as difference in sensitivity to poisoning by H2S due to differences in the detailed composition of the SZ and large change in apparent activation energy by change in cermet preparation. Finally, we will point out some elements, which seem important for any realistic and useful mathematical model of the H2/H2O/Ni/SZ electrode.