Mogensen, Mogens Bjerg1; Hansen, Karin Vels1; Holtappels, Peter1; Jacobsen, Torben4
1 Department of Energy Conversion and Storage, Technical University of Denmark2 Fundamental Electrochemistry, Department of Energy Conversion and Storage, Technical University of Denmark3 Imaging and Structural Analysis, Department of Energy Conversion and Storage, Technical University of Denmark4 Risø National Laboratory for Sustainable Energy, Technical University of Denmark
The concept of solid oxide electrochemistry, which we understand as the electrochemistry of cells based on oxide ion conducting electrolytes of non-stoichiometric metal oxides, is briefly described. The electrodes usually also contain ceramics. The chemical reactants are in gas phase, and the electrochemical reactions take place at elevated temperatures from 300 and up to 1000 C. This has as consequence that the region around the threephase- boundary (TPB), where the electron conducting electrode, the electrolyte and the gas phase reactants meet, is the region where the electrochemical processes take place. The length of the TPB is a key factor even though the width and depth of the zone, in which the rate limiting reactions take place, may vary depending of the degree of the electrode materials ability to conduct both electrons and ions, i.e. the TPB zone volume depends on how good a mixed ionic and electronic conductor (MIEC) the electrode is. Selected examples of literature studies of specific electrodes in solid oxide cells (SOC) are discussed. The reported effects of impurities - both impurities in the electrode materials and in the gases – point to high reactivity and mobility of materials in the TPB region. Also, segregations to the surfaces and interfaces of the electrode materials, which may affect the electrode reaction mechanism, are very dependent on the exact history of fabrication and operation. The positive effects of even small concentrations of nanoparticles in the electrodes may be interpreted as due to changes in the local chemistry of the three phase boundary (TPB) at which the electrochemical reaction take place. Thus it is perceivable that very different kinetics are observed for electrodes that are nominally equal, but fabricated and tested in different places with slightly different procedures using raw materials of slightly different compositions and different content of impurities. Further, attempts of quantitative general description of impedance and i-V relations, such as the simple Butler-Volmer equation, are discussed. We point out that such a simple description is not applicable for composite porous electrodes, and we claim that even in the case of simple model electrodes no clear evidences of charge transfer limitations following Butler- Volmer have been reported. Thus, we find overall that the large differences in the literature reports indicate that no universal truth such as “this is the rate limiting step of H2 oxidation in a Ni-zirconia cermet electrode...” will ever be found because the actual electrode properties are so dependent on the fabrication and operation history of the electrode. This does not mean, however, that deep knowledge of mechanisms of specific SOC electrodes is not useful. On the contrary, this may be very helpful in the development of SOCs.