Current electrolytes and catalysts

Stabilized zirconia (ZrO2) is recognized as the major candidate for the electrolyte material of solid oxide fuel cells (SOFCs) owing to its high ionic conductivity and chemical stability. The ionic conductivity originates from the oxygen vacancies introduced by doping 2+ or 3+ valence cations on the Zr4+ site. The conductivity shows the maximum at a certain dopant concentration. Although stabilized zirconia is a dominantly ionic conductor, electronic conductivity is not completely zero. Taking into account both the ionic and the electronic conductivities, the optimum electrolyte thickness is calculated to be 10–100 µm when the current density of 0.1–1 A cm−2 is generated at 1000 °C. Electrode materials should have high catalytic activity, high electronic conductivity, and high chemical and morphological stability. Currently, (La,Sr)MnO3 and Ni–YSZ cermet are the most popular cathode and anode, respectively. The crystal lattice of (La,Sr)MnO3 has a distorted perovskite type structure. The symmetry varies with oxygen partial pressure and temperature. Electrical conductivity of La1−xSrxMnO3 (x = 0–0.4) ranges from 100 to 300 S cm−1 at 1000 °C in air. Oxygen diffusion coefficient is 10−12 cm2 s−1, which is not enough to supply oxygen via the electrode bulk diffusion. Although catalytic activity is lower than a mixed conductor oxides such as (La,Sr)CoO3, (La,Sr)MnO3 has higher compatibility with YSZ electrolyte. Nickel is a superior catalyst for the electrochemical and chemical reactions when hydrogen or hydrocarbon gases are used as the fuel. The problem with the nickel anode is its morphological instability and thermal expansion mismatch with YSZ. Thus, nickel powders are used in a form of Ni–YSZ cermet. Electrical conductivity of the cermet electrode varies drastically around the percolation threshold. The optimum nickel content is around 40 vol%. Electrode reaction kinetics has been investigated intensively for the cathode and the anode. The major origin of the electrode overvoltage is assumed to be the shift of oxygen potential at the electrode/electrolyte contact, which is caused by the formation of oxygen potential gap or gradient to promote the rate limiting reactions. The current–voltage relationship and the electrochemical impedances are well explained under the assumption, and the reaction models were developed by the reaction order analyses. Keywords: electrolyte; conductivity; electronic conductivity; ionic conductivity; chemical stability; mechanical strength; cost; stabilized zirconia; YSZ; O2− ion conductor; crystal structure; fluorite type structure; phase relationship; oxygen vacancy; mobility; long term operation; activation energy; electron; hole; electrolytic domain; electromotive force; optimum thickness; open circuit voltage; oxygen permeation; efficiency; electrode material; catalytic activity; compatibility; morphological stability; thermal expansion; cathode material; LaCoO3; LaFeO3; LaMnO3; (La; Sr)MnO3; perovskite; oxygen nonstoichiometry; cation vacancy; diffusion coefficient; isotope diffusion coefficient; La2Zr2O7; SrZrO3; kinetic decomposition (unmixing; demixing); anode material; Ni; Ni-YSZ cermet; percolation threshold; fabrication; electrode reaction kinetics; porous electrode; electrode/electrolyte interface; gaseous diffusion; adsorption; dissociation; surface diffusion; charge transfer; steam reforming reaction; shift reaction; origin of the electrode polarization; local equilibrium; rate determining step; triple phase boundary (TPB); oxygen activity; Langmuir-type adsorption; cathode reaction kinetics; polarization curve; anode reaction kinetics; hydrogen oxidation; patterned nickel electrode; hydrocarbon oxidation; methane; cracking; steam/carbon ratio; carbon deposition