Ni-Based Solid Oxide Cell Electrodes

This paper is a critical review of the literature on nickel-based electrodes for application in solid oxide cells at temperature from 500 to 1000 °C. The applications may be fuel cells or electrolyser cells. The reviewed literature is that of experimental results on both model electrodes and practical composite cermet electrodes. A substantially longer three-phase boundary (TPB) can be obtained per unit area of cell in such a composite of nickel and electrolyte material, provided that two interwoven solid networks of the two solid and one gaseous phases are obtained to provide a three - dimensional TPB throughout the electrode volume. Variables that are used for controlling the properties of Ni-cermet electrodes are: (1) Ni/YSZ volume ratio, and (2) porosity and particle size distribution, which mainly affected by raw materials morphology, application methods and production parameters such as milling and sintering. The various electrode properties are deeply related to these parameters, but also much related to the atomic scale structure of the Ni-electrolyte interface, which in turn is affected by segregation of electrolyte components and impurities as well as poisons in the gas phase. The main emphasis will be on the following subjects: (a) electronic conductivity of cermets, (b) dimensional and thermodynamic stability including redox cycling, (c) thermal expansion coefficient matching, (d) chemical compatibility with stack components and gaseous reactants and (e) electrode reaction mechanism and polarisation resistance. A brief discussion of the main concepts in the modelling literature is given in context of the latter subject.