Generalized Ellingham Diagrams for Utilization in Solid Oxide Fuel Cells With an Emphasis on Interactions of Nickel Cermet Anode With Impurities in Coal Synthesis Gas

Generalized Ellingham diagrams have been constructed for those systems which are important in thermodynamic investigations on the chemical stability of nickel cermet anode against gaseous impurities contained in coal derived gases. In the same way as the original Ellingham diagram, the oxygen potential and temperature are adopted as the vertical and horizontal axes to make detailed examinations on related phase equilibria in terms of operational parameters of solid oxide fuel cells. As impurities, S, Se, As, and P are considered in their interactions with nickel as the metal component in cermet anode, and with zirconia, yttria, and ceria as the oxide component in the cermet anode. Since impurities come into cells as gaseous impurities, diagrams are constructed so as to fix the partial pressure of such gaseous species at a given value like 10(-6), 10(-9) and 10(-12) atm, while the partial pressure of main components in the cell operation such as O-2, H-2 or H2O is always fixed at 1 atm. The dominant areas of such gaseous species are displayed in an analogous way to the aqueous species in the Pourbaix diagram where equilibria between aqueous species and condensed phases are plotted in pE vs. pH. The multicomponent Ellingham diagrams for the Ni-M-O-H (M = S, Se, As, P) systems were constructed in a similar manner to the multicomponent Pourbaix diagrams and examined in terms of the reactivity of nickel anodes with phosphorus compounds in SOFCs, in terms of operational variables such as temperature, oxygen potential, overpotential under the anode polarization, etc. Comparison with available experimental observations is also made in terms of the normal chemical process or of the electrochemical process.