Sulfur poisoning on SOFC Ni anodes: Thermodynamic analyses within local equilibrium anode reaction model

Possible effects of sulfur on solid oxide fuel cell (SOFC) Ni anodes have been analyzed from the thermodynamic considerations of Ni-S-C-O-H systems by constructing the predominant area diagrams as the Ellingham diagram (oxygen potential vs temperature plot) with a parameter of partial pressure (10 ^-4 or 10^-6 atm) of sulfur-containing gaseous species such as H₂ S. Focus was made on the sulfur potential and its role on the sulfur adsorption on Ni, the sulfur dissolution into Ni, or the Ni sulfide formation, particularly on the eutectics formation. The important features of sulfur poisoning have been extracted and discussed as follows: (i) Under equilibrated conditions with 1-100 ppm contamination of H₂ S around 1073 K, Ni anodes are stable against sulfide formation up to a high utilization such as 90%. (ii) When the anode overpotential is strongly related to the change in oxygen potential in the vicinity of three-phase boundaries (TPBs), Ni can be sulfurized; in worse cases, Ni-S eutectic liquids can be formed. (iii) Several factors that can lead to serious damages are discussed in terms of diffusion of sulfur inside nickel, enhancement accumulation of sulfur at TPBs due to the electrochemical oxidation of H₂ S, and enrichment of sulfur in the lower temperature region in stacks.