Effect of Water Vapor Amount in Hydrogenous Atmospheres on Reducing Ability of the YSZ–NiO Fuel Cell Anode Material

Abstract

Anode ceramics of YSZ–NiO system for solid oxide fuel cells (SOFCs) has been investigated. A series of specimens were singly reduced in a hydrogenous atmosphere (the Ar–5 vol%Н₂ mixture) at 600°С under the pressure of 0.15 MPa or subjected to “reduction in the mixture–oxidation in air” (redox) cycling at 600°С. After both the treatment conditions, corresponding structures of YSZ–Ni cermets were formed and then subjected to dwell in “water vapor–Ar–5 vol% Н₂ mixture” atmosphere at 600°С under the pressure of 0.15 MPa. Additionally, behavior of the as-received material in this atmosphere was studied. It was revealed that small amount of water vapor in Ar–5 vol% Н₂ mixture (water vapor pressure below 0.03 MPa) does not affect the reduction of a nickel phase in YSZ–NiO ceramics but causes some changes in the YSZ–Ni cermet structure, in particular, growth of nanopores on tiny Ni particles. A higher concentration of water vapor in the mixture (water vapor pressure above 0.03–0.05 MPa) causes a converse change in the reduction kinetics. For as-received material, such amount of water vapor in the mixture is an obstacle for its reduction and causes reoxidation of a nickel phase at 600°С. For the material treated by redox cycling, better physical and mechanical properties were revealed after dwelling at 600°С in a water-depleted gas mixture. Contrary to this, after dwelling at 600°С in a water-enriched gas mixture, the material showed lower resistance against reoxidation. Based on the scanning electron microscopy and the data on the conductivity and strength, the dual effect of water vapor on durability of a nickel–zirconia anode is discussed.