Effect of densification processes on the properties of plasma-sprayed YSZ electrolyte coatings for solid oxide fuel cells

Atmospheric plasma spraying (APS) was employed to deposit yttria-stabilized zirconia (YSZ) electrolyte for cermet supported tubular solid oxide fuel cells (SOFCs) to reduce manufacturing cost. A post-spray densification process was conducted for the APS-sprayed YSZ coating in order to achieve a required coating density for gas tightness and improved electrical conductivity. The deposition of the YSZ layer was carried out at ambient atmosphere using commercial ZrO₂-8 wt.% Y₂O₃ powder. The densification of the YSZ coating was attained through impregnating yttrium and zirconium nitrate solutions into the porous coating, followed by thermal decomposition of nitrates within the pores of the coating through annealing treatment. The effects of impregnation number on gas permeability and electrical conductivity of the YSZ electrolyte have been examined. It was found that the gas permeability of the YSZ electrolyte layer after the densification treatment reaches a level of 2×10^-7 cm⁴ gf^-1 s^-1, which is a necessary value for practical operation of SOFCs. The test cell yielded an open circuit voltage of 1.0 V at 930 °C. Furthermore, the anisotropy of the electrical conductivity of the YSZ coating is clearly elucidated by comparing the present result to the reported data. The electrical conductivity of the as-sprayed YSZ coating showed a value of 0.53 S m^-1 from the through-thickness direction at 900 °C. The post-densification treatment using nitrate solution improved the electrical conductivity of the YSZ layer by ∼30%.