Morphological, structural and electrochemical analysis of sputter-deposited ceria and titania coatings for MCFC application

摘要

In order to protect the MCFC nickel cathode, TiO_2 and CeO_2 coatings were prepared by DC reactive magnetron sputtering. These oxides are stable thermodynamically whatever the cathode or anode gaseous conditions. Good quality, dense and homogeneous coatings were obtained at thicknesses lower than 1 mu m. The structure of the deposits, as analysed by XRD, was the expected one. In this work only dense nickel substrates were used. After their direct immersion in a Li_2CO_3-Na_2CO_3 carbonate eutectic at 650 deg C, which can be considered as extremely corrosive conditions with respect to the usual MCFC conditions, the coatings were affected. TiO_2 coatings were transformed into Li_2TiO_3, in agreement with thermodynamic predictions; however, they became progressively unstable, which was probably due to a problem of mechanical adhesion rather than to solubility. The thinner was the deposit, the higher was its conductance and the closer to that of a pure Ni electrode was its electrocatalytic activity. CeO_2 coatings were stable in a ceria form and their adhesion was better even though not fully satisfactory. These first preliminary results are promising regarding the direct contact of the coatings with the corrosive carbonate melt, but the improvement of the adhesion is one of the major problems to solve.