PROTECTIVE CO3O4 COATINGS FOR SOEC INTERCONNECTS SYNTHESIZED BY ATMOSPHERIC PRESSURE PECVD: OXIDATION AND ELECTRICAL PERFORMANCE

This PhD work investigates the use of Atmospheric Plasma-Enhanced Chemical Vapor Deposition (AP-PECVD) to produce Co3O4 protective coatings on AISI 441 interconnects for Solid Oxide Electrolyzer Cells (SOECs).
The first part of the manuscript is dedicated on the understanding of the growth mechanisms of Co3O4 coatings by AP-PECVD. The effects of deposition parameters (e.g., temperature, gas composition and atmosphere) on the coating morphology, stoichiometry, and composition are analyzed. Plasma energetic species, notably reactive oxygen-nitrogen species (produced after interaction with air), governed the film composition and crystallinity. The AP-PECVD technique enables the formation of pure, dense Co3O4 coatings with excellent substrate adhesion and minimal carbon contamination.
In the second part of the manuscript, the oxidation and electrical performances of the Co3O4 coated AISI 441 substrates are investigated under SOEC operating conditions (700 – 850 °C). Two AP-PECVD procedures were evaluated: fast vs. slow plasma nozzle displacement speed. Fast displacement speed promoted the formation of more homogeneous Co3O4 coatings, which significantly reduce the oxidation kinetics (factor 2 on the parabolic rate constant) and the Cr volatilization rate (by one order of magnitude) compared to uncoated AISI 441 substrates.
The oxide scale growth mechanism was examined using sequential oxidation test with labelled O2 molecules and short and long-term oxidation tests in air at 800 and 850 °C up to 5000 h of exposure. Co3-xMnxO4 grows outwardly by Mn oxidation and diffusion into the Co3O4 coating. Simultaneously, a Cr2O3 layer grows below it and a Cr-rich Reactive Layer (RL) is formed between Cr2O3 and the external Co-Mn spinel. It is demonstrated that the Cr2O3 and RL presented competitive growth, strongly correlated to the Mn/Co concentration ratio of the external Co-Mn spinel layer. Such behaviour is discussed.
Finally, in-situ Area Specific Resistance (ASR) measurements are conducted to investigate the electrical performance time evolution of the coated AISI 441 substrates. The Co3O4 coatings improve the electrical performance in comparison to uncoated AISI 441 substrate by exhibiting lower ASR values after long-term exposure and reduced ASR increasing rate. Such improvement is due to the lower growth rate of Cr-rich oxide layer and the drastic decrease of the volatilization rate of Cr. Several initial Co3O4 coating thicknesses (400-2000 nm) are assessed during this work. The 2000 nm-Co3O4 coating shows the best electrical performance, with a predicted lifetime below the maximum accepted ASR value (50 mΩ·cm2) of 5.8 years, higher than the minimum 4.5 years required.