MSE PhD Prospectus Defense: Kaixin Suo

TITLE: Advanced Surface/Interface Modifications for High Performance Solid Oxide Cell Systems

ADVISOR: Soumendra Basu, ME, MSE

COMMITTEE: Uday B. Pal, ME, MSE, Srikanth Gopalan, ME, MSE

ABSTRACT: Metal-supported solid oxide cells (MS-SOCs) offer high power density and strong mechanical stability. However, the large surface area of ferritic stainless-steel metal supports makes them prone to long-term degradation. On high temperature exposure, insulating Cr₂O₃ form on the air electrode side surface and release vapor-phase Cr (VI) species, leading to chromium poisoning of the air electrode. The complex geometry of the pore structure prevents conventional coatings methods from fully covering the internal surfaces, leaving them exposed to oxidation and chromium poisoning. To protect the surfaces of the porous metal support, alternating-current electrophoretic deposition (AC-EPD) was used to deposit CuNi₀.₂Mn₁.₈O₄ spinel coatings on SUS430. These coatings adhered well, slowed the growth of thermally grown oxides (TGO), and reduced chromium release. An area-specific resistance (ASR) model was developed combining conductivities of coatings with different Cr contents due to a Cr diffusion profile, temperature, chromium diffusion coefficients and TGO rates. This model showed that the EPD coatings effectively inhibit the out diffusion of Cr and reduce the area-specific resistance (ASR). The model predicts that after 50,000 hours of operation, the ASR of a coated sample would to be less than 1/10th of that of an uncoated sample. The metal supports also contain active layers consisting of a fine microstructure of YSZ and SUS430 grains. The spacing between grains of the two adjacent phases is too small to coat by the EPD method. Consequently, the volume expansion when the SUS430 forms surface Cr2O3, leads to stresses that form cracks in the YSZ grains. To alleviate this problem, atomic layer deposition (ALD) was investigated to achieve conformal coatings inside these fine-grained structures. The effects of ALD deposition parameters were studied for MnOₓ and CoOₓ ALD processes on flat SUS430 and Si, then adapted to porous substrates using a ‘static’ mode to improve precursor penetration. By alternating Mn–O and Co–O cycles, Mn–Co–O films with desired MnCo₂O₄ stoichiometry were achieved. Both as deposited film and annealed films formed crystalline spinel structures. Combining EPD and ALD coatings has the potential of protecting the metal supports at multiple scales, blocking chromium transport, reducing stress from oxidation, and offering a scalable way to improve the durability of MS-SOCs. This conjecture will be validated as part of future work. Additional future work will involve translating the Mn-Co-O ALD results to the Gd-Ce-O system to deposit GDC thin films in the Ni/YSZ fuel electrodes of solid oxide cells. It is conjectured that these coatings will inhibit Ni coarsening during cell operation, thereby reducing loss of Ni percolation, and minimizing performance degradation. Also, the structural stability of the YSZ electrolytes in SOCs will be examined by Raman spectroscopy as a function of the mode of SOC operation (SOFC, SOEC and RSOC) and current density.