Effect of SOFC Interconnect-Coating Interactions on Coating Properties and Performance

Abstract

The high operating temperature of solid oxide fuel cells (SOFCs) provides good fuel flexibility which expands potential applications, but also creates materials challenges. One such challenge is the interconnect material, which was the focus of this project. In particular, the objective of the project was to understand the interaction between the interconnect alloy and ceramic coatings which are needed to minimize chromium volatilization and the associated chromium poisoning of the SOFC cathode.This project focused on coatings based on manganese cobalt oxide spinel phases (Mn,Co) 3 O 4 , which have been shown to be effective as coatings for ferritic stainless steel alloys. Analysis of diffusion couples was used to develop a model to describe the interaction between (Mn,Co) 3 O 4 and Cr 2 O 3 in which a two-layer reaction zone is formed. Both layers form the spinel structure, but the concentration gradients at the interface appear like a two-phase boundary suggesting that a miscibility gap is present in the spinel solid solution. A high-chromium spinel layer forms in contact with Cr 2 O 3 and grows by diffusion of manganese and cobalt from the coating material to the Cr 2 O 3 . The effect of coating composition, including the addition of dopants, was evaluated and indicated that the reaction rate could be decreased with additions of iron, titanium, nickel and copper.Diffusion couples using stainless steel alloys (which form a chromia scale) had some similarities and some differences as compared to those with Cr 2 O 3 . The most notable difference was that the highchromium spinel layer did not form in the diffusion couples with stainless steel alloys. This difference can be explained using the reaction model developed in this project. In particular, the chromia scale grows at the expense of the alloy, the high-chromia layer grows at the expense of chromia scale and the high-chromia layer is consumed by diffusion of chromium into the coating material. If the last process (dissolution of high-chromium spinel phase) is faster than the second process (formation of highchromium spinel phase), the high-chromium layer may be consumed. The other important result of this mechanism is that it could result in a constant scale thickness if the scale forms at the same rate as it is consumed. This helps to explain the unexpected observation that the area specific resistance (ASR) of a SOFC with a (Mn,Co) 3 O 4 -coated ferritic stainless steel cathode becomes constant after long exposures.The project also evaluated the possibility of reducing the chromium content in a stainless steel alloy using experimental alloys. The conclusion of this evaluation is that at least 17-18% chromium is needed for good oxidation resistance is needed even if the alloy is coated with a spinel coating.Additional details on these findings are provided in a later section of this report and in the publications listed below. Submitted in Near FutureGOALI project with NexTech Materials on SOFC coating materials, National Science Foundation, ~$300,000, 10...

Transition Metal Spinel Oxide Coatings for Reducing Chromium Poisoning in SOFCs

“…In Figure 1a, the distance is referenced to the surface of the spinel pellet after removal of any remaining Cr 2 O 3 . Consistent with an earlier report [9], a two-layer interaction zone forms. The layer near the Cr 2 O 3 (to the right in Figure 1) contains approximately two chromium atoms per M 3 O 4 formula unit, which is the maximum concentration possible assuming that Cr forms a valence of 3+ or higher and Mn/Co form valences of 2+ or higher.…”

Pulsed Current Electrodeposition of Mn-Cu Alloys on SUS 430 Substrate as Coating Material for SOFC Interconnect Application

“…One of the supporting components of SOFC stack is interconnect. The interconnect conducts the electrical current between the two electrodes through the external circuit and simultaneously exposed to both high oxygen partial pressure (air) and low oxygen partial pressure (fuel) [3]. One of the candidates for interconnect material is ferritic steel.…”

Protective CuO Coating Synthesis via Electrodeposition of Copper on AISI 430 Ferritic Steel for SOFC Interconnect Applications

“…One of the supporting components of fuel cell stack is interconnect. The interconnect conducts the electrical current between the two electrodes through the external circuit and simultaneously exposed to both high oxygen partial pressure (air) and low oxygen partial (fuel) [2].…”