Strategies to Improve the Effectiveness of the Thin Film Coated Interconnects

Abstract

Interconnects made of ferritic stainless steel (FSS) are used in solid oxide cells (SOC), both in fuel cells and electrolyzers, owing to a coefficient of thermal expansion similar to that of ceramic components, high oxidation resistance, good thermal conductivity, and adequate electrical conductivity of Cr2O3 scale at high temperatures. However, corrosion of the FSS interconnects at operating conditions between 600 – 900°C leads to high degradation rates and is a critical area for improving the SOC's performance.The two significant issues with the FSS interconnects are 1. Chromia scale growth 2. Chromium evaporation. Protective chromia scale forms on the FSS upon exposure to high temperature, which grows continuously with time. The growing chromia scale increases the resistance across the cell, reducing cell efficiency. Additionally, the chromia scale reacts with oxygen in the presence of water vapour to form volatile Cr(VI) species such as CrO2(OH)2. These volatile Cr (VI) species interact with the cathode materials and get deposited at the triple-phase boundaries, hindering the oxygen electrochemical reaction on the cathode. This results in a decrease in the activity of the oxygen reduction reaction at the cathode referred to as cathode/ chromium poisoning.Despite developing speciality steels like Crofer 22H and ZMG232, chromium evaporation is very high for the long-term performance of SOFC. Applying coatings is a widespread solution to suppress chromium evaporation. Manganese cobalt oxide (MCO) based coatings are extensively studied for this purpose. Physical vapour deposition deposited metallic Ce/Co coatings, which form MCO up oxidation, have been extensively studied for up to 38000 hours. Most of these studies were performed on as-received coatings, as opposed to the conditions in the stack. Commonly stacks are conditioned at high temperatures, usually 100-250 °C above the operating temperatures, to ensure gas-tightness in the stack.To understand the influence of stack preheating on the coated interconnects, Ce/Co coated AISI 441 and Crofer 22 APU were pre-oxidized at 950 °C for 2 hours. After the pre/oxidation, the exposures were conducted in horizontal quartz tube reactors at 850oC for 1000 hours in air humidified with 3 % H2O. The oxidation kinetics of these coated steels was studied. Area-specific resistance (ASR) measurements were studied ex-situ after 1000 hours on these coated steels. The samples are characterized after pre/oxidation and exposure using scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDX). With the appropriate heat treatment, the interconnect lifetime can be increased by a factor of 5. The ASR of the pre-oxidized steels is lower than the as-received steels.