Reevaluating the Cr Evaporation Characteristics of Ce/Co Coatings for Interconnect Applications
Authors: Reddy Mareddy Jayanth , Svensson Jan-Erik , Froitzheim Jan 
Published: July 2021

Solid oxide fuel cell (SOFC) is a high-temperature energy conversion device that directly converts the chemical energy of the fuel into electrical energy via electrochemical reactions with a very high efficiency. Corrosion of the interconnects, which leads to high degradation rates, is one of the key issues impeding the commercial success of SOFC. Interconnects are used to provide electrical connection between the individual cells which are stacked together to deliver high power output. Recent developments in materials and manufacturing techniques have significantly reduced the operating temperatures, allowing the use of metallic interconnects. Chromia forming ferritic stainless steels (FSS) are preferred over the other materials owing to a coefficient of thermal expansion (CTE) similar to that of ceramic components, high oxidation resistance, good thermal conductivity, and adequate electrical conductivity of Cr2O3 scale at high temperatures. Nonetheless, the use of FSS as interconnect materials poses some significant challenges.

During the operation, the chromia scale grows continuously with time, which in turn increases the resistance across the cell. Moreover, chromia reacts with oxygen in the presence of water vapor to form volatile Cr(VI) species such as CrO2(OH)2. These volatile Cr(VI) species react and get deposited at the triple-phase boundaries hindering the oxygen electrochemical reaction on the cathode, leading to cell degradation, also known as chromium poisoning. Despite the various improvements to the steel composition, chromium evaporation is too high for the long-term operation of SOFC. Applying coatings in a widespread solution to suppress chromium evaporation.

Spinel structure-based coatings are considered promising owing to good adhesion and are effective barriers to volatile Cr species and oxygen. (Co,Mn)3O4 spinel oxides deposited using various techniques are widely reported in the literature and are known to reduce Cr evaporation and oxide scale growth. Metallic coatings deposited using physical vapor deposition (PVD) are easier and cheaper in terms of manufacturing and eliminate the need for extra heat treatment step for coating densification used in electrophoretic deposition, dip-coating, and screen printing.

PVD CeCo coated steels were reported to reduce the chromium evaporation by about 10 times compared to the uncoated on Sanergy HT, Crofer 22H, AISI 441. Nevertheless, the coated coupons investigated had the coating on the faces, but the edges were uncoated. The uncoated edges contribute to higher chromium evaporation than if the edges were coated. Thus, the reported chromium evaporation of the coated coupons is higher than the real chromium evaporation.

To understand the influence of the uncoated edges on the measured chromium evaporation, uncoated coupons and two sets of coated coupons, (1) coated coupons with coated edges and (2) coated coupons with uncoated edges were used in the study. The exposures were conducted in horizontal quartz tube reactors at 800 oC for 500 hours in air humidified with 3 % H2O. The airflow is set at 6 lit/min to achieve a flowrate independent regime in the reactor. To quantify chromium evaporation, Na2CO3 coated denuders were used to capture Cr(VI) species, which were replaced at regular intervals.

Although the edges made up to only 4% of the total area, the contribution to chromium evaporation from the edges is significant. The chromium evaporation of the coupons with coated edges is about 50 times lower than the chromium evaporation of the uncoated coupons.