Effect of coating density on oxidation resistance and Cr vaporization from solid oxide fuel cell interconnects

Abstract

Manganese cobalt spinel oxides are promising materials for protective coatings for solid oxide fuel cell (SOFC) interconnects. To achieve high density such coatings are often sintered in a two-step procedure, involving heat treatment first in reducing and then in oxidizing atmospheres. Sintering the coating inside the SOFC stack during heating would reduce production costs, but may 2 result in a lower coating density. The importance of coating density is here assessed by characterization of the oxidation kinetics and Cr evaporation of Crofer 22 APU with MnCo 1.7 Fe 0.3 O 4 spinel coatings of different density. The coating density is shown to have minor influence on the longterm oxidation behavior in air at 800 °C, evaluated over 5000 h. Sintering the spinel coating in air at 900 °C, equivalent to an in-situ heat treatment, leads to an 88 % reduction of the Cr evaporation rate of Crofer 22 APU in air-3% H 2 O at 800 °C. The air sintered spinel coating is initially highly porous, however, densifies with time in interaction with the alloy. A two-step reduction and re-oxidation heat treatment results in a denser coating, which reduces Cr evaporation by 97 %.

Comparison of iron and copper doped manganese cobalt spinel oxides as protective coatings for solid oxide fuel cell interconnects

“…Uncoated Crofer 22 APU oxidized at 900 °C has a similar oxidation rate as reported by Huczkowski et al [37] (1×10 -12 g 2 cm -4 s -1 ). The oxidation rate of uncoated Crofer 22 APU at 800 °C is however somewhat lower than we have previously reported for the same alloy batch (4.2×10 -14 g 2 cm -4 s -1 [38]) and what was measured by Gavrilov et al [39] (5.6×10 -14 g 2 cm -4 s -1 ) and Molin et al [40] (2.84×10 -14 g 2 cm -4 s -1 ). This can be attributed to the pre-oxidation heat treatment (900 °C, N 2 -H 2 , 2 h and 800 °C, air, 2 h).…”

“…This can in part be attributed to differences in Cr evaporation from the coated and uncoated samples. We have previously shown that a MCFe coating reduces the Cr evaporation rate of Crofer 22 APU by a factor of 10-30 at 800 °C in air with 3 % H 2 O [38]. Decreasing the temperature from 800 °C to 700 °C is here shown to decrease the oxidation rate by a factor of 10, while the rate of Cr evaporation has been shown to decrease by only a factor of 2-3 in this temperature interval [41].…”

Microstructure and Electrical Properties of Fe,Cu Substituted (Co,Mn)3O4 Thin Films

“…Materials based on MnCo 2 O 4 are interesting for a broad range of applications, from room temperature to high temperatures (800 • C). They can be used in Li-ion battery electrodes [6,7], electrochemical supercapacitors [8] and as coating materials for steel interconnects for fuel cells [9][10][11]. The microstructure and the composition of the spinel strongly affect its performance and applicability in a specific technology.…”

Evaluation of electrodeposited Mn‐Co protective coatings on Crofer 22 APU steel

“…An effective solution is to coat the alloy surface with protective layers . Many different materials have been evaluated for a possible corrosion protection . Among the materials, Mn‐Co spinels have been proposed as being very effective in reducing chromium evaporation .…”