Cobalt-free protective coatings for solid oxide stack components

Abstract

Ferritic stainless-steel (FSS) components used in solid oxide fuel cells (SOFCs) stack are rich in chromium content. During stack operating conditions, chromia scales (CrO3), otherwise beneficial for FSS protection, are formed on the component surface. This continuous growth of chromia scale becomes problematic when in contact to water vapour, forming volatile chromium species like CrO3(g) and CrO2(OH)2(g). These volatile species, released from the protective chromia layer, can migrate, and deteriorate the performance of the cathode. To address this issue, a protective conducting coating needs to be applied onto the ferritic interconnector (ICs). Most successful researched protective coatings are composed of conductive metal oxides containing cobalt, more specifically spinel-type (A,B)3O4, due to their high electrical conductivity and chemical/structural stability. While cobalt-based spinel coatings e.g., (Co,Mn)3O4 have been effective protecting SOFCs ICs from oxidation and corrosion, there is a demand for research alternative materials as a protective spinel coating (e.g., Mn-Fe, Mn-Cu), that can offer similar or improved performance without the use of cobalt. Previous work has indicated that highly doped spinels by various cations known as high entropy oxides (HEO) can contribute to better coating performance. This study investigates several cobalt-free coatings compositions, primarily focusing on manganese-based spinel systems doped with different elements such as Cu, Fe, Ni, Ce and Y. These coatings were deposited using thermal spray process onto ferritic stainless-steel substrate and evaluated for their microstructural integrity and ability to hinder the growth chromia scale layer and high temperature corrosion under single atmospheric oxidations (SAO) at 650℃ and double atmospheric oxidation (DAO) at 600℃ over 1000h. Although few coating experienced spallation and phase separation after extended testing, most maintained protective properties, withstanding the harsh operating conditions im-posed by thermal cycling and high-temperature exposure. The area specific resistance (ASR) results showed that most coatings demonstrated stable and low area specific resistance over time averaging results bellow 5 mΩ∙cm2.