Strategies to Mitigate the Degradation of Stainless-Steel Interconnects Used in Solid Oxide Fuel Cells
Doktorsavhandling, 2020

Interconnects are a vital part of solid oxide fuel cells (SOFC), where they electrically connect individual cells to form a fuel cell stack. They are a main contributor to the overall stack cost and the limited life-time of fuel cells, and, therefore, improvements carried out on the interconnect level could further the commercialization of SOFCs.

The limited life-time of the interconnect is related to the material used today, ferritic stainless steels (FSS). FSS interconnects are more cost-effective than previously used ceramics, but they degrade under the conditions prevalent in an SOFC: high temperatures between 600 °C and 850 °C, and a p(O2) gradient. Certain corrosion phenomena that occur, such as Cr evaporation and continuous oxide scale growth, negatively impact cell performance due to cathode poisoning and increased electrical resistance, respectively. These phenomena have been found to be effectively mitigated by coatings, such as the (Co,Mn)3O4 (MCO) coating, or reactive element coatings, such as Ce.

The present thesis examines these coatings with regard to three aspects: (i) does the semi-conducting spinel coating affect the electrical resistance of the interconnect negatively, or is its conductivity negligible in comparison to the continuously growing Cr2O3 scale below it; (ii) does the coating self-heal if it is cracked even at intermediate temperatures, i.e. 650 °C and 750 °C, or do the cracks persist and increase Cr evaporation; and (iii) is the long-term stability of the state-of-the-art Ce/Co coating (10 nm Ce/640 nm Co) still effective after 35 000 h, or not. The second aspect is not only important to understand corrosion behavior, but it would also allow for large-scale roll-to-roll PVD coating, which is significantly more cost-effective than batch coating.

Another corrosion phenomenon that is elucidated within the scope of this work is the dual atmosphere effect. This effect leads to increased corrosion on the air-facing side of the interconnect if the FSS is exposed to a dual atmosphere, i.e. air on one side and hydrogen on the other side, compared to if the FSS is exposed to an air-only atmosphere. A new theory as to why the dual atmosphere effect occurs is proposed, and it is indirectly verified by means of excluding all other possibilities. Factors that influence the dual atmosphere effect are discussed, and it is shown how the dual atmosphere effect could, in part, be mitigated.

Cr Evaporation

Hydrogen

Area Specific Resistance

Dual Atmosphere

Solid Oxide Fuel Cell

Interconnect

Long-term

Deformation

Corrosion

HA1, Hörsalsvägen 4, Chalmers.
Opponent: Professor Sebastien Chevalier, University of Burgundy, France.

Författare

Claudia Goebel

Chalmers, Kemi och kemiteknik, Energi och material, Oorganisk miljökemi

Does the conductivity of interconnect coatings matter for solid oxide fuel cell applications?

Journal of Power Sources,; Vol. 383(2018)p. 110-114

Artikel i vetenskaplig tidskrift

C. Göbel, V. Asokan, S. Khieu, J.-E. Svensson, J. Froitzheim. Self-Healing Properties of Ce/Co-Coated Stainless Steel Under Simulated Intermediate Temperature Solid Oxide Fuel Cell Conditions.

K. O. Gunduz, A. Chyrkin, C. Göbel, L. Hansen, O. Hjorth, J.-E. Svensson, J. Froitzheim. The Effect of Hydrogen on the Breakdown of the Protective Oxide Scale in Solid Oxide Fuel Cell Interconnect

The effect of pre-oxidation parameters on the corrosion behavior of AISI 441 in dual atmosphere

International Journal of Hydrogen Energy,; Vol. 43(2018)p. 14665-14674

Artikel i vetenskaplig tidskrift

Nowadays the most widely known green energy conversion systems are batteries. However, also other green energy conversion systems exist, for example fuel cells. In a fuel cell hydrogen and oxygen react to form water, and during this reaction energy is set free. This chemical energy is harnessed as electrical energy. In comparison to batteries, fuel cells have some advantages, for example, they do not require recharging and instead are refueled, and hydrogen, as a fuel, has the highest energy density by mass compared to all other energy storage devices. Still fuel cells have not yet managed to become equally important as batteries in our daily life, amongst others, because of their cost and their limited life-times. Different fuel cell types exist, and the present work focuses on solid oxide fuel cells (SOFC), which are a highly efficient fuel cell type. The cost and the limited life-time of SOFCs can be greatly impacted by improving the interconnect. The interconnect is an important part of the fuel cell, where it electrically connects individual fuel cells to form a fuel cell stack. Nowadays, interconnects are made up of thin ferritic stainless-steel (FSS) plates that are formed into a corrugated form. This form allows the gases to pass through channels inside the fuel cell. Because of the very high temperatures in the fuel cell, between 600 °C and 850 °C, degradation of the interconnect occurs. During the exposure of the FSS interconnect to high temperatures, a Cr2O3 layer is formed on the surface of the FSS, and this layer continues to grow. This Cr2O3 layer protects the steel from further corrosion, but it also leads to some issues. For example, under humid conditions hexavalent Cr species evaporate from this layer and poison the cathode of the fuel cell. Additionally, the continued growth of the Cr2O3 layer can increase the interconnect resistance, which can lead to voltage degradation of the fuel cell. Both these degradation mechanisms can be mitigated by applying coatings to the interconnect. Spinel coatings, such as the (Mn,Co)3O4, have been shown to be highly effective at mitigating Cr evaporation, and reactive element coatings, such as Ce coatings have been shown to decelerate the Cr2O3 scale growth. The present work analyzes these coatings further and shows that the state-of-the-art Ce/Co coating (10 nm Ce/640 nm Co) is highly effective at reducing the degradation of the FSS even up to 4 years of exposure at 800 °C. Additionally it is shown that the coating does not increase the resistance of the interconnect, but instead the Cr2O3 scale is the main contributor of the overall resistance. There are many more factors influencing corrosion of interconnects in the SOFC, and thus, the performance of the fuel cell. The present work will elucidate some of these degradation mechanisms further.

Drivkrafter

Hållbar utveckling

Ämneskategorier

Materialteknik

Kemiteknik

Styrkeområden

Energi

Materialvetenskap

Infrastruktur

Chalmers materialanalyslaboratorium

ISBN

978-91-7905-358-1

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4825

Utgivare

Chalmers tekniska högskola

HA1, Hörsalsvägen 4, Chalmers.

Opponent: Professor Sebastien Chevalier, University of Burgundy, France.

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Senast uppdaterat

2020-10-05