Spinel Coating Body for SOFC Interconnectors
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Solution Overview
Problem
In solid oxide fuel cells (SOFCs), chromium (Cr) diffusion from ferritic stainless steel interconnectors reduces the surface area of the air electrode, leading to decreased reaction speed and output density, known as 'Cr poisoning', which is exacerbated by temperature changes and the peeling of coating films.
Innovation Solution
A coating body with a conductive member made of ferritic stainless steel, covered by a spinel-type transition metal oxide coating film, including a chromia layer and a co-continuous structure, is used to prevent Cr diffusion and maintain adhesion under severe temperature changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a coating film is applied to suppress Cr diffusion, then Cr poisoning is reduced, but the coating film peels under severe temperature changes
Solution Approach 1:
The coating film is divided into multiple functional layers: a spinel-type oxide layer (AB2O4) as the outer layer for Cr diffusion barrier, and a ferrite layer containing Cr as the inner layer for adhesion promotion. This segmentation allows each layer to specialize in one function, preventing the conflict between Cr suppression and adhesion maintenance under thermal cycling.
Solution Approach 2:
The ferrite layer acts as an intermediary between the spinel-type oxide coating and the ferritic stainless steel substrate. It contains Cr in a controlled manner and provides a transition zone that accommodates thermal expansion differences, thereby preventing peeling while still suppressing Cr poisoning through the outer spinel layer.
2Reliability
If Pt material is used as bonding agent, then electrical connection is achieved, but cost increases significantly
Solution Approach 1:
The patent replaces expensive Pt material with cheaper conductive ceramic materials (such as La-Sr-Co-Fe perovskite or spinel-type oxides) for the coating film. While these materials may have shorter service lives or require specific operating conditions, they significantly reduce cost while maintaining adequate electrical connection functionality.
Solution Approach 2:
The bonding system uses composite structures: a conductive ceramic coating film (La-Sr-Co-Fe perovskite or spinel oxide) applied on ferritic stainless steel interconnector. This composite replaces pure Pt with a ceramic-material combination that achieves similar electrical connection at lower cost, leveraging the conductive properties of the ceramic coating.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The coating body effectively suppresses Cr poisoning by reducing Cr diffusion and preventing film peeling, enhancing the durability and performance of SOFCs under varying temperatures.
Implementation Method 1
a phenomenon occurs in which Cr is discharged from the surface of the SUS material during the operation of the SFOC (i.e., under high-temperature atmosphere (e.g., 800° C.), and the discharged Cr is diffused around the SUS material
Implementation Method 2
The atmospheric temperature of the interconnector at the air electrode can greatly vary between the room temperature (when the SOFC is stopped) and high temperature that is the working temperature of the SOFC (e.g., 700 to 800° C.) by the activation and stop of the SOFC
Data Source
AI summary
Powders of respective metal elements (Mn, Co) constituting a transition metal oxide (MnCo2O4) having a spinel type crystal structure are used as a starting material of the coating film. A film of a paste containing the mixture of the powders is formed on the surface of the interconnector, and with this state, the paste is sintered to form the coating film. In the coating body, a chromia layer including Cr2O3, a first layer including elements of Mn, Co, Fe, Cr, and O, and a second layer including elements of Mn, Co, Fe, and O are provided in this order from the side close to the interconnector at the boundary between the coating film and the interconnector. With this structure, the coating film is difficult to be peeled even if the coating body is placed in a severe temperature change.