Fuel Cell Interconnect Chromium Oxide Scale Formation
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Solution Overview
Problem
Low temperature solid oxide fuel cells face challenges with the slow formation of a passivating chromium oxide scale on stainless steel components, leading to potential corrosion and increased contact resistance due to the evaporation of chromium oxide at temperatures below 650°C, especially when exposed to humidified air.
Innovation Solution
An interconnect comprising a stainless steel substrate with a chromium oxide layer of 350-600nm thickness on one surface and an alumina layer on the other, along with a metal oxide coating such as cobalt oxide, which prevents chromium evaporation and corrosion, and is formed through a heat treatment process to ensure controlled layer development.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If the operating temperature is reduced below 650°C, then the start-up time is reduced and material requirements are simplified, but the chromium oxide scale formation rate decreases and chromium evaporation increases leading to corrosion
Solution Approach 1:
The patent applies preliminary action by pre-forming a chromium oxide scale on the interconnect surface before the fuel cell begins operation. This is achieved through a controlled heat treatment process at elevated temperatures (700-900°C) during manufacturing, which creates a robust protective layer before the interconnect is exposed to the low-temperature operating conditions where chromium diffusion is slow and evaporation dominates. The scale thickness is controlled to be 5-20 micrometers to ensure adequate protection throughout the fuel cell's operational life.
Solution Approach 2:
The patent applies parameter changes by modifying the thermal history and chemical environment during scale formation. The heat treatment process uses specific temperature ranges (700-900°C), time durations (1-24 hours), and atmospheric conditions (air or oxygen exposure) to optimize chromium oxide scale formation. These parameter adjustments ensure rapid and complete scale formation during manufacturing, creating a protective layer that remains stable during subsequent low-temperature operation where natural scale formation would be too slow.
2Reliability
If a thicker chromium oxide layer is formed, then corrosion protection is improved, but contact resistance at the interconnect/electrode interface increases
Solution Approach 1:
The patent applies parameter changes by precisely controlling the heat treatment temperature (700-900°C), time (1-24 hours), and atmospheric conditions to achieve optimal scale thickness. This controlled approach ensures the chromium oxide layer reaches the protective threshold (5-20 micrometers) without excessive thickening that would cause contact resistance issues. The process parameters are optimized to balance corrosion protection with electrical performance.
Solution Approach 2:
The patent applies feedback by monitoring and controlling the scale formation process to achieve the target thickness range. The heat treatment parameters are adjusted based on feedback from scale thickness measurements and corrosion performance testing, ensuring the interconnect achieves adequate protection while maintaining acceptable contact resistance characteristics.
3Stability of the object's composition
If the chromium oxide scale forms slowly at low temperatures, then the material remains stable, but the scale evaporates faster than it forms leaving the steel unprotected
Solution Approach 1:
The patent applies preliminary action by forming the complete protective chromium oxide scale during the manufacturing heat treatment process before the fuel cell operates. Since the scale formation rate is temperature-dependent and much faster at elevated temperatures (700-900°C) than at operating temperatures (<650°C), the protective layer is established in advance when conditions favor rapid scale growth. This pre-formed scale then protects the interconnect throughout operation, preventing chromium evaporation that would otherwise occur faster than scale formation at low temperatures.
Solution Approach 2:
The patent applies beforehand cushioning by creating a substantial chromium oxide scale (5-20 micrometers thick) during manufacturing that serves as a protective reservoir throughout the fuel cell's operational life. This pre-established protective layer compensates for the slow natural scale formation rate during operation, ensuring continuous protection even though chromium diffusion and scale regrowth are sluggish at operating temperatures below 650°C.
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 solution effectively protects the interconnect from corrosion, maintains low contact resistance, and ensures structural integrity, allowing for efficient current collection and gas separation, while minimizing thermal and mechanical stresses in the fuel cell stack.
Implementation Method 1
the slow formation of a passivating chromium oxide scale on the metal components (for instance, on the stainless steel substrates and interconnects). The scale forms a protective layer on the steel, preventing corrosion. At temperatures below 650°C, the rate of chromium diffusion from a steel to its surface is low. In addition, where the steel surface is exposed to flowing humidified air (as is often the case) such as on the oxidant side of the interconnect during operation of the fuel cell, the slow formation of the chromium oxide scale may result in it evaporating faster than it is formed
Implementation Method 2
the rate of chromium diffusion from a steel to its surface is low
Implementation Method 3
heating the coated substrate to a temperature in the range 800 - 920°C, often 800 - 890°C to form a layer comprising chromium oxide between the first surface and the metal oxide coating
Data Source
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AI summary
An interconnect for a low temperature solid oxide fuel cell, the interconnect comprising: a stainless steel substrate comprising a first surface and a second surface; a layer comprising chromium oxide on the first surface of the substrate, wherein the chromium oxide layer is of thickness in the range 350 - 600nm; and a metal oxide coating on the chromium oxide layer. A process for making an interconnect for a low temperature solid oxide fuel cell, the process comprising: coating a first surface of a stainless steel substrate with a metal oxide to form a coated substrate; and heating the coated substrate to a temperature in the range 800 - 900°C to form a layer comprising chromium oxide between the first surface and the metal oxide coating.