Metallic interconnects corrosion

Corrosion and Protection of Metallic Interconnects in Solid-Oxide Fuel Cells... 

This chapter will provide an overview of oxidation and corrosion behavior of candidate oxidation-resistant alloys under SOFC operating conditions and discuss surface modifications for improved stability and performance of metallic interconnects. 

During SOFC operation, interconnects interact with surrounding gaseous environments on both the cathode and anode side, as well as with adjacent components such as sealing materials, electrodes, and electrical contact layers inserted between interconnects and electrodes. These interactions potentially cause corrosion of metallic interconnects and affect their stability and performance. 

The so-called E- and F-designs for stacks with planar anode substrate type cells and metallic interconnects constitute the work horses at Forschungszentrum Jiilich used for testing SOFC materials, cells and manufacturing processes in cell and stack development since its introduction in the year 2000 [1]. Ferritic interconnects were chosen since they offer a high electric conductivity and thus the potential for high power density in the stacks. On the other hand the ferritic material gives rise to chromium evaporation (which can poison the cathodes) and is prone to massive corrosion at temperatures above approximately 900°C [2],... 

Z. Zeng and K. Natesan. Corrosion of metallic interconnects for SOFC in fuel gases. Solid State Ionics 167, (2004) 9-16. 

Use of metal interconnect gives rise to a new situation that materials are not necessarily used in their most stable states. This means that the durability is determined not only by equihbrium properties but also by kinetic properties. Thus, corrosion becomes one of the main issues at lower temperatures. 

The electrolyte in this fuel cell is an ion exchange membrane (fluorinated sulfonic acid polymer or other similar polymer) that is an excellent proton conductor. The only liquid in this fuel cell is water thus, corrosion problems are minimal. Typically, carbon electrodes with platinum electrocatalyst are used for both anode and cathode, and with either carbon or metal interconnects. 

Metal interconnect-supported. Lawrence Berkeley National Laboratory (66), Argonne National Laboratory, and Ceres (67) have pioneered metal-supported cells to minimize mass transfer resistance and the use of (expensive) ceramic materials. In such cells, the electrodes are typically 50 im thick and the electrolyte around 5 tol5 im. While the benefits are obvious, the challenges are to find a materials combination and manufacturing process that avoids corrosion and deformation of the metal and interfacial reactions during manufacturing as well as operation. 

In the beginning, strong interest arose in utilization of metal interconnects instead of LaCrOs-based oxide interconnects [13]. Because of the severe corrosion of metals at high temperatures, operation temperature needs to be lowered. 

Lanthanum chromite has provided long lifetimes, as long as 69,000 h in Siemens Westinghouse tubular cells, at 900-1000°C. However, metallic interconnects have not yet shown equivalent lifetime performance. Improvements in metallic interconnect compositions and contact layers between cells/interconnects are still issues for materials development. In particular, the metal/ceramic interface in cells should have low corrosion, low contact resistance and low permeability of chromium species. Recent results have shown that optimised steels for SOFC applications are available and alkaline earth-free and cobalt-containing perovskites are the most suitable materials for contact layers however, their long-term performance under fuel cell operation conditions needs to be proven. 

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