Chromium poisoning, cathode

Newly developed alloys have improved properties in many aspects over traditional compositions for interconnect applications. The remaining issues that were discussed in the previous sections, however, require further materials modification and optimization for satisfactory durability and lifetime performance. One approach that has proven to be effective is surface modification of metallic interconnects by application of a protection layer to improve surface and electrical stability, to modify compatibility with adjacent components, and also to mitigate or prevent Cr volatility. It is particularly important on the cathode side due to the oxidizing environment and the susceptibility of SOFC cathodes to chromium poisoning. 

Fergus (2007), Effect of Cathode and Electrolyte Transport Properties on Chromium Poisoning in Solid Oxide Fuel Cells , Int.J. Hydrogen Energy, 32, 3664-3671. 

Vinke, L, and Lippert, H. (2009) Systematic study of chromium poisoning of LSM cathodes - single cell test. ECS Trans., 25 (2), 2889-2898. 

Chromium poisoning of the porous composite cathode. Effect of cathode thickness and current density. 

Another interesting finding is obtained by Matsuzaki and Yasuda [69] - they found that the chromium poisoning depends not only on cathode materials but also on electrolyte to be used with cathodes. From four possible combinations made with LSM and LSCF as cathode and YSZ and SDC as electrolyte, they found that the LSCP/SDC combination does not show any degradation imder the same... 

Lanthanum nickel-iron oxide cathodes, La(Ni,Fe)03, have attracted much attention because they show essentially no effects of chromium poisoning [59]. This is mainly because there is no chance of SrCr04 formation. From the thermodynamic point of view, there is a possibility that the La(Ni,Fe)03 perovskites react with chromium-containing vapors to form the La(Ni,Fe,Cr)03 solid solution [42]. Even so, such perovskites must still be electro-conductive, although some surface kinetic activity will be lowered due to the fact that lanthanum chromite has lower catalytic activity among the transition metal perovskite oxides. 

In recent years, an interesting but troublesome fact has been revealed, that is, Pt wire reacts with oxygen in air to form Pt02(g) and will be deposited on the three phase boundaries of LSM cathodes [80]. Usually, Pt cathodes are not better than perovskite cathodes so that the electrode activity may be lowered when such Pt will be deposited on cathodes. However, the situation for the chromium poisoning for the LSM cathode is quite different. When Pt is deposited on TPB of LSM cathodes, the cathode performance can be improved or lowered depending on the state of deposited Pt. This disturbs the effects of chromium on the LSM cathodes. This makes it difficult to examine the chromium poisoning in a laboratory scale. 

Paulson SC, Birss VI (2004) Chromium poisoning of LSM-YSZ SOFC cathodes I. Detailed study of the distribution of chromium species at a porous. Single-phase cathode. J Electrochem Soc 151(1) A1961-A1968... 

Konysheva E, Mertens J, Penkalla H, Singheiser L, Hilpert K (2007) Chromium poisoning of the porous composite cathode effect of cathode thickness and current density. J Electrochem Soc 154(12) B1252-B1264... 

A serious challenge in the use of LSM as a cathode material in intermediate temperature SOFC stems from the use of metallic interconnects. Many of these metals contain chromium, which forms a stable protective oxide (chromia) layer with reasonable conductivity (see Section 7.1.4 on interconnects for more details). However, chromia vapors can lead to serious poisoning of the cathode (21, 22). Although one might attribute this problem more to the interconnect material than to the cathode, the poisoning effect was found to depend strongly on the electrolyte/cathode material combination. 

Konysheva E, Penkalla H, Wessel E et al (2006) Chromium poisoning of perovskite cathodes by the ODS alloy Cr5FelY(2)0(3) and the high chnunium ferritic steel Crofer22APU. 

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],... 

A major degradation mechanism in SOFC is poisoning of the cathode by chromium from volatilization of the interconnect material. The chromium deposition has been attributed to both chemical and electrochemical mechanisms. For an electrochemical reaction, deposition can occur only where both ions and electrons are available, which, for a purely ionic conducting electrolyte and a purely electronic conducting cathode, can occur only at the three-phase gas-electrolyte-electrode interface. However, the introduction of ionic conductivity into the cathode or electronic conductivity into the electrolyte can allow deposition to occur away from this... 

Chromium evaporation is a significant failure mechanism for chromium containing stainless steel Interconnects as it can lead to poisoning of the fuel cell cathode and loss of performance. This failure mechanism is not able to be resolved from the ASR testing, however, careful post-test characterization of the coating microstructure enabled an assessment of the effectiveness of the coating to act as a chromium diffusion barrier. 

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