Electrical Resistance of Co-W Alloy Coated Ferritic Stainless Steels As Interconnector Parts of Solid Oxide Fuel Cells
Authors: Gan Lu , Montero Xabier , Murakami Hideyuki 
Published: December 2020

Introduction

Over the last decade, several attempts have been made to improve a solid oxide fuel cell (SOFC) system which is one of the most promising alternative power sources due to its high efficiency and low emission. The SOFC system consists of several parts, such as electrolytes, anodes, cathodes, and interconnectors. Recently, the operation temperature of the SOFC system has been reduced to 800 °C, which makes it possible to apply ferritic stainless steels (FSSs) for the interconnector parts. However, the formation of Cr containing oxide layers over time would jeopardize the electrical conductivity of FSS interconnectors. Furthermore, the evaporation of Cr from FSS would react with neighboring components, resulting in their degradation of catalytic activity and electrical conductivity.
To overcome these issues, the authors have proposed Co-W alloy electroplating on FSSs. By heat exposure in air, the Co-W coated FSS forms multilayered oxides and among them, a CoWO4 layer effectively suppresses the outward diffusion of Cr cations at high temperatures. However, our former work of the Co-W coated FSSs was conducted under the condition that the coated specimens were separated with other materials, thus for the practical application as interconnector parts, further investigation is required under the simulated condition as a practical SOFC system.
In this study, several cathodic half cells were built using the Co-W coated FSSs as interconnector parts and perovskites-type conductive oxides, and the operation condition was simulated by the electricity from a constant current power source. For the cathode materials, La0.8Sr0.2MnO2.97 (LSM) and La0.8Sr0.2Co0.8Fe0.2O2.97 (LSCF) were selected due to their high electrical conductivity and high reaction activity. The results of their electrical performance and microstructural changes of the half cells are discussed.

Results and discussions

The cross-sections of uncoated and Co-W coated LSM specimens after 500 h exposure in air at 800 °C are shown in the following image. The formation of a CoWO4 oxide layer brought a significant influence on the Cr outward diffusion.
In the uncoated samples, Cr diffuses into both LSM and LSCF specimens to form Cr containing particulate oxides such as SrCrO4 or MnCr2O4, showing the Cr-poisoning. Further, the Cr oxide layer was not continuous in the uncoated samples due to Cr evaporation. Thus, internal oxidation in the grain boundaries are observed by the BSE image.
On the other hand, the Co-W coated samples exhibited in a completely different manner. Similar to our previous study, after oxidation, the five-layered structure was formed beneath the LSM and LSCF coatings. In the five layered oxides, Cr is enriched just beneath the Co-W oxide layer to form a continuous and dense Cr oxide layer. Thus, the further oxidation of the substrate is suppressed by the presence of the CoWO4 layer, due to its role as an outward diffusion blocker of Cr which accordingly protects cathodic materials against Cr poisoning. In addition, the ASR measurement proved that the addition of W in the Co coating does not significantly increase the electrical resistance of the cathodic system compared with those of other Co based coatings.
It should also be noted that the LSM particles showed an intrusion effect into the oxide layer formed on the metallic surface. Co oxide reacts with Mn oxide in the LSM cathode layer to form a Co-Mn spinel which can improve the adherence between the cathode and the protective layer. This intrusion effect can also reduce the contact resistance and improve the electrical property. And this kind of Mn-Co spinels are typically used in the SOFC interconnects for their good CTE mismatch with the substrate stainless steel and high electric conductivity.

Conclusions

For the evaluation of Co-W alloy electroplated ferritic stainless steels, cathodic half cells were fabricated simulating the SOFC system, and the following conclusions are drawn.

The CoWO4 layer effectively suppressed the outward diffusion of Cr and protected cathode materials against Cr poisoning.

The ASR value by the Co-W coated specimens were almost comparative to those in Co-based alloy coatings, confirming that W addition did not significantly increase the ASR values.

LSM particles showed an intrusion effect into the oxide layer formed on the metallic surface. Co oxide reacts with Mn oxide in the LSM cathode layer to form a Co-Mn spinel which can improve the adherence between the cathode and the protective layer.

Figure 1