Electrodeposition of manganese/cobalt alloys for solid oxide fuel cell interconnect applications.

With the reduction of solid oxide fuel cell (SOFC) operation temperature from 1000°C to 800°C, it is possible to use stainless steel as interconnect. Chromia forming ferritic stainless steels are the most acceptable materials for interconnects application because of their cost effectiveness and good oxidation resistance, as compared to other candidates. However, excessive scale growth and chromium evaporation will degrade the cell performance rapidly. Highly conductive coatings that aide oxidation resistance and prevent chromium evaporation may prevent the problems. (Mn,Co)3O4 spinel is one of the most promising coatings for interconnect application on account of its high conductivity, good chromium retention capability, as well as good CTE match with fuel cell materials. The traditional deposition methods, slurry coating and physical vapor deposition, have been carried out for the interconnect application. However, these methods have several limitations. Electrodeposition of thin film metallic layers followed by controlled oxidation to achieve the desired spinel phase offers an additional deposition option that is both cost effective and adaptable to work piece geometry. This work presents binary Mn/Co alloys deposition by DC and pulse plating. The dramatic difference of deposition potentials of Mn (−1.18V) and Co (−0.28V) makes it quite difficult to co-deposit two metals. Potentiodynamic polarization and cyclic voltammetry were conducted to characterize total reactions occurring during deposition. Effects of current density and pulse cycle on the surface morphologies and compositions of coatings were studied. Mn content increases with the on-time increasing, and surface morphologies changes from flake like structure to crystalline structures with less pores. Area specific resistance (ASR) measurement is used to characterize the conductivity of the coating. In tests of 1200 h, ASR is quite stable with a slight increase. The ASR value at 40,000h was predicted to be 0.0460 Ω cm2, which is well below the industry accepted goal of 0.1 Ω cm2. Following and interconnect on-cell tests have substantiated that the coating by electrodeposition can successfully block chromium evaporation, maintain high conductivity and survive thermal cycles. Therefore, this simple and cost effective coating technique is a great option for SOFC interconnects. Furthermore, in-situ mass gain of deposition can be monitored by quartz crystal microbalance (QCM). As expected, little mass gain was found for pure Mn deposition due to its high chemical reactivity. Combined with electrochemical calculations, Mn reaction with water is found to be the largest barrier for Mn electrodeposition. Several suggestions were recommended for future studies