Schematic of a planar solid oxide fuel cell. The ferritic stainless steel interconnects are coated to prevent poisoning of the cathode by volatile chromium compounds. NexTech Materials has just completed a one-year demonstration of a successfull manganese cobalt oxide coating. Credit: Olivera Kesler, University of Toronto.
When our neighbors down the street at NexTech Materials (Lewis Center, Ohio) came in to work recently and saw that nothing had happened in the lab overnight, they knew they had reached a big milestone.
NexTech has been running a continuous test of it manganese cobalt oxide coatings on SOFC interconnects and reached the one-year milestone in March. The test (still going) has been running at expected and accelerated operating temperatures, and the coated interconnects have been subjected to hundreds of thermal cycles. This work is a continuation of testing NexTech reported on last year when it first began performing accelerated stability tests which, according to the company, predicted a service life of over 40,000 hours (equivalent to about 4.5 years) at 750°C. Thus, the actual one year of service begins to offer some pretty strong confirmation of the predicted durability of the interconnects.
SOFC interconnects physically and electrically connect the individual cells in series and collect the electricity generated by the cells. They are exposed to both the oxidizing and reducing sides of the cell (anode and cathode) at operating temperatures in the range of 650-800°C, so they must be chemically very stable, oxidation resistant, electrically conductive, etc. Chromium-rich ferritic stainless steels, like 441, are good candidate materials because they are mechanically stable at those temperatures, compatible with other SOFC components, inexpensive and their oxide scale is electrically conductive.
Unfortunately, at the cathode side, which is the air channel, volatile chromium oxides and chromia-oxy-hydroxide compounds form and can poison the cathode. These species can eventually reach the triple phase boundary in the electrolyte and impair the electrochemical reaction that is the essence of a fuel cell.
Matt Seabaugh (L) and Neil Kidner. Kidner is working on scaling-up NexTech's MCO interconnect coating process. Credit: NexTech.
The best approach to avoiding the problem has been to coat the interconnects, usually with MCO. According to a NexTech press release, there are a number of high-cost coating manufacturing methods tried that use high-cost manufacturing methods. NexTech engineers say they have developed a process based on technology licensed from Ceramic Fuel Cells Limited that uses “relatively inexpensive yet high-speed equipment, thereby reducing the coating cost by nearly 67%.” (NexTech’s website has a nice video showing coatings being applied.) NexTech CEO, Bill Dawson says the company sees the coating as a “significant technical and economic breakthrough to improve SOFC stack life while reducing SOFC stack costs.”
The NexTech work has been supported by a collaboration of commercial partners and the DOE SBIR program. Matt Seabaugh, director of NexTech’s commercial services division, Nexceris, says the project is a “demonstration of small business, technology integrators and government agencies collaborating to create new technologies and products for a world market.” The company says it has applied for US and international patents on its materials and is working with several SOFC stack manufacturers to assume “a portion or all of their interconnect business.”
For interconnects testing, when the “big news” is “no news,” that’s a good thing.