Novel coatings for stainless steel interconnects utilized in SOFCs

Intermediate temperature solid oxide fuel cells (IT-SOFC) operate at temperatures <800 degrees C, permitting the use of relatively inexpensive metallic interconnects, such as ferritic stainless steels. Long term SOFC operation is limited, however, by the resistivity and volatility of the protective chromia scale that forms on the steel surface. To overcome these limitations, protective and conductive coatings can be applied to the steel surface. Two novel methods of applying such coatings are studied in this work. In the first approach, Mn-Co spinel oxides are anodically electrodeposited from aqueous solutions containing Mn and Co divalent ions. By controlling the solution composition and deposition parameters, dense and crack free deposits can be obtained. The as deposited oxide coatings are nanocrystalline, with a defective NaCl-type structure. The spinel structure is obtained through subsequent annealing at temperatures in excess of 500 degrees C. Oxidation tests at 800 degrees C show significantly improved oxidation resistance compared with uncoated ferritic stainless steels. In the second approach, composite coatings consisting of dispersed conductive LaCrO3 particles in a Ni matrix are electrodeposited onto ferritic stainless steel substrates. Co-deposition is performed using a Wafts Ni plating bath containing suspended LaCrO3 particles. The effects of particle loading in the bath, deposition current density, solution pH and electrolyte hydrodynamics on the volume fraction of embedded particles in the coatings have been studied and optimized. Upon oxidation in air, NiO forms on the steel surface. NiO is not protective; however, a protective layer of chromia (Cr2O3) forms beneath. The LaCrO3 particles are incorporated into the chromia scale and provide a stable, conductive path through the oxide layer.