A typical solid oxide fuel cell stack includes multiple fuel cells separated by metallic interconnects (IC) which provide both electrical connection between adjacent cells in the stack and channels for delivery and removal of fuel and oxidant. The metallic interconnects are commonly composed of a Cr based alloy such as an alloy known as CrF which has a composition of 95 wt % Cr-5 wt % Fe or Cr—Fe—Y having a 94 wt % Cr-5 wt % Fe-1 wt % Y composition. The CrF and CrFeY alloys retain their strength and are dimensionally stable at typical solid oxide fuel cell (SOFC) operating conditions, e.g. 700-900 C in both air and wet fuel atmospheres. However, during operation of the SOFCs, chromium in the CrF or CrFeY alloys react with oxygen and form chromia, resulting in degradation of the SOFC stack.
Two of the major degradation mechanisms affecting SOFC stacks are directly linked to chromia formation of the metallic interconnect component: i) higher stack ohmic resistance due to the formation of native chromium oxide (chromia, Cr2O3) on the interconnect, and ii) chromium poisoning of the SOFC cathode.
Although Cr2O3 is an electronic conductor, the conductivity of this material at SOFC operating temperatures (700-900 C) is very low, with values on the order of 0.01 S/cm at 850 C (versus 7.9×104 Scm−1 for Cr metal). The chromium oxide layer grows in thickness on the surfaces of the interconnect with time and thus the ohmic resistance of the interconnect and therefore of the SOFC stack due to this oxide layer increases with time.
The second degradation mechanism related to the chromia forming metallic interconnects is known as chromium poisoning of the cathode. At SOFC operating temperatures, chromium vapor diffuses through cracks or pores in the coating and chromium ions can diffuse through the lattice of the interconnect coating material into the SOFC cathode via solid state diffusion. Additionally, during fuel cell operation, ambient air (humid air) flows over the air (cathode) side of the interconnect and wet fuel flows over the fuel (anode) side of the interconnect. At SOFC operating temperatures and in the presence of humid air (cathode side), chromium on the surface of the Cr2O3 layer on the interconnect reacts with water and evaporates in the form of the gaseous species chromium oxide hydroxide, CrO2(OH)2. The chromium oxide hydroxide species transports in vapor form from the interconnect surface to the cathode electrode of the fuel cell where it may deposit in the solid form, Cr2O3. The Cr2O3 deposits on and in (e.g., via grain boundary diffusion) the SOFC cathodes and/or reacts with the cathode (e.g. to form a Cr—Mn spinel), resulting in significant performance degradation of the cathode electrode. Typical SOFC cathode materials, such as perovskite materials, (e.g., LSM, LSC, LSCF, and LSF) are particularly vulnerable to chromium oxide degradation.