Crystal Structure, Chemical Stabilities and Electrical Conductivity of Fe-Doped Manganese Cobalt Spinel Oxides for SOFC Interconnect Coatings

Abstract

The application of ceramic coatings has been demonstrated as an effective technique to suppress the oxidation scale growth and chromium volatilization of ferritic stainless steels used for SOFC interconnects. Manganese cobalt spinel oxides are promising materials for this application. In this work, the effects of iron doping in the MnCo2-xFexO4 (x = 0–0.7) spinel oxide on the crystal structure, reaction with chromia, cation distributions, and electrical conductivity at high temperature were investigated. With increasing the iron content, the lattice parameters increase and the electrical conductivity decreases. During reaction with chromia, iron does not appear in the chromium-rich reaction layer even for the sample with heaviest doping (e.g. MnCo1.3Fe0.7O4). Neutron diffraction studies suggest that cobalt has a stronger preference for the tetrahedral sites while manganese and iron favor the octahedral sites.

Thermal expansion and electrical conductivity of Fe and Cu doped MnCo2O4 spinel

“…For example, [49] reported 34 S/cm at 800 °C in air for a sample sintered at 1200 °C and [78] reported 40 S/cm for a sample sintered at 1300 °C. As previously discussed by Liu et al [48] some of the discrepancies found in literature are most likely due to the high temperature at which many of the investigated samples have been sintered. According to the Mn3O4-Co3O4 phase diagram reported by Aukrust and Muan [65], the MnCo2O4 spinel is only stable up to ca.…”

“…Previous investigations of the cation distribution in MnCo2-xFexO4 have suggested both a mixed occupancy of Fe on tetrahedral and octahedral sites [48], and that Fe mainly occupies the latter [73]. Crystal field theory predicts that both Mn 3+ and Co 3+ have a stronger preference for the octahedral site than Fe 3+ [74].…”

“…1050 °C in air. Liu et al [48] showed that MnCo2O4 sintered at 1200 °C had an electrical conductivity of 51 S/cm at 800 °C and contained significant amounts of rock salt secondary phases. When the same sample was annealed at 1000 °C for 72 h, the rock salt phases disappeared and the electrical conductivity at 800 °C increased to 87 S/cm.…”

“…Since the distance between octahedral sites is shorter than the distance between tetrahedral sites in the spinel structure, hopping takes place primarily between multivalent cations on the former [45]. For MnCo2O4, hopping is believed to take place mainly between Mn 3+ /Mn 4+ pairs on the octahedral sites, with a possible contribution due to hopping between Co 2+ /Co 3+ pairs [47,48]. There is considerable scatter in literature regarding the magnitude of the electrical conductivity in this material, with values measured in air at 800 °C ranging from 34 to 132 S/cm [49][50][51][52][53].…”

Comparison of iron and copper doped manganese cobalt spinel oxides as protective coatings for solid oxide fuel cell interconnects

“…Both materials have appreciable electrical conductivity (60-90 S cm -1 at 800 °C [11,12]) and an acceptable thermal expansion coefficient (TEC) (11.4-14.4×10 -6 K -1 between 25 °C and 800 °C [13][14][15]). Attempts to improve the (Mn,Co) 3 O 4 coating material by substituting some of the Co with Fe or Cu have been reported [13,[16][17][18][19][20][21]. The TEC of MnCo 2 O 4 is reduced by replacing some of the Co with Fe, which in some cases may be advantageous depending on what type of cell the interconnect is connected to [15].…”