Degradation of Solid Oxide Fuel Cell Cathodes Accelerated at a High Water Vapor Concentration

Abstract

The influence of water vapor in air on power generation characteristic of solid oxide fuel cells was analyzed by measuring cell voltage at a constant current density, as a function of water vapor concentration at 800°C and 1000°C. Cell voltage change was negligible at 1000°C, while considerable voltage drop was observed at 800°C accelerated at high water vapor concentrations of 20 wt % and 40 wt %. It is considered that La2O3 formed on the (La0.8Sr0.2)0.98MnO3 surface, which is assumed to be the reason for a large voltage drop.

Manganese Oxide Formation in Lanthanum Strontium Manganite-Yttria-Stabilized Zirconia SOFC Cathodes

“…This finding is consistent with prior reports that, relative to dry cathode air, the increase in degradation rate in humid cathode atmospheres is significant at temperatures below 1173 K (900°C) [8][9][10][11][12] but essentially absent at higher temperatures. [8,12] Figure 4 shows TEM images of two cathodes after operation in humid air, one at 1198 K (925°C) (top) and one at 1073 K (800°C) (bottom). The dark-field scanning transmission electron microscopy (DF-STEM) images of the cathode/electrolyte interface are at the left, and the corresponding XEDS elemental maps at the right.…”

“…For example, it has been reported [9] that the ASR rise was only partially reversed on removal of H 2 O from the cathode gas at 1023 K (750°C), and that at levels of humidity below~5 pct, the effect appeared to depend on current density and temperature. In that study, [9] the ASR increased linearly at 1023 K (750°C) for 0 to 12.8 pct humidity at 0.41 A cm À2 , whereas others [8] reported no ASR rise with 3 pct humidity at 1073 K (800°C) and 0.200 A cm À2 . The details of the ''steam effect'' (and cathode losses in general) also depend on the composition of the LSM.…”

“…[7][8][9][10][11][12] Under these conditions, an increase in ASR occurs below 1173 K (900°C) [8][9][10][11][12] but not above this temperature. [8,12] The ASR rise on introduction of humidity consists of an initial fast transient followed by a slower increase. [8,9,11,12] The effect is seen in cathodes of LSM with Gd-doped CeO 2 (GDC) [11] but not in cathodes of (La,Sr,Co) ferrite (LSCF) with GDC.…”

“…[8,12] The ASR rise on introduction of humidity consists of an initial fast transient followed by a slower increase. [8,9,11,12] The effect is seen in cathodes of LSM with Gd-doped CeO 2 (GDC) [11] but not in cathodes of (La,Sr,Co) ferrite (LSCF) with GDC. [9] This difference might be associated with the surface oxygen exchange rate, which has been reported to be temperature dependent for LSM, but relatively independent of temperature for LSCF.…”

A high performance cathode for proton conducting solid oxide fuel cells

“…To this end, materials with good oxygen ionic and electronic mixed conductivities and with sufficient catalytic activities towards oxygen reduction, for example, La 0.6 Sr 0.4 Co 1Àx Fe x O 3Àd (LSCF), Sm 0.5 Sr 0.5 CoO 3Àd (SSC), Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3Àd (BSCF) and La 2 NiO 4 , are highly recommended. 11 Such traditional composite structured cathode materials, however, are unstable in the high water-containing atmosphere 19,20 and are harmful to the long-term stability of a P-SOFC. As shown by eqn (2), proton conduction, instead of oxygen ion conduction, is vital to speed up the cathode reactions for a P-SOFC.…”