Cationic Intermixing and Reactivity at the La₂Mo₂O₉/La_0.8Sr_0.2MnO_3−δ Solid Oxide Fuel Cell Electrolyte–Cathode Interface

Abstract

Among standard high-temperature cathode materials for solid oxide fuel cells, La0.8 Sr0.2 MnO3-δ (LSM) displays the least reactivity with the oxide-ion conductor La2 Mo2 O9 (LMO), yet a reaction is observed at high processing temperatures, identified by using XRD and focused ion beam secondary-ion mass spectrometry (FIB-SIMS) after annealing at 1050 and 1150 °C. Additionally, Sr and Mn solutions were deposited and annealed on LMO pellets, as well as a Mo solution on a LSM pellet. From these studies several reaction products were identified by using XRD and located by using FIB-SIMS on the surface of pelletised samples. We used depth profiling to show that the reactivity extended up to ∼10 μm from the surface region. If Sr was present, a SrMoO4 -type scheelite phase was always observed as a reaction product, and if Mn was present, LaMnO3+δ single crystals were observed on the surface of the LMO pellets. Additional phases such as La2 MoO6 and La6 MoO12 were also detected depending on the configuration and annealing temperature. Reaction mechanisms and detailed reaction formulae are proposed to explain these observations. The strongest driving force for cationic diffusion appears to originate from Mo(6+) and Mn(3+) cations, rather than from Sr(2+) .

Cationic Interdiffusion at the SOFC Electrolyte/Cathode Interface in La2Mo2O9/La0.8Sr0.2MnO3‐δ

“…Similarly, the Sr signal was also strong for ∼60 μm from the surface falling to a background level, indicating a Sr diffusion length in LM of ∼60 μm. Both Mo and Sr signals show similar profiles, in agreement with a SrMoO 4 type material produced as a reaction product at the surface of the pellet (as confirmed by XRD). As shown in Figure , the Mo/La and Sr/La profiles are not linear, which indicates that the reaction product is not pure SrMoO 4 but also contains La, whose content varies over depth (as suggested in ref.…”

“…Considering this as the diffusion length of Mo (∼50 μm at 1150 °C for 12 h), the bulk diffusion coefficient was estimated as ∼ 10 −10 cm 2 .s −1 (from L =2 ( Dt ) , as above for Sr diffusion in LM. Some spots with a higher concentration of Sr were observed, due to the presence of SrMoO 4 type and Mn 3 O 4 grains on the surface . Irregular surface roughness was also detected, which is likely to be the result of the appearance of both small and large grains as observed in the FIB‐SIMS ion image .…”

“…After annealing the LSM pellet covered with (NH 4 ) 6 Mo 7 O 24. 4H 2 O solution for 12 hours at 1150 °C, the presence of Mn 3 O 4 and SrMoO 4 type phases was detected by XRD . Irregular grain distribution was reported, where the large grains were composed of a SrMoO 4 type phase and the small grains composed of a Mn 3 O 4 phase.…”

“…A Sr(NO 3 ) 2 solution was deposited on the LM pellets and annealed at 1150 °C for 12 hours. After cooling, XRD was performed on the pellet at room temperature and four different phases La 6 MoO 12 , β‐LM, SrMoO 4 and La 2 MoO 6 were observed . As mentioned in section 2.1, samples were prepared for TOF‐SIMS analysis and a line scan measurement was carried out on Side A of the pellet.…”

“…Additionally LSM was found to be the least reactive perovskite‐type/perovskite related cathode material with LM, leading to the formation of Sr(MoO 4 ) only above 700 °C ,. In a previous paper, we have studied the reaction mechanism between LM and LSM by annealing the pellets of these two compositions in close contact to each other after coupling their polished faces together. Three different pellet couples were annealed at 1) 1050 °C for 12 hours, 2) 1050 °C for 36 hours and 3) 1150 °C for 12 hours.…”