Conductivity and oxygen reduction activity changes in lanthanum strontium manganite upon low-level chromium substitution

Abstract

On the timescale of solid oxide fuel cell (SOFC) system lifetime requirements, the thermodynamically predicted low-level substitution of chromium on the B-site of (La,Sr)MnO 3 could be a source of cathode degradation underlying more overt and well-known chromium poisoning mechanisms. To study this phenomenon in isolation, electronic conductivity (σ) and electrochemical oxygen reduction activity of the (La 0.8 Sr 0.2 ) 0.98 Cr x Mn 1−x O 3 model series (x = 0, 0.02, 0.05 or 0.1) were measured in air between 850 and 650 °C. Depending on the extent of chromium substitution and the measurement temperature, electrochemical impedance spectroscopy (EIS) results could be deconvoluted into a maximum of three contributions reflecting possible limiting processes such as oxide ion transport and dissociative adsorption. Chromium substitution resulted in lowered σ (from 174 S cm − 1 (x = 0) to 89 S cm − 1 (x = 0.1) at 850 °C) and a steady rise in associated activation energy (E a ) (from 0.105 ± 0.001 eV (x = 0) to 0.139 ± 0.001 eV (x = 0.1)). From EIS analyses, ohmic and polarisation resistances increased, whilst Ea for the overall oxygen reduction reaction also increased from 1.39 ± 0.04 eV (x = 0) to 1.48-1.54 ± 0.04 eV upon chromium substitution.Keywords level, conductivity, oxygen, reduction, activity, changes, chromium, lanthanum, substitution, strontium, manganite, upon, low Depending on the extent of chromium substitution and the measurement temperature, electrochemical impedance spectroscopy (EIS) results could be deconvoluted into a maximum of three contributions 2 reflecting possible limiting processes such as oxide ion transport and dissociative adsorption. Chromium substitution resulted in lowered  (from 174 S cm -1 (x = 0) to 89 S cm -1 (x = 0.1) at 850 °C) and a steady rise in associated activation energy (E a ) (from 0.105 ± 0.001 eV (x = 0) to 0.139 ± 0.001 eV (x = 0.1)). From EIS analysis, ohmic and polarisation resistances increased, whilst E a for the overall oxygen reduction reaction also increased from 1.39 ± 0.04 eV (x = 0) to 1.48-1.54 ± 0.04 eV upon chromium substitution.

Thermal Stress Analysis of Solid Oxide Fuel Cells with Chromium Poisoning Cathodes

“…23 The substitution of Cr in LSM lattice will also cause reduction in conductivity and oxygen reduction activity. 24 The similar results confirm that Cr is responsible for the cathodic degradation of SOFC. 25 We have demonstrated that Cr preferably adsorbed on the MnO 2 -terminated and La(Sr)-O-terminated (001) surfaces, and even trace of Cr in cathode will strongly decrease conductivity of the LSM cathode.…”

Electronic structure origin of conductivity and oxygen reduction activity changes in low-level Cr-substituted (La,Sr)MnO3

“…Details regarding the synthesis and structural characterisation of the (La 0.8 Sr 0.2 ) 0.98 Mn 1−x Cr x O 3 model series are provided elsewhere. 2 In brief, compositions were prepared by solid state synthesis using stoichiometric amounts of La 2 O 3 , SrCO 3 , Cr 2 O 3 , and Mn 2 O 3 . Room temperature powder X-ray diffractometry (XRD) revealed that all compositions were isostructural single phase perovskites with respect to the analysis, while observed Bragg peak splitting was consistent with rhombohedral crystal symmetry.…”

“…However, quantification of the extent of hybridisation revealed a decrease in the (C + D)/(e g + t 2g ) ratio at elevated temperature from 3.73 ± 0.02 to 3.57 ± 0.01 upon low-level chromium substitution, indicative of lowered O 2p-Mn 3d hybridisation, and qualitatively consistent with the previously demonstrated lower electronic conductivity at 850 • C of (La 0.8 Sr 0.2 ) 0.98 Mn 0.9 Cr 0.1 O 3 (89 S cm −1 ) compared to (La 0.8 Sr 0.2 ) 0.98 MnO 3 (174 S cm −1 ). 2…”

“…We previously measured significant decreases in electronic conductivity and electrochemical oxygen reduction activity in air between 650 and 850 • C upon low-level chromium substitution of (La,Sr)MnO 3 . 2 In seeking to further explain such changes with respect to their orbitallevel origin, we present here the electronic structure of the (La 0.8 Sr 0.2 ) 0.98 Mn 1−x Cr x O 3 series (x = 0, 0.05, or 0.1), which models the thermodynamically predicted low-level substitution of chromium on the B-site of (La,Sr)MnO 3 . O K-edge and Mn L-edge near-edge X-ray absorption fine structure (NEXAFS) spectroscopies and Mn L 3 -edge resonant photoemission spectroscopy (RPES) at room and elevated temperature were performed, representing the first time that soft X-ray synchrotron radiation has been used to gain insight into the chromium poisoning of (La,Sr)MnO 3 -based SOFC cathodes.…”