Structure and Relative Thermal Stability of Mesoporous (La,Sr)MnO3 Powders Prepared Using Evaporation-Induced Self-Assembly Methods
Robin Chao, Ratiporn Munprom, Rumyana Petrova, Kirk Gerdes, John R. Kitchin and Paul A. Salvador, “Structure and Relative Thermal Stability of Mesoporous (La,Sr)MnO3 Powders Prepared Using Evaporation-Induced Self-Assembly Methods”, Journal of the American Ceramic Society 95(7), 2339-2346 (2012). https://doi.org/10.1111/j.1551-2916.2012.05236.x
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Question
Solid oxide fuel cell (SOFC) cathodes are often limited by slow oxygen reduction, so raising the number of accessible active sites with high surface area electrocatalysts, for example by infiltrating nanoscale catalyst into porous cathodes, is attractive. Conventional solid state synthesis of Sr-doped LaMnO3 (LSM) gives only about 2 m2/g, and the best wet-chemical route the authors cite reached about 32 m2/g. The paper asks whether evaporation-induced self-assembly (EISA), a surfactant-templated sol-gel method compatible with infiltration, can make crystalline mesoporous LaMnO3 and LSM with high surface area, how processing variables control that area, and whether the mesoporosity survives at SOFC operating temperature (Chao et al. 2012).
Approach
Metal nitrates (La:Mn = 1:1 or La:Sr:Mn = 0.8:0.2:1) were dissolved in a water/ethanol solution of a structure-directing surfactant, acidified to pH 1 with nitric acid, and evaporated for 24 h at either high (about 60%) or low (about 20%) relative humidity (RH). Three templates of increasing molecular weight were compared: CTAB (346 g/mol), Pluronic P-123 (5580 g/mol) and Pluronic F-127 (12500 g/mol). Precursor loading was varied from 5 to 60 mmol. The gels were calcined at 700 C for 2 h, a temperature chosen because lower calcinations gave poorly crystallized material and because target SOFCs run at 700 to 800 C. Thermal stability was probed by a further 100 h anneal in air at 750 C. Characterization used powder X-ray diffraction, N2 adsorption with BET surface areas and BJH pore size distributions, and HAADF STEM (Z-contrast) imaging, with ICP on selected samples (Chao et al. 2012).
Key findings
All calcined products were well crystallized orthorhombic perovskites. Refined lattice parameters were a = 5.55, b = 5.70, c = 7.69 A (V = 244 A3) for LaMnO3 and a = 5.55, b = 5.59, c = 7.72 A (V = 242 A3) for the Sr-doped powder. Weak La2O3 (undoped) or SrCO3 (doped) peaks appeared; the authors attribute them most likely to early phase separation during drying, and suggest the Sr content of the perovskite is somewhat below the targeted 20% (Chao et al. 2012).
Surface area rose with precursor loading. For LaMnO3 at 60% RH it increased from 5 to 40 m2/g with P-123 (5 to 40 mmol) and from 10 to 50 m2/g with CTAB (5 to 20 mmol), with CTAB higher at every loading. The useful loading is capped by salt solubility: CTAB with 40 mmol precipitated. BJH analysis tied the area to mesopores of roughly 5 to 50 nm; with P-123 a pore population near 25 nm appeared only above 10 mmol (a sixfold area jump the authors read as a mesophase transition), whereas CTAB already showed 18 nm pores at 5 mmol. Aging at 20% instead of 60% RH lowered the area by about 20 m2/g, consistent with higher water content favoring more curved micelles and finer pores (Chao et al. 2012).
For LSM at 20 mmol loading, CTAB, P-123 and F-127 gave 46, 24 and 15 m2/g respectively, so lower molecular weight templates yield smaller pores and higher area. The text also lists maxima of 46, 36 and 42 m2/g for the three surfactants, which is not fully consistent with its statement that F-127 always gave lower area than P-123; the figure was not available to resolve this. HAADF STEM of a 46 m2/g CTAB-derived LSM particle showed a disordered, interconnected three-dimensional pore network with pores of 10 to 20 nm and walls 1 to 10 nm thick, larger and less ordered than typical low temperature EISA oxides, which the authors attribute to pore coalescence and densification at 700 C (Chao et al. 2012).
After 100 h at 750 C, LSM made with CTAB lost 35% of its surface area through collapse of 10 to 50 nm pores, while P-123 and F-127 powders lost only 11% and 6%. All annealed powders ended at 17 to 24 m2/g but retained substantial mesoporosity. The authors propose that the larger Pluronic surfactants produce thicker pore walls (implied by their lower initial area and pore volume), which improves thermal stability (Chao et al. 2012).
Significance
The work establishes a trade-off for templated perovskite electrocatalysts: small surfactants, high humidity and high precursor loading maximize surface area (up to about 50 m2/g, which the authors describe as the highest reported for wet-chemically prepared (La,Sr)MnO3), while large surfactants maximize the retention of mesoporosity at SOFC temperatures. Mesoporous LSM in the 20 to 25 m2/g range with promising stability was readily made with P-123 and F-127. Because the same EISA solutions can infiltrate SOFC cathodes, the results support EISA as a route to high surface area infiltrated cathode catalysts, although the authors caution that stability on supports and under electrochemical load still has to be tested (Chao et al. 2012).