III. RESULTS AND DISCUSSION

In Fig. 1 are representative SEM micrographs of the LSCF surfaces before and after heat treatment in air. Before heat treatment, LSCF exhibited dense and flat surfaces as shown in Fig. 1(a). However, after heat treatment, submicron precipitates were formed on the surfaces. These precipitates were more pronounced and exhibited higher areal density with increasing heat-treatment temperatures ranging between 600 and 900 °C. Interestingly, in the figure, the precipitates were not formed along the grain boundaries but on grain surfaces at 800 and 900 °C. The magnified images in Fig. 2 show more clearly the shapes of the precipitates, which are different from grain to grain. The typical shapes of precipitates are rods, triangles, or irregular polygons. For the rod type, the direction of the precipitate was either well-aligned in one direction or vertically crossed. The precipitates were found not only on polished surfaces but also on unpolished surfaces as shown in Fig. 2(d). The precipitates align directionally along the facets of the grains, and indicate that the formation of precipitates corresponds to the surface orientation of the underlying grains. The characteristic of the rod-type precipitates is close to the Widmanstatten morphology, of which an incoherent curved interface exhibits a fast growth rate while a semicoherent facet has slower growth.Reference Porter and Eastering¹¹ However, the formation of precipitates observed in this study is different from the Widmanstatten morphology in that the precipitates grow out of the surface.

FIG. 1.

SEM micrographs of LSCF of (a) before heat treatment and after heat treatment at (b) 600 °C, (c) 700 °C, (d) 800 °C, and (e) 900 °C for 50 h. All images are of the same scale.

FIG. 2.

Magnified images of the precipitates on LSCF surfaces after heat treatment at (a) 900 °C for 50 h, (b) 900 °C for 50 h, (c) 860 °C for 100 h, and (d) 890 °C for 100 h. (a)–(c) are polished surfaces while (d) is an unpolished surface. (a) and (b) show different location of the same sample.

Chemical analysis was conducted using EDS after heat treatment and the results were compared with the as-fresh sample. As Table I shows, EDS data indicate roughly constant atomic percentages of all elements before and after heat treatment. This indicates that the bulk composition remains unchanged and the formation of precipitates is limited to the top surface- or near-surface region. AES analysis was performed to investigate the compositional change of the surface region. Figure 3 shows differentiated AES spectra of La_MNN, Sr_LMM, Co_LMM, Fe_LMM, and O_KLL for as-fresh and heat-treated LSCF. After heat treatment, a substantial increase in peak intensity was observed in Sr while that of lanthanum (La), cobalt (Co), and iron (Fe) decreased. The elemental atomic percentages obtained from AES are given in Table I. The enrichment factor, which represents the relative change of concentrations in the surface region, is calculated based on Eq. (2). Sr concentration doubled while Fe concentration halved. It should be stressed that the concentration of Co, which has been thought to be catalytically active, was not detected on the surface. Enrichment of A-site cation and deficiency of B-site cation in the topmost surface can be understood by the formation of the Ruddlesden–Popper (RP) phase, in which A–O and B–O phases are formed in the top surface layer and subsurface layer, respectively.Reference Ruddlesden and Popper¹²

FIG. 3.

Differentiated Auger electron spectra of the (a) La_MLL, (b) Sr_LMM, (c) Co_LMM, (d) Fe_LMM, and (e) O_KLL before (blue solid line) and after (red dotted line) heat treatment at 800 °C for 50 h.

TABLE I.

Elemental atomic percentages (%) of pre-and post-heat-treated LSCF at 800 °C obtained from EDS and AES.

Chemical analysis of the precipitate was conducted using transmission electron microscopy (TEM). Figure 4(a) is a bright-field image of the cross section of the precipitate on the LSCF surface. The TEM micrograph shows the presence of a transition region below the precipitate on the top surface, which is marked by the brightest region in the figure. The EDS line scan was performed from the inner bulk region to the precipitate on the surface as shown in Fig. 4(a). The intensity of the Sr peak increases in the precipitate region compared with that of the bulk and the oxygen peak decreases slightly. In contrast, the peak intensities of the other cations decrease substantially in the precipitate. Thus the precipitate is a Sr–O compound. The decrease of Sr intensity in the subsurface regions indicates that Sr diffuses from the subsurface region to the surface to form the precipitate.

FIG. 4.

TEM bright-field image of the cross section of precipitate (a) and EDS line scan of selected elements (b).

Figure 5 shows the pO₂ dependence for the formation of precipitates. Samples were heat-treated at 800 °C for 50 h in pure N₂ and 0.1% O₂, which was balanced by argon (Ar). Compared with heat-treated samples in air at the same temperature [Fig. 1(d)], the formation of precipitates reduced considerably in 0.1% O₂ [Fig. 5(b)] and no precipitates were found in the pure N₂-treated sample [Fig. 5(a)]. These trends were similarly observed when the samples were heat-treated at higher temperatures in the same environments. Figures 5(c) and 5(d) show the heat-treated LSCF surface at 890 °C in N₂ and 0.1% O₂, respectively. The reduced formation of Sr–O precipitates indicates that in addition to temperatures, the oxygen partial pressure is a critical factor affecting the formation of Sr–O precipitates on LSCF surfaces.

FIG. 5.

SEM micrographs of LSCF surfaces after heat treatment for 50 h at (a) 800 °C/N₂, (b) 800 °C/0.1% O₂, (c) 890 °C/N₂, and (d) 890 °C/0.1% O₂.

The observed Sr enrichment on the LSCF surface after heat treatment further results in a decrease in surface transition metal concentration. This is in good agreement with previous works. Bucher et al.Reference Bucher and Sitte³ found Sr enrichment on the surfaces of LSCF after measuring surface exchange coefficients at 600–700 °C. HeideReference Heide⁴ observed Sr enrichment on La_1-xSr_xCo_yFe_1-yO₃ surfaces after heat treatment at 1000 °C in 1% O₂. Recently, Kubicek et al.Reference Kubicek, Limbeck, Fromling, Hutter and Fleig¹³ showed cation segregation behavior from pulsed laser-deposited La_0.6Sr_0.4CoO_3−δ thin film electrodes on yttria-stabilized zirconia single crystals. They reported the formation of chemically inhomogeneous surface as a result of Sr excess particles after thermal annealing. Similarly, Cai et al.Reference Cai, Kubicek, Fleig and Yildiz¹⁴ observed Sr-enriched surface from pulsed layer deposited La_0.6Sr_0.4CoO_3-δ thin film. They clearly show the presence of SrO-Sr(OH)₂ species on the surface using spatially resolved AES. While these previous studies on Sr enrichment mostly postulated the formation of SrO layers on the surface based on the results from chemical analysis, this study clearly shows the formation of submicron-sized Sr–O precipitates on the surface using SEM and TEM. The elemental enrichment phenomena and formation of AO precipitates on the surface was previously shown in other ABO₃ perovskite materials,Reference Rahmati, Fleig, Sigle, Bischoff, Maier and Ruhle^15–Reference Jung and Tuller¹⁸ but the formation of the Sr–O precipitate on polycrystalline LSCF surfaces was visualized for the first time in the present study.

Generally, elemental enrichment and precipitation are related to defect structures, especially the presence of point defects.Reference Horvath, Gerblinger, Meixner and Giber^19–Reference Szot and Speier²² As La_1−xSr_xCo_yFe_1−yO₃ exhibits a substantial oxygen deficiency,Reference Tai, Nasrallah, Anderson, Sparlin and Sehlin^23, Reference Stevenson, Armstrong, Carneim, Pederson and Weber²⁴ oxygen vacancies may contribute to Sr–O precipitation. Even though oxygen vacancies improve the oxygen reduction reaction on the surface and oxygen diffusion in the lattice, their presence reduces the stability of the structure by increasing repulsive forces between like atoms. The effect of oxygen vacancies on bonding strength was experimentally demonstrated by Wang et al.Reference Wang, Duncan, Wachsman and Ebrahimi²⁵ They found that the elastic modulus of ceria decreases drastically as the oxygen vacancy concentration increases. This is because oxygen vacancies weaken the attractive forces. The formation of oxygen vacancy in the lattice can be written as Eq. (3) using Kroger-Vink notation.

(3)

Figure 6(a) shows the schematic of the (100) plane of a cubic perovskite structure. Due to originally broken bonding, atoms on the surface are unstable compared with those in bulk. If oxygen ions are removed from the lattice according to Eq. (3), the shielding effect between cations will be removed and atoms in the surface region will become more unstable. Thus, to reduce the excess surface energy corresponding to unstable surfaces, it is probable that some cations near the surface region precipitate on the surface, yielding cation vacancies in the lattice as demonstrated in Fig. 6(b). As the binding of A–O is generally weaker than the binding energy of B–O in the ABO₃ perovskite structure, the precipitation of A-site cations will be more energetically favorable than the precipitation of B-site cations.Reference Tanaka and Misono^6, Reference Szot, Pawelczyk, Herion, Freiburg, Albers, Waser, Hulliger, Kwapulinski and Dec²⁶ Therefore, enriched Sr in LSCF is likely to precipitate into Sr–O to reduce the excess surface energy caused by oxygen vacancy formation.

FIG. 6.

Schematic diagram of (a) the formation of oxygen vacancies and (b) formation of cation and anion vacancies simultaneously.

LSCF was heat-treated at the same temperatures over various pO₂ ranges from pure N₂ to air. LSCF exhibits reduced formation of Sr–O precipitates with decreasing pO₂. A similar pO₂-dependence of elemental enrichment was reported by Dufour et al.Reference Dufour, Bertrand, Caboche, Decorse, El Anssari, Poirson and Vareille²⁷ They found that the ratio of Ba to Ti on the surfaces of BaTiO₃ became doubled after heat treatment at 1300 °C in air, but the ratio was decreased to 1.4 when surfaces were heat-treated in N₂. This indicates reduced Ba enrichment at lower pO₂. Szot et al.Reference Szot, Pawelczyk, Herion, Freiburg, Albers, Waser, Hulliger, Kwapulinski and Dec²⁶ studied the nature of the surface layer in ABO₃ perovskite structure materials such as SrTiO₃ and observed the formation of A–O layers at elevated temperatures, which is known as the RP phase.Reference Ruddlesden and Popper¹²

They proposed that the RP phase is likely to be more stable at high pO₂ and therefore the formation of A–O layers can be inhibited at reduced pO₂.

The formation of Sr–O precipitates on LSCF surfaces can be understood by a two-step mechanism: (i) the formation of oxygen vacancies and (ii) subsequent precipitation of enriched Sr. Step (i) increases with decreasing pO₂ but step (ii) occurs at oxidizing conditions, i.e., at high pO₂. As LSCF exhibits a high oxygen vacancy concentration even in oxidizing conditions, the overall reaction is likely to be controlled by step (ii). Konigstein et al.Reference Konigstein and Catlow²⁸ reported that thermodynamically stable compounds of Sr–O are SrO and SrO₂. The step (ii) for Sr–O precipitation, then, can be written as

(4)

(5)

Thus Sr–O precipitation would increase with increasing pO₂ consistent with our experimental observation.

The precipitate crystal structure was analyzed using low-angle XRD but could not be identified due to limited amount of the secondary phase. Therefore, it cannot be concluded decisively which reaction (formation of SrO or SrO₂) is more dominant. However, the reaction increasing hole concentration is likely to be thermodynamically unfavorable because the formation of holes increases the concentrations of Co⁴⁺ and Fe⁴⁺, which are unstable species at high temperatures.Reference Ishihara²⁹ A further systematic quantum mechanical study is required to conclude the dominant reaction mechanism.

As reported by Kubicek et al.Reference Kubicek, Limbeck, Fromling, Hutter and Fleig¹³ and Cai et al.,Reference Cai, Kubicek, Fleig and Yildiz¹⁴ the formation of Sr–O precipitates on the surface contributes to degradation of cell performance. The oxygen reduction reaction at the cathode consists of a series of elementary steps such as

1. (i) Gas phase diffusion

2. (ii) Adsorption of molecular oxygen [O₂(g) + s_ad → O₂ (ad)]

3. (iii) Dissociation into atomic oxygen [O₂(ad) → 2O (ad)]

4. (iv) Charge transfer [O(ad) + 2e^/ → 2O^//(ad)]

5. (v) Incorporation of oxygen ions [O^//(ad) + V_O^•• → O_o^x]

where s_ad is the oxygen adsorption sites on the surface. Sr–O is electronically an insulator.Reference Copeland and Swalin³⁰ The collected data of the surface exchange coefficients and the oxygen tracer diffusion coefficients of various metal oxides indicate that the electronic species on the surface are essential for oxygen exchange reaction.Reference De Souza and Kilner³¹ It is expected that the formation of Sr–O precipitate on the surface of LSCF forms a passive layer, reducing the concentration of electronic species and inhibiting the oxygen reduction reaction (ORR), especially charge transfer. In addition, for ABO₃ perovskite materials, it is generally believed that B-site transition metals play a critical role for catalytic activity.Reference Tanaka and Misono^6–Reference Shimizu⁹ Takeda et al.Reference Takeda, Kanno, Noda, Tomida and Yamamoto³² measured the cathodic polarization of La_0.7Sr_0.3MO₃ (M = Cr, Mn, Fe, Co) and found that La_0.7Sr_0.3CoO₃ exhibited the lowest polarization at 800 °C in air. Kilner et al.Reference Kilner, DeSouza and Fullarton³³ investigated the dependence of surface exchange coefficients (K_o) on the composition of La_0.8Sr_0.2Mn_1-yCo_yO₃ and found that K_o increased with increasing Co concentration. Kan et al.Reference Kan and Wachsman³⁴ studied the effect of catalyst impregnation on the catalytic activity for oxygen exchange for La_0.8Sr_0.2MnO₃ (LSM). They found oxygen exchange reaction was improved with the addition of Co on LSM surfaces via wet impregnation. These results indicate that Co is a catalytically active element for ORR. Consequently, deficiency of catalytically active transition metals, such as Co, due to Sr enrichment is likely to further reduce the ORR. Thus, Sr–O precipitation on LSCF surfaces can cause substantial increases in non-ohmic resistance (activation polarization). It can also lead to increases in ohmic resistance if Sr is enriched at the interface of LSCF and other cell components, forming secondary phases such as SrZrO₃ or SrCrO_4.Reference Mai, Becker, Assenmacher, Tietz, Hathiramani, Ivers-Tiffee, Stover and Mader^35, Reference Fergus³⁶