Chapter 2. Structural, electrical, and electrochemical characteristics of
2.2. Result and Discussion
Figure 2.1. X-ray diffraction (XRD) patterns of a) LnBa0.5Sr0.5Co1.5Fe0.5O5+ (Ln = Pr, Sm and Gd), and b) difference between the observed and calculated XRD profile of PBSCF. The space group of the cathodes was indexed based on Rietveld refinement. c) The chemical compatibility between LnBSCF- GDC composite, LDC buffer layer, and LSGM
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The crystal structures of LnBSCF were confirmed by X-ray diffraction (XRD) patterns, as shown in Figure 2.1. a). The patterns of LnBSCF show a single phase of a layered perovskite structure without any detectable impurities. Figure 2.1. b) exhibits the difference between observed XRD data and the calculated Rietveld refinement profile of PBSCF with a reliability factor of Rp = 0.16. The Rietveld refinement data of LnBSCF reveal that the diffraction peaks of the samples are a tetragonal structure (space group: P4/mmm), and the calculated lattice parameters are summarized in Table 2.1. The results show that PBSCF has the largest lattice volume, 115.84 Å3, at room temperature while SBSCF and GBSCF have volume of 113.85 Å3 and 113.42 Å3, respectively. Figure 2.1. c) exhibits the XRD of LnBSCF-Ce0.9Gd0.1O2- (GDC)/La0.4Ce0.6O2- (LDC)/La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM), suggesting there are no side reactions among all of the cathode composites, LSGM electrolyte, and LDC buffer layer. The LDC buffer layer has been adopted to prevent undesirable inter-diffusion of cations between the electrolyte and the electrode at the interface.[29]
Space group a [Å] b [Å] c [Å] V [Å3] Oxygen content
(5+) TEC [K-1]
PBSCF P4/mmm 3.8680 3.8680 7.7425 115.8388 5.88 22.24 x 10 -6
SBSCF P4/mmm 3.8672 3.8672 7.6130 113.8542 5.71 21.15 x 10 -6
GBSCF P4/mmm 3.8689 3.8689 7.5773 113.4200 5.56 18.95 x 10 -6
Table 2.1. Space group, lattice parameters, oxygen content, and thermal expansion coefficient (TEC) of LnBSCF (Ln = Pr, Sm, and Gd).
Microstructures of LnBSCF cathodes are displayed in Figure 2.2. a)-c). All of the cathode materials have a similar morphology with enough porosity for gas diffusion and a homogenous microstructure.
Figure 2.2. d) exhibits a cross-sectional image showing separately stacked figure of PBSCF- GDC/LDC/LSGM layer with approximately 10 μm thickness for the porous cathode and 2.4 μm for the LDC buffer layer.
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Figure 2.2. SEM images showing the microstructures of the a) PBSCF-GDC, b) SBSCF-GDC, c) GBSCF-GDC composite and d) a cross sectional view of the cathode, buffer layer, and electrolyte.
The thickness of the cathodes and the buffer layers was about 10 m and 2 m, respectively
Figure 2.3. Temperature dependence of the electrical conductivity of the LnBa0.5Sr0.5Co1.5Fe0.5O5+
(Ln = Pr, Sm, Gd) at various temperatures.
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The electrical conductivities of LnBSCF under various temperatures are presented in Figure 2.3.
Electrical conductivities decrease with increasing temperature, indicating typical metallic behavior.
The PBSCF shows the highest electrical conductivity, 1150 S cm-1, while SBSCF and GBSCF exhibit values of 727 S cm-1 and 462 S cm-1, respectively. The differences in electrical conductivity might originate from the crystalline structure and oxygen concentration. Smaller size Ln ions contain more oxygen vacancies and lower interstitial oxygen concentration in the A-site layer, resulting in perturbation of the O-Co-O interaction and carrier delocalization. In addition, smaller size Ln ions cause higher bending of O-Co-O bonds, leading to lowered symmetry and decreased overlap between the 3d orbitals of cobalt and 2p orbitals of oxygen.[20] Consequently, LnBSCF with the largest size of Pr3+ cations shows the highest electrical conductivity while LnBSCF with the smallest size of Gd3+
cation exhibits the lowest electrical conductivity. [30,31]
Figure 2.4. Oxygen content of LnBa0.5Sr0.5Co1.5Fe0.5O5+ (Ln = Pr, Sm, Gd) at various temperatures.
The oxygen content at room temperature was measured by iodometric titration. Oxygen content at room temperature is summarized in Table 2.1.
Oxygen content in a range of 100 to 900 oC for the LnBSCFs is exhibited in Figure 2.4. All of LnBSCFs start to lose oxygen from the lattice at about 300 oC. The oxygen content (5+) at room temperature, confirmed by iodometric titration, increased gradually from 5.56 to 5.88 with increasing radius of A-site lanthanide cations (Gd3+ < Sm3+ < Pr3+).[30,31] In other words, larger lanthanide cations decrease the difference of average cations in the A site, resulting in an increase of the oxygen coordination number and oxygen content, respectively.[20,32]
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Figure 2.5. a) Depth profile of the normalized isotope oxygen fraction in LnBSCF, b) fitted values of oxygen bulk diffusivity (D*), and the surface oxygen exchange coefficient (k) of LnBSCF
Isotope oxygen exchange was analyzed to investigate oxygen kinetics at the surface and the bulk diffusion of LnBSCFs. Figure 2.5. a) presents depth profiles of 18O2 in LnBSCFs annealed at 590 oC in 200 mbar 18O2. The normalized 18O2 concentration of the samples was not completely homogeneous for the measured surface areas because of the differences in surface terminations and/or the different grain orientations. By integrating the area of the image along the x direction, the normalized values of the 18O2 concentration along the y-axis can be calculated and fitted to Crank's solution. The oxygen bulk diffusion coefficient (D*) and oxygen surface exchange coefficient (k) are presented in Figure 2.5. b). The D* values of PBSCF, SBSCF, and GBSCF are 1.9 x 10-8, 1.7 x 10-8, and 7.1 x 10-9 cm2 s-1, respectively, and the k values of LnBSCFs are 5.0 x 10-7, 3.4 x 10-7, and 1.2 x 10-7 cm s-1, respectively.
As a result, PBSCF shows the highest D* and k among the investigated samples. This result corresponds to the finding layered perovskites containing Pr usually show the lowest defect energy of oxygen Frenkel disorder. In other words, a synergistic effect of low oxygen Frenkel energy (0.24 eV/defect) with appreciable antisite energy (~1 eV) in a system of LnBaCo2O5.5 layered perovskite oxides, leading to a high rate of permeation of mobile oxygen species.[33] Thus, it is reasonable to consider that PBSCF has the highest diffusivity and surface exchange coefficient among the conducted LnBSCF (Ln = Pr, Sm and Gd) in terms of the transport energy for mobile oxygen species.
Based on the oxygen kinetics, PBSCF shows the lowest polarization resistance, as described by the Adler-Lane-Steele (ALS) model.[34] In other words, the higher concentration of oxygen mobile species and superior oxygen kinetics for PBSCF are believed to be the key difference in the surface electrochemical reaction and the transport of oxygen ions along the zig-zag type trajectory in stacked layers.[4]
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Figure 2.6. a) Experimental and fitted impedance spectra of symmetrical cell with LnBSCF-GDC composite at 700 oC. b) Comparison of fitted charge transfer resistance (R2) and noncharge transfer resistance (R3) for LnBSCF-GDC at 700 oC. c) Arrhenius plot of polarization resistance for LnBSCF at various temperatures.
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Figure 2.6. a) exhibits the impedances measured with a symmetrical cell in air at 700 oC and the impedance patterns are simulated with the equivalent circuit model to identify different electrochemical factors from the non-ohmic resistance. Generally, the impedance of a symmetrical cell is the sum of the charge-transfer resistance (R2) and the non-charge transfer resistance (R3), as given in Figure 2.6. b).[35] The intercept with the real axis at high frequency is the ohmic resistance (R1) and the impedance at intermediate frequency, R2, is associated with the charge transfer process such as electron and ion transfer at the electrode and the electrolyte. On the other hand, the impedance of low frequency is equivalent to R3 related to a non-charge transfer process, for instance, oxygen surface exchange and gas phase diffusion on the surface of the electrode.[34,35] According to Figure 2.6. b), the PBSCF-GDC composite cathode shows the lowest resistance in both charge transfer (R2) and non-charge transfer resistance (R3). Figure 2.6. c) presents the relation between the impedance and temperature, as illustrated by an Arrhenius plot.
Figure 2.7. I-V curves and the power densities of single cells (LnBSCF- GDC/LDC/LSGM/LDC/PBM) under various temperatures: Ln = a) Pr, b) Sm, and c) Gd. d) Maximum power densities at 700 oC.
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Figure 2.7. a)-c) exhibit the power density and cell voltage as a function of the current density for the LnBSCF-GDC/LDC/LSGM/LDC/PBM cells from 500 to 700 oC. The maximum power density of PBSCF at 700 oC shows the highest value, 1.02, W cm-2 while SBSCF and GBSCF exhibit values of 0.85 and 0.72 W cm-2, respectively, as shown in Figure 2.7. d). As a result, the PBSCF-GDC cathode shows the best cell performance, which is consistent with the trend of electrical conductivity, oxygen kinetics, and ASR among LnBSCFs (Ln = Pr, Sm, Gd).