Chapter 2. Structural, electrical, and electrochemical characteristics of
2.4. Experimental
32
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).
33
into a cylindrical shape and sintered at 1475 oC for 5 hours. Sintered LSGM pellets were polished until their surfaces were equally flat and their thicknesses were about 240 to 250 μm. Additionally, the promising anode material PrBaMn2O5+ (PBM) was used as the anode for a single cell.[3]
To evaluate electrochemical properties such as area specific resistance (ASR) and maximum power densities, cathode materials and gadolinium doped ceria (GDC) were mixed at a weight ratio of 6:4 to maximize the performance.[27] Then mixed powder was blended with an organic binder (Heraeus V006) to prepare a slurry for the screen-printing method. The influence of different lanthanide ions on the surface morphology and stacked structure of the cathode on the electrolyte was observed with a field emission scanning electron microscope (Nova Nano SEM, FEI, USA). Chemical compatibility between the cathode materials and other components such as GDC, the lanthanum doped ceria (LDC) buffer layer, and the LSGM electrolyte was evaluated by XRD measurement of a screen-printed symmetric cell.
The electrical conductivities of LnBSCF (Ln = Pr, Sm, and Gd) in air were determined by a four- terminal DC arrangement. The electrical conductivities were measured using a BioLogic Potentiostat from 100 oC to 800 oC in 50 oC intervals. Oxygen content of LnBSCF in the range of operating temperature was investigated by a thermogravimetric analysis (TGA). It was performed from 100 oC to 900 oC with a heating/cooling rate of 2 oC min-1 in air using a thermogravimetric analyzer (SDT- Q600, TA Instruments, USA). Iodometric titration was performed to measure the oxygen content at room temperature.
Isotope oxygen exchange was measured using a closed circulation system with >96% 18O2, with the concentration of 18O2 being confirmed by using a mass analyzer (Anelva, M-100-QA-F). After the sample was polished using diamond paste, natural abundant oxygen at pressure of 200 mbar was introduced into the system and the system was heated to 590 oC. The sample was annealed for more than ten times longer than the time for isotope oxygen exchange and then quenched to the room temperature and the residual oxygen was removed from the system.[36] Isotope oxygen 18O2 at pressure of 200 mbar was introduced for oxygen exchange, and the sample was rapidly heated to the measured temperature. Finally, the sample was rapidly cooled to the room temperature after the exchanging time. The isotope oxygen diffusion profile was obtained by secondary ion mass spectrometry (SIMS) using an ATOMICA 4100 quadrupole-base analyzer, with the line-scan mode at the cross section of the sample. The oxygen bulk diffusion coefficient (D*) and the oxygen surface exchange coefficient (k) were calculated by the semi-infinite diffusion model.[37]
The LnBSCF cathode slurries were placed on the surface of a LSGM electrolyte by a screen- printing method with an area about 0.36 cm2 on both sides of symmetrical cells and one side of single cells. In order to prevent the chemical reaction between the cathode and the electrolyte, a LDC layer was prepared as a buffer layer by a screen-printing method. A slurry of cathode or PBM as an anode
34
material was painted on each side of a single cell and the cell was sintered at 950 oC for 4 hours after the painted slurries were dried in an oven. A Co-Fe catalyst was infiltrated on the surface of the anode material to enhance the performance and heated in air at 450 oC. Ag wires and paste were attached on the electrode as the current collectors. An alumina tube was used to fix the cell with a ceramic adhesive. Impedance spectra of the symmetric cell in air were measured in a temperature range of 700
oC to 500 oC in 50 oC intervals. In order to determine the electrochemical performance of single cells, humidified H2 was provided on the surface of the anode while air was supplied to the cathode as an oxidant. Single cell tests were conducted under the same temperature conditions by a BioLogic Potentiostat.
35 References
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