EN LA MADERA DE LA CUBIERTA INCLINADA

A major advantage of electrodes prepared by infiltration of cathode components into a porous scaffold of the electrolyte is that the porous scaffold can act as an oxygen- ion conductor, so that oxygen ions generated deep in the cathode can be conducted to the electrolyte interface. However, the electrode must still be electronically conductive and be capable of catalyzing the dissociation of oxygen molecules. Therefore, the active electrode

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materials must still be impregnated into the porous scaffolds. This is being done by infiltration. In our case, a solution of the nitrate salts was first prepared. For example, if the target material was to be LSCF, a saturated aqueous solution of La(NO3)36H2O,

Sr(NO3)24H2O, Fe(NO3)39H2O and Co(NO3)26H2O with desired molar ratio was

prepared. In each infiltration step, the scaffold was saturated with the solution at room temperature, after which the excess solution on the surface was removed. After allowing the cells to dry, they were heated to 873 K to decompose the nitrates. This procedure was repeated for the desired number of infiltration cycles. After the final infiltration step, the entire structure was heated to 1123 K to form the LSCF, perovskite phase.

The selection of calcination temperature is important for the infiltrated materials since the surface morphology of the calcined nanomaterials are sensitive to the

preparation condition. In previous work, it was shown that the surface area of infiltrated LSM changed as a function of calcination temperature, with the surface area decreasing by a factor of four when the calcination temperature was increased from 1123 K to 1523 K [10]. In addition to surface area reduction, high temperature treatment can also cause the solid-state reaction between the perovskites and the YSZ backbone, forming insulating phases [11,12]. Therefore, it is important to avoid high-temperature calcination of the infiltrated materials. However, if the calcination temperature is too low, the electronically conductive perovskite phase will not be formed. Chelating agents, such as citric acid [13], are often added with the infiltrating salt in order to promote formation of the perovskites at lower temperatures. The chelating agents allow better mixing of the metal cations. For most of the studies used in this work, the condition that we choose (1123 K), adding these chelating agents is not necessary. Figure 2.2 shows the XRDs of infiltrated LSF, LSM

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and LSCo prepared with the methods and conditions. The XRD results demonstrate that one can form the perovskite phases without forming insulating phases. The surface area of the 30 wt% LSM infiltrated cathodes was measured to be 2.5 m2 g-1.

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Figure 2.2: XRD result of the porous YSZ cathode infiltrated with LSM (a), LSF (b) and LSCo (c) after calcination at 1123 K. The perovskite phase peaks are labeled in red asterisks and the fluorite YSZ peaks are labeled with green triangles.

LSCo O

LSF LSM

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2.3 Atomic Layer Deposition (ALD)

As discussed in section 1.7.2, it is useful to study the effect of surface composition on the electrochemical performance of the infiltrated electrodes, independent of the surface morphology. Therefore, ALD was chosen to modify the surface composition. The home- built ALD setup is shown schematically in Figure 2.3. This setup was designed to allow two different precursors to alternatively react with the sample surface, with intermittent oxidizing steps.

Figure 2.3: An illustration of the in-house built ALD system. Heating system is not shown in the figure.

We found that there are three key design factors that were important for our process. First, it was important to be able to evacuate the sample so that we could use pressure-driven flow of the precursor molecules to saturate the sample. Since the vapor

30

pressures of the precursors used in most of our work were below a few torr, even at elevated temperatures, this meant that the maximum base pressure for the sample chamber was 200 millitorr.

To achieve a sufficient vapor pressure of the precursor, it was necessary to heat the precursor. Furthermore, to avoid condensation, all of the tubing between the precursor and the sample had to be heated as well. Because chemical vapor deposition will occur at too high a temperature, there exists a temperature window for ALD [14,15]. Ideally, the sample temperature should be high enough to allow oxidation of the adsorbed precursor.

Finally, to avoid diffusional limitations with low-vapor-pressure precursors, the sample dimensions had to be sufficient small and the exposure times sufficiently long. As suggested by Onn et al. [16]_{, a higher precursor vapor pressure results in a larger driving}

force. In most of our trials, we used between 30 s and 2 min for each dosing and evacuation steps, depending on the surface area of the substrates.

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2.4 References

1. R. E. Mistler and Eric. R. Twiname, Tape Casting Theroty and Practice, The American Ceramic Society, Westerville, (2000).

2. M. Han et al., J. Power Sources, 165, 757-763 (2007).

3. J. H. Myung, H. J. Ko, C. H. Im, J. Moon, and S. H. Hyun, Int. J. Hydrogen Energy,

39, 2313–2319 (2014).

4. Z. Wang, J. Qian, J. Cao, S. Wang, and T. Wen, J. Alloys Compd., 437, 264–268 (2007).

5. H. Moon, S. D. Kim, S. H. Hyun, and H. S. Kim, Int. J. Hydrogen Energy, 33, 1758– 1768 (2008).

6. C. S. Ni, J. M. Vohs, R. J. Gorte, and J. T. S. Irvine, J. Mater. Chem. A, 2, 19150– 19155 (2014).

7. M. Boaro, J. M. Vohs, and R. J. Gorte, J. Am. Ceram. Soc., 86, 395–400 (2003).

8. R. J. Gorte, S. Park, J. M. Vohs, and C. Wang, Adv. Mater., 12, 1465–1469 (2000)

9. H. Kim, C. da Rosa, M. Boaro, J. M. Vohs, and R. J. Gorte, J. Am. Ceram. Soc., 85, 1473–1476 (2002).

10. V. A. C. Haanappel et al., J. Power Sources, 141, 216–226 (2005).

11. H. He et al., J. Am. Ceram. Soc., 87, 331–336 (2004).

12. R. Küngas, F. Bidrawn, J. M. Vohs, and R. J. Gorte, Electrochem. Solid-State Lett.,

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13. A. V. Call, J. G. Railsback, H. Wang, and S. A. Barnett, Phys. Chem. Chem. Phys.

(2016).

14. H. C. M. Knoops et al., Electrochem. Solid-State Lett., 12, G34 (2009).

15. E. Granneman, P. Fischer, D. Pierreux, H. Terhorst, and P. Zagwijn, Surf. Coatings Technol., 201, 8899–8907 (2007).

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Chapter 3. Preparation of SOFC Cathodes by Infiltration into