Research objectives
Scope of the thesis
Introduction section provides fundamentals for understanding how SOFCs work with state-of-the-art technologies. The three types of conductors, which determine different oxygen reduction reaction mechanisms at cathodes, and theoretical fundamentals of charge and mass transport in mixed conductors that are one of the three conductors, are described in this section. In the experimental section, synthesis process for base materials and the experimental techniques to evaluate electrical conductivity, electrochemical performances and redox behavior of the materials are provided.
In the second part of the results, a systematic investigation of the effect of strontium on the layered perovskite PrBa1-xSrxCo2O5+d type ABB'O5. In the last part of the results, the compromise between electrochemical performance and thermal expansion is defined by replacing Co with Cu in PrBa1-xSrxCo2O5+d.
Solid Oxide Fuel Cells (SOFCs)
Basic operating principles of SOFCs
The fuel electrode must be able to withstand the highly reducing environment of the anode, while the air electrode must be able to withstand the highly oxidizing environment of the cathode under a high operating temperature. 5 The hydrogen oxidation reaction (HOR) at the anode and the oxygen reduction reaction (ORR) at the cathode takes place at the triple phase boundary (TPB) zone where the electrode (electronic phase), electrolyte (ionic phase) and gas phase (hydrogen, air) are in contact. Note that the resulting current, as it is traditionally defined, is in the opposite direction to the electron flow and the relevant splitting of hydrogen molecules into protons and electrons can be accelerated by adopting a catalyst. A reaction of the form given in equation (1.3.1.1) therefore takes place at the anode, where suitable catalyst choices can allow it to proceed at an increased rate.
Meanwhile, gaseous oxygen is delivered to the cathode, where a more complex reaction takes place with the net reaction shown in equation (1.3.1.2). Oxygen and hydrogen, when mixed together with enough activation energy, have a natural tendency to react and form water because the Gibbs free energy of H2O is less than that of the sum of H2 and 1/2O2 combined.
Advantages and drawbacks of SOFCs
Theoretical basics
- The chemical thermodynamics of SOFCs
- Perovskite-type oxide structures as cathodes
- A simple perovskite oxide
- A double perovskite oxide
- Oxygen reduction reaction mechanisms in SOFC cathode materials
- Electronic conductor
- Mixed Ionic and electronic conductor
- Two-phase composite mixed conductor
- Transport processes in SOFCs
- Charge transport
- Mass transport
The criterion called "tolerance factor t" can be used as a measure of the deviation of the ABO3 perovskite structure from the ideal cubic symmetry. 4 illustrates the phenomenological role of the cathode, electrolyte and gas in realizing the reaction in equation (1.4.3.1). 5 describes some of the mechanisms known or theorized in the literature that are important in determining the rate of oxygen reduction at the cathode.
Phenomenological roles of the electronically conducting (electronic) phase (α), gas phase (β), and ionically conducting (ionic) phase (γ) in achieving oxygen reduction.35. At the same time, products must be constantly removed to prevent the cell from "suffocating".
Experimental
Cell fabrication
Characterization
- Structural analysis
- Iodometric titration
- Thermal analysis
- Electrochemical analysis
- Coulometric titration
- Introduction
- Results and discussions
- Conclusion
Further crystal chemistry of the materials was analyzed by the Rietveld refinement method using the GSAS program.1 XRD patterns were collected at a slow scan rate of 0.2 o min-1 in the 2θ range from 20 o to 100 o. Changes in oxygen content as a function of temperature were obtained with initial values of oxygen content at room temperatures. The thermal expansion coefficient (TEC) values of the samples were measured from 100 to 800 oC with a heating/cooling rate of 5 oC min-1.
Ag paste (SPI Supplies, 05063-AB) was painted on both the outside and inside of the tube as electrodes. The oxygen non-stoichiometry of Pr1-xSrxCoO3-d using coulometric titration and its electrical conductivity in a wide range of oxygen partial pressure at 700 oC are also investigated. A Ni-GDC anode-supported cell is fabricated to evaluate the electrochemical performance of the Pr1-xSrxCoO3-d cathode material.
The magnitude of the weight change, which indicates a loss of oxygen from the lattice in Pr1-xSrxCoO3-d, decreases on the order of x = 0.7 > 0.5 >. The electrical conductivity of Pr1-xSrxCoO3-d is described by an Arrhenius plot in Figure. However, the electrical conductivity of Pr1-xSrxCoO3-d decreases significantly with a p(O2) of about 10-6-10-5 atm at 700 oC .
This suggests that the electrical properties are closely related to the decomposition of the material, which can be speculated from the oxygen non-stoichiometry in the isotherm data. The electrochemical performances of the Pr1-xSrxCoO3-d using a Ni-GDC anode supported cells are shown in Fig. These results are explained by the higher electrical conductivity of the Pr0.7Sr0.3CoO3-d, possibly due to the higher concentration of holes caused by electronic compensation rather than ionic compensation.
- Introduction
- Results and discussions
- Conclusion
Therefore, the present work focuses on the effects of strontium doping on the electrical properties and electrochemical performances of PrBa1-xSrxCo2O5+d (x and 1.0) in terms of its application as an IT-SOFC cathode material. The unit cell volume indicated in Table 3 decreases with increasing strontium content in PrBa1-xSrxCo2O5+d due to the substitution of smaller Sr2+ for Ba2+. All the diffraction patterns could be well indexed based on physical mixtures of PrBa1-xSrxCo2O5+d and GDC.
No severe reaction is observed in the binary mixed PrBa1-xSrxCo2O5+d-GDC after 4 hours of sintering at 1000oC. The microstructure of the PrBa1-xSrxCo2O5+d cathodes is examined by SEM image, as shown in Figure 17(f) indicates a well-sintered dense GDC electrolyte and the upper part shows porous PrBa1-xSrxCo2O5+d-GDC- composite cathodes.
For all PrBa1-xSrxCo2O5+d composite cathodes, sintered for 4 hours at 1000 oC, there is no obvious difference in grain size. Fuel cell performance for x ≤ 0.75 improves with increasing strontium content in PrBa1-xSrxCo2O5+d oxides. The electrical conductivities of PrBa1-xSrxCo2O5+d in air increase with increasing Sr content due to higher coordination number and oxygen content, followed by a smaller size difference between Pr3+ and Sr2+.
The area-specific resistances based on symmetric GDC cells decrease with strontium doping in PrBa1-xSrxCo2O5+d oxides. The electrochemical performance of PrBa1-xSrxCo2O5+d is evaluated using an anode-supported cell based on a GDC electrolyte with humidified H2 (3% H2O). Therefore, the samples with x = 0.5 and 0.75 in PrBa1-xSrxCo2O5+d-oxides are more suitable cathode candidate materials for IT-SOFCs in terms of electrochemical performance.
- Introduction
- Results and discussions
- Conclusion
These results show that the microstructure of the PBSCF-GDC50 composite cathode sintered at 950 oC for 4 hours in air can be considered optimal. To evaluate the electrocatalytic activity of PBSCF-GDC50 sintered at different temperatures, impedance spectroscopy of a symmetric cell under open circuit voltage (OCV) conditions was investigated. The section on the real axis at high frequency corresponds to the ohmic resistance of the cell (Rohm), while the low-frequency section gives the total resistance (Rohm+Rp).
Therefore, the ASR values are determined by the intercept of the impedance between the high frequency and the low frequency with the real axis of the Nyquist plot. The minimum ASR value reaches 0.053 Ω cm2 at 600 oC, obtained from PBSCF-GDC50 sample sintered at 950 oC, which is a much lower value than the ASR values of other samples, as shown in the inset of Fig. A possible origin of the lower ASR value of the material sintered at 950 oC could be microstructural improvement, as mentioned above.
The temperature dependence of the polarization resistance for the PBSCF-GDC50 electrode in GDC electrolyte with different heat treatments is illustrated by an Arrhenius plot in Fig. The relatively lower Ea value of PBSCF-GDC50 sintered at 950oC may suggest a lower chemical barrier to oxygen reduction compared to that of the other samples, subsequently resulting in high electro-catalytic activity.38-40 . a) Impedance spectra of PrBa0.5Sr0.5Co1.5Fe0.5O5+δ-GDC50 composite cathodes in symmetric GDC cells sintered at different temperatures and measured at 600 oC under OCV. However, an increase in the amount of GDC gradually softens these agglomerations and homogeneously disperses the particles of the PBSCF-GDCx electrode.
It is clear that the addition of the ionically conductive phase GDC to the PBSCF cathode results in a significant reduction in the non-charge transfer resistance (R3), while R2 remains virtually unchanged up to 50 wt% of the GDC, which consequently reduces the total polarization resistance lowers. (Rp = R2 + R3). This can be explained by the optimized ORR mechanisms by the addition of the ionically conductive GDC phase that extends the TPB (all interfaces between PBSCF and GDC). However, at
Tradeoff optimization of electrochemical performance and thermal expansion for Co-based
- Introduction
- Results and discussions
- Conclusion
The evolution of the PrBa0.5Sr0.5Co2-xCuxO5+d phase with respect to temperature is studied on the basis of in situ XRD measurements. SEM images of PrBa0.5Sr0.5Co2-xCuxO5+d-GDC composite cathodes are shown in Fig. In the PrBa0.5Sr0.5Co2-xCuxO5+d system, however, oxygen loss with increasing Cu content would be accompanied by the dominant formation of Cu2+ instead of the reduction of Co4+ ions.
The pO2 dependence of non-stoichiometric oxygen for PrBa0.5Sr0.5Co2-xCuxO5+d at 700 oC is shown in Fig. The pO2 dependence of electrical conductivity for PrBa0.5Sr0.5Co2-xCuxO5+d at 700 oC is shown in the same way in Fig. The electrical conductivity of PrBa0.5Sr0.5Co2-xCuxO5+d increases with pO2, indicating that these materials are a p-type electronic conductor under the given circumstances.
The temperature dependence of electrical conductivity for PrBa0.5Sr0.5Co2-xCuxO5+d is illustrated in fig. In the crystal structure of a PrBa0.5Sr0.5Co2-xCuxO5+d layered perovskite, Pr3+ and Ba2+ or Sr2+ ions occupy the alternate layers of the perovskite lattice along the c-axis. These results provide a reasonable explanation for the electrochemical performance data in Fig. b) Temperature dependence of PrBa0.5Sr0.5Co2-xCuxO5+δ (x = 0, 0.5 and 1.0)-GDC composite cathodes polarization conductance by Arrhenius plots.
Fuel cell performance decreases as the Cu content in PrBa0.5Sr0.5Co2-xCuxO5+d increases, which is consistent with the trend of improved ASR and electrical conductivity. The temperature dependence of the electrical conductivity for PrBa0.5Sr0.5Co2-xCuxO5+d (x = 0, 0.5 and 1.0) reaches the generally required value of a cathode material. Moreover, the isotherms of PrBa0.5Sr0.5Co2-xCuxO5+d (x = 0, 0.5 and 1.0) obtained from the coulometric titration experiment reveal the higher redox stability at lower pO2 with the higher amount of Cu doping.
