Solid Oxide Fuel Cells

Herein, I investigated several approaches to improve the electrochemical performance of the layered perovskite electrode at intermediate or low operating temperatures. Comparison of the value of bulk diffusion coefficient (and surface exchange coefficient (for the ceramic cathode materials for SOFCs. SEM images showing the microstructures of a) PBSCF-GDC, b) SBSCF-GDC, c) GBSCF-GDC composite and d) a cross-sectional view of the cathode, buffer layer and electrolyte.

Temperature dependence of the electrical conductivity of the LnBa0.5Sr0.5Co1.5Fe0.5O5+. Ln = Pr, Sm, Gd) at different temperatures. Scanning electron microscope image of (a) PBCO, (b) PBCSc and (c) a cross-section consisting of the GDC electrolyte with porous cathode and Ni–GDC anode. Smaller particles on the porous backbone are observed on the surface of PBSCF-infiltrated electrode. a) Impedance spectra of a symmetric cell with PBSCF backbone and PBSCF infiltrated electrode at 600 oC. b) Arrhenius plot of the polarization resistance for the symmetric cells at different temperatures is displayed.

Introduction to Solid Oxide Fuel Cells and Perovskite Cathodes

What is fuel cell?

Type of fuel cells

Basic fuel cell operation

The electricity generated by fuel cell is directly related to the kinetics of electrochemical reactions. In general, kinetics of electrochemical reactions are the most important features to determine the fuel cell performance. Therefore, the performance of fuel cells depends critically on the catalyst to increase the speed and efficiency of the reaction.

To maintain the charge balance of fuel cells, the ions and electrons produced in one cell must be transported from the electrode where they are generated to the other electrode where they are consumed. Electrons will flow from one electrode to the other through the external electrical conduction path. Delivery of reactants to the fuel cell facilitates the removal of byproducts out of the system.

Figure 1.2. Cross section of fuel cell illustrating major steps in electrochemical generation of

Introduction to Solid Oxide Fuel Cells (SOFCs)

As the ionic charge carrier in a SOFC system, generated oxygen ions move through the electrolyte and generate water and electrons through hydrogen oxidation reaction (HOR) at the anode. However, the reduced operating temperature causes poor oxygen ionic conductivity and low electrocatalytic activity for the ORR at the cathode. Mixed ionic and electronic conductors (MIECs) have been widely investigated for the IT-SOFC cathode materials.

Conventional pure ionic conductive standard electrode of SOFC can only reduce the oxygen gas at the three-phase boundary where gas phase, electrode and electrolyte contact at the same time. Therefore, ORR can occur not only at the three-phase boundary, but also at the two-phase boundary where the gas phase and the electrode make contact. AA'B2O5+) and mechanism for the bulk diffusion of mobile oxygen species through the pore channels [20].

Figure 1.4. (a) A standard SOFC cathode electrode and (b) mixed ionic-electronic conducting (MIEC) SOFC cathode materials

Theoretical Background

Thermodynamic of SOFC
Ionic conductivity of SOFC
Impedance of SOFC
Performance of fuel cell

Under conditions of constant temperature and constant pressure, the electrical work of the system is given by the negative difference of the Gibbs free energy. Therefore, the Gibbs free energy represents the magnitude of the reversible voltage for an electrochemical reaction. The difference in the concentrations of the chemical species in the fuel cell system changes the free energy of the system, resulting in a reversible change in the fuel cell voltage.

For the crystalline lattice of the ionic conductor, various defects, including vacancies and interstitials, will be generated in the materials. On the other hand, in the higher temperature region, the concentration of the intrinsic defects is comparable to or greater than that of the extrinsic defects. Since the concentration of the intrinsic defects will not be constant, the conductivity in this intrinsic region is given by.

Impedance is the measure of the resistance that a circuit offers to a current under the voltage. Impedance models have described preliminary evidence that the kinetics for fast ionic transport of mixed conductors is dominated by surface chemical exchange of O2 and solid state oxygen diffusion.[22] Oxygen reduction reaction is observed across the electrochemically active surface of the electrode material. Absorbed neutral oxygen converts electronic to ionic current across an active area of the electrode.

On the other hand, the surrounding material cannot maintain the initial bulk liquid state as the reactants are consumed at the electrode by the electrochemical reaction. Kim, “Electrochemical and thermodynamic characterization of PrBaCo2−xFexO5+δ(x infiltrated into yttria-stabilized zirconia scaffolds as cathodes for solid oxide fuel cells), J. Choi et al., “Highly efficient cathode materials and Low Temperature Stable Solid Oxide Fuel Cells: PrBa0.5Sr0.5Co(2-x)Fe(x)O(5+δ).”, Sci.

Jeong et al., “Effect of scandium doping on a layered perovskite cathode for low temperature solid oxide fuel cells (LT-SOFCs),” Appl.

Figure 1.8. Reversible voltage ( versus temperature for electrochemical oxidation of various fuels [9]

Structural, electrical, and electrochemical characteristics of

Introduction
Result and Discussion
Conclusions
Experimental

In this study, the effects of different Ln ions based on Sr and Fe co-doped layered perovskites, LnBa0.5Sr0.5Co1.5Fe0.5O5+ (Ln = Pr, Sm and Gd), are discussed in terms of the oxygen kinetics, crystalline structure and electrical and electrochemical properties.[28]. The Rietveld refinement data from LnBSCF show that the diffraction peaks of the samples have a tetragonal structure (space group: P4/mmm), and the calculated lattice parameters are summarized in Table 2.1. All cathode materials have a similar morphology with sufficient porosity for gas diffusion and a homogeneous microstructure.

Smaller size Ln ions contain more oxygen vacancies and lower interstitial oxygen concentration in the A-site layer, leading to disturbance of the O-Co-O interaction and carrier delocalization. In other words, larger lanthanide cations decrease the difference of average cations in the A site, leading to an increase in the oxygen coordination number and oxygen content, respectively [20,32]. The normalized 18O2 concentration of the samples was not completely homogeneous for the measured surfaces due to the differences in surface terminations and/or the different grain orientations.

In general, the impedance of a symmetrical cell is the sum of the load transfer resistance (R2) and the non-load transfer resistance (R3), as given in Figure 2.6. b).[35]. On the other hand, the low-frequency impedance is equivalent to R3 related to a charge-free transfer process, for example, oxygen surface exchange and gas phase diffusion at the electrode surface [34,35]. The influence of different lanthanide ions on the surface morphology and aggregated structure of the cathode in the electrolyte was observed with a field emission scanning electron microscope (Nova Nano SEM, FEI, USA).

The oxygen isotope diffusion profile was obtained by means of secondary ion mass spectrometry (SIMS) using an ATOMICA 4100 quadrupole base analyzer, with the line scan mode at the cross-section of the sample. A Co-Fe catalyst was infiltrated on the surface of the anode material to improve the performance and heated in air at 450 oC. In order to determine the electrochemical performance of single cells, humidified H2 was supplied to the surface of the anode while air was supplied as an oxidant to the cathode.

Adler, “Electrode Kinetics of Porous Mixed Conducting Oxygen Electrodes,” Journal of The Electrochemical Society, vol.

$Figure 2.1. X-ray diffraction (XRD) patterns of a) LnBa 0.5 Sr 0.5 Co 1.5 Fe 0.5 O 5+ (Ln = Pr, Sm and Gd), and b) difference between the observed and calculated XRD profile of PBSCF$

Figure 2.1. X-ray diffraction (XRD) patterns of a) LnBa 0.5 Sr 0.5 Co 1.5 Fe 0.5 O 5+ (Ln = Pr, Sm and Gd), and b) difference between the observed and calculated XRD profile of PBSCF

Scandium Doping Effect on the Layered Perovskite Cathode for Low-temperature

Introduction
Materials and Methods
Conclusions

Scandium doping effect on the layered perovskite cathode for low temperature solid oxide fuel cells (LT-SOFCs). Among these properties, the Goldschmidt tolerance factor is a useful indicator to evaluate distortion of the crystal structure [ 36 , 37 ]. The cathode slurries were screen printed on the surface of the electrolyte with an active cathode area of 0.36 cm2.

The microstructures of PBCO and PBCSc were investigated using scanning electron microscopy (SEM) as shown in figure. The similar microstructure between PBCO and PBCSc indicates that Sc3+ substitution does not affect the morphological properties of the cathode materials. Arrhenius plots of the polarization resistances of PBCO, PBCSc and SP-PBCSc are shown in figure.

Sengodan et al., “Oxygen-deficient layered double perovskite as an efficient and stable anode for direct hydrocarbon solid oxide fuel cells,” Nat. Yoo et al., “Development of double perovskite compounds as cathode materials for low temperature solid oxide fuel cells.” Angew. Yoo et al., “Development of double perovskite compounds as cathode materials for low-temperature solid oxide fuel cells,” Angew.

Kim, “Comparative characterization of thermodynamic, electrical and electrochemical properties of Sm0.5Sr0.5Co1-xNbxO3-δ (x and 0.1) as cathode materials in intermediate temperature solid oxide fuel cells,” J. Jun et al., “Correlation between fast oxygen kinetics and improved performance in Fe-doped layered perovskite cathodes for solid oxide fuel cells,” J. Li et al., “Scandium-doped PrBaCo2-xScxO6-δ-oxides as cathode material for medium-temperature solid oxide fuel cells,” Int. .

Sammells, “On the systematic selection of solid perovskite electrolytes for medium-temperature fuel cells,” Solid State Ionics, vol.

Figure 3.1. (a) XRD patterns of PBCO, PBCSc and SP-PBCSc. (b) The chemical compatibility between the cathode materials and GDC

Enhancement of Electrochemical Performance by the infiltration for Metal-

Introduction
Experimental
Conclusion

After sintering the PBSCF scaffold, the PBSCF solution was infiltrated on the surface about 15 wt. However, the higher magnification SEM image shown in Figure 4.2 (c)-(d) shows different microstructure of the electrode samples. Furthermore, the cathode microstructure still retains the porous morphology without detectable aggregation on the surface, demonstrating improved electrochemical performance without apparent degradation.

Infiltration of PBSCF nitrate solution deposits the smaller nanoparticle on the surface of the PBSCF backbone than the particles of the PBSCF electrode, resulting in expanded active surface area. LnBa0.5Sr0.5Co1.5Fe0.5O5+ δ (Ln=Pr, Sm, Gd) as Cathode Materials in Intermediate Temperature Solid Oxide Fuel Cells," Energy Technol., vol. Kim, "Chemically Stable Perovskites as Cathode Materials for Solid Oxide Fuel Cells: La-doped Ba0.5Sr0 .5Co0.8Fe0.2O3−δ",.

Kim et al., “Nanostructured double perovskite cathode with low sintering temperature for medium-temperature solid oxide fuel cells,” ChemSusChem, vol. 34;Structural, electrical and electrochemical characteristics of LnBa0.5Sr0.5Co1.5Fe0.5O5+δ (Ln=Pr, Sm, Gd) as cathode materials in mid-temperature solid oxide fuel cell energy technology. 34;Scandium doping effect on a layered perovskite cathode for low-temperature solid oxide fuel cells (LT-SOFCs)" Applied Science.

34; Investigation of the Effect of Fe Doping on the B Site of Multilayer Perovskite PrBa0.8Ca0.2Co2O5+δ for a Promising Cathode Material of Medium-Temperature Solid Oxide Fuel Cells " International Journal of Hydrogen Energy. Donghwi Jeong, Sangwook Ju , and Guntae Kim* "Improving Electrochemical Efficiency by Infiltration for Metal supported solid oxide fuel cells" Presentation preparation. 34; Composite effect of proton-conducting materials as cathode material in proton-conducting solid oxide fuel cells" Presentation preparation.

34; Effect of Cobalt Doping on Barium Stannate as a Cathode Material for Proton Conducting Solid Oxide Fuel Cells.” Submission Preparation.