Therefore, lowering the operating temperatures towards the intermediate temperature range (600–800 °C) is essential to overcome the aforementioned problems. In medium-temperature SOFCs (IT-SOFCs), however, the electrocatalytic activity against the oxygen reduction reaction at the cathode is significantly reduced, which in turn results in insufficient fuel cell efficiency. In this regard, the infiltration method could be an excellent method of making cathodes, considering its outstanding advantages in operating at intermediate temperatures.
This thesis mainly focuses on the fabrication of SOFC cathode by the infiltration method to achieve high fuel cell performance in the intermediate temperature range. Schematic illustration of (a) a conventional composite electrode composed of MIEC and an electrolyte material (b) a composite electrode fabricated by infiltration method. a) XRD patterns of infiltrated LBSCF-LSGM composite sintered at 800oC in air. SEM images of (a) cross-sectional image of a prepared LSGM cell, (b) porous LSGM scaffold, (c) infiltrated LBSCF-LSGM composite sintered at 800oC in air, (d) high-resolution image of infiltrated LBSCF-LSGM composite. a) Impedance spectra and (b) Polarization resistance as a function of temperature of a symmetrical cell in air.
Introduction
General Introduction of Solid Oxide Fuel Cell (SOFC)
- Basic of Fuel Cell
- Basic Operating Principle of SOFC
- Advantages and Challenges of SOFC
Fuel cells are the most attractive energy devices for solving the world's energy and environmental issues, as they can theoretically produce an unlimited amount of electricity with an infinite supply of fuel. In addition, fuel cells directly convert the chemical energy of fuels into electrical energy, which has a higher energy conversion efficiency than internal combustion engines. In hydrogen fuel cells, the hydrogen oxidation reaction occurs at the anode, as shown in Equation 1.1, while the oxygen reduction reaction occurs at the cathode, as shown in Equation 1.2.
Considering that the reaction product is only water, as shown in equation 1.3, the fuel cell will be an excellent solution to future energy and environmental problems if hydrogen is continuously supplied in an environmentally friendly manner. Fuel cells can be classified according to the type of electrolyte and operating temperature, as shown in Table 1.1.1. Solid oxide fuel cells (SOFCs) convert the chemical energy of fuel directly into electrical energy without limiting the efficiency of the Carnot cycle.
At the cathode, oxygen ions are produced by the oxygen reduction reaction (ORR), as shown in equation 1.4. A hydrogen oxidation reaction (HOR) then occurs by combining hydrogen with oxygen ions at the anode, producing water and electrons, as shown in Equation 1.5. Conventional SOFCs have much higher energy conversion efficiency than internal combustion engines because they directly convert the chemical energy of fuel into electrical energy.
Since the hydrocarbon fuel can be used directly without an external reforming process, SOFCs are flexible and convenient for fuel use. Nevertheless, the commercialization of SOFCs has been hampered by some problems arising from high-temperature operation, such as material durability issues, cost competitiveness, sealing and interconnection issues, and thermal stress between cell components.2 Therefore, research is actively being conducted to reduce operating temperature to the intermediate range (600-800oC).3-5 Intermediate temperature solid oxide fuel cells (IT-SOFC) can mitigate the disadvantages of high temperature operation while maintaining the advantages of conventional SOFC.
Theoretical Background
- Fundamental Thermodynamics of SOFC
- Performance of SOFC
- Electrochemical Reaction Mechanism in SOFC Cathode
- Materials for SOFC Cathode
The reversible standard potential (E0) for the electrochemical reaction can be defined as Equation 1.24 if the reactants and products are all in their standard states. The standard electrode potential can be calculated by substituting the standard state values (T = 273.15 K, p = 1 atm) into Equation 1.24. The current-voltage curves show the output voltage of the SOFC for a given current density.
In practice, real SOFCs deliver less power than ideal SOFCs due to more polarization losses.2 Power can be calculated as the product of current and voltage. The polarization losses, also known as overpotential () cause the difference between thermodynamically predicted fuel cell voltage and actual fuel cell voltage. The polarization losses are composed of activation polarization, which is responsible for the initial part of the voltage curve, ohmic polarization, which is responsible for the slow voltage drop in the middle region of the voltage curve, and concentration polarization, which is responsible for the final section of the voltage curve.
To overcome the activation barrier, a partial voltage drop must occur to increase the reaction rate. The relationship between the current density and the activation polarization can be described by the Butler-Volmer equation as The Butler-Volmer equation states that the voltage drop will be greater if we want to extract more electricity from the fuel cell.
Ohmic polarization losses can be minimized by reducing the thickness and increasing the conductivity of the electrolyte. This reduction mechanism involves several basic electrochemical steps, including adsorption, dissociation, surface diffusion, and charge transfer.8 The ORR reaction occurs at the electrochemically reactive site where the ionic conductive phase, the electronically conductive phase, and the gas are in contact. The simple schematic illustration of the ORR mechanism and the TPB site is shown in Figure 1.3.
To determine the structural stability of perovskite oxides, the tolerance factor (t) can be used as a measure of the deviation from the ideal cubic structure.12 The tolerance factor can be described as the following equation.
Advantages of Infiltration Method
Double perovskite oxides have the general formula of AA'B2O5+, where A, A', and B are trivalent lanthanide ion, alkaline earth metal, and first-row transition metal ion, respectively. Double perovskite oxides have received wide attention due to their much higher oxygen ion diffusion and a surface exchange coefficient than single perovskite oxides.17-18 In particular, the compounds LnBaCo2O5+ (Ln = La , Pr, Nd, Sm and Gd) indicate rapid oxygenation. kinetics and high mixed ionic and electronic conductivity.3,18-22.
A Nano-structured SOFC Composite Cathode Prepared via Infiltration of
Experimental
To make the LSGM porous layer, graphite as a pore former was added together with the LSGM powder and the same fabrication procedures as the dense LSGM slurry were performed. The dense and porous LSGM slurries were fabricated into a strip shape and then dried at room temperature. The resulting strips were laminated as a three-layer configuration and fired at 1500°C for 6 h in air.
The thickness of the dense layer of the electrolyte and the porous layer of the electrode after firing is approximately 100 μm and 30 μm, respectively. The pH value of the solution was adjusted to about 4 by adding ammonium hydroxide. To fabricate a single cell, LBSCF and PBMO solutions were infiltrated into each LSGM porous scaffold at room temperature in ambient air.
The infiltration process was repeated to a loading of 40 wt%,22-24 and the infiltrated cell was sintered in air at 800°C for 4 hours to form the desired perovskite structure. The microstructures of the LSGM and LBSCF were analyzed using a scanning electron microscope (SEM) (Nova SEM). To measure the electrochemical performance of an as-prepared cell, Ag paste was applied to the active electrode as a current collector and then Ag wires were attached to both electrodes.
AC impedance spectroscopy was used to evaluate the ASR of symmetrical cells at open circuit voltage (OCV) in air. For the single-cell test, a flow rate of 100 mL min−1 humidified H2 (3% H2O) and dry air was supplied to the anode and cathode, respectively.
Results and Discussions
SEM images of (a) cross-section of an as-prepared LSGM cell, (b) porous LSGM scaffold, (c) infiltrated LBSCF-LSGM composite sintered at 800oC in air, (d) high-resolution image of infiltrated LBSCF- LSGM composite. In the Nyquist plot of Figure 2.4(a), the ohmic resistance is the intersection of the real axis at high frequency and arises mainly from the electrolyte resistance. The charge transfer resistance is mainly related to charge transfer reaction during diffusion of oxygen ions at the electrode and the non-charge transfer resistance is related to oxygen surface exchange and gas diffusion from the electrode surface.31 The polarization resistances of infiltrated LBSCF-LSGM composite are and 0.049 cm2 at 700, 650 and.
This exceptionally high electrochemical performance can be attributed to the extensive TPB sites in relation to the LBSCF-LSGM microstructure shown in Figure 2.3, as well as to the high ORR catalytic activity of LBSCF. The variation of the polarization resistance as a function of the temperature in the air is shown in Figure 2.4(b). The apparent activation energy values of LBSCF-LSGM composite from high-frequency (R2) and low-frequency (R3) arc contribution are 113.14 and 134.84 kJ mol−1, respectively, over the measured temperature range.
As shown in Figure 2.4(c), the cell shows fairly stable performance over 50 hours without any observable degradation. a) Impedance spectra and (b) Polarization resistance as a function of temperature of a symmetrical cell in air. The inset shows an Arrhenius plot of the polarization resistance of high-frequency and low-frequency arc contributions from a symmetric cell. Moreover, PBMO is used as anode material based on its fast oxygen kinetics, good thermal stability and high electrical conductivity in reducing conditions.36–40 An LSGM electrolyte-supported single cell (LBSCF-LSGM/LSGM/PBMO-LSGM) was prepared using LBSCF -LSGM composite cathode and PBMO-LSGM composite anode via infiltration method.
The reasons for our high cell performance can be attributed to the microstructural advantages of the electrode, which expands electrochemically reactive sites. Therefore, the infiltrated LBSCF-LSGM composite could be an attractive cathode material for IT SOFCs. a) I-V curves and corresponding power density curves and (b) impedance spectra of a single cell in a temperature range of 550-700oC.
Conclusions
Seungtae Lee, Seona Kim, Sihyuk Choi*, Jeeyoung Shin*, and Guntae Kim* “A nano-structured SOFC composite cathode prepared via infiltration of La0.5Ba0.25Sr0.25Co0.8Fe0.2O3-δ into La0.9a0r. 8Mg0.2O3-δ for Three-Phase Extended Boundary Zone” J. Seona Kim, Seungtae Lee, Junyoung Kim, Jeeyoung Shin* and Guntae Kim* “Self-transforming configuration based on atmospheric-adaptive materials for solid-state cells O. I would like to express my deepest gratitude to all supporters who provide encouragement and support for the completion of my master's thesis.
The driving force to complete this article is the continued support of my advisor, Prof. He gave me endless support, immense knowledge, encouragement with patience and guidance in completing the dissertation. Seona Kim, Sangwook Joo, Chaehyun Lim, Ohhun Kwon, Changmin Kim, Donghwi Jeong, Jeongwon Kim, Gihyeon Kim, Hyojea Hwang, Hansol Lee, Arim Seong and Yejin Yang.
Last but not least, I would like to sincerely thank my family for their endless support, love and encouragement.
