Fundamentals of solid oxide cell (SOC)

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Fundamentals of solid oxide cell (SOC)

Solid oxide cell (SOC) is an electrochemical energy-conversion system which can operate reversibly as a fuel cell for power generation (solid oxide fuel cell or SOFC) and an electrolysis cell for hydrogen production (solid oxide electrolysis cell or SOEC). It is characterized by a solid, oxygen-ion conducting ceramic as its electrolyte which is sandwiched in between air/oxygen electrode and fuel/hydrogen electrode [8], [11], [13]. The use of all solid materials resulting in the compact and flexible cell configuration (e.g. planar, tubular, corrugated) [13]. In addition, no corrosive liquids are involved in the system. SOC is mainly used for stationary applications [14]. The illustrations of SOC and its basic operating principle as both SOFC and SOEC are shown in Fig. 1.1a-b.
As a fuel cell, the oxygen, either pure or containing in air, is reduced to oxygen ions in a process called oxygen reduction reaction (ORR) at the oxygen electrode. The oxygen ions are then transported through the ionic conducting electrolyte to the hydrogen electrode where they react with the fuel (normally hydrogen but can also be carbon monoxide (CO), hydrocarbons, or alcohols) to produce either water or carbon dioxide (CO2). The reduction-oxidation (redox) reaction on each electrode resulting in the flow of electrons from the hydrogen electrode to the oxygen electrode that produces direct-current (DC) electricity. On the other hand, DC electricity is required for the electrolysis of water at the hydrogen electrode, producing hydrogen and oxygen ions. The oxygen ions are then transported through the electrolyte and oxidized into oxygen gas at the oxygen electrode through a process called oxygen evolution reaction (OER). The overall reaction in SOFC and SOEC can be found in the two last columns in Table i in the Introduction part of this manuscript.
SOC normally operates at high temperatures between 800 to 1000 °C [11], [14]. Compared to the other types of fuel/electrolysis cells, SOC has the highest efficiency, especially in a combined heat and power (CHP) system, thanks to the high operating temperature. Moreover, an expensive catalyst (e.g. platinum) to assist the reaction is not necessary. An SOFC also has the capability for internal reforming, leading to flexibility on the choice of fuel [13], [14]. However, the high operating temperature means that the materials should not only exhibit excellent electrical and electrochemical properties, but also good chemical, thermal, and mechanical stability to ensure a longer lifetime of the system in operation. At the same time, from the techno-economic point of view, the cost of materials should also be low and they should be able to be fabricated with ease at low fabrication cost [11]. SOC also has an issue regarding the slow start-up and shutdown cycles which limits its portability. Thus, it is of current interest to reduce the operating temperature down to 650-750 °C (intermediate temperature SOC, IT-SOC) and even lower to below 650 °C (low-temperature SOC, LT-SOC) [15]. Indeed, lowering the operating temperature brings down the cost of materials and prolongs the lifetime of the system. However, it also reduces the kinetics of the reactions taking place in the system because of the higher polarization losses from the electrolyte and electrodes at lower temperature, as will be explained in more detail in §1.2. Hence, new materials developments have been extensively studied as an effort to address this issue.
The detailed explanations on each component in an SOC are given hereafter.


As previously mentioned, the electrolyte of an SOC should be able to conduct only oxygen ions, hence, high ionic conductivity and low electronic conductivity are required for this purpose. The oxygen ions are transported through the electrolyte via an oxygen vacancy hopping mechanism which is a thermally-activated process [8]. Moreover, it should also be fully dense with no open porosity to avoid any cross-diffusion of the fuel and oxidant compounds. Since the electrolyte is in contact with both oxygen and hydrogen electrodes, it should be chemically inert, have a similar thermal expansion coefficient (TEC), and have good mechanical properties with respect to the electrode materials. The electrolyte should also be chemically and structurally stable under both oxidizing and reducing environments. Reliable mechanical properties (i.e. high tensile strength and toughness) are also necessary to be able to withstand thermal and mechanical stresses during cell fabrication and operation [8], [11].
Several common materials for SOC are the fluorite structure oxides (e.g. stabilized zirconia ZrO2, doped ceria CeO2, doped Bi2O3) and perovskites (e.g. doped LaGaO3) [8], [11]. The dopant enhances the ionic conductivity and phase stability of the electrolyte materials. The material that has been widely used in practice is 3-10 mol% yttria-stabilized zirconia (Y2O3-ZrO2, YSZ) due to its excellent stability. Among this family of material, 8 mol% YSZ (8YSZ) with cubic structure has the highest ionic conductivity. However, compared to other ceramics, the ionic conductivity of 8YSZ is far from the best and it is not good enough for operation at intermediate and lower temperature as indicated in Table 1.1. Moreover, there is also an issue of reactivity when lanthanum-based material is used for the electrode, such as the commonly used La1-xSrxCo1-yFeyO3-δ (LSCF), forming an insulating phase La2Zr2O7. For these reasons, another electrolyte material is preferred especially for the development of IT-SOC and LT-SOC. One such material is the gadolinium-doped ceria (Ce1-yGdyO2-y/2, GDC). The GDC with the addition of 10 mol% gadolinium as a dopant has been found to be a good candidate for electrolyte due to its sufficiently high ionic conductivity, around 4-5 times higher than YSZ [16], [17]. Moreover, GDC is also chemically stable in operation and compatible with various electrode materials. The main drawback of GDC is the reduction of Ce4+ to Ce3+ in a highly reducing environment especially at a temperature above 600 °C, which leads to the lattice expansion, mechanical instability, and internal short-circuit due to high electronic conductivity.

Air/oxygen electrode

The air/oxygen electrode is where the ORR/OER takes place. As will be explained in more detail in §1.2, the reactions involve three different species which are gas, electrons, and ions at reaction sites called triple-phase boundary (TPB). Thus, the material for the oxygen electrode should be porous to allow gas transport. The ORR/OER is considered to be responsible for the majority of cell voltage losses, thus the choice of material for oxygen electrode with high electrocatalytic activity is important to enhance the cell performance and efficiency. The electrode should also have high electronic conductivity to ensure the electron flows to/from the reaction sites from/to the external electrical circuit. High oxygen ion conductivity is not compulsory, but its presence can significantly extend the reaction sites and, as a consequence, increased the kinetics of reactions and electrode performance. The oxygen electrode should also have similar TEC with the electrolyte to ensure compatibility. Good chemical stability during fabrication and operation is also important. The properties, i.e. electronic and ionic conductivities, thermal expansion coefficient, oxygen diffusion coefficient, and surface exchange coefficient, of several oxygen electrode materials are listed in Table 1.2.
A pure electronic conductor that is used the most as oxygen electrode material is strontium-doped lanthanum manganite, La1-xSrxMnO3 (LSM). It has a perovskite-type structure, high electronic conductivity, and negligible ionic conductivity. Thus, the TPB where the reactions take place is restricted only at the electrode/electrolyte interface where the gas, electrons, and ions can be present at the same time. To extend the reaction site beyond the electrode/electrolyte interface, a composite of pure electronic conductor, i.e. LSM, and pure ionic conductor, i.e. YSZ, can be used as the oxygen electrode material. Another option is by using mixed ionic and electronic conductor (MIEC) such as La1-xSrxCo1-yFeyO3-δ (LSCF) or rare-earth nickelates Ln2NiO4+δ (Ln = La, Pr, Nd). The mixed conducting properties of MIEC arise from the presence of either oxygen vacancy (oxygen under-stoichiometry such as for LSCF) or interstitial (oxygen over-stoichiometry such as for rare-earth nickelates) resulting in high oxygen ion conductivity in addition to the high electronic conductivity.

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Fuel/hydrogen electrode

Similar to the air/oxygen electrode, three species are also involved in the reactions to produce either water or hydrogen at the fuel/hydrogen electrode. Thus, similar requirements also apply for hydrogen electrode, i.e. it should be porous, highly catalytic for water or hydrogen production reactions, has high electronic conductivity and preferably also has high ionic conductivity, has similar TEC with the electrolyte, and is chemically stable. If hydrocarbon is used as the fuel in SOFC, the material should also possess high catalytic activity for hydrocarbon cracking, sulfur tolerance, and resistance to coking [11].
The most common material used for the hydrogen electrode is Ni-YSZ cermet. Normally, it is fabricated by mixing NiO with YSZ and reduction treatment is required before cell operation. Ni metal exhibits high catalytic activity and high electronic conductivity (2 x 104 S cm-1) [11]. The addition of YSZ, which is a pure ionic conductor, not only extends the reaction sites but also prevents Ni coarsening during operation.

Sealing materials and interconnects

The seal is necessary to prevent the mixing between gases in the oxygen and hydrogen electrodes. It is normally applied on the electrolyte, thus similar TEC between the sealant and electrolyte is important. Moreover, it should also be chemically inert with respect to the other cell materials and chemically stable under operating conditions. The sealing material is normally made of glasses or glass-ceramic composites.
Interconnects are mainly used to provide an electrical connection between the stack of cells. It should have high electronic and thermal conductivities, high creep resistance, good mechanical properties, similar TEC with the other components, and structural and chemical stability under operating conditions. In a high-temperature SOC, ceramic materials are normally used for interconnections. However, the electronic and thermal conductivities of ceramic materials are low and they are also expensive. Hence, this is also one reason to lower the operating temperature to allow the use of cheaper metallic interconnects.

Oxygen reduction/evolution reaction

Triple-phase boundary (TPB)

The processes that occur at the oxygen electrode of SOC involve three different species: electrons, oxygen ions, and oxygen gas. In the simplest configuration, a porous electronic-conducting oxygen electrode is in contact with an ionic-conducting electrolyte and both phases are exposed to the oxygen-containing gas phase. The electrode is connected to an electrical circuit at some distance away from the electrode-electrolyte interface. For the oxygen electrode of SOC, the electrical circuit plays a role to conduct electrons to (for SOFC) or from (for SOEC) this interface. The electrolyte provides the sink for the oxygen ions from the oxygen reduction reaction (ORR for SOFC operation) or the source of oxygen ions for oxygen evolution reaction (OER for SOEC operation). Thus, the ORR/OER should take place in the vicinity of the electrode-electrolyte-gas phases interface. Since three phases are involved, these active regions where the reactions take place are called the triple-phase boundary (TPB).

Table of contents :

Structure of the thesis
1.1 Fundamentals of solid oxide cell (SOC)
1.1.1 Electrolyte
1.1.2 Air/oxygen electrode
1.13 Fuel/hydrogen electrode
1.1.4 Sealing materials and interconnects
1.2 Oxygen reduction/evolution reaction
1.2.1 Triple-phase boundary (TPB)
1.2.2 Factors affecting oxygen reduction/evolution reaction (ORR and OER)
1.3 Ruddlesden-Popper phase: rare-earth nickelates
1.4 Degradation of several MIEC oxygen electrode materials for SOC
1.4.1 Degradation mechanism of LSCF oxygen electrode
1.4.2 Degradation mechanism of rare-earth nickelates
2.1 Electrolytes for symmetrical and complete cells
2.2 Fabrication techniques
2.2.1 Sol-gel auto-combustion to prepare LPNO powder for screen printing
2.2.2 Screen printing (SP)
2.2.3 Electrostatic spray deposition (ESD)
2.2.4 Design of the LPNO oxygen electrode
2.3 Microstructural characterization techniques
2.3.1 Scanning electron microscopy (SEM)
2.3.2 Focused ion beam-scanning electron microscopy (FIB-SEM) tomography
2.4 Structural characterization techniques
2.4.1 Laboratory X-ray diffraction (XRD): room temperature and in-situ XRD
2.4.2 Synchrotron-based X-ray: Lamellae preparation by plasma-FIB, micro-X-ray diffraction (μ XRD), and micro-X-ray fluorescence (μ
2.5 Electrochemical characterization technique: Electrochemical impedance spectroscopy (EIS)
2.6 Long-term durability tests
2.6.1 Long-term test on the symmetrical cell
2.6.2 Long-term test on the complete cell
3.1 Introduction
3.2 Preliminary experiments
3.2.1 Optimized deposition parameters by SP and ESD SP layer: binder for the ink ESD layer: solution stabilization, deposition time, and hierarchical microstructure
3.2.2 Electrochemical performance of coral-type La2-xPrxNiO4+δ (x = 0, 1, 2)
3.2.3 Reproducibility
3.3 Structural characterization of LPNO as a powder, SP layer, and ESD layers
3.4 Equivalent electrical circuit model for EIS data fitting
3.5 Influence of the electrode/electrolyte interface quality on the performance of the LPNO electrode
3.6 Influence of the secondary higher-order phase on the electrochemical performance of the ESD layer
3.7 Influence of the CCL microstructure on the performance of the electrode
3.8 Influence of the various architectural designs on the performance of the electrode
3.9 Summary and conclusions
4.1 Introduction
4.2 Durability study on the symmetrical cell
4.2.1 Pre-test measurements
4.2.2 Long-term measurement in electrolysis and fuel cell modes (idc = ±300 mA cm-2)
4.3 Preliminary durability study on the complete cell
4.3.1 Preliminary durability test on CCell 1: GDC barrier layer by screen printing
4.3.2 Preliminary test on CCell 2: GDC barrier layer by RF magnetron sputtering
4.4 Summary and conclusions
5.1 Introduction
5.2 Post-mortem microstructural characterization: SEM and 3D reconstruction by FIB-SEM
5.2.1 Characterization of the samples operated in the electrolysis mode
5.2.2 Characterization of the sample operated in fuel cell mode
5.3 Post-mortem structural characterization: synchrotron-based μ-XRD and μ-XRF
5.3.1 Pristine cell Laboratory XRD characterizations for ESD and SP layers Synchrotron-based μ-XRD and μ-XRF characterizations for ESD-SP double-layer electrode
5.3.2 Thermally-aged cell
5.3.3 Long-term test in electrolysis mode (anodic polarization): symmetrical and complete cells Anodic side of the symmetrical cell (sample 3 or SCell-anodic) Anodically-aged oxygen electrode of the complete cell (sample 4 or CCell 1)
5.3.4 Long-term test in fuel cell mode (cathodic polarization): symmetrical cell
5.4 Summary and conclusions


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