Solid Oxide Cells: Basic Operating Mechanisms and Materials

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Literature Review on Solid Oxide Cells: Technology Fundamentals and Degradation Phenomena

This chapter is dedicated to the detailed description of the context in which the thesis has been carried out. A specific attention is given to the presentation of the fundamentals of Solid Oxide Cells (SOCs), including the state-of-the-art materials and architectures. Moreover, the basic operation with the physical processes occurring in SOCs are described as well as their impact on the performances during operation. Furthermore, the detailed reaction mechanisms for both electrodes are discussed analyzing the existing literature in terms of modeling and experimental results. Finally, the issues concerning the durability of SOCs are presented and the underlying causes of degradation are introduced and discussed. Based on this bibliography review, the objectives and the strategy adopted for the thesis are defined.
In the first section of the chapter, an overview of the technological, thermodynamic, and operational characteristics of SOCs is given. The second section is dedicated to the review of the detailed reaction mechanisms for the two electrodes, whereas the third section is devoted to the analysis of the degradation mechanisms for this type of technology. Finally, in the last section of this introduction, the justification of the applied approach is given.

Solid Oxide Cells: Basic Operating Mechanisms and Materials

SOCs Principle

SOCs are electrochemical devices that can directly convert fuel into electricity (fuel cell mode – SOFC) or electricity into fuel (electrolysis mode – SOEC). In recent years, the interest in SOCs has significantly grown thanks to their wide range of technological applications that could offer innovative solutions for the transition toward a renewable energy market. For example, when operated in electrolysis mode, these electrochemical devices could be coupled with wind turbines or solar panels for clean production of hydrogen [Godula-Jopek 2015, Lehner 2014]. Besides, in fuel cell mode, SOCs are typically used for the generation of heat and electricity for residential micro-Combined Heat and Power systems (µCHP) [Arsalis 2019, Kendall 2016]. The operating principle of SOCs is represented in Figure I-1.
SOFC mode and produced in SOEC mode (eq. (I-1b)). Between the two electrodes, the solid electrolyte ensures the migration of oxygen vacancies ( ••) from one electrode to the other.
To increase the ionic conductivity in the solid electrolyte, SOCs are usually operated at high temperature (700ºC – 900ºC), which offers many advantages. Indeed, the high operating temperature allows the direct conversion of carbon-containing fuels so that SOCs can be directly used to produce electricity and heat from hydrocarbons, such as methane, in SOFC mode [Morel 2005] and to perform the co-electrolysis of steam and carbon dioxide in SOEC mode [Aicart 2014]. In addition, thanks to the high operating temperature, SOCs present very high efficiency [Godula-Jopek 2015] and allow the utilization of non-expensive catalysts [Shaikh 2015]. Moreover, in SOFC mode, this technology is especially relevant for cogeneration applications. Finally, as already mentioned, it can be noticed that SOCs can be also operated alternatively in fuel cell and electrolysis modes for reversible utilization.

SOCs Architecture

Two main types of architectures have been proposed for SOCs: tubular and planar. In the tubular configuration, one of the electrodes is situated in the internal face of a cylinder and the other one on the external part (Figure I-2a). This configuration presents the advantages of having high mechanical robustness and simplicity in the sealing between the electrodes [Blum 2005, Larminie 2003]. Nevertheless, it has been shown that the performances for this type of architecture can be limited due to the difficulty for the reactants to reach the electrochemical active sites and to significantly high ohmic losses associated to the design and the complex connection between cells [Hino 2004].
Even if some projects on tubular SOCs are currently carried out [Yoshikawa 2017], nowadays the most common type of layout for this technology is the planar one [Wen 2002] (Figure I-2b). In this design, the manufacturing procedure is significantly simpler than the tubular one and the ohmic losses are much less pronounced. The cells are piled-up one on top of the other using metallic interconnects to form high power density stacks. The fuel and the air can be distributed in a cross-flow, a parallel co-flow or a counter-flow configuration. The main difficulty consists in ensuring the sealing between the different components to avoid any leakage especially from the hydrogen electrode side. Nevertheless, it has been shown that this type of configuration allows reaching the highest performances [Ni 2008].
Figure I-2: Different architectures for SOCs: a) Tubular design [Larminie 2003] – b) Planar design. To ensure the mechanical robustness in the planar configuration, different types of solutions have been found. In general, the thickness of one of the cell’s layers is increased and the layer serves as support for the rest of the cell. Depending on which layer is used as a substrate it is possible to have:
(i) oxygen or hydrogen electrode supported cells, (ii) electrolyte supported cells and (iii) metal interconnect supported cells. Each one of these solutions presents some advantages and drawbacks. Oxygen electrode supported cells have been reported to allow good performances, even if the specific cost is more elevated than in the other configurations [Irshad 2016]. Electrolyte supported cells are classically used at very high temperature (> 850 – 900ºC) to limit the ohmic losses inside the electrolyte [Kendall 2016]. Very recently, this type of cells has attracted significant interest because of the relative simplicity in the manufacturing and the high mechanical strength. Moreover, the use of highly conductive materials for the electrolyte has allowed working at lower operating temperatures [Jais 2017] and new experimental results at the cell level show promising behaviors in terms of durability [Schefold 2020]. The metal supported cells consist in using porous metallic interconnects to increase the mechanical robustness and the resistance to thermal and redox cycling, while reducing the manufacturing costs. Promising performances have been obtained for this type of technology [Blennow 2011, Dogdibegovic 2019b], even if it remains still immature, especially in terms of durability [Brandon 2017, Dogdibegovic 2019a]. Finally, the hydrogen electrode supported cells have been reported to be a good compromise between manufacturing costs, performances, and mechanical robustness [Irvine 2013]. For this reason, nowadays, the most common design for planar SOC is the hydrogen electrode supported cell configuration even if electrolyte supported cells and metal supported cells are being extensively studied as promising alternatives.

Materials Selection

To ensure the reliability of SOCs, the choice of the materials used for the two electrodes and the electrolyte is of central importance. Indeed, they must fulfill a large number of requirements such as:
– Compatibility on the thermal expansion coefficient (TEC) between the different layers,
– Chemical stability in the reducing and oxidizing environments of the cathode and the anode respectively,
– High electro-catalytic activity for the reactions taking place at the two electrodes,
– High ionic conductivity and negligible electronic conductivity for the electrolyte,
– Low chemical reactivity between the different layers (including the interconnectors),
– Mechanical robustness,
– Competitive fabrication costs with low environmental impact.
These strict requirements must be satisfied considering also the high operating temperature of SOCs making the selection of the materials even more complex.

Electrolyte

The electrolyte in SOCs is classically made of fluorite-type ceramic oxides (e.g. ZrO2, CeO2, Bi2O3 and ThO2). Di- or tri-valent rare-earth or alkaline-earth cations (such as Ce, Mg, Y, Nd, Sm, Yb and Sc) are used as substitution elements to stabilize the ceramic in the cubic crystalline phase throughout a large range of temperatures [Kharton 2004]. The substitution also creates oxygen vacancies in the lattice of the material leading to high ionic conductivities. With a fixed concentration of dopant, the evolution of the ionic conductivity can be described using a classical Arrhenius-type law as given in eq. (I-2). = ∙− ∙ (I-2)
In whichis a pre-exponential factor (S•cm-1), is the activation energy (J•mol-1), is the universal gas constant (J•K-1•mol-1) and is the operating temperature (K).
To date, the most typical electrolyte material for hydrogen electrode supported cells is Zirconia stabilized in the cubic phase adding 8 %mol. of Yttria ((ZrO2)0.92(Y2O3)0.08 – 8YSZ) [Suciu 2018].
The choice of 8YSZ represents a good compromise between the ionic conductivity and the 11
mechanical robustness. Indeed, the ionic conductivity of YSZ exhibits a maximum around 8.5 – 9 %mol. of Yttria [Nakamura 1986], whereas lower doping levels, such as the tetragonal Zirconia doped with 3 %mol. of Yttria (3YSZ), correspond to higher mechanical strength [Ghatee 2009]. Typical evolutions of the ionic conductivity for the 8YSZ and 3YSZ as a function of the operating temperature are shown in Figure I-3 (considering , = 44.7 S•cm-1, , = 466 S•cm-1,, = 69’837 J•mol-1 and, = 82’616 J•mol-1 [Lay-Grindler 2013]).
Figure I-3: Ionic conductivity of 8YSZ and 3YSZ as a function of the operating temperature [Lay-Grindler 2013].

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Oxygen Electrode

The oxygen electrode was classically made of a porous composite of 8YSZ and Lanthanum Strontium Manganite (LSM) [Jiang 2008]. With these materials, the oxidation/reduction of oxygen occurs at the so-called Triple Phase Boundaries lines (TPBs) where the electronic, ionic and gas phases meet. Nowadays, this composite has been advantageously replaced by Mixed Ionic and Electronic Conductors (MIEC) for which the reaction can be extended from the vicinity of TPBs to the whole surface area of the porous electrode resulting in a substantial increase of the performances [Adler 2004]. Among the different compounds, the Lanthanum Strontium Cobaltite Ferrite (LaxSr1-xCoyFe1-yO3-δ – LSCF) perovskite is the most employed material for SOCs application [Gómez 2016, Laguna-Bercero 2012, Menzler 2010]. In addition, it has been recently proposed to replace LSCF by a composite of LSCF and Gadolinium-doped Ceria Oxide (GdxCe1-xO2-δ – GDC) as an alternative oxygen electrode, in order to mitigate the mismatch in thermal expansion coefficients between the electrode and the electrolyte [Dusastre 1999] and to enhance the electrode performances, especially in electrolysis mode [Effori 2019].
To mitigate the reactivity of LSCF and YSZ, a barrier layer of GDC is usually added between the electrolyte and the electrode to avoid the direct contact between the two materials [Ferreira-Sanchez 2017, Knibbe 2010a, Laurencin 2017].

Hydrogen Electrode

The typical hydrogen electrode is constituted of a porous cermet of nickel and YSZ, in which the Ni serves as electronic conductor/reaction catalyst, the YSZ as ionic conductor and the pores ensure the gas transport [Jiang 2004, Menzler 2010]. Porous cermet electrodes are used to extend the electrochemical active area in the thickness of the electrode. The hydrogen electrode is also used as mechanical support of the cell. Therefore, the electrode is usually structured into regions with different microstructures to separate the electrochemical and the mechanical functions.
In contact with the electrolyte, the thin functional layer (FL) presents a fine microstructure in which the electrochemical reactions take place at the TPBs. This layer is associated with a thick current collector substrate (CC) with a coarser microstructure which ensures the gas diffusion to the FL and guarantees the collection of electronic current. It can be noticed that in some cases pores formers are added in the substrate to favor the diffusion to the active sites [Moussaoui 2020, Pecho 2015]. In the functional layer of the cermet, the YSZ presents typically the same amount of Yttria as in the electrolyte (8YSZ). Nevertheless, in some cases 3YSZ is used to take advantage of the higher mechanical robustness [Ghatee 2009] even if in this case the electrode exhibits a lower ionic conductivity (cf. Figure I-3). It can be noticed that for the CC layer the choice of 3YSZ is the most common due to the better mechanical properties.

Thermodynamic Description of SOCs and Nernst Equation

As a general matter, SOCs are electrochemical devices that allow the direct conversion of the chemical energy contained in the fuel in electricity and vice versa. To investigate the operation of SOCs it is important to obtain the standard voltage of the cell, corresponding to the difference between the standard potentials at the two electrodes. With this objective, it can be useful to analyze the thermodynamically reversible processes occurring in an ideal cell.
A solid oxide cell operating in SOFC mode can be described with the scheme reported in Figure I-4. Based on this representation, it is possible to apply the first law of thermodynamic to obtain an energy balance of the system as reported in eq. (I-3).  − = ∙ ℎ ( , )− ∙ℎ ( , )−ℎ ( , ) (I-3)
In which all the quantities are referred to an unitarian molar flow of fuel (mol•s-1).
In eq. (I-3), represents the specific reversible heat produced in the cell (J•mol-1),the specific reversible electrical work delivered by the cell (J•mol-1) and /the stochiometric coefficients corresponding to one mole of fuel for the product and the oxidant, respectively. The terms ℎ , ℎ , ℎ are the specific enthalpies of the product, the oxidant and the fuel as a function of the absolute temperature and at standard pressure of 1 atm (expressed in J•mol-1).

Table of contents :

General Introduction
1. Motivation
2. General Objective
3. Layout of the Manuscript
I. Literature Review on Solid Oxide Cells: Technology Fundamentals and Degradation Phenomena
I.1 Solid Oxide Cells: Basic Operating Mechanisms and Materials
I.1.1 SOCs Principle
I.1.2 SOCs Architecture
I.1.3 Materials Selection
I.1.4 Thermodynamic Description of SOCs and Nernst Equation
I.1.5 Losses at the Cell Level and Impact on Cell’s Performances
I.2 Literature Review on the Reaction Mechanisms for the Two Electrodes
I.2.1 Reaction Mechanism for the Hydrogen Electrode
I.2.2 Reaction Mechanism for the Oxygen Electrode
I.3 Durability of SOCs
I.3.1 Durability Results Reported in Literature
I.3.2 Main Degradation Mechanisms
I.3.3 Concluding Remarks
I.4 Conclusions of This Chapter: Objectives and Methodology of the Thesis
II. Durability Experiments and Post-Test Characterization Techniques
II.1 Long-term Tests in Electrolysis Mode at Different Operating Temperature
II.1.1 Cell Description
II.1.2 Description of the Test Benches
II.1.3 Experimental Techniques
II.1.4 Testing Protocols
II.1.5 Tests Results
II.2 Long-term Tests in Collaboration with External Partners
II.2.1 Cells Description
II.2.2 Durability Tests
II.3 Physico-Chemical Characterizations and 3D Reconstructions
II.3.1 Sample Selection
II.3.2 Sample Preparation
II.3.3 Characterization Techniques
II.4 Conclusion of This Chapter
III. Development and Validation of the Multi-Scale Modeling Tools
III.1 Micro-Scale Models for the Hydrogen Electrode
III.1.1 Models Description
III.1.2 Experimental Characterization
III.1.3 Models Validation and Discussion
III.1.4 Concluding Remarks and Model Adaptation for the Degradation Studies
III.2 Micro-Scale Model for the Oxygen Electrode
III.2.1 Experimental Characterization
III.2.2 Microstructural Reconstruction for Cell I
III.2.3 Description of the Modeling Tools
III.2.4 Model Validation and Discussion
III.2.5 LSCF-GDC Composite Electrodes
III.2.6 Concluding Remarks
III.3 Macro-Scale Model at the Cell level and Multi-Scale Integration
III.3.1 Macro-Scale Model Description
III.3.2 Multi-Scale Integration
III.3.3 Multi-Scale Model Validation
III.4 Conclusion of This Chapter
IV. Hydrogen Electrode Degradation: Impact of Polarization and Initial Microstructure on the Ni Evolution
IV.1 3D Reconstructions and Image Processing Techniques for Microstructural Computations
IV.1.1 Samples Extracted from Cell-A and Cell-B
IV.1.2 Methodology for the Determination of the Microstructural Parameters
IV.2 Results of the Microstructural Analysis
IV.2.1 Microstructural Properties of the Reference Cells
IV.2.2 Microstructural Evolution of the Aged Cells
IV.3 Impact of Ni Evolution on the Cell Performances
IV.3.1 Preliminary Remark: Adaptation of the H2 Electrode Model
IV.3.2 Role of Ni Agglomeration on Degradation
IV.3.3 Role of Ni Depletion on Degradation
IV.4 Suggested Mechanism of Ni Depletion
IV.5 Conclusion of This Chapter
V. Oxygen Electrode Degradation: LSCF Demixing and Interdiffusion at the GDC/8YSZ Interface
V.1 Characterization of the Pristine Cell
V.1.1 Detection of SrZrO3 After Sintering
V.1.2 Inter-Diffusional Layer Between the GDC and the YSZ
V.1.3 Crystalline Phases Distribution Across the Electrolyte and Barrier Layer
V.2 Characterization of the Aged Cells
V.2.1 Preliminary Remark: Proposed Mechanism for the LSCF Destabilization
V.2.2 LSCF Destabilization: Sr Release from the Material as a Function of Temperature and Polarization
V.2.3 Evolution of the IDL After Aging in Electrolysis Mode: Preliminary Characterizations and Discussion
V.3 Conclusion of This Chapter
VI. Conclusions and Perspectives
VI.1 General Conclusions
VI.2 Perspectives
Appendix A Cells Description and Experimental Conditions
Appendix B Development of the Micro-Kinetic Model for the Hydrogen Electrode Reaction Mechanism: Model I Based on the Oxygen Spillover Mechanism
Appendix C Development of the Micro-Kinetic Model for the Hydrogen Electrode Reaction Mechanism: Model II Based on the Hydrogen Spillover Mechanism
Bibliography

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