SOCs principle of functioning, components materials and architectures
Basis of functioning – SOCs are high temperature electrochemical devices usually composed of ceramic and metallic materials. The operating mode of SOCs relies on the electrochemical reactions occurring between oxidizing and reducing species. In the SOFC mode, the current is generated through the oxidation of the fuel (e.g. hydrogen, methane, carbon monoxide) and the reduction of oxygen, while in SOEC mode, the respective reactions are reversed to produce hydrogen and/or carbon monoxide from steam, carbon dioxide and oxygen under an electrical current. The gases produced in SOEC mode can be stored as tanks of chemical energy that can be subsequently used on demand to produce electricity. SOCs have recently attracted the green energy market attention thanks to their reversibility that makes these devices particularly compatible to be coupled with intermittent RES (e.g. wind turbines or solar panels) [Eriksonn2017, Staffell2019]. Moreover, high operating temperatures result in a very high efficiency, fuel flexibility and a co-production of high-quality heat in SOFC mode [Eguchi2002, Guaitolini2018]. This last feature makes SOFC as one of the most promising technology for clean residential energy supply through micro combined heat and power (micro-CHP) systems [Hawkes2007].
SOCs are usually composed by a dense ionic conductor electrolyte sandwiched between two porous electrodes exhibiting electronic or mixed conduction properties. The schematic operating principles of a SOC are shown in Fig. I-1. In SOFC mode, hydrogen is oxidized at the so-called hydrogen electrode (which behaves as an anode) to produce steam (forward reaction of Eq. (1)) and oxygen is reduced at the so-called oxygen electrode (which behaves as a cathode) to produce oxygen ions (forward reaction of Eq. (2)) (Fig. I-1a). The global chemical reaction yields the formation of steam (forward reaction of Eq. (3)). In SOEC mode, the backward reactions are activated and the hydrogen and oxygen electrodes behave as a cathode and an anode, respectively (Fig. I-1b). The reduction of steam produces oxygen ions which are oxidized. Regardless the operating mode, oxygen ions migrate through the dense electrolyte membrane.
To allow the correct functioning of SOCs and to maximize their efficiency, each cell component must have precise characteristics.
Electrolyte – The electrolyte is the core of a SOC since it allows the passage of oxygen ions between the hydrogen and oxygen electrodes and ensures the gas tightness between the two electrode layers [Singhal2000]. Therefore, the choice of the electrolyte material is of fundamental importance. An electrolyte material must obey the following characteristics [Mahato2015]:
i) High ionic conductivity,
ii) Chemical stability over a large range of temperature and oxygen partial pressure,
iii) Chemical compatibility with respect to the other component materials (to avoid the formation of secondary phases),
iv) Reliable mechanical properties (high strength and toughness),
v) Coefficient of thermal expansion (CTE) compatible with those of the other cell components,
vi) Low cost and environmental friendly.
Many kinds of materials can be found in the literature as candidates for a SOC electrolyte, from the classical fluorite oxide based structure (e.g. ZrO2 or CeO2) to more recent compounds such as lanthanum-based perovskites [Fergus2006, Singhal2000]. However, despite the wide possibility, Yttria Stabilized Zirconia (YSZ) remains the most common used electrolyte material thanks to its excellent ionic conductivity, good chemical stability and its adapted CTE regarding the usual electrode materials [Hossain2017, Ivers-Tiffée2001]. Two types of YSZ electrolytes can be used depending on the level of doping: with 3 % mol of yttria (3YSZ) the zirconia presents a tetragonal phase with high mechanical properties, while, passing to 8 % mol of yttria (8YSZ), the zirconia change in a cubic phase with higher ionic conductivity ( ). Indeed, is an increasing function of the dopant content as described in Eq. 4.
Accordingly, two moles of dopant create one mole of oxygen vacancies. These crystalline defects allow the migration of oxygen ions through the volume of the material [Singhal2000]. Despite its lower mechanical strength, 8YSZ is the most common electrolyte for SOCs since it presents a good compromise between ionic conductivity and mechanical resistance [Suciu2018]. As expected, the ionic conductivity of 8YSZ is thermally activated (Fig. I-2) [Zhang2007] and presents a value of 0.1 S•cm-1 at 1000 °C [Chen2002]. This characteristic explains the necessity of operating SOCs at high temperatures in order to maximize their performances.
As well as the electrolyte, the electrodes materials have to meet precise requirements [Singhal2000]:
i) High electronic conductivity,
ii) High catalytic activity for the electrochemical reactions,
iii) Chemical stability to avoid the reactivity with the other cell components,
iv) CTE matched with the electrolyte,
v) Sufficient porosity to allow the passage of the gas through the volume of the electrode,
vi) Low cost and environmental friendly.
Hydrogen electrode – The reaction in the hydrogen electrode occurs at the Triple Phase Boundary lengths (TPBls), where the oxygen ions, the hydrogen gas and the electrons meet [Zhu2003]. For this reason, the use of a material with a mixed ionic and electronic conductivity is favored, since it provides active sites in all the electrode volume. The common hydrogen electrode materials are composites made of an ionic conductor phase and a transition metal that provides the electronic conductivity [Shaikh2015]. In order to have a good CTE compatibility, the ionic phase is usually the same used for the electrolyte. Up to now, the most used transition metal is Nickel since it presents a good compromise between prize and catalytic activity [daSilva2017, Shaikh2015]. Thus, the most common composite material is the cermet (i.e. ceramic and metal composite) Ni-YSZ [Minh1993]. Since, a SOC is usually structurally supported by the H2 electrode, the mechanical properties of the cermet are extremely important [Pihlatie2009]. Moreover, a particular attention has to be paid during the hydrogen electrode manufacturing in terms of initial microstructural properties (such as the porosity level, the particle size diameter, the tortuosity etc.) to enhance the electrode performances and to limit the material degradation [Monaco2019].
Oxygen electrode – One of the most common material for the oxygen electrode is the composite composed of La1-xSrxMnO3-δ (LSM) and YSZ. Indeed, the mix of ionic and electronic conductor phases increases the number of the electroactive sites of an order of magnitude with respect to a simple electronic conductor (Fig. I-3a) [Sun2010]. Mixed Ionic and Electronic Conductors (MIECs) become of increasing interest for several decades. Indeed, their conduction properties combined in a single-phase compound results in higher performances, since the electrochemical reactions can take on all the surface of the electrode particles (Fig. I-3b) [Sun2010].
One of the most commonly used O2-electrode material for Intermediate Temperature SOC (IT-SOC) is La1-xSrxCo1-yFeyO3-δ (LSCF) [Menzler2010, Jiang2008, Sun2010]. A shown in Fig. I-4, the cell performances increase at low operating temperature (≤ 800 °C) using a LSCF electrode with respect to the traditional LSM-YSZ composite electrode [Tietz2006]. However, the use of LSCF-based electrodes implies the presence of an additional layer to mitigate the formation of non-conducting secondary phases with the YSZ electrolyte [Anderson2004]. For this reason, a thin barrier layer of Ce1-xGdxO2-δ(CGO) is usually added between these two materials [Sanchez Ferreira2017]. Moreover, recent studies have shown that adding a high ionic conducting material like CGO into LSCF is liable to enhance the electrode performances [Aziz2020, Dusastre1999, Hwang2005]. Indeed, it has been reported [Leng2008, Wang2005] that the porous LSCF-CGO composite exhibits a higher effective ionic conductivity than the LSCF electrode. Moreover, a more compatible CTE with the electrolyte material reduces the risk of mechanical fractures by using a composite electrode [Laurencin2015, Wang2005].
Architecture – SOCs present two main types of architecture: planar or microtubular (Fig. I-5) [Jamil2015].
Planar SOCs are compact devices that present multiple layers with square or circular shape [Birss2017]. The cell can be self-supported or ‘externally’ supported. In the first case, SOCs can be classified as anode-, electrolyte- or cathode-supported, depending on which layer provides the mechanical strength. In the second case, an additional layer of metallic or ceramic material is added, while the other cell components are deposited as thin layers [Birss2017]. An electrode-supported or external supported structure is usually preferred since the electrolyte is maintained as thin reducing substantially the related Ohmic losses [Nozawa2008].
Microtubular SOCs (MT-SOCs) are made of concentric tubes which dimension goes from a few millimeters to sub-millimeter size [Panthi2014]. As well as planar SOCs, an electrode- or electrolyte-supported architecture can be designed [Lawlor2013]. MT-SOCs can be designed in much smaller geometry than planar SOCs leading to some advantages such as a higher thermal shock resistance and an increased power per unit volume [Lawlor2013]. Both the possibility of a small size device and the resistance to multiple thermal cycling made this architecture very interesting for small portable fuel cell applications [Kendall2010]. However, the difficulty in positioning the sealing due to the reduced dimensions of the tubes enhances the risk of gas leakage [Lawlor2009].
Operation in steady state condition
The efficiency of a SOC is represented by the corresponding polarization curve (i-V) showing the evolution of the cell voltage as a function of the current density (Fig. I-6a). The Open Circuit Voltage (OCV) corresponds to the cell voltage at the equilibrium state =0 (i = 0). This potential depends on temperature as well as the gas conditions and is expressed by the Nernst’s equation in the case of a SOC fed by hydrogen and oxygen:
Regardless of the operating mode, the cell voltage deviates from OCV under current (Fig. I-6a) since additional losses exist [O’Hayre2016, Patcharavorachot2008]. In SOEC mode, a sufficient cell voltage or current must be applied to trigger the non-spontaneous electrolysis reactions. In SOFC mode, the existence of a current implies that the cell voltage serves the irreversible phenomena related to the electrochemical reactions. Without emphasizing on any detailed electrode reaction mechanism, any electrochemical process implies mass transport of multi-components in a gas phase, mass transport through solid phases and charge transfer steps. Thus, additional losses are generated in a SOC that modify the cell voltage and are divided in three main contributions: Ohmic losses ( ℎ ( )), activation overpotentials ( ( )) and concentration overpotentials ( ( )) [O’Hayre2016] (Fig. I-6b).
Table of contents :
Chapter I. Bibliography
1.1 General Aspects of SOFC/SOEC
1.1.1 SOCs principle of functioning, components material and architecture
1.1.2 Operation in steady state condition
1.2 Review on the reaction mechanisms: electrodes functioning and degradation phenomena
1.2.1 Hydrogen electrode reaction mechanism
1.2.2 Oxygen electrode reaction mechanism
1.2.3 Main degradation phenomena
184.108.40.206 LSCF demixing: study of the reaction mechanism
1.3 Objective and Methodology
Chapter II. Experimental results
2.1 Symmetrical cells testing for model validation
220.127.116.11 Three-electrode configuration
18.104.22.168 Current collecting
2.1.1 Hydrogen electrode
22.214.171.124 Studied cell
126.96.36.199 Experimental set-up
188.8.131.52 Testing conditions
184.108.40.206 Electrochemical characterizations
2.1.2 Oxygen electrode
220.127.116.11 Materials and studied cells
18.104.22.168 Experimental set-up
22.214.171.124 Testing conditions
126.96.36.199 FIB-SEM reconstructions.
188.8.131.52 Electrode 3D reconstructions and electrochemical characterizations
2.2 Long-term tests: study of the LSCF decomposition
2.2.1 LSCF symmetrical cells: effect of the anodic polarization
2.2.2 Complete cells: effect of the steam content
184.108.40.206 Studied cells, experimental set-up and testing conditions
220.127.116.11 Durability curves under dry and humid air
2.3 Conclusions of the chapter
Chapter III. Electrochemical model
3.1 Model description
3.1.2 Reaction mechanism
3.1.3 Kinetic rates and transport phenomena
3.1.4 Thermodynamic description
3.1.5 Boundary conditions and input parameters
3.1.6 Extension to the Cyclic Voltammetry simulation
3.2 Elementary model calibration and validation (model-II)
3.2.1 Model calibration on polarization curves
3.2.2 Model validation
18.104.22.168 Impedance diagrams
22.214.171.124 Effect of the oxygen partial pressure
3.3 Analysis of the reaction mechanism
3.3.1 Effect of polarization and temperature on the reaction pathway under air condition
3.3.2 Impact of the ratio on the impedance response
3.3.3 Effect of oxygen partial pressure on the reaction mechanism at OCP
3.4 Cyclic Voltammetry study contribution
3.4.1 Validation model-II for the CV computation: effect of temperature and scan rate
3.4.2 Calibration of model-I
3.4.3 Comparison between model-I and model-II for the CV response
126.96.36.199 Microstructural effect: from a porous to a dense electrode
188.8.131.52 Cyclic voltammetry peaks evolution
184.108.40.206 Impact of the ohmic losses
3.5 Conclusions of the chapter
Chapter IV. Impact of the LSCF demixing
4.1 Impact of the LSCF decomposition on the impedance diagrams
4.1.2 Analysis of the degradation at OCP
4.1.3 Analysis of the degradation under polarization
4.2 Impact of the LSCF demixing on the cyclic voltammetry response
4.3 Conclusions of the chapter
Chapter V. Conclusion and Outlook