The Relation of Microstructure, Materials Properties and Impedance of SOFC Electrodes: A Case Study of Ni/GDC Anodes
Abstract
:1. Introduction
2. Impedance Model for Porous SOFC Electrodes
2.1. Porous Electrode Impedance Models in Literature
2.2. Electronion Conduction and Electrochemical Reactions in a Porous Electrode
2.3. Equivalent Circuit Representation of Reactions in Porous Electrodes
2.4. The Need for Parameter Constraints in “Thick” Electrodes
2.5. Treatment of Interfacial Resistances and Wiring
2.6. Simulated Impedance Spectra
2.7. Distribution of the Local Overpotential η and Electrochemically Active Thickness λ
2.8. Prediction and Minimization of Gas Diffusion Resistances
2.9. Gas Diffusion ASR for Typical Testing Conditions
3. Materials and Methods
3.1. Symmetric Cell Fabrication
3.2. Impedance Spectroscopic Characterization and EIS Fitting
3.3. SEM Analysis of Screen-Printed Electrodes
4. Results and Discussion
4.1. Gas Diffusion Impedance
4.2. Chemical Capacitance
4.3. Effect of Electrode Thickness
4.4. Temperature Dependence and Comparison with Model Studies
4.5. Enhancing Electrolyte-Electrode Interface Adhesion and Ionic Transfer Resistance by a Thin Film GDC Layer
4.6. Why Ceria-Based Anodes are Optimal for Metal-Supported Cells
5. Conclusions
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
Appendix A. Equivalent Circuit Fitting in Zview
Parameter Name in Zview | Parameter Name in Equation (A1) | Fixed/Free Parameter | Unit |
---|---|---|---|
DX | None | fixed (11) | None; selector for circuit model “Bisquert 2” |
DX-R | None | fixed (0) | Ωcm |
DX-T | None | fixed (0) | F/cm |
DX-P | None | fixed (1) | 1 |
DX-U | Rion | free | Ωcm |
DX-A | None | fixed (0) | F/cm |
DX-B | None | fixed (1) | 1 |
DX-C | Rreact | free | Ωcm3 |
DX-D | Cchem,eff | free * | F/cm3 |
DX-E | None | fixed (1) | 1 |
DX-F | L | fixed (thickness measurement) | cm |
Qint-P | pint | may be free or fixed to a value 0.8–1 | 1 |
Qint-T | Qint | free | Fractional capacitance |
Particle Packing Model
Appendix B. List of Used Variables
Symbol | Meaning | Unit |
---|---|---|
Gibbs free energy of H2 oxidation | J/mol | |
δ | oxygen deficiency, e.g., in Ce0.9Gd0.1O1.95-δ | 1 |
μx | chemical potential, species x | J/mol |
electrochemical potential, species x | J/mol | |
τx | tortuosity, phase x | 1 |
εx | volume fraction, phase x | 1 |
Aspec | specific surface area (of the GDC phase) per electrode volume | cm2/cm3 |
ASRsurf | ASR of the oxygen exchange (H2 oxidation) reaction normalized to the GDC surface area | Ωcm2 |
ASRtot | measurable ASR of a porous electrode normalized to the geometric area | Ωcm2 |
jx | (effective) electrical current density per electrode area, carried by species x | A/cm2 |
jsurf | electrical current density of released oxygen ions on the GDC surface | A/cm2 |
jreact | (effective) density of reaction current per electrode volume | A/cm3 |
σeff | effective ionic/electronic conductivity | S/cm |
z | charge number | 1 |
F | Faraday’s constant | C/mol |
Δ(x) | perturbation of quantity x relative to the equilibrium with the gas phase and grounded electrical potential | Unit of x |
pseudoelectrical potential, species x | V | |
η | local overpotential in the porous electrode | V |
ηgas | concentration overpotential of the gas phase | V |
Cchem | chemical capacitance of the GDC phase | F/cm3 |
Cchem,eff | effective chemical capacitance of the porous electrode | F/cm3 |
Rion,Reon | effective ion/electron conduction resistance inverse of effective conductivity | Ωcm |
Rint | ionic electrode/electrolyte interface resistance | Ωcm2 |
D | diffusion coefficient | cm2/s |
Appendix C. Mathematical Isomorphisms and Differences to the Adler-Lane-Steele Model
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Gas Phase Pressures (mbar) | Dbinary (cm2/s) [60] | ASRdiff (Ωcm2) | Jlimit (Acm−2) |
---|---|---|---|
25 H2 + 25 H2O + 950 Ar | 6.3 (H2-Ar); 2 (H2O-Ar) | 0.27 | 0.75 |
500 H2 + 500 H2O | 7.2 (H2-H2O) | 0.005 | 16 |
990 H2 + 10 H2O | 7.2 (H2-H2O) | 0.14 | 32 |
45 H2 + 25 H2O | 110 (H2-H2O) | 0.006 | 21 |
150 O2 + 850 N2 | 1.7 (O2-N2) | 0.01 | 2.3 |
10 O2 + 990 N2 | 0.15 (O2-N2) | 0.15 | 0.16 |
Parameter | Value (Prediction) | Value (Fit, Mean) | Reference |
---|---|---|---|
GDC phase ion conductivity | 25 mS/cm (650 °C) 60 mS/cm (750 °C) | - | GDC10 polycrystal [67] |
GDC phase volume fraction | 0.3 | - | Porosity estimate from SEM cross-sections |
GDC phase tortuosity | 3 | 2.7 ± 0.4 | Estimate from SEM cross-sections [40,62,68,69] |
Effective ion conductivity | 2.5 mS/cm (650 °C) 6 mS/cm (750 °C) | 2.5 ± 0.15 mS/cm (650 °C) 5.3 ± 0.5 mS/cm (750 °C) | |
Effective chemical capacitance | 90 F/cm3 (650 °C) 260 F/cm3 (750 °C) | 112 ± 4 F/cm3 (650 °C) 321 ± 6 F/cm3 (750 °C) | Thermogravimetry [50,51], Equation (14) |
GDC surface ASR | 6 Ωcm2 (650 °C) 4 Ωcm2 (750 °C) | - | Thin film electrodes (initial performance) 650 °C: [18]; 750 °C: [44] |
Specific GDC surface area | 5 μm2/ μm3 | - | Particle packing model, Appendix A |
Electrochemical reaction resistance (Rreact) | 120 µΩcm3(650 °C) 80 µΩcm3 (750 °C) | 70 ± 10µΩcm3 (650 °C) 36 ± 5 µΩcm3 (750 °C) | |
Electrode ASR (6-μm-thickness) | 0.23 Ωcm2 (650 °C) 0.13 Ωcm2 (750 °C) | 0.17 ± 0.02 Ωcm2 (650 °C) 0.08 ± 0.01 Ωcm2 (750 °C) | Equation (29) |
Electrochemically active thickness | 5.4 µm (650 °C) 6.4 µm (750 °C) | 4.4 ± 0.4 µm (650 °C) 4.2 ± 0.4 µm (750 °C) |
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Nenning, A.; Bischof, C.; Fleig, J.; Bram, M.; Opitz, A.K. The Relation of Microstructure, Materials Properties and Impedance of SOFC Electrodes: A Case Study of Ni/GDC Anodes. Energies 2020, 13, 987. https://doi.org/10.3390/en13040987
Nenning A, Bischof C, Fleig J, Bram M, Opitz AK. The Relation of Microstructure, Materials Properties and Impedance of SOFC Electrodes: A Case Study of Ni/GDC Anodes. Energies. 2020; 13(4):987. https://doi.org/10.3390/en13040987
Chicago/Turabian StyleNenning, Andreas, Cornelia Bischof, Jürgen Fleig, Martin Bram, and Alexander K. Opitz. 2020. "The Relation of Microstructure, Materials Properties and Impedance of SOFC Electrodes: A Case Study of Ni/GDC Anodes" Energies 13, no. 4: 987. https://doi.org/10.3390/en13040987
APA StyleNenning, A., Bischof, C., Fleig, J., Bram, M., & Opitz, A. K. (2020). The Relation of Microstructure, Materials Properties and Impedance of SOFC Electrodes: A Case Study of Ni/GDC Anodes. Energies, 13(4), 987. https://doi.org/10.3390/en13040987