Impedance spectroscopy of symmetric cells for SOFC research

Transcription

1 Impedance spectroscopy of symmetric cells for SOFC research Shany Hershkovitz, Sioma Baltianski and Yoed Tsur Department of Chemical Engineering Technion, Haifa, Israel

2 Outline A short update on SOFC technology Impedance spectroscopy A very short introduction Analysis strategies Falling in love with a model could be dangerous Some measurements on symmetric cells

3 SOFC short update Sought for stationary applications Electrolyte of dense 8YSZ or ScSZ Anode: porous cermet (Ni/8YSZ or Ni/GDC) Cathode: (La,Sr)(Co,Fe)O 3 δ (LSCF) is replacing good-old LSM for lower operating T In the planar design: two key technologies: Anode-supported: 1.5 A/cm 700mV, T=600 C (Juelich) Electrolyte-supported: Lower performance but higher robustness

4 Major problem: degradation rate in stacks Target: several 10,000 of operating hours. Present: degradation rates of ~5 to 10 mv/kh, or 0.5-1%/kh. Most critical degradation mechanisms: Ni microstructure: agglomeration, etc. Ni response to fuel impurity (e.g., S) Cathode microstructure: sintering Cathode material changes, especially Cr poisoning Thermomechanical problems; redox cycles.

5 A very short introduction to IS f Signal Supply I V Z V ( ω) Z( ω) = where ω= 2πf I( ω)

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7 Kramers-Kronig Both the real and imaginary parts are frequency dependant, and related through the Kramers-Kronig transforms This can (and should!) serve as validation 2 xz ( x) ωz ( ω) Z ( ω) Z ( ) = dx 2 2 π x ω 2 ω xz ( x) ωz ( ω) Z ( ω) = dx 2 2 π x ω 0 0

8 Distribution function of relaxation times (DFRT) Any system with a finite number of time constants can be put into the following form: n n gk Z( ω) = Z( ) + R where gk = 1 1+ iωτ k= 1 k k= 1 Distribution of time constants is the extension: n. Then we have: g( τ ) dτ Z( ω) = Z( ) + R where g( τ ) dτ = 1 1+ iωτ 0 0 This is a general representation of an inhomogeneous Fredholm equation of the second kind. A simpler case is to set the impedance in the form of the an homogenous Fredholm equation of the first kind and thus: Z ( ω ) = R 0 g( τ) dτ 1+ iωτ

9 Equivalent circuits and DFRT Equivalent circuits are the most common approach for analyzing impedance data. In many cases one can find the DFRT from a given equivalent circuit. An equivalent circuit like this: has a DFRT of two delta functions. In most real cases: distributed elements should be used -> DFRT with peaks. Disadvantage- Equivalent circuits are not unique. Falling in love with the model.

10 Discrepancy-complexity plot 0 [Baltianski and Tsur, J. Electroceramis 2003] Log(Discrepancy) We take the discrepancy between the prediction of -2 2 the model and the data (e.g., χ ) on a log scale vs. the model s complexity (# of adjustable parameters). -4 Each point represents a solution. Look for the knee in this plot. Complexity Surprisingly amount of info can be inferred from that!

11 ISGP in action

12 Brief introduction to Solid oxide fuel cells In order to study the electrical properties of the interface between the cathode and the electrolyte a system of a symmetric cell was examined by impedance spectroscopy. Pt contact Cathode- LSCF Electrolyte- GDC Cathode- LSCF Pt contact

13 Symmetric Cell-LSCF GDC LSCF Electrolyte thickness

14 Symmetric Cell-LSCF GDC LSCF O 2 /Ar varying concentrations at constant flow rate

15 Symmetric Cell-LSCF GDC LSCF In general a DFRT composed of four different peaks is a characteristic model for this system: 3 peaks within the measured frequency range and one for Z( ). Arrhenius Plot of the conductivity of each peak. Four different activation energies are calculated.

16 Finite Length Warburg (FLW) Element Impedance related to diffusion of particles in a finite length region can be described using FLW. Applying ISGP on synthetic FLW with white noise yields two closely positioned peaks The unique shape of the DFRT can help identify diffusion processes even when the impedance spectra do not show it clearly.

17 Real system case study- Pt GDC Pt The resulting DFRTs of GDC consist of four different peaks, which have been observed at all temperatures and all oxygen partial pressures. It is also clear that the two closely positioned peaks mentioned above as a sign of Warburg diffusion process (peaks II+III) can be observed.

18 Summary It is better to analyze IS results by finding the distribution function of time constants Symmetric cells as a tool. ISGP in action For LSCF/GDC/LSCF system: (more experiments are needed, but:) the peak at the lowest frequencies probably stems from a process in the LSCF, while the two other peaks resemble Warburg behavior, mostly from the interface.