IWE slide: 1, 9/17/2009

Transcription

1 International Symposium on Diagnostics Tools for Fuel Cell Technologies Impedance Spectroscopy as a Diagnosis Tool for SOFC Stacks and Systems André Weber, Dino Klotz, Volker Sonn, Ellen Ivers-Tiffée () Adenauerring 20b, Karlsruhe, Germany source: slide: 1, 9/17/2009

2 Control an Diagnosis of SOFC-Stacks and Systems Stack Monitoring by Impedance Spectroscopy Control of parameters critical for a failure free operation of stack and system stack performance and efficiency stack temperature(s) reformer temperature(s) steam actual to steam carbon to carbon ratio / λratio POx -value / λ POx -value fuel (reformate) composition oxidant and fuel flow rates oxidant and fuel temperatures at gas inlet oxidant and fuel pressures inlet at gas inlet exhaust gas composition (remaining CO, HC s)... Monitoring of the internal resistance of the stack by electrochemical impedance spectroscopy source: ENBW, Delphi slide: 2, 9/17/2009

3 Impedance Spectroscopy Materials, (Model-) Electrodes and Single Cells Electrochemical Impedance Spectroscopy H. Gerischer, Elektrodenpolarisation bei Überlagerung von Wechselstrom und Gleichstrom. Z. Elektrochem., 58, 9, 278, 1954 source: see above slide: 3, 9/17/2009

4 Impedance Spectroscopy Materials, (Model-) Electrodes and Single Cells Electrochemical Impedance Spectroscopy Analysis of Solid Electrolytes by Impedance Spectroscopy H. Gerischer, Elektrodenpolarisation bei Überlagerung von Wechselstrom und Gleichstrom. Z. Elektrochem., 58, 9, 278, 1954 J. E. Bauerle, Study of solid electrolyte polarization by a complex admittance method, J. Phys. Chem. Solids 30, 2657, 1969 source: see above slide: 4, 9/17/2009

5 Impedance Spectroscopy Materials, (Model-) Electrodes and Single Cells Electrochemical Impedance Spectroscopy Analysis of Solid Electrolytes by Impedance Spectroscopy Analysis of Electrode Microstructure and Degradation Behaviour H. Gerischer, Elektrodenpolarisation bei Überlagerung von Wechselstrom und Gleichstrom. Z. Elektrochem., 58, 9, 278, 1954 J. E. Bauerle, Study of solid electrolyte polarization by a complex admittance method, J. Phys. Chem. Solids 30, 2657, 1969 T. Kawada, N. Sakai, H. Yokokawa, M. Dokiya, M. Mori, T. Iwata, Characteristics of Slurry-Coated Nickel Zirconia Cermet Anodes for Solid Oxide Fuel Cells, J. Electrochem. Soc., 137, 3042, 1990 source: see above slide: 5, 9/17/2009

6 Impedance Spectrum of an Anode Supported Single Cell anode supported single cell (Forschungszentrum Jülich) Ni-8YSZ substrate (50x50x1mm³) Ni-8YSZ anode functional layer 8YSZ electrolyte (~ 10 µm) CGO interlayer (~ 7 µm) LSCF cathode (10x10mm²) spectrum: -Z (f) Z / Ω cm² Z / Ω cm² Z / Ω cm² f / Hz 2 or more electrochemical processes??? high resolution impedance data analysis required!!! cell type: ASC el. area: 1 cm² fuel: H 2 (9.4% H 2 O), 250 sccm oxidant: air, 250 sccm source: slide: 6, 9/17/2009

7 Impedance Data Analysis Distribution of Relaxation Times (DRT) ideal: R 1 R 2 C 1 C 2 γ R 1 R ges τ 1 τ 2 R 2 R ges τ γ real: Process 1 (RQ/CPE) Process 2 (RQ/CPE) τ 1 τ 2 τ γτ () Z pol( ω ) = R dτ 1+ jωτ pol 0 γ(τ): Distribution function of relaxation times H. Schichlein et al., Deconvolution of Electrochemical Impedance Spectra for the Identification of Electrode Reaction Mechanisms in Solid Oxide Fuel Cells, J. Appl. Electrochemistry, 32, 8, 875, (2002) source: slide: 7, 9/17/2009

8 Impedance Spectrum of an Anode Supported Single Cell Distribution of Relaxation Times Z / Ω cm² Z / Ω cm², g(f) / Ω sec spectrum: -Z (f) DRT: g(f) source: Z / Ω cm² high resolution data analysis by the DRT 5 processes resovable perform impedance measurements at varying operating conditions! A. Leonide et al., J. Electrochem. Soc, 155 (1), pp. B36-B41, (2008). f / Hz cell type: ASC el. area: 1 cm² fuel: H 2 (9.4% H 2 O), 250 sccm oxidant: air, 250 sccm slide: 8, 9/17/2009

9 Analysis of Electrochemical Processes in an ASC Variation of Operating Temperature g(f) / Ohm*sec T / C P 1A P 2C P 2A P 3A ASR / Ω cm² temperature/ C R 2A +R 3A : anode electrochemistry R 2C : cathode electrochemistry R 1A : gas diffusion E a,2a+3a =1.30 ev E a,2c =1.43 ev source: up to 5 different electrochemical processes resolvable impedance values in between 10 and 1000 mω cm² A. Leonide et al., J. Electrochem. Soc, 155 (1), pp. B36-B41, (2008) f / Hz 1000 K/T T = C fuel: H 2, 250 sccm p(h 2 O) = bar ox.: air, 250 sccm slide: 9, 9/17/2009

10 Analysis of Electrochemical Processes in an ASC Impedance Model of a Single Cell Z / Ω cm 2 Abbreviation P 1C Q( ω) = Q 3A Q 2A Q 1C -0.1 ( jω) n Y0 α tanh ( jωt) ZW ( ω) = RW α ( jωt) P ( ) Z 3A 0 ZG ω = P 2A P k+ jω 2C P 1A f r, ASR R Hz, mωcm 2 R 3A R 2A R 2C R 1A R 1C CPE2 CPE3 CPE1 dependence p(o 2 ) cell type: ASC el. area: 1 cm² fuel: H 2 (9.4% H 2 O), 250 sccm oxidant: air, 250 sccm T = 717 C OCV gas diffusion (<< 10 mω cm² in air) impedance spectrum CNLS-Fit anodic cathodic Z / Ω cm 2 electrode process / physical origin P 2C Hz, mωcm 2 p(o 2 ), T oxygen surface exchange kinetics and O 2- - diffusivity P 1A Hz, mωcm 2 p(h 2 ), p(h 2 O) gas diffusion (anode substrate) P 2A P 3A source: khz, mωcm khz, mωcm 2 p(h 2 ), p(h 2 O),T p(h 2 ), p(h 2 O), T A. Leonide et al., J. Electrochem. Soc, 155 (1), pp. B36-B41, (2008). gas diffusion coupled with charge transfer reaction and ionic transport (AFL: anode functional layer) slide: 10, 9/17/2009

11 Electrochemical Impedance Spectroscopy for Stacks Impact of Cell and Stack Size anode supported single cell 1 cm² active electrode area single cell 100 cm² mω impedance of a few mω per cell gradients in temperature, gas composition etc. ASR: Ω cm² 80 mω... 2 Ω gas conversion impedance additional noise and error sources source: slide: 11, 9/17/2009

12 Electrochemical Impedance Spectroscopy for Stacks Testing Equipment 3 kw el SOFC stack 60 cells a 100 cm² U stack = 42 V I stack = 71.4 A target values 1260/ Power Booster IM6 + PP 2xx VersaSTAT4 + Power Booster CLB 500 TrueData-EIS frequency range 1 mhz... 1 MHz 10 µhz khz 10 µhz khz (10 µhz... 1 MHz) 10 µhz khz 200 µhz khz impedance range mω 10 µω... 1 kω 1 µω... 1 kω n.s. n.s. 0.1 mω Ω accuracy (error at 1 mω / 100 khz) 1 % 30 % / 30 (@ 10 mω) 0.25 % (f, Z not specified) n.s. 2 % / 2 1 % / 1 max. bias voltage ± 5 / 10 / [V] max. bias current [A] ± 40 / 20 / max. power diss. [kw] n.s. 0.5 limitations due to the testing equipment 150 source: websites and datasheets of manufacturers slide: 12, 9/17/2009

13 Analysis of Electrochemical Processes in an ASC Gas Conversion Impedance (at decreased fuel flow rate) 0.04 gas diffusion (anode substrate) 250 ml/min g(f)/ωs gas conversion (gas channel) cathode (+ 2 nd peak of anode gas diffusion) anode 150 ml/min 100 ml/min 50 ml/min k 100k 1M f/hz the gas conversion impedance will be included in the stack impedance source:, A. Leonide, presented at European SOFC Forum, Lucerne 2008 slide: 13, 9/17/2009

14 Electrochemical Impedance Spectroscopy for Stacks 2-dimensional Impedance Model oxidant (air) cathode electrolyte anode H % H 2 O fuel H % H 2 O U R 2C (x) R 1C (x) R 0 (x) P 3A (x) based on the impedance model of a single cell 1 single cell impedance units along a gas channel impact of the gas utilization on the local impedance is considered R 2A (x) R 1A (x) dynamics of gas conversion have to be taken into account 1 A. Leonide et al., J. Electrochem. Soc, 155 (1), pp. B36-B41, (2008). D. Klotz et al., accepted for publication in ECS Transactions (2009) source: slide: 14, 9/17/2009

15 2-dimensional Impedance Model Local Impedance Spectra and Stack Impedance U cell = 750 mv source: simulated fuel utilization along the gas channel D. Klotz et al., accepted for publication in ECS Transactions (2009) simulated local impedance spectra and impedance spectrum of the stack slide: 15, 9/17/2009

16 2-dimensional Impedance Model for Stacks Space and Time Dependence of the Fuel Utilization β f t time [s] / ms operating parameters U cell U ampl. f AC L v gas 0,725 V 70 mv 50 Hz 10 cm 3 m/s β f,in 20% β f,out 80% source: slide: 16, 9/17/2009

17 Experimental Sulzer Hexis Stack Test Bench Sulzer Hexis stack test bench 5 cell stack 100 cm² electrode area operating on pipeline gas desulfurization (disengageable) catalytic partial oxidation controlled gas flows controlled stack temperature (not thermal self-sustaining) variable interconnect / flow field geometry testing of different MEAs and MICs possible Impedance spectroscopy? source:, presented at Solid State Ionics 15 (2005) slide: 17, 9/17/2009

18 Sulzer Hexis Stack Test Bench Modifications for Impedance Spectroscopy initial state impedance / mω phase / EIS-Equipment Zahner IM6 & PP200 potentiostat impedance range: 1 µω... 1 kω frequency range: 10 µhz khz dc current: 20 A frequenzy / khz Significant interferences at frequencies above 10 khz source:, presented at Solid State Ionics 15 (2005) slide: 18, 9/17/2009

19 Sulzer Hexis Stack Test Bench Modifications for Impedance Spectroscopy Modifications adaptation of voltage probes & current lines to decrease mutual inductances impedance converters for dc voltage metering multiplexer for single cell / stack impedance measurement impedance / mω modified phase / frequenzy / khz Low interferences over the whole frequency range source:, presented at Solid State Ionics 15 (2005) slide: 19, 9/17/2009

20 Stack Diagnosis Impedance Spectra of Stacks and Cell Units -Z / Ohm Stack Stack A: Sulzer Hexis MEAs & MICs 2002 Stack B: Sulzer Hexis MEAs & MICs cell stack electrode area: 100 cm² oxidant: air, 1000 g/h temperature: 930 C fuel: natural gas, 20 g/h fuel processing: CPO, λ POx : 0.28 Z / Ohm Single Cell Units 1 to 5 Stack A cell 1 cell 2 cell 3 cell 4 cell source:, presented at Solid State Ionics 15 (2005) Z / Ohm Impedance spectra of the stack provide an averaged ohmic and polarisation resistance Impedance spectra of the individual cell units provide more detailed information Z / Ohm Stack B Z / Ohm Z / Ohm cell 1 cell 2 cell 3 cell 4 cell 5 slide: 20, 9/17/2009

21 System Diagnosis Impedance Data Analysis by parametric extended DRT Im Z / Ω impedance spectra Re Z / Ω khz 10 Hz gas conversion impedance 12 mhz DRT γ(τ) / Ω s log(1s/τ) γ(τ) τ L L R L source:, presented at Solid State Ionics 15 (2005) τ RQ, n RQ, R RQ RQ - element Z(DRT) Series resistance inductance slide: 21, 9/17/2009

22 System Diagnosis Impact of CPOx Reformer Temperature on cell voltage and n RQ U / V Cell 1 voltage cell 1 no dependency in cell voltage T pox / C n CPE RQ exponent n RQ Variation in fuel composition is detected theory: n 1 for pure H T pox / C source:, presented at Solid State Ionics 15 (2005) slide: 22, 9/17/2009

23 Conclusions and Outlook Future Research Activities - Diagnostic Tools for FC-Technologies Impedance spectroscopy is a powerful tool to analyze SOFC stacks but The available testing equipment hardly fulfills the requirements The stack testing facilities have to be designed for impedance spectroscopy Complex impedance models are required to understand the stack impedance To do s: Development of impedance analyzers for (SOFC-) stacks Development of tools for stack impedance modeling and data analysis Standardized testing procedures for stack impedance measurement and data analysis source: slide: 23, 9/17/2009

24 Impedance Spectroscopy and Impedance Data Analysis -References H. Schichlein, M. Feuerstein, A. C. Müller, A. Weber, A. Krügel and E. Ivers-Tiffée, "System identification: a new modelling approach for SOFC single cells", Proceedings of the Sixth International Symposium on Solid Oxide Fuel Cells (SOFC-VI), pp (1999). H. Schichlein, Experimentelle Modellbildung für die Hochtemperatur-Brennstoffzelle SOFC, Aachen: Verlag Mainz (2003). E. Ivers-Tiffée, A. Weber and H. Schichlein, "Electrochemical impedance spectroscopy", in W. Vielstich, H. A. Gasteiger, and A. Lamm (Eds.), Handbook of Fuel Cells - Fundamentals, Technology and Applications, Chichester: John Wiley & Sons Ltd, pp (2003). E. Ivers-Tiffée, A. Weber and H. Schichlein, "O2-reduction at high temperatures: SOFC", in W. Vielstich, H. A. Gasteiger, and A. Lamm (Eds.), Handbook of Fuel Cells - Fundamentals, Technology and Applications, Chichester: John Wiley & Sons Ltd, pp (2003). H. Schichlein, A. C. Müller, M. Voigts, A. Krügel and E. Ivers-Tiffée, "Deconvolution of electrochemical impedance spectra for the identification of electrode reaction mechanisms in solid oxide fuel cells", Journal of Applied Electrochemistry 32, pp (2002). V. Sonn, A. Leonide and E. Ivers-Tiffée, "Towards Understanding the Impedance Response of Ni/YSZ Anodes", ECS Transactions 7, pp (2007). A. Leonide, V. Sonn, A. Weber and E. Ivers-Tiffée, "Evaluation and Modelling of the Cell Resistance in Anode Supported Solid Oxide Fuel Cells", ECS Transactions 7, pp (2007). A. Leonide, V. Sonn, A. Weber and E. Ivers-Tiffée, "Evaluation and modeling of the cell resistance in anode-supported solid oxide fuel cells", J. Electrochem. Soc. 155, p. B36-B41 (2008). source: slide: 24, 9/17/2009

25 Acknowledgements Many Thanks to our Partners and Funding Organizations Fraunhofer Institut für Silicatforschung NSA source: & Partners slide: 25, 9/17/2009