Riegraf, Matthias

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www.DLR.de  • Chart 1Matthias RiegrafGerman Aerospace Center (DLR), Stuttgart, Germany12.10.2016Sulfur poisoning effect on Ni/CGO anodesMotivation and aim of the workExperimental procedureResults Short-term sulfur poisoning Long-term sulfur poisoningSummary and conclusionswww.DLR.de  • Chart 2OutlineMotivation and aim of the workExperimental procedureResults Short-term sulfur poisoning Long-term sulfur poisoningSummary and conclusionswww.DLR.de  • Chart 3Outline• Sulfur-containing impurities in dieselreformate used for APU• Ni-based anodes in state-of-the-art MSC• Higher sulfur tolerance of Ni/CGO than Ni/YSZ• CGO: MIEC + catalytically active for H2 oxidation [2 – 4]• Mechanism is not fully understoodwww.DLR.de  • Chart 4Elucidation of sulfur poisoning mechanism on Ni/CGO-basedanodesGoal:Motivation and aim of the work[1] Kavarucu et al., Journal of Power Sources, 217, (2012), 364; [2] Primdahl, Mogensen, Solid State Ionics,152, (2002), 597; [3] Nakamura, J.Electrochem. Soc., 155, (2008), B563; [4] Chueh et al., Nat. Mater., 11, (2011), 1550 3 6 9 12 1560708090100 Ni/CGO10Voltage U/U0 / %time / hNi/8YSZ2 ppm H2S, 850 °C, 225 mA/cm2CGOYSZOXOOXOeeH2O2H O2NiH2 H O2 CO CO2CH4 O2YSZeO2-H O2OH-Active TPB Active surfaceNi/YSZ based anodes Ni/CGO based anodesMotivation and aim of the work• Is Ni pure electronic conductor? [1 – 3]• Sulfur poisoning of pure CGO electrode possible [4]• Sulfur poisoning of CGO or Ni?[1] Chueh et al., Nat. Mater., 11, (2011), 155; [2] Feng et al., Nat. Commun., 5, (2014),1; [3] Chueh et al., Solid State Ionics, 179, (2008), 1036. [4] Mirfakhraei et al., J. Power Sources, 243, (2013), 95;www.DLR.de  • Chart 5Motivation and aim of the workExperimental procedureResults Short-term sulfur poisoning Long-term sulfur poisoningSummary and conclusionswww.DLR.de  • Chart 6Outlinewww.DLR.de  • Chart 7Experimental Procedure Systematic parameter study of short-term poisoning at different current density and temperatures Long-term experiments In-situ monitoring with electrochemical impedance spectroscopyNiCu5/CGO40 anode (25 µm, HEXIS)• 160 µm 6ScSZ electrolyte• 70 µm LSM/ScSZ cathodeNi/CGO10 anode (25 µm, Kerafol)• 160 µm 10Sc1CeSZ electrolyte• 65 µm LSM/ScSZ cathode0 5 10 15 200.650.700.750.800.85time / htime / hVoltage / V850 °C, 0.5 A•cm–201020304050 H2S concentration / ppm0 2 4 60.760.780.800.82Voltage / Vwww.DLR.de  • Chart 8T = 850°C , i = 0.5 A•cm–2Anode gas: 97 % H2, 3 % H2O + 1 – 20 ppm H2SPoisoning with increasing H2S concentrationCompleteregeneration after 24 hImpedancemeasurementExperimental Procedure:Testing protocol short-term poisoningExperimental Procedure: Testing matrixwww.DLR.de  • Chart 9 > Lecture > Author  •  Document > DateCurrent density / A•cm-2OCV 0.25 0.5 0.75800 X850 X X X X900 XTemperature/ °C• 1 l/min: 97 % H2, 3 % H2O + 1, 2, 5, 10, 20 ppm H2S• Variation of temperature and current density• Ni/CGO10 and Ni/CGO40-based anodesMotivation and aim of the workExperimental procedureResults Short-term sulfur poisoning Long-term sulfur poisoningSummary and conclusionswww.DLR.de  • Chart 10Outlinewww.DLR.de  • Chart 11Ni/CGO10: Influence of current densityT = 850°CAnode gas: 1 l/min: 97% H2, 3% H2O + 1, 2, 5, 10, 20 ppm H2S• Saturation effect similar to sulfur coverage on Ni• Temkin isotherm: )R/sRT/)a(h(pp 0S00HSH 1 exp22  Sulfur poisoning of Ni surface?www.DLR.de  • Chart 12Ni/CGO40: Influence of current densityT = 850°CAnode gas: 1 l/min: 97% H2, 3% H2O + 1, 2, 5, 10, 20 ppm H2S• Mitifation effect at high current densities• pH2O decreases charge transfer resistancewww.DLR.de  • Chart 13Ni/CGO10: Influence of temperature• Temperatur mitigation effect on sulfur poisoning• Sulfur desorption / Bulk diffusion?T = 850°CAnode gas: 1 l/min: 97% H2, 3% H2O + 1, 2, 5, 10, 20 ppm H2S10-1 100 101 102 103 104 1050.000.050.100.150.20 0 ppm 20 ppm–Zim / •cm2f / HzOCV, 850 °C10-2 10-1 100 101 102 103 104 1050.000.050.100.150.200.250.30OCV, 850°C  0 ppm 20 ppm–Zim / •cm2f / Hzwww.DLR.de  • Chart 14T = 850°C, OCV, 97 % H2, 3 % H2O + 1 – 20 ppm H2SNi/CGO40-based cell:• Influence of sulfur on anode surfaceprocess at ~ 50 Hz• No influence on LF processNi/CGO10-based cell:• Sulfur influence at ~ 0.2 Hz• TPB charge transfer process of Ni/YSZ: ~ 103 Hz• Characteristic frequency of surface chargetransfer process of pure CGO10Ni/CGO10 vs. Ni/CGO40: Impedance analysisChange of peak frequency of sulfur-affected process by 2 orders of magnitudewww.DLR.de  • Chart 15Ni/CGO10 vs. Ni/CGO40: Influence oftemperatureNi/CGO10 Ni/CGO40• Ni/CGO10: f = 1/(2ߨRC)• T ↑ R ↓  f ↑• Ni/CGO10:  f ↓  C ↑ = Capacitance is strongly temperature-dependent• Differential imgainary impedance plot Zim(1 ppm)-Zim(0 ppm)• Isolation of sulfur influence on anode charge transfer resistancewww.DLR.de  • Chart 16Ni/CGO10 vs. Ni/CGO40: Influence of currentdensity• Ni/CGO10: Larger frequency shift (>1 order of magnitude)• Capacitance is also dependent on pO2Ni/CGO10 Ni/CGO40• Differential imgainary impedance plot Zim(1 ppm)-Zim(0 ppm)• Isolation of sulfur influence on anode charge transfer resistancewww.DLR.de  • Chart 17• Chemical capacitance at CGO surface due to mixed valent Ce ions and charged adsorbates (e.g. OH−)• Rate-limiting step:	Ce4+ + OHCGO −	 → 	Ce3+	 ൅ 	OHCGO [1,2] • Ce3+ concentration in CGO10 bulk increases with temperature anddecreasing pO2 [3,4]• Gd doping reduces amount of Ce3+ through stabilization of Ce4+• Reflected by lower electronic conductivity• Reason for lower capacitance valueCe3+ Ce4+Discussion: Ni/CGOPeak frequency shift of the same process by 2 orders of magnitude[1] Feng et al., Nat. Commun., 5, (2014),1; [2] Chueh et al., Solid State Ionics, 179, (2008), 1036; [3] Chueh et al. Phys. Chem. Chem. Phys. 11, (2009), 8144 [4] Decaluwe, J. Phys. Chem. C, 114 (2010),19853.. Motivation and aim of the workExperimental procedureResults Short-term sulfur poisoning Long-term sulfur poisoningSummary and conclusionswww.DLR.de  • Chart 18Outline0 200 400 600 800 10000.40.50.60.70.8900 °C, 0.75 A•cm–2 0 ppm 1 ppm 10 ppmVoltage / Vtime / hwww.DLR.de  • Chart 19• Sulfur-inducedirreversible degradation• 10 ppm > threshold?• Combination of lowpotential and sulfurinfluence?Long-term sulfur poisoningMore experimentsneededT = 900 °C, 0.75 A•cm–2 , 97 % H2, 3 % H2O + 0, 1, 10 ppm H2SNiCu5/CGO40 anode0.2 0.3 0.4 0.5 0.6 0.7-0.050.000.050.100.150.2010 ppm H2S, 900°C, 0.75 A•cm–2 4 h 870 h–Zim / •cm2Zre / •cm20.2 0.3 0.4 0.5 0.6 0.7-0.050.000.050.100.150.200 ppm H2S, 900°C, 0.75 A•cm–2–Zim / •cm2Zre / •cm2 4 h 870 hwww.DLR.de  • Chart 20• Increasing Ohmic resistancePost mortem analysis necessaryT = 900 °C, 0.75 A•cm–2 , 97 % H2, 3 % H2O + 0, 1, 10 ppm H2SNiCu5/CGO40 anodeLong-term sulfur poisoning: Impedance analysis10-2 10-1 100 101 102 103 104 105 1060.000.020.040.0610 ppm H2S, 900 °C, 0.75 A•cm–2–Zim / •cm2 4 h 870 hfrequency f / Hz• Slight increase in anode chargetransfer resistancewww.DLR.de  • Chart 21Long-term sulfur poisoning: FIB/SEMTest TPB density [µm-2]Percolation NiPercolation CGO Percolation pore0 ppm 13.75 97.83 98.11 99.311 ppm 13.78 96.83 95.25 99.8710 ppm 14.07 94.87 96.35 99.64• No signficant influence ofsulfur on anode microstructure• Electrical Conductivity loss ofCGO40 for Redox cycling [1]Same for sulfur?[1] Iwanschitz et al., PhD Thesis, RWTH Aachen, (2012)FIB/SEM analysis and 3D reconstruction by Universität OldenburgMotivation and aim of the workExperimental procedureResults Short-term sulfur poisoning Long-term sulfur poisoningSummary and conclusionswww.DLR.de  • Chart 22Outlinewww.DLR.de  • Chart 23• Extensive characterization of sulfur poisoning of Ni/CGO10 and Ni/CGO40-based anodes• Reversible short-time poisoning• Ni/CGO10 vs. Ni/CGO40: Capacitance of anode surface processchanges by ~ 2 orders of magnitude• Ni/CGO40: Long-term stability demonstrated for 1 ppm H2S, irreversible degradation at 10 ppm• Next step: Sulfur poisoning of reformate-fuelled SOFC• Investigation of model electrodes required for deeperunderstanding of the mechanismSummary and conclusionswww.DLR.de  • Chart 24• Financial support from the German Ministry of Education andResearch (BMBF) in the framework of the “SOFC Degradation” (Verbundvorhaben SOFC Degradation: Analyse der Ursachen und Entwicklung von Gegenmaßnahmen) project. Thank you for your attention!Acknowledgments

Sulfur poisoning effect on Ni/CGO anodes

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