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Mitigation of Chromium Impurity Effects and Degradation in Solid - - PowerPoint PPT Presentation

Nov 23, 2022 15 likes •298 views

Mitigation of Chromium Impurity Effects and Degradation in Solid Oxide Fuel Cells Ruofan Wang, Zhihao Sun, Yiwen Gong, Uday Pal, Soumendra Basu and Srikanth Gopalan Division of Materials Science and Engineering Boston University 1 Outline

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Mitigation of Chromium Impurity Effects and Degradation in Solid Oxide Fuel Cells

Ruofan Wang, Zhihao Sun, Yiwen Gong, Uday Pal, Soumendra Basu and Srikanth Gopalan Division of Materials Science and Engineering Boston University

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Outline

  • Introduction
  • Cell Fabrication
  • Summary of Test Conditions
  • Electrochemical Degradation
  • Microstructural Evolution
  • Degradation Mechanisms
  • Development of Oxide Protective Coatings
  • Summary
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Introduction

  • Background

– Chromium (Cr) poisoning of cathode in solid oxide fuel cells (SOFCs) is considered to be one of the major reasons for performance degradation – For different cathode materials, the mechanisms of Cr-poisoning are complex.

  • Project Goals

– Compare the degradation phenomena in LSM, LSF, and LNO (La2NiO4) - based cathodes caused by Cr- poisoning – Through the comparative study, investigate the mechanisms of Cr-poisoning in these three types of cathodes in realistic full cell operating conditions – Design mitigating strategies based on applying protective coatings to ferritic stainless steel interconnects

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Cell Fabrication

LSM: (La0.8Sr0.2)0.95MnO3-x LSF: (La0.8Sr0.2)0.95FeO3-x GDC: (Gd0.10Ce0.90)O2-x

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Summary of Test Conditions

Conditions Cathode Atmosphere Current Condition Cells

1 Dry Air Open Circuit LSM-1 LSF-1 2 Humidified Air (10% H2O) Open Circuit LSM-2 LSF-2 3 Dry Air Galvanostatic (0.5 A/cm2) LSM-3 LSF-3 4 Humidified Air (10% H2O) Galvanostatic (0.5 A/cm2) LSM-4 LSF-4

  • General test conditions:

– Fuel: 98% H2+2% H2O (300 cc/min): Fixed – Oxidant: Air (1000 cc/min) – Interconnect: Crofer 22 H mesh (used as cathodic current collector in cell tests)

  • Conditions varied in the study:
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Electrochemical Degradation: V-i

Condition 1: Dried Air + OCV Condition 2: 10% Humidified Air + OCV Condition 3: Dried Air + 0.5 A/cm2 Condition 4: 10% Humidified Air + 0.5 A/cm2 LSM-Based cells LSF-Based cells

LSM-1 LSM-2 LSM-3 LSM-4 LSF-1 LSF-2 LSF-3 LSF-4

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Electrochemical Degradation: V-i

LSM-based cell performance vs. Time LSF-based cell performance vs. Time

  • Cr-poisoning is more deleterious in LSM-based cell than that in LSF-based cell.
  • In the case of LSM-based cell:

– Current load (0.5 A/cm2) accelerates the degradation – Presence of humidity in air promotes degradation under current load

  • In the case of LSF-based cell:

– Current load (0.5 A/cm2) slightly improved the cell performance (presumably due to cell break-in) – In humidified air, performance deteriorated under OCV condition but improved under current load

Wet Air + OCV Dry Air + 0.5 A/cm2 Wet Air + 0.5 A/cm2 Dry Air + OCV

2.7% 2.4%

  • 29.0%
  • 92.5%

0.7%

  • 9.5%

5.7% 9.6%

  • 100.0%
  • 90.0%
  • 80.0%
  • 70.0%
  • 60.0%
  • 50.0%
  • 40.0%
  • 30.0%
  • 20.0%
  • 10.0%

0.0% 10.0%

Performance Change Performance Change in 120 h in Different Conditions LSM-Based LSF-Based

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Electrochemical Degradation: EIS

Conditions LSM-Based LSF-Based Condition 1: Dried Air + OCV Condition 2: Humidified Air + OCV Condition 3: Dried Air + 0.5 A/cm2 Condition 4: Humidified Air + 0.5 A/cm2

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Electrochemical Degradation: EIS

LSM-based cell structure LSF-based cell structure

Air + 10% H2O Dried Air Dried Air Air + 10% H2O

  • EIS consistent with the V-i results. In 10% humidified air, it shows increasing polarization of LSM-based

cell and decreasing polarization of LSF-based cell.

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Microstructural Evolution: LSM-Based

LSM-1: Dry Air + OCV LSM-3: Dry Air + Current LSM-4: Humidified Air + Current LSM-2: Humidified Air + OCV

Cr-containing deposits are Cr,Mn-rich, suggesting (Cr,Mn)3O4 spinel phases

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Microstructural Evolution: LSM-Based

  • Cr intensity at cathode/electrolyte interface: LSM-4 > LSM-3 > LSM-2 ≈ LSM-1
  • Cr deposition was promoted by current and extended to TPB’s away from the cathode/electrolyte interface.

Criterion for quantifying Cr distribution in LSM Cross section of LSM-based cathode Cr-enrichment profile in the LSM-based cathode

* Wang, R., Pal, U. B., Gopalan, S., & Basu, S. N. (2017). Journal of The Electrochemical Society, 164(7), F740-F747.

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Microstructural Evolution: LSF-Based

25 μm Cr Mapping Cr Mapping 25 μm Cr Mapping Cr Mapping

LSF Paste LSF Paste LSF LSF-GDC GDC

YSZ

LSF LSF-GDC GDC

YSZ

LSF-2: 10% Humidified Air + OCV LSF-1: Dried Air + OCV 25 μm 25 μm

LSF Paste LSF Paste LSF LSF-GDC GDC

YSZ

LSF LSF-GDC GDC

YSZ

LSF-3: Dried Air + 0.5 A/cm2 LSF-4: 10% Humidified Air + 0.5 A/cm2

Most of Cr is distributed at the surface of cathode OCV condition: Cr distribution is homogeneous in the bulk of cathode Cr is distributed at the surface of cathode and also cathode/electrolyte interface

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Microstructural Evolution: LSF-Based

25 μm 25 μm 25 μm 25 μm

LSF Paste LSF Paste LSF Paste LSF Paste LSF LSF-GDC GDC

YSZ

LSF LSF-GDC GDC

YSZ

LSF LSF-GDC GDC

YSZ

LSF LSF-GDC GDC

YSZ

LSF-2: 10% Humidified Air + OCV LSF-1: Dried Air + OCV LSF-3: Dried Air + 0.5 A/cm2 LSF-4: 10% Humidified Air + 0.5 A/cm2

Cr Line Scan Sr Line Scan Cr Line Scan Sr Line Scan Cr Line Scan Sr Line Scan Cr Line Scan Sr Line Scan

Cr and Sr profiles do not match at the cathode/electrolyte interface

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Microstructural Evolution: LSF-Based

25 μm

LSF Paste LSF Paste LSF Paste LSF Paste LSF LSF-GDC GDC

YSZ

LSF LSF-GDC GDC

YSZ

LSF LSF-GDC GDC

YSZ

LSF LSF-GDC GDC

YSZ

LSF-2: 10% Humidified Air + OCV LSF-1: Dried Air + OCV LSF-3: Dried Air + 0.5 A/cm2 LSF-4: 10% Humidified Air + 0.5 A/cm2

Sr:Cr ≈ 1:2 (At%)

LSF contact paste

LSF current collective layer LSF current collective layer

LSF contact paste

LSF Paste LSF Paste

LSF contact paste

LSF current collective layer

LSF contact paste

LSF current collective layer

25 μm 25 μm 25 μm

Dense Sr-Cr-O phase Dense Sr-Cr-O phase

Sr:Cr ≈ 1:1 (At%)

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Microstructural Evolution: LSF-Based

25 μm

LSF Paste LSF Paste LSF Paste LSF Paste LSF LSF-GDC GDC

YSZ

LSF LSF-GDC GDC

YSZ

LSF LSF-GDC GDC

YSZ

LSF LSF-GDC GDC

YSZ

LSF-2: 10% Humidified Air + OCV LSF-1: Dried Air + OCV LSF-3: Dried Air + 0.5 A/cm2 LSF-4: 10% Humidified Air + 0.5 A/cm2 25 μm 25 μm 25 μm

LSF-GDC YSZ GDC LSF-GDC YSZ GDC Major amount Cr2O3 Major amount Cr2O3

Minor amount Sr,Cr-containing deposits Minor amount Sr,Cr containing deposits

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Degradation in LNO Cathodes

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Degradation Mechanisms

  • Effect of humidity on Cr evaporation:

Equilibrium Partial Pressure of Cr vapor species over Cr2O3 scale

Equilibrium Partial Pressure of Cr in 10% Humidified Air Equilibrium Partial Pressure of Cr in Dry Air

  • Cr vapor pressure in 10% humidified air is ~2-order-of-magnitude higher than that in dry air*.

* Wang, R., Würth, M., Pal, U. B., Gopalan, S., & Basu, S. N. (2017). Journal of Power Sources, 360, 87-97.

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Degradation Mechanisms

  • Effect of humidity on Cr distributions:

2SrCr2O4(s) + 2H2O(g) + 3O2(g)=2SrCrO4(s) + 2CrO2(OH)2(g) ----- (1)

  • r SrCr2O4(s) + 4H2O(g) + 2O2(g)=Sr(OH)4(s) + 2CrO2(OH)2(g) ----- (2)

Evaporation of Cr-deposits on the LSF surface:

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Oxide Protective Coatings

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XRD: a) CuMn1.8O4 powders b) after reduction anneal c) after 1h oxidation anneal

EPD Reduction annealing (1000 °C, 24 h) Oxidation annealing (850 °C, 1 h)

a b c

EPD Coating of CuMn1.8O4

  • Z. Sun et al, Journal of Power Sources,

378 (2018), 125-133.

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Cr Diffusion and Microstructure Evolution

750 ºC 100 h 750 ºC 950 h <1 μm ~ 2.1 μm 850 ºC 100 h 850ºC 100h + 800ºC 400h ~ 13.5 μm ~7.1 μm Particle Reaction layerNeedle structures

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TEM Analaysis of Protective Coatings

Mn Cu O

Mn Cu O Cr

Needle structures: Mn3O4 Particles in dense layer: Cr2O3

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Reaction between Cr2O3 and CuMn1.8O4 powders (800 °C, 10 h, in air)

Solubility of Cr2O3 in CuMn1.8O4

Cr2O3

Solubility

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1 10 100 500 600 700 800 900

Conductivity (S/cm) Temperature (o C)

(Cu,1.8Mn)1.3Cr1.7O4 (Cu,1.8Mn)1.8Cr1.2O4 (Cu,1.8Mn)2.4Cr0.6O4 Solubility limit

Electrical Conductivity of (Cu,Mn,Cr)3O4

* Zhu et al, Mater. Sci. Eng. A 348 (2003) 227–243

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Coating on complex geometry (mesh) and Electrochemical tests – LSM cells

Commercial CuMn2O4

Uncoated interconnect

Bare Commercial coating BU Coating

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Summary

  • LSM, LSF-GDC, and LNO-based cathodes have been tested against

chromium poisoning under load, and in the presence of 10% humidity

– LSF-GDC and LNO cathodes show excellent tolerance towards chromium poisoning compared to LSM – The differences in the mechanisms of degradation are still being worked out

  • High quality CuMn spinels have been applied using EPD to

complex geometries of ferritic stainless steel interconnects.

– The coatings are very effective in providing a barrier to Cr attack on LSM cathodes – The combination of LSF-GDC or LNO with CuMn protective coatings should provide excellent long term stability against Cr poisoning

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Publications

  • “Roles of humidity and cathodic current in chromium poisoning of Sr-doped LaMnO3-based cathodes in solid
  • xide fuel cells,” R Wang, M Würth, UB Pal, S Gopalan, SN Basu, Journal of Power Sources 360, 87–97
  • Chromium Poisoning Effects on Performance of (La,Sr)MnO3-Based Cathode in Anode-Supported Solid

Oxide Fuel CellsR Wang, UB Pal, S Gopalan, SN Basu, Journal of The Electrochemical Society 164 (7), F740-F747

  • Effect of Humidity and Cathodic Current on Chromium Poisoning of Sr-Doped LaMnO3-Based Cathode in

Anode-Supported Solid Oxide Fuel Cells, R Wang, M Würth, B Mo, UB Pal, S Gopalan, SN Basu, ECS Transactions 75 (42), 61-67

  • Chromium Poisoning of Cathodes in Solid Oxide Fuel Cells and its Mitigation Employing CuMn1.8O4 Spinel

Coatings on InterconnectsR Wang, Z Sun, Y Lu, UB Pal, SN Basu, S Gopalan, ECS Transactions 78 (1), 1665-1674

  • Mitigation of chromium poisoning of cathodes in solid oxide fuel cells employing CuMn1.8O4 spinel coating
  • n metallic interconnect, R Wang, Z Sun, UB Pal, S Gopalan, SN Basu, Journal of Power Sources 376, 100-

110

  • CuMn1.8O4 protective coatings on metallic interconnects for prevention of Cr-poisoning in solid oxide fuel

cells, Z Sun, R Wang, AY Nikiforov, S Gopalan, UB Pal, SN Basu, Journal of Power Sources 378, 125-133

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Acknowledgement

  • The financial support from U.S. Department of Energy, Office of Fossil

Energy, through Award # DE-FE0023325 is gratefully acknowledged.

  • Steve Markovich and Shailesh Vora

Thank you! Questions?