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

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 • Introduction • Cell Fabrication • Summary of Test Conditions • Electrochemical Degradation • Microstructural Evolution • Degradation Mechanisms • Development of Oxide Protective Coatings • Summary 2

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 (La 2 NiO 4 ) - 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 3

Cell Fabrication LSM: (La 0.8 Sr 0.2 ) 0.95 MnO 3-x LSF: (La 0.8 Sr 0.2 ) 0.95 FeO 3-x GDC: (Gd 0.10 Ce 0.90 )O 2-x 4

Summary of Test Conditions • General test conditions: – Fuel: 98% H 2 +2% H 2 O (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: • Cathode Current Conditions Cells Atmosphere Condition LSM-1 1 Dry Air Open Circuit LSF-1 LSM-2 Humidified Air Open Circuit 2 (10% H 2 O) LSF-2 LSM-3 Galvanostatic 3 Dry Air (0.5 A/cm 2 ) LSF-3 LSM-4 Humidified Air Galvanostatic 4 (10% H 2 O) (0.5 A/cm 2 ) LSF-4 5

Electrochemical Degradation: V-i Condition 1: Condition 2: Condition 3: Condition 4: Dried Air + 0.5 A/cm 2 10% Humidified Air + 0.5 A/cm 2 Dried Air + OCV 10% Humidified Air + OCV LSM-1 LSM-2 LSM-3 LSM-4 LSM-Based cells LSF-1 LSF-2 LSF-3 LSF-4 LSF-Based cells 6

Electrochemical Degradation: V-i Performance Change in 120 h in Different Conditions LSM-based cell performance vs. Time LSF-based cell performance vs. Time 9.6% 5.7% 10.0% 2.7% 2.4% 0.7% 0.0% -10.0% -9.5% Performance Change -20.0% -30.0% -29.0% -40.0% LSM-Based -50.0% LSF-Based -60.0% -70.0% -80.0% -90.0% -92.5% -100.0% Dry Air Wet Air Dry Air Wet Air + 0.5 A/cm 2 + 0.5 A/cm 2 + OCV + OCV  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/cm 2 ) 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/cm 2 ) slightly improved the cell performance (presumably due to cell break-in) – – In humidified air, performance deteriorated under OCV condition but improved under current load 7

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/cm 2 Condition 4: Humidified Air + 0.5 A/cm 2 8

Electrochemical Degradation: EIS LSM-based cell structure LSF-based cell structure Air + 10% H 2 O Dried Air Dried Air Air + 10% H 2 O  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. 9

Microstructural Evolution: LSM-Based LSM-1: Dry Air + OCV LSM-3: Dry Air + Current Cr-containing deposits are Cr,Mn-rich, suggesting (Cr,Mn) 3 O 4 spinel phases LSM-2: Humidified Air + OCV LSM-4: Humidified Air + Current 10

Microstructural Evolution: LSM-Based Criterion for quantifying Cr Cross section of LSM-based Cr-enrichment profile in the distribution in LSM cathode LSM-based cathode  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. * Wang, R., Pal, U. B., Gopalan, S., & Basu, S. N. (2017). Journal of The Electrochemical Society, 164(7), F740-F747. 11

Microstructural Evolution: LSF-Based LSF-3: Dried Air + 0.5 A/cm2 LSF-1: Dried Air + OCV LSF Paste LSF LSF-GDC GDC LSF Paste LSF LSF-GDC GDC YSZ YSZ 25 μ m 25 μ m Cr Mapping Cr Mapping Most of Cr is distributed at the OCV condition: surface of cathode Cr distribution is homogeneous in the LSF-4: 10% Humidified Air + 0.5 A/cm2 LSF-2: 10% Humidified Air + OCV bulk of cathode LSF Paste LSF LSF-GDC GDC LSF Paste LSF LSF-GDC GDC YSZ YSZ 25 μ m 25 μ m Cr is distributed at the surface of Cr Mapping Cr Mapping cathode and also cathode/electrolyte interface 12

Microstructural Evolution: LSF-Based LSF-3: Dried Air + 0.5 A/cm2 LSF-1: Dried Air + OCV LSF Paste LSF LSF-GDC GDC LSF Paste LSF LSF-GDC GDC YSZ YSZ 25 μ m 25 μ m Cr Line Scan Cr Line Scan Sr Line Scan Sr Line Scan LSF-4: 10% Humidified Air + 0.5 A/cm2 LSF-2: 10% Humidified Air + OCV LSF Paste LSF LSF-GDC GDC LSF Paste LSF LSF-GDC GDC Cr and Sr profiles do not match at the cathode/electrolyte YSZ YSZ interface 25 μ m 25 μ m Cr Line Scan Cr Line Scan Sr Line Scan Sr Line Scan 13

Microstructural Evolution: LSF-Based LSF-3: Dried Air + 0.5 A/cm2 LSF-1: Dried Air + OCV LSF contact paste LSF contact paste LSF Paste LSF Paste LSF LSF-GDC GDC LSF Paste LSF LSF-GDC GDC Dense Sr-Cr-O phase YSZ YSZ 25 μ m 25 μ m LSF current collective layer LSF current collective layer LSF-4: 10% Humidified Air + 0.5 A/cm2 LSF-2: 10% Humidified Air + OCV LSF Paste LSF Paste LSF LSF-GDC GDC LSF Paste LSF LSF-GDC GDC Sr:Cr ≈ 1:2 (At%) LSF contact paste YSZ YSZ 25 μ m 25 μ m LSF contact paste LSF current collective layer Dense Sr-Cr-O phase LSF current collective layer Sr:Cr ≈ 1:1 (At%) 14

Microstructural Evolution: LSF-Based LSF-3: Dried Air + 0.5 A/cm2 LSF-GDC LSF-1: Dried Air + OCV LSF Paste LSF LSF-GDC GDC LSF Paste LSF LSF-GDC GDC YSZ YSZ GDC 25 μ m 25 μ m LSF-4: 10% Humidified Air + 0.5 A/cm2 LSF-2: 10% Humidified Air + OCV LSF LSF-GDC LSF Paste GDC LSF Paste LSF LSF-GDC GDC YSZ YSZ YSZ 25 μ m 25 μ m LSF-GDC Major amount Cr 2 O 3 Minor amount Sr,Cr containing deposits GDC Major amount Cr 2 O 3 Minor amount Sr,Cr-containing deposits YSZ 15

Degradation in LNO Cathodes 16

Degradation Mechanisms  Effect of humidity on Cr evaporation: Equilibrium Partial Pressure of Cr Equilibrium Partial Pressure of Cr Equilibrium Partial Pressure of Cr vapor in Dry Air in 10% Humidified Air species over Cr 2 O 3 scale  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. 17

Degradation Mechanisms Evaporation of Cr-deposits on the LSF surface: 2SrCr 2 O 4 (s) + 2H 2 O(g) + 3O 2 (g)=2SrCrO 4 (s) + 2CrO 2 (OH) 2 (g) ----- (1)  Effect of humidity on Cr distributions: or SrCr 2 O 4 (s) + 4H 2 O(g) + 2O 2 (g)=Sr(OH) 4 (s) + 2CrO 2 (OH) 2 (g) ----- (2) 18

Oxide Protective Coatings 19

EPD Coating of CuMn 1.8 O 4 a EPD b Reduction annealing (1000 °C, 24 h) c XRD: a) CuMn 1.8 O 4 powders Oxidation b) after reduction anneal annealing c) after 1h oxidation anneal (850 °C, 1 h) Z. Sun et al, Journal of Power Sources , 378 (2018), 125-133.

Cr Diffusion and Microstructure Evolution 750 º C 100 h 750 º C 950 h Particle <1 μm ~ 2.1 μm 850 º C 100h + 800 º C 400h 850 º C 100 h ~ 13.5 μm ~7.1 μm Reaction layerNeedle structures

TEM Analaysis of Protective Coatings Needle structures: Mn3O4 Particles in dense layer: Cr2O3 Cr Mn Mn Cu O Cu O 23

Solubility of Cr 2 O 3 in CuMn 1.8 O 4 Cr 2 O 3 Solubility Reaction between Cr 2 O 3 and CuMn 1.8 O 4 powders (800 ° C, 10 h, in air) 24

Electrical Conductivity of (Cu,Mn,Cr) 3 O 4 Conductivity (S/cm) 100 (Cu,1.8Mn)2.4Cr0.6O4 (Cu,1.8Mn)1.8Cr1.2O4 (Cu,1.8Mn)1.3Cr1.7O4 Solubility limit 10 1 500 600 700 800 900 Temperature ( o C) * Zhu et al, Mater. Sci. Eng. A 348 (2003) 227–243 25

Coating on complex geometry (mesh) and Electrochemical tests – LSM cells BU Coating Commercial coating Commercial CuMn2O4 Bare Uncoated interconnect 26

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 27