Chromium Reactivity and Transport

Michael Krumpelt, Terry A. Cruse, Brian Ingram, Di-Jia Liu

7th Annual SECA WorkshopPhiladelphia, 9/12-14

Chromium Transport According to Hilpert(J. Electrochem. Soc. 143 (1996) 3642)

2

Flow-channel Hydrodynamics

3

CathodeCathode

ElectrolyteElectrolyte

InterconnectInterconnect

OO2(g)2(g)HH22OO(g)(g)

CrOCrO22(OH)(OH)2(g)2(g)

CrCr22OO33

CathodeCathode

ElectrolyteElectrolyte

InterconnectInterconnect

OO2(g)2(g)

HH22OO(g)(g) CrOCrO22(OH)(OH)2(g)2(g)

Three Cell Configurations Were Tested

4

E-Brite E-Brite, Au Separation Au

With and without externaldosing

Schematic of a Fixture with ribbed Flow-field

5

Metallic Interconnect– ANL

• E-Brite• Au Barrier Between E-Brite

and Cathode• Au

– PNNL• Au• Au with External Cr Dosing

LSM Contact PasteLSM CathodeLSM/8YSZ Active Cathode8YZS ElectrolyteNi/8YSZ Anode

Cell Potentials versus Time

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600 700 800 900 1000Time (Hrs)

Cel

l Pot

entia

l (V)

Au Interconnect, Baseline (PNNL)

E-Brite Interconnect Au Foil Spacer (ANL)

E-Brite Interconnect (ANL)

Au Interconnect External Cr Dosed (PNNL)

6

Au Between E-Brite and Cathode

Direct Contact

0

0.25

0.5

0.75

1

0 0.25 0.5 0.75 1Zreal (Ω∗cm2)

-Zim

ag ( Ω

∗cm

2 )

Initial

120 Hrs

264 Hrs

504 Hrs

Temperature 800CE-Brite InterconnectAir ~3% H2O, 160 sccmFuel N2:H2 (~1:1) ~3% H2O, 400 sccmActive Cathode Area 4.6 cm2

-1.15 A

0

0.25

0.5

0.75

1

0 0.25 0.5 0.75 1Zreal (Ω∗cm2)

-Zim

ag ( Ω

∗cm

2 )

Initial

24 Hrs

264 Hrs

504 Hrs

Temperature 800CE-Brite InterconnectAir ~3% H2O, 160 sccmFuel N2:H2 (~1:1) ~3% H2O, 400 sccmActive Cathode Area 4.6 cm2

-1.15 A

7

HEWAXS at Argonne APS is a Powerful Tool for Microscopic XRD Investigation

8

HEWAXS at APS

Focused X-rayBeam

2 µm x 50 µm

Investigation for Cr poisoned SOFC

Cr2O3 and MnCr2O4 were unambiguously identified

Cr2O3

MnCr2O4

9

Profiles across Cathode

1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

0 20 40 60 80 100 120Depth (micron)

Am

plitu

de (a

. u.)

LSM

YSZ

Cr2O3

MnCr2O4

Upper Cathode

Mixed Cathode

Contact Paste Anode

Electrolyte

10

Formation and Transport of Chromium Compounds

Cr2O3: Cr2O3 + O2 +2H2O 2CrO2(OH)2 ∆G = +84KJ2CrO2(OH) + 6ē + 3V Cr2O3 + 3O2-o&&

MnCr2O4:

1. 7.5 Cr2O3 + 5 La.8Sr.2MnO3 4 LaCrO4 + 5 MnCr2O4+SrCrO4 + ¾ O2 ∆G = -297KJ

2. 2CrO2(OH)2 + MnO MnCr2O4 + 2H2O + 1/2O2

11

Incremental Chromium Transport in MACOR fixture

K2Cr2O7 was identified as a condensate in the exhaust tube

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500

Time (Hrs)

Cel

l Vol

tage

(V)

12

Chromium Mass Balances

Oxyhydroxide formation:- theoretical: 15 mg- measured: 0.22 mg

Cr2O3 deposits:- E-Brite with gold foil: not detectable- E-Brite: 0.12 mg- E-Brite in Macor: 1.5 mg

13

Summary

Hilpert’s oxyhydroxide mechanism can account for a cell potential degradation of 5% with E-Brite current collectors

Spinel forming alloys or spinel coatings will largely prevent the chromium transport

Alkali and perhaps alkaline earth oxides can dramatically increase it

14

15

Cr diffusion across surface of oxide (800°C)

Cr deposited from vapor on LSM surface (1000°C)

2 transport mechanisms for Cr: vapor and solid state diffusion

Both mechanisms are material and temperature dependent. Cr transport into the cathode is greatly reduced by lowering the operating temperature to a range suitable for metallic components (650-750°C). However it does not stop solid state transport through the contact paste. Barrier layers or coatings solve this problem.

16

Solutions for Uncoated Interconnect

Choose a contact paste that does not have significant solid state diffusion or vapor deposition (Ag), or a contact paste that reacts with the chromia scale to form an effective barrier.

Accept degradation rate or develop Cr tolerant cathodes.

Coating decreases Cr vaporization rate by ~100x.

Contact paste can be Ag, LSM, LNF, etc.

Contact paste and/or current collection layer can include Cr getters such as Co to further protect the cathode.

Solutions for Coated Interconnect