SOFC Operation on Hydrogen and Coal Gas in the Presence of Phosphorus, Arsenic and Sulfur

Impurities

SOFC Operation on Hydrogen SOFC Operation on Hydrogen and Coal Gas in the Presence of and Coal Gas in the Presence of Phosphorus, Arsenic and Sulfur Phosphorus, Arsenic and Sulfur

ImpuritiesImpuritiesOlga A Marina, Larry R Pederson, Danny J Edwards,

Chris W Coyle, Jared Templeton, Mark Engelhard, Z Zhu

8th Annual SECA Workshop, San Antonio, TX, August 9, 2007

Contract Manager: Heather Quedenfeld

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OutlineOutlineOutline

ObjectiveExperimental ResultsPost-mortem cell investigations using SEM/EDS, TEM, EBSD, XPS, ToF-SIMS, andXRDSummary

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Objective and ApproachObjective and ApproachObjective and Approach

Evaluate performance of SOFCs operating on coal gas assuming warm gas clean-up

Phosphorus, arsenic, and antimony are potentially themost harmful to the Ni/YSZ anode because of possiblesecond phase formation with NiStudies also were performed with key contaminants inhydrogen to aid in poisoning mechanism understandingAdsorption studies on small Ni, Ni/YSZ, Ni/SDC couponswere performed to determine the interactions between Ni and P or As as a function of exposure time

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Schematic of Button Cell Test StandsSchematic of Button Cell Test StandsSchematic of Button Cell Test Stands

Ni-YSZ Anode/Current Collector

Air Air inlet tube

I-V wires

‘K’ type thermocouple

I-V wires

Alumina outer tube with circular openings

fuel in

fuel out

LSM current collector with Au gauze

LSM cathodeSDC layer

YSZ

Aremco seal

Alumina fuel inlet tube

Ni grid current collector

Test Conditions:

• 700 and 800°C

• Fuel (at equilibrium): H2/H2O = 97/3 or H2/CO/CO2/H2O = 30/23/21/26

• Initially preconditioned in H2 at 0.7 V for 24-30 hours followed by 0.5 A/cm2 in H2 or coal gas for ~100 hours

• All impurities were added after water bubbler

• Tested at a current of 0.5 A/cm2

Test Conditions:

• 700 and 800°C

• Fuel (at equilibrium): H2/H2O = 97/3 or H2/CO/CO2/H2O = 30/23/21/26

• Initially preconditioned in H2 at 0.7 V for 24-30 hours followed by 0.5 A/cm2 in H2 or coal gas for ~100 hours

• All impurities were added after water bubbler

• Tested at a current of 0.5 A/cm2

Standard Ni/YSZ anode:

• 50 vol% Ni (solids) active anode (~ 8 µm)

• 40 vol% Ni (solids) bulk anode ~ 800 µm thick

Standard Ni/YSZ anode:

• 50 vol% Ni (solids) active anode (~ 8 µm)

• 40 vol% Ni (solids) bulk anode ~ 800 µm thick

Ni gauze

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SOFC Performance at 800oC in Coal Gas with 2 ppm of PH3

SOFC Performance at 800SOFC Performance at 800ooC in Coal Gas C in Coal Gas with 2 with 2 ppmppm of PHof PH33

PH3 exposure leads to cell degradation,

~0.13-0.2%/hr increase in ASR

This degradation is irreversible

Cells continue to degrade after PH3removal from the fuel

PH3 exposure leads to cell degradation,

~0.13-0.2%/hr increase in ASR

This degradation is irreversible

Cells continue to degrade after PH3removal from the fuel

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 100 200 300 400 500 600 700

Time (Hours)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

AS

R (

Ohm

/cm

2 )

T=800oC

2 ppm PH3

H2coal

ASR

i

E

Cur

rent

Den

sit

y (A

/cm

2 ) or

Cel

l Vol

tag

e (V

)

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Effect of Phosphorus on Cell PerformanceEffect of Phosphorus on Cell PerformanceEffect of Phosphorus on Cell Performance

Power loss onsetoccurs more rapidlyat 700oCAfter onset,degradation ratesdo not appear tobe significantly different at 700 and800oC

Power loss onsetoccurs more rapidlyat 700oCAfter onset,degradation ratesdo not appear tobe significantly different at 700 and800oC

% Power retention calculated after the impurity introduction over power before impurity introduction

40

50

60

70

80

90

100

110

0 200 400 600 800

Time (Hours)

Pow

er R

eten

tion

(%)

H2 800o

H2 700ocoal 700o

coal 800oi=0.5 A/cm 2

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More Rapid ASR Increase in Coal GasMore Rapid ASR Increase in Coal GasMore Rapid ASR Increase in Coal Gas

50

100

150

200

0 200 400 600 800

Time (Hours)

AS

R In

crea

se (

%)

H2 800o

coal 800o

H2 700o

coal 700o, 0.3 A/cm 2

i=0.5 A/cm 2

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Effect of Arsenic on Cell PerformanceEffect of Arsenic on Cell PerformanceEffect of Arsenic on Cell Performance

Slow degradation (~1%/100 hr) in the presence of 1 ppm of As in both H2 and coal gas

Slow degradation (~1%/100 hr) in the presence of 1 ppm of As in both H2 and coal gas

1 ppm of AsH31 ppm of AsH3

0

20

40

60

80

100

120

0 200 400 600 800

Time (Hours)

Pow

er R

eten

tion

(%)

T=800oCi=0.5 A/cm 2

As in H 2

As in coal

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Effect of P and As on Cell ResistanceEffect of P and As on Cell ResistanceEffect of P and As on Cell Resistance

ROhmic and Relectrodic increase with time of exposure to PH3

Change in R ohmic is more significant for AsH3 exposure

ROhmic and Relectrodic increase with time of exposure to PH3

Change in R ohmic is more significant for AsH3 exposure

-0.2

-0.1

0

0.1

0.2

0 0.1 0.2 0.3 0.4 0.5

Z' (Ohm cm 2)

-Z"

(Ohm

cm

2 ) 0 hrs282 hrs

800oCcoal gas+2 ppm of PH 3

i bias = 0.5 A/cm 2

1

100

1000

1010e4 10

0 10

1000

1

10e4

-0.2

-0.1

0

0.1

0.2

0 0.1 0.2 0.3 0.4 0.5

Z' (Ohm cm 2)

-Z"

(Ohm

cm

2 ) 0 hrs 526 hrs

800oCcoal gas+1 ppm of AsH 3

i bias = 0.5 A/cm 2

10e4

10e3

1 0.1

1010

0

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Effect of Sulfur on Cell Performance:Baseline for Tests with ContaminantsEffect of Sulfur on Cell Performance:Effect of Sulfur on Cell Performance:Baseline for Tests with ContaminantsBaseline for Tests with Contaminants

Rapid initial degradation (ca. 15%) during first 30 hours in both H2 and coal gas

2nd stage degradation rate (0.06%/hr in H2

and 0.26%/hr in coal gas)

Rapid initial degradation (ca. 15%) during first 30 hours in both H2 and coal gas

2nd stage degradation rate (0.06%/hr in H2

and 0.26%/hr in coal gas)

0

20

40

60

80

100

0 20 40 60 80 100 120

Time to H 2S exposure (hours)

% P

ower

Ret

entio

n

coal gas, cell #2

coal gas, cell

hydrogen, cell #1

T=800oC1 ppm of H 2S

i = 0.5 A/cm 2

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Synergistic Effect of P+As+SContaminants in Coal Gas

Synergistic Effect of Synergistic Effect of P+As+SP+As+SContaminants in Coal GasContaminants in Coal Gas

In the presence of S, initial drop is ca. 15% and appears to be independent of other impurities

Effect of S is predominant over first 30 hours

S-related 2nd stage degradation is suppressed by the presence of P and As

2nd stage degradation is similar to that in P

In the presence of S, initial drop is ca. 15% and appears to be independent of other impurities

Effect of S is predominant over first 30 hours

S-related 2nd stage degradation is suppressed by the presence of P and As

2nd stage degradation is similar to that in P

1 ppm of AsH3, 1 ppm of H2S, 2 ppm of PH31 ppm of AsH3, 1 ppm of H2S, 2 ppm of PH3

0

20

40

60

80

100

0 200 400 600 800

Time (Hours)

Pow

er R

eten

tion

(%)

T=800oCCoal gasi=0.5 A/cm 2

P

S-PS-P-As

S

As

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Ni Contact Layer Surface Under the Fuel Inlet after 381 hr Test with 2 ppm of PH3

Ni Contact Layer Surface Under the Fuel Ni Contact Layer Surface Under the Fuel Inlet after 381 hr Test with 2 Inlet after 381 hr Test with 2 ppmppm of PHof PH33

Spectrum P (at%) Ni (at%)

Spectrum 1 31.00 69.00Spectrum 2 30.89 69.11Spectrum 3 32.67 67.33Spectrum 4 24.69 75.31

381 hr test in H2/H2O=97/3 with 2 ppm of PH3 at 800oC381 hr test in H2/H2O=97/3 with 2 ppm of PH3 at 800oC

Ni

Ni

NiP

Ni3P(TEM & XRD confirmed)

Ni3P(TEM & XRD confirmed)

All surface Ni under the fuel inlet is converted into Ni3P. Surface Ni away from the fuel channel is present as the metal.

All surface Ni under the fuel inlet is converted into Ni3P. Surface Ni away from the fuel channel is present as the metal.

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Cross Section of Top Ni/YSZ Layer after 381 hr Test with 2 ppm of PH3

Cross Section of Top Ni/YSZ Layer after Cross Section of Top Ni/YSZ Layer after 381 hr Test with 2 381 hr Test with 2 ppmppm of PHof PH33

Based on intensity maps, Ni is coarsened and converted into Ni3P within the top 60 μm.

Based on intensity maps, Ni is coarsened and converted into Ni3P within the top 60 μm.

P_K

Zr_L

Ni_KNi-Red, YSZ-green

Ni3P

381 hr test in H2/H2O=97/3 with 2 ppm of PH3 at 800oC

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Current Collecting Wire after 381 hr Test with 2 ppm of PH3

Current Collecting Wire Current Collecting Wire after after 381 hr Test with 2 381 hr Test with 2 ppmppm of PHof PH33

Outer layer of the wire is converted into phosphides

Outer layer of the wire is converted into phosphides

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EBSD Reveals Ni5As2 Formationafter 70 hr Test with 1 ppm of AsH3

EBSD Reveals NiEBSD Reveals Ni55AsAs22 FormationFormationafter 70 hr Test with 1 after 70 hr Test with 1 ppmppm of AsHof AsH33

70 hr test in H2/H2O=97/3 with 1 ppm of AsH3 at 800oC70 hr test in H2/H2O=97/3 with 1 ppm of AsH3 at 800oC

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Ni has two phase regions, one rich in As (ca. 45 μm) and the other rich in P (150 μm).

Ni has two phase regions, one rich in As (ca. 45 μm) and the other rich in P (150 μm).

NiNi55AsAs 22--NiNi33PP NiNi33PP NiNi

Ni-As–Magenta Ni-P–PurpleYSZ–teal Ni–Red

Cross Section of Top Ni/YSZ Layer after 700 hr Test in Coal Gas with PH3 + H2S + AsH3

Cross Section of Top Ni/YSZ Layer after 700 hr Cross Section of Top Ni/YSZ Layer after 700 hr Test in Coal Gas with PHTest in Coal Gas with PH33 + H+ H22S + AsHS + AsH33

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Surface Layer after 700 hr Test in Coal Gas with PH3 + H2S + AsH3

Surface Layer after 700 hr Test in Coal Gas with Surface Layer after 700 hr Test in Coal Gas with PHPH33 + H+ H22S + AsHS + AsH33

Ni is converted into a mix of Ni5As2 and Ni3P

Ni is converted into a mix of Ni5As2 and Ni3P

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Ni Grain Surfaces Uniformly Covered with P Throughout Ni/YSZ

Ni Grain Surfaces Uniformly Covered with P Ni Grain Surfaces Uniformly Covered with P Throughout Ni/YSZThroughout Ni/YSZ

ToF-SIMS line-scan of the cross-section of button SOFC after test at 800oC in coal gas with H2S, PH3 and AsH3.

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

0.00

0.01

0.02

0.03

0.04

0.05

0.06

Bottom side

I / I to

tal

Scan Distance (mm)

PO3

-

PO2

-

NiO2H- X 8

Top sideWith NiP print

High resolution photoemission spectra of the P 2p region

This is likely an adsorption layer undetectable by SEM/EDSThis is likely an adsorption layer undetectable by SEM/EDS

Ni contactlayer

Active anode

Wide scan,200 μm step

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Surface Probe Shows Presence of Adsorbed Sulfur on Ni

Surface Probe Shows Surface Probe Shows Presence of Adsorbed Sulfur on NiPresence of Adsorbed Sulfur on Ni

Ni/YSZ anodes tested in coal gas with H2S have higher S concentrations than the reference.

No second Ni-S phase found

Ni/YSZ anodes tested in coal gas with H2S have higher S concentrations than the reference.

No second Ni-S phase found

1 ppm H2S

850 hrs1 ppm H2S

850 hrs

No impurities

(Reference)No impurities

(Reference)

PH3+H2S+AsH3

700 hrsPH3+H2S+AsH3

700 hrs

NiO2Hx+

clusters

SO4-

HSO4-

O2-

S-

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Post Test Analysis Summary: Ni Interactions with P, S, and As

Post Test Analysis Summary: Post Test Analysis Summary: Ni Interactions with P, S, and AsNi Interactions with P, S, and As

S, P, and As adsorb on the Ni surfaceAffinity for Ni increases in the order S < P < As

Phosphorus displaces absorbed sulfurArsenic displaces adsorbed phosphorus

Unlike sulfur, P and As form secondaryphases, nickel phosphides and nickelarsenides. This transformation is irreversible.

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SummarySummarySummary

Ni/YSZ anodes were tested in simulated coal gas with 1 ppm of H2S, 2 ppmof PH3 and 1 ppm of AsH3 in various combinations at 700-800oC for up to 800 hours.

Initial cell degradation, ~ 15 % of the power density, is attributed to the presence of sulfur (effect on the anode resistance).

Second stage degradation, 0.01-0.07%/hr, is P and As related and is due to the increase in the ohmic resistance and electrode resistance.

Ni contact paste, wires and Ni in the top Ni/YSZ later under the fuel inlet were severely affected by the presence of 2 ppm of PH3. Surface and bulk (~ 150 μm) Ni3P was formed during 700 hours test at 800oC.

Ni3P showed extensive agglomeration and self-crystallization followed by the densification of the upper Ni layer. Similar grain orientation of Ni3P suggests partial liquefaction and re-solidification as a single, continues grain.

A nickel deficient layer was formed in the upper part of the Ni/YSZ affecting the Ni percolation.

Surface Ni5As2 was formed after exposures to 1 ppm of AsH3 at 800oC. Arsenide formation alone had insignificant degradation over 700 hour test because of strong Ni-As interaction near the surface (and not at the active interface).

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AcknowledgementsAcknowledgementsAcknowledgements

Support for this work is provided by the US Department of Energy, Office of Fossil Energy, National Energy Technology Laboratory through the SECA Program.We would like to acknowledge NETL management team for helpful discussions.

Pacific Northwest National Laboratory is operated for the US Department of Energy by Battelle.