Carbon and Sulfur Tolerant anodes for SOFCs

Stylianos G. Neophytides

FORTH Institute of Chemical Engineering Sciences

Hydrogen days 2014, Prague 2-4 April, 2014

ΙΤΕ/ΕΙΧΗΜΥΘ

Outline

• Introduction to SOFCs and and the Internal steam reforming process

• Carbon tolerance• NiAu/YSZ

• NiAu/GDC, NiAuMo/GDC

• Physicochemical characterization, catalytic, electrocatalytic and AbientPressure Photoelectron spectroscopy experiments

• Sulfur tolerance• NiAu/GDC, NiAuMo/GDC

Cathode reaction: ½ O2 + 2e- = O2-

Electrical Energy (e- )

Anode reactions: H2 + O2- = H2O + 2e-

CO +O2- = CO2 + 2e-

CnH2n+2 + (3n+1)O2 = nCO2 + (n+1)H2O + (6n+2)e-

Basic Operational principles

Internal reforming proceeds through the water produced by the fuel at the anode

CH4+H2O → CO+3H2

H2+O2-→ H2O+2e-

ADVANTAGES

H2 is directly produced in the SOFC

H2 is readily oxidized for the production of electricity

DISADVANTAGES

The exposure of the anode in high CH4/H2O may result in C deposition

Low CH4/H2O ratios cause a decrease in the cell’s Nerstpotential

Objectives

Development of anode electrocatalysts active for catalytic CH4 steam reforming and resistive to graphitic carbon formation and sulphur poisoning

Internal Methane steam reforming reaction

Common SOFC materials[1]

Anode

• Ni based cermets, Ni/GDC, Ni/YSZ

Cathode

• Sr-doped LaMnO3 (LSM)

• La0.6Sr0.4Co0.2 Fe0.8O3 (LSCF)

• Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF)

• Sm0.5Sr0.5CoO3

Electrolyte:

• ZrO2(Y2O3) (YSZ)

• La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM)

• Samaria doped Ceria (SmDC)

• Scandia doped ceria (ScDC)

• Scandia stabilized zirconia (ScSZ)

[1] C. Sun and U. Stimming, Journal of Power Sources 171 (2007) 247.

• dissociate O2

• high electronic and ionic cond.

• thermal expansion coefficient

• dense

• high ionic conductivity

• electronic insulators

• H2 electrooxidation• CH4 steam reforming

Basic SOFC designs-ConfigurationsPlanar: Anode-Supported

Basic SOFC designs-Configurations

Electrolyte-Supported

Cathode-Supported

Metal-Supported

Flat design Tubular design

Basic SOFC designs-ConfigurationsPlanar: Electrolyte-Supported

Experimental electrochemical reactor

Pt wire connected toWorking electrode

Inlet

Outlet

YSZ tube

Counter electrode contact

Quartz tube

Ni-Au/YSZ anode

Carbon Tolerant Ni-Au SOFC Electrodes operating under Internal Steam Reforming Conditions, Ilias Gavrielatos , Vasilis Drakopoulos and Stylianos G. Neophytides, Journal of Catalysis 259 (2008) 75–84

The combustion synthesis method

PrecursorsNi (NO3)2, ΗAuCl4, ZrO(NO3)2

Y(NO3)3, CH4N2O

363°K

873°K combustion

Electrode/gas phase interface

The electrode-electrolytethree phase boundary

Cross section images of reduced NiAu/YSZ

Electrokinetic measurements under internal steam reforming conditions on Ni/YSZ

0 200 400 600 800 1000

0

-100

-200

-300

-400

-500

-600

-700

-800

-900

-1000

-1100

yCH

4

= 20 %

yH

2O= 6.2%

T=8500C

rate

s , μ

mol/s

ec

VW

C ,

mV

I , mA

VWC

0

1

2

3

4

5

6

7

rH

2

rCO

rCO

2

Carbon Tolerant Ni-Au SOFC Electrodes operating under Internal Steam Reforming Conditions, Ilias Gavrielatos , Vasilis Drakopoulos and Stylianos G. Neophytides, Journal of Catalysis 259 (2008) 75–84

0 10 20 30 40 50 60

0

30

60

90

120

150

180

210

240

270

300

330

360

390

T=850°C

yCH4

=22%

yH2O

=6.5%

Vcell

=-500mV

FT=109cc/min

Cu

rre

nt,

mA

Time, hrs

0

-200

-400

-600

-800

-1000

-100 0 100 200 300 400 500 600 700 800

T= 850°C

yH

2

= 10%

yH

2O= 4.2%

FT= 80cc/min

VW

C ,

mV

I , mA

before

after

Short term stability test

Electrocatalytic performance under 10%H2

before and after the stability test

Pulse of O2 at T< 450 K , O2/C=2

20o/min, Ft=26 μmoles/sec

TPO following CH4 dissociative adsorption Ni(1%at Au)-YSZ

500 600 700 800 900 1000

0

2

4

6

8

0

2

4

6

8

10

0

25

50

75

100

H2 , n

mole

s/se

c

Temperature , K

CO

, n

mole

s/se

c T=673 K

T=723 K

T=773 K

T=923 K

CO

2 , n

mole

s/se

c

Simultaneous evolution of CO and H2

Decomposition of CHxO species at

elevated Temperature

N.C.Triantafyllopoulos,S.G.Neophytides, J. Catalysis 239, 187-199 (April 2006)

REACTION MECHANISM

CH4 CH2ad + 2Had CH2ad Cc + 2Had

Cc + 2Oad CO2CH2ad → H2COad

H2COad CO + H2

rOX

rC

To avoid carbon deposition

rOX>rC

H2O, O2, O2-

Ni/GDC

NiAu/GDC

NiAuMo/GDC

Study of the synergistic interaction between nickel, gold and molybdenum in novel modified NiO/GDC cermets, possible anode materials for CH4 fueled SOFCs, Niakolas, D.K., Athanasiou, M., Dracopoulos, V., Tsiaoussis, I., Bebelis, S., Neophytides, S.G. Applied Catalysis A: General 456 , pp. 223-232 (2013)

• Commercial NiO/GDC powder as the

support, hydrogen tetrachloroaurate

(HAuCl4) and ammonium

heptamolybdate [(NH4)6Mo7O24

4H2O]

• Adjustment of pH and Temp. of the

suspension

• NH3 as precipitant agent

• Filtering, drying and final calcination at

850 & 1100 °C

Deposition Co-Precipitation

(a) (b)(a) (b)

•Au0 varies 10-50 nm

• Calcined at 850°C

•Au0 varies 10-150 nm

• Calcined at 1100°C

•MoOx species could not be detected

SEM-BSE on Au-Mo-NiO/GDC

0 100 200 300 400 500 600 700 800

0,0

0,1

0,2

0,3

d(Δ

wt.

%)/

dT

em

p

Temp, 0C

Ni/GDC

3wt% Au-Ni/GDC

10wt% Mo-Ni/GDC

3wt% Au-10wt% Mo-Ni/GDC

3wt% Au-30wt% Mo-Ni/GDC

Ramp from room temp

up to 850 0C with

5 0C/min and 10% H

2/Ar

H2-TPR on Au-Mo-NiO/GDC

Study of the synergistic interaction between nickel, gold and molybdenum in novel modified NiO/GDC cermets, possible anode materials for CH4 fueled SOFCs, Niakolas, D.K., Athanasiou, M., Dracopoulos, V., Tsiaoussis, I., Bebelis, S., Neophytides, S.G. Applied Catalysis A: General 456 , pp. 223-232 (2013)

TPR-XRD on Au-Mo-Ni/GDC

42 43 44 4536 37 38 39 40 41 38 40 42 44 46 48 50 52

(b)

Ni

(b)

NiO

Ni

AuNiO

(c)

AuNiO

Ni

(a)

(2')(2)

(1')

NiO

Au

NiO

NiO

Inte

nsity

(a.u

.)

Au

(1)

Mo

(c)

(a)

440oC

315oC

366oC

418oC

468oC

546oC

325oC

370oC

420oC

472oC

534oC

490oC

539oC

588oC

635oC

Au

NiNi

Ni

NiO NiO

GDC

(b)

AuNiO

Ni

Au

GDC

NiO

8000C

6350C

250C

5880C

3840C

4400C

4900C

5390C

8000C

5980C

250C

3250C

3700C

4200C

4720C

5340C

8000C

5960C

5460C

4680C

4180C

3660C

3150C

280C

Au

(c)

GDCNi

Ni

NiO

2θ (deg)

NiO

(a)

2Φ

Ni/GDC

NiAu/GDC

NiAuMo/GDC

0 100 200 300 400 500 600 700 800

0,00

0,02

0,04

0,06

0,08

0,10

0,12Ramp from room temp up to 850

0C

11% CH4/Ar

gca

rbo

n/g

cat.

Temp, 0C

Ni/GDC

3wt.% Au-Ni/GDC

10wt.% Mo-Ni/GDC

3wt.% Au-10wt.% Mo-Ni/GDC

TGA on Au-Mo-NiO/GDC

Activation energies of CH4 dissociation on various NiAu/GDC powders

The activation energy of CH4 dissociation increases with increasing Au content

• Binary and ternary cermets are active.

• Less active for H2production and carbon deposition, compared to Ni/GDC.

• Synergy between Ni, Auand Mo for decrease of carbon deposits.

Sample Ni/GDC 10wt.%Mo 3wt.% Au 3wt.Au -10wt.% Mo

rH2 (mmol m-2 s-1) 1.2 1.0 0.5 0.3

rCarbon (mmol m-2 s-1) 0.116 0.127 0.041 0.039

TGA-MS on Au-Mo-NiO/GDC

0 1 2 3

0,00

0,05

0,10

0,15

0,20

0,25

gc

arb

on/g

ca

t.

Time, min

Ni/GDC

10wt.% Mo-Ni/GDC

3wt.% Au-Ni/GDC

3wt.% Au - 10wt.% Mo-Ni/GDC

T = 7500C

20% CH4, 10% H

2O

Stability of the Ni/GDC and NiAu/GDC under S/C=0.5 at 0.5A/cm2

90 100 110 200 250 300 350 400 450500

600

700

800

900

1000

1100

1200O.C.

(2b) (2c)

(1e)(1d)(1c)(1b)

(2a)

Ce

ll V

olt

ag

e (

mV

)

Time (hrs)

(1a)

O.C.

Pure H2feed

S/C=0.5

T=850oC

Au doped Ni/GDC as a new anode for SOFCs operating under rich CH4 internal steam reforming, Niakolas D.K., Ouweltjes J.P., Rietvelt G., Dracopoulos V., Neophytides S.G., International Journal of Hydrogen energy, 35(15), 7898-7904, (2010)

Electrokinetic behavior of NiAu/YSZ and NiAu/GDC

0 200 400 600 800 1000

0

-100

-200

-300

-400

-500

-600

-700

-800

-900

-1000

-1100

yCH

4

= 20 %

yH

2O= 6.2%

T=8500C

rate

s , μ

mol/s

ec

VW

C ,

mV

I , mA

VWC

0

1

2

3

4

5

6

7

rH

2

rCO

rCO

2

NiAu/YSZ

NiAu/GDC

During current application CO is being oxidized into CO2 only through WGS reaction

Mathematical modeling of Ni/GDC and Au-Ni/GDC SOFC anodes performance under internal methane steam reforming conditions, Souentie, S., Athanasiou, M., Niakolas, D.K., Katsaounis, A., Neophytides, S.G., Vayenas, C.G. Journal of Catalysis 306 , pp. 116-128 (2013)

Ambient pressure X-ray photoelectron spectroscopy shows massive reduction of CeO2 into Ce2O3 and enhancement in the current

On the active surface state of nickel-ceria solid oxide fuel cell anodes during methane electrooxidation, Papaefthimiou, V., Shishkin, M., Niakolas, D.K., Athanasiou, M., Law, Y.T., Arrigo, R., Teschner, D., (...), Zafeiratos, S. Advanced Energy Materials 3 (6) , pp. 762-769 (2013)

0 5 10 15 20 25 30 35 40

0,00

0,02

0,04

0,06

0,08

0,10

0,12

0,14

gc

arb

on/g

ca

t.

Time, min

T = 750 0C

20kPa CH4

10kPa H2O

TGA 20% CH4, 10% H2O

Ni/GDC

NiAu/GDC

REACTION MECHANISM NiAu/GDC

CH4 CH2ad + 2Had CH2ad Cc + 2Had

Cc + 2Oad CO2CH2ad → H2COad

H2COad CO + H2

rOX activated on Ce2O3

rC activated on Ni

To avoid carbon deposition

rOX>rC

H2O, O2, O2-

Ni activated decompomposition

Comparison of NiAu/YSZ and NiAu/GDC

• CH4 activation takes place on Ni

• CHxO formed on Ni is an intermediate and decomposes into CO and H2

• Au hinders CH4 dehydrogenation Ni into C deposites

• Under lean S/C NiAu/YSZ is selectively electrooxidizing H2 due to the low coverage of CHxO

• CH4 activation takes place on Ce2O3

• CHxO is formed on Ce2O3 and decomposes on Ni into CO and H2

• Au hinders CH4 dehydrogenationNi into C deposites

• Under lean S/C NiAu/GDC is selectively catalyzing CH4 partial electrooxidation due to the abundant formation of CHxO

Sulfur Tolerance

Effect of H2S on Ni/GDC under H2 and reforming conditions

0 20 40 60 80 100 120 140 160 180 200 220 240

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

T = 850 0C, I = 40mA

H2O = 5%, H

2S = 10ppm

CH4 = 2.5%

S/C = 2

Pote

ntial, V

Time, min

10ppm H2S in H2

Stability in the presence of H2S

10ppm H2S/H2

0 20 40 60 80 100 120 140 160 180 200 220

0.0

-0.1

-0.2

-0.3

-0.4

-0.5

-0.6

-0.7

-0.8

-0.9P

ote

nti

al

(V)

Time (minutes)

Ni/GDC

3Au-Ni/GDC

3Mo-Ni/GDC

3Au-3Mo-Ni/GDC

T = 850 0C, I = 40mA

Ftotal

= 100cc/min

10ppm H2S in H

2

Stability in the presence of H2S

CH4 SR, S/C=2 + 10ppm H2S

0 20 40 60 80 100 120 140

0.0

-0.1

-0.2

-0.3

-0.4

-0.5

-0.6

-0.7

-0.8

-0.9P

ote

nti

al (V

)

Time (minutes)

Ni/GDC

3Au-Ni/GDC

3Mo-Ni/GDC

3Au3Mo-Ni/GDC

T = 850 0C, I = 40mA

Ftotal

= 100cc/min

H2O = 5%, H

2S = 10ppm

CH4 = 2.5%

S/C = 2

Stability in the presence of H2S CH4 SR, S/C=0.13 + 10ppm H2S

0 20 40 60 80 100 120 140 160 180 200

0.0

-0.1

-0.2

-0.3

-0.4

-0.5

-0.6

-0.7

-0.8

-0.9

Po

ten

tia

l (V

)

Time (minutes)

Ni/GDC

3Au-Ni/GDC

3Mo-Ni/GDC

3Au3Mo-Ni/GDC

T = 850 0C, I = 40mA

Ftotal

= 100cc/min

H2O = 5%, H

2S = 10ppm

CH4 = 38%

S/C = 0.13

• The introduction of Au modified Ni/YSZ into a carbon tolerant catalyst by the formation

of NiAu1%at/YSZ syrface alloy

• Modification of commercial NiO/GDC with D.P. and/or D. CP. of Au and/or Mo resulted

in new binary and ternary materials, possible anodes in CH4 plus H2S fueled SOFCs

• Binary and ternary samples show catalytic activity though lower than the undoped

Ni/YSZ

• Synergistic interaction between Ni, Au and Mo towards the decrease of carbon

deposition for the catalytic CH4 dissociation and steam reforming reactions

• Synergy is attributed to the formation of Ni-Au-Mo solid solution

Conclusions – Current status

Research Team

• Dr Dimitris Niakolas

• Dr Nikos Triantafyllopoulos

• Dr Ilias Gavrielatos

• Michalis Athanasiou

Acknowledgements

• FCH-JU project Understanding and minimizing anode degradation in hydrogen and natural gas fuelled SOFCs, Acronym:ROBANODE

• FCH-JU project Innovative SOFC Architecture based on Triode Operation, Acronym:T-cell