17 nov08 jorge frade

Guidelines for materials selection and optimization

in electrochemical conversion of fuels

J.R.Frade

University of Aveiro, PortugalUniversity of Aveiro, Portugal

Economia de HEconomia de H2: Un desafio para Portugal, Vimeiro, Nov.17, 2008: Un desafio para Portugal, Vimeiro, Nov.17, 2008

Fuel cells

Clean; High η

Hydrocarbons

Partial oxidationor Reforming

e.g. CH4+1/2O2 → CO+2H2

Electricity Chemicals


Purification

H2

Effects of fuel/air atmospheres on cell materials

-Redox behaviour-Stability limits under reducing conditions

-Structural changes-Effects on transport properties, etc.

-Effects on carbon deposition-Effects of contaminants (S-tolerance) etc.?

-Thermochemical or thermomechanical failure

-…

Interactions between cell materials-Processing-Operation

-Maintenance, thermal cycling, etc.


α


Reducing cnditions imposed by H2

2H2 + O2 ↔ 2H2O;Po2 = (pH2O/pH2)2 exp(-∆G/RT)

-27

-23

-19

-15

-11

0.7 0.8 0.9 1

103/T; K-1

log

(Po

2/a

tm)

CH4; 99%

Conversion

CH4;

1%

H2;

99%

H2; 1%

1000ºC 900ºC 800ºC 700ºC

α


Reducing cnditions imposed by CH4, ...

0.0

0.2

0.4

0.6

0.8

1.0

-30 -20 -10

log(Po2/atm)

Mo

le f

rac

tio

ns

0

0.2

0.4

0.6

0.8

1

1.2

Th

eore

tica

l op

en c

ell v

olt

age

T=1100 K; H2O/CH4=0.02

H2

CO

CO2

H2O


Guidelines for redox tolerance of metallic electrocatalysts in H2 atmospheres

Nb

MnCr

Co

FeMo

Cu

W

Ni

10

1000 600 500

Ir

H2/H2O

10-1

8001200

-30

-25

-20

-15

-10

-5

0

0.5 0.7 0.9 1.1 1.3

103/T; (K-1)

log

(Po

2/a

tm)

T (ºC)


Stability limits of SOFC cathode materials and catalysts under reducing conditions

-25

-20

-15

-10

-5

0

1000 1100 1200 1300

T (K)

log

(pO

2/a

tm)

LaMnO3

LaFeO3

highest CO Yield

LaCoO3

La2NiO4

(Sr,La)CoO3 −δ

2LaMnO3 ↔ La2O3 + 2MnO + 1/2O2

2LaCoO3 ↔ La2CoO4 + CoO + 1/2O2

...

2 0 3 0 4 0 5 0 6 0 7 0

2 θ ,

Inte

nsity

, a.u

.

L S M C T 2

L S M C T 5

L S M C T 4

L S M C T 4

L S M C T 2s i n t e r e d a t 1 7 7 3 K i n a i r

s i n t e r e d a t 1 7 7 3 K i n a i r

s i n t e r e d a t 1 7 7 3 K i n a i r

a n n e a l e d a t 1 2 2 3 Kp ( O 2 ) = 1 1 0 - 1 8 a t m

a n n e a l e d a t 1 2 2 3 Kp ( O 2 ) = 1 1 0 - 1 8 a t m

M n O

Structural changes and property degradation under reducing conditions

LSMCT2 = (La0.55Sr0.45)0.95Mn0.5Cr0.3Ti0.2O3-δ;

LSMCT4 = (La0.35Sr0.65)0.95Mn0.5Cr0.1Ti0.4O3-δ...


- 1 5 - 1 0 - 5 0

l o g p ( O 2 ) ( a t m )

- 4 0

- 2 0

0

2 0

4 0

6 0

α, µ

V/K

0 . 0

0 . 5

1 . 0

1 . 5

log

σ (S

Scm

-1)

A

B

1

2

3

T = 1 2 2 3 K

- 1 5 - 1 0 - 5 0

( L a 0 . 7 5 S r 0 . 2 5 ) 0 . 9 5 M n 0 . 5 C r 0 . 5 O 3 - δ

( L a 0 . 5 5 S r 0 . 4 5 ) 0 . 9 5 M n 0 . 5 C r 0 . 3 T i 0 . 2 O 3 - δ

( L a 0 . 3 5 S r 0 . 6 5 ) 0 . 9 5 M n 0 . 5 C r 0 . 1 T i 0 . 4 O 3 - δ

( L a 0 . 2 5 S r 0 . 7 5 ) 0 . 9 5 M n 0 . 5 T i 0 . 5 O 3 - δ

- 4 0

- 2 0

0

2 0

4 0

1 . 2

1 . 3

1 . 4

T = 1 0 7 3 K

C

D

8 . 2 8 . 6 9 . 0 9 . 4

1 0 4 / T , K - 1

- 2 8

- 2 2

- 1 6

- 1 0

log

p(O

2) (a

tm)

R e g i m e 1( L a 0 . 7 5 S r 0 . 2 5 ) 0 . 9 5 M n 0 . 5 C r 0 . 5 O 3 - δ

( L a 0 . 3 5 S r 0 . 6 5 ) 0 . 9 5 M n 0 . 5 C r 0 . 1 T i 0 . 4 O 3 - δ

( L a 0 . 2 5 S r 0 . 7 5 ) 0 . 9 5 M n 0 . 5 T i 0 . 5 O 3 - δ

- 3 1

- 2 3

- 1 5

- 7

1

( L a 0 . 2 5 S r 0 . 7 5 ) 0 . 9 5 M n 0 . 5 T i 0 . 5 O 3 - δr e g i m e 1r e g i m e 2r e g i m e 3

8 . 0 8 . 5 9 . 0 9 . 5 1 0 . 0 1 0 . 5

B

A

L a 0 . 8 5 S r 0 . 1 5 C r O 3 / L a C r O 3 + S r O + C r 2 O 3

L a M n O 3 / L a O + M n O

M n 3 O 4 / M n O

L a M n O 3 / L a O + M n O

M n 3 O 4 / M n O

T i O 1 . 9 9 5Stability range estimated from

sudden changes in conductivity and Seebeck coefficient.

Stability limits

T(K) 104/T log(pO2) at pH2:pH2O=1:1

η=0 η=0.1V η=0.2V

973 10.28 -20.8 -18.8 -16.7

1073 9.32 -18.3 -16.4 -14.5

1173 8.53 -16.2 -14.5 -12.8


Typical dependence of ionic and electronic conductivity of CGO20 solid electrolytes


-4

-3

-2

-1

-25 -20 -15 -10 -5 0

log(Po2/atm)

log

{ σe/(

S/c

m)}

n-type

p-type

Ionic

700ºC

Predictions of methane conversion of ionic transport number and open cell voltage of a

CGO-based cell


0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0

CH4 conversion

t I,av

CH4

700ºC

900ºC

0.0

0.4

0.8

1.2

0.0 0.2 0.4 0.6 0.8 1.0

CH4 conversionO

pe

n c

ell

vo

lta

ge

(V

)

CH4

700ºC

900ºC

α


Predictions of carbon deposition

0.0

0.2

0.4

-30 -20 -10

log(Po2/atm)

Mo

le f

ract

ion

s

0

1

2

Ch

emic

al a

ctiv

ity

of

carb

on

T=1100 K; H2O/CH4=0.02

2CO → C + O2

CO

CO

2

Strategies to minimize the risks of carbon deposition:-Steam reforming-Substitution of metallic catalysts by alternative catalysts

α


PdS

NiS

CoS

FeS

PtS

RuS

Cu2S

-6

-4

-2

0

0.8 0.9 1.0 1.1 1.2

103/T; K-1

log

(pH

2S

/pH

2)

M+H2S↔MS+H2

Better S-tolerance

Poorer S-tolerance

Is thermodynamics useful to predict Sulphur tolerance?

Thermal expansion mismatchin a thin La10Si5AlO26+x electrolyte layer on a thick LSMCT

substrate, with firing at 1250ºC

ε = ∆L/Lo = ∆α.∆Tσ= ε E/(1-ν)

Typical for ceramics: E>100 GPa and ν≈0.3. For ∆α≈2x10-6 K-1, and ∆T≈1000ºC

ε ≈ − 0.002; σ ≈ - 300 MPa (delamination!)


Thermal expansion mismatch and ionic conductivity at 800ºC, in air


Composition α.10 6; Κ−1 σ I ;S/cm T, K Ea(kJ/mol)

La2Ni0.8Cu0.2O4+δ 13.3 0.11* - -

LSFC 12.9 0.01 1073-1223 206

LSM 11.7 1.3×10-5 - -

LSFC - 50 wt% CGO 12.6 0.04 1023-1223 82

LSM - 50 wt% CGO

11.7 0.03

1023-1223

104

La10Si5AlO26.5

9.1 0.08

973-1223

56

Dense tubular membranes of SrFe0.7Al0.3O3-δ with Al2O3 addition.

Chemical expansion under air/5%H2

0

0.2

0.4

0.6

600 700 800 900 1000

103/T; K-1

10

2 x∆

L/L

o

-22

-20

-18

-16

-14

-12

log

(pO

2/at

m)

LSMCT5

LSMCT0


ε = ∆L/Lo; σ = ε E/(1-ν)for ε=0.005; σ>200MPa (risks of fracture!)

EDS spectra showing cation diffusion accross the CGO buffer layer in

apatite/CGO/ LSMCT4 and LSGM/CGO/LSMCT4 arrangements

Interdiffusion between cell materials

4 . 0 4 . 5 5 . 0 5 . 5 6 . 0 6 . 5

E , k e V

Inte

nsit

y, a

.u.

e l e c t r o l y t e b u l kn e a r e l e c t r o l y t e / i n t e r l a y e r i n t e r f a c e

L a

C r

G dC e

e l e c t r o l y t e : L a 1 0 S i 5 A l O 2 6 . 5

i n t e r l a y e r : C e 0 . 8 G d 0 . 2 O 1 . 9

e l e c t r o d e : L S M C T 4

e l e c t r o l y t e : L S G M

i n t e r l a y e r : C e 0 . 8 G d 0 . 2 O 1 . 9

e l e c t r o d e : L S M C T 4

M n

A

B

L a

L a

L a

L a

L a

L a

L a

L a

L a

C eC e

C e

C eC e

C r

C rC r

M n

M nM n

G d


17 nov08 jorge frade

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