6565280 solid oxide fuel cell

8/9/2019 6565280 Solid Oxide Fuel Cell

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Materials for SOFCs, Pa 1

SOFC Lecture 2

Stacks

Materials Requirements

Defect Structure

Electrical Transport

Materials for the cause


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Fuel Cell Geometries


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Fuel Cell StacksCell puts out 1 V

Need MW of Power

Cells are bundled

SWPC tubular cell stack design

IntroducesINTERCONNECTS:

connects anode-

to-cathode of series

connected cells


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Fuel Cell Components


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High Electronic Conductivity

Chemically Stable at operating temperature in oxidizing conditions and

ALL processing temperatures and pressures.

Thermal expansions must match other components and NOT

Lead to cracking anywhere in cell- NO PHASE CHANGE-

All the way to theprocessing temperature.

Electronic Transference Number should be high, but some ionic

Conductivity can help minimize activation polarizations.

Working properties need only be at operating pressures.

Must be able to process into a porous form that allows gas transport to

reaction sites and that minimizes overpotentials. Must maintain mechanical

Integrity.

Must be compatible with processing and materials of other components.

Must be catalytically active with respect to the electrochemical

reduction of the oxidant.

SOFC Cathode


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High Electronic Conductivity.

Chemically Stable at operating temperature in reducing to weakly oxidizing

conditions, and ALL processing temperatures and pressures.


lead to cracking anywhere in cell- NO PHASE CHANGE- all the way to

theprocessing temperature. Also over a range of oxygen partial pressures.

Electronic Transference Number should be high, but some ionic

conductivity can help minimize activation polarizations.

Working properties need only be at operating pressures.

Must be able to process into a porous form that allows gas transport to

reaction sites and that minimizes overpotentials. Must maintain mechanical

integrity. No long term microstructural changes during operation.


Must be catalytically active with respect to the electrochemical

oxidation of the fuel. Must be tolerant of fuel impurities and cannot poison over

time of operation.

SOFC Anode


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High/adequate electronic conductivity over large pressure range.

Chemically stable at operating temperature in oxidizing and reducingconditions and at ALL processing temperatures and pressures.

Thermal expansion must match other components and NOT

Lead to cracking anywhere in cell- NO DISRUPTIVE phase change-

All the way to theprocessing temperature and over wide pressure range.

Must be able to process into dense form that is gas (H2 and O2) impermeable.

Must be compatible with processing and materials of other components. In

particular, chemical interactions must be minimized.

Properties must be insensitive to oxygen partial pressure.

Must have some strength and fracture toughness (> 400 MPa).

Low Cost and ease of fabrication.

SOFC Interconnect


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Historical (tubular) SOFC Materials


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SOFC Lecture 2

Stacks


Defect Structures

Electrical Transport (ionic and electronic)



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Conductivity SOFCs must conduct (transport) charge!

Electrolyte: something that conducts ions only

Interconnect: must conduct only electrons

Cathode/anode: must conduct electrons

some ion conduction can be a plus

In solid ceramics, DEFECTS Conduct charge

i=(Ni )(Zie)( i)


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Defect Chemistry

Assume we all know defect chemistry See Kingery Chapter 4 and Kelley Chapter 9 Kroger-Vink notation uses quasichemicals and effective

charges: only valid relative to a REFERENCE crystal

structure.

MSZeff

M is the mass or species

S is the site in the crystal M occupies

Zeff is the effective charge of M on S

CaZr''

VO

YZr'

ZrZrx

OOx


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N t i hi t i D f t Ch i t


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Non-stoichiometric Defect Chemistry

ZrO2x

OOx ZrO2

VO

+

1

2 O2(g)+2ecb '

Kred= VO[ ]ecb '[ ]

2pO2

1/ 2

Electronic Disorder

(insulator; band-to-band)

Reduction

(oxygen Vacancies)

O2(g)

ZrO2

2OOx

+VZr' ' ' '+4hvb

Kox =OOx[ ]

2

VZr' ' ' '[ ]hvb[ ]

4pO2

1

Defect Populations are a function of Oxygen Pressure

Is there anything else we need to worry about?


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Electroneutrality Condition

Kred=VO

[ ]ecb '[ ]

2

pO21/ 2

Kox=OOx[ ]

2

VZr' ' ' '[ ]hvb[]

4

pO21

Ki =VZr''''[ ]VO

[ ]2

Ke =ecb'[ ]hvb

[

4 VZr' ' ' '[ ]+ecb'[]=2 VO[ ]+hvb[

4 VZr' ' ' '[ ]+ecb'[]=2 VO[ ]+hvb[

2 VZr' ' ' '[ ]=VO[

As long as we know all K values, we know defect populations

What do the concentrations vs pO2 look like?

ecb'[ ]=hvb[

4 VZr' ' ' '[ ]=hvb[

ecb'[ ]=2 VO[

B S h ti


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Brouwer Schematic

for an MO Electrolyte

E t i i D i


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Extrinsic DopingGenerating Vacancies in ZrO2

CaO ZrO2 CaZr'' +OOx +VO

goes to completion

Y2O3 doping

Yttria stabilized Zirconia

CaO doping

Calcia stabilized Zirconia

Y2O3ZrO

2 2YZr'

+3OOx

+VO

goes to completion

4 VZr' ' ' '[ ]+YZr'[ ]+ecb'[]=2 VO[ ]+hvb[4 VZr' ' ' '[ ]+2 CaZr' '[ ]+ecb'

[]=2 VO[ ]+hvb[

CaZr' '[ ]=VO[

YZr'[ ]=2 VO[

YZr'[ ]= hvb[2 CaZr' '[ ]=hvb[

Ytt i D d Zi i


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Yttria Doped Zirconia

Brouwer Diagram


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SOFC Lecture 2

Stacks


Defect Structures

Electrical Transport (ionic and electronic)



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Conductivity and Defects

VO

= (NVO

)(2e)(VO

)

NVO

= VO[ ]=

1

2YZr

'[ ]

VO

= A

Te(

Hm

kT )

hvb = (Nhvb

)(e)(hvb )

Nhvb = h

vb

[ ](pO

2

)m

hvb =

C

Te(

E,p

kT)

ecb

' = (Necb

' )(e)(ecb

' )

Necb

' = ecb'[ ](pO2 )m

ecb

' =B

T

e(

E,n

kT)

tot=i=i VO+ecb' + hvb


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Conductivity and Defects

VO

Nhvb


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Transference Number

tot=i =i VO

+ecb'

+ hvb

ti

=i

tot

tecb' =

ecb

'

VO+ecb

' +hvb

tVO =

VO

VO+ecb

' +hvb


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Electrolytic Domain

electrolytic domain is when tVO

>0.99

or, generally, when tion >0.99

Electrolytic Ionic Electronic


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Electrolytic, Ionic, Electronic

Conduction Domainselectrolytic domain is when t

VO >0.99 or, generally, when tion >0.99

ionic domain is when tVO >0.5 or, generally, when tion >0.5

electronic domain is when telec (=te+th )>0.5


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Some Electrolytes


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Yttria stabilized Zirconia


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Generalized Transport Equations

ji = - (Ni)(Zi e)(i) Di(Zi e) Ni

Current density = drift + diffusion

(flux) (electric field) (chemical potential)

i=i0+kTln(Ni )+Zi e

The gradients are connected through the electrochemical potentials

i =kTNi

Ni+Zi e


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Nernst Equation and Current Density

Di =

kTi

Zi e

ji = - (Ni)(Zi e)(i) kT(i ) Ni

ji = - (Ni)(i ) {(Zi e)+kT

NiNi}

ji = - (Ni )(i ) i

Cell Output Voltage and


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Cell Output Voltage and

Electronic Defects

je=

-(Ne )(e)(e)

kT(e )Ne=

0 (electrolyte, e = 0)

e =kTNe

Ne+e=0 ; =

kTNe

eNe


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Electronic Conductivity for YSZ

Ke =ecb'[ ]hvb

[ ]=NeNh =AeEG

2kT

ecb

' =(Necb

' )(e)(ecb

' )

Necb

' = ecb'[ ](pO2 )

m


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Ionic Conductivity for YSZ

VO

= (NVO )(2e)(VO

)

NVO

= VO[ ]=

1

2YZr

'[ ]

VO

=A

Te(

Hm

kT)


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Ohmic Loss and Temperature

Vohm = IR

R =CgeometryVO =

Cgeometry

VO

Vohm = ICgeometry

VO

VO =(NVO)(2e)

A

T e

(Hm

kT)

SOFC El t l t


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High Ionic Conductivity

Low Electronic Transference Number

Properties must be relatively pO2 independent.

Chemically Stable at operating temperature

AND at both fuel and oxidizer oxygen partial pressures.


Lead to cracking anywhere in cell- NO PHASE CHANGE-

All the way to the processing temperature.

Must be able to process into a gas tight, dense form.

Must have some fracture toughness (> 400 MPa).


Low Cost and ease of fabrication.

SOFC Electrolyte


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High Ionic Conductivity

Low Electronic Transference Number

Properties must be relatively pO2 independent.

SOFC Electrolyte

ZrO2 Fluorite

CeO2 Fluorite

(La,Sr)(Ga,Mg)O3-x Perovskite

Bi2O3

A2B2O7Pyrochlores


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Why would we think Ceria is good?


35/35

Materials for SOFCs Pa 35

Why dont we use Ceria?

6565280 solid oxide fuel cell

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