Anodes for Direct Hydrocarbon Solid Anodes for Direct Hydrocarbon - - PowerPoint PPT Presentation

Anodes for Direct Hydrocarbon Solid Anodes for Direct Hydrocarbon Solid Oxide Fuel Cells (SOFC Oxide Fuel Cells (SOFC’ ’s) s) Challenges in Challenges in – – materials selection and materials selection and deposition deposition

Venkatesan Venkatesan V. Krishnan

• V. Krishnan

Department of Chemical Engineering Department of Chemical Engineering IIT Delhi IIT Delhi

Barriers to the hydrogen economy Barriers to the hydrogen economy

• How to make hydrogen (SR, electrolysis, Solar, Borohydride)?
• Loss of efficiency in making hydrogen
• Storage and Transportation?
• SOFC’s can work with any ‘fuel’
• Let us work in parallel with the Hydrogen advocates by using

hydrocarbons directly with SOFC’s

• Do not forget – well to wheels efficiency!

DIRECT OXIDATION OF DIRECT OXIDATION OF HYDROCARBONS AND OXYGENATES HYDROCARBONS AND OXYGENATES

Reactions: Cathode: (x + y/4 - z/2) O2 + (4x + y- 2z) e (2x + y/2 - z) O2- Anode: CxHyOz + (2x + y/2 - z) O2- (y/2) H2O + x CO2 + (4x + y - 2z) e Overall: CxHyOz + (x + y/4 - z/2) O2 (y/2) H2O + x CO2

Natural Gas, Propane/ Butane (LPG), Naphtha, Diesel, Alcohols, Natural Gas, Propane/ Butane (LPG), Naphtha, Diesel, Alcohols, Syngas (from coal and biomass) Syngas (from coal and biomass) WHAT HAPPENS IN THE ANODE IN SUCH CASES ? WHAT HAPPENS IN THE ANODE IN SUCH CASES ?

• Direct oxidation;

Direct oxidation; CH

CH4

4 + 4O

+ 4O2

2-

CO CO2

2 + 2H

+ 2H2

2O + 4e

O + 4e

• Internal reforming;

Internal reforming; CH

CH4

4 + H

+ H2

2O

O CO + 3H CO + 3H2

2

• Syngas oxidation;

Syngas oxidation; CO + H

CO + H2

2 + O

+ O2

2-

CO CO2

2 + H

+ H2

2O

O

• Coke deposition (thermal, catalytic);

Coke deposition (thermal, catalytic); CH

CH4

4

CH CHx

x + H

+ H2

2

WHY DO WE NEED NEW ANODES WHY DO WE NEED NEW ANODES

• Incompatibility of Ni

Incompatibility of Ni-

• YSZ anodes with any fuel other

YSZ anodes with any fuel other than H than H2

2 –

– reforming essential; reforming essential; coking with dry coking with dry hydrocarbons hydrocarbons Reference: Toebes et al. Reference: Toebes et al.

• Poisoning of Ni

Poisoning of Ni-

• YSZ by

YSZ by sulfur compounds sulfur compounds

• Poor Redox tolerance

Poor Redox tolerance of Ni

• f Ni-
• based anodes

based anodes

• YET, must satisfy the basic needs of SOFC anodes

YET, must satisfy the basic needs of SOFC anodes – – – – Electronic conductivity Electronic conductivity – – Ionic conductivity Ionic conductivity – – Good catalytic activity Good catalytic activity – – Compatibility of Compatibility of CTE CTE’ ’s s with that of electrolyte with that of electrolyte

EFFECT OF COKING ON Ni EFFECT OF COKING ON Ni-

• YSZ ANODES

YSZ ANODES

20% CO/7% H2 550°C. Ni particles

Coking due to decomposition and/or Bouduard Reaction

In H2, 800oC, 3 hrs In CH4, 800oC, 3 hrs

Toebes et al. (2002) Toebes et al. (2002)

ALTERNATIVE ANODES ? ALTERNATIVE ANODES ?

Perovskites, Irvine et al. Perovskites, Irvine et al. Boukamp Boukamp, et al. , et al.

A AB BO O3

3

Lanthanides/ Alkali earth Metal ions Oxygen ions Transition metal ions

Cu/CeO Cu/CeO2

2/YSZ

/YSZ Gorte et al. (2000)

Gorte et al. (2000)

Ce Ce0.9

0.9Gd

Gd0.1

0.1O

O1.95

1.95 combined with

combined with (La (La0.8

0.8Sr

Sr0.2

0.2) (Cr

) (Cr0.8

0.8Mn

Mn0.2

0.2)O

)O3

3-

• δ

δ

Barnett et al. (2002) Barnett et al. (2002)

Irvine et al. (2001) Irvine et al. (2001)

Titania Titania-

• Niobia, Nb

Niobia, Nb2

2TiO

TiO7

7-

• x

x

Irvine et. al. (2003) Irvine et. al. (2003) –

– (La (La0.8

0.8Sr

Sr0.2

0.2) (Cr

) (Cr0.5

0.5Mn

Mn0.5

0.5)O

)O3

3-

• δ

δ

Why perovskites??? Why perovskites???

• Ionic and electronic conductivity can be tailored

Ionic and electronic conductivity can be tailored

• Good Hydrocarbon oxidation catalytic activity has been

Good Hydrocarbon oxidation catalytic activity has been reported. reported.

• No coking

No coking

• Good thermal and redox stability

Good thermal and redox stability

• And yet, concern remains

And yet, concern remains – – do we have adequate do we have adequate electronic conductivity under reducing conditions? electronic conductivity under reducing conditions?

Review of work on alternate anodes Review of work on alternate anodes -

• Gorte et al.

Gorte et al.

• Anodes based on Copper – conductor; Ceria – electrocatalyst

– Cu and CeO2 deposited on pre-formed porous anode substrates – Intermediate temperature SOFC’s

• Considerable data on butane, methane, diesel, propane –

– Anode stable over long periods of time – Direct electrochemical Oxidation observed in button cells – Workable with fuel with existing sulfur levels

• Technology transfer to Franklin Fuel Cells, PA, US

– Demonstration level, with diesel, gasoline and ethanol

• Performance tends to be lowered due to Copper sintering;

enhancement necessary by carbon deposition

• Little/ no data on redox tolerance (deposition technique may be

advantageous, use of Vol % < 20%)

Gorte et al., cont Gorte et al., cont’ ’d d

0.0 0.3 0.6 0.9 1.2 1.5 0.0 0.2 0.4 0.6 0.8 0.00 0.04 0.08 0.12 0.16 0.20

V P I, A/cm2 A B C C

0.0 0.3 0.6 0.9 1.2 1.5 0.0 0.2 0.4 0.6 0.8 0.00 0.04 0.08 0.12 0.16 0.20

V P I, A/cm2 A B C C

1E

• 03

1E

• 01

1E +01 1E +03 1E +05 5 10 15 20 25 30

Volume Percent Cu, % Effective Conductivity, S/cm

Overall ‘connectivity’ of Cu is good, leading to about 4300 S/cm (from 4-point probe)

Compare with Ni-YSZ made by solid state methods

Performance, Conductivity with Cu content Performance, Conductivity with Cu content

• Carbon enhancement
• ccurs even at 25% Vol

Cu; constant, independent

• f Cu
• But sufficient

conductivity is attained with less than 10% volume Volume fraction of Cu

σ σ

, Conductivity, S/cm

• Max. P D, W/cm2

(w) carbon Conductivity (w/o) carbon Performance

Overall conductivity not the problem; the issue is proper ‘connectivity’ at the 3- phase boundary

1000 2000 3000 4000 5000 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.00 0.05 0.10 0.15 0.20 0.25

Irvine et al. (2003) Irvine et al. (2003) (La

(La0.75

0.75 Sr

Sr0.25

0.25 Cr

Cr0.5

0.5 Mn

Mn0.5

0.5 O

O3

3-

• δ

δ),

), LSCM LSCM

Conductivity of perovskite at 850oC

• in air, 28 S/cm
• in reducing atmosphere, 1.12 S/cm
• Max. P. D. (W/cm2) = 0.47, 900oC
• Max. P. D. in Methane = 0.3 W/cm2

(at 950oC)

Irvine et al., Niobia Irvine et al., Niobia-

• Titania rutile system (2001)

Titania rutile system (2001)

Reduction

• Max. conductivity under reduction, 300 S/cm
• Max. conductivity under reduction, 300 S/cm
• Good catalytic activity towards methane

Good catalytic activity towards methane Oxidation Oxidation

Barnett et al. (2002) Barnett et al. (2002) – – LSCM LSCM-

• GDC composite

GDC composite

• Usage of LSCM – high electronic

conductivity in reducing conditions, as well

• Compatible with GDC

electrolytes

Small qty of Ni + Small qty of Ni + Ce Ce0.9

0.9Gd

Gd0.1

0.1O

O1.95

1.95 –

– ionic conduction ionic conduction (La (La0.8

0.8Sr

Sr0.2

0.2) (Cr

) (Cr0.8

0.8Mn

Mn0.2

0.2)O

)O3

3-

• δ

δ −

− electronic conduction electronic conduction Compared to Ni Compared to Ni-

• GDC, performance in C

GDC, performance in C3

3H

H8

8 is significantly higher

is significantly higher

Focus of research at Chemical Eng. Dept., IIT Focus of research at Chemical Eng. Dept., IIT-

• D

D

• Nature of work essentially fundamental

Nature of work essentially fundamental -

• ‘

‘button cells button cells’ ’

• Aim is to contribute to existing knowledge

Aim is to contribute to existing knowledge ‘ ‘pool pool’ ’, via publications , via publications and patents by addressing problems involving Electrochemistry, and patents by addressing problems involving Electrochemistry, Materials and Catalysis Materials and Catalysis

• We look forward to collaborate with

We look forward to collaborate with ‘ ‘developmental laboratories developmental laboratories’ ’, , e.g., test our ideas on a larger scale e.g., test our ideas on a larger scale – – CGCRI, BHEL R&D CGCRI, BHEL R&D What we are involved in What we are involved in -

• SOFC component fabrication

SOFC component fabrication – – tape casting, sintering, painting, tape casting, sintering, painting, impregnation impregnation

• Characterization of components

Characterization of components – – bi bi-

• layers, catalysts

layers, catalysts – – SEM, SA/ SEM, SA/ PSD, Porosity, XRD, Thermal Analysis PSD, Porosity, XRD, Thermal Analysis

• Testing of Electrical Conductivity

Testing of Electrical Conductivity

• Fuel Cell Testing

Fuel Cell Testing

FUEL CELL REACTOR AND OPERATION FUEL CELL REACTOR AND OPERATION

Fuel Air Cathode Lead (Ag/ Pt) Anode Lead, Au Ag/ Pt mesh

LSM-YSZ Cathode Anode

Ceramabond, adhesive 1/2” OD Alumina tube

Tape Casting (non Tape Casting (non-

• Aq

Aq) )

1.2mL PEG Plasticizer 2.0g PVB Binder 0.5mL Oleic A cid Dispersant 10mL EtO H 20mL M EK Solvent 15g YSZ Powder Quantity Component Component function 1.2mL PEG Plasticizer 2.0g PVB Binder 0.5mL Oleic A cid Dispersant 10mL EtO H 20mL M EK Solvent 15g YSZ Powder Quantity Component Component function

Characterization of tape Characterization of tape

SEM of Graphite – as received SEM of sintered bilayer

Electrolyte

TG /DTA of Electrolyte Tap e

13 1 3.5 14 1 4.5 15 1 5.5 16 1 6.5 17 1 7.5 20 400 600 80 1 000 1200 14 00 1600 Tem p (C) weight (mg)

• 40
• 30
• 20
• 10

10 20 30

uV

tga DTA

Less than 500oC

Anode Preparation by Perovskite Deposition Anode Preparation by Perovskite Deposition

This method is totally different from the mixing and solid This method is totally different from the mixing and solid-

• state

state synthesis techniques listed in most literature. synthesis techniques listed in most literature. Moving away from the 3 Moving away from the 3-

• phase boundary limitation to a 2

phase boundary limitation to a 2-

• phase boundary situation

phase boundary situation Impregnation & calcination

Final Anode Structure Porous anode matrix

Three Phase Boundary

back

e- e- e- e- e- e- Two Phase Boundary Two Phase Boundary-

• a new approach

a new approach

fuel electrolyte anode

back

SEM and XRD both confirm that the required phase is formed inside the pores. But conductivity is very low i.e., 10 But conductivity is very low i.e., 10-

• 2

2 S/cm.

S/cm.

Impregnated La Impregnated La CrOx CrOx on preformed anodes

• n preformed anodes

Perovskites deposited

LaCrO3 La0.75Sr0.25CrO3 La0.9Sr0.1CrO3

Starch Starch

Morphology of bi Morphology of bi-

• layers

layers

• Investigation of pore sizes –

choice of pore-formers, like Graphite, Starch, PMMA to name a few

• Porosity
• Pore size distribution

Which are dependent on –

• Particle sizes
• Particle size distribution
• Particle ‘shapes’

Tailoring the porosity of anode Tailoring the porosity of anode

Removal of pore formers by 950oC, completely

5 10 15 20 25 30 35 40 45 50 200 400 600 800 1000 1200 1400 1600 Tem p C % red in weight

• 30
• 25
• 20
• 15
• 10
• 5

5 10 uV TGA D TA

TGA/DTA of Anode, in air; TGA/DTA of Anode, in air; Pore former is 50% Graphite (Alfa Pore former is 50% Graphite (Alfa Aesar Aesar) )

TGA TGA-

• DTA of anode pellet with

DTA of anode pellet with 30% graphite in N 30% graphite in N2

2 atm

atm

• 2

2 4 6 8 10 200 400 600 800 1000 1200 1400 Temp C % w eig h t L o s s

• 30
• 25
• 20
• 15
• 10
• 5

5 uV TGA DTA

Removal of pore formers

• nly begins at 1100oC

Sintering temperature is usually 1450 Sintering temperature is usually 1450o

• C up to 1550

C up to 1550o

• C

C Pore structure may have a strong dependence on temperature Pore structure may have a strong dependence on temperature

• f removal of pore former
• f removal of pore former

Work in Progress Work in Progress

• Preparation and testing of – chromites, titanates,

niobates and vanadates

• Impregnation of perovskites onto pre-formed

anode substructure

• Conductivity data on oxides as electrocatalysts

under reducing conditions – redox stability of

• xides, as well
• Systematic investigation of pore and particle

morphology

• Testing of anode supported cells on various fuels