Development of Nano-Structured Solid Oxide Fuel Cell Electrodes

G. Schiller, S.A. Ansar, M. Müller

German Aerospace Center (DLR), Institute of Technical Thermodynamics, Pfaffenwaldring 38-48, D-70569 Stuttgart, Germany

17th Annual International Conference on Composites/Nano Engineering (ICCE-17),Honolulu, July 26-31, 2009

Outline

Introduction Principle of planar SOFCDLR spray concept for SOFC

Nanostructured Cathode by using Deposition from liquid precursors RF Plasma Technology (TPCVD) Phase purity

Structure

Nanostructured Anode by using Agglomerated nanoparticlesDC Plasma Technology Suspension plasma spraying (SPS)

Solution precursor plasma spraying(SPPS)

Conclusion

Design and Principle of a Planar SOFC

Air

ReactionProducts

Fuel Gas

CathodeElectrolyteAnode

Bipolar Plate

UnconvertedAir

O2

H O2H O2

O2O2

H2

Load

Anode

Cathode

SolidElectrolyte

O2-

O2O2

H O2

H2H2

O2-

O2- O2-

+

- A

E

M

SOFC Metal-Supported Cell

30 m

25 m

35 mBipolar plate

Bipolar plate

porous metallic substrateanodeelectrolyte

contact layercathode current collectorcathode active layer

protective coating

not used airoxygen/air

air channel

fuel channel

fuel brazing not used fuel + H O2

(not in scale)

Plasma Deposition Technology

Thin-Film Cells

Ferritic Substrates and Interconnects

Compact Design with Thin Metal Sheet Substrates

Brazing, Welding and Glass Seal as Joining and Sealing Technology

Plasma Spray Laboratory at DLR Stuttgart

Experimental Setup

~Tasks:

spray angle 10-20°homogeneous spray (narrow droplet size distribution)

Present solution:

“air assist atomizer”:disintegration of liquid stream in gas jet

Atomization

Problems:

small dimensionsextremely high temperature

precursorgas

atomizing gas

z-adjustableRF plasma torch

peristalticpump

liquidprecursor

injection probecentral gassheath gas

500 KHz generator

plasma

coating

x-y-movablesubstrate

vacuumpump

window

vacuumreactor

Applied Precursor Solutions and Desired Synthesis Products

Synthesis product Precursor Concentration

A La0.9Sr0.1MnO3 (LSM)La(NO3)3 • 6 H2OSr(NO3)2Mn(NO3)2 • 4 H2O

0,9 M0,1 M1,0 M

B La0.5Sr0.5MnO3 (LSM)La(NO3)3 • 6 H2OSr(NO3)2Mn(NO3)2 • 4 H2O

0,5 M0,5 M1,0 M

C La0.65Sr0.3MnO3 (ULSM)La(NO3)3 • 6 H2OSr(NO3)2Mn(NO3)2 • 4 H2O

0,65 M0,3 M1,0 M

D Pr0.65Sr0.3MnO3 (UPSM)Pr(NO3)3 • 5 H2OSr(NO3)2Mn(NO3)2 • 4 H2O

0,65 M0,3 M1,0 M

E La0.8Sr0.2FeO3 (LSF)La(NO3)3 • 6 H2OSr(NO3)2Fe(NO3)3 • 9 H2O

0,8 M0,2 M1,0 M

F La0.8Sr0.2(Co,Fe)O3 (LSCF)

La(NO3)3 • 6 H2OSr(NO3)2Co(NO3)2 • 6 H2OFe(NO3)3 • 9 H2O

0,8 M0,2 M0,5 M0,5 M

G La0.58Sr0.4Fe0.8Co0.2O3(LSFC)

La(NO3)3 • 6 H2OSr(NO3)2Fe(NO3)3 • 9 H2OCo(NO3)2 • 6 H2O

0,58 M0,4 M0,8 M0,2 M

H Pr0.58Sr0.4Fe0.8Co0.2O3(PSFC)

Pr(NO3)3 • 5 H2OSr(NO3)2Fe(NO3)3 • 9 H2OCo(NO3)2 • 6 H2O

0,58 M0,4 M0,8 M0,2 M

X-Ray Diffraction Patterns of La0.9Sr0.1MnO3

25 35 45 55 65 75 852 Theta

Inte

nsity

(arb

itrar

yun

its)

LaMnO3

La2O3

25 35 45 55 65 75 852 Theta

Inte

nsity

(arb

itrar

yun

its)

LaMnO3

La2O3

LaMn perovskiteKey parameter:

Mn loss depends onradial distance fromplasma jet axis

Top: TPCVD coatingfrom central part of plasma jet

Bottom: TPCVD coatingfrom cooler margin

stationary substrate

X-Ray Diffraction Patterns of Pr0.58Sr0.4Fe0.8Co0.2O3

Top: TPCVD coatingfrom central part of plasma jet

Bottom: TPCVD coatingfrom cooler margin

stationary substrate

Microstructure of TPCVD Perovskite Coatings

Functional Principle of DC Plasma

Powder Jet

Subs

trat

e

Plasma GunParticle Melting and Acceleration

Particle Impingement

Pla

sma

Gas

es

ArH2N2He

Splat Layering

Coating

Particle Injection

Vacuum Plasma Spraying of SOFC Cells

NiO+YSZ Powder

Co-precipitation and Spray-Drying

22 vol%NiO + YSZ

Agglomerated

Agglomerate size: -50+10 µm

Primary particle size: 20-80 nm

Feedstock Powder for DC Plasma Spraying

As-Sprayed Anode Structure and XRD

2 µm

20 40 60 802 theta (°)

Z N

FeDeposit

Powder

ZZ

Z

NN

NiO+YSZ Plasma Sprayed Deposit

Particle size in deposit was 60-220 nm compared to 20-80 nm for the feedstock powder

Ni(II)O and cubic-YSZ were the phases detected by XRD 34 38 42 462 theta (°)

Z

N

Z

PowderDeposit

N= Ni(II)O

Z= Cubic-zirconia

Permeability of Anode

0

1

2

3

4

5

AS Reduced AS Reduced

Conven. NiO+YSZ Nano NiO+YSZ

Per

mea

bilit

y C

oeff.

(10

-14 m

²)

0

100

200

300

400

500

600

0 100 200 300 400 500

Time (h)

σ (Ω

/cm

)

0100200300400500

0 15 30 45

Time (h)σ (Ω

/cm

) Conventional AnodeNano Anode

Con

d(1

/ohm

.cm

)

Con

d(1

/ohm

.cm

)

Conductivity of Nano-Anode

Conductivity of nano and conventional anode were comparable for first 30 to 40 hrs

Conductivity of nano anode increased for extended period of test time- possible cause could be Ni particles phase I sintering

Test Atmosphere

2 slm Ar+5 vol% H2

Electrochemical Testing

1.017 V

1.074 V

230

292 309

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0 100 200 300 400 500 600 700 800 900Current density i [mA/cm²]

Cel

l vol

tage

U [V

]

0

200

400

600

800

Pow

er d

ensi

ty p

[mW

/cm

²]

Reference cell with conventional anode (); Cell with nanostructured anode after 100 h (Δ) and after 1500 h (◊) of operation 800°C - Cell area 12.6 cm² - Gases: 0.5 slm H2+0.5 slm N2/2.0 slm air).

• Cells containing a nano anode show 34% higher power density• Anodic polarization at OCV of nano anode was 0.42 Ωcm² instead of 0.72 Ωcm²

for conventional anode • 3.33%/kh degradation rate for cells with nano anode is comparable to cells having

conventional anodes – No evidence of additional degradation due to nanomaterials

Nano Anode Structure after SOFC Operation

2 µm

2 µm

After 100 h of operation After 1500 h of operation

Limited grain growth and sintering Particle size: 60 to 220 nm after 100 h and 95 to 390 nm after 1500 hExpected mechanisms are phase I or gas-phase sintering

Suspension and Solution Precursor Plasma Spraying

Suspension Plasma SprayingInjection of nano-particles in plasma by suspending them in a liquid.

Solution Precursor Plasma SprayingIn-flight nano-particles synthesis by chemical reaction of metal salt precursors in plasma.

Suspension Plasma Spraying

YSZ 40 nm particles and development of stable suspension using Zeta potential

Development of stable suspensions

05

1015202530

0 2 4 6 8 10 12Dispersant [wt.%]

zeta

-pot

entia

l [m

V]

Nanostructured porous YSZ depositPorosity 35%

Outlook:Additon of NiO for anode and LSM for cathode

Conclusion

Nanostructured cathodes and anodes for SOFC were prepared by applying plasma deposition processes (RF and DC plasma)Two approaches were applied:- Introduction of pre-synthesized nanoparticles as agglomerates or

suspensions into the plasma- In-situ synthesis of nanomaterials and deposits from solutions of

metal saltsTPCVD cathodes initially exhibited undesired secondary phases whichwas overcome by adjusting the chemical composition of the precursormaterial. The microstructure was columnar type with very high openporosityFor 1500 hours of operation only limited growth of nanosized particleswas observed in SOFC anodesFurther improvement of the microstructure of anodes is in progressusing DC plasma suspension and solution precursor plasma spraying

Synthesis from Liquid Precursors in an RF Plasma

Why liquid precursors?cost reduction

continuous feeding of material

homogeneous distribution in the plasma

synthesis of thermally instable materials

simple adjustment of stoichiometry

Why r.f. plasma?

large volume, low jet velocity, i.e. more complete synthesis

axial material injection

electrodeless, i.e. oxidizingconditions possible

central gassheath gas

induction coil

flange

exit nozzle

ceramic tube

quartz glas tube

gas distributor

polymer body

injection probe

precursors andatomization gas

X-Ray Diffraction Patterns of La0.58Sr0.4Fe0.8Co0.2O3

2Theta30.0 40.0 50.0 60.0 70.0 80.00.0

100.0

200.0

300.0

400.0

500.0

600.0

Abs

olut

e In

tens

ity

LSFC auf Cr5Fe, TPCVD, zentrum, HF0224[48-124] La0.6 Sr0.4 Co0.8 Fe0.2 O3 / Cobalt Strontium Iron Lanthanum O

[5-602] La2 O3 / Lanthanum Oxid

2Theta30.0 40.0 50.0 60.0 70.0 80.00

400

800

1200

1600

2000

Abso

lute

Inte

nsity

LSFC auf Cr5Fe, TPCVD, Rand, HF0224[48-124] La0.6 Sr0.4 Co0.8 Fe0.2 O3 / Cobalt Strontium Iron Lanthanum O

[34-396] Fe - Cr / Iron Chromium

Top: TPCVD coatingfrom central part of plasma jet

Bottom: TPCVD coatingfrom cooler margin

stationary substrate

IUR

cmS

AR

lel

ΔV

I = 10 mA

0

100

200

300

400

500

600

700

800

900

0 10 20 30 40 50 60Operation Time / h

Tem

pera

ture

/ °C Temperature

l

Atmosphere:Purge gas (Ar / 5vol% H2)

Electrical Conductivity Measurement

0.5 slpm H2 + 0.5 slpm N2

2,0 slpm Air

Gas volume flowanodecathode

12.57 cm2Effective area800°COperating temperature

Air

H2 + N2

1 SOFC2 Gold sealing3 Platinum net4 Glass sealing5 Housing6 Weight

Electrochemical Testing of Full Cells