The unification of paste rheologies for the co-extrusion of solid oxide fuel cells

The Unification of Paste

Rheologies for the

Co-extrusion of a Solid

Oxide Fuel Cell

Jonathan Powell1, Stuart Blackburn2, 3

1. Department of Metallurgy and Materials, 2. Department of Chemical Engineering and

3. Interdisciplinary Research Centre in Materials Processing

Outline

Introduction to solid oxide fuel cells.

Co-processing of a solid oxide fuel cell.

Experimental method.

Results and discussion.

Conclusion.

Introduction

SOFCs are highly efficient and provide

environmentally friendly and sustainable power.

Electrochemical conversion of chemical potential

energy into electrical potential energy.

Current SOFC manufacturing methods involve

multiple processing steps to form and sinter the

layers.

This leads to higher manufacturing costs.

Electrolyte

Fuel electrode

Air electrode

Silicone

sealant

Air

outlet

Insulation

Exhaust

Water heat

exchanger

Air inlet

manifold

Heat exchanger

Tubular fuel

cell

Cathode

Fuel

manifold

Fuel/air

premix

Anode

Cell rack (one of 40)

Ref: T.Alston, K.Kendall et al, J. Power Sources, 71, 1-2, 271, 1998C

Co-Processing

Self supported tubular SOFCs can be co-

processed.

– Co-extruded

– Co-sintered

This reduces the number of processing steps

and it should therefore reduce the

manufacturing costs.

Schematic of the co-extrusion

process

Method

Rheology of two end-member pastes is characterised.

– Solids loading varied.

Two pastes with equivalent rheology obtained.

Void volume approximated.

Packing theory used to predict the void volume of

intermediate pastes.

Solids loading and formulation for an intermediate paste

of equivalent rheology is obtained.

Materials

Electrolyte

– Yttria Stabilised Zirconia powder

HSY8

Daiichi Kigenso Kagaku Kogyo Co., Japan

0.5 μm

Anode

– Black NiO

Cerac, USA

-325 mesh

– Activated carbon

Norit, UK

– HSY8

Organic binder

– FMG, UK

Paste Preparation

Pastes mixed using two methods;

– Twin roll milling machine.

– Z-Blade kneader.

Anode pastes unable to mix in the z-blade

due to insufficient shear.

Paste Characterisation

Capillary rheometry

Extruded through 6.0 mm dies with lengths

8.0, 24.0 and 47.3 mm.

Extrudate velocities ranging from 5.000 to

0.125 mm.s-1.

90o entry angle.

Barrel diameter 20.3 mm.

Benbow Bridgwater equation

Paste flow modelled using Benbow/Bridgwater six parameter equation;

– P is the total pressure drop.

– P1 and P2 are the pressure drop due to convergent flow and plug flow.

– D0 and D are the barrel diameter and die diameter.

– L is die length.

– V is the extrudate velocity.

– σ0 is the convergent flow yield stress.

- t0 is the die wall yield shear stress.

– β and α are the velocity factors.

– V is the extrudate velocity.

– m and n are the velocity exponents

nm VD

L

D

DVPPP t

0

0021 4ln)(2

Results

0

5

10

15

20

25

30

35

40

45

0 1 2 3 4 5 6

Extrudate Velocity (mm/s)

Ex

tru

sio

n P

ress

ure

(M

Pa)

Model (L/D=1.4)

Experimental (L/D=1.4)

Model (L/D=4.0)

Experimental (L/D=4.0)

Model (L/D=8.0)

Experimental (L/D=8.0)

Results

0.00

0.02

0.04

0.06

0.08

0.10

0.12

10 11 12 13 14 15 16 17 18 19 20

Liquid Content (wt%)

1/α

(M

Pa.s

.m-1

)-1

Electrolyte – Z-Blade

Anode – Twin Mill

Electrolyte – Twin Mill

Equivalent to void volume of randomly packed powders

Rheology is dependent

upon the mixing

method. The measured

void volume is

unaffected. Void = 16.6 wt%

Void = 10.7 wt%

Properties of end components

Anode paste Z-blade mixed

electrolyte paste

Liquid wt % 17.7 11.7

σ0 MPa 1.0 0.7

α MPa.s.m-1 23.3 23.3

β MPa.s.m-1 5.3 10.6

τo MPa 0.1 0.1

NiOYSZ

RBV1.00

0.00

2.00

YSZ

3.00

2.00 RBV

Anode

1.00

Electrolyte Anode

NiO

YSZ

RBV 1.00

0.00

2.00

2.61

2.04

1.88

1.84 1.78 1.72 1.69

1.67YSZ NiO

Carbon3.70

2.61

1.88

A

BC

Electrolyte

Anode

2.61

2.04

1.88

1.84 1.78 1.72 1.69

1.67YSZ NiO

Carbon3.70

2.61

1.88

A

BC

Properties of intermediate paste

Anode paste Z-blade mixed

electrolyte paste

Intermediate

Paste

Liquid wt % 17.7 11.7 9.3

σ0 MPa 1.0 0.7 0.81

α MPa.s.m-1 23.3 23.3 26.28

β MPa.s.m-1 5.3 10.6 13.15

τo MPa 0.1 0.1 0.13

Conclusions

Void volume of powder mix can be accurately

measured using rheological data of a paste.

The method outlined is effective in predicting the

required liquid content to obtain a specific paste

rheology.

Using a tri-modal chart it should be possible to

predict the RBV any number paste formulations and

materials of various porosities.

Future Work

Obtain a complete tri-modal charts for anode

and cathode materials.

Co-extrude and co-sinter a complete solid

oxide fuel cell.

Aknowledgements

EPSRC for the funding of this project.

FMG for their advice on paste formulation.

Paul McGuire for his help with the modelling

of paste flow.