OCT. 22-24, 2006 COMSOL USERS CONF. 2006 BOSTON, MA 1 Use of COMSOL Multiphysics for Optimization of an All Liquid PEM Fuel Cell MEA George H. Miley (Speaker),

OCT. 22-24, 2006COMSOL USERS CONF. 2006

BOSTON, MA 1

Use of COMSOL Multiphysics for Optimization of an All Liquid PEM Fuel

Cell MEA

George H. Miley (Speaker), Nuclear, Plasma

and Radiological Engineering

E. D. Byrd Electrical & Computer Engineering

University of Illinois at Urbana-Champaign

Urbana, IL 61801 USA


BOSTON, MA 2

Outline

NaBH4/H2O2 Fuel Cell

Description of Model Physical Layout Electrical Considerations Mass/Momentum Balance Considerations

COMSOL Application Mode coupling Pressure Differential Simulations and

Results Land Area vs. Permeability and

Conductivity Simulation and Results Conclusions


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NaBH4/H2O2 Fuel Cell

Use in fuel cells is a relatively new development H2/H2O2 and NaBH4/H2O2 cells were investigated at

NPL Associates, Inc., the University of Illinois (UIUC), and elsewhere

Have shown great results, demonstrating the general feasibility of a peroxide based cell

Excellent potential for space applications due to high power density and air (oxygen) independence.


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UIUC/NPL Direct Peroxide Fuel Cells

The sodium borohydride/hydrogen peroxide reactions.

eHOHNaBOOHNaBH 8824 2224

OHeHOH 222 222

Anode:

Cathode:


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15-W NaBH4/H2O2 Test Fuel Cell as assembled.

The 15-W cell shown here uses an integrated cooling channel to dissipate the waste heat generated in the relative small 25-cm2 active area.

An optimized version of this small cell generated 36-W at ~ 60ºC, representing the highest power density reported to date for a small fuel cell working at sub-100C.

Test Cells - Compact 1-30 W Power Units

Flow rate of approximately 200 cm3/minMinimal pressure drop even with parallel flow due to low flow rateTemperature rise of approximately 15°CHeat flux is approximately equal to electrical power (500-W)


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The 500-W UIUC/NPL NaBH4/H2O2 Fuel Cell Stack

The active area per cell was 144 cm2 and 15 cells were employed to provide a total stack active area of

2160 cm2.


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UIUC/NPL Direct Peroxide Fuel Cells


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Objectives for COMSOL modeling

Gain insight into behaviors governing flow and current distributions

Determine space (diffusion layer parameters, conductivity effects, flow channel and land dimensions) for detailed optimization physics

Guide future design improvements


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Model Description- geometry

Physical Layout Based on repetitive

cross section of MEA and flow channels.

Outlined area represents the physical model.

Portion of graphite plates included to see the current density in the plate and to be able to vary their conductivity.


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Model Description - electrical

Standard Electrical Model DC current conduction - applies to each section with different

conductivity (graphite, diffusion layers, membrane)

Butler-Volmer Equations

0

eqamda

ffaa

aaaa eewNaBH

wNaBHii

1

04

4,0

eqcmdc

ffcc

cccc eeOwH

OwHii

2

022

22,0

Anode: Cathode:


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Modified Bulter-Volmer

The Butler-Volmer equation was modified to obtain an alternative version that is more robust when solving numerically in Comsol. In this version, the hyperbolic identity of Eq. 2-5 is used to form Eq. 2-6.

(2-5) (2-6)

)exp()exp(21)cosh( xxx

cccc

aaaa

fOwH

OwHii

fwNaBH

wNaBHii

cosh2

cosh2

022

22,0

04

4,0


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Model Description – conservation equations

Mass Balance

Momentum Balance – Darcy’s Law

pk

v

iRiDi u


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COMSOL Application Mode Coupling


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Parameters used

Necessary parameters (other than exchange current and equilibrium potentials, discussed next) were acquired through experimental means or published values These include the conductivities, permeabilities, diffusion coefficients, and viscosities given in the following table.


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Parameter set 1

Parameter Value Parameter Value σ_Nafion 15 S/m p_in 1.013e5 Pa

σ_Diffusion 2500 S/m p_diff 500 Pa σ_Graphite 16670 S/m D_H2O2 3.47e-9

m2/s κ 1.22e-11

m2 D_NaBH4 3e-9 m2/s

a 1.5 cP D_NaBO2 1.23e-9 m2/s

c 1 cP drag 3


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Parameter set 2 - determination of the exchange current density and reversible

potential

A Hydrogen half-cell was constructed and used to determine the exchange current density and additional parameters such as the Tafel slope in the Butler-Volmer eqns.. The reversible potential of each cell half was determined using the Gibb’s Free Energies applied to the reactants and products in each reaction.


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Model verification: I-V Curve calculated for the reference case agrees well with corresponding experiment

– model next used to explore design changes


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Simulations – Pressure Differential

Vary the pressure differential between the two flow channels.

Reasons Different flow

velocities create different pressure differences

Different locations have different pressure drops


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Simulations – Pressure Differential- Higher values optimal

Results Low pressure drops

cause less permeation in the diffusion layer, causing mass transport losses.

High pressure drops allow reactants to easily reach under the land area.

Reactant permeation under flow channel depends on fluid velocity and location along channel.


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Simulations – Land Area selection

Current collector land area width to flow channel width ratio is varied (collector + channel widths = constant). Land Area Width varied while also varying

diffusion layer permeability. Land Area Width varied while also varying

diffusion layer conductivity.


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Simulations – Land Area – high permeability give flexibility in width

Max Power at different Permeability with varying land areas.

Low Permeability diffusion layers have optimum current collector land area to flow channel ratio.

High Permeability diffusion layers function well with wide current collector widths.

Permeability

Maximum Power vs. Land Area Width


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Simulations – Land Area – optimum with high conductivity and equal width design

Max Power at different Conductivity with varying land areas.

High conductivity diffusion layers have are optimum with equal width current collectors and flow channels.

Low conductivity diffusion layers function better with wider land areas and narrower flow channels.

Conductivity

Maximum Power vs. Land Area Width


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Conclusions

Simulations performed of all-liquid PEM fuel cell using COMSOL Multiphysics.

Normalization uses data from half cell for io and Vrev. Pressure differentials, conductivities, permeabilities, and current

collector widths varied in the simulations. Cell performance varies with different flow velocities and along the

flow channels. Optimum current collector widths predicted for diffusion layers with

known conductivities and permeabilities. Model is very useful for optimization in region around normalization. Simulations narrow region for experimental studies to zone near

optimum performance. Greatly reduces time and expense of experimental studies.


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Acknowledgement

We would like to thank: NPL Associates, Inc. for their support with starting

the project. E. Byrd wishes to acknowledge fellow researchers

N. Luo, J. Mather, G. Hawkins, and L. Guo for their help.

This research was supported by DARPA SB04-032. Continuing studies are supported by DARPA/AFRL.


BOSTON, MA 25

Thank You

Dr. George. H. Miley

UIUC

Phone: (217) 333-3772

email: [email protected]

Ethan D. Byrd

UIUC

email: [email protected]

For more information please contact:

Documents

OCT. 22-24, 2006 COMSOL USERS CONF. 2006 BOSTON, MA 1 Use of COMSOL Multiphysics for Optimization of an All Liquid PEM Fuel Cell MEA George H. Miley (Speaker),