Harbin Institute of Technology

Design and Analysis of MEMS-based direct methanol fuel cell

Yuan ZhenyuHarbin Institute of Technology

2010.10

Presented at the COMSOL Conference 2010 China

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Contents

Application of COMSOL

Summary

Principle

Background

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µDMFC for speaker and gadget charger, Sony (Japan), 2009

µDMFC for notebook, Ultracell (USA), 2007

µDMFC for cell phone, Motorola (USA), 2008

µDMFC for PDA, Hitachi (Japan), 2005

BackgroundDue to the unique advantages, micro direct methanol fuel cells (µDMFCs) have been considered as the most promising candidates for the micro power sources of mobile devices.

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MEMS Center, HIT

0.64cm2，open circuit voltag:520mV，power density:5.9mW/cm2

fuel cell stack，open voltage:2.75V，

power density:6.8mW。

Open voltage:650mVpower density:15.9mW/cm2

Powered LED to work for 5h

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contents

Application of COMSOL

Summary

Principle

Background

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Anode CathodeMEA

Methanol

CO2e-H+

O2

H2O

Methanol

CO2e-H+

O2

H2O

Methanol

CO2e-H+

O2

H2O

Methanol

CO2e-H+

O2

H2O

Methanol

CO2e-H+

O2

H2O

Methanol

CO2e-H+

O2

H2O

Methanol

CO2e-H+

O2

H2O

Methanol

CO2e-H+

O2

H2O

Methanol

CO2e-H+

O2

H2O

Methanol

CO2e-H+

O2

H2O

Methanol

CO2e-H+

O2

H2O

Methanol

CO2e-H+

O2

H2O

Methanol

CO2e-H+

O2

H2O

Methanol

CO2e-H+

O2

H2O

Methanol

CO2e-H+

O2

H2O

Methanol

CO2e-H+

O2

H2O

Methanol

CO2e-H+

O2

H2O

Methanol

CO2e-H+

O2

H2O

Methanol

CO2e-H+

O2

H2O

Methanol

CO2e-H+

O2

H2O

Methanol

CO2e-H+

O2

H2O

Methanol

CO2e-H+

O2

H2O

Methanol

CO2e-H+

O2

H2O

Methanol

CO2e-H+

O2

H2O

3 2 2 6 6CH OH H O CO H e

2 23 6 6 32

O H e H O

3 2 2 23 22

CH OH O CO H O

（1）current collector material （2）structure design （3）mass transfer of MEA （4）theoretical analysis

COMSOL Multiphysics

anode:

cathode:

overall:

Principle

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contents

Summary

Principle

Application of COMSOL

Background

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3.1Two-dimensional,two-phase mass transport——mass transport，structure of

DMFC

3.2 µDMFC Three-dimensional model

A novel cathode——structure，stability

A air-breathing cathode flow field——water-flooding

Anode structure ——optimize structure

3.Application of COMSOL

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3.1A two-dimensional two-phase mass transport model ：

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Mathematical model

• continuous equation

two-phase mass transport in the diffusion layer ( )l l lS u

( )g g gS u

•momentum transport equationrl

l ll

Kk p

u

rgg g

g

Kkp

u

3rlk s

31rgk s

• Mass transport equation , , , ,

effi l i l i l l i lD C C S u 1.5 1.5

, ,effi l i lD D s

, , , ,effi g i g i g g i gD C C S u 1.51.5

, , 1effi g i gD D s

•pressure difference 0.5cos /c g l cp p p K J s

2 3

2 3

1.417 1 2.120 1 1.263 11.417 2.120 1.263

s s sJ ss s s

0 9090 180

oc

o oc

Leverette function

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Mathematical model

• momentum transport equation

two-phase mass transport in anode channel l l

l l l l l l lpt

u

u u u g

CO2 velocity—— g l slip u u u

• continuous equation( ) 0l l u

2

2

11 0CO

CO gt

u

• Mass transport of methanol —— 0effMeOH MeOH MeOH lD C C u 1.5eff

MeOH MeOHD D

two-phase mass transport in cathode channel•momentum transport equation——

H2O velocity——

•continuous equation

•Mass transport of O2——

1

1 1 1g gg g g g g g gp

t

uu u u g

g lu u ( 1 ) 0g g u 2

20H O

H O lt

u

2 2 20eff

O O O gD C C u 2 2

1.51effO OD D

34

dl slip slip l

b

C pd

u u 16Red

b

C

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Mathematical model

•Mass transport of methanol （Concentration and electro-osmotic）——

mass transport in PEM

•Mass transport of H2O——

The transportations of electron and proton

, , , 6peff m

MeOH cross MeOH mem MeOH mem d

iiN D C nF F

2 ,H O cross diN nF

, 0s eff s , 0m eff m

Electrochemical kinetics• Anode—— , exp( )m aclref a

m arefm acl

C Fi i sC RT

,

,

01

refm acl m

refm acl m

C CC C

• Cathode—— 2

2

2

,1 exp( )O cclref cc O cref

O ccl

C Fi i sC RT

• Current density—— p ci i i

Heat transfer• Anode catalyst layer—— ( )6

a aacl a a

H Gq iF

• Cathode catalyst layer—— ( ) ( )4 6c c a a

ccl c c c aH G H Gq i i i

F F

0.1 /refmC mol L

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Comparison of the modeling results and experimental results

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Model result effect of different methanol concentrations

Effect of the operating temperature

, , , 6peff m

MeOH cross m mem m mem d

iiN D C nF F

diffusion mosiselectroos

mosis

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Model resultCO2 content in anode flow field

Current density Flow rateTemperature

•CO2content increase with the current density and temperature•The anode flow rates have the effect of the removal rate of the CO2

• in accord with the results of Yang[1]and Liao[2]

[1] H. Yang, T. S. Zhao, Q. Ye. J. Power Sources, 2005, 139: 79-90 [2] Q. Liao, X. Zhu, X. Y. Zheng, et al. J. Power Sources, 2007, 171: 644-651

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Simulation Result

•stainless steel mesh

100%cell ath volt fuel

cell c

V iGH E i

Temperature distribution in the µDMFC with different current collector materials: (a) silicon; (b) stainless steel; (c) PMMA.

excellent qualities of the silicon and stainless steel in heat transfer, uniform temperature distributions were achieved in their corresponding cells

smaller methanol concentrationin the anode catalyst layer, obtain the better performance

•Journal of Power Sources 195 (2010) 7338–7348（IF=3.792）

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3.2 µDMFC three-dimensional Model3.2.1An air-breathing direct methanol fuel cell with a novel cathode shutter current collector

Current collector

Inlet

Diffusion layer

PEM

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MEMS Center, Harbin Institute of Technology Harbin, 150001, China

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The mass fraction of oxygen simulation results with the two types of collector.

The velocity simulation results in diffusion layer with two types of collectors.

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Test result

All experiments of the stack were performed with air self-breathing at room temperature on the cathode. Flow rates of 2 mL/min for 2 M methanol solution were supplied bya peristaltic pump.

international journal of hydrogen energy 35 ( 2010 ) 5638-5646(IF=3.945)

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3.2.2a three-dimensional two-phase-flow model for self-breathing µDMFCs

structure

In order to ensure that the oxygen from air could enter the electrochemical reaction sufficiently and is well-distributed, the intension of this paper is to design a self-breathing μDMFC with a new cathode spoke structure

a similar back interface (air-contacting interface) structure to conventional perforated cathode, its front interface (electrode-contacting interface) is equipped with a certain number of blade-form channels which are connected with self-breathing circular openings to form a “spoke” unit.

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Schematic of simulation domains of the three-dimensional model

Governing equations

•electronic current ——,( )l eff l a cS i ,( )s eff s a cS i

•Multicomponent mixed gas diffusion ——

3

1{ ( )ij j

i g i j i gj j

MD wn w w M w

M M

（Maxwell-Stefan）

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Governing equations• oxygen concentration ——

2 2O OD c R c （ ） gu

• mass transport equation of the liquid-phase0.5( )( ) ( )) 0l rl

c wl l

Kk cos J s RM K

（

• The gas flow in the cathode flow channel is expressed by N-S equation:

( ( ) ) ( ) 0Tg g g cp

t

g

g g g g

uu u u u

• The gas velocity in the diffusion layer rgg

g

Kkp

gu

• Butler-Volmer equation is used for explaining the relation between current and potential:

2

2

0, 0

( )( ) exp( )o c S lc

o ref

c Fi ic RT

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Result and discussion compare of different cahtode

•increases the efficiency of oxygen mass transportmake the concentration more equal

•promotes the water transfer to PEM

• declining the inner resistanceamong PEM

•Enchancing Electrocatalytic Activity•Promoting the air vaporization

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compare of different cahtode

•The latter shows a lower content than the former•indicating a higher discharge capability

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Model result different numbers of blades

•the eight-blade cathode showsthe highest•The distributions of thefour-bladeand the eight-blade are relatively uniform

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Test reult cathode structure

Performance comparison Durability number of blade

Performance comparison

two-blade——13.33mW/cm2

four-blade——14.79mW/cm2

eight-blade——14.01mW/cm2

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Methanol concentration distributions at the catalyst layers under different flow fields are 586.58、585.29、602.03和590.93mol/m3

single serpentine field flow has best performance

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Current density distributions at the catalyst layers along x direction under the conditions of different open ratios.

Current density distributions at the catalyst layers along x direction under the conditions of different channel depth.

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Comparison of the line pressure drop at the interfaces between the diffusion layers and two single-serpentine flow fields

Methanol concentration distributions

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contents

Application of COMSOL

Principle

Summary

Background

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4.summary

outlook：•Methanol crossover•Analyses of the fuel cell stack assembly pressure

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THANKS！