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Diagnosis of PEMFC operation using EIS

International Symposium on

DIAGNOSTIC TOOLS FOR FUEL CELL TECHNOLOGIES

Trondheim, Norway, June 2009

Hydrogen and

Fuel Cells Group

Electrical Research Institute

Félix Loyola, Ulises Cano-Castillo

ucano@iie.org.mx

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MEA STACK SYSTEM

quality performance reliability

degradation state of health

long life

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Characterization using d.c. techniques:

voltammetry – electrochem. active area

linear voltammetry – H2 crossover (int. short circuit)

polarization curves – performance

0 0,25 0 ,50 0,75 1,00-0,0005

0

0,0005

0,0010

0,0015

0,0020

E (Volts)

I (A

mps

/cm

2)

M6 00 s_t C ruce H 2_2 B.corM6 00 C ruce H2_A_ 2.cor

0 0,25 0 ,50 0,75 1,00-0,00 05

0

0,00 05

0,00 10

0,00 15

0,00 20

E (Volts)

I (Am

ps/c

m2)

M6 00 s_t C ruce H 2_ 2B.corM3 20 C ruce H 2_A_ 2.cor

Cruce H2 Cruce de H2

MEA i max (A/cm2) (mol/s-cm2) (mL/min-cm2)

M600 s_t 0.0013523 7.008E-09 0.01030

M600 0.00080397 4.166E-09 0.00612

M320 s_t 0.0010818 5.606E-09 0.00824

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Interest on EIS as applied to Fuel Cells

MEA development

• platinum activity follow-up

• bulk membrane resistance

• ionic conductivity at CL (through distributed element model)

Stack

• sensitivity to operating conditions

Systems control

• Potential flooding/drying detection

-0.04

-0.035

-0.03

-0.025

-0.02

-0.015

-0.01

-0.005

0

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07

Z'

Z''B i2 DE

B i2 SM

C i2 DE

C i2 SM

0

0.2

0.4

0.6

0.8

1

0 0.05 0.1 0.15 0.2 0.25

A/cm2

V

A

B

C

D

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- Combination of experimentally determined kinetic parameters with EIS-determined parameters for PEMFC dynamic model

Steady-state and transient models

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Facts & warnings:

• EIS highly sensitive to changes within the fuel cell

• EIS response is strongly dependent on design

• If gradients (heterogeneities) along active area exist,

EIS will pick them up

• “Dry” and excess of water may coexist in different

regions of FC (as well as other effects)

• EIS should not be seen as a black or white result but

as a color pallette (i.e. interpreted as such)

Q = can EIS bu used as a diagnostic technique during operation near

flooding/drying conditions?

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water dynamics near dry/wet limit

Dehydration Stage: (previously conditioned and purged)

• Tcell = 40 °C

• Cathode: P: 10psi; flow: air 0.5 L/min

• Anode: P= 10psi; gas exit closed

• t = 1hr, EIS for ohmic resistance @ Eoc

Rest Stage (no humidification):• Tcell = 40 °C

• Cathode, P = 10psi, Flow: air 0 L/min (just after dehydration process)

• Anode, P = 10psi, gas exit closed.

• t = 1hr, EIS for ohmic resistance @ Eoc

MEA: 25cm2, Gore, Pt = 0.7 mg/cm^2, carbon paper, DL = 50µµµµ,

GFF: simple serpentine

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0

10

20

30

40

50

60

70

80

0 20 40 60 80 100 120 140

Time (min)

Drying process 3

Redistribution ofwater

cell’s ohmic resistancedetermined by high frequency intercept

redistribution of

water from bulk

membrane

*low conductivity final current collectors plates used

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DLCLM

DLCLM

DLCLM

after conditioning

after high air flux

CL w/H2O gradient

after 1 hr rest

H2O redistributes in CL

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Experimental approach:

- Initial conditioning

- Operating near the limit of drying/flooding

- EIS (1Hz to 1KHz), 10mV (Ecell = 0.4V)

• low T & stoich’s

• dead ended configuration

• purge stages

• dry feeds

• use of O2

Real life is cruel for EIS:

Practical operating conditions are hardly under steady-state, it

depends on specific application and design

Own 50 cm2 MEA, GFFc = 4ps, GFFa pch. 0.7 mg Pt/cm2, Nafion NRE-212.

GDL-30-BC (C paper w/MPL). T=343.15 K (70°C) & 69 kPa (10 psi)

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2 3 4 5

-3

-2

-1

0

Z'

Z''

4SP EIS 78.z4SP EIS 79.z4SP EIS 80.z

4SP EIS 81.z4SP EIS 82.z

4SP EIS 83.z4SP EIS 84.z

gradual drying

out-of-phase shifts

in-phase

gradual drying

100 101 102

-10

0

Frequency (Hz)

thet

a

phase angle

increases and

shifts right

Drying

cathode stoichiometry = 4

not humidified

both Z” & Z’ increase

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2.0 2.5 3.0 3.5

-1.5

-1.0

-0.5

0

Z'

Z''

4SP EIS 63.z4SP EIS 64.z4SP EIS 65.z

4SP EIS 66.z4SP EIS 67.z

4SP EIS 68.z

out-of

-pha

se

grad

ual f

lood

ing

100 101 102 103

-14

-4

Frequency (Hz)

the

ta

Flooding

gradual flooding

1Hz

3.98Hz

5.01

6.30

7.94

10.00

12.58

freq. @

max. Imflooding

1

2

3

4

5

6

EIS: every 20 mins.

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Rs CPEa

Rct a

CPEc

Rct c

Rs = total ohmic resistance

Rct a = anode charge transfer resistance

CPEa = non-ideal double layer capacitance anode

Rct c = anode charge transfer resistance

CPEc = non-ideal double layer capacitance anode

X

CPE = distributed element, diffusional process

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0

0.5

1

1.5

2

2.5

3

3.5

4

0 20 40 60 80 100 120

t (min)

Rs o

hm

Rs (flooding)

Rs (drying)

resistive losses increase during drying

15

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 20 40 60 80 100 120

t (min)

Rct

oh

m

Rct (flooding)

Rct (drying)

kinetic losses increase during drying and flooding

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0

0.01

0.02

0.03

0.04

0.05

0.06

0 20 40 60 80 100 120

t (min)

CP

Ec

CPE (Flooding)

CPE (drying)

(sp/o

hm

)

during drying CPE impedance

increases (distributed nature of CL?)

during flooding CPE

impedance reduces

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100

1 01

102

103

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

F reque ncy (H z)

Z''

4 SP E IS 6 3 .z4 SP E IS 6 4 .z4 SP E IS 6 5 .z4 SP E IS 6 6 .z4 SP E IS 6 7 .z4 SP E IS 6 8 .z

100

1 01

102

-15

-10

-5

0

Frequency (H z)

theta

4SP EIS 63.z4SP EIS 64.z4SP EIS 65.z4SP EIS 66.z4SP EIS 67.z4SP EIS 68.z

100

1 01

102

103

- 15

- 10

-5

0

Freque ncy (H z)

theta

4 S P EI S 78 .z4 S P EI S 79 .z4 S P EI S 80 .z4 S P EI S 81 .z4 S P EI S 82 .z4 S P EI S 83 .z4 S P EI S 84 .z

100

1 01

102

103

-0 .60

-0 .35

-0 .10

Freque ncy (H z)

Z''

4S P EIS 78.z4S P EIS 79.z4S P EIS 80.z4S P EIS 81.z4S P EIS 82.z4S P EIS 83.z4S P EIS 84.z

Flooding Drying

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Conclusions:

- During drying, both in-phase and out-of-phase

impedance content increase

- During flooding only out-of-phase content increases

- For both cases it appears that there is one single

frequency threshold (~10Hz) from which out-of-phase

content starts to shift to:

• smaller frequencies for flooding

• larger frequencies for drying

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Phase angle vs. Imaginary content:

- θ seems to better define initial drying/flooding process

(one single frequency?)

- θ can be associated with concentration profiles (ac

voltammetry?)

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Recommendations:

• Minimize FC design effects (ε, GFF, GDL, CL, etc.), better

base-line during testing

• specific frequencies might be design-dependent: need

further studies

• dry/flooding case: comparison of states only as a short

time forcast

• Different FC sizes might need different approaches for

diagnosis

• Isolation of true dry and true flooding conditions is only

possible if homogenous internal conditions are achieved

• Structural effects should be studied (carbon support as a

conducting grid, i.e. additional capacitance or inductance

effects?)

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• Properties of components are needed particularly

substack layers (i.e. capillary properties, etc.)

This presentation was AH1N1 freeP!

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