Theory and Application of Electrochemical Impedance Spectroscopy for Fuel Cell Characterization

Wagner N., Schiller G., Friedrich K.A. Deutsches Zentrum für Luft-

und Raumfahrt e.V. (DLR)

Institut für Technische Thermodynamik Pfaffenwaldring 38-40, 70569 Stuttgart

Presentation

outlineIntroduction

MotivationTypes

of Fuel

Cells

Experimental set-up

for

different types

of FCs

Modeling

of fuel

cells

with

equivalent

circuits

and microstructure

of fuel

cells

electrodes

Impedance

models

of porous

electrodesDifferent applications

of EIS in FC research

Contributions

to performance

loss

of PEFCEIS on segmented

SOFC

EIS measured

on Ag-gas

diffusion

electrodes

Conclusion

and Outlook

Motivation

Characterization of Fuel Cells by Electrochemical Impedance Spectroscopy:

Determination of electrode structure and reactivity, separation of electrode structure from electrocatalytical

activity

Determination of electrochemical active surface (locally resolved)Determination of reaction mechanism and separation of different overvoltage contributions to the fuel cell performance lossDetermination of degradation mechanism of electrodes, electrolyte and other fuel cell components (bipolar plates, end plates, sealings, etc.)Determination of optimum operation condition (e.g. gas composition, temperature, partial pressure), cell design (flow field) and stack design

Schematic

representation

of main

types

of fuel

cells

AFC80 °C

PEM80 °C

PAFC200 °C

MCFC650 °C

SOFC1000 °C

O2

H2

AlkalineFC

PhosphoricAcidFC

MoltenCarbonate

FC

SolidOxide

FC

PolymerElectrolyt

MembraneFC

H2

OH-

H+

H+

CO3

-2 O-2

O H O2 2

H H O2 2

O H O2 2

H H OCO CO

2 2

2

H H OCO CO

2 2

2

CO O2 2 O2Current

Load

Oxidant

Anode

Tem perature

Charge carrierin electrolyte

Cathode

Fuel gas

Schematic

representation

of main

types

of fuel

cells

AFC80 °C

PEM80 °C

PAFC200 °C

MCFC650 °C

SOFC1000 °C

O2

H2

AlkalineFC

PhosphoricAcidFC

MoltenCarbonate

FC

SolidOxide

FC

PolymerElectrolyt

MembraneFC

H2

OH-

H+

H+

CO3

-2 O-2

O H O2 2

H H O2 2

O H O2 2

H H OCO CO

2 2

2

H H OCO CO

2 2

2

CO O2 2 O2Current

Load

Oxidant

Anode

Tem perature

Charge carrierin electrolyte

Cathode

Fuel gas

Experimental set

up and cells

used

for

EIS

Fuel

„half“

cell

with

liquid electrolyte

Segmented

and single

PEFC cell

(polymer electrolyte)

Test cell

for

SOFC (short

stack)

(Solid Oxide Electrolyte)

Fuel

cell

overvoltage

and current

density

/ voltage

characteristic

Cathode

d +(r )P

oten

tial

Current density (Current/Surface?)

0, Cathode

ct,C

d +(r )

ct,AAnode

Cell

Voltage

(UC

)

Hydrogen Oxidation Reaction (HOR):

H2 = RT/2F i/i*

Oxygen Reduction Reaction (ORR):

O2/air = RT/[(1-)2F] [ln i - ln i*]

Ohmic loss

= iR

Transport limitation (diffusion)

d = -

RT/2F ln (1 - i/ilim )

Fuel cell voltage

UC

= U0

-

ct,H2

-

ct,O2/air

-

d

-

U0

0Cathode

Electrochemical

Impedance

Spectroscopy: Application

to Fuel

Cells

Electrochemical

Impedance

Spectroscopy: Application

to Fuel

Cells

Schematic representation of the different steps and their location during the electrochemical reactions as a function

of distance from the electrode surface

N. Wagner, K.A. Friedrich, Dynamic Response of Polymer Electrolyte Fuel Cells in „Encyclopedia of Electrochemical

Power Sources“

(Ed. J. Garche

et al.), ISBN-978-0-444-52093-7, Elsevier

Amsterdam, Vol.2, pp. 912-930, 2009

Overview

of the

wide

range

of dynamic

processes

in FC

10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103 104 105 106 107 108

microseconds milliseconds seconds minutes hours days months

Electric double layercharging

Charge transfer fuel cellreactions

Gas diffusion processes

Membrane humidification

Liquid watertransport

Changes in catalyticproperties / poisoning

Temperatureeffects

Degradation and ageing effects

Time / s10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103 104 105 106 107 108

microseconds milliseconds seconds minutes hours days months

Electric double layercharging

Charge transfer fuel cellreactions

Gas diffusion processes

Membrane humidification

Liquid watertransport

Changes in catalyticproperties / poisoning

Temperatureeffects

Degradation and ageing effects

Time / s

Bode representation of EIS measured at different current densities, PEFC operated at 80°C with H2

and O2

at 2 bar

Phase o Impedance / m

Frequency / Hz

0

20

40

60

80

10

20

15

30

50

10m 100m 1 10 100 1K 10K 100K

Diffusion RMCharge transfer

of ORR

O

V=597 mV; i=400 mAcm-2

V=497 mV; i=530 mAcm-2

V=397 mV; i=660 mAcm-2

+

V=317 mV; i=760 mAcm-2

Charge transfer

of HOR

PEFC: Schematic

Diagram (cross section)

Common Equivalent

Circuit

for

Fuel

Cells

Cdl,a

RM

Rct,a

Cdl,c

Rct,c

Common Equivalent

Circuit

for

Fuel

Cells

Cdl,a

RM

Rct,a

Cdl,c

Rct,cZdiff

Diffusion of O2

Common Equivalent

Circuit

for

Fuel

Cells

Cdl,a

RM

Rct,a

Cdl,c

Rct,c ZdiffZdiff

Diffusion of H2

?

SEM micrograph

of PEFC elctrode

(Pt/C+PTFE)

TEM micrograph

of Carbon

Supported

Platinum

Catalyst

SEM-picture

of Silver-Gas

Diffusion Cathode

SEM picture

of PTFE/C powder

Field

of application

of porous

electrodesBatteries

and supercaps

Process

fluids

Hydro- gen

GDE

Packed

bed

cathodeMembrane

Auxiliary

supply

Current

collector

Water purification

and treatment

(Bio)-Organic

synthesis

Fuel

Cells

O22H

O22H O ,membranereaction layerdiffusion layer

flow field/current collector

electrons

l c i o ee e tr cal p w r

r t np o o s

a h danode c t o e

Electrolysis

(Water, NaCl, HCl, etc.)

NaCl H2 O

NaOH

Cl-Na+

OH-+ -

Cl2

O2

imag

inar

y pa

rt /

real part /

0

-3

-2

-2.5

-1

-1.5

-0.5

-1 -0.5 0 0.5 1 1.5 2

C=500mFPore

Nyquist representation

of Impedance

of RC- transmission

line, model

of a flooded

pore

R

C

R = 3 Ω C = 0.5 F

RCiCi

RiZ

coth)(

R0 R0

= R/3 = δL/3πr2

δ

= specific

electrolyte

resistancer = pore

radius

L = pore

lenght

Lr

100 mHz

Nyquist representation

of porous

electrode impedance

with

faradaic

impedance

element

imag

inar

y pa

rt /

real part /

0

-3

-2

-2.5

-1

-1.5

-0.5

-0.5 0 0.5 1 1.5 2 2.5

C=500mFC+Rpor(3 Ohm)C//R(1.5 Ohm)

r

c rct

r = 3 c = 500 mFrct

= 1.5

Agglomerated

Electrodes

Hierarchical model (Cantor-block model)

metal side

electrolyte sideionic current

Gas (backing) side

electrolyte sideionic current

M. Eikerling, A.A. Kornyshev, E. Lust J. Electrochem. Soc., 152

(2005) E24

ll

l/al/alz

lz/az

n = 0

n = 1

ll

l/al/alz

lz/azn = 2

S.H. Liu, Phys. Rev. Letters, 55(1985) 5289T.Kaplan, L.J.Gray, and S.H.Liu, Phys. Rev. B 35 (1987) 5379

mZZe

Current collector GDL

electrolyte pores

porous layer

Zs1 ZsnZsi

ZpnZpiZp1

Z q1 Zqi Zqn

H. Göhr in Electrochemical Applications/97, www.zahner.de

Cylindrical homogeneous porous electrode model (H. Göhr)

Ions (H+, OH -,..)

I I

Por

e

Ele

ctro

de, p

orou

s lay

er

Electrolyte Zq

Zp ZS

Zo

Zn

Current (e-)

Electrochemical

Impedance

Spectroscopy: Experimental Set-up

Electrochemical workstation

PEFC

Flow contollerPressure regulator

Humidifier

Bode diagram of measured EIS at different cell voltages

Phaseo

Impedance /

Frequency / Hz

0

20

40

60

80

10m

30m

100m

300m

1

3

10m 100m 1 10 100 1K 10K 100K

O

E=1024 mV; I=0 mA

E=841 mV; I=1025 mA

E=597 mV; I=9023 mA+

E=317 mV; I=17510 mA

EIS at Polymer Fuel

Cells

(PEFC): Contributions to the cell impedance at different current densities

0.001

0.041

0.081

0.121

0.161

0 100 200 300 400 500 600 700

Current density /mAcm-2

Cel

l im

peda

nce

/Ohm

Rdiffusion

Rmembrane

R anode

R cathode

0,001

0,01

0,1

1

10

0 22 117 236 656

Current density / mAcm-2

Cel

l im

peda

nce

/ O

hm

EIS at Polymer Fuel

Cells

(PEFC): Contributions to the overal U-i characteristic determined by EIS

200300400500600700800900

10001100

0 100 200 300 400 500 600 700 800

Current

density

/ mAcm-2

Cel

l vol

tage

/ mV

E0

EC

EA

EM

EDiff.

Cdl,a

RM

RA

Cdl,c

RK

CN

RN

0

5

10

15

20

25

30

35

40

0 2 4 6 8 10 12 14 16 18

Current / A

Pore

ele

ctro

lyte

resi

stan

ce /

mO

hm

0

200

400

600

800

1000

1200

Cel

l vol

tage

/ m

V

Evaluation of EIS with the porous electrode model Summary of current density dependency of pore resistance elements

Pore Electrolyte

Resistance

AnodePore Electrolyte

Resistance

Cathode

Segmented

SOFC cell

design

with

segmented

bipolar plates

16 Segments withfuel

gas channels Capillary for gas chromatography

Current

probe 16 Segments withair

flow

channels

Voltageprobe

Metallic housing

Thermo- couple

SOFC

13 14 15 16

9 10 11 12

5 6 7 8

1 2 3 4

fuel

gas air

22

20 lnHO

OHrevrev pp

pzFRTUU

Nernst equation:

Produced water:S4: 0.61%, S8: 0.72%, S12: 0.78%, S16: 3.30%

OCV distribution of ASC at 800°C and simulated reformate (50% H2 + 50% N2 + 3% H2 O, 0.08 SlpM/cm² air)

Fuel gas Air

Voltage

EIS at OCV, ASC with

segmented

cathode, 77.44 cm2

Dry

hydrogen

Hydrogen+ 3% H2

O

Bode Diagram of EIS, measured

at PEFC, 75°C, 0.5 Acm-2 Variation of gas flow

rates

0

15

30

45

60

75

90

2

5

10

20

1 3 10 100 1K 10K

O

1.5H; Air = 1.2; E = 644 mV+

1.5H; Air = 1.5; E = 675 mV

1.1H; Air = 2; E = 653 mV

1.5H; Air = 2; E = 654 mV active surface area 50 cm2

Impedance / m Phase / °

o

EIS on PEFC, 80°C, 5 A, cathode fed with different gas composition, λ=1.5, N111 IP CCM (Ion Power Inc.)

100m 1 2 5 10 30 100 300 1K 3K

8

10

20

15

25

|Z| / m

0

15

30

45

60

75

90|phase| / o

frequency / Hz

Air 5 A

50% He+50% O2 5 A50% N2+50% O2 5 A

Oxygen 5 A

10 15 20 25

0

-10

-5

5

Z' / m

Z'' / m

Air 5 A

50% He+50% O2 5 A50% N2+50% O2 5 A

Oxygen 5 A

Reactive

Mixing

and Rolling (RMR) GDE Production

Technique

for

AFC Electrodes

Schematically

representation

of cell

voltage

and potentials

in an alkaline

fuel

cell

AFC

Cathode

with

ORRO2 + 2 H2

O + 4 e-→ 4 OH-

Anode 2 H2 + 4 OH-

→ 4 H2

O + 4 e-

Current

density

Cell

Voltage

[V]

-0.83

+1.36

+0.40

ΔE0

= 1.23V

Current

density

/ potential characteristic

Cathode

Pot

entia

l

Current density

0, Cathode

k,C

d

U0

0Cathode

Reference

CE RE ODCHg/HgO

SEM picture

of cross section

of silver

membrane

with 0.2µm pores

diameter

CV‘s

(1 mV/s) from

-700 mV to 450 mV vs. Hg/HgO

bei 80°C in 10 M NaOH, O2

-600 -400 -200 0 200 400

0

-400

-200

200

Potential / mV

Current / mA

0.2um

0.8um

0.45um

1.2um3.0um

5.0um

Vergleich Impedanzspektren, aufgenommen in 10 M NaOH bei 80°C, -700 mV vs. Hg/HgO

nach 60 Minuten

1 2 5 10 30 100 300 1K 3K 10K 30K

500m

700m

1

1.5

|Z| /

0

15

30

45

60

75

90|phase| / o

frequency / Hz

1.2um

0.45um

3.0um

0.2um0.8um

5.0um

0.6 0.8 1 1.2 1.4 1.6

0

-800

-600

-400

-200

200

400

Z' /

Z'' / m

1.2um

0.45um3.0um 0.2um0.8um5.0um

Equivalent

circuit

with

relaxation

impedance

and measurement

at -700 mV, 1.2 µm membrane

100m 1 3 10 30 100 1K 3K 10K

600m

550m

800m

1

1.5

|Z| /

0

15

30

45

60

75

90|phase| / o

frequency / Hz

1 2

4

6

7

8

3 5

1 2 20.17 s 50.84 m

2 394 m3 2.167 mF

868.3 m 4 763.4 m5 1.779 mF

1.007 6 1

102.6 m 0

7 566.7 m8 213.2 nH

Conclusion

Determination of the individual potential losses during fuel cell operation

Determination of degradation mechanism and performance loss

Improvement of fuel cell performance and stability by understanding instead of trial and error

Determination of critical

operation

conditions

of fuel

cells