Theory and Application of Electrochemical Impedance Spectroscopy for Fuel Cell Characterization

Norbert Wagner, Andreas K. Friedrich German Aerospace Center, Institute for Technical Thermodynamics

Pfaffenwaldring 38-40, 70569 Stuttgart

9th International Microsymposium on Electrochemical Impedance Analysis

June 02-05, 2011, Otok Sv. Andrija (Red Island) – Rovinj, Croatia

Presentation outlineIntroduction

MotivationTypes of Fuel CellsExperimental set-up for different types of FCs

Examples of porous electrodesImpedance models of porous electrodes

Different applications of EIS in FC researchContributions to performance loss of PEFC (single cell)Time dependent EIS

CO poisoning of PEFC-anodesEIS measured on Ag-gas diffusion electrode (half cell)EIS measured on SOFC (segmented cell)

Conclusion and OutlookEIS with fuel cell stacksEIS on batteries (Li-Sulfur, Li-Air (Metal-Air) for determination of kinetics, degradation, SOC, SOH

Motivation

Characterization of Fuel Cells by Electrochemical Impedance Spectroscopy:

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

Thermodynamic Date of Selected Fuel Cell Reactions (Standard Conditions @25 °C)

Fuel Cell Reaction z H0 (kJ/mol) G0 (kJ/mol) U0 (V) theo

Hydrogen H2 + ½ O2 H2O 2 -286.0 -237,3 1,229 83,0 % CO CO + ½ O2 CO2 2 -283.1 -257,2 1,066 90,9 % Formic acid HCOOH + ½ O2 CO2 + H2O (l) 2 -270.3 -285,5 1,480 105,6 % Formaldehyde CH2O (g) + O2 CO2 + H2O (l) 4 -561.3 -522,0 1,350 93,0 % Methanol CH3OH + 3/2 O2 CO2 + 2 H2O (l) 6 -726.6 -702,5 1,214 96,7 % Methane CH4 + 2 O2 CO2 + 2 H2O (l) 8 -890.8 -818,4 1,060 91,9 % Ammonia NH3 + ¾ O2 ½ N2 + 3/2 H2O (l) 3 -382.8 -338,2 1,170 88,4 % Hydrazine N2H4 + O2 N2 + H2O (l) 4 -622.4 -602,4 1,560 96,8 % Zinc Zn + ½ O2 ZnO 2 -348.1 -318,3 1,650 91,4

W.

Vielstich

in Handbook

of Fuel Cells

Vol. 1, (W. Vielsich, H.A. Gasteiger, A. Lamm eds.) John Wiley & Sons, London, 2003

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

Schematic diagram of the U-i characteristic of PEFC and Electrochemical Impedance Measurements

Cel

l vol

tage

Current

density

Ruhespannung (ohne Stromfluß)i

AnodeUR acAn

)(

iCathodeUR ac

Cath

)(

iCellUR cd

Cell

)(

i

U(Cell)

U-i measured

i

n

U n

U = iRM

Cathodic Overvoltage

Anodic Overvoltage

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 a Silver membrane surface, 3 kx magnification

5 µm Pore radius 0,2 µm Pore radius

Multi-layer Gas Diffusion Electrodes with different porous layers

Dry

sprayedC/PTFE

Reactive

Mixing and Rolling

Ag-PTFE

N. Wagner, T. Kaz, DE 101 12 232 A1, 2002

SEM micrograph of a cross section of SOFC

Anode

Electrolyte

Cathode

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

Why porous electrodes?

•Enlargement

of active

electrode

surface•Lowering

of overvoltage

at same

current

input

(electrolyzer) or

output

(fuel

cell)•Increasing

of power density

(galvanic

cells)

•Increasing

of storage

capacity

(supercaps)•Lowering

catalyst

loading

by

increasing

active

surface

-100

-80

-60

-40

-20

0

20

40

60

80

100

-80 -55 -30 -5 20 45 70

Overvoltage / mV

Cur

rent

den

sity

/ m

Acm

-2 i0 = 1 mAcm-2

i0 = 10 mAcm-2

b = 25 mV/decade

HER

HOR

Butler-Volmer equation

for

hydrogen

oxydation

(HOR)

and hydrogen

evolution

reaction

(HER)

η1

η2

i = 100 mAcm-2

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

Simple pore

model

with

faradaic

processes

in pores

RC-transmission

line

of a flooded

pore

R. De Levie, Electrochim. Acta, 8(1963) 751

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

Evaluation of the U-i characteristics from EIS

100

300

500

700

900

1100

0 200 400 600 800

Current density /mAcm-2

Cel

l vol

tage

/mV

measured

curve: Un

= f(in

)calculated

curve: Un

= in

Rn

(without

integration) calculated

curve

using

method

II: Un

= an

i2n

+bn

in

+cnx calculated

curve

using

method

I: Un

= an

in

+bn

RU

In n

Integration method

I:

U UU

I

U

II I

n n n n n n

112 1 1

( ) ( )

Integration method

II:

U a I b I cn n n n n n 2 with:

aR R

I Inn n

n n

1

12 ( )

b R a In n n n

1 12

c U a I b In n n n n n

1 12

1

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

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

Comparison of EIS measured at 5 A, 80°C, λ=1.5, cathode fed

with

different gas composition

10 15 20 25

0

-10

-5

5

Z' / m

Z'' / m

Air

50% He+50% O2

50% N2

+50% O2

Oxygen

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

50% He+50% O2

50% N2

+50% O2

Oxygen

Reforming of Methane

Methane Compres. Reformer Shift-reactor HT

Shift-reactor LT

CO-cleaning

PEM-Fuel Cell Heat

E-Energy

b) Cat-Burner

Residual Gas

a) Reformer-heating

CH4 + 2H2O => 4H2 + CO2 (CO)

9% CO

0,5% CO

3% CO

H2, CO2<

0,005%COAir (O2)

Methane Compres. Reformer Shift-reactor HT

Shift-reactor LT

CO-cleaning

PEM-Fuel Cell Heat

E-Energy

b) Cat-Burner

Residual Gas

a) Reformer-heating

CH4 + 2H2O => 4H2 + CO2 (CO)

9% CO

0,5% CO

3% CO

H2, CO2<

0,005%COAir (O2)

Time resolved EIS - CO poisoning of Pt-anode

Nyquist plot

of EIS measured

at different times

during

poisoning of the Pt-anode with CO

Time progression

of cell voltage and overvoltage

in galvanostatic mode of PEFC operation

(217 mAcm-2)

Pt-anode , H2

+ 100 ppm CO at 80°C

200

300

400

500

600

700

800

0 3000 6000 9000 12000

Time / s

Cel

l vol

tage

/ m

V

0

100

200

300

400

500

600

Ove

rv. C

O /

mV

a

b

c d e f

Imaginary part / m

Real part / m

0

-200

-100

100

200

0 200 400

a

bc

de

f

Time resolved EIS - CO poisoning

Frequency / Hz

5

10

|Phase|0

0

15

30

45

60

75

90

10m 1 100 10K

Time / ks

Impedance / m

Frequency / Hz

5

10

10

30

100

300

10m 1 100 10K

Time / ks

Bode plot of EIS during CO poisoning of the Pt-anode at 217 mAcm2, H2 +100 ppm CO

Appearance of voltage oscillations during galvanostatic operation of PEFC with H2 +CO

Appearance of voltage oscillations during galvanostatic operation of PEFC with H2 +CO

impe

danc

e

time

frequency

drift affected data

time course of theinterpolated data

impedance atsingle frequencies

Improved evaluation techniques Time course interpolation

Requirements

•

Series measurement•

Time for each

•

measured frequency AND

for each spectrum

10m 50m

real part / Ohm

-10m

10m

0

imag

. par

t / O

hm

10m 50m

real part / Ohm

-10m

10m

0

imag

. par

t / O

hm

10m 50m

real part / Ohm

-10m

10m

0

imag

. par

t / O

hm

Results of the improved evaluation techniques

I. Only

real-time drift compensation

II. Additional time course

interpolation

III. Z-HIT refinement

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

Oxygen Reduction in Alkaline Media

Cathode

of the

alkaline

fuel

cellCathode

in metal-air

batteries

Cathode

in the

electrolyzer

for

chlorine

production (ODC, Oxygen

Depolarizing

Cathode)

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

Cell voltage and potentials in an electrolyzer for chlorine production

with

ODC E0

= 0.96 V

Anode4 Cl-→2 Cl2

+ 4 e-

Cathode

with

ORRO2

+ 2 H2

O + 4 e-→ 4 OH-

Current

density

Cell

Voltage

[V]

-0.83

+1.36

+0.40

Cell voltage and potentials in an electrolyzer for chlorine production

without

ODC E0

= 2.19V

with

ODC E0

= 0.96 V

Anode4 Cl-→2 Cl2

+ 4 e-

Cathode

with

ORRO2

+ 2 H2

O + 4 e-→ 4 OH-

Cathode

(conventional)4 H2

O + 4 e-

→ 4 OH-

+ 2 H2

Current

density

Cell

Voltage

[V]

-0.83

+1.36

+0.40

ΔE0

= 1.23V

O2

O O

NaOH 30% (H2

O)

NaOH 32%

Na+

Na+

Na+

Na+

NaOH 2 H2 O

4 NaOH NaOH

e-

OH-

e-

e-

e-

e-

Electrical

Circuit

OH-

O2

O O

O

Net AgMembrane

Silver GDE

O-2

OH

H

NaCl

solution (Brine)

Na+

Cl2

Cl-

e-

Anode

Impedance Measurements during Oxygen Reduction Reaction (ORR) in 10 N NaOH, on Silver Electrodes

at Different Current Densities

10m 100m 1 3 10 100 1K 10K 100K500m

600m

550m

800m

1

1.5

|Z| /

0

15

30

45

60

75

90|phase| / o

frequency / Hz

bbs3126 500 mAbbs3126 450 mAbbs3126 400 mAbbs3126 350 mAbbs3126 300 mAbbs3126 250 mAbbs3126 200 mAbbs3126 150 mAbbs3126 100 mAbbs3126 50 mA

0.6 0.8 1 1.2 1.4

0

-600

-400

-200

200

400

Z' / mΩ

Z'' / mΩ

bbs3126 200 mAbbs3126 150 mA

bbs3126 100 mA

bbs3126 50 mA

bbs3126 500 mAbbs3126 450 mAbbs3126 400 mAbbs3126 350 mAbbs3126 300 mAbbs3126 250 mA

Evaluation of EIS measured during ORR Equivalent circuit and Rct = f(i)

200

400

600

800

-500 -400 -300 -200 -100current/mA

R / m

N

1

2

3

4

5

6

1 170.8 m2 5.521 ms-1/2

19.38 s-1

3 61.37 mF

942.8 m 4 1

309.9 m 3.18 m

5 508.6 m6 73.35 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

Outlook

Using the existing models for development and characterization of catalysts and electrodes, optimization of fuel cell structure (flow field, bipolar plate, GDL) Combination and extension of existent and new models

Application of EIS to fuel cell stacks measurements, simultaneously recording of up to 16 parallel impedance spectra of 16 different

cells from

the stack

EIS on batteries (Li-Sulfur, Li-Air (Metal-Air) for determination of kinetics, degradation, SOC, SOH, BMS

Experimental EIS set-up for stack measurements