Application of Electrochemical Impedance Spectroscopy for Fuel Cell Characterization

Dr. Norbert Wagner DLR, Institut für Technische Thermodynamik, Stuttgart

Kronach Impedance Days 2010 Kloster Banz, April, 14th – 16th , 2010

Presentation outline

Introduction and motivationExamples of porous (technical) electrodes

Theory and models of porous gas diffusion electrodesImpedance models

Application of Göhr‘s porous electrode modelEIS measured at PEFCEIS measured during oxygen reduction on silver in alkaline solution

OutlookExperimental set up for EIS applied for stack measurements

Electrochemical kinetic and electrode structure

-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

Increasing power output (P=I·U) at constant cell voltage (overvoltage) by:

•

enlargement of active electrode surface using porous electrodes (electrode structure)

•

increasing i0

(electrode material with high catalytic activity)

I= Surface·i= Surface·i0

·exp{(αRT/zF)η}

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

NaCl

Cl2

H2 O

NaOH, H2

Cl-Na+

OH-+ -

Electrolysis

(Water, NaCl, etc.)

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

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- O2

-

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

Temperature

Charge carrierin electrolyte

Cathode

Fuel gas

Measuring methods used for fuel cell and fuel cell components characterization : in-situ und ex-situ methods

In-situ measuring methodsCurrent-voltage characteristic (U(i))Electrochemical Impedance Spectroscopy (EIS)

Local and time resolvedCyclic Voltammetry (CV)Current interruption (CI))Chronopotentiometry (CP) und Chronoamperometry (CA)Current density distribution

Ex-situ measuring methods used for fuel cell and fuel cell components characterization

Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM)Energy dispersive X-ray spectroscopy (EDS)X-ray photoelectron spectroscopy (XPS) X-Ray Diffraction (XRD)Thermal gravimetric analysis (TGA) Porosimetry (Hg-Porosimetry)Measurement of the specific surface area (BET-measurement)Determination of gas permeability

Electrochemical Impedance Spectroscopy: Application to Fuel Cells

Current I

U/I - Characteristicof a Fuel Cell

Cel

l vo

ltag

e U

Potential excitation signal - E(t)Cu

rrent respo

nse signal- I(t)

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

The Metal-Electrolyte Interface

The Metal-Electrolyte Interface

Double layer

capacity

(Cdl

)+ -

The Metal-Electrolyte Interface

Double layer

capacity

(Cdl

)

Faraday-Impedance

(ZF

)

Impedance spectra of a simple electrochemical system (ZF =Rct ): Nyquist representation

Imaginary part /

Real part /

0

-8

-6

-4

-2

2

103 7

Rct

=10

Cdl

=1 mF

Rel

=1

2fmax

=max

=1/Cdl

Rct

Rel Rel

+Rct

fmax

=15.9 Hz

Rct

Impedance spectra of a simple electrochemical system (ZF =Rct ): Bode representation

Rel

=1Rct

=10

Cdl

=1 mF

2fmax

=(1/Rct

Cdl

)(1+Rct

/Rel

)1/2

at =2f=1: ZC

=1/Cdl (------)

PhaseImpedance /

Frequency / Hz

0

20

40

60

80

1

2

5

10

10m 100m 1 10 100 1K 10K

Rel

+Rct

Rel

Rct

fmax

=52.8 Hz

Rel

Schematic representation of different steps during electrochemical reaction as a function of distance

from electrode surface

Redbulk

Mass transport

n e-

Oxad*

Redad*

Charge transfer

Ox* Ox* Oxbulk

Red* Red*

Mass transport

Chem. reaction

Chem. reaction

Adsorption

Ads. Des.

Des.

Electrode Double layer Reaction layer Diffusion layer

Ox + ne- ↔ Red

Multi-layer Gas Diffusion Electrodes with different porous layers

Carbon-PTFE

Layer(Dry

sprayed)

Ag-PTFE Layer(Rolled

Layer)

SEM micrograph of a cross section of SOFC

Anode

Electrolyte

Cathode

SEM micrograph of a porous silver membrane

R = electrolyte resistance inside the pore per unit lengthC = interface capacitance per unit length

Simple pore model of interface charging RC-transmission line of a flooded pore

RCiCi

RiZ

coth)(

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

Nyqusit 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

Nyqusit 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

Theory of Agglomerated Electrodes

metal side

electrolyte sideionic current

Gas (backing) side

electrolyte sideionic current

M. Eikerling, A.A. Kornyshev, E. Lust, J. Electrochem. Soc., 152 (2005) E24

mZZe

metalelectrolyte pores

porous layer

Zs1 ZsnZsi

ZpnZpiZp1

Zq1 Zqi Zqn

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

Cylindrical homogeneous porous electrode model (H. Göhr) I

Cylindrical homogeneous porous electrode model (H. Göhr) IIIons (H+, OH -,..)

I I

Por

e

Ele

ctro

de, p

orou

s lay

er

Electrolyte Zq

Zp ZS

Zo

Zn

Current (e-)

qsp ZZZ )(Z* =Z ZZ Z

p s

p s

( )Z#

=

C = cosh S = sinhZ ZZ

p s

*

Z ZZ

p s

*

P = q0

= v = sp

p

ZZZ

ZZ o

* Z ZZ

p s*

qn

= ZZn

* s = = 1-p ZZ Z

s

p s

C C p s S p q s q

S q q C q qn o

n o n o

( ) ( )( ) ( )

1 21

2 2

Z = Z#+Z*

Authors Reference Model and systemJ. -P Candy, P Fouilloux, M. Keddam, H. Takenouti

Electrochim. Acta, 26(1981) 1029 Ni in alkaline solution

R. De Levie Electrochim. Acta, 8(1963) 751 Transmission line model, J.S. Newman and C.W. Tobias J. Electrochem. Soc., 109(1962) 1183 Steady-stateJ. Giner, C. Hunter J. Electrochem. Soc., 116(1969) 1124 Flooded-agglomerate model, Pt-GDE, OCR in

alkaline solutionK. Mund, F.v. Sturm Electrochim. Acta, 20(1975) 463 HOR on Ni in alkaline

solutionS. Sunde, Electrochim. Acta, 42(1997) 2637 Composites, SOFCP. Björnbom Electrochim. Acta, 32(1987) 115 Steady state modelR. Holze, W. Vielstich J. Electrochem. Soc., 131(1984) 2298 OCR in alkaline

solutionT.E. Springer, I.D. Raistrick J. Electrochem. Soc., 136(1989) 1594 Flooded-agglomerate and thin film model,

differential element of a pore wallH. Göhr Poster ISE Erlangen, 1983 Homogeneous porous model, Pb

in sulfphuric

acid

G. Paasch, K. Micka, P. Gersdorf Electrochim. Acta, 38(1993) 2653 Macrohomogeneous

porous electrode modelW. Scheider J. Phys. Chem., 79(1975) 127 Model with pore branchingS. Srinivasan, H. D. Hurwitz, J. O'M Bockris J. Chem. Phys., 46(1967) 3108 Thin

film modelM. Kramer, M. Tomkiewicz J. Electrochem. Soc. 131(1984) Stochastic approach with interpenetrating

networkA. Winsel, E. Bashtavelova J. Power Sources, 73(1998) 242 Agglomerate-of-spheres modelM. Tomkiewicz, B. Aurian-Blajeni J. Electrochem. Soc. 135(1988) 2743 True effective medium approachH. Keiser, K.D. Beccu, M.A. Gutjahr Electrochim. Acta, 21(1976) 539 Various geometries of single pore, Ni-GDE

Brief Overview of Porous electrode models and Applications

Electrochemical Impedance Spectroscopy: Experimental Set-up

Electrochemical

workstation

PEFC

Flow

contollerPressure

regulator

Humidifier

Bode diagram of measured EIS at different cell voltages (current densities) I

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

Bode diagram of measured EIS at different cell voltages (current densities) II

Phase oImpedance / m

Frequency / Hz

0

20

40

60

80

10

20

15

30

50

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

Diffusion

RM

Charge 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

N. Wagner, K.A. Friedrich, Dynamic Operational Conditions. In: J. Garche, C. Dyer, P. Moseley, Z. Ogumi, D. Rand and B. Scrosati, editors.

Encyclopedia of Electrochemical

Power Sources, Vol. 2. Amsterdam: Elsevier, 2009, pp. 912-930

Common Equivalent Circuit for Fuel Cells

Cdl,a

RM

Rct,a

Cdl,c

Rct,c ZdiffZdiff

Diffusion of H2

EIS at Polymer Fuel Cells (PEFC): Common equivalent circuit and boundary case

Cdl,a

RM

Rct,a

Porous electrode with pore electrolyte resistance (Rpor ) and surface layer resistance (RS )

Cdl,c

Rct,c

CN

RN

Cdl,a

RM

Rct,a

Cdl,c

Rct,c

Equivalent

circuit

of the PEFC: anode and cathode

simulated

without

pores, without

diffusion

(valid

for

example

at lower

current

densities)

Bode diagramm of the EIS, measured at the PEFC at 80°C, symmetrical gas supply of the cell

Phase oImpedance /

Frequency / Hz

0

20

40

60

80

10m

100m

1

10

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

O2 /O2 H2 /H2

EIS at Polymer Fuel Cells (PEFC): Contributions to the cell impedance at different current densities

0

0.04

0.08

0.12

0.16

0.2

0 100 200 300 400 500 600 700

Current density /mAcm-2

Cel

l im

peda

nce

/Ohm

0

0.2

0.4

0.6

0.8

0 200 400 600Current density /mAcm-2

Cell

impe

danc

e /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

RC

CN

RN

Evaluation of EIS with the porous electrode model I

I

I

I

00.020.040.060.080.1

0 200 400 600 800

Current density /mAcm-2R

p,a;

Rct

,a; R

por,a

/Ohm

21

21

,

,

,,,

tanh

)(

act

apor

actaporap

RR

RRR

Porous electrode resistance (Rp, a ), charge transfer resistance (Rct, a ) and electrolyte resistance (Rpor, a ) in the pore of the anode at different current densities

Evaluation of EIS with the porous electrode model i-V characteristic and current dependency of pore electrolyte resistance

of the anode and cathode

■

Rel,por,Anode♦

Rel,por,Kathode

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

Cell

volta

ge /

mV

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 100K

500m

1

2

1.5

5

|Z| /

0

15

30

45

60

75

90|phase| / o

frequency / Hz

45 mA40 mA

35 mA30 mA

25 mA20 mA

15 mA

10 mA

5 mA

100 mA95 mA90 mA85 mA80 mA75 mA70 mA65 mA60 mA55 mA50 mA

1 2 3 4 5

0

-4

-2

Z' /

Z'' /

-50

5 mA

10 mA

15 mA20 mA

100 mA

i / 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

Outlook

Further improvement of porous electrode models

Combination and extension of existent and new models

Application of EIS to segmented cells

Experimental validation of models usingPEFC and DMFC electrodes with different porous structureGas Diffusion Electrodes (GDE) for Oxygen Consumption Reaction (OCR) in alkaline solution using different gas compositions

Experimental EIS set-up for stack measurements