eis in fuel cell science - german aerospace center eis dechema 2015... · 2015-11-17 ·...

EIS in Fuel Cell Science

Dr. Norbert WagnerGerman Aerospace Center (DLR)Institute for Engineering ThermodynamicsPfaffenwaldring 38-49, 70569 Stuttgart, Germany

www.DLR.de • Chart 1 EIS in Fuel Cell Science, Norbert Wagner, 06.11.2015

Electrochemical Impedance SpectroscopyFundamentals and Applications5.-6. November 2015Frankfurt am Main

Presentation outline

• Introduction• Motivation• Types of Fuel Cells• Experimental set-up for different types of FCs

• Microstructure of fuel cells and modeling of fuel cells with equivalent circuits

• Impedance models of porous electrodes

• Different applications of EIS in FC research• Contributions to performance loss of PEFC• Degradation mechanism of PEFC• Time dependent EIS

• CO poisoning of PEFC-anodes• Flooding of PEFC cathodes

• EIS measured on Ag-Gas Diffusion Electrodes (GDE) during Oxygen Reduction in alkalineelectrolyte (AFC)

• EIS measured at fuel cell stack (SOFC)

• Conclusion

EIS in Fuel Cell Science, Norbert Wagner, 06.11.2015www.DLR.de • Chart 2

Motivation

Characterization of Fuel Cells by Electrochemical Impedance Spectroscopy:

• Determination of electrode structure and reactivity, separation of electrode structure from electrocatalytically activity

• Determination of reaction mechanism (kinetic) and separation of different overvoltage contributions to the fuel cell performance loss

• Determination 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


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)

Pot

entia

l

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 ac

An

)(

i

CathodeUR ac

Cath

)(

i

CellUR 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 / s

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 / 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


N. Wagner in „Pem Fuel CellDiagnostic Tools“ , Haijaing Wang, Xiao-Zi Yuan, Hui Li (Eds.), 2011


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

O22

H

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

H2O

NaOH, H2

Cl-

Na+

OH-+ -

Electrolysis (Water, NaCl, etc.)

PEFC: Schematic Diagram (cross section)


Common Equivalent Circuit for Fuel Cells

Cdl,a

RM

Rct,a

Cdl,c

Rct,c ZdiffZdiff

Diffusionof H2


SEM micrograph of PEFC elctrode (Pt/C+PTFE)


TEM micrograph of Carbon Supported Platinum Catalyst


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

O22

H

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 H2O

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 ΩC = 0.5 F

RCiCi

RiZ

coth)(

R0 R0 = R/3 = δL/3πr2

δ = specific electrolyte resistancer = pore radiusL = pore lenght

L

r

100 mHz


c = interface capacitance per unit length

rct = interface charge transfer resistance per unit lenght

r = electrolyte resistance inside the pore per unit length

Simple pore model with faradaic processes in poresRC-transmission line of a flooded poreR. De Levie, Electrochim. Acta, 8(1963) 751

ct


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


Thin-film model and agglomerate plus thin-film model of a porous electrode

I.D. Raistrick, Electrochim. Acta, 35(1990) 1579


Agglomerated Electrodes Hierarchical model(Cantor-block model)

metal side

electrolyte sideionic

current

Gas (backing) side

electrolyte sideionic

current

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

l

l

l/al/alz

lz/az

n = 0

n = 1

l

l

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


m ZZe

Current collector GDL

electrolyte pores

porous layer Z s1 Zsn Zsi

Zpn Zpi Z p1

Z q1 Zqi Z qn

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 la

yer

Electrolyte

Zq

Zp ZS

Zo

Zn

Current (e-)

Bode diagram of EIS at different cell voltagesmeasured at PEFC (H2/O2) at 80°C,

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

Ce

ll im

pe

da

nc

e /O

hm

Rdiffusion Rmembrane R anode R cathode

0,001

0,01

0,1

1

10

0 22 117 236 656

Current density / mAcm-2

Ce

ll im

pe

da

nc

e /

Oh

m


Evaluation of the U-i characteristics from EIS

100

300

500

700

900

1100

0 200 400 600 800

Current density /mAcm-2

Ce

ll v

olt

ag

e /m

V

measured curve: Un = f(in)calculated curve: Un = inRn (without integration)

calculated curve using method II: Un = ani2n+bnin+cn

x calculated curve using method I: Un = anin+bn

RU

In n

Integration method I:

U UU

I

U

II I

n n n n n n

1

1

2 1 1( ) ( )

Integration method II:

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

aR R

I In

n 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 1

2

1


EIS at Polymer Fuel Cells (PEFC):Contributions to the overal U-i characteristic determined by EIS

200

300

400

500

600

700

800

900

1000

1100

0 100 200 300 400 500 600 700 800

Current density / mAcm-2

Cel

l vo

ltag

e / m

V

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

Po

re e

lect

roly

te r

esis

tan

ce /

mO

hm

0

200

400

600

800

1000

1200

Ce

ll v

olt

ag

e /

mV

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

Pore Electrolyte Resistance AnodePore Electrolyte Resistance Cathode


Improved Evaluation Techniques for Time ResolvedElectrochemical Impedance Spectroscopy (TREIS)

• Real time drift compensation

• Time course interpolation

• Z-HIT compensation


imp

ed

an

ce

time

frequency

drift affected data

time course of the

interpolated data

impedance atsingle frequencies

Improved evaluation technique:Time course interpolation

Requirements

• Series measurement• Time for each • measured frequency

ANDfor each spectrum


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)


Appearance of voltage oscillations duringgalvanostatic operation of PEFC with H2+ 100ppm CO


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 vo

ltag

e /

mV

0

100

200

300

400

500

600

Ove

rv.

CO

/ m

V

a

b

cd e f

Imaginary part / m

Real part / m

0

-200

-100

100

200

0 200 400

a

bc

d

e

f


Measured and simulated time resolved EIS - CO poisoning

Imaginary Part / m

Real Part / m

0

-200

200

0 100 200 300 400 500 600

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

Time


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


Simulated EIS with Relaxation Impedance: Theory

Imaginary part / m

Real part / m

0

-40

-20

20

40

10020 40 60 80

= 1 s; Rk = 10 m; R = 100 mRM = 5 m; Cdl = 10 mFZF (0) = RF = 9,1 m

1mHz

100 kHzCdl

RM

ZF

Cdl

RM

Rk Lk

R

Faraday-impedance: ZF = R/(1+ R/Zk)Relaxation imp.: Zk = (1+ i) / IF dlnk/dETime constant: = RkLk

Relaxation resistance: Rk = 1 / IF dlnk/dERelaxation inductivity: Lk = Rk


Time resolved EISFlooding the cathode at 2 A, dead end


Time resolved EIS, flooding the cathode at 2 A, 80°C; timedependency of the cell voltage and impedance elements

R ct / m

100

200

300

Time / Ks0 105 2015 25

500

550

600

650

700

750

800

850

0 5 10 15 20 25

Vol. produced water [ ml ]

Ce

ll v

olt

ag

e [

mV

]Rct, C

Cdl,c

RM

W

W / ms-1/2

0

200

400

600

Time / Ks0 105 2015 25


Change of cell voltage during constant load at 500 mA cm-2

131211

10

98

67

3 54

1 2

Current density

Cell voltage

Elapsed time / h

Cur

rent

den

sity

/ m

A c

m-2

, C

ell v

olta

ge /

mV

170 µV/h


Bode Plot of EIS measured during PEFC operation over 1000 h at 500 mA cm-2

Impedance /

Frequency /

|Phase|0

0

15

30

45

60

75

90

7

10

20

15

30

25

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

m

Hz

Time


9

10

11

12

13

14

15

16

17

0 200 400 600 800 1000 1200

Elapsed time / h

Imp

ed

an

ce / m

Oh

m

Cdl,a

RM

RA

Cdl,c

RC

CN

RN

Variation of RC during PEFC operation at 500 mA cm-2

New start up

after 24 h

Cathode


Variation of RA during PEFC operation at 500 mA cm-2

0

1

2

3

4

5

6

7

8

9

10

0 200 400 600 800 1000 1200

Elapsed time / h

Imp

ed

an

ce /

mO

hm

New start upafter 24 h

Anode

Cdl,a

RM

RA

Cdl,c

RC

CN

RN


Evaluation and Analyse of Voltage Loss by EIS

0

20

40

60

80

100

120

140

Vo

lta

ge

los

s /

mV

Irreversible voltage loss Reversible voltage loss Overall cell voltage loss

Anode68.8 mV

Anode 5.4

Cathode43.2 mV

Cathode20 mV

Reversible

voltage loss

Irreversiblevoltage

loss48.6 mV

M. Schulze, N. Wagner, T. Kaz, K.A. Friedrich, Electrochim. Acta 52 (2007) 2328–2336

AFC: Impedance Measurements during ORR in 10 N NaOH, on Silver Electrodes at Different CurrentDensities, i< -50 mAcm-2

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

500m

1

2

1.5

5

|Z| /

0

15

30

45

60

75

90|phase| / o

frequency / Hz

453 50 mA453 45 mA

453 40 mA453 35 mA

453 30 mA453 25 mA

453 20 mA453 15 mA

453 10 mA453 5 mA

1 2 3 4 5

0

-3

-3.5

-2

-2.5

-1

-1.5

-0.5

1

0.5

1.5

Z' /

Z'' /

50 mA

45 mA40 mA

35 mA30 mA

25 mA20mA

15 mA

10 mA

5 mA

Bode representation Nyquist representation


Impedance Measurements during ORR in 10 N NaOH, on Silver Electrodes at Different CurrentDensities, i> -50 mAcm-2

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

600m

800m

1

2

1.5

|Z| /

0

15

30

45

60

75

90|phase| / o

frequency / Hz

453 500 mA453 450 mA

453 400 mA453 350 mA453 300 mA453 250 mA

453 200 mA

453 150 mA

453 100 mA

453 50 mA

0.6 0.8 1 1.2 1.4 1.6 1.8 2

0

-1

-0.5

0.5

Z' /

Z'' /

500 mA

450 mA400 mA350 mA300 mA250 mA200 mA

150 mA 100 mA 50 mA

Bode representation Nyquist representation


Adsorptions- and heterogeous reaction impedance

Definition of Zad/het :

Zad/het = RT(k-i)/n2F2csA(k2+2)

With A=electrode surface, k= first order reactions rate,F=Faraday constant, cs=surface concentration and angular frequency =2f.The heterogenous reaction impedance can be converted into a parallel combination of Rad/het and Cad/het :

Rad/het = RT/(n2F2csAk)

Cad/het = n2F2csA/(RT)


Electrode Model with cylindrical , homogeneous pores and complex Faraday-impedance

Zq=


Evaluation of EIS measured during ORREquivalent circuit and Rad = f(i)

0

2

4

6

-100 -80 -60 -40 -20current/mA

R /

Rad Cad

Rct

Cdl

Rpor

Rel

L


U-i characteristic and current density dependency of impedance elements Rad and Rct

0,9

0,92

0,94

0,96

0,98

1

1,02

1,04

1,06

1,08

1,1

-0,10 -0,08 -0,06 -0,04 -0,02 0,00

Current density / Acm-2

iR-c

orr

. Po

ten

tia

l vs

. NH

E / V

0,00

1,00

2,00

3,00

4,00

5,00

6,00

7,00

8,00

Rad

; Rct

/ O

hm


Current density dependency of kad, Rad and Rct, determined from EIS evaluation

0

1

2

3

4

5

6

7

8

-0.100 -0.080 -0.060 -0.040 -0.020 0.000

Current density / Acm-2

Rad

; R

ct

/ O

hm

0

10

20

30

40

50

60

70

80

Rea

ctio

n r

ate

con

stan

te /

s-1

kad=1/CadRad


M. Lang

Impedance Spectroscopy at SOFC short stack

Cell 5

Cell 4

Cell 3

Cell 2

Cell 1

U5

U4

U3

U2

U1

Voltage probes

Current probes

I

Potentiostat/Electronic Load

Comparison of the sequential measurement principle for impedance spectra data acquisition with the synchronous parallel approach


http://www.zahner.de/products/addon-cards/pad4.html

Nyquist impedance diagram of the five individual cells within the SOFC short stack at OCP (1.19 V), measured synchronously


Operation under dry fuel gas (50 % H2 + 50 % N2 and air at 750 Cº. Symbols: measurement data, solid lines: model fit

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



Norbert Wagner, “Electrochemical power sources – Fuel cells”, in Impedance Spectroscopy: Theory, Experiment, and Applications, 2nd Edition, Edited by Evgenij Barsoukov and J. Ross Macdonald, John Wiley&Sons, Inc., 2005, pp. 497-537, ISBN: 0-471-64749-7

eis in fuel cell science - german aerospace center eis dechema 2015... · 2015-11-17 ·...

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