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