Analysis of high temperature polymer electrolyte membrane
fuel cell impedance during break-in
Søren Juhl Andreasen and Søren Knudsen Kær
Department of Energy Technology, Aalborg University, Denmark
sja@et.aau.dk
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Outline
•Introduction to HTPEM fuel cells
•Experimental setup
Fuel cell control system
Impedance system
•Experimental results
Cells examined
Voltage vs. time
Impedance vs. time
•Summary and outlook
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Introduction
High temperature PEM fuel cells
High Temperature PBI based PEM Fuel Cell
Membrane polymer:
PBI (polybenzimidazole)
Proton conductor :
H3PO4 (Phosphoric acid)
Fuel cell temperature: 120-200 oC
Typical operating range:160-180oC
Advantages
•Less complex polymer
•CO tolerant up to 2-3%
•No humidity control = Simple stack and system design
•Cooling possible at all ambient conditions
Disadvantages
•Lower cell voltage than LTPEM
•Long start-up time because of high temperature
•Liquid water should not e present
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Status - HTPEM Fuel Cell Break-in
The initial hours of operation are usually known as the ”break-in” time of the fuel cell
Usually recommended break-in time is 0 to 100 hours, using a certain current density path,
temperatures and stoichiometries, depending on manufacturer.
During break-in an increase of fuel cell voltage performance is typically experienced.
Break-in is costly and inconvenient to the stack/system manufacturer / integrator
Better understanding is required regarding the mechanisms taking place during break-in in order
to provide proper guidelines for optimal usage of HTPEM fuel cells.
Only a few references are available regarding proper break-in / activation of HTPEM fuel cells,
they are mainly related to pure hydrogen operation.
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Typical break-in voltage performance
Typical recommended break-in conditions for a HTPEM fuel cell:
Constant ”low current” 0.2 A/cm2, 160oC operating temperature, operation for 10-100 hours
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Experimental setup
HTPEM heated single cell assembly, straight flow channels
Two National Instruments based control systems:
•Automated fuel cell control system (Labview)
•Impedance measurement system able to superposition
signals onto fuel cell current
Hardware:
•Fuel Cell Control System
NI PCI 6401 AO DAQ card
NI PCI 6229 AI DAQ card
NI PCI 4351 AI DAQ card
TDI Power RBL 488
Bürkert 8711 MFC H2
Bürkert 8711 MFC CO
Bürkert 8711 MFC CO2
Bürkert 8712 MFC Air
230VAC controlled electrical heater
2 Type T thermocouple (cathode/anode)
•EIS measurement system
NI PCI 6259 DAQ card
Load signal switching relays
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Break-in impedance plot
Selected MEAs:
4 MEAs from DPS:
Dapozol G77 membranes (PBI)
Varying catalyst loading
Varying GDL thickness
Varying polymer content in CL
2 MEAs from BASF:
Celtec P2100
Celtec P1000*
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BASF Nyquist plot - pre/post break-in
Impedance behaviour:
Both P1000 and P2100 cells show
quite dramatic changes in most of
the impedance spectrum, both in
high,
intermediate
and
low
frequencies.
Membrane resistance increases
during break-in due to acid removal
The main changes contributing to
these changes are expected to be,
acid reallocation i.e. the combined
movement of acid into and away
from the gas difusion and catalyst
layer.
Water content and production is also
expected to play an important role
and needs further investigation.
sja@et.aau.dk
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DPS Nyquist plot - pre/post break-in
Impedance behaviour:
Not as dramatic difference between
pre and post break-in impedance
comparred to BASF membranes due
to different membrane types and
production methods. Less changes
are occuring at intermediate and low
frequencies.
Generally a slightly higher increase
in
membrane
resistance
is
experienced.
Only slight differences of the chosen
variations in catalyst loading and
polymer content in the catalyst layer,
with
the
most
promesing
performance in the MEAs with the
least PBI content in the CL
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Real part of impedance vs. break-in time
Real impedance behaviour:
Small impedance changes occur at
high frequencies during the first
couple of hours of operation
A steady drop in the real part of the
low and intermediate frequency
impedance is experienced during
the first 10 hours of operation
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Imaginary part of impedance vs. break-in time
Imag impedance behaviour:
The
intermediate
frequency
imaginary
impedance
show
changes that settle after 15-20
hours operation.
Low
frequency
imaginary
impedance does not shows
significant changes related to
break-in.
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Real part impedance behaviour
1kHz:
Both BASF MEAs show an initial short
term change, followed by a more
steady 1000 Hz real part impedance
change due to changes in membrane
resistance. The DPS MEA does not
exhibit the initial ”fast” change in high
frequency resistance.
The variations in catalyst loading and
PBI content in the catalyst layer does
not seem to by affect the slope of the
break-in impedance development.
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Real part impedance behaviour:
Slightly faster settling of the DPS fuel
cells MEAs is seen at this frequency,
which may well also be attributed to
the differences in not only phosphoric
acid content in the GDL, but also the
GDL type. The BASF GDL being
woven and the DPS carbon paper
type.
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Imag part impedance behaviour:
Both
BASF
MEAs
have
a
characteristic
10-20%
drop
in
imaginary part of the impedance after
10 hours. The DPS MEAs show a
more stable behaviour during this
time.
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Summary and Outlook
Conclusions
•Impedance changes occurring with different time constants are identified, from fast initial 1 hr decreases and increases to slower
20+ hr changes, all affecting fuel cell performance.
•High frequency impedance decrease is primarily related to membrane resistance, and the presence of phosphoric in the catalyst.
The changes are expected to be due to phosphoric acid re-allocation / water contribution.
•Intermediate and low frequency impedance changes are expected to be related to the increased catalyst activity due to removal of
phosphoric acid from the catalyst layer, more dramatic effect in the BASF MEAs, which initially show quite high content of
phosphoric acid in the MEA and GDL, compared to the DPS MEAs that are post treated with phosphoric acid.
•A quantitative analysis of the varying polymer content and the effect of catalyst loading on MEA performance has shown that higher
polymer content in the CL requires longer break-in possibly due to not only more polyer present in the catalyst layer, but also
phosphoric acid, blocking active areas.
•Presence of high amounts of phosphoric acid in the MEA could block catalytic sites and cause problems with anode starvation (high
anode potentials, and increased carbon corrosion), if high current densities are used in a very early stage of fuel cell lifetime. In turn
this could decrease lifetime
•Measuring selected impedances during break-in can be used as a guideline to determine proper break-in time.
Future work
•The long term effects of operation with/without break-in should be examined.
•Further tests should be conducted with reformate gas break-in, looking at the effects of water, CO, CO2 and residual fuel in the
gas, and how it affects the break-in impedance.
•Further detailed tests also using polarization curves and impedance measurements at different current densities should be
examined.
sja@et.aau.dk
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Acknowledgements
Ackowledgements
The authours would like to gratefully acknowledge the financial support from the EUDP program and the Danish Energy
Agency for sponsoring the project :COmmercial BReakthrough of Advanced Fuel Cells -(COBRA)
Thank you!
sja@et.aau.dk
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Analysis of high temperature polymer electrolyte membrane
fuel cell impedance during break-in
Søren Juhl Andreasen and Søren Knudsen Kær
Department of Energy Technology, Aalborg University, Denmark
sja@et.aau.dk
17