Monitoring of membrane failure due to pinhole formation...Professor Horst Cerjak, 19.12.2005 3...

Monitoring of membrane failure due to pinhole formation

Viktor Hacker, Eva Wallnöfer

Department of Chemical Engineering and

Environmental Technology, TU GRAZ, Austria

Peter Prenninger

AVL List GmbH, Austria

Trondheim, June 23rd,2009

Professor Horst Cerjak, 19.12.20052

Diagnostic Tools for Fuel Cell Technologies

Overview

1. Introduction degradation of PEMFC membranesinfluence of operating conditions

2. In-situ Degradation Studiesoperating conditionscharacterization methods

3. Conclusions



Degradation of PEMFC Membranes –impacts of degradation

membrane decomposition (release of HF, SO2, CO2, CO, C-F-compounds)membrane thinninghigher gas permeabilityplatinum particle deposition in the membraneperformance loss of the MEAdecrease of life time



Degradation of PEMFC Membranes –causes

thermal degradation

mechanical degradation

chemical degradation



Degradation of PEMFC Membranes –influencing factors

o material properties

o cell assembling

o operating conditions

membrane thicknessgas pressuretemperaturegas humiditycell potential



In Situ Degradation Studies



Operating Conditions5 to 1 fuel cells (in series), 25 cm2 each operated under the same conditions up to 1300 h

permanent operating 24h/day, 7 days/weekinterruptions only for electrochemical characterizationsevery one or two weeks, one cell was removed (SEM analysis)

gas flow:H2: λ = 1.5 (at OCV: 300 ml min-1)Air: λ = 2.2 (at OCV: 300 ml min-1)

MEAs:pt loading: A: 0.4 mg cm-2, C: 0.6 mg cm-2

membrane: bilayer membrane, reinforced with PTFE, thickness: 35 µmactivation of the MEAs ( 6 h at 0.4 and 0.6 V)



Characterisation Methods

performance (UI)cell potential (CP)membrane resistance (MR)fluoride emission rate (FER)pinhole detection (PD)membrane thickness and condition (SEM)



0

2

4

6

8

10

12

14

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50Potential / V

Hyd

roge

n D

iffus

ion

Cur

rent

Den

sity

/ m

A c

m-2

0

2

4

6

8

10

12

14

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50Potential / V

Hyd

roge

n D

iffus

ion

Cur

rent

Den

sity

/ m

A c

m-2

0

2

4

6

8

10

12

14

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50Potential / V

Hyd

roge

n D

iffus

ion

Cur

rent

Den

sity

/ m

A c

m-2

0

2

4

6

8

10

12

14

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50Potential / V

Hyd

roge

n D

iffus

ion

Cur

rent

Den

sity

/ m

A c

m-2

0

2

4

6

8

10

12

14

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50Potential / V

Hyd

roge

n D

iffus

ion

Cur

rent

Den

sity

/ m

A cm

-2

0

2

4

6

8

10

12

14

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50Potential / V

Hyd

roge

n D

iffus

ion

Cur

rent

Den

sity

/ m

A cm

-2

Hydrogen Diffusionanode: H2 flow / cathode: N2 flowstandard conditions H2 diffuses through the membrane and gets oxidisedwith an increasing potentialthe hydrogen diffusion current is limited by diffusion to < 5 mA cm-2

if there is a pinhole, the current increases with increasing potential

Po te n tia l / V

0 h 110 h 280 h 450 h 620 h 790 h 960 h 1130 h



0

10

20

30

40

50

60

70

80

90

0 100 200 300 400 500 600 700 800 900 1000 1100time / h

hydr

ogen

diff

usio

n cu

rrent

den

sity

be

twee

n 0.

2 am

d 0.

5 V

/ mA

cm

-2

Hydrogen Diffusion - Results

the formation of a pinhole can not be forecasted end of membrane lifetime:time interval, at which the hydrogen diffusion current density is in the range between 4 and 5 mA cm-2

standardlow humiditylow temperaturehigh pressure45 mA90 mA135 mA405 mA90 mA, low humidity



Cell Performancethe cumulated performance of the stack was investigated; H2: λ = 1.5 / Air: λ = 2.2no gas pressure; T: 70 °Cpolarisation curves: changes with operating time

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.0 0.2 0.4 0.6 0.8 1.0 1.2Current Density / A cm-2

Pot

entia

l / V

standardPo ten tia l / V

0 h 110 h 280 h 450 h 620 h 790 h 960 h 1130 h

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1


Pot

entia

l / V

low humidity (40 % rH)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1


Pot

entia

l / V

low temperature (40 °C)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1


Pot

entia

l / V

45 mA cm-2

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1


Pot

entia

l / V

135 mA cm-2



Cell Performance - Results

maximum power density changes with operating time

the performance loss is related to the membrane degradation, but also influenced by the electrode degradation

the formation of the first pinhole is not exactly related to a certain loss of performance




OCV decreases with the operating time (non-linear)

Cell Performance - Results

OCV

normalised OCV (to 0.95 V)

the formation of the first pinhole is related to a loss of 30 - 70 mV (mostly ~ 50 mV) of the initial OCV of the “stack”because of the increasing gas diffusion through the degradated membrane

a certain influence of the electrode degradation may exist




FER in the anode and cathode exhaust water was nearly the same, even though the degradation of the anode side was higher (→ SEM investigations)

FER was slightly decreasing with operating time

Fluoride Emission Rate - Results

0.000

0.002

0.004

0.006

0.008

0.010

0 100 200 300 400 500 600 700 800

Time / h

FER

/ m

g F

- cm-2

h-1

Anode Standard

Cathode Standard



Fluoride Emission Rate - Results

accumulation of the total anode and cathode FER:

the formation of the first pinhole is related to a certain cumulated FER:




the resistance does not correlate clearly to

operating conditions

operating time

pinhole formation

membrane thickness (SEM)

a slight increase of the resistance could be observed at higher degradation/ long operating time in most cases

Membrane Resistance - Results

the resistance is influenced by

the loss of proton conducting, hydrophilic functional groups

the structural changes of the hydrophobic phase

the thinning of the membrane

change of the resistance with the operating time


5.0E-03

6.0E-03

7.0E-03

8.0E-03

9.0E-03

1.0E-02

1.1E-02

1.2E-02

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300time / h

mem

bran

e re

sist

ance

/ Ω



Scanning Electron Microscopy - Results

membrane thickness the thinning of the

membrane is not clearly related to operating conditions

membrane thinning occurs under moderate and harsh conditions

pinhole formation is not related to a significant membrane thinning

5

10

15

20

25

30

35

0 100 200 300 400 500 600 700 800 900 1000 1100time

mem

bran

e th

ickn

ess

/ µm

standardlow humiditylow temperaturehigh pressure45 mA90 mA135 mA405 mA



Conclusions a low cell current and a low gas humidity accelerate membrane degradationa lower temperature and higher cell currents slow membrane degradationthe current density influences the impact of other parameters (e.g. gas humidity)the end of lifetime of the MEA was indicated by the detection of the first pinhole (at moderate performance losses)an exceed of a certain value of cumulated FER could indicate theformation of pinholes a drop of OCV below a certain value could indicate the formation of pinholesmembrane thinning at the anode side occurred at long-time operation even under moderate operating conditionsthe membrane resistance did not correlate clearly to the membrane degradation, the formation of pinholes and the membrane thinning



future research activities

evaluate and understand the complex interactions of fuel cell operating conditions, membrane parameters and membrane lifetimeobserve membrane and electrode degradation separately find a (simple) analysis instrument to observe membrane ageing during fuel cell operation

thank you for your kind attention!

Monitoring of membrane failure due to pinhole formation...Professor Horst Cerjak, 19.12.2005 3...

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