Synthesis and durability of CNT based MEAs for PEM fuel cellsNanoduramea

SusEn Seminar, Espoo 22 September 2011Dr. Pertti KauranenVTT Technical Research Centre of Finland

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Presentation lineout

1. Motivation2. Partners 3. Work packages4. Materials and methods5. Highlights by the partners6. Summary

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1. Motivation (2008)* Durability of carbon supported Pt catalyst and Nafion@ membrane

electrolyte are the key factors limiting more wide spread use PEMfuel cells.

* The catalyst life is limited by growth of the Pt nanoparticles on thecarbon support and corrosion of the support itself.

* Degradation of the Nafion@ membrane can be caused by local dryingor overheating due to uneven current distribution or membrane puncturing due to mechnical failure.

* Durability of the membrane is a complex function of humidity andtemperature.

* Degradation of recast membrane in the catalyst layer is more severethan that of the bulk membrane.

* Corrosion is caused by peroxide intermadiates of the oxygen reductionreaction.

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2. Partners

Dr. Pertti Kauranen

Prof. Esko Kauppinen

Prof. Göran Lindbergh

Prof. Svein Sunde

Prof. Eivind Skou

Dr. Magnus Thomassen

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3. Work packages (WP)

WP1 Synthesis of CNT,

CNF and CNB, By Aalto

WP3 Surface scientific characterizationBy Aalto, VTT, NTNU and SDU

WP2 Pt and Pt alloy deposition By Aalto, VTT and NTNU

WP4 Ex-situ electrochemical

characterization By NTNU, SINTEF and SDU

WP5 MEA preparation

By KTH and IRD Fuel Cells

WP6 Single cell testing and

effluent analysis By VTT, KTH, SINTEF and SDU

WP7 Post mortem analyses

By Aalto, KTH, NTNU and SDU

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4. Materials and methods

1. Carbon supports- carbon blacks (e.g. Vulcan XC-72), commercial- single and few walled carbon nanotubes by Aalto University- graphitized multi walled carbon nanotubes, commercial- graphitized carbon nanofibres (VGCF), commercial

2. Surface treatment of carbon- acid treatment- plasma treatment- polyaniline treatment

3. Platinum deposition- impregnation, polyol, electrodeposition

4. Physical characterization- XRD, TG, FTIR, XPS, ESR, NMR, SEM, EDS, TEM

5. Electrochemical characterization- CV, EIS, CO stripping, EQMB, start-stop on RRDE and in PEMFC single cells

5.1. Aalto: Pt on graphitized carbon nanofibers (GNFs)

1. Preparation of 2g batches of catalyst with ∼20% Pt on Showa-Denko GNFs by the polyol method for IRD FC to prepare MEAs. Activated in 2M HNO3 /1M H2SO4 1:1 at120°C, 6hTransmission electron microscopy (TEM) and scanning electron microscopy (SEM) ESA: ∼30 m2/g

2. Deposition of ∼ 20% Pt on several batches of Showa-Denko GNFs with different activation treatments carried out by VTTCharacterization by Raman spectroscopy and EDS-SEM.

Pt on few-walled carbon nanotubes (FWCNTs)

Preparation of catalysts with ∼20% Pt on FWCNTs (grown by CVD at Aalto Univ.) by the polyol method with different activation treatments.

Activated in 2M HNO3 /1M H2SO4 1:1 at120°C for: 0, 2, 4 and 6h.Characterization by Raman spectroscopy, TEM and EDS-SEM.ESA: 40-60 m2/g

untreated 4h 6h

9

5.2. NTNU: Effect of Chlorides on Pt dissolution by EQMB

Potentiostatic at 1.2 V (RHE)

10

Improved Polyol Deposition Method (Ex-Situ)15 % Pt/XC-72 by In-Situ Polyol (old) 20 % Pt/Xc-72 by Ex-Situ Polyol

5.3. SDU:Thin film Rotating Ring Disc Electrode

11

CV on Thin film RDE

12

Potential (V) vs. RHE

Cur

rent

(uA

)

Pt 20% on XC-72 Vulcan, BASF

0.0 0.5 1.0

V

-500

0

500

µA

Peroxide formation on carbon supports

13

14

5.4. KTH:PEMFC Degradation due to Catalyst Support Corrosion

Alejandro Oyarce

Applied electrochemistrySchool of Chemical Science and Engineering

Royal Institute of Technology (KTH)

Applied electrochemistry

15

Fuel cell performance

Humidifier temperature:83 oC Humidifier temperature:64 oC

Conditions: Nafion 115 (127μm), Cell temperature 80 oC, gases:H2 (CE/RE)/ O2 or Air (WE), flow rates: 120 ml/min (CE/RE), 60 ml/min (WE). Sweep rate: 1 mV/sEIS: Frequency: 100kHz-0.1Hz, amplitude:5% of idc.

galvanostatic measurement at 0.2 A/cm2

Humidifier temperature:77 oC

At 0.2 A/cm2 the decrease in voltage performance is:-7% decrease for O2/H2-15% decrease for air/H2

At 0.2 A/cm2 the decrease in voltage performance is:-5% decrease for O2/H2-13% decrease for air/H2

At 0.2 A/cm2 the decrease in voltage performance is:-4% decrease for O2/H2-12% decrease for air/H2

The results are an indication that the mass transport losses upon the support degradation are mainly due to changes in the electrode morphology, specifically the large decrease in porosity

16

Carbon agglomeration of Pt/Vulcan

Fresh-Pt/Vulcan low RH Degraded-Pt/Vulcan low RH Degraded-Pt/Vulcan at high RH orhigh current densities

Fresh-Pt/Vulcan at high RH or high current densities

O2H2O

O2O2

O2

H2OH2O

H2O

17

Carbon nanofibers (Pt/GNF)

After degradation

Before degradation

Materials and Chemistry

Humidification of gases with membrane humidifiers

Fuel cell under test

70 °C, 100%RH

Heated 2μm filter and sample gas

line

FTIR spectrometer for detection of CO2, CO, SO2, CHOH, CHOOH and HF

in cathode exhaust

5.5. Sintef: In situ carbon corrosion measurement by FTIR

Carboncorrosion

• 70°C, 100% RH100 mL/min H2/Air

• 0.6 – 1.5 V, 40 mV/s, 300 cycles• 1 minute FTIR resolution

Recovery

• H2/N2, 100 mL/min OCV for 10h• Pt area CV every 2h

Reactivation

• H2/H2, 100 mL/min• -0.2 – 0.2 V, 20 mV/s, 3 cycles

Materials and Chemistry

Results – accellerated carbon corrosionPotential cycling 0.6-1.5V vs RHE, 300 cycles

19

Pt / Vulcan Pt / CNF

0

2

4

6

8

10

12

14

0

20

40

60

80

100

120

‐50 0 50 100 150 200 250

Carbon

 loss / wt%

CO2 c

oncentratio

n / p

pm

Time / min

CO2 ppm ‐MEA1CO2 ppm ‐MEA2C loss ‐MEA1C loss ‐MEA 2

0

2

4

6

8

10

12

14

0

20

40

60

80

100

120

‐50 0 50 100 150 200 250

Carbon

 loss / wt%

CO2 c

oncentratio

n / p

pm

Time / min

CO2 ppm ‐MEA 1CO2 ppm ‐MEA 2C loss ‐MEA 1C loss ‐MEA 2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Cell V

oltage / V

Current Density / mAcm‐2

InitialAfter corrosionInitial ‐MEA2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Cell V

oltage / V

Current Density / mAcm‐2

Initial ‐MEA 1After corrosion ‐MEA 1Initial ‐MEA 2After corrosion ‐MEA 2

135 mΩcm-2 96 mΩcm-2

Materials and Chemistry

Results – real start stop cycling15 min purge cycles for 20 h at OCV

20

Pt / CNF

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Cell V

olta

ge / V

Current density / A cm2

Before start/stopAfter start/stop

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Cell V

olta

ge / V

Current density / A cm2

Before start/stopAfter start/stop

Pt / CB

0

10

20

30

40

50

60

70

80

90

100

0 100 200 300 400 500 600 700 800

CO2co

ncen

tratio

n / pp

m

Time /min

Vulcan XC72CNF

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5.6. VTT: Analysis of GNF surface treatments by FTIR

Acid treatments ↑

Polyaniline treatments ↓

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Multisinglecell

Optimization and testing of the setup and components for the new 3 cm2 multisinglecellInstead of start-stop operation testing with automotive drive cycleResult: Problems in MWCNT ink preparation

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6. Summary

1. Vulcan XC-72 and FWCNT are not stable at potentials above 1.4 V (RHE) and during start-stop cycling.

2. Temperature and high relative humidity promote carbon corrosion.3. The pore structure of Vulcan collapse due to carbon corrosion leading to electrode

thinning and mass transport limitations.4. Graphitization of the carbon support and especially use of graphitized nanofibres

can improve the catalyst stability by a factor of 5 under severe operating conditions.5. Potential cycling at high potentials and presence of even traces of chlorides promote

platinum dissolution.6. Acid treatment has only minor effect on graphitized GNF surface. 7. Oxide functionalization is harmful for MWCNT.8. N-doping by PANI decomposition was not successful.9. Three summer schools were organized.10. The two PhDs educated have been hired by Nordic FC industries.11. It makes perfect sense to join forces in Nordic FC&H2 research.

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