Preparation of Metallic Nanocatalysts/NafionFilm by a Drying Process

JAEYOUNG LEE, WOOKUM LEE, HYUNGRYUL RIM, HONGKI LEE

Hydrogen Fuel Cell RIC, Woosuk Univ., Wanju-gun, Jeonbuk 565-902, Korea

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2

Stack

Anode: H2, MeOHCathode: O2, air

Fuel

BP BP

MEA

Membrane

Catalyst

Electrode

Water

Heat

Electric Power

Heat Exchanger

Converter/Inverter

BOP/Control

A fuel cell is a device that converts the chemical energy froma fuel into electricity through a chemical reaction of positivelycharged hydrogen ions with oxidizing agent..

1. Introduction

3Pt Nanocatalysts in PEMFC

Loading of Metal Nanoparticles into a Polymer

A metallic precursor dissolved along with a polymer in a solvent is reduced to the metallic nanoparticles during stirring, heating and evaporating solvent

A precursor dissolved in a monomer is reduced

to the metallic nanoparticles during polymerization

A colloidal solution of metal nanoparticles is mixed

with either a monomer or a polymer solution

One-step dry process

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One-Step Dry ProcessVaporized metal complex and a polymer film co-exist under N2.

The metallic precursor is reduced and forms nanoparticles.

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Objective

A new method for the preparation of Pt nanocatalystloaded in polymer electrolyte membrane or loaded on carbon supporters by using our drying process.

Pt loading characteristics were investigated by TEM, SEM, XRD and UTM analyses.

Water uptake, Cation conductivity and fuel cell performance were evaluated.

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2. Experimental

Materials

Platinum(II) bis(acetylacetonato)(Johnson Matthey Materials Technology) Palladium(II) bis(acetylacetonato)(Johnson Matthey Materials Technology)

⇒ They were purified by the recrystallization in acetone. Nafion Membrane (Dupont, NR212)

- Polymer name: sulfonated tetrafluorethylene copolymer - Film thickness: 50.8 μm⇒ In order to remove impurities in the Nafion NR212, it was treated at

80oC for 2 hr in 0.5M-sulfuric acid solution and then residual acidwas removed by boiling at 80oC for 1 hr in diionized water. It wasboiled once more at 80oC for 1 hr in deionized water, and thendried at 25oC vacuum oven for 36 hr.

One-Step Dry ProcessVaporized metal complex and a polymer film co-exist under N2.

The metallic precursor is reduced and forms nanoparticles.

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One-Step Dry Process

(One-Step Dry process) (Pt-loaded Nafion Film)

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Fig. EDS spectra for the corresponding area on the cross-sectional Pt/Nafion film prepared by exposure to Pt(acac)2 for 30 min. 10

3. Results and discussion

Fig. TEM images for Pt nanoparticles on the cross-sectional Pt/Nafionfilm prepared by exposure to Pt(acac)2 for (a) 5 min, (b) 10 min, (c) 30min and (d) 60 min, respectively. Prime (') was captured at 0.2 μm depthand double prime (") was done at 6 μmdepth from the upper surface.

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Fig. XRD patterns for Pt/Nafion film prepared by adry process.

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Fig. Water absorption characteristics of (a) Nafion film and (b) Pt/Nafionfilm prepared by exposure to Pt(acac)2 for 30 min.

(a) (b)

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Fig. Effect of Pt-loading time on tensile strength of Pt/Nafion filmprepared by a dry process.

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Table. Cation conductivity of Nafion and Nafion/Pt-5 films.

Sample RH (%) Temperature(℃)

Conductivity(mS/cm) Test method

Nafion film

101.5 90.1 111.3

4-prove

100.0 80.0 79.7100.4 70.0 59.5100.5 60.1 48.397.5 49.6 44.6

101.1 40.3 14.5

Nafion/Pt-5 film

101.5 89.8 123.9

4-prove

100.0 80.0 88.8100.4 70.0 66.2100.5 60.0 53.897.5 50.0 49.7

101.1 40.0 16.1

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Fig. I-V curves for Nafion and Nafion/Pt-5 films.

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Preparation of Pt Nanoparticles on GDL

1. Mixing carbon graphite/nafion/surfactant with a sonicator.2. Spray coating on a GDL .3. Penetrating the Pt complex into the GDL in a drying process.4. Simultaneously reducing the Pt complex in the same step of the

penetration process. 5. Observing the Pt nanoparticles by SEM and analyzing the atomic

content by EDS.

One-step drying processfor the preparation of Pt-nanocatalyst.

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Fig. (a) SEM image and (b) EDS spectrum for the Pt (1.25 at%) loadedcarbon black layer prepared by spray-coating three times. (c) magnifiedSEM image and (d) Pt elemental mapping for the (c). 18

Fig. TEM images of Pt nanoparticles loaded on different weight ofnafion/carbon black: (a) 1.2 mg/cm2, (b) 2.4 mg/cm2 and (c) 3.6mg/cm2. The loading time and temperature conditions were for 30min at 180oC.

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Fig. (a) SEM and (a’) EDS for Pd (5.4 wt%) nanoparticles. (b) SEM and(b’) EDS for Pt-Pd (13.7 wt%) nanoparticles.

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Fig. (a) TEM and (b) the magnified image for Pd (5.4 wt%) (c) TEM and(d) the magnified image for Pt-Pd (13.7wt%) nanoparticles.

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A simple drying process was developed for thepreparation of a Pt/Nafion membrane to be used forPEMFC. Sublimed Pt(acac)2 was spontaneously reducedto Pt nanocatalysts on the nucleophilic groups in a Nafionmembrane.

As the exposure time to Pt(acac)2 increased, nanoparticlesize and its number density increased. And the particlesize at the surface area was larger than that at the deeperarea.

XRD patterns for Pt nanoparticles clearly showed thepreferential crystallographic orientation-face centeredcubic (FCC) structures.

From these results, we concluded that our dry processcould be applied to prepare Pt nanocatalyst/Nafionmembrine for a fuel cell.

4. Conclusion