Nanostructured Ti0.7

Mo0.3

O2

Support Enhances Electron

Transfer to Pt : High-Performance Catalyst for Oxygen

Reduction Reaction

1

Seonbaek Ha

Professor : Carlo U. Segre

12. 06. 2013

Department of Chemical and Biological Engineering

Illinois Institute of Technology

Outline

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Introduction and background of proton exchange membrane fuel cell

Challenges toward reality in fuel cell

The understanding of the paper

- Experimental design

: the electrochemical improvement of using Ti0.7Mo0.3O2

- Application of X ray absorption spectroscopy

: XANE, EXAF

Background of PEMFCs (Proton Exchange Membrane Fuel Cells)

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Anode

Cathode

Polymer electrolyte

Catalyst and catalyst support

Fuel

Source : US DOE

Anode : H2 (fuel) 2H+ + 2e- Eo = 0 V

Cathode : ½ O2 + 2H+ + 2e- 2H2O Eo = 1.229V

Overall : ½ O2 + H2 2H2O

Polymer Electrolyte Membrane Fuel Cell

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20 wt % Pt/CCatalyst : platinum

Support : Carbon black

Challenges : Catalytic activity

- oxygen reduction reaction (ORR)

Stability (durability)

- carbon corrosion

- loss of Pt during operation

Challenges of Catalyst and Catalyst Support in PEMFCs

• Carbon corrosion reaction

C + 2H2O CO2 + 4H+ + 4e- (E = 0.207 V vs. RHE)

High Surface Area (BET measurement)

(~50 – 800 m2/gcarbon)

High Porosity

(20 – 100 nm pore sizecarbon)

Electronic conductivity

( > 1 S/cmcarbon)

Stable under electrochemical conditions

potential cycling at 1.0 – 1.5 V

cyclic voltammetry (CV), linear sweep voltage (LSV)

Stable in acidic media (pH = 1, 2)

0.1 M HClO4, 0.5M H2SO4

Conducting metal oxides are a promising candidate due to higher stability

Approach to Find New Catalyst Support in PEMFCs

• Van Thi Thanh Ho et al., “Nanostructured Ti0.7Mo0.3O2 Support Enhances Electron Transfer to Pt : High-Performance Catalyst for Oxygen Reduction Reaction”, J. Am. Chem. Soc., 133 (2011) 11716-11724

• Target : high activity of Pt/Ti0.7Mo0.3O2 catalyst as compared Pt/C

: higher stability of Pt/Ti0.7Mo0.3O2 catalyst

• Electrochemical Measurment

NHE (reference electrode), 0.5M H2SO4, 0.1M HClO4,

7 μL of catalyst ink with 0.5 wt% Nafion and 6.2 mg of Pt/mL

Cyclic Volammetry ( 0.05 – 1.10 V, 25 mV/sec)

ORR measurement (0 – 1.1V, 1 mv/sec) at 1600 rpm

• Sample preparation

12 mM MoCl5 and 28 mM TiCl4 in Teflon-lined autocave (at 200 °C, 10 ° C/min, 2hr)

Ti0.7Mo0.3O2 and hexachloroplatinic acid in ethylene glycol + NaOH (pH – 11)

sonication for 30min and then heated (160 °C) in micro wave oven

• Analysis Instruments : XRD, TEM, XANES, EXAFS

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The Research Purpose and Experiment Preparation

of Pt/Ti0.7Mo0.3O2 in PEMFCs

Metal Doped TiO2

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Anatase TiO2 Rutile TiO2

200 °C 750 °C Annealing

temperature

BET Surface area 230 m2/g (232 m2/g, carbon) unknown

TiO2 phase

2.8 ·10-4 S/cm (Ti0.7Mo0.3O2)

1.7 ·10-7 S/cm (undoped TiO2)

Electrical

conductivityunknown

Deli Wang et al., J. Am. Chem. Soc. 9 (2010)10218 -10220Van Thi Thanh Ho et al., J. Am. Chem. Soc., 133 (2011) 11716-11724

Ti0.7Mo0.3O2 Ti0.7W0.3O2

Introduction to X-Ray Absorption Spectroscopy

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ln (I0/I) = µ(E) ·x

µ : absorption coefficient

X : sample thickness

Borh Atomic Model

Energy of absorbed radiation

at edge

Binding energy of electrons

in the K, L, M,.. shells of

the absorbing elements

Jens Als-Nielsen et al., Elements of Modern X-ray Physics

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XANES : ± 10 eV of edge

(x-ray absorption near edge structure)

K, L, M edge

NEAXFS : within 10 eV – 50 eV of edge

(near edge x-ray absorption fine structure)

EXAFS : 50 eV – 1000 eV above edge

(extended x-ray absorption fine structure)

E0 : binding energy

No

rmal

ized

Ab

sorp

tion

• Coordination number

• Oxidation state

• Geometry

Schematics of X-Ray Absorption Spectroscopy

• Local electronic and atomic structure

of sample

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Average valence state of Mo in

Ti0.7Mo0.3O2 = 5.75

XANE Result of Support Material

Mo : 4d5 5S1

MoO3 :1s 4d (at pre-edge)

Ebbinghaus, S.et al., J. Solid State Chem.156 (2001)194

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Decrease in d-band vacancy of Pt/Ti0.7Mo0.3O2 facile e- donation from Ti0.7Mo0.3O2 to Pt

XANE Results of Catalyzed Support

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EXAFS Results of Catalyzed Support

FCC Pt bulk

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ECSA (electrochemical surface area)

ORR Activity of Catalyzed Support

QPt [C/cm2]= Qtotal - Qdl

Pt/Ti0.7Mo0.3O2 – the highest performance

Qpt

Qdl

~ 8%

degradation

~ 25.8% degradation

Stability Evaluation of Catalyzed Support

~ 50.6% degradation

Conclusion

• Higher ORR activity and higher stability of Pt/Ti0.7Mo0.3O2

• X ray spectroscopy has predictable value in guessing electrochemical

performance of catalyst/support for the application of PEMFC

• Strong metal/support interaction between Pt and Ti0.7Mo0.3O2