1

Kenichi ShimizuResearch Seminar Nov. 8 2007

Study of Electrochemical Catalysts in Fuel Cells

2

Study of Electrochemical Catalysts in Fuel Cells

I. Study of catalyst effect in anodic oxidation of organic fuels.

II. Synthesis of Pt on single-walled carbon nanotube for cathodic reduction of oxygen.

3

Fuel Cells A system that harvests electrical

energy directly from a spontaneous redox reaction.

Electricity will be generated continuously as long as fuel is supplied.

Sutton, G.W. Direct Energy Conversion. Inter-University Electronics Series, Vol.3, NY, 1966.

4

F + zH2O wCO2 + vH+ + ve- + xI (E1)

yO2 + vH+ + ve- zH2O (E2)

F + yO2 wCO2 + zH2O + xI (Ecell = E1+ E2)

Fuel Cells

Sutton, G.W. Direct Energy Conversion. Inter-University Electronics Series, Vol.3, NY, 1966.

5

Experimental design by William R. Grove

Grove, W.R. Phil. Trans. 1843, 133, 91-112.

Gas Voltaic Battery (1842)

molkJHmolkJGOHOH

molkJHmolkJGOHeHO

molkJHmolkJGeHH

rxnrxn /8.285/2.2372

1

/8.285/2.237222

1

/0.0/0.022

222

22

2

6

After >100 years of Nothing Alkaline hydroxide

fuel cell was developed for Apollo mission in the 1950’s.

Hoogers G.; Fuel cell technology handbook, CRC press, NY, 2003, pp2-1.

0

2000

4000

6000

8000

10000

# pu

blic

atio

n

200620011996199119861981

Year

9733

7

Types of Fuel Cells

Solid oxide fuel cell.

Molten carbonate fuel cell.

Polymer Electrolyte Membrane (PEM) fuel cell.

Microbial fuel cell.

Hoogers G.; Fuel cell technology handbook, CRC press, NY, 2003, 2-1.Fuel Cell Technology. Reaching Towards Commercialization, Springer, Germany, 2006, 277-293.Meier, F.; et al.; J. Membr. Sci. 2004, 241, 137.

www.news.cornell.edu

Nafion®

8

Polymer Electrolyte Membrane Fuel Cell

Nafion®

Meier, F.; et al.; J. Membr. Sci. 2004, 241, 137.

9

Zero (low) emission. No mechanical parts. Higher fuel efficiency.

GM Sequal fuel cell vehiclehttp://www.fueleconomy.gov/feg/

Automotive Application of Fuel Cell

http://www.fueleconomy.gov/feg/

10

Generates little or no air pollution.

Sustainable fuel source. Anaerobic digester gas

Quiet.

www.fuelcellenergy.com

Stationary Power Plant

11

Fuel cells could supply larger energy density than the conventional battery system.

Very quick recharge.http://pr.fujitsu.com/en/news/

http://www.physorg.com/news6542.html

Portable power sources

12

Limitations of Fuel Cells Fuel availability and storage.

Use of Hydrogen as anode fuel

Low power density. Kinetic limitations.

High cost. Pt catalyst. Polymer electrolyte membrane.

O’Hayre, R. et al. Fuel Cell Fundamentals. John Wiley & Sons. N.Y. 2006.

Part 2

Part 1

13

Study of Electrochemical Catalysts in Fuel Cells

I. Study of catalyst activity in anodic oxidation of organic fuels.

II. Synthesis of Pt on single-walled carbon nanotube for cathodic reduction of oxygen.

14

Limitations of Fuel Cells Fuel availability and storage.

Use of Hydrogen as anode fuel

Low power density. Kinetic limitations.

High cost. Pt catalyst. Polymer electrolyte membrane.

O’Hayre, R. et al. Fuel Cell Fundamentals. John Wiley & Sons. N.Y. 2006.

15

Fuel Availability and Storage

molkJHmolkJGOHCOOOHCH

molkJHmolekJGOHeHO

molkJHmolekJGeHCOOHOHCH

rxnrxn /4.726/9.70222

3

/4.857/1.7123662

3

/0.131/2.966

2223

22

223

Heath, C.E.; Worsham, C.H.; The Electrochemical Oxidation of Hydrocabons in a Fuel Cell. In Fuel Cell, Young, G.J. Ed.; Reihold Publishing Corp.: NY, 1963, Vol. 2;, pp 182.

CH3OH Fuel Cell H2 Fuel Cell

Fuel Storage Easy Difficult

Fuel Availability High Low

Cost ($/kWh) 0.02 0.15

Efficiency (%) 97 83

Power (kW/g-Pt) 0.2 0.6

16

Lower power density

O’Hayre, R. et al. Fuel Cell Fundamentals. John Wiley & Sons. N.Y. 2006.

17

Kinetic Limitations for Fuel Cells

0

0.2

0.4

0.6

0.8

0 0.02 0.04 0.06 0.08 0.1Ampere

Cel

l p

ote

nti

al (

V)

1. Potential drop is observed due to activation energy.

2. Ohmic resistance of the cell is proportional to the applied amperage.

3. Potential starts dropping at higher current due to mass transport.

1 2

O’Hayre, R. et al. Fuel Cell Fundamentals. John Wiley & Sons. N.Y. 2006.

3

18

Basic Operation of a Fuel Cell

O’Hayre, R. et al. Fuel Cell Fundamentals. John Wiley & Sons. N.Y. 2006.

Anode

Fuel

1 13

H2OProduct

2

Electrolytee.g. Nafion®

H+

Anode Cathode

1. Activation energy.

2. Ohmic resistance

3. Mass transport.

e- e-

3

Air

3

19

Activation Energy Activation loss of cell potential is due to the

electrochemical reactions

For hydrogen fuel cell Oxygen reduction at cathode.

For fuel cells with organic fuel Anodic oxidation of organic fuel.

20

Activation Energy Activation loss of cell potential is due to the

electrochemical reactions

For hydrogen fuel cell Oxygen reduction at cathode.

For fuel cells with organic fuel Anodic oxidation of organic fuel.

21

Activity of Catalysts in Anodic Oxidation

1. Methanol oxidation.

Overview of how catalysts are evaluated in anodic oxidation using cyclic voltammetry.

2. Formic acid oxidation.

Evaluate kinetic effects of PtRu and PtBi catalysts on anodic oxidation.

22

1. Methanol OxidationCH3OH

HCHO CO

HCOOH

CO2

H2O M-OHabs+ H+

H2O M-OHabs+ H+

Christensen, P.A. et al. J. Electroanal. Chem. 1993, 362, 207-218.

23

Cyclic Voltammetric response of Methanol Oxidation

PtRuCNT

0

0.4

0.8

1.2

1.6

0.2 0.4 0.6 0.8 1 1.2E /(V vs. NHE)

i /(m

A)

24

PtRuCNT

0

0.4

0.8

1.2

1.6

0.3 0.5 0.7 0.9 1.1E /(V vs. NHE)

i /(m

A)

Interpretation of Forward Peak

CH3OH + H2O CO2 + 6H+ + 6e-

Slow reaction kinetics

• Deactivation of catalyst surface• Mass transport

Sufficient reaction kinetics

Onset potential

25

Interpretation of Reverse PeakPtRuCNT

0

0.4

0.8

1.2

1.6

0.3 0.5 0.7 0.9 1.1E /(V vs. NHE)

i /(m

A) Not sufficient

potential• Reviving catalyst surface • Mass transport

CH3OH + H2O CO2 + 6H+ + 6e-

Kinetically controlled reaction

Manohara, R.; Goodenough, J.B. J. Alloy. Compd. 2001, 315, 118.

26

Guidelines for Evaluating Catalysts using Cyclic Voltammetry

Catalyst efficiency (if/ib). Current density (A/cm2)

Study of catalyst effects on an electrochemical reaction.

Mass activity (A/g-Pt) Study of catalyst system.

PtRuCNT

0

0.4

0.8

1.2

1.6

300 500 700 900 1100E /(mV vs. NHE)

i /(m

A)

Liu, Z. et al.; J. Phys. Chem. B 2004, 108, 8234.

if

ib

27

Cyclic Voltammetric Evaluation of Catalysts in Methanol Oxidation

Yen, C.H.; Shimizu, K.; Lin, Y.-Y.; Bailey, F.; Cheng, I.F.; Wai, C.M.; Energy & Fuels 2007, 21, 2268.

Mass activity

28

Kinetic Effects of Carbon Nanotube Supported Binary Metal Catalysts; PtRuCNT

and PtBiCNT

2. Formic Acid Oxidation

29

Application of Formic Acid as a Fuel

Fuel Cell Formic acid Methanol Hydrogen

Efficiency (%) 95 97 83

Cost ($/kWh) -- 0.02 0.15

Fuel Storage Easy Easy Difficult

Fuel Availability High High Low

Cell Potential (V) 1.45 1.21 1.23

Fuel Crossover Low High High

Heath, C.E.; Worsham, C.H.; The Electrochemical Oxidation of Hydrocabons in a Fuel Cell. In Fuel Cell, Young, G.J. Ed.; Reihold Publishing Corp.: NY, 1963, Vol. 2;, pp 182.Kang, S.; et al.; J. Phys. Chem. B, 2006, 110, 7270.

30

Fuel Crossover

DMethanol = 5 x10-6 cm2s-1. Creates short circuit.

Fu

elPEM

Ai

r

H+

(Anode) (Cathode)

Meier, F.; et al.; J. Membr. Sci. 2004, 241, 137.Mauritz, K.A.; Moore, R.B.; Chem. Rev.2004, 104, 4535.

www.news.cornell.edu

H+

H+

H+

31

Formic Acid Oxidation Direct electrochemical oxidation to CO2.

Chemical pathway involves spontaneous dissociation of formic acid to water and CO.

Rice, C.; et al.; J. Power Sources, 2003, 115, 229.

32

-1

0

1

2

3

4

5

0.1 0.6 1.1E /(V vs.NHE)

mA

/cm

2

Pt CNT

Cyclic Voltammetric Response of Formic Acid Oxidation

1. HCOOH CO2 + 2e- E0 = + 0.17 V

2. HCOOH COads + H2O

3. H2O OHads + H+ +e-

4. COads + OHads CO2 + H+ + e-

1

4

1

33

PtRu CNT and PtBi CNT

Pt42Ru58CNT Pt38Bi62CNT

20 nm 50 nmAtomic ratio of Pt:Ru is 1:1.4.Atomic ratio of Pt:Bi is 1:1.6.

Image is courtesy of Clive, H. Yen.

34

Catalytic Effect of Ru

0

2

4

6

8

10

12

0.1 0.3 0.5 0.7 0.9 1.1E /(V vs. NHE)

i /(m

A/c

m2,

PtR

uCN

T)

0

1

2

3

4

5

i /(m

A/c

m2,

PtC

NT

)

PtRuCNTPtCNT

1 M H2SO4

0.1 M HCOOH

CO2

CO

intermediate

HCOOHRxn 1

Rxn 2

35

Evaluation of PtRu CNT using Peak Currents

-1

0

1

2

3

4

5

0.1 0.3 0.5 0.7 0.9 1.1E /(V vs. NHE)

i /(m

A/c

m2)

mA/cm2 (1) (2) (3) (1)/(3)

PtCNT 2.84 1.61 5.31 0.53

PtCBc 3.72 0.99 11.3 0.33

PtRuCNT 11.6 N/A 8.91 1.3

PtRuCBc 6.05 N/A 6.01 1.0C Commercial Catalyst

Higher current ratio suggests higher catalytic efficiency of PtRu pair.

(1)

(2)

(3)Pt CNT

1 M H2SO4

0.1 M HCOOH

36

Catalytic Effect of Bi

0

0.3

0.6

0.9

1.2

0.1 0.3 0.5 0.7 0.9 1.1E /(V vs. NHE)

i /(m

A/c

m2 ,

PtB

iCN

T)

0

1

2

3

4

5

i /(m

A/c

m2 ,

PtC

NT

)

PtBiCNTPtCNT

1 M H2SO4

0.1 M HCOOH

CO2

CO

intermediate

HCOOHRxn 1

Rxn 2

37

A/cm2 (1) (2) (3) (2)/(3)

PtCNT 2.84 1.61 5.31 0.30

PtCBc 3.72 0.99 11.3 0.09

PtBiCNT N/A 0.73 0.52 1.4

C Commercial catalyst

Evaluation of PtBi CNT using Peak Currents

-1

0

1

2

3

4

5

0.1 0.3 0.5 0.7 0.9 1.1E /(V vs. NHE)

i /(m

A/c

m2)

(1)

(2)

(3)Pt CNT

1 M H2SO4

0.1 M HCOOH

38

Eac (kJ/mole) CI (90%)

PtCBa 20.4 N/A

Ptb 20.9 1.6

PtCBc 21.1 2.1

PtCNT 17.5 2.5

PtRuCBc 21.0 3.9

PtRuCNT 22.2 4.3

PtBiCNT 45.5* 3.0

PtBi(III)b 42.3* 7.2C Commercial catalyst

a Lovic, J.D.; et al.; J. Electroanal. Chem., 2005, 581, 294.b Wilson, J.R.; et al.; J. Electrochem. Soc., 1984, 2369.

90% confidence interval

Activation Energy of Formic Acid Oxidation

39

Review of PtBiCNT Peak currents

Low peak current density suggests slower kinetics.

Current ratio suggests high catalyst efficiency.

Activation energy PtBiCNT requires the highest activation energy.

Bi keeps Pt free from CO poisoning but lowers overall catalytic activity.

40

Third Body Effect

Cao, D. et.al.; J. Phys.Chem. 2005, 109, 11622.

41

Third Body Effect Replace neighboring

Pt with the secondary metal catalyst.

Does not provide the three Pt binding site for CO.

Pt stays free of CO poisoning.

Casado-Rivera, E.; et al.; Chem. Phys. Chem. 2003, 4, 193.Gojković, S.Lj.; et al. Electrochimica Acta 2003, 48, 3607-3614.

42

Tafel Analysis

Exchange current, j0;

Equilibrium Potential, Eeq;

Tafel Slope, β;

PtCNT

-16

-12

-8

0.1 0.5 0.9E (V vs. NHE)

ln l

j/(A

/cm

2 )l β

Eeq

lnlj0l

Red Ox Aif

ib

kf

kb

B

1 M H2SO4

0.1 M HCOOH

43

Tafel AnalysisTafel Slope (mV/dec)

j0

(µA/cm2)

Eeq

(V vs. NHE)

PtCB 150 -- --

PtCNT 91 ± 9 3.6 ± 0.6 0.30 ± 0.02

PtCBc 80 ± 4 5.3 ± 1.3 0.23 ± 0.05

PtRuCNT 32 ± 5 11.8 ± 5.9 0.29 ± 0.01

PtRuCBc 39 ± 3 2.9 ± 0.6 0.26 ± 0.02

PtBiCNT 96 ± 3 4.3 ± 2.5 0.25 ± 0.07C Commercial catalyst

Lovic, J.D.; et al.; J. Electroanal. Chem., 2005, 581, 294.

±: 90% confidence interval

44

Conclusion of Formic Acid Oxidation

PtRuCNT improved catalytic efficiency by enhancing the reaction kinetics. No significant change in activation energies. Highest exchange current was observed.

CO2

CO

intermediate

HCOOHRxn 1

Rxn 2

45

Conclusion of Formic Acid Oxidation Addition of Bi could suppress Pt poisoning

by CO. Improved current ratio (catalytic efficiency). Activation energy was significantly higher.

CO2

CO

intermediate

HCOOHRxn 1

Rxn 2

46

Study of Electrochemical Catalysts in Fuel Cells

I. Study of catalyst activity in anodic oxidation of organic fuels.

II. Synthesis of Pt on single-walled carbon nanotube for cathodic reduction of oxygen.

47

For Oxygen Reduction;

O2(g) + 4H+ + 4e- 2H2O(l)

Synthesis of Pt-SWNT

48

Catalyst Requirement Stable. Adequate electrical conductivity. High surface area. High catalytic activity.

Liebhafsky, H.A; Cairns, E.J. Fuel Cells and Fuel Batteries. John Wiley & Sons Inc., N.Y., 1968, pp384.

49

Methods of Synthesis

Direct supercritical CO2 deposition.

Water-in-hexane microemulsion.

Water-in-supercritical CO2 microemulsion.

Electro-less deposition of Pt.

50

Electro-less deposition of Pt onto SWNT

Reduction of Pt2+ in Methanol/Water Solution

51

Single-Walled Carbon Nanotube Metal impurities does

not diffract on XRD. 29 wt% Fe present. Hydrophobic in nature.

TEM

20 nm

52

Pt-SWNT Synthesis 3 to 5 mg of unpurified

SWNT. Methanol/Water (1:1

v/v). Aqueous Pt2+ salt. Inspired by Choi et al.

Choi, H.C.; et al. J. Am. Chem. Soc. 2002, 124, 9058.

53

-0.1

-0.1

0.0

0.1

0.1

0.2

0.1 0.3 0.5 0.7 0.9 1.1E /(V vs. NHE)

i /(

Am

p/m

g P

t)

Pt-SWNT in Methanol Oxidation

Oxidation of 0.1M methanol in 1M sulfuric acid. Anodic peak current: 446 mA/mg-Pt for Pt-SWNT vs. 111 mA/mg-Pt for PtCB. Pt surface area: 351 cm2/mg-Pt for Pt-SWNT vs. 107 cm2/mg-Pt.

Pt-SWNT (1:10 C:Pt) Commercial PtCB

-0.1

0.0

0.1

0.2

0.3

0.1 0.3 0.5 0.7 0.9 1.1E /(V vs. NHE)

i /(A

/mg-

Pt)

54

Wt% Pt and % Utilization >90 % conversion of

Pt2+/Pt0 was obtained from UV-vis analysis.

50.5 wt% corresponds to only 5 % conversion.

24.534.4

50.5

0

20

40

60

1:4 C:Pt

1:6 C:Pt

1:10C:Pt

Wt%

Pt o

n P

t-S

WN

T

55

Spontaneous Reduction of Pt(II)

Pt nanoparticles can be formed through reduction of Pt2+ by alcohols.

Wang, X. et al. Nature 2005, 437, 121-124.

56

3 to 5 mg of SWNT

Pt2+

Shaking and ultrasonic agitation

Reduction of Pt by SWNT in aqueous solution

57

Reduction of Pt by SWNT

-0.1

-0.05

0

0.05

0 0.5 1E /(V vs. NHE)

i /(

mA

)

as received SWNTPt-SWNT

Hydrogen adsorption/desorption (circled region) indicates Pt(0).

2.8 wt% Pt in Pt-SWNT.

Deposition efficiency is 16 %.

2200 cm2/mg-Pt.

107 cm2/mg-Pt from the commercial PtCB.

EDS

Count

s

58

Methanol Oxidation

Pt-SWNT

-3.0E-05

-1.0E-05

1.0E-05

3.0E-05

5.0E-05

0.1 0.3 0.5 0.7 0.9 1.1E /(V vs. NHE)

i /(

Am

p)

w/o Methanolw/ Methanol

59

Reduction of OxygenPt-SWNT

-0.6

-0.4

-0.2

-0

0.2

0 0.5 1E /(V vs. NHE)

i /(

A/m

g-P

t)

w/ Nitrogen

w/ Oxygen

Commertial PtCB

-0.1

-0.05

0

0 0.5 1E /(V vs. NHE)

i /(A

/mg

-Pt)

Ep

(V)

ip

(A/mg-Pt)

Pt-SWNT 0.68 0.52

PtCB 0.57 0.05

60

Chronoamperometry

0200400600800

100012001400

0 10 20 30Time (s)

i /(A

/mg

-Pt)

Commercial PtCB w/ NitrogenCommertial PtCB w/ OxygenPt-SWNT w/ NitrogenPt-SWNT w/ Oxygen

@ 600 mV

14 times higher catalytic activity

61

Synthesis Pt-CB and Pt-MWNT

-11

-8

-5

-2

1

0 0.5 1E /(V vs. NHE)

i /(m

A)

w/ Nitrogen

w/ Oxygen-200

-150

-100

-50

0

50

0 0.5 1E /(V vs. NHE)

i /(m A

)

This synthetic method was applicable to other carbon supports.

Carbon black substrate (80-100 mesh, 100 % carbon)

Unpurified multi-walled carbon nanotube (95 % purity)

62

Conclusion for Pt-SWNT Direct deposition of Pt onto SWNT

without added reducing agent. Prepared catalyst was 14 times more

active towards O2 reduction. Inactive towards methanol oxidation.

Will not be affected by methanol crossover.

Applicable to other carbon substrates.

63

Current/Future work Investigate reaction mechanism for Pt-

SWNT synthesis in aqueous solution. Possible improvement on Pt utilization.

Currently 16 %.

Synthesis of bimetallic catalysts. Application to fuel cell.

64

Acknowledgement Dr. Frank Cheng Dr. Chien Wai and his research group

Clive Yen, Shaofen Wang, Byunghoon Yoon, Dinesh Thanu Dr. Peter Griffiths and his research group Dr. Garry Knerr Department of chemistry and office staff Cheng group

Tina Noraduon, Derek Laine, Simon McAllister, Rubha Ponraj, Yu Qun Xie, and Chris Roske Department of Agricultural engineering for power press Department of Forest Product for heat press Tom Williams and Franklin Barely for XRD, TEM, SEM, and EDS. Maria Paulina Viteri Espinel

Financial support Electric Power Research Institute (EPRI) Innovative Small Grants Program Dr. and Mrs. Renfrew Summer scholarship

65

Additional References1. Cheng, S.; Liu, H.; Logan, B.; Environ. Sci. Technol. 2006, 40,

364.

2. Zhou, L.; Gunther, S.; Imbihl, R.; J. Catal. 2005, 230,166.

3. Park, K.W.; Choi, J.H.; Sung, Y.E.; J. Phys. Chem. 2003, 107, 5851.

4. Matsumoto, T.; et al.; Chem. Comm. 2004, 840.

5. Park, K.-W.; Choi, J.-H.; sung, Y.-E.; J. Phys. Chem. 2003, 107, 5851.

6. Tang, H.; Chen, J.H.; Wang, M.Y.; Nie, L.H.; Kuang, Y.F. Yao, S.Z.; Appl. Catal. A 2004, 275, 43.

7. Casado-Rivera, E.; Volpe, D.J.; Alden, L.; Lind, C.; Downie, C.; Vázquez-Alvarez, T.; Angelo, A.C.D.; DiSalvo, F.J.; Abruña, H.D.; J. Am. Chem. Soc. 2004, 126, 4043.

8. Huang, J.; Yang, H.; Huang, Q.; Tang, Y.; Lu, T.; Akins, D.L.; J. Electrochem. Soc. 2004, 151(11), A1815.

9. Roychowdhury, C.; Matsumoto, F.; Zeldovich, V.B.; Warren S.C.; Mutolo, P.F.; Ballesteros, M.; Wiesner, U.; Abruña, H.D.; DiSalvo, F.J.; Chem. Mater. 2006, 18, 3365.