1

Ph. D., Seminar - 1

Ch. Venkateswara Rao

SEARCH FOR A VIABLE OXYGEN REDUCTION ELECTRODE

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Contents

Significance of oxygen reduction reaction in fuel cells Critical issues in oxygen reduction reaction

Oxygen reduction electrocatalysts - Noble metal based electrocatalysts - Non-noble metal based electrocatalysts

Pyrolyzed macrocycles (N4–metal chelates) as viable option

Conclusions

3

Importance of electrochemical reduction of oxygen

Fuel Cells

Metal-Air batteries

Industrial electrolytic processes

4

Thermal Energy Mechanical Energy

Chemical Energy Electrical Energy

Fuel CellsFuel Cells

Fuel Cell

ICE

Direct Energy Conversion Vs. Indirect Technology

5

Batteries - Needs recharging - Toxic chemicals - Low energy density

Internal combustion engines - Carnot limitations - Moving parts and hence friction - Noisy

Batteries/Internal Combustion Engines/Fuel CellsBatteries/Internal Combustion Engines/Fuel Cells

Energy Conversion

(devise/fuel)

Efficiency (%)

Fuel Cells (H2/PEMFC) 50-60

Internal Combustion

Engine / Gasoline C2H5OH

20-25

Diesel Engine / Diesel

25-30C. K. Dyer, J. Power Sources, 106 (2002) 245

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Efficiency

Cleanliness

Unique operating characteristics

Planning flexibility

Reliability

Future development potential

Fuel Cells – AdvantagesFuel Cells – Advantages

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Different Types of Fuel Cells

Characteristic

PEMFC

(Proton Exchange

Membrane Fuel Cells)

DMFC

(Direct Methanol Fuel Cells)

AFC

(Alkaline Fuel

Cells)

PAFC

(Phosphoric Acid Fuel

Cells)

SOFC

(Solid Oxide Fuel Cells)

MCFC

Molten Carbonate Fuel Cells)

Operating temp (oC)

60 – 80 60 – 80 100 –150 180 – 220 750 - 1050 650

Fuel

H2 (pure or reformed)

CH3OH H2 H2 (reformed)

H2 and CO reformed &

CH4

H2 and CO

reformed & CH4

Charge carrier

in the electrolyte

H+ H+ OH- H+ CO32- O2-

Poison

CO>10 ppm

Adsorbed intermediates

(CO)

CO, CO2 CO>1%

H2S>50 ppm

H2S>1ppm H2S>0.5 ppm

Applications Transportation, Portable Space, Military Power generation, Cogeneration

Low Temperature

Fuel Cells

Medium Temperature

Fuel Cells

High Temperature

Fuel Cells

Fuel Cells

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Chemical and Electrochemical data on various fuelsChemical and Electrochemical data on various fuels

FUEL

G0 kcal/mol

E0theoretical

(V)

E0max

(V)

Energy density (kWh/kg)

Hydrogen -56.69 1.23 1.15 32.67

Methanol -166.80 1.21 0.98 6.13

Ammonia -80.80 1.17 0.62 5.52

Hydrazine -143.90 1.56 1.28 5.22

Formaldehyde -124.70 1.35 1.15 4.82

Carbon monoxide -61.60 1.33 1.22 2.04

Formic acid -68.20 1.48 1.14 1.72

Methane -195.50 1.06 0.58 -

Propane -503.20 1.08 0.65 -

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2H2 4H+ + 4e-

O2 + 4 H+ + 4 e-

2 H20

Low Temperature Fuel CellsPEMFC & DMFC

PEMFC

DMFC

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

Methanolfrom Tank

Anode Cathode

Fuel

10Performance losses in PEMFC & DMFC MEAs operating at 80 oC

3

Activation losses

Ohmic lossesConcentration

losses

H2 2 H+ + 2 e- ; Eo = 0.0 V

CH3OH + H2O CO2 + 6 H+ + 6 e-; Eo = 0.02 V

½ O2 + 2 H++ 2 e- H2O; Eo = 1.23 V

PEMFC DMFC

3/2 O2 + 6 H++ 6 e- 3 H2O ; Eo = 1.23 V

H2 + ½ O2 H2O; Eo = 1.23 V CH3OH + 3/2 O2 CO2 + 2 H2O; Eo = 1.21 V

(At Anode)

(At Cathode)

T. R. Ralph and M. P. Hogarth, Platinum Met. Rev., 46 (2002) 146

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Difficulties in PEMFC & DMFCDifficulties in PEMFC & DMFC

Sluggish oxygen reduction kinetics

Methanol crossover (in DMFC)

Electrocatalysts

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Reaction pathways for oxygen reduction reaction

Path A – direct pathway, involves four-electron reduction O2 + 4 H+ + 4 e- 2 H2O ; Eo = 1.229 V

Path B – indirect pathway, involves two-electron reduction followed by further two-electron reduction

O2 + 2 H+ + 2 e- H2O2 ; Eo = 0.695 V

H2O2 + 2 H+ + 2 e- 2 H2O ; Eo = 1.77 VHalina S. Wroblowa, Yen-Chi-Pan and Gerardo Razumney, J. Electroanal. Chem., 69 (1979) 195

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High oxygen adsorption capacity

Structural stability during oxygen adsorption and reduction

Stability in electrolyte medium

Ability to decompose H2O2

High conductivity

Tolerance to CH3OH (in DMFC)

Low cost

Essential criteria for choosing an electrocatalyst for oxygen reduction

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Noble metal based electrocatalysts

Pt

Pt alloys -- PtFe, PtCo, PtNi, PtCr

Carbon supported Pt and its alloys

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Why Pt ?Why Pt ?

High work function ( 4.6 eV )

Ability to catalyze the reduction of oxygen

Good resistance to corrosion and dissolution

High exchange current density

Oxygen reduction activity as a function

of the oxygen binding energy

J. J. Lingane, J. Electroanal. Chem., 2 (1961) 296

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Why carbon as an electrode support ?Why carbon as an electrode support ?

Chemical properties

- Good corrosion resistance

- Available in high purity

- Forms intercalation compounds

Electrical Properties

- Good Conductivity

Mechanical Properties

- Dimensionally & Mechanically stable

- Low modulus of elasticity

- Light weight & adequate strength

- Availability in variety of physical Structures

- Easily fabricated into Composite Structures

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Most promising Electrocatalyst – 20 wt% Pt/C

Difficulties with Pt

Slow ORR due to the formation of –OH species at +0.8 V

Scarce and expensive

O2 + 2 Pt Pt2O2

Pt2O2 + H+ + e- Pt2-O2H

Pt2-O2H Pt-OH + Pt-O

Pt-OH + Pt-O + H+ + e- Pt-OH + Pt-OH

Pt-OH + Pt-OH + 2 H+ + 2 e- 2 Pt + 2 H2O

Development of mixed potential (in DMFC)Cyclic voltammograms of the Pt electrode in helium-deaerated () and O2 sat. (- - -) H2SO4

Charles C. Liang and Andre L. Juliard, J. Electroanal. Chem., 9 (1965) 390

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Why Pt alloys are more active for oxygen reduction ?

Shortening of Pt-Pt interatomic distance

Surface roughening

Increased d-band vacancies

Kin

e tic

cu

rren

t d

ensi

ty (

mA

/cm

2 )

Ni, Fe, Co atom% Kinetically controlled current densities for the ORR at 0.76 V as a function of the composition of alloy catalysts

Activity increases in the order: PtNi < PtCo < PtFe

Catalyst Pt-Pt distance (Å)

Roughness %

Pt

Pt53Ni47

Pt49Co51

Pt51Fe49

2.77

2.64

2.69

2.77

5.8

8.3

12.5

7.7

S. Mukerjee, S. Srinivasan, M. P. Soriaga, and J. McBreen, J. Electrochem. Soc.,142 (1995) 1409

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Proposed mechanism for oxygen reduction on Pt alloys

Increase of 5d vacancies led to an increased 2 electron donation from O2 to surface Pt and weaken the O-O bond

As a result, scission of the bond must occur instantaneously as electrons are back donated from 5d orbitals of Pt to 2* orbitals of the adsorbed O2

T. Toda, H. Igarashi, H. Uchida and M. Watanabe, J. Electrochem. Soc., 146 (1999) 3750

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Performance of DMFC MEAs operating at 80 oC Performance of PEMFC MEAs operating at 80 oC

Pt alloys offer a performance gain of 25 mV compared to Pt/C

Development of mixed potential (in DMFC)

Expensive

Difficulties

PtCr/CPtNi/CPtCo/CPtFe/CPt/C

T. R. Ralph and M. P. Hogarth, Platinum Met. Rev., 46 (2002) 3

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Non-noble metal based electrocatalysts

Transition metal oxides

Transition metal carbides

Transition metal chalcogenides

Transition metal macrocycles

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PerovskitesLn 1-x SrxCoO3, Ln 1-x SrxMnO3 (Ln = La, Nd; x = 0 to 0.9), LaNiO3, SrRuO3

SpinelsCo3O4, Mn3O4, Ni2CoO4, MnCo2O4, CdCo2O4

Pyrochlores

Pb2Ru2O7, Pb2Ru 1.95 Pb 0.05 O7-, Pb2Ru 1.95 Pb 0.05 O7-/CBi2Ru2O7, Pb2Ir2O7

Transition metal oxides

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Why Pb-Ru pyrochlores are preferred ?

stable in acid medium

activity comparable to platinum

Active site

alkaline medium – O' (bonded only to Pb cations)

acid medium – O (bonded to Pb and Ru cations)

Pb2Ru2O7 [Pb2O'. Ru2O6]

Pb

Ru

O

O'

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Mechanism for oxygen reduction reaction

Difficulty

- lower stability under fuel cell conditions

Pyrochlores (in acidic medium)

Ru3+OH- + O2- Ru3+O2- + OH-

Ru3+O2- + H2O + e- Ru3+OOH- + OH-

Ru3+OOH- + H2O Ru3+OH- + H2O2

Ru3+OOH- + H2O Ru3+OH- + 2 OH-

rds

J. B. Goodenough, R. Manoharan and M. Paranthaman, J. Am. Chem. Soc., 112 (1990) 2076

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Transition metal carbides

WC, TaC, TiC, B4C

Pt like behavior for the chemisorption of oxygen

Difficulties

- Synthesis is expensive

- Low corrosion resistance under acidic conditions

Lower activity compared to Pt

E, m

V v

s. N

HE

I (mA/cm2)

F. Mazza and S. Trassatti, J. Electrochem. Soc., 110 (1963) 847

Cathodic polarization curves for O2 reduction on various carbides

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Transition metal chalcogenides

Chevrel phase compounds - general formula, M6X8

, Ru MoxRuySz, RhxRuySz, RexRuySz, MoxRuySez

MoxRhySz, MoxOsySz, WxRuySz

RuxSy, RuxSey, RuxTey

Carbon supported catalystsCharacteristic features

Metal cluster - reservoir of electronic charge carriers

Capacity to provide neighbouring binding sites for reactants and intermediates

Volume and bond distances are flexible

High conductivity

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Mechanism for oxygen reduction reaction

Schematic representation of molecular oxygen reduction on the RuxXy catalysts

Cleavage of O-O bond occurs due to the large interatomic distance (2.7 Å) and leads to the formation of H2O

e

N. Alonso Vante, W. Jaegerman, H. Tributsch, W. Honle and K. Yvon, J. Am. Chem. Soc., 109 (1987) 3251

Crystal structure of RuxXy catalysts

O2 + 4 H++ 4 e- 2 H2O

• Ru

O X = S, Se, Te

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EXAFS results for the Ru K-edge spectrum of samples in oxygen atm. under potential variation

Sample Element Parameter Electrode potential (V) Vs. NHE

Forward scan Backward scan 0.08 0.33 0.53 0.78 0.53 0.33 0.08

RuxSey

O

Ru

R (Å)

CN

R (Å)

CN

R (Å)

CN

2.13 2.13 2.09 2.01 2.12 2.12 2.17

0.9 0.7 0.5 0.6 0.3 0.3 0.4

2.37 2.37 2.37 2.39 2.35 2.34 2.33

0.8 0.8 0.8 0.9 0.6 0.3 0.2

2.65 2.65 2.64 2.66 2.64 2.64 2.64

5.9 5.5 5.4 4.8 6.0 6.1 6.4

RuxTez

O

Ru

R (Å)

CN

R (Å)

CN

R (Å)

CN

2.05 2.04 2.04 2.07 2.01 2.02 2.04

1.5 1.5 1.9 2.8 2.7 1.9 0.5

-- -- -- -- -- -- --

0.1 0.2 0.3 0.4 0.4 0.4 0.5

2.63 2.63 2.65 2.68 2.65 2.65 2.64

3.1 3.3 3.2 1.7 2.4 2.9 3.8

RuxSy

O

Ru

R (Å)

CN

R (Å)

CN

R (Å)

CN

2.18 2.18 2.18 2.18 2.19 2.18 2.18

1.9 1.6 1.9 2.3 1.9 2.0 1.8

2.38 2.39 2.39 2.39 2.39 2.37 2.38

2.4 2.4 2.3 2.3 2.2 2.3 2.4

2.72 2.73 2.73 2.74 2.72 2.71 2.71

0.7 0.6 0.6 0.6 0.6 0.7 0.7

Se

Te

S

Effect of Chalcogens on the activity of Ru clusters to catalyze ORR

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Influence of selenium

Tafel plots for the ORR, as obtained from RDE experiments in O2 saturated 0.5 M H2SO4

A: 14.3 mol% SeB: 5.27 mol% SeC: 0 mol% SeD: metallic Ru

A: 0 Mol% SeB: 10.01 Mol% SeC: 14.3 Mol% Se

XRD-spectra of catalysts prepared with different amounts of selenium

High current densities

Inhibition of formation of Ru oxides

Lower amount of H2O2 production (< 3 vol%)

Enhanced stability towards electrochemical oxidation

Ru

RuOx

Mol% Se

Tafel slope

/mV dec-1

Overpotential // mV at 10 A cm-2

14.3

10.0

5.3

1.8

0

96.6

101.5

120.0

128.4

146.2

330.0

322.5

317.5

327.0

342.5

Tafel slopes and over potentials for Ru-based cluster catalysts with different Se contents

M. Bron, P. Bogdanoff, S. Fiechter, I. Dorbandt, M. Hilgendorff, H. Schulenburg and H. Tributch, J. Electroanal. Chem., 500 (2001) 510

Potential dependent hydrogen peroxide production of Ru based cluster catalysts with different selenium content

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Transition metal macrocycles

Square planar complexes with the central metal atom symmetrically surrounded by four nitrogen atoms

Structural analogues of metabolic systems

Delocalization of ‘’ electrons – high conductivity

Stability in both acidic and basic media

-linked face-to-face metal porphyrin

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catalyst

Mass activity at

0.7 V vs. NHE (mA/mg)

FeTPP

CoTPP

FePc

CoPc

RuPc

RuTPP

MnTPP

OsTPP

CrTPP

CoTAA

0.06

0.08

0.07

0.05

0.04

0.02

0.01

0.007

0.007

0.005

Oxygen reduction activities of various catalysts

Why Fe- and Co- containing macrocycles appear to be the best for oxygen reduction ?

Redox potential (V vs. SCE)

OR

R a

ctiv

ity

(V v

s. S

CE

)

FeTPP.. CoTPP

CoOEP.

Volcano plot

Jose H. Zagal, Coord. Chem. Rev., 119 (1992) 89

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Mechanism of the disintegration of metal macrocycle

Adverse effect of H2O2 on catalytic activity

x

x

x

x

+ H2O2, + O2

-x - M

M M M

K. Weisener, Electrochimica Acta, 31 (1986) 1073

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catalyst

Metal loading (wt%)

ORR activity at 0.7 V vs. NHE

FeTPP/Vulcan XC72R heat treated at 600oC

CoTPP/Vulcan XC72R heat treated at 600oC

FePc/Vulcan XC72R heat treated at 500oC

CoPc/Vulcan XC72R heat treated at 600oC

FeTMPP-Cl/BP heat treated at 800oC

FeTPP/CoTPP heat treated at 600oC

2.0

2.0

2.0

1.9

2.0

2.0

3.9 102 (0.06)#

3.1 98 (0.08)

4.0 78 (0.07)

3.1 58 (0.05)

5.1 127 (0.11)

3.0 69 (----)

Remarkable oxygen reduction activities of pyrolyzed Fe- and Co- based catalysts

* The catalytic activity was determined by taking the difference between the current measured at 0.7 V vs. NHE when the electrode is rotating at 1500 rpm and when it is stationary.

(mA/cm2) (mA/mg)

How to increase the oxygen reduction activity ?

Pyrolysis of the carbon supported metal macrocycles

# The values shown in bracket are the activities of non-heat treated catalysts

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Visualization of the reaction of the porphyrin with the carbon during heat treatment

Effect of heat-treatment

Active species for oxygen reduction --- MN4Cx (M = Fe, Co)

Improving the dispersion of supported macrocycle

Formation of a highly active carbon, which modify the electronic structure of the central metal

Retention of metal-N4 coordinate

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Methods of preparation

Heat-treatment of metal porphyrins and phthalocyanines adsorbed on carbon supports (scheme – 1)

Pretreatment of carbon with nitrogen containing media and exploiting these materials as supports for metal salts followed by heat treatment (scheme – 2)

Heat-treatment of metal included nitrogen containing polymers, which was adsorbed on carbon (scheme – 3)

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Carbon support

refluxing under Ar

filtration and wash with H2O

drying at 75 C

complex/carbon

Metal complex Solvent

heat-treatment under Ar

MN4Cx

+

Metal porphyrin

Scheme - 1

(i) General procedure for the preparation of metal porphyrins phthalocyanines

(ii) Adsorption of metal complex on carbon and thermal treatment

    G. Faubert, G. Lalande, R. Cote, D. Guay, J. P. Dodelet, L.T. Weng, P. Bertrand and G. Denes, Electrochimica Acta 41 (1996) 1689

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(i) Modification of carbon support

Carbon black

refluxing

filtration and wash with H2O

drying at 75 oC

HNO3 treated carbon

+ X wt% HNO3

HNO3 treatment

NH3 treatment

Carbon black NH3 NH3 treated carbon

Scheme - 2

(ii) Addition of ‘M’ ions

Modified carbon + Metal salt solution

ultrasonication for 1 hr

drying at 75 oC

M-based catalyst

heat-treatment under Ar

MN4Cx

H. Wang, R. Cote, G. Faubert, D. Guay and J. P. Dodelet, J. Phys. Chem. B 103 (1999) 2042

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Scheme - 3

Solution of polymer and metal salt e.g., polyacrylonitrile and cobalt acetate in DMF

carbon support

evaporation under Ar to remove solvent

solid

heat-treatment under Ar

MN4Cx

    S. Lj. Gojkovic, S. Gupta and R. F. Savinell, J. Electrochem. Soc. 145 (1998) 3493

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Evidence for the formation of CoN4

Co K edge (A) XANES and (B) EXAFS spectra of (a) cobalt phthalocyanine (PcCo), (b-e) PcCo on Vulcan XC-72 [(b) untreated sample; (c-e) sample heated to (c) 700 °C, (d) 800 °C, and (e) 1000 °C], and (f) Co metal

A B Co-Co

Co-N

CoN4

M. C. Martins Alves, J. P. Dodelet, D. Guay, M. T. Ladouceur and G. Tourillon, J. Phy. Chem. 96 (1992) 10898

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Evidence for the formation of FeN4

Fe K-edge XANES spectra of neat FePc (A), neat (FePc)2O (B), nonheat-treated (FePc)2O/KB (C), and heat-treated samples at 500 (D), 600 (E), 700 (F), 800 (G), 900 (H), and 1000 °C (I)Curve J is of Fe2O3 for comparison purpose

FeN4

FT EXAFS spectra of heat-treated(FePc)2O/KB at 600 °C

Fe-N

Fe-O

H. J. Choi, G. Kwag and S. Kim, J. Electroanal. Chem., 535 (2002) 113

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Can the pyrolyzed macrocycles be a viable option for theoxygen reduction in PEMFC & DMFC ?

Comparable activity with platinum

Structural stability during oxygen adsorption and reduction

H2O2 decomposition

Methanol insensitivity

Low cost

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Polarization curves obtained at 80 °C in H2/O2 fuel cell

Oxygen reduction activity

Relative intensities of various FeNxCy+ ions as a function of the pyrolysis temperature for FeTMPP/C

ToF-SIMS measurements

Even with 40% active sites (FeN4Cx), this heat-treated catalyst exhibited comparable activity with commercial Pt catalyst

Is there any scope to increase the activity of Macrocyclic complexes ?

FeN4Cx+

FeN2Cx+

FeN3Cx+

FeN1Cx+

M. Lefevre, P. Bertrand and J. P. Dodelet, J. Phy. Chem. B 104 (2000) 11238

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Precursor Carbon support

Heat-treatment (oC)

n % H2O2

Fe acetate

FeTMPP

FeTMPyP

FeTPPS

FeNPc

FeTMPP-Cl

Fe(phen)3

FeTPP/CoTPP

CoTMPP

Pyrrole black

R B carbon

Vulcan XC72R

Vulcan XC72R

Printex XE2

Black Pearls

Vulcan XC72R

No support

Vulcan XC72R

800

800

800

800

500

200-1000

800

600

800

3.90

3.96

2.7

2.7

3.5

3.45 - 4.0

3.7

4.0

4.0

5

2

15

15

25

28 – 0

15

0

0

Number of electrons transferred (n) and vol% H2O2 released in ORRat the maximum activity for Fe-based catalysts

20% Pt/C (commercial catalyst) 3.9 < 5

M. Lefevre and J. P. Dodelet, Electrochimica Acta, 48 (2003) 2749

44

Cu

rren

t (A

mp

)

Potential (V vs. NHE)

Comparison between Pt-based catalysts, RuxSey and Fe- based catalysts

Cyclic voltammograms in O2 -saturated aq. H2SO4

A. K. Shukla and R. K. Raman, Annu. Rev. Mater. Res., 33 (2003) 155

RuxSey/C

Fe-TMPP/C at 800 oC

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Conclusions

Though oxygen reduction reaction is quite often exploited in many fold, the mechanism remained less understood

There are several intricacies involved in bringing out the processes with the very many existing electrocatalysts

There is an abrupt need for the transition to non-noble metal electrodes

Pyrolyzed macrocyclic systems (N4 – metal chelates) appears to be a viable option as cathode electrocatalyst materials for oxygen reduction

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