Ph. D., Seminar - 1

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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 (N4metal chelates) as
  viable option
• ? Conclusions

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Importance of electrochemical reduction of oxygen
? Fuel Cells ? Metal-Air batteries ?
Industrial electrolytic processes
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Fuel Cells
Direct Energy Conversion Vs. Indirect Technology
ICE
Thermal Energy
Mechanical Energy
Chemical Energy
Electrical Energy

Fuel Cell
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Batteries/Internal Combustion Engines/Fuel Cells

• Batteries
• - Needs recharging
• - Toxic chemicals
• - Low energy density

Internal combustion engines - Carnot
limitations - Moving parts and hence
friction - Noisy
Energy Conversion (devise/fuel) Efficiency ()
Fuel Cells (H2/PEMFC) 50-60
Internal Combustion Engine / Gasoline C2H5OH 20-25
Diesel Engine / Diesel 25-30
C. 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 Advantages
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Different Types of Fuel Cells
Fuel Cells
Low Temperature Fuel Cells Medium Temperature Fuel Cells High Temperature 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 COgt10 ppm Adsorbed intermediates (CO) CO, CO2 COgt1 H2Sgt50 ppm H2Sgt1ppm H2Sgt0.5 ppm
Applications Transportation, Portable Space, Military Power generation, Cogeneration
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Chemical 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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Low Temperature Fuel CellsPEMFC DMFC
Cathode
Anode
Fuel
O2 4 H 4 e- ? 2 H20
PEMFC
2H2 ? 4H 4e-
DMFC
Methanol from Tank
CH3OH H2O ? CO2 6 H 6 e-
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DMFC
PEMFC
CH3OH H2O ? CO2 6 H 6 e- Eo 0.02 V
H2 ? 2 H 2 e- Eo 0.0 V

(At Anode)
3/2 O2 6 H 6 e- ? 3 H2O Eo 1.23 V
½ O2 2 H 2 e- ? H2O Eo 1.23 V
(At Cathode)
H2 ½ O2 ? H2O Eo 1.23 V
CH3OH 3/2 O2 ? CO2 2 H2O Eo 1.21 V
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Activation losses
Ohmic losses
Concentration losses
Performance losses in PEMFC DMFC MEAs operating
at 80 oC
T. R. Ralph and M. P. Hogarth, Platinum Met.
Rev., 46 (2002) 146
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Difficulties 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 V
Halina S. Wroblowa, Yen-Chi-Pan and Gerardo
Razumney, J. Electroanal. Chem., 69 (1979) 195
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Essential criteria for choosing an
electrocatalyst for oxygen reduction

• 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

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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 ?

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

Kinetic current density (mA/cm2)
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
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 lt
PtCo lt PtFe
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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PtCr/C PtNi/C PtCo/C PtFe/C Pt/C
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
Difficulties
? Development of mixed potential (in DMFC) ?
Expensive
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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Transition metal oxides
Perovskites Ln 1-x SrxCoO3, Ln 1-x SrxMnO3 (Ln
La, Nd x 0 to 0.9), LaNiO3, SrRuO3
Spinels Co3O4, Mn3O4, Ni2CoO4, MnCo2O4,
CdCo2O4 Pyrochlores Pb2Ru2O7, Pb2Ru 1.95 Pb
0.05 O7-?, Pb2Ru 1.95 Pb 0.05 O7-?/C Bi2Ru2O7,
Pb2Ir2O7
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Why Pb-Ru pyrochlores are preferred ? ?
stable in acid medium ? activity comparable
to platinum
O'
Pb ?
Ru ?
O
Pb2Ru2O7 ? Pb2O'. Ru2O6
Active site ? alkaline medium O' (bonded only
to Pb cations) ? acid medium O (bonded to Pb
and Ru cations)
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Mechanism for oxygen reduction reaction
Pyrochlores (in acidic medium)
rds
Ru3OH- O2- ? Ru3O2- OH- Ru3O2- H2O
e- ? Ru3OOH- OH- Ru3OOH- H2O ? Ru3OH-
H2O2 Ru3OOH- H2O ? Ru3OH- 2 OH-
Difficulty - lower stability under
fuel cell conditions
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
? Lower activity compared to Pt
E, mV vs. NHE
Difficulties - Synthesis is
expensive - Low corrosion resistance
under acidic conditions
I (mA/cm2)
Cathodic polarization curves for O2 reduction on
various carbides
F. Mazza and S. Trassatti, J. Electrochem. Soc.,
110 (1963) 847
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Transition metal chalcogenides
? Chevrel phase compounds -
general formula, M6X8
MoxRuySz, RhxRuySz, RexRuySz, MoxRuySez MoxRhySz,
MoxOsySz, WxRuySz RuxSy, RuxSey, RuxTey Carbon
supported catalysts
, Ru
Characteristic 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

• Ru
• O X S, Se, Te

O2 4 H 4 e- ? 2 H2O
e
Crystal structure of RuxXy catalysts
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
N. Alonso Vante, W. Jaegerman, H. Tributsch, W.
Honle and K. Yvon, J. Am. Chem. Soc., 109 (1987)
3251
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Effect of Chalcogens on the activity of Ru
clusters to catalyze ORR
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
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Influence of selenium
? High current densities ? Inhibition of
formation of Ru oxides ? Lower amount of H2O2
production (lt 3 vol) ? Enhanced stability
towards electrochemical oxidation
Tafel plots for the ORR, as obtained from RDE
experiments in O2 saturated 0.5 M H2SO4
Tafel slopes and over potentials for Ru-based
cluster catalysts with different Se contents
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
Ru
A 0 Mol Se B 10.01 Mol Se C 14.3 Mol Se
RuOx
A 14.3 mol Se B 5.27 mol Se C 0 mol Se D
metallic Ru
XRD-spectra of catalysts prepared with different
amounts of selenium
Potential dependent hydrogen peroxide production
of Ru based cluster catalysts with different
selenium content
M. Bron, P. Bogdanoff, S. Fiechter, I. Dorbandt,
M. Hilgendorff, H. Schulenburg and H. Tributch,
J. Electroanal. Chem., 500 (2001) 510
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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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Why Fe- and Co- containing macrocycles appear to
be the best for oxygen reduction ?
Oxygen reduction activities of various catalysts
Volcano plot
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
.
.
.
CoOEP
CoTPP
FeTPP
ORR activity (V vs. SCE)
Redox potential (V vs. SCE)
Jose H. Zagal, Coord. Chem. Rev., 119 (1992) 89
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Adverse effect of H2O2 on catalytic activity
x
x
H2O2, O2
M
M
M
-x
- M
x
x
Mechanism of the disintegration of metal
macrocycle
K. Weisener, Electrochimica Acta, 31 (1986) 1073
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How to increase the oxygen reduction activity ?
? Pyrolysis of the carbon supported metal
macrocycles
Remarkable oxygen reduction activities of
pyrolyzed Fe- and Co- based catalysts
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 (----)
(mA/cm2)
(mA/mg)
The values shown in bracket are the activities
of non-heat treated 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.
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Effect of heat-treatment
? 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
Visualization of the reaction of the porphyrin
with the carbon during heat treatment
Active species for oxygen reduction --- MN4Cx (M
Fe, Co)
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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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Scheme - 1
(ii) Adsorption of metal complex on carbon and
thermal treatment
(i) General procedure for the preparation of
metal porphyrins phthalocyanines
Metal complex
Solvent

Carbon support
refluxing under Ar
Metal porphyrin
filtration and wash with H2O
drying at 75 ?C
complex/carbon
heat-treatment under Ar
MN4Cx
    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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Scheme - 2
(i) Modification of carbon support
(ii) Addition of M ions
HNO3 treatment
Modified carbon Metal salt solution

X wt HNO3
Carbon black
ultrasonication for 1 hr
refluxing
drying at 75 oC
filtration and wash with H2O
M-based catalyst
drying at 75 oC
heat-treatment under Ar
HNO3 treated carbon
MN4Cx
NH3 treatment
NH3
Carbon black
NH3 treated carbon
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
B
A
Co-Co
CoN4
Co-N
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
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
FeN4
Fe-N
Fe-O
FT EXAFS spectra of heat-treated (FePc)2O/KB at
600 C
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
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 the oxygen 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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Oxygen reduction activity
ToF-SIMS measurements
FeN4Cx FeN2Cx FeN3Cx FeN1Cx
Polarization curves obtained at 80 C in H2/O2
fuel cell
Relative intensities of various FeNxCy ions as a
function of the pyrolysis temperature for
FeTMPP/C
? 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 ?

M. Lefevre, P. Bertrand and J. P. Dodelet, J.
Phy. Chem. B 104 (2000) 11238
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Number of electrons transferred (n) and vol H2O2
released in ORR at the maximum activity for
Fe-based catalysts
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
20 Pt/C (commercial catalyst) 3.9 lt 5
M. Lefevre and J. P. Dodelet, Electrochimica
Acta, 48 (2003) 2749
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Comparison between Pt-based catalysts, RuxSey and
Fe- based catalysts
Current (Amp)
Fe-TMPP/C at 800 oC
RuxSey/C
Potential (V vs. NHE)
Cyclic voltammograms in O2 -saturated aq. H2SO4
A. K. Shukla and R. K. Raman, Annu. Rev. Mater.
Res., 33 (2003) 155
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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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Thank you