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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
6
Efficiency
Cleanliness
Unique operating characteristics
Planning flexibility
Reliability
Future development potential
Fuel Cells – AdvantagesFuel Cells – Advantages
7
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
8
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 -
9
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
11
Difficulties in PEMFC & DMFCDifficulties in PEMFC & DMFC
Sluggish oxygen reduction kinetics
Methanol crossover (in DMFC)
Electrocatalysts
12
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
13
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
14
Noble metal based electrocatalysts
Pt
Pt alloys -- PtFe, PtCo, PtNi, PtCr
Carbon supported Pt and its alloys
15
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
16
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
17
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
18
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
19
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
20
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
21
Non-noble metal based electrocatalysts
Transition metal oxides
Transition metal carbides
Transition metal chalcogenides
Transition metal macrocycles
22
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
23
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'
24
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
25
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
26
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
27
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
28
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
29
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
30
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
31
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
32
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
33
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
34
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
35
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)
36
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
37
(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
38
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
39
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
40
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
41
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
42
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
43
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
45
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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