Sep. 20, 2006 Ph.D., Seminar - II T. Maiyalagan T. Maiyalagan CYD01006 CYD01006 Electro-catalytic supports for Electro-catalytic supports for noble metals for exploitation as noble metals for exploitation as electrodes for fuel cells electrodes for fuel cells
78
Embed
Sep. 20, 2006 Ph.D., Seminar - II T. Maiyalagan CYD01006 Electro-catalytic supports for noble metals for exploitation as electrodes for fuel cells.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Sep. 20, 2006
Ph.D., Seminar - II
T. MaiyalaganT. MaiyalaganCYD01006CYD01006
Electro-catalytic supports for noble metals Electro-catalytic supports for noble metals for exploitation as electrodes for fuel cellsfor exploitation as electrodes for fuel cells
Role of nitrogen on the carbon Role of nitrogen on the carbon nanotube nanotube supported anodes for methanol oxidationsupported anodes for methanol oxidation
Electrode Methanol oxidation
potential at +50 mA/cm2 (V)
Untreated (U) 604
Nitrogen functionalized (N)
554
Sulfur functionalized (S) 633
S.C. Roy et al., J. Electrochem. Soc., 144 (1997) 2323
Nitrogen functionalization in carbon Nitrogen functionalization in carbon supportsupport
Current – potential curve for sulfur functionalized (S), nitrogen functionalized (N) and un-functionalized (U) carbon supports
Pt bound strongly to nitrogen sites so higher dispersion, avoids sintering
Addition of nitrogen increases the conductivity of the material by raising the Fermi level towards the conduction band
The presence of nitrogen generates catalytic site and this catalytic site is responsible for increased activity of methanol oxidation
Cyclic voltammograms in 0.5 M H2SO4 and 1 M CH3OH Scan rate : 50mV/s
Electrocatalytic activity of the catalystElectrocatalytic activity of the catalyst
Electrocatalyst
Anodic scan peak
potential (V) vs Ag/AgCl
Anodic peak current density
(mA cm-2)
Bulk Pt Bulk Pt 0.16
20 % Pt/C 0.762 1.3
Pt/TiO2
nanotube0.680 13.2
Model presentation of M-OH transfer and spillover upon metallic part of electrocatalyst.
Strong metal support interaction (SMSI) OH adsorption on TiO2 facilitates CO oxidation
Pt-CO + OH(ads) → Pt∙∙∙CO2 + H+ + e-
OH(ads) forms on TiO2 surface and oxidizes Pt-CO
T. Maiyalagan, B. Viswanathan and U. V. Varadaruju J. Nanosci. Nanotech 6 (2006) 2067
Promoting effect of TiOPromoting effect of TiO22 nanotubenanotube support support
TiO2 nanotubes have been explored as support for Pt
High platinum dispersion was obtained on these TiO2 nanotube supports by the reduction of the Pt ions with H2 at 873 K
The resulting electrodes were tested for methanol oxidation reaction and shows higher catalytic activity than the commercial catalyst
Strong metal support interaction (SMSI)
COads on the Pt sites reacts with surface hydroxyl species (OHads), at the adjacent TiO2 sites facilitates CO oxidation
TiO2 nanotube morphology was also assumed to be responsible for the higher specific activity of methanol oxidation
Salient FeaturesSalient Features
M.S. Antelman, Encyclopedia of chemical electrode potentials, Plenum, New York, 1982
Metal oxides - promising CO toleranceMetal oxides - promising CO tolerance
Pt + H2O PtO + 2 H+ + 2 e- Eo = 0.99V
Mo + 3H2O MoO3 +6 H+ + 6 e- Eo = 0.25V
W + 2H2O WO2 + 4H+ + 4 e- Eo = -0.05V
W + 3H2O WO3 + 6H+ + 6 e- Eo = 0.09V
Lower values of Eo for the Mo/MoOx, and W/WOx than that for Pt /PtO enhance the ability of OH adsorption Metal oxides forms hydrogen bronzes provides H2 spill over and oxygen spill over mechanism Ability to act as redox centers Ionic and Electronic conductivity
Metal oxides supports to Pt must fulfill the requirement of forming O-containing surface species at
low potentials
Electro-oxidation of methanol on Electro-oxidation of methanol on PtPt/WO/WO33 nanorods catalysts nanorods catalysts
P. K. Shen et al., Catalysis Today 38 (1997) 439
Electro-oxidation of methanol on Pt/WOElectro-oxidation of methanol on Pt/WO33 catalysts catalysts
10 gm Phosphotungstic acid + 30 ml methanol
WO3/Alumina membrane
WO3 nanorods(light blue powder)
H3PW12O40/Alumina membrane
Calcined at 873 K 3h
HF
ImpregnationImpregnation
Template Synthesis of WOTemplate Synthesis of WO33 nanorods nanorods
73 mM H2PtCl6
Evaporated to drynessH2 atm, 623 K, 4h
Pt/WO3 nanorods (dark powder)
500 1000 1500 2000 2500 3000
WO3 nanorods
Bulk WO3
HPW calcined
% T
ran
sm
itta
nce
Wave number (cm-1)
990 cm-1 is for as(W-Od)
1080 cm-1 is for as (X-Oa)
894 cm-1 is for as(W-Ob-W)
817 cm-1 is for as(W-Oc-W)
The strong absorption between 500 and 1000 cmThe strong absorption between 500 and 1000 cm-1-1 can be associated with the W-O-W can be associated with the W-O-W
stretching modes ( characteristic peak of tungsten oxide )stretching modes ( characteristic peak of tungsten oxide )
FTIR spectraFTIR spectra
UV spectrum
600 700 800 900
717.632
808.284 WO3 nanorods
In
tens
ity (a
.u)
Raman shift (cm-1)
717.6 and 808 cm717.6 and 808 cm-1-1 : O-W-O stretching modes : O-W-O stretching modes
Raman spectrum
UV and Raman spectrumUV and Raman spectrum
10 20 30 40 50 60 70 80
(001
)
(221
)
(220
)
(120
)
(011
)
Inte
nsi
ty (
a.u
.)
2 (degrees)
H3PW
12O
40
WO3 nanorods
XRD pattern of tungsten oxide nanorods
X- Ray diffraction patternX- Ray diffraction pattern
Mag:3000K
SEM/EDX imagesSEM/EDX images
0 500 1000 1500
0
200
400
600
800
1000
1200
W
WCu
CuPt
Pt
Cu
(b)
O
W
W
Co
un
ts
Energy (KeV)
200 nm 100 nm
TEM/EDX imagesTEM/EDX images
xH+ + xe- + WO3 ---------------> HxWO3
Bulk WO3 WO3 nanorods
(formation of hydrogen tungsten bronze)(formation of hydrogen tungsten bronze)
Electrochemical studiesElectrochemical studies
overlap between reoxidation of hydrogen tungstenoverlap between reoxidation of hydrogen tungsten bronze and desorption of hydrogen on Pt, difficult tobronze and desorption of hydrogen on Pt, difficult to estimate the active surface area of Ptestimate the active surface area of Pt
WO3 + xPt-H ---------- >HxWO3 + xPt-
H HxWO3 --------------->xH+ + xe- +
WO3
Pt + H+ + e- ---------- > xPt
Hydrogen spill-over effect on Pt/WOHydrogen spill-over effect on Pt/WO33 nanorods nanorods
Electrocatalytic activity of the catalystElectrocatalytic activity of the catalyst
Catalyst Pt loading g/cm2 Specific activity mA/cm2
Pt/Vulcan 20 23
Pt/WO3 nanorods
20 60
Metal oxide nanorods as supports for Pt electrodes in the electrooxidation of methanol in acid electrolyte has been investigated
WO3 was demonstrated to promote electrocatalytic oxidation of methanol by interacting with Pt via hydrogen spillover or through the formation of highly conductive tungsten bronzes
Improvement of performance may be due to the presence of OHads groups on the oxide surface, that should facilitate oxidation of poisoning CO intermediates
The promotional activity of WO3 nanorod is related to the W(VI)/W(IV) redox couple acting as a surface mediator for the oxidation of surface methanolic residues
The nanorods morphology of the base-metal oxide is promoting the activity for platinum
Poly (o-phenylenediamine) - high aromaticity and high thermal stability
- catalyst for electrochemical reduction of dioxygen
- sensor for many chemical species, ladder polymer
NH
HN
NH
HN *
*
n
Polymers matrices usedPolymers matrices used
Alumina membrane
Electrochemical
polymerization
Conducting composite
Matrix = Alumina membrane
Graphite electrode(Nafion coating)
Dissolution
Polymernanostructure
Nanotubes
0.6 µm-thickness 200 nm pore diameter
Template assisted electrochemical synthesis of Template assisted electrochemical synthesis of conducting polymer nanotubesconducting polymer nanotubes
Schematic view of an electrochemical cell for the formation of nanostructured materials. RE, reference electrode; AE, auxiliary electrode; WE, working electrode (template membrane with a deposited Nafion contact layer).
WE
RE
AE
Experimental setupExperimental setup
Graphite 1 cm2
Coating of 5 wt% Nafion,Al2O3 Membrane hot pressed on
Graphite at 393 K, 3 min.
Graphite /Naf/Al2O3
GR/Naf/Al2O3/PoPD
GR/Naf/PoPDtemp-Pt
Dissolution of Al2O3 in 0.2M NaOH, Followed
by immersion in 1% HBF4 (10 min)
0.5 M OPD / 0.5 M H2SO4
Scanned between -0.2 V to 1.2 V vs Ag/AgCl at 50 mV/s
GR/Naf/Al2O3-PoPD-Pt
Electro reduction in 0.01 M H2PtCl60.8 V to -0.2 V
Template assisted electrochemical synthesis of Template assisted electrochemical synthesis of Pt/PPt/PooPD nanotubulesPD nanotubules
Cyclic voltammograms during the electropolymerization of o-phenylenediamine in 0.5M oPD + 0.5M H2SO4 solution (v = 50mVs−1)
500 1000 1500 2000 2500 3000 3500
PoPD nanotubule
3112.982158.3
2493.9
1469.9
Inte
nsity
(a.u
)
Raman shift (cm-1)These bands are assigned to stretching of Aminobenzene units of azo groups
N-H stretching C=Stretching
812 cm−1 1,4-disubstitude ring
FT-IR and Raman spectrumFT-IR and Raman spectrum
200 400 600 800
transition
364.3
Wavelength (nm)
A
bso
rban
ce
PoPD nanotubule
UV spectrumUV spectrum
Graphite/PoPD nanotube AFMAFM
TEM image of Pt/PoPDGraphite/Pt-PoPD nanotube
Electron microscopic images of Pt/PoPD nanotubulesElectron microscopic images of Pt/PoPD nanotubules
Cyclic Voltammograms of GR/Naf/PoPD Temp in 0.5 M H2SO4 ( after the dissolution of the template)
-0.2 0.0 0.2 0.4 0.6 0.8 1.0
-80
-60
-40
-20
0
20
40
60
80
100
Cu
rren
t d
en
sit
y (
mA
/cm
2 )
(b) Nanotubule electrode (with methanol)
E/V vs Ag/Agcl
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2-15
-10
-5
0
5
10
15
20
25
(c) Film electrode (with methanol)
Cu
rren
t d
en
sit
y (
mA
/cm2 )
E/V vs Ag/Agcl
-0.2 0.0 0.2 0.4 0.6 0.8 1.0-12
-10
-8
-6
-4
-2
0
2
4
6
8
10
C
urr
en
t d
en
sit
y (
mA
/cm2 )
(a) Nanotubule electrode (without methanol)
E/V vs Ag/Agcl
Electrocatalytic activity of the catalystElectrocatalytic activity of the catalyst
100 200 300 400 500 600
10
20
30
40
50
60
70
80
90
Film electrode
Tubule electrode
An
od
ic p
eak c
urr
en
t d
en
sit
y (m
A/c
m2 )
Pt Loading ( µg/cm2)
Plot of anodic peak current density as a function of Platinum loading on Nanotubule and conventional electrodes. (Current densities were evaluated from CV run in 0.5 M H2SO4 / 1M CH3OH at 50 mV/s )
S.No
Electrode Onset
Potential
(V)
Activity*
Forward sweep Reverse sweep
I( mA cm-
2)
E( mA cm-
2)
I( mA cm-
2)
E( mA cm-
2)
1
2
GR/Naf/PoPD Temp –Pt
GR/Naf/PoPD Conv –Pt
+0.1
+0.2
84
13.3a
0.82
0.70
--
6. a
---
0.45
* Activity evaluated from cyclic voltammogram run in 0.5 M H2SO4 / 1M CH3OH scanned between -0.2 and 1 V vs Ag/AgCl a peak current density (mA cm-2)
Electrocatalytic activity of the catalystElectrocatalytic activity of the catalyst
0
50
100
150
200
250
300
350
0 20 40 60 80 100 120
Time (min)
Cu
rre
nt
de
nsi
ty (
mA
/cm
2) GR/Naf/PoPD Temp –Pt
GR/Naf/PoPD Conv –Pt
GC/ 20 % Pt/C (E-TEK)
Stability of the electrodesStability of the electrodes
Template synthesis of conducting poly (o-phenylenediamine) yielded cylindrical nanotubules with outer diameter matches with pore diameter of the template used
Polymer nanotubules have adequate conductivity and allows both the incorporation of metal particles and an easy accessibility of the methanol to the catalytic sites.
Polymer nanotubules electrode have higher electrocatalytic activity than the conventional polymer film electrode.
Salient FeaturesSalient Features
Nanocomposites Organic-inorganic hybrid materials: “Compounds which contain both inorganic and organic moieties within the same micro structure”
Organic moiety - flexibility
Inorganic moiety - thermal stability and structural rigidity
Active moiety- catalysisNanocomposites with catalytic activity
• PPY-SiO2 and PANI-SiO2 hybrid nanocomposites offer large surface area - Exploited as host for entrapping large amount of metal particles with desirable catalytic activity
Electro-oxidation of methanol on Electro-oxidation of methanol on Pt supported Pt supported PEDOT/ VPEDOT/ V22OO55 nanocomposites nanocomposites electrodeselectrodes
Poly (3,4, ethylenedioxythiophene) (PEDOT)• Has wide potential range • Low band gap• Electroactive • Enhanced stability compared to polyaniline & polypyrrole• Excellent environmental stability • High electrical conductivity • Transparency in thin oxidized film • and is always in the p-doped state, i.e. conductive • Applications
– solid electrolytic capacitors,– antielectrostatic agents,– transparent electrodes in light emitting diodes, – and underlayers for the metallization of printed circuit boards
Vanadium(V) oxide xerogel (VXG)
Exhibiting high proton conductivity, as well as electronic conductivity arising from the presence of V(IV) ions
B. FolKesson et al., J. Electroanal. Chem., 267 (1989) 149
Comparison of the potentials at which methanol is usually oxidized electrochemically and the normal redox couples. The normal potentials of the probable intermediates in the methanol oxidation are also included
This PEDOT-V2O5 based organic inorganic nanocomposites could be a good Catalyst support for methanol oxidation
PEDOT is meant to substitute the carbon usually mixed with the inorganic oxide-based electrodes to improve their electronic conductivity; the PEDOT thus functions as electronic conductor
+13.4 A0
V2O5.nH2O gels
100 ml aqueous solution of hydrogen peroxide (10%)
0.75ml EDOT
Ammonium persulphate
PEDOT/V2O5 nanocomposite
Dried at 60 0 C
V2O5 Powder(1 g, or 5.5 10-3 mol)
Synthesis of poly(3,4-ethylenedioxythiophene)/ Synthesis of poly(3,4-ethylenedioxythiophene)/ VV22OO5 5 nanocomposites nanocomposites
PEDOT/V2O5 nanocomposite
20% Pt-PEDOT/V2O5nanocomposite
5 %H2Pt Cl6 (35ml) Stirring at 800C for 30 min to allow dispersion
CooledFiltered and washed with distilled waterDried in air oven at 125 C for 4h
0.1 N NaOH added to bring the pH 10-11
20% Formaldehyde
Stirred at 70C for 1 hour
Preparation of Pt /PEDOT/ VPreparation of Pt /PEDOT/ V22OO55 nanocomposite nanocomposite
FT-IR spectra of (a) V2O5 (b) V2O5 Xerogel and (C) PEDOT/V2 O5 nanocomposites
2000 1800 1600 1400 1200 1000 800 600 400
(V=O)
(V=O)
asym
(V-O-V)
asym
(V-O-V)(c)
(b)
(a)
(V=O)
asym
(V-O-V)
(a) V2O
5
(b) V2O
5 Xerogel
(c) PEDOT-V2O
5
Wavenumbers (cm-1)
% T
rans
mitt
ance
OH
After intercalation
Extra peaks --C-O-C stretching vibrations
FTIR spectraFTIR spectra
X-Ray diffraction patterns of (a) V2O5 and (b) PEDOT-V2O5 nanocomposite
5 10 15 20 25 30 35 40
d=1
3.4
d=3
.44
d=7
.01
(110
)
(002
)
(001
)
(310
)(0
11)(4
00)
(110
)
(101
)(0
01)
(200
)
(b)
(a)
(a) V2O
5
(b) PEDOT-V2O
5
Inte
nsi
ty
2 theta
After intercalationIncorporation of PEDOT in V2O5 increases the interlayer spacing to 13.4 A0
X- Ray diffraction patternX- Ray diffraction pattern
Thermogravimetric curves of (a) V2O5 (b) V2O5 xerogel and (c) PEDOT-V2O5 nanocomposite
first one step 8 % weight loss below 120 C----- due to water
second one step16.2 % weight loss up to 420 C ----combustion of organic polymer
mass gain 2.6 % up to 650 C ---- formation of orthorhombic V2O5
Thermal studiesThermal studies
SEM images of PEDOT/ VSEM images of PEDOT/ V22OO55 nanocomposite nanocomposite
Layered structure
SEM images of PEDOT/ VSEM images of PEDOT/ V22OO55 nanocomposite nanocomposite
SEM/EDX images of Pt-PEDOT/ VSEM/EDX images of Pt-PEDOT/ V22OO55 nanocomposite nanocomposite
EDX mapping of Pt on nanocomposite
Average particle size of 4.52 nm
XRD and TEM images of Pt-PEDOT/ VXRD and TEM images of Pt-PEDOT/ V22OO55
nanocompositenanocomposite
-0.2 0.0 0.2 0.4 0.6 0.8 1.0-0.10
-0.08
-0.06
-0.04
-0.02
0.00
0.02
0.04
VO2+/V3+
Cu
rrem
t (m
A)
Potential (mV)
PEDOT-V2O
5
V5+/VO2+
V5+
Cyclic voltammograms in 0.5 M H2SO4 Scan rate : 50mV/s
Electrochemical studiesElectrochemical studies
-200 0 200 400 600 800 1000 1200
-20
-10
0
10
20
30
20% Pt/C
20% Pt-PEDOT/V20
5
Potential (mV)
Cu
rren
t m
A/c
m2
Cyclic voltammograms in 0.5 M H2SO4 and 1 M CH3OH Scan rate : 50mV/s
Electrocatalytic activity of the catalystElectrocatalytic activity of the catalyst
Catalyst
Pt loadin
g g/cm
2
Methanol
oxidation
activity mA/cm2
Pt/(C6H4O2
S)0.4V2O5.0.5H2O10 28.4
Pt/Vulcan 10 15
Nearly two times higher activity for the same Pt loading
Methanol oxidation activity of Pt/ PEDOT-V2O5 catalyst prepared by formaldehyde reduction method
Electrocatalytic activity of the catalyst Electrocatalytic activity of the catalyst Stability of Stability of the electrodesthe electrodes
The Pt loaded on PEDOT-V2O5 nanocomposite has been evaluated as the electrode for methanol oxidation in acid medium
EDX mapping and TEM images shows fine dispersion of Pt particles in the nanocomposite leading to a better utilisation of the noble metal catalyst
Electrocatalytic activity of methanol oxidation reaction for Pt/PEDOT-V2O5 was higher than Pt/C
Salient FeaturesSalient Features
The salient points of the present investigation are:
The supports for noble metal electro-catalyst have a definite role in the development of electrodes for fuel cells
The functionalization of the support carbon materials is a mean to increase dispersion and also intrinsic activity of the noble metal sites
Oxide supports, though may lead to net loss of energy, can favour the removal of otherwise poisons for the noble metal electro-catalytic sites
Conducting polymers and their composites can be conceived as alternate supports for noble metal electrodes for effective dispersion and intrinsic activity