Embed Size (px)
Citation preview
Generation of HydrogenPeroxide In ORR Over
Low Loadings of Pt/C Catalysts
Raja SwaidanThe Cooper Union
Advisor: Dr. Branko N. Popov
Electrochemical Engineering
26 July 2007
Overview of Research
• Studied the selectivity of Pt catalysts supported on different types of porous carbons as a function of Pt loading on the Rotating Ring Disk Electrode (RRDE)
– Carbon supports included: Vulcan Carbon, Etched Vulcan Carbon, KetjenBlack, USC MFC, USC CC
•XRD, TEM for carbon surface and Pt dispersion analysis as well as Pt particle sizes
• B.E.T. Surface Area and distribution of mesoporous and microporousareas
• Comparative study of all Pt/C catalysts at one loading of Pt (µgPtcm-2) to
determine the effects of support properties like surface area and porosity on selectivity
• Studied the phenomena that lower loadings of Pt/C electrocatalysts demonstrate preference for 2e- reduction routes to H2O2
– Encounter a struggle between physical and electrochemical factors
– Fundamental question: Which plays the dominant role?
2
Oxygen Reduction Reaction (ORR)
Primary PEMFC Failure Mechanisms
Aggravated by H2O2
•Membrane thinning
• Reactant crossover
• Internal shorting
• Oxidation of H2 gas produces protons and electrons that transfer to cathode for ORR
O2 2H2O
2e- H2O2 2e- H2O
H2O2
Adsorb on catalyst
Flow into Electrolyte
Int.
• Cathode ORR
4H+ + 4e-
3
RRDE Technique
Pt Ring, 0.46V vs MMS
[Detection of H2O2intermediates that escape a series 2e- reduction to H2O]
Teflon
[Electrical separation between Disk and Ring allowing separate voltages on each]
Glassy Carbon Disk[ORR]
CATALYST
COAT
Electrolyte flow over the catalyst layer is increased with increasing RPM of electrode. Convection transports
H2O2 intermediates to the ring for detection and regulates catalyst’s O2 supply.
4
Low Pt/C Loading Phenomena
E-TEK 19.6% Pt/C Selectivity
900 RPM, 0.5M H2SO4, 25oC, 5 mV s
-1
0
2
4
6
8
10
12
14
16
0 0.2 0.4 0.6
Disk Potential [V]
Generated % H
2O
2
4 ugcm-2 8 ugcm-2 16 ugcm-2
20 ugcm-2 60ugcm-2
Selectivity, Vulcan Carbon
900 RPM
0
5
10
15
20
25
30
35
40
45
50
0 0.2 0.4 0.6
Disk Potential [V]Generated % H
2O
2
80 ug/cm^2
240 ug/cm^2
•Further testing reveals that E-TEK 60 µgPtcm-2 (240 µgVCcm
-2)loading yields the lowest peroxide.
The impression is that Pt is causing this phenomena. On the contrary, as shown here, it is the support which is the guiding factor.
5
Low Pt/C Loading Phenomena
• Further testing reveals that the Pt/KB 20 µgPtcm-2 (80 µgKBcm
-2)loading yields the lowest peroxide.
Selectivity, KB
900 RPM
0
5
10
15
20
25
30
35
40
45
50
0 0.1 0.2 0.3 0.4 0.5 0.6
Disk Potential [V]
Generated % H
2O
2
80 ugcm-2240 ugcm-2
20 wt.% Pt/KB Selectivity
900 RPM, 0.5 M H2SO4, 25oC, 5 mV s
-1
0
1
2
3
4
5
6
7
0 0.1 0.2 0.3 0.4 0.5 0.6Disk Potential [V]
Generated %H
2O2
4 ugcm-2 8 ugcm-2 12 ugcm-2 20 ugcm-2
Moreover, at any given loading, the catalyst with the support having larger B.E.T. surface area (KB) produces lower H2O2. This catalyst also reaches a
minimum in H2O2 production at a lower Pt loading.
6
Platinum UtilizationConcept of A Saturation Point
Electrochemically Active Pt Surface Area [ECSA]Note: Electrode Geometrical Surface Area is 0.5 cm2
0
10
20
30
40
50
60
70
0 20 40 60 80 100 120 140
Pt Loading onto Working Electrode [µgPtcm-2]
ECSA
[m2/g
Pt]
240 µgVCcm-2
80 µgKBcm-2
E-TEK 20 wt.% Pt 20 wt.% Pt/KB
Ignore absolute values of ECSA (different Pt deposition methods and support modifications)
7
A Model
= H2O2= H+ = O2 = H2O= C = Pt = e-
H+ from Anode
2 e-
released 2 more e-
released
O2 Gas Feed
Legend:
• O2 reduces to H2O2intermediate upon contact with active Pt/C particle
• H2O2 is likely to encounter nearby Pt/C particles in the agglomerates and reduce to H2O before being swept to the ring for oxidation and detection
Effect of Agglomeration of Pt/C Catalyst on Hydrogen Peroxide Formation
M. Inaba et al.
8
S.E.M. Images of Catalyst Layer
4 ugPtcm-2 20 ugPtcm
-2 60 ugPtcm-2 100 ugPtcm
-2
Agg. ~ 20 µm Agg. ~ 130 µm Agg. ~ 130 µmAgg. ~ 60 µm
E-TEK (Low B.E.T. Surface Area Support)
4 ugPtcm-2 12 ugPtcm
-2 20 ugPtcm-2 40 ugPtcm
-2
Agg. ~ 40 µm Agg. ~ 130 µm Agg. ~ 130 µmAgg. ~ 100 µm
Pt/KB (High B.E.T. Surface Area Support)
9
Carbon Support VS
Physical Concept
10
A link between the two is clear, but which is the dominant player?
The Carbon Support
Agglomerate Size, Peroxide, and Real Pt
Surface Area
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Disk Potential [V]
Generated % H
2O2
ETEK Pt/C, 20 Pt/KB, 20
ETEK Pt/C, 60
• Note: Electrode geometrical area is 0.5 cm2
• Note: Numbers in legend have units µgPtcm-2
Agglomerate Size, Peroxide, and Real Pt
Surface Area
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Disk Potential [V]
Generated % H
2O2
ETEK Pt/C, 20 Pt/KB, 20
ETEK Pt/C, 60
• Note: Electrode geometrical area is 0.5 cm2
• Note: Numbers in legend have units µgPtcm-2
60 µm
130 µm
130 µm
Agglomerate Size
The link pivots upon properties of the carbon support
11
Conclusions
ECSA Saturation Point
Pt/C Agglomerate
Size
H2O2
Formation
Low Loading Phenomena
• Carbon Support is steering, or causing, the physical phenomena
• Increasing particle agglomeration parallels decreasing H2O2 levels
• At the S.A. saturation point, particle agglomeration reaches maximum and H2O2 level reaches a minimum
The Carbon Support, Not Pt
• Pt—C Interactions
• Pt Particle Dispersion on Carbon
• B.E.T. Surface Area
•Mesoporous, Microporous Area Distributions
Future Work
• Compare selectivity of each of the aforementioned carbon supports at a given loading, holding Pt and carbon amounts on electrode constant.
•Highlight which support properties are impacting the selectivity of the catalyst
12
MaxMax
Min
Acknowledgments
Many thanks to
Dr. Gang WuDr. Prabhu GanesanDr. Branko N. Popov
Members of the Fuel Cell DivisionMy Family
for the intellectual enlightenment and encouragement that allowed me to make the best of my first research experience.
The NSF-REU fund is greatly appreciated.
13
