Synthesis and Evaluation of
Electrocatalysts for Fuel Cells
Jingguang Chen
Center for Catalytic Science and Technology (CCST)
Department of Chemical Engineering
University of Delaware
Newark, DE 19711
[email protected]
PEM Fuel Cells: H2/Methanol/Glycerol
Cathode: 6e- + 3/2 O2 + 3H2O Æ 6OH6OH- + 6H+ Æ 6H2O
O2 and H2O feed stream
Flow of
protons
Cathode
Electrolyte (Ionomer Membrane)
Anode
Electron
flow
via circuit
H2O and CH3OH feed stream
Anode: CH3OH + H2O Æ CO2 + 6H+ + 6e-
CHEG-Related Challenges:
Electroatalyst Issues: Cost of Pt or Pt/Ru; CO poisoning
Membrane Issues: Low operating temperature (< 100 oC); cost of Nafion
Transport Issues: Water management; three-phase region
Fuel Issues: H2 transportation and storage
Electrocatalyst Issues in PEM Fuel Cells
Anode Reaction: CH3OH + H2O Æ CO2 + 6H+ + 6eAnode Electrocatalysts: Pt; Pt/Ru
Issues: Cost of Pt/Ru; CO poisoning
Cathode Reaction: 6e- + 3/2 O2 + 3H2O Æ 6OHCathode Electrocatalysts: Pt; Pt alloys
Issues: Cost and low activity of Pt; stability of alloys
Reduction of Pt Contents in Electrocatalysts
Bulk Metal Prices in the United States in 9/2005
Price
($/kg)
Pt
W
29,200
1.10
Ti
V
Cr
24.00 45.20 1.36
Mn
Fe
Co
0.52
0.50
39.00
www.metalprices.com
Potential advantages of bimetallic and carbide catalysts:
z
Reduce cost
z
Enhance activity
Ni
13.82
Bimetallic and Carbide Catalysts
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zr
Nb
Mo
Tc
Ru
Rh
Pd
Ag
Hf
Ta
W
Re
Os
Ir
Pt
Au
Metal carbides
Pt-metal alloys
• Chemical/electronic properties are often tunable:
Alloying with carbon or with another metal
Research Approach: Combining
Model Surfaces, Theory and Electrocatalysis
Single Crystal
Model Surfaces
- UHV studies
- DFT modeling
Bridging
“Materials Gap”
- Thin films
- Supported catalyst
Bridging
“Pressure Gap”
-Electrochemical
evaluation
-
Objective: Do carbides possess the necessary
activity AND stability for PEM fuel cells?
OUTLINE OF PRESENTATION:
-
UHV Studies and DFT Modeling on Carbide Films
on Single Crystal Surfaces
-
Electrochemical Evaluation on Polycrystalline
Tungsten Carbide Thin Films
-
Synthesis of Tungsten Carbide Films on Carbon
Substrates (fiber, foam, cloth) Using PVD/CVD
-
Summary and Future Plans
Molecular Level Understanding
Using Advanced Techniques
Pt(111)
Single Crystals
•Well-defined single crystal surfaces
•Pchamber = 2 x 10-10 Torr
•Surface Spectroscopies:
–Elemental Composition (AES)
–Surface Order (LEED)
–Surface Structure (STM)
–Electronic Properties (NEXAFS;
Valence Spectroscopies)
–Gas Phase Products (Mass
Spectrometry)
–Surface Intermediates (Vib.
Spectroscopy)
DFT of Surface d-Band Center of Carbides
WC
W2C
Controlling properties of bimetallic and carbide surfaces:
Kitchin et al. Phys. Rev. Lett. 93 (2004) 156801
Kitchin et al. Catalysis Today 105 (2005) 66
Weigert et al. Topics in Catalysis 46 (2007) 349
Desirable/Prerequisite Properties
for DMFC Electrocatalysts
• Decompose CH3OH to produce H(ads)
• Decompose H2O to produce H(ads)
• Desorb CO at room temperature
Question:
Does tungsten carbide have these
properties?
Comparing C/W with Pt and Ru
CH3OH activity
per metal atom
H2O activity
per metal atom
CO desorption
(K)
C/W
(111)
O/C/W
(111)
Pt/CW
Pt
(111)
Ru
(0001)
0.28
0.24
0.18
<0.03
<0.03
0.18
0.10
0.06
330
284
330
Negligible
460
475
C/W, O/C/ and Pt/C/W are more active then Pt/Ru toward
dissociation of methanol and water
CO desorbs from tungsten carbides at lower temperatures than
Pt/Ru – more CO-tolerant
J. Phys. Chem. B 105, 10037 (2001) [CH3OH on C/W(111)]
J. Catal. 215, 254 (2003) [Pt/C/W(111)]
J. Phys. Chem. B 107, 2029 (2003) [C/W(110)]
J. Electrochem. Soc. 152 (2005) A1483 [Pt/C/W(110)]
OUTLINE OF PRESENTATION:
-
UHV Studies and DFT Modeling on Carbide Films
on Single Crystal Surfaces
-
Electrochemical Evaluation on Polycrystalline
Tungsten Carbide Thin Films
-
Synthesis of Tungsten Carbide Films on Carbon
Substrates (fiber, foam, cloth) Using PVD/CVD
-
Summary and Future Plans
O
C/W(111)
CH 3
CH 3
Synthesis of WC and W2C Films
O
W2C/WC
films
High S.A.
W2C/WC
powders
Electrochemical testing
Spectroscopic characterization
XRD Characterization of PVD Films
D:\86-122a1.udf Intensity
1400
101
1200
1000
Single phase W C
100
800
001
600
400
111
200
102
110
0
20
30
40
50
60
70
80
D:\86-132.udf Intensity
100000
Single phase W2C
C
10000
101
002
1000
102
C
103
100
110
112
201
100
10
1
20
30
40
50
Zellner & Chen, Surface Science, 569 (2004) 89
60
70
80
Desorption of CO from Polycrystalline WC
Polycrystalline
Surface
Pt
WC
~0.8ML Pt/WC
CO Desorption Temperature
(K)
Leading Edge
Peak
400
282
324
531
365
398
• TPD studies of adsorbed CH3OH – similar trend between
single crystal surfaces and polycrystalline films.
Electrochemical Testing of Carbide Films
In-Situ XPS and Electrochemistry
X-ray photoelectron
spectroscopy (XPS)
(1 x 10-8 Torr)
transfer chamber
(1 x 10-7 Torr)
Manipulator with
W-Foil working
electrode
gate valve
Pt metal
evaporation
source
gate valve
sputtering gun
three electrode
electrochemical cell (purged with N2)
In-Situ XPS and Electrochemistry
X-ray photoelectron
spectroscopy (XPS)
(1 x 10-8 Torr)
transfer chamber
(1 x 10-7 Torr)
Manipulator with
W-Foil working
electrode
gate valve
Pt metal
evaporation
source
gate valve
sputtering gun
three electrode
electrochemical cell (purged with N2)
Surface Preparation on Polycrystalline W
sputtering
gas:
C2H4
Pt-wire on
W Filament
Pt deposition
W Foil
∆
~1200 K WC Surface
∆
~600 K
Pt / WC Surface
• Decomposition of ethylene over hot filament
• Annealing by resistive heating to ~ 1200 K to form WC
• Analysis of surface composition using XPS
In-Situ XPS and Electrochemistry
X-ray photoelectron
spectroscopy (XPS)
(1 x 10-8 Torr)
transfer chamber
(1 Atm, N2)
Manipulator with
W-Foil working
electrode
gate valve
Pt metal
evaporation
source
gate valve
sputtering gun
three electrode
electrochemical cell (purged with N2)
X-ray photoelectron
spectroscopy (XPS)
(1 x 10-8 Torr)
transfer chamber
(1 Atm, N2)
Manipulator with
W-Foil working
electrode
•
•
•
gate valve
Pt metal
evaporation
source
gate valve
Deaerated room
temperature electrolyte
0.05 M H2SO4 (supporting)
0.2 M CH3OH
(fuel molecule)
sputtering gun
three electrode
electrochemical cell (purged with N2)
In-Situ XPS and Electrochemistry
X-ray photoelectron
spectroscopy (XPS)
(1 x 10-8 Torr)
transfer chamber
(1 x 10-7 Torr)
Manipulator with
W-Foil working
electrode
gate valve
Pt metal
evaporation
source
gate valve
sputtering gun
three electrode
electrochemical cell (purged with N2)
Cyclic Voltammetry (CV) of Pt, WC, Pt/WC
Conditions:
0.2 M CH3OH in
0.05 M H2SO4
Synergistic Effect:
Pt-modified WC
shows higher
activity and stability
for electro-oxidation
of methanol
Weigert et al. J. Phys. Chem. C, Letters, (2007)
Chronoamperometry (CA) of WC and
Pt/WC Electrocatalysts for Fuel Cells
• Steady-state current of
WC and 0.8 ML Pt/WC
higher than Pt
• Needs in-situ study
of active phases and
stability of WC under
fuel cell conditions
Weigert & Chen, J. Phys. Chem. C, Letters, 111 (2007) 14617
OUTLINE OF PRESENTATION:
-
UHV Studies and DFT Modeling on Carbide Films
on Single Crystal Surfaces
-
Electrochemical Evaluation on Polycrystalline
Tungsten Carbide Thin Films
-
Synthesis of Tungsten Carbide Films on Carbon
Substrates (fiber, foam, cloth) Using PVD/CVD
-
Summary and Future Plans
Phase-Pure WC: PVD, CVD and TPR
• PVD and CVD
synthesis of pure WC
on various carbon
substrates for fuel cell
testing
• Supported WC
particles produced by
temperature
programmed reaction
(TPR)
PVD and TPR: Weigert et al. J. Vac. Sci. Technol (2007)
CVD: Beadle et al. Thin Solid Films (2007)
SEM of Tungsten Carbide Films
Non-Reactive
450 °C
Non-Reactive
1040 °C
100 nm
Reactive
450 °C
Reactive
1040 °C
Fuel Cell Test Conditions
•
•
•
•
•
2M methanol solution as fuel feed: 4
mL/min
Air as oxidant: 500 sccm
Serpentine channel flow-field
Temperature range of (50 – 70 °C)
– Temperature of cell, fuel, and
oxidant streams
Emphasis on anode performance
– Stability
• Open circuit potential (OCV)
– Effects of:
• Fuel concentration
• Fuel flow rate
• Operating temperature
Pt/WC as Anode in Fuel Cells
•
Higher current density
possible at 2M
•
Improved current/power
densities with Temp.
•
Reaction-limited
behavior
– Linear behavior of
polarization curves
– Limitation of power
densities by catalyst
surface area
E.C. Weigert et al. J. New Materials Electrochem. Systems (submitted)
OUTLINE OF PRESENTATION:
-
UHV Studies and DFT Modeling on Carbide Films
on Single Crystal Surfaces
-
Electrochemical Evaluation on Polycrystalline
Tungsten Carbide Thin Films
-
Synthesis of Tungsten Carbide Films on Carbon
Substrates (fiber, foam, cloth) Using PVD/CVD
-
Summary and Future Plans
Disadvantages of H2 and Methanol
-
H2 Fuel Cells: CO-free H2; transportation; storage
-
Methanol Fuel Cells: toxicity; production of methanol
Potential Advantages of Oxygenates
CH3
O
H
CH2
O
H
CH3
methanol
ethanol
CH2
O
CH2
O
glycol
H
H
CH2
O
H
CH2
O
H
CH2
O
H
glycerol
Direct electrooxidation to e- and H+ without reforming reaction
CH3
O
H
CH2
O
H
CH3
CH2
O
H
CH2
O
H
CH2
O
H
CH2
O
H
CH2
O
H
Photoelectrochemical Cell (PEC):
An electrochemical cell containing at least one
photoelectrode through which it is able to convert light
energy into electrical and/or chemical energy
Load
e-
3 Main Components
1. Photoelectrode
OX
2. Counterelectrode
3. Electrolyte
RED
Counterelectrode Photoelectrode
(Cathode)
(Anode)
Carbides as potential counterelectrode in PEC
Electrolyte
Redox Species
Conclusions and Future Efforts
Case Study: Carbides as less expensive and
more active electrocatalysts
Single Crystal
Model Surfaces
- UHV studies
- DFT modeling
Bridging
“Materials Gap”
- Thin films
- Supported catalyst
Bridging
“Pressure Gap”
- Half-cell studies
- Full-cell studies
Other Research Efforts Related to Energy:
- Carbides as Alternative Anode Electrocatalysts
- Bimetallic Alloys as Alternative Cathode Electrocatalysts
- Fuel Cells Using Biomass-Derived Molecules (glycerol, glycol)
- Production of H2 from Reforming of Oxygenates
- Photoelectrochemical (PEC) for H2 Production from Water
UD Energy Institute: Assessment of Fuel Cells and Electrocatalysts
Basic Scientific
Challenges
UD Assets
•
Immediate Needs
•
Identification of
alternative
Now
electrocatalysts
•
Production and storage
of hydrogen
•
5-10
Years •
•
•
Long
Term
Combination of solar
devices with fuel cells
for H2 production from
H2O
Direct utilization of
biomass-derived
molecules as fuels
Systems Integration
Combination of solar
devices with fuel cells
for production of
chemicals from H2O
and CO2
•
•
Center for
Catalytic
Science and
Technology
UD Fuel Cell
Center
IEC
Energy Impact
& Implications
•
More efficient
and
environmentfriendly
utilization of
fuels
External Connections:
R&D Æ
Deployment
•
Fuel cell
companies in
Delaware and
in other States
UD Needs
•
•
New faculty
with expertise
in membrane
Team of faculty
interested fuel
cells and PEC